Absolute paleobathymetry of Upper Cretaceous chalks based on ostracodes - Evidence from the Demopolis Chalk (Campanian and Maastrichtian) of the northern Gulf Coastal Plain

Journal Article · Wed May 01 00:00:00 EDT 1991 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1991)019<0449:APOUCC>2.3.CO;2· OSTI ID:5541786

Puckett, T M ^[1]

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1. Geological Survey of Alabama, Tuscaloosa (USA)

The presence of abundant and diverse sighted ostracodes in chalk and marl of the Demopolis Chalk (Campanian and Maastrichtian) in Alabama and Mississippi strongly suggests that the Late Cretaceous sea floor was within the photic zone. The maximum depth of deposition is calculated from an equation based on eye morphology and efficiency and estimates of the vertical light attenuation. In this equation, K, the vertical light attenuation coefficient, is the most critical variable because it is the divisor for the rest of the equation. Rates of accumulation of coccoliths during the Cretaceous are estimated and are on the same order as those in modern areas of high phytoplankton production, suggesting similar pigment and coccolith concentrations in the water column. Values of K are known for a wide range of water masses and pigment concentrations, including areas of high phytoplankton production; thus light attenuation through the Cretaceous seas can be estimated reliably. Waters in which attenuation is due only to biogenic matter-conditions that result in deposition of relatively pure chalk-have values of K ranging between 0.2 and 0.3. Waters rich in phytoplankton and mud-conditions that result in deposition of marl-have K values as great as 0.5. Substituting these values for K results in depth range of 65 to 90 m for deposition of chalk and depth of 35 m for deposition of marl. These depth values suggest that deposition of many Cretaceous chalks and marls around the world were deposited under relatively shallow conditions.

• OSTI ID: 5541786
• Journal Information: Geology; (USA), Vol. 19:5; ISSN 0091-7613
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
ALABAMA
COASTAL REGIONS
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LIMESTONE
DEPOSITION
MARLSTONE
MISSISSIPPI
AQUATIC ORGANISMS
CRETACEOUS PERIOD
DEPTH
EQUATIONS
GEOLOGIC FORMATIONS
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GULF COAST
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580000* - Geosciences