Map Showing Outcrop of the Coal-bearing Units and Land Use in the Gulf Coast Region

United States Geological Survey Open File Report 97-172

[ Introduction | References | Stratigraphy ]
[ Map | Map Legend | Inset Map ]

Description of Map Units

The Jackson Group is present across the entire Gulf Coast coal region. In Georgia, the unit is exposed but not shown on this map because it is mostly marine and is represented by weathered material at the surface and therefore of minimal interest in coal studies. In Alabama and Mississippi, the upper Jackson Group consists of the Yazoo Clay--green and gray calcareous clay containing some glauconitic sand and fossiliferouslimestone and coquina beds. At the base of the Jackson Group in Alabama and Mississippi is the Moodys Branch Formation--which is composed of clayey quartz sand with shells embedded in glauconite, sandy limestone beds, and thin lignite beds a few inches thick in the lower part of the unit (Williamson, 1976; Bicker, 1969; Osborne and others, 1989). In Louisiana, the Jackson Group consists of (in ascending order): the Moodys Branch, Yazoo, Danville Landing, and Mosley Hill Formations, which consist of light gray to brown lignitic clays with interbedded limonitic sands or lignite; near the base, calcareous, glauconite, and fossiliferous beds may weather to black soil (Snead and McColluh, 1984). In Tennessee and Kentucky, the Jackson Group is undivided and consists of sand with layers of gray clay, silt, and lignite, and except for some isolated inliers within the blanket glacio-fluvial deposits of the Mississippi embayment, is exposed only along the banks of the Mississippi River (Miller and others, 1966; Olive, 1980; McDowell and others, 1981). In Arkansas, the Jackson Group is exposed along the flanks of Crowley's Ridge and south of the Arkansas River. The Jackson Group in Arkansas is composed of sandy clay; silt; glauconitic and fossiliferous sandy clay; and a few lignite beds in the southeastern part of the state (Wilbert, 1953; Guccione and others, 1986). In Texas, the Jackson Group consists of light-colored, fossiliferous, glauconitic sand, sandy clay, green tuffaceous clay, and lignite beds, and is divided into several formal and informal units. The main units from the base of the Jackson Group are: Caddell Formation--clay and fossiliferous, glauconitic sand; "Wellborn Formation"--sand, clay and lignite; Manning Formation--sand, clay, well-developed lignite beds; and Whitsett Formation--tuffaceous, argillaceous sand (Barnes, 1992). Lignite is well-developed in central and south Texas where it is currently being mined from the Manning Formation and the undivided lower Jackson respectively. [ Top ]

The Claiborne Group is exposed across most of the Gulf Coast coal region. In Georgia, the Claiborne consists of shale, mud, and silt that grade laterally into carbonate deposits. The group has been divided into two formations: the lower part is the Tallahatta Formation and the upper part is the marine Lisbon Formation (Galloway and others, 1991). In Alabama, the Tallahatta Formation consists of siliceous clay, that is interbedded with layers of fossiliferous clay and glauconitic sand and becomes coarse and mixed with fine gravel to the southwest. The upper part of the Claiborne Group of Alabama is composed of the Gosport Sand/Lisbon Formation--glauconitic sands with lenses of clay and calcareous, glauconitic, fossiliferous clay, silt, and sand. In Mississippi, the Claiborne is mapped as several formations. The lowermost is the Meridian Sand/Tallahatta Formation--glauconitic clay, with lenses of sand. It becomes predominately sand in the western part of the state. The Meridian Sand/Tallahatta Formation is overlain by the following units: Winona Formation and the Zilfa Shale--highly glauconitic sandy clay with some glauconitic sand; the Kosciusko Formation--irregularly bedded sand, clay, and some quartzite; the Cook Mountain Formation--marl, limestone, glauconitic sand, and clay to the southeast, becoming mostly clay in the northwest; and the Cockfield Formation--irregularly bedded, lignitic clay, sand, and some lignite beds (Bicker, 1969; Dockery, 1996). In Louisiana and Arkansas, the Claiborne is composed of the following formations, starting from the base: the Carrizo Formation--sand, mud (often included in the Wilcox Group); the Cane River Formation--clay with basal glauconitic, fossiliferous silt; the Sparta Formation-massive sand interbedded with some thin beds of lignite or lignitic sand and shale; the Cook Mountain Formation--clay and fossiliferous marl in the lower part and sideritic, glauconitic, clay in the upper part; and the Cockfield Formation--lignitic clay, silt, and sand, with some sideritic glauconite (Snead, 1984). In Tennessee and Missouri (subsurface), the Claiborne Group contains the basal Memphis Sand--sand, silt, and minor lignite; overlain by the Cook Mountain Formation--clay, silt, and sand; and the Cockfield Formation--sand, silt, clay, and lignite. The Cockfield and the overlying Jackson Group are sometimes grouped together (Hackman and Meissner, 1983; Parks and Carmichael, 1990). In Kentucky, the Claiborne is undivided and consists of interbedded silt, clay, sand and minor lignite beds. In Texas, the Claiborne changes facies across the state and includes a number of different names for time-equivalent strata. In northeast, east, and south Texas the basal Claiborne is marked by the Carrizo Formation--sand and mud. In northeast and east Texas, above the Carrizo, the lower part of the Claiborne is equivalent to the Cane River in Arkansas and Lousiana. These units have gradational contacts and are divided into the lower shale-dominated Reklaw Formation, overlain by the sand and silt dominated Queen City, and the shale-dominated Weches Formation. The middle part of the Claiborne Group contains the Sparta Formation, overlain by the shale-dominated Cook Mountain/Stone City Formation. The upper part of the Claiborne in northeast and central Texas contains the sand-dominated Yegua Formation which is equivalent to the Cockfield Formation in the eastern part of the Gulf Coast coal region and contains significant but as of yet unmined lignite. In the southern part of Texas, above the Carrizo Formation, is the sand-dominated Bigford Formation, which is overlain by the El Pico Clay. These units contain significant non-banded coal deposits which are presently being mined. In south Texas, the upper middle part of the Claiborne consists of the sand-dominated Laredo Formation, and is overlain by the Yegua Formation (Sellards and others, 1966; Barnes, 1992). [ Top ]

