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A misfit Cryogenian diamictite in the Vrede domes, Northern Damara Zone, Namibia:
Chuos (Sturtian) or Ghaub (Marinoan) Formation? Moraine or Palaeovalley?
P. F. Hoffman
1,2,
*, E. J. Bellefroid
3
, P. W. Crockford
4
, A. de Moor
5
, G. P. Halverson
4
,
E. B. Hodgin
2
, M. S. W. Hodgskiss
4
, B. K. Holtzman
6
, G. R. Jasechko
7
, B. W. Johnson
1
,
and K. G. Lamothe
4
1
School of Earth & Ocean Sciences, University of Victoria, Victoria, BC V8T 2K4, Canada
2
Department of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
3
Department of Geology & Geophysics, Yale University, New Haven, CT 06520-8109, USA
4
Department of Earth & Planetary Sciences, McGill University, Montreal, QC H3A 0E8, Canada
5
Department of Geosciences, Hamilton College, Clinton, NY 13323, USA
6
Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
7
Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada
*Correspondence: <paulfhoffman@gmail.com>
1216 Montrose Ave. Victoria, BC V8T 2K4, Canada
Abstract: A wedge-shaped mass of glacigenic diamicite up to 220 m thick is exposed in the southwestern
part of the more southerly of two structural domes on Vrede Farm, in the northwestern corner of the
Northern Zone of the Damara Belt. The clasts and matrix of the diamictite are sourced mainly from the
crystalline basement, which is characteristic of the older Cryogenian (Sturtian) Chuos Formation. The
Chuos glaciation coincided with crustal stretching and uplift of basement blocks, while the younger Cry-
ogenian (Marinoan) Ghaub glaciation occurred during passive-margin subsidence. The Ghaub glaciation
generated diamictite derived nearly exclusively from carbonate rocks of the directly underlying Abenab
Subgroup. However, the apex of the wedge of diamictite at Vrede is directly overlain by the basal Edia-
caran Keilberg ‘cap dolostone’ Member of the Karibib Formation. Accordingly, the diamictite was as-
signed to the Ghaub Formation, despite its ‘misfit’ composition. To test this assignment, we measured 20
closely-spaced stratigraphic sections around the Vrede domes, including 15 sections in the southern
dome, where we ‘walked out’ the thin (<4.3 m) Rasthof Formation, the lithologically-distinctive post-
Chuos ‘cap carbonate.’ We show unequivocally that the wedge of diamictite stratigraphically underlies
the Rasthof ‘cap carbonate’ and is therefore Chuos (Sturtian) in age, consistent with its composition. The
apex of the wedge is truncated by the sub-Ghaub erosion surface, which is overlain by 0-5.5 m of car-
bonate diamictite (Ghaub Formation) and the Keilberg ‘cap dolostone’ Member. The wedge of diamictite
has an aspect ratio of 0.15 and did not fill a palaeovalley because no incision is observed in the underlying
Ugab Subgroup. We conclude that it originated as a moraine-like buildup, presumably at a stable ice mar-
gin or grounding line, and was ultimately buried by basinal argillaceous sediments of the Narachaams
Member during the nonglacial interlude between the Sturtian and Marinoan glaciations. It was erosionally
decapitated during the Ghaub glaciation by an ice sheet that left little diamictite in the Vrede domes area.
Our work illustrates the importance of measuring multiple, closely-spaced, stratigraphic sections for the
interpretation of Cryogenian glacial stratigraphy.
Key Words: Neoproterozoic; Sturtian glaciation; diamictite; moraine.
To cite this article: Hoffman, P.F. Bellefroid, E.J. Crockford, P.W. de Moor, A. Halverson, G.P. Hodgin, E.B.
Hodgskiss, M.S.W. Holtzman, B.K. Jasechko, G.R. Johnson, B.W. & Lamothe, K.G. 2016. A misfit Cryogenian
diamictite in the Vrede domes, Northern Damara Zone, Namibia: Chuos (Sturtian) or Ghaub (Marinoan) For-
mation? Moraine or Palaeovalley? Communications of the Geological Survey of Namibia, 17, 1-16.
