RECONSTRUCTING LATE TRIASSIC ENVIRONMENTAL CONDITIONS USING PALEOSOL MINERALS ASSEMBLAGES FROM THE CHINLE FORMATION, ARIZONA, USA

The Chinle Formation, located in the Petrified Forest National Park, is composed of alluvial channel and overbank deposits. The Chinle Formation is comprised of several members which include (from youngest to oldest): the Mesa Redono, Blue Mesa, Sonsela, Petrified Forest, and Owl Rock members. One of the most distinctive features of the Chinle Formation is the presence of ancient soils (paleosols) that formed on the overbank muds. Soils that are preserved in the rock record provide information about the Earth’s ancient climate because they form at the intersection of the lithosphere and atmosphere.

In order to reconstruct environmental conditions during Chinle deposition, the mineral assemblages and abundances from each of the members were determined using x-ray diffraction. Clay minerals were identified using oriented clay mounts, and mineral abundance was semi-quantitatively determined using the area under the 100% intensity peak. It was found that the Mesa Redono and Blue Mesa members of the Chinle Formation contain relatively large amounts of kaolinite and diminished amounts of feldspar. Kaolinite is absent or in low abundance in the Sonsela, Petrified Forest, and Owl Rock members. However, these units have relatively larger amounts of feldspar. Smectite becomes volumetrically significant in the Blue Mesa member, but has the highest concentration in paleosols of the Sonsela member. Additionally, calcite is absent from the Mesa Redono, Blue Mesa, and Sonsela members, but becomes volumetrically significant in the Petrified Forest, and Owl Rock members. These trends in paleosol mineralogy indicate that the environmental conditions changed during Chinle deposition - the lower part of the section experiencing wetter conditions, and the upper part of the section experiencing dryer and hotter conditions.

The paleosols’ clay mineralogy was then compared to the abundance of volcanic grains in adjacent sandstones. There seems to be no relationship between the amount of volcanic sandstone framework grains and the amount of smectite present. This suggests that the increase in clay is a result of increased weathering in response to climate change rather than volcanic activity.