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Ammonites from the Upper Part of the Pierre Shale and Fox Hills Formation of Colorado

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Ammonites from the Upper Part of the Pierre Shale and Fox Hills Formation of Colorado PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3388, 45 pp., 24 ®gures, 1 table February 19, 2003 Ammonites from the Upper Part of the Pierre Shale and Fox Hills Formation of Colorado NEIL H. LANDMAN1 AND WILLIAM A. COBBAN2 ABSTRACT The upper part of the Pierre Shale and Fox Hills Formation were deposited in the Late Cretaceous (Maastrichtian) Western Interior Seaway. They crop out in a belt that roughly par- allels the Front Range of the Rocky Mountains from Douglas to Weld County, Colorado. These rocks consist of sandy shales and sandstones and are overlain by the nonmarine Laramie For- mation. A sparse assemblage of ammonites is present consisting of Coahuilites sheltoni BoÈse, 1928, Sphenodiscus pleurisepta (Conrad, 1857), Trachybaculites sp. cf. T. columna (Morton, 1834), Hoploscaphites birkelundae Landman and Waage, 1993, Hoploscaphites sp. cf. H. bir- kelundae, Jeletzkytes dor® Landman and Waage, 1993, and Jeletzkytes sp. cf. J. dor®. Hoplos- caphites birkelundae and Jeletzkytes dor® de®ne the H. birkelundae Zone in the Western Interior, which represents the lower part of the upper Maastrichtian. These rocks are thus equivalent in age to the Fox Hills Formation in Niobrara County, Wyoming, and older than the type Fox Hills Formation in north-central South Dakota. An analysis of the ratio of 87Sr/86Sr in a belemnite from this zone in Morgan County, Colorado, yields a value of 0.707790 6 0.000008 (2-sigma SE), nearly identical to that of a bivalve from the same zone in Niobrara County, Wyoming (McArthur et al., 1994). The western shoreline of the seaway during the time of H. birkelundae extended as far west as northwestern Colorado and southwestern Wyoming. INTRODUCTION ferred to as the Pierre Shale and Fox Hills Uppermost Cretaceous marine strata along Formation (®g. 1). These units represent the Rocky Mountain Front Range are re- thick sedimentary accumulations relatively 1 Curator, Division of Paleontology (Invertebrates), American Museum of Natural History. e-mail: landman@ amnh.org 2 Research Associate, Division of Paleontology (Invertebrates), American Museum of Natural History. Home ad- dress: 70 Estes St., Lakewood, CO 80226. Copyright q American Museum of Natural History 2003 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3388 Fig. 1. Outcrop of the upper transition member of the Pierre Shale (lined) and Fox Hills Formation (dotted) along the Front Range of the Rocky Mountains, Colorado, with localities of most of the fossils mentioned in the text. 2003 LANDMAN AND COBBAN: COLORADO AMMONITES 3 Fig. 2. Chart documenting the terms used to describe the upper part of the Pierre Shale and Fox Hills Formation by various authors, including ourselves. The thickness of the units is not drawn to scale. rich in marine fossils that were deposited inter®ngering of shales and sandstones in this along the western margin of the Western In- part of the section. It also re¯ects the paucity terior Seaway. of good outcrops in the area. We brie¯y re- We describe the ammonites from the upper view some of the most important papers on part of the Pierre Shale and Fox Hills For- the subject to provide a background for our mation in northeastern Colorado. Ammonites study (®g. 2). are somewhat rare in this part of the section One of the ®rst descriptions of the upper and, to make matters worse, there are few part of the Pierre Shale and overlying Fox exposuresÐeven these are disappearing due Hills Formation in the Denver Basin was giv- to urban sprawl. Our study is largely based en by Eldridge (1896). He characterized the on a collection of ammonites housed at the Pierre Shale as ``a great body of plastic clays, Colorado School of Mines (CSM), which carrying small, lenticular bodies of impure was assembled over the last 70 years, sup- limestone and, at a horizon about one-third plemented by material from the U.S. Nation- the distance from base to summit, a zone of al Museum and the American Museum of sandstone from 100 to 300 ft [30.5 to 91.4 m] Natural History. thick'' (ibid.: 69). He identi®ed a ``zone tran- sitional to Fox Hills'' marking ``a change STRATIGRAPHIC SETTING from the pure clay of the one to the arena- The relationship of the Fox Hills Forma- ceous shales of the other'' (ibid.: 71). Ac- tion to the underlying Pierre Shale and over- cording to him, the Fox Hills Formation con- lying Laramie Formation in northeastern sists ``mainly of soft, friable, arenaceous Colorado has always been controversial. This shales, with occasional interstrati®ed bands of is due to the complex vertical and lateral var- clay. The entire formation has a yellowish