Submitted 24 March 2016
Introduction
Vrede domes (Maloof, 2000) comprise a
pair of doubly-plunging anticlines located in the
northwest corner of the allochthonous Northern
Zone of the Pan-African Damara Belt (Miller,
1983, 2008) of Namibia (Fig. 1, 2). The domes
form topographically prominent inliers (Fig. 3,
4) of Swakop/Otavi Group carbonate and
coarse-grained clastic rocks that dip steeply
outward beneath semipelitic schist, carbonate-
clast debrite and conglomerate of the synorogen-
ic, middle Ediacaran, Kuiseb Formation (Frets,
1969; Maloof, 2000; Schreiber, 2006). Maloof
(2000) interpreted the domes as interference
folds, produced by successive orthogonal crus-
tal-thickening events in the Kaoko (east-west
shortening) and Damara (north-south shorten-
ing) belts, at the junction of which the domes are
situated. The domes are 1.5 km wide and 5.0 km
in combined length, and lie at the foot of the
Early Cretaceous Etendeka volcanic escarpment
on the west bank of the Huab River. They are
accessible by 4WD vehicle from the track from
the north through Vrede Farm and from the
south via the track between the farms Bethanis
and De Riet.
Figure 1: Location of the Vrede domes area (black rectangle) at the junction of the Northern Zone of the Damara
Belt and the Eastern Zone of the Kaoko Belt.
Maloof (2000) recognized the presence
in both domes of two Cryogenian glaciations,
represented by glacial-paraglacial diamictites of
the Chuos (Sturtian) and Ghaub (Marinoan)
formations, and their respective cap-carbonate
sequences, the Rasthof and basal Karibib (Keil-
berg Member) formations. Maloof (2000) also
recognized an older diamictite of uncertain
origin and limited extent in the core of the south
dome. He showed that the Rasthof Formation,
although less than 4.4 m thick in the domes, is
key to distinguishing the two Cryogenian dia-
mictites. However, the Rasthof Formation is ab-
sent at the southern closure of the south dome
(Fig. 3, 4), where a southwestward-thickening
wedge of polymictic boulder diamictite (fine-
grained matrix-supported conglomerate) up to
220 m thick appears. The wedge includes mas-
sive and stratified diamictites, as well as subor-
dinate sorted conglomerate and clast-free silt-
stone. The predominance of crystalline basement
debris is characteristic of the Chuos Formation.
In contrast, the Ghaub Formation is dominated
by carbonate-clast diamictite. Regionally, the
consistent lithologic contrast in diamictites re-
flects the tectonic transition from crustal stretch-
ing during the Chuos glaciation to passive-
margin thermal subsidence during the Ghaub
glaciation (Hoffman & Halverson, 2008). Yet,
the basement-dominated diamictite wedge in the
south dome is directly overlain by the Karibib
Formation, including a well-developed Keilberg
Member, which is the basal Ediacaran cap dolo-
stone regionally affiliated with the younger
Ghaub glaciation (Hoffmann & Prave, 1996;
Hoffman & Halverson, 2008; Hoffman, 2011).
Accordingly, the Chuos-like diamictite body in
the south dome was provisionally assigned to the
Ghaub Formation (Maloof, 2000; Schreiber,
2006; Hoffman & Halverson, 2008) despite its
atypical derivation and composition.
As the stratigraphy in the domes exhibits
considerable local variability, we mapped and
measured 20 sections over a strike-length of
10.5 km around the periphery of the domes (Fig.
3, 4). The sections clearly distinguish the Chuos
and Ghaub diamictites and demonstrate une-
quivocally that the wedge of diamictite in the
south dome belongs to the Chuos Formation. We
use the stratigraphies of the underlying and over-
lying units to show that the Chuos wedge, which
has an aspect ratio of 0.15, or 15% grade
(0.22/1.5 km), was a moraine-like body with
positive topographic relief and did not fill a pal-
aeovalley. This exercise illustrates the im-
portance of measuring multiple, closely-spaced,
stratigraphic sections for the interpretation of
Cryogenian glacial stratigraphy.
Figure 2: Geology of the southwestern border of the Kamanjab inlier at the junction of the Damara and Kaoko belts
(modified after Frets, 1969). The southwest-dipping thrust system (barbed black lines) that parallels road C39 (pur-
ple line) separates the allochthonous Northern Zone to the southwest from the autochthonous-parautochthonous
Northern Margin Zone to the northeast (Miller, 2008). Vrede domes occur within the red rectangle, which shows the
area of Fig. 3. The Huab River drainage is indicated by the yellow band. The Ugab Subgroup (Tonian) is limited to
the Northern Margin Zone. Abbreviations in legend: Cr. Cryogenian; Ed. Ediacaran; eK, Early Cretaceous.
Methods
Formation contacts were mapped on en-
largements of national 1:50,000 monochrome air
photographs (frames 746-5-298, 746-5-299,
746-6-378 and 746-6-379) and polychrome sat-
ellite imagery from Google Earth (Fig. 4). Key
units like the Rasthof Formation were walked
out. Stratigraphic sections were measured (in the
palaeovertical) with a folding 2 metre scale,
usually unfolded to 1.5 m for convenience. This
was facilitated by generally steeply dipping stra-
ta, rugged topography, and minimal vegetation
due to average annual precipitation of 15-20 cm.
