FIRST RECORD OF POST-MIDDLE DESMOINESIAN (LATE CARBONIFEROUS) BRACHIOPODS IN THE GREAT BASIN (USA): IMPLICATIONS FOR FAUNAL MIGRATION IN RESPONSE TO LATE PALEOZOIC PALEOGEOGRAPHY

Journal of Paleontology, Mar 2007 by Pérez-Huerta, Alberto

ABSTRACT-

A brachiopod fauna from the Hogan Formation near Skunk Springs, western Utah, in the eastern Great Basin, USA, includes only the new taxon Duartea bruntoni n. sp., and other brachiopod taxa described for the first time in North America. The faunas are considered to be late Moscovian (late Desmoinesian) in age, which represents the first record of post-middle Desmoinesian brachiopod faunas within the Pennsylvanian of the Great Basin. Systematic analyses of the faunas suggest strong affinities with brachiopods previously described in South America and Russia, confirming the idea of faunal migration into the Great Basin. The migration of these faunas appears to correlate with the development of new oceanic currents in response to paleogeographic changes related to the formation of the Late Paleozoic Pangea.

INTRODUCTION

MID-CARBONIFEROUS TIME marks the beginning of the dominant climatic and paleogeographic conditions present during the Late Paleozoic. The onset of largescale glaciation on Gondwana (e.g., Mu et al., 1999) and the renewed closure of the subequatorial oceanic gateway (Raymond et al., 1990; Saltzman, 2003) are recorded at this time. Brachiopod faunas are an important source of information about the biotic response to these changes (Raymond et al., 1990; Kelley and Raymond, 1991; Dietl and Kelley, 2001). Previous studies based on database analyses of brachiopod genera suggest migration of faunas from southern high latitudes toward the subequatorial regions in the early Pennsylvanian with the closure of this subequatorial oceanic gateway (e.g., Raymond et al., 1990). This hypothesis is supported by the analysis of early Pennsylvanian faunas from western North America because affinities with faunas from South America were reported in previous studies (Derby, 1874; Kozlowski, 1914; Reed and Sedgwick, 1933; Dresser, 1954; Mendes, 1959; Branisa, 1965; Samtleben, 1972; Langenheim, 1990). It is not known, however, if such faunal affinities were present in the Great Basin region of North America and whether they extended into the late Pennsylvanian. The Great Basin regional disconformity places overlying lower Permian (Wolfcampian) in direct contact with middle Desmoinesian deposits (see Snyder et al., 1990). Previous studies described faunas from the late Desmoinesian (Moffet, 1986) and Missourian-Virgilian (Lipman, 1982) from areas located nearby (e.g., Arrow Canyon in southern Nevada), but not within the limits of the Great Basin.

This paper provides a systematic and taxonomic study of the first record of post-middle Desmoinesian (late Moscovian) brachiopods from the Great Basin. Additionally, it provides a preliminary discussion of affinities of faunas and patterns of faunal migration in response to paleogeographic changes.

GEOGRAPHIC AND GEOLOGIC SETTING

Brachiopod faunas described herein were found at a locality in Millard County, Utah, near the border between Nevada and Utah in the eastern part of the Great Basin (Fig. 1). Fossils were collected in Pennsylvanian strata of the Hogan Formation near Skunk Springs within the Conger Range (see Hintze and Davis, 2003) (Fig. 1). A stratigraphie section was measured in a core of an anticline because abundant brachiopod faunas were observed in a Pennsylvanian carbonate sequence (Fig. 2). Analyses of these brachiopods suggested a late Moscovian-early Kasimovian age (late Desmoinesian-early Missourian) based on other faunas recorded in southwestern and Midcontinent regions of North America (e.g., Dunbar and Condra, 1932; Sutherland and Harlow, 1973). The conodont species Gondolella bella Ellison, 1941 has been found at the stratigraphic levels SPOIV (two Pa elements) and SPOVI-SPI (fragments of Pa elements) (Fig. 2). This record gives a late Desmoinesian age to these deposits (Mendez, personal commun., 2002). The sequence is about 250 m thick and contains marine carbonates ranging from mudstones to grainstones. Limestones of wackestone-packstone facies with interbedded shales containing abundant brachiopods, bryozoans, and crinoid fragments are typical in the lower part of the sequence. The upper section is characterized by shallowing-upward, intermediate cycles (25-45 m in thickness),with limestones organized as mudstone-wackestone-packstone-grainstone/bindstone packages. At the top of the succession, there is a covered interval of 10 m, and just above that there are strata that may be Permian because of the occurrence of the fusulinid Triticites Girty, 1904 (see St. Aubin-Hietpas, 1983).

MATERIAL EXAMINED AND REPOSITORY

The results presented here are based on the study of more than 400 brachiopods extracted from 300 kg of rocks collected in the field. Most of the shells occur as poorly preserved calcareous specimens or molds. Specimens illustrated and described are housed in the collections of the Museum of Paleontology of the University of California, Berkeley (USA), with specimen numbers prefixed herein UCMP, ranging from the numbers 155632-155813 and 156600-156603 for the holotypes.