9781565760967 Backmatter.Pdf by Guest on 25 September 2021 EVOLUTION of FLUVIAL STYLES in the EOCENE WASATCH FORMATION, POWDER RIVER BASIN, WYOMING

Home , Crevasse splay, Meander scar, Overbank, Paleosurface, Sedimentary basin, Tamur River

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PETER D. WARWICK US.Geological Survey, Reston, Virginia 22092 AND ROMEO M. FLORES US. Geological Survey, Denver, Colorado 80225

ABSTRACT:Vertical and lateral facies changes in the lower part of the Eocene Wasatch Formation in the Powder River Basin, Wyoming represent an evolution of fluvial systems that varied from meandering to anastomosing. The meandering facies in the lower part of the study interval formed in a series of broad meanderbelts in a north-northwest-flowing system. Upon abandonment this meanderbelt facies served as a topographic high on which a raised or ombrotrophic Felix peat swamp developed. Peat accumulated until compaction permitted encroachment of crevasse splays from an adjoining transitional facies which consists of deposits of a slightly sinuous fluvial system. Crevasse splays eventually prograded over the peat swamp that was partly covered by lakes. Bifurcation, reunification, and transformation of crevasse channels into major conduits produced an anastomosing system that was characterized by diverging and converging channels separated by floodbasins drowned by lakes and partly covered swamps.

INTRODUCTION LITHOFACIES VARIATIONS

Fluvial environments for Tertiary coal-bearing strata in In decreasing order of abundance rocks in the study in- the Rocky Mountain region have been proposed by a num- terval comprise: 1) interbedded claystone, siltstone, and ber of workers in recent years (Jacob, 1972; Beaumont, limestone; 2) sandstone; 3) carbonaceous shale; and 4) 1979; Galloway, 1979; Ethridge and others, 1981). These limestone and ironstone lithofacies. The interval consists studies described facies for meandering trunk-tributary sys- mainly of sandstone lithofacies in the lower part with in- tems that perennially drained the coal basins. Although coal terbedded claystone, siltstone, and sandstone lithofacies in deposits are commonly associated with facies of a mean- the upper part. dering fluvial system, Flores (198 1), and Flores and Hanley (1984) suggested that they occur in both meandering and Sandstone lithofacies. - anastomosed systems in the Paleocene Fort Union Forma- Three types of sandstone lithofacies can be recognized tion of the Powder River Basin. The purpose of this paper on the basis of their morphology and internal sedimentary is to document the evolution of these fluvial systems and structures. Type 1 lithofacies consists of en echelon lenticu- to relate their facies to the occurrence of economic coal lar sandstone bodies that range from 40 to 90 ft (1 2 to 27 deposits. m) thick and from 2 to 9.5 mi (3.2 to 15.2 km) in lateral The study interval, which averages 250 ft (76 m) thick, extent (thickness to width ratio from 1:260 to 1557). These includes the Felix coal bed and associated rocks in the lower sandstones have an erosional base and contain multiple part of the Eocene Wasatch Formation exposed in a 400 scoured surfaces floored by granule-size lag conglomerate mi2 (1,036 km2) area in the Powder River Basin, Wyoming which often includes fragments of mudballs, and coal spars (Fig. 1). Facies analysis was determined from 146 mea- and bands. The lower part of each unit includes a lag de- sured sections, 56 geophysical logs, and 118 published coal posit that is overlain by coarse-grained, planar-bedded thickness data. Vertical profiles and stratigraphic cross sec- sandstone which merges upward into large trough and tab- tions were utilized to establish successions and relation- ular (sets greater than 3 fl or 0.9 m wide) cross-bedded sand- ships between facies types. Paleocurrent direction of sandstone. Epsilon cross-stratification or lateral accretion units stone channels was determined from trough crossbeds (average dip angle of 15") and slumped blocks are common measured at 84 localities. in the lower portion of the sandstone bodies. The upper part A generalized description and origin of the rock types of of the sandstone contains abundant small trough crossbeds the Wasatch Formation were provided by Love (1952), El- (sets less than 1 ft or 0.3 m wide), ripple laminations, and liott (1976), and Toomey (1977) in conjunction with ura- convolute laminations and grades upward into claystone, nium exploration. A more specific analysis of the fluvial siltstone, carbonaceous shale, and coal lithofacies. facies of the lowermost part of the Wasatch Formation was Type 2 sandstone lithofacies occurs as lenticular bodies conducted by Ethridge and others (1981, 1983) in relation that range from 10 to 80 ft (3 to 24 m) thick and 0.2 to to coal exploration. These commodity related studies of the 2.5 mi (0.3 to 4.0 km) in lateral extent (ratio from 1:105 Wasatch Formation are concentrated in small areas in the to 1 : 165). This lithotype exhibits sedimentologic features basin. Seeland (1 976) proposed a basinwide depositional that are similar to type 1 sandstones but differs in that in- setting of the Wasatch Formation as a trunk-tributary sys- dividual sequences are thinner with less epsilon cross-strat- tem. Flores and Warwick (1984) suggested the tributaries ification, and more abundant tabular and trough cross-strat- were fed by alluvial fans along the western basin margin. ification. These changes in the fluvial style of the trunk system are Type 3 sandstone lithofacies occurs as tabular bodies stressed in this paper. generally less than 20 ft (6 m) thick and about 1 mi (1.6

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44m

0 50K \ o SM MEASURED SECTION O GEOPHYSICAL LOG O- SK FIG. 1.-Maps showing the localities of the Powder River Basin, Wyoming and study area in the basin. Rectangles 1, 2, and 3 represent areas of cross section shown in Figures 2, 3, and 4.

km) in lateral extent. This sandstone type, which varies from Limestone and ironstone lithofacies.- fine- to medium-grained , commonly caps a coarsening-upward sequence and is usually underlain by interbedded silt- The limestone lithofacies is a silty to mud-rich carbonate rock that usually is light gray. These rocks occur in tabular stone and shale. The sandstone consists of abundant ripple bodies that are generally less than 3 ft (0.9 m) thick and laminations , although small-scale trough and tabular can be traced laterally for hundreds of feet. The beds are crossbeds are locally common. Laterally, the sandstone type grades into interbedded siltstone, claystone, and limestone commonly massive, but some display ripple laminations and burrows. The ironstone lithofacies occurs in claystone as containing fossils of freshwater mollusks. reddish brown discontinuous lenses a few inches thick. The Siltstone lithofacies.- facies contains plant fragments and occasionally grades into or replaces limestone. The siltstone lithofacies is commonly dark to light gray and ranges from a few inches to 30 ft (9 m) in thickness. Carbonaceous shale and coal lithofacies.- This lithofacies displays gradational to sharp contacts with underlying and overlying deposits. The siltstone exhibits Carbonaceous shale lithofacies is a dark gray to black some flat bedding and usually contains burrows and ripple laminated claystone which contains abundant macerated plant laminations. This lithofacies contains common whole and fragments and ranges from a few inches to as much as 3 ft fragmented freshwater gastropod and bivalve fossils. It is (0.9 m) in thickness. This lithofacies is interbedded with rooted where overlain by carbonaceous shale and coal litho- claystone, siltstone , sandstone and coal lithofacies . Car- facies. bonaceous shale , however , is commonly laterally transitional into coal beds. Claystone lithofacies.- The coal lithofacies is commonly less than 3 ft (0.9 m) The claystone lithofacies ranges from black to light gray thick and can be traced for only a few miles. The thickest and comprises units which rarely exceed 30 ft (9 m) in coal lithofacies in the study interval is the Felix bed, which thickness. In some localities the claystone contains grada- is as much as 40 ft (12 m) thick. It is uniformly thick (>lo tional or sharp upward transitions into silstone and sand- ft or 3.0 m) over a 2,100 mi2 (3379.5 km2) area. The coals stone, and in other locations it underlies coal beds and car- range from lignite to sub-bituminous with the Felix bed rep- bonaceous shale as rooted seatrock. Clay stones commonly resenting the highest grade. Petrographic analysis of mac- overlie carbonaceous shale and coal beds. These units com- erals in the Felix bed indicate a direct relationship between monly contain siderite nodules and freshwater mollusk fos- huminite and inertinite macera1 contents of the lower and sils as well as thin laminations marked by plant fragments. upper parts of the bed. Huminite is abundant (92-99 per-

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cent) in the lower part of the bed and decreases in the upper nels. The claystone, siltstone, limestone, and ironstone part where inertinite is more common (as much as 35 per- lithofacies that grade vertically and laterally into the chan- cent). The huminite at the base of the coal bed represents nel sandstones represent overbank floodplain deposits ad- woody plant remains, and the increased inertinite near the jacent to the streams. These sediments include overbank top of the bed reflects oxidized plant material that was pre- levee deposits of interbedded rooted claystone, siltstone, served in a huminite groundmass with other macerals. and sandstone (type 3) lithofacies that immediately flanked the meanderbelt facies. The levee deposits grade into coars- FACIES RELATIONSHIPS ening-upward crevasse splay deposits of claystone, siltstone, and sandstone (type 3) lithofacies toward the flood- The vertical and lateral variations of the lithofacies in the plain. These floodplain deposits are commonly interbedded study interval are shown in Figures 2, 3, and 4, and rep- with carbonaceous shale and coal lithofacies that formed in resent cross sections across the length and width of the study adjacent backswamps. Thus, the lower part of the study area. The study interval in the cross sections shows a gen- interval is dominantly characterized by meanderbelt chan- eral fining-upward sequence. The lower part of the interval, nel facies with subordinate floodplain-backswamp facies. below the Felix bed consists of abundant (>50 percent) type The middle part of the interval is dominated by type 2 1 sandstone lithofacies with subordinate amounts of clay- sandstone and coal lithofacies (Figs. 1, 3, 4). The type 2 stone, siltstone, limestone, and ironstone lithofacies (Fig. sandstone lithofacies is laterally juxtaposed against the Fe- 2). The type 1 sandstone lithofacies are channel deposits of lix bed, which overlies the meanderbelt facies. The pres- a meandering stream as indicated by the en echelon ar- ence of some lateral accretion units in this sandstone type rangement, lateral accretion units, and slumped block de- indicate that it represents a channel deposit of a locally sin- posits. The extensive vertical and lateral distribution of the uous stream. This fluvial system was coeval with the Felix sandstone lithofacies also indicates deposition in a broad peat swamp which formed on abandoned deposits of the meanderbelt system. Multiple scour surfaces within the meandering stream systems. The Felix peat swamp was sandstone bodies suggest recurrence of cut and fill in the probably an ombrotrophic or raised deposit, as is indicated meanderbelt resulting from reoccupation by fluvial chan- by the decrease in huminite toward the top of the coal bed

SOUTH NORTH II Ill IiIiiiIIil I Ill II Ill1 I I I I I I

COAL AND CARBONACEOUS LIMESTONE SHALE a TYPE 1 SANDSTONE * MOLLUSK FOSSILS 1-1 1-1 TYPE 2 SANDSTONE xx IRONSTONE 0 TYPE 3 SANDSTONE bV CLINKER SILTSTONE AND SHALE I STRATIGRAPHIC SECTION FIG. 2.-A north-south cross section showing environmental-stratigraphic framework of the study interval along the Powder River. Note that the Felix bed splits into a number of beds in the southern area. Vertical exaggeration = 150X.

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(Warwick, 1985). Thus, the middle part of the study in- sediments. Thin coal and carbonaceous shales representing terval is dominated by channel-backswamp facies that rep- backswamp deposits are interbedded with the floodplain de- resent a transition between the underlying meanderbelt- posits. The floodplain-backswamp facies pass laterally into dominated facies and the overlying anastomosed fluvial fa- type 2 sandstones bounded by rooted, fine-grained levee cies. sediments. Type 2 sandstones are both thick and narrow The upper part of the study interval, above the Felix bed, and represent channel deposits of small, locally sinuous consists of abundant floodplain deposits of claystone, silt- streams, an interpretation also indicated by the general ab- stone, sandstone (type 3), limestone, and ironstone litho- sence of lateral accretion units. The presence of lateral ac- facies (Figs. 2, 3, 4). The floodplain deposits are mainly cretion units is only a function of preservation, however, coarsening-upward sequences of crevasse splays, which and is not a good indicator of channel sinuosity. In addi- contain abundant freshwater mollusk fossils. The fossils tion, this sandstone type as shown in Figures 2, 3, and 4 suggest debouching of crevasse splays into ponded waters seems to comprise contemporaneous bodies that are ar- or lakes on the floodplains. Widespread type 3 sandstone ranged in patterns very similar to those of modern anasto- directly above the Felix bed, as seen on Figure 4, indicates mosed streams, described by Smith (1983). Thus, the areal lakes formed in former sites of peat deposition probably distribution of coeval, small, vertically aggrading, slightly due to peat compaction. These lakes filled with crevasse sinuous channels separated by small floodplains drowned

NORTHWEST SOUTHEAST II II I I IIIIIIIIIIII I I I II I II I

COAL AND CARBONACEOUS LIMESTONE SHALE a

.::...o .::...o .e:::: 17.e:::: ..... 2 TYPE 1 SANDSTONE * MOLLUSK FOSSILS 1-j TYPE 2 SANDSTONE xx IRONSTONE

I]TYPE 3 SANDSTONE bv CLINKER SiLTSTONE AND SHALE 1 STRATIGRAPHIC SECTION

FIG. 3. -A northwest-southeast cross section showing environmental-stratigraphicframework of the study interval along the Wild Horse Creek. Note that the thickest part of the Felix bed is underlain by type 1 sandstone. Vertical exaggeration = 75x.

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WEST EAST I I I II II II I I 1 I 11

......

COAL AND CARBONACEOUS LIMESTONE SHALE .a:::: f3.a:::: :: TYPE 1 SANDSTONE * MOLLUSK FOSSILS [-I TYPE 2 SANDSTONE xx IRONSTONE

[-] TYPE 3 SANDSTONE LV CLINKER

- __ SILTSTONE AND SHALE I STRATIGRAPHIC SECTION FIG. 4.-A west-east cross section showing environmental-stratigraphic framework of the study interval along the North Prong Wild Horse Creek. Note the widespread type 3 sandstone above the Felix bed. Vertical exaggeration = 150X.

by lakes suggests deposition in an anastomosed belt of the Figure 5 is an isolith map of sandstones in the meander- alluvial plain. belt facies, probably representing a north-flowing trunk in a meandering stream system. The map shows a broad belt SUMMARY OF EVOLUTION OF FLUVIAL STYLES in which areas of sandstone concentration are found to the The vertical succession and lateral relationship of facies east and west of the study area. Based on the 25 ft (7.5 m) documented in this study suggest deposition in a continuum and greater isolines, the sandstones are elongate bodies ori- of fluvial environments. The vertical sequence of mean- ented in north-south and northwest-southeast directions. dering and anastomosed stream deposits from the lower to These bodies are interpreted as channel deposits of a mean- upper parts of the study interval, respectively, can be in- dering stream which may or may not be contemporaneous terpreted according to Walther's Law of Facies (1894) as as indicated by the en echelon arrangement of the type 1 products of laterally juxtaposed environments. The pres- channel sandstones. Thus, it could be presumed that these ence of local erosional breaks below channel sandstones in bodies are offset in which one body was being formed while the vertical succession indicates removal of parts of the flu- the other was being abandoned. The en echelon deposi- vial record and represents short periods of time. These sur- tional pattern as seen resulted from lateral shifts of stream faces indicate erosion that occurred during the migration of channels into adjoining low-lying areas in the alluvial plain, environments. which resulted from differential compaction of fluvial sed-

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1 1 101. N

44. 30'

44.15'

\ ...... "'O'...... MEASURED SECTION 10 - 24 Ft ...... :3 +GEOPHYSICAL LOG [YLJ 25 - 50 Ft ...... kZ-3: a>25 Ft SANDSTONE O O O Ow 5 Mi 108. 15' O 5 Km 105.45' FIG. 5.-Isolith map of sandstone lithofacies below the Felix bed rep- I 3> resenting the meanderbelt-dominated facies. Rose diagram indicates pa- FIG. 6.-Isolith map of the Felix bed and laterally equivalent channel leocurrent directions. N = number of observations. sandstone of the transition facies between the meanderbelt and anastomosed-belt facies. Stippled pattern indicates area where the coal bed is iments in a process similar to that proposed by Ferm and split by sandstone. Cavaroc (1969). Walker and Cant (1979) suggested that the en echelon pattern of channel sandstones is common for deposits of meandering streams. Crossbed measurements from channel sandstones indicate the direction of flow was toward the north. The trunk meandering stream consisted of a meanderbelt of both active and abandoned channels. The meanderbelt shifted its position on the alluvial plain through time by floods breaching the levee and diverting flow to create an avulsion route. The avulsion is followed by development of a new meanderbelt and abandonment of the old meanderbelt. The abandoned meanderbelts eventually became a topographic high on which a poorly drained backswamp formed. This is indicated by the accumulation of the Felix bed, which is uniformly thick above the meanderbelt facies (Fig. 6). As noted by Ferm and Staub (1984) in Appala- chian coal beds, areas where the Felix bed substrate is sandstone and relatively uncompactible, the swamp surface was topographically high free of detrital influx. This condition, in addition to the development of a raised or domed (ombrotrophic) Felix peat swamp, provided an ideal setting for organic accumulation. The thicker parts of the Felix bed represent peat deposition on a raised platform in compari-

FIG. 7.-Isolith map of the sandstone lithofacies above the Felix bed representing the anastomosed-belt facies. Rose diagram indicates paleocurrent directions. N = number of observations.

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son to adjacent areas where the coal bed is thin and split minated by drowning. The lakes were gradually filled with (Fig. 6). Where the detritus underlying the Felix bed is fine- sediments of crevasse splays, and the associated feeder grained and readily compactible, the resulting low-lying areas channels served as avulsion routes and major conduits. These received water-borne clastic sediments that interrupted peat channels bifurcated and rejoined as they prograded to form accumulation. The low-lying swamps to the west of the raised as an anastomosed pattern. Thus, the trunk stream during Felix peat dome seem to have controlled aggradation of a this period was transformed into a multichannel system of north-flowing, slightly sinuous trunk stream, as indicated an anastomosed stream, as indicated by an isolith map of by the sandstone body lateral to the Felix bed (Fig. 6). The the sandstones above the Felix bed (Fig. 7). The areal dis- north-trending trunk stream flowed along the western edge tribution of the sandstones greater than 25 ft (7.6 m) thick of the swamp and was responsible for the influx of sedi- shows merging and diverging bodies similar to channel de- ments that produce splits in the Felix bed. posits of modern anastomosed streams described by Smith A combination of vertical stream aggradation and sub- (1983) for the Columbia River in western Canada, by Flo- sidence as well as compaction of the peat allowed the chan- res and Hanley (1984) for Paleocene coal-bearing deposits nel to flow into the adjoining swamp. In areas where the in the Powder River Basin in Wyoming, and by Rust and peat swamp subsided below the water table, lakes formed others (1984) for Carboniferous coal-bearing deposits in Nova and the accumulation of peat in the Felix swamp was ter- Scotia, Canada.

:ies

Aiiuvial Fan Meander Belt Facies FIG. 8.-Depositional models showing the evolution of fluvial styles of the north-south trunk system varying from meandering (A) to anastomosed (C) stream. An intermediate stage (B) reflects a period of accumulation of thick peat on abandoned meanderbelt deposits. The western part of the alluvial basin plain received detritus from alluvial fans draining the ancestral Big Horn Mountains.

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The paleogeographic succession and evolution of fluvial tions: Society of Economic Paleontologists and Mineralogists Special systems of the study interval are summarized in Figure 8. Publication 31, p. 169-190. The transition (Fig. 8B) of the trunk system from the mean- , AND HANLEY,J. H., 1984, Anastomosed fluvial deposits: Upper Tongue River Member, Paleocene Fort Union Formation, northern dering stream (Fig. 8A) to the anastomosed stream (Fig. Powder River Basin, Wyoming, in Rahmani, R. A., and Flores, R. 8C) is similar to the avulsion-controlled process described M., eds., Sedimentology of Coal and Coal-bearing Sequences: Inter- by Smith and Cross (1985) for the lower Saskatchewan River. national Association of Sedimentologists Special Publication 7, p. 85- The transition period, which followed the abandonment of 104. , AND WARWICK,P. D., 1984, Dynamics of coal deposition in meanderbelts, provided a stage for accumulation of thick intermontane alluvial paleoenvironments, Eocene Wasatch Formation, peat. The slightly sinuous stream along the west edge of Powder River Basin, Wyoming, in Houghton, R. L., and Clausen, E. the Felix peat swamp probably represents deposition during N., eds., 1984 Proceedings of the Symposium on the Geology of Rocky a transition period between development of meandering and Mountain Coal: North Dakota Geological Society Special Publication anastomosed streams. Anastomosis may have been con- 84-1, p. 184-199. GALLOWAY,W. E., 1979, Early Tertiary-Wyoming intermontane basins, trolled by lowering of base level by compaction of sedi- in Galloway, W. E., Krietler, C. W., and McGowen, J. H., eds., ments and/or basin subsidence. Depositional and Ground-water Flow Systems in the Exploration for Uranium: Bureau of Economic Geology, The University of Texas at ACKNOWLEDGMENTS Austin, A Research Colloquium, p. 197-213. JACOB,A. F., 1972, Depositional environments of parts of the Tongue Portions of this work were derived from a Ph.D. disser- River Formation, western North Dakota, in Ting, F. T. C., ed., De- tation completed by P. D. Warwick at the University of positional Environments of the Lignite-Bearing Strata in Western North Kentucky under direction of J. C. Ferm. Financial support Dakota: North Dakota Geological Survey Miscellaneous Series 50, p. 43-62. was provided in part by the U.S. Geological Survey, Wy- LOVE,J. P., 1952, Preliminary report on the uranium deposits in the oming Geological Survey, Geological Society of America, Pumpkin Buttes area, Powder River Basin, Wyoming: U.S. Geological and the University of Kentucky Department of Geology. Survey Circular 176, p. 37. RUST,B. R., GIBLING,M. R., AND LEGUN,A. S., 1984, Coal deposition REFERENCES in an anastomosing-fluvialsystem: The Pennsylvanian Cumberland Group south of Joggins, Nova Scotia, Canada, in Rahmani, R. A., and Flo- BEAUMONT,E. A., 1979, Depositional environments of Fort Union sed- res, R. M., eds., Sedimentology of Coal and Coal-bearing Sequences: iments (Tertiary, northwest Colorado) and their relation to coal: Amer- International Association of Sedimentologists Special Publication 7, ican Association of Petroleum Geologists Bulletin, v. 63, p. 194-217. p. 105-120. ELLIOT,E. S., 1976, Environment of deposition of the Wasatch Forma- SEELAND,D. A., 1976, Relationships between early Tertiary sedimen- tion, Powder River Basin, Wyoming: Unpublished M.S. Thesis, Wright tation patterns and uranium mineralization in the Powder River Basin: State University, Dayton, Ohio, 94 p. Wyoming Geological Association 28th Field Conference Guidebook, ETHRIDGE,F. G., BURNS,L. K., ALEXANDER,W. G., CRAIG,G. N. 11, p. 221-230. AND YOUNGBERG,A. D., 1983, Overburden characterization and post- SMITH,D. G., 1983, Anastomosed fluvial deposits: Modem examples bum study of the Hoecreek, Wyoming underground coal gasification from western Canada, in Collinson, J. D., and Lewin, J., eds., Modem site and comparison with the Hanna, Wyoming site: U.S. Department and Ancient Fluvial Systems: International Association of Sedimen- of Energy Contract Report DE-AS20-8 1LC10705, 185 p. tologists Special Publication 6, p. 155-168. , JACKSON,T. J., AND YOUNGBERG,A. D., 1981, Flood basin se- SMITH,N. D., AND CROSS,T. A., 1985, Avulsion-controlled fluvial evo- quence of a fine-grained meanderbelt subsystem: The coal-bearing lower lution: The Cumberland Marshes, east-central Saskatchewan (abs.): Third Wasatch and upper Fort Union Formations, Powder River Basin, Wy- International Fluvial Sedimentology Conference Abstracts volume, oming, in Ethridge, F. G., and Flores, R. M., eds., Recent and An- p. 36. cient Nonmarine Depositional Environments: Models for Exploration: TOOMEY,W. J., 1977, A stratigraphic analysis of the Wasatch Formation, Society of Economic Paleontologists Special Publication 3 1, western Powder River Basin: Unpublished M.S. Thesis, Wright State p. 191-289. University, Dayton, Ohio, 96 p. FERM,J. C., AND CAVAROC,V. V., JR., 1969, A field guide to Allegheny WALKER,R. G., AND CANT,D. J., 1979, Facies models 3, sandy fluvial deltaic deposits in the upper Ohio Valley: Ohio Geological Society and systems, in Walker, R. G., ed., Facies Models: Geological Association Pittsburgh Geological Society, 27 p. of Canada Reprint Series 1, p. 23-3 1. , AND STAUB,J. R., 1984, Depositional controls of minable coal WALTHER,J., 1894, Lithogenesis der Gegenwart, Beobachtungun uber bodies, in Rahmani, R. A., and Flores, R. M., eds., Sedimentology die Bildung der Gesteine ander heutigen Erdoberflache: Dritter Teil of Coal and Coal-bearing Sequences: International Association of einer Einleitung in die Geologie als historische Wissenchaft, Jea: Ver- Sedimentologists Special Publication 7, p. 257-289. lag Gustav Fischer, p. 535-1055. FLORES,R. M., 1981, Coal deposition in fluvial paleoenvironments of WARWICK,P. D., 1985, Depositional environments and petrology of the the Paleocene Tongue River area, Powder River Basin, Wyoming and Felix coal interval (Eocene), Wasatch Formation, Powder River Basin, Montana, in Ethridge, F. G., and Flores, R. M., eds., Recent and Wyoming: Unpublished Ph. D. Dissertation, University of Kentucky, Ancient Nonmarine Depositional Environments: Models for Explora- 333 p.

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ROMEO M. FLORES AND CHARLES L. PILLMORE US.Geological Survey Denver, Colorado 80225

ABSTRACT:The Raton basin is bounded on the west by the ancestral San Luis highlands. The highlands were probably uplifted along a high-angle reverse fault zone marginal to the basin. Coarsening-upward megacycles and types of facies sequences and facies associations within the Upper Cretaceous Vermejo Formation and Upper Cretaceous and Tertiary Raton and Poison Canyon Formations indicate that both allocyclic and autocyclic mechanisms influenced alluvial sedimentation in the Raton basin. The megacycles comprise a coal-rich, fine detritus-dominated facies and an overlying coal-poor, sand-dominated facies. The coal-rich, fine detritus-dominated facies was deposited by braided, meandering, and anastomosed fluvial systems during periods of increased basin subsidence associated with a stable source area. The sand-dominated facies was deposited by alluvial fans and by braided and high-bedload meandering fluvial systems during periods of tectonic uplift in the source area and reduced basin subsidence. The basin alluvial paleoarchitecture (spatial arrangement, patterns, interconnection, distribution, and evolution of the fluvial systems) was mainly controlled by allocyclic mechanisms of uplift in the source area and subsidence in the basin. Climatic change and the presence of vegetation played only minor roles in controlling the alluvial paleoarchitecture of the Raton basin. In this alluvial environment, basin subsidence (an allocyclic mechanism) and fluvial avulsion and abandonment (autocyclic mechanisms) served as the major controlling factors in the accumulation of thick coal beds.

INTRODUCTION crop diagrams. The cross sections show the lateral continuity of major clastic wedges of the SD facies and their One of the most important controls on siliciclastic sedi- vertical and lateral relationships with the FDD facies. The mentation is tectonism, either the uplift of source areas or outcrop diagrams provide information on stratigraphic-en- basin subsidence, or both. The response of sedimentary vironmental facies variations at individual outcrop loca- processes to tectonism is commonly represented by down- tions. These diagrams also record the successions of en- slope changes in the patterns of alluvial systems (McLean vironmental changes, as well as variation of temporally and Jerzykiewicz, 1978; Steel and Aasheim, 1978). An ex- equivalent depositional settings, for both the SD and FDD ample of this response is the arrangement of vertical sed- facies. imentary successions expressed as coarsening-upward and fining-upward sequences (Steel and others, 1977; Heward, Basinal Facies Variations 1978). Three coarsening-upward megacycles occur in the Facies variations from west to east across the basin are Upper Cretaceous Vermejo and Upper Cretaceous and Pa- leocene Raton and Poison Canyon Formations in the Raton shown in the cross sections in Figures 1 and 2. Both cross sections show three clastic wedges of the SD facies, which basin of Colorado and New Mexico (Fig. 1). Each megacycle is made up of a lower, coal-rich, fine detritus-dom- thin eastward and coarsen upward. Figure 1 displays the inated (FDD) facies and an upper, coal-poor, sand-domi- lateral variation of three coarsening-upward megacycles in nated (SD) facies. The megacycles are as much as 760 ft the western part of the basin. The first coarsening-upward megacycle, as much as 300 ft (90 m) thick, includes the (230 m) thick and generally thicken westward toward the ancestral San Luis highlands (Pillmore and Flores, 1984). FDD facies of the Vermejo Formation and the overlying SD facies of the basal Raton Formation (Lee, 1917). In the Westward-thickening clastic wedges of the SD facies are traceable across the width of the basin. western part of the basin, the basal Raton Formation is a conglomerate bed resting unconformably on the FDD facies The purpose of this paper is to analyze the facies sequences and associations of the Vermejo, Raton, and Poi- of the Vermejo Formation. Where present, the basal Raton son Canyon megacycles to determine the controls of sedi- conglomerate consists of quartzose sandstone with chert pebbles derived in large part from the Upper Cretaceous mentation. Repetition of the coarsening-upward megacycles Dakota Sandstone (Flores, 1984). The conglomerate re- may reflect cyclic sedimentation common to an alluvial plain, as recognized by Beerbower (1964). Analysis of a facies flects the initial uplift of the source area, an allocyclic within the megacycles will help establish the mechanism of mechanism. The same relationship between the Vermejo cyclicity. Cyclic sedimentation resulting from changes in Formation and the basal Raton is observed in other parts energy supply or seidment input into a sedimentary system, of the basin; however, in some areas, especially near Ra- which may be caused by tectonic, sea level, or climatic ton, New Mexico, and Trinidad, Colorado, the conglom- changes, is called allocyclic (Beerbower, 1964). Cyclic erate is not present. In these areas, the lower or first mega- sedimentation that results from redistribution of the total cycle consists of the Vermejo FDD facies passing into and interfingering with channel and interchannel deposits of the energy and sediment input into a sedimentary system, caused by channel avulsion, bar migration, and crevassing, is called Raton SD facies. The second coarsening-upward megacy- autocyclic (Beerbower, 1964). cle overlies the SD facies of the basal Raton Formation, is as much as 350 ft (100 m) thick, and consists of the FDD STRATIGRAPHIC-DEPOSITIONAL FACIES VARIATIONS facies overlain by an SD facies. The SD facies of this megacycle, described by Pillmore and Flores (1984), was Stratigraphic facies variations are best displayed by gen- identified by Lee (19 17) as a barren zone consisting mostly eralized cross sections across the basin and by detailed out- of sandstones and a few thin minor coal beds. The Creta-

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I 1

-ElN.MEX. INDEX MAP

p , 5,üOMI O 50KM

DOMINATED FACIES

MINATED FACIES

FIG. 1.-Diagrammatic cross section of the western part of the basin, displaying coarsening-upward megacycles (1-3) that contain clastic wedges (coal-poor, sand-dominated facies) thickening to the west. Location shown on the inset map.

ceous-Tertiary boundary clay, a chronostratigraphic marker Flores, 1984) and is three times as thick as the first and (Pillmore and others, 1984; Pillmore and Flores, 1987), second megacycles. This may suggest more protracted ba- is located either at or just below the base of the barren zone. sin subsidence and fdling than during deposition of the FDD The third coarsening-upward megacycle, as much as 1,000 facies of the two older megacycles. The basin-axis thick- ft (300 m) thick, comprises the FDD facies above the bar- ening of the FDD facies in all three megacycles may be a ren zone of the Raton Formation upward through the SD response to crustal downwarping. facies in the overlying Poison Canyon Formation. This megacycle differs from the lower two megacycles in that it Facies Sequences and Associations contains a coal-rich FDD facies three times as thick as the other megacycles. The facies sequences (vertical successions of facies) and The sand-dominated facies of the three megacycles prob- associations (assemblages of facies occurring together as ably represents deposits formed during recurring allocyclic genetically related units) of the Vermejo, Raton, and Poi- episodes (tectonic) with reduced basin subsidence. These son Canyon Formations are best analyzed from outcrops episodes accompanied uplift, unroofing, and erosion of the along the westem basin margin, as well as within the basin granitic core rocks in the source area. Marked thickening proper. The western basin margin includes lithofacies prox- of the sand-dominated facies to the west suggests a source imal to the source area, and the basin proper consists of in that direction. Recurrent uplifts of the source area were lithofacies distal to the source area. Photographs and dia- probably caused by repeated movement along an inferred, grams of outcrops are used to illustrate the detailed facies ancient, high-angle, reverse fault zone at the western mar- sequences and associations and the influence of the cyclic gin of the basin. A younger reverse fault zone to the east mechanisms. The stratigraphic position and basinal distri- has been recognized by Ouellette and Boucher (1983) along bution of these facies sequences are shown in Figures 1 the northwestern margin of the basin. Uplift along the fault and 2. zone in the source area is reflected in the change from fine Basin-margin lithofacies.- to coarse detritus. The FDD facies of the megacycles probably represents The basin-margin facies sequences and facies associa- deposition during subsidence induced by differential com- tions of the second coarsening-upward megacycle in the paction and related isostatic adjustment that resulted from Raton and Poison Canyon Formations are shown in Figures sediment loading. Basin subsidence is probably indicated 3 and 4. Figure 3 shows a sequence of lenticular sandstones by thickening of the FDD facies upward and along the basin interbedded with fine-grained sediments of the Raton For- axis (Fig. 2). The FDD facies of the third megacycle con- mation overlain by sheetlike sandstones of the Poison Can- tains coal beds as much as 12 ft (4 m) thick (Pillmore and yon Formation. The lenticular sandstones have erosional

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I I I c 'D BASIN AXIS

300 COAL-RICH, FINE-DETRITUS DOMINATED FACIES

COARSENING-UPWARD .c:;.. v >.:- :::..:.+.>.:- .: ...... MEGACYCLE

MINATED FACIES

FINE-DETRITUS DOMlN ATED

VERMEJO FORMATION

PIERRE SHALE

;IG. 2.-Diagrammatic cross section showing the extent of the coarsening-upward megacycles (1-3) across the width of the basin. Location shown on Figure 1.

bases, fine upward, contain mud-draped lateral accretion ity of these tributary systems at the basin margin may be a surfaces, and are locally channeled with mudstone and silt- function of availability of sediment load from upstream. For stone infills. The sandstones contain trough crossbeds and instance, the presence of alluvial fans upstream may pro- convolute laminations in the lower part and ripple lami- vide high bedload sediments locally to the braided streams nations in the upper part. In addition, the lenticular sand- during flash floods. The close proximity of the two fluvial stones are arranged en echelon. Facies associations (Fig. 3) systems indicates that neither basin subsidence nor tectonic are displayed by the lenticular sandstones passing laterally uplift (allocyclic mechanisms) greatly influenced the over- and vertically into the sheetlike sandstones. The sheetlike all proportion of coarse-to-fine sediments that accumulated. sandstones are multiscoured, floored by granule- to gravel- Tectonic uplift (allocyclic mechanism), however, may have size particles, and infilled by coarse sandstones that include controlled widespread development of braided streams along random successions of trough and planar crossbeds. Both the basin margin. The coarse detrital influxes (sand-domi- lenticular and sheetlike sandstones laterally merge with fine- nated) pervasive along the basin margin may have had their grained sediments and coaly carbonaceous shale. point sources in mountain-front alluvial fans generated by The lenticular sandstones are interpreted as deposits of a uplift along reverse faults. meandering fluvial system, whereas the sheetlike sand- Figure 4 shows the basin-margin lithofacies of the Poison stones were probably deposited by a braided fluvial system. Canyon and Raton Formations, representing a stratigraph- The vertical succession of facies sequences suggests that ically higher part of the second coarsening-upward mega- the meandering streams gave way to braided streams through cycle. This lithofacies consists of sheetlike, coarse-grained, time. Yet, the lateral facies associations suggest that the arkosic sandstones interbedded with conglomerates and silt- meandering and braided streams may have coexisted within stones. The sheetlike sandstones contain trough and planar a 2- to 3-mi-wide (3.2- to 4.8-km) belt. The contemporane- crossbeds, are multiscoured, and are floored by lag gravels.

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FIG. 3.-Lithofacies of the Raton and Poison Canyon Formations in the basin margin, consisting of channel, channel piug (CP), and overbank deposits of meandering and braided streams. The diagram of the outcrop was constructed from the measured section shown.

FIG. 4.-Photograph and outcrop diagram of the Raton and Poison Canyon Formations along the basin margin, comprising braided stream deposits. The diagram was constructed from the measured section shown.

