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
Both marine (Bijl et al., 2009; Hollis et al., 2009) and terrestrial
(Carpenter et al., 2012; Contreras et al., 2014; Macphail et al., 1994)
temperature proxy records suggest that the midlatitude to high-latitude
southwest Pacific was extremely warm (25–35
Schematic overview of regional lithostratigraphic units and their approximate ages (see McGowran et al., 2004, for a detailed overview). Relative thickness of lithological units is not to scale. PFZ: planktonic foraminifer zone. Absolute ages in million years ago (Ma); tie points derived from previous work are marked with *.
The Otway Basin consists of several east–west-oriented troughs on the
northwestern side of the Tasmanian Gateway. It borders the Bass and
Gippsland basins to the east and the Murray Basin to the northwest
(Gallagher and Holdgate, 2000;
Holdgate and Gallagher, 2003). The formation of the Otway Basin is related
to the breakup between Australia and Antarctica, starting in the late
Cretaceous (Cande and Stock, 2004). Extensive deposits were
formed in the Otway Basin during post-rift subsidence in the Cenozoic with
several depositional centers. During the early Paleogene, the depositional
center was located close to the present-day shoreline
(Gallagher and Holdgate, 2000). The
Wangerrip Group of the Otway Basin consists of Paleocene and Eocene strata,
which overlie the upper Cretaceous Sherbrook Group or unconformably overlie
the lower Cretaceous Otway Group in the shallower parts of the basin (see
Fig. 2 for a schematic overview of regional lithological units and
Holdgate and Gallagher, 2003, for a detailed overview of the regional
Cenozoic geology). Wire line logs and seismic analyses show that up to
1000
Arditto (1995) further subdivided the Wangerrip Group into eight third-order
transgressive–regressive sequences, the lowermost of which, the Pember
Mudstone, represents the transgression from the Pebble Point Formation to
the Dilwyn Formation. Lithologically, the Pebble Point Formation is
characterized by meter-scale mud-to-sandstone alternations interpreted to
represent high-energy, shallow-marine environments (e.g.,
Keating, 1993). The Pebble Point Formation is further divided into three
lithostratigraphic units, the Margaret (bottom), Buckley and Cobble Cove
(top) members (Keating, 1993; Holdgate and Gallagher,
2003). The homogeneous silty claystones of the Pember Mudstone overlie the
Cobble Cove member. Several other, younger, distinct units are recognized
within the Dilwyn Formation, including the Rivernook-A and Rivernook beds
consisting of glauconitic sandstones. Together with thin macrofossil-bearing
intervals designated as “
Several pronounced marine ingressions, ranging in age from Maastrichtian to Eocene, have been identified in the composite sedimentary succession for southern Australia, representing the northern neritic flank of the AAG (McGowran, 1991, 1978; McGowran et al., 2004). The ingressions are named as follows: Ceduna (Maastrichtian in age), Kings Park (late Selandian), Pebble Point (late Thanetian), Rivernook-A and Rivernook (earliest Ypresian), and Princetown and Burrungule (Ypresian). The oldest Paleogene sediments in the Otway Basin are assigned to the Pebble Point Formation (Holdgate and Gallagher, 2003). The calcareous macro- (Eglington, 2006; Stilwell, 2003) and microfossils (McGowran, 1970, 1965; Taylor, 1964) from the marine ingressions in the Otway Basin have allowed low-resolution correlation with regional and global biostratigraphic zonal schemes.
The Burrungule Member ingression of the Dilwyn Formation represents the
highest known Ypresian stratum in the Otway Basin. There is no
biostratigraphic evidence anywhere in the AAG for the interval between
latest Ypresian and latest Lutetian; this well-documented hiatus is known as
the “Lutetian gap” (
Pollen and dinoflagellate cyst biostratigraphy has been instrumental in detailed correlations of these marginal marine formations within the Otway Basin and with other southern Australian basins (Harris, 1971; Partridge, 1999). The Pebble Point Formation, Pember Mudstone and Dilwyn Formation were previously dated as middle or late Paleocene to early Eocene in age based on planktonic foraminifera, pollen, spore and dinocyst assemblages (Harris, 1971; McGowran, 1965; Partridge, 1999; Taylor, 1965, 1964).
Foraminifera and calcareous macrofossils in the Pebble Point Formation are
largely restricted to the Buckley and lower Cobble Cove members (Holdgate and
Gallagher, 2003; Keating, 1993). The key planktonic foraminifer is
Planktonic foraminiferal species identified in the Rivernook-A Bed (Dilwyn
Formation) higher in the sequence include
The Rivernook assemblage contains the highest relative abundance of
planktonic foraminifera of any assemblage in the neritic Ypresian in the
AAG. The diverse assemblage includes, amongst others,
Casing used for continuous sampling of the upper Pember Mudstone at
Point Margaret. Photo of a casing (no. 27; 47.63–48.16
Using samples dredged from the edge of the Ceduna Plateau (northern AAG,
The existing stratigraphic assessment suggests the presence of extensive late Paleocene to early Eocene deposits in the Otway Basin (Fig. 2). However, detailed correlation with sites on the east side of the Tasmanian Gateway remains challenging due to the small number of tie points and diachroneity in bio-events due to a closed Tasmanian Gateway.
