Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations
1971 Early Eocene fanglomerate, northwestern Big Horn Basin, Wyoming Steven Roger Bredall Iowa State University
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Recommended Citation Bredall, Steven Roger, "Early Eocene fanglomerate, northwestern Big Horn Basin, Wyoming" (1971). Retrospective Theses and Dissertations. 16670. https://lib.dr.iastate.edu/rtd/16670
This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Early Eocene fanglomerate, northwestern Big Horn Basin, Wyoming
bV
Steven Roger Bredall
A Thesis Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of MASTER OF SCIENCE
Major Subject! Earth Science (Geology)
Signatures have been redacted for privacy
Iowa State University Of Science and Technology Ames, Iowa 1971 ii
TABLE OF CONTENTS
Page
INTRODUCTION 1
Objectives 1
~revious Investigations 1 Pield and Laboratory Procedures 3 GEOLOGIC SETTING 5
Regional Geology 5
Stratigraphy and Structure 10
PETROG~HY 12
Carbonate Pebble and Cobble Conglomerate 12
Hixed Igneous and 14etamorphic Pebble 15 and Cobble Conglomerate
Sandstone 19 Mudstone, Siltstone, and Carbonaceous Shale 23 SEDIMENTARY STRUCTURES 26 Geometrv of Rock Bodies and 26 Composite structures
Conglomerates 26 Sandstones 30 Mudstones, Siltstones, and 31 Carbonaceous Shales
Cross-Stratification 31 other Structures 35 iii Page
ANAL~SIS OF DATA 40
First Occurrence of Conglomerate 40 Clast Lithology
~-iaximum Clast SizE' and Li tho logy 44
:'laximUITI Quartz Grain Size 45
Paleocurrents 45 Mudstone Chemical Analvsis 51
DISCUSSION AND INTERPRETATION 57
Provenance 57 Depositional Environments 59
Alluvial Pan 59 Alluvial Plain 63
CONCLUSIONS 67
ACKNOWLEDGE~ENTS 69
REFERENCES CITED 70 iv
LIST OF FIGURES Page Fig. 1. Geologic map of the area of study with 7 locations of measured sections and resul~ant mean vectors of paleocurrent indicators. Fig. 2. Distribution of the average size of the 14 ten largest clasts of the conglomerate and their dominant lithologies.
Fig. 3. Classification of sandstones. 22 Fig. 4. Photographs of rock bodies and composite 28 structures.
Fig. 5. Photographs of cross-stratification. 34 Fig. 6. Photographs of other structures. 37
Fig. 7. Graphic sections of Willwood fanglomerate. 42
Fig. 8. Distribution of average size of the ten 47 largest quartz grains in sandstone. Fig. 9. Profile distribution of free iron, 56 manganese, aluminum, organic carbon, and clay «9¢) in a selected red mudstone sequence. Fig. 10. Block diagrams of Willwood fanglomerate. 66 Diagrams are idealized to show suggested major stratigraphic relations. v
LIST OF TABLES Page Table 1. Stratigraphy of the area of study 8 Table 2. Percentages of mineralogical 16 components, grains, matrix, and cement in sandstones Table 3. Mineralogical composition of two red 24 mudstones Table 4. Paleocurrent data 49 Table 5. Quantitative data on free iron, 52 manganese, aluminum, organic carbon, and clay in a selected red mudstone sequence 1
INTRODUCTION Objectives This report presents a study of the conglomerate facies of the Willwood Formation exposed along the east flank of the Beartooth Mountains in the northwestern part of the Big Horn Basin, Wyoming. The Eocene willwood Formation was deposited in response to regional uplifts surrounding the Big Horn Basin (Fisher, 1906; Van Houten, 1944; and Neasham, 1970). Determination of the source and distribution of the conglomerate facies and its relation to uplift should aid in the interpre tation of the depositional history of the Willwood Formation. The study of the conglomerate facies was initiated with the following objectives! (a) to describe the facies and its distribution, (b) to determine the provenance of the sediments, and (e) to determine the environment or environ ments of deposition.
Previous Investigations A fluviatile mode of origin for the Willwood Formation was first postulated by Fisher (1906) and Loomis (1907) and later confirmed by Sinclair and Granger (1911 and 1912). Van Bouten (1944 and 1948) presented valuable information on the stratigraphy of the Willwood and the origin of red-banded deposits in the Rocky Mountain area. Rohrer and Gazin (1965) provided data for a more precise definition of the contact 2 of the Gray Bull and Lysite faunal zones of the willwood in the Iratman Mountain area of the Basin while Neasham (1967) discussed stratigraphic and petrologic data from the Sheep Mountain area. The marginal Willwood conglomerate facies has been de scribed in exposures along the western margin of the Big Horn Basin (Fisher, 1906; Van Houten, 1944; and Neasham, 1970). Rouse (1937) in a study of the Beartooth Mountains, described its lithology near Line Creek (Fig. 1). Pierce (1965a and b) has mapped and dated the Willwood deposits as early Eocene. Most recently, Neasham (1970) presented a detailed exam ination of the Willwood Formation in which he suggested that the conglomerate facies in the Beartooth region was deposited on a series of coalescing alluvial fans. Conglomerate sequences have often been interpreted as alluvial fan deposits because of their tectonic settings (Miall, 1970). Much work has been carried out on recent fanglomerates in the southwestern United States where Trowbridge (1911) supplied criteria for recognition of alluvial fan deposits. Blissenbach (1954) furnished detailed descriptions of alluvial fan deposits and their modes of origin whereas competence of stream transport on alluvial fans was discussed by Lustig (1963). Bluck (1964) obtained measurements on size and sphericity of clasts. Excellent summaries on alluvial fan sediments and depositional proc- 3
esses were provided bv Allen (1965) and Bull (1968). Bluck (1965), Miall (1970), Laming (1966), Steidtmann (1971), and Wilson (1970) have focused attention on ancient conglomerates utilizing modern alluvial fan features as models for their interpretation.
Field and Laboratory Procedures Measuring sections, sampling, and collection of infor mation concerning sedimentary structures were accomplished over a period of tilree months during the summers of 1970 and 1971. Over two hundred sandstone samples and fifty conglomerate framework and matrix samples were examined in hand specimen. Grain counts were made on 22 sandstone and 4 conglomerate matrix (sandstone) thin sections. Sampling localities were chosen to adequately cover the geographic and stratigraphic occurrence of the conglomerate facies as completely as time and terrain would allow. A total of 1269 paleocurrent directions were determined from cross-stratification, clast imbrication, and conglomerate and sandstone channel trends. Most exposures of trough cross-stratification were sufficient to determine orientation of axes of troughs although directions of maximum dip were measured on some trough flanks as well as all tabular cross stratified units. Clast imbrication was observed at four localities with a total of 58 readings taken. 4
Conglomerate clast size and composition counts were determined in the field where 200-300 clasts were examined and lithologies determined at each locality. The largest
size method described by Steidtmann (1971) was used to obtain a clast size index. The long axes of the ten largest clasts at each locality were measured and averaged and the result was used as the maximum clast size index for that locality. Percentages of mineral components, framework grains, matrix (particles smaller than .02 rom), and cement were determined from 250 point counts per thin section of 22 sandstones. Four conqlomerate matrices (sandstones) were examined in tile same manner to determine percentages of runeral components. Feldspar composition was determined by both thin section analysis and a staining technique described by Reeder and McAllister (1957) while plagioclase composition was determined by the Slemmons method (1962) in samples with plagioclase grains large enough to permit determinations. Size, sorting, and shape were qualitatively determined using visual comparison charts of Russell and Taylor (1937), Powers (1953), and Williams, et ale (1954). A largest quartz grain size index was obtained from 16 sandstone samples. The long axes of the ten largest quartz grains in each thin section were measured and aver aged with the average used as an index of maximum quartz grain size for that locality. 5
GEOLOGIC SETTING
Regional Geology The area of study occupies approximately 100 square miles in Park County in northwestern Wyoming. It is in the northwestern part of the Big Horn Basin within the Clarks Fork drainage basin (Fig. 1). Lake Creek, Bennett Creek, and Line Creek are primary tributaries to the Clarks Fork of the Yellowstone River which is present in the south. Willwood conglomerate exposures trend north-south paralleling the Beartooth Mountain front. A gently sloping pediment surface immediately north of the Wyoming-Montana border roughly delineates the northern extent. The town of Clark is in the east central part of the area. Rocks exposed along the steeply dipping east flank of and in the Beartooth Mountains range in age from Precambrian through Cretaceous (Table 1). The uplifted Precambrian core of the Beartooths is exposed west of the high-angle Bear tooth Fault (Pierce, 1965b) with middle Paleozoic rocks forming prominent flat-irons on the Beartooth flank.
