JURASSIC UNCONFORMITIES IN THE WESTERN INTERIOR: CONFUSING LITHOSTRATIGRAPHY AND CHRONOSTRATIGRAPHY

The need to distinguish between the nature (lithostratigraphic expression) and value (geologic age and duration) of unconformities is a classic stratigraphic principle. The succession of Jurassic unconformities (J-0 through J-5) proposed for the Western Interior Jurassic section provides good examples of confusion of lithostratigraphy and chronostratigraphy. Three of the unconformities (J-0, J-2 and J-5) were proposed and maintained, not as lithostratigraphic boundaries, but as chronostratigraphic boundaries. Thus, by definition and subsequent practice, J-0=beginning of the Jurassic, J-2=Early-Middle Jurassic boundary and J-5=Middle-Late Jurassic boundary. The placement of J-5 within strata of the upper San Rafael Group well exemplifies this. This placement was based, not so much on the lithostratigraphic evidence of an unconformity, but on data that indicated the Middle-Upper Jurassic time boundary is within the Summerville Formation, which "forced" placement of J-5 within that unit. Thus, the J-5 unconformity was placed at an untraceable surface at the base of the "Tidwell Member," which then became the San Rafael Group-Morrison Formation contact, and thereby trivialized a long recognized lithostratigraphic boundary. Instead, lithostratigraphic evidence indicates a regional unconformity at the base of the Salt Wash Member of the Morrison Formation, within Upper Jurassic strata, so this unconformity appears not to represent a geologically substantial hiatus. The J-5 unconformity at the Salt Wash base marks an important tectonic reorganization of the Jurassic Western Interior basin. Before J-5, Middle Jurassic eolian, shallow marine, lacustrine and arid coastal plain deposits of the San Rafael Group were deposited as part of a westward-thickening sedimentary prism in a retroarc foreland basin. After J-5, during Morrison time, deposition changed to fluvial deposits derived from the uplifted arc terrane to the west and locally from the Mogollon highlands to the south. Confusing lithostratigraphy and chronostratigraphy resulted in failure to recognize this important tectonosequence boundary and thus a misappreciation of Jurassic depositional history.