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Australia: The Land Where Time Began |
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Elatina Glaciation, Late Cryogenian (Marinoan Epoch), South Australia –
Sedimentary Facies and Palaeoenvironments
The Elatina Glaciation from the late Cryogenian (Marinoan Epoch), South
Australia, is named after the Elatina Formation of the glaciogenic
Yerelina Subgroup, which covers ~200,000 km2 in the Adelaide
Geosyncline as well as on the cratonic Stuart Shelf and is up to ~1,500
m thick. Many facies that are like those of the glaciations in the
Phanerozoic mark the Elatina glaciation:
·
Basal diamictite that displays glacitectonites with penetrative
deformation of preglacial beds, which indicates grounded ice or scouring
by icebergs.
·
Glaciomarine diamictites that contain many faceted and striated clasts
up to several metres across of intrabasinal and extrabasinal origin.
·
Laminated siltstone and mudstone that contains scattered, ice-rafted
dropstones in outer marine shelf environments.
·
Sandstones that were deposited in fluvial, deltaic and inner marine
shelf settings.
·
Tidalites that were deposited in estuaries and tidal deltas during
interstadial rise of sea level, with cyclic tidal rhythmites that
recorded the annual oscillation of sea level and displaying ripple marks
that were wave-generated, which together indicate long-lived, extensive
open seas.
·
Siltstone that contains acicular crystal pseudomorphs which implies the
formation of evaporite minerals in littoral deposits.
·
Permafrost regolith of frost-shattered quartzite breccia ≤20 m deep,
that displays large-scale periglacial structures that include primary
sand wedges, which are 3 m deep that indicate a frigid seasonal climate
near sea level.
·
Periglacial sandstone that covers 25,000 km2 and contains
primary sand wedges near its base.
A spectrum of settings is recorded in these deposits that range from
permafrost regolith and periglacial sand sheet on the Stuart Shelf in
the west, to fluvial, deltaic, and inner marine-shelf in the western and
central parts of the Adelaide Geosyncline, and outer marine-shelf in the
north and southeast. It is indicated by the widespread and persistent
rainout of fine-grained sediment and ice-rafted debris that the sea was
not frozen over during the Elatina Glaciation. At present the only age
that can be applied to the Elatina Glaciation is a maximum and minimum
age limits of between ~604 Ma and ~580 Ma, respectively, as no direct
determination is available. Deposition within 10o of the
palaeoequator is indicated by high quality palaeomagnetic data for red
beds from the Elatina Formation, which is supported by positive fold
tests on soft sediment slump folds that demonstrate the early
acquisition of magnetic remanence. The Nuccaleena Formation “cap
carbonate” overlies disconformably to unconformably the Yerelina
Subgroup in the Adelaide geosyncline and on the Stuart Shelf.
The presence of Glaciomarine deposition, grounded ice and permafrost
near sea level at near palaeolatitudes, a strong seasonal palaeoglacial
climate, and widespread open seas implies a paradoxical palaeoclimate
and palaeogeographic setting for the Elatina Glaciation.
A stimulus for worldwide multidisciplinary research on Cryogenian
glaciogenic successions has been the strong evidence for a
non-atualistic climate in the late Cryogenian in South Australia. In
recent years the term “Marinoan Glaciation” for the younger of the 2
Cryogenian glaciations that left their mark in the record of the
Neoproterozoic of most continents. In order to correlate these
glaciations of the late Cryogenian with the glacial episode of the
Marinoan Epoch in South Australia various lithostratigraphy and
chemostratigraphic arguments have been employed (Coats & Preiss, 1987;
Preiss, 2000), though the correlations have not been tested by direct
dating. Unfortunately, there is no presentation that is available that
discusses the current understanding of the stratigraphy, sedimentary
facies, and palaeoenvironments of the of the late Cryogenian glaciogenic
succession in its de facto “type region” of the Adelaide Geosyncline. In
volumes of the Geological Survey of South Australia and the Geological
Society of Australia (Coats & Preiss, 1987; Lemon & Gostin, 1990;
Preiss, 1993) that have limited distribution beyond Australia and these
do not include diverse stratigraphic and sedimentological studies and
detailed palaeomagnetic analyses of this succession that were carried
out over the past 2 decades. According to Williams et
al. it is highly desirable
that a synthesis that is up-to-date is in the international literature.
This paper has the aim of meeting this need by bringing together the
findings of pioneering stratigraphic studies by many workers from South
Australia and recent research in order to demonstrate the wide range of
glacial facies and palaeoenvironments that are associated with the
Cryogenian glaciation in the Adelaide Geosyncline region. According to
Williams this is timely as there is currently widespread interest in
glacial environments from the Neoproterozoic and the Ediacaran System
and Period with its global Stratotype Section and Point (GSSP)
purportedly placed at the base of the Nuccaleena Formation “cap
carbonate” which overlies glacial deposits from the late Cryogenian in
the Flinders Ranges in South Australia (Knoll et
al., 2004, 2006; Preiss,
2005). Also discussed are the appropriate terminology for the
glaciogenic succession in the late Cryogenian in South Australia, and
its palaeomagnetism and limited geochronology. The glacial palaeoclimate
and Palaeogeography of the region, with important implications for the
global climate of the late Cryogenian, are illuminated by the
sedimentological and palaeomagnetic data.
Geological setting and stratigraphy
The glaciogenic succession from the late Cryogenian that is in this
paper mostly accumulated in the sedimentary basin that dates to the
Neoproterozoic to the Early Palaeozoic age that is now marked by folded
strata of the Flinders Ranges and Mount Lofty Ranges in South Australia.
There are several terms that have been applied by various authors to
this basin such as “Adelaide Geosyncline”, “Adelaide Rift”, and
“Adelaide Rift Complex”. The Adelaide Geosyncline concept was discussed
(Preiss & Forbes, 1987) which stated (p. 15) that “although rifting
played an important role in the early stages of development, it had
little relevance during the later stages, and terms such as the Adelaide
Rift and
Adelaide Aulacogen are not considered applicable to the whole basin
throughout its evolution.” In this paper Preiss & Forbes, (1987, p.15)
employing “Adelaide Geosyncline” herein “as a formal term in the broad
and non-genetic sense of geosyncline as originally defined, i.e., a
complex basin of very thick sediment accumulation.”
Rifting of a craton from the Precambrian in the Early Neoproterozoic
initiated the Adelaide Geosyncline, which is now represented in the west
by the Gawler Craton and by the Curnamona Province in the east. A number
of inliers in the margins of the basin have exposures of the basement.
Sm-Nd isochrones 867 ± 17 Ma and 802 ± 35 Ma (Zhao & McCulloch, 1993;
Zhao, 1994) and a U-Pb baddelyite age of 827 ± 6 Ma (Wingate et
al., 1998), were yielded by
mafic dykes that are regarded as feeders for volcanic rocks near the
base of the succession from the Neoproterozoic. The subsequent
development of the Adelaide Geosyncline as a deeply subsiding basin
complex from the Neoproterozoic to Cambrian was marked by a succession
of major rift cycles early in the development, though rifting on a large
scale was less important in its later history, including the Late
Cryogenian (Preiss, 2000).
In the Adelaide Geosyncline the Neoproterozoic-Cambrian succession was
deformed by the Delamerian Orogeny at 514 to 490 Ma (Drexel & Preiss,
1995; Foden et al., 2006).
The following major subdivisions of the Adelaide Geosyncline region as
was identified by Preiss (2000) are based on differing responses to
deformation, though they also partially reflect primary sedimentary
facies belts:
·
Cratonic platforms on the Stuart Shelf in the west and the Curnamona
Province in the east, with thin, little deformed Neoproterozoic to
Cambrian cover.
·
Meridional Torrens Hinge Zone of gentle folding.
·
Central Flinders Zone of broad dome and basin structures.
·
North Flinders Zone of arcuate, open to tight folds.
·
Nackara Arc of long, arcuate, folds that are relatively upright.
·
Fleurieu Arc marked by major orogenic shortening by thrusting and tight
folding.
The Delamerian Origen is the term used for the zone of Delamerian
deformation.
The term “Adelaide Fold Belt” has sometimes been used in recent
literature for an orogen, though it should not be used for the
sedimentary basin.
The Neoproterozoic stratigraphy of the Adelaide Geosyncline regions has
been discussed by Preiss (1987a, 1993 and 2000) and Preiss et
al. (1998). According to
Williams et al. the full
sequence can be divided into 12 sequence sets, and the lithospheric
classification can be refined so that group and subgroup boundaries
coincide with the most significant sequence boundaries. Each subgroup
is, therefore, a discrete depositional entity, whereas formations that
are within subgroups are characterised by particular facies associations
and may have boundaries that may be gradational and/or diachronous.
Boundaries of sequences may represent significant time gaps in the
record and may have origins that are tectonic or eustatic, or both.
