Abstract

In the Bull Flat area of Grand Staircase (Escalante National Monument, Utah, United States), sedimentological analysis of the tip line area of a Late Cretaceous normal fault reveals that the fault was active during the deposition and preservation of a sag pond deposit. Sag ponds deposits are probably under-recognized in the rock record, and, as demonstrated in this study, they are helpful in chronicling activity of ancient faults. The sag pond sequence is located just below the contact between the upper and capping sandstone members of the Wahweap Formation, and it occurs within close proximity to other features of seismogenic origin. The sag pond deposit is a 2.2-m-thick succession of carbonaceous, interlaminated mudstones and siltstones intruded by sublithic sandstone seismogenic dikes and sills. At least two seismic events are required to produce the observed sequence. The initial event ruptured the surface, probably tilting the underlying strata, and produced the down-drop for the sag pond. After infilling of the sag pond, a second seismogenic event mobilized sediment in the underlying upper member and generated the dikes and sills that intrude the sag pond deposit. Subsequently, a minor channel scoured the sag pond deposit with flow parallel to the fault trace. Infilling of the sag pond ended before deposition of the capping sandstone member.

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