Low-latitude glaciation in the Neoproterozoic of Oman
Ben Kilner

Conall Mac Niocaill 
Martin Brasier

Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
ABSTRACT
Although Earth is widely believed to have undergone a series of extreme low-latitude
snowball glaciations during the Neoproterozoic (ca. 1000–543 Ma), only one reliable paleomagnetic result, from Elatina, South Australia, places glacial rocks close to the equator.
We report new paleomagnetic data from the Neoproterozoic Huqf Supergroup of Oman
that pass fold and reversal tests and yield a paleopole at 52.38S, 074.48E (N 5 25 sites;
a95 5 7.38). This paleopole places the Muscat region of Oman at a latitude of 138 in the
late Neoproterozoic and provides the first direct evidence that both glacial and overlying
cap carbonate units were deposited in the tropics. The presence of glacial-interglacial
cyclicity within the Huqf Supergroup indicates that areas close to the equator may have
been largely free of ice at the time of deposition, a result that is inconsistent with the
classic snowball Earth model. Our result precludes the possibility that contrasting lithologies mark a phase of rapid plate motion and provides the first evidence for low-latitude
glaciation in Arabia. A series of magnetic reversals in the Fiq tillite and the overlying
Hadash dolomite, in northern and central Oman, correlates well with a similar sequence
in the Mirbat Formation in southern Oman and indicates that recovery from glacial conditions took place over long time scales (possibly .105–106 yr).
Keywords: Oman, Neoproterozoic, paleomagnetism, glaciation, Marinoan, Arabia.
INTRODUCTION
The late Neoproterozoic interval was
marked by the most extreme climate change
known in Earth’s history, possibly including
several episodes of global refrigeration (e.g.,
(Hoffman et al., 1998; Kennedy et al., 1998;
Evans, 2000; Hoffmann et al., 2004). These
have generally been grouped into two major
glacial phases: the Sturtian, ca. 745 Ma to 670
Ma (Lund et al., 2003) and the Marinoan, ca.
600 Ma (Evans, 2000), but there is still considerable uncertainty about their precise chronology. Probable Marinoan glacial deposits in
Tasmania have been dated as ca. 580 Ma, and
have been correlated with the Gaskiers Formation of Newfoundland (Calver et al., 2004).
In contrast, the Ghaub Formation of Namibia
is dated as 635 6 1.2 Ma, and is also bracketed as a Marinoan glaciation (Hoffmann et
al., 2004). Thus the Marinoan glaciation event
was either a long-lasting event of global significance, or incorporates multiple nonsynchronous smaller events, and many of the
proposed correlations between different continents are incorrect (e.g., Hoffmann et al.,
2004).
Nevertheless there is abundant stratigraphic
evidence for major glaciations at that time,
and several workers have argued for a completely frozen ocean during these events, temporarily arresting the hydrological cycle and
potentially causing catastrophic extinction, in
a so-called snowball Earth (Kirschvink, 1992;
Hoffman et al., 1998; Hoffman and Schrag,
2002). The snowball Earth model predicts a
long-lasting (;106–107 yr) ice-covered ocean,
followed by a runaway greenhouse world in
which cap carbonates are deposited in a relatively short period of time (103–104 yr). Evidence supporting low-latitude glaciation includes the ubiquitous occurrence of tillites in
late Neoproterozoic sedimentary successions
and the apparent conformity of these glacial
deposits with overlying putative warm-water
carbonates (Hoffman and Schrag, 2002). To
date, however, there is only one paleomagnetic
result, from the Elatina Formation of South
Australia, that directly places the Marinoan
glaciation in equatorial latitudes (Schmidt and
Williams, 1995; Sohl et al., 1999; Evans,
2000).
