Abstract

The Upper Pennsylvanian (Virgilian) Halgaito Formation (HF) is an ∼ 125–155-m-thick succession of carbonate and carbonate-cemented siliciclastic strata exposed along Cedar Mesa on the Colorado Plateau in southeastern Utah. Defining the stratigraphic standing of the HF has been problematic due to differing paleoenvironmental and paleogeographic interpretations. This stratigraphic confusion is likely because the HF and overlying Cedar Mesa Sandstone lie at the interface between the underlying, predominately marine carbonate Honaker Trail Formation and the overlying, alluvial-eolian Organ Rock Formation. This study uses a combined ichnological, paleopedological, and sedimentological approach to refine the paleoenvironmental and paleogeographic history of the HF. The lower, marine portion of the HF is predominately fossiliferous packstone and calcareous sandstone, containing abundant Scalichnus and Thalassinoides. Marine sandstone beds contain shallow rhizoliths and indicate a relative sea-level drop that was punctuated by at least four, small-scale transgressions. The upper, continental section contains predominately eolian siltstone with siliciclastic Entisols and mottled Inceptisols. These paleosols contain large rhizohaloes and calcareous rhizocretions that are commonly associated with abundant Naktodemasis bowni. The uppermost ∼ 40 m of strata are laminated and crossbedded siltstone that contain little to no paleopedogenic development and few ichnofossils. These eolian beds were likely sourced from the erosion of underlying marine strata of the Hermosa Group as the Elephant Canyon Seaway regressed northward during the Pennsylvanian. Thin, coarse-grained fluvial strata can be observed throughout the HF and were likely sourced from the highlands of the Uncompahgre Uplift.

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