Cenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia

Journal Pre-proof Cenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia Elizabeth M. Mahon, Malcolm W. Wallace PII:

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DOI:

https://doi.org/10.1016/j.marpetgeo.2020.104243

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JMPG 104243

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Marine and Petroleum Geology

Received Date: 1 August 2019 Revised Date:

16 December 2019

Accepted Date: 13 January 2020

Please cite this article as: Mahon, E.M., Wallace, M.W., Cenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/j.marpetgeo.2020.104243. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Cenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia

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Elizabeth M. Mahon*, Malcolm W. Wallace

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School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia

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*Corresponding Author. E-mail address: [email protected] (E. Mahon), [email protected] (M. Wallace)

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ABSTRACT The Gippsland Basin contains some of the largest hydrocarbon accumulations in

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Australia, and has been in production since the 1920’s. These hydrocarbons are

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trapped by large growth anticlines offshore, in reservoirs of the Cretaceous to

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Eocene-aged Latrobe Group. Despite the obvious importance of these growth

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anticlines, the timing of their formation, and the overall Cenozoic tectonic history of

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the basin is not well understood.

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Here, we present a detailed growth strata analysis of the faults and anticlines

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within the Cenozoic sediments of the Gippsland Basin. This indicates two major

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phases of tectonism in the basin: 1. Late Cretaceous to Eocene extension, and 2.

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Oligocene to Holocene compression. Detailed analysis of the extensional phase

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indicates the development of numerous normal growth faults, which display an

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overall reduction in the magnitude of extension from the Late Cretaceous to the

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Eocene, commonly terminating at the top of the Latrobe Group. The shift to

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compressional tectonism occurred at approximately the Eocene-Oligocene boundary

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(~34 Ma). A major and widespread episode of compression then occurred, with

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evidence of growth on anticlines and reverse faults beginning in the early Oligocene.

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This previously unrecognized early Oligocene event produced significant growth (20

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to 50% total growth) of the major anticlines which host hydrocarbon accumulations.

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The early Oligocene event represents the first phase of the compressional tectonic

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regime that continues to the present day in SE Australia. It appears likely that this

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Oligocene event affected other basins in SE Australia and probably contributed to 1

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uplift of the Eastern Highlands. The underlying tectonic cause for this Oligocene

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compressional regime is enigmatic and may be related to far field tectonic

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processes.

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A second pulse of compressional tectonism and anticline growth occurred during

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the mid-Miocene and is generally more significant in the onshore regions of the

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basin. The youngest phase of compressional tectonism beginning in the Late

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Miocene (~10 Ma) is also more intense in onshore regions and is marked by an

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unconformity that is widespread in SE Australia.

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Key words: Gippsland Basin; SE Australia; growth strata; basin inversion; seismic; extension; compression; tectonics

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1. Introduction The Gippsland Basin has been one of the most prolific hydrocarbon-producing

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regions in Australian history, containing some of the largest hydrocarbon

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accumulations in Australia. From the discovery of oil onshore in 1924, to the

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discovery of giant oil and gas fields offshore in the 1960’s, the Gippsland Basin has

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contributed extensively to the history of hydrocarbon exploration and production in

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Australia. A great deal of geological research and exploration was carried out in the

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basin between the 1960’s and the 1990’s. However, despite this attention, there is

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relatively little published literature on the basic geology of the basin. The position of

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the basin on the south-eastern corner of the Australian continent has meant it has

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been subjected to multiple phases of tectonic movement, including rifting along the

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southern margin of Australia, rifting on the eastern margin of Australia, and Neogene

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compressional tectonics (Dickinson et al., 2001). There has been some focus on

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early basin formation (e.g. Gunn, 1975; Threlfall et al., 1976; Etheridge et al., 1985;

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Lowry, 1988; Lowry and Longley, 1991), and on young tectonism (e.g. Dickinson et

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al., 2001, 2002). However, the general Cenozoic tectonic history of the basin has

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not been well documented.

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Hydrocarbon accumulations in the Gippsland Basin are trapped offshore in broad

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northeast-southwest trending anticlines. Despite the importance of these structures

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as traps for hydrocarbon accumulations, the timing of the formation of these

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anticlines is quite poorly constrained. The timing of anticline growth in the Gippsland

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Basin has been estimated by various authors to a time period anywhere from the

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Late Cretaceous to the Miocene (e.g. Brown, 1986; Smith, 1988; Maung and

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Nicholas, 1990; Rahmanian et al., 1990; Duff et al., 1991; Lowry and Longley, 1991;

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Johnstone et al., 2001; Holford et al., 2011).

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Sedimentation concurrent with tectonism provides a method to identify the history

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of structures, and is often referred to as ‘growth strata’ (Childs et al., 2003; Jackson

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et al., 2017). Measurements of thickness changes of sediment across a structure

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(fault or fold), in conjunction with chronostratigraphic data (e.g. paleontological

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information), can provide accurate information regarding the history of growth on the

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structures (Childs et al., 2003). To date, no studies have quantified tectonic growth

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episodes on structures in the Gippsland Basin. In this study, we have quantitatively

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analysed the growth history of structures to determine the timing and intensity of the

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various structural episodes in the Gippsland Basin. We document a hitherto

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unidentified episode of major tectonism during the Oligocene that appears to

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represent the onset of compressional tectonism in South Eastern Australia.

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2. Geological Setting

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This Gippsland Basin, located on the southeast corner of the Australian continent,

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occupies a unique position in Australia’s tectonic history. It is part of a series of rift

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basins which formed along the southern margin of Australia, as Australia and

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Antarctica rifted apart in the Late Jurassic to Early Cretaceous (Etheridge et al.,

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1985). The present day Gippsland basin displays a roughly ESE-WNW-oriented

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main depocentre, the Central Deep, flanked by the Northern and Southern Terraces,

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and the Northern and Southern Platforms (Abele et al., 1988). The Northern Terrace

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is bounded by the Lake Wellington Fault System to the north, and the Rosedale

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Fault System to the south. The Southern Terrace is constrained by the Darriman

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Fault System to the north, and the Foster Fault System to the south (Fig. 1).

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93

94 95

Fig. 1. The main structural features of the Gippsland basin (modified from Abele et

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al., 1988). Seismic lines used in this study are displayed in red.

