Title: Characteristics and Origin of Proximal Volcanic Breccias in Hydromagmatic Tertiary Basaltic Centres, in South-East Tasmania.
By: Isabella J. von Lichtan
Supervisor: Dr. Sharon Allen
Three basaltic centres (Apsley, Firthside, and Inglewood) in South-East Tasmania, show a range of eruptive mechanisms from a dry explosive and effusive style to dry and wet hydromagmatic. The source of water for these eruptions were shallow aquifers within siltstone, or at siltstone/dolerite, sandstone/dolerite contacts.
Apsley volcanic centre represents the lower portion of a diatreme, with a series of matrix-supported volcanic sandstones and breccias. Facies are distributed concentrically, with near vertical contacts between facies. The outer facies are the most poorly sorted, lithic-rich and fine grained (sandstones), and the inner most are medium sorted, juvenile- and lithic-rich breccias. These facies are interpreted as being formed by hydromagmatic explosions caused by the interaction of magma with groundwater. Country rock lithic clasts consist of dolerite, sandstone, and siltstone suggesting the aquifer source occurs at a contact with these rock types.
Firthside volcanic centre consists of six facies groups, and stratigraphy combined with contact relationships suggest the sequence is: sedimentary facies, altered juvenile-rich facies, scoria facies, coherent basaltic facies, polymictic breccia dykes, and flat lying juvenile-poor facies. The sedimentary facies are made up of massive beds of poor to medium sorted, polymictic clasts. The beds are wedge shaped and some contain rounded boulders at the base. This facies probably represents part of a fluvial sequence. Altered juvenile-rich facies and scoria facies are of primary eruptive volcanic origin. Altered juvenile-rich facies are crudely stratified with medium to thick beds of well to poorly sorted, angular, dominantly juvenile clasts, which are altered and have infilled vesicles. When matrix is present, it includes grains and crystals of volcanic and sedimentary origin. Most of the beds in this facies show draping. This facies is interpreted to be a phreatomagmatic fall deposit containing lithic clasts from the subsurface and surface explosions. In comparison, the scoria facies are clast-supported made up of thick beds of medium sorted, highly vesicular juvenile bombs deposited by fallout from a dry explosive magmatic eruption. This sequence shows a change in the magma: water ratio, from a wet to dry eruption. Coherent basaltic facies is very-fine grained, highly fractured, and intrudes the scoria facies. Cross cutting relationships suggest it is synchronous to, or postdates scoria cone formation. Polymictic breccia dykes are massive, matrix-supported, poorly sorted, and juvenile-poor. These cross cut both the scoria faces and coherent basaltic facies suggests it postdates both facies. Flat-lying juvenile-poor facies are massive and poorly stratified beds with poorly sorted polymictic clasts in a siltstone matrix. This facies has been interpreted as cohesive debris flow deposits and are probably generated by slope failure.
Inglewood volcanic centre represents a tuff-ring remnant showing a sequence of clast-supported to matrix-supported generally planar bedded basaltic and lithic lapilli. Clast-supported facies are thinly stratified with some sandwave beds, medium sorting, and dominated by basalt lapilli with 30% country rock lithic (dominantly dolerite) clasts. This facies is interpreted to have been formed by relatively dry surges, and subordinate ash fall, from phreatomagmatic explosions that involved a limited amount of water during fragmentation. In comparison, matrix-supported facies are finer grained, thinly stratified with well-developed planar bedding, poorer sorting, and a high proportion of country rock lithic (both dolerite and sandstone) clasts. It also includes sand sized quartz grains in the matrix. This facies is interpreted as having been formed by eruptions that contained a higher magma: water ratio than the clast-supported facies. The country rock lithic clasts suggest the the aquifer source for the clast-supported facies is dolerite, and for the matrix-supported facies is a dolerite/sandstone contact.
von Lichtan, I.J., 2000. Characteristics and Origin of Proximal Volcanic Breccias in Hydromagmatic Tertiary Basaltic Centres, in South-East Tasmania. Geology Department, University of Tasmania, unpublished honours thesis.