The Huqf Supergroup of Oman (Fig. 1) preserves a continuous Neoproterozoic to lower
Paleozoic sedimentary sequence that contains
evidence for at least two large glacial events;
i.e., the tillites of the Ghubrah and Fiq Formations. The age of the Huqf Supergroup is
constrained by a U-Pb zircon age from an ash
unit in the lower part of the group, which has
been dated at 711.8 6 1.6 Ma (Allen et al.,
2002), whereas at the top of the unit a date of
544.5 6 3 Ma suggests deposition into the
Early Cambrian (Amthor et al., 2003). The
Ghubrah therefore fits in the Sturtian glacial
bracket, between 750 and 695 Ma, and is
thought to correlate with other glaciogenic
sections, such as the Pocatello Formation of
Idaho (Fanning and Link, 2004). The Fiq Formation unconformably overlies the Ghubrah
and Saqla Formations in Oman. It has previously been assigned a Marinoan age, based on
carbon isotope comparisons with other sec-
Figure 1. Geology, sampling sites, and outcrops of Precambrian rocks in eastern Arabia. A: Major outcrops of Huqf Supergroup.
B: Outcrops within Jebel Akhdar, with major
wadis shown. C: Jebel Samhan. D: Huqf
area. Sampling sites are shown in B–D.
UAE—United Arab Emirates.
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; May 2005; v. 33; no. 5; p. 413–416; doi: 10.1130/G21227.1; 4 figures; Data Repository item 2005073.
413
Figure 2. Examples of
demagnetization behavior observed in samples
from Huqf Supergroup
(A–D). NRM—natural remanent magnetization. On
orthogonal plots, solid
symbols are horizontal,
and open symbols are
vertical projections. All
plots shown in geographic coordinates. Solid dots
are projected onto horizontal axis, open onto
vertical axis. E shows
stereographic
projections of site means for B
component (see text for
details). On stereographic projections, solid symbols represent lower
and open symbols represent upper hemisphere
projections.
tions (Burns and Matter, 1993; Brasier et al.,
2000). However, given the uncertainties in age
of the Marinoan period, and the identification
of several d13C spikes, the Fiq Formation can
only be difinitively assigned a post–712 Ma,
pre–544 Ma age. The thickness of the sequence between the Fiq and the Fara Formations suggests that it is more likely a Marinoan (600–580 Ma) than latest Precambrian
event (Calver et al., 2004). This does not exclude the possibility that the Fiq Formation
could correlate with older events, such as the
ca. 635 Ma Ghaub Formation of Namibia
(Hoffmann et al., 2004).
The glaciomarine Fiq diamict incorporates
marine interglacial cycles (Leather et al.,
2002; Kellerhals and Matter, 2003), and is
overlain by a dolostone cap carbonate, the
Hadash Formation. This grades into the Masirah Bay Formation, a silty carbonate, which
is abruptly overlain by the Khufai limestone.
In south Oman, the Mirbat Formation, comprising diamict, overlain by a thin carbonate
and turbiditic siltstones and sandstones, is also
included in the Huqf Supergroup. Although
predominantly unaltered, the Huqf Supergroup has undergone some tectonism. In the
Jebel Akhdar in northern Oman (Fig. 1B),
much of the folding is Cretaceous in age, with
some evidence for early Paleozoic deformation (Blendinger et al., 1990; Gass et al.,
1990). In the Huqf region of eastern Oman
(Fig. 1D), strike-slip faulting began in the late
Precambrian and continued through much of
the Paleozoic (Husseini and Husseini, 1990).
In south Oman (Fig. 1C), the Mirbat Formation shows little sign of deformation.
414
PALEOMAGNETIC RESULTS
Drilled cores and hand samples for paleomagnetic analysis were collected from a total
of 44 sites across Oman in the 3 main locations of the Huqf Supergroup (Fig. 1). In the
Fiq Formation, six sites (20 samples) were
sampled, mostly located in sandstone interbeds within the tillite. Where possible, samples of tillite clasts were also collected. The
Hadash Formation was sampled at 21 sites (60
samples) and the Masirah Bay Formation at 5
sites (36 samples). In the Mirbat Formation,
samples of the dolostone, tillite, and siltstone
were collected from 12 sites (31 samples).
Where possible, samples were taken from either side of local fold structures. The samples
were cut into 2.5-cm-diameter cores. Measurements of natural remanent magnetism
(NRM) were carried out in a field-free room
with a 2G cryogenic magnetometer: 404 specimens were demagnetized by using standard
progressive thermal and alternating field techniques. During thermal treatment, the samples
were systematically reoriented to detect spurious, laboratory-induced magnetization. In
order to identify the characteristic remanent
magnetism (ChRM), the components of demagnetization for each specimen were analyzed by the least-squares methods (Kirschvink, 1980).
