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Facies and early diagenetic influence on the depositional magnetization of carbonates

¹ Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149

Rock-magnetic data from a core (Clino) on the margin of Great Bahama Bank show that both depositional facies and early diagenesis contribute to preservation of depositional remanent magnetization. This primary magnetization, carried by single-domain magnetite, and resultant directional data recorded in carbonates are influenced by diagenetic alteration that is largely controlled by facies and to a lesser degree relative sea level. Although polarity does not change, there is usually inclination shallowing relative to the axial dipole value, and the inclination values are more dispersed as a result of cementation. Four magneto-diagenetic zones were identified in Clino: from top down, these are (I) a low-Mg calcite, a cemented, shallow-water facies with dispersed inclinations, relatively high remanence coercivity, and magnetostatic interaction; (II) an upper-slope facies, moderately cemented, that has original aragonite mineralogy, produced more statistically consistent inclination angles, lower magnetostatic interactions and coercivity, and higher remanence intensity; (III) a cemented, lower-slope facies having characteristics of both the overlying zones, but containing some dolomite and abundant fractures; and (IV) a partially dolomitized, basal lower-slope interval with magnetic characteristics (dispersed inclination, weaker intensity, higher coercivity, higher magnetostatic interaction) similar to the shallow-water facies. The relatively uncomplicated diagenetic history, excellent age control, and depositional facies provide a spectrum of magnetization destructive processes and characterization of polarity data from platform carbonates.

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