RELATIONS OF THE E~1BAR AND CHUGWATER FORMATIONS IN CENTRAL WYOMING.

By D. DALE CoNDIT.

FIELD DATA. overlying red beds composing the Chugwater The information set forth in this paper was formation. obtained in field work during the seasons of The Embar has in recent years attracted at­ 1913 and 1915. During 1913 the writer was tention on account of its yield of petroleum in engaged in the detailed mapping of the phos­ the Wind River basin and also on account of its phate beds of the Embar formation on the phosphate beds, which· have been traced and northeast slope of the Wind River Mountains mapped by the United States Geological Sur­ and in the Owl Creek Mountains as far east as vey throughout the Wind River and Owl Creek Bighorn River canyon. In 1915 the mapping mountains in the course of land classification. was continued eastward as far as Holt, from The stratigraphy of the Embar formation as which a reconnaissance examination was made found in the Wind River and Owl Creek moun­ east and north along both flanks of the Bighorn tains has been outlined in a reconnaissance 2 Range as far as the latitude of Tensleep. A report by Eliot Blackwelder, and those areas visit was also made to the west end of the will be only briefly reviewed in this paper, the Rattlesnake Mountains and neighboring points object of which is a consideration of the forma­ in Natrona County, to the Conant Creek anti­ tion in its eastward extensions from the Owl cline, in the eastern part of Fremont County, Creek Mountains into the Bighorn Mountains and to the Sheep Mountain anticline, in the and from the Wind River range into the Bighorn Basin. Rattlesnake Mountains. (See Pl. LXI.) As found in typical development, the Embar GENERAL CHARACTER OF THE EMBAR consists of two principal parts, of ·which the FORMATION. upper is largely shaly and the lower is chiefly The term Embar was introduced by Darton 1 limestone, but includes phosphatic and cal­ to designate strata believed to be of " P ermo­ careous shale and nodular chert. The name Carboniferous" age and lying between the Ten­ Park City has been adopted by the United sleep sandstone (Pennsylvanian) and the Chug­ States Geological Survey for the lower part, on water formation (red beds of Triassic~ age) in the the recommendation of Blackwelder, who iden­ Owl Creek Mountains of Wyoming. The type tifies these beds with the Park City formation locality is at Embar post office, about 25 miles of Utah. He has also suggested the name Din­ west of Thermopolis, where the formation is woody formation for the upper shaly part, from about 250 feet thick and consists largely of Dinwoody Canyon, in the Wind River Moun­ shaly to massive marine limestone, for the tains, where the formation, about 200 feet thick, most part fossiliferous. Followed laterally consists of pale-green to white clay and shaly eastward, the marine limestones gradually give limestone weathering brown and containing ob­ place to red shales containing many gypsum scure pelecypod shells. The Park City and Din­ beds. In the Bighorn Mountains, 70 miles woody beds as found at these localities are de­ east of the type locality, the transformation of scribed by Blackwelder in a bulletin on the the Embar beds is so complete that they can stratigraphy of the Wind River Mountains, only with difficulty be distinguished from the submitted for publication by the United States

1 Darton, N.H., GeologyoftheBighornMountains: U. S. Geol. Survey 2 A reconnaissance of the phosphate deposits in western Wyoming: Prof. Paper 51, p. 35, 1906. U.S. Geol. Survey Bull. 470, pp. 452-481,1911. 263 264 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916.