Paleozoics in turn are overlain bv a thick Mesozoic sequence which is steeply dipping to overturned. Quaternary deposits lie unconformably on Precambrian, Paleozoic, and Mesozoic units at various localities. 6 Fig. 1. Geologic map of the area of study with locations of measured sections and resultant mean vectors of paleocurrent indicators. 7
R21E EXPLANATION
QUATERNARY DEPOSITS o Unconsolidated or poorly consolidated deposits of silt, sand, and bravel ~n stre ~m valleys and terraces; pediment deposits; - loose rock debris of land sq.de origin and boulders , sand, and silt of DlorAine deposits.
WILLWOOD FORMATION
Varicolored sandstone and mudstone: ~om e carbonaceous shale with thin coa l lenses; thick conglomerate at base which wedges out eastward.
FORT UNION FORMATION II Thin-bedded sandstone and conglomera te ; drab La brown mudstone with some red mudstones mid Cl1rbOnACeous s hales.
MESOZOIC ROCKS. UNOIIIDED
:·Iill.nl y sandstones and mudstones; from oldest to youngest includes Dinwoody I Chugwater, Gypsum Spring, Sundance, Horrision, Cloverly. 'nIermopolis I Howry, Frontier I Cody, Mesaverde, Meeteets e, and Lance (1) Formations. Z • CLARK I"- 10 I- ~ PALEOZOIC ROCKS. UNOIIIDED
~ Mainly carbonates nnd same sandstones; tram o ldest to youngest includes F1 11 hend, Gros' Ventre, Gallatin, Bighorn, Beartooth Butte, Jefferson, ntree Forks, Madison, Amsden, Tensleep, and Park City Formation• •
PRECAMBRIAN ROCKS. UNOIIIIDEO Mainly granitic gneiss '
FAULT
MNOR L..OW-o¥IGLE F..ut..T Savteeth on upper plate
0 ••••• 0' LOCATION OF MEASURED SECTIONS
AEllULTANT MEAN VECTOR OF P'ALEDCUAAENT NDICATORS AND LOCALITY NUMBER
AFTER PERCE (194S!5) AND CASELLA (1864) SCALE o 2 3 MILES . I I I
.!I 0 2 3 KILOMETERS I EJ SlVDY AREA LOCATDN
...... ::>X... ATE MEAN DECLNATION. 1_
GEOLOGIC MAP OF STUDY AREA 8
rable 1. Stratigraphy of the area of study
AGE FORMATION LITHOLOGIES a
Tertiary Fort Union Cretaceous monzonite porphyry Lance, Meeteetse, Mesaverde, Frontier sandstoneb Jurassic Sundance glauconitic pelecypod biosparrudite Triassic Chugwater Permian Park City b Pennsylvanian Tensleep Sandstone sandstone Mississippian Madison silicified algal biolithite limestonec ------aLithologies listed are those found as clasts in the Willwood Formation. bIdentified as undifferentiated sandstone in the field. cIdentified as undifferentiated carbonate in the field. 9
Table 1. (Continued)
a AGE FORMATION LITHOLOGIES
Devonian Three Forks and dolomiteC Jefferson Ordovician Bighorn dolomiteC Cambrian Gallatin intramicrite Pilgrim Limestone oosparite Precambrian igneous and pink and gray metamorphic granitic gneiss pink and gray migmatite biotite gneiss amphibolite quartzite quartz dolerite pegmatite 10
Stratigraphy and Structure The Willwood Formation lies with angular unconformity on the Paleocene Port Union Formation and the Upper Cretaceous Meeteetse Formation (Fig. 1). Quaternary alluvium, terrace gravels, pediment deposits, and moraine deposits in turn unconformably overlie the Willwood. Near to the mountain front the Willwood is steeply dipping up to 75°E to over turned. It flattens out to the east becoming horizontal to gently dipping (l-4°W) in exposures south of Clark and north and east of Sugarloaf Butte. The remaining exposures south of Bennett Creek have dips from 6-l3°E near their eastern extents as the Willwood flattens out. The conglomerate facies is composed of approximately 65-75% conglomerate near the mountain flank. Toward the east, sandstone units interbedded with the conglomerate become increasingly abundant, thicker, and more widespread as conglomerates rapidly thin and pinch out away from the Beartooth Mountain front. Conglomerates do not occur east of Sugarloaf Butte, in the three small exposures south of Clark, or in the exposures south of the Clarks Fork. Mud stones, siltstones, and carbonaceous shales comprise approximately 5-10% of the conglomerate facies and display little variation in abundance throughout the area. The exposed thickness of the conglomerate facies varies from approximately 2,500 feet near the mountain flank (Fig. I, 11
Sec. E-E') to as little as 300 feet near Sugarloaf Butte (Fig. 1, Sec. g_gl), 200 feet in one large exposure south of the Clarks Fork, and 20-60 feet in the three small exposures south of Clark. The conglomerate facies is composed of two distinct lithologic assemblages. Carbonate pebble and cobble conglom erate is present in exposures south of Bennett Creek while mixed igneous and metamorphic pebble and cobble conglomerate occurs in exposures north of Bennett Creek (Fig. 1). The b~o conglomerates interfinger in the northeast part of the large exposure immediately south of Bennett Creek. 12
PETROGRAPHY
Carbonate Pebble and Cobble Conglomerate Carbonate conglomerate consists of rounded to well rounded pebbles, cobbles, and boulders in a sandstone matrix. From 80-100% of the clasts are composed of carbonate which include (Folk, 1968) silicified algal biolithite (Missis sippian, Madison Formation), glauconitic pelecypod biosparrudite (Jurassic, Sundance Formation), oosparite (Cambrian, Pilgrim Limestone), intramicrite (Cambrian, Gallatin Formation), and undifferentiated carbonates (Paleozoics). Minor clast types are undifferentiated sand stones and pink and gray granitic gneiss (Precambrian). The average maximum diameter of clasts measured at 21 locations is 12.5 em while the maximum size examined was 97.3 em. Fig. 2 shows the distribution of the carbonate conglomerate and the average size of the ten largest clasts at each locality. Clasts are very poorly sorted, rounded to well-rounded, granule to boulder size with cobbles and pebbles the most common size fraction. Disc, roller, and spherical shaped clasts are most prevalent. The sandstone matrix is medium to coarse grained, rounded to well-rounded litharenite with calcite the primary cementing agent. Iron oxide and silica cement are present in minor amounts. The average composition of matrix is common quartz 44.5%, metaquartzite 1.5%. chert 9.5%, 13 Fig. 2. Distribution of the average size of the ten largest clasts of the conglomerate and their dominant lithologies. 14
R201 R211 T"J _..L __ ~O~A.!:...A_-f WYOMING
TsaN
TS7N
EXPLANATION
Average size of ten I a r ge s tel a s t s I n conglomerate 34 Locality number o •10 20 A v era g e S I Z e (c m)
75 85
- 0__ • 85-95 f 7. carbonate clasts J- .;:::=::= > 95 * 75-85 f :& 85-95 % Igneous and meta- A morphlC clasts .. >95
SCALE N 1/7. 0 2 I I miles 5 5 0 29 I k II 0 met e r I> L R103W t-L ...J 15 feldspar 6.2%, and miscellaneous components 38.3% (Table 2). Miscellaneous components consist primarily of fragments of carbonate rocks and minor amounts of calcareous quartz sandstone. Feldspars consist of an average of 88.0% ortho clase and microcline and 12.0% plagioclase. Plagioclase feldspars analyzed by the Slemmons method (1962) were distributed throughout the andesine range with some oligo- clase also present.
