New Mexico Geological Society Annual Spring Meeting — Abstracts

Several historically problematic ?Permian to ?Cretaceous outcrops in west-central New Mexico are readily correlated using precise Upper Triassic lithostratigraphy. In particular, outcrops of mudstone and limestone in sections 19 and 20, T13N, R13W and section 13, T13N,
R14W near Cottonwood Canyon have been described as either: (1) Permian karsted topography and cave infilling developed on the Permian San Andres Formation or (2) a siliciclastic tongue of the San Andres Formation overlain by San Andres Formation limestones. Likewise, limited outcrops of dark greenish-gray carbonaceous shales and bentonitic mudstones near Carrizo Arroyo in section 6, T6N, R2W have previously been assigned to the Mancos Shale. Both of these outcrops actually belong to the Upper Triassic Bluewater Creek Formation of the Chinle Group.

Most of the Bluewater Creek Formation throughout west-central New Mexico and eastern Arizona is dominated by lithofacies of: (1) slightly bentonitic mudstone and siltstone "red beds" with lenses of sandstone; and (2) laminated to ripple-laminated and trough crossbedded micaceous, bench-forming litharenites with minor intraformational conglomerates. However, a third siliciclastic lithofacies composed of grayish bentonitic mudstones and dark, carbonaceous shales occurs locally at the base of the Bluewater Creek Formation throughout west-central New Mexico and eastern Arizona. This lithofacies records localized but regionally persistent ponding in the paleotopography generated during the Tr-3 unconformity, a paleotopography only partially filled by Shinarump Formation deposition in response to base-level rise.

The Cottonwood Canyon outcrops clearly pertain to this lithofacies and in fact rest on highly pedoturbated sandstones of the basal Chinle "mottled strata." A 1.0-m-thick micritic limestone capping this section lacks body fossils and is only slightly bioturbated, indicating that this is a lacustrine limestone, unlike most Chinle Group limestones, which are typically pedogenic in origin. Heteropolar vertebrate coprolites, fragmentary amphibians, and fossil plant debris strongly indicate that this outcrop is similar to other Bluewater Creek Formation outcrops.

The Carrizo Arroyo deposits are strikingly similar in lithology and rest directly either on thin (<5.0 m) Bluewater Creek Formation red beds or the "mottled strata," which in turn overlie the Moenkopi Formation. These deposits cannot belong to the Mancos Shale for the following reasons: (1) the Dakota Group, which should be present beneath the Mancos, is absent; (2) stringers of coarse-grained clastics and diagenetic selenite are more commonly associated with the complex deposits of the third lithofacies of the terrestrial Bluewater Creek Formation than the fairly uniform shallow marine setting of the Mancos Shale; (3) the strata rest directly on mottled strata and include bluish-gray bentonitic mudstones and siltstones, lithologies typical of the Bluewater Creek formation throughout the Colorado Plateau; (4) there is no indication of Cretaceous marine fossils, either in situ or as float, in the area; (5) nearby limestones more closely resemble lacustrine limestones of the Chinle Group, such as those of Cottonwood Canyon.

These two, newly identified outcrops of the Bluewater Creek Formation represent deposition of fine-grained siliciclastics and lacustrine limestones in paleotopography generated during the development of the Tr-3 unconformity, a paleotopography that was not completely filled until the latest Carnian during deposition of the middle and upper Bluewater Creek Formation.