RIVERS, LOESS, AND VEGETATION; THE EARLY PERMIAN HALGAITO FORMATION IN THE VALLEY OF THE GODS, SOUTHEAST UTAH

During the late Paleozoic, the Uncompahgre Uplift sourced river systems of the Paradox Basin, headed towards the Panthalassa Ocean. In the Valley of the Gods area, sections 11 km apart were measured in the Early Permian Halgaito Formation, from the McKim Limestone to the overlying Cedar Mesa Formation, divided into two packages about 55 and 25 m thick.

The lower package has lenticular channel bodies of very fine sandstone and siltstone with trough cross-beds, ripple cross-lamination (commonly climbing), plane beds, and mudstone lenses. Carbonate-pebble conglomerates, desiccation cracks and adhesion features are present. The channel bodies represent broad, shallow sandbed rivers with seasonal flow. Plant material includes impression remains of the conifer Walchia, stems of calamitaleans, tree ferns, and rarely lycopsids, and root traces. Diplichnites cuithensis trackways are present in one channel body, as well as other abundant vertical and horizontal burrow ichnofossils. Laminated, bioturbated sandstone and shale a few metres thick represent shallow lakes. Sheets of red, weakly stratified siltstone and very fine sandstone are identified as loess. In places, they form 4 m stacks incised by channel bodies with steep and overhanging margins. Elsewhere, loess lenses are present within channel bodies and lake deposits.

The upper package has a high proportion of loess sheets up to 2 m thick, stacked into composite units and penetrated by deep root traces. They are intercalated with thin channel bodies and poorly developed paleosols. Even thin loess layers are traceable for several kilometres. Aeolian sandstone of the Cedar Mesa Formation abruptly overlie the package.

The succession records progressive aridification from rivers and lakes with plant and animal fossils to stacked, rooted loess sheets below aeolian dunes. A key feature is the dynamic nature of the landscape, with interaction between rivers, lakes and eolian sediment as drainage adjusted to aeolian aggradation on floodplains and in channels. Vegetation played an important role in stabilizing the landscape after aeolian events.