LOWER TRIASSIC OOLITES IN TETHYS: A SEDIMENTOLOGIC RESPONSE TO THE END-PERMIAN MASS EXTINCTION

Permian through Early Jurassic time has been characterized as an oolite-poor interval, with relatively high continental emergence identified as a possible reason for the paucity of oolites. Within this interval, however, oolite deposition peaked during the Early Triassic.

Lower Triassic oolites are found in a recurring facies association with anachronistic carbonates in the Tethyan region in a belt extending from northern Italy through Turkey into South China. The oolites can be regarded as “disaster deposits” that formed in the aftermath of the end-Permian mass extinction, just as basal Triassic stromatolites and other microbialites have been interpreted as “disaster forms.” In Early Triassic time, minimal grazing and bioturbation by marine benthos allowed for the widespread development of biomats and related structures. The concomitant decline of skeletal carbonate production reduced a major sink for seawater carbonate, thereby promoting the calcification of microbial structures and the inorganic precipitation of ooids (the so-called “default” mechanism of ooid formation). In addition to extinction-related ecologic controls on oolite deposition, widespread marine anoxia may have contributed to elevated carbonate saturation levels, so that oolite deposition was the product of ecologic and environmental controls operating interdependently.

Separate evidence for regarding certain oolites as “disaster deposits” comes from the Silurian of Sweden, where widespread oolites occur in close stratigraphic proximity to Wenlock-Ludlow extinction events. Like the Permian-Early Jurassic, the Middle Ordovician-Devonian was an oolite-poor interval. Although the vast majority of oolite deposits are unrelated to biologic crises, we suggest that upon closer examination many oolites, especially those in otherwise oolite-poor periods, may prove to be extinction-related sediments.