University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Earth and Atmospheric Sciences, Department Papers in the Earth and Atmospheric Sciences of 5-1998 Paleocene Mammalian Biostratigraphy of the Carbon Basin, Southeastern Wyoming, and Age Constraints on Local Phases of Tectonism Ross Secord University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/geosciencefacpub Part of the Earth Sciences Commons Secord, Ross, "Paleocene Mammalian Biostratigraphy of the Carbon Basin, Southeastern Wyoming, and Age Constraints on Local Phases of Tectonism" (1998). Papers in the Earth and Atmospheric Sciences. 170. https://digitalcommons.unl.edu/geosciencefacpub/170 This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in the Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Paleocene mammalian biostratigraphy of the Carbon Bash, southeastern woming, and age constraints on local phases of tectonism Ross Secord Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071-3006, U.S.A. ABSTRACT Mammalian fossils from two principal collecting areas in the lower Hanna Formation of the Carbon Basin comprise three faunas, the Grayson Ridge, Halfway Hill, and Sand Creek faunas. The Grayson Ridge and Halfway Hill faunas are diverse, consisting cumulatively of 29 mammalian species, at least two of which are new. The faunas are approximately equivalent in age and are either latest lbrrejonian or earliest Tiffanian, or possibly sample both NALMAs. In any event, the faunas are very close in age to the lbrrejonian-Tiffanian boundary. Strata bear- ing the Grayson Ridge and Halfway Hill faunas were truncated by erosion, resulting in a previ- ously unrecognized intraformational unconformity (IFU). The Sand Creek fauna consists of a meager sampling of mammalian fossils from directly above the unconformity and provides a Ti3-Ti5 age (middle or late Tiffanian) for strata overlying the unconformity. Fortuitous stratigraphic positioning of the faunas allowed age constraints to be placed upon two local phases of deformation. Simpson Ridge anticline, which separates the Hanna and Carbon basins, resulted from the first phase. Based upon lithologic correlation of the lower Ferris Formation at Simpson Ridge to its type section, development of Simpson Ridge began in the Lancian or Puercan time. Based upon fossil ages in the lower Hanna Formation, which onlaps the anticline, most, or all, of Simpson Ridge had formed by the early Tiffanian. A younger episode of deformation occurred no earlier than Ti4 (middle Tiffanian) and overprinted the folding of Simpson Ridge. Additionally, fossils from above and below the IFU suggest a hiatus of 1 to 3.5 m.y. in the lower Hanna Formation. Age constraints and orientations of Simpson Ridge anticline and an overprinting syncline suggest a local change in the direction of maximum shortening from early Paleocene to late Paleocene or Eocene. INTRODUCTION Orogenesis that led to the structure of the Cretaceous. The basins were separated by local up- present day Rocky Mountains began in the Late lift that began in the latest Cretaceous or early Pa- Cretaceous during what is known as the Laramide leocene and culminated in the formation of a large orogeny and lasted into the early Eocene. Basement- north-northeast trending anticline, known as involved thrusting led to large-scale uplifts. Debris Simpson Ridge (Figs. 1 and 2). shed off the uplifts into newly forming basins re- The Hanna Formation is a thick sequence of sulted in deposition of thick Late Cretaceous and terrestrially derived sandstone, shale, and coal, fill- Paleogene sedimentary sequences. The Hanna and ing the central parts of the Hanna and Carbon ba- Carbon basins are relatively small basins located sins. The formation has only been recognized in in southeastern Wyoming formed during the and around the Hanna, Carbon, and the northern Laramide orogeny (Fig. 1). The Hanna and Carbon Laramie basins, and at the northern end of the basins were not distinct depositional centers until Medicine Bow Mountains (Fig. 1). The Hanna For- the early Paleocene, as this study demonstrates, but mation ranges in age from latest lbrrejonian or rather comprised a single basin during the Late earliest Tiffanian at its base, to at least late Tiffanian Rocky Mountain Geology, v. 33, no. 1, p. 119-154, lzfigs., 11 tables, May, 1998 119 R. SECORD (Ti5-Ti6) in the northern and eastern Carbon Basin, which is the focus of this study (see Fig. 3 for geochronologic scale). Vertebrate fossils are not com- mon in most parts of the Hanna Formation and preservation is generally poor. Fossils are, how- ever, occasionally found in local abundance, and recent discover- ies of fragmentary mammalian remains in the Hanna and Car- bon basins have greatly increased knowledge of Paleocene mamma- lian life in southeastern Wyo- ming. Mammalian fossils were first discovered in the Carbon Ba- sin by a field party from The Uni- versity of Wyoming led by Dr. Jason A. Lillegraven in the sum- mer of 1980 in response to a re- port of a new vertebrate locality Figure 1. Map showing locations of Carbon and Hanna basins, surrounding by M~.Bd G~~~~~~.