Paper No. 236-2
Presentation Time: 1:15 PM
DEPOSITIONAL AND CLIMATIC HISTORY OF THE LATE TRIASSIC CHINLE FORMATION: A BASIN SCALE COMPARISON, SOUTHWESTERN UNITED STATES
To date, rigorous scientific investigations of the Late Triassic Chinle Formation of the Southwestern United States have almost exclusively been focused in Petrified Forest National Park (PEFO), Arizona. This study, located 180 km northwest of PEFO, provides an external counterpart that allows deconvolution of the Chinle depositional and climatic trends at the basin scale. A continuous, 190 m thick section is well exposed at Ward’s Terrace located adjacent to the eastern bank of the Little Colorado River in northern Arizona. Preliminary zircon U-Pb analyses suggest dates consistent with the uppermost Blue Mesa and Owl Rock Members at PEFO. This indicates that documented trends in climate and deposition, particularly aridification and subsidence rates, are also captured at Ward’s Terrace. Meter-scale, fluvial aggradational cycle (FAC) analysis has yielded 77 FACs within 2 hectometer-scale, composite sequences. From base to top, the lowermost composite sequence is characterized by a transition from drab colored and weaker developed paleosols to more oxidized, better-developed paleosols with abundant pedogenic carbonate. The lowermost composite sequence is capped by limestones, which are possibly lacustrine in origin. The uppermost composite sequence is characterized by a progressive increase in pedogenic carbonate development from stage I at the base to stage IV at the top.
These trends are similar with those observed at PEFO, where humid climatic conditions precede semi-arid to arid climatic conditions through the Late Triassic. Composite sequences in PEFO are also comparable. At PEFO, composite sequences have been documented to be the result of Cordilleran arc plutonism, and associated changes in paleoflow are suggested to coincide with a rapid acceleration in basin subsidence. At Ward’s Terrace a marked rise in base level is similarly observed at the transition between composite sequences; this may also be the result of foreland basin tectonism.