The Neoproterozoic succession of the Adelaide Geosyncline was divided
(Mawson & Sprigg, 1950; Sprigg, 1950), which they termed the
Adelaide(an) Syncline system, in 4 chronostratigraphic units or series
rank: Willouran, Torrensian, Sturtian and Marinoan. These series, like
the lithostratigraphic subdivisions, have boundaries that are now
recognised as major sequence boundaries and therefore the
chronostratigraphic significance that was implied by the original
definitions remains valid.
In the Adelaide Geosyncline region the glaciation of the late Cryogenian
is recorded by diverse tectonic and glaciosedimentary settings, which
span permafrost regolith and periglacial-aeolian on the cratonic
platform in the west, through fluvial, deltaic and inner marine shelf
further east to outer marine shelf to the north and southeast. The
deposits of the late Cryogenian are all included in the Yerelina
Subgroup (Preiss et al.,
1998) which represents the 10th of the 12 sequence sets.
Within the Yerelina Subgroup the stratigraphy and suggested correlations
a possible division into 3 sequences marked by erosional boundaries and
a proposed sea level curve (Preiss et
al., 1998; Preiss, 2000).
During the glaciation of the late Cryogenian there are no volcanism
events that are known of, and active rifting on a large scale, such as
characterised deposition in the Willouran, Torrensian, Sturtian times,
was not important. Source regions include the Gawler Craton and the
Curnamona Province, though the Provence of much glacial detritus remains
to be identified.
Terminology
“Marinoan Glaciation” is a term that has been shown to be useful in
South Australia, but based on recent stratigraphic evidence and
geochronological data from other parts of Australia as well as other
continents it is now not appropriate. The Marinoan Series was defined
chronostratigraphically (Mawson & Sprigg, 1950) for strata between the
top of the Brighton Limestone (~650 Ma) and the base of the Cambrian.
The corresponding term for time is the Marinoan Epoch (Preiss, 1987b),
therefore includes the Ediacaran Period. The Cryogenian glaciogenic
succession has attracted worldwide attention in recent years. As a
result of this the “Marinoan Glaciation” being applied indiscriminately
to glacial deposits dating to the late Cryogenian from elsewhere in the
world that are perceived, rightly or wrongly, to correlate with the
glaciation from the late Cryogenian in South Australia. The Ediacaran
Glaciation in Western Australia (Corcoran & George, 2001) and on other
continents (Fairchild & Kennedy, 2007) also occurred during the Marinoan
Epoch. Therefore, Williams et al.
agree with Preiss (2000) that usage of the term “Marinoan” for
glaciation in the late Marinoan is invalid.
In order to avoid confusion with terminology Williams et
al. employed the term
“Elatina glaciation” as was proposed originally by Mawson (1949a)
following his discovery of many faceted and striated clasts that
contained diamictite, which he termed “Elatina tillite”, at Elatina
along Elatina Creek in the central Flinders Ranges. The Elatina
Formation was defined by subsequent mapping which showed it to be a
widespread lithostratigraphic unit of the glaciogenic succession in the
Marinoan Series (Coats & Preiss, 1987), and it was recommended by
Williams et al. that the term
“Elatina glaciation“ should be adopted in South Australia and the term
“Marinoan glaciation” should be discarded. Presumptions and circularity
of arguments concerning the global extent, correlations and nature of
those glaciations would be helped if naming Cryogenic glaciations on
other continents after the principal respective glaciogenic formations.
In the description and naming of diamictite units of glaciogenic
successions from the Proterozoic in the Adelaide Geosyncline (Howchin,
1928, Mawson, 1949a, b), as it has for diamictites in many other
countries (Hambrey & Harland, 1981). As applied in South Australia at
the present formal usage of the term “Tillite” for the
lithostratigraphic terminology of Cryogenian formations that contain
diamictites (Coats & Preiss, 1987; Preiss et
al., 1988) reflects the early
practice. It implies that all diamictites have a glaciogenic component,
rather than that they were deposited from ice. It is common for faceted
and striated clasts, many of extrabasinal origin, to be contained in
Cryogenian diamictites in the Adelaide Geosyncline. Together with many
dropstones in associated laminated sediments that are fine-grained,
confirm that glacial processes were involved in their genesis.
Age of glaciation
Difficulties of correlation are compounded by the Elatina glaciation not
being accurately dated. U-Pb zircon dating of igneous rocks that are
associated with diamictites in the Namibia (Ghaub Formation) and on King
Island, Tasmania (Cottons Breccia), respectively, that were assumed to
be correlative with the Elatina glaciogenic succession, were the basis
for suggested ages of glaciation of 635 ± 1.2 Ma (Hoffmann et
al., 2004) and 575 ± 3 Ma
(Calver et al., 2004).
Cottons Breccia is, however, of uncertain genesis and it is not clear
what its relationship to the Elatina formation is. An older age limit of
663 ± 4 Ma (Zhou et al.,
2004) and a younger age limit of 635.2 ± 0.6 Ma was given (Condon et
al., 2005) for the Nantuo
glaciation in China, which they equated with the Elatina glaciation.
An estimated age of ~600 Ma for the Elatina glaciation was based
on chemostratigraphy (Walter et
al., 2000). The correlation that has been claimed between the
glacial deposits in Namibia and South Australia (Hoffmann et
al., 2004) assumes
synchronous glaciations during the Cryogenian. It is still to be
securely determined whether the glaciations were synchronous or
diachronous (e.g., Lund et al.,
2003; Zhou et al., 2004;
Kendall et al., 2006) and how
many glaciations in the Neoproterozoic that are represented in the
geological record.
Constraints have been placed on the age of the Elatina glaciation by
geochronological studies of the strata in the Adelaide Geosyncline. A
detrital zircon U-Pb age of 657 ± 17 Ma was obtained for a single grain
from the Marino Arkose Member in the upper part of the underlying
Upalinna Subgroup (Ireland et al.,
1998; Preiss, 2000). An age of 643 ± 2.4 Ma was given by Re-Os dating
for black shale from the Tindelpina Shale Member (Lowermost Tapley Hill
Formation) that overlies glacial deposits from the Sturtian glaciation
(Kendall et al., 2006).
Furthermore, an age of ~658 Ma was given by U-Pb SHRIMP dating of
a tuffaceous layer from the upper part of the Sturtian glaciogenic
succession (Fanning & Link, 2006). The age of the Bunyeroo Formation
from the Ediacaran may be approximated as 590-580 Ma based on Rb-Sr
whole rock data and chemostratigraphy (Compston et
al., 1987; Walter et
al., 2000).
It is indicated by the above findings that only maximum and minimum age
limits of ~640 and 580 Ma, respectively, can be applied at present to
the Elatina glaciation. Accepting their age of 643 Ma for the Tindelpina
Shale Member, as has been pointed out (Kendall et
al., 2006), and also an age
of 635 Ma for the Elatina glaciation requires high rates of
sedimentation for the >4 km of interglacial strata in the Central
Flinders Zone. The Elatina glaciation and the beginning of the Ediacaran
Period could, alternatively, be younger than 635 Ma.
Glaciogenic deposits
Great variation is shown by the Yerelina Subgroup in the Adelaide
Geosyncline. The Elatina Formation is 30-130 m thick in the western and
central areas
, though in the North Flinders Zone the subgroup is thickest and
more complete, >1,500 m, and the Nackara Arc, >1,000 m thick.
Lithological descriptions and sections that have been measured showing
the suggested facies relationships and correlations are given by (Coats
& Preiss, 1987; Lemon & Gostin, 1990; Preiss, 1993; Preiss et
al., 1998). Broad
correlations are possible, though there is no continuity between these
areas. The waxing and waning of glacial conditions are recorded by the
thicker successions of the Yerelina Subgroup, whereas about the time of
the main glacial stage, the deposition of the Elatina Formation began,
and the glacial conditions persisted to the end of the deposition of the
Elatina Formation. Therefore, in the west the margins of the basin were
initially subjected to exposure, periglacial conditions and erosion,
while the deeper more distal areas recorded the gradual onset of glacial
conditions and accumulated a substantial thickness of glaciogenic
deposits.
The Nuccaleena Formation overlies the Yerelina Subgroup, the contact
being a disconformity to very low angle unconformity (Preiss, 2000;
Knoll et al., 2006). The
Nuccaleena Formation forms an important marker unit and is discussed
below.
Yerelina Subgroup, North Flinders Zone and Nackara Arc
Fortress Hill formation (1070 m)
The onset of the glacial conditions is marked by the fortress Hill
Formation. It is comprised of grey-green (weathering reddish brown)
pyritic siltstone, that is finely laminated, that contains scattered
pebbles and cobbles and gritty lenses. The formation is ~1,070 m thick
at the type section located 1 km north of Mount Saturday. There is a
progressive increase upwards in the material that has been ice-rafted,
and some of the clasts are faceted and have been interpreted as drop
stones. Included among the clast lithologies are granite, pink and grey
quartzite, pale grey limestone, oolitic limestone, and dolostone.
Gumbowie Arkose (45-90 m)
The Gumbowie Arkose overlies the Fortress Hill Formation disconformably,
at a possible sequence boundary, in the Nackara Arc. The Gumbowie Arkose
is a white to pale yellow cross-bedded, coarse-grained feldspathic
sandstone and arkose that contains some interbeds of pebbly sandstone,
sandy siltstone, and pebbly mudstone.
