Triassic Stratigraphy of Southeastern Idaho and Adjacent Areas By BERNHARD KUMMEL A SHORTER CONTRIBUTION TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 254-H Studies in southeastern Idaho, western Wyo11ting, and southwestern Montana, revealing one of the thickest and most complete marine sequences of Lower Triassic faunal zones in the world UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954 UNITED STATES 'DEPARTMENT OF THE INTERIOR. Douglas McKay, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price $1 (paper cover) CONTENTS Page )lbstract------------------------------------------------------------------------------------------------------ 165 Introduction----------------~--------------------------------------------------------------------------------- 165 I>inwoodyformation___________________________________________________________________________________________ 167 VVestern VVyoming_________________________________________________________________________________________ 168 Southeastern Idaho________________________________________________________________________________________ 169 Southwestern ~ontana_____________________________________________________________________________________ 170 VVoodsideformation____________________________________________________________________________________________ 170 Southeastern Idaho---------------------------------------------------------------~------------------------ 170 VVestern VVyoming ___________ ~--------------------------~------------------------------------------------T- 171 Southwestern ~ontana_____________________________________________________________________________________ 171 Northern Utah __________________________________________________________________________________________ ·__ 171 Thaynesformation---------------------------------------------------------------------~----------------------- 171 Southeastern Idaho________________________________________________________________________________________ 172 Fort Hall Indian Reservation to Grays Range ___________________________ :__________________________________ 172 Eastern Bear Lake to Sublette Ridge____ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 17 3 Bear River Range_____________________________________________________________________________________ 175 VVestern VVyoming ____________________________________________________________ :____________________________ 17g Southwestern ~ontana_____________________________________________________________________________________ 177 Central 'Vasatchand Uinta ~ountains_______________________________________________________________________ 177 Post-Thaynes Triassic------------------------------------------------------------------------------------------ 179 Central VVasatchand Uinta ~ountains_______________________________________________________________________ 179 Southeastern Idaho________________________________________________________________________________________ 180 VVestern VVyoming_________________________________________________________________________________________ 181 Chugwater formation of central VVyoming_ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ ___ __ _ _ _ __ __ _ __ __ _ _ _ __ __ _ _ _ _ _ _ __ .181 Age and correlation of formations of Triassic age__________________________________________________________________ 182 Literature cited------------------------------------------------------------------------------------------------ 188 Index-------------------------------------------------------------------------------------------------------- 191 ILLUSTRATIONS [Plates 34-40 in pocket] Pan PLATE 34. Composite correlation diagram of formations of Triassic age in ~ontana, Idaho, VVyoming, and Utah. 35. Correlation of Dinwoody and VVoodside formations between Paris, Idaho, and the VVind River ~ountains, VVyo. 36. Correlation of Dinwoody and Woodside formations between Provo, Utah, and Henry, Idaho. 37. Correlation of formations of Triassic age in southwestern ~ontana. 38. Correlation of the Thaynes and overlying formations of Triassic age between Fort Hall, Idaho, and Swift Creek, Wyo. 39. Correlation of the Thaynes and overlying formations of Triassic age between the Bear River Range, Idaho, and Muddy Creek, Wyo. 40. Correlation of outcrop sections of the Thaynes formation in the central VVasatch and Uinta Mountains, Utah. 