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cross-bedded facies in the Eureka Syncline is subtidal
sand shoals in the Oosterschelde estuary in the
Netherlands that similarly contain mud-draped bundles
of foresets typically arranged in semi-diurnal thick-
thin pairs (Boersma and Terwindt
1981
; de Boer et al.
1989
) and in which neap-spring-neap cycles have been
identified (Visser
1980
).
Analogues for tidal channel deposits developed in
the Moodies Group in the Dycedale Syncline are
developed in the inner river-dominated but marine-
influenced zones of Holocene tide-dominated estuaries
(Dalrymple et al.
1992
). Channel bank sediments in
modern tidal channels consist of interlaminated
sand and mud comparable to those in the upper parts
of fining-upward packages in the Moodies Group
(Eriksson et al.
2006
). An important difference
between Holocene tidal channel deposits and those in
the Moodies Group is the coarseness of the sediment
particularly at the base of the packages where pebbles
of extrabasinal origin are ubiquitous. Eriksson et al.
(
2006
) inferred a proximal source area in a tectonically
active basin to explain this distinction.
Cross-bedded sandstones that comprise the major
portion of parasequences in the Upper Mount Guide
Quartzite have analogues in the form of subtidal sand
waves in many Holocene settings including the Bay of
Fundy (Dalrymple
1984
). Comparable modified ripple
types to those present in the thin-bedded facies of the
upper Mount Guide Quartzite are present on Holocene
tidal flats including the North Sea, northwestern
Australia, the Bay of Fundy and The Wash, and develop
as a result of ebb runoff and emergence (Klein
1977
).
Eolian modification of tidal flats is reflected in the
preservation of adhesion warts and ripples (Kocurek
and Fielder
1982
) and inversely graded wind-ripple
stratification (Hunter
1977
).
In the absence of direct evidence for the existence
of a barrier island in the Elatina Formation, an alterna-
tive depositional setting to an ebb tidal delta is a tide-
dominated delta such as the Fly River
,
Yangtze and
Amazon deltas
.
Millimeter- to decimeter-scale, sand-
mud alternations are present in the delta front/prodelta
settings of all three deltas (Jaeger and Nittrouer
1995
;
Dalrymple et al.
2003
; Hori et al.
2002
; Harris et al.
2004
)
.
In the case of the Amazon Delta, a neap-spring
signal is discernable in the tidal laminites (Jaeger and
Nittrouer
1995
). Similar Holocene tide-dominated
delta analogues are inferred for rhythmites of the
Coronation Formation.
15.7
Summary
Some of the singular qualitative criteria used previously
to support a tidal origin are equivocal but repetitive
associations of structures may warrant, a tidal interpre-
tation. For example, stacked meter-scale parasequences
in the Upper Mount Guide succession containing
acceleration-deceleration cycle capped by thinly bedded
sandstones with a variety of modified ripples and other
exposure indicators are strongly suggestive of a tidal
origin. In the Moodies Group, the association in some
stratigraphic intervals of bimodal-bipolar paleocurrent
patterns, tidal bedding, rare herringbone cross bedding
and mudstone-draped foreset bundles strongly support
the existence of tides in the early Precambrian Era.
The most convincing evidence for tidal forcing in
the Precambrian is provided by rhythmites that dis-
play semi-diurnal, fortnightly (neap-spring-neap) and,
in some cases, monthly (perigee-apogee) hierarchical
bundling patterns. Data of these types presented
earlier strongly indicate the existence of tides during
deposition of the Elatina-Reynella and Moodies suc-
cessions. Data on rhythmites from the Witwatersrand
succession are more “noisy” but are suggestive of tidal
forcing.
The land-ocean interface in the Precambrian
was likely much different to most coastlines that exist
today. In the absence of rooted land plants, point
sources of sediment supply to the ocean were unlikely.
Instead, the land-ocean interface was probably in the
form of braid deltas with tidal modification taking
place in river channels (Moodies Group), on tidal
flats within embayments between delta lobes (Moodies
Group), in delta front and prodeltaic settings (Elatina
and Coronation), and on the shallow shelf (Moodies
and Upper Mount Guide). Data from cross beds in the
Moodies Group and the Upper Mount Guide reveal
that bedforms were of comparable scale to those
existing in Holocene estuaries supporting the conclu-
sion that tidal current velocities were similar to those
existing today in spite of a closer Earth-Moon distance
at least in the Archean Era. Similarly there is no evi-
dence for tidal ranges on the order of tens of meters as
inferred previously by Von Brunn and Hobday (
1976
)
for the 2.9 billion year old Mozaan Group in South
Africa based on thicknesses of inferred progradational
tidal-flat cycles. The lack of barrier-beach facies in
association with examples discussed may indicate that
