Alternate ecological State shifts during the Triassic-Jurassic transition: observations from the Pucara group, Peru

Chert and carbonate deposits of the Pucará Group in central Peru preserve continuous deposition of shallow marine ramp sediments across the Triassic-Jurassic transition. The formations of the Pucará provide an opportunity to examine the contribution of metazoan sedimentation throughout a time interval rarely preserved in the rock record, including marine ecosystems before and after the end-Triassic Mass Extinction. This work presents microfacies analyses from three sites within the Yauli Dome of the central Peruvian Andes and finds facies dominated by metazoan biocalcifiers (such as gastropods and bivalves); by metazoan biosilicifiers (such as demosponges); a mixture of the two; or facies lacking obvious metazoan input. The distribution of the defined microfacies within the stratigraphy of the three sites suggests that the biocalcifiers and biosilicifiers represent two alternate stable ecological states. 87Sr/86Sr analyses reveal values that are congruous with previously published values of seawater for the Rhaetian and Hettangian stages (~208.5-199.3 Mya), adding further age constraints to these deposits. These geochemical analyses also provide support for Mn/Sr (g/g) screening as a method for evaluating diagenetic overprinting of marine carbonates. The eruptions of the Central Atlantic Magmatic Province are likely a primary driver of the decrease in biocalcifier sedimentation from the basal Chambará Formation into the middle Aramachay Formation. This examination of these chert and carbonate deposits predicts fine-scale sequence stratigraphic patterns to be tested in future work.