Abstract: Diagenesis of non-tropical carbonates of the Otway Basin, Australia

Stelios Nicolaides

The Oligo-Miocene Heytesbury Group of the Otway basin, southeastern Australia, is a non-tropical carbonate shelf succession. Subsurface samples consist of bioclastic grainstones to wackestones which commonly contain bryozoa, benthonic and planktonic foraminifera, echinoderms, ostracods, brachiopods, and molluscs.

Six morphological types of carbonate cement and two non-carbonate cement types have been recognized: (1) scalenohedral calcite spar is associated with the inter- and intraparticle pore space of bryozoan fragments and has a bright to dull cathodoluminescence; (2) blocky equant calcite spar, commonly found in the interparticle porosity, is dully luminescent; (3) syntaxial calcite overgrowths on echinoderm particles have a zoned cathodoluminescence, with an initial non-, an intermediate brightly, and an outer dully luminescent rim; (4) euhedral dolomite, mainly observed in the interparticle pore space of the Port Campbell Limestone, has a zoned dull/non-dull cathodoluminescence; (5) bladed-prismatic calcite spar is associated with planktonic foraminifera test walls; (6) fibrous calcite s ar fills intraparticle pore space; and (7) glauconite and iron-oxides are observed mainly associated with intraparticle porosity. The morphology of the cement crystals depends on the mineralogy and fabric of their substrate and the various morphological types of carbonate cement in general do not represent individual cementation events.

The carbonate cements appear to have formed in the shallow burial environment from meteorically-derived diagenetic fluids. Pressure solution was the main cement-producing mechanism which commenced at burial depths of less than 100 m. The calcite-rich and aragonite-poor character of the original sediments has caused the scarcity of interparticle meteoric cements.

Cenozoic non-tropical carbonates are probably better analogues for meteoric diagenesis of ancient, calcite-rich, shallow-water, tropical successions, than are modern aragonitic tropical carbonates.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994