THE CLAY MINERALOGY OF PALEOSOLS IN THE CHINLE FORMATION AT PETRIFIED NATIONAL FOREST: INSIGHTS INTO WEATHERING AND CLIMATE DURING THE LATE TRIASSIC

Pedogenically formed clay minerals are useful indicators of climatic conditions because they are the most abundant weathering product at Earth’s surface and their formation is largely temperature and moisture dependent. Thus, changes in mean annual temperature (MAT) and precipitation (MAP) should be observable through stratigraphic changes in clay mineral assemblages. The Chinle Formation at Petrified Forest National Park is a thick succession of pedogenically modified overbank deposits within which a bulk paleosol geochemical climate model has revealed a significant climatic shift during the late Triassic. This bulk geochemical model, although useful for documenting shifts in paleosol chemistry, reveals little about changes in the climate-dependent weathering pathways that drive them. This study uses quantitative X-ray diffraction to characterize the clay minerals in the Chinle formation at Petrified Forest and documents a marked change in both clay types, abundances, and structure.

Clays make up an average of 65% of Chinle paleosols and include the following clays in decreasing order of abundance: mixed layer illite/smectite, illite-mica, kaolinite, palygorskite, and chlorite. The basal portion of the Chinle contains abundant neoformed kaolinite comprising 11% of the whole rock whereas in the upper Chinle kaolinite is almost absent. The sudden loss of kaolinite in paleosols younger than 215 Ma reflects a climate shift involving increased aridity. Further up section, palygorskite comprises 5% of uppermost Chinle paleosols indicating a continuing decline in precipitation as western Pangaean environments evolved into a desert during the Jurassic. The pedogenic threshold from kaolinite to smectite occurs around 1000 mm of precipitation in modern environments and palygorskite formation is restricted to areas with MAP less than 300 mm/yr. Mixed layer illite-smectite is the most abundant clay mineral in Chinle paleosols. The proportion of illite to smectite interlayers decreases up-section, averaging 24% illite layers. The results of the study support the assertion that quantitative x-ray diffraction analysis of clay minerals in paleosols can be a powerful tool for understanding climate controlled chemical reactions.