Abstract

The lithostratigraphic boundaries of the Upper Cretaceous Dakota Formation in southwestern Utah have long been problematic. Detailed studies in recent years have helped characterize the sedimentology and stratigraphy of the Dakota Formation in the Kaiparowits Plateau region, but outcrops to the west on the Markagunt Plateau remain relatively little studied. Recent geologic mapping on the Kolob Terrace adjacent to Zion National Park indicates that the stratigraphic positions of both the lower and upper contacts of the Dakota Formation should be revised. This reinterpretation will likely lead to improved understanding of lateral facies changes within basal Cretaceous strata across southern Utah, which in turn should provide new insights into the nature, timing, and interplay of tectonic and eustatic depositional events in the Cretaceous foreland basin of the Sevier orogen.

In southern Utah, the lower contact of the Dakota Formation has traditionally been placed at the base of a laterally persistent unit of quartzite- and chert-pebble conglomerate and conglomeratic sandstone, coincident with the basal Cretaceous unconformity. Strata that overlie the conglomeratic unit consist of a relatively thin sequence of drab to variegated, bentonitic mudstone locally containing carbonate nodules and barite crystals, interbedded with minor sandstone, organic shale, and volcanic ash. Recent pollen analyses indicate an Albian or older age for these strata; radiometric dating results from an ash layer are pending. Age and lithologic similarities suggest that these strata and the underlying conglomeratic unit are correlative with the Lower Cretaceous Cedar Mountain Formation. Thus, the basal Dakota contact should be placed above the mudstones, at the base of the overlying coal-bearing sequence dominated by fluvial (overbank) sandstone and siltstone.

On the Markagunt Plateau, the upper contact of the Dakota Formation has traditionally been placed just above the upper of two laterally continuous, relatively thick coal zones within the Dakota. However, this convention places a ledgy sequence of interbedded sandstone, siltstone, organic shale, and thin coal within the lower part of the Tropic Shale, below more typical Tropic outcrops characterized by slope-forming, septarian nodule- and ammonite-bearing, fine-grained sandstone and shale. Invertebrate faunal assemblages that include Crassostrea and Inoceramus pictus, as well as palynomorph assemblages, indicate that the ledgy sequence includes shallow marine, brackish, and fresh-water deposits of Cenoma-nian age. This sequence is therefore more appropriately considered to be an upper member of the Dakota rather than basal Tropic Formation, and the contact should be placed at the base of the slope-forming marine shale and sandstone above the highest coal and ledge-forming sandstone. In the vicinity of Zion National Park, the upper Dakota-Tropic interval records a change from estuarine swamp/marsh depositional environments to a more open-marine environment around the time of the Cenomanian-Turonian boundary.