J-5 UNOCONFORMITY: A TECTONOSEQUENCE BOUNDARY IN THE JURASSIC WESTERN INTERIOR BASIN

The Upper Jurassic section in the Western Interior contains a regionally pervasive unconformity that marks a significant tectonic reorganization of the depositional system. This is the J-5 unconformity, located at the base of the Salt Wash Member of the Morrison Formation, and above the Tidwell Member of the Summerville Formation or older strata. Strata beneath the unconformity are eolian, lacustrine, shallow marine and tidal flat deposits of the San Rafael Group and its equivalents. These strata form a westward-thickening sedimentary prism that was deposited in a retroarc foreland basin that extended from Oklahoma-Nebraska to Idaho-Nevada, with major sediment sources including the craton to the east. San Rafael Group strata are mostly of Middle Jurassic age, but their uppermost part may be as young as earliest Late Jurassic (Oxfordian). The Morrison Formation strata immediately above the J-5 unconformity are fluvial deposits derived mostly from the uplifted arc terrane to the west. Physical stratigraphic evidence demonstrates a sharp depositional break between Morrison arkosic channelform sandstones and conglomerates and San Rafael Group eolianites or gypsiferous sabkha deposits. However, the temporal hiatus represented by this J-5 unconformity is a short one geologically, less than or equal to about a stage/age. The J-5 unconformity can be traced throughout the Western Interior and thus represents a regionally significant tectonosequence boundary.