Origin of a cool‐water, Oligo‐Miocene deep shelf limestone, Eucla Platform, southern Australia

Abstract

The Abrakurrie Limestone is an areally extensive, bryozoan‐rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine‐sand‐ to granule‐sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free‐living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well‐cemented layers with erosional tops, which are interpreted as hardgrounds. The limestone is interpreted as a cool‐water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion. The vertical sequence correlates in a general way with the global sea‐level model for the mid‐Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool‐water carbonate deposition elsewhere (c. 2.5 cm kyr‐1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.

Cementation scenarios for New Zealand Cenozoic nontropical limestones

“…Consequently, Class II limestones are typically relatively soft, open, and porous, and make an ideal building stone, such as the "Oamaru stone" (Hayward 1987). They are analogous to many of the Oligocene-Miocene limestone occurrences in southern Australia described by James & Bone (1991).…”

Late Cretaceous to Early Eocene Sedimentation in the Sinop–Boyabat Basin, North‐Central Turkey: A Deep‐Water Turbiditic System Evolving into Littoral Carbonate Platform

“…The abundance of the delicately branched cyclostome bryozoans seems to preclude a high-energy environment. Bryozoans are small colonial organisms with little tolerance to strong waves, living in shallow to moderately deep waters, commonly in excess of 70 m (James & Bone, 1991;Tucker, 2001). Coralline red algae are encrusting and binding organisms which prefer clear, generally shallow (< 25 m) and low-turbidity waters, and that thrive in high-energy reefal shoals and bank-edge environments (Wray, 1978;Adams et al, 1984;Adams & MacKenzie, 1998).…”

“…This evidence and the lack of distinct stratification support the notion of a reefal platform with moderate levels of hydraulic energy and limited transport of skeletal sand by waves and tidal currents, but with a strong erosional impact of frequent storms (cf. James & Bone, 1991;Jones & Desrochers, 1992;Wright & Bruchette, 1996).…”

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“…Rare reefs also occur in the Pliocene-Quaternary section (Ryan et al, 2009), and conditions were favorable for late Quaternary reef development even farther south to 28°S (Collins, 2002). However, sedimentation rates, even in temperate water carbonates, can be high (>40 cm/ky), comparable to the lower end of the spectrum of tropical carbonate platform growth rates (James and Bone, 1991).…”