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Multiple deformation around Taemas Bridge 831
appear to be restricted to the Currowong Syncline (Lyons
et al. 2000; Glen et al. 2002; Packham) and Jesse Syncline
(Glen & Watkins 1999) and grade into sub-concordance
north and south, respectively, along strike. Consistent, very
gentle to gentle, tilts over many kilometres (plus or minus
some faulting: Glen & Watkins 1999) seem much more
prevalent. The Warrumba Volcanics and Dulladerry
Volcanics are clear examples representing the early Late
Devonian phase of the unconformity movement in the
Cowra Trough area. These upper Middle to lowermost
Upper Devonian (Packham) volcanics thicken continuously
from 0 m to up to ~2500 m over about 20 km northwards
beneath the Hervey Group (after mapping by
Conolly 1965; Raymond & Pogson 1998; Raymond et al.
2001). The data indicate tilt components of ~6 and ~8 for
the respective volcanics to the north. Significantly, the
discordance components across strike of the dominant
sub-meridional folds are similar or less and are likewise
consistently orientated, implying no folding of the
volcanics prior to deposition of the Upper Devonian.
(5) Sub-upper Middle Devonian unconformity Nonconformable
relationships of the Dulladerry Volcanics and
coeval Comerong Volcanics/Boyd Volcanic Complex with
underlying Emsian–Eifelian and Early Devonian granites
(Packham) represent a further and older erosion phase,
some time within the early Eifelian to early Givetian
interval. Angularity cannot be assessed from these
relations and so must be sought elsewhere. The sites
involving Upper Silurian to lower Middle Devonian strata
mentioned in point (1) would incorporate any movement of
this age. These, together with known conformable
relations of lower Middle Devonian Hatchery Creek
Conglomerate (discussed in Hood & Durney 2002) and
Boogledie Formation (Packham) with underlying Lower
Devonian limestones, suggest generally low to no angularity
for this phase.
(6) Moura Formation The single case of consistently
higher discordance angle noted by Packham—the gentle to
moderate 26 and 42 local-means of Powell et al. (1980)
between the ?Siluro-Devonian Moura Formation and
Upper Devonian in the Parkes Syncline—may not
represent ca Middle Devonian movement as the tilting
may have occurred prior to or during intrusion of the
late Early Devonian Eugowra Granite.
What are the relationships between folds in Upper
Devonian and Lower to Middle Devonian strata?
Figure 2 Early Carboniferous multiple folding of the Hervey
Group, Gilgandra–Cowra–Yass Zone. Interpreted fold groups are
identified by line symbols; faults and regolith are omitted for
clarity. B, Bumberry Syncline; H, Hervey Syncline; K, Koorawatha
Syncline; M, Mandagery Syncline; P, Parkes Syncline.
Geology after Conolly (1965) and references in Durney (2000);
interpretation by Durney.
Where the preserved Upper Devonian does not rest directly
on the older Devonian units, folds and cleavages should be
structurally correlatable between the two successions if
they had both been affected by the same folding (as
opposed to tilting) at a later time. To minimise possible
complications from (i) different degrees of folding due to
different underlying crustal competencies and (ii) spatial
variation of dominant fold trend, we compare folds and
cleavage in large areas from the same structural zone—the
Gilgandra–Cowra–Yass Zone of Scheibner and Basden
(1998)—as mentioned briefly in Hood and Durney (2002).
These are (i) the predominantly Lower Devonian rocks of
the Black Range Synclinorium (Hood & Durney 2002) and
(ii) the Upper Devonian Hervey Group to the north
(Figure 2: south of the Cowra Trough in Figure 1). Here, we
summarise the correlation for seven independent structural
attributes, with between one and four orientation
subsets each.
(1) Fold trend As shown in Figure 2, the Hervey Group
contains an identical range of fold trends and very similar
orientationally groups [northeast–north-northeast, northnortheast–north
(possibly separate north-northeast and
north trends in the Koorawatha Syncline) and northnorthwest–west-northwest]
to those in the Black Range
Synclinorium (northeast, north-northeast–north, northnorthwest–northwest
and west-northwest–west: Hood &
Durney 2002 figure 13).
(2) Fold sequence Overall fold sequence in the Hervey
Group has not yet been fully established owing to a predominance
of dome–basin interference patterns in these rocks
(Durney 2000). However, a reliable time relation (in Scott
2000 p. 60) was obtained from steep tilting of northeast-foldrelated
vein arrays on the east limb of the dominant north–
north-northeast-trending Mandagery Syncline (Figure 2)