Geological Survey. The somewhat meager accompanying diagram (fig. 26). It is believed evidence at hand seems to indicate that the that the Embar includes strata ranging in Dinwoody beds are of Triassic age and are to age from Pennsylvanian to Triassic, and that be correlated with the Woodside shale and its beds in the Owl Creek Mountains (see Pl. Thaynes limestone of southeastern Idaho. LXII, A) grade laterally into beds that Toward the east the Dinwoody beds thin were included in the lower part of the Chug­ and change to gypseous greenish or brownish water formation as interpreted by Darton in shales devoid of fossils. Outcrops of this char­ the Bighorn Mountains. The beds as followed acter are found in the vicinity of Bighorn River northeast approach the Chugwater more and canyon near . Thermopolis. Still farther east more closely in lithologic character, and a the beds become increasingly gypseous and difficulty arises in determining just how far the assume a lithology that can only with difficulty name Embar as a formational designation be distinguished from that of the Chugwater. should be carried. Certainly the Embar facies There is likewise a gradual transformation in is recognizable for some distance northward the Park City beds as followed eastward from into the Bighorn Range, and throughout this the Owl Creek and Wind River mountains. part of the field the term Chugwater should be These changes may be summarized briefly as restricted to the overlying red beds, probably follows: (1) Increasing thickness; (2) lithologic of true .Triassic age, rather than to all the ~eds, changes, including thinning of limestone beds as the term is applied in the Laramie Range and and an increasing amount of shale almost en­ Black Hills, where strata apparently as old as tirely of red color; (3) the appearance of beds of Pennsylvanian are included. massive white sediment-free gypsum in all but FACIES OF THE EMBAR FORMATION. the basal portion; (4) the gradual eastward dis­ appearance of marine fossils except in the basal ILLUSTRATIVE SECTIONS. beds of the formation; (5) the increasing preva­ The lithologic character of the Embar in its lence of calcareous conglomerates at the prin­ typical development in the Wind River and cipal limestone horizons; (6) the disappearance Owl Creek mountains and in its gradual .east­ of concentrated beds of phosphate rock. With ward gradation into gypsiferous red beds in the these changes the equivalents of the Park City Bighorn Mountain region (Pl. LXII, B) is illus­ and Dinwoody units as found in the Bighorn trated by the series of sections in figure 25, and Mountains so little resemble the typical Embar ·also by figure 26, a diagrammatic cross section of the Owl Creek Mo~ntains that the relation from the Wind River Range northeastward to would hardly be suspected. The same is true the Bighorn Mountains. The first section in of the Park City beds in the Rattlesnake Moun­ figure 25, measured in Bighorn Canyon near tains as compared with those in the Wind River Thermopolis, is with slight modifications repre­ Range. The one member common to all locali­ sentative of the marine facies of the Wind ties is the nodular chert of the upper part of the River and Owl Creek mountains. The second Park City beds. It is largely through the section was measured near the head of No persistence of this rock that the relations of the Wood Creek, T. 41 N., R. 89 W., near the . Embar and Chugwater beds have been deter­ southwest extremity of the Bighorn Mountains. mined. The conditions of deposition here were evi­ The eastward thinning of the Dinwoody for­ dently intermediate between the marine con­ mation and its apparent disappearance at ditions of the west and the shallow-water or about the head of No Wood Creek, in the south­ lacustrine conditions that prevailed farther ern part of the Bighorn Mountains, may be· east. The third section, measured at the south attributed to nondeposition, although it is pos­ end of the Sheep Mountain anticline, near sible that the beds have been eroded, or, less Greybull, illustrates conditions not greatly probably, that they are present but so closely different from those indicated at the No Wood resemble the Chugwater red beds in lithology Creek locality. The presence of fossils at sev­ as to be unrecognizable. eral horizons records temporary marine inva­ The character and relations of the Embar and sions which alternated with partial emergence associated strata in the Wind River, Owl Creek, or shoaling, thus favoring evaporation and con­ and Bighorn mountains are represented in the centration into beds of gypsu~ and possibly U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE LXI

Base from U. S. G. S. map of Wyomin g A HOLN a co e.l\l"TIMOAE , ,...O. MAP OF CENTRAL AND NORTHWESTERN WYOMING SHOWING OUTCROPS OF EMBAR FORMATIO

