UNUSUAL LOWER TRIASSIC (SMITHIAN-SPATHIAN) SEAFLOOR PRECIPITATES, MICROBIALITES, AND CARBONATE CEMENTS FROM THE LOWER ALWA FORMATION, WADI ALWA MEGABLOCK, OMAN MOUNTAINS, OMAN: IMPLICATIONS FOR THE BIOTIC RECOVERY FROM THE PERMIAN-TRIASSIC MASS EXTINCTION

The Early Triassic was one of the most unique intervals of Phanerozoic time. Widespread evidence suggests that a mosaic of unusual environmental conditions were present during the period, and likely had an effect on shaping the timing and direction of the recovery from the Permian-Triassic mass extinction. Recent studies have revealed a variety of unusual Lower Triassic sedimentary textures and features that are typically associated with Proterozoic-age rocks (anachronistic facies), including seafloor calcium carbonate precipitates, shallow subtidal microbialite bioherms, wrinkle marks, and flat pebble conglomerates. The Smithian-Spathian lower Alwa Formation (found within the Wadi Alma Megablock) from the Oman Mountains, Oman, contains variety of unusual carbonate features and textures, including thrombolites, seafloor precipitates, and cavity-filling cements. The lower Alwa Formation is an ammonoid-bearing, pelagic Hallstatt-type limestone deposited on an isolated carbonate platform in the Neotethys. The lower Alwa Formation exhibits a thrombolitic texture in places, and the occurrence of stromatactis cavities throughout much of the unit implies that the mud may have had a gel-like consistency that allowed the formation of lateral cavity systems prior to complete lithification. The limestone also contains small seafloor precipitates that occur as crusts and occasional fans (typically < 1 cm high). The precipitates are interpreted as having a primary origin (as opposed to a diagenetic origin) based on draping of other sediments over the fans and the inclusion of ammonoid fossils within some precipitate fans or crusts. Slumping following lithification led to fracturing of the limestone and the precipitation of large, botryoidal aragonite cement (raggione) in fractures that crosscut primary fabric. The presence of both widespread microbialite and unusual calcium carbonate cements is likely related to unusual oceanic conditions during the Early Triassic. The sustained development of anoxic, alkaline deep waters in Early Triassic oceans and the upwelling of those waters likely led to the precipitation of inorganic calcium carbonate cements and the enhanced growth of microbial carbonate. These conditions would have also had a deleterious effect on local biota and impeded biotic recovery.