Abstract

Middle Cretaceous (Turanian to early middle Coniacian) paleogeography of Utah is described from the context of changes in the stratal architecture of four substage (1- to 2-m.y.) chronostratigraphic units, termed stratigraphic sequences. The four stratigraphic sequences are bounded by regional base-level rise unconformities or by base-level rise-to-fall turnaround points in conformable stratal successions that record intermediate-term base-level cycles. The four stratigraphic sequences are arranged in an offlapping pattern, and they step progressively basinward (eastward) to a late Turonian regressive maximum. This offlap geometry began and ended with basin-scale transgressions initiated in the early Turanian and latest Turonian. The end of the regional transgression and the beginning of the four offlapping stratigraphic sequences is the turnaround point of a long-term base-level cycle, 4.5 m.y. in duration. The next transgression is the end of this cycle and the beginning of the second of four long-term base-level cycles recorded by Upper Cretaceous strata of Utah.

Regional variations in the stratal architecture of intermediate-term stratigraphic sequence are related to southward migration of depocenters along the western margin of the mid-Cretaceous seaway. These basin-wide changes in sediment supply and accommodation are superimposed on progressive eastward shoreline progradation. With respect to the thrust belt axis, oblique and transverse depositional patterns of migrating depocenters in the foreland are predictably related to high and low accommodation and sediment supply regimes, respectively. In the late Turonian, unique high-accommodation and sediment-supply conditions combined in central Utah and are recorded by maximum shoreline regression of the youngest Turonian sequence.