Sequence Stratigraphy and New Subsurface Mapping Strategies in the Middle and Upper Pennsylvanian of the Bend Arch, North-Central Texas

CLEAVES, ARTHUR W., Oklahoma State University, Stillwater, OK

Subsurface mapping of Middle and Upper Pennsylvanian rocks in north-central Texas using gross sandstone isolith, total interval isopach, and structural contouring on the tops of carbonate marker units has served as a valuable tool for defining vertically persistent depositional systems depocenters and determining the distribution of potential reservoir rock. Current research on sequence stratigraphy applicable to the region, however, calls into question the overall validity of the traditional approach employed to construct these maps. This standard method (format subdivisions) involves identifying regionally widespread, presumably marine transgressive carbonate marker units and constructing maps based on the lithologies included between the tops or the markers. Important assumptions m de here include the expectation that the carbonate interval represents the maximum inundation of the shelf for a given transgressive-regressive cycle and the requirement that all lithologic changes within the cycle are fully conformable and related to facies changes. Previously published diagrams of upper Strawn and Canyon Group cyclicity also strongly imply that no disconformities are present between any of these 11 cycles.

Sequence stratigraphic analysis of the Strawn-Cisco outcrop zone suggests that numerous disconformities are within the section, as indicated by widespread soil zones, incised valley-fill sandstone bodies, and meteoric diagenetic exposure zones in carbonate banks. Phospatic black shale units, not shelly carbonates, represent the interval of maximum inundation. Such "core" shales should be mapped as the genetic unit boundaries and be used to construct glacial-eustatic cycle intervals. One should use great care in identifying distinct subsurface highstand facies tract assemblages and lowstand assemblages within a given cycle. These assemblages must be mapped separately, otherwise genetically unrelated, disconformable facies elements will be grouped together. All of the major marine trans ressions and many of the deltaic regressive sequences were glacially forced coastline changes and not simple delta progradational facies change.

AAPG Search and Discovery Article #91011©1991 AAPG Southwest Section Meeting, Abilene, Texas, February 9-12, 1991 (2009)