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The Gippsland Basin initially formed as a northeasterly trending en echelon graben system (Willcox et al., 1992; Power et al., 2001) as part of a pre-rift

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depression that extended along the southern margin of Australia, linking the Otway,

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Bass and Gippsland Basins (Rahmanian et al., 1990). The basement consists of

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granites and metamorphics of the Lachlan Fold Belt (Evans, 1986; Willcox et al.,

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1992). Deposition of the Strzelecki Group began during the Early Cretaceous in the

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grabens as an immature, syn-rift, volcanoclastic unit, deposited from braided

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streams and alluvial fans (Threlfall et al., 1976; Rahmanian et al., 1990; Tosolini et

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al., 1999). By the end of the Early Cretaceous, rifting shifted to southwest of

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Tasmania, resulting in uplift and an associated unconformity in the Gippsland Basin,

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and the compartmentalisation of the Otway, Bass and Gippsland Basins (Rahmanian

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et al., 1990; Power et al., 2001). The Strzelecki Group underwent complex wrench-

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style deformation at this time (Threlfall et al., 1976; Willcox et al., 1992). During the

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Late Cretaceous, Zealandia rifted off the eastern side of southeastern Australia, 4

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opening the Tasman Sea (Mortimer et al., 2019). An angular unconformity

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associated with this event separates the Strzelecki Group from the overlying Latrobe

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Group. This unconformity is particularly distinct on the terraces, but becomes

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conformable in the Central Deep (Threlfall et al., 1976; Featherstone et al., 1991).

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The Gippsland Basin then underwent a shift from active rifting, to a post-rift,

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thermal sag phase, during which normal faulting continued but gradually waned

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(Evans, 1986; Willcox et al., 1992), creating accommodation space for the

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siliciclastic Latrobe Group. The Latrobe Group has been subdivided into formations

120

by a number of authors (Bodard et al., 1986; Bernecker and Partridge, 2001). Here,

121

we have used a simplified onshore stratigraphic scheme for the Latrobe Group (Fig.

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2).

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South-southeast trending canyons up to 650 m deep (Holdgate et al., 2003)

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incised into the Central Deep during the Early Eocene, forming the Tuna-Flounder

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and the Marlin-Turrum formations (James and Evans, 1971; Hocking, 1972). The

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ages of incision and infill of these features is constrained by palynological data from

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well penetrations, and indicates that the Tuna canyon incised during the M diversus

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zone (early Eocene) and the Flounder Formation infilled it during the P asperopolus

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zone (mid Eocene), after which the Marlin channel incised during the P asperopolus

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zone, cutting into the southwestern side of the Tuna-Flounder Formation, and

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partially filled with the Turrum Formation during the N asperus zone (Late Eocene).

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Due to the transgressive nature of the upper Latrobe Group, the upper boundary

133

of the Group is diachronous, and is oldest offshore and youngest onshore. The top of

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the Latrobe Group is characterized by an interval of condensed marine sediments

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known as the Gurnard Formation, a thin unit of glauconitic sandstone, deposited

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discontinuously across the eastern portion of the basin (James and Evans, 1971).

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Some authors have interpreted the top of the Latrobe Group as a regional

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unconformity (Rahmanian et al., 1990; Holdgate et al., 2003; Gibson-Poole et al.,

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2007; Holford et al., 2011) although we favour a diachronous condensed marine

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origin for this boundary.

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The Seaspray Group was deposited over the Latrobe Group, and is dominated by

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non-tropical shallow and deep-water carbonates (James and Evans, 1971; Gallagher

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et al., 2001; Wallace et al., 2002), with laterally equivalent brown coals and clastic 5

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interseam sediments onshore. The basal unit, the Lakes Entrance Formation,

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comprises the first fully marine sedimentation onshore Gippsland Basin (Holdgate

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and Gallagher, 2003). It consists of shelfal marine muds and limestones, which

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experienced extensive submarine canyoning in its upper intervals in the central deep

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of the basin (James and Evans, 1971; Wallace et al., 2002). Overlying the Lakes

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Entrance Formation is the Gippsland Limestone, which consists of fossiliferous

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limestone and marl, and their lateral equivalent coastal plain coals and interseam

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clastics in the Latrobe Valley (Gallagher and Holdgate, 1996).

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Structures in the Gippsland Basin reflect the multi-phase tectonic history of the

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Basin. The basin displays predominantly northwest trending extensional faults in

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deeper parts of the basin, with northeast trending anticlines and some reverse

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faulting occurring at shallower depths. These anticlines form traps for the major

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hydrocarbon reservoirs in the upper Latrobe Group offshore. Timing of the initiation

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of compressional tectonics in the Gippsland Basin (which resulted in growth of these

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anticlines) is poorly constrained, and is interpreted as beginning in the Late

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Cretaceous (Duff et al., 1991), early Eocene (Smith, 1988; Johnstone et al., 2001),

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early-mid Eocene (Brown, 1986; Lowry and Longley, 1991; Holford et al., 2011), or

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Late Eocene-Miocene (Maung and Nicholas, 1990; Rahmanian et al., 1990). The

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basin bounding extensional fault systems have experienced inversion to varying

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degrees along their lengths (Dickinson et al., 2001). A young phase of

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compressional tectonism occurred in the late Miocene, resulting in uplift of the

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Strzelecki Ranges and folding of onshore Palaeogene coals (Dickinson et al., 2001;

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2002).

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Fig. 2. Major stratigraphic subdivisions of the Late Cretaceous to present sediments

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in the Gippsland basin (modified from Norvick et al., 2001).

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3. Methodology 2D and 3D seismic and well data are available over most of the basin. There are

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many 3D seismic datasets located offshore, which have been merged into a

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‘Megacube’ seismic volume. This merged 3D seismic was used to undertake seismic

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interpretation of faults and horizons. The volume is SEG reverse polarity 7

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(otherwise referred to as ‘European’ polarity), full stack, and approximately zero-

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phase. Interpretation of 2D seismic was undertaken on the nearshore and onshore

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areas (where 3D data was unavailable). Data was obtained from the Geological

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Survey of Victoria, Australia.

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Well control in the Gippsland Basin is generally good, with over 400 exploration

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wells, and many more development wells available. Well logs include gamma ray,

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sonic, neutron porosity, and neutron density. Biostratigraphic data for the

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Seaspray Group is derived from planktonic foraminiferal analysis. A zonation

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scheme for offshore wells was first developed by Taylor (1965a, b, 1966) and later

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refined by him (e.g. Taylor, 1975, 1977, 1979). This zonation scheme consists of a

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series of lettered “zonules” (Fig. 2) (discussed in detail in Abele et al., 1988) which

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can be correlated to international planktonic foraminiferal zonation schemes

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(Gradstein et al., 2012). Onshore, various biostratigraphic schemes have been used

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(mainly Carter zones – Carter, 1958a, b) and these can also be correlated to the

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international scheme of Gradstein et al (2012). Foraminiferal data is derived from a

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series of unpublished open file biostratigraphic reports that are available for many of

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the onshore and offshore wells of the Gippsland Basin (largely authored by C. Abele

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in the onshore and D.J. Taylor in the offshore). The onshore wells Wurruk Wurruk-1

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and Boole Poole 1 were the subject of a detailed foraminiferal/biostratigraphic study

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(Holdgate and Gallagher, 1997) and these cored wells can be correlated to nearby

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wells with electric logs and seismic data.

