Molluscan Record From a Mid-Cretaceous Borehole in Weston County, Wyoming By WILLIAM A. COBBAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1271 Brief descriptions, illustrations, and stratigraphic ranges of the more common fossils UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1984 UNITED STATES DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Cobban, William Aubrey, 1916- Molluscan record from a mid-Cretaceous borehole in Weston County, Wyoming. (Geological Survey Professional Paper; 1271) Bibliography: 21 p. 1. Mollusks, Fossil. 2. Paleontology Cretaceous. 3. Paleontology Wyoming Weston County. 4. Borings Wyoming Weston County. I. Title. II. Series. QE801.C55 1983 564'.11'0978714 82-600120 For sale by the Branch of Distribution U.S. Geological Survey 604 South Pickett Street Alexandria, VA 22304 CONTENTS Page Abstract ............................................ 1 Introduction .......................................... 1 Acknowledgments ....................................... 1 Stratigraphy summary .................................... 1 Preservation of fossils ..................................... 2 Stratigraphic distribution and age .............................. 2 Molluscan zones ........................................ 2 Megafossil content ....................................... 4 Characteristic fossils ...................................... 8 References cited ........................................ 19 ILLUSTRATIONS [Plates follow index] Page PLATES 1-5. Molluscan fossils FIGURE 1. Ranges of bivalves and ammonites ...................... 3 HI MOLLUSCAN RECORD FROM A MID-CRETACEOUS BOREHOLE IN WESTON COUNTY, WYOMING By WILLIAM A. COBBAN ABSTRACT at intervals for investigations of the foraminifera, dino- A core borehole in the Osage oilfield on the west flank of the Black flagellates, and palynomorphs. Unfortunately, the di­ Hills uplift in eastern Wyoming penetrated, in decending order, most ameter of the cores is small (47 mm), but, nevertheless, of the Carlile Shale, all of the Greenhorn Formation, and the upper an impressive fossil record was obtained. part of the underlying Belle Fourche Shale. Molluscan fossils are abundant in parts of the core and indicate an age span of early Conia- ACKNOWLEDGMENTS cian to the middle Cenomanian. Most of the fossils are bivalves and ammonites; gastropods are scarce. E. A. Merewether of the U.S. Geological Survey pro­ Fossils in the cores indicate the following zones Lower Coniacian posed this project, selected the borehole site, and Crem.noceramusl waltersdorfensis supervised the coring. He also split most of the cores Upper Turonian and collected most of the megafossils as well as the Scaphites corvensis samples for micropaleontological studies. R. E. Burk- S. nigricollensis holder, also of the Geological Survey, photographed the S. whitfieldi megafossils and prepared some of the specimens. S. warreni Middle Turonian All illustrated specimens are stored in the National Collignoniceras woollgari Museum of Natural History in Washington, D. C., and Lower Turonian have USNM catalog numbers. Mytiloides mytiloides Mytiloides aff. M. duplicostatus Upper Cenomanian STRATIGRAPHIC SUMMARY Sciponoceras gracile Dunveganoceras albertense A graphic log of the borehole showing formational D. pondi assignments and lithologic contacts was presented by Middle Cenomanian Merewether (1980, fig. 4). The top of the borehole is Acanthoceras amphibolmn probably a little below the top of the Carlile Shale. Here the Carlile consists of three members. The upper INTRODUCTION member, Sage Breaks, consists of calcareous and non- calcareous shale with scattered limestone concretions. A cored borehole was drilled by the U.S. Geological About 85 m were penetrated in the borehole. Survey in 1976 in the Osage oilfield on the west flank Merewether (1980, p. 13) noted thicknesses of 91-94 of the Black Hills uplift in eastern Wyoming m in wells in the Osage oilfield. The Turner Sandy (Merewether, 1980). This borehole, in the SW1/4NW1/4 Member, the middle part of the Carlile, extends from sec. 30, T. 46 N., R. 63 W., Weston County, was drilled depths of 85-140 m and consists of interbedded fine- to a total depth of 270.8 meters. The purpose was to to very fine grained sandstone, siltstone, and noncal- obtain fresh samples for sedimentologic and paleon- careous shale with sandy or silty limestone concretions. tologic analyses as well as to obtain geophysical logs The underlying Pool Creek Member extends from 140- for references to subsurface studies of mid-Cretaceous 163 m and consists of calcareous shale. Calcareous shale rocks in the Powder River Basin. The site of the and thin beds of limestone make up the Greenhorn For­ borehole was selected because of its proximity to meas­ mation, which extends from 163 to 240 m. Noncalcare- ured outcrop sections of mid-Cretaceous rocks. Cores ous shale with several beds of bentonite comprise the from the borehole were split along bedding planes for underlying Belle Fourche Shale of which the upper 31 molluscan fossils, and samples of the cores were taken