REVISION OF THE BECKWITH AND BEAR RIVER FORMATIONS OF SOUTHEASTERN IDAHO. By G. R. MANsFIELD and P. V. RouNDY. INTRODUCTION AND SUMMARY. exposed in the region of their type localities. In the detailed geologic mapping of the Descriptions of the Beckwith formation in the Wayan and Montpelier quadrangles, in south­ areas where it has been studied show consider­ eastern Idaho and adjacent territory, it has able variety in lithologic character. The fossil been found necessary to apply new names to content is ·meager, and there is uncertainty strata hitherto referred to the Beckwith and about the upper limit of the formation. Bear River formations or to portions of the The Bear River. formation has long been Laramie as mapped by the Hayden Survey. known and has been studied by many geolo­ The two quadrangles are contiguous, and the gists. It has a distinctive fauna and nearly Montpelier quadrangle lies south of the Wayan. as distinctive a lithology. The lack of agree­ They occupy the corner of the State and include ment between the beds in the Wayan quad­ a very narrow strip in northeastern Utah and rangle heretofore called Bear River and the a somewhat broader strip in western Wyo­ typical Bear River is so marked as to raise the ming. They lie between parallels 42° and 43° question whether any of these beds can prop­ N. and meridians 111° and 111° 30' W. The erly be assigned to that formation. The beds Wayan 30-minute quadrangle, the topographic in the Wayan quadrangle resemble somewhat map of which has not yet been published, con­ the Kootenai of Montana and Canada and may sists of four 15-minute quadrangles, the Free­ in part represent that formation. They may dom, Lanes Creek, Crow Creek, and Slug Creek. furnish a connecting link between the Kootenai Topographic maps of these areas are now avail­ and the Bear River. able. The strata in this area that were formerly The formations to be discussed extend from called the Bear River are here assigned to the the northeastern part of the Montpelier quad­ Wayan formation, of Cretaceous, possibly rangle northward through the eastern part of Lower Cretaceous age, and the so-called Beck­ the Wayan quadrangle and thence northward with is divided into seven formations, of which an undetermined distance, possibly including the lower two are marine formations of Juras­ a considerable part of the Caribou Range. sic age, and the remaining five are nonmarine They include about 17,000 feet of strata, unless formations assigned to the Gannett group, of there are unrecognized repetitions by folding or Cretaceous (?) age. This paper gives a state­ faulting. ment of the stratigraphic problems involved Two large groups of beds are recognizable and a description of the formations. The in the field. At first, in accordance with earlier writers are indebted to Mr. T. W. Stanton for interpretations, these groups were assigned examination of the fossils and for helpful sug­ respectively to the Beckwith formation, of gestions. Cretaceous and Jurassic age, and the Bear GENERAL STRATIGRAPHY OF THE AREA. River formation, of Upper Cretaceous age. On the maps of the Hayden Survey both are The stratigraphic sequence in southeastern included in the Laramie. There is, however, Idaho is remarkably full, all the great Paleo­ considerable lack of ~greement both lithologic­ zoic and later systems being represented. The ally and faunally between these formations as Paleozoic formations are identical in name and exposed in the district under discussion and as number with the corresponding formations of 30830°-17---6 75 76 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. northern Utah described by Richardson,! those of the Utahsection. The subjoined table except that the Threeforks limestone has not shows the general character of the Mesozoic been recognized. The Idaho formations dif­ formations of the region and the relations of fer somewhat in thickness and character from the formations here discussed to the others. · Mesozoicjormations in southeastern Idaho. Thickness System and series. Formation. Character. (feet). Sandstones, shales, carbonaceous shales, Cretaceous (Lower Cretaceous?). Wayan formation. limestones, some conglomerate _______ . 11,800 Unconformity ryg~e sandstone ................ 100 Draney limestone ..•.... _....... _ 200 Cretaceous (?). Gannett group. Bechler conglomerate ..... _~ . ___ 1, 775 Peterson limestone_ .... ___ ..... _ 205 Ephraim conglomerate_ ... ____ .. 1,025 --- 3,305 - Greenish-gray sandstone, with massive Stump sandstone. calcareous bed at base _. __ .... _______ . 200-600 Jurassic. Preuss sandstone. Red sandstones and shales ..... _... __ . _. 1,300 Unconformity Twin Creek limestone. Whitish shaly limestone, some massive beds .. --·-·.·--- ............. ··--·--- 3,500± Subdivided into four members: Red sandstone, with heavy grit at base, sue- Jurassic or Triassic. Nugget sandstone. ceeded by cherty limestone and red shale. Raymond Canyon section ....· . 