U-Pb dating of redeposited volcanics in non-marine sedimentary strata: case studies from the early Mesozoic.

IRMIS, RANDALL; MUNDIL, ROLAND; MARSICANO, CLAUDIA; MANCUSO, ADRIANA

Los Angeles

Congreso; 73rd Annual Meeting Society of Vertebrate Paleontology; 2013

Radioisotopic dating of vertebrate fossil assemblages typically relies on primary volcanic deposits interbedded with the fossiliferous strata, such as airfall tuffs and ashes. However, the nature of non-marine sedimentary systems means that these units are often altered or destroyed before they can be preserved by final burial. Therefore, find suitable material to analyze, particularly in settings with lower sedimentation rates, can be particularly difficult. The recent application of CA-TIMS analyses to individual detrital zircons from redeposited sediments (e.g., fluvial sandstones containing volcanic detritus) has greatly improved our ability to provide absolute age constraints for vertebrate fossil assemblages in the form of maximum depositional ages. When multiple ages are consistent with stratigraphic superposition, and combined with other geochronologic data, they can precisely date important fossiliferous bearing beds that were previously poorly constrained by the lack of primary volcanic deposits. We have recently successfully applied these methods to a variety of early Mesozoic non-marine records that preserve important data of vertebrate evolution. Samples from the Middle Triassic Chañares Formation of northwestern Argentina reveal that its assemblage of early dinosauromorphs and cynodont synapsids is likely to be no older than middle Ladinian, and therefore only a few million years older than the oldest dinosaurs. Our recent work in the Upper Triassic Chinle Formation of northern Arizona has demonstrated that the major biotic turnover in the middle of the unit is dated middle Norian, rather than Carnian-Norian boundary. Finally, new analyses from the La Quinta Formation of Venezuela demonstrate that a critical bonebed of a new taxon of early ornithischian dinosaur, which was previously dated to anywhere between the Late Triassic and Middle Jurassic, has a maximum depositional age of earliest Jurassic. These data collectively demonstrate that U-Pb zircon ages of redeposited volcanics from mixed sources can be used to construct a precise time framework in order to study major evolutionary events on vertebrate history.