David K. Beach, Robert N. Ginsburg
Shallow cores from nine locations across northwestern Great Bahama Bank contain multiple, often closely spaced, subaerial unconformities. These unconformities separate Plio-Pleistocene carbonate rocks into discrete depositional units. Interpretation of these and similar stacked unconformities in the Bahamas has been controversial. However, their recognition is crucial to understanding Plio-Pleistocene bank history and reflects a record of multiple sea-level changes.
Recognition of unconformities is based largely on petrographic evidence. This evidence includes diagenetic features produced by karstification, calichification, residual soil development, and erosion; as well as depositional features reflecting marked changes and/or hiatuses in sedimentation. Diagenetic features below unconformities include staining; dense laminated and/or unlaminated micrite; soil-zone pisoids; microcodium; needle-fiber cement; solution pipes; increased induration; rhizomorphs; macroborings; and microborings. Extensive but localized leaching commonly occurs overlying unconformities. Depositional features include marked facies changes; paleosol; and filling of solution pipes and vugs by overlying paleosol and sediments. Basal and capping sections of depositional units are often mud rich. Basal sections commonly contain lithoclasts (usually blackened) and abundant fossils (notably molluscs, including articulated bivalves, and corals, particularly Porites porites).
Using the observed features, all unconformities are ranked according to certainty of existence. Rankings do not necessarily equate to degree of alteration nor to relative length of exposure. Correlation of depositional units between core locations supports identification of unconformities. The methods used and evidence cited should be particularly applicable to recognition of subaerial unconformities in other late Tertiary-Quaternary carbonates.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994