Geological note: New field evidence resolving the relationship between the Grampians group and the Rocklands Rhyolite,western Victoria

The Rocklands Rhyolite is a latest Silurian to Early Devonian sequence of silicic ignimbrite, lava, volcanic sedimentary rocks and dykes in western Victoria. These volcanic rocks lie west of the Grampians Ranges, which consist of a thick succession of quartz sandstone of presumed Silurian age called the Grampians Group. The previously unresolved stratigraphic relationship between these two sequences of rocks is clarified by an exposed contact between steeply dipping Grampians Group cut by quartz veins, and overlying undeformed rhyolite. The implications of this relationship are that the Grampians Group is older than the Rocklands Rhyolite and that parts of the sandstone succession were locally deformed prior to volcanism. Furthermore, other outlying areas of sandstone and rhyolite, previously correlated with the Grampians Group and Rocklands Rhyolite, respectively, display different timing relationships and are proposed to be significantly younger.