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The Lance Formation Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana

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The Lance Formation Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana The Lance Formation Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana U.S. GEOLOGICAL SURVEY BULLETIN 191 7-I Chapter I The Lance Formation Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana By CAROL WAITE CONNOR A multidisciplinary approach to research studies of sedimentary rocks and their constituents and the evolution of sedimentary basins, both ancient and modern U.S. GEOLOGICAL SURVEY BULLETIN 1917 EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN U.S. DEPARTMENT OF THE INTERIOR MANUEL LUJAN, JR., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government UNITED STATES GOVERNMENT PRINTING OFFICE: 1992 For sale by the Book and Open-File Report Sales U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Connor, Carol Waite. The Lance Formation petrography and stratigraphy, Powder River Basin and nearby basins, Wyoming and Montana / by Carol Waite Conner. p. cm. (U.S. Geological Survey bulletin ; 1917-1) (Evolution of sedimentary basins Powder River Basin ; ch. I) "A multidisciplinary approach to research studies of sedimentary rocks and their constituents and the evolution of sedimentary basins, both ancient and modern." Includes bibliographical references. Supt. of Docs, no.: I 19.3:1917 1. Sandstone Powder River Basin (Wyo. and Mont.). 2. Geology, Stratigraphic Cretaceous. 3. Lance Formation (Colo. and Wyo.). I. Title. II. Series . III. Series: Evolution of sedimentary basins Powder River Basin ; ch. I. QE75.B9 no. 1917-1 [QE471.15.S25] 557.3 s dc20 [552'.5] 91-37738 CIP CONTENTS Abstract II Introduction II Stratigraphy 12 Subsurface data 12 Thickness 15 Structure 15 Lithology 15 Petrography 16 Paleocurrent data 18 Summary and interpretation 112 References cited 115 PLATES [Plates are in pocket] 1. Map showing outcrop, drill-hole, and cross-section locations of the Lance Formation in the Powder River Basin and Bull Mountain Basin and outcrop locations of the Lance Formation in the Bighorn Basin and part of the Wind River Basin 2. Cross section A-A', Lance Formation and Fox Hills Sandstone combined, Powder River Basin and Bull Mountain Basin 3. Cross section B-B', Lance Formation and Fox Hills Sandstone combined, Powder River Basin and Bull Mountain Basin 4. Cross sections C-C', D-D', E-E', and F-F', Lance Formation and Fox Hills Sandstone combined, Powder River Basin and Bull Mountain Basin 5. Isopach map of the undifferentiated Lance Formation and Fox Hills Sandstone, Powder River Basin and Bull Mountain Basin 6. Structure map of the top of the Lance Formation, Powder River Basin and Bull Mountain Basin 7. Map showing characteristics of and boundaries between the three petrographic groupings used in this report, Lance Formation, Powder River Basin and Bighorn Basin 8. Paleocurrent map, Lance Formation, Powder River Basin and nearby basins FIGURES 1. Index map of the study area 13 2. Chart showing the method of picking some Cretaceous and Tertiary stratal boundaries in the subsurface of the Powder River Basin 14 3-5. Photographs of: 3. Longest log-like sandstone concretion seen in the Lance Formation, Powder River Basin 16 4. Log-like concretion with planar cross-bedding, Lance Formation, Powder River Basin 17 5. Log-like concretion with typical contorted cross-bedding, Lance Formation, Powder River Basin 18 Contents III 6. Ternary diagram of sandstones of the Lance Formation, Powder River Basin and Bighorn Basin 18 7-9. Photomicrographs of: 7. Sandstone from the Lance Formation from the northern part of the Powder River Basin 111 8. Sandstone from the Lance Formation from the southern part of the Powder River Basin 111 9. Sandstone from the Lance Formation from the central part of the Powder River Basin 112 10. Isopleth map showing percent chert in thin sections, Lance Formation, Powder River Basin and Bighorn Basin 113 11. Paleogeographic map of the region surrounding the present-day Powder River Basin during Lance time 114 TABLES 1. Estimates of constituent abundance in sandstones from the Lance Formation, Powder River Basin, according to the number of points identified on a thin section 19 2. Constituent abundance (in percent) in sandstones from the Lance Formation, Powder River Basin and Bighorn Basin 110 IV Contents EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN The Lance Formation Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana By Carol Waite Connor Abstract Cross-bed measurements, channel orientations, and orientations of well-cemented sandstones of log-like form were The uppermost Cretaceous Lance (Hell Creek) Formation used to determine stream-flow directions. Of particular signifi­ was studied in the Powder River Basin and nearby basins of cance is the evidence of eastward-flowing streams that is found Wyoming and Montana. The Lance Formation is overlain by on both sides of the Bighorn Mountains and on the eastern side the continental Paleocene Fort Union Formation. In the of the Powder River Basin; this indicates that neither the Bighorn and Wind River basins, it is underlain by the con­ Bighorn Mountains nor the Black Hills had emerged by Lance tinental Meeteetse Formation, and in the Powder River Basin, time. the Lance is underlain by the marginal marine Fox Hills Both paleocurrent data and the location of coarse detri­ Sandstone. Channel sandstones make up about one-third of tus imply sources to the west of the area of study, and the Lance; the rest of the formation is composed of thinner petrographic data indicate three source areas in that direction. sandstones and finer grained interfluve sedimentary rocks. The The very angular volcanic rock fragments in the northern coarsest material within the studied area is along the west side Powder River Basin were probably introduced aerially from of the Bighorn Basin and in the east end of the Wind River the Elkhorn Mountains Volcanics in western Montana. The Basin. Within the Powder River Basin, the combined Lance distinctive green hornblende in the central Powder River Basin and Fox Hills thicken from less than 700 ft in the north to more might have come from erosion of the Precambrian core of the than 3,300 ft in the south; there is little change in thickness Beartooth Mountains or from volcanics on the northern side of from east to west. Maximum structural relief of the Lance in the the Beartooths. Rock fragments of intrusive origin in the Powder River Basin is more than 6,000 ft. southern Powder River Basin probably came from the Granite Petrographic studies of sandstones from the Lance Mountains, south of the present Wind River Basin. Formation from around the Powder River Basin show that the Deposition of the Cretaceous Lance Formation ended at mineralogy is different in the northern, central, and southern the time of the Cretaceous-Tertiary boundary event, now parts of the basin. In the northern part of the basin, the thought by many to have been an asteroid impact. Previous abundance of rock fragments (average 57 percent) exceeds studies have indicated that rainfall amounts increased monocrystalline quartz (average 34 percent), and plagioclases dramatically following this boundary event. This was exceed potassium feldspars. Many of the rock fragments are accompanied initially by a greater influx of sandy sediments volcanic. In the southern part of the basin, the composition is and eventually by development of widespread peat- exactly reversed: monocrystalline quartz (average 57 percent) accurm'Iating swamps in the upper part of the Paleocene exceeds rock fragments (average 34 percent), and potassium Fort Union Formation. feldspars exceed plagioclases. Many rock fragments in the southern part of the basin are of intrusive origin. Between these areas, the sandstones are more like those in the northern part of the basin but are distinguished by the presence of a green INTRODUCTION hornblende. No significant vertical changes in mineralogy were noted. The Lance Formation in Wyoming and the age- equivalent uppermost Cretaceous Hell Creek Formation in Montana have not previously been studied in any detail except in small areas. This study of the Lance and Hell Manuscript approved for publication August 5, 1991. Creek was planned to collect information on stratigraphy, Lance Formation, Wyoming and Montana 11 structure, petrography, and provenance throughout the (Bohor and others, 1984); the Teapot Dome site is approxi­ Powder River Basin. mately 30 mi (48 km) north-northeast of Casper, Wyoming The original research plan for the formation was (Wolfe and Izett, 1987). The K-T boundary has generally abandoned during the first field reconnaissance in the been placed at the base of the lowest persistent lignite or Powder River Basin. Exposures were found to be generally coal bed that overlies the highest occurrence of dinosaur poor and widely scattered, and because of the commonly remains; this placement of the K-T boundary was proposed low dip and gentle terrain, "complete" sections covered as by Calvert (1912). many as 20 mi (32 km). The desired detailed measurement No original geologic mapping was undertaken for this of clusters of stratigraphic sections at approximately 50- study because of the size of the study area and because of mile (80-km) intervals was deemed impractical. Instead, time constraints. Previously mapped formation boundaries field work focused on collection of sandstone samples of the were used as a guide in the field, and where the Lance had Lance and Hell Creek for subsequent petrographic analysis been mapped in combination with a unit below, the position and on measurement of cross-beds and orientation of of the base of the Lance was roughly estimated based on the channel sandstones. The alternate plan proved fruitful, and thickness of the Lance reported in adjacent areas.
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