Facies and Diagenesis of Triassic (Austrian Alpine) and Upper Permian (New Mexico-Texas) Platform-Margin Carbonates: A Comparison

S. J. Mazzullo, Harald Lobitzer

Platform-margin carbonates of the Austrian Middle and basal Upper Triassic (Carnian) (Wettersteinkalk) and Upper Permian (Capitan reef, Guadalupian) are similar in terms of facies development, biota, and syndepositional diagenetic fabrics. These similarities are noteworthy because of their occurrence across the Permian-Triassic boundary, and the fact that these carbonates are distinct from younger (Upper Triassic Dachsteinkalk) depositional systems. The Upper Permian and Middle to basal Upper Triassic were deposited as subdued rimmed platforms with component lagoonal, reef-flat, reef, and fore-reef facies belts. The reef facies appear to represent upper slope marginal mounds, and consist mainly of sponges, solitary corals, Tubiphytes, and various encrusting biota; typical y, this facies is pervasively marine cemented. The reef-flat facies consist of similarly lithified biograin-stones deposited in high-energy wave-base zones along the platform margins. By contrast, steep rimmed platforms with massive framework coral buildups and relatively less syndepositional marine cements dominate in Upper Triassic units (e.g., Dachsteinkalk, Steinplatte reef). Proximal fore-reef beds in the Upper Permian and Middle to basal Upper Triassic are characterized by carbonate-clast debris cemented by coarse crystalline carbonate. The precipitation of this cement ("grossoolith") may have occurred during early shallow burial of the fore-reef beds. Similar depositional and diagenetic facies are poorly developed in the Upper Triassic, instead being represented mainly by bedded m crites.

The similarity of Permian and Triassic facies development is an integral aspect of the evolution of carbonate platforms through time, and the rapid change to steep rimmed platforms in the Late Triassic records a major shift in platform depositional style that persists into the Holocene. Although the reasons for such changes are unknown, they immediately postdate the marked change in land floras recognized throughout Eurasia.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.