New Zealand Journal of Geology and Geophysics abstracts
Blue Spur Conglomerate: auriferous Late Cretaceous fluvial channel deposits
adjacent to normal fault scarps, southeast Otago, New Zealand
B. G. Els*
J. H. Youngson
D. Craw
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
*Present address: Department of Earth Science, University of Pretoria, Pretoria
0002, South Africa.
Abstract Latest Cretaceous Blue Spur Conglomerate
at Gabriels Gully was deposited in a northeast-southwest-trending valley
incised >200 m into metamorphic basement within the Tuapeka Fault Zone.
Clast imbrication unequivocally indicates paleoflow towards the southwest,
across the Tuapeka Fault Zone from the uplifted block. Early schist- and
quartz-rich fluvial sediments in the valley and transverse mass flow deposits
derived from the valley sides are preserved on a strath terrace on the southeast
side of the valley. These sediments were incised by the paleoriver and partially
reworked into the base of a deeper channel cut into the underlying schist
before deposition of greywacke-rich fluvial conglomerates. There is a strong
enrichment of gold on and just above schist at the base of the incised channel,
and subordinate enrichment at and above the basal scours of the overlying
fluvial conglomerate packets. Imbricated clasts at less well exposed Blue
Spur Conglomerate at Wetherstons, Forsyth, and Waitahuna Gully suggest that
southwest-flowing rivers in incised paleovalleys crossing the Tuapeka Fault
Zone also deposited those outliers. The four incised paleovalleys were probably
fed by a single river that successively abandoned the valleys in favour of
a new course around the propagating tip of the Tuapeka Fault Zone. Structural
and geomorphic relationships along the Tuapeka Fault Zone suggest that it
is multi-stranded and segmented on the scale of a few kilometres, rather
than being a single continuous structure. Independent and concomitant extension
on collinear and overlapping strands, respectively, was the primary control
on fluvial incision into intrafault zone blocks and on gold concentration
within the valleys. Transverse topographic highs across the fault zone, between
sediment sinks on the downthrown side, are probably the result of distributed
deformation amongst and between strands. These relative highs controlled
the location, size, and orientation of the depositional systems, and prevented
the development of axial drainage along the Tuapeka Fault Zone itself. Valley
incision and associated gold-concentrating processes will not have extended
significantly downstream from the Tuapeka Fault Zone. The best gold concentrations
will lie on intrazone blocks that have undergone the most favourable combination
of sediment accumulation, fluvial incision, and re-concentration cycles,
and not necessarily close to the absolute footwall.
Keywords Blue Spur Conglomerate; Gabriels Gully; fluvial
gold placers; Tuapeka Fault Zone; fault segmentation
G01022 Received 6 July 2001; accepted 16 September 2002; published 21
March 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 123-139
0028-8306/03/4601-0123 $7.00/0 © The Royal Society of New Zealand
2003
PDF file of entire paper (subscribers only): Print-quality (8542K)
| screen-quality (387K)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