The geographical coordinates of the base and top
of each section are given in Table 1.
Table 1. Geographical coordinates of measured sections
Section Coordinates at base of section Coordinates at top of section
I 20° 23' 37.8" S; 14° 09' 19.2" E 20° 23' 30.20" S; 14° 08' 55.1" E
II 20° 23' 33.3" S; 14° 09' 32.1" E 20° 23' 33.5" S; 14° 08' 53.6" E
III 20° 22' 13.7" S; 14° 09' 38.5" E 20° 22' 10.6" S; 14° 09' 54.4" E
IV 20° 21' 45.1" S; 14° 09' 15.9" E 20° 21' 38.1" S; 14° 09' 10.0" E
1 20° 22' 59.1" S; 14° 09' 02.0" E 20° 23' 05.0" S; 14° 08' 59.2" E
2 20° 13' 18.4" S; 14° 09' 02.5" E 20° 23' 15.9" S; 14° 08' 55.6" E
3 20° 23' 44.2" S; 14° 08' 52.4" E 20° 23' 43.3" S; 14° 08' 43.1" E
4 20° 23' 59.3" S; 14° 08' 52.4" E 20° 23' 58.0" S; 14° 08' 46.3" E
5 20° 24' 13.7" S; 14° 08' 54.4" E 20° 24' 15.0" S; 14° 08' 42.6" E
6 20° 24' 20.0" S; 14° 08' 58.5" E 20° 24' 31.4" S; 14° 08' 49.9" E
7 20° 24' 20.1" S; 14° 08' 58.7" E 20° 24' 31.8" S; 14° 09' 00.2" E
8 20° 24' 16.7" S; 14° 09' 06.9" E 20° 24' 18.6" S; 14° 09' 12.2" E
9 20° 24' 09.8" S; 14° 09' 10.3" E 20° 24' 10.3" S; 14° 09' 13.0" E
10 20° 24' 03.0" S; 14° 09' 13.1" E 20° 24' 06.8" S; 14° 09' 16.1" E
11 20° 24' 00.5" S; 14° 09' 19.2" E 20° 24' 02.7" S; 14° 09' 16.8" E
12 20° 23' 59.9" S; 14° 09' 32.7" E 20° 24' 03.1" S; 14° 09' 26.9" E
13 20° 23' 54.2" S; 14° 09' 40.0" E 20° 23' 55.1" S; 14° 09' 45.9" E
14 20° 23' 27.5" S; 14° 09' 50.3" E 20° 23' 29.1" S; 14° 10' 01.3" E
15 20° 23' 08.9" S; 14° 09' 44.4" E 20° 23' 06.9" S; 14° 09' 47.7" E
16 20° 22' 27.0" S; 14° 09' 58.1" E 20° 22' 35.5" S; 14° 10' 04.4" E
17 20° 22' 10.9" S; 14° 09' 54.4" E 20° 22' 12.9" S; 14° 10' 02.1" E
18 20° 21' 51.4" S; 14° 09' 46.2" E 20° 21' 50.4" S; 14° 09' 55.1" E
19 20° 21' 35.3" S; 14° 09' 34.4" E 20° 21' 30.2" S; 14° 09' 35.5" E
20 20° 21' 38.1" S; 14° 09' 10.0" E 20° 21' 25.4" S; 14° 09' 01.1" E
The lithological divisions used to meas-
ure, describe and graphically present the meas-
ured sections (Fig. 5, 6) are described in Table 2.
The measured sections have not been corrected
for tectonic strain, which is most likely to be
significant for the rheologically weak Nara-
chaams Member (Fig. 6). Stratigraphic nomen-
clature follows SACS (1980), modified by
Hoffmann & Prave (1996), Schreiber (2006) and
Miller (2008). The Cryogenian period has yet to
be formally defined. In this paper, we place the
Tonian–Cryogenian boundary at the base of the
Chuos Formation, rather than at the provisional
chronometric boundary of 850 Ma. We correlate
the Cryogenian–Ediacaran boundary with the
base of the Keilberg Member (Karibib For-
mation), following Narbonne et al. (2012).
Table 2. Carbonate lithofacies
Lithofacies Description Palaeoenvironment
Microbialaminite Lamination crinkly to undulatory with small-scale
channels and unconformities, intraclast breccias,
polygonal tepees and tepee breccias
Upper littoral and supratidal zones
Grainstone Massive to bedded arenite in beds >0.2 m thick,
commonly crossbedded, oolitic and/or intraclastic
grains, selectively replaced by authigenic chert
Shallow sub-littoral and lower litto-
ral zones of continuously breaking
waves
Stromatolite Laminated microbial growth structures with
mounded or columnar forms, commonly associat-
ed with grainstone
Shallow sub-littoral and lower litto-
ral zones of intermittently breaking
waves
Ribbonite Thin bedded (<0.2 cm) lutite or fine-grained are-
nite with low-angle cross-stratification and wavy
or current rippled surfaces
Weakly agitated bottom water above
storm wave base but below the zone
of breaking waves
Rhythmite Parallel flat-laminated lutite, lacking structures
related to wave action or traction currents. Para-
llel-sided graded beds (turbidites) may be present
Quiet bottom water below storm
wave base
Tonian
Ugab Subgroup
Both domes are cored by Tonian strata
that predate the Chuos glaciation (Fig. 2).
Maloof (2000) assigned the pre-Chuos suc-
cession, comprising 0.35-0.45 km of mixed clas-
tics and carbonates, to the Ombombo Subgroup
(Otavi Group) of the Northern Platform. He
subdivided it into four units, Ombombo-1
through 4. Schreiber (2006) retained Maloof’s
subdivisions but reassigned them to the Ugab
Subgroup (Swakop Group), consistent with
stratigraphic usage in the Northern Zone of the
Damara Orogen. Hoffman & Halverson (2008)
retained the carbonate-dominated units 3 and 4
in the Ugab Subgroup, but correlated the coarse-
clastic unit 2 with the Naauwpoort Formation,
which crops out in the Austerlitz, Welwitschia
and Summas Mountains inliers to the east of
Vrede domes. However, the bimodal terrestrial
volcanic assemblage that generally overlies
coarse-clastic rocks in the Naauwport Formation
elsewhere is absent in the domes. As no major
disconformity, which could have truncated the
Naauwpoort volcanics, is evident between units
2 and 3 (Fig. 5), we follow Schreiber (2006) in
placing all four pre-glacial units within the Ugab
Subgroup, designated U1-U4. In the Summas
Mountains, the Ugab Subgroup overlies the
Naauwpoort volcanics, dated at 746 ± 2 Ma at
the top (Hoffman et al. 1996; Hoffman & Hal-
verson, 2008), but the base of the Ugab Sub-
group could be regionally diachronous. The
litho- and chemostratigraphy of the Ugab Sub-
group in the Vrede domes is currently under in-
vestigation by one of us (KGL) as part of a
M.Sc. thesis project at McGill University.
Unit U1
Maloof (2000) recognized unit U1 only
in the core of the south dome (Fig. 3-5). Struc-
turally it overlies white limestone-marble tecton-
ite, within which there is a coherent, mappable,
carbonate-clast diamictite of unknown origin.
On the western and northern flanks of the dome,
unit 1 consists of black and maroon limestone
interbedded with quartz-arenite and conglomer-
ate. It is characterized by refolded, metre- to
decametre-scale, flexural-flow folds, reflecting a
rheological contrast between weak limestone
and quartz-arenite under greenschist-grade met-
amorphic conditions. On the eastern plunge of
the south dome, the carbonate is a more compe-
tent dolomite, which includes interstratified rib-
bonite, stromatolite and grainstone lithofacies
(Table 2, Fig. 5). The top of unit 1 is a signifi-
cant flooding surface, where coarse-grained
conglomerate is overlain by thin-bedded argilla-
ceous siltstone of unit 2.
Unit U2
Unit U2 is a generally coarsening-
upward sequence of mixed terrigenous and car-
bonate lithologies (Fig. 5). Conglomerates with
granitoid and quartzite clasts overlie sharp ero-
sive contacts and grade upward into thin-bedded
micaceous and locally feldspathic sandstones
and quartz-wackes, with variable amounts of
brown-weathering authigenic dolomite, com-
monly in beds exhibiting medium-scale cross-
bedding. These form multiple, decametre-scale,
fining-upward cycles. These coarse-grained clas-
tic sediments grade distally into cycles of dolo-
mite ribbonite, stromatolite and grainstone.
Stromatolite units are typically pinkish in colour
and massive in appearance, with a subtle lamina-
tion that defines bush-like columnar stromato-
lites, with a strongly diverging style of branch-
ing characteristic the stromatolite form-genus
Tungussia (Hofmann, 1969). The top of unit 2 is
a significant flooding surface where deeper-
water lithofacies abruptly overlie conglomerate
or dolomite grainstone (Fig. 5).
Units U3 and U4
The conformable base of unit 3 marks an
upward shift to carbonate-dominated strata from
clastic-dominated in all but section II (Fig. 5).
