--> Abstract: Evaluation of {87}Sr/{86}Sr Chronostratigraphy with Unaltered, Partially Altered, and Completely Altered Oyster and Brachiopod Shells, Seroe Domi Formation, Curacao, Netherlands Antilles, by B. W. Fouke, C. J. Beets, W. J. Meyers, and G. N. Hanson; #90987 (1993).

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FOUKE, BRUCE W., Department of Earth and Space Sciences, SUNY Stony Brook, Stony Brook, NY; C. J. BEETS, Geomarine Center, Free University, Amsterdam, Netherlands; and WILLIAM J. MEYERS and GILBERT N. HANSON, Department of Earth and Space Sciences, SUNY Stony Brook, Stony Brook, NY

ABSTRACT: Evaluation of {87}Sr/{86}Sr Chronostratigraphy with Unaltered, Partially Altered, and Completely Altered Oyster and Brachiopod Shells, Seroe Domi Formation, Curacao, Netherlands Antilles

Modern brachiopod (Thecidellina barretti) and oyster (Osfrea frons) shells from Curacao have been petrographically and geochemically compared with texturally unaltered, partially altered, and completely altered specimens from the Seroe Domi Formation in order to evaluate whether "pristine" shells have retained their original {87}Sr/{86}Sr for use in chronostratigraphy and ancient seawater reconstructions. The Seroe Domi shells exhibit depictions in Sr, {18}O and {13}C and enrichments in Mn, Mg, and Fe with respect to their modern counterparts that might be expected from meteoric water-rock interaction, yet the data fit more closely with binary mixing curves. In addition, significant Sr depletion in texturally unaltered ancient shells compared to modern specimens suggests that the foss lized shells determined to be pristine using plane-light, cathodoluminesence, and SEM petrographic techniques have experienced partial geochemical alteration.

The hyperbolic asymptotes of mixing curves calculated from unaltered, partially altered and completely altered Seroe Domi Formation shells have been compared with modern shell and groundwater compositions to place maximum and minimum constraints on the original Sr isotopic compositions of the fossilized shell material. Modelling of four O. frons specimens from Subunit 1 (the basal 30 m of the Seroe Domi Formation) suggest that the shells had an initial {87}Sr/{86}Sr of 0.70879 (+/- .00002), which implies a depositional age of 13.8 +/- 1.4 Ma (alter Hodell et al., 1991). A single O. frons from the lower part of Subunit 2 (the upper 320 m of the Seroe Domi Formation) has an {87}Sr/{86}Sr of 0.70903, yielding

an age of 4.8 +/- 0.5 Ma. And modelling of four specimens of T. barretti and three specimens of O. frons from the upper 100 m of Subunit 2 suggests initial Sr isotope ratios of 0.70907 and 0.70905 respectively, yielding an age of 2.5 +/- 0.6 Ma. These {87}Sr/{86}Sr chronostratigraphic interpretations of the Seroe Domi Formation are consistant with field relationships, biostratigraphy, and magnetostratigraphy, demonstrating that Subunit 1 is Middle Miocene in age while Subunit 2 represents deposition during the Late Miocene and Pliocene.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.