Engineers Australia

Intrusions in the Sydney Basin with reference to the Greenacre Dyke

H. Baxter-Crawford

Abstract

Doleritic to basaltic dykes ranging in width from centimetres to several metres and with strike lengths tracing up to several kilometres on the mainland intrude the sedimentary country rocks of the Sydney Basin. Exposure of the Greenacre Dyke in the middle of the Enfield Brickworks presented a unique opportunity to collect textural and structural data from a dyke in Sydney. Evidence collected from the 3D exposure of the Greenacre dyke indicates the magmatic source was located towards the coast, possibly offshore. Published coastline mapping suggests a higher prevalence of dykes than have been discovered within the Basin proper. The limited intersections of dykes away from the coast is likely due to the dense coverage of urban development in greater Sydney and that few dykes intersected in excavations and tunnels have been publicly documented. This paper presents the data collected for the Greenacre Dyke and uses indicators of emplacement mechanism to postulate a change in the regional geological understanding of dykes in the basin. A review of the dyke orientations in the Sydney Basin was undertaken with a view to investigate possible magmatic origins. The orientation of the Greenacre Dyke, dykes presented on the Sydney 1:100 000 geological sheet and other published or mapped dykes were traced to see if they coalesced to a single origin, indicative of the magmatic source. The result was multiple dykes radiating from a series of point locations; locations that align along the remnants of the Gerringong Volcanic Ridge, some 10 to 20 km off the present Sydney coast line. This volcanic arc was active in the late Permian, through to the early Triassic. Age dating of the dykes implies a Jurassic age, a period that coincides with a gap in the stratigraphic record of the Basins sedimentary sequences. Apatite fission track studies suggest the Basin was buried some 2.5 to 3 km deeper than present during the Jurassic before later uplift and erosion to its present position. This paper discusses the possibility that the Gerringong Volcanic Ridge was active during the Jurassic and is the source of the intrusive dykes. It discusses whether the heat and pressure generated by 3 km burial depth was sufficient to have reactivated the assumed inactive magmatic source, but prevent extrusive volcanism. Evidence from the Greenacre dyke is presented that indicates the country rocks were in a semi-ductile phase during intrusion and how this relates to the depth of Basin burial at the time.