Fluvial and Marginal Marine Architecture of the John Henry Member, Straight Cliffs Formation, Kelly Grade of the Kaiparowits Plateau, South-central Utah

Understanding how changes in fluvial and marginal marine architecture correlate with shifts in marine shoreface architecture is a crucial step to understanding the effect of relative sea level changes on terrestrial sedimentary systems. Three measured sections (each >230 m), 2367 paleocurrent measurements, and examinations of lateral facies relationships were made over 3 km of continuous outcrop in the John Henry Member of the Upper Cretaceous Straight Cliffs Formation in the Kelly Grade area of the Kaiparowits Plateau, southern Utah. Three outcrop facies associations (OFA) were interpreted from stratigraphic base to top: (OFA1) tidally influenced fluvial channel belts and coastal plain coal mires, (OFA-2) lagoon fill, isolated distributary channels, laterally restricted (<500 m wide) channel belts, and bay-head deltas, and (OFA-3) laterally extensive (>500 m) channel belts, channel-belt complexes, and floodplains. Fluvial and marginal marine architecture trends show: (1) an upward decrease in marine-influenced and laterally restricted sinuous channel belts, and (2) an upward increase in the lateral extent of downstream accreting channel belts and channel-belt complexes. Ratios of net sand to gross sediment are greatest in OFA-3 (0.42) and least in OFA-2 (0.23). Each outcrop facies association correlates with trends of net transgression and net regression observed depositionally down-dip in shallow marine transgressive-regressive cycles. These correlations and the absence of sequence boundaries do not conform to previously interpreted, simplified sequence stratigraphic models of fluvial and marginal marine architecture.