Paleosols as an indicator of ancient landscapes, climates and stratal response during the Triassic: the Salt Anticline Region of Utah.

Abstract

Modern soils are excellent indicators of environmental conditions, and paleosols can be used as proxy for past conditions. This dissertation documents the response of Triassic paleosols and fluvial sediment accumulation to 1) landscape dynamics influenced by subsurface salt movement and 2) variations in paleoclimatic conditions in eastern Utah. The first chapter is an introduction to the research. The second chapter examines the influence of salt tectonics on cyclic stratigraphy, paleosol morphology, and sedimentation rates. The distribution of paleosols are documented within a sequence stratigraphic framework to evaluate whether salt tectonics influenced patterns fluvial sediment dispersal. The Chinle Formation is divided into two sequences that are in turn composed of decameter-scale fluvial aggradational cycle sets (FACSET) and meter-scale fluvial aggradational cycles (FAC). Facies and paleosol distributions within this stratigraphic framework suggest that a minibasin initially increased and then decreased in diameter and that the axis of subsidence drifted to the northwest through the Late Triassic. The third chapter documents how ancient slopes, a geomorphic expression of halokinesis, controlled paleosol development through a traditional catena perspective. Paleoslopes on a major sequence boundary that extends across the minibasin control associated paleosol development. Pedotopographic influences included both downslope redistribution of sediment and change in soil hydrology. The fourth and final chapter utilizes a multi-proxy paleosol approach to reconstruct climatic and atmospheric conditions during the deposition of Triassic fluvial sediments and the formation of paleolandscapes associated with halokinesis. Paleoclimate reconstruction supports the occurrence of a global pluvial episode during the Carnian (late Middle Triassic) that contrasted with mostly arid climates during the Middle to Late Triassic. Arid (and near arid) episodes in the latest Moenkopi and upper Chinle contradict monsoonal seasonality, whereas the lower Chinle pluvial suggests a non-continental climate similar to the modern eastern United States. Drastic changes in rainfall and soil types indicate that the Middle to Late Triassic in eastern Utah was an interval of dynamic climate change that may partially be tied global carbon dioxide levels and tectonism.