The Wilcox Group is exposed across most of the Gulf Coast coal region. In Georgia, the Wilcox is composed of the following units in ascending order: the Nanafalia Formation--shale, lignitic mud, silt and glauconitic sand; the Tuscahoma Formation--glauconitic and lignitic mud and sand; and the Bashi Marl Member of the Hatchetigbee Formation--sandstone interbedded with carbonaceous clay and silt (Murray, 1961; Zapp, 1965; Lawton and Marsalis, 1976). In some places in Georgia, parts of the Midway Group have been mapped with the Wilcox. In Alabama, the Wilcox is composed of, in ascending order: the Nanafalia Formation--sandy fossiliferous clay, glauconitic quartzose sand, and gravelly sand with lignite beds; the Tuscahoma Formation--carbonaceous silt and clay with thin lignite beds; and the Hatchetigbee Formation--carbonaceous clay, silt, and glauconitic, calcareous, fossiliferous sand (Osborne and others, 1989). In Mississippi, the lower units in the Wilcox Group are similar to those in Alabama: the Nanafalia--silt, clay, and sand, with lignite beds becoming more numerous upwards; overlying the Tuscahoma Formation--sand, clay, shale, and common lignite beds. The Bashi Formation--glauconitic fossiliferous sand with fossiliferous concretions, separates the lignite-bearing Tuscahoma and the overlying Hatchetigbee Formation--sand, shale, carbonaceous shale, and lignite (Williamson, 1976; Bicker, 1969; Dockery, 1996). In Louisiana, the Wilcox has been divided into numerous stratigraphic formations that are used for surface mapping; they are generally indistinguishable in the subsurface (Pope, 1981). The lower Wilcox formationsare time-equivalent to Midwayan strata elsewhere in the Gulf region, but because they are lithologically similar to Wilcox strata, the upper Midwayan-age strata in Louisiana are mapped as Wilcox Group (Murray 1961). The lowermost Wilcox unit is the Naborton Formation which contains clay, mud, sand, and lignite beds that are currently mined. This unit is overlain (in ascending order) by the Dolet Hills, Cow Bayou, Converse, Lime Hill, and Hall Summit Formations--all containing variable amounts of sand, mud, and glauconite. The Hall Summit contains lignitic clays. The Marthaville, Pendleton, and Sabinetown Formations, overlying the Hall Summit Formation, are generally equivalent to the Wilcox strata in other parts of the Gulf region and are lithologically similar to the lower part of the Wilcox in Louisiana, but contains fewer lignite beds (Murray, 1961). In Tennessee and Missouri, the Wilcox Group from the base upward, consists of: Old Breastworks Formation--clay, silt, and lignite; Fort Pillow Sand--sand and minor clay; and the Flour Island Formation--clay, silt, sand, and lignite (Parks and Carmichael, 1990). In Kentucky, the Wilcox is undivided and consist mostly of sand and silty clay with minor lignite (Olive, 1980). The Wilcox is undivided in the subsurface of southernmost Illinois and consists of sand and mudstone (Cushing and others, 1964). In Arkansas, the Wilcox Group is undivided and consists of sand, mudstone, carbonaceous shale, lignite, lenses of bentonitic clay, and lenses of quartzose gravel (Spooner, 1935). In northeastern Texas, the Wilcox Group is undivided and consists of silt, sand and clay with local beds of clay, minable lignite, and quartz sand. In the east-central part of the state, the Wilcox Group is divided into three formations. The lowermost Hooper Formation--mudstone, sandstone locally glauconitic, and local lignite beds; Simsboro Formation--sand, mudstone, clay, and mudstone conglomerate; and the Calvert Bluff Formation--mudstone, with sandstone, minable lignite beds, ironstone, and glauconitic in the uppermost part. In southern Texas, the Wilcox Group is mapped undivided or is called the Indio Formation and consists of sand, shale and lignite beds (Barnes, 1992). [ Top ]

Midway Group

The Midway Group is exposed across the Gulf Coast coal region, but only the uppermost lignite-bearing Naheola Formation in Alabama and Mississippi is shown on this map. The Naheola Formation in the lower part consists of silt, clay, and fine sand, with lignite beds; the upper part consists of glauconitic sand, silty clay, carbonaceous shale, and sandy fossiliferous marl (Osborne and others, 1989; Williamson, 1976). [ Top ]

Navarro Group

The Olmos Formation occurs in southern Texas and northeastern Mexico where it is composed of mud, sand, and coal beds which are currently mined in Mexico (Barnes, 1992).

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