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The conglomerates have basal erosional surfaces, are crudely ening-upward megacycle) are shown in Figures 5 and 6. horizontally bedded, are imbricated, and fine upward. The Figure 5 exhibits facies sequences that include a multi- conglomerates consist of framework-supported gravel, and scoured, erosional-based, lenticular sandstone laterally jux- gravel-size particles may be present in associated trough taposed with a tilted sequence of sandstone, siltstone, and crossbedded sandstones. The conglomerates make up 20- mudstone that is marked by a sharp, curvilinear basal con- 50 percent of the succession and occur more frequently in tact. These facies sequences are underlain and overlain by the upper part of the succession. The sandstones and con- coal beds as much as 7 ft (2 m) thick. The lenticular sand- glomerates grade upward into siltstones that contain very stones display lateral accretion surfaces delineated by mud- rare carbonaceous detritus. stone and siltstone drapes. Internally, these lenticular sand- The facies succession of sandstones and conglomerates stones contain large-scale trough crossbeds with convolute is interpreted as deposits of proximal braided streams. The laminations in the lower part and small-scale trough crossbeds crudely horizontally bedded, framework-supported, imbri- with ripple laminations in the upper part. The lenticular cated conglomerates represent longitudinal bar deposits. Such sandstones are associated with erosional-based deposits of bars are deposited as coarse bedload in response to low- mudstone, siltstone, and silty sandstone. ering of flood stages (Leopold and Wolman, 1957). Gravel The facies sequences and associations suggest deposition imbrication indicates deposition during high flow, and as- in a meanderbelt of a high-sinuosity meandering stream. sociated silty sandstone was probably deposited during The lenticular sandstones indicate that this meandering stream waning stages of flow. Reworking of the longitudinal bars was formed by slow, lateral accretion of point bars. Avul- during lower flood stages is indicated by associated trough- sion and abandonment (autocyclic mechanism) of the crossbedded sandstones. Paucity of carbonaceous detritus meandering stream are indicated by coal beds that overlie in associated fine-grained sediments suggests that the prox- the lenticular sandstones. Oversteepening of meander cut- imal braid belt was composed mainly of active braid tracts banks produced slumped and rotated overbank deposits of with a few vegetated passive tracts. This proximal braided- mudstone, siltstone, and silty sandstone. The margins of stream model resembles the gravelly braid belt of the Don- the meanderbelt are often defined by channel plugs of fine- jek River in Canada (Williams and rust, 1969). grained detritus. The abandoned meanderbelt deposit formed by an autocyclic mechanism may have served as a platform Basin-proper lithofacies. - on which swamps formed and peat accumulated, undis- The basin proper lithofacies consists of coal-bearing and turbed by detrital influx and vegetation sustained by ground coal-barren deposits, the former deposit being the most water discharged through underlying permeable and trans- common. The sequences and associations of the FDD fa- missive channel sands. cies of the Raton Formation (lower part of the third coars- Figure 6 shows facies sequences and association in the

FIG. 5.-Photograph and outcrop diagram of the Raton Formation in the basin proper, containing channel and slumped cutbank (SC) deposits of meandering streams. The diagram was constructed from the measured section shown.

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FIG. 6.-Photograph and outcrop diagram of the Raton Formation in the basin proper, consisting of a succession of channel and crevasse-splay deposits of anastomosed streams. The diagram was constructed from the measured section shown.

coal-bearing Ration Formation which are different from those mosed streams described by Clough (1983) and Smith and of the meanderbelt. Here, the facies sequence consists of Cross ( 1985). interbedded, fining-upward, lenticular sandstones; coars- The facies sequences and associations of the barren zone ening-upward mudstones, siltstones, and sandstones; and of the Raton Formation (SD facies of the second megacy- coals and carbonaceous shales. The lenticular sandstones cle) are shown in Figure 7. The facies sequence comprises have erosional bases and a thickness-to-width ratio ranging abundant (as much as 75 percent), multistory, lenticular from 1:2 to 1:120; internally, they are trough crossbedded. sandstones interstratified with subordinate mudstones, silt- Several of the lenticular sandstones occur as isolated, con- stones, and minor coaly carbonaceous shales. The lenticu- temporaneous bodies within a 3- to 4-mi-wide (4.8- to 6.4- lar sandstones have erosional bases, fine upward, are mul- km) belt and are encased in thick successions of coarsen- tiscoured, and are trough crossbedded with convolute ing-upward sequences. These sequences consist of rippled lamination in the lower part and ripple lamination in the mudstones and siltstones that are bioturbated in the lower upper part. Although the lenticular sandstones lack lateral part and rippled (ripple drift and asymmetrical) and con- accretion surfaces , they show an en echelon arrangement. volute laminated in the upper part. The coarsening-upward The facies associations of the barren zone represent de- sequences are overlain by thin, discontinuous coals and car- position in meandering streams that rapidly incised, filled, bonaceous shales. and avulsed (autocyclic), as indicated by the en echelon These facies relationships suggest deposition in a cre- deposition of lenticular sandstones. Multiple scouring and vasse-splay-dominated alluvial environment as indicated by subsequent infilling by channel sandstones indicate re- vertically accreted successions of thick coarsening-upward working of old meanderbelt deposits by younger streams. sequences. The crevasse splays probably debouched into The meandering fluvial system was a high-bedload stream, subaqueous floodbasins, as evidenced by the bioturbation as evidenced by the abundance of sand and the scarcity of of the deposits. The lenticular sandstones associated with fine-grained detritus. The input of sand in the fluvial sys- the crevasse-splay sediments indicate deposition in crevasse tem was probably the result of tectonic uplift (allocyclic channels that enlarged into major conduits with increased mechanism) of the source area. Lateral migration of the discharge. These channels were separated by small flood- meandering streams may account for the occurrence of mi- basins that were, in turn, infilled by crevasse-splay, lake, nor fine-grained detritus. Most of the fine-grained detritus and backswamp deposits. One of these channels was a was probably reworked and subsequently retransported and pathway of avulsion (autocyclic mechanism) and was likely redeposited downstream. transformed into an active , laterally aggrading, meandering The basin proper lithofacies sequences and associations stream. This alluvial setting is very similar to the anasto- of the noncoal-bearing Poison Canyon Formation (SD fa-

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FIG. 7.-Photograph and outcrop diagram of the barren zone of the Raton Formation in the basin proper, comprising multistory channel and overbank deposits of high-bedload meandering streams. The diagram was constructed from the measured section shown.

FIG. 8.-Photograph and outcrop diagram of the Poison Canyon Formation in the basin proper, containing channel, overbank, and channel plug (CP) deposits of meandering streams. The diagram was constructed from the measured section shown.

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cies of the second megacycle) are illustrated in Figure 8. stones, since feldspar is less prone to chemical decompo- The facies sequence is composed of lenticular arkosic sand- sition and is better preserved in a cold or arid climate stones interstratified with siltstones and silty sandstones. The (Fuchtbauer, 1974). lenticular sandstones have erosional contacts at their bases. Texturally, the sandstones are coarse-grained to conglom- ALLUVIAL PALEOARCHITECTURE AND SUMMARY eratic. Within these sand bodies, lateral accretion surfaces are delineated by siltstones. The sandstones exhibit trough Facies sequences and associations described for the basin and planar crossbeds in the lower part and ripple lamina- margin and basin proper can be integrated into a model that tions in the upper part. The lenticular sandstones, which shows temporal deposition of a facies in the coarsening- are stacked en echelon, are juxtaposed with scour-based, upward megacycles. The paleoarchitectural model displays muddy siltstones and rooted silty sandstones. the patterns of a paleodrainage network, including the amount The facies associations of the Poison Canyon Formation of interconnection and areal distribution in a downslope di- in the basin proper suggest deposition in a meandering flu- rection. The alluvial paleoarchitecture and the basinal evo- vial system. Lateral accretion of deposits is indicated by lution within the Raton basin are discussed with the aid of point bar sandstones formed in meanderbelts. Abandoned the depositional models in Figures 9A and 9B. meanders infilled by fine-grained detritus (channel plugs) The depositional model for the FDD facies (Fig. 9A) shows are associated with the meanderbelt deposits. Scarcity of the relationships of coeval-tributary braided and meander- organic remains in related floodplain deposits may indicate ing streams along the basin margin. The braided streams deposition in a less humid climate. This observation is sup- were probably controlled by increased influx of bedload ported by the abundance of feldspar in the arkosic sand- sediments due to either floods from alluvial fans, proximity

A AB AN DONED

F:AULT ZONE\

FIG. 9. -Depositional models illustrating basin alluvial paleoarchitecture and evolution of fluvial systems during deposition of (A) coal-rich, fine detritus-dominated facies and (B) coal-poor, sand-dominated facies.

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to the source area, or high gradient imposed by local tec- stream continuum from braided to meandering streams. The tonism. In contrast, the meandering streams probably re- extensive braiding of bedload streams at the basin margin sulted from the increase of mixed-load sediments due to reflects a response to tectonic uplift (allocyclic mechanism) either distance from source area or low gradient. Sparse along a fault zone, which promoted formation of mountain- vegetation, as indicated by organic remains in some carbo- front alluvial fans that were point sources of bedload sed- naceous shales of floodplain deposits, may have retarded iments. The conglomerates of the proximal braided system widespread braiding of bedload streams but may not have deposits represent distal alluvial fan deposits. influenced bank stabilization and formation of mixed-load The downslope continuum of the braided streams con- streams. sisted of high-bedload meandering streams choked with side The tributary system in the basin margin merged down- channel and mid-channel bars. The abundance of bedload slope into the basin proper fluvial systems and peat-forming and lack of overbank floodplain sediments indicate devel- swamps and floodplains. The fluvial systems in the basin opment of slightly to moderately sinuous channels that were proper consisted of meandering and anastomosed streams rapidly filled and abandoned. The fluvial channels devel- that were choked with mixed-load and suspended-load sed- oped on older channel fills and on low-lying floodplains, iments. These fluvial systems, shown in Figure 9A, may the positions of which were determined by the positions of have formed contemporaneously, with either a meandering previously deposited channel sands. This process of chan- stream or an anastomosed stream being dominant. The nelization is responsible for reworking meanderbelt and dominance of one type of fluvial system over the other type overbank floodplain sediments. The reworking and lateral is a function of the stage of evolution and the nature of accretion of deposits by the meandering fluvial system may autocyclic mechanism. That is, vertical aggradation in the have been caused by reduced basin subsidence. alluvial plain was primarily developed by anastomosed The depositional models of braided and meandering flu- streams and lateral aggradation by meandering streams. vial systems (Figs. 9A and 9B) mainly apply to the second Lateral aggradation was dominant when the alluvial plain and third coarsening-upward megacycles. In these mega- was near base level. Rapid vertical aggradation occurred cycles, the relationship of the alluvial paleoarchitecture to during periods when differential compaction of sediments the presence or absence of coal or the remains of land veg- or basin subsidence had lowered the alluvial plain signifi- etation is not consistent with that observed by Schumm (1968) cantly below base level. The latter allocyclic mechanism and Cotter (1978). Schumm proposed that vegetation con- was significant during deposition of the thick sequences of trols stream patterns by: (1) retarding erosion, altering sed- the FDD facies. The vertical aggradation in the alluvial plain iment yields, and changing total runoff, discharge, and flood was initiated by crevassing, which occurs when the levee peaks; (2) decreasing bedload grain size and intensifying of a meandering stream is breached during floods. A cre- production of fine sediments; and (3) stabilizing banks. Thus, vasse splay formed on the low-lying floodplain and pro- the occurrence of vegetation tends to favor development of graded as feeder (crevasse) channels bifurcated. Progra- meandering instead of braided streams. Cotter ested this dation, bifurcation, and reunification of crevasse channels hypothesis in Paleozoic rocks of the central Appalachian produced anastomosis. The crevasse channels served as basin and observed changes in stream patterns consistent avulsion routes (autocyclic mechanism) as progradation en- with the speculation of Schumm. In addition, Cotter found sued. Vertical aggradation continued in channel and over- that, in older Paleozoic units in which land vegetation was bank-floodplain environments. Peat-forming swamps were, not an important factor, a sheet-braided fluvial style dom- at this stage, restricted by the size of floodbasins and by inated the entire downslope continuum. rapid detrital influx from crevasse splays. The aggradation The test of the hypothesis of Schumm and observations to near base level would transform branches of the anas- of Cotter on the relationship of vegetation to stream pat- tomosed channels into major conduits and lead to channel terns yielded mixed results in our study. The development migration and lateral accretion of deposits by meandering of meandering and anastomosed streams in the FDD facies streams. Thus, the deposits of a meandering fluvial system in the basin proper is consistent with the observations of were superimposed on older deposits of anastomosed streams Schumm and Cotter. The coexistence of tributary braided (Fig. 9B). Sedimentation on the alluvial plain by a mean- and meandering streams within a short distance along the dering stream proceeded until the next stage of crevassing Raton basin margin, however, is an enigma. Here, the veg- and avulsion (autocyclic mechanism), together with anas- etation, which may be indicated by the presence of some tomosis, was repeated. coal beds, was insignificant in controlling the pattern of Although peat-forming swamps occurred in both mean- meandering streams. The downslope changes in alluvial pa- dering and anastomosed fluvial systems, poorly drained leoarchitecture associated with the SD facies is especially swamps were better developed on abandoned meanderbelts . problematic. In this facies, the development of meandering In addition, portions of the anastomosed belts, which were streams in the basin proper in relation to the presence of abandoned while an anastomosed channel was being trans- some vegetation, as indicated by a few thin coal beds, is formed into a meandering channel, also served as platforms inconsistent with the observations of Schumm and Cotter. on which poorly drained swamps formed. These abandoned The meandering pattern and downslope change from braided areas were ideal settings for peat-forming swamps because to meandering streams formed despite some presence of they were far removed from detrital influx. vegetation. The sparse vegetation in the SD facies of the Figure 9B is a depositional model of the SD facies that second megacycle may be a function of either sedimenta- succeeded the FDD facies. The model suggests a down- tion controlled by tectonism rather than by climatic change

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or controlled by an oxidized environment, because the fa- With examples from Westphalian D-Stephanian B coalfields, northern cies is enclosed by coal-bearing intervals that contain thick Spain, in Miall, A. D., ed., Fluvial Sedimentology: Canadian Society coal beds. The sparse vegetation of the same facies in the of Petroleum Geologists Memoir 5, p. 669-702. LEE,W. T., 1917, Geology of the Raton Mesa and other regions in Col- third megacycle, however, may be due in part to climatic orado and New Mexico, in Geology and Paleontology of Raton Mesa change. and other regions in Colorado and New Mexico: U.S. Geological Sur- Finally, basin subsidence (allocyclic mechanism) and vey Professional Paper 101, p. 2-221. fluvial avulsion and abandonment (autocyclic mechanism) LEOPOLD,L. B., AND WOLMAN,M. G., 1957, River channel patterns- Braided, meandering, and straight: U.S. Geological Survey Profes- are the major factors in controlling the accumulation of thick sional Paper 282-B , p. 39-85. coal beds in an alluvial environment. Although coal beds MCLEAN,J. R., AND JERZYKIEWICZ,T., 1978, Cyclicity, tectonics, and formed in swamps of meandering and anastomosed fluvial coal-Some aspects of fluvial sedimentology in the Brazeau-Paskapoo systems, the meandering system offered the ideal setting, Formations, Coal Valley area, Alberta, Canada, in Miall, A. D., ed., where abandoned meanderbelts served as swamp platforms Fluvial Sedimentology: Canadian Society of Petroleum Geologists Memoir 5, p. 441-468. free of detrital influx. Extensive detrital influx caused by OUELLEITE,R. G., AND BOUCHER,D. A., 1983, Frontal thrust structure, tectonic uplift of the source area (allocyclic mechanism) south-central Colorado, in Bally, A. W., ed., Seismic Expression of chocked peat-forming swamps. Structural Styles: American Association of Petroleum Geologists Stud- ies in Geology Series No. 15, p. 3.2.2-25-3.2.2-28. PILLMORE,C. L., AND FLORES,R. M., 1984, Field guide and discussions ACKNOWLEDCMENTS of coal deposits, depositional environments, and the Cretaceous-Ter- tiary boundary, southern Raton Basin, in Lintz, J., Jr. , ed., Western We thank R. C. Johnson of the U.S. Geological Survey, Geological Excursions: Geological Society of America Annual Meet- H. G. Churnet of the University of Tennessee, and R. G. ing, v. 3, p. 1-51. Goodwin of the University of Nebraska for their technical , TSCHUDY,R. H., ORTH,C. J., GILMORE,J. S., AND KNIGHT,J. reviews. We also thank V. H. Sable of the U.S. Geological D., 1984, Geologic framework of nonmarine Cretaceous-Tertiary boundary sites, Raton basin, New Mexico and Colorado: Science, v. Survey for her assistance in editing and preparation of the 223, no. 4641, p. 1180-1182. manuscript. , AND FLORES,R. M., 1987, Stratigraphy and depositional environments of the Cretaceous-Tertiary boundary interval and associated REFERENCES rocks, Raton basin, New Mexico and Colorado, in Fassett, J. E. and Rigby, K. B., Jr., eds., Cretaceous-Tertiary Boundary of the San Juan BEERBOWER,J. R., 1964, Cyclothems and cyclic depositional mechanisms and Raton Basins, Northern New Mexico and Southern Colorado: Geo- in alluvial plain sedimentation, in Merriam, D. F., ed., Symposium logical Society of America Special Paper 209, p. 11 1-130. on Cyclic Sedimentation: Kansas State Geological Survey Bulletin 169, SCHUMM,S. A., 1968, Speculations concerning paleohydrologic controls p. 31-42. of terrestrial sedimentation: Geological Society of America Bulletin, CLOUGH,S. R., 1983, Facies development and evolution of fluvial chan- V. 79, p. 1573-1588. nels in the Cumberland marshes of Saskatchewan, Canada: Unpub- SMITH,N. D., AND CROSS,T. A., 1985, Avulsion-controlled fluvial evo- lished M.S. Thesis, University of Illinois at Chicago, 80 p. lution-The Cumberland marshes, east-central Saskatchewan (abs.): COTTER,EDWARD, 1978, The evolution of fluvial style, with special ref- International Fluvial Sedimentology Conference, Colorado State Uni- erence to the central Appalachian Paleozoic, in Miall, A. D., ed., Flu- versity, Fort Collins, p. 36. vial Sedimentology: Canadian Society of Petroleum Geologists Memoir STEEL,R. J., AND AASHEIM,S. J., 1978, Alluvial sand deposition in a 5, p. 361-383. rapidly subsiding basin (Devonian, Norway), in Miall, A. D., ed., Flu- FLORES,R. M., 1984, Comparative analysis of coal accumulation in Cre- vial Sedimentology: Canadian Society of Petroleum Geologists Memoir taceous alluvial deposits, southern United States Rocky Mountain ba- 5, p. 385-412. sins, in Stott, D. F., and Glass, D. J., eds., The Mesozoic of Middle , MAEHLE,S., NILSEN,H., ROE,S. L., AND SINNANGR,A., 1977, North America: Canadian Society of Petroleum Geologists Memoir 9, Coarsening-upward cycles in the alluvium of Hornelen Basin (Devon- p. 373-385. ian) Norway-Sedimentary response to tectonic events: Geological So- FUCHTBAUER,HANS, 1974, Sediments and sedimentary rocks I: John Wiley ciety of America Bulletin, v. 88, p. 1124-1134. and Sons, New York, 464 p. WILLIAMS,P. F., AND RUST,B. R., 1969, The sedimentology of a braided HEWARD,A. P., 1978, Alluvial fan sequence and megasequence models- river: Journal of Sedimentary Petrology, v. 39, p. 649-679.

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3790711/9781565760967_backmatter.pdf by guest on 25 September 2021 AN APPLICATION OF STATISTICAL MODELLING TO THE PREDICTION OF HYDROCARBON RECOVERY FACTORS IN FLUVIAL RESERVOIR SEQUENCES

CHRISTOPHER R. FIELDING’ AND RICHARD C. CRANE Sedimentology Branch, BP Petroleum Development Ltd., Britannic House, Moor Lane, London EC2Y 9BU, UNITED KINGDOM

ABSTRACT:Fluvial channel sandstones commonly form hydrocarbon reservoirs in oil- and gas-bearing basins. Where such sands were the product of deposition in broad, extensive channel systems, their lateral extent is often greater than the area of the producing oil or gas field. In such situations, lateral pinchout of reservoir sandstone bodies is unlikely to reduce the gross reservoir volume of the field. Where fluvial sands were deposited in more laterally restricted, perhaps meandering channel belts, however, lateral pinchouts are common. In the case of laterally restricted fluvial reservoirs, the spacing and distribution of producing wells becomes critical, particularly where numbers of laterally offset, “shoestring” sandstone bodies within a vertical sequence are isolated from each other by floodplain/floodbasin fines. A method has been developed for predicting the proportion of laterally discontinuous fluvial reservoir sandstones that can be accessed by specified development well patterns. The method is based on (1) relationships between channel belt width, channel depth and deposit thickness for various fluvial regimes; and (2) a statistical description of reservoir sandstone beds encountered in previously drilled wells, and is designed to be generally applicable. A Fortran IV computer program has been written to allow rapid and easy usage of the method.

INTRODUCTION RELATIONSHIP BETWEEN SAND BODY GEOMETRY AND FLUVIAL CHANNEL TYPE Fluvial channel sandstones have been found to constitute oil and gas reservoirs in a number of hydrocarbon prov- The first stage in the construction of the predictive model inces, particularly in North America (e.g., Brown, 1979; is the establishment of sandstone thickness versus deposit Putnam & Oliver, 1980; Mossop & Flach, 1983). Most flu- width relationships for various types of fluvial channel. If vial channel deposits occur in laterally confined, elongate the thickness of a channel sandstone could be said to be belts, recording the limited capability of the depositing equivalent to the depth of its formative channel, that thick- channel to expand in a direction perpendicular to its paleo- ness could be used directly to predict channel belt width flow. There is , however, considerable variation in modem from relationships described below. For most ancient flu- fluvial channel morphology (Allen, 1965; Fig. 1) and con- vial deposits this is not the case, however, since channel sequently in the geometry of ancient channel belt deposits. deposits accrete in the vertical sense with time, as well as In the development of oil and gas fields with fluvial res- spreading laterally. Collinson’s (1978) published figure 2, ervoirs, particularly those with multiple pay intervals , ques- which relates ancient channel sandstone thickness to chan- tions arise as to the continuity of those beds across the area nel belt width using a 1 : 1 channel depth: preserved sand- of the field and the likely effectiveness of planned devel- stone thickness ratio, is therefore rather oversimplified. opment well patterns (Fig. 2). In some cases, it may be possible to accurately determine Modern fluvial channels appear to obey certain “natural original channel depths in subsurface wells (e.g., from con- laws” which, while complex, allow an understanding of the tinuous core, or nearby outcrops). A recent predictive study processes and parameters which control channel morphol- by Lorenz and others (1985) employed such a technique. ogy and ultimately, deposit geometry. Based on observa- For instances where such detailed data are not available, tions of channel dimensions and geometry, Carlston, however, a thickness: depth relationship has been distilled Schumm and others were able to define equations which from published information (Fig. 3). Sandstone thicknesses linked those parameters for meandering and other channels determined from wireline logs may then be assigned a to their hydrological characteristics (reviewed by Collin- formative channel depth. son , 1978). Extending this concept , Collinson ( 1978) sug- Published data relating modern and ancient fluvial chan- gested that, using such equations, the width of ancient nel belt widths to channel depths have been compiled for channel belt deposits could be predicted from a knowledge various channel morphological types (Figs. 1, 4) ranging of channel bankfull depth or sandstone body thickness. from braided systems through meandering to anastomosed, This paper aims to further expand the concept of channel straight and incised channels. Broad positive relationships belt width prediction from basic sandstone thickness data, between channel depth and channel belt width are identi- by examining available dimensional information on modern fiable, which are to some extent governed by the different and ancient fluvial channel deposits. The results of this study, channel types. Five such relationships have been distilled together with simple statistical techniques , have been ap- from the data set to represent the spectrum of fluvial chan- plied to distributions of sandstone bed thickness within nels. These have been named Cases la, lb, 2a, 2b, 3 (Fig. exploration wells to develop a model for predicting the 4) - efficacy of a planned pattern of development wells in an Cases la and 3 are, respectively, the least and most op- oil/gas field. timistic situations with regard to hydrocarbon recovery, as they represent the lowest and highest width/depth ratios of all the collected data. ‘Present address: Department of Geology and Mineralogy, University From a sandstone body thickness as estimated from wire- of Queensland, St. Lucia, Queensland, Australia 4067 line logs, it is thus possible to estimate channel belt (i-e.,

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A MEANDERING B LOW SINUOSITY LOW SINUOSITY 3 MEANDERING CHANNEL .BRAIDED MORPHOLOGV I i ANASTOMOSED ! /

l V I V

C ANASTOMOSED D BRAIDED

I Y

O 10 20 30 40 SANDSTONE BODY THICKNESS (rn)

FIG. 3.-Channel depth vs. sandstone bed thickness relationships for

CHANNEL BELT SWAMP deposits of various fluvial channel types. The two outer lines enclose the 0SANDDEPOSITS MARSH limited available data compiled from published studies. A median line OVERBANKI CHANNEL TRANSPORT (channel depth = 0.55 sandstone bed thickness) is taken as a reasonable a FLOODBASIN FINES DIRECTION representation of the data. Where more reliable data are available on the local relationship between the two parameters in any study, these should FIG. 1 .-Three-dimensional block models showing possible fluvial be used to generate the required figures in preference to the crossplot. systems and the morphology of sand bodies which might be deposited by them. A) meandering stream, B) low sinuosity stream, C) anastomosed stream, and D) braided stream. Each has different implications for reservoir geometry. The graphs of Figure 3 and 4 relating preserved sandstone bed thickness to formative channel belt width via sandstone body) width for various potential channel mor- channel depth have been combined in Figure 5 for a gen- phological cases (Figs. 3, 4). To some extent, the range of eralized case. This is Case 2a of Figure 4 and the median cases considered should reflect the degree of confidence in line of Figure 3. the sedimentological interpretation of the considered reservoir sequence. Any available data on the interval of in- THE STATISTICAL MODEL terest should be used in channel belt width computations. The next stage in the development of the model is the assembly of a statistical description of sandstone beds in 1\ n the intervals of concern, in terms of bed thickness and width. Assuming that continuous core is not available for the interval of interest, wireline logs may be employed as the basis for geological interpretation, calibrated by any cores which may be available. For many sequences, a gamma ray log, which measures natural gamma ray emissions from rocks downhole, represents the closest imitation of sediment grain size. In some instances, however, this will not be the case, perhaps due to high concentrations of (radioactive) K-feldspar in sandstones; in such cases an alternative wireline log such as neutron porosity, spontaneous potential, or resistivity should be used. Well results must first by simplified to block logs describing only two facies: (1) sandstones of 2 m or greater thickness, and (2) all other sediments, predominantly siltstones and claystones with thin sandstones (Fig. 6). The 2- m cutoff is employed so as to eliminate thin, non-channel sandstone (crevasse-splay deposits). Thicker non-channel FIG. 2.-Three-dimensional block diagram illustrating the relative effectiveness of different well spacings in penetrating isolated, laterally re- sandstones of interdistributary minor mouth bar or levee or- stricted, fluvial reservoir sandstones. The channel belt sandstone bodies igin are common in coastal plain sequences (Fielding, 1984). are of the order of 100’s of meters wide and several meters thick. Wells These may be distinguished from channel deposits by their A and B collectively penetrate only three of the four sandstones illus- coarsening- or “cleaning” upward character on well logs trated. A closer well spacing entails a third well, C, which penetrates the fourth sandstone. Well spacing must be measured perpendicular to pa- and eliminated from the model (channel deposits typically leocurrent; wells A and D penetrate the same sand bodies because they show “blocky,” ”spiky, ” or fining-upward log responses; overlie one another with respect to paleocurrent. Fig. 6). Other sandstones, which for whatever reason (low

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SANDSTONE BODY THICKNESS 30- VS. CHANNEL BELT WIDTH - MOST LIKELY CASE

20 - /* SANDSTONE BODY THICKNESS (mi

10-

I I I 10 100 1Ooo 10,Ooo CHANNEL BELT WIDTH (rn) FIG. 4.-Channel depth vs. channel belt width relationships for various types of modem and ancient fluvial channel deposits (data from 45 FIG. 5.-Sandstone bed thickness vs. width perpendicular to paleo- published sources). current (generalized case, constructed by using Case 2a from Fig. 4 and the median line of Fig. 3). Case la: the upper limit of all data collected (Le., lowest width/depth ratio)-channel belt width (m) = (0.01 channel de~th)~.~.This relationship describes deposits of incised, straight and non-migrating channels and is an extreme case. CONSTRUCTION OF SIMPLIFIED WELL LOG Case lb: the upper bounding line for meandering channel deposits (i.e., lowest width/depth ratio)-channel belt width (m) = 0.95 (channel GAMMA RAY LOG INTERPRETATION SIMPLIFIED This relationship also describes deposits of channels which WELL LOG have not been allowed to migrate laterally, but excludes highly in- O API 150 cised and narrow, vertically accreted types. FLOODBASIN Case 2a: the best fit line for all the collected data-channel belt width 2000 ...... "...... (m) = 12.1 (channel depth)' 85. This relationship is a geometric mean ,.:::::: CHANNEL ;.;;;?:...... 15.5m of all fluvial channel types, and describes a situation where a variety ::.::...... of channel types are said to exist or alternatively may be used when FLOODBASIN,WITH o DEPTH the formative channel type is not known. CREVASSE SPLAY (m) Case 2b: the published empirical relationship for modem, truly meandering streams (Collinson, 1978)-channel belt width (m) = 64.6 CHANNEL (channel depth)'.54.This relationship describes deposits of channels 2050 which have been allowed to develop fully meandering profiles. FLOODBASIN Case 3: the lower bounding line of all data collected (Le., highest width/ depth ratio)-channel belt width (m) = 513 (channel This LEVEE relationship describes deposits of laterally unrestricted, (i.e., braided) fluvial systems. FLOODBASIN Cases la and 3 are, respectively, the least and most optimistic situations :... . with regard to hydrocarbon recovery, as they represent the lowest and 2100 013.5m...... : . . <:. CHANNEL highest width/depth ratios of all the collected data. ki ...... : '. FLOODBASIN

SANDSTONE CUT-OFF porosity, high clay content) are considered non-reservoir , can also be removed from the model at this stage. FIG. 6.-Example of a hypothetical fluvial sequence, showing the Multiplying the thickness of an identified reservoir bed method of construction of simplified block logs. In this case sandstones are first identified by using an independently determined cutoff value on by its predicted channel belt width gives the cross-sectional a Gamma Ray log. Sandstones of non-channel origin are then eliminated, area of that sandstone perpendicular to paleocurrent . Cross- leaving the three channel sandstone bodies displayed along with their sectional areas of all the sandstones identified in control thicknesses in the right-hand column. wells may then be calculated and totalled (Fig. 7a). If, in a given sequence, there are y1 sandstone beds sl, s2,...... s,- 1, s, with thicknesses tl, t2,...... tnPl,t, meters and cross-sectional widths wl...... w, meters, then the total sandstone through the interval of interest will penetrate a set of sand- cross-sectional area in communication with the test wells is stone beds equivalent to those found in the test wells (Fig. E.i"=ltiwi(Fig. 7a). 7b). Thus, between two proposed wells of spacing W me- The total cross-sectional area of sandstone existing in the ters, there exists a total sandstone cross-sectional area of succession is dependent on their statistical distribution. Given x;=1 tiw. that wells previously drilled in an area have penetrated a The "success rate" for penetrating sandstones in the in- representative sequence, then any hypothetical well drilled terval of interest between two proposed wells a certain dis-

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well p well p well p+l I I 4 Wn b tk tn

tn-1 I S,-1

I -I

Cross-sectional area of sandstones intersected (a) Cross-sectional area of all sandstones present (b)

n n

i=l i=l Success rate = a/b FIG. 7.-The statistical description of sandstone beds in the average well, based on log data from available wells. (a) Cross-sectional area of sandstones intersected. The average well p intersects n channel belt sand bodies. Their individual widths (w)are computed from the observed thickness (t) using graphs of Figures 3 and 4. The total cross-sectional area of channel belt sandstone bodies perpendicular to paleocurrent is then equal to the sum of all individual cross sections. For this computation the order of occurrence of the sandstones in the well is of no significance. (b) Cross-sectional area of all sandstones present. The average spacing between proposed wells is W, and across this distance the distribution of sandstones fits the average statistics of the field. The total cross-sectional area is thus equal to the thickness of each sandstone times W, since sand bodies of each thickness are inferred to occur everywhere between the wells. The success rate or proportion of the total reservoir sandstone volume in a given interval which will be penetrated by a planned pattern of production wells of a certain spacing may then be calculated by dividing the total (a) by the total (b).

tance apart is then calculated by dividing the cross-sectional as the proportion of the total channel belt sandstone res- area of sandstone hit by the total cross-sectional area of ervoir volume in a given interval which will be penetrated sandstones in existence. Since cross-sectional area is pro- by planned development wells a certain distance apart (cf. portional to volume, the “success rate” may also be re- Fig. 2). garded as representing reservoir volumes. It is possible that It is important to note, though, that further sandstones some of the channel sandstone bodies in a given reservoir could be in hydraulic communication with wells through interval will be sufficiently wide that they cover more than erosional interconnections betweeri channel belt deposits. the distance between two adjacent wells. This must be com- Theoretical simulation studies indicate that meandering pensated for in the model by limiting the width of sand- channel belt sandstone bodies are essentially isolated if they stones considered to the distance between adjacent wells (in comprise less than about 50 percent of the succession (Al- a direction perpendicular to that of regional paleoflow). len, 1978; Crane, 1982; Fig. 8). Studies of Eocene fluvial deposits in the Ebro basin by Atkinson (pers. commun.), however, suggest that sequences with far lower channel sand ti minimum (wi, fractions may possess considerable interconnection be- i= 1 tween channel sandstones. In the present study, consider- Then success rate = n ation of interconnection is based on theoretical studies only, and it is acknowledged that the current data base is poor.

I= 1 For field development cases involving secondary and/or tertiary hydrocarbon recovery, the equation for “success rate” where “success rate,” expressed as a percentage, is defined may be rewritten as

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50 its, the presence of which could increase the lateral extent and connectivity of channel belt sandstones, have 5 40 P not been incorporated into the model since they are very 30 difficult to quantify. m 2. It is assumed that sandstone bodies of a predicted width v) = 20 are homogeneous units with respect to fluid movement n0 and this need not always be the case. 3. Because channel belts are themselves often sinuous, individual reservoir sandstone bodies may intersect with each other at some point up or down paleocurrent, thereby increasing connectivity. 4. The effect of faulting, which, depending on the throws and angles of throw of faults and sands fraction of the sequence, could either increase or decrease the success rate, has not been modelled in this study. 5. The statistical distribution of sandstone bodies is specified by the method used but not the actual location in space of any particular sandstone body. 6. The method is presently applicable only to a purely gravity-driven fluvial system where there is no disrup- tive tectonic control on deposition across the area of interest (were such an influence demonstrable, the model framework would require adjustment). 0.0 0.2 0.4 0.6 0.8 1.o The accuracy and resolution of the method are dependent sands fraction I) on a number of factors, including: the number of test wells FIG. %-Sandstone body interconnection as a function of the net sands available; the validity of the chosen depth vs. width, and fraction of the sequence (after Crane, 1982). The ordinate denotes the thickness vs. width relationships; and the user’s choice of average number of channel belts in one interconnected domain. A domain is a set of channel belt sandstones which, during deposition, have eroded most appropriate depth vs. width relationship. Here, the into each other. The abscissa records the sands fraction, which is the sedimentological interpretation of a sequence, in terms of quantity of channel belt sandstones as a fraction of the total sequence environmental setting and channel type, whether based on thickness. core, log or even regional geological data, is of the utmost importance in defining the operating parameters for the model. It is essential that any application of this model to (Wi3 w) tiw fluvial reservoir sequences should first take into account I their sedimentologic context. n A HYPOTHETICAL CASE STUDY i= 1 To demonstrate the application of the model, a hypo- since for effective recovery, reservoir sand bodies must be thetical example has been devised and examined. The hy- simultaneously penetrated by at least two development wells. drocarbon accumulation of interest is contained within mul- A VAX computer has been programmed to execute the tiple reservoir beds of fluvial channel origin, which are above computations enabling rapid delivery of results and interbedded with fine-grained lithologies and occasional allowing a number of sensitivities to be easily tested, in- sandstones of non-channel origin. Figure 9 shows simpli- cluding: fied well logs for the five exploration and appraisal wells drilled in the field; on these logs the producing formation 1. variable well spacing, has been reduced to reservoir and non-reservoir facies, and 2. variable channel belt orientation (and thus effective sandstones believed to be of non-channel origin have been spacing in a planned well pattern), deleted. 3. variable channel morphology (and hence sand body width/ The individual sandstone beds vary in thickness from 2 thickness ratio), to 65 m, suggesting a variety of channel types and channel 4. the presence of multistory sandstone bodies. Channel in- belt dimensions. Some of the thicker units may be com- terconnection has also been considered, and lateral posite bodies but are treated as the product of deposition in thickness variations and the possible presence of fine- a single channel belt for the purposes of this example. Fig- grained channel “plugs” are accounted for by the sta- ure 9 also shows the net “sands fraction” of the reservoir tistical description of test wells. sequence in each well, which varies from 0.23 to 0.33. The model does have a number of limitations, however, Individual reservoir sandstones may therefore be consid- which are worth pointing out: ered to be isolated within non-reservoir facies. Figure 10 illustrates the planned pattern of development 1. Channel belt “wings” or overbank/minor delta depos- wells for the hypothetical field. Dipmeter and orientated

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CASE STUDY SIMPLIFIED WELL LOGS TABLE1 .-COMPUTED SUCCESS RATES FOR THE CASE STUDY, - SHOWING THE EFFECT OF VARIABLE WELL SPACING. WELL SPACING IN METERS. ALL CASE VALUES IN PERCENT. WELL 1 WELL 2 WELL4 WELL5 AB Well Spacing Case IA Case 1B Case 2A Case 2B Case 3 2000 - i.AcHiric 113 500 16 60 94 99 100 875 9 43 88 97 1 O0 rn 20 2100 950 8 41 87 97 1 O0 - 1500 5 32 80 94 100 I13 -5 -2 I24 2200 - 20

I25 -2 =7 model, together with 500 m and 1500 m for comparison. 2300 - 550 124 The model has been run for all five width-depth cases 25 575 0.23 57! I40 ’ 161 132 (Fig. 4). The sequence in question is known to have been 0.2: -2 0.2! -9 deposited on a coastal plain, fed by distributary channels I 2400 - 60í I15 19: - of variable sinuosity, and so the “best fit” Case 2a may be 0.3: -2 116 I20 475 considered a realistic “most likely” situation. 151 2500 - -3 -9 0.32 The resultant success rates (defined as “the proportion of 113 I15 10 the total reservoir volume in a given interval which will be

-2 penetrated by a planned pattern of production wells of a 2600 35 - - 31 certain spacing”) suggest that the planned well pattern will

-5 allow a large proportion of the recoverable hydrocarbon re- -2 serves to be accessed (Table 1). Increasing the density of 2700 i - 4 wells would increase the success rate by 7 percent, while FIG. 9.-Simplified block logs for the five exploration and appraisal decreasing the number of producing wells to a spacing of wells in a hypothetical oil/gas field. Only inferred channel belt sandstones are shown in column A with their thickness in meters. Column 1500 m would reduce the penetrated hydrocarbon volume B gives (from top to bottom) (1) total reservoir formation thickness, (2) by 7 percent. total reservoir sandstone thickness, and (3) net sands fraction (2 divided by 1). CONCLUSIONS This paper describes a technique for predicting the pro- core data have suggested a northerly inclined paleoslope, portion of total reservoir volume in an oil or gas field which and in a north-south direction the maximum distance be- will be penetrated by development wells a certain distance tween wells is 875 m. Allowing for uncertainty in the inter- apart, where the reservoirs are laterally restricted, fluvial pretation of regional paleoflow, and for local variability, channel sandstones. the maximum distance between wells in any direction is A series of relationships has been defined, from pub- 950 m. These spacings are used as input parameters to the lished data relating fluvial channel deposit width, thickness and formative channel depth for various channel morpho-

NORTH = logical types. These relationships, together with measured INTERPRETED o 5001Ooo sandstone bed thickness distributions and channel sand PALAEOCURRENT DIRECTION fractions from exploration and appraisal wells, form the basis of a statistical description of the interval of interest. From

PLANNED this description, the predicted total cross-sectional area of sandstone intersected by wells a certain distance apart can

”+ / be calculated. Dividing this figure by the total cross-sectional area present in the sequence (also from the statistical description) gives a “success rate” for a given well spacing. OIUGAS - Since cross-sectional area is proportional to volume, the FIELD BOUNDARY “success rates” refer also to reservoir volume. The model is in the form of a Fortran computer program for VAX machines, designed for rapid and easy usage, and is generally applicable to fluvial reservoir sequences.

