Paper No. 4
Presentation Time: 8:50 AM
TECTONIC AND CLIMATIC CONTROLS ON CYCLIC ALLUVIATION AND BIOLOGIC TURNOVER WITHIN THE LATE TRIASSIC CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, ARIZONA
Based upon a framework of high-precision U-Pb zircon geochronology, we provide a sedimentologic and paleopedologic assessment of the mechanism(s) that may account for both large-scale alluvial cyclicity and a pronounced biologic reorganization preserved within the Late Triassic Chinle Formation at Petrified Forest National Park (PEFO), Arizona. The most conspicuous sedimentologic trend within the Chinle Formation at PEFO is the organization of alluvial deposits within 103 meter- to decameter-scale fluvial aggradational cycles (FACs), and 2 fining-upward, hectometer-scale composite sequences. Each composite sequence consists of interbedded channel sandstones, conglomerates and overbank mudrocks that stack within an overall fining-upward trend, and have a corresponding upward decrease in the proportion of volcanic rock fragments. U-Pb age dates from tuffaceous beds constrain the boundary between the composite sequences to ca. 218 Ma. Paleocurrent indicators within the lower composite sequence suggest varied, but dominantly eastward paleo-flow, whereas indicators within the upper composite sequence indicate consistently northerly paleoflow. Compilation of age versus thickness trends for FACs indicate that the transition in paleoflow direction is nearly coincident with the onset of rapid acceleration in basin subsidence. Taken collectively, these observations are consistent with long-period cyclic alluvial response to pulses of foreland tectonism.
A marked change in the character of paleosols at ca. 216 Ma document a relatively abrupt transition from humid to dry conditions. Paleosols prior to 216 Ma are characterized by drab coloration and weaker development, and paleosols after this time are more brightly colored, better-developed and contain abundant pedogenic carbonate. This climatic transition coincides with a regional turnover in both terrestrial vertebrates and plants, i.e., the Adamanian-Revueltian faunal turnover. The occurrence of both the biologic reorganization and climatic shift near the 218 Ma composite sequence boundary indicates that Late Triassic ecologic conditions across what is now northeastern Arizona may have been influenced by the foreland tectonic processes that likely accounted for composite sequence-scale patterns of sediment accumulation.