Abstract

During late Miocene time the central part of the north Jamaican coast consisted of a carbonate bank bordered to the north by a slope on which pelagic chalk accumulated. Deformation between late Miocene and middle Pleistocene time uplifted the bank edge, subaerially exposing some of the slope deposits of cherry chalk, and down-faulted the lower part of the slope deeper into the Cayman Trench. Chalk from the down-faulted part of the Miocene slope has never been subaerially exposed, and has undergone very little chemical alteration. In contrast, subaerially exposed chalk has undergone varying degrees of chemical alteration. Except for depletion in 13 C, vadose chalk diagenesis yields a chemical product nearly identical to some limestones recovered by JOIDES and produced during marine burial diagenesis. Chert from the down-faulted part of the slope consists of opal-CT porcellanite nodules. In contrast, subaerially exposed nodules are composed of quartz with a similar isotopic composition to the opal-CT precursor. It is hypothesized that the porcellanite --> quartz conversion was initiated either by temperature increase as the Miocene slope was uplifted through the warm surface waters of the ocean, or by changes in pore fluid chemistry once interstitial sea water was replaced with meteoric water. In either case, the isotopic composition of the microquartz nodules in this case appears to record the conditions of formation of the opal-CT precursor.

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