ACKNOWLEDGMENTS FIG. 10.-Map of the hypothetical oil/gas field, showing the locations of planned development wells (black circles). By projecting lines in a We acknowledge the permission of the British Petroleum down-paleocurrent (i.e., north-south) direction, the effective well spac- Company to publish this paper. Valerie Muscutt, Carol ings may be determined. The maximum spacing in this orientation is 875 m. Varying the orientation of the projected lines to account for the pos- McGraill, Jenny Huggett and Selina Britton of BP London sibility of a variable paleocurrent direction gives a maximum spacing be- are thanked for their help, encouragement and technical tween wells of 950 m. support. Cathy Fielding typed the manuscript. S. R. Brit-

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ton, J. W. Buckee, B. C. Els, Yinan Qiu, P. J. Smith and CRANE,R. C., 1982, A computer model for the architecture of avulsion- C. A. Wright reviewed earlier versions of the manuscript. controlled alluvial suites: Unpublished Ph. D. Dissertation, University of Reading, 543 p. FIELDING,C. R., 1984, Upper delta plain lacustrine and fluviolacustrine REFERENCES facies from the Westphalian of the Durham coalfield, NE England: Sedimentology, v. 31, p. 547-567. ALLEN,J. R. L., 1965. A review of the origin and characteristics of recent LORENZ,J. C., HEINZE,D. M., CLARK,J. A., AND SEARLS,C. A., 1985, alluvial sediments: Sedimentology, v. 5, p. 89-192. Determination of widths of meander-belt sandstone reservoirs from , 1978, Studies of fluviatile sedimentation-An exploratory vertical downhole data, Mesaverde Group, Piceance Creek Basin, Col- quantitative model for the architecture of avulsion-controlled suites: orado: American Association of Petroleum Geologists Bulletin, v. 69, Sedimentary Geology, v. 21, p. 129-147. p. 710-721. BROWN,L. F., 1979, Deltaic sandstone facies of the Mid Continent, in Mossop, G. D., AND FLACH,P. D., 1983, Deep channel sedimentation in Hyne, N.J., ed., Pennsylvanian sandstones of the Mid Continent: Tulsa the Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Al- Geological Society Special Publication 1, p. 35-63. berta: Sedimentology, v. 30, p. 493-509. COLLINSON,J. D., 1978, Vertical sequence and sand body shape in al- PUTNAM,P. E., AND OLIVER,T. A., 1980, Stratigraphic traps in channel luvial sequences, in Miall, A.D., ed., Fluvial Sedimentology: Cana- sandstones in the Upper Mannville (Albian) of east-central Alberta: dian Society of Petroleum Geologists Memoir 5, p. 577-588. Bulletin of Canadian Petroleum Geology, v. 28, p. 489-508.

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YINAN QIU, PEIHUA XUE, JINGSIU XIAO Scientific Research Institute of Petroleum Exploration and Development, P.O. Box 910, Beijing, China

ABSTRACT:Fluvial sandstone bodies play an important role in forming hydrocarbon reservoirs, especially in lake basins. They possess more than one-half of the oil in place in known oil fields in Chinese Mesozoic and Cenozoic lake basins. The strong reservoir heterogeneity and restricted lateral continuity of the fluvial sandstone bodies, however, result in some difficulties in secondary and tertiary oil recovery. Almost all types of fluvial deposits documented from studies of modern river patterns have been identified in ancient Chinese lake basins. The identification of depositional facies models that relate to fluvial sandstone body geometry in the basins has been possible because of close well spacing and complete coring through pay zones. Six depositional-facies models have been identified: (1) short- coursed braided river, (2) long-coursed braided river, (3) highly sinuous meandering river, (4) low sinuosity meandering river, (5) straight distributary channel river, and (6) confined-valley channel. Each depositional group is identified by its depositional character and respective heterogeneities. The depositional composition of the sandstone unit is a result of the depositional processes that took place during its formation in the stream. These processes and heterogeneities thus formed vary from one facies to another and depend on such depositional processes as lateral accretion in point bars, vertical accretion in braided bars, aggradation in confined-valley channels, and channel fill in straight distributary channels. Although there are variations in lateral continuity of sandstone bodies among fluvial types, all are rather discontinuous in lateral dimension. Only those sandstone bodies deposited during the transition from a period of tectonic uplift to one of subsidence, and during episodes of change from lacustrine regression to transgression, result in a wide areal distribution of the size of coalesced sandstone bodies. The vertical density of channel sandstone bodies of 50 percent and 30 percent may be empirically used to predict laterally extensive and isolated fluvial sandstone bodies, respectively, but much care should be exercised in the prediction of lateral continuity of sandstone units when the density of channel sandstone bodies ranges between 30 and 50 percent.

INTRODUCTION distances along the depression’s depositional axis. Gener- ally, sandstone bodies formed in highly sinuous streams are Fluvial sandstone bodies are important hydrocarbon res- not volumetrically significant as petroleum reservoir rocks. ervoirs, particularly in lake basins. Clastic rock reservoirs Third, straight channels characterize distributary channels contain more than 90 percent of the oil in place in Meso- on deltaic plains and on terminal fans, the latter frequently zoic-Cenozoic lake basins in China; nearly half of the res- formed during the “dying” stage of the lake basins. Fourth, ervoirs are developed in fluvial sandstone bodies. The larg- some confined channel-fill sandstone bodies , believed by est oil accumulations in each lake basin, such as Daqing some to be similar to those of the anastomosing pattern pro- and Shengtuo fields, generally occur in the basin’s largest posed by Smith (1983), have been identified in the longi- ancient fluvial-deltaic deposit complex, in which units of a tudinal system that formed during periods of active tecto- fluvial and deltaic origin are nearly equal in volume. Flu- nism in the region of the basin. A generalized model of vial reservoirs, however, are the most common reservoir deposition for a lake basin is shown in Figure 1. type for “secondary” oil accumulations in those parts of the In many Chinese oil fields, it has been proven that the eastern China faulted lake basins that have never been deeply heterogeneity of fluvial reservoirs has caused some diffi- buried. These “shallow” clastic deposits originated in streams culties for oil recovery. In secondary or tertiary oil recovery during periods in which the lake shrank or ceased to exist. processes, the reservoir heterogeneity can be briefly de- Most ancient lake basins in western China existed in an arid scribed from two prospectives: ( l) in-layer heterogeneity, to semi-arid climate and oil fields developed in rocks formed and (2) areal heterogeneity. They determine the vertical and in these depositional basins are almost entirely in sandstone lateral conformance factor of injected fluids and the effec- reservoirs that originated as fluvial and alluvial fan depos- tiveness of the recovery attempt. The in-layer heterogeneity its. More fluvial reservoir sandstones undoubtedly remain of a sandstone reservoir concerns (1) vertical variations in to be detected in the ancient lake basins of both western permeability, (2) the position of the rock unit in the sand- and eastern China. stone body that possesses maximum values of permeability, Although to date ancient counterparts of virtually all river (3) the ratio of vertical to horizontal permeability in a res- patterns documented from modem rivers have been discov- ervoir body, and (4) the frequency, location, and dimension ered in ancient lake basins, they do vary in a manner spe- of discontinuous shaly barriers to flow. Areal heterogeneity cific to environmental factors and to individual basins. First, mainly concerns a sandstone reservoir’s lateral continuity braided rivers are most extensively developed in smaller and directional permeability. Both types of heterogeneity intermontane and foremontane lake basins and in major are the result of the lithologic and sedimentologic architec- components of crosscutting “transverse depositional sys- ture of the rock unit. Therefore, it is important to recognize tems” in those larger lake basins with a steep depositional the origin of fluvial sandstones by depositional processes slope and short transport distance. Many of the rivers in so that they may be compared to characteristics of reservoir these basins are braided along their entire course to the point heterogeneity to permit construction of a comprehensive of their intersection with the lake’s shoreline. Second, large- reservoir model that links depositional processes to the scale meandering rivers are rather rare in China’s ancient characteristics of reservoir performance. These models can lake-basin deposits except in those cases where a mean- be used to improve responses to production tests in fluvial dering river system formed to flow longitudinally for long sandstone reservoirs. On the basis of our experiences with

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are among the unit’s most important lithic constituents that influence fluid flow in a sandstone body. Because mud drapes commonly occur at an oblique intersection to the regional bedding and can only be preserved in the upper part of the point bar, the geometry and number of drapes can strongly influence the lateral flow of both hydrocarbons and injected fluid. In almost all the examples of point bar reservoirs we have studied, mud drapes have been identified but with much variation in the amount of their preservation. Under conditions of strong intermission and unexpectedness of flood events with relatively rapid rates of sediment deposition, as in low sinuousity meandering rivers, mud drapes are frequently broken into intraclasts by subsequent floods. On the other hand, in high sinuousity meandering rivers characterized by relatively infrequent floods and relatively slow rates of deposition, drapes are commonly preserved. Al- Legend though mud drapes frequently hinder the effectiveness of AF Alluvial fan DF Deltaic front waterflds, they may increase the volume of reservoir swept BR Braided river FD Fan-Delta in its upper units during gas injection tests. MR Meandering river SF Sublacustrine fan The lateral continuity of sandstone units in a genetic unit DP Deltaic plain ID Interdeltaic bay WL Water level is essentially determined by the width of the river’s meander belt. Obviously, the width/thickness ratio of a highly sinuous sandstone body is greater than that of a relatively Shoreline Lacustrine sandbody straight unit.

FIG. 1.-A generalized model of depositional systems in a lake basin. Case I. The highly sinuous sandstone bodies-Subzone PI2 in the eastern part of Sardu field, Daqing oil province.- examples from ancient Chinese lake basins, this paper suggests a six-fold division of depositional process-reservoir A typical cored point-bar sequence is illustrated in Figure heterogeneity models for fluvial sandstone bodies. The 2. A sharp base with an erosional contact with the under- models are: (1) meandering stream-high sinuosity, (2) lying rock is common. Overlying the erosion surface are meandering stream-low sinuosity, (3) braided stream-long lag deposits; the medium-grained sandstone contains scat- course, (4) braided stream-short course, (5) distributary- tered pebbles derived from the sediment of the source area. straight, and (6) confined channel filled. The unit contains a few intraformational pebbles of mudstone. The subrounded shape and scarcity of mud pebbles imply that the pebbles have been transported an uncertain MEANDERING MODELS distance and that rate of deposition of the sediments was The main sand units formed in meandering rivers are parts rather slow. Thereafter, fine- to medium-grained sandstone of point bars, of which the characteristic bed pattern is lat- with large-scale trough or tabular crossbedding, fine-grained eral accretion. The gradual growth of a series of lateral ac- sandstone with small-scale crossbedding, and siltstone to cretion units combines to form a point bar. In general, lat- fine-grained sandstones with ripple crossbedding are de- eral accretion results in a vertical bar sequence in which the veloped in ascending order. Uppermost units contain struc- topographically lowest components at the unit base are tureless mudstone, pedogenetic carbonate concretions and transitional upward to the topographically highest compo- rootlets that collectively indicate formation on a floodplain. nents. Lag deposits at the bar base contain the bar’s coars- Internal values of permeability occur in a sequence of est grains and the grain size generally becomes finer up- gradually decreasing-upward numbers. Maximum values are ward to the bar’s overbank deposits. Consequently, the located along the unit base within the coarsest lag deposits. vertical variation of permeability values within a lithified The maximum permeability (in lowermost interval)/mini- point bar declines upward and the values result in a striking mum permeability (commonly in topmost interval) ratio, permeability contrast between the maximum values near the which is usually called “permeability contrast’’ in reservoir point bar base and minimum values near its top. This con- engineering, ranges from 8 to 50, and is 10-20 in general. trast in permeability within a point bar is very common in The “variation coefficient of permeability” (kv)* is about rock reservoirs formed in sandstone bodies of the mean- 0.6-1.3 and mostly 0.8-1.0 dering type, regardless of their sinuosity, and it usually re- A complete point-bar sequence is usually 5 to 7 m thick, sults in waterflood performance in which injected water and it can commonly be divided into 3 to 5 subcycles. A preferentially and rapidly advances along the reservoir’s lowermost components and within a thin interval. Mud drapes formed in the upper part of the point bar *note: Kv = bk/K, bk is mean square deviation and K is average value sequence as a result of deposition between two flood events of permeability.

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Grain size Permability E 5 @I sequence (d so) (md )- 5; 0.1 0.2 0.3 (I) 200 400 1000 2000 4000 8000

mmm m 151 181 White Grey Purply red Orange Gre-yish yellow Greyish green Dark brown

...... Da& grey Tabular cross Massive S.S. Low angle Trough cross Parallel lamination bedded cross bedded bedded n m U /31 Horizontal. Ripple cros Mud ston e Mud Pebble Clastic pebble Massive mud Plant debris lamin ation bedded fragment

FIG. 2.-Lithofacies sequence of high sinuousity meandering sandstone body (Subzone PI, +2, Well 213-23, Daqing).

trend of thinning in bed thickness (3.0-0.1 m), an upward thickness of a complete point bar sequence is 5-7 m and fining in grain size for each subsequent subcycle, and the that it is a measure of the bank full depth of the ancient presence of silty and clayey streaks between the subcycles PI2 river. We further assume 5”-10” as the dip angle of the in the upper half of the bar sequence collectively indicate lateral accretion surface, which indicates that the river’s bank that the cycles are as a series of continuously deposited lat- full width ranged from 90 to 120 m. Thus, the width of the eral accretion bodies and that the mud streaks are the pre- meander belt is about 10-15 times as wide as the channel served parts of intercalated inclined mud drapes (Fig. 2). width. Our inference is that the course of the ancient PI2 We assume an “equal elevation of the top bed surface river was highly sinuous. underlying the nearest marker unit” and an analogous response of logging to utilize drill hole logs from closely spaced Case 2. Meandering sandstone bodies of low sinuosiíy- development wells in the identification of sandstone units Subzone P12-3 in the western part of Sardu field, Daqing formed in individual meander belts. With this method of oil province. - identification of sandstone units on drill hole geophysical The vertical rock sequence illustrated in Figure 4 is in- logs, we suggest that the subzone PI2, as shown in Figure terpreted to be that of a typical point bar similar to that of 3, is characterized by a rather simple sandstone geometry Subzone P12. Important differences can be distinguished and that it has good continuity. Our interpretation is that a between the two cases, however. In the bar’s lag deposits, genetic unit of meander belt is about 800-1000 m wide there are many intraformational pebbles associated with the with a width/thickness ratio 130: 170. We assume that the pebbles derived from the sediment of the source area. Most

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I Density of channel sandbody h7% Legend 0-- 200 400 M Boundary of meander belts

FIG. 3.-Geometry of high sinuousity meandering sandstone body, (Subzone PI2, Daqing).

intraclasts are composed of mudstone breccia, and their di- ILithofaciezI'I BGrain size Permeability ameters range to as much as 7-8 cm; no regular grain fabric El I is evident. Intraclasts are abundant at the base of the point bar and through the lower half of the bar sequence. The clasts clearly mark the boundaries between lateral accretion bodies (subcycles). The close association of rock type and sedimentary structures between most intraclasts and mud streaks preserved between some lateral accretion bodies probably indicate that the bar intraclasts were derived from previously deposited mud drapes. The composition of some mudstone fragments is identical to that of the structureless mudstone of flood plain deposits and is most likely derived from bank caving. In addition to brecciated mud clasts, an additional distinctive feature of low sinuosity meandering sandstone bodies is the presence of an irregular and high rugged erosional basal scour surface, as interpreted from the correlation of drill hole logs from closely spaced wells (Fig. 5). Collectively, these facts imply that, as compared with P12, the P12-3 river was subject to sudden floods, strong channel-base erosion, high rates of sediment deposition, and frequent channel migration. FIG. 4.-Lithofacies sequence of low sinuousity meandering sand- As a result, two features of in-layer heterogeneity in low stone body (Subzone PIzc3, Well L7-28, Daqing). sinuosity meandering units are evident which contrast with

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25 26 27 28 S144 3-30 31 32 33 34

4-B20

O 200 400 M c-- Density of channel sandbody 60%

FIG. 5 .-Geometry of low sinuousity meandering sandstone body, (Subzone P12+3,Daqing).

those of bars developed in highly sinuous sandstone bodies. units, however, may act as a relatively continuous barrier In Case 2 the uppermost meander-belt sandstone body cuts to fluid flow. into and was developed upon the underlying bar unit to form As shown in Figure 5, it is likely that composite sand- a thicker composite sandstone body. The internal hetero- stones with good lateral continuity are composed of nu- geneity of the composite unit was reduced because of the merous genetic units of the meander belt. These individual apparent juxtaposition and connection of the upper and lower units are very narrow in width (about 150-400 m, accord- sandstone units. Although the permeability contrast in the ing to log correlation) and they vary in thickness and grain composite unit is 10-20 and as much as 40, approximately size corresponding to the varied hydraulic conditions of the equal to that of case 1, the variation coefficient of per- ancient PI2-3 river phases. A single, complete point-bar se- meability is apparently declined to 0.6-0.7 and is never over quence ranges from 5 to ll m thick, thus the width/thick- 1. The second contrasting feature is poorer preservation of ness ratio of a meander belt sandstone body is only 30-60. mud drapes in low sinuosity meandering bodies than in high Additionally, we infer a 150-200 m of bankfull width. From sinuosity units. The flood plain mudstone between the stacked these data, a conclusion could be logically made that the

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ancient P12-3 river was a low sinuousity meandering stream ben. As a result, the channel sandstone bodies of individual with frequent avulsion and migration phases and that it fre- streams generally do not coalesce laterally to form wide- quently changed hydraulic conditions. spread rock units. As a result, in this scenario, many isolated sandstone bodies are embedded in structureless and thick mudstone of the flood plain. BRAIDED MODELS Long-coursed rivers, such as those developed along a ba- The main sandstone bodies deposited in a braided river sin’s longitudinal axis and during the waning stage of a lake are mid-channel bars developed dominantly by vertical ac- basin, may develop widespread, multilateral sandstone,bodies cretion. Vertical accretion is an accumulation process in as a result of sufficient supply of sediments and rather slow which topographic elements of the surface do not shift lat- rates of basin subsidence. erally but instead build upward. Mid-channel bars that ac- cumulate vertically during a series of flood events are char- Case 3. Short-coursed braided sandstone bodies-SaII2 acterized by a unique in-layer heterogeneity that contrasts and Sa113 in Shentuo field, Shengli oil province.- with that of bars formed in meandering streams. Each flood event is characterized by its hydraulic force and capacity Subzones Sa112 and Sa113 contain the main rock reser- to transport grains of varied sizes. Mid-channel bars are voirs with high productivity (from tens to more than 100 deposited through multiflood events such that internal bed- t/d oil/well) in Shentuo field, Shengli oil province. They form and grain-size stratification exist but do not grade reg- consist of a series of interbedded pebbly medium- to coarse- ularly between unit boundaries. In turn, internal variations grained sandstone units. The reservoir units are easily iden- of permeability are controlled by characteristics of the re- tified as being of a fluvial facies that extends between the sultant stratification. northern alluvial fans (marked by abundant debris flow de- An important characteristic of internal heterogeneity of posits) and southern lacustrine deltas (with typical river- the braided bars is that few mud streaks (drapes) are de- mouth bars). Tectonically, the reservoirs are located along posited and preserved due to the stream’s small suspended the steep depositional slope and their formation was con- load/bedload ratio. Mud streaks, which may form in braided trolled by a series of active major basin-boundary faults. bars and constitute barriers to fluid flow, are formed in The median grain size of the fluvial sandstone units ranges abandoned channels. The lateral continuity of abandoned from 0.3 to 0.6 mm with very poor sorting. The fine grav- channel fills however, seems impossible to extend more than els, almost all derived from source rocks of granite, are a channel width, which when compared with the entire di- developed throughout the section. Few intraformational clasts mension of a braided bar, is of little significance to fluid can be seen. Vertically, the sequence of grain size is dis- flow. Few significant barriers to fluid flow exist in braided ordered and some abrupt changes are evident. Coarsest grains sandstone bodies. Thus, the vertical permeability is ap- are not confined to unit bases (Fig. 6). Although crossbeds proximately equal to the horizontal permeability. This per- are well developed in the sandstone, parallel lamination is meability character is very important for the cross flow of not uncommon. Most crossbeds seem to be tabular with fluid when secondary or tertiary oil recoveries are at- low (I5”) foreset dips; some tabular and trough crossbeds tempted. In waterfloods, the injected water always ad- with high foreset dips are evident. We did not identify rip- vances rapidly along the lowermost interval with little ple or horizontally laminated siltstone or very fine-grained thickness to the zone being swept, even though the maxi- sandstone as the top stratum of the point bar. Some sand- mum permeability pathway is not generally at the unit’s stone sequences, however, are capped by a thin-bedded base. These data indicate a great effect of gravity on fluid gravel which in turn is overlain by the flood-derived mudflow due to the density differentiation between oil and water. stone. Few mud streaks are identified within the sandstone Although a braided river is commonly considered to be bodies. We infer from these sedimentary features that the able to deposit continuous sandstone bodies due to less co- fluvial sandstone bodies were deposited by a braided river hesive stream banks and frequent channel migration, a system near its source, that it was subject to strong inter- number of braided sandstone bodies has been identified in mittent floods and had a low suspended load/bedload ratio, Chinese lake basins in which these sandstone characteristics and that they resulted from a deposition pattern of vertical were not apparently dominant. Both a multilateral sand- accretion. We believe that this pattern is similar to that of stone body with a wide areal distribution and an isolated the Donjek type proposed by Miall (1978). sandstone body with little continuity can be formed in braided The geometry of Subzones Sa112 and SaII3, as delineated rivers depending on the river’s tectonic setting. For ex- by production wells (closest well spacing is only 10 m), is ample, short-coursed braided rivers, extensively developed obviously narrow and elongated with a width of 200-600 at the strongly faulted side (transverse to the strike of the m and a width/thickness ratio of 40-70 (Figs. 7, 8). A few tectonic framework) of faulted lake basins in China, com- multilateral sandstone bodies may be as wide as 2000 m. monly produced isolated sandstone bodies. The internal vertical permeability heterogeneity varies In faulted basins, several key factors influence the chan- greatly and seems to conform to the changes in grain size. nel sandstone bodies deposited. Those factors are the ba- Maximum permeability intervals are commonly located in sin’s steep slope (maybe as much as 10 percent), the stream’s the middle to lower part of the reservoir and are not along short course (I20 km from source area to depocenter), unit bases. Five to 20 of permeability contrast and 0.88 rather small-scale rivers (depth < 10 m), and very active of average variation coeffiGent of permeability indicate that basin margin faults with high subsidence rates in the gra- the braided sandstone bodies have slightly less heteroge-

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light green and purply-red mudstone. The sandstone units L ithofaaes ;rain size P ermeabi1 it y can be classified as arkose and graywacke. They have a median grain size of 0.3-0.4 mm and contain pebbles in 4 sequence (Darcy ) amounts that vary with degrees of poor to moderate sorting. The depositional sequence of NgI sandstone units has an 7 -0.1 0.20.3 0.4 erosional base that is overlain by lag deposits of conglom-

I erate or conglomeratic sandstone (Fig. 9). Most pebbles were originally derived from the source area of the sediments and a few are of intraformational mudstone fragments. Crude imbrication of pebbles can sometimes be identified. Coarse- to medium-grained sandstone that contains pebbles constitutes most of the sequence. Although no clear depositional cycles can be recognized, a crude fining-upward sequence can be identified in some units. Large-scale tabular and trough crossbeds with 20"-30" foreset dips are developed throughout the section; parallel bedding is locally evident. The massive flood mudstone marked by abundant plant de- 9 ry...... bris, rootlets and carbonate concretions, directly overlies .. .. .'...... the crossbedded rocks. Some thin-bedded (10-20 cm) siltstone occurs with other rock types. Mud streaks have not been found, a phenomenon that may suggest that lack of suspended-load sediments characterized the streams. Thickness of individual sandstone bodies is generally 5- 8 m but ranges upward to 13 m. Some multistory composite units may be more than 20 m thick. Laterally, channel sandstone bodies change rapidly in thickness or elevation. Coalescence of clastic units is extensive, as interpreted from drill hole log signatures (Fig. 10). The very good continuity of reservoirs is evident in production performance by the large volumetric influx of very active edge water. This phenomenon indicates the existence of a large connected water body beyond the zone of oil saturation of the reservoir. It FIG. 6.-Lithofacies sequence of short-coursed braided sandstone body (Subzone SaII:'4 Well 2-3-51502, Shengtuo field). is this good reservoir continuity that marks an important distinction between long-coursed and short-coursed braided sandstone reservoir bodies. The internal heterogeneity of NgI sandstone bodies is neity than meandering ones. In addition, the vertical/hor- similar to that of Case 3 discussed above in the rather dis- izontal permeability ratio measured on a core plug is par- ordered variation of permeability, permeability contrast of ticularly large, ranging from 0.8 to 0.9 due to the lack of 4- 14 and as much as 30, and average variation coefficient clay minerals in the rock's matrices. of permeability 0.74. Silt streaks are believed to be present, Case 4. The long-coursed braided sandstone bodies- and they undoubtedly act as barriers to fluid flow, for the plant debris commonly associated with silt laminae is abun- Subzone NgI of Gongdong field, Dagong oil province.- dant in most sandstone units. The Lower Neogene Guantao Formation (Ng) was widely deposited in Bohai Bay Basin overlying a regional unconformity developed upon Eocene rocks at a time when the STRAIGHT DISTRIBUTARY CHANNEL MODELS basin's floor was rather flat. There was sufficient erosion The distributary channels of lacustrine deltas commonly of the surrounding mountains, however, to supply a large are straight because of their small size and rather low en- volume of sediments to this basin. Consequently, fluvial ergy where they intersect the lake. Meandering distributary deposits, dominantly of braided and low sinuosity mean- channels may be able to develop, especially in the larger dering streams, were extensively developed and cover a large delta complex of longitudinal systems. In the latter case, area of the basin. the in-layer heterogeneity is similar to that of the fluvial Subzone NgI, has been identified as consisting of braided meandering model described above. sandstone units. A main secondary oil reservoir in Gong- Straight distributary channels consist dominantly of ac- dong field is at the upstream reach of the northern depo- tive channel fill. Straight channel-fill sediments appear to sitional system, and it is one of the basin's largest braided be crudely segregated by gravitational forces during their systems with a course length of 120 km developed along accumulation, so that the process of this deposition results the basin's depositional axis. in crude sediment cycles in which grain size decreases up- NgI is composed of grey and brown conglomeratic sand- ward from a unit's base; permeability values decrease up- stone, and coarse- to fine-grained sandstone intercalated with ward as grain size decreases. The degree of lithologic and

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1 Isopach of sandstone 2 Pinch out of sandbody 3 Direction of source area 4 Contact with alluvial fan 5 Well 6 Section in Fig.8

FIG. 7.-Geometry of short-coursed braided sandstone bodies, (Subzone Sa@, Shengtuo field).

NW Well spacing (m) SE

Density of channel sandbody 25.9-33.3 % FIG. 8.-Correlation section of subzones SaII,,SaII,, the short-coursed braided sandstone bodies, Shengtuo field.

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therefore permeability variation, however, is by far less than 5 I I Lithofacies IGrain size1 Permeability those of point bars formed by lateral accretion. An additional important characteristic of straight distributary channel sandstone bodies is the low width/thickness ratio, which results in a very discontinuous lateral geometry of the rock unit. The low ratio seems to be much more important than the internal heterogeneity of permeability in determining the success of secondary and tertiary recovery attempts. The most difficult problem for recovery attempts is that it is impossible to drain oil effectively from these units in the absence of a very dense well spacing, more dense than that normally used in recovery attempts from most sandstone reservoirs. Case 5. The sandstone body reservoirs in a set of straight distributary channels-the Subzone SII13-16in Sardu field, Daqing oil province. - SI113-16is a deltaic unit of the basin’s largest depositional system of Early Cretaceous age in Songliao basin. This depositional system contains the majority of reservoirs in Daqing. As shown in Figure 13, most of the deltaic-plain channel sandstone bodies of SI113-,6are very narrow, are elongate with a N-S trend, intersect the lake shore, and conform to the regional direction of stream sediment transport. This geometry of zone sandstone bodies iilustrates the straight pattern of the ancient SI113-16distributary channels. Inter- pretation of their lithofacies composition indicates that they

I I L originated as channel fills. In addition to common sedimentary characteristics of these sandstone bodies, such as a basal scour marked by mudstone pebbles, fine-grained and small-scale crossbedded to FIG. 9.-Lithofacies sequence of long-coursed braided sandstone body (Subzone NgI, Well G212, Gongdong field). ripple crossbedded sandstone, two subtypes of lithofacies sequence can be identified. The first lithofacies subtype is a thick sandstone interval overlain by a thinner layer of rock

10-18 10-20 234 10-25 10-27 232 10-32 E

Density of channel sandbody 53%

FIG. 10.-Correlation section of Ng, the long-coursed braided sandstone bodies, Gongdong field.

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derived from suspended sediments (Fig. 11). Lack of significant vertical variation of grain size in the sandstone unit L ith of acies Grain size Permeability implies rapid sedimentation in channels; the fact that the top stratum is composed mainly of mud-size grains suggests rapid abandonment of the distributary channel. Moreover, we suggest this sequence indicates that the main distributary channels carried a large volume of sand, The second lithofacies subtype is composed of a thin sandstone interval overlain by a thicker unit composed of interbedded siltstone to very fine-grained sandstone and mudstone in centimeter-scale thick beds (Fig. 12). The mudstone is developed with well defined flat laminae. This lithologic sequence is formed in subordinate distributary channels that carried sandy sediments which did not fill the channel. The process of channel filling was therefore pro- longed and influenced repeatedly by the main distributary L channels from which some sandy suspended sediments pe- x- riodically overflowed into the secondary channels. Straight distributary channel sandstone bodies always feature an FIG. 12.-Lithofacies sequence of straight distributary channel sand- asymmetric fining-upward sequence as opposed to the uni- stone body (slowly abandoned, Subzone SIII4,Well Z5 1-5203, Daqing). formly fining-upward sequence characteristic of most point bars in sinuous systems. summary, the internal heterogeneity of Subzone su13-16 In most sandstone bodies in Subzone SI113-16are 100-150 m is characterized by: (1) normal rhythmic (cyclic), grain size wide and change abruptly to mudstone of the interdistribu- variations and resultant permeability values, (2) a basal in- tary facies. The thickness of an individual sandstone ge- terval of maximum permeability corresponding to a coarse netic unit ranges from 3 to 5 m, and we assume a width/ basal rock, (3) a great permeability contrast in the unit of thickness ratio of 20-40 by comparison to their modem as much as 30-50, and (4) a smaller variation coefficient equivalents. Thus, the fluvial architectural system of this of permeability about k 0.5. zone is composed of many isolated channel sandstone bod- Some multistory composite sandstone bodies are devel- ies of various dimensions imbedded in massive mudstone. oped as a result of vertical stacking. Since downcutting is not great in distributary channels, the complete removal of the mud deposits at the top of the underlying channel sand- THE CONFINED CHANNEL-FILLING MODELS stone bodies is rare. Thus, these multistory sandstone bod- The anastomosed river pattern as defined by Smith (1983) ies are commonly separated by thin mud barriers and each has not been definitely identified yet in Chinese ancient lake genetic unit is an isolated reservoir unit for fluid flow. basins. Although some fluvial sandstone bodies have been tion of drill hole logs has shown (Fig. 13) that reported to be of that pattern (Chen, 1984), the relation is not accepted by all. We suggest, however, that many of the “anastomosed” channels reported in the Chinese literature n should be cautiously called confined channel-filling depos- E Lithofacies Permeability W its, as described here. Examples are found in some oil fields, 5L such as the Zone Y10, the Lower Jurassic of Maline field a2 sequence I in Shanganning basin, northwestern China. The Lower Jurassic sequence in Shanganning basin was 8U 300 500 1000 3000 I1 formed on a post-Triassic erosional surface, which had geo- 13 morphologically formed as a stream valley system flowing 855 to the coast. During the earliest depositional period, this 8 -13- valley system was continuous because of the relatively sta- 18: ble equilibrium between valley downcutting and tectonic - 1’3 ,. r.. uplift. As a result, a set of confined channel-fill sandstone ...... units were deposited to form strata about 300 m thick and, 9 e..”.;G. the main reservoir in Maline field is the upper part of this . . , ... sandstone suite. An interesting phenomenon of these res- 860 ervoir sandstone bodies, which may be indirectly regarded as the evidence by which they are identified as confined channel-filling deposits, is that the mineralogical and pe- trophysical properties of sandstone bodies deposited in different paleotributary valleys contrast greatly due to their FIG. 11.-Lithofacies sequence of straight distributary channel sand- different source terranes, even though in some cases the stone body (rapidly abandoned, Subzone SIII2,Well Z5 1-5203, Daqing). distance between two tributary valleys is less than 10 km.

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200 ...... -7- -- A --_ I’ 1 ::l. Y .’. qp. . ’.?.i.‘ . ” *-> ” Density of channel sandbodv - [ 27 % \ -- -‘A-/-.

- 200 350‘- \ 150 ‘, 200 C C’ I.

E’

FIG. 13 .-Geometv of straight distributary channel sandstone body (Subzone SI115+,6,northern Daqing).

