Middle Permian deep-marine carbonates are an important exploration play and significant contributor to production from the Delaware Basin, but the stratigraphy, sedimentology and tectonic controls on sedimentation remain poorly understood. The well-exposed Leonardian (Cisuralian)-Guadalupian stratigraphy mapped in outcrops from the Guadalupe and Delaware Mountains is not commonly applied in nearby subsurface correlation of this interval in the Delaware Basin. In outcrop, the (Leonardian) Bone Spring Limestone and the lower part of the Cutoff Formation (Shumard Member) represent carbonate turbidites that are the basinal equivalents to carbonate ramp deposits of the Victorio Peak Limestone. The Bone Spring Limestone is overlain by a 141-m thick succession of mostly remobilized carbonate turbidites mapped as the Cutoff Formation. These mass transport deposits correlate to the lower San Andres Formation on the shelf. The Cutoff Formation is overlain by a thin interval of organic-rich siltstone assigned to the Pipeline Shale Member of the Brushy Canyon Formation. This thin siltstone drape forms a distinctive wireline log signature that enables correlation of this interval across the Delaware Basin. In the subsurface, the Cutoff Formation is generally not recognized and the carbonate-rich interval below the Brushy Canyon Formation is correlated to the Bone Spring Limestone.
Outcrop to subsurface correlations indicate the upper part of the Bone Spring Limestone in the subsurface Delaware basin is correlative to the upper Cutoff Formation (Williams Ranch Member) in outcrop. This interval includes the Avalon Sandstone as well as the Bone Spring 1-3 Sandstones. Subsurface cores from this complicated interval reveal skeletal carbonate debris flows, carbonate mass transport deposits, thin, discontinuous sandstones and organic-rich siltstone drapes. Conspicuous lateral and vertical changes in lithology and thickness complicates correlations and, in part, explains the miscorrelation between the outcrop and subsurface. Understanding these complications provides an exploration opportunity.
The proportion of carbonate debris flows increases as the upper Cutoff interval thins basinward, which is consistent with the down-profile evolution of mass failures from rafted blocks, to slumps, to debris flows. Organic-rich siltstones drape the paleo-topography developed on top of the remobilized carbonates and help define mass transport events. The drapes record periods of quiescence between failures as both hyperpycnal flows generated from river floods and/or suspension settling of airborne silt cored marine algae flocculates. Thin, skeletal-rich sandstones commonly occur at the base of carbonate mass transport events and record sand deposition from subaqueous flows introduced during periods of relative sea-level fall, the record of which is poorly preserved in shelf strata. The minor upper Cutoff sand deposition precedes the onset of a major period of deep-marine siliciclastic deposition recorded by the Delaware Mountain Group. In this sense, the upper Cutoff Formation is more genetically related to the DMG than the underlying Bone Spring Limestone.
Biostratigraphy can independently validate the proposed outcrop to subsurface correlations because the global stratotype section for the base of the Roadian Stage and Guadalupain Series is placed at the contact between the lower and upper Cutoff Formation in the Southern Guadalupe Mountains. This chronostratigraphic boundary is defined by changes in fusulinids, amminoids and conodonts. The proposed revisions provide a more robust stratigraphic framework and predictable deep-marine system model that relates lithology and thickness distributions to paleo-topography on top of mass transport deposits and more subtle, lower order tectonic controls on basinal sedimentation. These revisions should aid future exploration and development of Middle Permian deep-marine carbonates in the Delaware Basin.