Narooma Terrane
The Narooma Accretionary Complex or Narooma Terrane is a geological structural region on the
Mallacoota
.
It has attached itself to the Lachlan Fold Belt and has been considered as either an exotic terrane or as a part of the fold belt. Rocks are turbidites, block in matrix mélange, chert, and volcanics. The accretionary complex itself could either be the toe of a subduction zone, or an accretionary prism.[1]
It was moved by the Pacific Plate westwards for about 2500 km until it encountered the east coast of Gondwana.[2]
It is part of the Mallacoota Zone according to Willman,[3] 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, dilational veins and boudinage. Some of the rock appears as mylonite. Underneath the high strain is chert from the Late Cambrian to Late Ordovician.[1]
The lowest part of the stack is block in mélange. The blocks are mostly turbidite, but also includes chert, and some
white mica. The strike direction of the cleavage is 330°. The interpretation of the mélange is that it is either an olistostrome or an upwelling. The mélange was underplated beneath the chert layer. Pockets of underplated material are expected to form low angle detachments.[1]
The inland zone has a chevron folded structure with reverse faults.[4]
Stratigraphy
From the stratigraphic point of view the terrane comprises the Wagonga Group. This consists of the Narooma Chert overlain by the Bogolo Formation.[5]
History
Formation
Deep sea chert (Narooma Chert) was deposited on the
Paracordylodus gracilis and Acodus cf. A. comptus.[6] The terrane gradually approached the continental margin and began to include sediments derived from the continent, such as sandstone, siltstone, argillite and shale as well as chert bands.[5]
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.[5] A low angle oblique imbrication
formed.
Rocks have become more deformed closer to the coast as the
References
- ^ a b c d 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
- ^ Ian G. Percival and Richard A. Glen: Early Ordovician geography of East Gondwana February 2003
- ^ 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
- ^ 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
- ^ a b c 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
- ^ 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