The lithology, sedimentology, and reservoir variability of Sedimentary features described herein could be reason- this type of channel sandstone body can be characterized ably interpreted to mean that the Y10 channel sandstone by using Zone Y 10 as an example. bodies are the result of aggradation in confined valleys during flash flood events. Each thin subcycle is the result of Case 6. Confined channel-jill sandstone bodies-Zone a flood event of great magnitude and short duration. More- Y10 in Maline field, Shanganning basin.- over, flat laminated muddy siltstones sharply overlying the The Y 10 reservoir zone is chiefly composed of sandstone top of subcycles illustrate the existence of rapid and inter- and conglomerate, which collectively constitute more than mittent channel abandonment. Their excellent preservation 70 percent of the total thickness of the section. Both are could be considered the result of the relative balance be- mineralogically immature and they are rich in clay min- tween channel aggradation and tectonic basin-floor subsi- erals. In places, the reservoir contains clayey conglomer- dence. ates with the pebbles floating within the clay matrices. The The geometry of confined channel sandstone units is typ- conglomerates are probably debris flow deposits. ically that of a “shoestring. ” The thickness of the units can The lithofacies sequence contains a number of thick fin- be several times that of the river’s depth due to multiple ing-upward cycles; a number of subcycles can be recog- episodes of aggradation and depositional stacking. Conse- nized (Fig. 14). Each subcycle contains a fining-upward quently, no empirical value of width/thickness ratio could sequence that is capped by centimeter, thick flat or ripple be established for purposes of prediction. laminated siltstone or muddy siltstone. Most cycles are ap- As shown in Figure 15, the Subzone Y10-2 apparently proximately 1.0 m thick but some range to 4.5 m. consists of two main channel sandstone bodies which had All pebbles are derived from one source area, and the a width of 400-800 m in its coarser parts. units contain little evidence of grain fabric. Carbonaceous Sandstones of the Y 10 zone have been subjected to min- plant stems are very common near the bottom of the coarser eral diagenesis. Permeability variation in the zone cannot subcycles. The fact that it is very rare to detect two sets of be used as a guide to general depositional characteristics of crossbeds in the sandstone units through a core tens of cen- reservoir heterogeneity. The in-layer heterogeneity of the timeters long means that the crossbed sets must be of a sandstone bodies of the confined channel-fills , however, large scale. Parallel lamination has been identified in some suggests they should be similar to that of multistory normal sandstone. cycles. The smaller variation in grain size in zone units

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The width of a genetic unit in a river belt is controlled Permeability by the river’s paleohydraulic character, and it may there- L ithofacies fore differ from one to another depending on the river’s size. Generally, the width/thickness ratio of fluvial sand- sequence (md) stone bodies can be arranged in a decreasing order: (1) high sinuousity meandering, (2) braided, (3) low sinuousity meandering, and (4) straight channel sandstone bodies. The 100 200 300 examples described above are given in this order. The fact, however, that lake-basin rivers are generally small in scale x with a depth of no more than several meters and with widths of tens to hundreds of meters indicates that the width of a genetic unit composed of fluvial sandstone bodies cannot exceed several hundred meters. From a practical standpoint, the variability in width of a genetic unit produced by a river pattern in a lake basin will have an insignificant effect on oil production. The degree to which multiple genetic units are connected to form a composite multilateral sandstone body has proven, however, to be the most significant factor in determining the effectiveness and size of rock reservoirs in Chinese lake basins. 0.0 Allen (1978) proposed a theoretical density of channel ’O’ sandstone bodies based on his simulation research, which relates degree of connection of multiple fluvial units. He concluded that 50 percent is a critical value. In our examples, the densities of channel sandstone bodies are estimated using data derived from a large number of production wells. We have found that the density of channel sand bodies in one dimension can be used to predict the connection in three dimensions. A value of over 50 percent, such as in Cases 2 and 4, confirm Allen’s hypothesis. We expect these units to be well connected. Values of less than 30 percent, such as in Cases 3 and 5, suggest to us that only isolated genetic units will be expected. Values between 30 to 50 percent, however, should be used carefully because of their uncertainty. In Case 1, the density of channel sandstone bodies is 47 percent, assuming a rock interval consisting of four layers of sandstone bodies, and that one FIG. 14.-Lithofacies sequence of confined channel-filling sandstone layer will be effectively connected. This tells us that it is body (Zone YlO, Well L68, Maline field). important to select a proper stratigraphic interval for estimation. Ideally, the thinner the interval, the more accurate the results should be. It is impossible in practice, however, seems to produce a rather small permeability contrast, and to correlate in detail thin intervals in the subsurface, be- this relation is similar to that of straight distributary channel cause only a few evaluation wells can be used for prediction sandstone bodies. In those beds where debris flow deposits during the early stages of development drilling. exist at the unit bases, the interval maximum permeable It should be emphasized that many fluvial units in lake values may be developed in the middle-lower part of the basins are poorly connected. The small scale of rivers and reservoir. Internal thin-bedded muddy siltstone units are rapid subsidence of the lake basin floor result in restricted barriers to fluid flow where preserved, and they are gen- units. If we assume 1000-2000 yrs as the avulsion period erally a really extensive relative to the lateral continuity of and 0.005-0.0025 m/yr as the basin’s rate of subsidence, the overall sandstone body. fluvial sandstone bodies produced in the same genetic channel belt at a 5 m-thick scale cannot be effectively connected. Such a rate of subsidence is realistic during the main LATERAL CONTINUITY period of deposition in the Mesozoic-Cenozoic lake basins The geometry of fluvial sandstone bodies always occurs of China. We believe it is these conditions that produced in elongated shapes that are substantially determined by the most of the fluvial units in China that are poorly connected geomorphology of the originating rivers. In general terms, and laterally restricted sandstone reservoirs. Some excep- a good lithologic continuity can be expected along the tions have been found, such as in Cases 2 and 4. In those channel’s trend. Therefore, the lateral continuity-the width cases, however, units were deposited at a time either when of sandstone bodies-is the key to determining the effec- the basin changed from being uplifted to a stage of subsi- tiveness of the fluvial petroleum reservoir. dence or from a stage of lake regression to transgression.

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FIG. 15 .-Geometry of Subzone Y lo2, the confined channel-filling sandstone body.

TABLE1 .-THE DIFFERENCES IN RESERVOIR HETEROGENEITY OF SAND BODIES DEPOSITED IN VARIOUS RIVER PATTERNS

The High The Low The Short- The Long- Sinuosity Sinuosity coursed coursed The Straight The Confined Meandenng Meandering Braided Braided Distributary Channel Sequence in uniformly fining single or multi- disordered disordered or asymmetric fining multiple, grain size upward fining upward roughly fining upward asymmetric fining upward. upward The location of bottom bottom uncertain bottom or bottom botom or middle to maximum middle-lower lower permeability Variation 0.8-1.0 (1.3)* 0.6-0.7 (1.0) 0.4-1.0 0.5-1 .O r+ 0.5 (0.8) similar to the coefficient of straight distributary permeability Permeability 10-20 (50) 10-20 (40) 5-20 (40) 4-14 (30) 30-50 contrast Internal muddy mud drapes of mud drapes, channel channel overbank, channel channel filling, barriers lateral accretion, overbank, rich filling, rare filling, rare filling, rather rather continuous rich continuous Width/thickness ratio of a genetic unit 130-170 30-60 40-80 r 100 20-40 Very small *The value within brackets is the maximum.

During these events, subsidence of the basin started at a be subdivided into six models based on “depositional pro- very slow rate, thus a widely distributed fluvial sandstone cesses-reservoir heterogeneity” and the depositional and body with multiple connections was easily formed where tectonic conditions in a lake basin. The comparison be- sediment supply was sufficient. tween them is listed in Table l as follows: The small scale of rivers in a lake basin indicates that the width of fluvial sandstone bodies is in the magnitude CONCLUSIONS of hundreds of meters or less. The connection of multiple From the standpoint of the in-layer heterogeneity and genetic units becomes the key link for forming a continuous continuity for oil recovery, fluvial sandstone reservoirs can sandstone body.

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The density of channel sandstone bodies could be em- REFERENCES pirically used to predict their degree of connection. Values ALLEN,J. R. L., 1978, Studies in fluviatile sedimentation: An exploratory of over 50 percent indicate a good connection; only isolated quantitative model for the architecture of avulsion-controlled alluvial sandstone bodies can be expected when the value is less suites: Sedimentary Geology v. 21, p. 129-147. than 30 percent. Care should be underscored when the value CHEN,Y. L., 1984, A preliminary analysis on the types of river system ranges between 30 and 50 percent. depositional characteristics and development results of the Guantao Reservoir in Gudao Oilfield: in Chinese, Petroleum Exploration and Development, v. 11, p. 66-72. ACKNOWLEDGMENTS MIALL, A. D., 1978, Lithofacies types and vertical profile models in braided river deposits: A summary in Miall, A. D., eds., Fluvial Sedimentol- The authors acknowledge the encouragement and dis- ogy, Canadian Society of Petroleum Geologists Memoir 5, p. 597- cussions of Professor Qin Tonglou and of their colleagues 604. at Daqing, Shengli, Changqing and Dagang oil fields. Sin- SMITH,D. G., 1983, Anastomosed fluvial deposits: Modern examples from Western Canada: in Collinson J. D., and Lewin J., eds., Modern cere thanks are due to Dr. T. Fouch who revised, and put and Ancient Fluvial Systems, International Association of Sedimen- into an understandable form, the English text. tologists Special F’ublication 6, p. 155- 168.

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JOHN MELVIN Standard Oil Production Company, 5400 LBJ Freeway, Suite 1200, Dallas, Texas 75240

ABSTRACT:The hydrocarbon reservoir at Endicott Field, located offshore from the Sagavanirktok Delta in northeast Alaska, is the Kekiktuk Formation which is late Mississippian in age. It rests directly on weathered basement of weakly metamorphosed argillite. The lowermost Kekiktuk, known as Zone 1 in Endicott Field, is not recognized at the type section in the northeastern Brooks Range. It consists of coarsely interbedded mudstones, siltstones, coals and very fine- to fine-grained sandstones. Facies relationships suggest that deposition took place upon an undulating peneplaned basement surface in a number of environments. Those included: well and poorly drained swamps; deeper, swamp-plain lakes; partially and wholly abandoned fluvial channels, and a number of overbank environments including levees and possibly crevasse splays/lacustrine deltas. The overall setting was a low-lying swamp plain with numerous lakes, traversed by sluggish, mixed- to suspended-load streams. In the Endicott Field area, the streams appear to have been more dominant in middle Zone 1 time; however, the distinction between streams and overbank environments was not marked, and the area was characterized by common overbank flooding and channel abandonment. By end-Zone 1 time, the entire area appears to have been blanketed by swamps and swamp-plain lakes.

INTRODUCTION scribed by Brosge and others (1962) from near Lake Peters The Endicott Field is located in the Beaufort Sea 1-4 mi in the northeastern Brooks Range. Nilsen (1981) described (1-6 km) offshore from the Sagavanirktok delta in north- how in that location the Kekiktuk may be divided into three east Alaska (Fig. 1). The field was discovered in January members. The basal member, 1 to 3 m thick, represents a 1978 with the drilling of Sohio Sag Delta 4 well and con- thin interval of mixing of weathered basement rocks and tains about one billion reservoir barrels of oil. Production other transported clasts. Overlying this basal member is an from the field is scheduled to start in early 1988. The res- interval of massively cropping out fining-upward couplets ervoir is made up of Mississippian fluvial sandstones of the of pebble/cobble conglomerate and sandstone 58 m thick. Kekiktuk Formation, which is the lowermost of three for- The highest member of the Kekiktuk at the type section is mations comprising the Endicott Group (Fig. 2). In the dis- 20 m thick and is composed of several fining-upward cycles covery well, the Kekiktuk was encountered at 9779 ft (2981 of conglomerate, sandstone and shale. m) subsea. The trap comprises a tilted fault block within a The Kekiktuk Formation in the subsurface at Endicott Field southwesterly plunging antiform that is truncated updip by is somewhat different from the type section. It also rests a major unconformity, known as the Lower Cretaceous Un- directly on basement rocks, and where that contact has been conformity, and bounded to the north by the Niakuk Fault cored, about 1 m of altered and weathered basement ma- Zone. terial can be seen, similar to that described at the type section by Nilsen (1981). Above that, the Kekiktuk is subdivided into three distinct zones that can be easily identified GEOLOGIC SETTING on wireline logs and correlated with core material (Fig. 2). The Endicott Group crops out in the Brooks Range of They are numbered from the base upward. Zone 1 rests northern Alaska where it has been studied extensively by directly on the weathered basement and is composed of U.S. Geological Survey workers. In the central and western mudstones, siltstones, coals and fine-grained sandstones. Brooks Range, it forms an allochthonous (overthrust) ter- The base of Zone 1 is defined by a downward increase in rain, whereas in the northeastern Brooks Range and the North bulk density from about 2.2 to 2.4 g/cm3 to greater than Slope subsurface the Endicott Group is considered to be 2.7 g/cm3 (K. Woidneck, pers. commun.). Its top is de- autochthonous (Nilsen, 198 1; Nilsen and Moore, 1982; fined by the base of the “barrel”-shaped gamma curve pro- Moore and Nilsen, 1984). The allochthonous Endicott sed- file that typifies Zone 2, (Fig. 2) along with the highest iments represent a regressive-transgressive sequence, the occurrence of the uppermost Zone 1 coal or shale section. lower part of which is composed of marine deposits of the Zone 2 consists primarily of medium- to coarse-grained Hunt Fork Shale and the Noatak Sandstone. These rocks sandstones with subordinate mudstone and granule con- are overlain by fluvial deposits of the Kanayut Conglom- glomerate and is characterized by a blocky gamma curve erate (Moore and Nilsen, 1984). The Kanayut is Devonian profile. Zone 3 is composed of varying amounts of sand- in age, and thus is older than the autochthonous lower Mis- stone and shale with subordinate coal and conglomerate. Its sissippian Kekiktuk Formation (lower Endicott Group) at lower contact with Zone 2 is transitional and difficult to its type section in the northeastern Brooks Range. The Kek- determine on logs; the top is defined by an upward increase iktuk of the subsurface at Endicott Field is still younger, in bulk density from about 2.5 g/cm3 in the Kekiktuk to since palynological studies suggest a Visean (Late Missis- 2.8 g/cm3 in the overlying calcareous Kayak or Itkilyariak sippian) age for those rocks (J. E. Williams, pers. com- Formations (K. Woidneck, pers. commun.). mun.). In all areas the fluvial coarse clastics of the lower The Kekiktuk in the subsurface differs from that in the Endicott Group are overlain by shales of the Kayak For- type section in that (i) it is generally finer-grained and (ii) mation (Fig. 2) which represent a marine transgression. it contains the distinctive lowermost interval of mudstone, The type section of the Kekiktuk Formation was first de- siltstone, coal and sandstone defined as Zone 1, which is

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DEPOSITIONAL ENVIRONMENTS General.- Since the discovery of Endicott Field in 1978, a number of wells have penetrated the Kekiktuk Formation, as have other wells in the area surrounding the field (Fig. 1). ENDICOTT Downhole logging has enabled Zone 1 to be identified in those wells, but only limited coring has taken place -. . throughout its entire thickness. Palynological studies of ditch cuttings and core material in this zone do not reveal the Miles - presence of any marine palynomorphs (J. E. Williams,

O 10 30 L. Ravn, pers. commun.) and it is considered to have 20 3 1- 10- 16 R. Kilometers been deposited entirely within a continental setting. FIG. 1 .-Location map of Sagavanirktok Delta area, showing Endicott Those cores that have complete recovery from Zone 1 Field and wells that have penetrated Kekiktuk Formation (open circles). have yielded important information regarding lithofacies , Where Zone 1 has been cored is indicated by black circles. vertical grain size distribution and sedimentary structures, allowing considerable appreciation of depositional processes and environment to be gained. Figure 3 illustrates the sedimentary sequence in Zone 1 where it has been com- not present in the type section. Zone 1 ranges in thickness pletely cored in two wells at Endicott Field (Fig. 1). Those from about 30 m (100 ft) to over 61 m (200 ft) across the wells are approximately 1.2 km (0.72 mi) apart. Endicott Field area and is the subject of this paper. The This lower Kekiktuk sequence is dominated by fine-grained sedimentological development of the entire Kekiktuk For- sediments (mudstone, siltstone, coal and fine-grained sand- mation at Endicott Field in relation to these three zones will stone) arranged in sequences exhibiting a variety of vertical be presented by the author elsewhere. grain-size trends (Fig. 3). The general fine-grained and coaly

~ TERTIARY SAGAVANIRKTOK Fm. I COLVILLE I CRETACEOUS TOROK Fm. (HRZ) KUPARUK RIVER Frn.

KINGAK Fm. JURASSIC LIi SAG RIVER Frn. Y 3 TRIASSIC SHUBLIK Fm. I- Y PERMIAN Y W Y PENN- SY LVANIAN LISBURNE Gp. II

ITKILYARIAK F MISSISSIPPIAN 31KAYAK Fm. ENDICOTT Gp. KEKIKTUK Fm - Fran Base PRE- FRANKLINIAN BASEMENT MISSISSIPPIAN I FIG. 2.-Stratigraphic column showing position of Kekiktuk Formation at base of Endicott Group, overlying Franklinian Basement, and subsurface reference section from Endicott Field showing tripartite zonation of Kekiktuk Formation.

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Well A Well B metres metres ZON ZON PDS ZON- ZON PDS WDS (with shallow lake infill in upper part) Lacustrine PDS

PDS Lacustrine

Levee WDS

Overbank PDS WDS PDS PDS

WDS Partially Abandoned Channel Ch. Margin Slump? Abandoned Abandoned Channel Channel 'Oxbow Lake) Partially Abandoned Overbank Channel

Abandoned PDS Yiw?- WDS WDS

PDS PDS

Abandoned WDS Channel Lacustrine PDS Swamp

FIG. 3.-Lithostratigraphic sections through lower Kekiktuk (Zone 1) based on complete core coverage in two wells at Endicott Field. Note predominance of lacustríne/paludal sequences in lowest and highest parts and abandoned channels (fining-upward) in middle part of the section. (WDS: well drained swamp; PDS: poorly drained swamp). For legend see Figure 4.

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character of these rocks suggests deposition upon a low- ment. There also the sequence commences with a sharp, lying plain, within which a number of subenvironments are scoured basal contact that is overlain by about 1 m of coarse represented. These subenvironments are described below. sandstone containing abundant mudchips and coalified plant debris. That deposit, however, is abruptly overlain by a Partially abandoned channel deposits. - thick sequence (about 9 m) of thin (10 to 50 cm), fine- to Zone 1 is characterized by fine-grained lithologies and a very fine-grained sandstones separated by very thin (1-3 distinct lack of sandstone-dominated fluvial channel de- cm) silty partings. The lowermost of those thin beds con- posits. There is evidence for limited channel activity, how- tains dense concentrations of mudchips (18-19 m level, Well ever. In Well A (Fig. 3), the interval between 30 m and B, Fig. 3). Following channel cutting and deposition of the 39 m is reproduced in detail in Figure 4. The sequence basal coarse sand, partial abandonment of the channel con- commences with a medium- to coarse-grained crossbedded fined deposition within the channel to predominantly sus- sandstone overlying a sharp erosional base, suggesting ac- pension fallout of fine sand introduced during flood events tive migration of dune forms on the base of a channel. The in the nearby main channel. Complete abandonment is dis- crossbedded sandstone is only about 1.7 m thick, and fines counted due to the lack of very fine-grained, argillaceous upward into featureless, medium-grained sandstones that sediment. Similar interpretations have been made on out- are also underlain by sharp, locally scoured bases. Those crop sections in the Carboniferous of Nova Scotia by Ger- sandstones are overlain by a thin interval of siltstone with sib and McCabe (1981). interbeds of fine- to very fine-grained sandstone (Fig. 4). Wholly abandoned channel deposits. - This entire fining-upward interval is interpreted as point- bar deposits within a fluvial channel (Fig. 4). That se- Ethridge and others ( 1981) discussed how abandoned quence is truncated by another erosionally based medium- channel deposits are difficult to recognize in cores, because to coarse-grained sandstone about 1.8 m thick which is they consist of fine-grained, parallel laminated, organic-rich abruptly overlain by about 3 m of fine-grained sandstones clay stone and siltstone with very fine-grained sandstones that are intensely mottled, penetrated by roots and possibly that are not unique to that environment of deposition. bioturbated (Fig. 4). Nonetheless, the recognition of such subenvironments is The composite sequence described above is about 10 m important to an understanding of the three-dimensional ge- thick (Fig. 4) and is thought to represent a partially aban- ometry of the interval and distribution of permeability bar- doned channel. Following active point-bar deposition (rep- riers within hydrocarbon reservoirs. A few wholly aban- resented in the lower part), the bar top was scoured, pos- doned channel deposits (?oxbow lakes) have been recognized sibly by a chute channel which was actively infilled and within the lower Kekiktuk cores examined in this study. then abandoned, resulting in gradual accumulation of bio- In Well A (Fig. 3), the sequence of deposits between 23 turbated, root mottled fine-grained sand. This indicates that m and 29 m is thought to represent a wholly abandoned relatively long periods of time passed between depositional channel or oxbow lake fill. It is reproduced in detail in Fig- events, allowing complete destruction of any primary sed- ure 5. The sequence commences with a granule conglom- imentary structures by plants and possibly also burrowing erate resting upon a sharp scoured base. This is critical evi- organisms. dence for channel incision into underlying sediment. The The interval between 17 m and 27 m in Well B (Fig. 3) conglomerate is abruptly overlain by about 2 m of dark grey is similarly thought to represent partial channel abandon- mudstone within which are a number of very thin (1-2 mm) silty laminae. The silty laminae are undisturbed apart from local water escape structures. The laminated mudstone passes Lithology upward into a black, root mottled (unlaminated), carbona- ...... Sandstone ceous mudstone containing siderite nodules and interbed- Siltstone ded thin coals. The passage from laminated to mottled car-

Mudstone bonaceous mudstone indicates infilling of the oxbow lake

Carbonaceous Mdst. to a level where vegetation could maintain a hold in the Coat sediment, and poorly drained swamp conditions (q.v.) became established. Infilling of the abandoned channel con- Gravel tinued, and the swamp deposits were overlain by two small Sed. Structs. 1 ) Rootlets (about 1 m) coarsening-upward sequences composed of grey 4 Q a Plant debris argillaceous siltstone that is intensely mottled as a result of 85 ? Mottled water escape and rootlets (Fig. 5). These coarsening-up- 22 Contorted ward units probably represent the progradation of sediment - -= Parallel lamination deposited rapidly by flood events associated with a nearby //// Cross lamination channel (crevasse-splay or overbank deposits). Such incur- MA Ripple lamination sions represent only an interruption of the poorly drained --z?=: - Mudchips swamp conditions, which resumed until the oxbow lake was compietely infilled (or invaded by the next cycle of channel FIG. 4.-Detail of section between 30 m and 39 m in Well A (Fig. 3) showing partially abandoned channel sequence. Note that the only pre- formation and infilling)’ served evidence of Drimarv current activitv is confined to the ven base Other dXdoned Channel deposits have been identified of the section. in the lower Kekiktuk using similar criteria as above, for

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Meters channel banks as sheet flow with no breaches or crevasse channels; and crevassing, by which flood waters are trans- 6 ferred to the flood basin via crevasse channels cut in the PDS =? levee crest (Coleman, 1969). Fielding (1984) has stated how LL WDS the relative importance of crevasse, as opposed to over- W -PDS bank, processes is still poorly understood. He has identified ?L - 4 a number of depositional facies from the Upper Carbonif- a WDS - erous of the Durham Coalfield, England, some of which he z PDS attributes to (i) coarse-grained overbank deposits, (ii) me- 0 dial crevasse splay/minor delta and (iii) distal crevasse splay/ rn I- 2 minor delta. All can occur in sequences as much as 10 m X thick, and all are composed of interbedded claystone, silt- 0 I ( Lacu stone and fine-grained sandstone (Fielding, 1984). The cri- e* teria for distinguishing between these facies and the sub- CHA NNEL LAG environments they represent are related in part to lateral facies associations as well as to internal characteristics of the facies themselves, which creates problems in vertical facies and sequence analysis of cores. The sequence described above from the Kekiktuk For- mation (Fig. 6) is interpreted as follows. The lower 5 m of FIG. 5.-Detail of section between 23 m and 29 m in Well A (Fig. thinly interbedded sandstones and silty mudstones repre- 3) showing wholly abandoned channel (?oxbow lake) sequence. Note ab- sents pulsatory sedimentation associated with floods. The rupt shift in grain size from base-of-channel lag conglomerate to over- sandstone layers may have formed as overbank deposits by lying mudstones representing progressive lacustrine/swamp infill. (WDS: well drained swamp; PDS: poorly drained swamp). For legend see Figure sheet flow of sediment-laden flood-waters. Alternatively, 4. the minor thickening-upward sequences may represent stacked, distal crevasse-splay deposits (Elliott, 1974; Field- ing, 1984). The upward increase in root mottling in these example at the very base of Zone 1 in Well A (Fig. 3), and thin beds is indicative of shallowing waters. The sharp-based, in Well B between 15 m and 17 m (Fig. 3). ripple cross-laminated fine-grained sandstone is the thickest Natural levee and crevasse-splayllacustrine delta deposits.- These subenvironments of the flood basin are grouped together, because of their depositional similarities and close association in the natural environment. The added difficulties inherent in differentiating such subenvironments in -intense root mottling, cored sequences were highlighted by Ethridge and others water escape (1981). Figure 6 is a detailed illustration of the sequence between 46 m and 55 m in Well A (Fig. 3). It is about 10 m thick, commencing with a coarsening-upward transition from silt- LEVEEICREVASSE stones with thin (1 cm) interbeds of very fine-grained sand- CHANNEL stone to thin (1-3 cm) very fine- to fine-grained sandstones with silty partings. The thin-bedded sandstones appear to be arranged in minor thickening-upward sequences with some STACKED OVERBANK beds 15 cm thick (Fig. 6). They also display an overall ?CREVASSE SPLAY upward increase in degree of root mottling. This lower interbedded interval is overlain by a sharp-based, fine-grained sandstone 1.3 m thick characterized by small-scale, ripple cross-lamination. That sandstone is overlain by about 4 m .+ * ::: of fine-grained, argillaceous, intensely mottled sandstone. ,.-I-: :. Primary bedding is severely interrupted in this upper inter- ..-.. val by abundant root mottling. There is sufficient evidence, however, to show that the sediment was originally depos- ido ited as thin beds of fine sandstone and siltstone (Fig. 7). Elliott (1974) has described the characteristics of a number of minor coarsening-upward sequences in the fluvio- FIG. 6.-Detail of section between 46 m and 55 m in Well A (Fig. deltaic environment, and he attributes them to a variety of 3) showing proximal overbank sequence. Note upward increase in (i) grain flood-generated processes. Such processes include over- size, (ii) bed thickness, (iii) intensity of mottling. For legend see Figure bank flooding, where sediment-laden waters flood over the 4.

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ceous mudstones and siltstones (Fig. 8a). No current-generated sedimentary structures have been observed, and burrows and other faunal remains are rare to absent, suggesting restricted circulation and anoxic bottom conditions. Al- though these rocks contain abundant black organic matter, the preservation of delicate lamination implies that the water was too deep to support a permanent rooted plant com- munity such as might be expected in a swamp, and the fine sediment grain size suggests low depositional energy levels. Lacustrine deposits in the lower Kekiktuk commonly occur abruptly above coals and black carbonaceous mudstones of the poorly drained swamp environments (see below). They also very commonly pass upward into such deposits (Fig. 3). Well drained swamp deposits.- Two types of swamps have been reported in the Atchaf- alaya Basin of Louisiana by Coleman (1966), namely well drained and poorly drained swamps. The type of swamp is determined by the level of the groundwater table relative to the depositional interface. When the river floods and the groundwater table rises in the channel margin area, the depositional interface is inundated across the entire backswamp area. During dry periods, river and groundwater table are lower and water drains from the topographically higher well drained swamps. Water continuously covers the topographically low, poorly drained swamps (see Coleman, 1966; Weimer, 1977, p. 13). In the lower Kekiktuk Formation (Zone 1) at Endicott Field, both types of swamp deposits are recognized. The well drained swamp deposits are characterized by pale grey to light olive brown, silty mudstones and muddy siltstones (Fig. 8b). The rock is characteristically crumbly and breaks apart along curved surfaces that are highly polished and grooved (“slickensided”). Such breaks may be caused by expansion of roots that were later removed by oxidation FIG. 7.-Core photograph of levee deposits that occur at top of Figure (Weimer, 1977). A common explanation of such features 6. Note highly mottled appearance and locally well preserved roots (ar- is that the slickensides form around roots and rootlets dur- rowed). ing compaction, these being areas of low strength within the sediment pile (Fielding, pers. commun., 1986). In many places, these Kekiktuk mudstones are traversed with a fine and the coarsest bed of the sequence. As such it may rep- network of hairline fractures filled with kaolinite. Siderite resent traction-dominated deposits of a minor crevasse is also very common in these deposits. Although these silty channel. The highest interval of this sequence, namely the mudstones do show abundant evidence of oxidation, in places 4-m-thick mottled silty sandstone with rootlets (Fig. 6) was root mottling is preserved (Fig. 8c). Such occurrences may probably originally deposited by pulsatory overbank flow, indicate “intermittently drained’’ swamp conditions. These but deposition diminished allowing colonization by plant sediments of the well drained swamp environment are much life to dominate. This mottled sandstone thus represents the less common in Zone 1 of the Kekiktuk Formation at En- paleotopographically highest part of the sequence, and the dicott Field than poorly drained swamp deposits, described entire 10-m sequence (Fig. 6) is interpreted as the progra- below. dation of vegetated, shallow (?subaerial) levee deposits over a minor crevasse channel overlying deeper water, subaque- Poorly drained swamp deposits. - ous distal levee or crevasse-splay deposits. The poorly drained swamp deposits of the lower Kek- The sequence between 15 m and 23 m in Well A (Fig. iktuk are characterized by an intimate association of coal, 3) shows characteristics similar to the one described above. and black, highly carbonaceous mudstone (Fig. 3). The coal It is similarly interpreted as a stacked crevasse-splay, or is variably argillaceous and generally dull in luster. In places prograding levee, deposit. it displays poorly developed banding and a somewhat more shiny luster. The mudstones are extremely carbonaceous, Lacustrine deposits. - with abundant plant remains including stems and rootlets. Lake floor sediments in the lower Kekiktuk Formation In Well B (Fig. 3), a large imprint of Stigmaria was iden- are composed of dark grey, finely laminated, very carbona- tified on a mudstone bedding plane in Zone 1. Pyrite is

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FIG, 8.-Core photographs illustrating various aspects of lacustrine/swamp deposits in lower Kekiktuk at Endicott Field: (a) very well laminated, dark grey to black, carbonaceous mudstones of lacustrine environment. Note: core cut in a deviated well; (b) unlaminated, rubbly, light grey silty mudstone of well drained swamp environment; (c) detail of another well drained swamp siltstone showing abundance of root traces; (d) poorly to non-laminated, dark grey to black carbonaceous mudstones of the poorly drained swamp environment. Note: core cut in deviated well.

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quite common, both as nodules and disseminated through- covered from farther afield at BP Sag Delta 31-10-16 well, out the rock. These black mudstones are similar to the la- and ditch cuttings combined with palynological data and custrine mudstone described above, with the important dis- wireline logs have revealed its presence to the northwest at tinction that they exhibit no lamination but instead are the Reindeer Island Stratigraphic Test (RIST) well (Fig. 1). intensely mottled throughout (Fig. 8d). The lithologies in Zone 1 in those wells are similar to those Lack of drainage in the subenvironment characterized by that have been discussed in this article; hence, the flood these coals and carbonaceous mottled mudstones is inferred basin environment extended over a considerable area. from the preservation of the abundant organic material. This The evolution of Zone 1 depositional environment may suggests stagnant, reducing conditions, with highly re- be elucidated by examining its internal stratigraphy and fa- stricted circulation of the ponded water in what must have cies organization. The basement of pre-Mississippian de- been some of the lowest elevations of the overall lower formed argillite that directly underlies the Zone l deposits Kekiktuk depositional environment. Similar rocks have been of Endicott Field must have been an extremely 'low-lying, described from the lower Tertiary Wasatch Formation of undulating peneplane. Figure 3 shows how a large propor- the Powder River Basin in Wyoming by Ethridge and oth- tion of the Zone 1 depositional sequences is characterized ers (198 1), and are analogous to today's organic-rich fresh- by either coarsening-upward trends or by the superposition water clays described from the lowest-lying areas of the of shallow-water swamp deposits over deeper water lake Atchafalaya Swamp in Louisiana (Coleman, 1966; Krinitz- floor deposits. In either case, the sequences are interpreted sky and Smith 1969). Poorly drained swamp deposits are as indicating progressively shallowing water. This suggests abundant throughout Zone 1 in the lower Kekiktuk at En- that, following drowning of the basement surface, deposi- dicott Field, but it is important to note that they are most tion occurred by the infilling of topographic lows upon the common in the lowest, and very highest parts of this strati- lower Kekiktuk swamp plain. Those lows were reflected in graphic interval (Wells A, B. Fig. 3). an abundance of lakes and poorly drained swamps, and in- The poorly drained swamp deposits are commonly infill took place by either overbank flooding or crevassing of terbedded with the rarer, well drained swamp mudstones. streams which traversed the swamp plain. In places, particularly in the upper part of Zone 1 in both Those streams are interpreted from the sharply fining trends Wells A and B (Fig. 3), these mixed swamp deposits over- observed in the vertical sequence (Fig. 3). The paucity of lie laminated, dark grey lacustrine mudstones, suggesting primary sedimentary structures and the fine grain size typ- that the lakes slowly filled to a level where plants could ical of those trends suggests that the streams were char- take root and flourish in a swamp environment (e.g., Well acterized by mixed to suspended sediment load and prone A, 56 m-58 m, 68 m-77 m; Well B, 67 m-73 m: Fig. 3). to widespread flooding. The greatest channel activity oc- In almost all places, the coals and carbonaceous mudstones curred around middle Zone 1 time in the Endicott Field of the poorly drained swamp environment are also abruptly area. The sedimentologic evidence and generally low pro- overlain by well laminated, lacustrine deposits (Well A, 68 portion of recognizable channel deposits suggest that even m; Well B, 67 m, 73 m: Fig. 3). This suggests that the then there was only limited fluvial channel activity. The accumulation of swamp peats commonly ceased by drown- existence of an environment that was prone to widespread ing as a result of markedly increased rates of subsidence, flooding is substantiated by consideration of the coals of particularly toward the end of Zone 1 time. McCabe (1984) the poorly drained swamp environment. has noted how a majority of coals are apparently derived Flores (1983) has observed that frequent interruptions of from swamps which were eventually drowned. Lacustrine backswamps in a flood basin by crevasse splay and other sediments having relationships similar to those described overbank sediments, as well as frequent avulsions of mean- above in the lower Kekiktuk Formation between swamp de- der belts, appear to produce unfavorable conditions for very posits and deeper water have been observed by Fielding thick peat (i.e., coal) accumulations. Examination of the ( 1984) and Haszeldine ( 1984) in the Upper Carboniferous cored sequence in Zone 1 (Fig. 3) shows that it can be of northern England. divided informally into an upper interval characterized by lacustrine and swamp deposits and a lower interval characterized by a greater diversity of environmental settings DISCUSSION (channel, overbank, lacustrine, and swamp plain deposits). Consideration of lithology, lithofacies and lithofacies as- The coals in that lower Zone 1 interval are generally thin sociations, as well as palynology and paleontology, sug- (as much as 25 cm), argillaceous and everywhere associ- gests that the entire Zone 1 succession in the lower Kek- ated with an abundance of extremely carbonaceous mud- iktuk Formation at Endicott Field was deposited within a stone. This association suggests that the poorly drained flood basin environment. A number of distinct paleoenvi- swamps of lower Zone 1 were exposed to regular inunda- ronments can be recognized, namely: flood basin lakes, well tion by mud- and silt-laden floodwaters and that there was drained swamps and poorly drained swamps, as well as par- little distinction between the fluvial channels and the flood tially and wholly abandoned fluvial channels. Natural levee basin. By implication, the streams of the flood basin did and crevasse-splay/lacustrine delta deposits are also rep- not have very high or well developed restraining levees. resented, although specific distinction between them is nec- The relatively diverse number of paleoenvironments inter- essarily difficult considering the nature of the evidence preted from lower Zone 1 cores included sluggish streams available. The most completely cored sections of Zone 1 containing a mixed to suspended sediment load, prone to occur at Endicott Field, but other cores have been re- widespread overbank (and possible crevasse) flooding.

