T122 New Insights into Triassic-Jurassic Transition Events and End-Triassic Mass Extinction

Monday, 28 October 2013
8:00 AM - 12:00 PM
Room 603

ANDEAN EVIDENCE FOR A LONG-TERM STATE SHIFT IN BENTHIC ECOLOGY FOLLOWING THE TRIASSIC/JURASSIC MASS EXTINCTION

Paper number: T122-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

New field investigations in the Central Peruvian Andes reveal ecological dominance of siliceous sponges across widespread shallow shelf habitats following the Triassic/Jurassic mass extinction. Six separate field sites in the Yauli Dome region (near La Oroya) provide records of chert and dolomite sediment accumulated on shoreface to outer-shelf habitats. The Aramachay Formation of the Pucara Group has been shown to represent the entire Hettangian and lower Sinemurian stages of the lowest Jurassic, and the sites can be correlated by ammonites and tuffs. At 70 – 180 m thick in this region, the Aramachay Formation presents profoundly more extensive records than other Triassic/Jurassic boundary sites around the world. Sponge body fossils are abundant in situ on bedding planes and cliff faces, and established persistent level bottom horizons traceable for hundreds of meters. Individual body fossils are most abundant in inner shelf settings, while compressed horizons of sponge material are persistent in mid-shelf deposits. The deepest site records sediment hashes of transported spicules in bedded cherts.

The siliceous sponge-dominated cherty deposits of the Aramachay Formation are a dramatic change from the carbonate biocalcifier-dominated dolomitic wackestones of the conformably underlying Upper Triassic Chambara Formation. The Aramachay Formation represents a facies change consistent with disturbances in Triassic/Jurassic carbonate records around the world (Greene et al., 2012, Earth Science Reviews 113:72-93). More importantly, facies change in the Central Peruvian record shows complete revolution in the metazoan contribution to sediment accumulation across the shelf that persisted for approximately two million years. The sponges likely responded both to habitats vacated by extinction (which selected against carbonate biocalcifiers), and to increased concentration of siliceous acid following weathering of the Central Atlantic Magmatic Province basalts. Comparison to strikingly similar facies in Nevada (USA), to more compressed records in the Austrian Alps, and to the more diverse record in the Moroccan High Atlas show that the ecological expansion of siliceous sponges to dominate shelf systems is a globally relevant ecological phenomenon and a response to the Triassic/Jurassic transition.