REGIONAL LOWER JURASSIC UNCONFORMITY ON THE SOUTHERN COLORADO PLATEAU, USA

On the southern Colorado Plateau of Utah-Colorado-Arizona, the significant change in depositional systems, from the eolian erg of the Upper Triassic-Lower Jurassic Wingate Sandstone, to the fluvial braidplain of the overlying Kayenta Formation, is marked by a regional unconformity. However, diverse published sources have described the Wingate-Kayenta contact as interfingering and/or gradational. We examined this contact over southern Utah, SW Colorado and NE Arizona, an area of at least 100,000 km², to conclude that it is a regional surface of unconformity between: (1) Wingate strata that are very fine- to fine-grained, well sorted, quartzose and (mostly) crossbedded eolian sandstone that weathers to a “slick rock” cliff; and (2) Kayenta strata that are fine- to medium-grained subarkosic arenite (in many beds micaceous), lenticular to tabular, cross bedded fluvial sandstone with some beds of intrabasinal conglomerate and thin interbeds and lenses of red-bed mudrock. The Wingate-Kayenta contact is typically marked by an incised surface characterized at the base of the Kayenta Fm (base of Springdale Member) by intrabasinal conglomerate. This conglomerate includes rip-up clasts of sandstone from the underlying Wingate Sandstone. In some areas of SE Utah and SW Colorado, no conglomerate is present, although the contact is still a sharp surface between the contrasting sandstones of the two formations. At some outcrops (example: Lisbon Valley), we can document meters of stratigraphic relief over 100 m of strike at the sandstone-sandstone contact between the Wingate and Kayenta fms. Therefore, we reject claims of a conformable contact and support the conclusion of those workers who have identified the Wingate-Kayenta contact as a regional unconformity. The duration of the hiatus represented by this unconformity can be calibrated to be much of Hettangian time (~ 3 my), as the Wingate is no younger than earliest Hettangian (based on its stratigraphic relationship to the Moenave Fm) and biostratigraphy demonstrates an early Sinemurian age for the lower Kayenta Fm.