Pepuarta Tillite (120-197 m)
This is a sparse diamictite that is comprised of scattered pebbles,
cobbles and boulders in a grey massive, laminated, calcareous siltstone
matrix. Faceted and striated boulders are up to 2.5 m across (Binks,
1971). Included in cast lithologies are grey and pink granite, granite
gneiss, grey porphyry, quartz granule conglomerate, various quartzites,
and vein quartz. Scattered clasts up to boulder size, are contained in
the laminated siltstone facies, which is consistent with deposition from
floating ice. The glacial maximum of the Yerelina Subgroup is marked by
the Pepuarta Tillite and the correlative Mount Curtis Tillite.
Mount Curtis Tillite (90)
The Fortress Hill Formation in the North Flinders Zone is overlain
sharply by sandstone and conglomerate that is comparable with the
Gumbowie Arkose which is suggested by Williams et
al. may record a lowering of
relative sea level with a sequence boundary at their base. The Mount
Curtis Tillite, which is a diamictite which is comprised of sparse
erratics of pebble to boulder size in grey-green, massive and laminated,
sandy siltstone matrix, follows these units. Some of the clasts are
faceted and striated. Clast morphologies are mostly quartzite, limestone
and dolostone, and less commonly granite and porphyry. Granite boulders
are up to 3 m x 8 m (Dickson et
al., 1951).
Grampus quartzite (45-60 m)
The Grampus Quartzite overlies unconformably the Pepuarta Tillite;
possibly at a sequence boundary that defines a 3rd genetic
sequence of the Yerelina Subgroup. It is comprised of pinkish-grey
medium grained and coarse-grained feldspathic quartzite, with local thin
dolomite beds.
Balparana Sandstone (130 m)
This is a pale grey and brownish grey medium-grained feldspathic
sandstone, that has interbeds and lenses of calcareous siltstone and
pebble conglomerate. The pebbles are mostly comprised of vein quartz,
with some quartzite, siltstone and granite clasts.
Ketchowla Siltstone
(≥170 m)
This is a pale grey (weathering brown), laminated to ripple
cross-laminated, siltstone that is highly calcareous (Preiss, 1992). It
contains scattered granules and pebbles and local concentrations of
cobbles and boulders, mostly quartzite, including some up to 1 m across.
The clasts were interpreted as glacial erratics that had been deposited
from floating ice. The outer marine shelf deposition under glacial
conditions that are generally waning is represented by the Ketchowla
Siltstone.
Elatina Formation, Central Flinders Zone (30-200 m)
The lithostratigraphy and sedimentology of the Elatina Formation in the
Central Flinders Zone have been described (Lemon, 1988; Lemon & Gostin,
1990). In a peripheral sink around the active and emergent Enorama
Diapir (Lemon, 1985), deposits that were relatively thick (~100 m)
accumulated. Glacial advance to sea level that was followed by a
punctuated glacial retreat is attested to by the nature of the basal
unconformity and the main sedimentary facies. The characteristic reddish
hue of the Elatina Formation is the result of haematite pigmentation
that is finely dispersed, which is important for palaeomagnetic studies,
though it is absent from the more distal deposits of the Nackara Arc and
North flinders Zone.
Basal Unconformity
The Elatina Formation overlies unconformably the Trezona Formation and
the Yaltipena Formation, which is preglacial, in the Central Flinders
Zone. At outcrop, little relief is shown by the sub-Elatina surface,
with the exception of some possible Karst solution of the Trezona
Formation. Frost shattering may be indicated by a regolith of breccia
that is ≤1 m thick that overlies the limestone surface in places. The
Trezona Formation, about 50 km north of Enorama Creek, is >450 m thick,
has been eroded and the Elatina Formation rests on the Enorama Shale
(Brenchley-Gaal, 1985).
The Yaltipena Formation (≤100 m) is a clastic red bed unit of limited
extent (Lemon & Reid, 1998) that largely coarsens upwards. It displays
languid and symmetrical marks, which commonly have ladder-like
interference patterns, desiccation cracks and shale-clast conglomerate.
A shallow water environment is indicated by these features, which imply
the lowering of sea levels that heralds the approaching glacial advance.
Basal Boulder Diamictite
The top few metres of the Yaltipena Formation, that was poorly
lithified, was truncated and deformed the Trezona Bore by boulder
conglomerate and a massive boulder diamictite that was up to 5 m thick.
The basal diamictite contains extrabasinal boulders that were forced
into the top of the Yaltipena Formation, here, as well as other places.
These structures that display evidence of penetrative deformation were
interpreted as Type A glaciotectonics (Evans et
al., 2006)
which indicate the advance of grounded ice (Lemon & Gostin, 1990;
Lemon & Reid, 1998) or scouring by icebergs.
Sandstone Facies
Overlying the basal diamictite and conglomerate is about 5 m of
channelled, cross-bedded, coarse-grained sandstone that had a pebble
conglomerate at its base, which were interpreted as fluvial deposits
(Lemon & Gostin, 1990). The sandstone grades upwards into flaser-bedded,
muddy and silty sandstone, which was followed by several sandstone beds
up to 1 m thick. Large ball and pillow structures are shown by the
Sandstone ascribed (Lemon & Gostin, 1990) to rapid influx of the sand
bodies and their collapse into the underlying muds. In the Elatina
Formation the main arenaceous unit is comprised on 20-40 m of
feldspathic sandstones that are white, pink to red-brown, and poorly
sorted that extend beyond the peripheral sink around the Enorama Diapir
to areas where they overlie directly the basal unconformity. Contained
within these sandstones are subangular to subrounded quartz grains
(60%), about (20%) feldspar grains (sodic plagioclase, orthoclase, and
microcline) as well as lithic fragments and heavy mineral grains (Coats
& Preiss, 1987). The absence of distinct bedding and the presence of
thin discontinuous granule layers is a characteristic feature of the
sandstones. Slumped cross-bedded sets up to 1 m thick that contain
fining-upwards beds, are shown by some exposures. There are small
indistinct, dewatering structures as well as rare pebbles to outsize
boulders. The sandstones are generally bimodal, being marked by a
dominance of coarse silt to fine sand and sand that is very coarse to
granule fractions and depletion of medium and coarse sand fractions,
which suggests there was aeolian winnowing in the source area. It is
suggested there was subaqueous deposition in the source area (Preiss,
1987b; Lemon and Gostin, 1990).
Diamictite Facies
The extensive sandstone unit grades upwards into reddish grey, laminated
siltstone and red mudstone containing rare dropstones that pass
laterally into diamictite with sparse clasts. No evidence of slumping is
shown by the diamictite, which together with the random scattering of
large clasts suggests that deposition was from floating ice and a return
to glacial conditions.
To the west of the Enorama Diapir, in the upper third of the Elatina
Formation, reworking by currents of diamictite in shallow marine
conditions it is indicated by the common occurrence of lag gravel layers
that are capped by cross-laminated sandstones within what were otherwise
massive units. It is common to find in the diamictite at the top of the
Elatina formation in the Trezona Bore-Enorama Creek area, faceted and
striated clasts of vesicular basalt. The whole of the Elatina Formation
being deposited under conditions that were glacially influenced is
attested to by the presence of a basal diamictite with glacitectonites
and a glaciogenic diamictite immediately below the Nuccaleena Formation.
Deposits east of diapiric islands
East of the Elatina Formation it is ~120 m thick and displays some
similarities with the formation that is described above west of the
Diapiric islands. There is a thin basal conglomerate that is overlain by
massive diamictite carrying boulders that are of extrabasinal origin.
Succeeding this is ~50 m of pink sandstone that has indistinct bedding.
Ripple cross-laminated fine to very fine-grained sandstone that has rare
lonestones dominates the remaining section. Further east, in the
Chambers Gorge area, the sandstone facies contain cross-bedding of large
scale that thickens to ~200 m, which suggests a delta was present at the
western margin of the Curnamona Province (Coats & Preiss, 1987).
Nature of clasts in diamictite facies
In the diamictites many of the clasts are faceted and striated (Mawson,
1949a; Dalgarno & Johnson, 1964; Coats & Preiss, 1987; Lemon & Gostin,
1990), with the striations typically paralleling the long axis of the
clasts. Linear series of chattermarks occur at the Bulls Gap section on
a faceted boulder of quartz that is 75 cm in diameter. In the
diamictites, about 60% of the clasts, including those composed of
basalt, dolostone and heavy-mineral banded sandstone and other
lithologies were derived from the
underlying formations and diapiric islands within the sedimentary
basin. At the top of the Elatina Formation the common occurrence of
faceted and striated clasts of vesicular basalt in the diamictite is an
indication that glacial erosion of rafts of identical basalt that were
up to 1 km across were present within the diapiric islands a few
kilometres to the east (Coats & Preiss, 1987; Lemon & Gostin, 1990). It
is therefore suggested by Williams et
al., that grounded ice
persisted on the diapiric islands to the end of Elatina deposition.
About 40% of the clasts are of extrabasinal origin, and are comprised of
granite gneiss, red porphyritic dacite, schist, metaquartzite, vein
quartz, iron formation, gritty quartzite, as well as other sedimentary
rocks. It is indicated by the wide variety of clasts that derivation was
from a cratonic and metamorphic basement. Some may have been derived
from the Curnamona Province, and lithologies have been identified (Lemon
and Gostin, 1990) that may be matched with basement rocks in the Iron
Knob area of the Gawler Craton. According to Williams et
al. it is not likely that
derivation was from the west of the Flinders Zone, however, as the
presence on the Stuart Shelf of a preglacial palaeosols, a permafrost
regolith, and it is implied by the presence of a conformably overlying
periglacial-aeolian sand sheet that the cratonic region to the west was
not eroded by the glaciers of the Cryogenian.
Elatina Formation, Hallett Cove area (120 m)
Pale brownish grey sandstone of the preglacial Wilmington Formation is
overlain disconformably by the Reynella Siltstone Member of the Elatina
Formation in the Hallett Cove area of the Fleurieu Arc near the
southwestern margin of the Adelaide Geosyncline (Thomson, 1966; Dyson &
von der Borch, 1986; Coats & Preiss, 1987). The Reynella Siltstone
Member is well exposed in a 120 m thick coastal section of Marino Rocks,
which is 2.5 km to the north of Hallett Cove and is comprised of the
following facies in ascending stratigraphic order:
·
A massive dark red siltstone that contains rare granules and angular
fragments of dolostone.
·
Siltstones and fine-grained sandstone including Tidalites that display
herringbone cross-bedding, flaser bedding and cyclic tidal rhythmites.
·
Calcareous and dolomitic sandstone that contains angular, granular- to
pebble-sized intraclasts of limestone and dolostone that are obviously
derived from calcareous interbeds. Tepee-like structures and dolostone
interbeds with stromatolytic laminae are also present. Acicular crystal
pseudomorphs after evaporite minerals occur in some beds and in small
vugs within red siltstone.
·
At the top of the section there is massive, granule-bearing siltstone.
Marino Rocks the Reynella Siltstone Member displays no conclusive
evidence of glacial deposition, though it includes a siltstone with a
quartzite erratic that has been interpreted as a dropstone, 7 km to the
south-southwest (Coats & Preiss, 1987). The gritty, calcareous Marino
Arkose Member in the Hallett Cove area was correlated with the
glaciogenic deposits at Elatina 400 km to the north (Mawson, 1949a, p.
120), who regarded the southern deposits to be “the echo, so to speak”
of “distant glaciation”, though he did not recognise the Reynella
Siltstone Member. No evidence of glacial influence is contained in the
Marino Arkose Member, and this member is the basal part of the
Wilmington Formation. Williams et
al. concurred with Forbes & Preiss (1987) that the Saecliff
Sandstone which, in the Hallett Cove area, overlies the Reynella
Siltstone Member, is a partial lateral equivalent of the Nuccaleena
Formation.
Tidal Deposits
Evidence of tidal activity
In the Elatina Formation Tidalites are common, notably in a rhythmite
unit at Pitchi Richi Pass that is ~10 m thick, and represents ~70 years
of deposition, and not in the Reynella Siltstone Member at Marino Rocks
(Williams, 1989, 1991, 1998a, 2000). Tidal deposition is indicated by
the following features:
1.
Reynella Siltstone Member, herringbone crossbedding in sandstone.
2.
Reynella Siltstone Member, flaser bedding, consisting of trough
cross-laminated, fine-grained sandstone with flasers of mudstone in the
troughs.
3.
Cyclic rhythmites that are accreted vertically in which most cycles are
5 to 60 mm thick and are comprised of up to 16 laminae of fine-grained,
feldspathic sandstone and siltstone that are normally graded and thicker
near the centre of each cycle. Red mudstone bands form the boundaries of
the cycles. The laminae have been interpreted as increments that
represent the lunar day, and the tidal ranges (or greater tidal heights)
are reflected by the thicker laminae, and faster tidal currents marking
the spring phase of the fortnightly tidal cycle. The thin laminae are a
reflection of the small tidal ranges, and the mudstone bands the slack
water around the times of neap tides. The cycles are comparable with the
neap-spring (fortnightly) cycles that have been identified in modern
tidal deposits, including those on Glaciomarine settings (Smith et
al., 1990; Cowan et
al., 1990).
4.
Sublaminae of unequal thickness
within the rhythmites represent semidiurnal increments, best seen
in the Reynella Siltstone Member. Such alternation of thick and thin
laminae are present in tidal deposits of the present, and record the
“diurnal inequality” of successive semidiurnal tides and tidal currents,
which reflects a mixed semidiurnal-diurnal tidal pattern (de Boer et
al., 1999; Dalrymple et
al., 1991).
5.
The similarity of long term (years to tens of years) patterns and
laminae and neap-spring cycle thickness with the pattern of modern tidal
records. Regular changes in the thickness in the neap-spring cycles in
the Elatina rhythmites, e.g., are comparable with variations in
fortnightly tidal heights for tides at Townsville, Queensland. The
“monthly inequality” of heights and ranges that result from the
elliptical lunar orbit is reflected in this pattern.
Facies model for tidal rhythmites
At Pitchi Richi Pass the rhythmite unit overlies diamictite and
sandstone, and is succeeded by fine- to medium-grained sandstone and
diamictite that contains scattered granules and small pebbles, with rare
clasts that are up to 10 cm across. At Warren Gorge 20 km to the north a
correlative rhythmite is overlain by a similar diamictite that is 6 m
thick (Jablonski, 1975). Very thin neap-spring cycles (0.5-3.0 m) are
shown by continuous cores from 3 drill holes through the Elatina
rhythmite unit (Williams, 1991) to be confined to the base of the unit
and grade downwards into muddy fine-grained sandstone. Within the limits
of exposures and drill holes, the neap-spring tides can be traced
laterally over several hundred metres. It is indicated by rare
directional structures such as ripple cross-lamination and slumped beds
that there was movement to the present east, away from the presumed
nearby western margin of the basin.
Deposition on prograding, distal ebb-tidal delta is involved in a facies
model for the Elatina rhythmites (Williams, 1989, 1991, 2000).
Fine-grained sediment is entrained in such a setting by ebb-tidal
currents and is transported mainly in suspension by periodic turbid
currents and jets (özsoy,
1986) to deep water offshore by the main ebb channel. Such jets
transform to plumes with increasing distance of transport (Powell, 1990)
and suspended sediments settle out to form normally graded laminae in
distal settings. The potential sediment load of ebb-tidal currents and
the effectiveness as an agent of sediment entrainment and deposition of
the tide are related directly to tidal range (or maximum tidal height)
and speed of the current (FitzGerald & Nummedal, 1983; Boothroyd, 1985).
The relative thickness of successive laminae and neap-spring cycles,
however provide a proxy tidal record.
The adjacent hinterland on the Stuart Shelf was a periglacial region
with minimal fluvial activity interfering with periodic tidal processes,
during the deposition of the Elatina rhythmites. Tidal estuaries and
inlets that favoured the deposition of cyclic tidal rhythmites in
nearshore settings and on distal tidal deltas may have been produced by
the drowning of coastal areas through rising sea levels as was discussed
(Williams, 2000). At Pitchi Richi Pass the rhythmite unit that was
underlain and succeeded by the glaciogenic deposits was interpreted as
marking a high stand of relative sea level during temporary glacial
retreat. That the glaciation occurred close to sea level is confirmed by
the structural proximity of Tidalites, and that glaciation occurred
close to sea level is confirmed by diamictite.
At Marino Rocks the Reynella Siltstone Member contains some neap-spring
cycles that are relatively thick and a variety of sedimentary structures
which suggest a setting that is more variable, and at times a more
proximal setting than that that has been envisaged for the Elatina tidal
rhythmites at Pichi Richi Pass. The tidalites in the Reynella Siltstone
Member may have accumulated on upper and lower delta slopes as well as
in estuarine settings like those in the Bay of Fundy, Canada (Dalrymple
et al., 1991).
Slump structures and ripple marks
At Pichi Richi Pass and Warren Gorge the Elatina rhythmites display
slightly sinuous, discontinuous, near cuspate folds that are
near-parallel and are spaced 13-50 cm apart and with amplitudes of 3-5
cm (Williams, 1996) that are draped by symmetrical and interference
ripple marks that are generated by waves. Some cuspate folds have crests
that are truncated or show delicate scouring, which indicates that the
folds were present during deposition. At Warren Gorge the ripple marks
are more common than at Pichi Richi Pass, which implies there was a more
distal, deep water setting for Pichi Richi Pass.
The folds were interpreted as gravity slides on a tidal delta that was
triggered by stresses generated by storm waves (Williams, 1996). Similar
cuspate folds, which were also interpreted as gravity slides, that
underlie deposits that were generated by wave action are displayed by
deposits in Newfoundland that date to the Neoproterozoic which were
generated by wave action (Myrow & Hiscott, 1991). On the lower slopes of
submarine fans, fan deltas and deltas (e.g., Prior & Coleman, 1980;
Prior & Bornhold, 1989), were marked by gravity slides that were
slightly sinuous, discontinuous and en echelon compressional ridge
s of dimensions that were similar to the soft-sediment folds at
Elatina are ubiquitous.
Quantitative analysis of tidal rhythmites
Time series analysis of stratigraphic logs of lamina and neap-spring
cycle thickness that were obtained from 3 drill cores from the Elatina
rhythmite unit, that were supplemented by data from the Reynella
Siltstone Member rhythmites, has provided the most complete palaeotidal
set that is available for the Precambrian (Williams, 1989, 1991, 1998a,
2000). In late Cryogenian times it is indicated by the rhythmites
that there were 13.1 ± 0.1 lunar months/year, 400 ± 7 solar days/year
and 21.9 ± 0.4 h/solar day, with a mean Earth-Moon distance 96.5 ± 0.5%
of the present distance.
The non-tidal, annual oscillation of sea level is also recorded by the
Elatina rhythmite unit. This oscillation is the result of the
interaction of the physical processes that include changes in the
temperature of the water, pressure changes of winds and atmosphere (Komar
& Enfield, 1987). The annual oscillation of sea level in low and
moderate latitudes mostly results from seasonal changes in the heat
content of the sea, with winds being a minor importance (Roden, 1963;
Pattullo, 1996; Wunsch, 1972; Mellor & Ezer, 1995). It is indicated by
the Record of the annual oscillation in the Elatina data that the late
Cryogenian global ocean was not frozen over during the interstadial of
the Elatina glaciation, because if there was complete ice cover it would
have isolated the sea from seasonal changes of temperature and winds
(Williams, 2004; Williams & Schmidt, 2004).
Permafrost regolith
During the Neoproterozoic, the arenaceous Pandurra Formation that dates
to the Mesoproterozoic (1,424 ± 51 Ma; Fanning et
al., 1983) formed a broad
structural high, the Pernatty Upwarp, in the central part of the Stuart
Shelf. On the irregular surface of silicified Pandurra Formation
(quartzite), and locally on the Tapley Hill Formation, from the
Cryogenian, the Cattle Grid Breccia developed (Williams & Tonkin, 1985;
Williams, 1986; Coats & Preiss, 1987). Silicification of the Pandurra
Formation predates the Elatina glaciation, and as it is not evident in
the poorly lithified sandstone that is encountered in deep drill holes
appears to be a superficial effect. It is implied by this probable
palaeosol, though it has been modified by cryogenic processes during the
Elatina glaciation, that the cratonic Stuart Shelf was not eroded by the
glaciers of the late Cryogenian. In the now abandoned and partially
filled Cattle Grid open pit, as well as other pits near Mt Gunson, where
it hosts a stratabound copper deposit the Cattle Grid Breccia is exposed
(Tonkin & Creelman, 1990). A periglacial origin for the Cattle Grid
Breccia and structures that were associated with it was first proposed
(Williams, 1981) during an inspection of the Cattle Grid mine in January
1981, after which Williams continued to study the structures that were
revealed by mining until the late 1980s when excavation ended.
The Cattle Grid Breccia is overlain conformably by the Whyalla
Sandstone, which is capped by the Nuccaleena Formation. It is
demonstrated by the stratigraphic relationships that the Cattle Grid
Breccia and the Whyalla Sandstone both formed during the Elatina
glaciation (Coats, 1981; Coats & Preiss, 1987; Preiss, 1993; Preiss et
al., 1998). The post-glacial
marine transgression that submerged most of the Stuart Shelf (Forbes &
Preiss, 1987) is marked by the Nuccaleena Formation, which indicates
that the Cattle Grid Breccia and the Whyalla Sandstone formed near sea
level.
Brecciated bedrock
The Cattle Grid Breccia is 0.5 to 20 m thick, and averages 5 m. The top
≤2 m is comprised of angular to subrounded quartzite clasts mostly 2-10
cm across, with some blocks up to 40 cm, which are oriented randomly in
a sandy matrix. In places there is crude horizontal bedding. Channels
form that are up to 10 m wide and 2-3 m deep, which contain
fragments and
well-rounded quartzite pebbles, that occur locally at the contact with
the overlying Whyalla Sandstone. The sedimentary breccia passes
downwards into in situ
breccia in which there are tabular clasts of cobble to boulder size that
are roughly horizontally oriented, marking the original bedding of the
Pandurra Formation, and clasts which are adjacent have matched sides.
The in situ breccia has
little or no sandstone matrix and passes downwards into the solid
bedrock.
It is common to find anticlinal structures 0.5 to 4 m high and 0.3 to
4.0 m wide in the in situ
breccia (Williams & Tonkin, 1985). There are near vertical axial planes
in most anticlines, though there are some structures that are
asymmetrical, and overturned in extreme cases. Associated with some of
the asymmetrical anticlines are moderate- to high-angle reverse faults.
Locally, there are diapiric-like mounds that are truncated at the
contact with the Whyalla Sandstone.
The Cattle Grid Breccia was compared with block fields (Williams &
Tonkin, 1985), which are level or sloping gently (<10o)
surfaces that are covered with angular blocks, that are distributed
widely in polar regions (Embleton & King, 1975; White, 1976; Washburn,
1980). The block fields are generally agreed to have developed by frost
shattering in situ, of
adjacent, well-jointed bedrock and the expansion that results exerts
pressure in all directions. According to Williams et
al. both horizontal and
vertical expansion of the breccia profile that is developing and the
frost heaving and frost thrusting that results is attested to by the
anticlines and reverse faults within the Cattle Grid Breccia.
The in situ breccia is
composed of tabular fragments that are near-horizontal of bedrock that
extends to depths of a few metres to 11+ m below the surface of the
ground, occurs in southeast England, Spitzbergen, and Melville Island in
the Canadian Arctic (Murton, 1996). In southeast England small open,
folds are exhibited which are capped by an involuted layer 0.5 – 2.0 m
thick comprised of clayey beds and chalk diamicton with sub-rounded to
rounded clasts of chalk. These features are similar to those of the
Cattle Grid Breccia. Continuous permafrost more than 500 m thick with a
mean annual ground temperature of about -17oC underlies the
breccia on Melville Island.
Primary sand-wedge polygons
At the top of the Cattle Grid Breccia V-shaped sandstone wedges that are
downwards pointing, 15 cm – 4 M in apparent maximum width, and 3.5 – 3.0
m deep are common (Williams & Tonkin, 1985; Williams, 1986, 1994;
Williams Schmidt, 2004).
The wedges consist of fine- to very coarse-grained sandstone displaying
a divergent or fan-shaped lamination with laminae near the margins of
the wedge paralleling the breccia contact and laminae near the centres
of the wedge is dipping steeply. In the breccia the relict bedding is
turned upwards adjacent to most wedges. The surface of the Cattle Grid
Breccia has grooves that are several metres wide and up to 1 m deep that
overlie large wedges which outline polygons ~1 – 30 m across has been
revealed by mine workings. There are 2 generations of wedges that were
recognised.
At the present sand wedge polygons that are closely comparable with
those in the Cattle Grid Breccia and Whyalla Sandstone are forming in
the arid Arctic and dry valleys of the Antarctic (Péwé, 1959; Washburn,
1980). In the upper part of the permafrost such wedges develop from
vertical, thermal contraction cracks that are ~1–5 mm wide and several
metres deep that outline polygons ~10–30 m across. Following rapid
temperature drops during repeated severe winters the contraction cracks
form. In polar periglacial regions seasonal changes of temperature may
reach depths of more than 15 m, whereas the upper permafrost diurnal
temperature fluctuations may affect the upper permafrost only to a depth
of ≤1 m (Embleton & King, 1975). It has been confirmed by observations
over several decades that under climates that are strongly seasonal the
wedges are forming actively at high latitudes. The contraction cracks
are filled by windblown sand in arid periglacial areas, and the primary
sand wedges that result show lamination that is deeply dipping. Such
wedges can be distinguished from ice wedges, which form in periglacial
areas that are relatively humid where water freezes in the cracks which
may then be replaced on thawing by sediment that commonly exhibits
horizontal layering. During summer expansion lateral pressure causes
upturning of permafrost adjacent to the wedges.
At Mount Gunson it is indicated by the geometry, dimensions and internal
fabric of wedges that they are primarily sand wedges. Further features
that are shared by the sand wedges in the Cattle Grid Breccia and in
Antarctica (Péwé, 1959; Black, 1973) include their development in
bedrock rubble that was produced by the action of frost and the presence
of vertical wedge-in-wedge. Structures that are interpreted as
periglacial sand wedges or fossil-ice wedges are known to occur on other
continents (Deynoux, 1982; Spencer, 1085; Williams, 1986; Moncrieff &
Hambrey, 1990).