iation in lithology associated with repetitive cast, but while the shales are generally of a 4 AMERICAN MUSEUM NOVITATES NO. 3388 grayish-yellow the sandstone itself has a pro- in ®gure 3. The portion of the Fox Hills For- nounced tint of green'' (ibid.: 71). He esti- mation exposed at this locality is 166 ft (50.6 mated the thickness of the formation as be- m) thick and consists of three massive sand- tween 800 and 1000 ft (243.8 and 304.8 m), stone units separated by thinner shalier units. diminishing to 500 ft (152.3 m) near Golden, The uppermost sandstone, approximately 40 Jefferson County, Colorado. According to ft (12.2 m) thick, is the Milliken Sandstone him, the top of the formation is capped by a Member and forms the top of the formation. ``persistent and characteristic sandstone, usu- Lovering et al. (1932) elevated the base of ally about 50 ft [15.2 m] thick'' (ibid.: 71). the Fox Hills Formation to approximately He described this sandstone as transitional to 250 ft (76.2 m) below the base of the Lara- the Laramie Formation. mie Formation (®g. 2). They de®ned the base Henderson (1920) de®ned the Milliken of the Fox Hills Formation as ``the horizon Sandstone Member as the top unit of the Fox below which the section is predominantly Hills Formation, describing it as a massive gray marine clay shales and sandy shales of cliff-forming sandstone. He named it after Pierre age, and above which the section the town of Milliken, Weld County, Colora- changes rapidly to a buff to brown sandstone do, where it is well exposed. containing numerous large gray to brown, Mather et al. (1928: 90), in a geologic hard, sandy concretions'' (ibid.: 702). They study of northeastern Colorado, identi®ed considered the top of the Fox Hills Forma- what they called the ``higher beds'' of the tion ``as the horizon above which the section Pierre Shale as showing ``a progressive re- is composed predominantly of fresh- and duction in the quantity of sand and a con- brackish-water deposits accompanied by sequent increase in the purity of the shaly coals and lignitic shales, and below which it strata'' (®g. 2). They estimated the thickness is predominantly marine'' (ibid.: 703). They of these beds as between 2500 and 5200 ft brie¯y described four localities where these (762 and 1,585 m) and observed ``no sharp relationships could be observed. break, either faunal or lithologic, to mark the Lavington (1933: 403) introduced the term contact between the Pierre and Fox Hills for- ``transition zone'' for the upper part of the mations'' (ibid.: 93). They stated that the Pierre Shale lying ``between the mappable boundary was ``drawn primarily to indicate group of sandstones and sandy concretion the contrast between the nongritty shale of beds of the Fox Hills sandstone . and the the upper Pierre and the sandy shale or sand- lower beds which contain typical Pierre fos- stone . of the overlying Fox Hills For- sils'' (®g. 2). He described this zone as con- mation'' (ibid.: 93). They described the Fox sisting of ``gray, blue, and buff shales and Hills Formation as consisting of ``soft crum- sandy shales, scattered buff calcareous con- bly sandstone and sandy shale with here and cretions, sandstones and several thin gray-to- there a bed or two of ®rmly indurated mas- buff concretionary limestone beds'' (ibid.: sive sandstone'' (ibid.: 92) and considered 403). He estimated the thickness of the tran- the Milliken Sandstone Member, where pre- sition zone as between 650 and 875 ft (198.1 sent, as the top of the formation. They esti- and 266.7 m) in Lincoln County, southeast mated the thickness of the formation as be- of Denver. He followed the suggestion of tween 1,200 and 1,800 ft (365.8 and 548.6 Lovering et al. (1932) and restricted the Fox m) near the eastern margin of Larimer Coun- Hills Formation to ``the mappable group of ty and characterized the boundary with the buff sandstones and sandy concretion beds at overlying Laramie Formation as transitional the top of the Fox Hills, as previously de- representing ``the inter®ngering of brackish- ®ned'' (ibid.: 405). water and fresh-water beds with marine LeRoy (1946) also followed the usage of sandstone and shale'' (ibid.: 93). Lovering et al. (1932) in de®ning the Fox Mather et al. (1928) presented three mea- Hills Formation in a stratigraphic study of sured sections of the Fox Hills Formation in the Golden-Morrison area, Jefferson County, northeastern Colorado. The section at Wild- Colorado. He interpreted the Fox Hills For- cat Mound in the NW¼ sec. 26, T4N, R6W, mation as ``a transitional depositional phase south of Milliken, Weld County, is illustrated between marine (Pierre) and nonmarine (Lar- 2003 LANDMAN AND COBBAN: COLORADO AMMONITES 5 Fig. 3. Stratigraphic section of the Fox Hills Formation and overlying Laramie Formation at Wildcat Mound, sec.
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