Geology Reference
In-Depth Information
Fig. 15.7
Flat-topped ripples
from tidal-flat caps to
parasequences in the Upper
Mount Guide Quartzite
Fig. 15.8
Washed-out
ripples from tidal-flat caps to
parasequences in the Upper
Mount Guide Quartzite
to shallow-marine followed by progradational deltaic
environments (Tankard et al.
1982
; Winter and Brink
1991
). Intercalated diamictites are interpreted as gla-
ciogenic deposits (Tankard et al.
1982
; Crowell
1999
).
Rhythmically interbedded facies (Fig.
15.13
) sampled in
core from the base of the progradational component of
the formation are interpreted as hemipelagic, bottomset
deposits of a prograding delta the proximal equivalents
of which were eroded along the upper sequence
boundary (Winter and Brink
1991
). The rhythmically
bedded facies consist of vertically accreted alter-
nating fine-grained sandstone/siltstone and mudstone
couplets. Alternating, thick-thin sandstone pairs record
semi-diurnal, dominant and subordinate currents.
Cyclic variations in laminae thickness permit identifi-
cation of neap-spring-neap cycles. Bar graphs of sand-
stone laminae thickness reveal thickening and thinning
trends that are interpreted as neap-spring-neap cycles
(Fig.
15.14
; cf. Kvale et al.
1999
).
Rhythmically bedded facies in the Central Rand
Group (Fig.
15.15
) are preserved in outcrop at the top of
the Randfontein Formation (Fig.
15.10
). Most laminae
are normal graded and range in thickness from 1 to
8 mm. Sandstone laminae are separated by siltstone/
mudstone partings. Bar graphs of sandstone laminae
thickness (Fig.
15.16
) reveal a hierarchy of lamina-
tions that are interpreted as semi-diurnal (thick-thin
pairs) and possible neap-spring-neap tidal periodicities
