Basin Dynamics and Bayhead Deltas: Recognition of Their Deposits and Possible External Controls from Their Development, Campanian, Utah
Estuaries are common features on transgressive coastlines and are thought to have a high preservation potential due to their position within incised valleys. Despite recent advances in estuarine facies models they are still not widely recognized in the stratigraphic record, and the external controls on their genesis are not well understood. Bayhead deltas develop at the innermost part of wave-dominated esturaries; their deposits provide critical sequence-stratigraphic markers because they typically develop during the shift from transgression to regression. This study presents (A) detailed sedimentologic analyses of 3 superb bayhead delta deposits from Campanian strata in the Uinta Basin (middle Castlegate Sandstone in the Willow Creek and Little Park Wash areas, and Neslen Formation near Coal Canyon), and (B) their relationship to reduced accommodation during the Sevier-Laramide transition using a 400 km correlation and isopach maps. The following criteria for bayhead delta identification in outcrop are proposed: (1) presence of small-scale clinoforms (5-15 m height; 100-1000 m length), (2) overall coarsening-up pattern, (3) down-clinoform decrease in grain-size and paleocurrent energy, (4) increasing abundance of mud interbeds toward the clinoform toe, (5) basinward- directed paleocurrent with definite fluvial input, (6) more pronounced tidal influence toward clinoform toe, (7) brackish trace and body fossils and (8) general position within a transgressive interval. Additionally, this study proposes that a fundamental change in basin dynamics during the transition from Sevier- to Laramide-style deformation reduced basin-scale accommodation and lowered the basin slope. The reduced accommodation and lower slope facilitated the development of incised valleys during sea-level fall, and the development/preservation of estuarine deposits within the valleys during sea-level rise.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009