Petrography and Depositional Systems of Tonkawa Format Interval (Virgilian), Woods County, Oklahoma

Michael W. Fies, Arthur W. Cleaves, II

The Tonkawa format is a carbonate-bounded genetic unit of regional scope that was deposited on the northern shelf of the Anadarko basin during the Late Pennsylvanian. A facies tract that includes fluvial-deltaic and slope terrigenous clastic depositional systems occurs between the format-bounding Haskell and Avant limestones. This interval records the southwestward progradation of a major high-constructive delta system from eastern Kansas.

Sandstone units in the study area have been grouped into four vertical subintervals (packages) based on data from eight cross sections and 1,100 well logs. The lowest three packages contain most of the net sandstone thickness of the format; they represent episodes of deltaic lobe switching. Cores and wireline-curve characteristics indicate that barfinger, distributary-channel fill, crevasse-splay, marine reworked delta-front, and interdistributary-bay facies are prominent in Woods County. Higher, with the fourth package, delta-lobe destructive facies such as barrier sandstone units and transgressive sheet sandstones predominate. Updip stratigraphic pinch-out of bar finger crevasse splay and barrier-bar sandstones toward the northeast comprise the dominant hydrocarbon-trapping mechanis .

Petrographically, the Tonkawa sandstone units may be defined as mature, fine sandstone, sublitharenite. Shale clasts and phyllite-schist grade grains are the most significant rock fragment components. Maximum porosity is roughly 14% and is wholly secondary. Authigenic calcite and calcite comprise the major pore-filling materials.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.