Geologic units in Marengo county, Alabama

Alluvial, coastal and low terrace deposits (Holocene) at surface, covers 23 % of this area

Varicolored fine to coarse quartz sand containing clay lenses and gravel in places. Gravel composed of quartz and chert pebbles and assorted metmorphic and igneous rock fragments in streams near the Piedmont. In areas of the Valley and Ridge province gravel composed of angular to subrounded chert, quartz, and quartzite pebbles. Coastal deposits include fine to medium quartz sand with shell fragments and accessory heavy minerals along Gulf beaches and fine to medium quartz sand, silt, clay, peat, mud and ooze in the Mississippi Sound, Little Lagoon, bays, lakes, streams, and estuaries.

Wilcox Group; Tuscahoma Sand (Paleocene) at surface, covers 17 % of this area

Light-gray to light-olive-gray laminated and thin-bedded carbonaceous silt and clay interbedded with fine sand; thin lignite beds occur locally. Lower part of the formation includes beds of fossiliferous, glauconitic fine quartz sand containing speroidal sandstone concretions, gravel and clay pebbles.

Selma Group; Demopolis Chalk (Cretaceous) at surface, covers 16 % of this area

Light-gray to medium-light-gray compact, brittle chalk overlain by abundantly fossiliferous chalky marl, very clayey chalk, and calcareous clay (Bluffport Marl Member). In south-central Montgomery County the Demopolis is split into two eastward extending tongues by a westward-extending tongue of the Cusseta Sand Member of the Ripley Formation. The lower tongue is pale-olive to yellowish-gray silty to finely sand, micaceous, fossiliferous chalk that eastward becomes more sandy and merges with the Cusseta in central Bullock County. The upper tongue is yellowish-gray clayey, very finely sandy, micaceous chalk that merges with the Ripley in southeastern Montgomery County.

Midway Group; Porters Creek Formation (Paleocene) at surface, covers 15 % of this area

Dark-gray massive plastic clay in western AL with a thin bed of glauconitic shell marl at the top (Mathews Landing Marl Member). Becomes calcareous eastward grading into light-greenish-gray calcareous, micaceous, clayey fine to medium sand, medium-gray sandy, calcareous clay and white to light-gray thin bedded partly clayey, fossiliferous limestone. East of Crenshaw County, owing to lithologic similarity, beds correlative with the Porters Creek are included in the Clayton Formation.

Selma Group; Ripley Formation (Cretaceous) at surface, covers 13 % of this area

Light-gray to pale-olive massive, micaceous, glauconitic, fossiliferous fine sand; sandy calcareous clay; and thin indurated beds of fossiliferous sandstone.

Midway Group; Naheola Formation (Paleocene) at surface, covers 8 % of this area

The Naheola Formation is restricted to western AL and pinches out in western Butler County. Descriptions of the members of the formation follow in descending order. Coal Bluff Marl Member - glauconitic sand, thin-bedded silty clay, and sandy fossiliferous marl; Oak Hill Member - laminated silt, clay, and fine sand; contains a prominent bed of lignite near the top. The Coal Bluff Marl Member in Sumter County and in parts of Marengo County is mostly cross-bedded fine to coarse sand that is indistinguishable from the overlying lower beds of the Nanafalia Formation. Therefore, in these areas, the contact between the two is mapped at the top of the Oak Hill Member of the Naheola.

Wilcox Group; Nanafalia Formation (Paleocene) at surface, covers 5 % of this area

Members of the Nanafalia Formation follow in descending order. Grampian Hills Member - medium-gray massive clay, claystone, sandy fossiliferous clay, and fossiliferous fine sand. "Ostrea thirsae beds" - glauconitic, abundantly fossiliferous, quartzose fine to medium sand. Gravel Creek Sand Member - pale-yellowish-orange to moderate-reddish-brown micaceous cross-bedded fine to very coarse sand containing gravel and clay pebbles in some exposures. Gravel Creek Member is absent locally and near the base may contain thin beds of lignite. Updip deposits in northern Henry County and southern Barbour County include beds of alternating medium-gray and white clay, carbonaceous clay, white and grayish-yellow fine to coarse sand and lenses of bauxite and bauxitic clay. Sand beds commonly are cross-bedded, gravelly, and contain numerous clay pebbles. The sequence of beds is often obscured by weathering and the collapse of beds into sinkholes in the underlying Clayton Formation.

High terrace deposits (Pleistocene) at surface, covers 2 % of this area

Varicolored lenticular beds of poorly sorted sand, ferruginous sand, silt, clay, and gravelly sand. Sand consists primarily of very fine to very coarse poorly sorted quartz grains; gravel composed of quartz, quartzite, and chert pebbles.

Midway Group; Clayton Formation (Paleocene) at surface, covers 0.6 % of this area

White to yellowsih-gray argillaceous limestone occurs in the upper part (McBryde Limestone Member): the lower part is medium-gray fossiliferous calcareous silt, glauconitic sand and thin beds of sandy limestone and calcareous sandstone (Pine Barren Member). At the base of the formation in southeast AL is a gravelly medium to coarse sand containing clay pebbles. The formation thins west of Wilcox County and westward from Thomaston in eastern Marengo County is mapped with the Porters Creek Formation. The formation is generally deeply weathered and fresh exposures are rare. In western areas exposures consist of weathered white to yellowish-gray argillaceous, fossiliferous sandy limestone, ferruginous sand, and fossiliferous sandstone. In eastern areas exposures consist of residual accumulations of chert boulders, moderate-reddish-orange sand, and clay containing masses and thin layers of iron minerals (limonite-goethite).

Selma Group; Mooreville Chalk (Cretaceous) at surface, covers < 0.1 % of this area

Yellowish-gray to olive-gray compact fossiliferous clayey chalk and chalky marl. The unconformable contact at the base is characterized by a bed of glauconitic, chalky sand containing phosphate pellets and molds of fossils. The Arcola Limestone Member at the top consists of two to four beds of light-gray brittle, dense, fossiliferous limestone seperated by beds of light-gray to pale-olive calcareous clay.