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The chronostratigraphic framework for the Latrobe Group is largely derived

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from palynological spore-pollen and dinoflagellate zonation schemes (James and

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Evans, 1971; Partridge, 1971; Stover and Evans, 1973; Stover and Partridge,

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1973; Partridge 1976; Partridge, 1999; Warne et al., 2003; Partridge, 2006) in

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combination with numerous unpublished palynological reports that are available

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for most petroleum wells from the Gippsland Basin. These biostratigraphic

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zonation schemes have here been updated to the Gradstein 2012 time-scale

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(Gradstein et al., 2012; Korasidis et al., 2018, 2019). Specific wells of importance

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in this study were Barracouta 3 (Stover and Partridge, 1971), Veilfin 1 (Hannah and

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Macphail, 1985) and Teraglin 1 (Macphail, 1983).

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Seismic horizons were interpreted across each structure, as closely spaced as

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data allowed. Regional correlations tied to biostratigraphic data were correlated to

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each structure and used as biostratigraphic datums for age determination on each

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structure. The thicknesses (in two-way-time) of the syn-tectonic sedimentary

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packages were measured at two locations across each structure. For growth

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anticlines, the crest and hinge locations were used, with preference given to a hinge

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location that was not separated from the crest by faulting. Intervals were measured

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vertically, and intervals that have been eroded at the crest of the structure were not

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included. For growth faults, the footwall and hanging wall were measured as close to

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the fault as possible. In total, ten different structures were quantitatively assessed for

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structural growth; the Mackerel, Fortescue, Salmon, and Barracouta extensional fault

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blocks, and the Dolphin, Barracouta, Golden Beach, Darriman and Baragwanath

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anticlines, and the Baragwanath fault. The specific structures studied were selected

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based on their location in the basin, with the aim of gaining a representative spread

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of geographic areas and structure types. Faults located in the Central Deep of the

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basin were targeted for analysis. Measurements were taken from the centre of the

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fault traces, in order to minimise any fault-tip effects which may skew the fault growth

225

analyses. Normal faults that have been inverted were also avoided for growth strata

226

analysis.

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Thickness measurements of sediment packages across each structure were used

228

to determine the Expansion Index (EI). The EI is a ratio of the thickness of growth

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sediments between the upthrown (or on-structure) and downthrown (or off-structure)

230

locations (e.g. anticline crest and hinge respectively) (Thorsen, 1963). An EI of 1

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indicates no growth, while an EI of >1 indicates structural movement and growth

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sedimentation (Jackson et al., 2017). (1)

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Expansion Index = a / b (Thorsen, 1963)

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Where: a = twt of individual downthrown package

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b = twt of individual upthrown package

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(twt = two-way-time)

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242 243

The EI displays the expansion, or growth of individual sediment intervals.

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However, it is difficult to assess the relative amount of structural growth from EI data.

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This is because different thickness (twt) intervals are used for each calculation based

246

on which horizons can be correlated accurately. For example, a relatively thin unit

247

may display a very large expansion index and appear as a significant episode of

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growth on the plot. However, because the unit is relatively thin, it will represent a

249

very small proportion of total growth on the structure and may be unrepresentative in

250

terms of the overall growth history. Hence we have calculated another function

251

which we have called the Growth % (Fig. 3). The Growth % provides the proportion

252

of total growth on the structure for each interval and this function appears to provide

253

a more quantitative and representative picture of the relative growth on the structure

254

over time. The equation used is as follows. (2)

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Growth % = ((a – b) / (A – B)) *100

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Where:

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a = twt of individual downthrown package

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b = twt of individual upthrown package

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A = twt of total downthrown section

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B = twt of total upthrown section

263 264

(twt = two-way-time)

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Each growth analysis diagram displays the measured sediment packages and biostratigraphic datums.

267 268

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269

270 271 272

Fig. 3. Diagram illustrating the variables used in the calculation of the Growth % equation (Equation 2).

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4. Uncertainties and Limitations Studies of structural growth on anticlines and faults have used either sediment

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thickness (e.g. Thorsen, 1963) or two-way-time on seismic profiles (e.g. Mansfield

277

and Cartwright, 1996; Jackson et al., 2017), depending on the data available

278

(seismic, outcrop or wells) and the purpose of the study. Several authors have noted

279

that fault growth plots for faults commonly appear similar whether presented in depth

280

or two-way time, especially if sonic velocity varies gradually with depth and there are

281

no significant lateral velocity variations (Baudon and Cartwright, 2008; Omosanya et

282

al., 2015).

283

In the Gippsland Basin, sonic velocities in the Seaspray Group generally increase

284

gradually with depth. Lateral variations can be present and are related to submarine

285

canyons (Wallace et al., 2002). Using typical velocity gradients for the Seaspray

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Group (Wallace et al., 2002), we estimate that on the structures with the thickest

287

growth, Growth % plots would underestimate the oldest growth by around 15%.

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This would mean that our growth plots slightly underestimate the significance of the

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oldest (Oligocene) growth in the Seaspray Group. This would not significantly affect 11

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the timing estimates for structural growth presented in this study.

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in velocity due to the presence of submarine canyons are unlikely to affect our

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structural growth estimates because such large-scale canyons are visible on seismic

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images and we have avoided measuring across them. A potential source of lateral

294

velocity variation that might affect growth calculations would be the presence of

295

different lithologies on structural highs (e.g. higher carbonate contents on highs).

296

However, there are no completely off-structure wells to provide a lithologic

297

comparison. Additionally, lateral facies change of this nature would be expected to

298

affect most of the Seaspray Group section and changes in growth rates would still be

299

visible on growth analysis plots. Velocity gradients in the Latrobe Group tend to be

300

lower than those in the Seaspray Group, with consequently lower errors.

301

Lateral variations

Another source of error in estimates of growth on structures is the effect of

302

differential compaction (Mansfield and Cartwright, 1996). Sediments on structural

303

highs are subjected to lower depths of burial and hence less compaction than off-

304

structure sediments. This effect is somewhat related to sonic velocity (as sonic

305

velocity is largely controlled by compaction and porosity), and has the same

306

outcome, resulting in underestimation of the growth history of more deeply buried

307

strata (i.e. Oligocene growth will be underestimated in the Seaspray Group).

308

Interstratal variations in lithology and compaction tend to be cancelled out by the

309

analysis of several different geographic regions and structures.

310

Differential compaction has occurred in the Latrobe Group due to the presence of

311

lateral facies changes. This is particularly evident in the upper interval of the Group,

312

as a result of sand-rich barrier systems. This form of differential compaction has

313

been taken into account in this study, as the sandy barrier systems of the Latrobe

314

Group can be easily recognised on seismic profiles (e.g. Fig. 9 Barracouta Fault,

315

Fig.11 Dolphin Anticline).

316

A source of uncertainty in the relative age of structural growth is the spacing of

317

interpreted seismic horizons and age datums. In the seismic data from the deeper

318

water Seaspray Group, it is commonly difficult to interpret seismic reflectors over

319

large distances due to many small (<50 m deep) erosional features that are related

320

to small-scale submarine canyon systems. Widely-spaced seismic horizons will

321

result in less detailed age determinations for specific growth events.

322 12

323 324

5. Results Analysis of growth across a range of structures was undertaken in the Gippsland

325

Basin. These structures fall into two clear groups: those that occur within the Latrobe

326

Group and those that occur within Seaspray Group. These structures are clearly

327

evident on seismic data as large anticlines and faults (Figs. 4, 5).

328 329

Fig. 4. Top Latrobe Group two-way-time-structure map over the 3D Megacube area

330

(offshore Gippsland Basin). Faults analysedin this study are shown in black. The

331

Dolphin, Golden Beach and Barracouta anticlines (analysed here) are also shown.

332

The white line (Z-Z’) is shown as a seismic section on Fig. 5.

333

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334 335

Fig. 5. Megacube arbitrary line Z-Z’ (located on figure 4). Latrobe Group faults

336

analysed are depicted in a hard black line. Grey dashed lines indicate bends in

337

seismic profile. See Fig. 4 for line location.

338 339

5.1 Latrobe Group Structures

340

Virtually all faults within the Latrobe Group have growth strata which indicate a

341

normal sense of movement. Normal growth faults are evident on seismic profiles by

342

a thickening of sedimentary packages into the hanging wall of the fault. These

343

normal growth faults are more common offshore, with faults generally terminating at

344

or close to the Top Latrobe Group reflector.

345

The Mackerel fault block is a northwest trending block, which dips to the

346

southwest. It is bound by northwest trending normal faults, which both dip to the

347

northeast. The fault block contains three well penetrations - Halibut 1, Trumpeter 1

348

and Mackerel 3. The growth strata on the southern bounding fault of this fault block

349

were analysed (Fig. 6). At this location the Top Latrobe Group horizon is affected by

14

350

marine erosion on the northern side of the fault block. This is associated with post-

351

Latrobe Group erosion.

352

The Expansion Index plot shows multiple intervals of increased growth, with a

353

maximum EI of 1.5, in the base F longus. The Growth % plot for the Mackerel fault

354

block shows four distinct phases of growth, with an overall reduction in magnitude of

355

growth over time. A feature not readily apparent on the expansion index plot is a

356

large amount of growth at the base of the interval analysed (Late Cretaceous – T

357

lilliei time). On this structure, 39% of the total growth occurred during the mid-Late

358

Cretaceous (T lilliei), 45 % during the late Late Cretaceous (F longus), while only

359

17% occurred during the late Palaeocene (Upper L balmei). The fault terminates at

360

the Top Latrobe Group horizon, when growth was less than 5%.

361

362 363

Fig. 6. 3D Megacube inline 4253 with biostratigraphic datums and showing Mackerel

364

fault block growth analysis. See Fig. 1 for line location.

15

365

The Fortescue fault block is a west-northwest trending fault block, which dips to

366

the south-southwest. It is bound by west-northwest trending normal faults, which

367

both dip to the north-northeast. It contains the wells Fortescue 1, Fortescue 3, and

368

West Halibut 1. The southern fault of this fault block was analysed (Fig. 7).

369

The Expansion Index shows three phases of increased growth. The Growth % plot

370

displays correlative phases of increased total growth to the EI, with the largest

371

growth occurring during the Late Cretaceous (Lower F longus). On this structure,

372

42% of the total growth occurred during the Late Cretaceous (Lower F longus), 21%

373

occurred during the early Palaeocene (Lower L balmei), another 21% occurred

374

during the late Palaeocene (Upper L balmei), and 16% occurred during the middle

375

Eocene (P asperopolus). This again shows a pattern of reduction in growth over

376

time, terminating at the Top Latrobe Group horizon with the cessation of normal

377

faulting.

378 379

Fig. 7. Megacube inline 3807 with biostratigraphic datums and showing Fortescue

380

fault block growth analysis. See Fig.1 for line location.

16

381

The Salmon fault block trends northwest, and dips to the southwest. It is bound by

382

two northwest dipping normal faults, and contains the wells Salmon 1 and Vielfin 1.

383

The southern fault of this fault block was analysed (Fig. 8). The Expansion Index plot

384

has a variable character, with every individual interval displaying growth except the

385

uppermost Latrobe Group (N asperus), which has an EI = 1 indicating no growth.

386

The Growth % plot shows a slightly variable growth character, with growth

387

gradually waning with time. On the Salmon block, 30% of the total growth occurred in

388

the late Palaeocene (Upper L balmei), 46% in the early Eocene (M diversus), 15% in

389

the early-mid Eocene (P asperopolus), and 9% in the late Eocene (Middle N

390

asperus). Normal faulting appears to have ceased at the Top Latrobe Group (middle

391

Eocene - P asperopolus time), with sediments infilling accommodation space (and so

392

showing growth) until just after this time, during the late Eocene (N asperus).

393

394 395

Fig. 8. Megacube inline 2989 with age datums and showing Salmon fault block

396

growth analysis. See Fig.1 for line location.

397

The Barracouta fault block trends west-northwest, and is located on the southern

398

side of the Barracouta anticline. It is bound by a normal fault to the south, and by a

399

(currently) reverse fault to the north. The southern fault was analysed (Fig. 9). The

400

Expansion Index plot has a variable character, though there appears to be two 17

401

phases of increased growth; during the middle Palaeocene (L balmei time), and the

402

mid Eocene (P asperopolus to N asperus time).

403

The Growth % plot for the Barracouta Fault block again shows growth gradually

404

reducing with time. The majority of the growth (73%) occurs in the late Palaeocene

405

(L balmei), and 31% of the total growth occurs during the mid-Eocene (P

406

asperopolus). By late Eocene time (middle N asperus) growth transitions from 18%

407

to negative (i.e. apparent compression). The apparent compression is likely due to a

408

facies change within the Latrobe Group.

