Dissected paleotopography and base-level changes in a Triassic fluvial sequence

At one stratigraphic level in Petrified Forest National Park in Arizona, fluvial mudrocks of the Chinle Formation are truncated by areally extensive deep scours that are filled dominantly by pedogenically modified mudrocks. Color differences between paleosols, developed on scoured and scour-fill deposits, facilitate recognition of the dissected interval. The mudstone-filled scour system records a period of channel incision and widespread gullying caused by lowered base level. Although it is difficult to document subsidence rates in the Chinle basin, the tectonic setting suggests that the enormous scour system could be the result of fluctuations in regional thermal activity during Late Triassic time. Similar scour and fill should occur in other fluvial sequences; however, because they both truncate and are filled by mudrocks, identification can be difficult. Recognition of them is important because they are indicators of base-level fluctuations and thus are useful in unraveling the depositional history of a basin.