Mixed Igneous and Metamorphic Pebble and Cobble Conglomerate Mixed igneous and metamorphic conglomerate is composed of rounded to well-rounded pebbles and cobbles in a sand- stone matrix. Counts indicate that 82-99% of the clasts are igneous and metamorphic and include pink and gray granite qneiss, pink and gray migmatite, amphibolite, quartzite, biotite gneiss, quartz dolerite, pegmatite (all Precambrian), and monzonite porphyry (Cretaceous, Pierce, 1965b) described by Harris (1959) and Casella (1964). Minor clast types include undifferentiated carbonates and sandstones, silicified algal biolithite (Mississippian, Madison Formation), and intramicrite (Cambrian, Gallatin Formation) • The maximum clast size examined was 33.8 em while the average maximum clast diamete~ from 18 locations was 10.6 cm. The distribution of mixed igneous and metamorphic conglomerate and maximum clast size is shown in Fig. 2. Clasts are very 16 Table 2. Percentages of mineralogical components, grains, matrix, and cement in sandstones ;0 a % % % % % Sample Grains Matrix Cement Conunon Metaquartzite Chert guartz A-2 73.3 2.9 23.8 34.5 7.5 15.5 D-11 82.3 6.6 11.1 33.5 11.5 15.5 d-13 69.2 4.5 26.3 33.0 12.0 7.5 d-22 73.0 27.0 29.0 15.0 10.0 E-23 63.3 2.8 33.9 36.5 13.0 8.0 E-27 70.9 3.5 25.6 31.5 13.0 5.0 E-44 73.5 0.7 25.8 26.0 12.0 3.0 F-4 78.1 2.0 19.9 32.5 8.5 6.5 F-33 70.4 4.6 25.0 42.0 8.0 5.0 F-70 72.7 2.5 24.8 43.0 9.0 15.5 F-73 63.9 6.4 29.7 65.0 2.0 7.0 F-95 62.7 4.1 33.2 56.0 2.0 14.0 f-9 61.4 4.3 34.3 42.0 9.0 2.0 G-9 76.0 6.9 17.1 36.5 11.0 4.5 G-15 66.2 3.0 30.8 32.0 16.0 3.0 g-6 71.1 4.3 24.6 42.5 12.0 9.0 H-2 74.6 0.7 24.7 35.0 9.5 4.5 h-17 71.4 6.1 22.5 32.0 16.5 3.5 h-18 64.5 1.0 34.5 34.0 12.5 7.0 h-20 59.0 2.4 38.6 34.0 11.0 3.0 h-21 73.3 5.5 21.2 35.0 4.5 3.5 h-23 66.7 9.7 23.6 32.0 12.5 1.0 Mean 69.9 3.8 26.3 37.6 9.9 7.0 b A_14 67.0 2.5 8.5 b E_l 22.0 0.5 10.5 b G_9 32.0 1.5 13.5 b G-l2 29.0 2.5 18.5 Mean 37.5 1.7 12.8
a Sample designation - Letter refers to measured section; number refers to unit. (Sample A-2 is from the second measured unit of section A-A'). b Samples from matrix of conglomerates. 17
% % Misc. Feldspar Igneous and Metamorphic Carbonate Rock Sandstone Rock Mica Rock Frasments FraS!!!ents Fragments 25.5 9.0 3.5 4.5 18.0 2.5 14.0 5.0 27.0 7.5 8.5 4.5 28.0 10.5 7.0 0.5 28.0 3.0 11.5 32.5 8.0 1.5 8.5 31.5 5.0 15.5 7.0 24.0 1.5 16.5 10.5 19.0 1.5 21.0 3.5 23.5 9.0 11.5 1.0 13.0 0.5 8.0 16.5 3.5 31.5 12.5 4.0 34.0 12.0 23.5 19.5 3.5 2.5 26.5 10.0 42.5 8.5 32.5 13.5 2.0 35.5 5.5 5.5 34.0 15.0 2.5 0.5 38.0 14.5 2.5 2.0 26.5 24.5 3.5 27.3 8.2 6.9 2.7 0.5 11.0 11.0 1.5 7.5 49.5 5.5 3.0 47.5 6.0 44.5 5.5 26.0 4.7 15.1 1.1 0.8 18 poorly sorted, rounded to well-rounded, granule to cobble size with pebbles and cobbles the most prevalent. Clast shapes are similar to those of the carbonate conglomerate. The matrix is fine to coarse grained, rounded to well rounded arkose cemented primarily bv calcite with small amounts of iron oxide and silica. The average composition is common quartz 30.5%, metaquartzite 2.0%, chert 16.0%, feldspar 46.0%, and miscellaneous components 5.8% (Table 2). Miscellaneous components are entirely igneous and meta morphic rock fragments. The feldspars of the two conglomerate assemblages provide evidence of provenance. Feldspars from igneous and metamorphic conglomerate consist of an average of 65.7% orthoclase and microcline and 34.3% plagioclase. Comparison to feldspars of the carbonate conglomerate reveals a markedly higher ratio of plagioclase to orthoclase and microcline in the mixed igneous and metamorphic conglomerate. Plagioclase feldspars from the igneous and metamorphic conglomerate are restricted to ranges of anorthite between 13-16% and 33-38%. Anorthite percentages are consistent with those of granitic gneiss, miqmatite, and biotite gneiss (Eckelmann and Poldervaart, 1957) which are the dominant Precanmrian rock types in the eastern Beartooth Mountains. Further, plagioclase is the dominant feldspar in these Precambrian rocks (Eckelmann and Poldervaart, 1957). 19
These differences in feldspar percentage and composition suggest two distinct sources for the main conglomerate assemblages.
Sandstone Grain types counted in sandstone thin sections include common quartz, metaquartzite, chert (micro-crystalline quartz), feldspar, igneous and metamorphic rock fragments, carbonate rock fragments, calcareous quartz sandstone fragments, and mica (Table 2). Percentages of framework grains, matrix (particles smaller than .02 rom), and cement (primarily calcite, with minor amounts of silica and iron oxide) were also determined and are presented in Table 2. Quartz is the dominant mineral in all but four of the samples examined and averages 37.6%. Feldspars are dominant when quartz is not and two samples have a quartz-feldspar ratio close to unity. Inclusions of zircon and rutile needles, bubble trains, and vacules are present in the quartz grains. Un- dulatory and non-undulatory quartz was observed in all samples. Metaquartz averages 9.9% while chert averages 7.0%. Feldspars from sandstones average 27.3% and display the same characteristics as those from conglomerate matrices. Feldspars from exposures south of Bennett Creek (area of carbonate conglomerate) possess an average of 75.1% orthoclase and microcline and 24.9% plagioclase. Those north of Bennett 20
Creek (area of mixed igneous and metamorphic conglomerate) contain an average of 60.2% potash feldspar and 39.8% plagioclase. Plagioclase feldspars also have the same range of anorthite percentages as previously discussed for conglomerate matrices. Thus, the relatively higher ratio of plagioclase to orthoclase and microcline in sandstones associated with mixed igneous and metamorphic conglomerates and differences in plagioclase composition suggest the same differences in source rock litllology as discussed for conglomerate matrices. Miscellaneous components average 18.2% and include rock fragments of igneous and metamorphic origin, carbonate \ fragments, calcareous quartz sandstone, and mica. Carbonate fragments occur primarily in sandstones south of Bennett Creek with sandstones north of Bennett Creek containing a markedly lower percentage. Igneous and metamorphic rock fragments comprise the bulk of miscellaneous constituents in sandstones north of Bennett Creek. Heavy minerals were observed but were not identified. Most of the sandstones are lithic arkose (Fig. 3). Qualitative determinations show sandstones close to the mountain front are generally fine to coarse grained, angular to rounded, and poorly sorted. Laterally toward the east they grade to fine to medium grained, sub-angular to rounded, and moderately sorted sandstones. 21 Fig. 3. Classification of sandstones (after McBride, 1963). 22
QUARTZ. QUARTZrTE. CHERT
-~Ir--_--1~_ LITHIC SUBAAKOSE
LITHIC ARKOSE FELCSPATHIC LITHARENITE
FELDSPAR ROCK FRAGMENTS AND 10% 10% ACCESSORY MINERALS
COMPOSITION 23
Considerations of the importance revealed by examination of sandstones are: (a) The high feldspar percentage which is indicative of proximity to the source area (Folk, 1968). (b) The abundance of carbonate rock fragments which is also indicative of proximity to the source area. (c) Differences in feldspar composition which mirror conglomerate lithology and are diagnostic of source rock type.
