Abstract
Multiple stable isotope investigations from upper Eocene to lower Oligocene deep-water marine sequences record the transition from global greenhouse to the icehouse conditions (Oi-1 glacial). While Southern Ocean high latitude deep sea records of this transition are well known, their shallow marine equivalents are rare and have the potential to record the eustatic and oceanic consequences of Paleogene glacial variability. The well-known high paleolatitude (~55°S) neritic carbonate sequence at Browns Creek and Castle Cove in the Otway Basin in southeast Australia spans the Eocene-Oligocene boundary. During this time the area lay on the northeastern margin of the Australo-Antarctic Gulf facing the evolving Southern Ocean. The importance of this record has been hampered by a lack of a consistent stratigraphy and contradictory microfossil interpretations. To reconcile these issues we combine new bio-, chemo- and lithostratigraphic analyses of the outcrops and a new core (Colac-2) with pre-existing data to revise the stratigraphy. This confirms the middle/upper Eocene boundary is near the base of the section. The overlying upper Eocene siliciclastic strata are truncated by an unconformity (of ~0.8 Ma in duration) and overlain by glauconitic sand (the Notrostrea greensand) deposited after ~35.9 Ma. Subsequently deepening to middle to outer neritic depths deposited cyclic carbonates. Shallowing after ~35 Ma deposited laterally variable calcareous siliciclastic facies. These strata were tilted and eroded prior to 34 Ma leading to shallow water facies that may have been subaerially exposed during uplift. Brachiopod strontium isotope dates and an 0.5‰ carbon isotope excursion above this unconformity suggests the top of the Browns Creek and the base of the Castle Cove section correlate to Eocene-Oligocene transition (EOT-1) at ~34 Ma. The subsequent persistence of positive C/O isotope values above this level records the transition to the Oi-1 glaciation at ~33.7 Ma. Strong cyclicity in the inner shelf Castle Cove limestone is interpreted to record the commencement of obliquity dominated glacio-eustacy during the Oi-1 glacial phase. The shallowing from outer to inner shelf palaeodepths from the late Eocene to the early Oligocene is likely related to the onset of cryosphere expansion, however, palaeodepth estimates are complicated by the onset of regional compressional tectonism at the Eocene/Oligocene boundary that caused localized tilting and an unconformity with possible antisiphoning effects in this near-field site.
Highlights
There has been a long-term change to a cooler, ice sheet-prone planet since the Cretaceous period
We suggest that the earliest Oligocene tempestite cycles in the Castle Cove are likely a manifestation of obliquity-dominated (41 kyr) sea level variability that persisted during the Early Oligocene Glacial Maximum (Pälike et al, 2006) and that these strata were deposited from ~33.5 to 33.2 Ma (Fig. 13)
We combine new bio, chemo- and lithostratigraphic analyses of the outcrops and subsurface core (Colac-2) at high latitude Otway Basin, southwest Australia with pre-existing data to revise the stratigraphy of this section
Summary
There has been a long-term change to a cooler, ice sheet-prone planet since the Cretaceous period. A driver implicated in this change was the opening of the Southern Ocean and the onset of the Antarctic Circumpolar Current (ACC) during the early Oligocene thermally isolating Antarctica (Kennett, 1977). CO2 estimates in the late Eocene decline from > 1200 to ~900 ppmv just prior to the Eocene/Oligocene boundary before falling to > 700 ppmv during Oi (Pearson et al, 2009; Zheng et al, 2013) suggesting CO2 decline was the primary driver of glacial expansion on Antarctica (Pearson et al, 2009; Pagani et al, 2011)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