409

410 411

Fig. 9. Megacube inline 2057 with biostratigraphic datums and showing Barracouta

412

fault block growth analysis. See Fig. 1 for line location.

413 414

5.2 Seaspray Group Structures

415

Growth anticlines and reverse faults dominate the structures of the Seaspray

416

Group, both in the onshore and offshore regions. On seismic data, thickening of

417

sedimentary packages is evident adjacent to anticlinal structures and thinning over

418

the crest of the anticline. The same is apparent for reverse faults. In contrast to the 18

419

underlying Latrobe Group, there is no evidence of normal faulting within the

420

Seaspray Group.

421

The Barracouta anticline (Fig. 10) is one of the largest structures in the offshore

422

Gippsland Basin. It is a northeast trending structural high, has numerous well

423

penetrations, and contains significant hydrocarbon reserves in the upper Latrobe

424

Group. Basal Seaspray Group sediments which onlap the structure fall within the P

425

tuberculatus palynological zone.

426

The Expansion Index plot for the Barracouta anticline displays a negative EI value

427

at the uppermost Latrobe Group interval, indicating thickening of sediments onto the

428

crest (caused by the presence of sandy barrier facies on the crest of the anticline).

429

This indicates anticline growth likely had not begun yet. Immediately above the

430

Latrobe Group, in the Oligocene to early Miocene, EI values increase abruptly up to

431

9 indicating a large amount of growth. EI values subsequently remain relatively low

432

to the top of the interval analysed, in the mid Miocene (just above Taylor zone C).

433

Above this, the crest of the structure has experienced erosion, evident by truncated

434

reflectors. While growth rate cannot be quantified due to this erosion, it indicates the

435

Barracouta anticline continued growing.

436

The Growth % plot for the Barracouta anticline shows a similar pattern. Growth is

437

negative in the uppermost Latrobe Group, due to pre-anticline facies distribution in

438

the Latrobe Group. Beginning quite abruptly above the middle N asperus-Top

439

Latrobe Group reflector is a significant pulse of growth. Here, 64% of the total

440

growth of the Barracouta anticline occurred in the Oligocene-early Miocene - the

441

majority of which occurred in the Oligocene. Moderate growth occurred during the

442

mid-Miocene, remaining below 20%. Growth was reduced significantly in the late

443

Miocene, remaining below 5%. Subsequent Late Miocene-early Pliocene growth is

444

inferred by erosion of the anticline crest after this time.

445

19

446

447 448

Fig. 10. Megacube inline 2079 with key horizons, showing Barracouta anticline

449

growth analysis. See Fig.1 for line location.

450

The Dolphin anticline (Fig. 11) trends northeast, and is located south of the

451

Barracouta anticline. Basal Seaspray Group sediments which onlap the structure are

452

interpreted as Taylor zone J1. The Expansion Index plot for the Dolphin anticline

453

displays growth in the upper Latrobe Group (middle N asperus, Late Eocene).

454

However, this apparent structural growth is due to a lateral facies change within the

455

Latrobe Group, rather than structural growth (a middle N. asperus sandy barrier is

456

present in the syncline). The EI remains above 1 until the mid Miocene (Taylor zone

457

C). After this time it drops below 1, indicating no growth of the structure. Above this,

458

the Dolphin Anticline has experienced erosion at the crest, evident by an angular

459

unconformity at or close to the seafloor. This erosion is indicative of anticline growth,

460

though it can’t be confidently quantified.

461

The Growth % plot for the Dolphin anticline indicates 48% of the total growth

462

occurred during the Oligocene to Early Miocene (between the N asperus and Taylor

463

zone G times). Growth on the structure gradually reduced during the Miocene, 20

464

although 37% of the total growth occurred during this time (Taylor zone G to Taylor

465

zone C). Above this, growth becomes negative. This is likely reflecting lateral

466

sedimentation changes, and not structural movement. Above this, an erosional

467

surface is evident, indicating growth of the anticline again.

468

469 470

Fig. 11. Megacube arbitrary line with biostratigraphic datums, showing Dolphin

471

anticline growth analysis. See Fig.1 for line location.

472

The Golden Beach anticline (Fig. 12) is located close to the present-day coastline,

473

and trends northeast. It has multiple well penetrations, including Golden Beach 1.

474

Basal Seaspray Group sediments are dated at Taylor zone K. The Expansion Index

475

plot for the Golden Beach anticline displays EI values of 1 in the upper Latrobe

476

Group, indicating no anticlinal growth. Above this, the EI is highly variable in value,

477

ranging from just above 1, up to almost 6. The largest EI occurs in the mid Miocene

478

(just above Taylor zone G). EI does not drop below 1, indicating growth to the top of

479

the interval analysed, in the mid Miocene. Above this, erosion is evident by an 21

480

angular unconformity just below the sea floor, again, indicating continued anticline

481

growth.

482

The Growth % plot shows almost no growth in the upper Latrobe Group, then a

483

sudden onset of growth immediately above the Top Latrobe Group reflector, in the

484

basal Seaspray Group. 59% of the total growth occurs in the Oligocene-early

485

Miocene, the majority of which occurs in the Oligocene. The Growth % is variable

486

after this time, with 26% occurring in the early-mid Miocene (Taylor zone G to Taylor

487

zone C). Growth % remains positive to the top of the interval analysed.

488

489 490

Fig. 12. 2D seismic line GGS185AE-02 with biostratigraphic datums and showing

491

Golden Beach anticline growth analysis. See Fig. 1 for line location.

492

The Darriman Anticline (Fig. 13) is located in the southern area of the onshore

493

Gippsland Basin. It is associated with one of the main bounding faults of the

494

southern margin of the basin, the Darriman Fault System. Measurements were taken

495

on the northern side of the anticline. Expansion Indices for the Darriman Anticline are

496

variable, although they do not display the very high indices of offshore anticlines. 22

497

Except for a short interval in the mid Miocene, EI values remain above 1, indicating

498

continued growth of the anticline. Indices are largest immediately above the Latrobe

499

Group reflector (approximately Eocene-Oligocene boundary), and during the mid

500

Miocene. The present day land surface is an angular unconformity. This indicates

501

continued growth on the structure which post-dates the youngest strata in the section

502

(~10 Ma).