Mudstone, Siltstone, and Carbonaceous Shale Red mudstones (lOR 3/4) are the most abundant fine clastic and ,,~ere examined using standard X-ray diffraction techniques described bv Raup (1962). Neasham (1970) observed tllat basin margin mudstones contained a high percentage of feldspar with illite and montmorillinite dominant and kaolinite subordinate among the clay minerals. Table 3 shows the results of the semi-quantitative analyses of two samples. Results agree closely with Neasham as a high feldspar percentage and abundant illite and monbnori1linite with subordinate kaolinite are present. Olive gray (SY 4/1), yellowish gray (5Y 7/2) I and olive bro\'ln (5Y 5/6) mudstones are also present in the conglomerate facies. The dominant siltstones are greenish gray (5GY 4/1), olive gray (5Y 4/1), and reddish orange (lOR 6/4). They are poorly bedded and
~enerally unfossiliferous with the exception of two localities where gastropods were noted in gray and red siltstone. 24
Table 3. Mineralogical composition of two red mUdstones
Sample P-2 A-1M % % Quartz 43 41 Feldspar 29 37 BULK SAMPLE Calcite tr
Clay and ~tica 27 22
Kaolinite 9 15
Illite 32 42 ORIENTED CLAY SAMPLE Montmorillinite 35 28
Mixed Layer 12 9 Illite-Montmorillinite
Others 12 6 25
Carbonaceous shales are black eNl) and contain fossil leaves and thin coal seams up to 1 1/2 inches ~lick. Finelv disseminated grains of unidentified yellow sulfide mineral occur throughout the carbonaceous shale units. Mudstone, siltstone, and carbonaceous shale samples were not examined in thin section. 26
SEDIMENTARY STRUCTURES
Geometry of Rock Bodies and Composite Structures
The geometry of rock bodies and composite sedimentary
structures reflect larger aspects of Willwood deposition.
These include tabular deposits of conglomerate, sandstone, mudstone, and siltstone, channel fills of conglomerate and
sandstone, and minor tabular carbonaceous shales distal to
major sediment accumulation.
Cong1omerat~
Geometry of conglomerate bodies varies from massive
tabular units with interbedded thin tabular bodies and lenses
of sandstone to less massive tabular units interbedded with
tabular sandstones and mudstones, to channel deposits cut
into sandstone and mudstone, and stringers and localized
lenses. ~assive tabular units range in thickness from 100-
800 feet (Fig. 4c) and in lateral extent along depositional
strike from 3,000 feet to approximately 1 1/4 miles. These bodies occur closest to the Beartootll Mountains where they
form prominent hogbacks and cuestas. Less massive tabular
units range in thickness from 6 to 30 feet and lateral extent up to 3,000 feet, although 300 to 800 feet is most common
(Fig. 4d). Tabular conglomerates exhibit planar erosional basal surfaces with underlying units. These bodies rapidly pinch out to the east along depositional dip as tabular 27 Fig. 4. Photographs of rock bodies and composite structures. a. Channel-fill of conglomerate radiating from massive tabular conglomerate and cut into underlying sandstone. b. Channel-fill of conglomerate (1) cut into underlying sandstone (2). c. Massive tabular conglomerates (1) interbedded with tabular and lenticular sandstones (2) exposed near the Beartooth Mountain front (approximately 350 feet shown in photograph). d. Tabular deposits of conglomerate (1), sandstone (2), and mudstone (3) close to the mountain front (sandstone unit (2) is approximately 18 feet thick). 28 29 sandstones become more abundant. Channel fills of conglomerate cut into underlying sandstone are exposed in localized areas (Pigs. 4a and b) • They can be traced from Jl1assive tabular conglomerates into shoestring-l:ike bodies ':Y'hich progressively narrow in \4idth and thin in thickness away from the Beartooth front as they breakup into distributaries. i\t the point of separati.on from tabular masses, the channels are generally about 15-20 feet thick and up to 60 feet \~id.e but rapidly decrease in wi~th and uepth as well as clast size until fills can not be differentiated from underlying sandstones. Upper surfaces of fills are slightly convex upward while lower bonding surfaces are sharply concave upward and smooth. Both upper and lower bounding surfaces are sharply defined when the fill is in sandstone but less well-defined in conglomerate. Conglomerate-filled channels distal to and not traceable to massivE.; tabular conglomerates are similar to those described above. Nidths and depths are similar and decrease, as does clast size, to the east. Upper bounding surfaces are smooth and slightly convex upward while lO\ver surfaces are concave upward and irregular. Irregular lower bounding surfaces indicate a flow re~fime with a higher erosional capacity than that associated with the conglomerate fills previously described. Lenses of conglomerate occur in tabular sandstones and 30 mudstones interbedded with tabular conglomerates. These
~odies are from 2 to 20 feet thick. The lateral extent of
these bodies is proportional to their thickness; the maximum being 200 feet for the thickest units and as little as 8 feet for the thinnest. Inclined pebbles and cobble stringers enclosed in sandstone are sometimes only as thick as individual clasts up to a maximum of 18 inches.
Sandstones
Sandstone body geometry varies from tabular units interLeuded 'I"",i tIl tabular conglomerates and mudstones to channel fills and lenses. Tabular units are commonly 10-30 feet thick where interbedded with conglomerates (Pigs. 4c and d) and are more abundant and b~icken to 100 feet or more toward the east with the thickest units exposed at Sugarloaf
Butte (Fig. 1). Tabular bodies extend laterally along depositional strike up to approximately 3,500 feet with
1,000-2,500 feet the most common. As in tabular conglomerates, tabular sandstones exhibit a discontinuous appearance.
LOCRlly, tabular sandstones separate into recognizable channels. The sandstone fills are lense-shaped in cross section with upper and lower bonding surfaces poorly defined due to the presence of sandstone above and below. Channel fill thickness ranges from I or 2 feet to a maximum of 5 feet.
Nidths are about 10-15 feet and decrease to b'1.e east.
Sandstone lenses occur in tabular conglomerates. They 31 vary in thickness from 1 or 2 feet to 20 feet and in lateral extent to 80 feet.
:v1uds tones, S i 1 ts tones, and Carbonaceous Shales l\tudstone, siltstone, and carbonaceous shale bodies occur as tabular units throughout t..l--te area. f\luds tone units are primarily associated with tabular conglomerates and sand stones close to the Beartooth .f\1ountain front (Fig. 4d) and with tabular sandstones in the extreme eastern part of the area. Three-hundred feet is the maximum thickness where associated with lenticular conglomerates (Pig. 7, Sec. E-E'). Most commonly, mudstone unit thickness is between 3 and 30 feet with a maximum lateral extent along depositional strike of approximately 2,500 feet. Siltstone and carbonaceous shale occur in isolated localities and are commonly 1 to 10 feet in thickness. Siltstones grade into tabular sandstones and have lateral dimensions of a few feet up to 500 feet. Carbonaceous shales occur at only three localities with their true maximum lateral extent not determined because of recent erosion around all exposures. However, stratigraphic relations suggest a maximum lateral extent of approximately 250 feet.