In the Bighorn Mountains and n orth of Cody, w h ere the Embar formation Is thi n or possibly lacking, the outcrop of he Ch ugwater for mation 1. I ncl u d d s JO 0 lO

tO 0 )0 40 :;o J{ilom t :•s =-1

1916

Embar formation Embar and Chugwater formations RELATIONS OF THE EMBAR AND CHUGWATER FORMATIONS, WYOMING. 265

Sheep ~ol.!ntain; ant1CI1ne

Bighorn River· Near head of canyon No Wood Creek

LEGEND Ten sleep Canyon

LimestoneI =. - NodularI chert

-

RedII shale

200' Gypsum

Sandstone

160' -...... --

·······

. , ,,

FIGURE 25.-Sections of the Embar formation in central Wyoming. 266 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. other salts. The fourth section, measured at horizons, one in the upper part and the other Tensleep Canyon, is representative of the for­ near the base. The lower bed is of consider­ mation as found on the west slope of the Big­ able value in the Wind River Range but is horn Mountains. Red shales with many thick missing in the Owl Creek Mountains. beds of gypsum and insignificant beds of lime­ Section of Park City formation on South Fork of Little Wind stone make up the section. Similar conditions River, on the northeast slope of the Wind River Mountains. are recorded on the east slope of the same Ft. in. Limestone, chalky in upper portion, dolomitic range. The local detailed evidence of the and cherty, grayish below... _____ ._. __ ...... 12 0 several facies outlined is presented below. Limestone, brownish gray, crystalline, fossilifer- ous; Leioclema, Fenestella, Spirijerina pulchra, MARINE FACIES OF WIND RIVER AND OWL Derbya, and small crinoid segments plentiful._. 22 0 CREEK MOUNTAINS. Chert, nodular, in bluish-gray shaly matrix...... 14 0 Shale, siliceous, bluish gray and cherty near top, The Park City portion of the Embar forma­ drab to sepia brown, with phosphate bands, in tion in the Wind River and Owl Creek moun­ lower part. ______.. _. ___ ...... _- . - - . . 37 8 tains consists largely of limestone with calca~ Upper phosphate bed: reous shale and nodular chert. The general Phosphate, dark, oolitic, sandy; Pugnax only sequence, in ascending order, is (1) dolomitic fossil recognized (tricalcium phosphate, 41.14 per cent)_ . __ . ___ ... __ . _. _...... - . - 1 4 gray fossiliferous limestone, resting with slight Phosphate, sandy; Productus nevadensis and unconformity on the Tensleep sandstone; (2) Pugnax common (tricalcium phosphate, argillaceous shaly limestone and sandy calca- 43.74 per cent) ...... _... _...... -. 1 6

Oinwoody Canyon Bi~horn River canyon Wind River Mts Owl Creek Mts reet~l~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

FIGURE 26.-Diagrammatic cross section from Wind River Mountains to Bighorn Mountains, Wyo. reo us shale of yellowish color; (3) limestone Limestone, brownish gray, thin bedded; Productus with geodes, chert masses, and many fossils, nevadensis and Spirifer aft. S. cameratus plenti- ful. In the lower portion are abundant speci­ among which are Productus nevadensis, Leio­ mens of Chonetes aff. C. geinitzianus __ . _...... 12 0 clema, Spirijerina pulchra, and Productus sub­ Limestone, massive, yellowish brown, crystalline; horridus; (4) phosphatic shale of sepia-brown many fossils, Leioclema especially abundant .. _ 11 0 color, with one or more layers of granular phos­ Limestone, dolomitic, interbedded with lami­ nated chert and sandy shale .... _. _... . - .... -- 84 0 phate rock and phosphatic limestone at the Limestone, gray, coarsely crystalline, massive; base; (5) nodular greenish phosphatic chert abundant molds of diminutive pelecypods and interbedded with many shale laminre; (6) sili­ gastropods ... _...... -...... -...... 31 0 Lower phosphate bed: ceous greenish limestone grading up into gray Limestone, phosphatic, gray, with dark spots; resistant limestone, with abundant fossils, in­ Lingulidiscina utahensis present .. - ...... 1 0 cluding Derbya, Pseudomonotis, Leioclema, Phosphate rock, calcareous, dark, granular and Spirijerina pulchra. These and other (tricalcium phosphate, 36.51 per cent) .. -.. 6 Limestone, phosphatic, dense textured, dark; forms mentioned in this paper were identified a fossil, probably Composita, present._ .. -. 10 by G. H. Girty. Phosphate rock, calcareous; green granules of The detailed stratigraphy in the Wind River glauconite(?) abundant; phosphatic shells Mountains is further illustrated by a section of Lingulidiscina abound (tricalcium phos- phate, 48.13 per cent). _ ...... -- ...... -- 1 0 measured on South Fork of Little Wind River Phosphate rock, dark, calcareous; few traces in T. 1 S., R. 2 W. Wind River meridian. It of fossils (tricalcium phosphate, 39.92 per will be noticed that phosphate occurs at two cent) _ ... --- .... - .. ----... - - - . ------· · 8 U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE LXII

