STRATIGRAPHY, UNCONFORMITIES, AND PALEOGEOGRAPHY OF THE UPPER TRIASSIC CHINLE GROUP, SOUTHWESTERN U.S.A

All nonmarine Upper Triassic strata in the western U.S.A. pertain to the Chinle Group. Detailed litho- and biostratigraphy define three Chinle sequences. The basal sequence (Shinarump-Blue Mesa) was deposited on a surface (Tr-3 unconformity) developed principally between Moenkopi (Anisian) and early Chinle (Carnian) deposition, a hiatus of at least 10 Myr. Basal Chinle strata are fluvial channel deposits (Shinarump Fm.) and pedogenically modified strata ("mottled strata"=Zuni Mountains Fm.) that are overlain by redbeds and relatively wet floodplain deposits. The surface overlying the Shinarump-Blue Mesa sequence, the Tr-4 unconformity, represents a briefer (<1Myr?) hiatus, and has a subtle but complex paleogeography. During the ensuing sequence (Moss Back-Owl Rock) basal channel deposits (Sonsela Mbr. of Petrified Forest Fm. and equivalents) are overlain by better drained floodplain (Painted Desert Mbr. and equivalents) and paleosol-dominated strata (Owl Rock Fm.). During this time the Chinle basin's trunk drainage migrated northward and Norian strata onlap remnants of the Ancestral Rocky Mountain (ARM) highlands. There is no evidence for a separate "Dockum" basin in West TX-eastern NM during Chinle deposition. Overlying the Moss Back-Owl Rock sequence is another unconformable surface, the Tr-5 unconformity. Strata above the Tr-5 unconformity in the Four Corners region (Rock Point Fm., Wingate Ss. and equivalents-Rock Point sequence) are best removed from the Chinle Group as they represent an entirely different lithosome (siltstone and fine-grained sandstones lacking bentonitic strata) and depositional regime (eolian and eolian-influenced deposition). This sequence reflects the latest burial of ARM highlands by Mesozoic strata. Each of these sequences corresponds to third-order eustatic inflections, suggesting that regional base level and eustasy are linked. Regionally persistent sand bodies, especially in the lower Painted Desert Mbr., may reflect additional, lower amplitude base-level inflections.