Geology and structure of the Maryamma District

This article is published to show the geological sub-structure of the Maryamma District in which the Wyangala dam is located.

"The Geology and Structure of the Maryamma District

South of Wyangala Dam, NSW

Lloyd White 2005

University of New South Wales

The Maryamma district is located approximately 40km southeast of Cowra, NSW. The field area is situated on the western side of the Hill End Trough, in the Eastern Lachlan Fold Belt. The field area consists of the Ordovician Adaminaby Group and Kenyu Formation, which have been intruded by the I-type Streamville Granodiorite and S-type Wyangala, Reids Flat and Wywurri granites of the Wyangala Batholith.
The lithologies of the study area have been regionally metamorphosed to greenschist biotite facies. The Wyangala Granite shows north-south layering, and the roof of the pluton is exposed along its eastern, to northeastern boundary.
The study area is located in the most deformed part of the Wyangala Batholith and characterised by the presence of numerous shear zones and folded granite. Shear zones occur in the Wyangala and Reids Flat granites, but are more prevalent in the Wyangala Granite. The shear zones found in the study area are along strike from structures immediately north of the field area, and are classified as part of the Wyangala Fault System. Lower-strain shear zones occur in the western side of the Wyangala Granite in the field area. Two-to-three high-strain shear zones have been identified and characterised along the eastern margin of the Wyangala Granite. Shear zones were delineated based on grain-size reduction, the presence of S-C fabrics and mylonite to ultramylonite development. Microstructural analyses of the granites, and the acute angle between S and C fabrics indicate that strain intensity increased from west to east in the Wyangala Granite. Strain intensity is also thought to decrease progressively south, as shear zones appear to merge into the dominant high-strain


Wyangala Dam Shear Zone.
The Maryamma district displays a complex structural history, consisting of at least four deformation events. The earliest deformation occurred in the Benambran orogenic event, and led to the formation of a pervasive, axial planar cleavage and tight isoclinal north-south trending and doubly plunging folds in the Adaminaby Group.
Granite emplacement occurred in the latest Early Silurian and is possibly associated with the oblique opening of the Hill End Trough. The Wywurri Granite intruded the Reids Flat Granite as a nested pluton in the same period, suggesting the crystallisation of the Reids Flat Granite was relatively quick. Continued east-west shortening in the subsequent Bowning-Bindian events led to the development of S-C foliations in the Wyangala and Reids Flat Granites, and weak S foliations in the Wywurri Granite. The differences in foliation development in the respective granites may be explained by mineralogical differences, or longer crystallisation histories in the more deformed granites. The development of both brittle to ductile structures in the Wyangala Fault System probably began in the Bindian orogeny, and continued with reactivation in the Tabberabberan. The slip direction obtained from S-C fabrics indicate that the Wyangala Batholith was possibly rotated dextrally in association with sinistral, strikeslip movement on the Wyangala Fault System. East-west oriented brittle-ductile mylonites with an apparent sinistral sense of movement occurred after the development of the dominant north-south oriented mylonites associated with the


Wyangala Fault System.
The Wyangala Granite was folded after the development of mylonite zones. Folding occurred in response to a long history of faulting and reactivation along the Wyangala Mount Darling Fault. Fault propagation originally began within the granites at depth.
A blind reverse, east-over-west backthrust developed adjacent to the Wyangala/Mount Darling Fault leading to the propagation of trishear zones from the deep seated fault tips. The combination of fracturing associated with faulting and trishear led to the development of tight isoclinal folds in the Wyangala Granite. The folding of S and C fabrics indicates folding occurred late in the region’s geological history and is probably associated with faulting in the Tabberabberan and Kanimblan orogenic events."

What this indicates is hard rocky valleys with shear sides surrounding basins through which waterways of the Abercrombie and Lachlan Rivers made their way down stream into the Murrumbidgee and Murray/Darling System.

Exploring this country is not easy with many precipitous climbs, however, the resulting views are worth the effort.

Many of these ridge lines are heavilly timbered and subject to lightning strikes and subsequent bushfires.

But the views are worth the climb!