AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Depositional Systems and Sequence Stratigraphic Relationships of the Upper Clear Fork-Lower San Angelo Interval (Permian), Eastern Shelf, Midland Basin, Texas
(1) Geosciences, Texas Tech University, Lubbock, TX.
(2) El Paso Exploration & Production, Houston, TX.
The Choza Formation of the upper Clear Fork Group and overlying San Angelo Formation of the Pease River Group are late Early Permian (Leonardian) units that outcrop across west - central Texas on the Eastern Shelf of the Midland Basin. An abrupt lithofacies transition from the mudstone - dominated Choza to the overlying Duncan Sandstone of the San Angelo was recognized early on as a significant change in sedimentation on the Eastern Shelf. However, the recent view of this interval interprets this relationship as being conformable, composed of a facies mosaic of muddy prodeltaic to sandy delta front and distributary channel environments. This study was undertaken to re-investigate the sedimentology of these units and to assess the sequence stratigraphic relationship between them.
The upper Choza of the north Texas study area is dominantly red, blocky, silty mudstone. Root mottling is observed with soil textures (peds, slickensides) developed, but not extensively so. Nodular and thin-bedded gypsum horizons along with thin beds of gypsum-bearing dolomitic silt/sandstones and dolomites with salt hopper casts provide unequivocal evidence for sabkha environments. Together the Choza represents an arid coastal system of terrestrial mudflats and marginal marine brine ponds, tidal flats, and tide influenced wadi mouths.
The overlying lower Duncan is composed of: 1) a lower isolated to laterally amalgamated single story channel complex composed of ripple-laminated fine sand with muddy overbank and planar laminated splays, 2) an upper laterally and vertically amalgamated channel complex composed of fine sand often observed with a basal gravel lag component in up to 1m trough and planar laminated bar forms; local incision into underlying units can be significant. The Duncan represents a coarsening-upward fluvial complex set from a lower suspended load-dominated fluvial channel complex into an upper bed load-dominated fluvial channel complex.
The transition from arid marginal marine to braided fluvial environments represents a significant basinward shift in facies with no genetic relationship between Choza and Duncan depositional systems. Sequence stratigraphic interpretation of this succession places the Choza in the HST, the lower channel complex as the Falling Stage, and the upper channel complex as the LST. Regional correlation of this interval indicates it is the expression of the late Leonardian 2nd order sequence boundary on the Eastern Shelf.