METEORIC DIAGENESIS OF CHINLE SANDSTONES, PETRIFIED FOREST NATIONAL PARK (ARIZONA, USA): A RECORD OF LATE TRIASSIC CLIMATE CHANGE

Tuffaceous fluvial sandstones of the Late Triassic Chinle Formation at Petrified Forest National Park (PEFO), Arizona contain a well-preserved paleoclimate record. Petrographic analysis of 32 Chinle sandstones provides insights into this record and evaluates the linkage between climate and meteoric diagenesis during the Late Triassic in western North America.

Chinle sandstones consist of lithic subarkose, lithic arkose and feldspathic litharenite composed of poorly to moderately sorted, angular to subrounded, fine to coarse grained sand. Chinle sandstones show distinct stratigraphic variations in the abundance of matrix and framework grains, and intensity of feldspar weathering and mineralogy of clay cements. The abundance of volcaniclastic grains indicates waxing and waning episodes of volcanic activity. Feldspar weathering intensity is high in the Lower Chinle. Lower Chinle feldspars exhibit partial or complete dissolution whereas Upper Chinle feldspars are mostly unweathered. Matrix abundance gradually increases through lowermost Chinle, reaches its highest abundance in the middle portion of the Chinle, and then decreases through the upper portion of the Chinle. Clay cements vary from kaolinite at the base of the Chinle to smectite elsewhere, with the exception of chlorite cement within a thin interval of the Blue Mesa Member (lower portion of the Chinle).

Climate records from paleosol geochemistry indicate an overall transition from humid to dry conditions during Chinle deposition. This trend is also reflected in the meteoric diagenetic features of Chinle sandstones. During deposition of the Mesa Redondo and Blue Mesa members, elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite while feldspars were partially or completely dissolved. A transition over time to dryer conditions favored the formation of smectite cement and reduced feldspar dissolution.