| Description |
The John Henry Member of the Straight Cliffs Formation contains regressive and transgressive marginal marine strata deposited along the shoreline of the Late Cretaceous Western Interior Seaway. Four main facies associations - wave-dominated facies, coastal plain sandstones and mudstones, sandstone-rich tidal deposits, and carbonaceous lagoonal mudstones - reflect deposition in regressive shorefaces and transgressive tide-dominated barrier island systems. Regressive-Transgressive (R-T) cycles are defined by two main transgressive surfaces, the first (wave ravinement) marking the maximum transgression and beginning of regression, and the second (tidal ravinement) recording the onset of transgression. Wave ravinement surfaces are underlain by tidal and coastal plain deposits and overlain by distal to proximal lower shoreface deposits, forming the base of the regressive interval. The shoreface deposits thin at the base onto the wave ravinement surface and are eroded at the top by tidal ravinement, which marks the onset of the transgressive portion of the cycle. Transgressive deposits are either sandstone or mudstone dominated and thicken landward to form successions more than 20 m thick. R-T cycles stack progradationally in the lower John Henry Member and exhibit net retrogradational stacking through the middle and upper John Henry Member. The John Henry Member at Left Hand Collet Canyon contains two distinct tidal facies associations: mudstone rich units and sandstone rich deposits. Mudstone dominated facies reflect deposition in a microtidal, wave-dominated lagoon. Sandstone rich deposits reflect erosion and deposition in a mesotidal barrier island system with frequent migratory inlets and efficient longshore transport. Along-strike correlations, ~20 km to the southeast to Rogers Canyon, show variations in the upper John Henry Member. This interval at Left Hand Collet is characterized by tidal ravinement into proximal lower to middle shoreface and sandstone dominated tidal deposition during transgression. At Rogers Canyon, it is dominated by lower and upper shoreface deposits with little tidal ravinement. These variations are attributed to increased sediment supply via longshore drift from a delta to the north of Left Hand Collet as well as a more embayed coastal morphology. An accretionary shoreline trajectory with high rates of sediment supply is necessary to preserve transgressive deposits, while high basinal energy at the onset of transgression is required to erode the shoreface. Finally, transgressions form significant landward thickening, sandstone rich facies with discernible trends that may be useful in reservoir characterization and modeling studies. |
