BLACK BLUFF STRUCTURAL MAP

··. 4. 5.1 Structure of the l-1ount Read Volcanics/Dundas stratigraphic equivalent

These rocks occupy three main outcrop areas on the Black Bluff mc:>.p (See Map 4) : -

(1) A roughly triangular shaped area approximately 1 km north of the summit of Black Bluff, which is bounded at its eastern edge by the major NMV-trending Loongana Fault, discussed in Section 4.3 herein. No detailed mapping has been carried out in this area.

(2) A d01mthrown fault block apparently occupying the core of a gently N-NNE plunging open upright Sj�cline in the vicinity of 4100/4125 at the central northern boundary of the map. Two vertical cleavages, of Nand NMV strike respectively, are expressed as preferred

orientations of fine sericitic mica in the matrix of lithic wackes near 4078/4126. The cleavage of NMV strike is later (see specimen UTGD 48407).

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(3) The large area to the west and south1�est of Mt. Tor. The strati­ graphy of this area was discussed in Section 3.2.3 herein. The available bedding information is consistent 1�ith a moderately to steeply SE-dipping and younging sequence apparently unrelated to Tabberabberan structures. HoNever, a steeply dipping to vertical N-striking cleavage is present at most outcrops, and is correlated with the 111\'est Coast Range/Valentine Peak trend11 of Williams

(1978). In the vicinity of 4059/4106, this cleavage crenulates an earlier cleavage which strikes NE and dips steeply SE, sub-parallel to layering in interlayed thin bedded quartz-felspar crystal tuffs and mudstones (see specimen UTGD 48409 and UTGD 48415). This earlier cleavage is correlated 1�ith a group of NE-trending

Tabberabberan structures which dominate the Vale of Belvoir (North) and (South) maps and are also evident in the southern half of the Black Bluff map. These structures are referred to herein as the "Belvoir trend". They were not recognized as a separate distinct set by lnlliarns (1978), 1�ho in fact grouped them together with the "West Coast Range/Valentines Peak trend".

The only other structures in this area are minor folds with SE­ plunging axes near the bridge over the Leven River at 4044/4122, and at 4050/4111, and steeply dipping cleavage of the same strike at 4060/4122. The relative age of these structures is uncertain, but they may possibly be correlatd lfith the 11Deloraine/Rail ton trend11 of Williams (1978). They are parallel to strikes of steeply dipping faults lfhich offset the basal surface of the Denison Subgroup

stratigraphic equivalent south-southwest of Mt. Tor. Possible drag effects around 4090/4052 indicate sinistral strike-slip movement on these faults.

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4.5.2 Structure of the Denison Subgroup equivalent

Discussion in this section mainly concerns two subareas, BBl and BB2, coverage of 1�hich is shmm on Fig. 4.4. Lack of time has precluded detailed mapping of the Denison Subgroup stratigraphic equivalent ��est of these subareas, and the structure of the unmapped area has been interpreted from aerial photographs.

(1) Subarea BB1

The plot of bedding data for the whole subarea (Fig. 4.19A) indicates that the dominant feature is folding on gently Sl'/-plunging axes. Examination of Map 4 confirms the presence of open upright folds on this trend, with half-wavelengths of the order of 1-2 km, although photo interpretation of the area immediately northwest of subarea BB1 has indicated fold plunge to the NE. This folding is correlated with

the "Belvoir trend" of this study. However, the pattern in Fig. 4. 19A also suggests the presence of folding on axes which plunge gently to between SE and SSE. This is confirmed by a plot of bedding data from

a smaller subarea, BB1(b), within subarea BB1 (see Fig. 4.19C, and

Fig. 4 . 4 for areal coverage). Folding on this trend is correlated 1dth

the "Deloraine/Rail ton trend" of Williams (1978). Fig. 4 .19B is a plot of bedding data from another smaller subarea, BB1 (a), 1�hich covers approx-

. imately the northern half of BBl. The pattern is similar to that in Fig. 4. 19A, except that there is some indication of folding on approx­

imately S-trending axes. The latter is correlated with the "1'/est Coast Range/Valentines Peak trend" of Williams (1978).

The most commonly developed cleavage in subarea BB1 is st�eply dipping to vertical and strikes N (see Map 4) . Minor upright horizontal folding on the same trend is present in thin bedded quartz arenites at 4119/4107, at the north1�est end of the summit ridge of Black Bluff. This cleavage is also correlated with the "11/est Coast Range/Valentines

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Peak trend" of 1'/illiams (ibid.). Cleavage of "Belvoir trend" (NE strike) is apparently absent. n�o steeply dipping or vertical cleavages, of approximately N and�'/ strike respectively, are present in quartz arenites at 4112/4109 and 4114/4097. The �'/-striking cleavage at

these t1�0 localities is later, and is correlated with the "Deloraine/ Railton trend" of Williams (ibid.).

(2) Subarea BB2

Fig. 4.20A shows the dominant folding on SN-SSN trending axes in this subarea. Steeply dipping to vertical cleavage of similar strike

also begins to appear in places (see Map 4), and minor folding on the same trend is present at 4102/4077 and 4104/4070. Folding on this trend is more clearly demonstrated by a plot of bedding data and minor fold hingelines from a smaller subarea, BB2(a), within subarea BB2 (see

�. Fig. 4.20B, and Fig. 4.4 for coverage). All of these structures are correlated with the "Belvoir trend" of this study.

The only other cleavage recorded in this subarea is steeply W-dipping or vertical and strikes N, and is correlated with the

"West Coast Range/Valentines Peak trend" of Williams (1978). The pattern of the bedding plot in Fig. 4.20A suggests that folding on �'/-SE trends persists into this subarea, although no associated cleavage has been recognized. Another small subarea, BB2(b), in the vicinity of 4077/4064 within subarea BB2, displays minor folding on gently plunging N-W�'l trending axes. The correlation of this folding is problematical, as

the only other major structures of similar trend i n the Black Bluff region are those developed in the limestone of the Gordon Subgroup

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4.5.3 _{Minor structures in the Denison}

Cleavage is not extensively developed in these rocks on the Black Bluff map, its presence apparently being mainly controlled by the existence of favourable lithologies. The best development of cleavage

is seen in fine volcanic wacke conglomerates of Lithofacies I, close to the base of the sequence near 4131/4103 just southwest of Paddys Lake. These rocks have a single steeply dipping cleavage of MV-NMV strike

("Deloraine/Rail ton trend"), which consists of t1�0 main fabric elements:­ (1) Short, discontinuous dark seams (cf. "cleavage folia'' of Powell,

1969, 1977) which anastomose around and truncate the edges of the clastic grains.

(2) Very well developed preferred orientation of intergrown quartz and sericitic mica (including "beards" on the clastic grains) in the matrix of the parts of the rock between the dark seams.

The overall fabric is very similar to the intermediate stage bet1�een "rough disjunctive" and "smooth disjunctive" cleavage of P01�ell (1979)

(compare Fig. 4.21 with Fig. 4.6 ) , and is also similar to the fabrics described and illustrated by Powell {1969, 1977) from the Early Proterozoic Siamo Slate, nor�hern Michigan, U.S.A. The quartz-mica

beards are so well developed as to be at least equal in importance with the seams in defining the appearance of cleavage in outcrop and hand specimen.

As was the case in certain tuffs in the Mount Read Volcanics/Dundas Group stratigraphic equivalents on the Smiths Plains map, discussed in Section 4.4.2, some of the oriented quartz-mica intergrowth in these rocks appears to have ·formed as extensional fracture infill. Thus, different segments of the volcanic quartz clasts in Fig. 4.22A and B appear to have moved relative to each other, the opening spaces being filled with oriented quartz-mica. In the case of the grain in Fig. 4.228

this appears to have followed brittle tensional failure. The seam running sub-parallel to the length of the grain appears to be a pressure dissolution surface (tectonic stylolite). The grain in

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Fig. 4.22A however, appears to have been an embayed volcanic quartz clast, and the areas of oriented quartz-mica now occurring within it may have partly formed by recrystallization of the embayment material rather than purely by extensional fracture infilling. There has however been some relative extensional strain, especially bet1�een the two smaller segments of the grain.

Some of the quartz-mica beards appear to be actually intergrown with the edges of the quartz grains which they abut against. This effect can be seen on the l01�er boundary of the large quartz grain in Fig. 4. 23.

The formation of the beards in this case appears to have involved quartz of the original detrital grains, i.e. not just quartz in diagenetic overgrowths, as 1�as the case with the beard structures described by Boulter (1977). It may even be possible that some reaction mechanism

of the matrix clays with the quartz clasts was involved. Note that the

beard on the top side of the large quartz grain in Fig. 4.23 becomes progressively more quartz-rich as the boundary of the grain is approached.

This points to active diffusion of quartz away from the clast during

formation of the beards. Diffusion transfer of quartz has also consider­

ably modified a group of smaller quartz clasts just above the large quartz

grain in Fig. 4.23. The diffused material is seen in Fig. 4.23 as elongate dark areas consisting of fine granoblastic quartz, extending

between the group of small grains and the large grain below. For

comparison, a remarkably similar fabric was illustrated by Cowan (1974,

his Fig. 3A) in "metagraYI�acke semischist" from the Franciscan Complex, California.

165. In some of the more hematitic rocks, matrix hematite has

apparently also been involved in the formation of beards. Fig. 4.24A and B show quartz-mica-hematite beards 1�hich have formed in the matrix of a hematitic fine (?) chert breccia 1vith quartz arenite matrix

(Lithofacies III) from 4138/4089, 2 km southeast of the summit of

Black Bluff. A single vertical cleavage of ffiq-Nffiq strike is present at this outcrop.

4. 6 VALE OF BELVOIR (NOR TI-l) STRUCTURAL �lAP

4.6.1 Structure of the Mount Read Volcanics stratigraphic

These rocks occupy two isolated outcrops on the Black Bluff Range, and extensive areas on and to the east of Bonds Range. The structure of the basal parts of the Denison Subgroup stratigraphic equivalent around the boundaries of the two outcrop areas on the Black Bluff Range suggest that they occupy the cores of structural domes. These domes are

suggested to have arisen by oblique cross-folding on NE and N trends, inferred partly from the presence, in the southern outcrop area, of vertical cleavages parallel to these two trends (seP. Map 5). The dominant cleavage on the Black Bluff Range is vertical to steeply ffiq­

dipping and strikes approximately NE. This cleavage is correlated with the "Belvoir trend" of this study. Where the cleavage of N strike also occurs, it is later than the cleavage of NE strike, and is correlated with the "1\'est Coast Range/Valentines Peak trend" of Williams l.�.ns).

On Bonds Range the dominant cleavage in quartz porphyries in the �fount Read Volcanics �tratigraphic equivalent is once again vertical to steeply ffiq-dipping 1dth NE strike. Cleavage of N strike has not been detected in these rocks an)"'vhere on this part of Bonds Range. H01vever,

at the northern end of the range, a second, later, vertical cleavage

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The two cleavages and the relationship between them are particularly visible in outcrop� in the vicinity of 4123/4040 (specimen UTGD 48427). Dome and basin structure in the Denison Subgroup stratigraphic

equivalent at the northern end of Bonds Range is suggested to be due to folding on trends parallel to the strikes of the two cleavages.

4.6.2 Minor structures in the Mount Read Volcanics equivalents

As noted in Section 3.2.4 herein, the dominant rock type in the Mount Read Volcanics stratigraphic equivalent on Bonds Range and the southern Black Bluff Range is altered coarse quartz porphyry, some of which may represent hypabyssal intrusive material. The fine grained groundmass in these rocks has commonly suffered considerable pre­ tectonic sericitic alteration. This fine groundmass sericite has often developed a tectonite fabric during the later cleavage development,

in a similar manner to that described i n the Mount Read Volcanics/ Dundas Group stratigraphic equivalents on the Smiths Plains map

(Section 4.4.2). The large quartz phenocrysts have usually absorbed considerably less strain than the incompetent matrix, with the result that the foliation defined by crystallographic preferred orientation of

fine sericite in the matrix "Nraps" arol.!nd the phenocrysts.

The large quartz phenocrysts themselves appear to have deformed by a number of different mechanisms. Brittle fracture is a common

feat-··t'e, and is very obvious in the grains illustrated i n Figs. 4. 2SA

and 4.26. Many of the fractures appear to be of tensile type, and have subsequently opened during continuing strain. The fractures in the grain in Fig. 4.2SA, and those Nhich are oriented at a high angle to the prism faces of the grain in Fig. 4.26, appear to be of this type. HoNever, the fracture whose overall trend is sub-parallel to the prism faces i n Fig. 4.26 i s oriented at a high angle t o the local compressive strain

171.

(Z) direction, and relative simple shear movements have taken place along it. In s ome cases, later pressure dissolution has occurred al ong fractures with such orientations, changing their morphol ogies to that of tectonic stylolites. It is notable that the material filling the extensional fractures in the grain in Fig. 4.26 is oriented quartz­ sericite intergrowth, probably with essentially the same bulk composition as the rock matrix. This points to the operation of pressure solution processes (including dissolution, solution transfer and reprecipitation)

in the matrix.

Not all sites of reprecipitation contain quartz-mica intergro1�th, hm�ever. Some sub-planar veins consist of essentially pure quartz, in fibrous form with grain preferred orientations sub-perpendicular to the vein walls (see Fig. 4.25B). The overall attitude of such veins is usually perpendicular both to cleavage and to the dip direction in the cleavage surface. The veins probably represent reprecipitation sites of silica dissolved from the matrix, and from favourably oriented grain boundaries and internal microfracture surfaces of the large quartz

phenocrysts.

Brittle failure is not the only deformation mechani�m in the large quartz phenocrysts. Thus , in phenocrysts with mean grainsize of 5 mm

in altered quartz porphyries on the northern part of Bonds Range, intra­ granular strain has often been absorbed by formation of deformation lamellae in t1�0 or more crystall ographic orientations (see Figs . 4.27 and 4.28). It is notable that these grains are heavily embayed, and are also loaded with primary fluid inclusions which are often concentrated along growth z ones. In the grain in Fig. 4.27, the deformation lamellae appear to be partly localized by the embayments, which implies that the presence of the latter had the effect of mechanically weakening the grain.

176.

The grains may have also been hydrolytically 1�eakened by the presence

of the copious fluid inclusions. However, these very coarse phenocrysts

are the only quartz grains in the Mount Read Volcanics/Dundas Group

stratigraphic equivalents in the whole Black Bluff region in which this type of intragranular strain effect has been observed. This suggests that the dominant control may be grainsize in the manner suggested

by the analysis of deformation mechanism maps by IVhite (1976) . Examin­

ation of l�ite's diagrams indicates that at particular conditions of

temperature and differential stress, coarse grained quartz may fall in the. field of dislocation creep, while fine quartz falls in the field of Coble Creep (1�hich may be partly or completely replaced by a field of pressure solution, as demonstrated by Rutter, 1976).

Some of the large phenocrysts, most noticeably in quartz

porphyries on Bonds Range east of Lake Lea (e.g. specimen UTGD 48426)

appear to have also suffered considerable diffusion of quartz, particu­ larly _{on crystal faces oriented at a high angle to the cleavage trace} on vertical sections perpendicular to cleavage. A similar effect was

noted in Section 4 . 5.3 in discussion of quartz-mica beards in volcanic

wacke conglomerates at the base of the Denison Subgroup stratigraphic

equivalent near Black Bluff. However, the diffusion is much more pronounced in these rocks. A commonly occurring texture is illustrated in Fig. 4.29, where an almost hourglass-shaped area of diffused and recrystallized quartz has formed in the area between two adjacent large phenocrysts. Much of the recrystallized quartz has a granoblastic fabric with only weak grain elongation fabric, but it is intergrown

with presumably recrystallized matrix mica and opaques having strong

lattice preferred orientation. The quartz content of the hourglass­

shaped area decreases away from each of the two phenocrysts and is at a minimum approximately at the midpoint bet1�een them .

179.

It is doubtful that the structure il lustrated in Fig. 4.29 is purely a dilational feature; that is, it is definitely not envisaged

that the two phenocrysts were originally together. The amount of

extensional strain in the space bet\'leen the phenocrysts is probably

only a small to moderate fraction of the present distance between them. The main reason for this conclusion is the observation that the strong

preferred orientation of mica, opaques and, to a less extent, quartz ,

in the hourglass-shaped area rapidly becomes very much weaker laterally

away from the area bet\'leen the phenocrysts. It is therefore suggested

that the structure has formed mainly by in situ recrystalli zation of the

rock matrix during moderate extensional strain, with stress-controlled diffusion of quartz away from the boundaries of the two phenocrysts

tO\'Iards the mid-point bet\'leen them . A similar mode of origin is

envisaged for the quartz-mica-iron oxide beard structure between the t\'IO

large quartz phenocrysts in Fig. 4.28, although in this case the dilation may be a higher percentage of the present space between the

phenocrysts .

Tectonic preferred orientations of fine matrix sericite are best

devcloP.ed in rare sedimentary parts of the Mount Read Volcanics strati-

graphic equivalent on the Vale of _Belvoir (North) and (South) maps. The best developed examples occur in strongly cleaved volcanic quartz

wackes near 4123/4040 on the northern Bonds Range. n"o sub-vertical cleavages , respectively of NE strike (local s 1 , "Belvoir trend") and

NNE strike (local S2, "Bonds Range trend"), are present in outcrops at

this locality. The appearance of the sericite fabrics is illustrated in

Fig. 4.30A and B . The development of mica films bet\'leen the quartz

clasts in Fig. 4. 30A appears to have been associated \'lith concomitant

mutual pressure dissolution of the quartz grain boundaries. The sections