| Embayment Megagroup | |
| Type: | Mega Group |
| Age: | Eocene–Cretaceous |
| Period: | Eocene–Cretaceous |
| Prilithology: | Sandstone, Clay |
| Otherlithology: | Siltstone, Lignite |
| Subunits: |
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| Thickness: | upto ~1000' |
| Extent: | Gulf Coast Plain to Southern Illinois |
The Embayment Mega Group is found from the Gulf of Mexico north to southern Illinois.
The Mounds Gravel is found in southern Illinois. It is composed of chert and quarts pebbles.[1]
The Grover Gravel (Rubey, 1952) is found in western Illinois. Named for Grover, Missouri. It is composed of chert pebbles, quartz pebbles, as well as a matrix of quartz sand.[1]
The Wilcox Formation is a sand formation that is part of the Embayment Megagroup. It runs from southern Illinois to the State of Mississippi. It thickens moving southward, starting at about ~20' thick in outcrops in Illinois to 250' thick in Mississippi.[2]
The Porters Creek is found in southern Illinois. It is massive in nature and lacks bedding or has weak bedding surfaces. it is composed of over 80 percent montmorillonite. This clay is mined commercially in Pulaski County, Illinois.
The Clayton Formation is made up of clay. The clay is green to buff in color and is silty throughout. At the formation's base it becomes sandy with pebbles. The pebbles are made of chert and are black or white in color.[2]
The Owl Creek Formation is a geologic formation ranging from Tennessee to Mississippi. It preserves fossils dating back to the Cretaceous period. Ornithomimids and ceratopsid remains are known from the formation. The ceratopsid tooth could represent a chasmosaurine.[3]
The McNairy Formation was formed as a deltaic deposit. The upper and lower parts consists of cross-bedded white to light grey micaceous sand. The middle section is made up of grey to blackish silt, with beds of lignite. This formation is up to 450'.[2]
The Post Creek Formation was once called the Tuscaloosa Formation. It has been renamed due to inconsistencies with dating the formation. It is composed of chert gravel and has moderate cementations by silica or kaolinite.[4]
The Baylis Formation is Cretaceous in age and found in the western sections of the Embayment Megagroup. It rests unconformably on Carboniferous age rock. It is composed of medium to fine grained quartz sand. The upper part of the formation is the Kiser Creek Member. It also contains lenses that may contain, clay, silt and pebbles. The lower member is the Hadley Gravel Member and contains mostly pebbles.[2]