Cross-Stratification Large-scale cross-stratified units consist of sets with an individual thickness qreater than 5 em. Conglorn- 32 erates and sandstones in the area exhibit three main types of large-Rcale tabular cross-stratification. Tabular cross stratification made up of solitary sets '-lith non-erosional planar basal surfaces (alpha type of Allen, 1963) occur in less than 10% of the tabular sandstones examined. Construc tion of these units probably occurs in shallow water by the building of solitary banks with straight or curving leading edges above slip-off faces. Structures of this type are commonly found in braided streams (Allen, 1963). Tabular cross-stratification made up of solitary sets similar to ti1e previous type but with basal planar erosion surfaces (beta type of Allen, 1963) is present in approximately 15% of tabular sandstones and sandstone filled channels (Fig. Sa).
Allen (1963) attributes their construction to the building of solitary banks as previously described but with bank advancement occurring erosivelv and swiftly under more changeable flow regimes than the above type. £abular cross stratification with solitary sets bounded by irregular basal erosional surfaces (gamma type of Allen, 1963) occur in approximately 15% of tabular conglomerates. The construction of solitary banks as described above in rapidly changeable flo\V' regimes is suggested as the origin for this type (Allen,
1963). Harms and Fahnestock (1965) attribute this type of cross-stratification to high gradient, braided, ephemeral streams. 33 Fig. 5. Photographs of cross-stratification. a. Large-scale tabular cross-stratification (beta type of Allen, 1963) developed in sandstone. b. Larqe-gcale trough cross-stratification (pi type of Allen, 1963) developed in sandstone. c. Large-scale trough cross-stratification (pi type of Allen, 1963) developed in pebble-sized conglomerate. 34 35
Large-scale trough cross-stratification (Figs. Sb and c) is the most extensive type of cross-stratification occurring in tabular conglomerates and sandstones and conglomerate filled c:lannels. It is made up of interfering, grouped sets Witil a basal scoop-shaped erosional surface open at one end (pi type of Allen, 1963). Symmetrical cosets are the ryeneral case '-lith fiome outcrops not permitting an adequate detenni nation of degree of synunetry. Allen (1963) suggests that
~1eir origin is due to migration of trains of large-scale asymmetrical dunes with curved crests. "-{arms and Fahnestock
(1965) explain their formation by the filling of elongate scours by irregularly-shaped migrating dunes. A relatively high flow regime is required for formation of trough cross stratification in gravel-sized material while the upper part of the lower-flow regime is postulated for sand-sized material (Harms and Fahnestock, 1965).
Other Structures Horizontal stratification is present in both tabular sandstones and conglomerates (Figs. 6a and b) and typically displays graded ~edding in each sedimentation unit. Individual units in sandstones are from 1/2-2 inches thick while those in conglomerates range in thickness frOM 1-6 inchf>s. The basal surfacp is usually planar and erosional. Harms and Fahnestock (1965) explain horizontal stratification 36 Fig. 6. Photographs of other structures. a. Horizontal stratification developed in sandstone. b. Horizontal stratification in conglomeratic sandstone grading upward into sandstone with slightly curved horizontal stratification. c. Clay blebs and balls (larger ones indicated by arrows) occurring in sandstone. d. Contact imbrication (lower one-third of photo) in close up of carbonate pebble and cobble conglomerate. e. Close-up of graded bedding sequence commonly present at top of conglomerate unit grading upward to sandstone. f. Secondary calcium carbonate concretions. 37 38 in sand-sized material as the result of plane-bed or low standing wave transport with upper flow regime necessary for formation. Horizontal stratification in conglomerates also marks the middle to upper portion of the upper flow regime (Gwinn, 1964). Although both sandstones and conglomerates display abundant horizontal stratification and cross-stratification, the majority of conglomerate deposits and many sandstones do not exhibit any stratification features or these features are so poorly defined as to be unrecognizable in the field. Harms and Fahnestock (1965) observed that stratification is commonly poorly defined and difficult to see in sediments deposited in the upper flow regime due to the high rate of transport and lack of opportunity for local sorting. The size and shape of clasts may mask crude stratification in massive conglomerates. Clay blebs and balls occur in tabular sandstones (Fig. 6c). They range in maximum diameter from less than one inch to about 18 inches and are generally composed of greenish- gray clay. They are angular to rounded with laminations in the enclosing sandstones continuous around the blebs and balls. Contact imbrication (Laming, 1966) in conglomerates occurs in less than 10% of tabular units (Fig. 6d) and stringers enclosed in tabular sandstones. Conglomerates deposited by debris floods also lack clast imbrication 39
(~iall, 1970). Imbrication is commonly poorly defined in sheetflood and violent streamflood deposits and where clast shape does not allow such development as in spherical clasts (Blissenbach, 1954). Graded bedding occurs commonly within each sedimentation unit of horizontal stratified tabular sandstones and conglom erates. It is also present in tabular pebble and cobble conglomerates grading upward to sandstone (Fig. 6e). Graded bedding results from the decline in competency of the dizpersing agent taking place over an extended period of time (Pettijohn, 1957). Secondary calcium carbonate concretions ranging in size from 1/2 inch to 2 inches in diameter were observed (Fig. 6f). Concretions of the same type but broken into irregular shaped fragments were also observed and reflect transportation and deposition of previously formed concretions in Willwood strata. 40
ru7~LYSIS OF DATA
First Occurrence of Conglomerate Clast Lithology The principle of first occurrence has been used by
previous workers (~iall, 1970 and Wilson, 1970) to illustrate
tile stripping of source areas and consequent deposition of conglomerate clasts in inverted stratigraphic sequence. First occurrence of clasts of a given lithology reveals
the progressive denudation of b~e Beartooth Mountains as they were uplifted during the Laramide orogeny_ Pebble, cobble, and boulder-sized clasts were examined where exposures were accessible. Masking of first occurrence by more abundant carbonato clasts that were not able to be differentiated as to formational derivation in the field made tile task tedious and not as complete as the writer had hoped. However, significant trends in clast first occurrence are present and reflect depositional features of the conglomerate.
Fig. 7 shows b~e first occurrence of clasts on graphic sections.