Magnetic behavior and NRM intensity varied both between and within lithologies. Typically, the fine red siltstone units of the Fiq
and Mirbat Formations have the highest initial
NRM, ;30 mA/m. The dolostones of the
Hadash and lower Mirbat Formations and the
carbonate silts of the Masirah Bay units range
between 0.5 mA/m and 30 mA/m. Although
in many specimens patterns of demagnetization were too poor to confidently assign a
trend, in more than 30% of specimens, stable
components of magnetization were revealed: a
low-stability component A with a maximum
unblocking temperature (Tub) of 400 8C, and
a higher-stability component B, with a maximum Tub of ;600 8C (Fig. 2; GSA Data Repository Table DR11 presents the site mean
data and associated statistical parameter).
Component A is identified in 28% of specimens. In some specimens, it is the ChRM,
present to peak temperature and trending toward the origin on orthogonal demagnetization plots. Component A points down at a
shallow angle in a northerly direction. After
tilt correction, the a95, a cone of 95% confidence describing the spread of a set of data,
increases slightly, whereas k, the precision parameter, decreases. Component A therefore
fails a fold test, indicating that it was imprinted after folding. In some specimens, a similar
low-temperature component is identified
pointing south and up. When inverted, this fits
with component A, passing a reversal test.
The direction of component A is statistically
indistinguishable from the present earth field
in Oman and is therefore interpreted as a magnetism of recent origin, probably a chemical
remanent magnetization acquired during
weathering over at least the past 780 k.y.,
since the time of the last magnetic reversal.
Component B is identified in 21% of specimens, commonly between 400 and 600 8C,
and in some cases at lower temperatures, always as a ChRM. Component B points either
at a shallow angle upward and to the north, or
in the opposing direction, down and southward. After bedding correction, the dispersion
of the data decreases significantly, and the
data pass both the McElhinny (1964) fold test
and the Enkin (2003) fold test at the 95% confidence level (Fig. 2). Given that a number of
sites contained only a few specimens, we also
ran the fold test excluding those sites with
fewer than three specimens. The results were
similar, with positive fold tests using both the
McElhinny (1964) and Enkin (2003) tests.
Given that the earliest folding in Oman is
Cambrian–Ordovician in age (Blendinger et
al., 1990; Gass et al., 1990), this result indicates that component B is either primary or
an early magnetic reset acquired prior to
folding.
For this study, sites with a negative inclination and declination close to north are as1GSA Data Repository item 2005073, Table
DR1, site mean directions from the Huqf Supergroup, Oman, is available online at www.
geosociety.org/pubs/ft2005.htm, or on request from
[email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301-9140,
USA.
GEOLOGY, May 2005
Figure 3. Stratigraphy
showing interpreted correlation between global
events and Precambrian
units in Oman. Also
shown is current accepted nomenclature for Huqf
Supergroup (Leather et
al.,
2002). Sampling
scope of this study is
highlighted.
Magnetostratigraphies of Jebel
Akhdar, Huqf region, and
Jebel Samhan are shown.
Sample sites are labeled,
and site mean directions
used in our analysis are
displayed. Dec—Declination; Inc—Inclination.
signed a normal polarity, and south-down vectors are assigned a reversed polarity. This
assumes that Arabia was in the Southern
Hemisphere in the late Precambrian, an argument consistent with Gondwana assembly. A
Northern Hemisphere location would simply
involve switching these polarity assignments.
The south-down directions cluster well with
the north directions after inversion, implying
at least one reversal. Figure 3 shows a composite magnetostratigraphy based on reversals
identified in the combined north Oman sections and the Mirbat Formation. In north
Oman, a reverse polarity horizon is identified
in the Fiq Formation in a site in Wadi Mistal,
;400 m below the base of the Hadash Formation; ;300 m higher in the unit, a site in
Wadi Sahtan demagnetizes with normal polarity, indicating a reversal in the upper part of
the Fiq Formation. Component B in the Hadash Formation is predominantly of reverse
polarity. However, above the solid dolomite,
in the lowermost of four dolomite-silt interbeds, a reversed polarity horizon is present in
Wadi Bani Awf. In the Huqf region, where the
dolomite is thinner, one site 2 m from the base
of the outcrop is normal. This pattern is mirrored by the Mirbat dolomite with reversals at
the base, and one normal polarity horizon in
the lowermost of two dolomite-silt interbeds.
A normal polarity horizon is identified ;150
m from the base of the middle Mirbat member,
in site Mir03, with a similar normal horizon
in one site, Msb04, in the Huqf region. All
sites above this, in the middle Mirbat Formation, have reverse polarity. The presence of
reversals at the same stratigraphic level in distinct basins is a strong indication, when combined with the fold test, that the magnetization
was acquired during deposition, and as such,
we interpret component B as a primary component of magnetization.
GEOLOGY, May 2005
DISCUSSION AND CONCLUSIONS
Given that our sampling sites extend over
68 of latitude we report our result in terms of
a mean pole. A comparison of the corresponding paleopole for component B (52.38S,
74.48E; N 5 25 sites; k 5 16.7; a95 5 7.38),
rotated into African coordinates, with those of
a similar age from other elements of Gondwana (taken from Meert, 2003), also rotated
into African coordinates, is presented in Figure 4A. If the various elements that made up
Gondwana had been assembled at this time
the poles should also converge. Geological evidence suggests that the collision between
Arabia and North Africa had occurred by the
Cambrian (Meert, 2003). The disparity between the Neoproterozoic Gondwana poles
and the paleopole from this study suggests
that the eventual collision between Arabia and
North Africa took place after ca. 600 Ma (Davies et al., 1980; McWilliams and McElhinny,
1980; Schmidt and Williams, 1996; Sohl et
al., 1999; Meert, 2003). The distribution of
paleopoles also implies that the paleogeography at the time was characterized by a series
of separate land masses, rather than one large
supercontinent (Fig. 4B).
This result provides a high-reliability datum
for low-latitude (;13.08 calculated for a location at Muscat, Oman) glaciation and supports extensive late Neoproterozoic cooling.
Our result is the first to directly indicate that
deposition of both the tillite and the overlying
cap carbonate took place at the same latitude
in the tropics. We note, however, that magnetic reversals are present in both the tillite
and the overlying cap carbonate. The identification of a sharply defined reversal toward
the top of the cap carbonate may indicate that
deposition of this unit could have taken place
over time scales of at least 105–106 yr, or, alternately, there are significant variations in the
rates of deposition between the solid dolomite
lower in the Hadash Formation and the overlying dolomite-silt interbeds that form the upper part of the Hadash Formation. Similar reversals have been described for the Puga cap
carbonate in the Amazon craton (Trinidade et
al. 2003), though these have proved controversial because the mean direction is close to
the present earth field for the area and there
are no field tests to constrain the age of
magnetization.
Sedimentary cyclicity in the low-latitude
Fiq Formation is comparable with modern
glacial-interglacial alternations, and the presence of wave ripple marks indicates openocean deposition (Leather et al., 2002; Kellerhals and Matter, 2003). This implies that
tropical latitudes were not continuously covered by ice during the Marinoan snowball episode, meaning that both CO2 and methane
could be freely released from the oceans to the
atmosphere at this time, consistent with some
less extreme climate models (Crowley et al.,
2001).
ACKNOWLEDGMENTS
This work was supported by a Natural Environment Research Council studentship NER/S/A/2001/
06063 to Ben Kilner. We thank Rob Van der Voo
and Dave Evans for helpful comments on an earlier
draft, and Hilal Al Azri and Dave Willis and Judy
Willis for support in Oman. Joe Meert and Phil
Schmidt provided extremely helpful reviews that
were of great benefit in revising the manuscript.
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Manuscript received 24 September 2004
Revised manuscript received 19 January 2005
Manuscript accepted 19 January 2005
Printed in USA
GEOLOGY, May 2005