Carbon isotope chemo-stratigraphy represents a powerful stratigraphic tool
for the Paleogene. It allows marking the position of several transient
(10–100
To date, stable carbon isotope chemo-stratigraphy has not yet been established for the sedimentary successions of the Otway Basin. In addition, high-resolution carbon isotope analyses may allow a detailed assessment of the completeness of the sedimentary archives. Therefore, to refine regional stratigraphy and set the stage for further high-resolution paleoclimate studies, we here combine carbon isotope chemo-stratigraphy with pollen and dinocyst biostratigraphy to date the Paleocene and Eocene from the Latrobe-1 core and dinocyst biostratigraphy from the Point Margaret outcrop section (Fig. 1).
Stratigraphy of the Point Margaret section. Main lithostratigraphic
units are indicated, as are all sample positions, relief, lithology and the
dinocyst zonations of Partridge (1999), Wilson (1988) and Crouch et
al. (2014). Colors in the lithological column are used to indicate
approximate sediment coloring. Important local bio-events are also given.
Carbon isotope values of bulk organic matter are given in per mill versus
(Vienna) Pee Dee Belemnite (VPDB) (
Stratigraphy of the Latrobe-1 borehole. Basic lithostratigraphic
units are indicated, as are all sample positions, lithology, dinocyst and
pollen zonations of Partridge (1999) and dinocyst zonations of Wilson (1988),
Bijl et al. (2013b) and Crouch et al. (2014). Colors in the lithological
column are used to indicate approximate sediment coloring. Important local
bio-events and, where available, absolute age tie points are also given.
Carbon isotope values of bulk organic matter are given in per mill versus PDB
(
We targeted coastal outcrops of the Pebble Point Formation and Pember
Mudstone at Point Margaret (coordinates: 38
In addition, the uppermost
The Latrobe-1 core (coordinates: 38
Total organic carbon (TOC) measurements and bulk organic carbon isotope
analyses (
A total of 121 samples – 33 samples from the Latrobe-1 drill core and 88
samples from the Point Margaret outcrop – were analyzed. Detailed
determination for pollen, spores and dinocysts was performed for the
Latrobe-1 and for dinocysts in the Point Margaret material. Methods followed
those described in Sluijs et al. (2003) and included spiking with exotic
marker spores (
The carbonate concentration of 50 Point Margaret samples was determined at
the University of Southampton using a UIC CM5015 coulometer coupled to an
AutoMateFX autosampler and carbonate digestion system. Approximately
0.5
A detailed lithological description of the Point Margaret outcrop was made
during the field expedition in February 2016 (Fig. 4). Lower-resolution
logging efforts exist for outcrops nearby (e.g., Arditto, 1995), notably
from Pebble Point and Buckley's Point to the east of the Point Margaret
outcrop. Some lateral differences in lithology are expected between each of
these outcrops given the highly dynamic, shallow-marine depositional
environment (Keating, 1993; Holdgate and Gallagher,
2003). We determined the positions of the different members composing the
Pebble Point Formation at Point Margaret as they were described from other
outcrops by Keating (1993) and Holdgate and Gallagher
(2003) (Fig. 1c, Fig. 4). The unconformity between the early Cretaceous
chloritic volcanogenic sandstones of the Otway Group and the coarse
arenaceous sandstones of the lowermost Pebble Point Formation is situated at
a stratigraphic height of 1.7
The unconformity marking the boundary between the Margaret Member and the
Buckley Member is at the top of a
The Buckley and Cobble Cove members contain some calcareous macrofossils that
are mostly concentrated in two beds (around 15 and 18
The identification of the different members within the Dilwyn and Pebble
Point Formation is more challenging in the Latrobe-1 borehole (Fig. 5). Our
lithological description is based on the initial wire line logging
interpretation of Esplan (1971) supplemented by observations of Eglington
(2006) and our own observations during discrete sampling. The Pebble Point
Formation comprises sandstones, minor shales and carbonate cemented units
recorded between borehole depths 344.4 and 396.2
Conformably overlying the Pebble Point Formation, the Pember Mudstone is
present in cores between 305.4 and 344.4
Fine-grained lithologies throughout the Point Margaret outcrop and Latrobe-1
borehole are relatively rich in organic matter, with average TOC percentages
of
At Point Margaret, the sediments of the Pebble Point Formation are also
typified by relatively high (average
In the Pember Mudstone of the Latrobe-1 borehole, the patterns are very
similar to those from Point Margaret. High terrestrial input dominates the
In the Point Margaret samples, palynological residues are continuously
dominated by terrestrial material (average