A. EMBAR AND ASSOCIATED FORMATIONS ON SOUTH FLANK OF OWL CREEK MOUNTAINS NEAR SHOT GUN CREEK, WYO., IN T. 7 N., R. 1 E. Upturned shales of Chugwater formation at left; resistant upturned I imestone of Park City formation at right, underlain by Tensleep sandstone, which forms the highest hill in the foreground at right.

B. RED BEDS IN MIDDLE OF EMBAR FORMATION ON EAST SLOPE OF BIGHORN MOUNTAINS NEAR BARNUM, WYO., IN T. 42 N., R. 84 W. At top of hill is calcareous conglomerate. The beds below consist of structureless friable sandy red shale with irregular lamince of limestone, much distorted and faulted. These beds rest on even, undisturbed layers of limestone that form the basal part of the Embar formation. Thickness to top of hill is about 90 feet. U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE LXIII

A. BED AND VEINLETS OF GYPSUM IN LOWER B. NODULAR CHERTY SHALES OVERLAIN BY LIME­ PART OF EMBAR FORMATION NEAR SOUTH END STONE IN UPPER PART OF EMBAR FORMATION OF SHEEP MOUNTAIN ANTICLINE, WYOMING. NEAR HEAD OF NO WOOD CREEK, WYO., IN T. 41 N., R. 89 W. About 17 feet above the 4-foot layer of gypsum is limestone filled with marine fossils. The veinlets of gypsum are At the base is phosphatic limestone in the geologic position of in structureless red shale. the principal phosphate bed found elsewhere. RELATIONS OF THE EMBAR AND CHUGWATER FORMATIONS, WYOMING. 267

