Cool-water carbonate sedimentation and diagenesis associated with a tertiary paleotopographic high: Kangaroo Island, south Australia

Kangaroo Island presently lies at the entrance of St. Vincent Gulf, separating the relatively quiet embayment from the energetic Southern Ocean to the south. During much of the Cenozoic the island was a paleotopographic high that likewise protected depositional environments within St. Vincent Basin from open ocean waves and swells. The ∼40 m-thick Eocene-Oligocene Kingscote Limestone accumulated in a shallow-water paleoseaway on Kangaroo Island and is divisible into four, unconformity-bounded units. These units can, on the basis of microfaunal biostratigraphy, be correlated with sediments in the generally deeper and quieter St. Vincent Basin. While Kingscote Limestone echinoid-rich Eocene carbonates are distinctly different from coeval marls and biosiliceous spiculites in the basin, Oligocene, bryozoan-rich Kingscote limestones are broadly similar to inboard carbonates. Eocene sediments are interpreted to have accumulated in a wide, storm-dominated, flooded seaway during late Eocene eustatic highstand. Strata are characterized by two types of tempestites; (1) submetre to metre-scale graded beds rich in clypeasters and, (2) decimetre-scale, reworked Fibularia beds. Oligocene sediments are interpreted to have been deposited in a narrower, tide-dominated strait, due to a somewhat lower eustatic sea level. These cross-bedded carbonates occur as truncated, high-energy, shallowing-upward, metre-scale cycles. Such cycles are capped by hardgrounds that contain microcrystalline and inclusion-rich fibrous cements. Some cycles were also exposed to meteoric diagenesis, resulting in aragonite dissolution. Subsequent transgression is marked by sediment infiltration and local cementation.; The whole sequence was finally cemented following Pleistocene uplift and exposure. The Eocene-Oligocene unconformity is, as everywhere in the basin, a profound hiatus, represented in the Kingscote Limestone by hardground formation and subaerial erosion. Correlation of all sea level events suggests that most resulted from eustasy, with only minor breaks related to local tectonics.