Sedimentology and paleoecology of the Lower Member of the Lower Triassic (Smithian-Spathian) Union Wash Formation, east-central California

Abstract

Most paleoecological and paleobiologic studies of the postextinction interval following the end-Permian crisis have been focused on rocks deposited in shallow shelf, open marine settings. Lower Triassic marginal marine environments have not been examined closely but represent a crucial piece of the postextinction puzzle since they provide an end member to which studies of open marine environments can be compared. The Lower Member of the Lower Triassic (Smithian-Spathian) Union Wash Formation is a mixed carbonate-siliciclastic succession deposited in environments ranging from intertidal to lagoonal and provides a glimpse into Early Triassic marginal marine settings and faunas. Fossil components of the member are dominated by echinoid spines and debris, which are interpreted to represent a parautochthonous fossil assemblage concentrated primarily in tidal-dominated distributary channels and as grain-rich mouth bars. This Early Triassic echinoid assemblage affords an opportunity to examine the paleoecology of echinoids during the postextinction interval and to speculate about how the extinction and recovery relate to long-term paleobiological trends. Echinoids of the Lower Member of the Union Wash Formation inhabited environments very proximal to the intertidal zone, and this distribution may have provided a survival mechanism in the wake of the end-Permian mass extinction by allowing echinoids to avoid shelfal stresses such as shallow-water anoxia and euxinia. These shallow-water echinoid forms would likely seed the diversification of all later echinoid taxa, and indeed, all later shelfal and deep-sea echinoids can be traced back to post-Paleozoic shallow-water forebears.