Stratigraphically important late Paleocene dinocyst taxa include amongst
others
Both pollen and spores and dinocyst assemblages were analyzed for the
Latrobe-1 borehole. Samples are continuously dominated (although to a
variable degree) by terrestrial organic material: pollen, spores and plant
debris. Stratigraphically important pollen and spore taxa include
We correlate our dinocyst and pollen assemblages from the Latrobe-1 borehole
and our dinocyst assemblages from the Point Margaret section with the
regional (Gippsland Basin, South Australia) zonations of Partridge (2006,
1999). For practical purposes, we assume that stratigraphic events at the
Latrobe-1 and Point Margaret sections are synchronous, as our sites are only
A few specimens of
The base of this zone is defined by the last occurrence (LO) of
In this study, we use the FO of
The
At Point Margaret, the lowermost
The FO of
The
The only occurrence of
The base of the
The base of this subzone is marked by the top of the previous zone, and the
top is based on the FO of
The FO of
This subzone is based on the FO of the eponymous pollen species, here
recorded between 305.2 and 299.64
The base of the
The base and top of this subzone are marked by the FO of
The FO of
The FO of
The
Key early to middle Paleocene dinocyst species used in the Paleocene dinocyst
zonation of New Zealand (Crouch et al., 2014) include
The lowermost beds of the Pebble Point Formation, the Margaret Member,
contain a few sparse
A large negative CIE of
The
In the Latrobe-1 borehole, a minimum thickness of 7.3
In the Latrobe-1 borehole, we find
The first occurrence of
Some late Paleocene to early Eocene dinocyst bio-events may be used both in
the SW Pacific and AAG. This may include the FO of the (sub)tropical dinocyst
genus
In the absence of these dinocyst marker species, the first consistent
occurrence of the pollen species
Relatively large sample gaps in the Latrobe-1 dataset imply that the FO and LO of early Eocene dinocysts cannot be directly tied to those recorded at Site U1356. However, it should be noted that none of the recorded events are out of stratigraphic order and therefore may be considered consistent with the zonation established for the Southern Ocean (Bijl et al., 2013b). By extension, these events may be employed for approximate age determinations until new, high-resolution stratigraphic constraints become available for the lower Eocene of the Otway Basin.
Two expanded sedimentary successions, the Latrobe-1 borehole and the Point Margaret outcrop, in the Otway Basin of southeast Australia were studied for pollen and dinocyst biostratigraphy as well as bulk organic matter carbon isotope chemo-stratigraphy. The combination of these techniques allows us to identify the Paleocene–Eocene boundary in both successions and to create a stratigraphic framework for the latest Paleocene and early Eocene at the respective localities.
Based on previous calcareous microfossil evidence (McGowran, 1970, 1965), the
Buckley Member within the Pebble Point Formation predates the
Paleocene–Eocene boundary by a maximum of
The first occurrence and first acme of
The presence of early Eocene sediments in the Latrobe-1 borehole is
established with the FO and LO of
Our results indicate the presence of late Paleocene, PETM, early Eocene and also MECO sediments in the Otway Basin, southeast Australia. Combined with the excellent spatial coverage of seismic and wire line log data for the Otway Basin, it is now possible to readily identify these critical climate intervals in existing boreholes and outcrops. We anticipate that forthcoming records from these sediments will play an important role in resolving the long-standing climate-model proxy-data discrepancy around the Tasmanian Gateway.
Data used in this paper will be available
in the online database Pangaea
(
PKB designed the research. GH, JF, JP, PKB, SG and SMB collected the samples. CCMR EPH, THD and JF generated pollen, dinocyst and isotope data. JF assembled the data and wrote the paper with input from all coauthors.
The authors declare that they have no conflict of interest.
This research was funded through NWO VENI grant no. 863.13.002 to Peter K. Bijl. Jörg Pross acknowledges financial support by Heidelberg University. Stephen J. Gallagher was partly funded by the ARC Basin Genesis Hub. We thank Natasja Welters, Giovanni Dammers and Arnold van Dijk (Utrecht University) and Rineke Gieles, Marcel van der Meer and Ronald van Bommel (Netherlands Institute for Sea Research (NIOZ)) for technical assistance. Wim Hoek (Utrecht University) and Marcel van der Linden and Johan van Heerwaarden (NIOZ) are thanked for help developing the casing technique of sampling. We thank Erica Crouch and an anonymous reviewer for their constructive comments. Edited by: Luke Mander Reviewed by: Erica Crouch and one anonymous referee