Geology Reference
In-Depth Information
Internal structures in the first two sand-wave types
consist of ripple stratification and grain flows (sand
flows; Hunter
1977
) that typically are associated in
acceleration-deceleration flow cycles. Ripple strati-
fication predominates in the large-scale trough cross-
beds. Paleocurrent data display a bimodal-bipolar
pattern with a prevailing mode to the southwest and a
subordinate mode to the northeast. Tabular cross-bed
sets and cosets are interpreted to be the product
of migration of Type I and Type II megaripples of
Dalrymple et al. (
1978
), whereas large-scale trough
cross-bed cosets reflect migration of sinuous-crested
sand waves. Migration of megaripples on sand waves
or sand ridges produced the compound cross-bedded
cosets (cf. Dalrymple et al.
1978
). Allen (
1980
) inter-
preted the hierarchy of E surfaces to represent erosion
surfaces generated by the movement of superimposed
bedforms (E1 and E2) and a change in flow dynamics
within a tidal regime (E3). Comparable sigmoidal
reactivation surfaces bounding acceleration and dece-
leration flow cycles to those developed in this facies
have been identified from tidal sand-wave deposits and
have been related to fluctuating tidal current velocities
(Boersma and Terwindt
1981
; Kreisa and Moiola
1986
). The above criteria, together with a lack of expo-
sure features and the dominant westerly paleocurrent
mode indicate that this facies was deposited in a sub-
tidal setting dominated by tidal flow to the southwest.
The thin-bedded arenite facies contains a variety of
structures: (1) asymmetric, slightly asymmetric, and
symmetric ripples and megaripples; (2) modified rip-
ples including ladder-back, round-crested, flat-topped,
washed-out forms (Figs.
15.7
and
15.8
); (3) inversely
graded stratification and adhesion ripples and warts
(cf. Kocurek and Fielder
1982
); and (4) desiccation
cracks. The variety and types of preserved sedimen-
tary structures within the thin-bedded arenite facies
indicate that the depositional interface frequently was
emergent to intertidal and possibly supratidal condi-
tions. Comparable parasequences to those developed
in the Upper Mount Guide Quartzite have also been
described from the Quilalar Formation higher up in the
Haslingden Group (Fig.
15.4
; Jackson et al.
1990
).
Fig. 15.2
Tidal rhythmites from the Elatina Formation, South
Australia, showing five complete thickening and thinning
(neap-spring-neap) cycles consisting of between 10 and 14 graded
sandy to silty laminae. Cycles are bounded by thin mudstone
partings that developed during the neap phases of the tidal cycle
(Published with permission of G.E. Williams)
felsic dykes constrain the age of the Upper Mount
Guide Quartzite to between 1,800 and 1,740 Ma
(Page
1983a, b
).
Facies in the Upper Mount Guide Quartzite are
arranged in parasequences (cycles) between 0.5 and
12 m thick and consist of cross-bedded arenites capped
by thin-bedded arenites (Fig.
15.5
). The parasequences
are interpreted to record shoaling from subtidal-sand-
wave to tidal-flat conditions (Eriksson and Simpson
1990
; Simpson and Eriksson
1991
). Qualitative
evidence for tidal processes is recorded in both facies.
Three types of sand-wave deposits are recognized:
(1) tabular cross-bed sets and cosets (0.5-0.3 m thick)
consisting of planar, tangential, and trough cross strata
(Fig.
15.6
); (2) compound cross-bed cosets (up to
10.0 m thick) characterized by three hierarchical orders
of bounding surfaces (E1, E2, E3); and (3) large-scale
trough cross-bed cosets (up to 5.0 m thick). Medium
sand is the predominant grain size.
15.3.3 Witwatersrand Supergroup,
South Africa
The gold-bearing Witwatersrand Supergroup (Fig.
15.9
)
is upward of 7 km thick (Tankard et al.
1982
) and