Carbonate conglomerate, located SOUtil of Bennett Creek, typically displays the following first occurrence pattern. Clasts of pelecypod biosparrudite from the Jurassic, Sundance Formation occur near the base of the exposures. Also present ncar the base of exposures are clasts of silicified algal biolithite (Condonophyous austini) derived from the ~issis- 41 Fig. 7. Graphic sections of Willwood fanglomerate. 42
GRAPHIC SECTIONS OF WILLWOOD FANGLOMERATE
EXPLANATION
LITHOLOGY
10 0°1 CARBONATE CONGLOMERATE CLAST FIRST OCCURRENCE L.:....£jro:ol CONGLOMERATE -IGNEOUS-METAMORPHIC CLASTS 1 - MONZONrrE PORPHYRY
~ SANDSTONE L.J 2 - JURASSIC SUNDANCE FORMATION o 100 200 FEET 3- UNDIFFERENTIATED PALEOZOIC CAROONATE . , D MUDSTONE I I I MISSISSIPPIAN MADISON FORMATION UNrrTHIO COLOR SY4/2 / SGY4 1 lOY4/2 SYS/2.7/2.8/2 o o o SGY6/1.712 o 7 o o o SYRS/4.7/2 o o N'I.3.4 10YRS{S D N7.8.9 ? ? o o . . . , .. G-G . . o o o o 0 o o o o o o o 4 o o '0 o o . 0 • c::::I::::!:::;:e~ o 0 , 0 • 0 o t:::;:::z::>- - 0 o 0 o o o 0 o o o 0<' . - 0 o S '0 o o .0 o I 0 o o o o o o o -= I o o o o o .0 o o o o o o o 000 o o o o o o o F-F' o o ? ? o o o o o o o o o o o 000 o o o o o • 0 o o o S 0 o o o o -0 o o o o o • 0 o o ,0 • 0 o 0- 0 o - 0 o o 3 o o o o . 0 o o o o '-' t::::::=O=~~O::::>O o o o o o o o 4 o o H-H' o o o o o o o o o o o o o o 0-==::J===-=--° 4 o o o <, . o ° o o o o o a o o o o o o o o a o o o o o 0 o 0 0 o 0 o o 0 o o o o o ° o o o o o o 0 o o o o o o o o o o o ? ? o 0 2 o 0 , 0-0' .-\ -A E- E' 43 sippian, ·Iadison Fonnation and grayish-green clasts of intramicrite originating from ~~e Cambrian, Gallatin Formation. Higher in most sections, white to gray oosparite clasts of the Cambrian, Pilgrim Limestone are present. Toward the top of some exposures clasts of granitic gneiss and migmatite occur and represent dissection of the Precam brian core of the Beartooth I1ountains. Mixed igneous and metamorphic conglomerate typically displays the following first occurrence pattern. In lOl¥'er portions of sections clasts of silicified algal biolithite (Mississippian, fladison Formation) as well as undifferentiated carbonate from Paleozoic formations are present. Higher in most sections intramicrite from the Cambrian, Gallatin Formation occurs. Near the top of most sections, clasts of monzonite porphyry, derived from Cretaceous intrusives emplaced contemporaneously ,,>lith uplift (Rouse, 1937 i Eckel mann and Poldervaart, 1957; and Harris, 1959), are present. Cla.sts of monzonite porphyry are the youngest clasts present and occur only in exposures north of Bennett Creek (Fig. 1). Although correlation between measured sections is inexact due to the discontinuous nature of tabular deposits, the following lateral aspects of first occurrence are inportant to consider. Pelecypod biosparrudite clasts (Jurassic) appear lowest in exposures nearest the Beartooths immediately south of Bennett Creek and are encountered 44 stratigraphically higher in exposures near section A-A' (Fig. 1). Silicified algal biolithite (Mississippian) is first encountered at its lowest stratigraphic position near sections A-A' and F-F' ,~'here Lake Creek makes a sharp turn to the south, stratigraphically higher in the exposure in~ediately north of Clarks Fork (sec. D-D') and in its ,highest stratigraphic position in exposures immediately south of Bennett Creek (Fig. 1). Intramicrite (Cambrian) occurs in its lowest stratigraphic position in exposures east of the point. ''lherc Lake Creek turns south (sec. F-F') and in hiqher stratigraphic horizons of exposures west of Lake Creek (Fig. 1). The lateral shifting pattern of clast first occurrence sU9gests either (a) deposition of clasts by one dispersal system with a constantly changing position or (b) several dispersal systems acting more or less independ entlv causing changes in deposited sequences. ~o lateral pattern of first occurrence was observed in the mixed igneous and metamorphic conglomerate. r.1aximum Clast Size and Li tho logy Fig. 2 ShO\'15 the distribution of the averaqe maximum diameter (long uxia) of the ten largest clasts of the conglomerate and their dominant lithologies. A decrease in clast size to the east is apparent. Distribution of the dominant clast lithologies reveals two distinct areas of 45 deposition. One area, south of Bennett Creek, is dominated primarily by carbonate conglomerate. The other, north of Bennett Creek, is dominated bv igneous and metamorphic conglomerate. Thus, two separate and distinct conglomerate lithologic assemblages are present indicating two distinct areas of deposition. Maximum Quartz Grain Size The average size, in phi units, of the ten largest quartz grains (long axis) measured in thin section is distributed as shown in Fig. 8. Although the localities of Fig. 8 are unevenly distributed and do not cover the entire area, a general decrease in size away from the mountain front is apparent. Steidtmann (1971) showed that maximum quartz grain size of alluvial fan sandstones decreased away from the source area and was a function of the decrease in stream competence down the depositional slope. Paleocurrents A total of 1269 paleocurrents direction indicators (Table 4) were obtained from large-scale trough and tabular cross-stratification, clast imbrication, and conglomerate and sandstone channel trends. Since conglomerate channels radiating from tabular deposits are indicative of deposition in an alluvial fan environment (Steidtmann, 1971), 46 Fig. 8. Distribution of average size of the ten largest quartz grains in sandstone. 47 .201 R211 T9S .... ------T- MONTANA --I ....' .y' 14 TSIN TS7N EXPLANATION Average sIZe of ten largest quartz grains In sandstone 14 Locality number o -1-2 Average size I I I (If un I tS) Rl02W r- R'03W ...L SCALE -I 1/2 mIles N .5 0 I kllomete r S t T56N L Rl03W ...L Rl02W .J 48 paleocurrent determinations were made in two stratigraphic horizons per exposure. The purposes of obtaining readings in this manner are (a) to identify radial sediment dispersal patterns if they exist, and (b) to detennine general dispersal patterns for the entire area. The vector mean method of H.eiche (1938) 'vas used to analyze paleocurrent data. Table 4 shows the results of this analysis and includes the number of readings (n), azimuth of the resultant vector mean (x), and the magnitude of the resultant vector in percent (L) for each locality. Azimuths of the resultant vector means are shown in Fig. 1. Resultant vectors indicate radial patterns of dispersal in some stratigraphic horizons \'1hile others show a trend in one main direction. More significant trends may not be expressed due to the relatively small sample size and distribution of locations where readings were taken. Small sample size resulted from Ca) lack of good exposures, and (b) readings taken at adequate intervals to prohibit "interference" between localities. Average azimuths for the two lithologically distinct areas were calculated. Areas dominated by carbonate conglomerate, south of Bennett Creek, have an average azimuth of 084 degrees while areas dominated by igneous and meta morphic conglomerate, north of Bennett Creek, have an average azimuth of 107 degrees. Paleocurrent data as \'1el1 as 49 Table 4. Paleocurrent data Number of Azimuth of Magnitude of Loca1itya readings resultant vector resultant vector n x L (%) 1 44 101 47.4 2 43 058 86.8 3 33 089 50.0 4 43 093 54.3 5 41 159 71.8 6 47 116 58.3 7 50 113 52.1 8 42 088 86.3 9 47 129 80.3 10 51 104 69.7 11 54 047 84.7 12 50 054 82.6 13 48 089 83.7 aLoca1ities grouped and divided by underlining are from the same stratigraphic horizon. For example, localities 11, 12, and 13 are in the same stratigraphic horizon. n n x = arctan (£ n. sin X. /:E: n. cos X.), where i=1 1 1 i=l 1 1 n. = number of observations in each class 1 X. = mid-point azimuth of the ith class interval 1. n n L = (R/n) 100, where R = (~n. cos X.;) 2+ <'E:, n. sin X.) 2 1.=· 1 1. .... 1.