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The Tonkawa sandstone (Missourian-Virgilian) in the Anadarko basin consists of upper, middle, and lower sandstone units, separated by intervening shales. The lower Tonkawa sandstone consists of coarse to pebbly sandstones deposited in thick, coarsening-upward sequences. The unit is up to 400 ft (120 m) thick and the sandstones grade to shale from the southeast to the northwest. Gross sand isopachs outline northwest-southeast oriented linear "thicks" that extend for tens of miles. The middle Tonkawa sandstone consists of thin- and thick-bedded fine-grained sandstones displaying wavy and flaser bedding, a convolute laminae, and some small-scale cross-laminae. These sandstones range in thickness from 100 ft (30 m) to 400 ft (120 m). The "thicks" in the middle Tonkawa extend nly for a few miles and do not show a preferred directional orientation. The upper Tonkawa sandstone caps the entire sequence and is overlain by oolitic limestones.
We interpret the lower Tonkawa sandstone to represent a submarine-fan complex for the following reasons. The regional setting indicates that the sandstones were deposited tens of miles from the shelf edge. Graded beds, convolute laminae, and load casts indicate transportation by sediment gravity flows. The areal geometry of sandstones indicates channelized flows that coalesced to form an overall fanlike geometry.
The middle Tonkawa sandstones are interpreted as a sequence deposited on the basin slope. Regional setting indicates that the unit is located immediately basinward of the shelf edge and that it progrades over the lower Tonkawa sandstone. Deposition as sheets or aprons is suggested by lateral continuity, at least for several miles, of individual beds. Small-scale cross-laminae, wavy and flaser bedding, and sharp bed contacts suggest bottom-current activity. Presence of short numerous channels is suggested by the thick beds of middle Tonkawa sandstones that extend for only a few miles.
Although the sedimentology and areal geometry of the upper Tonkawa are not discussed in this paper, the regional setting and lithologic character suggest shallow water (nearshore) deposition. The entire Tonkawa clastic sequence is capped by shallow water marine limestone.
The lower and middle Tonkawa sandstones represent offshelf deposition in an interior basin setting. The present study shows that the understanding of the geologic setting and the integration of all available data, rather than sedimentologic data alone, are required for proper identification. The following formations in the Anadarko basin also appear to contain fan and/or slope facies: Springer-Morrow (Springerian-Morrowan), Red Fork (Desmoinesian), and Cleveland, Marchand, and Cottage Grove (all Missourian).
If the deep water sandstones in interior basins are indeed as common as they appear to be in the Anadarko basin, then it is crucial for stratigraphic exploration that we recognize and document such deposits in other interior basins as well.
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