Facies Variations Within Shoreface Packages in the John Henry Member of the Upper Cretaceous Straight Cliffs Formation, Southern Utah

Jessica L. Allen and Cari Johnson
Geology and Geophysics, University of Utah, Salt Lake City, UT

The John Henry Member of the Upper Cretaceous Straight Cliffs Formation in southern Utah records several atypical variations of complete shallowing-upward packages of marginal marine deposits. These deposits are exposed within a series of dip and strike oriented canyons that have been measured and mapped, both physically and with panoramas. Shoreface deposits within the John Henry stray from normal depositional models in the distal lower shoreface, middle shoreface and upper shoreface facies. The distal lower shoreface is not characterized by a gradually coarsening upward section, rather it is sharp based. Here, this regressive sharp-based shoreface is interpreted as a result of high sedimentation and normal progradation rather than a base level fall. Additionally, it is common that the middle shoreface is absent within the section. Amalgamated hummocky cross-bedded lower shoreface sandstones transition directly into trough cross-bedded upper shoreface deposits; supporting a high sedimentation rate interpretation. Finally, these shallowing-upward sections record upper shoreface units that are usually eroded by a base level change. The upper shoreface is composed of three facies; trough cross-bedded, bioturbated and unidirectional trough cross-bedded sandstones. Trough cross-beds are bidirectional and areas have a laminated to scrambled texture indicative of wave-dominated upper shoreface deposits. Bioturbated sandstones are interpreted as locally protected areas. Both of these depositional units are interpreted as reworked sediment of prior points of fluvial input. The third facies is interpreted as a distributary mouth bar and records the preservation of an area of fluvial input. Due to the massive thickness of lower shoreface packages and the interactions between the upper shoreface facies, this depositional system is interpreted as a wave-dominated deltaic deposit.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas