CAPÍTULO 2: MARCO DE REFERENCIA
2.2 La dimensión religiosa
MRF 2 comprises two facies associations; 1) intercalated basaltic andesite volcaniclastic breccia, volcanogenic sandstone and siltstone (VBX I/VSST), interbedded with black mudstone, and 2) rhyolite volcaniclastic breccia in volcaniclastic sandstone (VBX II/VSST).
In general, the VBX I/VSST facies type coarsens upsection, with coarse-grained breccia dominating the upper parts of stratigraphic profiles (e.g. Fig. 3.2). Throughout, basaltic andesite clasts are the main components. Intervals of the quartz-bearing and relatively pumice-poor VBX II/VSST facies association occur at a number of intervals in the upper Marianoak Formation, but most commonly towards its top.
Facies
Association
VBX
I/VSST:
basaltic
andesite
volcaniclastic
breccia/volcaniclastic sandstone
The VBX I/VSST facies association overlies MRF 1 stratigraphy and laterally has a highly variable thickness that ranges from ~130 m in hole R10032 to a maximum of 600 m in hole R10063 (Figs. 3.1 and 3.2).
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Facies 5: Volcaniclastic sandstone and siltstone.
This facies comprises graded beds of 1-15 m thickness. Beds generally have poorly sorted polymictic bases of medium to coarse grained volcaniclastic sandstone with light grey to dark grey, angular to sub-rounded clasts (0.5-1.0 mm, 5%) and minor quartz crystal fragments in a sericite-altered matrix that grades into volcaniclastic siltstone (Fig. 3.10). Beds within this sequence can be separated by ~1- 20 m thick, dark grey and pyritic mudstone beds (Figs. 3.1 and 3.2).
Figure 3.10. Poorly sorted polymictic volcaniclastic sandstone containing fine grained basaltic andesite clasts and less abundant quartz clast, set in a sericite-altered fine grained matrix. R10063: 610 m. XPL
Facies 6: Polymictic basaltic andesite breccia
This facies comprises ~15-20 m thick graded beds with polymictic bases of very coarse grained volcaniclastic breccia dominated by dark green basaltic andesite clasts (1 mm-5 cm, 10-15%), minor light grey angular to sub-rounded aphanitic clasts and mudstone lithic clasts (3-4 cm, 1-2%) that grades to stratified volcaniclastic sandstone and siltstone top (Fig. 3.11).
54 These framework components are set in a volcaniclastic sandstone matrix with minor angular to sub-rounded and fractured volcanic quartz (0.5-1 mm, 1-2%) and minor euhedral feldspar crystals (1-2%) that grades to volcaniclastic sandstone (Figs. 3.12 A & B). The facies is interbedded with quartz-carbonate altered black mudstone beds of ~1-8 m that are in turn interbedded with volcaniclastic siltstone.
Facies 7: Dark grey mudstone
At the top of MRF 2 in holes R10032 and BP 272, mudstone with bed thicknesses of 160-220 m is interbedded with minor volcanogenic sandstone and siltstone of VBXI/VSST (Fig. 3.1 & 3.2). It is generally massive to weakly cleaved, dark grey in colour, with intense quartz-carbonate veining and fine, locally pyritic, lamination. A very thick interval crops out to the west of Rosebery Township, forming the principal lithofacies in what is conventionally referred to as the Chamberlain Shale (Brathwaite, 1970; Green, 1983; Parfrey, 1993).
Interpretation
Although VBX I/VSST is strictly polymictic, the predominance of basaltic andesite clasts throughout the facies association indicates derivation from a relatively uniform volcanic source. The general upsection increase in grain size suggests progradation of the sediment input system with time.
The clast textures and shape (i.e. angular to sub-rounded) along with a lack of pumice fiamme suggests that original clasts were dense and therefore are possibly derived from an effusive eruption, with variable reworking from the source. Thick beds of 15-20 m with normally graded and stratified tops suggests their deposition from water supported gravity flows, most probably high concentration density mass-flow currents or debris flows (Cas and Wright, 1991). The increased abundance of large volcanic clasts with minor mudstone lithic-clasts towards the base of beds implies density sorting, and the mudstone rip up lithic clasts at the base of the beds are interpreted
55 to be scoured and eroded from unconsolidated mud substrate during the lateral transport of mass flows (Allen, 1991; McPhie et al., 1993).
Mudstone beds preserved between the mass-flow deposits record below wave base pelagic sedimentation. They represent periods of basin starvation and probable volcanic inactivity, the great thickness of some the intervals reflecting particularly prolonged hiatuses.
Figure 3.11. VBX I geometry and textures in R10063. In A) & B) Basaltic andesite (Bas-And) clasts dominated breccias grading to volcaniclastic sandstone (e.g., C).
56 Figure 3.12 (A). Polymictic volcaniclastic breccia dominated by basaltic andesite (Bas-And) clasts set in quartz-phyric volcaniclastic sandstone matrix and (B). Photomicrograph of polymictic volcaniclastic breccia with angular to sub-rounded basaltic andesite (Bas-And) clasts and quartz crystal fragments. R10063: 416.40 m. XPL.
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Facies 8: Rhyolitic volcaniclastic breccia with volcaniclastic sandstone and siltstone
The rhyolitic volcaniclastic breccia interbedded with volcaniclastic sandstone and siltstone VBX II/VSST intercalates with VBX I/VSST mainly, but not exclusively, within the upper parts of coarse sequence of MRF 1 except in hole 397R (Figs. 3.1 & 3.2). Individual bed thicknesses are about 2-17 m with very coarse bases grading to stratified siltstone top beds ~ 1m except in hole R10035 where a 60 m thickness is attained (Fig. 3.2).It is composed of coarse and poorly sorted 1-6 cm quartz-phyric, monomictic, rhyolitic clasts and minor 1-6 cm feldspar-phyric pumice fiamme that grade into volcaniclastic sandstone and stratified siltstone (Fig. 3.2). It also comprises angular to sub-rounded quartz crystal fragments (2-3 mm, 7-10 %), euhedral feldspar crystals (0.25-0.5 mm, 1-2%), and former glass shards in a strongly sericite altered matrix (Fig. 3.13). Unlike the well graded beds in most of the holes (R10035, BP 273 & R10063) locally jigsaw fit texture of quartz phyric rhyolite clasts with planar and curviplanar margins in intensely silicified matrix is observed in hole R10032.
Figure 3.13. Photomicrograph of rhyolitic volcaniclastic breccia, VBX II/VSST, showing quartz (Qtz), feldspar (Fsp) crystal fragments and rhyolitic clasts (Rhy) from hole R10032 at 226 m. XPL
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Interpretation
The VBXII/VSST facies association is composed of dominantly monomictic quartz-phyric rhyolite clasts that are poorly sorted, normally graded with stratified tops indicative of its deposition from water supported gravity flows, possibly high concentration density currents soon or after eruption (Gifkins, 2001). The paucity of pumice clasts and dominance of monomictic rhyolitic clasts indicates explosive eruption is unlikely and it is possibly sourced from reworking of effusive volcanism of rhyolitic composition. The local jigsaw fit textures with minor curviplanar clast morphology of monomictic quartz-phyric rhyolitic clasts at the base of hole R10032 is consistent with quench fragmentation in a subaqueous environment (McPhie et al., 1993; Cas and Wright, 2012).