503

The Growth % plot shows minor growth in the upper Latrobe Group, similar to

504

Dolphin 1. This is again interpreted as a result of lateral facies changes within the

505

Group. Overall, the Growth % reflects a similar pattern to EI, displaying a period of

506

increased growth immediately above the Latrobe Group reflector, in the Oligocene

507

basal Seaspray Group, and again in the mid Miocene. Growth in the Oligocene to

508

early Miocene accounts for 37% of the total growth of the anticline, while 33% of

509

growth occurred in the mid Miocene (Taylor zone G to Taylor zone C), and 21 %

510

occurred in the late mid Miocene.

511

512 513

Fig. 13. 2D seismic line GCR87B-101 and GCR87B-101a with biostratigraphic

514

datums and showing Darriman anticline growth analysis. See fig. 1 for line location. 23

515

The Baragwanath anticline is a large anticline trending roughly east-west across

516

the onshore section of the Gippsland Basin, and has many well penetrations (Fig.

517

14). The Expansion Index plot for the Baragwanath Anticline shows EI’s consistently

518

>1, which range from 1.5 to 3.5. The interval with the largest EI occurs in the

519

Oligocene (EI =3.5). Above this, sediments are truncated by surface erosion at the

520

crest of the structure, indicating continued structural growth.

521

The Growth % plot for the Baragwanath Anticline shows a small (EI =2) amount of

522

apparent growth in the upper Latrobe Group (again, due to a stratigraphic feature- an

523

unconformity over the Latrobe Group). Growth % increases above this, and although

524

not with the distinct sudden onset characteristic of the offshore structures, the time

525

of maximum growth – 41% occurs during the early Oligocene. On this structure,

526

13% of growth occurs during the early Miocene, and 27% during the mid Miocene.

527

528 529

Fig. 14. 2D seismic line GHG-85a-4 biostratigraphic datums and showing

530

Baragwanath anticline growth analysis. See Fig. 1 for line location.

531

The Baragwanath Anticline has two similar-trending reverse faults along its

532

southern side, which roughly parallel the trend of the anticline. The easternmost of

533

these two faults was analysed for growth rates (Fig. 15). The Expansion Index plot

534

for the Baragwanath Fault shows only minimal growth, with EI values only slightly

24

535

above 1. The interval with the largest EI is immediately above the Top Latrobe Group

536

reflector (in the early Oligocene), where EI=1.2.

537

The Growth % plot shows negative to low growth in the upper Latrobe Group

538

during the early Oligocene (Upper N. asperus and older), then sudden, significant

539

growth during approximately the early Oligocene, gradually reducing to almost zero

540

by the end Oligocene. Oligocene growth accounts for 60 % of the total growth of the

541

structure. In the early Miocene growth increased suddenly again, and with the

542

exception of a short period of no growth in the mid Miocene, growth continued.

543

Miocene growth contributed 44 % to the total growth.

544

545 546

Fig. 15. 2D Seismic line GHG85a-3 with biostratigraphic datums, and showing

547

Baragwanath Fault growth analysis. See Fig. 1 for line location.

548 549 550 551

6. Discussion Growth % plots for the Gippsland Basin in the Late Cretaceous and Cenozoic display two major phases of tectonic activity. The Latrobe Group is exclusively 25

552

dominated by normal growth faulting, indicating an extensional tectonic regime.

553

Growth across these faults is evident from the oldest intervals analysed (Late

554

Cretaceous – T lilliei), to the youngest Latrobe Group intervals analysed (Late

555

Eocene – end Lower N asperus). In contrast, the Seaspray Group is dominated by

556

reverse growth faults and growth anticlines, indicating a compressional tectonic

557

regime. Growth on these compressional structures appears to begin at

558

approximately the Eocene-Oligocene boundary (~34 Ma), and continues to the

559

youngest intervals analysed (late Miocene – Taylor zone C). Thus, the transition

560

from extensional to compressional regimes occurs during the Latest Eocene to early

561

Oligocene (~34 Ma). Fig. 16 shows a summary of the growth history of all structures

562

analysed in this study in the Gippsland Basin.

563

564 565

Fig. 16. Plot of structural growth in the Gippsland Basin from the Late Cretaceous to

566

the late Miocene, based on growth % plots from each of the structures analysed.

567

Structures appear to undergo a shift from dominantly extensional to dominantly

568

compressional at around the Eocene-Oligocene boundary ~34 Ma. C, E2, G, H1

569

from Taylor foraminiferal zonation scheme. 26

570 571 572 573

6.1 Extensional Structures Extensional structures in the Gippsland Basin are constrained to the Latrobe

574

Group (and older) sediments. Normal faults in the Latrobe Group show clear

575

thickening of sediments into the hanging wall of these faults. In general, structural

576

growth reduces in magnitude over time, ceasing in the mid to late Eocene (Fig. 16).

577

The Growth % plots for these faults have a generally irregular, ‘pulsed’ appearance.

578

This pulsed or episodic growth character may be due to intermittent fault movement

579

related to the building and release of strain on the fault (Ren et al., 2013).

580

Alternatively, the pulsed appearance might be due to accommodation of strain

581

shifting to other faults in the region.

582

Episodic fault displacement created accommodation space for Latrobe Group

583

sediments in stages, resulting in alternating periods of growth and no growth of

584

sediments across the fault. This episodic fault behaviour has been documented on

585

faults in other basins (Wallace, 1987; Nicol et al., 2005; McClymont et al., 2009;

586

Jackson and Rotevatn, 2013). There appears to be no uniformity in the timing of

587

displacement on the normal faults, nor any relationship to location in the basin.

588

Displacement events on extensional faults appear to have occurred at different times

589

in different parts of the basin. The only observable trend in normal fault growth

590

across the basin is a general waning during Latrobe Group deposition.

591

The data presented here show that extensional faults terminate at or very close to

592

the Top Latrobe Group reflector. However, the Top Latrobe Group is a diachronous

593

horizon, being oldest in the Central Deep to the east and youngest onshore to the

594

west. Given the shift from an extensional to a compressional regime appears to be a

595

regional event, it is reasonable to assume that the youngest sediments showing

596

offset by normal faulting are indicative of when extensional faulting ceased. Thus

597

extensional tectonics in the Gippsland Basin appears to have ended in the latest

598

Eocene.

599

27

600

6.2 Compressional Structures

601

Analysis of growth strata from this study indicate that the tectonic regime in the

602

Gippsland Basin changed from extensional to compressional during latest Eocene to

603

earliest Oligocene time (~34 Ma). The major outcome of this compression was the

604

formation of broad, roughly northeast-southwest trending anticlines in the Central

605

Deep, which today host major hydrocarbon accumulations in the basin (Dickinson et

606

al., 2001). This compressional regime continued into the Neogene, and in addition to

607

anticline formation, reactivation and inversion of previously extensional faults within

608

the Latrobe Group occurred. The Baragwanath Fault (a reverse fault, Fig. 15)

609

displays episodic growth similar to normal faults in the Latrobe Group, as opposed to

610

the more continuous growth behaviour evident on the anticlines.

611

The anticlines studied here generally display two major pulses of growth. The first

612

began at approximately the Eocene-Oligocene boundary (~34 Ma) and continued to

613

the early Miocene (~20 Ma). Where finer scale correlation across structures allows

614

for a more precise timing (e.g. Darriman Anticline, Fig. 13), a large amount of this

615

growth is observed to occur in the early Oligocene. During the Oligocene to early

616

Miocene, anticlinal growth occurred on both onshore and offshore structures, with

617

the magnitude and proportion of growth varying between structures. A second phase

618

of anticlinal growth occurred in the mid-Miocene, and appears to have largely

619

affected onshore anticlines. Offshore structures, however, did not experience this

620

second pulse of increased growth so strongly (or at all in some cases), and instead

621

appear to have remained in a low growth phase from the early Miocene (~20 Ma),

622

until at least the Miocene-Pliocene boundary. At this time erosion is evident,

623

indicating the anticlines began growing again.

624

Concurrent with early anticline growth was deposition of the Seaspray Group. The

625

lower Seaspray Group shows onlap onto the major anticlines and thinning of upper

626

intervals over anticline crests (e.g. Fig. 10, Barracouta anticline), clearly indicating

627

anticlines were growing at the same time deposition was occurring. Despite these

628

compressional events, the basin was still subsiding. Sediments of the Seaspray

629

Group were deposited in deep marine environments (in the offshore part of the

630

basin), characterised by marine calcareous muds and limestones (Holdgate and

631

Gallagher, 2003). The offshore Seaspray Group is characterized by abundant 28

632

submarine canyons (Wallace et al., 2002) indicating an outer shelf and slope setting.

633

A common theme evident on Growth % charts for each structure is a very large and

634

sudden onset of growth in the early Oligocene. This first phase of compression,

635

occurring from the early Oligocene to early Miocene, displays the most growth

636

across both onshore and offshore structures. Brown (1986) suggested that the

637

Dolphin anticline likely had relief in the early stages of post-Latrobe Group

638

sedimentation, due to thinning of sediments toward the anticline. Dickinson et al.,

639

(2001) also noted the onlap and thinning of Oligocene sediments across some of the

640

large offshore anticlines, indicating structural growth at that time. After deposition of

641

the Latrobe Group, there was a period of low sedimentation evident from

642

palynological data showing condensed intervals (and localised, brief unconformities),

643

and deposition of the glauconitic Gurnard Formation. The sudden onset of the first

644

early Oligocene compressional phase may in part reflect reduced sedimentation at

645

the top Latrobe Group-lower Seaspray Group boundary. However, there is no

646

evidence of onlap or thinning of Latrobe Group sediments over the anticlines. In

647

addition, faulting remains extensional until the late Eocene, which suggests that

648

compression and subsequent anticline growth did not occur before this.

649

Previous interpretations of the major hydrocarbon-bearing anticlines of the

650

Gippsland Basin beginning to develop in the early to mid-Eocene appear to be based

651

on the interpretation of large, incised canyons (i.e. the Marlin and Tuna canyons) in

652

the Latrobe Group as resulting from fluvial incision (e.g. Hocking, 1972; Brown

653

1986). This interpretation of deeply incised fluvial canyons implies significant uplift of

654

the basin and hence possible compression during the mid-Eocene. This

655

interpretation first appears in some earlier papers on the Gippsland Basin (Hocking,

656

1972; Threlfall, 1976) at a time when seismic and well data were limited in the

657

Gippsland Basin. There is no evidence to suggest that the Marlin-Turrum and Tuna-

658

Flounder canyons are the result of fluvial incision, and are perhaps of submarine

659

incision origin (as are the abundant canyons in the overlying Seaspray Group). If

660

these canyons were of submarine origin, they are likely unrelated to tectonic uplift.

661

Importantly, there is no evidence for regional unconformities of mid Eocene age

662

within the Latrobe Group. Regardless of the origin of the mid-Eocene canyon

663

systems, there is also no evidence for anticline growth during Latrobe Group time.

29

664

Furthermore, the canyon fill is cut by normal faults (showing extension occurring

665

after infilling) and is subsequently deformed by folding.

666

In the offshore Gippsland Basin, the Oligocene compressional event essentially

667

ceased in the early Miocene, with the Dolphin, Barracouta and Golden Beach

668

anticlines experiencing significantly reduced growth. This low growth of structures

669

persisted to the youngest sediments analysed, in the late Miocene. It is assumed to

670

continue to at least the Miocene-Pliocene boundary, as on seismic lines, structural

671

highs appear peneplained (e.g. Dolphin anticline – Fig. 11). Where seismic

672

resolution allows, this appears as an angular unconformity overlain with horizontal

673

strata. This unconformity coincides approximately with the Miocene-Pliocene

674

boundary, and is particularly evident on the Golden Beach structure (Figs. 12,17).

675

Dickinson et al., (2001, 2002) discussed the Miocene-Pliocene unconformity in

676

detail, observing it in the Murray, Otway, Torquay and Port Phillip Basins, as well as

677

the Gippsland Basin. They also documented erosion of anticlines from shallow

678

seismic data.

679

In the onshore Gippsland Basin, structures underwent a second phase of growth,

680

beginning in the middle Miocene and lasting until the youngest sediments analysed,

681

in the late Miocene. This second phase of compression is not seen in offshore

682

structures. This pattern of more intense structural development onshore continues

683

into the younger tectonic regime. The Late Miocene-Pliocene episode of tectonics is

684

also more intense in onshore settings than in the offshore. In the onshore Latrobe

685

Valley, hundreds of meters of uplift and erosion occurred, with significant uplift of the

686

Strzelecki Ranges (Dickinson et al., 2001, 2002). While the Late Miocene-Pliocene

687

tectonic episode did affect the offshore, it was significantly less intense than in the

688

onshore.

689

30

690 691

Fig. 17. Interpretation of 2D seismic line GGSI85AE-02, which crosses the Golden

692

Beach structure at the northeastern end of the seismic line. Note the thinning of the

693

Oligocene-Miocene interval over the anticline.

694 695 696

6.3 Cenozoic tectonics in SE Australia Australia is commonly regarded as having an insignificant record of Cenozoic

697

uplift and tectonics, with ancient landscapes dominating the geomorphology (e.g.

