| Annona Chalk | |
|---|---|
| Stratigraphic range: Cretaceous | |
 Outcrop east of Clarksville, TX (c. 1910) | |
| Type | Sedimentary |
| Sub-units | Austin Group |
| Underlies | Marlbrook Marl |
| Overlies | Ozan Formation |
| Thickness | 30 Meters |
| Lithology | |
| Primary | Chalk |
| Location | |
| Region | Arkansas |
| Country | United States |
| Type section | |
| Named for | Annona, Red River County, Texas [1] |
| Named by | Robert Thomas Hill |
The Annona Chalk is a geologic formation in Arkansas, Texas, Louisiana, and Oklahoma. [2] It preserves fossils dating back to the Cretaceous period. The formation is a hard, thick-bedded to massive, slightly fossiliferous chalk. It weathers white, but is blue-gray when freshly exposed. The unit is commercially mined for cement. Fossils in the Annona Chalk include coelenterates, echinoderms, annelids, bivalves, gastropods, cephalopods, and some vertebrate traces. [3] The beds range in thickness, up to over 100 feet in depth in some areas (such as at White Cliffs)., [4] but thins to the east and is only a few feet thick north of Columbus, Arkansas and is completely missing to the east. The break between the Annona Formation and the Ozan Formation appears to be sharp with a few tubular borings up to a foot long extending down from the Annona in to the Ozan. [5]
