8th International Nannoplankton Association Conference
Charles C. Smith:
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. The Arcola Limestone represents a thin, yet geographically widespread, unit consisting of from one to four beds of white to pale grey, indurated Thalassinoides-bored, calcisphere wackestone/packstone, each varying in thickness from a few centimetres to a maximum of 0.8m. These indurated beds are separated by thin, extensively burrowed and bioturbated, glauconitic and phosphatic, quartzose silty and sandy chalky marl. In outcrop exposures, the Arcola extends from near Tupelo in SC Lee County, Mississippi, about 365km SE-ward to near Downing in EC Montgomery County, Alabama. At both its western (Tupelo) and eastern (Downing) termini, the Arcola is represented by isolated, discontinuous, 1cm- to 2cm-diameter nodules of calcisphere-rich limestone. These discontinuous nodules pass successively into one, two, then three continuous indurated beds toward WC Alabama, reaching its maximum development and thickness of four indurated limestone beds, 4.37m thick, at Hatcher's Bluff along the Alabama River, SW of Selma in EC Dallas County. Individual limestone beds are believed to have resulted from blooms of a calcified, free-floating cyst or encysted benthic reproductive stage in the life-cycle of an extinct planktonic alga, perhaps related to living dinoflagellates. Limestone-marl pairs undoubtedly resulted from productivity and dilution cycles on a broad, shallow shelf dominated by hemipelagic sediment accumulation.
In the shallow subsurface, the Arcola generally varies from 3 to 5m 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 pale colour and indurated nature of the Arcola, is no longer recognisable within the massive chalky marl of the Selma Group, yet the distinctive tiny calcispheres have been traced over a 30 000km2 area of the Alabama Coastal Plain.
Calcareous nannofossils recovered from the upper 'typical' Mooreville Chalk, the Arcola Limestone Member, and from the overlying lower Demopolis Chalk are exceptionally abundant, diverse, and well-preserved. Biostratigraphic studies indicate that the upper Mooreville, Arcola, and approximately the lower 2m of the Demopolis Chalk contain Bukryaster hayi and thus are assigned to the lower part of the Calculites ovalis Biozone (CC19a) of Early Campanian age. The overlying 2 to 3m of lower Demopolis sediment contain neither B. hayi nor Ceratolithoides aculeus and thus are assigned to the upper part of the C. ovalis Biozone (CC19b). Overlying Demopolis strata contain common C. aculeus and are assigned to the Ceratolithoides aculeus Biozone (CC20). These data establish the Arcola as the most geographically-extensive, time-synchronous lithostratigraphic unit within Mississippi and Alabama.
Copyright © 2000, most recent revision July 26, 2000Tania Hildebrand-Habel (email@example.com)