Structure of the Upper Devonian Merimbula Group, Lachlan Orogen, Southeastern Coastal New South Wales


  • Christopher Fergusson


The Upper Devonian Merimbula Group (NSW South Coast) is dominated by northerly-trending folds developed at several scales and indicative of buckling. In the northern area (west of Nowra), moderately to gently dipping limbs of the Budawang Synclinorium are exposed. In upper Ettrema Creek, distinct thinbedded graded sandstone-mudstone are affected by abundant mesoscopic folds. Farther south, the southern Budawang Synclinorium along Bumbo Creek consists of three map-scale folds with locally common mesoscopic folds of upright, open folds and angular monoclinal folds. The western contact with the underlying Adaminaby Group is repeated by a steep fault. The southern end of the Budawang Synclinorium is a simple, upright, open syncline, with a faulted western limb. In the Pambula area (north of Eden), the eastern Bald Hills anticline is an upright, gentle fold whereas the western Box Range syncline has a wide flat-lying keel with common angular monoclines. Shortening is greatest in the southern Budawang Synclinorium (35%) and is 19-20% west of Nowra and 12% or less in the Pambula area. Locally, rigid granitic basement has resulted in reduced shortening in the overlying Merimbula Group.