QUATERNARY SALT DISSOLUTION IN THE MOAB–SPANISH VALLEY, UT PLEISTOCENE AND HOLOCENE EVIDENCE

Thick fluvial sediments (>123 m) have accumulated in grabens formed by dissolution-induced collapse of salt-cored anticlines in the north-central portion of the Colorado Plateau. Moab–Spanish Valley in east-central Utah is such a feature on the Moab Valley salt-cored anticline. Preliminary field observations and limited research indicate that Pleistocene and Holocene deformation is ongoing.

More than 123 m (405 ft) of fluvial gravels and sands have accumulated in the northern end of the Moab–Spanish Valley (referred to as the Moab Valley) just west of the Colorado River, as shown in the boring log from Atlas Minerals ATP-1. These deposits rest on cap rock, the insoluble residue on top of the leached salt diaper formed by the Paradox Formation of Pennsylvanian age.

The Colorado River course across Moab Valley also suggests on-going salt dissolution. The river enters the valley at a high “paradoxical” angle before flowing across the valley axis and reversing direction as it crosses the proposed main dissolution area. The Moab Wash tributary from the west becomes a braided channel forming a delta as it enters a “sink” in the valley prior to joining the Colorado River.

Terraces are conspicuously missing from the center of Moab Valley although gravel strath terraces exist along the Colorado River both upstream and downstream of the valley. Terrace absence from the valley interior is hypothesized to be caused by subsidence and burial of alluvial material. Formation of downstream terraces require transport and aggradation of a gravel bed load through the valley, however changes in the sediment source could also explain these observations. Terraces exist at two elevations immediately after the Colorado River exits Moab Valley at The Portal, suggesting that dissolution and subsidence have been episodic. Pedogenic carbonate accumulation in the upper terrace indicates a middle Pleistocene age of 380 ky.