Geologic units in Clark county, Arkansas

Alluvium (Holocene) at surface, covers 29 % of this area

Alluvial deposits of present streams

Stanley Shale (Early Mississippian) at surface, covers 11 % of this area

Stanley Shale - Includes Chickasaw Creek Chert equivalent of Harlton (1938) near top and Hatton Tuff Lentil and Hot Springs Sandstone Member near base

Wilcox Group (Early Eocene) at surface, covers 9 % of this area

Wilcox Group

Jackfork Sandstone (Early Pennsylvanian- Morrowan) at surface, covers 8 % of this area

Jackfork Sandstone

Terrace deposits (Late Pleistocene) at surface, covers 7 % of this area

Alluvial deposits on one or more terrace levels

Nacatoch Sand (Late Cretaceous) at surface, covers 6 % of this area

Nacatoch Sand

Midway Group (Paleocene) at surface, covers 6 % of this area

Midway Group

Brownstown Marl (Late Cretaceous) at surface, covers 5 % of this area

Brownstown Marl

Arkadelphia Marl (Late Cretaceous) at surface, covers 4 % of this area

Arkadelphia Marl

Atoka Formation - Lower part (Middle Pennsylvanian- Atokan) at surface, covers 4 % of this area

Atoka Formation - Lower part

Johns Valley Shale (Early Pennsylvanian- Morrowan) at surface, covers 3 % of this area

Johns Valley Shale

Ozan Formation (Late Cretaceous) at surface, covers 3 % of this area

Ozan Formation

Tokio Formation (Late Cretaceous) at surface, covers 2 % of this area

Tokio Formation

Sartoga Chalk (Late Cretaceous) at surface, covers 2 % of this area

Sartoga Chalk

Marlbrook Marl (Late Cretaceous) at surface, covers 2 % of this area

Marlbrook Marl

Arkansas Novaculite (Early Mississippian to Late Devonian) at surface, covers < 0.1 % of this area

Includes upper division (Mississippian), middle division (Mississippian and Devonian), and lower division (Devonian)