Outer-Ramp Limestones of the Zuloaga Formation, Astillero Canyon, Zacatecas, Mexico

Abstract A thick section of Zuloaga Limestone (Oxfordian) crops out in Astillero Canyon about 30 km south of Concepcion del Oro, Zacatecas, Mexico and about 50 km southeast of the type locality of the Zuloaga Formation. Here the Zuloaga Formation is predominantly a carbonate mudstone-wackestone with minor amounts of grainstone. The section is divided into the following facies, in order of their volumetric importance: (1) skeletal mudstone and wackestone, (2) pellet mudstone and wackestone, (3) pellet packstone-grainstone, (4) ooid-peloid grainstone-packstone, (5) ooid grainstone, and (6) algal boundstone. In addition, there are a few thin dolomite beds. Northward toward the Coahuila Peninsula and eastward toward the Tamaulipas Arch the Zuloaga Limestone thins and contains more grainstone and evaporite. The Zuloaga Limestone was deposited on a carbonate ramp which sloped gently southward from the Coahuila Peninsula and westward from the Tamaulipas Arch. On the inner part of the ramp, carbonate sediment accumulated in peritidal environments. The area of Astillero Canyon was on the “outer ramp”, probably 150 km south of the southern shoreline of the Coahuila Peninsula. At this locality, Zuloaga limestones were deposited predominantly in low-energy subtidal environments. The thickness of the Zuloaga Formation indicates that this was a relatively rapidly subsiding part of the epicontinental basin, but carbonate accumulation kept up with subsidence and shallow-water regimes prevailed. Measured sections of the Zuloaga Limestone on both limbs of the same anticline are difficult to correlate, indicating that lithic units are not laterally extensive. The vertical sequences suggest that shallow-water depositional environments frequently shifted from place to place as the epicontinental shelf continuously subsided. Thus, the Zuloaga strata at Astillero Canyon lack the predominately regressive characteristics of the age-equivalent Smackover Limestone in South Texas. Assuming that the grainstone and algal-stromatolite represent deposition in the shallowest water, the stratigraphic section at Astillero Canyon may be divided into at least eight major “shoaling-upward” sequences.