Abstract

A new quantitative approach uses the 87Sr/86Sr compositions of pristine and chemically altered marine invertebrate low-Mg CaCO₃ skeletons to determine the depositional and diagenetic age of mixed carbonate and siliciclastic depositional systems tracts. This technique has direct application to marine and terrestrial sedimentary systems that contain brachiopod and bivalve skeletal low-Mg CaCO₃, significantly expanding the range of shell material suitable for 87Sr/86Sr age dating.

Texturally and chemically altered low-magnesium calcite skeletal material records the progressive stages of meteoric diagenesis of the Seroe Domi Formation limestones on Cura$cLao, Netherlands Antilles. The elemental and isotopic geochemical composition of these altered shells is compared with taxonomically equivalent unaltered Recent shells, and quantitatively modelled with respect to mechanical mixing and water-rock interaction processes. This permits reconstruction of the 87Sr/86Sr of the original shells and their replacement calcites for use with Sr isotope age models. Results indicate that deposition of the Seroe Domi Formation during the Miocene-Pliocene-Pliestocene was punctuated by periods of subaerial exposure caused by the combination of tectonic uplift and eustatic lowering of sea level.

Ostrea frons oyster shells from the base of the 350 m-thick Seroe Domi Formation exhibit 87Sr/86Sr versus 13C and 1/Sr covariations suggesting that the shell compositions represent a mechanical mixture of original shell calcite and replacement calcite. The asymptotes of hyperbolic mixing curves fitted with unaltered and completely-altered shell compositions indicate that the oysters had an initial 87Sr/86Sr of 0.70877 (±0.00002). This implies that deposition occurred during the Middle Miocene (13.8 ± 1.4 Ma) and is consistent with foraminifer biostratigraphy. Modeling also suggests that the replacement calcite precipitated from meteoric water that was depleted in 13C, Sr and Mg, and enriched in Mn. An 87Sr/86Sr of 0.70883 in completely altered O. frons shells implies that subaerial exposure took place no later than the Middle to Late Miocene (11.2 ± 1.4 Ma).

Texturally unaltered and completely altered shells of Thecidellina barretti brachiopods and O. frons oysters from near the top of the Seroe Domi Formation have indistinguishable 87Sr/86Sr compositions. The unaltered shells contain Sr isotope ratios of 0.70904 to 0.70906, which suggest a Pliocene depositional age consistent with scleractinian coral biostratigraphy and that shell alteration took place soon after deposition during the Plio-Pleistocene.