The basement in Oklahoma consists of igneous rocks, mostly granitic, whose surface is a profound erosional discontinuity overlain by marine sediments. Basement deforming tectonism was active in the southern part of the state forming the Southern Oklahoma Aulacogen, which is commonly interpreted as a Cambrian rift. This feature contains thick sequences of bimodal igneous rocks with unusual sheet granites. The lithologic and erosional discontinuities that mark the transition from the igneous crust to overlying Paleozoic marine sediments probably represents the pre-Flood boundary, and the tectonism of the Southern Oklahoma Aulacogen may reflect activity at the onset of the Flood in the southern midcontinent.