Aberystwyth University
Glacitectonic deformation in the Chuos Formation of northern Namibia
Busfield, Marie E.; Le Heron, Daniel P.
Published in:
Proceedings of the Geologists' Association
DOI:
10.1016/j.pgeola.2012.10.005
Publication date:
2013
Citation for published version (APA):
Busfield, M. E., & Le Heron, D. P. (2013). Glacitectonic deformation in the Chuos Formation of northern
Namibia: Implications for neoproterozoic ice dynamics. Proceedings of the Geologists' Association, 124(5), 778-
789. https://doi.org/10.1016/j.pgeola.2012.10.005
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Manuscript Draft
Manuscript Number: PGEOLA-D-12-00028R1
Title: Glacitectonic deformation in the Chuos Formation of northern
Namibia: implications for Neoproterozoic ice dynamics
Article Type: Special Issue: Glacitectonics
Keywords: Neoproterozoic; glacitectonism; ductile deformation; Snowball
Earth; Otavi Mountainland
Corresponding Author: Miss Marie Elen Busfield,
Corresponding Author's Institution: Royal Holloway
First Author: Marie Elen Busfield
Order of Authors: Marie Elen Busfield; Daniel P Le Heron
Abstract: The Chuos Formation is a diamictite-dominated succession of
Cryogenian age, variously interpreted as the product of glaciomarine
deposition, glacially-related mass movement, or rift-related sediment
remobilisation in a non-glacial environment. These interpretations have
wide ranging implications for the extent of ice cover during the
supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland,
northern Namibia, detailed analysis of soft sediment deformation
structures on the macro- and micro-scale support glacitectonic derivation
in response to overriding ice from the south/south-east. Overall, the
upward increase in strain intensity, predominance of ductile deformation
features (e.g. asymmetric folds, rotational turbates and necking
structures, clast boudinage, unistrial plasmic fabrics) and pervasive
glacitectonic lamination support subglacial deformation under high and
sustained porewater pressures. In contrast, soft sediment structures
indicative of mass movements, including flow noses, tile structures, and
basal shear zones, are not present. The close association of subglacial
deformation, abundant ice-rafted debris and ice-contact fan deposits
indicate subaqueous deposition in an ice-proximal setting, subject to
secondary subglacial deformation during oscillation of the ice margin.
These structures thus reveal evidence of dynamic grounded ice sheets in
the Neoproterozoic, demonstrating their key palaeoclimatic significance
within ancient sedimentary successions.
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Glacitectonic deformation in the Chuos Formation of northern Namibia: 1
implications for Neoproterozoic ice dynamics 2
3
Marie E Busfield
1
* & Daniel P Le Heron
1
4
1
Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, 5
TW20 0EX 6
*Corresponding author. E-mail: Marie.Busfield.2011@live.rhul.ac.uk 7
Abstract 8
The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously 9
interpreted as the product of glaciomarine deposition, glacially-related mass movement, or 10
rift-related sediment remobilisation in a non-glacial environment. These interpretations have 11
wide ranging implications for the extent of ice cover during the supposedly pan-global 12
Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of 13
soft sediment deformation structures on the macro- and micro-scale support glacitectonic 14
derivation in response to overriding ice from the south/south-east. Overall, the upward 15
increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric 16
folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) 17
and pervasive glacitectonic lamination support subglacial deformation under high and 18
sustained porewater pressures. In contrast, soft sediment structures indicative of mass 19
movements, including flow noses, tile structures, and basal shear zones, are not present. The 20
close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan 21
deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary 22
subglacial deformation during oscillation of the ice margin. These structures thus reveal 23
evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key 24
palaeoclimatic significance within ancient sedimentary successions. 25
