Depositional Environments and Diagenesis of the Tonkawa Format (Yirgilian) in Woods and Part of Woodward Counties, Oklahoma

Abstract

The study area is located on the northern shelf of the Anadarko Basin. It comprises some 1420 square miles of Woods and part of Woodward Counties, Oklahoma. The Tonkawa format represents the earliest regressive-transgressive couplet of the Douglas Group in the Virgilian Series. It is bounded above and below by the Haskell and Avant Limestones, respectively. Tonkawa sandstones are widespread over much of northern and northeastern Oklahoma and extend into eastern Kansas, where surface exposures are present. In the study area, the sandstones have been divided into four stratigraphic "packages", each representing different episodes of deposition. A package consists of one or more sand bodies that are laterally equivalent. The interval thins dramatically to the north and northwest as it approaches the carbonate shelf area. Isopach thicknesses vary from about 160 feet adjacent to the carbonate shelf to greater than 420 feet in the south and southeast parts of the study area. The Tonkawa sandstones probably represent a highconstructive lobate delta system. A net sandstone isolith map of the format suggests a northeast source area. The thinning of the interval probably represents the northern edge of the system in this area. Deltaic facies represented by the cores studied include prodelta, delta front, distributary-mouth bar, interdistributary bay, crevasse splay, and distributary channel. Structure maps of the top of the limestones show a basinward dip to the south and southwest ranging from 28 to 37 feet per mile. A series of north-northeast trending anticlinal and synclinal structures are present on both limestones. These tend to be broad and gentle. No faulting was identified. The dominant trapping mechanism in the Tonkawa sandstones is stratigraphic pinching out of sandstone bodies in an updip direction. Most of these sands probably represent distributary-mouth bar deposits. The sands present in the cores are identified as subrounded, fine to very fine grained, sublitharenites. Metamorphic and sedimentary clasts make up the rock fragment component. Muscovite is the most abundant detrital constituent in the sands. Porosity ranges from 0 to 14% and is almost wholly secondary. Authigenic calcite and siderite constitute the major pore filling constituents. Authigenic chlorite, kaolinite, and illite clays are present, but only in minor amounts.