Nadeem Ahmad¹, Brian D. Keith², Peter R. Vail³
(1) Indiana University (Bloomington), Bloomington, IN
(2) Indiana University, Bloomington, IN
(3) Rice University, Houston, TX

Abstract: Deposition, sequence stratigraphy, and hydrocarbon potential of a sandy debris flow and turbidity current deposit in an interior cratonic ramp setting, Middle Mississippian Carper sandstone (Borden Group), eastern Illinois Basin, USA

The Middle Mississippian Carper sandstone (Borden Group) of the eastern Illinois Basin was deposited on a locally steepened ramp in the North American cratonic interior. Integrated study of the cores, well logs, cuttings, and outcrops from southern Indiana reveals a unique process sedimentology and genetic stratigraphy, indicating deposition by sandy debris flows and turbidity currents on the slope and base-of-slope of the steepened ramp. Sand is a very fine-grained to coarse silt-sized sub-arkose to quartz-arenite with 5-15% clay matrix. Trace fauna of Nereites-Zoophycos affinity is present. The facies present are hemi-pelagic mudstone, laminated mudstone, parallel-laminated sandstone, ripple cross-laminated sandstone with contorted bedding, and massive sandstone of characteristic plastic rheology.

Facies tract mapping, based on the sets of facies assemblages recognized in the cores and their characteristic wireline log motifs, allows the mapping of depositional systems as distinct paleogeomorphic "elements" and systems tracts. Gravity flows formed toe-of-slope prograding to aggrading ramp, distal ramp-lobes/sheets, and back-stepped slope aprons. Carper sandstone is interpreted to have been deposited as lowstand systems tract during the falling stage, low still-stand stage, and early rising stage of relative sea level without subaerially exposing the proximal ramp margin. Wireline log motifs, isopach maps, stratigraphic relationships, and sediment dispersal patterns point to a north-south locally steepened ramp and a sag that coincide with the gravity and magnetic anomaly associated with the Proterozoic basement structures. Unique process sedimentology, massive sand-rich facies, structural setting, and worldwide proven analogues make the Carper sandstone an attractive play for shallow exploration (250-400m).

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana