Geologic units in Decatur county, Tennessee

Coffee Sand (Cretaceous) at surface, covers 35 % of this area

Loose fine-grained sand, light-gray, sparsely glauconitic, locally interbedded with laminated lignitic clay. Thickness 25 to 200 feet; thins northward.

Silurian Formations, including Decatur Limestone, Brownsport Group (Lobelville Formation, Bob Limestone, Beech River Formation), Wayne Group (Dixon Formation, Lego Formation, Waldron Shale, Laurel Limestone, Osgood Formation) and Brassfield Limestone (Silurian) at surface, covers 20 % of this area

Silurian Formations - A complete section of Silurian formations is not common because of pre-Chattanooga and/or pre- Cretaceous erosion. Where preserved, Silurian formations are remarkably uniform in thickness and are characteristically light olive-gray to greenish-gray with variable reddish-brown color in some area; Decatur Limestone - Thick-bedded, medium- to coarse-grained limestone, gray with reddish-brown grains. Thickness 0 to 70 feet; Brownsport Group which includes 1) Lobelville Formation - Shale with thin beds of limestone. Thickness 0 to 40 feet; 2) Bob Limestone - Thick-bedded, medium-grained limestone, locally oolitic. Thickness 0 to 25 feet; and 3) Beech River Formation - Shale with thin beds of limestone. Thickness 0 to 60 feet; Wayne Group which includes: 1) Dixon Formation - Green and reddish-brown argillaceous limestone, shale, and mudstone. Thickness 0 to 40 feet; 2) Lego Limestone - Even-bedded, olive-gray limestone with scattered reddish-brown grains. Thickness 0 to 30 feet; 3) Waldron Shale - Greenish-gray fossiliferous shale. Thickness 0 to 5 feet; 4) Laurel Limestone - Even-bedded, gray limestone with scattered reddish-brown grains. Thickness 0 to 30 feet; 5) Osgood Formation - Greenish- and reddish-gray shale and argillaceous limestone. Thickness 0 to 15 feet.; and Brassfield Limestone - Thin-bedded cherty limestone, locally glauconitic. Thickness 0 to 20 feet.

Alluvial deposits (Quaternary) at surface, covers 15 % of this area

Sand, silt, clay, and gravel. In flood plain of Mississippi River more than 100 feet thick; in smaller streams generally less than 20 feet thick.

Devonian Formations, includes Pegram Formation, Camden Formation, Harriman Formation, Flat Gap Limestone, and Ross Formation (Devonian) at surface, covers 12 % of this area

Devonian Formations - Characterized by marked north-south facies variations. Because of pre-Chattanooga and/or pre-Cretaceous warping and erosion, the distribution and thickness of Devonian formations is very irregular. Includes Pegram Formation - Thick-bedded, gray limestone and gray sandstone. Thickness 0 to 15 feet; Camden Formation - Light-gray novaculitic chert and tripolitic clay; and minor siliceous limestone. Thickness 0 to about 100 feet; Harriman Formation - Light-gray novaculitic chert and tripolitic clay; and minor siliceous limestone. (Harriman and Camden are differentiated paleontologically.) Thickness 0 to 50 feet; Flat Gap Limestone - Thick-bedded, coarse-grained limestone, gray with red and brown grains. Thickness 0 to 55 feet; Ross Formation - Siliceous limestone; gray and variegated shale; and medium-grained glauconitic limestone. Thickness 0 to 75 feet.

Fort Payne Formation and Chattanooga Shale (Mississippian) at surface, covers 11 % of this area

Fort Payne Formation - Bedded chert and calcereous and dolomitic silicastone; minor coarse-grained limestone and shale. Thin green shale (Maury) at base. Thickness about 200 feet. Chattanooga Shale - Black carbonaceous shale, fissile. Thickness 0 to 70 feet.

High-level alluvial deposits (Quaternary-Tertiary) at surface, covers 3 % of this area

Iron-stained gravel, sand, silt, and clay; variable in thickness but generally less then 60 feet thick.

Coon Creek Formation (Cretaceous) at surface, covers 2 % of this area

Fossiliferous, micaceous sand, silty and glauconitic; locally fossiliferous sandy clay at base. Siderite concretions common in upper part. Thickness about 140 feet.

Ordovician Formations including Richmond Group (Mannie Shale, Fernvale Limestone), and Nashville Group (Hermitage Formation) (Ordovician) at surface, covers 0.6 % of this area

Ordovician Formations - Richmond Group (including Mannie Shale - Shale with thin beds of argillaceous limestone. Thickness 0 to 20 feet and Fernvale Limestone - Thick-bedded, coarse-grained limestone with vari-colored grains. Thickness 0 to 20 feet and Nashville Group (including Hermitage Formation - Gray shale and thin-bedded to laminated, sandy and argillaceous limestone. Maximum exposed thickness 80 feet.)

Alluvial deposits (Quaternary) at surface, covers 0.5 % of this area

Sand, silt, clay, and gravel. As much as 60 feet thick in flood plains of Tennessee and Cumberland Rivers; in smaller streams generally less than 20 feet thick.

Sardis Formation (Cretaceous) at surface, covers 0.2 % of this area

Quartz sand and glauconite sand, argillaceous and locally fossiliferous. (Mapped with Kcc north of Beech River.) Maximum thickness 70 feet.

Eutaw Formation (Cretaceous) at surface, covers 0.1 % of this area

Grayish-green sand, fine-grained, glauconitic, micaceous; interbedded with gray laminated clays which commonly contain carbonized or silicified wood. (Mapped with Coffee except in Hardin County and southeastern Decatur County.) Thickness 0 to 180 feet; thins northward.

McNairy Sand (Cretaceous) at surface, covers < 0.1 % of this area

Predominantly sand, in places interbedded with silty light-gray clays. Fine-grained sand at base, locally contains heavy minerals. Thickness about 300 feet.

Silurian Formations, including Decatur Limestone, Brownsport Group (Lobelville Formation, Bob Limestone, Beech River Formation), Wayne Group (Dixon Formation, Lego Formation, Waldron Shale, Laurel Limestone, Osgood Formation) and Brassfield Limestone (Silurian) at surface, covers < 0.1 % of this area

Silurian Formations - Characteristically light olive-gray to greenish-gray, with variable reddish-brown color in some areas. Individual formations are generally uniform in thickness, except where truncated by pre- Chattanooga erosion. Most formations are slightly thicker (than indicated) in Wells Creek Basin. Includes Decatur Limestone- Thick-bedded, medium- to coarse-grained limestone, gray with reddish-brown grains. Thickness 0 to 70 feet; Brownsport Group (which includes Lobelville Formation - Shale with thin beds of limestone. Thickness 0 to 30 feet; Bob Limestone - Thick-bedded, medium-grained limestone, locally oolitic. Thickness 0 to 25 feet; and Beech River Formation - Shale with thin beds of limestone. Thickness 0 to 60 feet); Wayne Group (which includes Dixon Formation - Green and reddish-brown argillaceous limestone, shale, and mudstone. Thickness 0 to 40 feet; Lego Limestone - Even-bedded, olive-gray limestone with scattered reddish-brown grains. Thickness 0 to 30 feet; Waldron Shale - Greenish-gray fossiliferous shale. Thickness 0 to 5 feet; Laurel Limestone - Even-bedded, olive-gray limestone with scattered reddish-brown grains. Thickness 0 to 30 feet; Osgood Formation - Greenish- and reddish-gray shale and argillaceous limestone. Thickness 0 to 15 feet); and Brassfield Limestone - Thin-bedded cherty limestone, locally glauconitic. Thickness 0 to 50 feet; generally about 20 feet.