Other periglacial forms
The Cattle Grid Breccia and the Whyalla Sandstone are both involved in
the following structures (Williams & Tonkin, 1985; Williams, 1986):
·
In the top of the Cattle Grid Breccia sags of the Whyalla Sandstone form
that are as much as 2 m deep.
·
Rare diapiric tongues of breccia penetrate upwards into the lowermost
few metres of the Whyalla Sandstone.
·
Tabular blocks of quartzite that are up to 1 m long that are inclined
steeply within the Whyalla Sandstone 1-3 m above the top of the Cattle
Grid Breccia.
These structures were interpreted as periglacial involutions,
periglacial injections and frost-hardened blocks, respectively Embleton
& King, 1975; Washburn, 1980). It is suggested by the breccia tongues
that liquefaction of the topmost breccia as a result of permafrost
thawing in the Cattle Grid Breccia. In the Whyalla Sandstone probable
water-escape structures include sinuous trails that are near vertical of
disrupted bedding above sand wedges, and vertical columns of sandstone
that are well cemented that are 30-40 cm in diameter. In the basal
Whyalla Sandstone the presence of these structures, together with
primary sand wedges, shows that periglacial Cryogenic activity continued
for some time following the burial of the Cattle Grid Breccia.
The presence of liquid water and liquefied sediment at the top of the
permafrost horizon is implied by many features of the topmost Cattle
Grid Breccia and the lowermost Whyalla Sandstone (Williams & Tonkin
1985; Williams, 1998b):
·
Reworked breccia horizon.
·
Channel forms that contain well-rounded pebbles.
·
Thermokarst microtopography.
·
Periglacial involutions.
·
Diapiric breccia injections.
·
Water-escape structures.
An active layer at the top
of the permafrost is attested to by these features. The active layer was
defined (Washburn, p. 57) as the “layer of ground above permafrost which
thaws in summer and refreezes in
winter”.
Periglacial aeolianite
The Whyalla Sandstone, which is flat lying and 165 m thick (Coates &
Preiss, 1987; Preiss, 1993; Williams, 1998b) covers 25,000 km2
in outcrop and subcrop on the Stuart Shelf. The formation consists
mostly of medium- to very coarse-grained quartzose sandstone that is
moderately well assorted, bimodal, with rounded, frosted quartz grains
that are 1-2 mm across and subrounded to subangular, fine (50 -200 μm)
grains of quartz, feldspar, and lithic fragments. Bands of rounded
granules, and small pebbles of quartz and lithic fragments, that occur
locally. Commonly, the sandstone is intercalated with gritty or granule
bearing siltstone, and in places at contacts with underlying or
overlying formations and in the east the transition to the Elatina
Formation, there are argillaceous beds. The sandstone weathers yellowish
grey and light grey. Reddish brown and pale greyish red to pale red
colours predominate below depths of 60 m, which results from ultrafine
haematitic pigment in the matrix and haematitic staining on detrital
grains. In the formation, regional fining is towards the southeast of
the present.
Sedimentary structures
The Whyalla Sandstone displays a variety of structures (Williams,
1998b):
·
The principal stratification type is represented by low angle (≤15o)
strata.
·
Large-scale cross-bedded sets that are up to 7 m thick occur mainly in
the central area of the sandstone body. Cross-stratal inclinations are
mostly up to 20-23o and rarely up to 30o, with
most being inclined towards what is the southeast of the present.
·
Inversely graded subcritically climbing translatent strata, which are
diagnostic of aeolian deposition (Hunter, 1977). Grainfall laminae and
grainflow deposits and pin-stripe lamination.
·
2 generations of primary sand wedges at the base of the formation.
·
Drop involutions, periglacial injections and frost-heaved blocks that
involve the Cattle Grid Breccia and the Whyalla Sandstone.
·
Water-escape structures, thermokarst microtopography, and small-scale,
soft sediment tensional faults which suggest collapse caused by snow
melt.
Depositional environment
An origin that is predominantly aeolian is indicated by the primary
sedimentary structures, and it is implied by the periglacial structures
that the climate was cold and arid. The preponderance of low-angle
strata that display locally periglacial cryogenic structures that accord
with a periglacial sand-sheet environment (Koster, 1988; Dijkmans &
Törnqvist, 1991), with most dune forms occurring near the centre of the
sand sheet.
Cross-bedding attitudes and the regional fining direction indicated by
palaeomagnetic data for the Elatina Formation that the winds were
palaeowesterly to palaeonorthwesterly surface winds near the
palaeoequator during the Elatina Glaciation. It is implied by the
subdued topography and the absence of glaciation in the late Cryogenian
of the Gawler Craton in the palaeowest and palaeonorthwest (Coats &
Preiss, 1987; Preiss, 1983) that the wind direction, rather than the
katabatic winds blowing off an ice cap or ice sheet has been recorded.
The contact between the Yerelina Subgroup and the Nuccaleena Formation
was interpreted as a sequence boundary that was marked by a
disconformity to unconformity of very low angle (Preiss, 2000).
Throughout the Central Flinders Zone, the Elatina-Nuccaleena contact
varies from generally sharp to gradational over a thickness of ~0.5. In
Enorama Creek, at the Ediacaran GSSP the gradation is from silty
dolostone downwards into reddish, fine-grained sandstone that is
~0.2-0.3 m thick, which has an irregular, probably erosional base over
reddish siltstone ~0.3 m thick. The siltstone is poor in clasts, though
grades down into diamictite of the Elatina Formation. This interface
between the sandstone and the siltstone may be a sequence boundary that
marks the base of the Nuccaleena Formation, with the sandstone possibly
being a local manifestation of the Seacliff Sandstone.
Cap carbonate
Contact with the Yerelina Subgroup
A persistent marker through the Adelaide Geosyncline region is the
Nuccaleena Formation, and particularly its dolostone member (Forbes &
Preiss, 1987) that represents the cap carbonate of the Yerelina
Subgroup. The contact between the Nuccaleena Formation and the Yerelina
Subgroup is interpreted as a sequence boundary that is marked by a
disconformity to a very low angle unconformity (Preiss, 2000). The
Elatina-Nuccaleena contact varies from generally sharp to what appears
to be gradational over a thickness of ~0.5 m. At the Ediacaran GSSP the
gradation if from silty dolostone downwards into reddish, fine-grained
sandstone ~0.2-0.3 m thick, which has an irregular base that is probably
erosional, over reddish siltstone ~0.3 m thick. The siltstone is poor in
clasts, but grades down into diamictite of the Elatina Formation.
Williams et al. suggest the
interface between the sandstone and the siltstone may be a sequence that
marks the base of the Nuccaleena Formation, with the sandstone possibly
being a local manifestation of the Seacliff Sandstone.
To the east, at Chambers Gorge, the Elatina-Nuccaleena contact is an
unconformity that is of very low angle. At this sequence boundary
erosion is very pronounced in the North Flinders Zone where several
different stratigraphic units of the Yerelina Subgroup are overlain by
the dolostone member (Ambrose, 1973; Preiss, 2000). The primary
stratigraphic evidence is consistent with isostatic rebound of the
Curnamona Province in the east and possibly an appreciable delay between
the end of the Elatina glaciation and the following marine
transgression, though the erosional contact seems to be a peneplained
surface that lacked major relief (Knoll et
al., 2006).
Sedimentary facies
The Nuccaleena Formation, at its type section near Mount Saturday, is
comprised of 10 m of cream, well-bedded dolostone that is overlain by up
to 60 m of purple shale that contains rare dolostone bands. The
Nuccaleena Formation, however, usually lacks the upper shale member. The
range in thickness of the lower, dolostone member is from several metres
along the western margin of the Adelaide Geosyncline, and on the Stuart
Shelf, 5-17 m in the Central Flinders Zone and the Curnamona Province
where it forms part of the cratonic cover succession. Along the southern
Torrens Hinge Zone, in the Hallett Cove area, and in some areas of the
Nackara Arc, laminated, pale bluff dolostone that is characteristic of
the Nuccaleena Formation intertongues with the Seacliff Sandstone, which
is up to several hundred metres thick (Forbes & Preiss, 1987; Dyson,
1992; Preiss, 2000). The dolomitic member that is characteristic of the
Nuccaleena Formation typically commences with a pink, silty dolostone
that passes upwards into buff, well laminated to massive, micritic
dolostone, which in places, forms linear tepee-like structures that are
up to 1.5 m high and are associated with growth faults and sheet veins
(Plummer, 1979; Gammon et al.,
2005). In turn, this dolostone grades upwards through silty dolostone,
flaggy lenticular dolostone that is interbedded with red shale, to red
shale (blue Shale in the eastern Nackara Arc). The top of the Nuccaleena
Formation is defined by the last appearance of dolostone (Lemon & Gostin,
1990). The uppermost shale, which is believed to possibly record the
point of maximum flooding (Preiss, 200), is succeeded by the Brachina
Formation.