The carbonate is exclusively dolomite and can
be divided into five basic depth-dependent litho-
facies: rhythmite, ribbonite, stromatolite, grain-
stone and microbialaminite (Table 2), in order of
decreasing water depth. The lithofacies are or-
ganized in generally shoaling-upward cycles. A
major flooding surface overlain by ribbonite de-
fines the base of unit 4. This serves as a strati-
graphic marker below the base of the Chuos
Formation diamictite (Fig. 5).
Cryogenian
Chuos Formation
The Chuos Formation ranges from 0 to
219 m in thickness within Vrede domes (Fig. 6).
The lenticular shape of the main mass of the
Chuos Formation in Figure 6 is an artifact of the
curvature of the line of sections (Fig. 3). The
actual geometry is a southwestward-thickening
wedge, with a zero isopach that intersects the
line of sections between sections 1-2 and 12-13
(Fig. 3). Smaller masses of carbonate diamictite
occur in the North Dome in sections 16 and 18.
The dominant lithology is polymictic matrix-
supported diamictite in which subrounded peb-
bles and boulders of basement rocks, quartzite
and dolomite, in variable proportions, are dis-
persed in a green argillaceous matrix. Less
abundant carbonate diamictite has a detrital car-
bonate wackestone matrix, with subrounded
clasts of dolomite, chert and subordinate base-
ment debris. Variations in clast size, composi-
tion and concentration, as well as matrix compo-
sition, define distinct bodies of diamictite (Fig.
6). Stratified diamictites are distinctly-bedded
intervals of fine-grained clastics with rafted de-
bris (dropstones). Intervals of laminated or mas-
sive siltstone and mudstone lacking rafted debris
also occur. Clast-supported roundstone con-
glomerate (orthoconglomerate) is a minor com-
ponent of the Chuos Formation (Fig. 6). The
base of the formation is a sharp erosion surface
with local shattering (brecciation) of underlying
dolomite (Ugab Subgroup). The top of the for-
mation is a flooding surface sharply overlain by
black-and-tan laminated dolomite of the Rasthof
Formation.
The Chuos Formation is inferred to have
been deposited in the grounding-zone of a dy-
namic and unstable ice-mass. Massive diamictite
likely includes both melt-out and subaqueous
rain-out deposits - tectonic strain makes them
difficult to distinguish. Stratified diamictite ac-
cumulated in subaqueous periglacial environ-
ments at greater distance from the grounding
line. Lenses of size-sorted orthoconglomerate,
intimately associated with massive diamictite,
imply channelized subglacial meltwater flow.
Clast-free siltstone and argillite could represent
ice-free interglacials (Le Heron et al. 2013) or
glacial maxima, when thick shelf ice suppressed
outlet glaciers (Dowdeswell et al. 2000). Alter-
nations between basement-dominated and car-
bonate-dominated diamictite, can be accounted
for by ice-stream switching, a well-established
phenomenon on Quaternary glaciated continen-
tal shelves (e.g. Dowdeswell et al. 2006). No
primary carbonate or wave-generated sedimen-
tary structures were observed within the Chuos
Formation in Vrede domes.
We have no idea what part of the Stur-
tian glaciation the Chuos Formation in Vrede
domes represents. The volume of sediment could
have been deposited in a few centuries by a tem-
perate tidewater glacier in a tectonically active
area (Koppes & Hallet, 2006). The estimated
duration of ~58 Myr for the Sturtian low-latitude
glaciation globally (Rooney et al. 2014, 2015)
implies that the thickest section of the Chuos
Formation in Vrede domes accumulated either
extremely slowly (<0.004 mm/yr on average), or
represents only a miniscule time fraction of the
total glacial epoch.
The high aspect ratio (0.15) of the dia-
mictite wedge in the South Dome suggests either
confinement in a palaeovalley or a moraine-like
buildup with positive relief. These alternatives
are distinguishable through reciprocal thickness
variation in the older and younger strata, respec-
tively. We return to this issue after the younger
strata have been described.
Rasthof Formation
The Rasthof Formation is the postglacial
cap carbonate of the Chuos glaciation. Its aver-
age thickness in Vrede domes is only 2.4 m,
with extremes of 0.6 and 4.3 m. Its base and top
are sharply defined. A thin basal interval (0.2-
0.3 m) of dolomite ribbonite, olive-tan in colour
and not everywhere present, grades into the
characteristic coal-black weathering, finely-
laminated dolomite, easily recognizeable in
float. Evidence of subaerial exposure is lacking,
but small-scale irregularities and crinkling sug-
gest a subaqueous microbial origin for the lami-
nation. This is confirmed by excellent examples
of rollup structures (Pruss et al. 2010) in section
20 (also present in section 12), implying that the
laminae were pliable but cohesive at the sedi-
ment-water interface, presumably due to micro-
bial binding of sediment grains. Black microbial
dolomite is sharply overlain by green argillite of
the basal Narachaams Member.
