Figure 8. Model used to define assessment units in the USGS assessment of Tertiary strata in Gulf of Mexico onshore and State water areas. For the Blue Stratigraphic Unit, Assessment Unit (AU) 1 is the stable shelf structural area; AU 2 defines the main unit expansion and thickening zone; and AU 3 comprises sediments deposited in slope and basin floor depositional environments. After Warwick et al. (2007a).