| Popis: |
The Arcola Limestone, the upper member of the Mooreville Chalk of Mississippi and Alabama, is the most distinctive lithostratigraphic unit within the Mesozoic or Cenozoic outcrop belt of the eastern Gulf Coastal Plain area. Geographically, the Arcola extends from near Tupelo in northeastern Mississippi, about 365 km southeastward to near Downing in east-central Montgomery County, Alabama. The Arcola, which stratigraphically separates the Mooreville Chalk from the overlying and lithologically near identical Demopolis Chalk, varies in thickness from a few centimeters at its western and eastern termini to a maximum known thickness of 4.37 m at Hatchers Bluff along the Alabama River southwest of Selma, Dallas County, Alabama. Lithologically, the Arcola consists of from one to four beds of white to light-gray, bored and indurated calcisphere limestone, each varying from a few centimeters to a maximum of 0.8 m in thickness, separated by thin, moderately glauconitic and phosphatic, fossiliferous, quartzose silty chalky marl. In the shallow subsurface, the Arcola generally varies from 3 to 5 m in thickness and can be identified and mapped by its thin yet distinctive electric-log character. Further downdip, the electric-log marker as well as the characteristic light-color and indurated nature of the Arcola is no longer recognizable within the massive chalky marl of the Selma Group, yet the distinctive tiny calcispheres have been traced over a 30,000 km2 area of the Alabama Coastal Plain. Throughout its outcrop, as well as in numerous subsurface samples, the Arcola consistently lies within the uppermost few meters of the top of the lower Calculites ovalis Nannofossil Zone (CC19a) of middle Campanian age, thus representing the most geographically extensive time synchronous lithologic unit known within Mississippi or Alabama. |