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Floodwaters invaded numerous floodbasin lakes, which NW SE consequently silted up to become well and poorly drained - I - swamps. This setting is illustrated in Figure 9. -Upper (more restricted) palynomorph assemblage The localized infilling of the irregular lower Kekiktuk depositional surface is reflected in the nature of the vertical sequences as discussed above. Infill of the lower Kekiktuk basin can also be considered at a more regional level. Lower FIG. 10.-Schematic NW-SE biostratigraphic section across Endicott Zone 1 deposition has been discussed above. The marked Field area, showing distribution of palynologic assemblages in Zone 1. increase in coals and carbonaceous mudstone in the upper Note onlapping effect of upper restricted assemblage. Datum is top Zone parts of this zone (Fig. 3) suggests that in later Zone 1 time 1. Black bar ornament indicates dominance of swamp deposits. a more restricted depositional environment became established that was composed of shallow lakes and poorly drained swamps. Palynological evidence strongly supports the idea mentally restricted, of the two palynomorph assemblages of the onset of a widespread swamp environment in latest (Fig. 10). Locally, subsidence rates may have fluctuated Zone 1 time. quite dramatically, as suggested by the repeated drowning Two major palynomorph assemblages can be recognized of the peat swamps by lakes, discussed previously (Fig. 3). within Zone 1 in a number of wells at Endicott Field Those fluctuations in subsidence rates may have been re- (R. L. Ravn, pers. commun.), and they appear to have lated to compactional effects, or they may reflect initial re- stratigraphic significance within the zone (Fig. 10). Fol- sponses to tectonic instability in the Kekiktuk basin in the lowing initial drowning of the basement surface, the irreg- region of Endicott Field. That instability manifested itself ular topography became levelled in early Zone 1 time by ultimately in the termination of Zone 1 depositional patterns depositional infill of the hollows as a result of widespread and the abrupt introduction of coarse-grained clastics of Zone flooding. The variety of depositional subenvironments at 2 (Melvin, 1985). that time supported a relatively diverse spore assemblage. Subsequently, the lower Kekiktuk swamp plain became in- capable of supporting any active sedimentation other than ACKNOWLEDGMENTS lacustrine fill and the development of poorly drained swamps. This paper is published with the permission of Standard These upper Zone 1 deposits are characterized by the higher Oil Production Company. The data were originally pre- of the two palynomorph assemblages, which is more resented as part of a broader paper on the sedimentological stricted in its character but more widespread in its occur- evolution of the Kekiktuk Formation at Endicott Field dur- rence (Fig. 10). Coals in the upper Zone 1 interval are more ing the 1985 Meeting in Anchorage, Alaska of the Pacific abundant and tend to be thicker than in the lower parts of Sections of the AAPG/SEPM. My thoughts on the Kek- the zone (Fig. 3), possibly reflecting a marked reduction in iktuk Formation have benefitted from discussions with argillaceous clastic input in latest Zone 1 time. These better A. S. Knight, J. E. Williams, K. Woidneck and R. L. Ravn developed coals may have formed in clear water as floating to whom I owe my thanks. I also appreciate the construc- or raised bogs (see McCabe, 1984, for discussion). Cer- tive comments of C. R. Fielding and B. P. J. Williams who tainly, the onset of such widespread swamplands reflects reviewed the paper. I thank Leta Smith (drafting) and Rob- diminished gradient in the lower Kekiktuk depositional ba- in Gardner (typing) for their patient assistance in the pro- sin, and that in turn limited the effectiveness of any rivers duction of this paper. that may have existed. Lower Kekiktuk (Zone 1) sedimentation came to an end with widespread subsidence. Increasingly larger areas of the REFERENCES low-lying basin became inundated by swamps as indicated BROSGE,W. P., DUTRO,J. T., MANGUS,M. D., AND REISER,H. N., 1962, by the onlapping nature of the higher, but more environ- Paleozoic sequence in the eastern Brooks Range, Alaska: American Association of Petroleum Geologists Bulletin, v. 46, p. 2174-2 198. COLEMAN,J. M., 1966, Ecologic changes in a massive fresh-water clay sequence: Transactions of the Gulf Coast Association of Geological Partially abandoned Societies, v. XVI, p. 159-174. , 1969, Brahmaputra River: Channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239. ELLIOTT,T., 1974, Interdistributary bay sequences and their genesis: Sed- imentology, v. 21, p. 611-622. ETHRIDGE,F. G. , JACKSON,T. J., AND YOUNGBERG,A. D., 1981, Flood- basin sequence of a fine-grained meander belt subsystem: The coal- bearing lower Wasatch and upper Fort Union Formations, southern Poorly-drained Powder River Basin, Wyoming, in Ethridge, F. G., and Flores, R. M., eds., Recent and Ancient Nonmarine Depositional Environments: Models / I \ for Exploration: Society of Economic Paleontologists and Mineralo- Lake bottom Levee Well-drained swamp gists Special Publication 31, p. 191-209. FIELDING,C. R., 1984, Upper delta plain lacustrine and fluviolacustrine FIG. 9.-Schematic representation of sedimentation in Endicott Field facies from the Westphalian of the Durham coalfield, NE England: area during lower Kekiktuk (Zone 1) time. The area was dominated by Sedimentology, v. 31, p. 547-567. a very low-lying swamp plain with abundant lakes, abandoned meanders FLORES,R. M., 1983, Basin facies analysis of coal-rich Tertiary fluvial and sluggish silt-laden streams. deposits, northern Powder River Basin, Montana and Wyoming, in

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Collinson, J. D., and Lewin, G., eds., Modern and Ancient Fluvial dimentologists Special Publication 7, p. 13-42. Systems: International Association of Sedimentologists Special Publi- MELVIN,J., 1985, Sedimentological evolution of Mississippian Kekiktuk cation 6, p. 501-515. Formation, Sagavanirktok delta area, North Slope, Alaska (abs.): GERSIB,G. A., AND MCCABE,P. J., 1981, Continental coal-bearing sed- American Association of Petroleum Geologists Bulletin, v. 69, p. 669. iments of the Port Hood Formation (Carboniferous), Cape Linzee, Nova MOORE,T. E., AND NILSEN,T. H., 1984, Regional variations in the fluvial Scotia, Canada, in Ethridge, F. G., and Flores, R. M., eds., Recent Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, and Ancient Nonmarine Depositional Environments: Models for Ex- Brooks Range, Alaska: Sedimentary Geology, v. 38, p. 465-497. ploration: Society of Economic Paleontologists and Mineralogists Spe- NILSEN,T. H. , 1981, Upper Devonian and Lower Mississippian redbeds, cial Publication 31, p. 95-108. Brooks Range, Alaska, in Miall, A. D., ed., Sedimentation and Tec- HASZELDINE,R. S., 1984, Muddy deltas in freshwater lakes, and tecton- tonics in Alluvial Basins: Geological Association of Canada Special ism in the Upper Carboniferous coalfield of NE England: Sedimentol- Paper 23, p. 187-219. ogy, V. 31, p. 811-822. , AND MOORE,T. E., 1982, Fluvial facies model for the Upper KRINITZSKY,E. L., AND SMITH,F. L., 1969, Geology of backswamp de- Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks posits in the Atchafalaya Basin, Louisiana: U.S. Army Waterways Ex- Range, Alaska, in Embry, A. F., and Balkwill, H. R., eds., Arctic periment Station, Corps of Engineers, Vicksburg, Mississippi, Tech- Geology and Geophysics: Canadian Society of Petroleum Geologists nical Report S-68-8, 60 p. Memoir 8, p. 1-12. MCCABE,P. J., 1984, Depositional environments of coal and coal-bear- WEIMER,R. J. , 1977, Stratigraphy and tectonics of western coals, in Mur- ing strata, in Rahmani, R. A., and Flores, R. M., eds., Sedimentology ray, D. K., ed. , Geology of Rocky Mountain Coal: Colorado Geolog- of Coal and Coal-Bearing Sequences: International Association of Se- ical Survey Resource Series 1, p. 9-27.

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3790711/9781565760967_backmatter.pdf by guest on 25 September 2021 NUMERICAL SIMULATION OF GOLD DISTRIBUTION IN THE WITWATERSRAND PLACERS

M. NAMI Gold Exploitation Laboratory, Chamber of Mines Research Organization, Johannesburg AND C. S. JAMES Department of Civil Engineering, University of Witwatersrand, Johannesburg

ABSTRACT:The distribution pattern of gold deposits in a fluvial environment can be studied by either a descriptive or a deterministic approach. The deterministic approach is based on physical and mathematical modelling of the processes involved in the formation of the placer and can be applied to the system, channel reach and channel cross section scales. Two numerical models are presented which address the channel reach to cross section scales. These describe the fine particles in suspension in a compound channel reach and the associated patterns of deposition of these particles. The models are used to reproduce observed cross section distributions of gold concentration in the Carbon Leader and Composite Reefs. The results show that the observed distributions can be explained deterministically. The improved understanding of gold distribution gained by this approach will help to improve techniques for valuation and ore reserve estimation and thus lead to more economical exploitation of gold reserves.

INTRODUCTION these models and illustrates case studies from various underground exposures of the placer. An understanding of the distribution and concentration of any economically viable mineral in placer deposits is of fundamental importance to the mining of that deposit. As SPATIAL SCALES a result of advance knowledge concerning distribution pat- The following three-scale levels (Fig. 1) have been iden- terns of these minerals, considerable improvements can be tified on which deterministic analysis of gold distribution made to valuation of the ore body which leads to the de- can be conducted. velopment of more economic methods of mining. One of the most important, yet difficult, problems is the (i) The system scale considers whole fluvial systems ex- prediction of the distribution pattern of tracer minerals, such tending over distances on the order of kilometers. as gold, in a water-laid placer deposit. In order to predict (ii) The channel reach scale focuses on individual reaches distribution patterns of gold in a fluvial environment, two within the total system and would consider longitu- approaches can be employed: (i) the descriptive approach, dinal distributions over hundreds of meters and trans- and (ii) the deterministic approach. The descriptive ap- verse distributions over distances of less than tens of proach involves the identification of the sedimentary en- meters. vironment and the relationship between sedimentological (iii) The cross section scale considers detailed transverse characteristics and gold distribution. Using this approach, distributions at individual sections along a reach. it is possible to classify the placer into various sedimentary The distribution of gold at all scales is interrelated and units (facies) with specific characteristics, where each fa- the reach and cross section scale distributions, in particular, cies represents a unique process of sedimentation. Cogni- cannot be considered independently. It is important to make zance of sedimentary facies and incorporation of these fa- the scale distinctions, however, because the dominant cies in the present valuation techniques (e.g., kriging) results transport processes are different, and different mathemati- in considerable improvements in valuation and ore reserve cal models would be required. estimation of the gold-bearing placer deposits. Simulation of distribution of gold deposits is probably The deterministic approach is based on physical and easiest at the system scale. The system characteristics can mathematical modelling of the prevailing hydraulic condi- be described by average values of parameters, such as gra- tions and processes which were responsible for deposition dient, channel size, channel shape, etc. Local deviations and distribution of gold. In this case not only can sedi- from average conditions would not affect the overall dis- mentary units be classified, but relative distribution patterns tribution of gold concentration through the system. On the can also be estimated. Furthermore, the direction and mag- smaller scales, it is the local characteristics which deter- nitude of the trends in the placer can be determined with a mine the patterns of concentration distribution which are of high degree of confidence. interest. These local characteristics are not easy to predict. While the descriptive sedimentological approach is re- From a practical point of view, however, it is the smaller ceiving considerable attention in the Witwatersrand gold scale distributions which are important in planning mining mining industry, little emphasis has been placed on the de- operations, and for this reason simulation on the reach to terministic approach. In an effort to investigate the deter- cross section scale has been attempted first. ministic approach, a major program has been embarked upon by the Chamber of Mines Research Organization. Two numerical models have been developed to simulate the hy- TRANSPORTATION OF GOLD draulic transport and deposition processes responsible for It has been established (James, 1984) that the fine-grained the distribution and concentration of gold in Witwatersrand gold now present in the placer was originally transported placers. This paper describes the fundamental concept of in suspension. This was done by comparing the hydraulic

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TABLE1 .-MATHEMATICAL NOTATION

Cross-section C Local sediment concentration (arbitrary units) d Particle size (mm) UP Flow depth on plain (m) v ks Representative bed roughness (mm) S Hydraulic gradient P Probability of deposition of suspended particle U Transverse velocity component (m/s) SYSTEM SCALE CHANNEL REACH CROSS -SECTION U* Shear velocity (m/s) Longitudinal velocity (m/s) SCALE SCALE W Particle fall velocity (m/s) X Longitudinal direction FIG. 1.-Classification into system, reach and cross section scales. Y Vertical direction Z Transverse direction & Particle diffusivity (m’/s) conditions necessary to mobilize the largest quartz-density particles occurring in the placers with the conditions necessary for suspension of gold particles. Figure 2 shows the system as illustrated in Figure 3. During periods of over- combinations of flow depth and hydraulic gradient neces- bank flow, a strong interaction takes place between the fast, sary to transport 70-p gold particles in suspension, accord- deep, main channel flow and the relatively slow, shallow ing to Engelund’s (1973) criterion: flow over the plain sections. This interaction transfers mo- mentum and any suspended particles from the high con- w < O.@* (1) centration channel region to the lower concentration plain in which w is the particle fall velocity and u* is the shear regions, where suspended particles tend to settle because velocity associated with grain roughness (Table 1). Also of the reduced transport capacity of the flow. The presence shown are the combinations necessary to mobilize 17-mm of a transverse flow velocity component, such as would quartz-density particles according to the criteria of Shields occur if the channel were inclined to the principal flow di- and Meyer-Peter and Muller (see Graf, 1971). For these rection over the plain, would enhance the transfer process fairly typical particle sizes, it can be seen that the prevailing on one side of the channel and inhibit it on the other, re- hydraulic conditions were probably well in excess of those sulting in an asymmetrical distribution of deposition. necessary for suspension of gold. The transfer of material to the overbank section depletes Because suspension is believed to have been the domi- material in the channel and, therefore, must also affect the nant mode of transport of gold, the distribution of deposits distribution of deposits along the length of the channel. The must be closely related to the distribution of suspended gold three-dimensional problem of describing the distribution of during transport. The mathematical simulation models de- suspended material in a system as shown in Figure 3 has scribe the distribution of suspended particles under the in- been separated into two two-dimensional problems. The fluence of convective and diffusive transfer components. transverse distribution across the plain and the longitudinal Interaction between suspended particles and the bed ma- distribution down the channel are described by two separate terial is described by Einstein’s (1950) probability of ero- models. sion. The transverse distribution is described by the equation: NUMERICAL MODELS ac ac The models developed so far simulate the distribution of suspended and deposited material in a channel-overbank in which y and z are the vertical and transverse directions, respectively, EY and EZ are the diffusivities for sediment in = 2,1 mis these directions, C is sediment concentration, w is the par- y ticle fall velocity (positive downward) and u is the transverse velocity component. The solution to this elliptic equation is obtained numerically by the method of Successive

n ,Shields l,o Particles ,< 17 mm transported z Meyer Peter-Muller Flow direction with Zone of turbulent shearing puspension transport

L \/ .- I Deposition from

/ /w/ ater surface v

0,oo 1 0,005 0,009 Hydraulic gradient Channol FIG. 2.-Hydraulic conditions for gold suspension. FIG. 3.-Illustration of a compound channel system.

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Over-Relaxation (Smith, 1978) and gives the distributions Simulation of Gold Distribution of sediment concentration throughout the flow region over The two models described above have been used together the plain. This numerical solution procedure assumes initial to reproduce some observed distributions in several sections concentration values at grid points throughout the solution of the Carbon Leader Reef placer at Blyvooruitzicht Gold domain. These values are adjusted iteratively until equation Mine and the Composite Reef placer at Randfontein Estates (2) is satisfied simultaneously for all points within a spec- Gold Mine. Examples of these simulations follow. ified tolerance. Convergence is accelerated by applying an optimization procedure proposed by Apelt and Isaacs ( 1977). Carbon Leader Reef-east of No. 2 shaft pillar.- The distribution of deposits can be obtained from the concentration distribution according to the specified bed On the cross section shown in Figure 4, the channel shape and size can be clearly seen. The measured gold concen- boundary condition (equation 5). The values of EY and EZ are obtained from empirical results presented by other re- tration values plotted on the cross section show a distinct searchers. (Graf, 1971 ; Rajartnam and Ahmadi, 1981). correlation with the channel geometry, with high values on The longitudinal distribution is described by the equa- the overbank sections close to the channel edge. The dis- tion: tribution pattern is consistent with the transfer mechanisms described earlier. The distribution is asymmetric, suggesting the presence of transverse convection. This is supported o=v-+w--+- by the increasing trend of concentration within the channel acax acdy dy ("'z toward the bank with the greater concentration. The overbank distributions were reproduced using the - UEaZ + az (Ezz)(3) transverse distribution model by assuming gold particle characteristics and different combinations of uniform plain flow depth (Dp), gradient(s) and transverse convection (u) in which x is the longitudinal direction and v is the velocity until a reasonable fit was obtained (as shown). There are, in that direction. Longitudinal diffusion is considered neg- in fact, many different combinations of these parameters ligible compared with longitudinal convection. The last two which would give just as good a fit. Only one combination, terms in this equation can be evaluated by the transverse however, will give a satisfactory fit to the overbank distri- distribution model and treated as constants in the solution butions and also give the correct proportional relationship of the longitudinal model, making it a two-dimensional rather between overbank deposits and average channel deposition than a three-dimensional problem. Transverse transfer within relative to the plain deposition. The overall distribution thus the channel is ignored. The solution is obtained by an ex- found and the assumed parameter values are shown on the plicit numerical method in which concentration values at cross section in Figure 4. grid points on a vertical section are computed progressively downstream from known values at the preceding section. Carbon Leader Reef-western part of 5-22 raise, Bly- This yields the vertical and longitudinal distribution of con- vooruitzicht .- centration along the channel. The distribution deposits can The same procedure was used to reproduce another dis- again be determined from the concentration distribution and tribution measured in the Carbon Leader Reef. This cross the specified boundary condition. section, shown in Figure 5, has some interesting features. The initial condition for the longitudinal model and the The channel is not symmetrical, possibly because the sec- channel-plain interface boundary for the transverse model tion is at a bend, in which case significant transverse con- are specified in terms of concentration values over the flow vection would be expected, as is indicated by the very ex- depth. In both models the boundary condition at the water surface states that no transfer can take place across this surface, i.e., Measured - Simulated I 1 OOOl dC o E,- + wc = o (4) = 0,12 m ay I 1 ? ~0,013 I A- f ?+ At the channel bed or plain surface, the rate of transfer u =Ovoo6 across the boundary is defined by the probability (p)that """1 si ' ! a particle reaching the surface will deposit (entrainment from the bed is not accounted for), i.e., 0,6 O O 1 ím)

dC E, - + (1 - p)wC = o dY The probability (p)is estimated as the complement of Ein- stein's (1950) probability of erosion. The mathematical models and their numerical solutions FIG. 4.-Diagram of a section through the Carbon Leader east of the are described in detail by James (1984, 1985). No. 2 shaft pillar showing the measured and simulated gold distributions.

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CARBON LEADER REEF simulation, and the assumed set of hydraulic parameters is W 1 1face3 not as nearly unique as for the other simulations. The sim- 1 000- dgold 10pm ulated distribution is shown on the cross section in Fig- wgoid = 0.001 mis ks =0,19 mm ure 6. Dp =0,12 1 S =0,013 m DISCUSSION AND CONCLUSIONS U = 0.001 mis git 500 - The models presented in this paper provide a determin- 0 istic explanation of some gold distribution patterns ob- *. **_ served in the Witwatersrand placers. The improved under- n standing of gold distribution gained by developing and

01’ , I applying such models will lead to better economic exploi- 0,5 O O 1 ,O 2.0 (m) tation of the gold mine reserves by improving valuation techniques and advance planning of mining operations. Ex- amples of such improvements include selective mining, de- marcation of pay and nonpay areas in mines, appropriate Measured - Modelled blending of high- and low-grade ore for increasing mine life, and accurate siting of stabilizing pillars in deep-level FIG. 5.-Diagram of a section through the Carbon Leader at 5-22 raise, mining. showing the measured and simulated gold distribution patterns. Although many advances have been made in recent years in geostatistical techniques for valuation and ore reserve es- tensive gold deposition on the one side of the channel. Also, timation, these use sampling data without giving cogni- there appears to be a secondary channel a short distance zance to the distribution of gold. Many errors in geosta- from the main one on one side. Although there is consid- tistical applications result from the omission of geological erable scatter of data, the gold concentration appears to be and especially sedimentological information regarding the lower at the location of this secondary channel. The sec- placer deposit. As our knowledge of the hydraulic condi- ondary channel was accounted for in the simulation by pre- tions prevailing during formation of the placer advances, calculating the deposition probability appropriate to its depth, the use of deterministic models could provide quantitative and overriding the calculated values with this. The overall techniques for predicting gold distribution patterns, thereby simulated distribution is as shown on Figure 5. improving present valuation techniques. It must be emphasized that the simulation results pre- Composite Reef-85 west 4 .- sented here are reproductions of observed distributions, ob- This simulation is not as satisfactory as the others be- tained by selection of suitable hydraulic parameters (flow cause the channel geometry is not well defined. Only one depth, gradient, transverse convection, bed roughness), rather side of the channel is present (Fig. 6) due to faulting, and than predictions. The deterministic approach will be useful on this side the channel bed rises gradually to the overbank for predictions only when more knowledge is available con- level, making it difficult to define a distinct intersection cerning prevailing hydraulic conditions and the character- between channel and plain. On the other hand, more data istics of gold particles at the time of deposition. Certain were available and the gold concentration distribution rep- aspects of the existing models also still require experimen- resents an average obtained from a number of adjacent cross tal validation. It is also not entirely realistic to attempt sim- sections, and the scatter is, therefore, less. The absence of ulations at isolated sections of channels; sediment concen- one overbank distribution removes a constraint from the trations can vary along a reach even if flow is relatively uniform. The results do show, however, that observed dis-

ELSBURG REEF tributions can be explained deterministically, and the pa- dgold = 15~m rameter values used to obtain agreement are realistic. It is 0 Measured wgold = 0,002 mis - Modelled therefore believed that the deterministic approach has potential for improving our understanding of gold distribu- =O tion, thereby leading to more economical exploitation of gold reserves. O0

ACKNOWLEDGMENT

The work described in this paper forms part of a research I I I program of the Research Organization of The Chamber of Mines of South Africa. Their permission for its publication is gratefully acknowledged.

O 10 20 ím) REFERENCES

FIG. 6.-Diagram of a section through the composite reef at 85W4 APELT,C. J., AND ISAACA,C. T., 1977, On the estimation of the optimum showing the measured and simulated gold distribution patterns. accelerator for SOR applied to finite element methods: Computer Methods

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in Applied Mechanics and Engineering, v. 12, p. 383-391. JAMES,C. S., 1984, Numerical modelling of gold transport and deposi- EINSTEIN, H. A., 1950, The bed-load function for sediment transportation tion: Unpublished Ph.D. Dissertation, University of the Witwatersrand, in open channel flows: U.S. Department of Agriculture, Soil Conser- Johannesburg, South Africa, 257 p. vation Service Technical Bulletin No. 1026, 70 p. , 1985, Sediment transfer to overbank sections: Journal of Hy- ENGELUND,F., 1973, Steady transport of moderately graded sediment, draulic Research, v. 23, p. 435-452. Lyngby: Technical University of Denmark, Institute of Hydrodynamics RAJARATNAM,N. AND AHMADI,R. M., 1981, Hydraulics of channels with and Hydraulic Engineering, Progress Report No. 29, p. 3-12. flood plains: Journal of Hydraulic Research, v. 19, p. 43-60. GRAF,W. H., 1971, Hydraulics of Sediment Transport: McGraw-Hill SMITH,G. D., 1978, Numerical Solution of Partial Differential Equations: Book Company, N.Y., 573 p. Second edition, Oxford University Press, 304 p,

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CHARLES S. KINGSLEY Department of Geology, Anglo American Corporation, Welkom, South Africa

ABSTRACT:The Eldorado Formation is a wedge-shaped sequence of fanglomerates and arenites which thickens from 200 m in the west to 600 m in the east. The formation rests unconformably on the Aandenk Formation and is overlain by the Ventersdorp lava. The rapid change in thickness is due to intraformational unconformities. Four distinct alluvial fan cycles with radii of between 5 km and 25 km prograded from west to east; these cycles define four members, from the bottom upward: the Rosedale Member, the Van den Heeversrust Member, the EA zone and Uitkyk Member. The lower two members are crude fining-upward megasequences. The topmost two members of the formation in the west consist almost entirely of rudities which, 15 km to the east, grade into arenites. In the southern part of the area the entire Eldorado Formation is composed of clast-supported, poorly sorted polymictic conglomerates, which show crude stratification, and trough cross-bedded litharenites and subgreywackes. This is called the Welkom facies. In the northern part of the area oligomictic conglomerates and quartzarenites are commonly developed in the lower three members, especially in the EA zone; this is called the Loraine facies. These two facies interfinger. Sedimentation units commonly between 60 cm and 120 cm thick fine upward into matrix-supported conglomerates capped in places by trough cross-bedded arenites. Ephemeral sheet floods produced the gravels, whereas sands were deposited during waning flow conditions. Detrital pyrite, gold and uraninite occur sporadically and in low concentrations in the polymictic Rosedale Placer at the base of the formation. These heavy minerals are also concentrated at various horizons in oligomictic conglomerates of the Loraine facies. Pay zones as much as 400 m long are oriented from west to east perpendicular to the depoaxis of the basin. The amount of gold shows a sympathetic relation with the size and amount of pyrite grains which decreases sharply east of this axis. Each of the lower two members is interpreted as a geomorphological cycle which was deposited during uplift of the source area. In each case tectonism was followed by recession of the fan and retreat of the uplifted mountains. The upper two members were deposited during more frequent uplifts of the source area. Marginal unconformities and the wedging of units near the basin edge in the west suggest syndepositional folding and thrusting in the source area and accumulation of fanglomerates in a foreland molasse basin. Two entirely different sources, one oligomictic and the other polymictic, are postulated. Gravels and sands were distributed from west to east as alluvial fans from both sources. The sediments were redistributed longitudinally as gravel and sand bars on a broad braid plain down a southeastward paleoslope.

INTRODUCTION basin. The Welkom Goldfield lies at the southern tip of the Witwatersrand Basin (Fig. 1). This paper describes proxi- Vertical sequence analysis has only recently been applied mal-distal facies relationships over a 1000 km2 area in the in studies of the Witwatersrand Basin and a few authors Eldorado Formation and relates its depositional character- have attempted to investigate parts of the sequence toward istics to periodic uplift and syntectonic alluvial fan sedi- model building. In a broad sense Winter (1984) used the mentation. The proximal facies is dominated by fanglom- vertical sequence to draw some similarities between the erates and the distal facies by braid plain sands. The author structural styles of the Great Eastern Basin and those of the points out first the remarkable facies changes that take place Witwatersrand Basin. He approached his study from a tec- in the Eldorado Formation; second the thinning of this se- tonostratigraphic viewpoint using genetic sequences of strata quence due to intraformational basin edge unconformities ; (GSS), as defined by Busch (1971), in correlating strati- and, third that syntectonic sedimentation took place during graphic units. Eriksson and others (198 1) studied the lower folding and thrusting episodes in a foreland molasse basin. part of the Witwatersrand succession making use of se- The stratigraphic classification is shown in Figure 2. Only quence analysis to derive its model of sedimentation. In a a few reliable previous studies are notable. Winter (1964a) detailed sequence analysis of the Dagbreek Formation, found rapid facies changes in the Loraine area from con- Kingsley (1984) constructed the environmental model of this glomeratic to arenitic facies, both within the EA conglom- fining-upward megacycle. erates and the Uitkyk Member. Sims (1969) showed that In the present study of the Eldorado Formation the author the percentage conglomerate in the Van den Heeversrust has combined detailed sequence analysis with the concept Member decreases in a fanlike fashion eastward from 60 of tectonostratigraphy . Using this concept, GSS have been percent to 5 percent within 3.5 km. A study on clast size studied and reliable interrelationships among facies defined distribution by Kleynhans (1970) showed that the main source (Kingsley, 1979). Finally, the model of sedimentation and of the Eldorado sediments lay west of the goldfield and that tectonic control has been synthesized based on the observed the coarse material was distributed in a fanlike pattern, thus features and geometry of the sedimentary bodies. By using confirming Sims’s conclusion. a genetic approach to depositional cycles combined with In a more general sense a conceptual model of sedimen- tectonics, the sedimentation of the Eldorado sequence and tation of Witwatersrand placer deposits was proposed by more particularly the Eldorado placers is explained. This Pretorius (1976) in which each placer was considered to be synthesis was done from information accumulated from four the result of cyclic sedimentation, first deposited during underground sections and core from approximately 150 drill progradation, followed by recession of an alluvial fan. In holes. another description of the megasequences and facies changes The Eldorado Formation is about 500 m thick, covers the of the Witwatersrand Supergroup, Vos (1975) concluded entire Welkom Goldfield and extends much farther into the that the main depositional environment contained wet al-

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Heeversrust Member is typically a very gritty litharenite and subgreywacke in the lower part with minor quartzarenite beds that increase upward. Subgreywacke and polymictic conglomerates alternating with minor quartzarenites and oligomictic conglomerates characterize the overlying EA zone. Locally, the latter conglomerates are economically mineable deposits on Loraine Gold Mine. Overlying this unit is the Uitkyk Member consisting of polymictic boulder and cobble conglomerates. The lithofacies of the various units of the Eldorado For- mation vary in the different areas of the goldfield because n dramatic facies changes take place down the paleoslope, a phenomenon discussed in more detail later. Furthermore, in the Loraine area in the north, quartzarenites predominate )-J(HOLDINGS DlVlSlON over litharenites and subgreywackes, whereas in the south, (the Welkom area) litharenites and subgreywackes are in abundance. Similarly, oligomictic conglomerates are a feature of the Loraine area and polymictic conglomerates that of the Welkom area. These two contrasting facies interfinger. At the western edge of the basin the base of the Eldorado Formation is an angular unconformity which truncates all the underlying formations of the Central Rand Group. Un- conformities are also evident at the bases of the different members, the Uitkyk Member showing the most marked unconformity as it overlaps all the underlying members as well as the older formations (Fig. 3). Both McKinney (1964) and Winter (1964a) discuss the basin edge unconformities A and show how the increase in conglomerate westward may cause misinterpretations of the stratigraphy. These unconformities become disconformities eastward deeper into the basin. The unconformable relationships within the EA zone U O to are discussed later. Within the basin south of Welkom the km unconformity at the base is less angular, and the Eldorado BEATRI Formation overlaps the underlying formations of the Cen- P tral Rand Group over a distance of some 30 km. FIG. 1.-Index map of the Welkom Goldfield showing location of drill holes, shafts and two section lines (Figs. 4 and 9). DESCRIPTION AND FACIES CHANGES In the area of investigation two distinct facies are rec- luvial fans. He postulated a braided alluvial plan with coarse ognized, one of which is predominantly quartzarenite in the gravels grading downstream into braided channel sands and north (the Loraine facies) and the other which constitutes finally into lacustrine silts and muds. mainly litharenite and subgreywacke facies in the south (the Welkom facies). These two contrasting facies are developed in the lower three members, especially in the EA zone. STRATIGRAPHY These facies are best illustrated where quartzarenites and The Eldorado Formation is a wedge-shaped coarsening- litharenites of the EA zone interfinger to form the Rainbow upward sequence of fanglomerates and arenites located at Reefs on Loraine Mine No. 3 Shaft area (Winter, 1964a). the top of the Central Rand Group. It thickens from west The different lithofacies of the Eldorado Formation are re- to east from 200 m to 600 m over a distance of 5 km. The peated vertically through the sequence. A common feature rapid decrease in thickness westward is mainly due to basin of the Welkom facies is the presence of polymictic con- edge unconformities at the bases of the major conglomerate glomerates with a broadly similar pebble assemblage. In units. Four different sedimentary cycles are recognized in contrast, in the Loraine area subordinate oligomictic placer the Welkom Goldfield, namely the ED zone (Rosedale conglomerates occur at various horizons but particularly Member), the EC-EB zone (Van den Heeversrust Member), within the EA zone. Another feature of the Welkom facies the ‘EA’ zone, and the Uitkyk Member, each varying be- is the chloritic matrix of the immature arenites and con- tween 100 m and 150 m in thickness (Fig. 2). glomerates giving it a dark grey color, which is vastly dif- The Rosedale Member consists of a polymictic conglom- ferent from the mainly pyrophillitic component present in erate, the Rosedale Placer, at the base, succeeded by an the underlying Aandenk and Spes Bona Formations. alternation of quartzarenites and litharenites. The Van den Marked facies changes from conglomerates to arenites

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VENTERSDORP SUPERGROUP [LORAINE AREA/ \Formation / + Member C O E Eldor ado Uitkyk O I500 N T- - Aanden k va o - R 13 Spes Bona oa lao - A 300- 0 Dag breek 00 / L 'Harmony O. - go' ( ) 00 Loraine facies We1 kom R O ( Welkom facies) - A :o O4 200-0 0 I .....:.: . .: ,' N :.. ..I..::. ...- ..- REFERENCE St. Helena D : . -.. -. --...... - ... -' OO&b; /a n cong lom . G -:-

R ...... :. :.,.:...... - .. ..*: Virginia O E.....-. Rocedale U ...... -c - c D 'Iacers o-- Oligomictic. A<.::I.,: ...... o:.:. P -. &, ...-. rtcpB, Polymictic - ... Rosedale Placer Jeppestown Shale

FIG. 2.-Stratigraphic subdivision of the Central Rand Group and the Eldorado Formation showing its main lithologies.

can be demonstrated at many localities and in all directions of the EA zone reach much farther into the basin than the down the fan paleoslope. Facies changes affecting all the others. This phenomenon will be described in more detail members regionally are illustrated by the section line which later in this paper. goes from the proximal JR 5 drill hole to the distal BM 1 drill hole over 18 km (Fig. 4). Note how the conglomerates The Rosedale Member Geometry.- The isopach map of the combined Rosedale and Van den Heeversrust Members (Fig. 5) shows a distinct southeast-

-N.w JR5 HAK I DR6 17

350

:300

i250

:200

:150 ROSEDALE MEMBER :100

i 50

MEMBER O IC0 3COm &;>' 200 O 5 lOkm FIG. 3.-Unconformable relationship of the Eldorado Formation upon older formations on Loraine Mine. Note the angular unconformity un- FIG. 4.-Major facies changes in the Eldorado Formation from a prox- derneath the Uitkyk Member and the various intraformational uncon- imal conglomeratic facies to a distal arenitic facies over a distance of 18 formities within this formation (modified after Winter, 1964a). km. See Figure 1 for location of section line.

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and arenite. For example, the Rosedale Placer grades into a poorly developed grit some 20 km to the east of the western edge (Winter, 1964b). The RosedalelBeatrix Placer.- In the southern part of the area an oligomictic placer, the Beatrix Placer, is sporadically developed on the old paleosurface or pediment. This gold placer is older and does not form part of the Eldorado sequence proper and for this reason it is not discussed here. The actual base, the Rosedale Placer, is a polymictic cobble conglomerate or a pebbly quartzite as much as 5 m thick in the proximal area grading into a medium and small pebble conglomerate 1 m thick or less 15 km down the paleoslope. The conglomerate is absent in some distal areas due to the irregular paleosurface. Clasts are poorly sorted and commonly consist of the following types: quartz = 25 percent, quartzite (various varieties) = 25 percent, chert = 30 percent, yellow silicified siltstone/shale = 15 percent and minor felsic volcanics set in a dark grey, gritty, and argillaceous matrix. Quartz rarely exceeds 25 percent of the pebble population but increases relative to the other components downslope because the nondurable clasts were de- stroyed during abrasion. The matrix is typically dark grey litharenite and subgreywacke with a coarse-grained to gritty texture, similar to the lenticular arenitic intercalations within the conglomerate body itself. At some localities the lower 1 m of the conglomerate was sorted texturally, thereby up- grading it to a litharenite and in some cases a quartzarenite. As a result of this sorting, concentration of some heavy minerals such as pyrite, gold and uraninite took place. The detrital pyrite grains are as much as 10 mm in the proximal area in the west (Fig. 7a) but decrease to a maximum of 3 mm only 5 km downstream and to 1 mm in the distal area some 15 km downcurrent. Lithology and sedimentology.- w In the Welkom area the Rosedale Member may be sub- FIG. 5.-Isopach map in meters of the combined Rosedale and Van divided into two units, the lower one being dark grey and den Heeversrust Members. Note how the depoaxis curves from a north- siliceous with siltstone drapes on top of thin sedimentation south orientation on President Steyn Mine to a northwest-southeast direction on Free State Geduld Mine. units. Crossbedding is conspicuous (Fig. 7b). Coset units are between 30 cm and 100 cm thick and in many cases are capped by dark grey silty beds. Pyrite foresets are com- northwest trending depodxis, indicating that the maximum mon in places. The thickness of this lower unit varies from deposition took place in a strip stretching between FSG 30 m in the south to about 60 m in the north where the top (North) and the northern tip of President Steyn Gold Mine. becomes poorly defined. Some fine-grained to medium- This axis then swings southward through the southeastern grained arenite intercalations occur near the top. Arenites comer of President Brand Gold Mine, where the package in this lower unit are typically medium- to very coarse- of sediment thins and the axis becomes indistinct. grained quartzarenites with some litharenites and subgrey- The position of this axis parallels the distinct fan shape wackes. defined by the cumulative thickness of conglomerate and The color changes in the overlying unit to slightly yel- the maximum quartz clast size (MPS) depicted in Figure lowish-grey as a result of an increase in the pyrophyllite 6a. The distribution pattern of the Rosedale Placer as shown matrix but toward the top, grey quartarenites dominate again. by the MPS is the same as that of the fanlike pattern of the Gritty material is very common in the upper part of the unit. cumulative conglomerate thickness within this member. The In the Loraine facies several thin, mostly oligomictic con- Rosedale Placer is much more extensive, however, and glomerates occur at various stratigraphic horizons, largely covers almost the entire goldfield. in the lower part of the member where they are called the The Rosedale conglomerates are intimately associated with ED placers (Fig. 2). Although no clast size data are avail- facies changes. They interfinger northward, eastward and able, they seem to have been distributed radially on Loraine southward with the overlying arenites and grade into grit Mine. These conglomerates grade rapidly northeastward and

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n= 110 Xv 151"

L = 92

I V

FIG. 6.-a. Cumulative conglomerate thickness of the Rosedale Member and maximum quartz clast size of the Rosedale Placer both show a distinct fanlike distribution for the gravels to the east. Paleocurrents in intercalated and overlying arenities show a completely different direction of transport. b. Maximum quartz clast size as well as paleocurrents in the Van den Heeversrust Member indicate a dispersal pattern to the east.

eastward into grits and sands. Southeastward, however, down The Van der Heeversrust Member the regional paleoslope, as defined by paleocurrents in the arenites (Fig. 6a), this facies change is more gradual. Fig- The basal conglomerate of the Van der Heeversrust ure 8 shows some details of typical sequences in the Wel- Member rests disconformably on the Rosedale Member. This kom facies of this member. relationship is especially noticeable on the western part of

FIG. 7.-a. Large detrital pyrite grains at the base of the proximal, polymictic Rosedale Placer from the western edge of Holdings Division. b. Quartzarenite near the top of the Rosedale Member at Holdings Division showing a transverse view of trough crossbedding. Some pyrite foresets are faintly visible in the lower left corner.

