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Preliminary Results from the Mixed Carbonate-Siliciclastic, Lower Permian Halgaito Formation, Southeastern Utah: Application of Ichnopedofacies for Interpreting Environments of Deposition


The Upper Pennsylvanian (Virgilian) Halgaito Formation contains a succession of thinly interbedded, siliciclastic and carbonate deposits and is time-equivalent with the Lower Cutler Group. Mixed carbonate-siliciclastic systems are common in the geologic record, but few studies have focused on them. Mixed siliciclastic and carbonate systems may be vertically and laterally complex and often display rapid changes in lithofacies at a small scale. These rapid changes may prove challenging for oil and gas exploration, as they may be too small to be seen with standard subsurface method (i.e. seismic imaging, well logs). This study provides new data on the diversity, distribution, abundance, and ichnofossil-pedofacies-facies relationships within the Halgaito Formation and tests whether trace-fossil assemblages vary predictably with environmental conditions through a combined ichnopedologic approach. The Halgaito Formation displays three distinct depositional units. The base of the formation is dominated by primary carbonates and thinly bedded, fine-grained siliciclastics interpreted as turbidity deposits. Vertically oriented clam burrows similar in morphology to modern razor clam burrows dominate horizons within the carbonate beds. The uppermost clam burrow beds are overprinted by a weakly developed paleosol and are crosscut by rhizoliths that extend down an average of 12 cm, cross-cutting 7 cm of carbonate. Siliciclastic sheetflood deposits with some relatively thin trough cross-bedded channel deposits dominate the middle unit of the formation. The paleosols within this middle section are relatively well developed, showing subangular blocky pedogenic structure, few remnant sedimentary features, abundant carbonate nodules, and distinct upper gleyed horizons. These paleosol horizons average between 0.5 m and 1 m thick. The middle section contains pedogenic carbonate beds associated with siliciclastic paleosols, as well as abundant, deeply penetrating (up to 2 m) rhizoliths and associated rhizohalos. The uppermost section of the formation displays a transition to a channel-dominated system with thicker trough crossbedded sandstones and is associated with weaker paleosol development than seen in the middle section. Preliminary ichnopedological units seen in these groups include: 1) weakly developed overprinted carbonate ichnopedounits, 2) well-developed siliciclastic ichnopedounits, and 3) weakly to moderately developed siliciclastic ichnopedounits.