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Figure 15. The Frio Stable Shelf Oil and Gas Assessment Unit, with boundaries used to define the assessment unit. AU = assessment unit. The Frio outcrop (base of Miocene) is from Schruben et al. (1998), which is based on King and Beikman, 1974; the Early Cretaceous shelf margin is modified from Ewing and Lopez (1991); and the saltwater/freshwater interface (10,000 mg/L dissolved solids isoline) is from Pettijohn (1996).