698

Pain et al., 2012). Some authors have suggested Mesozoic ages for many Australian

699

landscape features like the Southeast Highlands (e.g. Vandenberg, 2010; Webb,

700

2017), while others have suggested much younger tectonic events have been

701

important in SE Australia (Dickinson et al., 2002; Holdgate et al. 2008). The origin of

702

the Southeast Highlands is historically a contentious area of research, with authors

703

proposing old origins for the uplift (uplift in the Mesozoic, e.g. Vandenberg, 2010),

704

very young origins (uplift in the Plio-Pleistocene, e.g. Andrews, 1910) or continuous 31

705

uplift since the Mesozoic (e.g. Wellman, 1979). However, some researchers have

706

argued for a largely post-Eocene origin for this uplift (e.g. Browne, 1969; Holdgate et

707

al., 2008). Recent research undertaken by Greenwood et al., (2017) on vegetation

708

type of the Highlands through geologic time is consistent with a significant episode of

709

post-Eocene uplift affected the SE Highlands. It has been suggested that up to 50%

710

of the present-day topographic relief in the Southeast Highlands formed in the last 10

711

Ma (Braun et al., 2009). Given that the younger tectonic events in the Gippsland

712

Basin (mid-Miocene and Late Miocene-Pliocene) appear to be more intense in

713

onshore positions, it appears likely that post-Eocene compressional tectonics

714

(including the early Oligocene event documented here) and uplift have affected the

715

SE Highlands inshore from the Gippsland Basin.

716

The Oligocene compressional event documented here appears to be a significant

717

event in the Gippsland Basin and almost certainly affected other areas in SE

718

Australia. Evidence for this compressional tectonism has been documented in

719

surrounding basins. The Bass Basin, located immediately southwest of the

720

Gippsland Basin, displays a series of hydrocarbon-bearing anticlines which formed

721

during the Oligocene-Miocene as a result of transpressional inversion (Cummings et

722

al., 2004; Leech et al., 2008). West of the Bass Basin, the Torquay Sub basin of the

723

Otway Basin displays anticlines which began growing in the Oligocene and

724

underwent a second phase of inversion in the Miocene (Young et al., 1991). An

725

angular unconformity is also evident in the onshore Torquay Sub basin, in the

726

Anglesea coal mine, which has been dated palynologically at ~39-40 Ma (Holdgate

727

et al., 2001). This unconformity is believed to be related to significant late Eocene

728

uplift, associated with the Otway Ranges (Holdgate et al., 2001).

729

There is some uncertainty surrounding the cause of the initiation of compressional

730

tectonism in SE Australia. Previous interpretations which have placed the timing of

731

compression earlier (early-mid Eocene) have linked it with the onset of fast sea floor

732

spreading in the Tasman Sea (e.g. Lowry and Longley, 1991; Norvick et al., 2001;

733

Holford et al., 2011; Holford et al., 2014). However, data presented here shows

734

compression likely initiated approximately 10-15 Ma later, at (or close to) the

735

Eocene-Oligocene boundary. At this time, there were a number of smaller tectonic

736

events which affected the continental margins of Australia. These include:

737

development of the Solomon Sea and Caroline Plate on the northeast margin of the 32

738

Australian Plate (Honza et al., 1987; Smith, 1990), oceanic spreading at the South

739

Tasman and Emerald Basins southwest of NZ (Lebrun et al., 2003), and the final

740

separation of Australia and Antarctica at the South Tasman Rise, directly south of

741

the Gippsland Basin (Hill and Exon, 2004; Scher et al., 2015). There is also evidence

742

for a significant unconformity and reverse fault movement on the Lord Howe Rise at

743

around the Eocene-Oligocene boundary (Sutherland et al., 2010). Identification of

744

the exact driver for the initiation of compression in SE Australia is difficult; however

745

far field plate tectonic events have been interpreted to produce local deformation by

746

transference of intraplate stresses over large distances in Australia (Dickinson et al.,

747

2001; Sandiford et al., 2004).

748 749 750

7. Conclusions The Cenozoic history of the Gippsland Basin is characterised by two major

751

tectonic regimes. The Late Cretaceous-Eocene period is characterized by

752

extensional tectonics that produced abundant normal growth faults. Analysis of

753

growth strata on these normal faults indicates a general waning of extensional

754

activity from the Late Cretaceous through to the Eocene. During the latest Eocene to

755

early Oligocene (~34 Ma), the tectonic regime changed to one of compression. This

756

compressional tectonic episode continues to the present day in the South Eastern

757

Australian basins. The large anticlines which host the giant oil fields of the Gippsland

758

Basin were all formed during the Oligocene to the Holocene.

759

Detailed analysis of growth strata from anticlines and reverse faults in the

760

Seaspray Group (Oligocene to Holocene carbonates) indicates a significant and

761

widespread episode of compression occurred during the early Oligocene, a period

762

not previously identified as being important tectonically. It appears likely that this

763

Oligocene compressional event also affected other South Eastern Australian basins

764

and was likely a significant component of uplift in the Eastern Highlands.

765

In onshore regions of the basin, a second major pulse of compressional tectonism

766

occurred during the mid-Miocene. In offshore regions, this mid-Miocene event is

767

more subdued, with continued low-level growth on the major anticlines. In nearshore

768

and onshore regions, the late Miocene-Pliocene is characterized by a significant 33

769

unconformity which corresponds to the previously documented episode of tectonic

770

uplift and erosion that began at around 10 Ma (Dickinson et al., 2002). Like the mid-

771

Miocene event, the Late Miocene event has a greater affect in onshore regions, with

772

major uplift occurring on features like the Baragwanath Anticline and Strzelecki

773

Ranges.

774 775 776

Acknowledgements We would like to thank IHS Markit for their donation of the Kingdom seismic

777

interpretation software. E. Mahon is supported by a Baragwanath Geology Research

778

Scholarship, and the Melbourne Research Scholarship as part of the Australian

779

Government Research Training Programme Scholarship, in addition to the AAPG

780

Foundation Grants-in-Aid Chandler and Laura Wilhelm Named Grant, and the

781

Geological Society of Australia (Victorian Division) Postgraduate Grant. We would

782

like to thank two anonymous reviewers and the editor Adam Bumby for their helpful

783

and constructive comments that significantly improved the manuscript.

784

34

785 786

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45

Highlights • • •

Estimates for timing of growth for structures in the Gippsland Basin are made A change from extension to compression occurs at ~ Eocene-Oligocene boundary A previously unrecognized early Oligocene tectonic event is documented for SE Australia

E.M.M and M.W.W. conceived the project, E.M.M. compiled the data, interpreted the seismic profiles and analysed the data. E.M.M and M.W.W. interpreted the results and wrote the paper.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: