A PALEOENVIRONMENTAL AND ICHNOLOGIC ANALYSIS OF THE UPPER MISSISSIPIAN PORTION OF THE REST SPRING SHALE OF INYO COUNTY, CALIFORNIA

The Upper Mississippian – Lower Pennsylvanian Rest Spring Shale of eastern California was deposited during the latter stages of the Antler Orogeny, within the deep foreland basin associated with the emplacement of the Roberts Mountain Allochthon (Laudon, 1989). The foreland basin was present along the eastern edge of Panthalassa and the western edge of the Kaskaskia Epicontinental Sea during the Late Mississippian, and provides a means to examine paleoenvironmental conditions at the juncture between open ocean and epicontinal sea within an active tectonic setting. The Rest Spring Shale was examined at the Willow Springs Canyon locality, Mazourka Canyon, Inyo Mountains, California. The Rest Spring Shale was previously studied by Laudon (1989), who conducted a regional sedimentologic analysis of the unit, and broke it up into 4 informal members based on lithology. The lowermost, Upper Mississippian unit was examined by this study via gamma ray spectrometry, trace fossil analysis, and petrographic investigation. Subunit I (the lowermost subunit) consists of black, laminated, silty shale that contains various trace fossils that are part of the Nerites ichnofacies, and are present as looping traces on bedding surfaces (Laudon, 1989); ichnogenera include Chondrites, Scalarituba, and Planolites. Th/U values from the Rest Spring Shale imply that the unit was primarily deposited under anoxic conditions. Therefore, the trace fossils found within the unit are thought to be the remnants of activity from doomed organisms that were dragged down into deeper, anoxic waters by turbidity currents, which also supplied a limited amount of oxygen to the environment (Follmi and Grimm, 1990). The abundance of bedding-plane traces over the 80m-thick study interval implies a tectonically-active setting that frequently introduced doomed émigré trace makers to the basin.