Geology Reference
In-Depth Information
consists primarily of conglomerate and sandstone with
subordinate mudstone and rare volcanic horizons
(Fig.
15.10
). Based on U-Pb SHRIMP dating of lavas in
the older Dominion Group, in the Jeppestown Subgroup
and in the overlying Venterdorp Supergroup (Fig.
15.10
;
Armstrong et al.
1991
), the age of the Witwatersrand is
constrained to ca. 3.0-2.7 billion years. Quantitative
evidence for tidal processes in the form of herringbone
cross bedding (Fig.
15.11
), bimodal-bipolar paleocur-
rent patterns and three-dimensional modified ripples
(Fig.
15.12
) have been reported from the Hospital
Hill and Johannesburg Subgroups (Eriksson et al.
1981
). Rhythmically interbedded sandstones and
mudstone developed in the Coronation Formation, and
below the Livingstone Reef (placer) at the base of the
Luipaardsvlei Formation (Fig.
15.10
) may provide
quantitative evidence in support of tides.
The Coronation Formation is an unconformity-
bounded sequence underlain and overlain by coarse-
grained sedimentary rocks of mostly braided-alluvial
origin. Internally, the Coronation Formation is a
coarse- to fine-grained siliciclastic unit that consists of
an upward-fining interval overlain by an upward-
coarsening interval. Facies stacking patterns are con-
sidered to represent a transition from braided alluvial
Fig. 15.5
Stacking patterns of facies in parasequences in the
Upper Mount Guide Quartzite of the Mount Isa Inlier (Based on
Simpson and Eriksson
1991
). Parasequences range in thickness
from 0.5 to 12 m and record shoaling from subtidal sandwaves
(
a
) to tidal flats (
b
)
Fig. 15.6
Medium-scale tabular-tangential cross-bed set in
Upper Mount Guide Quartzite showing variation in dip angle.
This facies represents the deposit of a simple subtidal sandwave
that developed under conditions of accelerating and decelerating
tidal current flow velocities
