Rocky Mountain Geology 33.1
An Introduction to Rocky Mountain Geology
The Editors
Keywords: An Introduction to Rocky Mountain Geology
In January, 1962, The University of Wyoming published the first issue of Contributions to Geology (vol. 1, no. 1). That single-issue volume was followed by 31 two-issue volumes and four special papers. The final issue of Contributions to Geology appeared in March, 1998 (vol. 32, no. 2). Contributions to Geology is now transformed to Rocky Mountain Geology, featuring a new format, a new editorial staff, and renewed emphasis on high-quality, refereed articles reporting original research on geology and paleontology of the greater Rocky Mountain region. Although continuity with Contributions to Geology will be maintained in terms of subscription price, volume number, and regional emphasis, significant changes involve more than a new name. Inside and out, Rocky Mountain Geology is strikingly different from Contributions to Geology. Furthermore, the editors have the enthusiasm, dedication, and institutional support required for production of a high-quality, semiannual geological journal. Our goal is to produce a premier journal focusing on all aspects of the geology and paleontology of the Rocky Mountain region and environs. We will continue publishing two issues a year — Spring and Fall. Procedures for submitting manuscripts to Rocky Mountain Geology are provided on the inside of the front and back covers. We encourage theme issues with guest editors. Our fundamental requirement for Rocky Mountain Geology is that a manuscript be an original scientific contribution, not work previously published or submitted to another journal. The present issue features papers on stratigraphy and paleontology of the Hanna and Carbon basins, south-central Wyoming. Our Fall 1998 issue, conceived and organized by Karl E. Karlstrom (University of New Mexico), will focus on lithospheric structure and geologic evolution of the Rocky Mountains from Wyoming to New Mexico. A theme issue on Proterozoic magmatism, organized by Carol D. Frost (University of Wyoming), is scheduled for Fall 1999. We highlight these future issues to emphasize two points. First, we are committed to diversity of subject matter. Second, we encourage colleagues to consider Rocky Mountain Geology as a viable outlet for publication of studies relevant to the region. We invite high-quality manuscripts for the Spring 1999 and future issues and welcome new subscribers (see inside of back cover). If you have questions about Rocky Mountain Geology, please contact any one of us.
A new important record of earliest Cenozoic mammalian history: geologic setting, Multituberculata, and Peradectia
J. J. Eberle and J. A. Lillegraven
Keywords: Mammalia, Cretaceous, Cretaceous-Tertiary boundary, evolution, Ferris Formation, Hanna Basin, Lancian, Marsupialia, Multituberculata, Paleocene, palynomorphs, Peradectia, Puercan, stratigraphy, Wyoming.
The type Ferris Formation of south-central Wyoming is thick, comparatively undeformed, and relatively fossiliferous. We documented more than 100 vertebrate-bearing, stratigraphically superposed fossil localities that span roughly 3,000 ft (c. 900 m) of continental strata of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age. Fossil mammals were recovered from 39 of the localities, 32 or 33 of which represent Puercan time. The mammalian fossils allowed a detailed biostratigraphic zonation of the Puercan section, which is thicker, by nearly an order of magnitude, than any other known of that age. Preserved in a 1,763 ft- (537 m-) thick section are mammalian assemblages that represent all three Puercan Interval-zones (i. e., Pu1–Pu3), originally defined elsewhere from principally non-superposed strata. The local strata underwent only minor deformation, and that occurred late in the regional Laramide orogeny, not before the late Paleocene. On the basis of mammalian faunas, we place the Lancian-Puercan boundary at approximately 2,050 ft (625 m) above the base of the type Ferris Formation; remains of dinosaurs occur to just above that level, in absence of Puercan mammals. The lowest stratigraphic occurrence of Protungulatum donnae, a placental mammal diagnostic elsewhere of the earliest Puercan, exists at the 2,075 ft (632 m) level. Taxonomic composition of palynological samples is compatible with our placement of the Lancian-Puercan boundary.
Previous workers assumed that advent of locally derived clasts in the Hanna Formation could be used to distinguish its outcrops from those of the underlying Ferris Formation. However, diverse pebbles from local sources also occur in the type Ferris Formation, even within its dinosaur-bearing parts. We have been unable to determine any combination of lithologic criteria that can be used reliably in the field to distinguish between outcrops of Ferris and Hanna Formations. We summarize important variations in depositional regime within Lancian-Puercan parts of the type Ferris Formation.
