{{Use dmy dates, date=September 2020 The Narooma Accretionary Complex or Narooma Terrane is a geological structural region on the

south coast South Coast is a name often given to coastal areas to the south of a geographical region or major metropolitan area. Geographical Australia *South Coast (New South Wales), the coast of New South Wales, Australia, south of Sydney * South Coast (Q ...

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

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 ...

that is the remains of a

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

or an oceanic

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

. It can be found on the surface around

Narooma Narooma is a town in the Australian state of New South Wales on the far south coast. The town is on the Princes Highway, which crosses the Wagonga Inlet to North Narooma. The heritage town of Central Tilba is nearby to the south. The name Naro ...

Batemans Bay Batemans Bay is a town on the South Coast region of the state of New South Wales, Australia. Batemans Bay is administered by the Eurobodalla Shire council. The town is situated on the shores of an estuary formed where the Clyde River meets the ...

and down south into

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 ...

near

Mallacoota Mallacoota is a small town in the East Gippsland region in the state of Victoria, Australia. At the 2016 census, Mallacoota had a population of 1,063. At holiday times, particularly Easter and Christmas, the population increases by about 8,000 ...

. It has attached itself to the

Lachlan Fold Belt The Lachlan Fold Belt (LFB) 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 (Australia), Victoria, also extending into T ...

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

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites were ...

s, block in matrix

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 cons ...

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

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

accretionary prism An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the do ...

. It was moved by the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

westwards for about 2500 km until it encountered the east coast 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 ...

.Ian G. Percival and Richard A. Glen: ''Early Ordovician geography of East Gondwana'' February 2003 It is part of the Mallacoota Zone according to Willman,C. E. Willman,* A. H. M. Vandenberg and V. J. Morand ''Evolution of the southeastern Lachlan Fold Belt in Victoria'' Australian Journal of Earth Sciences (2002) 49, 271–289 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 A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

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 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 eit ...

, dilational veins and

boudinage 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 hornfels, is stretched and deformed amids ...

. Some of the rock appears as

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a cl ...

. Underneath the high strain is chert from the Late

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

to Late

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

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

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

. 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 with lenses of

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

and

white mica White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...

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

olistostrome An olistostrome is a sedimentary deposit composed of a chaotic mass of heterogeneous material, such as blocks and mud, known as olistoliths, that accumulates as a semifluid body by submarine gravity sliding or slumping of the unconsolidated sedi ...

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.J. M. Miller, and D. R. Gray ''Fossil Subduction in The Eastern Lachlan Fold Belt: Significance and Tectonic Implications'' Geol. Soc. Aust. Abstracts 41, 298, 1996 URL=http://www.agcrc.csiro.au:80/publications/9596/monash/c138.html

Stratigraphy

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

History

Formation

Deep sea chert (Narooma Chert) was deposited on the

Pacific ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

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

Paracordylodus gracilis ''Paracordylodus'' is an extinct genus of conodonts in the clade Prioniodontida, also known as the "complex conodonts". The species ''P. gracilis'' has been recovered from the chert of the Narooma Terrane, a geological structural region on the so ...

'' and ''

Acodus ''Acodus'' is an extinct genus of conodonts. Species * ''A. acutus'' * ''A. campanula'' * ''A. crassus'' * ''A. delicatus'' * ''A. deltatus'' * ''A. erectus'' * ''A. firmus'' * ''A. jonesi'' * ''A. kechikaensis'' * ''A. neodeltatus'' * ''A. ...

cf. A. comptus''.Ian G. Percival, Yong–Yi Zhen and Barry D. Webby: ''Early Ordovician conodont distribution from craton to basin and island terranes in East Gondwana'' 15 February 2003 URL=92.htm The terrane gradually approached the continental margin and began to include sediments derived from the continent, such as

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' 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 ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

as well as chert bands.R. A. Glen, I. R. Stewart and I. G. Percival ''The Narooma Terrane: implications for the construction of the outboard part of the Lachlan Orogen'' Australian Journal of Earth Sciences Volume 51 issue 6 Page 859 - December 2004 URL=narooma_terrane.htm

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 is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes. *In roofing, imbrication is employed in the Imbrex and tegula system. *Imbrication (sedimentology) In sedime ...

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

trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from eros ...

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

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s. Inland the rocks have developed a scaly cleavage. The chert on the coast has developed a

dextral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...

shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...

.J. M. Miller, and D. R. Gray ''Subduction and Sediment Accretion By Underplating in the Eastern Lachlan Fold Belt'', Geol. Soc. Aust. Abstracts 40, 109–110, 1995 url=http://www.agcrc.csiro.au:80/publications/9596/monash/c131.html

References

Geology of New South Wales Terranes