CONFIRMED: SALT GRABEN OF THE PARADOX BASIN ARE QUATERNARY FEATURES LINKED TO REGIONAL RIVER INCISION – NEW RESEARCH ON MOAB-SPANISH VALLEY

The salt-tectonic features of the central Paradox Basin are well known, but their activity in the Quaternary is poorly known. In fact, localized salt deformation is active and superimposed on region-wide patterns of erosion. Our research uses fluvial deposits and terraces and luminescence and cosmogenic-burial dating to quantify the co-evolution of salt-tectonic and erosional processes in and around the Moab-Spanish Valley salt graben in east-central Utah.

The age and distribution of upland gravels along the eastern rims of Spanish Valley, as well as paleo-topographic reconstruction, indicate the faulting and subsidence forming the graben began after ~1.5 Ma. Upstream incision rates of drainages that approach and enter the graben subsequently accelerated from <50 m/my to >600 m/my in the middle Pleistocene, tracking both graben subsidence and known regional patterns of Colorado River incision. Chi-transformed stream profiles in the area are consistent with tributaries responding to this wave of middle-to-late-Pleistocene incision.

Then, tracking activity over the late Quaternary, a suite of five terraces and undifferentiated alluvial deposits <200 ky old are offset along fault zones and down-warped along the axis of Moab-Spanish Valley. Previous workers recognized that active subsidence is highest where the Colorado River crosses Moab Valley. Part of this subsidence is accommodated by fault-slip along the Kayenta Heights fault zone at the northeast graben margin, which we measure at 439 ± 49 m/my. Adjacent incision in Mill Creek, which crosses the fault zone into the graben, has kept pace with faulting at 529 ± 44 m/my, which is also similar to regional mainstem Colorado River incision rates.

It is known that salt walls and anticlines were already forming in the Paleozoic, but the Moab-Spanish Valley graben, and presumably other collapsed anticlines of the Paradox Basin, may be entirely-Quaternary features with significant, active deformation. Results support the prior hypothesis that salt-dissolution subsidence of Moab-Spanish Valley is coupled to incision and baselevel fall on the Colorado River via groundwater flow, which is responsible for the below-grade mass removal of salt.