We provide systematic description and discussion of multituberculate and peradectian components of the mammalian fauna. All reported taxa represent new records for the Hanna Basin and southern Wyoming in general, and the faunas help fill distributional gaps between species known to the north and south of central Wyoming. At least one species of multituberculate is recognized as new. Geographic range extensions include: (1) most southerly records of Cimolodon nitidus, Alphadon lulli, Mesodma ambigua, M. hensleighi, M. sp. cf. M. garfieldensis, and Catopsalis joyneri; and (2) most northerly records of Ptilodus sp. cf. P. tsosiensis and Taeniolabis taoensis. Within the Hanna Basin, no genera of multituberculates or peradectians from the Ferris Formation have been documented in strata both of Lancian and Puercan age; several examples of pseudoextinction, however, may exist through taxonomic artifact. Temporal range extensions include first: (1) Puercan records of Mesodma hensleighi and Ectypodus spp.; (2) records within Puercan Interval-zone Pu3 of Ptilodus sp. cf. P. tsosiensis; and (3) record in Puercan Interval-zone Pu2 of Catopsalis joyneri. In general, the Lancian multituberculate and peradectian faunas of the type Ferris Formation are similar to, although not nearly so diverse as, those from the type Lance Formation; the lower diversity almost certainly is an artifact of paucity of specimens available for study.
A new important record of earliest Cenozoic mammalian history: Eutheria and paleogeographic/biostratigraphic summaries
J. J. Eberle and J. A. Lillegraven
Keywords: biostratigraphy, Cretaceous, Eutheria, evolution, Ferris Formation, Hanna Basin, Lancian, Mammalia, Paleocene, Puercan, Wyoming.
We provide systematic description and discussion for each of 47 species of eutherian (“placental”) mammals from the type Ferris Formation of the western Hanna Basin, south-central Wyoming. All records are new for the basin, and for southern Wyoming in general. Although fossiliferous strata are both of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age, all but a few specimens (restricted to two genera) represent the Puercan. We describe five species of previously known genera as new, all of Puercan age. We also assign a previously described Puercan species to a new genus, to emphasize its distinctness. Ordinal categories are represented by Leptictida, Proteutheria, Condylarthra, and Cete. Except for fossils from lowest parts of the Puercan, the local early Paleocene fauna is biased taxonomically in favor of condylarths, animals of relatively large body size in early parts of the epoch. The taxonomic bias probably is due in part to fluvial size-sorting of specimens related to generally sandy, relatively high-energy depositional environments. The local Puercan mammalian fauna is of keen paleobiogeographic and biostratigraphic importance. Among the 59 species of Puercan mammals now known from the Hanna Basin (combining recently reported multituberculates and peradectians with eutherians considered here), at least 25 represent significant extensions of previously recognized geographic ranges (18 in northerly extent, 7 in southerly). Because many of those extended ranges involve major parts of the continent, confidence becomes greatly heightened in biostratigraphic utility of Puercan mammalian species for temporal correlations, even across vast distances of western North America. Composition of the Hanna Basin’s mammalian fauna confirms utility of most elements of the formalized interval-zones of Puercan time, established in the San Juan and Williston basins. We do, however, abandon use of Puercan Interval-zone Pu0 as impractical, and use an expanded concept of Pu1 instead. Other than that adjustment, we strictly apply original faunal criteria for recognition of Puercan Interval-zones Pu1 through Pu3 to zonation of the type Ferris Formation. The unusually great thickness of Puercan parts of the formation, combined with our high density of sampled fossiliferous localities, has led to recognition of many extensions of temporal ranges beyond those known previously for individual species. The biostratigraphic importance of this section, therefore, becomes elevated for potential studies in: (1) long-distance temporal correlation of Puercan strata; and (2) many aspects of biological evolution across North America during earliest Cenozoic time. Basal parts of Interval-zone Pu2 in the Ferris Formation uniquely show persistence of some mammalian species known elsewhere only in Pu1. Faunal change within Interval-zone Pu2 greatly exceeds that observed between Pu2 and Pu3, although the evolutionary distinctiveness of Pu2 and Pu3 are confirmed in the present study. The newly discovered mammalian assemblages of Puercan age from southern Wyoming exhibit virtually no geographic endemism. Although they share species both from northern and southern fossil-bearing realms of the North American continent, composition of the Ferris assemblages clearly is more similar to southern than to northern faunas under comparison.
Paleocene mammalian biostratigraphy of the Carbon Basin, southeastern Wyoming, and age constraints on local phases of tectonism
Ross Secord
Keywords: Paleocene mammalian biostratigraphy of the Carbon Basin, southeastern Wyoming, and age constraints on local phases of tectonism
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 Torrejonian or earliest Tiffanian, or possibly sample both NALMAs. In any event, the faunas are very close in age to the Torrejonian-Tiffanian boundary. Strata bearing the Grayson Ridge and Halfway Hill faunas were truncated by erosion, resulting in a previously 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.