Unusual intraclastic limestones in Lower Triassic carbonates and their bearing on the aftermath of the end-Permian mass extinction

Flat pebble conglomerates were a common carbonate facies in Cambrian to Ear ly Ordovician open marine settings, but they become extremely rare in these environments after this time. However, the Early Triassic witnessed an ana chronistic reappearance of flat pebbles, together with other intraclast typ es, in a range of carbonate depositional settings. In south China, flat peb ble conglomerates are encountered in storm-dominated, platform carbonates t o deep basinal settings, while prefossilized bivalve intraclasts and flat p ebbles are common in mid-ramp facies of northern Italy. The emplacement mec hanisms of the intraclast-bearing beds appear to have been diverse and to h ave included basinal turbidity Bows and storm-generated hyperconcentrated f lows: true storm beds, deposited under combined Bow conditions, are rare. T he cause of the widespread early lithification implied by the Early Triassi c intraclasts appears to have been twofold: suppression of bioturbation, al lowing the preservation of thin beds, and rapid submarine lithification. Bo th features appear to be a response to the widespread development of benthi c dysoxia/anoxia during and following the end-Permian mass extinction. This event appears to have temporarily recreated the conditions that pertained in Cambro-Ordovician shelf seas. Flat pebble conglomerates may, therefore, constitute a proxy indicator of stressed environmental conditions associate d with global anoxic/dysoxic events.