New Mexico Geological Society Annual Spring Meeting — Abstracts

Subsurface correlations tied to previous surface stratigraphic studies provide an improved stratigraphic framework for the western portion of the Orogrande Basin as well as the adjacent northeastern portion of the Robledo Shelf. Extensive work has been completed on outcrops containing Pennsylvanian strata within this area located primarily within south-central New Mexico; however, previous work has done little to integrate existing surface knowledge with the available subsurface database and previously unavailable fusulinid data. The focus of the presentation is an area located immediately north of Las Cruces and southeast of the San Mateo Uplift, between the Caballo and San Andres Mountains. The objectives of the study are to determine the subsidence and depositional history of the western part of the basin and adjacent eastern shelf as well as link previous surface stratigraphic work with subsurface findings.

A combination of well-log analysis, fusulinid biostratigraphy, cuttings descriptions and previous geochemical analyses and core descriptions creates a comprehensive subsurface data base that allows correlation of strata throughout the study area. Pennsylvanian strata within the Orogrande Basin are markedly cyclic, dominantly shallow-marine deposits of Morrowan to Virgilian age. The cyclic character of the well log response within the subsurface sections permits delineation of upward shoaling parasequence sets consisting predominantly of shale and limestone packages indicating periods of transgression and regression within the basin. The log response within the middle to upper Desmoinesian clearly shows a marker unit believed to be an unconformity truncating the stage. Consistent cyclicity within these strata results in confident lithostratigraphic correlation between wells, except with regard to the Morrowan. Presence of the Morrowan within the study area is subject to debate. While the fusulinid biozone Eoschubenella indicates early Atokan time (Clopine, 1992; Wilde, 1990), the fusulinid biozones Millerella and Eostaffella are diagnostic of the Morrowan. Unfortunately, lower Pennsylvanian fusulinid samples available in the subsurface sections are . limited. Thick Morrowan strata are present in surface outcrops at Vinton Canyon (75 meters thick) and Bishop Cap (44 meters thick) located in the southwestern portion of the basin. However, the Morrowan thins substantially to the north (Clopine, 1992), where it is only one meter thick in the Derry Hills section, and is completely absent in the Mud Springs section. Clopine (1992) attributes this substantial thinning to both the loss of the lower Morrowan at the widespread basal Pennsylvanian unconformity and to northward transgression during Morrowan time. In spite of the Morrowan-Atokan boundary problem, the remaining upper Atokan through Virgilian strata make up the bulk of the section within the study area and are much better constrained than the Morrowan and lower Atokan. Fusulinid biozones within these strata parallel the lithostratigraphic cycles and link subsurface and surface stratigraphy.

Findings are presented as fence diagrams, isopach maps, correlation diagrams, a composite Pennsylvanian type log for the study area and a compilation of biostratigraphic data available thus far. A north-south subsurface section consisting of the following four wells, Gulf Oil Co. Sierra K State No.1, Shell Oil Co. No. 1 Leeman Federal, Exxon Corp. No. 1 Beard Federal, and Exxon Corp. No.1 Prisor Unit Federal, demonstrates subsurface thickening and thinning trends within Pennsylvanian deposits. Fence diagrams and isopach maps linking previous surface work with subsurface sections show parallel strata thinning to the north and thickening dramatically basin ward to the east.