New Mexico Geological Society Annual Spring Meeting — Abstracts

Lower Chinle Group strata in west-central New Mexico and eastern Arizona comprise a 70-150 m thick tectonosequence consisting of (ascending) the "mottled strata," Shinarump and Bluewater Creek. Formations, and the Blue Mesa Member of the Petrified Forest Formation. The Mesa Redondo Formation laterally replaces the Bluewater Creek Formation in parts of Arizona and the San Pedro Arroyo Formation replaces the Bluewater Creek Formation in the southern Lucero uplift in central New Mexico. This tectonosequence rests disconformably upon the Moenkopi Formation and is disconformably overlain by the Sonsela member of the Petrified Forest Formation. These surfaces represent the Tr-3 and Tr-4 unconformities, respectively.

Mottled strata are color-mottled, pedogenically modified sandstones, conglomerates, siltstones, and mudstones that represent a cumulative pedon developed on pre-Chinle strata during a time of lowered base level. Shinarump Formation deposits consist of extrabasinal conglomerates and conglomeratic sandstones dominated by Paleozoic limestone and Precambrian chert and quartzite clasts. These coarse-grained clastics overlie either the mottled strata or, in places, the Moenkopi Formation and represent localized channel-drowning in response to rising base level. Thus Shinarump Formation deposits are the nonmarine reflection of the onset of a transgressive systems tract (TST).

Although dissimilar in the exact distribution of sediment types, the Bluewater Creek Formation and its lateral equivalents, the San Pedro Arroyo and Mesa Redondo Formations, are all red-bed-dominated units deposited in response to continued base-level rise. These units consist of sheet flood and lenticular channel sandstones and siltstones with abundant red-bed mudstones and minor paleosols. Where the Shinarump is absent, these strata mark the only TST equivalent in the sequence. Throughout eastern Arizona and west-central New Mexico the Blue Mesa Member overlies the Mesa Redondo and Bluewater Creek Formations with apparent conformity. The Blue Mesa Member thins across the outcrop belt from 77.7 m thick at the Petrified Forest National Park to 44.5 m thick at Fort Wingate (western Zuni Mountains) and 22 m at Prewitt (eastern Zuni Mountains). The Blue Mesa Member does not crop out in the Lucero uplift and instead appears to have been removed during the Tr-4 unconformity, after which the Sonsela Member was deposited. Where present, the Blue Mesa Member consists of highly bentonitic mudstones that represent a series of stacked paleosols and floodplain deposits. These low-energy deposits comprise 'the nonmarine equivalent of a high-stand systems tract (HST). Biochronology, including palynology and tetrapod biochronology, indicates that this tectonosequence was deposited over a relatively brief interval (<3 Myr) of late Carnian time.