Geology of Arkansas

The geology of Arkansas includes deep 1.4 billion year old igneous crystalline basement rock from the Proterozoic known only from boreholes, overlain by extensive sedimentary rocks and some volcanic rocks. The region was a shallow marine, riverine and coastal environment for much of the early Paleozoic as multi-cellular life became commonplace. At the end of the Paleozoic in the Permian the region experienced coal formation and extensive faulting and uplift related to the Ouachita orogeny mountain building event. Extensive erosion of new highlands created a mixture of continental and marine sediments and much of the state remained flooded even into the last 66 million years of the Cenozoic. In recent Pleistocene and Holocene time, glacial sediments poured into the region from the north, down major rivers, forming dunes and sedimentary ridges. Today, Arkansas has an active oil and gas industry, although hydraulic fracturing related earthquake swarms have limited extraction. Mining industries in the state also produce brines, sand, gravel and other industrial minerals.

The oldest rocks in Arkansas are igneous granites encountered in deep wells in the Ozarks and the northern part of the Mississippi Embayment, dated to be 1.4 billion years old. These felsic rocks likely formed sometime soon after the breakup of the supercontinent Columbia, into its component continents, including Laurentia, which today forms the stable basement rocks of most of North America.

The Precambrian, including the Archean and Proterozoic eons is poorly understood in Arkansas, but the entire state is believed to be underlain by deep, igneous and metamorphic crystalline basement rock. In the Ouachita region, geologists have found igneous and metamorphic Precambrian erratic boulders as well as metamorphosed igneous rock intrusions.

At the time of the Cambrian explosion, as multi-cellular became commonplace, Arkansas was primarily flooded by rivers and a shallow marine environment. In the Ozark region, calcareous, quartzose sand and clay deposited, while the Ouachita area witnessed the formation of alternating layers of sand, clay, silt and small amounts of lime mud.

Water levels may have deepened in Arkansas with sand and clay depositing in a deep marine offshore trough environment in the Ouachita region, which shifted to siliceous ooze and clay by the Devonian. A shallow, near shore environment prevailed in the Ozarks, where thin layers of carbonates were interrupted by periodic erosion during the Ordovician. During the Devonian, carbonate deposition continued, but with added emplacement of carbonaceous clay, siliceous ooze and some shallow marine sand deposits.

While a marine environment continued to prevail into the Mississippian, the period marked rapid clastic sedimentation in the Late Mississippian in the Ouachita region. The depositional environment in the Ozarks area remained much the same as in the Devonian for the Mississippian, but the region experienced rapid clastic sedimentation at the beginning of the Pennsylvanian. The Ozarks lack Early Pennsylvanian rocks.

Arkansas experienced widespread faulting in the Pennsylvanian, associated with the Ouachita orogeny mountain building event. As a result, the quickly accumulated sediments in the Ouachita deep marine trough were faulted, folded, experienced low-grade metamorphism, followed by uplift and the formation of numerous quartz veins.

A shallow environment of nearshore swamps formed in the Arkansas Valley region, bounded by newly formed faults, leading to subsequent coal formation from plant debris. The region experienced erosion during the Permian and widespread uplift of highlands as well as the continued formation of milky quartz veins.