Deposition and diagenesis
The sandstone above the Elatina Formation, and the dolostone of the
Nuccaleena Formation were interpreted as a transgressive tract (Gammon
et al., 2005). It is believed
that the primary carbonate was calcite that was deposited in a marine
shelf setting up to several hundred metres deep (Knoll et
al., 2006). Thin beds of
dolomicrite that has red brown shale partings and current ripples may be
turbidites (Kennedy, 1996). Contrary to the impression that was conveyed
by others (Halverson et al.,
2004; Knoll et al., 2006),
however, none of those features are present in the Nuccaleena Formation.
Based on δ13C values in 10 different stratigraphic sections
throughout the Adelaide Geosyncline, the distinctive carbon isotopic
profile of the Nuccaleena Formation decreases upwards from -1 to -2.5‰
at its base to -2 to -3.5‰ at its top (McKirdy et
al., 2001, and in
preparation). The profile in its shape and absolute values is broadly
similar to profiles of carbonates from elsewhere that date to Early
Ediacaran, notwithstanding its uncertain significance of its lateral
variability across the basin (1.5‰) that is yet to be determined more
certainly. Knoll et al.
(2006, p. 23) was led by this attribute to interpret the Nuccaleena
Dolostone Member as representing a “short-lived chemical oceanic event
accompanying Marinoan deglaciation and the sea level rise, and to be of
global extent”. Recent work has shown, however, that more than 90% of
the Nuccaleena Cap Carbonate is comprised of dolomicrospar with a
radical geochemical and isotopic zonation that could have formed only
via early diagenetic organogenic dolomitisation (McKirdy et
al., 2995; Gammon, 2006). It
does not appear likely that the Nuccaleena Formation records the
carbon-isotopic composition of the post-glacial ocean.
As was first noted (lemon, 1988) and (Lemon & Gostin, 1990), the
unusually large tepee-like structures in the Nuccaleena Formation do not
contain many of the characteristic features of peritidal tepees (e.g.
Assereto & Kendall, 1977) and evidently formed at a time when the host
sediment was submerged entirely. It was indicated by recent study of
such structures (Smith, 2001; Gammon et
al., 2005) that they
developed as much as 7 m below the sediment-water interface, and that
they as well as their associated sheet veins were the result of fluid
overpressure that was generated by the expansive crystallisation of
dolomicrite during early diagenesis.
Palaeoenvironment of the Elatina glaciation
Palaeogeography
A wide range of depositional environments, which together cover an area
of ~200,000 km2, that mark glaciation in the late Cryogenian
that occurs immediately before that Ediacaran:
·
In the west, a permafrost regolith that was blanketed by a periglacial
aeolian sand sheet that formed on the Stuart Shelf at the eastern margin
of the Gawler Craton. There was little or no glaciation in this cratonic
region, possibly because of its aridity and low relief, and wide areas
remained free of ice. Palaeowesterly to palaeonorthwesterly winds
prevailed and a climate that was frigid and strongly seasonal prevailed
near sea level.
·
To the east, in the Central Flinders Zone, fluvial, deltaic and inner
marine shelf settings of the Elatina Formation prevailed, with deposits
that were glaciomarine and local glaciers on emergent diapiric islands
extending to sea level.
·
In the North Flinders Zone, the thick (>1,500 m) glaciogenic succession
of the Yerelina Subgroup accumulated in an intracratonic sub basin in an
outer marine-shelf setting. To the east detritus was derived from
glacial erosion of crystalline basement, where basement rocks of the
Curnamona Province may have been exposed (Preiss, 1987b). To the north,
the Muloorina Ridge, that is now buried, may also have been a source
area.
·
To the southeast, in the Nackara Arc, an outer marine shelf formed by
subsidence along a passive margin and accumulated the glaciogenic
Yerelina Subgroup (>1,000 m), which was derived from glacial erosion of
crystalline basement to the northeast and east.
Major rifting or compressional tectonics, or any bordering terrain that
was highly elevated, have left no record. Only subdued extensional
tectonics, with tilting and down-to-basin faulting involved in the
northern sub-basin and southeastern outer marine shelf (Plummer & Gostin,
1976; Preiss, 1993). Because outer marine settings that were bordered by
cratons mark these regions, ocean currents and waves had little
influence. Under the protection of an ice shelf and icebergs, during
maximum glaciations, wave activity was reduced further, which allowed
the accumulation of massive and laminated calcareous diamictites of the
Mount Curtiss Tillite and Pepuarta Tillite under anoxic conditions. The
presence of wet-bedded glaciers with rainout of fine-grained sediments
and ice rafted debris, attested to widespread deposition of such
glaciomarine diamictites that contained extrabasinal erratics and the
thick mudstone and siltstone deposits that contained scattered
dropstones. The Fortress Hill Formation and Ketchowla Siltstone are
representative of outer marine shelf deposition under glacial conditions
that were waxing and waning, respectively, with the presence of icebergs
indicated by large glacial erratics in the Ketchowla Siltstone.
Grounded ice or icebergs deformed the preglacial Yaltipena Formation,
that were poorly lithified, and depositional diamictites at the base of
the Elatina Formation. Fluctuations of ice margins and sea level are
implied by the deposition of fluvial, deltaic and inner marine shelf
sandstones, and tidalites that originated in estuarine and tidal-delta
environments, then alternated with glaciomarine diamictites that were
deposited from floating ice.
Glacial conditions persisted, and within the basin, grounded ice on
diapiric islands, ice rafting and diamictite deposition continuing to
the top of the Elatina Formation.
It is indicated by the widespread occurrence and persistence of ripple
marks that were generated by waves in the Elatina tidal rhythmites, that
there was a continuously open sea over several decades, the signal of
the annual oscillation of the sea level in these rhythmites implies the
presence of extensive open seas for at least the ~70-year interval of
rhythmites deposition (Williams, 2004; Williams & Schmidt, 2004).
Importantly, it is indicated by the widespread occurrence of diamictites
at the top of the Elatina Formation that glacial conditions returned
after rhythmite deposition. Further evidence that the sea was not frozen
over during the deposition of the Yerelina Subgroup, which permitted the
rainout of fine-grained sediment and ice rafted debris, is provided by
the extensive glaciomarine diamictites with clasts of extrabasinal
origin, and thick formation of laminated siltstone with dropstones.
According to Williams et al.
the various igneous and high grade metamorphic erratics in the
glaciogenic succession were derived from adjoining cratonic areas, had
probably been transported by glaciers or ice streams that emanated from
ice caps that bordered the eastern margin of the Adelaide Geosyncline.
That the extensive cratonic region to the west of the Adelaide
Geosyncline was essentially free of ice is implied by the presence of
silcrete-type palaeosols, a permafrost regolith and an aeolian sand
sheet on the Stuart Shelf and the lack of evidence for glaciation of the
Gawler Craton during the late Cryogenian. It is suggested for most of
the extrabasinal clasts, there was a provenance to the east of the
present day, which includes the Curnamona Province. Their source terrain
has not yet been confirmed, however, and it is implied by some of the
sparsely pebbly nature of the diamictites that this facies was deposited
in settings that were relatively distant.
Palaeoclimate
The Periglacial structures are among the most reliable of palaeoclimate
indicators, as they formed through
physical weathering processes
and, therefore, their interpretation avoids uncertainties that arise
from the former nature of the atmosphere and biosphere and later
diagenetic alteration. Quantitative information on palaeoclimate that is
provided by periglacial structures include indications of mean annual
air temperature (MAAT), seasonal temperature range and mean annual
precipitation (Washburn, 1980; Karte, 1983), as well as longer-term
oscillatilons of climate, Primary sand wedges that have dimensions and
structure that are comparable with Mount Gunson of the present are
confined to arid periglacial regions that are marked by a climate that
is strongly seasonal (Péwé,
1959, Washburn, 1980; Karte, 1983). Such primary wedges are
indicative of a MAAT of <-12 to <-20oC and a mean monthly air
temperature (MMAT) that ranged from <-35oC in mid-winter to
+4oC in mid-summer (Karte, 1983). The Cattle Grid Breccia
that was 20 m thick attests to frost shattering at the top of the
permafrost zone and a frigid climate.
A mean horizontal growth rate of ~1mm/year that was determined for
certain modern sand wedges and ice wedges (Black, 1973; Washburn, 1980)
suggests that at Mount Gunson the sand wedges took up to ~4 ky to form
(Williams & Tonkin, 1985). In the Cattle Grid Breccia and the lowermost
Whyalla Sandstone the development of at least 4 generations of sand
wedges, punctuated by intervals that were marked by permafrost
deformation or aeolian sand deposition which suggests that there were
climate cycles of several thousand years duration that involved large
fluctuations of MAAT, that ranged from -12 to -20oC or lower
up to >0oC.
