AGS / Geology / General Geology / Geologic History
Age boundaries in millions of years (Ma)
Holocene (Present to .01 Ma): Frequent flooding of the Mississippi
and other rivers results in deposition of alluvium. Sand dike injection
and extrusion during major earthquakes occurs in northeastern Arkansas.
Erosion of the Interior Highlands and down-cutting by most streams
Pleistocene (.01 to 2.6 Ma): Local alpine snow packs form in the Boston Mountains and Ouachita Mountains. Repeated intervals of erosion and deposition of glacial outwash by the ancestral Mississippi and Ohio Rivers form the Eastern and Western Lowlands and Crowley’s Ridge. Sand and silt winnowed from the outwash form dunes and sheet-like deposits of loess. Erosion occurs over most of the Interior Highlands.
Tertiary 2.6 to 66.5 Ma
Pliocene - Oligocene (2.6 to 33.9 Ma): Erosion over most of the
Eocene (33.9 to 55.8 Ma): Marine and fluvial clastic sediments fill the Mississippi Embayment; transportation of some bauxite; swampy conditions favorable to the accumulation of plant debris (lignite). Erosion occurs in the Interior Highlands.
Paleocene (55.8 to 65.5 Ma): Shallow marine sea in the Mississippi Embayment; formation of nearshore reefs and layers rich in marine remains and accumulation of dark marine clays; development of bauxite on islands of exposed Cretaceous igneous rocks. Erosion occurs in the Interior Highlands.
Deposition of water-laid volcanic debris preceded the accumulation of sand, marl, and chalk in shallow marine seas during the Late Cretaceous. About 100 Ma, downwarping of the Mississippi Embayment resulted in the invasion of the region by a shallow sea. Igneous activity in central, southwestern, and eastern Arkansas; nearshore deposition of clastic and carbonate debris in the Early Cretaceous, along with deposition of gypsum and anhydrite in highly saline waters in southwestern Arkansas. Erosion occurs in the Interior Highlands.
Jurassic 145.5 to 201.6 Ma
Deposition of carbonate sand (often oolitic) in a shallow marine environment and some red clay and anhydrite in a shallowing sea; preceded by the accumulation of a thick sequence of salt beds (halite), some anhydrite, and red clay and silt in shallow highly saline waters. Present only in the subsurface in southern Arkansas. Erosion occurs in the Interior Highlands. Jurassic or Triassic: Intrusion of small localized bodies of magma in southern Arkansas, which crystallized to form igneous rocks.
Triassic 201.6 to 251 Ma
Accumulation of predominantly red clay and silt, sand and gravel, and minor beds of anhydrite in a non-marine environment. Present only in southern Arkansas (in the subsurface). Age is indefinite. Erosion occurs in the Interior Highlands.
Erosion occurs. Last interval of uplift of the Interior Highlands and end of milky quartz-vein formation.
Pennsylvanian 299 to 318 Ma
Ozark region: Deposition of clastic sediments occurs. Younger
Pennsylvanian rocks absent; very late normal faulting.
Arkansas Valley region: Rapid infilling of clastic sediments and development of growth faults along northern basin margin. As the basin shallowed, plant debris (now present as coal) accumulated in nearshore swampy areas.
Ouachita region: Rapid influx of clastic sediments in a deep marine trough followed by intense deformation, such as folding, faulting, and, at depth, low-grade metamorphism; uplift during the Ouachita orogeny. Concurrent formation of quartz veins.
Mississippian 318 to 359 Ma
Ozark region: Deposition and episodic erosion of shallow-water
platform carbonate debris, clay, sand, and siliceous ooze.
Ouachita region: Rapid influx of clastic sediments in the Late Mississippian following slow accumulation of siliceous ooze and clay in the Early Mississippian.
Devonian 359 to 416 Ma
Ozark region: Slow deposition of carbonate sediments, siliceous
ooze, carbonaceous clay, and some sand in shallow marine water,
interrupted by intervals of erosion.
Ouachita region: Slow accumulation of siliceous ooze and clay in a deep marine environment.
Silurian 416 to 444 Ma
Ozark region: Deposition of thin, shallow-water carbonate sediments,
interrupted by intervals of erosion.
Ouachita region: Slow influx of sand and clay in a deep marine environment.
Ordovician 444 to 488 MA
Ozark region: Deposition of shallow-water carbonate sediment
and minor sand interrupted by intervals of erosion.
Ouachita region: Prolonged deep water accumulation of clay, sand, carbonaceous sediment, and siliceous ooze.
Cambrian 488 to 542 Ma
Ozark region: Calcareous sediment, some quartzose sand, and
clay accumulate in shallow water.
Ouachita region: Sediments accumulated as alternating layers of clay, silt, sand, and minor lime mud.
Ozark region and northern part of Mississippi
igneous rocks (some approximately 1,400 Ma) have been encountered in
Ouachita region: Erratic boulders of late Precambrian igneous and metamorphic rocks and sparse tectonically emplaced bodies of metamorphosed igneous rocks (~ 1,000 Ma) are present in Paleozoic rocks. In these areas, there is no evidence of the succession of events during Precambrian time. Although not exposed elsewhere, Precambrian rocks are assumed to underlie the entire state.
Time scale from Palmer (1983). Geology from the AGS.
Walker, J. D., and Geissman, J. W., compilers, 2009, Geologic Time Scale: Geological Society of America, doi: 10.1130/2009. CTS004R2C