0 VOLCANIC FACIES IN STAVELY DRILLCORE PRELIMINARY OBSERVATIONS Report for Geoscience Australia Attn: Dr David Huston Jocelyn McPhie McPhie Volcanology www.mcphievolcanology.com.au March 2015
1 Introduction Geoscience Australia drilled several holes into poorly exposed, ~500 Ma volcanic rocks in the Stavely area, western Victoria. This report summarises my observations and preliminary conclusions based one day of reviewing selected drill core. The report includes rough graphic logs of the holes based on my observations and detailed written logs prepared by Geoscience Australia geologists. Stavely 17 This hole contains a very thick section (downhole 156 m) of massive to weakly graded sandstone interbedded with minor laminated mudstone. The thick beds and weak grading imply deposition below wave base from turbidity currents. There are no obvious volcanic components (I am guessing, given the fine grain size) so a basement provenance is inferred. Igneous rocks occur at three depths. The top two occurrences (23.3 to 26.2 m and 46 to 51 m) are moderately to strongly altered, feldspar-ferromagnesian-phyric, and probably intermediate in composition. The upper interval has a porphyritic texture (Fig. 1) and sharp contacts. The lower interval appears to be equigranular (~1 mm grain size; Fig. 2), has a sharp upper contact and brecciated lower contact. Both these intervals are interpreted to be intrusive, based on the contacts and the igneous lithology. The sharp passive contacts are consistent with the sandstone being lithified when intruded. The moderate to strong alteration that has affected the igneous intervals and the abundant veins in the adjacent sandstone presumably both relate to hydrothermal activity initiated by the intrusions. Figure 1. Altered weakly porphyritic andesite(?), Stavely 17, ~24 m.
2 Figure 2. Altered equigranular microdiorite(?), Stavely 17, ~48 m. The lowest occurrence is a narrow (10 cm), irregular interval of weakly altered basalt (Fig. 3). The basalt is unlikely to be a clast because it is far coarser than the average grainsize of the sandstonemudstone that contains it, and it has an intricately irregular shape that would not have survived transport in a turbidity current. It is likely to be a very small intrusion presumably connected to a larger nearby feeder (dyke or sill). The irregular shape suggests that the basalt intruded the sandstone-mudstone before it was consolidated. Figure 3. Narrow interval of basalt with irregular contacts with grey mudstone, Stavely 17, ~127.5 m.
3 Stavely 09 This hole intersected ~40 m of weakly altered, mafic igneous rocks in which there is no obvious evidence of bedding. The top ~24 m mainly comprises dark green, aphyric amygdaloidal basalt (Fig. 4). There are at least four narrow (<1.5 m wide) intervals of monomictic basalt breccia in which the clasts are separated by white siliceous cement (Fig. 5). These breccia intervals are probably hydraulic in origin, formed where over-pressured fluid has locally brecciated the coherent basalt. The siliceous cement is likely to have been deposited from the over-pressured fluid that caused the brecciation. Close to the top, there are two narrow intervals of dense (non-amygdaloidal) basalt with sharp contacts against the amygdaloidal basalt (Fig. 6). The sharp contacts and cross-cutting relationships suggest that these intervals are narrow dykes or sills. Figure 4. Dark green, aphyric amygdaloidal basalt, Stavely 09, 138.8 m. Figure 5. Monomictic basalt breccia with siliceous cement, Stavely 09, 145 m.
4 Figure 6. Aphyric dense basalt in sharp contact (arrow) with aphyric amygdaloidal basalt, Stavely 09, ~130 m. The lowermost ~15 m comprises equigranular, medium grained (1-2 mm) dolerite in which feldspar and a ferromagnesian phase can be recognised (Fig. 7). The dolerite has a sharp contact with the overlying aphyric basalt, and the top ~20 cm of the dolerite is very fine grained (Fig. 8). Below about ~160 m, the dolerite has a spotty appearance due to epidote and the grainsize is slightly coarser than above. Given the sharp chilled topmost contact, and the equigranular texture, it appears that the dolerite has intruded the aphyric basalt. Figure 7. Equigranular dolerite, Stavely 09, ~156 m.
5 Figure 8. Top, very fine grained (chilled) margin of dolerite, Stavely 09, ~150 m. The basaltic section would have accumulated close to a basaltic volcanic centre that was mainly producing lavas, and the lavas accumulated without interruption because there is no interbedded volcaniclastic or sedimentary facies. This section does not provide any constraints on the depositional environment. The dolerite intrusion could be related to the basaltic volcanic section, being part of the deeper magma feeder system of a basaltic volcano, or unrelated. Data on the compositions of the basalt and dolerite could help distinguish between these two possibilities. Geochronology could also help, providing apatite or zircon or another dateable mineral is present.
