New Mexico Geological Society Annual Spring Meeting — Abstracts

Lower Permian marine carbonates and calcareous shales of Hueco Formation lithology and red-bed siliciclastics of Abo Formation lithology intertongue extensively over a 120-m-thick interval in the southern Robledo Mountains of Dona Ana County, New Mexico. In the middle of this interval, more than 30 fossil tracksites occur at a single stratigraphic level over an area of at least 20 square km and thus define the Robledo Mountains megatracksite. Previous analyses of depositional cyclicity in these'strata suggested that the red beds containing the megatracksite are an asymmetrical depositional cycle of marine regression followed by a shorter transgression culminated by marine limestone deposited during maximum transgression. (Other, supposed symmetrical cycles of regression-transgression in these strata have not been observed by us in the field). According to previous analyses, most of the red-bed facies accumulated as offlap facies during regression.

These analyses, however, contradict current understanding of coastal depositional systems, which maintain that sediment aggradation takes place during rising base level that typically characterizes tmnsgression, not during the base-level fall and lowstand associated with most regressions. We propose a new model of Abo-Hueco depositional cyclicity consistent with this understanding and supported by detailed and extensive stratigraphic, sedimentologic and paleontologic data. Our model identifies the base of a red bed parasequence, such as the megatracksite interval, as an unconformity on top of highstand marine limestone that tops the preceding cycle. Varied facies shoreface sandstone, tidal-flat sandstone/siltstone and delta cliniforms immediately overlie this unconformity. They record irregular infilling during early base-level rise (transgression) of the incised topography developed during regression and lowstand. Continued transgression produced laminated and ripple-laminated sandstones/siltstones with tracks on extensive tidal flats developed on a surface of negligible topographic relief. Marked tidal cyclicity controtled tracksite formation and preservation. Continued base-level rise then produced calcareous shales capped by marine limestone that represents the maximum flooding zone. The overlying cycle (parasequence) then began with a marine regression that produced another unconformity surface.