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*Manuscript
Click here to view linked References
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1. Introduction 27
The concept of a Neoproterozoic icehouse has remained contentious since its inception in the 28
early 19
th
century (‗die Eiszeit‘ of Agassiz, 1837: cf. Allen and Etienne, 2008), with renewed 29
deliberation in recent years following proposal of the ‗snowball Earth‘ hypothesis 30
(Kirschvink, 1992; Hoffman et al., 1998; Hoffman and Schrag, 2002). This hypothesis has 31
centred on the recognition of broadly age-equivalent diamictite-dominated successions on 32
each continent, which are argued to be glaciogenic in origin (Hoffman et al., 1998; Hoffman 33
and Schrag, 2002). Many diamictites are sharply overlain by dolomotized carbonates, 34
interpreted as the record of rapid post-glacial climatic recovery (Shields, 2005). Compared to 35
younger icehouse intervals, diagnostic glacial indicators, including striated and faceted clasts, 36
subglacially striated pavements and extrabasinal clast assemblages, are notably scarce in the 37
Neoproterozoic (Etienne et al., 2007), and rarely occur together in any one glacial succession. 38
Consequently, Neoproterozoic diamictites have been argued to represent non-glacial, syn-39
tectonic sediment gravity flows (e.g. Eyles and Januszczak, 2004, 2007), associated with 40
widespread rift activity during break-up of the Rodinia supercontinent. 41
In Quaternary studies, detailed analysis of soft-sediment deformation structures has received 42
significant credence in discriminating between glacial and non-glacial successions (e.g. 43
Lachniet et al., 2001; Menzies and Zaniewski, 2003; van der Meer and Menzies, 2011). 44
However, with a few notable exceptions (e.g. Benn and Prave, 2006; Arnaud, 2008 , 2012), 45
such analyses are scarcely applied to Neoproterozoic deposits. To redress this, we present a 46
new macro- and micro-scale structural analysis of the Chuos Formation of Cryogenian age in 47
the Otavi Mountainland, northern Namibia (Fig. 1). The study will utilise standard 48
sedimentological and structural analysis of the diamictite succession in order to determine the 49
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genetic origin of the Chuos Formation, and assess the significance of soft-sediment 50
deformation structures as palaeoclimate proxies during Neoproterozoic glaciation. 51
1.1 Geological background 52
The Otavi Group is a carbonate-dominated succession of Neoproterozoic age housing two 53
diamictite horizons, the older Chuos and younger Ghaub formations (Fig. 1), each sharply 54
overlain by fine-grained carbonate deposits (Hoffmann & Prave, 1996; Hoffman & 55
Halverson, 2008; Miller, 2008). These horizons have been dated in turn to <746 ± 2 Ma 56
(Hoffman et al., 1996) and 635.5 ± 1.2 Ma (Hoffmann et al., 2004), through U-Pb zircon ages 57
of underlying and interbedded volcanic ash beds, leading to correlation with the purportedly 58
global Sturtian and Marinoan glaciations, respectively (Kennedy et al., 1998). In light of the 59
argued syn-rift derivation of the diamictite assemblages (e.g. Eyles & Januszczak, 2004, 60
2007), proponents of the glacial hypothesis have focussed largely on the younger Ghaub 61
Formation, considered to have accumulated during the ‗drift‘ stage of post-rift subsidence 62
(Hoffman & Halverson, 2008). The older Chuos Formation, by comparison, has received less 63
attention. 64
The glacial origin of the Chuos Formation was first proposed by Gevers (1931) due to its 65
lithological similarity to the Late Palaeozoic Dwyka Tillite (cf. Henry et al., 1986), and its 66
abundance of faceted and extrabasinal clasts. The stratigraphic position of the Chuos between 67
carbonate successions was used to support a glaciomarine origin (Martin 1965a, b; Hedberg 68
1979), in-keeping with the regional absence of subglacial striated pavements (Kroner & 69
Rankama, 1973). Alternative studies conversely describe the textural immaturity of the 70
diamictites, abundance of locally-derived erratic lithologies and their spatial and temporal 71
association with faults as evidence of high energy, rift-related submarine gravity flow 72
deposition (e.g. Hedberg, 1979; Miller 1983; Porada, 1983; Porada & Wittig 1983a, b; Martin 73