SAG POND DEVELOPMENT AT THE UPPER AND CAPPING SANDSTONE MEMBER CONTACT, UPPER CRETACEOUS WAHWEAP FORMATION, GRAND STAIRCASE – ESCALANTE NATIONAL MONUMENT, UTAH

The transition from the upper to the capping sandstone member of the Upper Cretaceous Wahweap Formation is marked by an abrupt change in petrology from lithic to quartz sandstones and a shift in paleocurrents from northeast to southeast modes. At Bull Flat in Grand Staircase -Escalante National Monument, examination of this contact near the tip line of a probable normal fault revealed the presence of a preserved sag pond filled with seismically disturbed strata. Sag ponds develop along the surface expression of active faults and are characterized by a limited areal extent, fine-grained fill and localized internal drainage. At the Bull Flat locality, sag pond deposits are characterized by carbonaceous mudstone and siltstone with interbedded lenses of fine- to medium-grained sandstone. The deposit can be traced for approximately 100 meters and reaches a maximum thickness of 5.0 meters. The carbonaceous mudstones and siltstones appear massive with rare, faint, diffuse, folded laminations. The sandstone lenses pinch and swell in thickness with irregular bases and tops and are best interpreted as sedimentary sills. Vertical dikes, containing inclusions of the mudstone, connect sills to the top of the sandstone underlying the sag pond deposit. The contact between the mudstone fill within the sag pond and the underlying sandstone is convolute. At least two seismic events occurred to generate the observed sequence. The initial event tilted underlying strata and produced the sag pond. The second event generated the dikes and sills, probably sourced from the underlying sandstone. The overlying capping sandstone member contains ripped up clasts of the sag pond and is characterized internally by convolute bedding. The generation of the convolute bedding may be related to the second seismic event that generated the dikes and sills or may be a discrete younger event.