m were penetrated in the borehole. MOLLUSCAN RECORD FROM MID-CRETACEOUS BOREHOLE, WYOMING PRESERVATION OF FOSSILS nian lies near the top of the Greenhorn Formation. Col- lignoniceras woollgari regulare (Haas), which occurs in With few exceptions, the mollusks in the cores occur the cores of the basal part of the Carlile Shale at depths crushed in dark-gray shale. Some specimens are com­ of 155-160.9 m, is a middle Turonian ammonite (Cobban pletely flattened. Other specimens, especially those in and Hook, 1979, p. 12). Mytiloides mytiloides (Mantell), more silty matrices, are not as crushed. A few speci­ found in cores from the upper part of the Greenhorn mens in gray siltstone beds in the Turner Sandy Mem­ Formation at depths of 164-171.4 m, is usually regarded ber of the Carlile Shale are almost undeformed. Some as a guide fossil to the lower Turonian although the shell material is usually preserved on most specimens. species overlaps the lower range of C. woollgari. Pyritic fossils are common in the lower part of the Mytiloides subhercynicus (Seitz) occurs in cores from Greenhorn Formation at depths of 216-230 m and 244- depths of 167.1-168.4 m. Inasmuch as this fossil is found 247.5m. with both the early (woollgari woollgari) and late (woollgari regulare) forms of C. woollgari, the bound­ STRATIGRAPHIC DISTRIBUTION ary between the lower and middle Turonian is probably AND AGE best placed at a depth of about 168 m. The boundary between the middle and upper Turo­ The ranges of most of the mollusks from the borehole nian lies somewhere between depths of 136-155 m. are shown in figure 1. Gastropods are extremely scarce, Megafossils were not found in the cores from those and none is shown. depths. A core from a depth of 155 m has Collig- The mollusks reveal an age span of the cores of mid­ noniceras woollgari regulare (Haas) of middle Turonian dle Cenomanian into early Coniacian. The middle age, and a core from a depth of 136 m has Scaphites Cenomanian age of the lowest cores is based mainly warreni Meek and Hay den of early late Turonian age. on the occurrence of the ammonite Acanthoceras at The boundary between the Turonian and Coniacian depths of 243.8-257.3 m. With the exception of A. hip- is placed at the lowest occurrence of Cremnoceramusl pocastanum (J. de C. Sowerby), most occurrences of waltersdorfensis (Andert) and C? rotundatus (Fiege) the genus are in rocks regarded as middle Cenomanian following the proposals by Kauffman, Cobban, and age (for example Kennedy and Hancock, 1976, p. 5.15; Eicher (1976 p. 23.16). This is at a depth of about 70 1977, p. 130-135). Cores from depths of 237.7-246 m m. The next lower control point is a core from a depth contain Inoceramus crippsi Mantell, a species found of 76 m that contains Mytiloides fiegei fiegei (Troger) mostly in lower and middle Cenomanian rocks of of late Turonian age. Europe. Cores from depths of 220.7-240 m contain In­ oceramus prefragilis Stephenson, a species restricted to the ammonite zones of Plesiacanthoceras MOLLUSCAN ZONES wyomingense (Reagan) and Dunveganoceras pondi Fossils from the lowest part of the core include Acan­ Haas in the Western Interior. These ammonites, which thoceras amphibolum Morrow and Inoceramus ruther- developed out of Acanthoceras, seem best assigned a fordi Warren, both restricted to the middle Cenomanian late Cenomanian age. The boundary between the mid­ zone of A. amphibolum. That ammonite has long been dle and upper Cenomanian is probably near a depth accepted as a guide fossil to one of the Western Interior of 240 m in the borehole. Cenomanian zones (Cobban, 1951a, p. 2197; Cobban and The boundary between the Cenomanian and Turonian Reeside, 1952a, p. 1017). The top of the zone is placed seems best placed at a depth of about 180 m, which at a depth of 248 m in the borehole. lies about at the top of the range of Inoceramus pictus Plesiacanthoceras wyomingense (Reagan) marks the J. de C. Sowerby and base of the range of Mytiloides. next Cenomanian zone above that of A. amphibolum Ammonites associated with I. pictus in cores from in the Western Interior. The zone of P. wyomingense depths of 181.1-184.7 m include Scaphites (Ptero- was originally recorded as the zone of Acanthoceras^ scaphites) minutus Moreman, Worthoceras vermiculum n. sp. (Cobban, 1951a, p. 2197), next as the zone of (Shumard), and Allocrioceras annulatum (Shumard), Acanthoceras'! sp. A (Cobban and Reeside, 1952a, p. which are restricted to the zone of Sciponoceras gracile 1017), later as the zone of Acanthocerast wyomingense (Shumard). This zone is usually regarded as the top (Cobban, 1958, p. 117-119), and finally as the zone of of the Cenomanian (for example Kennedy and Hancock, Plesiacanthoceras wyomingense (Merewether and Cob- 1976, p. 5.16; 1977, p. 134-136). ban, 1972, p. D59). On outcrops in the vicinity of the The boundary between the lower and middle Turo­ borehole, a disconformity separates the Belle Fourche i MOLLUSCAN FOSSILS METERS BIVALVES AMMONITES 0 FIGURE 1. Ranges of most of the bivalves and ammonites in the cores from the borehole in sec.