ABSTRACT

Mid-Tertiary limestones, exposed as sea cliffs along the coast of Victoria, southeastern Australia, accumulated on the inner part of a cool-water, distally steepened ramp or open shelf. All sediments are bioclastic, dominated by particles of bryozoans, echinoderms, benthic foraminifers, brachiopods, and molluscs. Shallow, grainy facies also contain coralline algae, quartz, and glauconite, while deep, muddy facies include ahermatypic corals, ostracodes, sponge spicules, planktic foraminifers, and terrigenous clays.

The fundamental depositional unit is a meter-scale, subtidal cycle or rhythm. Shallow-shelf cycles are shallowing-upward, cross-bedded or burrowed grainstone capped by a marine-cemented hardground or quartz-granule lag, while mid-shelf cycles are upward-coarsening and thickening, proximal to distal tempestites. Such cycles are interpreted to form by eustatically driven, climatically controlled oscillations in abrasion wave base and swell wave base. Deep shelf rhythms are interbedded bryozoan marl and calcareous clay, thought to be generated by climatically influenced fluctuations in carbonate productivity and terrigenous dilution.

Lowstand systems tracts are depositional or erosional. Sequence boundaries are complex, multigeneration, mostly submarine surfaces (condensed cycle boundaries) that can be traced offshore into multiple omission surfaces and conformable contacts. Basinward facies shifts result in deposition of condensed lowstand wedges of stacked, shallowing-upward grainstone cycles. Transgressive systems tracts are thick bryozoan marl-calcareous clay rhythms associated with facies backstepping and abrupt deepening. Such rhythmites typically grade stratigraphically upward into tempestite cycles. Highstand systems tracts are highly progradational, wedge-shaped rock bodies of shallowing-upward grainy cycles.

These limestones provide the critical link of stratigraphy between modern cool-water carbonate shelf deposits and the older Mesozoic and Paleozoic record of similar sediments.