no volcanicity to the depositional environment of the Braemar ironstone.
Deposition of the Pualco Tillite diamictites during the Sturtian glacial maximum was re-
stricted to the marine Baratta Trough and the main depocentre was in the Braemar area
Preiss, 1987. The diamictites have been inter- preted as glaciomarine sediments, deposited from
wet-based
glaciers originating
from the
Palaeoproterozoic to Mesoproterozoic Willyama basement Curnamona Cratonic Nucleus and
debouching into a marine basin Preiss et al., 1993. The lack of local detritus in the basal
diamictite and the generally regionally planar de- positional surface probably imply deposition of
the Pualco Tillite from an extensive floating ice- sheet Preiss, 1987. Reworking of the diamic-
tites, possibly by water currents, is indicated by interbedded quartzites, or the quartzites may
have been derived from a different, more mature sediment source than the associated diamictites.
The Pualco Tillite rests on a slightly irregular erosional surface developed on the Burra Group,
or locally on crystalline basement, over the whole Olary province Preiss, 1987. It contains
basement-derived material near Olary, probably shed from the exposed Willyama Inliers. An
800-m long slide block of granite in the Pualco Tillite adjacent to the MacDonald Fault and
lenticular granite conglomerates suggest an ac- tively rising fault scarp in the Olary region and
it is thus interpreted that the diamictites and conglomerates may have been deposited in deep
glacial valleys of a highland terrain Preiss, 1987. Faulting is less obvious at the other mar-
gins of the Barratta Trough, which may have been a halfgraben. The preservation of pre-
glacial regoliths in parts of the southern and central Flinders Ranges, as well as possibly on
the Stuart Shelf, suggests that the lowlands to the west were not severely glaciated Preiss,
1987. These sedimentological data imply that the Braemar ironstones accumulated in a basin
along the border of a continental glaciated high- land to the northeast and a low-lying weathered
landmass to the west.
8. Genesis
8
.
1
. Formation of the Braemar facies The role of glaciation in the formation of
Neoproterozoic ironstones has been emphasised by a number of authors Yeo, 1983; Urban et
al., 1992; Klein and Beukes, 1993; Graf et al., 1994. The occurrence of ironstones in several
Neoproterozoic glacial deposits inspired the hy- pothesis that during glaciation, the underlying
stagnant seawater was cut off from oxygen sup- ply and was rendered anoxic by organic matter
decomposition. Build-up of dissolved Fe oc- curred in the Proterozoic oceans during glacial
periods and deposition of Fe followed during transgressive interglacial periods e.g. Urban et
al., 1992; Klein and Beukes, 1993. In fact, the ‘snowball-type Earth’ theory suggests the pres-
ence of floating pack ice over most of the ocean surface at middle to high latitudes as well as
equatorial glaciation Kirschvink, 1992. Oxy- genation of ferriferous waters after glaciation
would drive the precipitation of ferric oxide in oxic and highly oversaturated surface waters cf.
Kaufman et al., 1991; Hoffman et al., 1998.
Previous authors have assigned the origin of the Braemar ironstones to chemical precipitation
in a lacustrine environment Preiss, 1987 or to physical accumulation of detrital Fe oxides
Whitten, 1970; Preiss, 1987. However, our compositional data show that the Braemar facies
was generated by the intermixing of chemical precipitates and terrigenous debris on a conti-
nental margin. Evaporation of waters in a playa- lake complex cf. Eugster and Chou, 1973 did
not produce Braemar ironstones as indicated by the palaeogeographic environment.
The possibility that ferriferous exhalations have influenced the formation of the Braemar
ironstone is supported for the following reason. While the rocks exhibit exceptionally low con-
centrations of As, Cu, Pb and Zn compared to upper crustal abundances values Taylor and
McLennan, 1981, they exhibit positive correla- tions of Fe with As, Cu, Sb, V and Zn, elements
typical of ironstones formed under hydrothermal influences e.g. Dymek and Klein, 1988; Duhig
et al., 1992; Lottermoser and Ashley, 1996; Ash-
ley et al., 1998. Thus the Fe of the Braemar ironstone originally derived from hydrothermal
exhalations. The Braemar ironstone facies consists of lentic-
ular laminated and diamictic ironstones and di- amictic ironstones are interbedded with Braemar
facies devoid of, and also with, dropstones Fig. 3c. Thus Fe deposition was clearly associated
with ice-melting and clastic sedimentation and occurred during openwater ocean circulation and
melting of icebergs. The presence of Fe oxide-rich diamictites indicates that refrigeration prevailed
during Braemar facies deposition, and was proba- bly maintained throughout deposition of the
Benda Siltstone as indicated by scattered drop- stones brought in by floating icebergs cf. Preiss,
1987. Thus the Braemar facies, including the dolostones, was deposited during the waning
stages of the Pualco glaciation, during a climate change from deep refrigeration to slightly warmer
temperatures. When the sea ice retreated, oxy- genation of the water column of ferriferous +
manganiferous, carbonate and CO
2
charged waters occurred cf. Kaufman et al., 1991; Hoff-
man et al., 1998. These waters precipitated car- bonate upon release of CO
2
to the atmosphere cf. Kaufman et al., 1991; Hoffman et al., 1998 and
hence the dolostones of the Braemar facies are interpreted as inorganic cold water deposits.
During glacial periods build-up of dissolved, reduced hydrothermal Fe occurred. A subsequent
transgressive event during an interglacial period and associated melting of floating ice led to the
oxidation of coastal seawater and the precipita- tion of dissolved Fe. Coprecipitation and adsorp-
tion of REE from the water column caused a coastal seawater REE signature of the chemical
sediments. Intercalations of Fe-poor siltstones and diamictites between the ferruginous facies
Fig. 3c would indicate an episodic decrease in Fe precipitation and dominating clastic sedimenta-
tion. These siliceous, aluminous sediments ob- tained
a detrital
REE signature.
Climate controlled regressions and transgressions of the
sea ice are the most likely reason for the presence of intercalated non-ferruginous clastic sediments
in diamictic and laminated ironstones. Redox po- tential differences kept Mn in solution, however,
increasing oxidising conditions led to the precipi- tation of Mn oxides or carbonates and their in-
corporation in clastic sediments and the resulting formation of manganiferous siltstones cf. Urban
et al., 1992; Manikyamba and Naqvi, 1995.
8
.
2
. Geotectonic setting The palaeolatitude of Neoproterozoic iron-
stones and associated glacial sediments has been controversial. Meert and Van der Voo 1994
argued, using
palaeomagnetic data,
that Neoproterozoic glaciations did not occur below
25° latitude. However, recent palaeomagnetic data indicate that the formation of Neoproterozoic
ironstones and associated glacial sediments oc- curred in low-latitude environments. The Mari-
noan glaciation in South Australia 650 – 600 Ma, including
permafrost, grounded
glaciers and
marine glacial deposition, occurred near the palaeoequator Schmidt and Williams, 1995.
Similarly, the formation of the Rapitan ironstone and associated glacial sediments ca. 725 Ma of
northwestern Canada occurred in a low-latitude environment Park, 1997. Young 1992, 1995
proposed that the Sturtian glaciation in the Ade- laide Geosyncline corresponds to the Rapitan
glaciation in northwestern Canada. Such strati- graphic correlations would imply that the iron-
stones
of the
Sturtian Braemar
facies ca.
750 – 700 Ma were deposited in a low-latitude environment. However, further geochronological
and palaeomagnetic studies will be required to establish whether the Sturtian and Rapitan glacia-
tions are diachronous or synchronous. In addi- tion, Rui and Piper 1997 stated that the
sedimentary cycles recognised in the Neoprotero- zoic successions of Australia and Canada were
probably driven by global eustatic changes and do not imply close proximity of the two regions.
9. Conclusions
