DIFFERENTIATING BETWEEN INHERITED AND PEDOGENIC MINERALS WITHIN PALEOSOLS: AN EXAMPLE OF RECONSTRUCTING PALEOCLIMATE DURING CHINLE FORMATION DEPOSITION

This study unambiguously identifies the products of pedogenesis in Late Triassic paleosols through the characterization of a pedogenically modified volcanic ash. The Chinle Formation of the Colorado Plateau is composed of pedogenically-modified fluvial sands and muds with a significant volcanic component. However, only one distinctive, air-fall volcanic ash layer has been identified in the Chinle Formation and resides within the basal Mesa Redondo Member. Although significantly modified by pedogeneisis, this horizon is identified as an ash layer based upon a well-preserved pyroclastic texture as well as a single age population of euhedral zircon (225.185±0.079 mya) that preclude post-depositional reworking.

Quantitative X-ray diffraction analysis indicates that the ash is now composed of 83% kaolinite, 6 % smectite, 9 % quartz, and 2% mica. Unlike other tonsteins, which are often found at the base of coal deposits, we interpret this tonstein to have formed pedogenically through recrystallization to kaolinite under wet climatic conditions that favor kaolinite formation over other clay minerals. Because the volcanic ash is devoid of an inherited detrital component, the neoformed weathering products give us insight into the minerals that are stable under prevailing climatic conditions.

Paleosols in the lower portion of the Chinle Formation are composed of between 5 to 20% kaolinite. We interpret this kaolinite to have formed pedogenically rather than to be detritally inherited because of the mineralogy of the tonstein. This is consistent with other paleoclimate interpretations which suggest that the lower Chinle Formation was deposited under conditions of high mean annual precipitation.