'FIGURE 18. Correlation chart of Triassic formations in Montana, Idaho, VVyoming, and Utah___________________________ 166 19. Diagram showing northeastward overlap of Triassic formation on Permian formations in Idaho and VVyoming___ 169 20. Correlation diagram of the units of the Thaynes formation between Fort Hall, Idaho, and ~ublette Ridge, VVyo. 174 , 21. Correlation of formations of Triassic age along the south flank of Uinta Mountains, Utah and Colo__________ 178 m • • A SHORTER CONTRIBUTION TO GENERAL GEOLOGY TRIASSIC STRATIGRAPHY. OF SOUTHEASTERN IDAHO AND ADJACENT AREAS By BERNHARD KuMl\IEL ABSTRACT studies have demonstrated that this area contains one '.rhick marine strata of Early Triassic age occur in eastern of the thickest Lower Triassic marine sections in the Idaho and adjacent areas. The area covered in this report lies world and one of the most complete sequences of Lower along the eastern margin of an ancient miogeosycline. Basin­ Triassic faunal zones. Lower Triassic deposits around ward (west) the strata are thickest and consist of fossiliferous marine rocks with no red beds. Toward the craton (east) the the world are generally thin and incomplete in respect strata thin and intertongue with red sandstones and shales. to the number of faunal zones present. Data on the The Dinwoody and Thaynes formations attain their thickest Idaho section will add much to our knowledge of North marine development in the region of the Fort Hall Indian Res­ American Triassic stratigraphy and paleontology; but, ervation. To the east, north, and south of Fort Hall, the Din­ as is probably more important, they are proving to be woody formation intertongues with the red Woodside and Chug­ water formations; the T:Q.aynes formation tongues out eastward a key in the interpretation and correlation of worldwide into the Ankareh and Chugwater formations. The zone of in­ Lower Triassic faunal zones. tertonguing for the Dinwoody formation follows a line from The Early Triassic seaways that covered much of southwestern Montana along the Idaho-Wyoming boundary and western North America were more widespread than turns sharply westward in northern Utah. East of this line those of Middle or Late Triassic time. The distribution red beds are predominant ; west of the line the red beds are absent or of minor thickness. A similar facies line for the of facies and thickness of Lower Triassic strata in Thaynes formation passes southward from the Idaho-Wyoming western North America follows a pattern similar that boundary through the 'Vasatch Mountains and does not turn of the late Paleozoic. Permian and Triassic strata of westward in northern "'Ctah as for the Dinwoody formation. N evacla, California, Oregon, and Washington are char­ New ammonite faunas of Gyronitan and Flemingitan age are acterized by the presence of volcanic rocks. These areas recorded from the Dinwoody formation. The Prohungarites zone, the youngest Scythian ammonite zone, is recognized in include the eugeosyncline ofKay (1951). Strata of the the upper part of the Thaynes formation. The post-Thaynes same age in eastern Nevada, Arizona, Utah, eastern Triassic rocks of this area are of continental origin and are • Idaho, western Wyoming, and southwestern Montana correlated with the Shinarump conglomerate and Chinle forma­ contain no Yolcanic rocks and belong to the miogeosyn­ tion of the Colorado Plateau. The marine miogeosynclinal helt cline of Kay ( 1951). The line marking the approxi­ was destroyed at the end of Early Triassic time. The eastern mate boundary of the·miogeosyncline and the more sta­ Nevada geanticline (Nolan 1943) probably extended northward into Idaho in the middle and late Triassic. ble platform area of the continent has been named the Wasatch line by Kay ( 1951), and it passes in an approx­ INTRODUCTION imate north-south direction through the area of south­ eastern Idaho and the adjacent regions discussed in this Throughout the world, data on stratigraphy and report. At the Wasatch line the Triassic strata begin paleontology of the upper Permian and Lower Triassic to thin, and the marine rocks with red beds intertongue are surprisingly incomplete when compared to higher eastward onto the platform area or the craton of Kay beds of the Mesozoic. Considering that one of the im­ (1951). portant era boundaries separates these hvo series, in1- Nolan ( 1943) described a late Paleozoic and early portant physical and paleobiological data should be Mesozoic geanticline in eastern Nevada. This geanti­ sought in upper Permian and Lower Triassic rocks. cline began to form in the Permian period and extended Although the presence of thick marine deposits of northward until by Late Triassic time it reached en­ Early Triassic age in southeastern Idaho has long been tirely across Nevada. It was at least .a partial barrier known, it is only recently that field and laboratory between the miogeosyncline and the eugeosyncline dur- 16fi 166 SHORTER CONTRIBUTIONS