=• 111. n = number of observations 50 'l'ab1e 4. (Continued) Number of Azimuth of Magnitude of I.Joca1itya readings resultant vector resultant vector n -x L (%) 14 ~ 45 054 83.0 15 52 077 68.3 16 38 066 86.7 17 49 052 42.8 18 51 117 72.6 19 44 108 59.4 20 41 109 66.7 21 46 091 54.5 22 40 131 79.0 23 47 122 73.7 24 42 083 64.3 25 42 065 70.2 26 48 141 67.1 27 50 114 58.3 28 41 106 62.7 Total 1269 088 62.3 Localities 1-16 728 084 69.3 Localities 17-28 541 107 57.5 51 conglomerate lithology suggest dispersal from two distinct source areas. The overall average azimuth of 088 degrees indicates that dispersal systems coming from the west deposited sediments in the area of study. Mudstone Chemical Analysis Neasham (1970) postulated pedogenesis on the Willwood alluvial plain as reflected by profile development of free iron, manganese, and aluminum within a mudstone sequence. Bull (1968) has observed soil profile development in modern alluvial fan sediments. In an effort to determine any aspects of soil formation within the area of study, quanti tative measurements were obtained on clay size «9¢), organic carbon, plus free iron, manganese, and aluminum (Table 5) in a red mudstone. Samples were collected at six inch increments from a mudstone interval 52 feet above the base of section E-E' (Fig. 1) within a sequence of interbedded tabular conglomerates, sandstones, and mudstones. Percent clay slnaller than 9¢ was obtained using a hydrometer technique described by Lambe (1967). Percentages of organic carbon were determined using the Wilkley-Black titration method modified by Peech et ale (1947). Percentages of free iron, manganese, and aluminum were analyzed with a Perkin-Elmer Model 305 Atomic Absorption Spectrophotometer and Recorder after extraction from samples by the sodium citrate-sodium Table 5. Quantitative data on free iron, manganese, aluminum, organic carbon, and clay in a selected red mudstone sequence Wt. % Free Wt. % Free Wt. % Free Wt. % Organic Wt. % Clay Sample Iron Manganese Aluminum Carbon «9¢) P-1 0.332 .0025 .0204 .039 2.139 P-2 3.112 .0112 .0683 .137 4.764 P-3 3.282 .0105 .0728 .110 13.249 P-4 1.106 .0084 .0321 .106 14.318 P-5 0.885 .0086 .0303 .014 10.334 U1 P-6 0.872 .0090 .0349 .123 12.358 to..) P-7 0.992 .0111 .0340 .132 14.185 P-8 1.980 .0150 .0360 .116 18.861 P-9 1.815 .0154 .0326 .079 5.643 P-10 1.288 .0156 .0277 .059 5.871 P-11 0.880 .0170 .0237 .113 6.204 P-12 1.025 .0165 .0250 .135 13.281 P-13 1.107 .0260 .0326 .083 3.270 P-14 1.116 .0250 .0312 .098 9.913 P-15 1.028 .0273 .0298 .074 1.914 Table 5. (Continued) Wt. % Free Wt. % Free Wt. % Free 'tit. % Organic Wt. % Clay Sample Iron ~anganese Aluminum Carbon «9¢) P-16 1.025 .0396 .0230 .029 9.862 P-17 1.717 .0187 .0254 .103 3.989 P-18 0.76J .0100 .0239 .085 3.431 P-19 0.568 .0077 .0195 .023 9.815 P-20 1.807 .0050 .0419 .003 5.977 P-21 1.252 .0052 .0371 .052 7.100 U1 w P-22 1.276 .0053 .0308 .055 3. 701 P-23 0.474 .0017 .0254 .077 8.953 P-24 0.278 .~030 .0174 .056 3.268 P-25 0.485 .0026 .0201 .028 2.843 Hean a 1.289 .0140 .0328 .081 8.272 aDoes not include samples P-l and P-25 which are sandstone. 54 dithionite method of Holmgren (1967). The vertical distribution of the analysis is shown in Fig. 9. Free iron has zones of accumulation at 0-12 inches and 36-48 inches; free manganese at 66-90 inches; free aluminum at 0-12 inches and 108-120 inches. Clay has zones of accumulation at 6-18 inches and 30-42 inches. Below 54 inches, percent clay smaller than 9¢ exhibits an alternating low-high distribution. Organic carbon accumulations are evident at 0-18 inches, 24-42 inches, and 54-66 inches. Neasham (1970) found a correlation between clay and free aluminum distribution. The sample correlation coefficient (Snedecor and Cochran, 1967) showed no direct relationship between clay distribution and free iron (r = .118), free manganese (r = -.053), free aluminum (r = .154), or organic carbon (r = .350) distribution. Therefore, profile distri bution of the chemical constituents is probably affected by factors other than clay content. 55 Fig. 9. Profile distribution of free iron, manganese, aluminum, organic carbon, and clay «9~) in a selected red mudstone sequence. 56 1.0 2.0 3.0 40 10 15 20 Wt X Free I ron I I I I Wt % Clay I I I -X-)(-l(- «9.) .01 .02 .03 .04 -A-A- Wt-% Free Manganese I I I I -.-a-.- Wt % Organic .05 .10 .15 .20 .02 .04 .011 .08 I I I Wt X Free Aluminum I I I I Carbon -0-0-0- -e-e- :' .s: ~ Sample CII 0 P-1 P-2 o • P-3 j~\ P-5 f / I 24 P-II . \ P-7 '-8 ~ I p-o P-lI ~. eo P-12 • k~ P-13 n P-14 1 I '-15 P-16 A • P-17 1111 P-1\I 108 '-20 P-21 120 p-u P-23 132 '-24 ,-u 138 57 DISCUSSION AND INTERPRETATION Provenance ',fWO major source rock lithologies are indicated by the distribution of conglomerate clast assemblages and sandstone grains. Carbonate clasts dominating conglomerates from exposures south of Bennett Creek are indicative of predom inantly carbonate source rocks. The relatively high percentage of carbonate grains in the matrix of the conglom erates and in sandstones as well as minor amounts of calcareous quartz sandstone further indicate source rocks that are predominantly carbonate. Igneous and metamorphic clasts dominating conglomerate assemblages in exposures north of Bennett ~rcek indicate an exhwned igneous and metamorphic source. Minor amounts of monzonite porphyry cla~ts, identical to intrusives emplaced in Precambrian and Cruru)rian rocks contemporaneous with major uplift of the Beartooths, also indicate an igneous and metamorphic source. The general decrease in ~aximum clast size and maximum quartz grain size to the east, volume increase of sandstones to the east, and wedging out of conglomerate units to the east indicate that detritus \Y'as deposited by current systems from the west. Paleocurrent data also indicate a source to the west. The above information indicates a source area with relief capable of supplying large amounts of rounded to well-rounded carbonate and igneous and metamorphic 58 boulders, cobbles, and pebbles, igneous and Inetamorphic and carbonate rock fragMents, and plagioclase feldspar in some areas. Sedinentary rocks of Paleozoic and Mesozoic age now exposed along the eastern flank of the Beartooth 110untains were a western source for some detritus including clasts of glauconitic pelecypod biosparrudite (Sundance Formation), silicified algal biolithite (Hadison Formation), intramicrite (Gallatin Formation), and oosparite (Pilgrim Limestone), as well as undifferentiated carbonates (mainly Paleozoics) and sandstones (from Paleozoic and t1esozoic formations). Paleozoic and rriesozoic sandstones were probably the source of most conglomerate matrix and sandstone detritus. The ratio of potash to sodic-calcic feldspars is higher in sandstones occurring south of Bennett Creek indicating a source from Paleozoic and r-1esozoic sandstones as well as igneous and metamorphic rocks. Igneous and metamorphic rocks exposed in the core of the Beartooth Mountains were a western source for clasts of pink and gray granitic gneiss, pink and gray migmatite, biotite gneiss, amphibolite, quartzite, quartz dolerite, and pegmatites. Clasts derived from Cretaceous monzonite porphyry also have their origin in the Beartooths. Plaqioc1ase grains from sandstones north of Bennett Creek have anorthite percentages from 13-16% and 33-38% which 59 are consistent with anorthite percentages of granitic gneiss, migmatite, and biotite gneiss (Eckelmann and Poldervaart, 1957), the dominant Precambrian rock types in the Beartooth l10un tains • Neasham (1970) concluded that Willwood clay minerals are largely detrital, derived from adjoining upland source areas. He found little variation in clay mineralogy of red, purple, and drab mudstones with marginal mudstones charac terized by illite dominance accompanied by abundant montmor illinite and subordinate kaolinite. Two red mudstones from the area of investigation are likewise characterized by illite and montmorillinite dominance and subordinant kaolinite. The writer agrees with Neasham's contention (1970) that the upland Will~tlood source area (more specifically the adjacent Beartooth Mountain area) was characterized by pedological modification of bedrock analogous to red-yellow Mediterranean type soils found in areas of semihumid climates under deciduous forest or shrub vegetation. Depositional Environments Alluvial Fan The location along the flank of the Beartooth Mountains, the coarseness of conglomerate clasts, the inverted stratigraphic first occurrence sequence of clasts, the decrease in clast size and quartz grain size to the east, 60 