~ili~~~tcol- uplifts, and distribution of Hanna Formation. Modified from Roberts (1989), letting of fossil vertebrates, how- Lillegraven (1994), and Brooks (1977). Dark areas show Laramide uplifts; arrows and teeth on faults toward hanging wall. ever, did not begin until 1994 when I devoted 14 weeks to col- lecting fossils and gathering geo- logic data for research leading to a Master of Science degree. A to- tal of 170 mammalian specimens consisting of isolated teeth, tooth fragments, and jaw fragments has been cataloged to date. Although the number of specimens is small, taxonomic diversity is high. At least 33 mammalian spe- cies are represented, comprising 7 orders and 16 families. The Hanna Formation in the Carbon Basin onlaps previously formed geologic structures and was itself deformed late in the Laramide orogeny. Because of this overlapping relationship and sub- sequent deformation, mamma- lian fossils from the lower Hanna Formation are critical in provid- ing age constraints on local phases of tectonism. The Grayson Ridge and Halfway Hill faunas have important biostratigraphic Key to Figure 2. implications as well. They pro- vide evidence for faunal changes near or across the boundary be- 120 Rocky Mountain Geology, v. 33, no. I, p. 119-154, 12 figs., ll tables, May, 1998 PALEOCENE BIOSTRATIGRAPHY OF CARBON BASIN Figure 2. Geologic map of Carbon and southeastern Hanna basins, including Simpson Ridge anticline, vertebrate collecting areas, and other locations discussed in text. Simpson Ridge anticline extends from the southern end of T21N to the northern end of T22N. The Ferris-Hanna contact marks the northern outer trace of the anticline. Modi- fied from Hansen (1986) and Dobbin et al. (1929). Rocky Mountain Geology, v. 33, no. I, p. 119-154,12figs., 11 tables, May, 1998 121 R SECORD uw Collection of Fossil Vertebrates, Departmental Scientific Collections, MAMMAL "AGES" Department of Geology and Geophys- ics, The University of Wyoming, I 54 Laramie Eo EARLY --, -. YPM-PU Yale Peabody Museum, Yale Univer- 55 WASATCHIAN sity, New Haven, and Princeton 56 CLARKFORKIAN c11 University, Princeton Ti5 57 W Miscellaneous b ---Ti4 NALMA North American land mammal "age" !2 !2 2 58 TIFFANIAN Ti3 w IFU intraformational unconformity u 59 Ti2 O gen. indet. genus indeterminate W 60 Ti1 m.y. million years 61 --Tj6 2 --Ti5 m meters 62 km kilometers 5 TORREJONIAN 2;--- - 63 Tj2 5 63.90t0.04 /64.11 t0.02 Tjl Dental Measurements 64 h3 PUERCAN 16: L length PUT 65 F '64.77t0.06 W width ' 66 LANCIAN AW anterior width of lower teeth 3 I PW posterior width of lower teeth Figure 3. Geochronologic chart showing correlations between relevant mammal "ages," epochs, and polarity L, length to first serration of chrons. Torrejonian zonal boundaries are not intended to represent durations or absolute ages of zones. Base chart H height of first serration of M, modified from Woodburne and Swisher (1995) with D depth below base of H to base of 'Zbrrejonian zones of Williamson (1996). exodaenodont lobe of M, S number of serrations of M, tween the 'Ibrrejonian and Tiffanian NALMAs and raise questions about the synchroneity of presumed moth Designations Tiffanian first appearances. D deciduous dentition ABBREVIATIONS I", 1" upper, lower incisor P", px upper, lower premolar Institutions M", Mx upper, lower molar AMNH American Museum of Natural History, New York 1Mb multituberculate lower blade (Sloan, 1987, p. 192) KU University of Kansas, Lawrence NMMNH New Mexico Museum of Natural STRATIGRAPHY OF HANNA FORnilATION History and Science, Albuquerque The Hanna Formation consists of sandstone, PU Princeton University, Princeton siltstone, conglomerate, carbonaceous shale, and TMM Texas Memorial Museum, Univer- coal. The formation reaches a maximum thickness sity of Texas, Austin in excess of 3,500 m in the northern Hanna Basin (Lillegraven and Snoke, 1996, fig. 13) and 325 m in USNM U.S. National Museum, Smithsonian the Carbon Basin (Brooks, 1977). The Hanna For- Institution, Washington mation in the Carbon Basin rests with depositional 122 Rocky Mountain Geology, v. 33, no. 1, p. 119-154,