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-S a sp9- VAN DEN HEEVERCRUST MEMBER

FACIES SEQUENCES ROSEDALE ROSEDALE MEMBER MEMBER

Ct h m LEGEND FIG. 9.-Correlation diagram between cored drill holes on President 36 0Quartzarenite Brand and southward, showing facies changes, especially in the Van den Litharenite 8 Heeversrust Member. See Figure 1 for position of section line. 35 a subqreywacke Mud drupe Conglomerate 34 a erates, called the EC placers, occur at various horizons within this member. An interesting feature is the change downslope in thick- FIG. 8.-Some detailed facies sequences in SP 9. Note siltstone cap- pings on top of some fining-upward sedimentation units (a and b). In JR ness of sedimentation units. The average thickness of con- 3 note how quartzarenite at 11 m grades into litharenite and subgrey- glomerate and overlying pebbly arenite beds decreases down wacke at 13 m underneath polymictic conglomerate (c). Positions of these the paleoslope over a distance of 15 km from 100 cm at columns are shown on Figure 9. Holdings Division to 50 cm at FSG. The significance of this change has not yet been clarified. Trough crossbedding is ubiquitous and sets very in thickness between 4 cm and Holdings Division, where the latter thins considerably due 8 cm and reach 12 cm rarely. Individual beds and coset to a marginal unconformity and is also noticeable on Lo- units are lenticular. Symmetrical and slightly asymmetrical raine Mine (Fig. 3). The distribution of MPS in both the ripples with a ripple length of 7 cm and a height of 1.2 cm Rosedale and Van den Heeversrust conglomerates describes have been observed in the arenites. distinctive fans which are geographically superimposed. At At two localities, Holdings Division No. 5 Shaft and FSG any specific locality, the mean size of the largest quartz No. 2 Shaft, detailed measurements of the MPS vertically clast is smaller in the Van den Heeversrust conglomerate through the gravel deposit have disclosed a very interesting than in the Rosedale conglomerate (Figs. 6a, 6b). The former is only well defined in a small area of about 110 km2 phenomenon. The MPS at both localities first increases gradually to a maximum of 12 cm at Holdings Division and covering Holdings Division and surrounding mines Presi- dent Brand and Free State Geduld. In these areas there is 8 cm at FSG halfway up the conglomerate and then decreases toward the top, producing a symmetrical distribu- a pronounced decrease in conglomerate thickness down the paleoslope due to marked facies changes from conglomerate into grit and arenite. The apices of both fans as de- VAN DEN HEEVERSRUSTFAN duced from these clast size distributions lay west of St. Hel- FSG 2 # ena Mine. The lower part of the Van den Heeversrust Member consists of a typical conglomeratic/subgreywacke sequence. The base is erosional in the proximal area, but in the medial to distal areas a thin transition zone 5 m thick can be rec- I10 ognized. The robust conglomerate changes within 5 km downfan into first a sequence of conglomerate/arenite units 105

and then into an arenite sequence with only a few pebble 100 lags which grade into grits beyond the limit of the conglomerate fan (Figs. 6b, 9). This unit, similar to others in PROGRA DÍÍG 95 the sequence, demonstrates how facies in the Eldorado For- FAN 90

mation change dramatically over a few kilometers. Because 85 of this phenomenon, the EC sandy unit distally becomes cm difficult to separate from the overlying EB unit. They have therefore been grouped together. Lithologically, there is no I I I I difference between this member and the Rosedale Member O 1000 2OOO WOO m except that in the lower part of the former, yellowish-grey FIG. 10.-Moving average of maximum quartz particle size (MPS) litharenites predominate, grading upward into quartzaren- across the Van den Heeversrust fanglomerate showing a distinct sym- ite. In the Loraine area subordinate oligomictic conglom- metrical distribution as emphasized by the curved lines at both localities.

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tion curve (Fig. 10). The significance of this clast pattern any valley feature in this position, so the thinning is attrib- will be discussed later. uted to deposition and not to postdepositional erosion. The depoaxis continues north of this feature. Conglomerate de- The EA Zone velopment in these two upper members again shows a fan- shape distribution with its proximal part in the west (Fig. The geometry of the upper two members is reflected in 12). Its elongated north-south shape is enigmatic, but it may the combined isopach map (Fig. 11) which depicts some be due to coalescence and interfingering of two fans during important differences from those in Figure 5. First, the de- deposition of the EA zone, a phenomenon which is dis- poaxis in the south migrated some 4.5 km westward from cussed in more detail below. President Steyn to President Brand Mine and in the north A marked change occurs at the base of the EA zone it changed its orientation from northwestward to northward. throughout the area, especially near the proximal western Second an anticlinal warp, oriented northeastward, devel- edge. Furthermore, there is also a marked lithological change oped during sedimentation of the upper two members; the within this zone between the Welkom facies and the Lo- geometry of the overlying Ventersdorp lava does not show raine facies. In the past this facies change has caused much confusion in local stratigraphic classification. In the proximal Welkom area it starts at the base with robust polymictic conglomerate with very few thin subgreywacke intercalations; previously, this was included in the so-called ‘Boulder Beds’ but it should not be confused with the true

FIG. 12.-Distribution of percentage conglomerate in the combined FIG. 11.-Isopach map of the combined EA zone and Uitkyk Mem- EA zone and Uitkyk Member. The close contour spacing on Free State ber. Note north-south orientation of depoaxis and the discontinuation of Geduld results from the facies changes mainly in the EA zone. The solid this axis on Free State Geduld (north) as a result of the northeast-south- line on western border of Loraine Mine is the subcrop position of the EA west anticlinal warp. zone.

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boulder beds of the Uitkyk Member overlying this unit. From the following general pebble assemblage: quartz = 20 per- Holdings Division southeastward down the paleoslope, the cent, quartzite = 35 percent, chert = 30 percent, yellow individual conglomerate beds in JR 5 deteriorate into con- siltstone/shale = 10 percent and some porphyry fragments. glomerate/subgreywacke units in HAK 1 and DR 6 and In a few cases rare granite and gneiss cobbles have been grit/sandy units in BM 1 (Fig. 4). In general, the maturity observed. The porphyries were derived from the Dominion of the sediments also increases, although nowhere are any Supergroup (McKinney , 1964) underneath the Witwaters- oligomictic conglomerates of any significance developed in rand Supergroup. These conglomerates are polymodal, the Welkom facies. composed of small pebbles as matrix in the proximal area The situation is quite different northward in the Loraine where the MPS is as much as 60 cm in diameter. They are facies. In a 2.5-km zone defined by drill holes KK 5 and dominantly clast-supported and mostly free of sand-size EV 3 the robust polymictic conglomerates decrease from matrix. Although bed thicknesses are difficult to determine, 90 percent to 50 percent and change into conglomerate/ it seems that individual beds are 1 m to 2 m thick and are subgreywacke beds (Figs. 1, 12). Simultaneously, some sheetlike in appearance. The beds show possible inverse small pebble oligomictic conglomerates and quartzarenites grading of clast sizes in a few places in the proximal grav- make their appearance as intercalations in the immature se- els, but generally no vertical particle size trend is apparent. quence. Farther northward, significantly in the general vi- The thickness of this unit varies between 100 m and 140 cinity of the anticlinal warp, the polymictic conglomerates m in the Loraine area, but its correlative reaches 190 m in disappear and grade into litharenite and subgreywacke beds, the Welkom area. In the former area the base of this unit while intercalations of the Loraine facies become more is an angular unconformity where it transgresses the Rose- prominent. Toward the northern part of Loraine Mine, oli- dale Placer toward the west (Winter, 1964a). This rela- gomictic conglomerates and quartarenites are well devel- tionship is also evident in drill holes RP 1 and KK 4 (Fig. oped near the western edge and predominate over, and in- 1), where as much as 120 m of the Van den Heeversrust terfinger with, the Welkom facies (Fig. 13). Here the pebble and Rosedale Members have been eroded on this uncon- size decreases rapidly and the conglomerates grade into formity prior to deposition of the EA zone. Local angular quartzites (arenites) , becoming more mature, mineralogi- unconformities even developed at bases of individual pla- cally and texturally, eastward and northeastward (Winter, cers in the EA zone. For example, an angular unconformity 1964a). Winter's observation fits very well with the re- has been recorded beneath the EA 12 placer, which changes gional pattern described above. westward within 40 m form a 10" angular unconformity to These oligomictic conglomerates contain heavy mineral a 60" unconformity (Fig. 14). and gold concentrations, which is the main reason why the Winter (1964a) made the following important observa- EA zone is economically viable in the Loraine facies, whereas tions in the Loraine area: the depoaxis of the EA zone is its correlative in the Welkom facies is not. The detrital py- oriented in a south-southeastward direction and coincides rite grains are much larger in the oligomictic conglomerates with the present synclinal axis; economical gold placers are and range to as much as 6 mm, whereas the pyrite grains only developed in a zone about 320 m wide along the syn- in the intercalated polymictic conglomerates have a maxi- clinal axis and westward as far as the respective subcrop mum size of only 2 mm (Winter, 1964a). The significance positions of the EA placers; individual beds thicken rapidly of this will be discussed later. from their respective subcrop positions toward the trough The polymictic conglomerates of the Welkom facies have axis and conglomerates grade within a few hundred meters into arenites east of the trough axis.

The Uitkyk Member ...... In the proximal Welkom area of Holdings Division, FSG and St. Helena Mines the 'typical' boulder beds of this UITKYK Qo member are hardly distinguishable from the underlying MEMBER massive conglomerates of the EA zone. This is the reason why misinterpretations occurred in the past. There are, however, a few parameters that are distinctly different: first,

EA ZONE

FIG. 13.-Schematic diagram showing the interfingering relationships in the EA zone of the mineralogically mature Loraine facies and the im- FIG. 14.-Basin edge unconformities are typically developed in the mature Welkom facies. Significantly, the maximum interfingering occurs EA zone at the bases of placers, illustrating syntectonic sedimentation above the anticlinal warp. during their deposition.

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at most localities there is an abrupt clast size increase above ments from five localities is 025" TN. Paleocurrent the base of the Uitkyk Member; second, there is a sharp information is sparse in the overlying members, but a few decrease of quartz clasts from more than 20 percent in the observations indicate that some of the quartzarenites might EA zone to 10 percent in the Uitkyk Member; concomi- have been distributed southeastward as well. tantly there is an increase in blue and green chert clasts. Southward and southeastward from the proximal area, the GOLD AND PYRITE MINERALIZATION boulder beds of this member change facies very rapidly over 4.1 km from a robust cobble and boulder conglomerate to In general, the polymictic conglomerates of the Eldorado a litharenitic facies (Fig. 4). Toward the north and north- Formation are not economically important. The only viable east, the robust boulder beds are more continuous and well gold ore carrier is the Rosedale Placer at the base of the developed, so that this member can be identified easily where sequence which contains low values of as much as 5 g/t it overlies the mainly sandy EA zone on FSG (North) and Au over 100 cm. At a few localities on the western edge Loraine. Therefore, the conglomerates change facies more of the basin on Holdings Division and FSG, however, some rapidly toward the south than toward the north. On the other sporadic high values as much as 30 g/t Au over 100 cm hand, the polymictic conglomerates of the EA zone spread are encountered. In these cases this placer has been mined much farther southeastward than northeastward. In distal on a local scale. positions, such as on Beatrix Gold Mine, Erfdeel Gold Mine The high gold values are associated with large detrital (drill hole EDPC 1) and Jeannette Gold Mine (drill hole pyrite grains (Fig. 7a) which are locally concentrated in WR 1), both members change into a sandy sequence and varying amounts as part of the conglomerate matrix. The distinction between them becomes more difficult. These fa- amount and size of the detrital pyrite decrease rapidly east- cies changes from robust conglomerate to almost 100 per- ward down the paleoslope and so does the gold value, so cent arenite take place within a distance varying between that within 1 km downcurrent this placer becomes com- 14 km and 18 km (Fig. 12). pletely uneconomical. Various placer deposits within the Eldorado Formation are economically viable in the Loraine area. Almost all of PALEOCURRENTS them are oligomictic conglomerates that occur at various Paleocurrent measurements were done on the lower two stratigraphic horizons within the sequence, although the best members, and general observations of paleocurrent vectors mineralized placers appear in the EA zone. Good descrip- were made on the upper two members. Foreset aximuths tions of these EA placers (reefs) are to be found in Winter and the orientation of trough axes were used as paleocurrent ( 1964a). They have undergone little sedimentological in- indicators. The mean vectors of measurements collected at vestigation, so that only their broad framework and crude 10 locations (Fig. 6a) illustrate a very consistent south- depositional model can now be deduced. eastern paleocurrent direction in the Rosedale arenites. The The EA placers are quartz cobble to large pebble con- vectoral mean for the whole area calculated from 110 mea- glomerates with a minor amount of black chert pebbles and surements is 151" TN. Unimodal patterns and high consis- rare quartzite pebbles. Regarding clast assemblage, the oli- tency ratios were obtained at all the localities. This direc- gomictic nature of the Loraine facies contrasts sharply with tion is not consistent with the eastward distribution shown any of the polymictic conglomerates in the Welkom area. Their matrix is quartzarenite which contains a variable de- by attributes of the conglomerates (Fig. 6a) and an explanation must be offered for this phenomenon. trital pyrite component. Down the paleoslope to the east, In places where arenite is intercalated with the basal con- the pebble size, pyrite grain size and amount drop dramat- glomerate, crossbedding in the former also indicates a ically, producing only a poorly pyrite-mineralized con- southeastern paleocurrent direction. Therefore, two differ- glomerate 1 km from the subcrop. Concomitantly, the gold ent directions of distribution, one for the arenite and an- values, which are around 1000 cm g/t over 100 cm in many other for the conglomerate, are indicated. In the upper 6 m places in the proximal area, drop to uneconomical levels of the Rosedale Member, a paleocurrent transition zone ex- eastward when one crosses beyond the synclinal axis. The ists in which an interfingering of beds derived from the most dramatic drop in gold value occurs east of the syn- northwest as well as from the southwest occurs. A definite clinal axis, a phenomenon which has not yet been studied switch in paleocurrent direction therefore takes place within in any detail. the arenites of the transition zone. It is significant that the base of this zone is marked by a definite textural change MODEL OF SEDIMENTATION as well. A sequence of medium-grained to very coarse- Environment Deposition grained arenites is overlain at the base of the transition zone of by either a conglomerate bed or a pebbly grit as much as The Eldorado conglomerates are interpreted as alluvial 20 cm thick. The rest of the zone above it also consists of fanglomerates because they constitute bodies of coarse de- pebbly and gritty arenite and thin conglomerate beds. bris with radial sediment dispersal patterns. Down the pa- Paleocurrents measured in the arenites above the base of leoslope, the gravels interfinger with braided alluvial sands the Van den Heeversrust Member in general trend north and that display trough crossbedding and lenticularity . Most northeast (Fig. 6b) and coincide with the distribution in- gravels are sheetflood deposits because they display crude dicated by the clast size distribution of the basal conglom- normal clast grading, crude plane bedding, and some are erate of this member. The vectoral mean of 46 measure- capped by thin gritty sands. In rare cases, inverse grading

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occurs, indicating possible debris flows. bution along a braid plain oriented similarly to that of the Four main pulses of debris influx coinciding with the four Rosedale cycle. Thus, the lower two members are crude members can be identified. In their proximal parts con- fining-upward sequences, each suggesting recession of the glomerate dominates the lower parts of the lower two mem- source area subsequent to rejuvenation. bers. Conglomerate and arenite are more complexly ar- Interpretation of the EA zone is more complex. Again, ranged in the upper two members and at any one locality there were two sources of sediment in the same areas that have much higher percentages of conglomerate than the lower produced two distinctly different facies interfingering with two members, indicating the progradational relationship of each other. The mature Loraine facies was derived from a the overlying fans with the underlying ones. homogeneous silicic source west of Loraine Mine and was The fans were distributed from west to east (Figs. 6, 12) distributed eastward in that area. Material of the Welkom but the regional paleoslope was toward the southeast, as facies was shed simultaneously from a heterogeneous poly- clearly indicted by the paleocurrent pattern of the Rosedale mictic source west of Holdings Division and St. Helena sandstones (Fig. 6a), suggesting a broad braid plain with Mines. The latter produced the polymictic gravels and min- shallow low sinuosity channels superimposed upon it where eralogically immature sands. The two distinct facies inter- these sands were redistributed. These mature sandstones were finger in a wide zone on FSG (North) and the southern part obviously derived from a silicic source in the northwest, of Loraine Mine over an anticlinal warp in the basin. The and as the polymictic Rosedale gravel fan retreated, the braid control on sedimentation exercised by this warp is a matter plain was established over the same areas. Figure 15 de- for further study. This interfingering relationship is espe- picts a physiographic model which combines the features cially evident in the alternating sequence of quartzarenite described above and which is comparable with the wet al- and subgreywacke, as well as in the alternation of poly- luvial fan/braid plain setting described by Rust (1979, Fig. mictic and oligomictic conglomerates of the EA zone. The 2), the model of the Hornli Fan (Bürgisser, 1984) and the differences in detrital pyrite grain sizes of the interfingering Van Horn Fan (McGowen and Groat, 1971). two facies at a particular locality emphasize the difference The intercalated arenites of the Van den Heeversrust fan in source areas. show the same distribution as those of the associated gravels, thus suggesting fan gravel and sand prograded over the Tectonic Control and Sedimentation braid plain sands mainly from west to east. The fan slope was probably steeper than that of the Rosedale fan, as shown The robust conglomerates which overlie the quartzaren- by the rapid pebble-size change and the consistent transport ites at the base of the Van den Heeversrust fan emphasize direction of the gravel and sand. Little paleocurrent infor- one of the transitions in the basin which is undoubtedly mation is available from the mature topmost arenites of this indicative of tectonism. This change marked a rejuvenation member, but the similarity of their lithology to that of the of the source area. Rosedale arenites suggests resorting and possible redistri- The gradual increase in clast size vertically across 27 m of this alluvial fan suggests progradation and the gradual decrease further upward is indictive of fan recession (Fig. 10). This explanation is in agreement with that by Heward (1978), where he describes the initial phases of fan-building off a steep scarp as the progradational phase (coarsening upward), and as the scarp erodes and retreats, the slope of the fan lowers, the fan recedes, a phase which is represented by fining of the clast size upward. This process was the beginning of a geomorphological cycle, a process that was probably repeated several times through the depositional history of the Eldorado Formation; the start of the main cycles is marked by the bases of the different members. The thickening of individual Loraine placers near the depoaxis, their rapid facies change and their basin edge unconformable relationships show that syntectonic sedimentation was the regular pattern during deposition of the EA placers. The occurrence of some oligomictic gold placers in the lower two members in the Loraine area indicates that ‘Loraine-type’ source rocks were already active periodi- cally during the early stages of Eldorado sedimentation. This source became more active and produced its maximum sed- u SECTION A-B iment input during deposition of the EA gold placers. This maximum sediment supply, in fact, is also true of the FIG. 15.-Physiographic model of sedimentation of the Eldorado sediments. Note how the alluvial fans distribute gravel and sand eastward polymictic source west of Welkom, which experienced onto an alluvial braid plain where the sediments are redistributed south maximum uplift during this time as well; rare basement gneiss and southeastward down the regional slope. and lower Witwatersrand quartzite cobbles are proof of this.

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Sediment from both sources, introduced by sheet floods and ence of mainly fining-upward stacked sequences give strength possibly some debris flows onto alluvial fans, intermixed to the proposal for their tectonic setting given above. These in the medial and distal areas and along the toes of the fans sequences show some resemblances to the Old Red Sand- were the sandy sediments were redistributed southeastward stone as shown in Table 1 of Heward (1978). down the regional paleoslope along a broad braid plain. The plate tectonic island arc/foreland basin model sug- Most of the mature sands and some fine-grained gravels of gested by Winter (1984) may well be compatible with the the Eldorado Formation were deposited in this latter envi- model of sedimentation described above. According to ronment (present in drill hole EDPC 1). Winter (1984), this model may, in very general terms, be The two lower members represent two cycles, each one compared with the Cretaceous island arc/foreland model of starting off with a source area uplift. During deposition of the Rocky Mountains. According to Miall (1978), the mar- the EA zone, several uplifts and active sediment input from gins of active foreland molasse basins are characterized by both the Loraine and Welkom facies obscure and interrupt internal unconformities, folds and faults. Internal angular the geomorphological cycle. These uplifts are reflected in unconformities , which die out laterally into bedding, in- the pebble sizes and assemblages, and three different cycles dicating the occurrence of localized pulses of uplift con- between 30 m and 50 m thick can be recognized. temporaneous with sedimentation, are very common in mo- The onset of the fourth major uplift produced the poly- lasse basins. All these features are typical of the Eldorado mictic boulder beds of the Uitkyk Member, which were gravels and sands, and it can best be interpreted as a fore- distributed from west of Welkom and covered the whole land molasse basin setting. area, thereby completely obliterating the oligomictic source west of Loraine Mine that had existed until that time. Fur- ACKNOWLEDGMENTS thermore, this marked syntectonic unconformity extends much farther westward across all older formations, so that I gratefully acknowledge the permission and financial the sedimentary basin eventually overlapped the fold felt in support granted by the management of Anglo American a manner very similar to the Cretaceous-Tertiary example Corporation to publish this paper. It was presented at the of Utah described by Miall (1978). This major uplift oc- Third International Fluvial Sedimentology Conference held curred in three phases and was followed by a period of at Fort Collins, Colorado in August 1985. I also thank G. standstill, erosion and recession of the source area. During Wheelock for access to unpublished data. the initial uplift the coarse gravels were distributed more actively northeastward in contrast to the EA polymictic REFERENCES gravels which spread out actively toward the east and southeast. This is the reason why the sandy distal equiva- BURGISSER,H. M., 1984, A unique mass flow marker bed in a Miocene streamflow molasse sequence, in Koster, E. H., and Steel, R. J., eds. lents of the Uitkyk Member in the southern part of the Wel- Sedimentology of Gravels and Conglomerates: Canadian Society of Pe- kom area overlie the polymictic conglomerates of the EA troleum Geologists Memoir l O, p. 147- 163. zone (Fig, 4). BUSCH,D. A., 1971, Genetic units in delta prospecting: American As- The rapid thinning of members and units of the Eldorado sociation of Petroleum Geologists Bulletin, v. 55, p. 1137-1 154. ERIKSSON,K. A., TURNER,B. R., AND Vos, R. G., 1981, Evidence of sequence westward and the increasing number of angular tidal processes from the lower part of the Witwatersrand Supergroup, marginal unconformities suggest uplift in the west by fold- South Africa: Sedimentary Geology, v. 29, p. 309-325. ing and thrusting, in preference to the classical fault-bounded GLOPPEN,T. G., AND STEEL,R. J., 1981, The deposits, internal structure through model of Pretorious (1976) which would have pro- and geometry in six alluvial fan-fan delta bodies (Devonian-Nor- way)-A study in the significance of bedding sequence in conglom- duced a thick sediment pile in the west. The present model erates, in Ethridge, F. G., and Flores, R. M., eds., Non-Marine De- is also at variance with the epeirogenic uplift model pro- positional Evironments: Models for Exploration: Society of Economic posed by Minter (1978) and Tankard and others (1982). Paleontologists and Mineralogists Special F’ublication 3 1, p. 49-69. The syntectonic unconformities of the Tertiary sequence in HEWARD,A. P., 1978, Alluvial fan sequence and megasequence models, the Spanish Pyrenees described by Riba (1976) display a with examples from Westphalian D-Stephanian B coalfields , northern Spain, in Miall, A. D., ed., Fluvial Sedimentology: Canadian Society remarkable similarity in scale and arrangement with the un- of Petroleum Geologists Memoir 5, p. 669-702. conformities of the Eldorado sequence. Miall ( 1978) pointed KINCSLEY,C. S., 1979, The Eldorado Formation: Sedimentological as- out that the position of the axis of rotation determines the pects of the lower succession in the central part of the Welkom Gold- type of basin edge unconformity. According to the various field: Internal AAC report, 87 p. , 1984, Dagbreek fan-delta: An alluvial placer to prodelta se- examples he gave, the axis of rotation during deposition of quence in the Proterozoic Welkom Goldfield, Witwatersrand, South the upper Eldorado must have been within the basin and Africa, in Koster, E. H., and Steel, R. J., eds., Sedimentology of very close to its edge. Syntectonic sedimentation is also Gravels and Conglomerates: Canadian Society of Petroleum Geologists strongly suggested on a smaller scale by the marginal un- Memoir 10, p. 321-330. conformities between the various EA placers. KLEYNHANS,J. J., 1970, A sedimentological study of the conglomerates in the Elsburg Stage on the Weikom and Western Holdings Gold Mines: In particular, the features of the lower two fans described Unpublished M. S. thesis, University of the Orange Free State, Bloem- here are different in two ways from the small fault-bounded fontein, 149 p. fans described, for example, by Gloppen and Steel (1981) MCGOWEN,J. H., AND GROAT,C. G., 1971, Van Horn Sandstone, West and Nemec and Steel (1984). Their cycles mainly show a Texas: An alluvial fan model for mineral exploration: Bureau of Economic Geology, University of Texas, Report of Investigation 72, coarsening-upward motif and the basin sequence is very 57 p. thick. The much thinner Eldorado sequence, the westward MCKINNEY,J. S., 1964, Geology of the Anglo American Group Mines wedging of individual units (and members) and the pres- in the Welkom area, Orange Free State Goldfield, in Haughton, S. H.,

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ed., The Geology of Some Ore Deposits in Southern Africa, Vol. I: SIMS,J. F. M., 1969, The stratigraphy and paleocurrent history of the Geological Society of South Africa, p. 451-506. upper division of the Witwatersrand System on the President Steyn MIALL,A. D., 1978, Tectonic setting and syndepositional deformation mine and adjacent areas in the Orange Free State Goldfield with spe- of molasse and other nonmarine-paralic sedimentary basins: Canadian cific reference to the origin of auriferous reefs, Vol. I and 11: Unpub- Journal of Earth Sciences, v. 15, p. 1613-1632. lished Ph.D. Dissertation, University of the Witwatersrand, Johannes- MINTER,W. E. L., 1978, A sedimentological synthesis of placer gold, burg, 181 p. uranium and pyrite concentrations in Proterozoic Witwatersrand sedi- TANKARD,A. J., JACKSON,M. P. A., ERIKSSON,K. A., HOBDAY,D. K. ments, in Miall, A. D., ed., Fluvial Sedimentology: Canadian Society HUNTER,D. R., AND MINTER,W. E. L., 1982, Crustal Evolution of of Petroleum Geologists Memoir 5, p. 801-829. Southern Africa: Springer-Verlag, New York, 523 p. NEMEC,W., AND STEEL,R. J., 1984, Alluvial and coastal conglomerates: Vos, R. G., 1975, An alluvial plain and lacustrine model for the Pre- Their significant features and some comments on gravelly mass-flow cambrian Witwatersrand deposits of South Africa: Journal of Sedi- deposits, in Koster, E. H., and Steel, R. J., eds., Sedimentology of mentary Petrology, v. 45, p. 480-493. Gravels and Conglomerates: Canadian Society of Petroleum Geologists WINTER,H. de la R., 1964a, The geology of the northern section of the Memoir 10, p. 1-31. Orange Free State Goldfield, in Haughton, S. H. ed., The Geology of PRETORIUS,D. A., 1976, Gold in Proterozoic sediments of South Africa: Some Ore Deposits in Southern Africa, Vol. I: Geological Society of Systems, paradigms and models, in Wolf, K. H. ed., Handbook of South Africa, Johannesburg, p. 417-448. Strata-Bound and Stratiform Ore Deposits: Elsevier, New York, v. 7, , 1964b, The geology of the Virginia section of the Orange Free p. 1-27. State Goldfield, in Haughton, S. H., ed., The Geology of Some Ore RIBA,O., 1976, Syntectonic unconformities of the Alto Cardener, Span- Deposits in Southern Africa, Vol. I: Geological Society of South Af- ish Pyrenees: A genetic interpretation: Sedimentary Geology, v. 15, p. rica, Johannesburg, p. 507-548. 213-234. , 1984, Structural style of the Rocky Mountains, Utah: Kaapvaal RUST,B. R., 1979, Coarse alluvial deposits, in Walker, R. G., ed., Fa- analogues: Geobulletin, Geological Society of South Africa v. 27, p. cies Models: Geoscience Canada Reprint Series 1, p. 9-21. 44.

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Conventions used: An underlined page number denotes the first page of a paper in which the word is used numerous times throughout the paper. Stratigraphic members are indexed under the formation of which they are a part. Geographic features and selected geologic structures show in parentheses the State or Country which the feature is located. A Alluvial fan deposit 5 1 Aandenk Formation 360, 361 Alluvial fan facies 252 Absaroka Mountains (WY) 264, 267 Alluvial fan wet 359 Absaroka Range (WY) 254, 265 Alluvial fill 259 Absaroka thrust (WY) 280, 284 Alluvial paleoarchitecture 3 11, 3 18, 3 19 Absaroka volcanic field (WY) 263 Alluvial plain 58, 202, 217, 229, 237, 243, 248, 297, 305, 307, 3 1 1, Acadia terrains 297 319 Acadian clastic wedge 297 Alluvial plain braided 360 Acadian compression 297 Alluvial plain cyclicity 229 Acadian deformation 287, 297 Alluvial plain lake 250 Acadian events 287 Alluvial processes 99, 100, 248 Acadian mountains 298 Alluvial sediment 99, 100, 229 Acadian orogeny 287, 297 Alluvial sedimentation 243, 282, 3 1 1 Acadian tectonics 298 Alluvial sequence 159, 279 Accretion 260 Alluvial slope 100 Accretion alluvial 253 Alluvial soil 166 Accretion bedding 179 Alluvial stratigraphy 243, 253 Accretion deposits lateral 183 Alluvial suites 253 Accretion downstream 63, 71, 72, 73, 295 Alluvial surface 163 Accretion flank 63 , 7 1, 72 Alluvial systems 31 1 Accretion lateral 2, 4, 56, 0,87, 93, 97, 98, B, 1 11, 179, 185, 187, Alluvial terrace 207 188, 193, 197, 201, 235, 237,253, 272, 279, 281, 291, 295, 315, Alluvial valley 87 318, 319, 329 Alluvium 99, 179, 187, 218 Accretion levee 112 Alluvium nonvolcanic 226 Accretion point bar 255 Ahus Rubra 106 Accretion surfaces 179, 186, 294, 313, 315, 316 Alphanumerical system 165 Accretion surfaces lateral 95, 98, 312 Alpine molasse 59 Accretion upstream 0 Amalgamation surface 159, 163 Accretion vertical 56, 101, 108, 193,237, 259,288,292,295,323,329, Amazon Basin (Brazil) 4 334 Amphibian remains 229 Accretionary bank 179 Amstrat grain size chart 84 Acritarchs 160 Anaheim, CA 1 Adrian, OR 257 Anastomosed belt 307, 318 Afton, WY 280 Anastomosed model 1 Aggradation 210, 229, 233, 339 Anastomosed system 28 1 Aggradation lateral 56 Anastomosis 3 19 Aggradation vertical 56, 235 Andesitic detritus 220 Airy-type isostatic model 282 Angle tributary 30 Alaska 343, 344 Angular unconformity 267, 268, 366, 369 Albegna River (Italy) 202 Ankylosaurids 160 Alberta (Canada) 4, 83, 149, 155, 159, 160, 166 Anticline 220 Alberta data 154 Anticline axes 220 Alberta Foothills (Canada) 90 Appalachian basin (PA) 319 Albuquerque, NM 93, 94 Appalachian belt 297 Algal binding 294 Appalachian coal beds 308 Algal mat 56 Appalachian molasse 59 Algal oncolites 172 Appalachian Plateau (WV) 229 Algal structure 263 Appalachian-Caledonian fold belt 298 Allegheny Group 229, 230 Appersett, England 32, 33 Allgold Creek (Canada) 206 Appersett Field (England) 33 Allochthonous terrain 343 Aprons 76 Allocyclic 6, 253, 279, 31 1, 312 Aquatic scavenger 161 Allocyclic controls 253, 254, 298 Aqueous setting 161 Allocyclic mechanism 253, 311 Aquitanian age 269 Allogenic controls 269 Arc-adjacent basins 2 17 Alluvial apron 297 Architecture -see Alluvial Alluvial architecture 2, 6, 113, 243, 253, 279, 282 Arenite 359, 360 Alluvial basin 252, 304 Arent 57 Alluvial channel 248 Aricha, Bangladesh 70 Alluvial cone 59, 250 Armorica 297 Alluvial cyclicity 229 Arno River (Italy) 198 Alluvial deposits 99, 102, 104, 162, 243, 287 Articular surfaces 161 Alluvial episodes 163 Articulated remains 165 Alluvial facies 230, 274 Arun River (India) 51, 53, 59 Alluvial fan 4, 51, 55, 59, 60, 100, 101, 102, 212, 226, 243, 247, 304, Ash 152, 183, 264 309, 311, 313, 318, 319, 359 Assam quake 58 Alluvial fan cycle 359 Atchafalaya Basin (LA) 348 37 1

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Atchafalaya River (LA) 112 Bar forms 73 Atchafalaya Swamp (LA) 350 Bar geometry 67 Athabasca Channel 83, 84 Bar gravel 75, 81, 105, 108 Athabasca Delta 84, 86 Bar headz, 180, 186 Athabasca delta plain 83, 86 Bar lateral 66, 73, 75, 9, 202 Athabasca Oil Sands 4, 11 1 Bar lateral migration 193 Athabasca River (Canada) 88 Bar load 121, 128, 130 Athabasca River delta 84, 85 Bar lobate 56 Atnarko River (Canada) 99, 100 Bar longitudinal 149, 153, 201, 315, 316 Atri Gur Flood Basin (Bangladesh) 73 Bar macroform 1 Attritional assemblages 161, 166 Bar medial 0,121 Auburn, WY 280 Bar mid-channel 69, 102, 106, 295, 334 Australia 28 1, 294 Bar mid stream 56 Australia Creek (Canada) 206 Bar migrating 63, 191, 31 1 Australian Hill (Canada) 206 Bar morphology 78 Autochthonous 343 Bar platform 76 Autochthonous assemblages 162, 166 Bar river mouth 334 Autochthonous attritional assemblages 162 Bar scroll 0,84 Autocyclic 6, 237, 253, 279, 311, 316 Bar second order 73 Autocyclic mechanism 311 Bar subaquatic 125 Autogenic 269 Bar tail 3, 104, 186 Avalanche face 33, 76 Bar third order 73 Avalanche sets 294 Bar top 104, 106, 191, 195, 346 Avalon terrain 296, 297 Bar tributary 0 Avon River (England) 192 Bar vegetation 104 Avulsion 2, 63, 102, 111, 118, 174, 195, 202, 236, 243, 248,249, 263, Bar vertical 171 269, 276, 279, 282, 316, 334 Barachois, Canada 288 Avulsion fluvial 31 1, 320 Bar-head 102, 104, 106 Avulsion node 248, 268 Barren zone 317 Avulsion pattern 266, 267 Basal Placer 361 Avulsion stage 119 Basal Raton conglomerate 311 Avulsive events 253 Basalt 100, 221 Axial drainage system 287 Basantpur, India 52, 55, 59 Aycross Formation 265 Basin and Range Province (OR-CA-NV) 218 Ayerbe, Spain 27 1, 272 Basin faulted 334 Basin fill 246 B Basin foreland 246 B Placer 361 Basin pull-apart 243, 246, 247 Babahovo, Ecuador 86 Basin sag 246 Babahovo point bar 86, 87 Basin subsidence 229, 253, 261, 269, 275, 284, 312, 334 Babahovo River (Ecuador) Basin tectonics 243 Backswamp 111,305, 306, 308, 316 Basin-fill 248 Backswamp core 111 Basin-fill model 243 Backswamp deposit 111 Baton Rouge, LA 111 Backwater deposits 106, 108 Battery Point Formation 287 Bagmati River (India) 56, 58 Bay of Bengal 63, 64 Bahaderabad, Bangladesh 0 Bay of Biscay 269, 270 Balan-Tiljuga (India) 59 Bay-lake 229 Balmoral flood channel (NZ) 139, 141, 142 Bear Creek, Canada 206 Balmoral glaciation (NZ) 141 Beartooth Mountains (MT-WY) 254, 264 Bangladesh 59, 63 Beartooth Range (MY-WY) 254 Bank benches 75 Beatrix Gold Mine (South Africa) 367 Bar 1, 4, 5, 22, 52, 56, 60, 0,3, 97, 98, 99, 121, 179, 210, 212, Beatrix Placer 362 214, 225, 270, 292, 295, 319, 322, 330, 331, 332 Beaufort Sea (AK) 343, 344 Bar abandonment 79, 80 Beaver Divide (WY) 264, 265 Bar apex 180 Beaver Divide conglomerate 265 Bar assemblage 0 Bed aggradation 68 Bar back 104 Bed forms 24 Bar bank-attached 102 Bed gravel 17 Bar body deposits 180 Bed load transport 153, 212, 234 Bar braid 66, 209, 329, 334 Bed microform 13, 15, 17 Bar channel 63 Bed microtopography 16 Bar channel edge 56 Bed morphology 22 Bar channel floor 56 Bed roughness 356 Bar chute 66 Bed sheer stress 13, 34 Bar coalesced 71 Bedform 1, 6, 195, 219, 270, 295 Bar cobble-gravel 103 Bedform channel floor 56 Bar compound 5 Bedform distribution 63 Bar crescent shaped 149, 155 Bedform linguoid 3 Bar deposit 3 Bedload fi, 63 Bar diagonal 66, 68, 73 Bedload discharge Bar evolution 78 Bedload mixed 5 Bar first order 73 Bedload movement 13

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Bedload pulse 18 Bridger Formation 263, 265, 267 Bedload suspended 5 Bridger gravel 265 Bedload transport 13 Bridger paleocurrent 266 Bella Coola basin (Canada) 99 Bridger sediments 264 Bella Coola River (Canada) % Bridger strata 262 Bella Coola valley-fill 100 Bridgerian 265 Bend, OR 222 Bridgnorth, England 192 Bengal basin (India) 73 British Columbia (Canada) 81, 84, 85, 99, 217 Bessel function 38 Bromsgrove, England 192 Bewdley , England 192 Brooks Range (AK) 343 Bhaber 51 Bruce Park, WA 85 Bhaber springs 59 Brule Formation 151, 152 Bhaber terraces 56 Bruneau, ID 257 Bhagalpur, India 52 Buffalo 59, 161 Bhimnagar, India 52 Buffalo, WY 304 Bhuyanpur, Bangladesh 69 Bunter Pebble Beds (England) 205 Big Chico Creek (CA) 224 Burhi Gabdak River (India) 56 Big Sand Draw 265 Burial 159, 172 Big Timber, MT 254 Burnham Creek (Canada) 206 Bighorn Basin (WY) 253, 264 Burnt Bridge Creek (Canada) 103 Bighorn Mountains (WY) 254, 255, 264, 304, 309 Burrow 114, 116, 117, 229, 280, 304 Bihar quake 59 Bijou Creek flood 151 C Bijou Creek (CO) 162 Cache thrust (WY) 280 Billings, MT 254 Caenagnathids 160 Bimodal 32 Cagayan basin (Philippines) 2 14 Bimodal distribution 40, 201 Calamus River (NE) 72 Bimodal ripple 2 Calcite concretions 258 Biocoenosis 160 Calcrete 56, 199, 201, 270 Biostratigraphic analysis 166 Calgary, Canada 1, 88, 159 Bioturbation 236, 27 1, 272, 274 Caliche 224, 232 Birds 160 Caliche horizons 233 Birkbeck electromagnetic sensor 14, 15 California 217, 224, 251 Birbeck samplers 18 Campanian 159, 160, 2 Birmingham, England 192 Canada 83, 84, 180, 205, 281, 287, 288, 297 Birpur, India 52, 57 Canadian Maritime province 297 Birpur-Basatpur-Purnea road (India) 52 Canadian Rockies 155 Birpur-Saharsa road (India) 52 Canal 51, 52, 59 Birpur-Supaul road (India) 55 Canis 162 Bivalve 304 Cannes de Roches Formation 288 Black Butte (ID) 259 Capillary zone 149 Blue Ridge Mountains, Va 230 Carbon Leader East (South Africa) 355 Blyvooruitzicht Gold Mine (S.Af.) 355 Carbon Leader Reef (South Africa) 353, 355, 356 Bog 351 Carboniferous 174, 251, 297, 309, 346, 347, 350 Bog deposit 279 Carcass 160, 161, 162 Bohai Bay Basin (China) 335 Carcass flotation 162 Boise, ID 257 Carnivore 161 Bonanza Creek (Canada) 205 Carnivore activity 172 Bonanza Creek valley (Canada) 205 Cascade Range (OR) 217 Bone assemblage 172 Cascade Range (CA) 224 Bone occurrences 172 Cascade uplift (OR) 217 Bone preservation 174 Cascade volcanic arc (OR) 217, 227 Bone weathering 161 Cascade volcaniclastic sequence 2 17 Bounding surfaces 2, 4, 74, 193, 194, 195, 295 Cascade volcanism 220, 224 Bovids 56 Cascade volcanoes 2 17 Bozeman, MT 254 Castelnuovo, Italy 198 Brachiopod 237 Castelnuovo Group 198 Brahmaputra River (Bangladesh) 4, 60, 0,249 Catalan chain 269 Braid plain 359 Catskill 59 Braid plain alluvial 368 Catskill Mountains (NY) 297 Braid belt 3 15 Catskill wedge (NY) 296, 297, 298 Braided stream sedimentation 224 Cenozoic 5, 149, 151, 217, 297, 329, 340 Braided system 3 Central Appalachians 229 Braidplain 166, 205, 210, 295 Central Rand Group 359 Braidplain deposit 287 Centrosaurus 165 Braidplain facies 287, 297 Ceratopsian component 165 Braidplain system 2 13 Ceratopsids 160 Breccias 223 Chamber of Mines Research Organization 353 Bridger alluvial plain 266, 267 Champsosaurs 160 Bridger Basin (WY) 264 Channel 1, 22, 24, 33, g,3, 63, 65, 73,3, 93, 95, 98, %, 130, 140, Bridger channels 263 141, 149, 160, 162, 169, 173, 174,179, 199, 201, 202, 211, 226, Bridger deposystem 264, 266 232, 236, 243, 248, 260, 265, 270, 275, 276, 281, 282, 288, 291, Bridger fluvial system 268 295, 303, 311, 315, 316, 321, 337, 338, 340, 342, 350, 354, 355

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Channel abandoned 5, 51, 113, 125, 162,169, 248, 288, 294, 295, 308, Channel lateral migration 163 334, 339, 343 Channel lateral movement 248 Channel abandonment 0,232 Channel low gradient 284 Channel active 99, 113, 125, 160, 172, 174, 181 Channel low sinuosity 138, 279, 281, 283, 284, 292, 295, 296, 368 Channel agradation 294 Channel macroform 102 Channel alluvial 13, 18 Channel main 27, 30, 66, 76, 106, 107, 212, 346 Channel anastomosed 2, 66, 73, 232, 295, 321 Channel margin 111, 172, 193, 270, 290, 291, 292 Channel architecture 1 Channel meander 141 Channel assemblages 160 Channel meanderbelt 88, 305 Channel asymmetrical 355 Channel meandering 2, a,66, 73, 69, 281, 319, 321, 340, 341 Channel avulsion 5, 69, 99, 163, 274 Channel meltwater 14 1, 180 Channel back Channel migration 2, 0,77, 104, 106, 108, 179, 187, 188, 243, 250, Channel bank 161, 347 252, 253, 332 Channel bankful 32 1 Channel morphology 1, 63, 99, 108, 141, 143, 321, 322, 325 Channel bar 83, 106 Channel movement first order 67 Channel bedform 84 Channel movement second order 68, 73 Channel belt 0,111, 171, 173, 174, 91 , 94, 19 236, 237, 243, Channel movement third order 68 249, 253, 321, 340 Channel mud 56 Channel belt assemblage 235 Channel multistorey 295 Channel belt asymmetric 249 Channel network 292 Channel belt braided 52 Channel plug 249, 314, 315, 317, 318 Channel belt facies 229 Channel relict 135, 147 Channel belt meandering 321, 324 Channel re-occupation 174 Channel belt orientation 325 Channel second order 0 Channel bend 186 Channel secondary 356 Channel boulder 138 Channel sequence 163 Channel boulder bed 147 Channel shallow 193, 195, 288 Channel braided 2, 52, 59, 0,95, 135,212, 225, 321, 340, 341, 360 Channel shifting 102, 289 Channel chute 66, 67, 93, 97, 98, 104, 186, 188, 201, 202, 346 Channel sinuosity 137, 188 Channel compound 354 Channel sinuous 3, 93, 99, 102, 257, 272 Channel confined 341 Channel slough 99, 186, 188, 202 Channel confluence 22 Channel slumped 291 Channel converging 303 Channel splay 232 Channel conveyance Channel splitting 188 Channel crevasse 3, 1 11, 174, 229, 255, 303, 316, 347, 348 Channel stacked 283 Channel cross bar 56- Channel straight 2, 321, 329, 340 Channel cut off 57, 80 Channel stream 224 Channel cutting 346 Channel system 263, 321 Channel deposit 104, 130, 136, 140, 232,253,264, 266,275, 281, 307, Channel third order 0 321, 322, 323, 346, 350 Channel traces 249 Channel deposition 63, 169 Channel traverse 201 Channel depth 321, 322, 323 Channel tributary 3 1, 33, 102, 174 Channel discharge 28, 73 Channel trough 175 Channel distributary 4, 329 Channel trunk 236 Channel diversion 106 Channel valley-confined 329 Channel environment 160, 165 Channel zone 9 Channel ephemeral 93, 295 Channel-bounding surfaces 4 Channel facies 166, 232, 236, 237, 319 Char 72 Channel facies crevasse 232, 235 Char topography 72 Channel feeder 3 19 Charcoal 99, 183 Channel fill 2, 67, 99, 105, 169, 172, 221, 223, 255, 256, 263, 264, Charleston, WV 229, 230 267, 269, 287, 329, 335 Charlotte Lake (Canada) 100 Channel fill active 335 Chapra (India) 52, 53 Channel fill abandoned 172, 173, 174 Chatra, India 51, 52 Channel fill facies 221 Chert 367 Channel first order 0 Chew marks 161 Channel floodway 32 Cheyenne Tableland (NE) 150, 151 Channel floor 56 Chico, CA 223, 224 Channel fluvial 4, 221, 222, 279, 287, 321, 322, 325, 326, 346, 350 China 329 Channel geometry 5, 279, 355 Chinji Formation 169, 170, 173, 175 Channel gradient 141 Chinji section 172 Channel gravel 101, 108 Chinle Formation 5 Channel hierarchy 64, 67, 68 Chita Parwela-Gabhir section 170 Channel high sinuosity 281, 287 Chronostratigraphic datum planes 5 Channel hydraulic geometry 27 Chronostratigraphic framework 170 Channel immature 267 Chute 56, 76, 77, 106, 187, 346 Channel incised 135,272, 321, 346 Chute bars 181 Channel infill 274 Chute cutoff 253 Channel interwoven 28 1 Clam 201, 199 Channel island 99 Clarkforkian 254, 255, 259, 260 Channel lag 90, 232, 235, 263, 347 Clarkforkian-Wasatchian 253 Channel lag deposits 236 Clast 153 Channel lag facies 230 Clast imbricated 152

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Clay 182, 312 Cutler Formation 174 Clay ball 114, 183 Cyclic sedimentation 359 Clay drapes 181, 232 Cyclicity 237 Clay plug 197, 201, 202 Cyclothems 229, 237 Clay rooted 114, 115, 118 Clear Creek (Canada) 205 D Clearwater River (ID) 14, 15 Dacitic detritus 220 Climate 243 Dagbreek Formation 359, 361 Clinker 305, 306, 307 Dag0 Hill (Canada) 206 Coal 1, 3, 5, 84, 162, 229, 279, 280, 303, 311 Dakota Sandstone 3 1 1 Coal barren deposits 315 Dalkhola, India 52 Coal bearing deposits 315 Dalles Formation 218 Coal fragments 230 Damaran Belt 360 Coal spars 303 Daqing Field (China) 329 Coal Valley (Canada) 90 Daqing Oil Province (China) 329 Coast Mountains (Canada) 99, 100 Darby thrust (WY) 280 Coast Plutonic Complex (Canada) 100 Darcy-Weisbach equation 135, 136, 138, 149, 153 Coast Ranges (CA) 224 Darcy-Weisbach friction factor 138, 141 Coastal plain 159, 217, 287 Darjeeling, India 52 Cody, WY 254 Daule, Ecuador 86 Colorado 18, 141, 149, 151, 254, 311, 312 Daule River (Ecuador) @ Colorado Plateau (CO-UT-AZ-NM) 5 Daule River point bar 86 Colorado-Wyoming 162 Daule-Babahovo River system (Ecuador) 83 Columbia Plateau (WA) 218, 220 Dawson City, Canada 205, 206 Columbia River (WA) 217, 220, 309 Death assemblages 161, 162 Columbia River basalt 220 Death Valley (CA) 251 Columbia River Basalt Group 220 Debris flow deposits 217, 340, 368 Colville Group 344 Debris flow facies 2 18, 220, 22 1, 226 Composite Reef (South Africa) 353, 356 Deer 56 Composite Reef placer (South Africa) 355 Deflection zone 28 Compressional terrain 247 Deglaciation 135 Concave bank 75, 79, 80, 76 Delta4, 5, 76, 102, 121, 124, 343, 347 Concretion 163, 232, 234, 335 Delta deposits lacustrine 111, 116 Conemaugh Group 229, 230 Delta development 121, 128 Confluence angle 28 Delta front deposits 130 Confluent channel 28 Delta gravel 76 Conglomerate glaebule 173 Delta lobe 251 Conglomerate intraformational 169, 172 Delta plain 85, 263, 329, 330 Conglomerate oncolitic 172, 173 Delta platform 125 Connochaetes 16 1 Delta river dominated 274 Conservation of mass 124 Delta system 121, 122 Continental fault system 267 Deltaic deposits 121, 131 Convolute laminations jiJ Depocenter 334 Continental Fault Zone (WY) 263 Deschutes Basin (OR) 221, 226 Cooper’s Creek (Australia) 294, 295 Deschutes Formation 217 Coprolite 159 Deschutes River (OR) 217 Cordilleran ice advances 207 Deschutes River ancestral 222 Cordillera 99 Desiccation 272, 295 Cordillera (Canada) 274 Desiccation cracks 172 Cordilleran region (Canada) 298 Desiccation surfaces 272 Core 111, 321 Detector vibration 25 Counterpoint bar 75 Devensian glacial limit 192 Couplet bar 21 1 Devensian Glaciation 19 1 Couplet pool 21 1 Devonian 205, 234, 275, 287, 343 Cow 56 Dharbanga, India 52, 58 Coyote skeleton 162 Dharleswari River (Bangladesh) 73 Crab 56, 58 Dhok Pathan Formation 169 Crawford-Meade thrust (WY) 284 Diagenesis 149 Cretaceous 83, 162, 166, 174, 224, 269, 279, 3 1 1, 337, 344, 369 Diatomite 220, 257 Cretaceous-Tertiary boundary 3 12, 369 Dichotomous group 101 Crevasse deposits 236 Dinoflagellates 160 Crevasse facies 235 Dinosaur carnivorous 160 Crevasse prograding 229, 236, 237 Dinosaur herbivorous 160 Crevasse splay 3, 4, 113, 23 1, 255, 256, 257, 259, 322, 346, 347, 350, Dinosaur limb bone 163 35 1 Dinosaur Provincial Park (Canada) 88, 89, 159 Crevasse splay deposits 162 Dinosaur remains 162 Crevasse subsystem 28 1 Dinosaur skeleton 166 Crevasse system 229, 237 Direction transverse 354 Crocodiles 160, 163 Disarticulated remains 165 Crooked River (OR) 222 Distal facies 275 Current lineation 290, 295 Distal zone 270, 272 Cut and fill 21 1 Distributary system 269 Cutbank 84, 106, 292, 315 Distributary system fluvial 269

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Distributary system radial 274 Eolian deposits 93, 94 ' Distributary system terminal 276 Eolian ripples 95 Distribution model transverse 355 Eolian sand 93, 98 Dome 251, 309 Eolian sediments 95, 192 Dominion, Canada 206 Epeirogenic uplift 369 Dominion Creek (Canada) 206 Ephemeral channel anastomosing 294 Dominion Mountain (Canada) 206 Ephemeral discharge 294, 297 Dominion Supergroup 366 Ephemeral drainage 297 Donjek deposits 214 Ephemeral hydrologic regime 166 Donjek River (Canada) 64, 66, 315 Ephemeral mud 292 Donjek River model 213, 214 Ephemeral mudflat 287, 297 Donjek River type (Canada) 334 Ephemeral water bodies 162 Draas 2 Epiclastic 149, 265 Drainage system 287 Epiclastic material 266 Drape 56, 182, 200, 201, 295, 315, 330, 334 Episodic transport 136 Drayton Valley (Canada) 155 Epsilon 56 Droitwich, England 192 Epsilon cross stratification 2, 3, 4, 87, 93, 183, 201, 263, 267, 271, 303 Dromeosaurids 160 Epsilon foresets 3 Drought 161 Erfdeel Gold Mine (South Africa) 360, 367 Drumheller, Canada 89, 90 Erosion surface 163, 172, 195, 230, 288, 291, 292, 295 Du Boys equation 135 Erosional scarp 97 Dune 2, 32,56, 68, 93, 102, 149, 155, 211, 212, 288, 346 Erosional surface 229, 23 1, 338 Dune bedform 295 Escape structures 295, 346 Dune migration 210, 263, 268 Estuaries 83, 166 Dune structure 145 Estuarine complex 159 Dunkard area (WV) 229 Europe 83, 229 Dunkard Basin (WV) 229 Eustacy 243 Dunkard Group 229 Eustatic 100 Dunkard sediments 229 Eustatic sea level 229 Dunkard time 229 Evaporite deposits 269 Durham Coalfield (England) 347 Evaporites 269, 297 Duschesnean 265 Evesham, England 192 Extrabasinal control 265 E Eyjafjalljokull (Iceland) 145 Eardington, England 191, 192 Earth's magnetic field 25 F Earthquake Lake (MT) 248 Fabric analyses 209, 210 East Fork River (WY) 14, 15, 17, 166 Facies 107, 246 Ebro Basin (Spain) 173, 269 Facies architecture 11 1, 118 Echooka Formation 344 Facies association 243, 347 Ecosystem 161 Facies belt 269, 275 Ecuador 83, 84, 86 Facies changes 31 1, 359, 360 Edmonton, Canada 88 Facies distribution 243 Edwards Stream (NZ) 142 Facies model 2, 3, 217, 253, 329 Eggshell 159 Facies type 108, 303 Eifelian-Givetian 297 Faden Paleovalley (NE) 150 Ejea, Spain 272 False bayou 1 13 Eldorado Creek (Canada) 206 False River (LA) 111 Eldorado Formation 359 False River region (LA) 111 EA zone 3 Fan 2,166, 217, 227, 243, 359 Rosedale Member 3 Fan alluvial 55, 210, 212, 217, 274, 275, 329 Vitkyk Member 359 Fan arid 59 Van Den Heeversrust Member 359 Fan delta 4, 197 Eldorado placer 359 Fan distributary 54 Elephant 56, 59 Fan gravel 143 Elk Mountain (WY) 264, 266 Fan head 5 1, 59 Ellensburg, WA 220 Fan lobe 247 Ellensburg debris 220 Fan prograding 364 Ellensburg Formation 217 Fan proximal 191, 368 Elmisaurids 160 Fan recession 368 Elsburg Reef (South Africa) 356 Fan slope 368 Embarras Channel 83, 84 Fan terminal 275, 329 Embarras distributary channel 83, 85 Fan-delta 4 Endicott Field (AK) 343 Fanglomerate 359, 360 Endicott Group 343, 344 Fanglomerate alluvial 367 England 162, 192, 298, 350 Fault 243 Entisol 269 Fault antithetic 243 Entwistle, Canada 155 Fault block 263 Eocene 14, 162, 205, 217, 253, 263, 283, 303 Fault movement 25 1 Eolian 3, 38, 0 Fault normal 248, 25 1 Eolian bed 93, 95 Fault strike-slip 243 Eolian bedform 2 Fault synthetic 243, 247 Eolian curves 95 Fault thrust 247

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Fault transfer 247 Flow divergence 122, 126, 128 Fauna 175, 255 Flow dynamics 22, 63 Faunal assemblages 282 Flow ephemerality 272 Faunal groups 160 Flow field 125, 126, 128, 130 Faunal level 263 Flow flood tidal 87 Faunal remains 159, 160 Flow fluvial-ebb tidal 87 Felix coal bed 303 Flow funneling 136 Felix peat swamp 303 Flow halocline 124 Femur 162 Flow hyperflood 222, 224 Ferrite 13 Flow indicator 279 Fiord 100 Flow lateral 330 Fish 160 Flow lava 217, 257 Fish scales 234 Flow overbank 103, 135, 140, 174, 348, 354 Fission-track data 257 Flow paleo 13 Flash flood 161 Flow peak 56, 64 Flat Creek (Canada) 206 Flow post flow 56 Flood 4, 17, 18, 27, 51, 93, 97, 102, 161, 179,201, 315, 330, 347 Flow pyroclastic 214, 224 Flood annual 104 Flow recovery 22 Flood channel 135 Flow regime 289, 294 Flood channel relic 139, 147 Flow resistance 136, 138, 141 Flood control 248 Flow reverse 28, 43, 46 Flood cycle 63 Flow separation 17, 2,122, 129 Flood cycle deposits 281 Flow separation zone 33 Flood dam-burst 18 Flow shallow 191, 218, 354 Flood deposits 135, 141, 217, 250, 294 Flow sheet 56, 347 Flood deposits meltwater 137 Flow stagnation 28, 34 Flood distribution 243 Flow strength 279, 281 Flood events 98, 135, 146, 330 Flow three dimensional 130 Flood flash 161, 166, 313, 339 Flow transport 161 Flood frequency plot 103 Flow transverse 354 Flood hydrograph 14, 15 Flow tributary 3 1 Flood periodicity 175 Flow turbulent 231, 264, 267 Flood waning 289 Flow two dimensional 130 Flood water 137, 248, 250, 347 Flow uniform 135, 143 Flood water dam 248 Flow velocities 30, 31, 35,160, 272 Flood wave 14, 17 Flow waning 37, 105, 172, 186, 201, 232, 292, 359 Floodbasin 111, 226, 229, 303, 316, 319, 347 Fluid velocity 27, 33 Floodplain 3,4, 5,U,2, 63,67, 68, 72,E, 102, 107,111, 161, 162, Flume 33, 162 --169, 179, 195, 202, 243, 253, 257, 259, 261, 275, 280, 282, 289, Flume channels 27 292, 306, 319, 322, 330, 332, 334 Flume transport 162 Floodplain abandoned 169 Flute casts 236 Floodplain architecture 5 Fluvial facies 224 Floodplain avulsion 5 Fluvial architecture 253, 282, 338 Floodplain channel 172, 174 Fluvial basins 217, 226 Floodplain crevassing 5 Fluvial beds 93, 95, 98 Floodplain deposit 102 Fluvial channel 276, 305, 321 Floodplain deposition 169 Fluvial cyclicity 253 Floodplain facies 1, 99, 173, 174 Fluvial deposit 1, 3, 46, 93, 95, 128, 159, 183, 197, 212, 229, 259, Floodplain fauna 175 274, 329, 343 Floodplain flora 175 Fluvial environment 83 Floodplain tuff 267 Fluvial estuarine deposit 88 Flood-waters 348 Fluvial facies 1, 5, 169,217, 218, 226, 274, 306 Flora 255 Fluvial facies model 1 Floral zones 254 Fluvial geomorphology 1, 2 Florence, Italy 197 Fluvial meander deposit 83, 87 Florida 166 Fluvial models 4 Flow 17, 27, 153, 182, 187, 356 Fluvial reservoirs 321, 329 Flow acceleration 32, 79 Fluvial sedimentation ancient 1 Flow active 172 Fluvial sedimentation modem 1 Flow andesite 223, 265 Fluvial sedimentology 1, 160 Flow bankfull 76 Fluvial style 217, 303, 305 Flow basalt 218, 223 Fluvial style anastomosed 304 Flow channel 140, 354 Fluvial style meandering 304 Flow chart 122 Fluvial style sheet-braided 3 19 Flow continuity equation 140 Fluvial system 149, 159, 166, 169, 217, 226, 235, 274, 295, 303, 310, Flow debris 136, 208, 209, 217, 263, 264, 267, 275, 334, 339, 367, 311, 353 369 Fluvialsystem anastomosed 303, 3 11, 3 19, 322 Flow debris subarea1 227 Fluvial system ancient 1 Flow deflection 27, 28, 33, 34, 79 Fluvial system braided 295, 311, 322, 323 Flow depth 137, 353 Fluvial system high bedload 3 1 1 Flow dimension 137 Fluvial system high sinuosity 295 Flow direction 279, 281, 354 Fluvial system low sinuosity 322 Flow discharge 139, 140 Fluvial system meandering 303, 311, 322

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Fluvial system modern 1 Georgia 166 Fluvial-estuarine system 87 Georgia estuaries 166 Fluvial lacustrine interface 121, 197 Geosynclinal trough 237 Fluvial-sandstone architecture 279 Germania, Ecuador 86 Fluvio-deltaic environment 263, 347 Giddings soil probe 113 Fluvio-paludal deposits 343 Gilbertian delta 4 Flying reptiles 160 Gillette, WY 304 Footprint 159 Gironde Estuary, France 87 Footwall 243, 247 Glacial diamict 191 Forbesganj, India 52, 57 Glacial processes 100, 101 Forearc basin 226 Glacier alpine 99 Foredeep 269 Glacier Bay (AK) 100 Foreland 243 Glacier outlet 143 Foreland basin 3, 246, 269, 275, 284, 297 Glaciofluvial deposit 2 14 Foreland molasse 283 Glaciofluvial sedimentation 2 17 Foreland molasse basin 359, 369 Glaciomarine processes 1O0 Foreland thrust basin 243 Glenns Ferry, ID 257 Foreset 4, 41, 43, 161, 197,210, 334, 367 Glenns Ferry basin 257, 259 Foreset asymptotic 29 1 Glenns Ferry Formation 253 Foreset drape 294 Glenns Ferry paleoclimate 257 Foreset slope 126, 130 Glenns Ferry sequence 260 Foreset tabular 295 Gloucester, England 192 Fort Collins, CO 1 Gogra River (India) 53 Fort Prével sediments 298 Gogri, India 53 Fort Union Formation 280 Gold 205, 353, 359 Fortran computer 326 Gold concentration 355 Fossil 165, ‘69, 223, 237, 282, 304 Gold deposit 353, 359 Fossil assemblage 159 Gold distribution 353 Fossil brackish 229 Gold reserves 353, 356 Fossil bone 162 Gold Run Creek (Canada) 206 Fossil distribution 234 Gold transport 353 Fossil localities 160 Gomati River (India) 57 Fossil marine 229 Gondwana Talchir deposits 213, 214 Fossil record 159, 160 Gondwana Talchir Formation 213 Fossil remains 162 Gongdong Field (China) 335, 337 Fossil trace 199, 201 Graben 25, 218, 221, 243, 334 Fossil vertebrate 169,234 Graben asymmetric 243 Foxe’s Flat (Canada) 179 Graben half 243 France 270 Graben intra-arc 221 Franklinian Basement (AK) 344 Graben slump 247 Fraser Plateau (Canada) 100 Grand Forks, Canada 206 Frasian 297 Grand View, ID 257, 258 Free State Geduld Mine (South Africa) 364, 365, 366 Granville, Canada 206 Free State Geduld No 2 Shaft (South Africa) 364 Grassland 59, 183 Frogs 160 Gravel 38, 151, 179, 201, 205, 257 Front Range (CO) 151 Gravel deposit 188, 206 Froude Number 28, 135, 140, 141, 143, 149, 154, 155 Gravel fluvial 207 Grave1 glaciofluvial 18 1 G Gravel imbricated 149 Gallatin County (MT) 22 Gravel massive 155, 207, 209 Gallatin River (MT) 22 Gravel pit 15 1 Game Hill Fault (WY) 279 Gravel stratified 209, 210 Game Hill (WY) 280, 283 Gravel stream 206, 207 Gandak River (India) 53 Gravel tabular 149 Ganges River (India) 3,0 Gravel terrace 206, 207 Gangetic plain (India) 51, 52, 56 Gravel white 207 Gannett Formation 59 Gravel yellow 207 Garpike 164 Grayling Creek (MT) 248 Gas field 321, 325, 326 Grayling Creek Fan (MT) 248 Gas reservoir 321 Great Britain 174 Gaspé (Canada) 205 Great Divide Basin (WY) 264, 265, 268 Gaspé Basin (Canada) 296 Great Eastern Basin (South Africa) 359 Gaspé Bay (Canada) 287, 288 Great Hungarian Plain 250 Gaspé Peninsula (Canada) 287 Great Malvern, England 192 Gaspé region (Canada) 297, 298 Great Plains 149 Gaspé Trough (Canada) 297 Great Valley Sequence 224 Gastropod 304 Greece 247 Gastropoda1 limestone 274 Green Ridge (OR) 221 Gauss magnetic epoch 257 Green River Basin (WY) 263, 279 Gaussian curves 128 Green River Formation 263, 265 Gaussian function 122 Laney Shale 263, 265 Geometry multistory 279 Wilkins Peak Member 265 Geomorphic topography 243 Groove casts 236 Geomorphological cycle 368 Gros Ventre Range (WY) 279

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Gros Ventre Mountains (WY) 280 Idaho 59, 85, 253, 280 Groundwater 149, 151, 191 Idaho Batholith (ID) 257 Groundwater table 348 Idaho-Wyoming 283 Guantao Formation 335 Idaho-Wyoming thrustbelt 280 Guarga thrust sheet 274 Idavada Volcanics 257 Guatemala 4, 217 Igneous intrusion 248 Guayaquil, Ecuador 85, 86 Ignimbrites 217 Guayaquil River (Ecuador) 86 Illuvial processes 114 Guayas River (Ecuador) 85 Imbrication 143, 209, 214, 210, 335 Gulf Coast 5, 231 India 53, 170, 213 Gulf of Guayaquil (Ecuador) 85 Indian River (Canada) 205, 206 Gulf of Mexico 112 Indo-Gandetic molasse 60 Indo-Gangetic plain (India) 52, 59 H Indonesia 2 19 Hacienda Monterray , Ecuador 86 Indus River (India) 60, 174 Hadrosaur skeleton 162 Inertinite 304 Hadrosaurids 160 Inn River (Germany) 18 Hagerman, ID 260 Insect activity 161 Hamilton, Canada 1 Interarc basin 226 Hanging wall 246, 247, 25 1, 275 Interdistributary facies 338 Hanumannagar, India 52 Interfluve 282 Harbin, China 332, 333 Interfluve deposits 28 I, 282 Harebell Formation 280 Intermontane basin 253, 254 Harmony Formation 360, 361 Intermontane Volcanic Belt (Canada) 100 Heavy minerals 359 Internal organs 159 Hebgen Lake (MT) 248, 249, 250, 251 Intertidal flat 83 Hebgen Lake earthquake 248 Intra-basinal control 267 Helley-Smith sampler 2 1 Intra-basinal structures 247, 248 Herbivore biomass 166 Intraclasts 33 1 Hester Creek (Canada) 206 Ireland 298 Heterolithic facies 229, 287 Irishman Creek (NZ) 142 Heterolithic unit 236 Isostatic 100 High Cascades (OR) 217, 221, 222, 224 Itkilyariak Formation 343, 344 High Lava Plains (OR) 222 Ivishak Formation 344 High Plains (NE) 153 Himalayan alluvial plain 169 J Himalayan foothills 169 Jaca Basin (Spain) 271 Himalayan front 5 1 Jaca Basin molasse 272 Himalayan rivers 59 Jackson, WY 280 Hoback Basin (WY) 279, 280 Jaganathganj , Bangladesh 69 Hoback Formation 279, 280 Jeannette Gold Mine (South Africa) 360, 367 Holdings Division (South Africa) 359 Jenni River (Bangladesh) 67 Holdings Division No 5 Shaft (S. Af.) 364 Jensen Creek, Canada 206 Holocene 83, 84, 99, 108, 112, 162, 198, 207, 218, 222, 224, 243, 249 Jeppestown Shale 361 Holt Heath, England 191 Jet flow 121 Hornli Fan (Central Europe) 368 Jogbani, India 52 Horseshoe Canyon Formation a Joggins section (Canada) 162 Horst 25 1 Jokulhlaup event 143 Huachi (China) 341 Jokulhlaups 135 Huminite 304, 305 Jokulsa River (Iceland) 146 Hunker Creek (Canada) 205 Jokulsargil River (Iceland) 145 Hunker Creek valley (Canada) 205 Jostedalsbreen ice cap (Norway) 124, 125 Hunker Summit (Canada) 206 Judith River Formation 83 Hunt Fork Shale 343 Jurassic 162, 248, 284, 338, 344 Hydraulic 13, 14 Hydraulic assemblages 161 K Hydraulic condition 334, 353, 354 Kaap-Vaal Craton (South Africa) 360 Hydraulic gradient 356 Kame 99 Hydraulic parameter 13, 14, 18, 159, 356 Kamla River (India) 54 Hydraulic transport 353 Kamlial Formation 170 Hydrocarbons 19 1 Kanayut Conglomerate 343 Hydrocarbon recovery 321 Kankar 56 Hydrocarbon reserves 326 Karoo (South Africa) 162 Hydrocarbon reservoir 321, 329, 343, 346 Katihar, India 52 Hyperbolic distribution 38, 44 Katla volcano (Iceland) 143, 145 Hyperbolic fit 39, 45 Kaulial section 170 Hyperconcentrated flood flow 217, 264, 267 Kayak Formation 343, 344 Hyperconcentrated stream flow 264 Keele, England 4 Kekiktuk cores 346 I Kekiktuk deposit 349 Iberian chain 269 Kekiktuk depositional basin 35 1 Ice 179, 180 Kekiktuk depositional environment 350 Iceland 135 Kekiktuk depositional surface 35 1 Ice-rafting 136 Kekiktuk Formation 343

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Kekiktuk swamp plain 350, 351 Lalong Oil Province (China) 335 Kenya (Africa) 161 Lambeosaurine hadrosaur 164 Kidderminster, England 192 Lambeosaurus 162 Kilaba floodplain (India) 57 Land mammal ages 254, 256, 260 Kinematic wave 13, 14 Landslide 247 King Solomon Dome (Canada) 206 Lapillistones 22 1 Kingak Formation 344 Laramide 254 Kishanganj, India 58 Laramie Basin (WY) 149 Klamath Mountians (CA) 224 Laramie Range (WY) 151 Klondike area (Canada) 205 Last Chance Creek (Canada) 206 Klondike deposits 207, 211 Latereto Quarry (Italy) 198, 200 Klondike Gravel 206, 207 Laurentian 297 Klondike placer 205 Lava 221 Klondike Plateau (Canada) 205 Law of Facies 307 Klondike River (Canada) 206 Leaf 76, 80, 114, 182 Klondike schist 207 Leaf layer 11 1, 114 Knik River (AK) 21 1 Lenticular Sandstone (WY) 2 Konaie River (Bangladesh) 67 Levee 3, 4, 52, 55, 56, 72, 104, 105, 111, 121, 125, 162, 169, 173, Kosi alluvial fan (India) 2 174, 232, 233, 234, 250, 253, 281, 283, 284, 322, 323, 343 Kosi bar (India) 56 Levee core 111, 116 Kosi barrage 52, 59 Levee deposits 111,233, 234, 236, 305 Kosi catchment basin (India) 51 Levee distal 115 Kosi fan (India) a Levee facies 172, 173, 174, 232, 236 Kosi fan channel 51 Levee natural 347 Kosi gorge (India) 51 Levee progradation 11 1, 236 Kosi meanders 59 Levee proximal 115 Kosi River (India) Levee sub-environment 112 Kosi River fan (Bangladesh) 275 Levee subsystem 281 Kosi River system (India) 52 Levee surficial 115 Kuparuk River Formation 344 Levee-crevasse progradation 23 1 Kursela, India 52, 58 Levee-floodplain deposits 229, 236 Kurtosis 45 Levee-splay 237 Levee-splay complex 28 1 L Levee-splay facies 232 La Barge, WY 280 Liard River terrace (Canada) 84 Lacey equation 154 Liard River (Canada) 83, 84 Lacustrine delta 334 Lignite 304 Lacustrine deposits 197, 276, 283, 348, 350 Limerinos model 141 Lacustrine facies 1 15, 2 17, 25 1, 274 Limpopo Belt (South Africa) 360 Lacustrine limestone 272 Linch Hill (Canada) 213, 214 Lacustrine regression 329 Lisburne Group 344 Lacustrine sediments 2 18 Lithofacies association 192, 195 Lacustrine transgression 329 Lithofacies gravel 192 Lacustrine zone 274 Lithofacies sandy 193 Lag 43, 102, 201, 279 Lithospheric extension 243 Lag channel base 159 Lithostratigraphy 169 Lag conglomerate 3 14 Little Wind River (IN) 187 Lag deposit 151, 163, 199, 230, 263, 279, 280, 303, 329 Lizards 160 Lag gravel 195, 200, 201, 313 Llandeilo, England 76, 80 Lahar 214, 219 Llandovery , England 76 Lake 4, 59, 99, 115, 116, 118, 126, 128, 230, 234, 235, 237, 247, 251, Load casts 152, 236 257, 260, 274, 303, 316, 335, 337, 343 Lobate sheets 188 Lake Alexandrina (NZ) 142, 143 Locride (Greece) 247 Lake Amboseli (Africa) 16 1 Lodgepole Creek (NE) 150 Lake basins 329 Lodgepole Creek Valley (NE) 150 Lake County Uplift (LA) 249, 250 Loess 141 Lake crater 226 Log 102, 106 Lake deposits 237, 279 Log data 324 Lake ephemeral 234 Log gamma ray 322, 323 Lake fjord valley 121 Log geophysical 308 Lake floodplain 233, 234 Log neutron 322 Lake flow field 121, 129 Log resistivity 322 Lake freshwater 234 Log spontaneous potential 322 Lake glacier 121, 126 Log vertical 169 Lake Gosiute (WY) 263 Log well 322, 323, 325 Lake oxbow 54, 59, 60, 84, 111, 113, 116, 250, 343 Log wireline 32 1 Lake Peters (AK) 343 Log-hyperbolic curve 39 Lake regression 340 Log-hyperbolic density 39 Lake Rudolf (Africa) 163 Log-hyperbolic distribution Lake swamp plain 343 Log-hyperbolic fits 45 Lake Tekapo

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Loraine facies 359 Meandering system 3 Loraine Gold Mine (South Africa) 360 Medial facies 275 Loraine Mine (South Africa) 359 Medial zone 270, 272, 273 Loraine Mine No 3 Shaft 360' Medicine Bow Mountains (WY) 154 Louisiana jiJ Medicine Bow Range (WY) 151 Lovett Hill (Canada) 205 Mediterranean Sea 269, 270 Lower Cretaceous Unconformity 343 Megaclast 151, 152 Luna, Spain 272 Megacycle 311 Luna depositional system 255 Megaforms 66, 71, 72 Luna System (Spain) 269 Megaripple 31, 56, 66, 181, 186 Luna System alluvium 270 Mercia Mudstone 191 Luna System deposit 269 Mesotidal Luna System drainage basin 274 Mesoforms 66 Luna System facies zones 270 Mesoreptiles 160 Luzon, Philippines 2 14 Mesozoic 282, 297, 329, 340 Mesozoic-Cenozoic 329 M Metapodia 162 Maastrichtian 280 Metolius River (OR) 222 Macerals 305 Micrites 274, 280 Macigno Formation 198 Micritic limestone 279, 280 Macroflora debris 162 Microtidal 83, 87, 89, 90 Macroform 3, 4, 5, 66, 149, 155 Microdeltas 18 I Macroform bar 4 Microfauna 25 1 Madagascar 58 Microflora 159 Maderganj, Bangladesh 68 Microplates 297 Madison River (MT) 248, 249 Microroot traces 1 16 Madras, OR 222 Microscale 17 Magnetic cobbles 26 Microvertebrate 162 Magnetostratigraphy 169 Mid-channel belt 235 Mahananda, India 52 Midland Valley (Scotland) 25 1 Mahananda River (India) a Midlands (England) 19 1 Main Terrace (England) 191, 192 Migration bedform 295 Malbaie Bay (Canada) 288 Migration lateral 0,173, 247, 249, 275, 28 I, 295 Malbaie conglomerates 298 Migratory herd 166 Malbaie Formation 205, 287 Miocene 56, 155, 162, 166, 169, 170, 174, 205, 217, 231, 257, 269 Fort Prével Member 287 Mississippi floodplain (LA) 249 Maline Field (China) 338, 339 Mississippi River (LA) 111, 248, 250 Mammal 160, 161 Mississippi Valley (LA) 111, 243 Mammal bone 161 Mississippian 297, 343 Mandibule 162 Modal fraction 40 Manning equation 135, 138, 153, 154 Modules 122 Mara (Dead) Kosi (India) 55 Molasse basin 271, 297, 369 Marine processes 100 Molasse belt 60 Marine transgression 343 Molasse sequence 298 Maritime Province 297 Molluscs 160, 303 Markanda River (Spain) 275 Monghyr, India 52, 59 Markov chain analysis 149, 191,205, 21 1, 234 Monongahela Group 229 Marl 270, 272, 274, 276 Monongahela sediments 229 Marsh 52, 59, 281, 322 Monongahela-Dunkard floodplains 234 Matrix traps 14 Monongahela-Dunkard Groups 229 Matuyama magnetic epoch 257 Monongahela-Dunkard paleochannels 23 1 McMurray Formation e Monongahela-Dunkard sediments 229, 237 McMurray point bar 88 Monogahela-Dunkard strata 230, 235 Meade thrust (WY) 280 Monsoon rains 63 Mean velocity 136, 137 Monsoon season 56 Meander abandoned 35 1 Montana 20, 22, 159, 243, 250, 254, 257, 304 Meander belt 111, 179, 249, 250, 260, 282 Monterray, Ecuador 86 Meander belt complex 230, 235, 236 Montevarchi Group 197, 198 Meander bend 14, E, 98, 113, 182, 186, 187 Monticello deposit 202 Meander bend migration 11 1 Monticello Group 197 Meander cutoff 78, 249 Moraine 142 Meander lobe 179 Moraine ridge 145 Meander loop 73, 75, 113, 138 Moraines neoglacial 124 Meander plain 159, 166 Moraines terminal 125, 136 Meander platform 75 Morien Group 151 Meander scar 250 Morien intraclast 151 Meanderbelt 303, 305, 310, 315, 316, 318, 319, 330, 331, 333 Morphology 243 Meanderbelt facies 303 Morrison Formation 4, 59, 275 Meanderbelt system 305 Saltwash Member 275 Meandering facies 303 Mortality aqueous 160 Meandering river deposits 87 Mortality attritional 160, 161 Meandering river lithofacies 87 Mortality catastrophic 160 Meandering sandbody 333 Mortality events 160

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Mortality mass 161, 165, 166 Neoglacial 99 Mortality natural 160 Nepal 59 Mosquito Creek (Canada) 207 Neural canal 164 Mother Lode 205 Neural spine 164 Mount Everest (India) 5 1, 53 Nevada 257 Mount Hood (OR) 218 New England 297 Mount Jefferson (OR) 221, 222 New Madrid (MO) 243 Mount Rainier (WA) 217, 220 New Madrid earthquake 250 Mount St. Helens (WA) 218, 224, 225 New Mexico 4, 32, 169, 311, 312 Mountain Home, ID 257 New Orleans, LA 112 Mozambique Belt (South Africa) 360 New York 297 Mt. John glacial phase (NZ) 141 New York promontory 296, 297 Mt. John moraine (NZ) 143 New Zealand 135 Mt. John flood channel (NZ) 139, 142 Newfoundland 288 Mt. John/Tekapo flood channel 141 Newtonian fluid flow 136 Mt. St. Helens (WA) 209 Niakuk Fault Zone (AK) 343 Mud 56, 57, 87, 88, 90, 95, 111, 180, 181, 194, 270, 274, 292, 331 Nijman River (Canada) 182 346, 347, 360 Nile syncline 220 Mud accumulation 295 Nile, WA 220 Mud balls 5, 85, 88 Niñemile Creek (Canada) 206 Mud barriers 338 Nipawin, Canada 179 Mud block 85 Nirmali, India 52, 55, 57 Mud channel-bank 258 Noatak Sandstone 343 Mud chips 85, 88 Node points 65 Mud clasts 232, 236 Nodosaurids 160 Mud convoluted 21 1 Nodule 114, 119, 236 Mud couplets 85, 87 Nodule calcareous 236 Mud deposits 338 Nodule vivianite 115 Mud drape 56, 72, 118, 179, 231, 287, 330 Nonmarine facies 282 Mud film 291, 294 North America 1, 159, 297, 321 Mud galls 303 North Bentinck Arm (Canada) 100 Mud glaciomarine 101 North Fork Toutle River (WA) 225 Mud gypsiferous 274 North Island (NZ) 142 Mud lacustrine 269, 272, 274 North Platte, NE 155 Mud laminated 193 North Platte River (NE) 149 Mud layer 271 North Platte River Valley (NE) 149, 150, 153 Mud partings 288 North Prong Wild Horse Creek (WY) 307 Mud pebbles 330 North Saskatchewan River (Canada) 88, 149, 155, 180 Mud pellets 116 North Slope (AK) 343 Mud plug 69 North Yorkshire, England 32, 33 Mud streaks 330, 331, 334 Northern Apennines (Italy) j9J Mud wormy 116, 117 Northwest Territories (Canada) 84, 88 Mudball 115 Norway 121, 124, 126 Mudball conglomerate 1 11, 114, 1 15 Nova Scotia (Canada) 151, 162, 288, 309, 346 Mudchip 346 Nugget Hill (Canada) 206 Mudclasts 287, 289, 291 Nusatsum alluvial fan 101 Mudcurl 289, 291, 294 Nusatsum River (Canada) 100, 101, 102 Mudflake 291, 293 NYE-Bowler lineament 254 Mudflat 297 Mudflow 51, 59 O Murphy, ID 257 Ochoco Mountains (OR) 222 Muzaffarpur, India 52, 58 Ogallala deposits 155 Myrdalsjokull (Iceland) 145 Ogallala fluvial systems 150 Myrdalsjokull ice cap (Iceland) 143 Ogallala gravels 149, 151 Ogallala Group 149 N Ogallala paleovalley 149 Naches, WA 217 Ogallala stream 149 Naches River (WA) 220 Ogallala valley-fill 149 Naches syncline 220 Ogilivie Mountains (Canada) 207 Naches-Wenas valleys (WA) 220 Ohio 174 Nagarbari, Bangladesh 70 Oil field 321, 325, 326 Nagri Formation 169 Oil reservoir 321 Nagri level 173 Oil/gas field boundary 325, 326 Nagri taxa 173 Old Appalachia 229 Nahanni Butte (Canada) 84 Old Brahmaputra River (Bangladesh) 0 Nahanni River (Canada) 84 Old Red Sandstone (Spitzbergen) 253, 297, 369 Namaqua Natal Belt (South Africa) 360 Oligocene 217, 231, 265, 267, 269, 274 Nebraska 149, 162, 254 Oligocene-Miocene 173 Nebraska panhandle 149 Oligomictic conglomerate 359 Nelson Paleovalley (NE) 150 Oligomictic source 359, 365 Neogene 217, 335 Ordovician 288 Neogene aprons 225, 226, 227 Ore 356 Neogene deposits 150 Ore body 353

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Oreana, ID 257 Paleogeology 265 Oregon 85, 217, 257 Paleohydraulic 1, 13, 27, 141 Organic debris 113 Paleohydraulic analysis 136, 138, 154 Ornithominids 160 Paleohydraulic character 340 Orographic barrier 226, 276 Paleohydraulic computations 137 Ostracode 229, 234, 237, 257, 263 Paleohydraulic conditions 160 Outwash channel 145 Paleohydrology 149, 150, 153 Outwash deposit 137, 195 Paleomagnetic reversal 5 Outwash environments 136 Paleontology 159, 350 Outwash glacial 191 Paleoshoreline 234 Outwash gravels 191 Paleoslope 249, 255, 263, 326, 359 Outwash plain 143 Paleosol 5, 162, 163, 169, 183, 217 Outwash sediments 141, 191 Paleosol floodplain 173 Overbank 112, 160, 174, 222, 237, 345, 354, 356 Paleosol horizon 173, 174, 229, 237, 279 Overbank clays 112 Paleosol levee 173 Overbank deposit 111, 183, 202, 251, 263, 264, 276, 283, 330, 346, Paleostream 232 347, 350 Paleosurface 362 Overbank distal 237 Paleovalley 149,214 Overbank dry 161 Paleovalley slope 155 Overbank environment 160, 165 Paleovalley-fill 150 Overbank facies 169, 172, 173, 229, 237, 269 Paleovelocity 136, 138, 141, 149, 153, 155 Overbank flooding 347, 350 Paleozoic 159, 229, 230, 312 Overbank proximal 237 Paludal deposits 283 Overbank sediments 253 Palynological studies 344 Overbank sequences 162 Palynology 350 OVIS 162 Palynomorph assemblage 35 1 Owl Creek Mountains (WY) 254 Palynomorphs 344 Paradise Hill (Canada) 206 P Paris thrust (WY) 280, 284 Pachycephalosaurids 160 Park Range (CO) 15 1 Pacific Northwest 218, 226 Paskapoo Formation a Pakistan 59, 159, 162, 165, 169, 170 Patna, India 52 Paleic surface 124 Peace River (Canada) 88 Paleobasin 360 Peat 3, 4, 281, 303, 315, 350 Paleocene 83, 90, 159, 254, 255, 279, 303, 304, 309, 311 Peat swamp 5, 162, 303, 351 Paleocene-Eocene 259 Pebble 23, 24, 26, 151, 181, 197, 255, 331, 335, 339 Paleochannel 145, 154, 159, 166, 179, 180, 182, 183, 221, 223, 227 Pebble cluster E Paleochannel braided 141 , Pebble discoidal 20 1 Paleochannel dendritic 141 Pebble granitic 151 Paleochannel system 135 Pebble metamorphic 15 1 Paleoclimate 254, 270 Pebble polymictic 230 Paleocurrent 1, 195, 197, 218, 202, 231, 259,263, 272, 283, 287,321, Pebble volcanic 15 1 359 Pediment 362 PaleoGrrent azimuth 23 1, 236 Pediment surface 164 Paleocurrent characteristics 287 Pedogenesis 159 Paleocurrent data 226, 230, 263, 266, 267, 276, 281 Pedogenic 161 Paleocurrent direction 88, 181, 183, 188, 191, 193, 232, 255, 259, 269, Pedogenic features 297 270, 279, 303, 308, 326 Pedogenic horizons 169, 171, 172, 229, 233, 274 Paleocurrent distribution 23 1 Pedogenic process 27 I Paleocurrent flow 180 Pelletal clay 116 Paleocurrent patterns 254, 287 Pelletal fabric 114, 115, 116, 117 Paleocurrent trend 230, 232, 236, 288 Pelletal mud 116 Paleocurrent variance 294 Pelletal sand 116, 118 Paleodepth 153, 155 Pelletal silt 1 16 Paleodischarge 140, 146, 149, 154, 155 Pelvis 162 Paleodrainage 149, 298, 3 18 Pembina River (Canada) 155 Paleodrainage system 160 Peneplane 350 Paleoecologic analysis 166 Pennsylvania 151, 162, 229, 230, 344 Paleoecology 160 Pennsylvania-Permian 229 Paleoenvironment 237 Permeability contrast 330 Paleoequator 296 Permian 162, 174, 229, 230, 344 Paleoflow 135, 149, 153, 155, 210, 255, 270, 271, 290, 295, 321, 324, Petrofacies 149 326 Petrologic synthesis 217 Paleoflow analysis 136 Picea Sitchensis 106 Paleoflow depth 141 Piceance Basin (CO) 265 Paleoflow direction 255 Piedrahita, Ecuador 86 Paleoflow discharge 146 Pierre Shale 311 Paleoflow parameters 139, 146, 147 Pinchout lateral 32 1 Paleoflow resistance 146 Pinedale meander (MT) 250 Paleoflow velocities 141, 146 Minedale meander belt (MT) 250 Paleogene 260, 280 Pivot point 25 1 Paleogeographic zones 274 Placer 353, 354, 359 Paleogeography 149, 275 Placer deposit 353, 356

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Planar tabular crossbeds 2 18 Puigdefabregas River (Canada) 182 Plane-bedded crossbeds 218 Pumiceous material 263, 266 Plant 119, 211, 230, 305, 346, 348 Pumice 143, 264 Plant debris 183, 331, 335, 346 Pumpkin Creek (NE) 150 Plant detritus 236 Pumpkin Creek Valley (NE) 150, 151 Plant stems 339 Purnea, India 52, 56 Plate tectonic theory 287 Pumea District (India) 59 Platte River (CO-NE) 3, 155 Pumea River (India) 51 Platte River system 153 Pyrenean Basin (Spain) 251 Playa lake 297 Pyrenean chain 276 Pleistocene 14, 37, 56, 75, 83, 84, 85, 99, 108, 112, 149, 169, 191, Pyrenean front 269 192, 197, 198, 205, 207, 214, 243, 251, 257 Pyrenean thrust belt 269 Pleistocene terraces 112 Pyrenean thrust sheet 275 Pleistocene till 164 Pyrenees (Spain-France) 269, 270, 369 Pleistocene-Holocene 2 17 Pyrite 236, 348, 359 Plesiosaur 160 Pyrite druses 115 Pliocene 37, 205, 217, 253 Pyrite foresets 362 Pliolite 28, 29 Pyroclastic 266 Plio-Pleistocene 160, 253 Pyroclastic debris 263 Plug 325 Pyroclastic flow 222 Plutonic clast 220, 263, 267 Pyroclastic volcanism 22 1, 223 Plutonic fraction 10 1 Plutonic lithologies 102 Q Plutonic rock 99, 108 Qingyang (China) 341 Plutonic terrain 99, 102 Qiqihaer, China 332, 333 Plutons 220 Quartz Creek (Canada) 206 Pocatello, ID 257 Quaternary 59, 99, 100, 102, 150, 159, 160, 166, 179, 193, 202, 217, Pocono clastic wedge 297 226, 227 Point bar 1, 56, 57, 59, 67, 72, 73, n,83, o,B,135, 165, 166, 172, Quebec (Canada) 287, 297 179, 201, 229, 259, 260, 279, 281, 289, 295, 318, 329, 346 Quebec Appalachians (Canada) 297 Pointr bank-attached 104 Point bar deposits 83, 179, 230 R Point bar estuarine 87 Radiocarbon dates 112 Point bar facies Radiometric ages 223 Point bar fluvial 83, 93 Radius 162 Point bar gravel 75, 81 Rainbow Reefs (South Africa) 360 Point bar lateral 63, 80 Randfontein Estates Gold Mine (South Africa) 355 Point bar migration 111 Raton, NM jiJ Poison Canyon Formation 311 Raton basin (NM) 311 Poison Canyon megacycle 3 1 1 Raton Formation 311 Pollen 183, 279 Raton megacycle 31 1 Polymictic 236, 237 Rattlesnake Creek (WA) 220, 221 Polymictic conglomerate 359 Rattlesnake Hills (WY) 264 Polymictic gravel 368 Raymond, WA 85 Polymictic source 359, 365 Reactivation surfaces 229 Polymodal 366 Readford, Canada 206 Pond 4, 55, 237 Red Canyon Fault (MT) 248 Pool-and-riffle 225 Red Deer, Canada 155 Populus trichocarpa 106 Red Deer River (Canada) 88, 155 Postglacial time 128 Red River (Canada) 88 Pottsville Group 229, 230 Redding, CA 224 Potwar Plateau (Pakistan) 169, 170 Redditch, England 192 Powder River (WY) 304, 305 Redmond, OR 222 Powder River Basin (WY) 303, 350 Reed 52 Power House terrace (England) 192 Reelfoot Lake (LA) 249, 250, 251 Pratomagno Fan (Italy) 197, 198, 201 Regina, Canada 180 Precambrian 124, 297 Reh 56 Predation 161, 162 Reid advance 207 Preida Hill, Canada 206 Reindeer Island Stratigraphic Test 350 President Brand Gold Mine (South Africa) 359 Reptilian fossil 162 President Steyn Gold Mine (South Africa) 359 Reptilian remains 162 Prével, Canada 288 Reservoir fluvial 322 Prineville, OR 222 Reservoir heterogeneity 339 Process-response system 128 Reservoir sandstones 325 Proglacial deltaic system 130 Reverse flow 33 Proglacial system 130 Reynolds number 28, 122 Progradational basin 275 Rhine River (Germany) 18 Prospect thrust (WY) 280 Rhino calf 162 Proterozoic 359 Rhinocerous 56, 59 Proximal facies 275 Rhone delta 232 Proximal zone 269 Rhythmite Prudhoe Bay (AK) 344 Rhyzolith 169 Pryor Mountains (MT) 254 Rib 162

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Rib transverse 136 Sacramento Valley (CA) 223, 224 Richter scale 248 Sacrum 162 Riffle z,102, 106 Sadlerochit Group 344 Rio Gallego (Spain) 271 Sag River Formation 344 Rio Grande (NM) Sagavanirktok Delta (AK) 343 Rio Grande conveyance channel Saharsa, India 52, 53, 56, 57 Rio Grande floodway Salamanders 160 Rio Grande valley 259 Salloomt River (Canada) 100, 101 Rio Puerco (NM) 0,294 Salt Range (Pakistan) 169 Rio Puerco alluvial valley (NM) 93 San Acacia, NM 37, 39 Rio Puerco point bar 0 San Luis highlands (NM) 3 11, 3 18 Ripple x,52, 56, 72, 88, 93, 102, 116, 210, 279, 288, 364 Sand-bed 17 Ripple adhesion 291 Sandur deposits 135 Ripple climbing 72 Sandur plains 125 Ripple crests 293 Sangre De Cristo Mountains (CO-NM) 312 Ripple drift 3 16 Santa Lucia, Ecuador 86 Ripple laminations 289, 290 Santonian 280 Ripple lunate 106 Sardu Field (China) 330, 33 1, 337 Ripple marks 231 Saskatchewan (Canada) 179, 180 Ripple sandy 37 Saskatchewan River (Canada) 179,310 Ripple structure 90 Saskatchewan River Valley (Canada) 179, 18 1, 188 Ripple system 124 Saskatoon, Canada 180 Ripple wave 29 1 Scale 163 Rist (AK) 344 Scale ganoid-type 164 River anastomosed 3, 63, 281, 338 Scotland 251 River bed deposits 130 Scour and fill 15 1 River braided 1, 3, 51, 52, 63, 73, 125, 193, 197, 226, 227, 276, 329 Sea level 100, 31 1 River channel 25 1, 275, 276 Seabee Formation 344 River Endrick (England) 183, 187 Sediment load 1 River ephemeral 4 Sediment transport 1, 26 River high sinuosity 1, 3 Sedimentary facies 224 River hinterland 166 Sedimentation rates 243 River hydrology 63 Sediment-transport waves 23, 24 River Klaralven (Sweden) 187 Separation zone 2 River low sinuosity 1, 3, 63, 191, 193, 197,281, 295, Serajganj, Bangladesh 0 River meandering 51, 63, 72, 73, 3,83, 179, 187, 197, 201, 279, 329, Serengeti (Africa) 161 330 Severn River (England) 191, 192 River perennial 59 Severn valley (England) 195 River plain deposits 130 Shale Sequence (WY) 279 River tributary 4 Shanganning Basin (China) 338, 339 River trunk 4, 5 Shareakhandi, Bangladesh 68 River Tywi (England) Shear forces 135 River Ure (England) 2 Shear layer 28, 30, 31, 32 River wandering 193, 202 Shear strength 13 Roaring River (CO) 18 Shear stress 13,23, 31, 33, 45, 72, 73 Rob Roy Creek (Canada) 206 Shear stress parameters 153 Rock Creek, Canada 206 Shear velocity 354 Rocky Mountain front 153 Sheep skeleton 162 Rocky Mountain House, Canada 155 Sheet flood 5, 115, 116, 217, 275, 369 Rocky Mountains (CO-WY) 149, 153, 155, 303, 369 Sheet flood deposits 223, 227, 292 Romghat River (India) 58 Sheet flood ephemeral 359 Root 346, 348 Sheet flood facies 224 Root burrows 114 Shelby tube 113 Root casts 258 Shell 88, 90 Root hairs 114, 115 Shengli Oil Province (China) 334 Root linings 114 Shengtuo Field (China) 329, 334 Root marks 232 Sherwood Sandstone 194 Root mottling 52, 114, 115, 172, 231, 347, 349 Shublik Formation 344 Root structures 233, 289 Siderite 236, 304, 348 Root traces 52, 114, 116, 117, 221, 232 Siderite nodules 346 Root wads 106 Siegenian 297 Rootlet 234, 294, 330, 348 Sieve analysis 94, 96 Rootlet horizon 274 Sieve deposits 275 Rootmarks 234 Siliguri, India 52 Rosedale conglomerate 359 Silurian 288, 296 Rosedale fan 368 Sisters, OR 222 Rosedale Placer 359 Sivapithecus 173 Rwenzori National Park (Africa) 161 Siwalik deposits (Pakistan) 169 Siwalik exposures (Pakistan) 169 S Siwalik formations 169, 173, 174 Sacral rib 164 Siwalik floodplain deposit 174 Sacral vertebra 164 Siwalik Group 117, 159, 170, 175 Sacramento River (CA) 224 Siwalik magn. time framework 169

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Siwalik Pakistan 5 Sternum 162 Siwalik sediments 5 Stigmaria 348 Siwalik sequence 174, 175 Stochastic element 121, 124 Skeletal remains 162 Stochastic model 128 Skewness 45, 129, 130 Stomach contents 159 Skogasandur (Iceland) 135 Storm periods 225 Skogasandur channels (Iceland) 143, 146 Stourport terrace (England) 192 Skogasandur flood channel (Iceland) 143, 144, 146 Stratigraphic architecture 160 Skull 162, 163 Stream 5, 13, 14, 26, 33, 37, 43, 79, 179, 187, 248, 272, 305, 309, Slate Creek (ID) 18 319, 329, 334, 335, 351 Slipface 37, 155 Stream anastomosed 307, 310, u,322 Slope 150 Stream avulsion 237 Slope tectonic 248 Stream bank 334 Slope transverse 243 Stream braided 3, 34, 60,149, 212, 214, 217, 231, 269, 270, 272, 276, Slough 104, 106 31 1, 322, 335 Slump 212 Streamcapture 266 Snail 56 Stream channel 307, 334 Snake River (ID) 14, 15, 257 Stream channel morphology 150 Snake River Plain (ID) 253, 257, 260 Stream competence 149 Soan Synclinorium (Pakistan) 169, 170, 174 Stream confluent 28 Socorro, NM 37 Stream Donjek type 153 Socorro Main Canal 37 Stream drainage 149 Sognefjord, Norway 124 Stream ephemeral 93, 288, 289, 295 Soil 183 Stream flow 153, 217, 218, 221, 226 Soil glaebules 172 Stream gravel bed 13, 14, 17, 25, 154 Soil horizon 172 Stream high sinuosity 263 Soil profile 162 Stream lateral migration 276 Solheimajokull (Iceland) 143, 145 Stream low sinuosity 231, 263, 288, 322 Solheimajokull terraces 146 Stream meandering 52, 75, 179,253, 263, 310, 2,322, 323, 334 Songliao Basin (China) 337 Stream meltwater 136 Sourdough Hill (Canada) 207 Stream migration 179, 255 Sourdough Hill, Canada 206 Stream mixed load 232, 318, 319 South Africa 162, 359 Stream pattern 253 South America 83, 84 Stream perennial 226 South Dakota 254 Stream power u South Esk River (Scotland) 187 Stream Scott type 205 South Fork Madison River (MT) 249, 250 Stream sinuous 275, 306, 310, 329 South Island (NZ) 135 Stream suspended load 343 South Pass deposition 267 Stream transverse 295 South Pass Formation 263, 265, 267 Stream trunk 4, 255 South Platte River (NE) 155 Stream turbulent 263 South Platte river system 153 Stream valley proximal 217 South Pyrenean thrust front 270 Stream water 219 South Saskatchewan River (Canada) 7 1, 180 Stream winding 205 South Wales (England) 75, 81 Stream-flow facies 2 19 Spain 251, 269 Stress 13 Specchiano, Spain 198 Stromatolite 173 Specchiano Quarry (Spain) 200 Subaqueous transport 294 Spes Bona Formation 360, 361 Subsidence asymmetric 243 Spirorbis 237 Subsidence differential 274 Spitsburgen 234, 298 Subsidence rate 282 Splay 111, 116, 117, 280, 281, 282 Subsidence tectonic 243 Splay core 111 Sulphur, Canada 206 Splay deposit 111 Sulphur Creek (Canada) 206 Splay distal 118, 23 1, 234, 237 Sun Kosi River (India) 51, 53, 59 Splay facies 237, 282 Supaul, India 52, 59 Splay lobe 229 Sursar River (India) 57 Splay lobe facies 232, 236 Suspended load 121 Splay progradation 1 11 Swale 105, 113 Splay proximal 115 Swamp 4, 5, 212, 281, 303, 319, 320, 322, 343 Spodosol 269 Swamp deposit 279, 351- Squaw Creek (MT) u Swamp facies 115, 117 St Helena Formation 361 Swamp plain 350 St Helena Mine (South Africa) 359 Swamp poorly drained 111 St. Georges de Malbaie, Canada 288 Swamp well drained 111 St. Lawrence promontory 287, 296, 297 Swiss molasse basin 297 Stagnation zone Syncline 217, 220 Standard deviation 45, 272 Syntectonic deposition 217, 267, 369 Standing water 160 Syntectonic sedimentation 2 17 Statistical modelling 321 Synvolcanic 224 Steepbank River (Canada) 89 Synvolcanic sedimentary sequences 2 17 Stem 348 System scale 353

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T Thrust tectonics 243 Tabernacle Butte (WY) 264, 266 Thuja Plicata 106 Tabqua reservoir (Norway) 121 Thurragil (Iceland) 145 Talchako basin (Canada) 99 Tibet 63, 246 Talchako River (Canada) Tibia 162 Tamar River (India) 51 Tidal fluctuation 83 Tamur River (India) 53 Tidal range 85 Tanzania (Canada) 16 1 Tiffanian 260 Taphonomic features 160 Tiger 59 Taphonomic investigation 165 Till 75, 99, 100, 179 Taphonomic megabias 175 Tinta fault (Canada) 205 Taphonomic potential 166 Tinta Trench (Canada) 205 Taphonomic processes 16 1 Tiptonville Dome (MO) 251 Taphonomy 159, 172 Tista fan (India) 53, 67, 64 Taphonomy vertebrate 159, !69 Tista River fan head (India) 51 Tattershall Thorpe (Canada) 2 13, 2 14 Toesets 294 Taxa 172 Tooth marks 173 Taxa aquatic 159 Torlesse stream (New Zealand) 14 Taxa lowland 159 Torok Formation 344 Taxa terrestrial 159 Torrejonian 280 Tectonic activity 248, 252 Tournaisian 297 Tectonic back tilt 248 Toutle River (WA) 225 Tectonic bank 249 Trace fossils 88 Tectonic control 243, 269, 325 Tracer minerals 353 Tectonic disturbance 136 Traction threshold 17, 15, 18 Tectonic episodes 269 Trail Hill (Canada) 205 Tectonic events 267, 287 Trail Section (Canada) 21 1 Tectonic framework 253 Transport bedload 2 Tectonic gradient 248 Transport bone 162 Tectonic movement 243 Transport pebble 21 Tectonic setting 243 Transport waves 21, 26 Tectonic slope 68 Transpressional terrain 247 Tectonic subsidence 284 Transverse convection 355, 356 Tectonic synthesis 21 7 Tree 106, ill, 183 Tectonic terrain 243 Tree roots 114, 115 Tectonic tilt 248, 249, 252 Tree stump 162 Tectonic uplift 146, 243, 338 Tree trunk 76, 199 Teepee Trail Formation 265 Triassic 99, 191, 192, 205, 338, 344 Teeth 159, 163 Trinidad, CO 3 1 I Tehama Formation 224 Trinidad Sandstone jiJ Tekapo channel (NZ) 135 Troodontids 160 Tekapo flood channel (NZ) 135 Tuff 220, 223, 264 Tekapo glacial phase (NZ) 141 Tunsbergdal basin (Norway) 128 Tekapo River (NZ) 142, 143 Tunsbergdal delta (Norway) 121 Tekapo Valley (NZ) 135 Tunsbergdal glacier (Norway) 124 Tekapo-Pukaki canal zone 143 Tunsbergdalbreen glacier (Norway) 125 Tephra 207, 221 Tunsbergdalsbre, Norway 12 1 Terai 51 Tunsbergdalsvatn Lake (Norway) 121 Terminal moraine 141 Turbidity 21 Termite 56, 58 Turbidity current 122, 126 Terrace 143, 179, 181 Turkey Brook (England) E Terrace aggradation 19 1 Turtles 160 Terrace bluffs 137 Tuscan Formation 217 Terrace braided 144 Tverrdal glacial valley (Norway) 124 Terrace deposits 137, 147 Twin Falls, ID 257 Terrace outwash 135 Tyrannosaurid dinosaur 164 Terrace sequence 135 Tyrannosaurids 160 Terrace surface 135 Tyrrell Museum of Paleontology 5 Tertiary 59, 88, 90, 88, 99, 112, 149, 159, 166, 224, 246, 251, 253, Tywi valley (England) 75, 76, 78, 80 269, 280, 297, 311, 350, 351, 369 Tertiary intrusions 2 1 8 U Teton Mountains (WY) 280 Texas Coastal Plain 248 Uganda (Africa) 16 1 Thalweg 40, 42, 57, 63, 84, 93, 98, 104, I 13, 118, 136, 186, 225 Ugnu Formation 344 Thalweg degradation I 19 Uintan 265 Thalweg meandering 1 12 Uncastillo Formation 269 Thescelosaurids 160 Unidirectional currents 195 Three Sisters (OR) 222 Unimodal E, 367 Threshold velocity 162 Unimodal ripple 45 Thrust faulting 243 Unio 199 Thrust front 247 United States 217 Thrust normal 247 Uplift 25 1

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Uranium 303 Wasatch Formation 265, 303, 350 Utah 59, 253, 257, 280, 369 Cathedral Bluffs 265 Wasatchian 253, 265 V Washakie Basin (WY) 264, 265 Valdamo Basin (Italy) 197, 198 Washington 83, 84, 85, 205, 217, 218, 220 Valley alluvial 150 Water buffalo 56, 58 Valley confined 205, 339 Water hyacinth 52 Valley slope 247 Water hyacinth bog 54, 59, 60 Valley tributary 338 Waterflood 330 Valley-fill 99, 108, 149, 214, 226, 251 Waterhole 161, 162, 173, 175 Valley-fill alluvial 100, 104, 108 Watertable 163 Valley-fill deposits 149 Wave 3 Valley-fill postglacial 99 Wave kinematic 2 Valley-wall 150 Wayout Formation 59 Van den Heeversrust conglomerate 359 Wealden Group 174 Van den Heeversrust fan (South Africa) 364, 368 Welkom facies 359 Van den Heeversrust fanglomerate 364 Welkom Formation 361 Van Horn Fan (South Africa) 368 Welkom Goldfield (South Africa) 359, 360 Van Pelt Paleovalley (NE) 150 Welsh plateau 75 Vax machine 326 Welsh rivers (England) 78, 80 Velocity longitudinal 354 Wenas Creek (WA) 220 Velocity maximum 34 West Sak Formation 344 Ventersdorp lava 359, 365 West Virginia 229 Ventersdorp Supergroup 36 1 Western Cascades (OR) 222 Vermejo Formation 311 Western Interior Foreland Basin 166, 284 Vermejo megacycle 3 1 1 Wetland 3 Vermillion River (IN) 186 White Channel deposits 205 Vertebral column 163 White Channel facies 209, 21 1, 213 Vertebrate 159 White Channel Formation 207 Vertebrate articulation 163 White Channel Gravels 205 Vertebrate assemblages 160, 166 White Channel outcrops 206, 213 Vertebrate fauna 165, 166 White Channel sediments 207 Vertebrate land 159, 160 White Channel sequence 205 Vertebrate localities 172 White River Formation 265 Vertebrate occurrences 172, 173 White Sandstone 279, 280 Vertebrate paleoecology 159 Whitehorse, Canada 206 Vertebrate preservation 163, 165, 169, 174, 175 Widdale Beck (England) 32, 33, 34 Vertebrate record 169 Widdale Beck-River Ure confluence 34 Vertebrate remains 159, 162, 173, 175, 269 Wiggins Formation 265 Vertebrate terrestrial 162 Wilcox’s 1830 survey 67 Vertical vortice 31 Wild Horse Creek (WY) 306 Vertisol horizons 234 Wildcat Ridge (NE) 151 Vibracore Wildebest 161 Virginia 230 Willapa Bay, WA 85 Virginia Creek (Italy) 14 Willapa River (WA) Virginia Formation 36 1 Willapa point bar 85 Visean 343 Williams River (Canada) 3 Volcanic ash 149, 161, 162 Willow Creek (Canada) 88, 89 Volcanic ash-fall 166 Willwood Formation 153, 175, 253 Volcanic centers andesitic 217 Willwood paleosols 255, 256, 260 Volcanic debris 219, 220, 267 Willwood sequence 253 Volcanic epiclast 263 Willwood sheets 259, 260, 261 Volcanic eruptions 4 Willwood streams 254 Volcanic rock 108 Wind River Basin (WY) 264, 267 Volcanic terrain 99, 108 Wind River fault (WY) 266 Volcaniclastic aprons 217 Wind River Formation 265 Volcaniclastic debris 220 Wind River Range (WY) 263 Volcaniclastic deposit 209, 214 Wisconsin McConnell advance 207 Volcaniclastic sequence 226, 227 Wisconsin Reid advance 207 Volcaniclastics 217, 220, 227 Witwatersrand placers (South Africa) 359 Volcanism 217, 263 Witwatersrand placer deposit (South Africa) 359 Volcano 217 Witwatersrand Supergroup 359, 366 Volcano subglacial 143 Wolds flood channel (NZ) 139, 141, 142 Volcanogenic material 263 Wolds glaciation (NZ) 141 Von Karman-Prandtl velocity law 138 Wolds moraine (NZ) 143 Vortex 28, 3 1 Wolverhampton, England 192 Wood 85, 90, 101, 106 W Wood Bay Formation 275 Wabash River (IN) 187 Worcester terrace (England) 192 Wagon Bed Formation 265 Worcestershire, England 19 1 Wales 192 Worm 56, 58 Waning flow 40 Wormy fabric 111 Warps 243 Wyoming 59, 149, 151, 153, 162, 175, 253, 264, 267, 304

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Y Yukon Cataclastic Complex (Canada) 205 Yakima, WA 220 Yukon gold 205 Yakima foldbelt (WA) 220 Yukon placer 205 Yakima River (WA) 220 Yukon River (Canada) 205 Yankton, SD 154 Yukon Territory (Canada) 205 Yellowstone Plateau (WY-MT) 264 York River Formation 27, 288 Yorkshire Basin (England) 248 Z Yukon (Canada) 207, 212 Zimbabwian Craton (South Africa) 360

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