MEANDERING FLUVIAL DEPOSITS OF THE CEDAR MOUNTAIN FORMATION: THE STRATIGRAPHIC RECORD OF THE EARLY CRETACEOUS FORELAND BASIN OF UTAH

The Cedar Mountain Formation of east-central Utah, U.S.A. is composed of meandering channel deposits of high (>1.5) and low (<1.3) sinuosity encased in flood plain mudstone. Fluvial style follows a predictable pattern of widely meandering, coarse grained deposits at the base that transition upward into similarly coarse grained, low sinuosity channels with limited lateral accretion.

The basal members of the formation are the Buckhorn Conglomerate and Poison Strip Members which have been described as dominantly braided systems. However, the upper surface of these members is exceptionally exposed over areas of up to 26 km² that reveal a meandering planform with sinuosities greater than 1.5. An interpretation that is bolstered by paleocurrent and accretion direction measurements delineating point bars up to 1 km².

The Buckhorn Conglomerate and Poison Strip Members are overlain by the Ruby Ranch Member which shows a significant decrease in the width of channel belts (average of 70 m) and a significant increase in the proportion of flood plain mudstone. Channel belts are exceptionally well-exposed throughout the field area and exhibit low sinuosity (<1.3).

The uppermost channels of the Ruby Ranch Member and the overlying Naturita Formation increase in width (average of 190 m) and retain a low sinuosity (<1.3) planform. There is also a reduction in the proportion of flood plain and coastal plain mudstone as well as an increase in tidal deposits signifying the incursion of the Early Cretaceous Western Interior Seaway into the study area.

The identification of a meandering planform for the Buckhorn Conglomerate and Poison Strip Members has implications for tectonic models for the Early Cretaceous. Models should account for the difference in paleohydraulics and paleoslope for braided vs meandering rivers. Additionally, the problem of coarse-grained deposits preserved at great distances from the thrust belt is more problematic due to the fact that these cobble to pebble conglomerates exist throughout the Cedar Mountain Formation and are not confined to the lowermost units. Additionally, the change from widely meandering fluvial deposits to narrow, low sinuosity deposits may indicate increasing accommodation as thrusting in the nearby Sevier fold and thrust belt continued throughout the Early Cretaceous.