Importance of Cement Stratigraphy in Studying Marine Carbonate Cements as Proxies for Paleo-Oceanic Conditions: Insights from the Devonian of the Canning Basin, Australia
W. Bruce Ward, Earthworks, West Redding, CT and Jay L. Banner, University of Texas
The use of coarse marine cements in Phanerozoic carbonate buildups as proxies for seawater conditions leads to major questions that arise from the high susceptibility of cements to alteration and from their occurrence as cavity fills instead of seafloor precipitates: 1) Are the cements marine? 2) Are they well preserved so that their compositions and textures record waters from which they precipitated? 3) What is the time between deposition of the host sediments and precipitation of the cements? 4) Do their compositions and fabrics reflect open-ocean compositions and conditions, or modified marine porewaters? Most studies on ancient marine cements rarely address 3 and 4. This is due to ambiguous compositions and timing relationships of many cements, and the erroneous assumption that all marine cements precipitate near the seafloor penecontemporaneously with deposition of their host sediment. Superposition, cross-cutting relationships, and correlation, the essence of stratigraphy, may constrain all 4 questions and show the temporal and areal extent that the cements represent. This is the case for Devonain marine cements in the Canning Basin.
Two essential steps of the stratigraphic method applied to marine cements are A) establishing micro- to macroscopic physical relation-ships that identify replacement textures and constrain timing relative to zones, pores, facies and depositional packages; and B) correlating cements to reveal their areal exent. This method reveals that the Canning Basin marine cements 1) range from being altered to texturally well-preserved with mm-scale zoning similar to modern cements, 2) range from occurring wholly within a cycle to cross cutting sequences, 3) have zones representing <102 to <107 years and 4) have zones that correlate 200 km, indicating those zones reflect basin-wide conditions.
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Presentation at: SEPM Seafloor Diagenesis Symposium, 1999 AAPG National Convention, San Antonio