Limest-me, granular, thick bedded, having small Ft. in. · cia ted Derbya and other fossils characteristic calcite vugs; granules of glauconite(?) present. 12 0 of the upper part of the Park City formation. Limestone and dolomite, argillaceous, gray, with No gypsum beds were discovered here, but cherty layers and localized green stains...... 31 0 Chert, white and gray, laminated...... 7 0 some have been noted by C. J. Hares at Alcova, Limestone, buff, dolomitic, resting on Tensleep about 35 miles to the southeast. sandstone ...... 4 0 SHALLOW-WATER FACIES. At certain points along the Wind River Mountain front the limestone beneath the lower Arid conditions, favoring concentration of phosphate bed is rich in cephalopod and gastro­ brines and deposition of thick beds of gypsum pod forms believed by Mr. Girty to indicate and probably other salts, prevailed in the Pennsylvanian age. 'I'he upper phosphate bed Bighorn Mountain region almost from the 1 contains the Spirijerina pulchra fauna, which is beginning of Embar time. · The basal 20 feet regarded as of Permian age. or so of strata resting on the nearly even upper surface of the Tensleep sandstone consist of TRANSITIONAL FACIES. dolomitic cherty limestones with a few marine The exposures at the Sheep Mountain anti­ fossils, overlain by green and red shales con­ cline and near the head of No Wood Creek taining locally a layer of impure fine-textured illustrate conditions intermediate between the curly brown limestone, apparently of algal marine facies of the west and the shallow­ Origin. Higher strata have little regularity water, arid conditions of the east. of sequence, but there is a predominance of In the Sheep Mountain region the fossilif­ arenaceous red beds which are gypseous erous limestones of the upper two-thirds of throughout and contain an increasing amount the formation show that deposition for the of gypsum toward the top. The thickest most part took place in sea water. A little gypsum beds in the upper part of the forma­ below the middle, however, persistent beds tion are locaUy 100 feet or more thick and con­ of gypsum (Pl. LXIII, A) alternate with the sist of numerous clean white layers 5 to 10 feet fossiliferous limestones, indicating intermittent thick, lacking mechanical sediments and alter­ restrictions of the sea and concentration of the nating with thin layers of red shale and water, which temporarily favored precipita­ laminre of dolomitic limestone, generally a few tion. The fine-textured arenaceous even­ inches thick. In the upper part of the forma­ bedded red shales of the lower part denote tion there is a persistent layer of nodular deposition for the most part at or a little below purplish-red chert in the midst of the gypseous sea level. The general absence of coarse beds. The chert bed bas been traced con­ sandy sediments here and elsewhere at the tinuously eastward from the Owl Creek Moun­ Park City horizon indicates that the land tains and is believed to lie at the same horizon areas supplying the sediments were reduced as the greenish chert above the phosphate bed nearly to base-level and that the products of of that region. erosion were carried by streams of low gradient. The gypsum beds are prominent at Tensleep The source of the sediment is a matter of Canyon, at Cherry Creek near Redbank, and at speculation but may have been land to the numerous other points on the west slope of east, for the beds thicken in that direction. the Bighorn Mountains, being 50 to 100 feet Evidence of depositional conditions equiv~ or more in thickness at a number of places. alent to those recorded near Greybull and at Deposits even thicker than this are found on the head of No Wood Creek is found at the the east slope of the mountains, along the sev­ west end of the Rattlesnake Range, inN a trona eral forks of Crazy Woman Creek, and to the County. The Park City beds, where observed south, along branches of Powder River. near the Jamison ranch, in T. 33 N., R. 88 W., One of the noteworthy features concerning are about 300 feet thick and consist largely the gypsum beds is that they are not persist­ of red shales with~ few impure limestone beds ent. They grade laterally into red beds and and one limestone member about 25 feet thick conglomeratic limestone of vesicular, tufa­ near the base. The upper part of the forma­ ceous texture. The gypsum and conglomerate tion contains much reddish-brown nodular beds lie approximately at the same horizon chert (Pl. LXIII, B), with which are asso- and are found in the same general vicinity, but 308300--17--,--18 268 SHOR.TER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. are not as a rule associated in any one section. would seem to indicate that they did, because The gradation from one to the other not un­ a certain amount of distortion is noticed even commonly