Sequence stratigraphy of the Dakota Formation (Cenomanian), southern Utah: interplay of eustasy and tectonics in a foreland basin

Abstract

The Dakota Formation in southern Utah (Kaiparowits Plateau region) is a succession of fluvial through shallow‐marine facies formed during the initial phase of filling of the Cretaceous foreland basin of the Sevier orogen. It records a number of relative sea‐level fluctuations of different frequency and magnitude, controlled by both tectonic and eustatic processes during the Early to Late Cenomanian. The Dakota Formation is divided into eight units separated by regionally correlatable surfaces that formed in response to relative sea‐level fluctuations. Units 1–6B represent, from bottom to top, valley‐filling deposits of braided streams (unit 1), alluvial plain with anastomosed to meandering streams (2), tide‐influenced fluvial and tide‐dominated estuarine systems (3A and 3B), offshore to wave‐dominated shoreface (4, 5 and 6A) and an estuarine incised valley fill (6A and 6B). The onset of flexural subsidence and deposition in the foredeep was preceded by eastward tilting of the basement strata, probably caused by forebulge migration during the Early Cretaceous, which resulted in the incision of a westward‐deepening predepositional relief. The basal fluvial deposits of the Dakota Formation, filling the relief, reflect the onset of flexural subsidence and, possibly, a eustatic sea‐level rise. Throughout the deposition of the Dakota Formation, flexure controlled the long‐term, regional subsidence rate. Locally, reactivation of basement faults caused additional subsidence or minor uplift. Owing to a generally low subsidence rate, differential compaction locally influenced the degree of preservation of the Dakota units. Eustasy is believed to have been the main control on the high‐frequency relative sea‐level changes recorded in the Dakota. All surfaces that separate individual Dakota units are flooding surfaces, most of which are superimposed on sequence boundaries. Therefore, with the exception of unit 6B and, possibly, 3B, most of the Dakota units are interpreted as depositional sequences. Fluvial strata of units 1 and 2 are interpreted as low‐frequency sequences; the coal zones at the base and within unit 2 may represent a response to higher frequency flooding events. Units 3A to 6B are interpreted as having formed in response to high‐frequency relative sea‐level fluctuations. Shallow‐marine units 4, 5 and 6A, interpreted as parasequences by earlier authors, can be divided into facies‐based systems tracts and show signs of subaerial exposure at their boundaries, which allows interpretation as high‐frequency sequences. In general, the Dakota units are good examples of high‐frequency sequences that can be misinterpreted as parasequences, especially in distal facies or in places where signs of subaerial erosion are missing or have been removed by subsequent transgressive erosion. Both low‐ and high‐frequency sequences represented by the Dakota units are stacked in an overall retrogradational pattern, which reflects a long‐term relative sea‐level rise, punctuated by brief periods of relative sea‐level fall. There is a relatively major fall near the end of the M. mosbyense Zone, whereas the base of the Tropic shale is characterized by a major flooding event at the base of the S. gracile Zone. A similar record of Cenomanian relative sea‐level change in other regions, e.g. Europe or northern Africa, suggests that both high‐ and low‐frequency relative sea‐level changes were governed by eustasy. The high‐frequency relative sea‐level fluctuations of ≈100 kyr periodicity and ≈10–20 m magnitude, similar to those recorded in other Cenomanian successions in North America and Central Europe, were probably related to Milankovitch‐band, climate‐driven eustasy. Either minor glacioeustatic fluctuations, superimposed on the overall greenhouse climate of the mid‐Cretaceous, or mechanisms, such as the fluctuations in groundwater volume on continents or thermal expansion and contraction of sea water, could have controlled the high‐frequency eustatic fluctuations.