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Precambrian and Lower Ordovician Rocks in East-Central Idaho

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Precambrian and Lower Ordovician Rocks in East-Central Idaho Precambrian and Lower Ordovician Rocks in East-Central Idaho GEOLOGICAL SURVEY PROFESSIONAL PAPER 889 Precambrian and Lower Ordovician Rocks in East-Central Idaho Precambrian Y Sedimentary Rocks in East-Central Idaho By EDWARD T. RUPPEL Precambrian Z and Lower Ordovician Rocks in East-Central Idaho By EDWARD T. RUPPEL, REUBEN JAMES ROSS, JR., and DAVID SCHLEICHER GEOLOGICAL SURVEY PROFESSIONAL PAPER 889 Chapters A and B UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:l975 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Main entry under title: Precambrian and Lower Ordovician rocks in east-central Idaho. (Geological Survey Professional Paper 889) Includes bibliographies. CONTENTS: Ruppel, E. T. Precambrian Y sedimentary rocks in east-central ldaho.-Ruppel, E. T., Ross, R. J., Jr., and Schleicher, D. Precambrian Z and Lower Ordovician rocks in east-central Idaho. Supt. of Docs. no.: I 19.16:889 1. Geology, Stratigraphic-Pre-Cambrian. 2. Geology, Stratigraphic-Ordovician. 3. Geology-Idaho. I. Ruppel, Edward Thompson, 1925- Precambrian Y sedimentary rocks in east-central Idaho. 1975. II. Ruppel, Edward Thompson, 1925- Precambrian Z and Lower Ordovician rocks in east-central Idaho. 1975. III. Series: United States Geological Survey Professional Paper 889. QE653.P73 551.7'15 74-30002 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02643-1 Precambrian Y Sedimentary Rocks in East-Central Idaho By EDWARD T. RUPPEL PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO GEOLOGICAL SURVEY PROFESSIONAL PAPER 889-A Definition and description of newly named and revised formations deposited in the Belt miogeocline CONTENTS Page Page Abstract --------------------------------------------------------------------------------- 1 Local descriptions and correlations near the Lemhi Range 18 Introd ucti on ----------------------------------------------------------------------------- 1 Elkhorn Ranch-Sawmill Gulch-West side Summary of earlier work------------------------------------------------------- 2 of Lost River Range------------------------------------------------ 19 Precambrian sedimentary rocks in the Lemhi Range------------ 4 Morgan Creek-------------------------------------------------------- 19 Yellowj acket Formation----------------------------------------------- 5 Ellis, Idaho------------------------------------------------------------------ 19 Lemhi Group------------------------------------------------------------------ 6 Twelvemile Creek and Perreau Creek---------------------- 19 Inyo Creek Formation--------------------------------------------- 7 Yellowjacket mining district---------------------------~---- 19 West Fork Formation------------------------------------------ 7 Carmen Creek------------------------------------------------------ 19 Big Creek Formation--------------------------------------------· 9 North Fork, Idaho, and vicinity--------------------------------- 19 Apple Creek Formation-------------------------------------- 10 Big Hole Pass------------------------------------------------------------- 20 Gunsight Formation------------------------------------------ 11 Head of Big Hole River and Big Hole Divide--------------- 20 Swauger Formation ----------------------------------------------------- 12 Bloody Dick Creek--------------------------------------------------- 20 The underlying rocks---------------------------------------------------------- 14 Lemhi Pass --------------------------------------------------------------- 21 Regional tectonic implications--------------------------------------------- 15 Grizzly Hill------------------------------------------------------------ 21 The overlying rocks---------------------------------------------------- 16 The Beaverhead Mountains south of Grizzly Hill-------- 21 Regional correlation --------------------------------------------------------- 18 References cited---------------------------------------------------------------- 22 ILLUSTRATIONS Page FIGURES 1-4. Maps: 1. Index of east-central Idaho and southwest Montana, showing localities mentioned in text------------------------------- 3 2. Type localities or areas, or reference section for the Inyo Creek, West Fork, Big Creek, Apple Creek, and Gunsight Formations·-------------------------------------------------------------------------------------------------------------------- 8 3. Reference section for the Swauger Formation------------------------------------------------------------------------------------------------------------ 13 4. Principal elements of the Belt basin reentrant and miogeocline, and area of deposition of Precambrian sedimentary rocks--------------------------------------------------------------------------------------------------------- 17 TABLES Page TABLE 1. Summary of previous descriptions of Precambrian sedimentary rocks of east-central Idaho------------------------------------------ 4 2. Revised sequence of Precambrian sedimentary rocks, Lemhi Range, Idaho------------------------------------------------------------------ 5 3. Tentative regional correlations of Precambrian sedimentary rocks------------------------------------------------------------------------· 18 PRECAMBRIAN AND LOWER ORDOVICIAN ROCKS IN EAST-CENTRAL IDAHO PRECAMBRIAN Y SEDIMENTARY ROCKS IN EAST-CENTRAL IDAHO By EDWARD T. RUPPEL ABSTRACT INTRODUCTION Sedimentary rocks of Precambrian Y age in east-central Idaho The approximate distribution of slightly meta­ crop out in a belt that reaches 150 miles (240 km) from the Snake morphosed sedimentary rocks of Precambrian age in River Plain northward to the Idaho-Montana line at Lost Trail east-central Idaho and adjacent southwestern Pass. In the northern part of this belt, the rocks are mostly fine­ grained, dark-colored, micaceous, feldspathic quartzites that Montana has long been known, for the antiquity of at closely resemble and are tentatively correlated with the Yellow­ least some of these rocks was recognized in early geo­ jacket Formation. Farther south, the Yellowjacket-like rocks are logical studies. But the relation of these old rocks to overlain by a thick sequence of lighter colored, fine-grained felds­ each other and to the better known rocks of the Belt pathic quartzites and argillitic siltites of the Lemhi Group Supergroup farther north in Idaho and Montana has (formerly called Lemhi Quartzite) that can be separated into a number of new mappable units, and the Swauger Formation at the remained uncertain. Perhaps the most appropriate top. The Lemhi consists of, in ascending order, the lnyo Creek, conclusion was that finally reached by C. P. Ross West Fork, Big Creek, Apple Creek, and Gunsight Formations. The (1963, p. 7), that the then-known relations and thickness of the Yellowjacket is not known, but probably exceeds correlations were tenuous, partly contradictory, and 10,000 feet (3,050 m); the Lemhi Group is about 20,000 feet (6,100 m) premature. To some extent, Ross' conclusions are still thick; and the Swauger Formation is at least 10,000 feet (3,050 m) thick. valid, but as a result of detailed mapping of these Regional considerations suggest that at least the sequence rocks in the central part of the Lemhi Range and re­ above the Yellowjacket has been moved from the original deposi­ connaissance mapping in surrounding areas, it is tional area perhaps as much as 100 miles (160 km) eastward on possible now to better define the sequence of rocks major flat surfaces of detachment, and the entire eastern and to suggest some regional relations~even while boundary of these Precambrian sedimentary rocks appears to be tectonic. acknowledging that my grip on some rock units is in­ Correlation of Precambrian rocks of east-central Idaho with the secure and that regional correlations are still some­ better known Belt Supergroup in western Montana and northern what premature and will remain so until more Idaho remains tenuous because of the different characteristics of detailed geologic mapping is available. the sequences and the likelihood that they were deposited under The Precambrian sedimentary rocks of east­ different conditions-the Belt Supergroup in the Belt basin, and the east-central Idaho rocks in the Belt miogeocline. Therefore, the central Idaho crop out in a belt that reaches about 150 Yellowjacket, Lemhi, and Swauger are removed from the super­ miles (240 km) from the south end of the Lemhi Range group, as is the Hoodoo Quartzite of central Idaho. However, at the Snake River Plain northward to the Idaho­ possible correlations are suggested by a few geologic events and Montana line at Lost Trail Pass-the approximate lithologic features that seem to be represented in both sequences. edge of the Idaho batholith. In the northern third of The Yellowjacket and Prichard are both basal sequences that differ greatly from the overlying rocks, and are possible this belt, where it is almost 50 miles (80 km) wide, the correlatives. The Apple Creek Formation is the only carbonate­ rocks are not well known; available descriptions bearing formation in east-central Idaho, and perhaps is equivalent suggest that they are similar from one place to to the middle Belt carbonate rock unit represented by theWallace another and that they are mainly rather dark, fine­ Formation and Helena Dolomite; the lnyo Creek, West Fork, and grained micaceous quartzites. This northern area Big Creek Formations
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