takes· place within a distance of a in exposures where there is much gypsum. few hundred yards. This mode of occurrence The possibility of the former presence of saline is in large part a result of the conditions of deposits leads to the query whether those salts deposition, which favored prolonged evapora­ may not still exist down the dip far below the tion in numerous small basins, the margins of surface. which · were above water from time to time, The evidence at hand seems to indicate that permitting a drying and cracking of the cal­ · the gypsum beds were deposited under condi­ careous muds . and slight erosion. The frag­ tions somewhat different from those prevailing ments were rolled along and cemented by cal­ during the deposition of the gypsum beds found careous deposits of tufaceous character. The in the upper part of the Chugwater, which are conglomerate pebbles, consisting of red shale, described by Branson.1 The red beds (Chug­ friable sandy material, limestone, and chert, water), which are coextensive with the Embar all of local origin, occlir at several horizons. in Wyoming and range from about 1,000 to A peculiar result of weathering of this rock is 1,400 feet in thickness, are believed by Branson the removal, partly by solution and partly by to be for the most part of marine origin, and the wind erosion, of many of the pebbles, leaving presence of the gypsum in the upper part is the calcareous matrix of vesicular texture with regarded as pointing to marine origin for that rounded cavities as much as 4 to 6 inches in part of the red beds in most of Wyoming. The diameter. reasons for his conclusions are set forth as The nonpersistence of the gypsum beds is in follows: part attributed to solution in recent times. 1. Uniformity in thickness of beds over wide areas. Evidence supporting this conclusion is seen 2. Uniformity in texture of rocks over wide areas. along the east slope, where sink holes are not 3. Ripple marking on horizontal beds through most of uncommon, and also in the absence of gyp­ the formation. sum in exposures extending a short distance 4. Chemical precipitation of limestone at the 800-foot level. up the mountain slope. This feature is espe­ 5. Chemical precipitate of gypsum near the top over cially noticeable near No Wood and Redbank, wide areas and at various levels in many places. where there is an a bundance of bedded gyp­ 6. Absence of sun cracks and fossils of land animals sum up to about the 6, 700-foot contour and excepting in the Popo Agle beds. none at all in higher exposures. Stra.tigraphic 7. Presence of undoubted subaerial evidences in the Popo Agie beds, with textures and materials like much of measurements above and below this elevation the rest of the red beds. show a corresponding difference in thickness. The succession of events is summarized as The irregularity of the gypsum beds is also follows: in part attributable to partial solution at 1. The red beds began under marine conditions, and the different times in the past, when it has been sea gradually became more and more charged with calcium favored by the ever-changing ground-water carbonate and magnesium carbonate until a dolomitic conditions. The chief evidence of this proc­ limestone was precipitated. ess is the greatly distorted character of the 2. Above the limestone the sea gradually filled with sand rocks in all but the basal part of the formation. until the sediments were exposed, and the Popo Agie beds were formed under subaerial conditions. The distortion is noticed even in exposures 3. The sea in upper Triassic time readvanced and some along the base of the mountains. The arena­ 200 feet of sandstone and shales filled the western margin. ceous red clays are structureless and loosely 4. Subaerial deposition, mainly of wind-blown sand, cemented. Thin laminre of limestone distrib- succeeded and lasted while beds varying from a few feet to . uted through the section are faulted and tilted 60 feet in thickness were deposited . 5. The sea readvanced, but concentration of calcium in edgewise blocks. The evident disturbance sulphate had been in progress for a long time and soon of these beds and the undisturbed character of resulted in widespread deposits· of gypsum. the underlying beds indicate that there has 6. Usually some sandstone and some thin layers of lime­ been a widespread collapse through the removal stone were deposited above the gypsum before the with­ drawal of the sea a~ the close of the period. of soluble parts of the beds, perhaps at differ­ ent times. Possibly the soluble parts included 1 Branson, E. B., Origin of the Red Beds of western Wyoming; Origin of thick gypsum and salt deposits: Geol. Soc. America Bull., vol. 26, salt as well as gypsum. In fact, the data No. 2, pp. 217-242, 1915. RELATIONS OF THE EMBAR AND CHUGWATER FORMATIONS, WYOMING. 26'9

Darton's conception of the history of Chug- the rule during the later part of the epoch, water red-bed deposition is as follows: 1 and aridity favored prolonged evaporation of In the latter part of Carboniferous [Pennsylvanian] the inclosed salt-water basins. time, and probably during the Permian also, there was SOURCE OF MATERIALS. a widespread emergence, resulting in shallow basins with very wide mud flats which occupied a large portion of The source of the materials of the Te:q.sleep, the Rocky Mountain province. In these regions were laid down the last deposits of the Pennsylvanian division Embar, and Chugwater formations is largely a and the great mass of red clay and sands constituting the matter of speculation. That the Tensleep Chugwater formation. These beds probably were de­ sandstone was at least in part deposited in the posited by saline water under arid-climate conditions sea is indicated by its beds of arenaceous and accumulated in a thickness of 1,000 feet or more. dolomite, some of which contain fossils. The The waters were shallow much of the time, and there were wide, bare wash slopes and mud flats, as is indicated following collection was obtained from beds a by the frequent mud cracks, ripple marks, and impres­ little above the middle of the formation on the sions of various kinds on many of the layers throughout east slope of the Bighorn Range in T. 44 N., the formation. R.84 W.: In contrasting the Embar of the Bighorn Crinoid stems. Mountains with the overlying Chugwater beds Nucula levatiformis var. obliqua. the following features are noted: Deltopecten sp. 1. Limestone is much more plentiful in the Astartella subquadrata? Embar, but, aside from that which occurs in Schizodus ovatus. Plagioglypta canna? the basal portion, it is in smaU patches later­ Lrevidentalium? sp. ally alternating with thick beds of gypsum. Bellerophon aff. B. sublevis. 2. There is ample evidence of frequent Euphemus? sp. emergence and slight erosion d.uring late Bucanopsis? sp. Embar time, a feature not observed in the Phanerotrema aff. P. grayvillense. Chugwater. This is especially true of expo­ Mr. Girty reports that the fauna is too small sures on the east slope of the Bighorn Range. to warrant positive statement as to its rela­ The conglomerates are, in part, composed of tions. It shows a new and very indefinite travel-rounded pebbles, denoting contempo­ combination of specimens, which may be late raneous erosion. There are, however, other Pennsylvanian or, less probably, Permian. conglomeratic beds regarded as breccias. Elsewhere reedlike, columnar plant remains These consist of angular pebbles of various have been observed in the Tensleep sandstone. materials bound together by a spongy calca­ It has been found that over the greater part reous, travertine-like cement. The brecciated of central Wyoming there is a general south­ portion may be in part of secondary origin, ward direction of inclination of the lines of produced by the solution of gypsum and pos­ deposition, suggesting a possible derivation of sibly salt beds and the resultant collapse of the sands from the north; but this view is not the strata. Breccias and conglomerates such supported by the data as to thickness, for the as these have not been observed in any part formation at the north end of the Bighorn of the Chugwater formation. Range is 30 to 40 feet thick and increases south­ 3. Although the evidence of contempora­ ward to 200 or 300 feet in the Owl Creek Range neous erosion in the Embar denotes tempo­ and to more than 500 feet in the Wind River rary emergence, it is believed that the mate­ Range. rials were for the most part laid down beneath The slight thickening of the Embar forma­ the surface of the sea. The persistence of the tion eastward from Bighorn River canyon into nodular chert and the similarity of certain the Bighorn :Mountains has been mentioned. other beds over nearly all the area indicate There is likewise a gradual increase to the that throughout · Emb;tr time the sea waters southwest, where the beds become more and frequently extended from the west over the more calcareous. The change is not consid­ entire region. Reductions of 'the sea were erable westward along the Wind River Moun­ tain front, and, in fact, the formation thins I Darton, N. H., Paleozoic and Mesozoic of central Wyoming: Geoi. Soc. America, Bull., vol. 19, pp. 465-466, 1908. from about 430 feet at Dinwoody Canyon to a 270 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. little more than 200 feet about 50 miles formation, comprises several distinct facies, each of which farther northwest, across the divide, on Buffalo is considered in detail, and some of the formational boundaries in the Bighorn Mountain region are redefined. Fork of Snake River at a locality measured In connection with the description of the gypsum and by Blackwelder. associated strata it is suggested that conditions were possibly also favorable for accumulation of salt beds. The ABSTRACT OF PAPER. chance of finding such deposits down the dip below the The Embar formation of central Wyoming, which lies surface is believed to be sufficient to merit further inves­ between the Tensleep sandstone and the Chugwater tigation.