The striking diagonal boundary across the heart of the state is the edge of the Mississippi Embayment, a wide trough in the North American craton where once, long ago, the continent tried to split. The crack has remained seismically active ever since. Just north of the state line along the Mississippi River is where the great New Madrid earthquakes of 181112 occurred. The gray streaks crossing the embayment represent the recent sediments of (from left to right) the Red, Ouachita, Saline, Arkansas, and White rivers.
The Ouachita Mountains are actually part of the same foldbelt as the Appalachian range, separated from it by the Mississippi Embayment. Like the Appalachians, these rocks produce coal and natural gas as well as various metals. The southwestern corner of the state yields petroleum from its early Cenozoic strata. And just on the border of the embayment, a rare body of lamproite (the largest of the red spots) is the only diamond-producing locality in the United States, open for public digging as Crater of Diamonds State Park.
Besides this small-scale map, I also have scanned the much larger and far more detailed state geologic map of Arkansas published by the U.S. Geological Survey in 1993. The mid-sized version is 1200x1050 pixels and weighs 700 KB. It looks pretty good: The small type is not legible, but you can check the colors against the explanation (1200x1200, 300 KB) to identify the different rock units.
The big version is 2000x1740 pixels and weighs 2 MB. But I've prepared an even bigger version in four pieces, each one 2300x2000 pixels: