significant terrestrial influences. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Ironstones; Geochemistry; Glaciation; Adelaide Geosyncline; Neoproterozoic; South Australia
1. Introduction
Glaciogenic and iron-rich rocks are intimately associated in Neoproterozoic sequences. Examples
are known from North and South America e.g. Young, 1976; Yeo, 1983; Urban et al., 1992; Klein
and Beukes, 1993; Graf et al., 1994, Africa e.g. Breitkopf, 1986; Bu¨hn et al., 1992, China Rui
and Piper, 1997, and South Australia Whitten, 1970. In some cases, the iron-rich rocks are
present as iron-rich clastic sediments e.g. South Australia; Whitten, 1970; China; Rui and Piper,
1997, whereas others occur as ironstones e.g. Klein and Beukes, 1993. However, despite nu-
merous research efforts, the reason for the com- mon association of iron- and carbonate-rich and
glaciogenic rocks in the Neoproterozoic, the ap- parent glaciation in low-latitude environments
during that time, and the source of chemical components and genesis of ironstones remain
controversial.
Neoproterozoic sedimentary rocks of the Ade- laide Geosyncline in South Australia and far west-
ern New South Wales host well preserved glaciomarine sequences and associated ferrugi-
nous units Preiss, 1987; Preiss et al., 1993; Fig. 1. These ferruginous rocks are rich in magnetite
andor hematite, iron-bearing silicates and car- bonates. The purpose of this paper is to describe
the geochemical composition of Braemar iron- stones, to establish the genetic processes responsi-
ble
for their
formation and
the palaeoenvironment of deposition, and to discuss
their genesis in light of other models for Neoproterozoic glaciogenic and iron-rich rock
occurrences.
2. Geology
The Adelaide Geosyncline in South Australia is a major, deeply subsident Neoproterozoic to
Cambrian sedimentary
basin which
overlies Palaeoproterozoic to Mesoproterozoic metamor-
phic basement rocks. It contains one of the most complete and well preserved Neoproterozoic suc-
cessions and displays evidence of two major glaci- ations during the Neoproterozoic, the Sturtian
: 750 – 700 Ma and Marinoan glaciation : 650 – 600 Ma; Preiss, 1987; Preiss et al., 1993. The
widespread Sturtian glaciation event is manifest in the Umberatana Group Preiss et al., 1998, and
particularly
in the
great thicknesses
of glaciomarine sedimentary rocks deposited in the
fault-controlled Baratta Trough, extending from the central Flinders Ranges to the Yunta-Olary
region in eastern South Australia Sumartojo and Gostin, 1976; Preiss, 1987; Preiss et al., 1993; Fig.
1. Much of the glaciogenic sedimentation in the Umberatana Group is characterised by diamictite,
laminated siltstone and orthoquartzite, but in places there are distinctive intercalated dolomitic
and ferruginous units Preiss, 1987; Preiss et al., 1993. In this paper, diamictite is a nongenetic
term referring to poorly sorted siliciclastic sedi- mentary rocks containing a wide range of class
sizes in an abundant fine-grained matrix in which the clasts are dispersed so that most of them are
not in contact cf. Panahi and Young, 1997.
The section in the Barratta Trough comprises the diamictite- and quartzite-dominated Pualco
Tillite 3300 m and overlying siltstone- and sand- stone-dominated Benda Siltstone 260 m; Preiss et
al., 1993. Both units pass laterally into the lentic- ular, ferruginous Braemar ironstone facies in the
Yunta-Olary region and the thinner Holowilena Ironstone 130 m in the central Flinders Ranges
Preiss, 1987; Preiss et al., 1993. In places, the Pualco Tillite passes vertically into the Braemar
ironstone facies also known as the Hoof Hearted Formation. The latter, although locally lenticu-
lar, is widespread in the Yunta-Olary region and a possible equivalent is present southwest of Broken
Hill in western New South Wales Preiss, 1987. In the Yunta area, four to six lenticular ironstone
units grade into the host diamictites and siltstones
with decreasing iron minerals, but also contain dolostone beds and quartzites Whitten, 1970;
Figs. 2 and 3. Throughout the Yunta-Olary re- gion, ironstone occurrences crop out prominently
Fig. 4A and are interbedded with diamictites, carbonate-rich rocks, quartzites in part with
heavy mineral lamination, siltstones and man- ganiferous siltstones. The distribution of the iron-
stone and associated ferruginous siltstones and diamictites is especially notable on aeromagnetic
images. The ironstones are particularly prominent at Razorback Ridge south of Yunta Fig. 2,
where the thickest iron-rich sub-units have been evaluated as a potential iron ore resource Whit-
ten, 1970. Although at some locations, there is only one prominently ferruginous horizon, at
many locations, there are several zones e.g. 3 or 4, separated by tens to hundreds of metres of
other strata, e.g. in the Bimbowrie Hill region and at Razorback Ridge Fig. 3.
Fig. 1. Location of the Adelaide Geosyncline in South Australia showing the inferred distribution of Sturtian ferruginous facies of the Umberatana Group in the Baratta Trough modified from Preiss et al., 1993.
Fig. 2. Outcrop of ferruginous facies Braemar ironstone facies and associated Sturtian glaciogenic rocks Pualco Tillite in the eastern part of the Adelaide Geosyncline modified from Rogers, 1978; Forbes, 1991.
3. Sampling and methods of analysis
