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Figure 14. Boundaries and areas used to define assessment units for the Frio and Anahuac Formations. The Frio outcrop (base of Miocene) is from Schruben et al. (1998), based on King and Beikman, 1974; the Early Cretaceous shelf margin is modified from Ewing and Lopez (1991); the saltwater/freshwater interface (10,000 mg/L dissolved solids isoline) is from Pettijohn (1996); salt diapirs are from Lopez (1995), Ewing and Lopez (1991), and Martin (1980); the USGS Oil and Gas Province Boundary is from USGS (2004); the Vicksburg Fault Zone is from Ewing et al. (1990); and the Frio unstable shelf edge is based on the Frio Fault Zone (Ewing, 1986, 1991b; Ewing et al., 1990) and unstable shelf areas (Paine et al., 1968, John et al., 1992 b,c,d).