Geology of Arkansas Ozarks Plateau Salem Plateau Dolostone

Geology of Arkansas

Ozarks Plateau

Salem Plateau Dolostone with interbedded shale in the Powell Formation The Salem Plateau contains the oldest rock at the surface - Ordovician dolostones, sandstones and limestones. This is also the lowest plateau surface with some of the highest elevations reaching between 800 and 1400 feet above sea level.

Dolostone in the Cotter Formation at Cotter, AR

Springfield Plateau Boone Formation forming buttresses along Bear Creek in Searcy County. The Springfield Plateau contains mostly Mississippian age limestone and chert from the Boone Formation at the surface. This plateau is intermediate in elevation with some of its highest elevations reaching 1800 feet above sea level. The plateau surface consists mostly of gently rolling hills or an undulating topography, however in some places the Boone Formation forms steep hillsides separated by ravine-like drainages.

The Springfield Plateau (foreground) near Marshall, Arkansas.

Boston Mountains Plateau Bloyd Formation near Phillips Ford The Boston Mts. Plateau contains the youngest rock at the surface - Early Pennsylvanian sandstones and shales with minor limestones. This is also the highest plateau surface with some of the highest elevations reaching 2200 -2500 feet above sea level.

"middle Bloyd sandstone" at Buzzards Roost

The geology of the Ozark Plateau Region is characterized by slightly southward-dipping sedimentary rock deposited mostly by shallow marine seas from the Ordovician through the Pennsylvanian Period. Caves and sinkholes are common in limestones of the Springfield Plateau with less karst development in the dolostones in the Salem Plateau. The Boston Mt. Plateau surface contains mostly sandstones and shales deposited by south/southwestward flowing streams during times of low sea level.

Arkansas River Valley The Arkansas River and surrounding low-lying land as seen from Petit Jean Mountain The Arkansas River Valley is a low-lying region surrounding the valley of the Arkansas River and its major tributaries. The highest point in Arkansas (Mount Magazine – 2753 ft. ) is found among several mountain ridges that rise above the lowlands in the southern portion of the river valley. The mountain ridges are separated by broad valleys. Surface rocks in this region consist of a sequence of coal-bearing sandstones and shales.

Sandstone and shale on Magazine Mountain

The Arkansas River Valley represents the northern extent of the Ouachita orogenic (mountain building) system in Arkansas. Once flat-lying, these Pennsylvanian sedimentary rocks have been compressed into well developed east-west trending open folds (anticlines and synclines) and faults, which gradually diminish northward into the Ozark Plateau Region. Shaded relief map of the Arkansas River Valley. Canoe and cigar-shaped structures represent anticlines and synclines.

Diagram showing structures in the river valley. Most of the higher elevations exposed in the river valley are actually what is left of the center if the syncline. Nearly all of the rocks from the surrounding anticlinal limbs have been eroded away.

Ouachitas The Ouachita Mountain Region consists of a series of sharp ridges, mostly east-west trending and often buckled and distorted, separated by narrow to broad valleys. Surface rocks from this region are mostly shales, sandstone, novaculite, chert and minor limestone.

Inclined bedding in the Stanley Shale in Hot Springs Arkansas

Folded strata in the Big Fork Chert near Hot Springs

Gulf Coastal Plain The West Gulf Coastal Plain is a south sloping plain of gently rolling hills and can be divided into two areas based on the age of the rocks. The area in orange consists of Tertiary clays, sands and silts with lignite deposits and Quaternary gravels, sands and clays. The area in green consists of Cretaceous sand, clay, gravel, marl, limestone, chalk and Quaternary sand gravel.

This part of the state contains mostly early Tertiary aged rocks, mostly sands, silts and clays, deposited in streams, swamps and shallow marine water. Lignite beds occur throughout the sequence. Quaternary alluvium and terrace deposits are present from rivers in the area. Underneath the Tertiary deposits are the sands, gravels, limestones, chalks and marls seen in the Cretaceous area.

The geology of the Cretaceous area within the West Gulf Coastal Plain is characterized by gently southward-dipping sedimentary rocks deposited mostly in shallow marine water of the Gulf of Mexico that once extended into Arkansas. Alluvium deposits from older and present day streams are common The Annona Formation at Whitecliffs near Millwood Lake

Gypsum beds in the De. Queen Limestone

Tokio Gravel

Mississippi River Alluvial Plain The Mississippi River Alluvial Plain is a relatively level plain with elevations varying between 100300 feet except for Crowley’s Ridge with some of its highest points reaching 400 -560 feet. This region contains mostly unconsolidated sediments such as sand, clay, silt, gravel and loess.

Loess canyon on Crowley’s Ridge

The geology of the Mississippi River Alluvial Plain is characterized by unconsolidated flat -lying alluvium from present day and older streams, overlying poorly consolidated Tertiary formations that are dipping slightly to the south. Quaternary terrace sand gravels on top of Crowley’s Ridge

Tertiary deposits