Triassic loessite of north-central Utah; stratigraphy, petrophysical character, and paleoclimate implications

Abstract

ABSTRACT Even though paleoloess (loessite) deposits serve as important stratigraphic markers and can imply specific environmental and/or climatic conditions, few have been described in detail. The Mahogany Member of the Triassic Ankareh Formation of north-central Utah contains a well-exposed example of an ancient loess. Although difficult to trace regionally, its distinguishing characteristics are uniform massiveness (lacking visible internal structure or bedding) over a 37 m thickness and a silt grain-size distribution. The upper 2.6 m of the loessite contains a distinct, vertical color zonation with lithologic changes indicating development of an immature paleosol. Whole-rock chemical analyses indicate trends in carbonate and clay (aluminum) that are consistent with a paleosol at the top of the loessite. A hand-held gamma ray spectrometer yields natural gamma counts that may distinguish the paleoloess as a stratigraphic unit, correlatable to the subsurface. Both the loessite and the superimposed immature paleosol (at the top of the loessite) can serve as important proxies for paleoclimate and may further represent a Middle Triassic record that is generally missing throughout much of the Western Interior. Analogous sedimentary deposits that have routinely been grouped as floodplain deposits may, on closer examination, be found to have physical characteristics indicative of loess deposition. Recognition of large, regional-scale loessites could be useful indicators of global climate changes and monsoonal influence associated with varying plate configurations.