In modern polar regions the shallow (≤1 m) influence of diurnal
temperature changes on permafrost (Embleton & King, 1975) and at high
elevations on the equator of the Pleistocene the lack of periglacial
wedges, where MAAT was well below 0oC for several millennia
and fluctuations of just a few oC were mainly diurnal
(Williams & Schmidt, 2004) militate against the claim (Maloof et
al., 2002), that was based on
numerical models that diurnal temperature fluctuations produced the 3 m
deep sand wedges at Mount Gunson during the late Cryogenian. It was
conceded (Maloof et al.,
2002) that experiments would be required to ascertain if the strain that
was caused by the variations of diurnal temperature were squandered in
microcracking. Moreover, the length of day of 21.9 h in the late
Cryogenian that derived from the Elatina-Reynella palaeotidal data set
indicates that the diurnal cycle was shorter and therefore less
effective as a geocryologic agent than the cycle of the present. An
actualistic approach, which was based on more than a century of
observations and research on periglacial geomorphology and processes
argue strongly that the primary sand wedges as well as the associated
suite of large-scale periglacial structures at Mount Gunson record a
periglacial climate that was strongly seasonal.
In summary, by analogy with the climate significance applied to
periglacial structures that are comparable in modern polar regions
(Washburn, 1980; Karte, 1983), at Mount Gunson the suite of periglacial
structures implies that the climate in South Australia in the late
Cryogenian displayed the following features:
·
MAAT near sea level of -12 to -20oC or lower.
·
Large seasonal temperature range from -35oC or lower in
mid-winter up to +4oC in mid-summer (MMAT range of up to ~40oC).
·
Aridity (<100 mm mean annual precipitation) and windiness.
·
Summer temperatures above freezing to produce an active layer at the top
of the permafrost.
·
Fluctuations in MAAT that rage from as low as -20oC or lower
up to >0oC on a 103 year timescale.
This frigid climate, that is strongly seasonal prevailed at sea level at
the margin of a marine basin within 100 of the palaeoequator.
The implications that MAAT rose above freezing, annually as well as
through long-term temperature changes is consistent with other evidence
that
long-lived open seas existed during the Elatina Glaciation.
Palaeolatitude
Currently, the most robust data that indicates a low palaeolatitude
during the Cryogenian glaciation comes from detailed palaeomagnetic
studies of red beds from the Elatina Formation in the Adelaide
Geosyncline and equivalent strata on the Stuart Shelf (Embleton &
Williams, 1986; Schmidt et al.,
1991; Schmidt & Williams, 1995; Sohl et
al., 1999; Williams &
Schmidt, 2004). The early timing of magnetic remanence for the Elatina
Formation is confirmed by fold tests that were executed on small folds
of demonstrable soft-sediment origin (Williams, 1996), which yielded
positive results at a high level of confidence (Sumner et
al., 1987; Schmidt et
al., 1991; Schmidt &
Williams, 1995) and by the presence of sequential magnetic reversals
(Schmidt & Williams, 1995; Sohl et
al., 1999). The remanence was
acquired during deposition through detrital remanent magnetisation (DRM)
carried by silt-sized grains of high-temperature titano-haematite as
well as soon after deposition during early diagenesis through chemical
remanent magnetisation (CRM) carried by ultrafine haematite in the
matrix and as coatings on grains. All the palaeomagnetic reliability
criteria of Van der Voo (1990) are satisfied by the data for the Elatina
Formation.
It is implied by the observation of mixed polarities within the
specimens from Elatina that the CRM was acquired at the time scale of
magnetic reversals; therefore, it is appropriate to employ sample
directions to provide an overall formation mean direction (Schmidt &
Williams, 1995). For the Elatina Formation, combined data (Schmidt &
Williams, 1995; Sohl et al.,
1999; Geological Society of America data Repository item) for 205
samples yielded a mean declination
D = 208.3o and a
mean inclination of I =
-12.9o (α95 = 4.2o ) and a
palaeolatitude of 6.5 ± 2.2o
(P.W. Schmidt, personal communication, 2006). The palaeomagnetic
remanence of Elatina has inclinations that are persistently low for
different lithologies and for samples that were acquired from Basinal
and cratonic tectonic settings. It is also shown by the remanence that
anisotropy of magnetic susceptibility (AMS) of 4% for 44 samples that
carry CRM and 2.1% of 21 samples that carry DRM (P.W. Schmidt & G.E,
Williams, unpublished data) that indicates slight magnetic foliation (Enkin
et al., 2003). It is implied
by these findings that there is minimal shallowing of inclination. It
was concluded that in the Adelaide Geosyncline the Elatina glaciation
took place at a palaeolatitude of <10o. This is consistent
with the palaeolatitude of 8.4 + 6.2/-5.7o that was
determined for the Yaltipena Formation that is immediately periglacial (Sohl
et al., 1999). Palaeomagnetic
data that is available accord with the geomagnetic field approximating a
geocentric axial dipole (GAD) of the Earth during the Proterozoic
(Schmidt, 2001; Williams & Schmidt, 2004; McElhinny, 2004) concluded
that the GAD model is a good first-order approximation probably for the
whole of geological time. Therefore, the low palaeolatitudes that were
determined for the Elatina Formation, as well as other glaciogenic
successions in Australia dating to the Cryogenian (Pisarevsky et
al., 2001, 2007) and other
continents (e.g. Park, 1997; Evans, 2000; Kempf et
al., 2000; Kilner et
al., 2005; Weil et
al., 2006) and the lack of
palaeomagnetic data for glaciogenic deposits that indicate high
palaeolatitude glaciation in the Cryogenian, present a major geophysical
and geological enigma.
Discussion and conclusions
The Elatina glaciation occurred in the Adelaide Geosyncline-Stuart Shelf
region of South Australia in the Cryogenian at a time that has been
undetermined between about 640 Ma and 580 Ma. A wide range of facies
displayed by the Yerelina Subgroup:
·
Permafrost regolith with many large scale cryogenic structures;
·
Periglacial aeolianite;
·
Littoral and neritic deposits that included tidalites and evaporites;
·
Basal diamictite;
·
Fluvial, deltaic and inner marine shelf sandstones;
·
Glaciomarine diamictites; and
·
Outer marine shelf mudstone and siltstone with drop stones that were ice
rafted.
The Yerelina Subgroup with its variety of facies has much in common with
the glaciogenic succession from the Cryogenian in the fjord region of
East Greenland (Moncrieff & Hambrey, 1990).
Insight into the climate and the extent of sea ice during the glaciation
of the late Cryogenian is provided by the various facies of the Yerelina
Subgroup. An in situ cold
climate near sea level (mean annual air temperature -12 to -20oC
or lower in mid-winter up to +4oC in mid-summer) is implied
by the cryogenic structures and changes in the longer-term (103
years) in mean annual air temperature. The advance of grounded ice or
icebergs is indicated by the presence of glacitectonites displaying
evidence of penetrative deformation of preglacial deposits that are
poorly lithified beneath the Elatina Formation. Ripple marks that were
wave-generated and the signal of the annual sea level oscillation that
are displayed by the Elatina rhythmites, and the widespread occurrence
of glaciomarine diamictites and thick mudstone/siltstone with drop stone
facies of the Yerelina Subgroup indicates the presence of extensive and
persistent open seas during the Elatina glaciation. Glacial deposition
continued to the top of the subgroup.
It is indicated by high quality palaeomagnetic data for red beds from
the Elatina Formation that this frigid, strongly seasonal
glacial-periglacial climate, in which there was grounded sea ice near
sea level, occurred within 10o of the palaeoequator. The
recognition of such a non-actualistic glacial climate in South Australia
in the late Cryogenian has been a catalyst for Cryogenian
glaciogenic successions around the world and has helped to
stimulate the development of global models, accompanied by a lot of
vigorous debate, to explain the underlying cause of such a paradoxical
climate (e.g. Williams, 1993; Hoffman et
al., 1998; Williams &
Schmidt, 2004; Hoffman & Schrag, 2002; Jenkins et
al., 2004; Fairchild &
Kennedy, 2007).
The Nuccaleena Formation from the Early Ediacaran, which overlies the
Yerelina Subgroup disconformably to unconformably, represents the cap
carbonate that was deposited during the marine transgression in
post-glacial times. The Nuccaleena Formation probably doesn’t record the
carbon-isotope composition of the post-glacial ocean as >90% of its
carbonate is comprised of dolomicrospar that formed through early
diagenetic organogenic dolomitisation.
According to Williams et al.
research on Elatina glaciation deposits in the future should aim at
determining an accurate age for the Yerelina Subgroup in South
Australia, preferably by identifying volcaniclastic layers, if present,
that have yet to be discovered, in order to test correlations that have
been proposed with other glaciogenic successions. Detailed study is also
required on detrital zircons and on the nature and provenances of the
extrabasinal clasts in order to illuminate the palaeogeography of the
Elatina glaciation within Australia and adjoining continents. Such work
will elucidate the temporal relationship of the Elatina glaciation to
the glaciation of the late Cryogenian elsewhere and thereby establish
whether it is part of a single, near-global event, or alternatively one
of a series of glaciations that were more local that affected different
parts of the Earth at slightly different times, some or all of which may
have been recorded in South Australia only as boundaries of sequences.
Williams, G. E., et al. (2008). "The Elatina glaciation, late Cryogenian
(Marinoan Epoch), South Australia: Sedimentary facies and
palaeoenvironments." Precambrian Research 163(3): 307-331.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |