Depositional Systems in the Wilcox and Their

Bureau of Economic Geology

Geological Circular 67-4

Depositional Systems in the Wilcox Group of Texas and Their Relationship to Occurrence of Oil and Gas

By W. L. Fisher and J. H.McGowen

REPRINTED FROM TRANSACTIONS OF THE GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES VOL XVII, 1967

The University of Texas at Austin October 1967 DepositionalSystems in the Wilcox Group of Texas and Their Relationship to Occurrence of Oil And Gas

W. L. Fisher J. H. McGowen Bureau of Economic Geology Bureau of Economic Geology The University of Texas at Austin The University of Texas at Austin

Abstract

Regional investigation of the lower part of the Wilcox Group in Texas in outcrop and subsurface indicates seven principal depositional systems. These include: (1) _ Mt. Pleasant Fluvial System developed updip and in outcrop north of the Colorado River; (2) Rockdale Delta System, present primarily in subsurface, chiefly between the Guadalupe and Sabine Rivers; (3) Pendleton Lagoon-Bay System in outcrop and subsurface largely on the southern flank of the Sabine Uplift; (4) San Marcos Strandplain-Bay System, occurring in outcrop and subsurface mainly on the San Marcos Arch; (5) Gotulla Barrier Bar System in subsurface of South Texas; (6) Indio Bay-Lagoon System developed updip and in outcrop of South Texas; and (7) South Texas Shelf System, an extensive system entirely within subsurface of South Texas. The Rockdale Delta System, consisting of large lobate wedges of muds, sands, and carbonaceous deposits, is the thickest and most extensive of the lower Wilcox depositional systems. It grades updip to the thinner terrigenous facies of the Mt. Pleasant Fluvial System. Deposits of the Rockdale Delta Sys- tem were the source of sediments redistributed by marine processes and deposited in laterally adjacent marine systems. Delineation of depositional systems and, more specifically, delineation of component facies of the various systems, permits establishment of regional oil and gas trends which show relationship of producing fields and distribution of potentially producing trends. ' Introduction mation and the upper part of the Wilcox Group are not considered here. The Wilcox Group (Lower Eocene) is a thick sequence of predominantly terrigenous clastic sediments We thank Josephine Casey, L. F. Brown, and P. T. and volumetrically a significant part of the large ter- Flawn, Bureau of EconomicGeology, and Alan J.Scott, rigenous fill of the western Gulf Coast Province. It Department of Geology, The University of Texas at is an economically important group of rocks, providing Austin, for reading the manuscript and offering many helpful suggestions. Gerald H. Baum, Texas Water oil, gas, and reservoirs as well as deposits fresh-water Development Board, made the extensive well- of lignite, ceramic clay, available and industrial sand. log library of that agency; numerous Gulf Coast Area of study includes approximately 40,000 square operators provided records of specific wells. Peter U. miles in the Texas Gulf Coastal Plain,extending from Rodda, Bureau of Economic Geology, The University of Texas Austin, has been helpful in the Sabine River on the east to the Rio Grande on at many ways during the project. were south,and in to including the course of Illustrations the a 100- 300-mile wide belt prepared W. Macon, and subsurface. under direction of James Bureau outcrop Subsurface control consisted of Economic Geology, The University of Texas at of approximately 2,500 electric logs, supplemented by Austin. lithologic control based on outcrop descriptions and cuttings and cores of strategically located wells. Fifty Terminology and Depositional stratigraphic dip sections and 15 strike sections were Systems prepared at spacings of 8 to 10 miles to determine three-dimensional relationships of principal lithologic The stratigraphic terminology employed is informal. units. Emphasis in outcrop study was on mapping of Reference is made to existing, defined, formal strati- principal rock units, interpreting primary sedimentary graphic units (chiefly in outcrop) where applicable. structures, and determining fossil composition. Establishment of a formal stratigraphic terminology throughout the extent of the Wilcox Group (outcrop This report preliminary, being part of a continuing, is awaits The larger study of the Wilcox Group in Texas which has and subsurface) further study. terminology been under way at the Bureau of Economic Geology used derives from the concept of depositional systems for the past two years; it is regional in context. Herein, or complexes, employed recently, for example, in con- consideration is restricted to the lower part of the nection with modern sediments and depositional en- Wilcox Group, including the sequence underlain by vironments of the South Texas coast (Hayes and Scott, marine muds and clays of the Midway Group and 1964) and in reference to the Frio Barrier Bar System normally overlain by marine or lagoonal muds of the of South Texas (Boyd and Dyer, 1964). In study of middle part of the Wilcox Group. The Carrizo For- modern sediments emphasis is generally placed not only on description of specific environments and two- dimensional distribution of sediments and environ- of facies, 1Publication authorized by the Director, Bureau of Economic ments but also on analysis and integration Geology, The University of Texas at Austin. with fabrication of facies tracts and three-dimensional

105 TRANSACTIONS— GULF COAST OF GEOLOGICAL SOCIETIES Volume XVII, 1967 106 ASSOCIATION

Figure 1. Principal depositional systems, lower part of Wilcox Group, Texas depositional models of known depositional complexes involves recognition of large-scale natural, genetic units or systems. These approaches are significant in in- comparable to modern depositional systems readily ap- terpretation of ancient sediments, especially in regional parent from physiographic characteristics. analysis. More emphasis is placed on lithologic and fossil composition, sedimentary structures, and dis- Principal Depositional Systems tribution, geometry, and relationship of component Seven principal depositional systems are recognized facies than on consideration of regional correlation and in the lower part of the Wilcox Group of Texas (Fig. persistence of locally established formal stratigraphic 1). The dominant depositional element is the large units. Rockdale Delta System, comprising large lobate wedges of muds, sands, and carbonaceous deposits, chiefly The principal unit here employed is the depositional lignites (Fig. 1). Maximum lower Wilcox sedimen- system., recognized by specific criteria and designated tation, with sequences up to 5,000 feet thick, occurred by a genetic term, e.g., delta system; a delta system during deposition of this system. It occurs principally includes one or more deltas. Components of a depo- in the subsurface and grades up depositional slope and sitional system are referred to simply as facies, c. g., up structural dip to thinner terrigenous clastic facies the delta front facies or distributary channel facies of of the Mt. Pleasant Fluvial System, present in outcrop a delta system. Smaller scaled components are desig- northward from the Colorado River. A large part of nated by such terms as units,beds, or deposits. Basically, the original fluvial system apparently was deposited the concept of depositional systems in ancient sedi- updip of the present outcrop with only a part of the ments, fabricated principally from integration of facies, system preserved in the East Texas Basin. FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 107

Copies of this map at original scale measuring, 36 x 14 are available on special order from Cambe Log Library, Inc., 718 Milam St., Houston, Texas

Southwestward transport of sands from the delta Mt. Pleasant Fluvial System system, principally by means of longshore currents dur- Mt. Pleasant System includes most of destruction, embayments The Fluvial ing periods of delta and local the lower part of the Wilcox Group in outcrop north marginal to the delta system resulted in distinct of the Colorado River and over the northern half of depositional systems laterally associated with the delta the Sabine Uplift (Figs. 1, 2). It extends down system. These systems include: on the east the Pendle- structural dip to a line running approximately from ton Lagoon-Bay System developed in embayments central Bastrop, through centralLee, northern Burleson, marginal to the delta system, on the southwest the southern Robertson, central Leon, northern Houston, San Marcos Strandplain-Bay System developed proxi- southern Cherokee, and central Nacogdoches counties. mal to the delta system, and the Gotulla Barrier South and downdip from this line the system grades Bar and complementary Indio Bay-Lagoon Systems to the Rockdale Delta System (Fig. 3); laterally it developed moredistally from the main areaof deltation. grades to embayed marine facies developed marginally Contemporaneous sedimentation in systems marginal to the fluvial and delta systems. Three more or less and lateral to the delta system was relatively slow with distinct facies, based on (1) sand body geometry, (2) sequences generally less than one-fourth the thickness sand isolith pattern, (3) internal sedimentary structures, of the delta system. Gulfward from the Gotulla Barrier (4) vertical sequence in structures and textures, (5) Bar System and in part lateral to the Rockdale Delta grain size, and (6) relative amounts of channel and System is the South Texas Shelf System, marked chiefly overbank deposits, are recognized (Figs. 1, 2, 3). These by muds and by relativelyminor sands. facies principally reflect different fluvial environments. 108 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII,1967

Texas Group, Wilcox System, Fluvial Pleasant Mt. distribution, subsurface and Outcrop 2. Figure FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 109

relationship showing ,

System Delta Rockdaie and System Fluvial Diagrammaticstratigraphicdip principalrecomponent3.PleasantMt.ofsectionu characterandofFig facies 110 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII,1967

Tributary channel facies individual sand bodies in this facies are slightly more extensive, and sands are slightly larger in mean grain In northernmost Wilcox outcrops, from central Van size than those of the tributary channel facies. Primary Zandt to out- County northeastward Texarkana and in sedimentary structures of sands, relationship of channels crop over the northern part of the Sabine Uplift, sand to clay and lignite deposits, out- in definite, Mt. mineral distribution deposits occur separated areas (c.g., lined above, are similar for the two facies. Pleasant, Reilly Springs, Simms deposits) ; clays and muds crop out in intervening areas. Isoliths of sand Highly meanderingchannel facies elongate with an dendritic, units outline bodies overall A distinct facies is developed in the Mt. Pleasant tributary pattern (Fig. 2). The fluvial facies defined System immediately depositional slope chiefly by this pattern extends from outcrop south- Fluvial down and down structural dip of the Mt. Enterprise-Elkhart ward in the subsurface to a line from southwestern fault zone and salt domes to southwest Smith to west-central Rusk counties. The tributary associated the (Figs. 1, 2). predominate; channel facies varies from about 300 to 800 feet thick, Within this facies sands overbank muds make up less than 10 percent of the commonly increasing in thickness downdip and toward total Sand are sands the axis of East Texas Basin; it of about facies. isoliths uniform with in the consists (=Simsboro of outcrop) forming 40 percent sand or channel deposits and about 60 per- the facies Sand persistent For such units as these, Pettijohn et al. cent mud or overbank deposits. The overbank (or units. suggested the term nonchannel) muds are chiefly laminated, light to (1965) multilateral. These bodies of multiple at moderate gray, containing minor amounts of carbona- are made up channels which trend various to of the Aggregate ceous matter. Individual sand units range from 5 angles strike sand isoliths. to about 80 feet thick and occur as channel deposits thickness of sand in the facies ranges from 600 1,100 tops and convex bases. Bases of to feet. Individual sand units, comprising with flat downward de- channels are characterized by channel lag of granules numerous superposed and multilateral channel are up to 30 miles wide. Areas along to fine gravel with a few angular clay pebbles. Sands posits, strike are cross-bedded; inlower parts of the channel deposits, between the more extensive sand units are made up sands display trough or festoon cross-beds and in upper of greater amounts of overbank mud deposits. Sands parts show wedge sets Upper of of the facies have a mean average about 0.22 mm and of cross-beds. parts in channel sands are horizontally bedded. Sands in this are much coarser than sands upslope fluvial facies. facies aremostly fine grained to locallymedium grained; Sand channel deposits that make up the multilateral mean sand grain size averages about 0.14 mm sand bodies cut down and laterally into underlying with and adjacent channel sands. are con- range in mean 0.11 to mm; Channels broadly of about 0.25 channel downward by lo- sands typically fine upward. Channels cut into vex and are characterized a. basal, are cally conglomeratic, displaying underlying and laterally adjacent overbank muds. zone festoon or trough Sinuous, elongate, narrow lignite deposits commonly cross-bedding representing channel lag deposition; this zone a thin occur laterally adjacent to channel sand deposits; is overlain by unit with tabular cross-beds, these apparently peatbogs in in turn overlain by a zone of wedge set cross-beds accumulated as depressions Allen, along higher stream terraces (a common occurrence of (epsilon bedding of 1963). The tabular and sands modern peat bogs in inner Gulf Coastal Plain) or in wedge cross-bedded probably represent lateral deposition the abandoned stream courses. Clays within and immedi- as toe andlower parts of point bars. The ately adjacent to sand channel deposits are chiefly horizontally bedded and rippled zones, characteristic kaolinitic; feldspars are mostly Clays of the upper parts of many point bar deposits, are rarely weathered. of preserved in and overbank— deposits are, by contrast, largely nonkaolin- this facies as the channel lag lower itic mostly illitic, chloride, and ; point bar deposits are succeeded disconformably by montmorillonitic channel of similar Within the feldspars of overbank muds are generally unweathered. deposits a sequence. preserved channel sequence only a slight degree of up- Slightly meanderingchannel facies ward fining is shown. The relatively small amount of clay in Down depositional this facies is highly kaolinitic. Lignite deposits, slope in the fluvial system a tran- typically associated with channel deposits in upslope sitional facies is characterized by uniform to slightly facies, are not preserved highly sand 1, fluvial in the meander- dendroid isolith (Figs. 2) patterns which are ing channel facies. Lateral continuity and permeability less than in tributary dendritic the channel facies. This of the sands make this facies a significant aquifer. change in pattern probably represents a slight meandering of the original stream system. Channel sand The highly meandering channel facies constitutes deposits are more numerous within this facies than in the most downslope facies of the Mt. Pleasant Fluvial the tributary channel facies; they make up about 60 System, grading down depositional slope or southeast- percent of the total facies with 40 percent represented ward to the distributary channel-marsh-swamp facies by overbank deposits. The facies ranges in thickness of the Rockdale Delta System. It thus heads the delta from about 500 to 800 feet; it occurs in outcrop from system, extending in outcrop from central Bastrop central Freestone, through western Henderson, into northward to southern Freestone counties and in sub- southern Van Zandt counties and extends downdip in surface from central Bastrop northeastward to western the subsurface to a line running from central Freestone, Nacogdoches counties (Fig. 1). Incentral Bastrop and through central Anderson, central Cherokee, and into western Nacogdoches counties this facies changes ab- northwestern Nacogdoches counties. In addition to ruptly to marine muds of the San Marcos Strandplain- containing a higher percentage of channel deposits, Bay and Pendleton Lagoon-Bay Systems, respectively. FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 111

Figure 4. Diagrammatic stratigraphic strike section of Mt. Pleasant Fluvial System, Wilcox Group, Texas

Within narrow belts marking these transitions, fluvial sediments redistributed and deposited in associated bar- sands of the facies, which were modified by marine rier bar, strandplain, bay-lagoon, and shelf systems. processes, are very well sorted and fine grained and The Rockdale Delta System occurs down structural locally redeposited as local beach, barrier bar, and dip of the fluvial system, with southern limit marked strandplain deposits. by a line extending from north-central Karnes northward to central Bastrop counties and an eastern limit Multistory sand bodies by a line from central Nacogdoches through northern A characteristic feature of sand bodies in the fluvial San Augustine and Sabine counties to the Sabine River; facies containing relatively high proportions of over- the downdip limit is beyond existing well control (Fig. of 3,500 to 5,000 feet are bank muds (tributary channel and slightly meandering 1). Maximum thicknesses central axis of the system situated chiefly vertical persistence or stacking (Fig. along the channel facies) is along drainage of the modern Brazos River and roughly 4). Successive sandbodies areroughly superposed with coincident with structural axis of the Houston Em- complementary interchannel areas void of significant bayment. Laterally and along existing structural strike channel deposits. This feature, termed multistorying the delta system generally, but not uniformly, thins to by Feofilova (1954) has been noted in several facies about 1,500 feet in the southwestern part of the system containing channel sand deposits (c. g., Mueller and and to about 1,000 feet in the eastern part. Wanless, 1957; Friedman, 1960; Potter, 1963; Brown Criteria for recognition of this part of the lower et al., 1967). Some variation in degree of stacking Wilcox Group as a delta system include: ( 1) com- occurs, but vertical persistence is sufficient to show a position andboundingrelationships of component facies, fluvial drainage pattern exhibited by composite se- (2) external geometry of skeletal sand facies, (3) quences similar to patterns of individual sand bodies. position in relation to laterally associated depositional Controlling factors involved inmultistorying are not well systems, and (4) contrasts in rates of deposition with known; occurrence in relatively high mud facies sug- thick, rapidly deposited terrigenous clastic sequences gests a possible control by differential compaction, (constructional units) alternating with thin, slowly de- though regional structure is, in at least some cases, posited, relatively persistent marine and marsh deposits the more fundamental control (L. F. Brown, personal (destructional units). Details of these criteria are dis- communication, 1967). cussed subsequently. Rockdale Delta System Principal facies The Rockdale Delta System, following designation of Four regional facies constitute the Rockdale Delta Echols andMalkin (1948),is the dominant depositional System. Down depositional slope and down structural system of the lower Wilcox Group in Texas, comprising dip of the highly meandering facies of the Mt. Pleasant areally about 60 percent and volumetrically about 80 Fluvial System, the lower Wilcox consists of an ex- percent of the known lower Wilcox (Fig. 1). It was the tensive facies of sand units alternating with mud and ultimate depositional site of most sediments transported silt units with numerousbeds of lignite (Fig. 3).Farther through the fluvial system and the principal source of downdip this facies changes rather abruptly to thick, 112 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES VolumeXVII, 1967 massive sands, which in turn grade down structural depositional origin. More detailed distinctions of delta dip and along structural strike to thick, uniform mud lignites or coals could probably be made in terms of sequences. The alternating sand and mud-lignite se- origin as fresh-water, brackish, or saline marsh ac- quences are interpreted as distributary channel and in- cumulations. terdistributary deposits accumulated on delta plains; (2) Delta front fades. Downdip the distributary the massive sand facies represents delta front deposits channel-marsh-swamp facies changes abruptly to a thick laid downchiefly as distributary mouthbars; the down- sand facies in which lignites are absent and mud con- dip thick mud sequence represents deposition as pro- tent is low (Fig. 3). These sands are mostly fine delta facies. Mud sequences along strike of the delta grained, generally much better sorted than fluvial and front sand facies are interpreted as interdelta facies. distributary channel sands. Commonly, they show thin, multidirectional, ripple cross laminae and other ( 1 ) Distributary channel-marsh-swamp facies. The current Plant and lignite fragments facies of features. detrital consisting alternating sand and mud-silt-lignite occur locally. Individual sand are generally units is units 200 an areally extensive facies of the Rockdale or more feet thick; in plan, external geometry is lobate Delta System (Fig. 3). It contrasts markedly with the or lunate. They are interpreted as distributary mouth immediate upslope fluvial facies chiefly in terms of bars representing delta front deposition. The high body sand geometry and relative proportion of channel percentageof sand in this facies resulted from deposition and nonchannel deposits. The fluvial facies show cross- of bedload of delta plain distributary channels debouch- bedding structures and textural features indicative of ing relatively quiet reservoirs; the in into marine-water channel lag and lateral point bar deposition highly muds were carried in suspension farther seaward, ac- meandering streams. The result is relatively persistent, uniform, cumulating as prodelta deposits. Relative cleanness, multilateral sand units with very few non- good sorting, and multidirectional current features channel deposits preserved. These sands change down indicate winnowing reworking sands in this isolated, and of structural dip and down depositional slope to environment by channel open-water currents sand bodies in both and elongate, narrow interspersed nonchannel under shoal conditions existing in delta en- muds and with similar the tribu- the front associated lignites, to vironment. Redistribution of delta front sands and tary channel and slightly meandering facies of the Mt. lateral coalescing of constructive delta front deposits Pleasant Fluvial System. These facies differ signifi- result in a more or less continuous, arcuate delta front cantly, however, in that the channels in the delta sand facies for most of the Rockdale deltas. system are thicker, symmetrical in cross section, generally nonerosive at base, and show a distributary (3) Prodelta facies. The farthest downdip sections rather than a tributary pattern in plan. Symmetry in of lower Wilcox penetrated by wells generally consist cross section, nonerosive bases, and thickness suggest of relatively uniform, thick sequences of dark muds accumulation of these sands as bedload deposits in (Fig. 3). These contain moderately large amounts of relatively straight, stabilized streams in which accumula- very finelycomminuted and disseminatedorganic matter, tion by downward accretion was afforded principally show laminations marked commonly by slight textural by compactional subsidence of the channel sands in a and color variations, and have a marine fauna ex- frameworkof interchannel muds. Such features charac- ceedingly low in numbers and species diversity. This terize certain modern delta plain distributary channels mud facies interfingers with and grades updip to the and contrast with the lateral deposition in point bars delta front sand facies and represents deposition from of meandering, highly sinuous streams. Muds and silts, suspension seaward from the delta front and river making up about 60 percent of this facies, apparently mouth as prodelta facies. Thickness and downdip accumulatedas subaerial levee, crevasse splay, lacustrine, extent of the facies are difficult to determine, primarily and flood basin interdistributary deposits. because few wells have penetrated significant thickness of the lower Wilcox in these downdip areas. However, Two kinds of lignites occur within the distributary facies, the few deep wells drilled indicate that the facies is channel-marsh both distinct from the elongate very thick and probably the thickest facies of the lignite deposits of the fluvial system. One type, which delta system; approximately 4,000 to 5,000 feet have is tabular in shape but only local in extent, probably been recorded in some deep wells. represents peat accumulation in local interdistributary depressions similar to origin of interdistributary peat in (4) Interdelta, subembayment facies. Sequences en- the Mississippi Delta System as discussed by Fisk countered in areas between principal delta lobes are (1960). A second type is also tabular in shape but much similar in some respects to the prodelta facies (Fig. 1). more areally extensive; this type probably developed They consist principally of mud, much of which is as a regional, landward facies of marine, delta de- laminated as in the prodelta facies. The interdelta struction. Further distinctions betweenfluvial anddelta facies is thinner than the prodeltafacies and in addition plain lignites are made on basis of grade and com- contains several thin, burrowed mud units or thin position. Fluvial lignites are more variable in com- units of fossiliferous silts and very fine-grained sands, position than delta lignites and contain high percentages commonly containing small amounts of glauconite. of woody materials consistent with a swamp origin. These thin units accumulated more slowly than the Delta lignites are slightly higher in grade and contain laminated muds and in areas not directly affected by relatively low percentages of woody material, suggesting deltation. The interdelta facies grades directly up organic production by marshes. Regional variations structural dip to marsh facies of the delta plain and in Wilcox lignites (Fisher, 1963) reflect differences in does not grade through the delta front sand facies. FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 113

Delta destructive units scale, though only approximate, subdivision of delta systems. Delineation of a thick terrigenous clastic sequence such as the Rockdale Delta System into component Individual deltas in Rockdale Delta System and task. individual deltas is a time-consuming Basically, To we have mapped 16 principal it involves and thin marine date and delineated recognition correlation of deltas within the lower Wilcox Rockdale Delta System units formed in process of marine of the destruction (Fig. 5). These exhibit the component facies and the delta mass. During periods of high constructive facies pattern as described above. Striking, and sig- influx, the above described tract of delta sediment nificant in another context later, is the delta front, considered facies (distributary channel-marsh-swamp, vertical stacking and persistence is established, and with progradation and of individual deltas the prodelta) of interdelta subembayments within the delta system. basin and compactional subsidence, a thick terrigenous Further, the Rockdale deltas show a marked coinci- With of sequence accumulates. cessation sediment dence with modern coastal plain streams (Fig. 5). influx through either a diminished source area or more commonly with upstream avulsion resulting in abandon- Through mapping of several delta destructional units ment of the previous drainage system and establishment we have been able to delineate three main phases of in the Rockdale System, in to out- of a new network, compaction of the water-saturated deltation addition lining principal individual deltas. The initial phase sediments of the constructed delta mass exceeds the of in relatively lobe, or deltation resulted three small deltas rate of deposition. At this point the delta situated roughly along drainage of the modernColorado, at least marginal portions of it, is subjected to marine Brazos, and Trinity Rivers (Fig. 5). These range in encroachment, involving reworking or destruction of area from 200 to 350 square miles with thicknesses on the constructive delta surface. Rate of sedimentation the order of 300 to 500 feet. A second or intermediate during destruction is very low, primarily involving period of delta building resulted in the largest, thickest, marine reworking of underlying delta sediments; as and most prograded deltas of the system. The larger a consequence, units are relatively very thin. In the deltas of this phase, up to 2,000 square miles in area, Rockdale Delta System, marine destructive units con- were constructed along the axes of the modern Guada- sist of very well-sorted, fine-grained, slightly glauconitic, lupe, Colorado, and Brazos Rivers; the smaller deltas, commonly bioturbated sand, shell detritus, and muds 500 to 700 square miles in area, developed along the either burrowed or containing a marine fauna. These axes of the modern Sabine and Neches Rivers. The units were deposited chiefly only on the lower parts Brazos and Colorado deltas (Fig. 5) are the thickest, of the abandoned delta plain. Landward destruction with maximum thickness of 1,500 feet, exclusive of the of the abandoned delta plain involved extensive marsh prodelta facies. A terminal phase of deltation in- growth, coincident with compactional subsidence, re- volved delta construction in all areas of intermediate sulting in laterally extensive lignite deposits. These phase deltation as well as construction of a large delta lignites contrast with the thinner, local lignites which situated primarily in Angelina, Houston, and Trinity formedas interdistributary peats. The marine destructive counties (Angelina Delta). These terminal deltas are units apparently represented deposition in environments comparable in size and thickness to the underlying, comparable to delta destruction environments in the intermediate phase deltas but do not extend as far modernMississippi System, with marine sands originat- down structural dip. ing under conditions similar to those of the Chandeleur Islands and the burrowed or fossiliferous muds as open Although individual deltas within the Rockdale sig- bay deposits similar to those of Breton Sound. System vary in size and sediment volume, a more nificant variation occurs in shape of the delta lobes Following recognition of delta destruction units and (Fig. 6). One type, shown by the intermediate phase their facies relationships in delta systems, those of delta constructed in the area of the Trinity River, the maximum areal extent are studied in detail; areal four intermediate and terminal deltas constructed in extent of these thin units can be determined only by the area of the Neches and Sabine Rivers, and to some careful correlation of closely spaced wells over a wide extent the three initial phase deltas, has an elongate, area. By this means the delta system is subdivided narrow shape resembling outline of the modern bird- and smaller delta units mapped. Determination of foot delta and certain of the initial deltas of older sand content of such subdivisions permits isolith delta complexes as delineated by Frazier (1967) in the mapping of skeletal elements of the delta (distributary Mississippi Delta System. The other type, shown by channel and delta front facies) and along with mapping the other presently mapped deltas of the Rockdale of component facies allows delineation of specific delta System, is more or less lobate or rounded in shape, lobes. Most delta destructiveunits are restricted areally similar to premodern, presently abandoned deltas of to a specific, underlying, constructive delta lobe. How- the Mississippi System. If a delta line is drawn in ever, as delta construction is a relatively rapid process the same manner in which bayline maps are commonly and delta destruction a slow process, destruction of made (Lowman, 1949), the elongate, narrow birdfoot one delta lobe may persist during the construction, deltas of the Trinity, Neches, and Sabine Rivers ex- subsidence, and subsequent initiation of destruction of tend considerably beyond the line, suggesting pro- an adjacent lobe so that destructional units of two or gradation into deeper waters, as is the case with the more lobes may be in part contemporaneous. Recog- modern, deep-water birdfoot delta of the Mississippi nition of widespread destructive units permits larger System (Fig. 6). These Rockdale deltas are associated 114 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII, 1967

margin distal at drawn outline lobal Texas; Group, Wilcox System, Delta Rockdale deltas, principal of facies

Distribution5.front delta Figure of FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 115

Figure 6. Comparison of size, distribution, and arrangement of principal delta lobes of the Mississippi Delta System(Recent, southeastern Louisiana) and Rockdale Delta System, Wilcox Group (Eocene, Texas) 116 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII, 1967

Figure 7. Comparison of component facies of Mississippi Delta System and Rockdale Delta System, Wilcox Group, Texas

with a sequence relatively high in mud, strenghtening (Yegua and Fayette) and Pleistocene delta systems the analogy with the sand-poor, deep-water birdfoot along these axes. Certainly, compactional subsidence delta of the Mississippi. The initial phase deltas is a possible factor, though the more important control similarly represent progradation over underlying thick probably is control by muds. tectonic. Structural is indicated By contrast, the lobate or rounded deltas of the occurrence of growth zones coincident with the System consist of pro- fault the Rockdale relatively large thicker fades of Rockdale Delta portions of sand, suggesting construction as shoal-water the System (Fig. 3). deltas analogous to the sand-rich lobate, shoal-water Comparison with Mississippi Delta System deltas of the modern Rhone (Kruit, 1955) and premodern Mississippi (Fisk, 1955, 1961). Certain similarities between the Eocene Rockdale and Vertical stacking or approximate superposition of the Recent Mississippi Delta Systems, both built into individual deltas within the system, persistence of in- the Gulf Coast Basin, have been noted. These interdelta subembayments, and coincidence of Rockdale clude similarities in shape, occurrence, and position deltas andmodern drainage have beenmentioned. The of shoal-water, lobate deltas and deep-water, birdfoot controlling factor or factors, like those of the multistoried deltas, and in vertical stacking of individual deltas. channel sands in upslope fluvial facies of the Mt. The two systems are comparable in area (Fig. 6). A Pleasant System, are not fully understood. Such features most significant similarity of the two systems is the are apparently common in fluvial systems containing distribution of component facies (Fig. 7), in which the relatively high percentage of muds and maybe common high sand delta front facies is bounded up-delta or in delta systems as well,though few ancient delta systems landward by a relatively high mud and organic facies have beenmapped and studied.Maps of delta systems in (distributary channel and marsh-swamp facies) and the Pennsylvanian of theEastern InteriorBasin (Wanless off-delta or seaward by a high mud facies (prodelta et al., 1963) and the modern and premodern deltas facies). Significance of such bounding facies in delta of the Mississippi System (Kolb and van Lopik, 1966; sequences was emphasized by Coleman and Gagliano Frazier, 1957) (Fig. 6) indicate some degree of ver- (1964); we judge delineation of such bounding facies tical stacking, though perhaps not as pronounced as along with determination of external geometry of the in the Rockdale Delta System. Significant in this re- skeletal elements (distributary channel and delta front gard is not only the approximate coincidence of Rock- sands) and delineation of delta destructive units as dale deltas and modern coastal plain drainage, but the best regional criteria for recognition of ancient also the coincidence of other Eocene delta systems deltas or delta systems. FISHER AND McGOWAN: WILGOX DEPOSITION IN TEXAS 117

Pendleton Lagoon-Bay System Caldwell, Guadalupe, and northern Wilson counties and extends a short distance downdip into the sub- Marginal to the Rockdale Delta System and along surface; the Hilbig facies of Glaypool (1933). part Uplift (chiefly Shelby it includes the southern of the Sabine The faunal and northern parts of San Augustine and Sabine and lithologic composition of the facies counties), the Wilcox Group is predominantly a mud is very similar to that of the Pendleton System. A facies. This facies includes parts or all of several similar origin is suggested, with deposition in embay- formally defined stratigraphic units (Pendleton,Martha- ments and on mudflats along the opposite margins ville, Hall Summit, Lime Hill, Converse, Cow Bayou, of downslope fluvial facies of the Mt. Pleasant System Dolet Hills,and Naborton Formations) ; it occurs part- and delta facies of the Rockdale System. In outcrop, ly in outcrop and partly in subsurface. The facies is muds of this facies are continuous below the downslope designated informally as the Pendleton Lagoon-Bay facies of the Mt. Pleasant System (highly meandering System (Fig. 1); it is approximately 1,000 feet thick channel facies or Simsboro Sand) northward to about in Texas, comparable in thickness to fluvial facies along midway between the Brazos and Trinity Rivers, where strike but thicker than updip fluvial facies and thinner the muds grade to fluvial and delta facies of earliest than downdip delta facies. The Pendleton System Rockdale deltation (initialphase,Fig. 5). The northern consists chiefly of laminated to locallybioturbated muds extent of this mud facies includes at least parts of with local lenses and beds of very fine to fine-grained, units formally designated the Hooper and Sequin For- sparingly glauconitic, massive to cross-bedded sands mations. Southwestward displacement of the facies and fine-grained, broken, massive to cross-bedded, is contemporaneouswith subsequent large scale deltation lignitic sands. Certain of the muds are gypsiferous; in the lower Wilcox Rockdale System (intermediate and clay-ironstone concretions are common. Lignites are terminal phases, Fig. 5). locally common, though they are relatively impure, Downdip and marginal to the shoal-water deltas of thin, and discontinuous. The system contains a few southwestern less Faunal the part of the Rockdale System (Guada- more or discrete units with marine fossils. lupe deltas) muds grade to a facies consisting of inter- is molluscan, with a foram- composition chiefly few bedded sands and muds. Sands are generally fine inifers and ostracodes; fossil occurrence is mostly in grained, sorted, sparingly glauconitic, the is well at least and sparingly glauconitic units. Species diversity commonly contain a few fragments. They moderate low but low to suggest highly detrital shell to not so as occur as elogate bodies parallel to depositional strike abnormal salinities, at least for fossiliferous units. Mol- to chiefly tabular or sands. comprise largely a and mud charac- sheet Individual sand and lusks bank infauna mud are approximately are terized by several forms. units 50 feet thick. Muds thick-shelled The fauna of similar to those of the updip mud facies but the Pendleton has been described Barry and commonly System by pyritic and slightly gypsiferous. Carbonaceous ma- (1942). Leßlanc terials are locally present as thinlignitic stringers. This The Pendleton System is underlain by marine clays interbeddedsand and mud facies is approximately 1,200 of the Midway Group (Porters Creek Formation) and feet thick, occurring chiefly in southwestern Gonzales, overlain by sands of the Carrizo Formation. A large Wilson, southwestern Karnes, and eastern Atascosa part of the facies is contemporaneous with delta facies counties. Composition of the facies and its position (Sabine deltas of the Rockdale System) to the west marginal to shoal-water deltas of the Rockdale System and south, though the upper part of the system is suggest an origin analogous to the Recent chenier sys- contemporaneous with other Wilcox depositional system of southwestern Louisiana. Muds accumulated in tems not considered here. To the west and north the low energy environments ranging from mudflat to open Pendleton System grades to the various fluvial facies bay, contemporaneous with periods of active deltation. of the Mt.PleasantSystem;it extends east into Louisiana During marine destruction of the adjacent sand-rich, but its extent in that direction has not been determined. shoal-water deltas, sands were redistributed laterally by prevailingly southwestward longshore currents with The lithologic andbiologic composition Pendle- of the deposition of the elongate to sheet sands in a strand- ton System and its situation marginal to the Rock- plain in marginal to the delta system. Transgression and dale Delta System suggest deposition low-energy regression of relatively thin marine units in this facies lagoons, bays, and mudflats marginal to sys- the delta were thus related to periods of active deltation and tem, comparable to marginal environments of the delta destruction. Such as Delta was subsurface sands the Poth Mississippi System. Deposition chiefly marine and Bartosch are typical of this facies. In reference conditions of varying degrees of restricted to under to their origin and area of principal occurrence, the open circulation; the system opened seaward primar- updip mud and downdip developed ily southeast with and sand-mud facies to the greater brackish fresh- southwestern margin of Rockdale the and along the the Delta water influence to west north where the System are designated the San Marcos Strandplain-Bay system grades to fluvial facies. System. San Marcos Strandplain-Bay System Cotulla Barrier Bar and Indio Bay-Lagoon Systems In outcrop south of the Colorado River, the highly Southwestward along depositional strike, sheet and meandering channel sand facies of the Mt. Pleasant elongate sand bodies of the San Marcos System grade System changes abruptly to a facies primarily of muds to facies of elongate sand bodies arranged parallel to which locally contain marine fossils (Fig. 1). This depositional strike. Sands are generally fine grained, facies persists in outcrop through southern Bastrop, very well sorted and locally glauconitic; internal 118 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII, 1967

structures are not known,due to lack of cores. Thickness of a distinctive lagoon-bay system is thus a complement of individual sand units ranges upward to 100 feet, with of the barrier bar system. aggregate sand thickness of 400 to 1,000 feet for the interpretation facies. Sand isoliths show Other features substantiate of these an extensive elongate sand South Texas, lower Wilcox facies as barrier bar and depositional belt unit oriented parallel to strike. This bay-lagoon systems. First, sand isolith patterns depict- of sand trends northeast-southwest and occurs in western ing LaSalle, the lagoonal side of the barrier bar system are Atascosa, southern Frio, northwestern south- generally more irregular than those of the seaward eastern Dimmit, and northwestern Webb counties (Fig. side; further, to mud more from this sand belt, in southern the transition from sand is 8). Updip occurring abrupt on the lagoonal than on the seaward side. Such Bexar,northern Atascosa, northernFrio, Zavala, western features are typical of modern barrier bar systems. Dimmit, and northwestern Webb counties and ex- Secondly, in is mud southeastern Frio and northeastern LaSalle tending to the outcrop, a predominantly facies counties, sand isoliths of the Cotulla Barrier Bar Sys- including the Indio most of Formation of formal tem are sharply indented on the lagoonal side; a less stratigraphic definition (Fig. 8). The facies is charac- pronounced indentation occurs on the opposite or sea- terized by thin-bedded, laminated to locally burrowed ward configuration inden- muds, calcareous side. Position and of these large cemented fine-grained sand tations suggest former existence of tidal channel, a concretions, and a few beds lenses of sand and or feature common to modern barrier bars. Under such massive clay. Muds are mostly gray to buff, gypsifer- through in conditions flood and ebb tidal surges move ous to calcareous, and locally pyritic. Lignites this a channel or inlet bar, accreting sand in the impure discontinuous, contrasting sig- in the facies are and form of tidal deltas both within the lagoon and onto nificantly with lignites of the fluvial and delta systems. the foreshore of the bar. Presence of anomalous thick- textures, fine Sands are of various though chiefly ness of sand in the Indio Lagoonal-Bay System updip grained; they flaggy but notably are massive or not of the terminus of the inferred tidal channel (in north- cross-bedded; marks are common in ripple the flaggy eastern Frio and southeastern Medina counties and cement but it beds. Chief is siliceous, is locally calca- extending outcrop) probably represents accumu- ferruginous associated into reous or with small ironstone lation as a lagoonal tidal delta (Fig. 8). Lobate concretions. Certain of the sands are very slightly extension of sand on the foreshore side glauconitic. of the bar and Marine fossils are common in this facies, immediately south of the inferred tidal channel (east- and like other lower Wilcox facies containing marine central La County) probably represents delta fossils, molluscan; Salle a composition is chiefly species diversity constructed by ebb tide transport of sand through the is low to moderate. Thick-shelled mollusks are most Assuming origin above common. tidal channel. of the described A few thin units contain significant num- features as a tidal channel and complementary tidal bers of foraminifers, mostly textulariids and glo- deltas, the position of the deltas in respect to the biginerids. Shark teeth and fish scales occur locally. as well as shape configuration Some units channel the and of the clay locally contain fossil plants. tidal channel substantiate the inference of a prevailing The lower Wilcox sand and mud facies of South southwestward transport of sediment by means of long- Texas described above are judged to have originated shore currents. Thirdly, lobate lagoonward extensions as extensive barrier bar and complementary lagoon-bay of sands in northeastern and north-central Dimmit systems and are here designated, respectively, the County (Fig. 8), without associated isolith indentations Cotulla Barrier Bar System and the Indio Bay-Lagoon indicative of tidal channels, possibly represent deposi- System. Source of sand deposited as the barrier bar tion as washover fans, with sand carried into the lagoon system was from shoal-water deltas of the Rockdale by storm surges. This feature is also common to mod- System situated to the northeast; sands were transported ern barrier bars (Andrews, 1966). laterally by prevailingly southwestward longshore cur- South Texas Shelf System rents. In the San Marcos Strandplain System, situated intermediate to the delta and barrier bar systems, the Downdip and seaward of the Cotulla Barrier Bar adjacent delta system served not only as a source of System a predominantly mud system characterizes the sands but also, through periods of active and inactive lower Wilcox of South Texas. This system extends deltation, as a control for the distribution of the sheet over an area of about 3,000 square miles in south- sands. Accumulation of sand in the barrier bar sys- western Karnes, northern Goliad, northern Bee, north tem was sufficiently distant from the delta system for Live Oak, McMullen, northwestern Duval, Webb, it to have had any direct control on sand distribution. Zapata, and western Starr counties (Fig. 1). It is The principal distinction in sand accumulation in these relatively uniform in thickness throughout much of its two systems is chiefly sheet or tabular sand units in extent, ranging from about 1,000 to 1,500 feet, but the San Marcos System and elongate sands in the it thickens northeastward toward the Rockdale Delta Cotulla System. Similar patterns in sand dispersal are System and southward along the Rio Grande. Principal shown in the modern and premodern marginal marine lithologic component is mud; aggregate thickness of sand systems southwest of the Mississippi Delta System. sand in the system is generally less than 200 feet (Fig. In addition, accumulation of sands in relatively nar- 1). Individual sand units vary in number, commonly row elongate belts in the barrier bar system, as opposed 8 to 10, and in thickness, generally 10 to a maximum to sheet sand accumulation in the strandplain system, of about 100 feet; average thickness of sand units is facilitated— development of extensive, landward mud about 30 feet. Muds are variable in composition, facies the Indio Bay-Lagoon System. Development though generally light to moderate gray to olive or FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 119

Figure 8. Sand/mud ratio map and sand isolith map of Cotulla Barrier Bar System and Indio Bay-Lagoon System, Wilcox Group, Texas 120 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII,1967 buff, and laminated to extensively burrowed; marine system. Within the various depositional systems of fossils and glauconite pellets are common. Muds are the lower Wilcox, several modes of sand transport and commonly calcareous, though a few are pyritic or deposition occur, each resulting in sand units with dis- carbonaceous. The sands are very fine to fine grained, tinct geometries, structures, and textures (Fig. 9). well sorted to muddy, slightly glauconitic, and com- Principally, the pattern involves dispersal through monly contain detrital shell fragments. Faunal diversi- several fluvial facies in which only a relatively small ty is moderate; composition is chiefly molluscan, but part of the sand was permanently stored or preserved, thick-shelled forms are not predominant as in other through separate delta facies in which bulk of deposi- lower Wilcox marine facies; foraminifers are common tion represented permanent storage. Marine redis- and a few echinoderms are present. tribution of a relatively small part of the sand deposited in the delta facies, of shoal-water deltas, system, its front chiefly Relative uniformity of this predominantly in of sand in marginal strand- fauna, and position resulted accumulation mud composition, its marine its plain, barrier bar, and shelf systems (Fig. 9). Sand to other depositional systems suggest deposition relative bodies of the fluvial system and distributary channel extensive, continental shelf; the on an shallow-water sand system regionally is here Texas bodies of the delta trend normal system designated the South Shelf Sys- to strike; of the of the Falls Formation, as depositional sand bodies delta front tem. Part City proposed of delta system and in the shelf sys- Hargis (1962), is included in this facies the those and described by tem trend roughly parallel to depositional strike; those system. of the barrier bar and strandplain systems closely Origin of the relatively thin sand units in the South parallel depositional strike. Texas Shelf System External is not clear. geometry The most upslope, preserved element of the lower of the sand units is not well known, but existing con- Wilcox sand dispersal pattern consists of elongate trol suggests both laterally persistent sheet sands, more channel sands with dendritic, tributary distribution; numerous updip the Bar Sys- toward Cotulla Barrier ratio or to sands tem, and elongate, northeast-southwest-trending of nonchannel overbank muds channel sand is high. Down depositional slope, ratio of nonchannel bodies. Source of sands was probably the Rockdale muds to sands decreases and individual sand bodies Delta System to the northeast like that of updip sand increase in width, a result of deposition by increasingly systems, of transport of the but mode and dispersal streams. A zone of highly meandering sands is in meandering unknown. Relatively sharp indentations stream as from the sand isoliths of the southwestern lobes of the deposition judged dominance of Guadalupe bar structures, a the Rockdale point resulted in fluvial facies made deltas of Delta System (Fig. 1) suggest chiefly of extensive, with possible submarine channeling of the deltas during up multilateral sand bodies very little associated nonchannel muds. Immediate of delta destruction with of periods possible dispersal bounding of this facies by a fault system (pres- sands across the shelf by southward-flowing turbidity upslope ent Mt. Enterprise-Elkhart system) or threshold currents. in the Wilcox re- day (Submarine channeling as Basin known to deposition with an upper (Tyler threshold) influence ported by Hoyt (1959) is associated in several and Mesozoic units, suggests Wilcox system.) Another explanation of these sands, other Genozoic a possible or at least a local gradient change and one we judge more probable, is accumulation as fall line that resulted in sand deposition. Once such a zone barrier bars duringperiods of initial marine destruction was established, weak bank conditions of streams tra- of more gulfward-projecting delta lobes. The origin versing it would contribute to a high degree of mean- thus is similar to origin of the Cotulla Barrier Bar A high with minor dering or lateral shifting. similar sand fluvial System but only accumulation downdip in patterns of Pennsylvanian and principal accumulation updip the Cotulla Sys- facies sand dispersal Upper in units in the Eastern Interior Basin is reported by Potter tem. of system possibly are Sheet sands the shelf (1963). related to marine transgression associated with periods of delta destruction; shelf edge, marine reworking of The next downslope element in the sand dispersal certain shelf sands was likely. Extensive mud depo- pattern is elongate sand bodies showing a distributive sition in this system was probably contemporaneous network; these are associated with relatively large with periods of active deltation in the Rockdale Delta amounts of nonchannel muds (Fig. 9). Channel sand System, though muds of the two systems are compo- accumulation was chiefly by downward accretion sitionally distinct; amount of mud deposition on the through compaction, as indicated by the generally non- shelf is small in relation to amount of contemporaneous erosive bases of the distributary channels and compac- prodelta mud deposition. tional features of underlying muds. of system oc- Sand Dispersal Patterns Principal deposition sand in the delta curs in the delta front facies, built up of distributary The dispersal pattern of coarser fraction sediments mouth bars. Where delta front sands are built as (generally sand) in a terrigenous clastic system provides elongate, narrow sand facies (bar finger sands of Fisk, significant clues to the origin of specific facies and 1961) in relatively high mud frameworks and under also provides a basis for the integration of various deeper water conditions (birdfoot deltas), sand storage facies and systems. Transport, dispersal, and deposition is chiefly permanent. Such is apparently the case of of the coarser fraction sediments normally involve higher the elongate delta fronts of deltas characteristic in the energy elements within a system, so that geometry of eastern part of the Rockdale System where the delta sand bodies effectively outlines skeletal units of the fronts are not associated with marginal barrier bar FISHER AND McGOWAN: WILGOX DEPOSITION IN TEXAS 121

Figure 9. Idealized sand dispersal system in various depositional systems, Wilcox Group, Texas 122 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII, 1967 and strandplain sands. Where the delta front is con- of several individual barrier bars with the main fea- structed as a lobate to rounded facies of sand-rich, tures and facies tract of the individual barrier bar pre- shoal-water deltas (typical of deltasin the southwestern served in the larger system. Essential difference in part of the Rockdale System), part of the sand may be the two is size. The magnitude of the Gotulla Barrier redistributed laterally by marine destruction of the Bar System here recognized in the lower Wilcox of deltas and transported by longshore currents. In such South Texas is closely comparable to other barrier bar— situations two main types of sand bodies accumulated, systems recognizedelsewhere in the Gulf Coast Basin though both have a common source. Adjacent to the Frio Barrier Bar System (Neogene) of South Texas delta system, accumulation generally occurred as rela- (Boyd and Dyer, 1964) and Terryville Barrier Bar tively thin, areally persistent sheet sands, making up (Jurassic) of southern Arkansas andnorthern Louisiana a strandplain system. Down longshore current and more (Thomas and Mann, 1966). distally from the delta system, sand accumulated as Secondly, the tectonic of the basin more narrowly restricted, elongate barrier bars with nature markedly complementary development of a landward, distinct, influences the distribution of component facies within lagoon-bay system. a depositional system, though composition and character of component facies of a given depositional system may A final pattern of sand dispersal,not shown onFigure be very similar in different structural basins. For ex- 9 but possibly occurring as a part of lower Wilcox ample, delta systems developed in relatively stable depositional systems, is transport and deposition by basins such as the Pennsylvanian delta systems of the means of turbidity currents. Sharp indentations in Eastern Interior Basin, commonly show extensive sand isoliths of certain of the shoal-water deltas in the progradation; sections within such systems show a verti- Rockdale System (Guadalupe and Colorado deltas) cal succession of component facies with prodelta muds mayreflect mud-filled, submarine canyons with cutting overlain by delta front sands, in turn succeeded by and subsequent filling associated with specific stages delta plain and fluvial facies. By contrast, delta sys- of delta construction and destruction. Such a feature tems in rapidly subsiding, unstable basins, such as the has been documentedin a higher Wilcox system (Hoyt, Gulf Coast Basin, commonly do not show pronounced 1959) and submarine channeling is well known from vertical succession of facies but rather pronounced several modern delta systems; similar features in lower lateral facies that tend to persist vertically. Within Wilcox systems are probable. Under such conditions any particular part of the delta system, therefore, a delta sands eroded during channeling wouldbe carried vertical sequence chiefly shows only a single component down depositional slope by turbidity currents and de- facies of the system. posited as fans at the first major flattening of the slope. Mode of dispersal visualized is similar to that Occurrence of Oil and Gas outlined by Walker (1967) for near-source submarine fans associated with the Pennine delta in the Upper A well-established generalization in Gulf Coast ge- Carboniferous of northern England. Verification of ology is that occurrence of oil and gas is regionally such fans in the lower Wilcox awaits more extensive controlled by depositional facies and locally entrapped drilling of very deep downdip wells. by structures coincident with depositional facies containing optimum reservoirs. The environmental control Depositional Systems and of oil and gas occurrence in terrigenous clastic rocks has been particularly by (1963). Basin Tectonics emphasized Rainwater Distribution of depositional facies in the context of We have used the concept of depositional systems the regional deposition system of which they are a not only as an approach to interpreting certain rock part explains control of known accumulations and pro- units and facies of the Wilcox Group but also as a vides a basis for; outlining regional, potentially pro- basis for establishing criteria for recognizing large scale, ductive trends. In the lower Wilcox of the Texas Gulf regionally developed depositional units. Many sedi- Coast, five, possibly six, main oil and gas trends are mentary basins, and notably the Gulf Coast Basin, defined in relation to depositional systems established contain very thick and large masses of terrigenous herein (Fig. 10). These include: (1) delta trend clastic rocks. Application of certain criteria for recog- (chiefly delta front constructional and marine de- nition of specific depositional environments and uti- structional sands), (2) strandplain trend, (3) barrier lization of detailed depositional models are facilitated bar trend (including tidal deltas), (4) shelf trend, (5) first by outlining major depositional systems involved shelf-edge trend, and (6) a possible submarine fan in the basin fill. trend not yet defined. The nature of a sedimentary basin, especially its Oil and gas accumulation in the lower Wilcox from structural or tectonic attitude, is critical to the concept north-central Karnes County northeastward to the of depositional systems in two ways: (1) volume of Sabine River is closely controlled by distribution of sediment or magnitude of sequences involved and (2) specific deltas of the Rockdale Delta System. Principal variation in distribution of component facies. First, in reservoirs are delta front sands and immediately as- a relatively unstable basin where subsidenceis generally sociated marine reworked sands of delta destruction; a response to sediment load (as we judge the case of existing fields are largely coincident with a relatively the Gulf Coast Basin during Wilcox deposition), sites narrow belt determined by the more distal fringes of of deposition of particular facies are maintained. Thus delta front sands (Fig. 10). Interfingering of delta front a barrier bar system, for example, can be constructed sands with adjacent marine prodelta muds provides FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 123

Figure 10. Producing and potential-producing oil and gas trends, lower part of Wilcox Group, Texas 124 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XVII, 1967 the multipay reservoirs, typical of many of the large, downdip of the major shoal-water deltas of the Rock- delta front fields (c.g., Falls City, Sheridan, Columbus, dale System, chiefly in Goliad, Victoria, Jackson, and Lake Creek, New Ulm, Quicksand Creek). Marine Wharton counties. Identification of such a trend will clays deposited over delta front sands during stages require extensive and very deep drilling. of marine destruction of delta lobes locally provide effective seals for sand reservoirs. Not only do known References Cited producing fields coincide with distribution of delta sand units, but numerous large scale structures situated Allen, J. R. L. (1963) The classification of cross- along structural trend of lower Wilcox production stratified units, with notes on their origin: Sedimen- within the delta system are notably nonproductive due tology, v. 2, pp. 93-114. to location in sand-poor interdelta subembayment facies Andrews, P. B. (1966) Facies and genesis of a hur- (Fig. 10). Growth faults and diapiric structures are ricane washover fan, St. Joseph Island,central Texas associated with zones of thick deposition such as the coast: Unpub. Ph. D. dissertation, Univ. Texas at delta front; structures associated with these features Austin, 238 pp. are locally important traps. Barry, J. 0., and Leßlanc, R. J. (1942) Lower Eocene Marginal to the delta trend is the strandplain trend, faunal units of Louisiana: Louisiana, Geol. Survey controlling oil accumulation chiefly in the lower Wil- Bull. 23, 208 pp. cox of northwestern Gonzales, Wilson, northwestern Boyd, D. R., and Dyer, B. F. (1964) Frio Barrier Bar Karnes, and eastern Atascosa counties. Areally ex- System of South Texas: Gulf Coast Assoc. Geol. tensive sheet sands interbedded with dominantly marine Socs., v. 14, pp. 309-322. muds result in a wide producing trend; this trend in- Brown, L. F., et al. (1967) Role of compaction in cludes the farthest updip lower Wilcox production. development of geometry of superposed elongate Sand units on the seaward side of the Cotulla Barrier sandstone bodies (abstract): Bull. Amer. Assoc. Pe- Bar System constitute the barrier bar trend which ex- troleum Geologists, v. 51, pp. 455-456. tends as a narrow trend from northern McMullen, Glaypool, C B. (1933) The Wilcox of central Texas: southwestward across La Salle through western Webb Abstract of Ph. D. Thesis, Univ. Illinois, Urbana, counties. Most of the established production from 12 pp. this trend is in east-centralLa Salle County associated Coleman J. M., and Gagliano, S. M. (1964) Cyclic mainly with a shelf side tidal delta of the Cotulla sedimentation in the Mississippi River deltaic plain: System. Providing entrapping structures exist, ad- Gulf Coast Assoc. Geol. Socs., Trans., v. 14, pp. ditional discoveries in the barrier bar trend are likely; 67-80. significantly, the trend occurs at shallow depths. Pro- Echols, D. J., and Malkin, D. S. (1948) Wilcox duction to date from this trend has been chiefly oil (Eocene) stratigraphy, a key to production: Bull. with relatively minor amounts of gas. Arncr. Assoc. Petroleum Geologists, v. 32, pp. 11-33. Several lower Wilcox fields have been developedin Feofilova, A. P. (1954) The place of alluvium in sedi- the sand reservoirs of the South Texas Shelf System. mentation cycles of various orders and time of its With exceptions of Green Branch and Dilworth Ranch, formation: Acad. Sci. U. S. S. R., Inst. Geol. Sci. however, fields are small. Individual sand units within Trans. (Akad. Nauk S. S. S. R., Inst. Geol. Nauk this extensive system tend to be sporadic in distribution Trudy), no. 151, Coal ser., no. 5, pp. 241-272. and not easily definable. Further, many sands are Fisher, W. L. (1963) Lignites of the Texas Gulf muddy with low permeabilities. Production to date has Coastal Plain: Univ. Texas, Bur. Econ. Geology been primarily gas with small amounts of oil. Rept. Invest. 50, 164 pp. Sand Recent Mississippi A final, main lower Wilcox trend is here referred Fisk, H. N. (1955) facies of Petroleum Cong., to as the shelf-edge trend (Fig. 10). Sands of this delta deposits: Proc. 4th World 1-C, trend are a part of the South Texas Shelf System, Sec. pp. 377-398. though their origin is not fully understood. They form Fisk, H. N. (1960) Recent Mississippi River sedi- a fairly well-defined, narrow trend extending from mentation and peat accumulation: 4th Intern. Cong. the southwestern margin of the Rockdale Delta System Carboniferous Stratigraphy and Geology (Heerlen, through central Bee, central Live Oak, northwestern 1958), Compt. Rend., pp. 187-199. Duval, southeastern Webb, eastern Zapata, and western Fisk, H. N. (1961) Bar-finger sands of the Mississippi Starr counties. Producing sands in this trend may delta:in Geometryof sandstone bodies: Amer. Assoc. have originated as barrierbars similar to elongate sand Petroleum Geologists, Tulsa, pp. 29-52. bodies updip on the remaining part of the shelf system Frazier, D. E. (1967) Recent deltaic deposits of the and in the Cotulla System. Their distinction and down- Mississippi River: Their development and chro- dip occurrence suggest possible shelf-edge reworking nology: Gulf Coast Assoc. Geol. Socs., Trans., this and cleaning as a result of the higher energies as- volume. with sociated such a zone. Friedman, S. A. (1960) Channel-fill sandstones in the If occurrence of submarine fan deposits, as con- middlePennsylvanianrocks of Indiana: Indiana Geol. sidered above, is indicated by very deep drilling, an Survey Rept. Prog. 23, 59 pp. additional lower Wilcox trend may be established. Hargis,R. N. (1962) Stratigraphy of the Carrizo-Wilcox Most likely area of such sand development would be of a portion of South Texas and its relationship to FISHER AND McGOWAN: WILCOX DEPOSITION IN TEXAS 125

production: Gulf Coast Assoc. Geol. Socs., Trans., Pettijohn, F. J., et al. (1965) Geology of sand and v. 12, pp. 9-25. sandstone: Conference, Indiana Geological Survey Hayes, M. 0., and Scott, A. J. (1964) Environmental and Dept. Geology, Indiana University, 205 pp. complexes, South Texas coast: Gulf Coast Assoc. Potter, P. E. (1963) Late Paleozoic sandstones of the Geol. Socs., Trans., v. 14, pp. 237-240. Illinois Basin: Illinois Geol. Survey, Rept. Invest. Hoyt, W. V. (1959) Erosional channel in the middle 217, 92 pp. near Yoakum, County, Texas: Gulf Wilcox Lavaca Rainwater,E. H. environmental control of Coast Assoc. Geol. Socs., Trans., v. 9, pp. 41-50. (1963) The oil and gas occurrence in terrigenous clastic rocks: Kolb, C. R., and van R. Lopik, J. (1966) Depositional Gulf Coast Assoc. Geol. Socs., Trans., v. 13, pp. environments of the Mississippi River deltaic plain — southeastern Louisiana: in Deltas in their geologic 79-94. framework: Houston Geol. Soc, Houston, pp. 17-61. Thomas, W. A., and Mann, C. J. (1966) Late environments, and Kruit, C. (1955) Sediments of the Rhone delta. I. Jurassic depositional Louisiana Grain size and microfauna: Kon. Arkansas: Bull. Amer. Assoc. Petroleum Geologists, Nederlands Geol. 50, Mijnbouwk Gen. Verhand, v. 15, pp. 397-499. v. pp. 170-182. Lowman, S. W. (1949) Sedimentary facies in the Gulf Walker, R. G. (1967) Turbidite sedimentary structures Coast: Bull. Amer. Assoc. Petroleum Geologists, v. and their relationship to proximal and distal depo- 33, pp. 1939-1997. sitional environments: Jour. Sed. Petrology, v. 37, Mueller, J. C, and Wanless, H. R. (1957) Differential pp. 25-43. compaction of Pennsylvanian sediments in relation Wanless, H. R., et al. (1963) Mapping sedimentary to sand-shale ratios, Jefferson County, Illinois: Jour. environments of Pennsylvanian cycles: Bull. Geol. Sed. Petrology, v. 27, pp. 80-88. Soc. America, v. 74, pp. 437-486.