The Narooma Terrane: implications for the construction of the outboard part of the Lachlan Orogen

Abstract

In its type area around Narooma, the Narooma Terrane in the Lachlan Orogen comprises the Wagonga Group, which consists of the Narooma Chert overlain by the argillaceous Bogolo Formation. Conodonts indicate that the lower, largely massive (ribbon chert) part of the Narooma Chert ranges in age from mid‐Late Cambrian to Darriwilian–Gisbornian (late Middle to early Late Ordovician). The upper Narooma Chert consists of shale, containing Eastonian (Late Ordovician) graptolites, interbedded with chert. Where not deformed by later faulting, the boundary between the Narooma Chert and Bogolo Formation is gradational. At map scale, the Narooma Terrane consists of a stack of imbricate thrust slices caught between two thrust faults that juxtaposed the terrane against the coeval Adaminaby Superterrane in Early Silurian time. These slices are best defined where Narooma Chert is thrust over Bogolo Formation. The soles of such slices contain multiply foliated chert. Late extensional shear bands indicate a strike‐slip component to the faulting. The Narooma Terrane, with chert overlain by muddy ooze, is interpreted to be an oceanic terrane that accumulated remote from land for ∼50 million years. The upward increase in the terrigenous component at the top of the Wagonga Group (shale, argillite, siltstone and sandstone of the upper Narooma Chert and Bogolo Formation) records approach of the terrane to the Australian sector of the Gondwana margin. Blocks of chert, argillite and sandstone reflect extensional/strike‐slip disruption of the terrane as it approached the transform trench along the Gondwana–proto‐Pacific plate boundary. Blocks of basalt and basalt breccia represent detritus from a seamount that was also entering the trench. There is no evidence that the Narooma Terrane or the adjacent Adaminaby Group formed in an accretionary prism/subduction complex.

Underplated seamount in the Narooma accretionary complex, NSW, Australia

“…comm. 1999to Glen et al, 2004. However, prior to the present study no detailed geochemistry of the basalts has been documented.…”

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Macquarie Volcanic Province from Percival and Glen (2007). Narooma Zone (Nar) from Glen et al (2004). Melville Point (MP) from personal observation and I. Stewart, personal communication.…”

Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation

“…Bedded chert and black shale occur interbedded with the Ordovician turbidites and presumably reflect periods of sediment starvation on parts of the mega-fan (VandenBerg & Stewart 1992;Colquhoun et al 2005). In the Narooma anticlinorium, on the south coast of New South Wales, an apparently continuous succession of Middle Cambrian to Eastonian age occurs and consists of chert and black mudstone, with no interbedded quartz turbidites (Glen et al 2004). This succession has been interpreted as having formed on a separate oceanic plate from the submarine mega-fan (Glen et al 2004).…”

“…In the Narooma anticlinorium, on the south coast of New South Wales, an apparently continuous succession of Middle Cambrian to Eastonian age occurs and consists of chert and black mudstone, with no interbedded quartz turbidites (Glen et al 2004). This succession has been interpreted as having formed on a separate oceanic plate from the submarine mega-fan (Glen et al 2004). Alternatively, it may represent either the distal oceanic part or an elevated area of the sea floor, as part of the same plate containing the mega-fan.…”

“…An apparently near-continuous Middle Cambrian to Late Ordovician sedimentary succession was involved in the subduction complex on the south coast of New South Wales (Bischoff & Prendergast 1987;Glen et al 2004). 40 Ar/ 39 Ar ages of 445−450 Ma for mica crystallisation, at Narooma and Bermagui, indicate that metamorphism, associated with the developing subduction complex, formed by this time and are about the same age as the youngest fossils at the top of the sedimentary succession (Offler et al 1998).…”