Identification of the Paleocene–Eocene boundary in coastal strata in the Otway Basin, Victoria, Australia

Abstract

Abstract. Detailed, stratigraphically well-constrained environmental reconstructions are available for Paleocene and Eocene strata at a range of sites in the southwest Pacific Ocean (New Zealand and East Tasman Plateau; ETP) and Integrated Ocean Discovery Program (IODP) Site U1356 in the south of the Australo-Antarctic Gulf (AAG). These reconstructions have revealed a large discrepancy between temperature proxy data and climate models in this region, suggesting a crucial error in model, proxy data or both. To resolve the origin of this discrepancy, detailed reconstructions are needed from both sides of the Tasmanian Gateway. Paleocene-Eocene sedimentary archives from the west of the Tasmanian Gateway have unfortunately remained scarce (only IODP Site U1356), and no well-dated successions are available for the northern sector of the AAG. Here we present new stratigraphic data for upper Paleocene and lower Eocene strata from the Otway Basin, southeast Australia, on the (north)west side of the Tasmanian Gateway. We analyzed sediments recovered from exploration drilling (Latrobe-1 drill core) and outcrop sampling (Point Margaret) and performed high-resolution carbon isotope geochemistry of bulk organic matter and dinoflagellate cyst (dinocyst) and pollen biostratigraphy on sediments from the regional lithostratigraphic units, including the Pebble Point Formation, Pember Mudstone and Dilwyn Formation. Pollen and dinocyst assemblages are assigned to previously established Australian pollen and dinocyst zonations and tied to available zonations for the SW Pacific. Based on our dinocyst stratigraphy and previously published planktic foraminifer biostratigraphy, the Pebble Point Formation at Point Margaret is dated to the latest Paleocene. The globally synchronous negative carbon isotope excursion that marks the Paleocene-Eocene boundary is identified within the top part of the Pember Mudstone in the Latrobe-1 borehole and at Point Margaret. However, the high abundances of the dinocyst Apectodinium prior to this negative carbon isotope excursion prohibit a direct correlation of this regional bio-event with the quasi-global Apectodinium acme at the PaleoceneEocene Thermal Maximum (PETM; 56 Ma). Therefore, the first occurrence of the pollen species Spinizonocolpites prominatus and the dinocyst species Florentinia reichartii are here designated as regional markers for the PETM. In the Latrobe-1 drill core, dinocyst biostratigraphy further indicates that the early Eocene (∼ 56-51 Ma) sediments are truncated by a ∼ 10 Myr long hiatus overlain by middle Eocene (∼ 40 Ma) strata. These sedimentary archives from southeast Australia may prove key in resolving the model-data discrepancy in this region, and the new stratigraphic data presented here allow for detailed comparisons between paleoclimate records on both sides of the Tasmanian Gateway.

Towards quantitative environmental reconstructions from ancient non-analogue microfossil assemblages: Ecological preferences of Paleocene – Eocene dinoflagellates

“…Relation of relative abundance of selected ecological groups and SA index to sedimentary carbonate content in weight percentage (wt%) and number of data points in each bin. PETM, F. reichartii is found at Bass River and Wilson Lake on the New Jersey Shelf and, remarkably also at Site 1172 in the Southern Ocean and the Otway Basin, Australia (Frieling et al, 2018a), which were located at paleolatitudes between 55 and 60°S (van Hinsbergen et al, 2015). The occurrence of F. reichartii and its apparent narrow high temperature tolerance (Fig.…”

Surface-circulation change in the Southern Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry

“…The Otway Basin contains a regionally thick sequence of shallow-marine Paleogene deposits (Gallagher et al, 1999;Gallagher and Holdgate, 2000). These deposits developed due to Paleocene-Eocene post-rift extension on the edge of the continental margin, causing subsidence of extensive troughs that served as depocentres of terrigenous sediment in deltaic and shallow marine environments (Krassay et al, 2004;Stacey et al, 2013;Frieling et al, 2018a). In southeast Australia, the middle Eocene to early Oligocene Nirranda Group uncomformably overlies the early Eocene Dilwyn Formation (Wangerrip Group) (Abele, 1994;Krassay et al, 2004;Tickell et al, 1993).…”

Tectonic, Oceanographic, and Climatic Controls on the Cretaceous‐Cenozoic Sedimentary Record of the Australian‐Antarctic Basin

“…Drill sites in the adjacent Eyre and Inner Duntroon subbasins reveal that sedimentary unit S2 is absent leading to the merging of horizons U1 and U2, corresponding to a break in the stratigraphic record of ~24 Myr (~83–59 Ma, based on spore‐pollen and foraminifera biozonation; Totterdell et al, ; Figure ). In the Otway and Sorell basins, U2 is dated by drill sites to latest Maastrichtian‐early Paleocene (~65 Ma; based on spore‐pollen biozonation; Krassay et al, ; Boreham et al, ; ~67 Ma onshore in the Otway Basin; Frieling et al, ; Figure ). Consequently, sedimentary unit S2 was likely deposited between ~94 and 65 Ma in the Bremer Subbasin, ~83 and 65 Ma in the central Ceduna, eastern Recherche, outer Duntroon subbasins, and offshore the Otway Basin, and ~80 and 65 Ma in the Sorell Basin.…”

“…We do not discount that at least some sediment erosion occurred along the Antarctic shelves during the onset of glaciation around 33.6 Ma, as reworked Eocene microfossils have been detected in early Oligocene glacial sediment units at IODP Site U1356 and U1360 (Bijl, Houben, Bruls, et al, ; Houben et al, ). In the Australian case, U3 transforms into a conformable horizon toward the shelves and onshore regions (e.g., Frieling et al, ; Totterdell et al, ), marking the transition from thin S3 terrigenous layers to S4 carbonate platforms, without any significant hiatus formation (e.g., Totterdell et al, ). Similarly, a thin drape of Eocene material may have been deposited along the Antarctic shelves.…”

“…In Figure 2. Overview of stratigraphic sequences and depositional environment of the (c) basins along the Australian southern margin (Blevin & Cathro, 2008;Frieling et al, 2018), (d) drill site U1356A, Antarctica (Escutia et al, 2011), and (e) the stratigraphic interpretation (this study). (a) Geological timescale (Gradstein et al, 2012), (b) deep-sea benthic foraminiferal oxygen-isotope curve (Linnert et al, 2014;Zachos et al, 2008), (f) regional tectonic (red), and oceanic (blue) events are shown.…”