SEDIMENTOLOGY AND PALEONTOLOGY OF LATE CRETACEOUS SAG PONDS, UPPER MEMBER, WAHWEAP FORMATION, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH
At the northernmost syndepositional normal fault, the deposits of a small sag pond are located at the boundary between the upper and capping sandstone members of the Wahweap Formation. Pond infill consists of laminated, gray mudstones and siltstones intruded by sandstone dikes and sills. Adjacent to the southernmost syndepositional normal fault, a larger and thicker sag pond deposit is located near the base of the upper member. This southern older sag pond preserves gray, seemingly structureless siltstones and mudstones. The simple fine-grained fill is cut by brittle faulting along the fault-side margin of the southern pond in contrast with the pervasive clastic intrusions in the younger sag pond. The geographic positions and thicknesses of the deposits suggest that the older more southern sag pond has a protracted history of extension, in comparison with the more northern sag pond locality.
The younger, northern sag pond deposit preserves small macerated flora, but no discernable invertebrate fauna. Either the pond was too small, was not active for long, or sedimentation rates were too high (or a combination of factors) to sustain a well-developed ecosystem. The older, larger sag pond fill comprises a series of fossil horizons consisting of juvenile gastropods, Lioplacodes, Planorbis, and Vivaparis and unionid bivalves, Unio, along with unidentifiable macerated plant fragments. The immature fossil assemblages suggest that the older, southern sag pond experienced frequent significant sedimentation events that prevented the invertebrate fauna from reaching maturity punctuated by periods of low sedimentation rates. It is likely that fault movements triggered these rapid catastrophic sedimentation events, but significant precipitation events cannot be ruled out.