The Narooma Accretionary Complex or Narooma Terrane is a geological structural region on the south coast

South Coast

South Coast is a name often given to coastal areas to the south of a geographical region or major metropolitan area.-Australia:*South Coast refers to the coast of New South Wales Australia south of Sydney....

 of New South Wales

New South Wales

New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 that is the remains of a subduction zone or an oceanic terrane

Terrane

A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

. It can be found on the surface around Narooma, Batemans Bay and down south into Victoria

Victoria (Australia)

Victoria is the second most populous state in Australia. Geographically the smallest mainland state, Victoria is bordered by New South Wales, South Australia, and Tasmania on Boundary Islet to the north, west and south respectively....

 near Mallacoota.
It has attached itself to the Lachlan Fold Belt

Lachlan Fold Belt

The Lachlan Fold Belt or Lachlan Orogen is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age. It dominates New South Wales and Victoria, also extending into Tasmania, the Australian Capital Territory and Queensland. It was formed in...

 and has been considered as either an exotic terrane or as a part of the fold belt. Rocks are turbidite

Turbidite

Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...

s, block in matrix mélange

Mélange

In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies...

, chert

Chert

Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...

, and volcanics. The accretionary complex itself could either be the toe of a subduction zone, or an accretionary prism

Accretionary wedge

An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...

..
It was moved by the Pacific Plate

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....

 westwards for about 2500 km until it encountered the east coast of Gondwana

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

.
It is part of the Mallacoota Zone according to Willman, which in turn is part of the Eastern Lachlan Fold Belt, which is part of the Benambra Terrane.

Components

The complex is made up of an imbricate stack in a sequence that is the same both at Narooma and Murruna Point, Batemans Bay. The top layer consists of turbidite sequence from the Early Ordovician. Below this is a high strain zone full of broken fragments. Special textures from the high strain zone include pressure solution

Pressure solution

In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress...

, dilational veins and boudinage

Boudinage

thumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...

. Some of the rock appears as mylonite

Mylonite

Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...

. Underneath the high strain is chert from the Late Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 to Late Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

..
The lowest part of the stack is block in mélange. The blocks are mostly turbidite, but also includes chert, and some pillow lava

Pillow lava

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...

 basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

. The blocks are at all different orientations, different sizes all mixed together. The sediments were deformed into these blocks before they turned into stone. Later deformation has developed a cleavage

Cleavage (geology)

This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...

 with lenses of chlorite

Chlorite

The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...

, quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 and white mica. The strike direction of the cleavage is 330°. The interpretation of the mélange is that it is either an olistostrome

Olistostrome

The term olistostrome is derived from olio - a dish of many ingredients or stew; a mixture of heterogeneous elements; hodgepodge. and stroma - a bed covering...

 or an upwelling. The mélange was underplated beneath the chert layer. Pockets of underplated material are expected to form low angle detachments.

The inland zone has a chevron folded structure with reverse faults.

Stratigraphy

From the stratigraphic point of view the terrane comprises the Wagonga Group. This consists of the Narooma Chert overlain by the Bogolo Formation.

Formation

Deep sea chert (Narooma Chert) was deposited on the Pacific ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 floor over a period of 50 million years from Late Cambrian to Ordovician. Fossils from the chert include the conodont

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

s Paracordylodus gracilis and Acodus cf. A. comptus.The terrane gradually approached the continental margin and began to include sediments derived from the continent, such as sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

, siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

, argillite

Argillite

An argillite is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is...

 and shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

 as well as chert bands.

Deformation

After formation the terrane was accreted to the Lachlan Fold Belt in the early Silurian. The rock was deformed in the Benambran Orogeny in early Silurian. A low angle oblique imbrication

Imbrication (sedimentology)

In sedimentology imbrication refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes...

 formed.
Rocks have become more deformed closer to the coast as the trench

Oceanic trench

The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....

 is approached. The country here was shortened between middle Silurian to Middle Devonian in the east west direction, with many folds

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 and thrust fault

Thrust fault

A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

s. Inland the rocks have developed a scaly cleavage

Cleavage

Cleavage may refer to:*Cleavage , partial exposure of the separation between a woman's breasts.**Cleavage enhancement, methods of making a person's breast cleavage look more substantial than it really is....

. The chert on the coast has developed a dextral shear.