Lower Atoka is an informal name applied to the lowermost depositional sequence of the Atoka Group, which is partly equivalent to the Big Saline Formation. Depositionally it reflects a transition from carbonate to terrigenous-dominated environments. Chert litharenites derived from the Ouachita structural belt and locally from the Muenster arch first prograded across the northern part of the Fort Worth basin during lower Atoka deposition.
Lower Atoka strata are interpreted to be a fluvially dominated fan-delta system which exhibits characteristics of both a fan delta and a high-constructive elongate delta. Fan-delta characteristics include tectonically active source, poorly developed alluvial plain, and interfingering and/or coexisting terrigenous and carbonate facies. High-constructive elongate delta characteristics include elongate, digitate sand-body geometry and a facies tract reflecting rapid progradation and aggradation.
Contemporaneous faulting within the basin was a major factor in distribution of lower Atoka facies. Sediment was confined to the downthrown sides of contemporaneous faults resulting in superposed fan-delta deposits. Stacking and coalescing of early fans constructed an alluvial plain. When sediment supplies exceeded fault movement, the fan deltas continued to prograde across the basin.
Cumulative oil and gas (equivalent) production from the lower Atoka is over 130 million bbl. Production is facies controlled and the distribution of fields coincides with the distribution of fan-delta lobes. Fields are aligned predominantly on the basinward side of major faults and they produce from fan-delta channel-fill and coarse-grained fan-delta plain facies.