1Structural Geology Regional Structure

The Karnak Quadrangle lies along the southern margin of the Illinois Basin and at the northern edge of the Mississippi Embayment. The Illinois Basin, which is the older structure, is expressed by the gentle northward dip of Mississippian bedrock formations, as shown on cross section A–A9 on map sheet 2. (The actual dip ranges from less than 1 ° to about 2° but is exaggerated 2:1 on the cross section.) This dip was imparted by uplift of the Pascola Arch, a structure south of Illinois that rose during Cretaceous time (Stearns and Marcher 1962). The Mississippi Embayment began to subside late in the Cretaceous Period and became an arm of the Gulf of Mexico. The Embayment overlies an ancient rift zone, the Reelfoot Rift, which probably controlled its later subsid-ence (Kolata and Nelson 1991). The McNairy Formation, a nearshore marine deposit of late Cretaceous age, dips and thickens southward into the embayment. The much younger Mounds Gravel, a fluvial deposit, overlies the McNairy with an angular unconformity. Little Cache Fault Zon