1, 900 Variable, yellow sugary sandstone, thin to massive bedded, or red or chocolate- Ankareh formation. colored shale and sandstone, or mottled clays and shale with some sandstone and limestone. __ . _.... _.... _......... 750± Triassic (Lower Triassic). Limestones, calcareous sandstones, and Thaynes limestone. shales .. _...... __ ...... _......•... _.. 2,000± Thin-bedded calcareous shales and lime- Woodside shale. stones .........· .•.................... 1,500 THE BECKWITH PROBLEM. is largely derived from the three principal The Beckwith formation has been identified published descriptions, which represent more at several localities in western Wyoming and or less separated districts and contain differ­ adjacent parts of Idaho by a number of geolo­ ences sufficient to warrant a brief discussion. gists, but present knowledge of the formation Their main features are set forth in the follow­ ing table, which includes also the correlations I Richardson, G. B., The Paleozoic section in northern Utah: Am. 1our. Sci., 4th ser., vol. 36, pp. 406-416, 1913. given in the present report: Tentative correlation of the Beckwith and Bear River formations in southeastern Idaho and southwestern Wyoming. Idaho-Wyoming border (Breger, U. S. Lincoln County, Wyo. (Schultz, Age. Southwestern Wyoming (Veatch, U. S. Geol. Survey Southeastern Idaho (Mansfield and Roundy). Geol. Survey Bull. 430, pp. 562-563, Prof. Paper 56, pp. 57-61, 1907). U.S. Geol. Survey Bull. 543, 1910). pp. 30, 52-55, 1914). Upper Cretaceous. Not recognized. Bear River. Bear River. t:i:i t:zj Unconformity----- 0 P:l Western area. Eastern area. ~ 1-1 1-3 Wayan formation: Feet. Ill Cretaceous (Lower Alternating sandstone and Cretaceous?). shales ............... 9, 000 Beckwith: Upper > and lower phases ~ Sandstones, shales, and Beckwith: UpJ?er mem­ t:1 limestones:...... 2, 800 Higher beds not recognized. ber time eqmvalent of merge. Predom­ inant r e d d ish t;d Unconformity---! I Lower Cretaceous and color but all light Not represented or in­ t;:l Dakota(?); consists of cluded with Bear River. > Gannett group: light-colored sand­ color; conglomer­ !;d ates in this sec­ t:d Tygee sandstone ....... 100+ stones and clays; 3,000 tion are white to Draney limestone ...... 200 feet. Cretaceous (?). yellow. Entire l'j< Bechler conglomerate .. 1,775 Beckwith: Feet . t:d Peterson limestone .... 205 Upper gray limestone .. 100 section a b o u t 4,000 feet. l'%j Ephraim conglomerate .. 1,025 Conglomerate .......... 850 0 ~ > Beckwith considered 1-3 1-1 equivalent of Veatch's 0 eastern section, but all ~ Beckwith: Lower red­ r/1 bed member c om­ Jurassic. Light-colored Stump sandstone........ 200-600 Lower gray band shale, sandstone, and 0 posed of sandy clays, Beckwith: Lower l'%j sandstone. 250-600 sandstones, and con­ part of section. clay with associated Preuss sandstone........... 1, 300 Red sandstone. 1, 200 beds of white to light­ r/1 Jurassic. glomerates; 2,500 feet; 0 Upper Jurassic. yellow and red con­ q glomerate and black 1-3 limestone; 900-2,400 P:: l'j feet. > Unconformity I 1-------- r/1 ~ Twin Creek limestone. Twin Creek. Twin Creek. Twin Creek. Twin Creek. t:d ~ 1-1 t:;:j > P:: 9 -::r -::r 78 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. Veatch, 1 who defines and describes the forma­ shale, sandstone, and clay. The sandstone is usually tion, divides it into two unnamed members­ light yellow, pink, or white, and the clay varies from " a lower red-bed member composed of inter­ yellow or light pinkish red to dark purple. With the sandstone, shale, and clay are associated beds of white to bedded sandy clays, sandstones, and conglom­ light yellow and red conglomerate and black limestone. era.tes 2,500 feet thick, and an upper member In places * * * conglomerate beds constitute an composed of rather light-colored interbedded important part of the formation. sandstones and clays with a thickness of 3,000 Fossils are not numerous, but Schultz ob­ feet or more." In the eastern exposures tained collections from four localities. These within his district these two phases merge. were examined by Stanton, 'Yho referred three "Just west of Hilliard these beds, while having of the collections to the Jurassic and stated that a predominant reddish cast, are a~ light in the fourth was not sufficient for age determina­ color. The conglomerates, which near Beck­ tion but suggested Jurassic. According to with are deep red, are here white to yellow." Schultz two of the lots "come from the top of Veatch appears not to have found fossils in the the Beckwith and indicate. that it is Upper Beckwith, but he refers to this formation col­ Jurassic." The thickness of the formation in lections of marine Jurassic fossils previously the region ranges from 900 to 2,400 feet. made by Stanton at two localities, one of Lithologically the descriptions given by which was outside of the district described. Veatch and Schultz agree fairly well, except On the evidence of these fossils he refers the that in Schultz's area the red conglomeratic lower part of the Beckwith to the Jurassic; phase seems not to be well developed.