WARD, W. BRUCE, and ANN COX, Department of Earth and Space Sciences, SUNY at Stony Brook, NY; MILTON H. KWONG, Department of Geological Sciences, University of Texas at Austin, TX.; WILLIAM J. MEYERS, Department Earth and Space Sciences, SUNY at Stony Brook, NY; and JAY L. BANNER, Department of Geological Sciences, University of Texas at Austin, TX

ABSTRACT: Upper Devonian Seawater Samples Encased in Calcite Cements: Single-Phase Fluid-Inclusion Salinities and Cement Compositions, Devonian Reef Complexes, Canning Basin, Western Australia

Primary single-phase fluid inclusions in pristine Upper Devonian marine fibrous and marine dogtooth calcites have final melting temperatures of -1.7 to -2.8 degrees C consistent with salinities ranging from normal seawater to hypersaline (up to 4.5 wt. % NaCl equivalents). These inclusions are judged to be primary because (1) they define concentric growth zones or occur as isolated individuals, (2) they occur in populations of exclusively single-phase inclusions in unaltered calcites, and (3) their salinities are similar within growth zones, but vary between zones. The fluid-inclusion salinities are consistent with previous interpretations that these calcites are geochemically pristine and texturally unaltered marine cements based on petrography, C, O, and Sr isotopes, and Mg content. /P>

Cements occurring in neptunian sills within Frasnian marginal-slope strata were examined from two localities 200 km apart (Dingo Gap and McWhae Ridge) in the Canning basin. In both localities cements have similar fabrics, compositions, and fluid-inclusion salinities. Dogtooth cements are interlayered with fibrous cements having fluid-inclusion-rich to inclusion-poor growth zones. Both the dogtooth and fibrous cements may be partially or totally replaced by inclusion-rich calcites containing microdolomite inclusions (equivalent to 1 to 15 mol% MgCO<3>) revealed by etching.Unaltered dogtooth and fibrous calcites (1 to 5 mol% MgCO<3>)

have (1) fine-scale (1- 5 micrometer) growth bands revealed by etching, (2) no cathodoluminescence, (3) no microdolomites, (4) low Fe and Mn abundances, and (5) for dogtooth crystals, sectoral zoning with respect to Sr and Mg. Dogtooth cements are preferentially unaltered compared to the fibrous cements.

The fabrics, compositions, and fluid inclusions support these cements as being low-temperature oxic marine precipitates. The elevated salinities are consistent with a model involving reflux from the platform interior of evaporitively concentrated seawater during marine-burial diagenesis.

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