6 Stavely 04 This hole intersected ~45 m of chlorite-sericite-epidote altered, amygdaloidal basalt in which there is no obvious evidence of bedding. The basalt is either aphyric (Fig. 9) or weakly porphyritic, having a locally visible population of altered ferromagnesian phenocrysts (~1 mm, <5 %). There are three intervals (56.5 to 59.5m, 62.1 to 63.6 m, 75.2 to 81.9 m) where the basalt contains highly irregular domains of black or red silicified, massive mudstone (Figs 10, 11, 12). In these domains, the basalt is locally brecciated and dismembered, forming clasts in mudstone matrix. From 81.9 to 88.4 m (below the lowest interval), the basalt contains narrow (mm cm), wispy and fluidal lenses of mainly red silicified mudstone. Figure 9. Weakly sericite-altered aphyric basalt, Stavely 04, 89 m. Figure 10. Fluidal lenses of silicified red mudstone in aphyric basalt, Stavely 04, ~57 m.
7 Figure 11. Red mudstone-matrix basalt breccia, interpreted to be blocky peperite, Stavely 04, ~78 m. Figure 12. Red mudstone-matrix basalt breccia, interpreted to be blocky peperite, Stavely 04, ~80 m. The intervals of basalt that contain significant mudstone are readily interpreted as peperite, a facies that forms from the mingling between molten magma (in this case, basalt) and unconsolidated sediment (in this case, mud, now mudstone). Peperite forms subsurface where magmas interact with sediment piles, and can be connected to dykes, sills or irregular intrusions of any dimensions. The peperite in this section implies that the basalt was largely intrusive but intrusion took place at shallow depths where mud remained unlithified. Peperite can occur in subaerial settings but given that the sediment in this case was mud, it is most likely that the basalt intruded mud that had accumulated on the seafloor. Substantial thicknesses of basalt can be emplaced by means of shallow intrusions. The Hellyer Basalt in the northern Mount Read Volcanics (western Tasmania) is typically >100 m thick and up to ~250 m thick. Intermingled mudstone (equivalent to the Que River Shale) occurs throughout sections through the Hellyer Basalt, and no evidence for conventional extrusive emplacement has yet been found.
8 Stavely 07 Only the top ~150 m of this long hole (total depth 501. 6 m) was available for review. There is no evidence of bedding in the drill core reviewed. Almost of all of the section comprises feldsparferromagnesian-phyric andesite(?). The andesite(?) locally has well-preserved relic perlitic texture in the groundmass and is weakly amygdaloidal. There could be two emplacement units of only slightly different andesite(?) (Fig. 13), separated by ~55 cm of mudstone at ~98.05 m (Fig. 14). The upper andesite(?) is cut by narrow (<1 m), irregular, green basaltic dykes at 35.2 m and 53.25 m (Fig. 15). The top contact of the lower andesite(?) (at ~98.6 m) with mudstone is sharp (Fig. 14) and the adjacent andesite(?) is not brecciated. It could be that the andesite(?) is intrusive but at very shallow levels because its groundmass was originally glassy. The intervals of porphyritic coherent facies (andesite and/or dacite) in this hole are substantial; clastic facies account for less than 1% of the total depth (~500 m). Such dimensions and the dominance of coherent facies typically correspond to the proximal parts of domes or dome complexes, and their intrusive equivalents (cryptodomes). Because the orientation of the hole in relation to regional bedding is not known, the actual dimensions of the units in Stavely 07 may be much smaller.
9 Figure 13. Feldspar-ferromagnesian-phyric andesite(?) in Stavely 07. Top: upper andesite, ~47 m; middle: lower andesite, ~120 m; lower: quartz-filled amygdales in the lower andesite, ~120 m.
10 Figure 14. Sharp contact between grey mudstone (LHS) and feldspar-ferromagnesian-phyric andesite(?)(rhs), Stavely 07, 98.6 m. The groundmass of the andesite(?) is perlitic. Figure 15. Narrow, irregular, green basaltic dykes intruding the feldspar-ferromagnesian-phyric andesite(?) at 35.2 m (top) and 53.25 m (lower) in Stavely 07.
