The Otway Basin is a northwest trending

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...

located along the southern coast of

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. The basin covers an area of 150,000 square kilometers and spans from southeastern

South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...

to southwestern

Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...

, with 80% lying offshore in water depths ranging from 50-3,000 meters. Otway represents a

passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creat ...

rift basin and is one of a series of basins located along the Australian Southern Rift System. The basin dates from the late

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

to late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

periods and formed by multi-stage

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing during the breakup of

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

and the separation of the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

and

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plates. The basin contains a significant amount of

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

and is a current source of commercial extraction.

Tectonic setting

The Otway Basin developed along the Australian Southern Rift System during late

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

breakup of eastern

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

began rifting away from

. The basin lies at the transition from a normal-obliquely

ed continental margin to the west to a

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continental margin to the southeast. This transition zone is dominated by transtensional faulting that contributes to the basin's complex structural and depositional history. The margin developed through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of

seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener an ...

between

and

Structural elements

The basin includes five significant

depocenter A depocenter or depocentre in geology is the part of a sedimentary basin where a particular rock unit has its maximum thickness. Depending on the controls on subsidence and the sedimentary environment the location of basin depocenters may vary wit ...

s. The Inner Otway Basin, Torquay Sub-basin, Morum Sub-basin, Nelson Sub-basin, and Hunter Sub-basin formed as a result of two major, basin-wide rifting phases. At the onset of major north-south

ing in the late

several east-west trending extensional depocenters developed in the onshore part of the basin to define the Inner Otway Basin. Renewed rifting in the late

was driven by a change in crustal extension style from north-south to northeast-southwest resulting in the structurally different, northwest-southeast trending Torquay, Morum, Nelson, and Hunter Sub-basins in the mid- and off-shore. Following each rifting episode the basin underwent compressional phases resulting in the inversion and wrenching of pre-existing structures.

Tectonostratigraphic evolution

Eight distinct and regionally mappable stratigraphic groups define the Otway Basin fill.

Initial rifting (Early Cretaceous)

Basin rift system initiated at the onset of north-south extension between the

and

plates in the late

. Growing extensional structures were filled with ~5000 meters of continental and fluvio-lacustrine sediments that comprise the Crayfish Group. In the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

the basin experienced widespread

thermal subsidence In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools an ...

where ~4000 meters of fluvial and lacustrine sediments of the

Eumeralla Formation The Eumeralla Formation is a geological formation in Victoria, Australia whose strata date back to the Early Cretaceous. It is Aptian to Albian in age. Dinosaur remains are among the fossils that have been recovered from the formation, particularl ...

were deposited in the progressively widening sag basin.

Compression and uplift (Mid Cretaceous)

The first phase of rifting ceased in the

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 M ...

at which time the basin underwent an erosive compressional period leading to the basin wide Otway

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

Renewed rifting (Late Cretaceous)

After a 6.5 Ma hiatus, rifting renewed and rift-related subsidence began in the

Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by t ...

. This rifting phase is marked by a change in crustal extension direction from north-south to northeast-southwest resulting in the deposition of large and deep depocenters in the mid- and off-shore basin. Syn-rift deposition includes ~1300 meters of fluvial and deltaic sediments that comprise the Shipwreck Group. A major

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

in the

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...

produced ~5000 meters of deltaic and marine Sherbrook Group sediments.

Continental breakup (Late Maastrichtian)

In the late

rifting transitioned to

marking the onset of post-rift

conditions within the Otway Basin. As

plate clearance continued and the

developed further the basin experienced widespread

leading to an increase in accommodation space. Deposition from the late

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...

to present day is marked by a succession of marine and carbonate accumulations of the Wangerrip, Nirranda, Heytesbury, and Whalers Bluff Groups, separated by distinct

unconformities An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

associated with basin-wide compression events.

References

{{reflist Sedimentary basins of Australia Mesozoic rifts and grabens