CONODONT BIOSTRATIGRAPHY OF THE VIRGIN LIMESTONE AT LOST CABIN SPRING: IMPLICATIONS FOR THE STUDY OF EARLY TRIASSIC BIOTIC EVENTS

The aftermath of the End Permian mass extinction is not yet well-understood, but low-diversity faunas, geochemistry, unusual lithologic features and evidence for anoxia suggest that environmental conditions were not conducive to biotic recovery during the Early Triassic (e.g., Hallam and Wignall, 1997; Payne et al., 2004; Pruss et al., 2006). The Lower Triassic of the western United States has been an invaluable natural laboratory for studying the aftermath because of its well-exposed mixed carbonate and siliciclastic strata. The Virgin Limestone Member of the Lower Triassic Moenkopi Formation at Lost Cabin Spring has received particular attention because of its well-exposed microbialite build-ups and because its upper Lower Triassic strata contain the transition to the biotic recovery (Fraiser and Bottjer, 2007) which began in earnest during the Anisian.

The Virgin Limestone Member section at Lost Cabin Spring has been assumed to be Spathian in age based on the occurrence of the ammonoid Tirolites in the Virgin Limestone Member near St. George, Utah about, 220 km to the northeast (Poborsky, 1954). Recent radiometric dating suggests that the Spathian represents 3 of the 5 million years of the Early Triassic (Ovtcharova et al., 2006). Given the attention that the Lost Cabin Spring section has received and its importance for understanding the mass extinction aftermath, it is vital to better constrain the timing of this section. Here we present preliminary results from an ongoing investigation of the conodont biostratigraphy of Lost Cabin Springs.

The top of the section is probably Anisian in age based on previously-reported strontium isotope results (Marenco, 2007). The Spathian-Anisian boundary is still uncertain, but the strontium isotope data suggests that the boundary is well above the sponge-microbe patch reefs that occur approximately 155 m from the base of the section. Our tentative identification of Triassospathodus homeri group P1 elements at 195 m supports our interpretation of the strontium isotope results and a Spathian age for the sponge-microbe patch reefs. Conodont elements from the base of the section are either Smithian or Spathian in age pending further analysis.