Brachiopod Paleontology and Paleoecology of the Lower Mississippian Lodgepole Limestone in Southeastern Idaho

Christensen, A.M., 1999, Brachiopod Paleontology and Paleoecology of the Lower Mississippian Lodgepole Limestone in Southeastern Idaho, in Hughes, S.S. , and Thackray, G.D. , eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of Natural History, p. 57-67. ABSTRACT Lodgepole Limestone strata (Lower Mississippianupper Kinderhookian to lower Osagean) is located throughout southeastern Idaho. The lower Paine Member of the Lodgepole Limestone represents basin deposits in the eastern part of a foreland basin, and the upper Woodhurst Member represents shallower deposition on a ramp directly west of a cratonic platform. A paleontological and paleoecological analysis was performed on the brachiopod samples and comparisons were made to other similar age localities. Four localities in southeastern IdahoBuckskin Hill, Little Flat Canyon, Gardner Canyon, and St. Charles Canyon were studied. Disarticulation, current stable positions, and size sorting of the fossils suggest quiet marine conditions with occasional storm events. The Paine Member was deposited below storm wave-base and the Woodhurst Member above storm and below fairweather wave-base. The fossils suggest deposition in a quiet, oxygenated environment with a soft substrate. The increasing number of individual specimens up section indicates the development of more stable conditions on the ramp through time. The Lodgepole Limestone contains thirteen different species of brachiopods from twelve genera. The population is composed of 37% impunctate spiriferids, 25% athyridids, 22% rhynchonellids, 14% punctate spiriferids, and 2% other (punctate orthids, strophomenids, and chonetids). Although there is no major lateral change between the four sections, the Buckskin Hill section may represent a paleo-high. Vertical changes in the sections show shallower water deposition and a significant increase in the number of fossils up section. The Lodgepole Limestones depositional environment and brachiopod content are very similar to the Banff Formation in Alberta, Canada. Only tentative correlations can be done with the midcontinent. INTRODUCTION Lower Mississippian (345-330 mya) strata of the Lodgepole Limestone (Madison Group) is found throughout southeastern Idaho in north to northwest-trending linear fault block mountains. A paleogeographic view of the western United States during this time would show the Antler highlands on the west, a foreland basin, and the western cratonic platform on the east (Poole and Sandberg, 1977) (Fig. 1). The Lodgepole Limestone in southeastern Idaho has been divided into the Paine and Woodhurst Members. The lower Paine Member accumulated in the eastern part of the foreland basin. The upper Woodhurst Member contains rocks consistent with deposition on a ramp directly west of the cratonic platform. The Lodgepole Limestone was deposited during a transgressive marine event during the Lower Mississippian (Craig and Varnes, 1979; Skipp and others, 1979). Brachiopods are generally the most abundant macrofossil found in the Lodgepole Limestone. Although no comprehensive study of the Lodgepole Limestone brachiopods has been made, brachiopods are well documented from Lower Mississippian rocks of western Canada and the midcontinent of the United States (Carter, 1968, 1972, 1987, 1988). The abundance of brachiopods in the Lodgepole Limestone, and descriptions of Lower Mississippian brachiopods from other localities made them a valuable macrofossil to study.