the increase in sandstone unit frequency and thickness away from the mountain front, plus certain sedimentary structures suggest deposition in an alluvial fan environment. (Steidtmann, 1971; Wilson, 1970; Sharp, 1948; Blissenbach, 1954; Tro\~bridge, 1911; Bluck, 1964 and 1965; and Neasham, 1970). The increase in thickness and frequency of sandstone units and the decrease in clast size away from the mountain front indicates a decrease in stream competence down the depositional slope (Lustig, 1963). At least two separate alluvial fan systems can be delineated based on clast petrology, stratigraphic relations, and paleocurrents. Stratigraphic relations are inexact among the five main Willwood exposures due to the discontinuous nature of deposits and locally covered intervals. However, stratigraphic equivalence between exposures of predominantly carbonate clasts and those of mainly igneous and metamorphic clasts is apparent. Therefore, two separate alluvial fans originating within distinct drainage basins in the source area were formed. Paleocurrent data also suggests development of two alluvial fans. The alluvial fan dominated by carbonate clasts, essen tially the three exposures south of Bennett Creek, was formed by stream- and sheetflooding as suggested by Neasham (1970). Streamflood deposits are represented by conglomerates containing large-scale trough-stratification and minor clast 61 imbrication with sheetflood deposits represented by massive to moderate tabular deposits usually lacking directional structures. Sheetflood deposits were formed when overloaded alluvial fan channels (streamfloods) became clogged and were diverted or overtopped so as to cover the fan surface with a sheet of coarse material (Allen, 1965). Moderate to violent stream- and sheetfloods are indicated by the variability in thickness of tabular conglomerates. The variability in stratification and imbrication laterally and vertically in the fanglomerate further suggests this range of depositional agents (Blissenbach, 1954). Uudstones were deposited as overbank material distal to major gra"rel accumulation. Braided distributary channels are represented by inclined pebble and cobble stringers enclosed in sand stone and trough cross-stratified sandstones (Bull, 196B and Doeglas, 1962). Shoestring-like conglomerate bodies radiating from tabular conglomerates indicate a radial distributary network characteristic of alluvial fans. Conglomerate lenses in the fanglomerate units indicate that flow was intermittent and a response to periods of high rainfall in the source area (Steidtmann, 1971). Carbonaceous shales were formed in isolated, paludal environments eiL~er on the fan surface in areas poorly drained by distributary channels and not affected by sheet floods or on the fanbase margin (Blissenbach, 1954) where 62 alluvial relief described by Neasham (1970) could be adequate to impede drainage and create water tables near the depositional surface thereby favoring a reducing environment and preservation of organic matter. Stream- and sheetfloods were also responsible for formation of fanglomerate deposits dominated by igneous and metamorphic clasts. The presence of only minor amounts of carbonate clasts suggests that the Precambrian core of the Beartooth ~1ountains had been dissected and was the major source for sediments deposited in tile fan. The presence of monzonite porphyry clasts suggests that fan development was near the end of primary uplift since the porphyry was intruded into Precambrian and Cambrian rocks during the orogeny. Alluvial fan development may be related to major tear faulting in the Beartooths, such as the Maurice Tear Fault (Casella, 1964), after major uplift was completed. Large-scale cross-stratification and interbedded lenticular sandstones and conglomerates suggest deposition by moderate stream- and sheetf1oods. The lack of many overbank mudstones further suggests these depositional agents (steidtmann, 1971). However, chaotic orientation of clasts in some localities suggest that violent sheetfloods occurred frequently. Clay blebs and balls in sandstones of both fanglomerates suggest redeposition of overbank material by ephemeral streams flowing across the surface of 63 bob~ fans (Karcz, 1969 and Steidtmann, 1971). Soil development upon the aggrading alluvial fan surface is suggested by the profile development of clay «9fZS), free _ iron, manganese, and aluminum (Fig. 9) within a mudstone interval. Textural variations could have developed by depositional or pedological processes. Below 54 inches the alternating high-low distribution of percent clay smaller than 9¢ is indicative of depositional processes in overbank material. Above this horizon, zones of accumulation of clay, free iron, aluminum, and to a lesser extent manganese indicate translocation of mobile components from higher levels and concentration in lower levels. Translocation was probably caused by fluctuations in the water table (de Leon, 1961; Neasham, 1970; and Mohr, 1944) and/or infiltration of run-off characterized by a rate of sedimentation sufficiently low to allow soil-forming processes to be effective thus modifying any new sediment (Nikiforoff, 1949). Calcium carbonate concretions support the contention of a fluctuating water table (fJIohr, 1944 and Van Houten, 1948). Alluvial Plain Outcrops east of Sugarloaf Butte and three small exposures south of the to\'ln of Clark exhibit features commonly found in alluvial plain sediments. The absence of very coarse detritus, distance from the mountain front, and small amount of overbank deposits all indicate deposition as part 64 of a broad expanse of sediments paralleling the trend of the Beartooth ~ountains (Steidtmann, 1971 and Allen, 1965). Trough cross-stratified sediments similar to those of braided streams described by Doeglas (1962) indicate that braided streams traversed the alluvial plain with ease ~nd reduced overbank deposition to a minimum. Fig. 10 shows block diagrams of the Willwood fanglomerate that are idealized to show suggested stratigraphic relations. 65 Fig. 10. Block diagrams of Willwood fanglomerate. Diagrams are idealized to show suggested major stratigraphic relations. 66 BLOCK DIAGRAMS OF WILLWOOD FANGLOMERATE T58N T57N...... EXPLANATION N QUATERNARY SCALE GJ FEET woo 1°0 ::r /: ~~TATION - CONGLOME • • • 1- --I RATE SAN - M [)STONE . FQ'RT UNION r::::::-:_{ORMATION r:r PAr:ZT PREC::= o ~ G' 67 CONCLUSIONS 1) Willwood sediments were deposited in two main alluvial fans, delineated by conglomerate clast lithology, stratigraphic relations, and paleocurrent data, during uplift of the Beartooth Mountains. 2) First occurrence patterns of conglomerate clasts illus trate the stripping of Mesozoic, Paleozoic, and Precambrian rocks from the uplifted Beartooths and deposition in inverted stratigraphi9 sequence. 3) Two main fanglomerate assemblages are present and consist of (a) primarily carbonate clasts derived from Paleozoic and Mesozoic formations, and (0) primarily igneous and metamorphic clasts derived from the Precambrian core of the Beartooth uplift and Cretaceous intrusives. 4) Sandstones contain high percentages of feldspar (mean 27.3%) and miscellaneous constituents (mean 18.2%) reflecting proximity to source area. Variations in feldspar composition mirror differences in conglomerate clast lithology. 5) High-gradient, overloaded, ephemeral streams originating in the Beartooth uplift produced stream- and sheetflood deposits containing large-scale tabular and trough cross-stratification, horizontal stratification, and clast imbrication. 68 6) Pedogenesis in a selected alluvial fan overbank mudstone is indicated by the profile development of clay smaller than 9¢, free iron, aluminum, and manganese. 69 ACKNOWLEDGE!-mNTS This study was supported by the National Science Foun dation grant GA-15996 awarded to Dr. Carl F. Vondra of the Department of-Earth Science, Iowa State University. His assistance in the field and suggestions during the prepa ration of the manuscript were important to the completion of the project. Special thanks to the members of my committee; Dr. John Lemish and Dr. Lyle V. A. Sendlein of the Department of Earth Science and Dr. David V. 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