Diagenesis of Upper Devonian Platform Deposits in Emanuel Range, Canning Basin, Western Australia

V. A. Pedone, William J. Meyers

The Emanuel Range consists of 600 m of peritidal and subtidal carbonate platform deposits of the Pillara Limestone (Givetian-lower Frasnian). Diagenesis of the lower 75 m contrasts markedly with the upper 520 m, principally in the occurrence of extensive dolomite. Dolomitization exquisitely preserves original fabric in the basal 35 m of peritidal deposits but obscures original fabric in the overlying 40 to 60-m section of subtidal stromatoporoid packstones and wackestones. In these lithologies, dolomite gradually decreases upward to where it is absent in overlying rocks. Changes in depositional lithology are not associated with the transition. Peritidal dolomite predates all calcite cements and compaction. Fabric-destructive dolomite occurs in association with incipient s ylolites and overlaps formation of secondary porosity and calcite cementation. Differences in texture and timing suggest that fabric-mimicking peritidal dolomite formed early in the diagenetic history, preceding burial, and that the fabric-destructive dolomite formed later in a shallow burial setting.

Formation of extensive secondary porosity by tectonic fracturing, insitu brecciation, and dissolution of calcitic fossils was enhanced in previously dolomitized strata, exhibiting only minor occurrences in nondolomitized rocks. Formation of secondary porosity occurred over an extended period, marked by a succession of calcite cements, generally following a sequence from slightly ferroan, luminescently zoned cements to nonferroan, nonluminescent cement. Beckite chalcedony and hematite/geothite postdate but are associated with the final calcite cement. The post-dolomite diagenetic sequence suggests initial cementation under reducing burial conditions to final cementation under strongly oxidizing conditions. Fluids moving through the extensive Late Carboniferous karst system in the platf rm may be the diagenetic agent for the later calcites, chalcedony, and iron oxides.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.