11 Stavely 02 This hole intersected roughly 70 m of both coherent and clastic facies. The coherent facies are predominantly amygdaloidal basalt that locally includes possible pillows (around 121 m) and is cut by a non-amygdaloidal basalt dyke (~118 m). There are two different clastic facies. In the lower ~ 20 m of the hole, there are intervals with a distinctive clastic texture comprising fluidal, amygdaloidal basalt clasts with chilled margins and minor angular basalt clasts ( fluidal-clast basalt breccia ). This breccia is monomictic and derived from weak fountains of highly vesicular basalt on the seafloor. The fountains break up into droplets of molten basalt that chill and fall out around the vent; some of the smallest droplets quench-fragment entirely, producing angular basalt clasts that are deposited with the fluidal clasts. This is a near-vent facies, usually found within tens of m of the vent(s) that produced it. The second clastic facies occurs near the top of the hole (~98 to 115 m) and comprises polymicitc volcanic breccia. Although polymictic, the clasts are mainly amygdaloidal basalt and ferromagnesianphyric andesite(?). The breccia is not bedded or internally organised. This breccia probably records a mass-wasting event from a local basalt/andesite source. Collectively, the association of facies in this hole are broadly proximal, comprising basaltic lavas and fluidal-clast breccia, and thick, coarse polymictic volcanic breccia that accumulated in proximity to an active basaltic-andesitic volcano. There are no non-volcanic sedimentary facies, implying that the accumulation rate was too high for such facies to be included. The fluidal-clast breccia, and of course, the pillows if confirmed, provide evidence of a subaqueous depositional setting.
12 Stavely 16 Almost all this hole (from ~288 to 354 m) comprises very thick, polymictic volcanic breccia (Fig. 16) that is lacking in any internal organisation. The true thickness is not known because the orientation of the drill hole in relation to regional bedding is not known. The lowermost ~3 m (from 354 to 357.7 m) of the hole intersected feldspar-ferromagnesian-phyric andesite(?) that could be a large clast or in situ andesite(?). The polymictic clast assemblage includes green and dark green feldspar-ferromagnesian-phyric andesite(?), highly vesicular green basalt(?), red basaltic scoria and red finely feldspar-phyric rhyolite(?). The latter clasts are particularly conspicuous in being very coarse (many are coarser than 20 cm) and abundant. Some of the largest rhyolite(?) clasts contain fractures that are filled by the matrix (Fig. 17), some occur in jigsaw-fit clusters, and some are weakly banded. Figure 16. Polymictic volcanic breccia in Stavely 16, 294-295 m.
13 Figure 17. Polymictic volcanic breccia in Stavely 16, 338-339 m. Note the matrix-filled fractures (arrow) in the red weakly feldspar-phyric rhyolite(?) clast in the lower core, and the cluster of red weakly feldspar-phyric rhyolite(?) clasts (arrow) in the upper core. The polymictic volcanic breccia in Stavely 16 does not provide any constraints on the depositional setting. The components indicate derivation from a volcanic source that included rhyolite, andesite and basalt compositions. The breccia contains a significant proportion of red and purplish red clasts, including red scoria. Redness in volcanic clasts or in lavas may be the product of thermal oxidation which is restricted to subaerial settings (thermal oxidation requires cooling of hot clasts or lava in contact with the atmosphere). One implication is that the source of the clasts was at least partly subaerial. Oxidation that produces reddening may also be the result of hydrothermal processes. If the red colour of the clasts in the breccia is hydrothermal in origin, then the source area included hydrothermally altered volcanic rocks (subaerial or submarine). The coarse grain size, angular clast shapes and lack of organisation suggest that the breccia is the product of catastrophic mass-wasting event involving the partial failure of a volcanic edifice. Given the high abundance of the red weakly feldspar-phyric rhyolite(?) clasts, and the presence of prepared fractures in these clasts, a dome-collapse event is plausible. Assuming the true thickness is in the order of a few tens of m or more, the thickness and coarse grain size imply relative proximity to the source (10 s m to perhaps 1-2 km?).
14 Recommendations Assuming there is an unlimited budget, additional research could be carried out on the Stavely drill holes. Prepare graphic logs to accompany the detailed written logs. Complete systematic petrography and geochemistry (especially immobile trace elements and mineral chemistry on secondary phases) on the main coherent units, and on selected clasts in the polymictic breccia units (e.g. in Stavely 02), in order to confirm field names, to identify original geochemical affinities, and to determine the mineralogy, textures and relationships among the secondary phases present. Complete systematic geochronology on the main units, in order to determine the ages and duration of volcanic activity, and to identify regional correlations.