Gravity and magnetic investigation of the Marulan Supersuite around the Marulan area
The Marulan Supersuite is a set of Early Devonian granitic intrusions in southeast Australia, intruded into the Bindook Group, which is a set of volcanic and volcanogenic sedimentary rocks of the same geological period. This igneous setting is emplaced within a composite of sedimentary rocks of the Ordovician Adaminaby Group, Silurian Bungonia and Mount Fairy Groups. The Ordovician and Silurian units on the west are thrusted against the igneous setting on the east by the Middle Devonian Yarralaw Fault, which is also suggested to serve as a structural barrier during the Early Carboniferous Kanimblan Orogeny. The Late Devonian sedimentary rocks unconformably overlie both the Early Devonian igneous units in the east and the Ordovician sedimentary rocks in the west of the Yarralaw Fault. Both, the Marulan Supersuite and the Bindook Group show a defining magnetic response, whereas their gravity response is indefinite. Surface geological and structural observations suggest that the Kanimblan Orogeny resulted in strong folding of the Late Devonian units to the west of the fault, whereas it has minimally affected the units east of the Yarralaw Fault and is only observed as broad open folds in the Late Devonian Strathaird Formation.
The aim of this study is to evaluate whether the Early Carboniferous Kanimblan Orogeny has caused widespread deformation of the geological units east of the Yarralaw Fault. To investigate this question, this study focused on the acquisition and analysis of potential field data of the Marulan Supersuite and its adjacent units in order to construct geological profile and 3D models.
The laboratory measurements showed that the Marulan Supersuite and the Bindook Group have a wide range of magnetic susceptibilities, 0.006–0.033 SI and 0.001–0.046 SI, respectively. The acquired density data demonstrated overlapping values for the Marulan Supersuite and the Bindook Group, spanning a range between 2.67– 2.87 g/cm3.
The gravity anomaly map produced by Geoscience Australia indicated a signal from a deeper gravity source; thus, a new gravity dataset was acquired in this study with a closer (500 m) sampling interval to investigate if the Marulan Supersuite can be better delineated between the volcanic units and the neighbouring sedimentary units. However, no significant improvement of the detailed gravity grid was observed.
The profile magnetic modelling in this study was applied in order to define the field relations and the contact characteristics between the different units in the study area. The results demonstrated that the contacts of the Marulan Supersuite with the Bindook Group are dipping to the east, whereas the Bindook Group units form into a tight synclinorium structure. Magnetic modelling further revealed that the Kerrawarra Dacite Member (west of the Marulan Granite) is a hinge of an anticline. Moreover, it showed that the Barrallier Ignimbrite forms a synclinal structure underlain by the Kerrawarra Dacite Member. Additionally, magnetic modelling of this study established that the Lumley Granite and the Torwood Granodiorite of the Marulan Supersuite underlie the folded Late Devonian Strathaird Formation in the southwestern study area.
To further illustrate the subsurface structure of the Marulan Supersuite the profile models were extended along strike to produce 3D geological models. The results showed that the Marulan Supersuite is tilted to the west with angles ranging from 14-38°. These models further exhibit less pronounced tilting of the axial planes of the Bindook Group folds. Furthermore, 3D modelling successfully demonstrated that the Marulan Supersuite is intruded into an axial plane of the Bindook Group.
The application of the potential field modelling in this study demonstrated that the deformational effects of the Kanimblan Orogeny do extend further east into the units of the Marulan Supersuite evidenced by the tilting of the Marulan Supersuite. However, this deformation is not as widespread as observed west of the Yarralaw Fault, supported by the low strain deformation in the Marulan Supersuite.