| Description |
A complex mixture of wave, tide, and fluvial energies form paralic strata, and although these units are important hydrocarbon reservoirs, they are complex and poorly understood. This study documents the architecture of an estuarine succession using outcrops of the Upper Cretaceous John Henry Member of the Straight Cliffs Formation, southern Utah (USA). Terrestrial LiDAR, photomosaics, 18 detailed measured sections, and 652 paleocurrent indicator measurements inform this stratigraphic analysis. The ~65-m-thick interval of interest records evolution of a mixed-energy to wave-dominated estuary, with basal elongate tidal bars overlain by carbonaceous bay fill, tidal flat deposits, a bayhead delta, and ultimately a coastal plain succession. A detailed interpretation of the ~8.5-m-thick by 550-m-wide bayhead delta outcrop highlights internal architecture as well as the relationship between the bayhead delta, the underlying tidal bar units, and the overlying coastal plain strata. Within the bayhead delta, beds are composed of very fine- to medium-grained trough cross-stratified, rippled (some climbing), planar laminated, planar cross-stratified sandstones, and interbedded mudstone/siltstone. These units thicken and coarsen vertically. Statistical analysis of the bayhead delta indicates that average bedding thickness, net to gross, amalgamation ratio and grain size increase down-dip, and vertically up-section. This study compares grain size analysis results to a published study of a heterolithic fluvial point bar to provide guidelines for subsurface differentiation of inclined heterolithic strata, and to better predict the impact on reservoir distribution and probable fluid flow pathways. Understanding the variety of expressions and reservoir behavior of IHS intervals will guide future studies of heterogeneous paralic reservoirs. |