The Rasthof Formation was deposited
during the highstand of Cryogenian postglacial
flooding in a permanently subaqueous environ-
ment. Water depth relative to wave-base is diffi-
cult to determine because of bottom stabil-
ization by microbial mats. The thickness of the
Rasthof Formation in Vrede domes does not ap-
pear to have been limited by accommodation, as
there is no evidence of shoaling, subaerial expo-
sure or erosion at the top of the formation.
Figure 3: Geology of the Vrede domes (modified after Maloof, 2000) showing locations of measured sections of
the Ugab Subgroup (sections I-IV, Fig. 5), and the Abenab Subgroup and lower Karibib Formation (sections 1-20,
Fig. 6). The white line labelled Ac is the interpolated zero isopach of the southwestward-thickening wedge of Chuos
Formation diamictite.
Narachaams Member
The Narachaams Member (Hoffman &
Halverson, 2008) conformably overlies the
Rasthof Formation and disconformably under-
lies the Ghaub Formation, or the Karibib For-
mation where the Ghaub is absent (Fig. 6).
Composed of fine-grained siliciclastic sediments
and deeper-water carbonate, in part authigenic,
the Narachaams Member represents a basinal-
facies equivalent of the upper Rasthof, Gruis and
Ombaatjie formations (Abenab Subgroup) of the
Northern Platform (Hoffman & Halverson,
2008). In the Northern Margin Zone, the Nara-
chaams Member is overlain by carbonate
rhythmite, turbidites and oolitic debrite of the
Franniaus Member, which is interpreted as a
glacioeustatic falling-stand wedge preceding the
Ghaub (Marinoan) glaciation (Hoffman & Hal-
verson, 2008). Within the Vrede domes, the Na-
rachaams Member ranges from 0 to 237 m in
thickness (Fig. 6). Three factors potentially ac-
count for the large variability: depositional onlap
against the Chuos diamictite, differential trunca-
tion beneath the sub-Ghaub disconformity, and
tectonic strain of the rheologically weak Nara-
chaams Member during the formation of the
structural domes.
The Narachaams Member is composed of the
following lithologies: (1) parallel-laminated
green argillite, locally pyritic; (2) argillite with
silty laminae and occasional low-angle cross-
stratification; (3) argillite with brown-
weathering concretions (authigenic) and graded
beds (turbidites) of marly dolomite or limestone,
and (4) marly dolomite ribbonite. Carbonate is
concentrated in the lower and upper parts of the
unit (Fig. 6). The unit was deposited mainly be-
low fairweather wave base and contains no evi-
dence of subaerial exposure.
Ghaub Formation
The glacigenic Ghaub Formation is
poorly represented in Vrede domes, ranging
from 0 to 5.5 m in thickness. Where present, it
has a sharp erosive contact with the underlying
Narachaams Member and an abrupt conformable
one with the overlying Keilberg Member. The
Ghaub Formation is well developed in section
17 (Fig. 6), where 0.5 m of massive carbonate
diamictite, in which clasts of dolomite and
quartzite are dispersed in a dolomite wackestone
matrix, is overlain by 2.6 m of parallel-
laminated quartz siltstone with rafted dropstones
of quartzite and dolomite, the latter as much as
24 cm in diameter. Thinner intervals of diamic-
tite and/or siltstone with lonestones occur in sec-
tions 3, 10, 12-13 and 18-20 (Fig. 6). In section
6, 2.0 m of stratified carbonate-clast diamictite
with possible Rasthof-derived clasts lies sharply
on polymictic basement- and Ugab-derived dia-
mictite of the Chuos Formation (Fig. 6). In three
sections, 7 and 15-16, the Ghaub Formation is
demonstrably absent and the Keilberg Member
disconformably overlies the Narachaams Mem-
ber.
The Ghaub Formation is identified with
the late Cryogenian Marinoan glaciation through
its association with a lithologically distinct cap
dolostone, the basal Ediacaran Keilberg Member
(Hoffmann & Prave, 1996; Narbonne et al.
2012). The glacial deposits are much thicker on
Bethanis Farm in the autochthonous Northern
Margin Zone, 20 km to the east (Fig. 2), where
the Ghaub Formation includes 30-240 m of
mainly massive limestone-clast diamictite
(Hoffman & Halverson, 2008). The more distal
character of glacigenic facies in Vrede domes -
laminated siltstone with lonestones - suggests a
location seaward of the grounding-zone wedge
exposed at Bethanis. Similar proximal-distal
relations occur 120 km to the east between
Fransfontein Ridge (Domack & Hoffman, 2011)
and the Summas Mountains dome (Hoffman &
Halverson, 2008).
Figure 4: Satellite image (Google earth) of the south dome on Vrede Farm, showing contacts (yellow lines) be-
tween units U1, U2 and U3-4 (combined) of the Ugab Subgroup, Chuos Formation (Ac), Rasthof Formation (white
line), Narachaams Member (An), Karibib Formation (Tk) and Kuiseb Formation (Mk). Black lines with dotted end-
points indicate measured sections, numbered as in Figures 5 and 6.
Ediacaran
Keilberg Member (basal Karibib Formation)
The Keilberg Member is the basal trans-
gressive unit of the Karibib Formation, ranging
in thickness from 6.0 m in section 2 to 19.3 m in
section 7 (Fig. 6). It consists of pale grey to pale
pinkish-grey, tan-weathering, micropeloidal do-
lomite. Its base is sharply conformable on the
Ghaub Formation, or disconformable on older
units where the Ghaub is absent. The top grades
into deeper water, marly limestone and/or dolo-
mite rhythmite, representing the maximum post-
glacial flooding. In Vrede domes, the Keilberg
Member is similar in facies to the ‘distal
foreslope’ on Fransfontein Ridge (Hoffman et
al. 2007): a sheet-crack cement zone occurs near
the base and tubestone stromatolite is absent.
Section 13 is representative of the vertical se-
quence of structures (Fig. 6). A thin diamictite
(0.3 m) with dolomite, quartzite and porphyritic
granite clasts (Ghaub Formation) is sharply
overlain by 0.2 m of pale tan dolomite ribbonite
with scattered lonestones (<0.02 m) of dolomite.
The ribbonite is gradationally overlain by 2.3 m
of pale pinkish micropeloidal dolomite with
buckled bedding-parallel cracks filled by fi-
brous-isopachous dolomite cement (Hoffman &
Macdonald, 2010). Sheet-crack cement forms a
continuous zone in the lower, but not basal, part
of the Keilberg Member. In section 13, the
sheet-cracked zone is overlain by 10.5 m of pale
tan-coloured micropeloidal dolomite with me-
chanical lamination including low-angle cross-
lamination. The uppermost 3.5 m of dolomite is
a ribbonite that becomes finer-grained and dark-
er coloured upward, grading into thin-bedded
marly rhythmite of the middle Karibib For-
mation.
Karibib Formation
On the Northern Platform, the Ediacaran
(post-Ghaub glaciation) part of the Tsumeb
Subgroup comprises the Maieberg, Elandshoek
and Hüttenberg formations (Hedberg, 1979;
Hoffman & Halverson, 2008). In the Northern
and Northern Margin zones, these formations
cannot be distinguished lithologically because
sequence boundaries are lacking in the foreslope
facies. The name Karibib Formation con-
veniently encompasses the entire Ediacaran car-
bonate sequence in those zones (Schreiber,
2006), which is greatly attenuated relative to the
Northern Platform. In the Northern Margin
Zone, the Karibib Formation ranges in thickness
from 468 m at Fransfontein to 327 m at Bethanis
(Hoffman & Halverson, 2008). It is thinner in
the Vrede domes, ranging from 127 m (section
20) to 5 m (section 4). Thinning is due to down-
slope condensation (section 20) and erosional
truncation beneath the Kuiseb Formation (sec-
tion 4). The maximum flooding zone of the low-
er Karibib Formation, directly above the Keil-
berg Member, is well exposed in the small
drainage at section 16. The Keilberg Member
(10.1 m) is conformably overlain by 6.4 m of
flaggy, tan-coloured, marly-dolomite rhythmite,
followed by 66.5 m of tan- to grey-coloured,
thin- to medium-bedded (0.3 m), non-marly do-
lomite rhythmite. No seafloor cement (crystal
fans) was observed. In more complete sections
to the north and south, increasingly numerous
dolomite debrites (rhythmite breccia) occur
stratigraphically upward. The Karibib Formation
is heavily silicified below the erosional discon-
formity at the base of the overlying Kuiseb For-
mation, which cuts all the way down to the
Keilberg Member in sections 4-7 (Fig. 6).
Kuiseb Formation
A major regional disconformity sepa-
rates Otavi Group carbonates from overlying
clastics of the Kuiseb Formation in the Northern
Zone and the stratigraphically equivalent Brak-
laagte Formation of the Northern Margin Zone
(Frets, 1969). Around Vrede domes, the Kuiseb
Formation is comprised of feldspathic and non-
feldspathic quartz-chert arenites (with ankerite
or siderite concretions), polymictic conglomer-
ate, green argillite and siltstone, and argillite-
hosted carbonate- and chert-clast debrite and
megabreccia.
Destruction of the carbonate platform
and burial under clastic sediments is related to
diachronous (southward-younging) abortive sub-
duction of the Northern Platform westward be-
neath the Dom Feliciano - Ribeira magmatic arc
of southeastern Brazil ~590 Ma (Stanistreet et
al. 1991; Goscombe et al. 2005; Oyhantçabal et
al. 2009; Faleiros et al. 2011; Chemale et al.
2012; Tupinambá et al. 2012; Alves et al. 2013;
Heilbron et al. 2013; Jung et al. 2014). In this
view, the Kuiseb Formation is a progradational
marine facies of the stratigraphically overlying
fluvial sandstones (‘molasse’) of the Renos-
terkop Formation, in which large-scale cross-
bedding indicates southeastward-directed sedi-
ment transport (Hoffman & Halverson, 2008).
Figure 5: Graphical logs of measured sections and tentative subdivision of the Ugab Subgroup in the Vrede domes
(see Fig. 3 for section locations). Datum is base of the Chuos Formation. See Table 2 for carbonate lithofacies de-
scription and interpretation. The continuity of unit U4 between sections I and II, above which the Chuos diamictite
is ~160 m and 5.5 m thick respectively, demonstrates that the wedge of diamictite (Fig. 6) is not accommodated by
an incised palaeovalley.
Figure 6: Graphical logs of measured sections of Cryogenian and earliest Ediacaran strata in the Vrede domes (see
Fig. 3 for section locations). Datum is base of the Keilberg Member, corresponding to the base of the Ediacaran Pe-
riod (Narbonne et al. 2012). See Table 1 for the geographical coordinates of each section and Table 2 for carbonate
lithofacies description and interpretation. The lensoidal form of the Chuos diamictite between sections 2 and 12 is an
artifact of the ovoid line of sections (Fig. 3). The projected shape of the diamictite body is a northeastward tapering
wedge with an apex between sections 5 and 6. Inset at base symbolizes the presence or absence of each Cryogenian
diamictite and its respective ‘cap carbonate.’ Only 5 of the 20 sections are ‘complete.’ The Narachaams Member has
not been decompacted and may have undergone differential tectonic strain between sections (e.g. between sections
14 and 15).
Chuos diamictite wedge: palaeovalley or moraine?
The wedge-shaped mass of diamictite in
the South Dome (sections 2-12) undoubtedly
belongs to the Chuos Formation as it strati-
graphically underlies the Rasthof Formation cap
carbonate (Fig. 6). The thickness (<220 m) and
high aspect ratio (0.15) of the wedge demand an
explanation. Was it confined within a palaeoval-
ley or was it a positive topographic feature like a
moraine or drumlin? A palaeovalley would be
indicated by incision of underlying strata. This is
not observed (Fig. 5). Section I of the Ugab
Subgroup, which underlies ~150 m of Chuos
diamictite (Fig. 4), is truncated little more than
sections II-IV, where the Chuos Formation is
thin or absent.
If the wedge of diamictite was a buildup
with positive topographic relief, it should have a
reciprocal relationship with the thickness of
overlying strata. The Rasthof cap carbonate
Formation forms a drape of near-constant thick-
ness, <4.3 m, except where it was erosionally
removed at the apex of the wedge (Fig. 6). The
Narachaams Member, in contrast, reciprocates in
thickness with the Chuos Formation. The recip-
rocal relationship (Fig. 6) would be more quanti-
tative if the argillaceous Narachaams Member
was decompacted. It may have undergone 50%
or more differential compaction, relative to the
diamictite and carbonate lithofacies. It is not
clear if the reciprocal relationship is due to dep-
ositional onlap or differential erosion beneath
the Ghaub glacial surface (Fig. 6). Stratigraphic
subdivision of the Narachaams Member is re-
quired to answer this question definitively. The
complete disappearance of the Narachaams
Member in sections 5-7 implies a significant role
for sub-Ghaub erosion. This is itself significant
given the paucity of Ghaub diamictite in the
Vrede domes. The magnitude of diamictite lo-
cally is no measure of the local depth of glacial
erosion.
Conclusions
The wedge of glacigenic diamictite
composed largely of basement debris in the
southern structural dome on Vrede Farm belongs
to the early Cryogenian (Sturtian) Chuos For-
mation. It stratigraphically underlies the cap car-
bonate (Rasthof Formation), which although thin
(0.6-4.3 m) is easily recognizable in outcrop and
float. The apex of the wedge is truncated at the
younger Cryogenian (Marinoan) Ghaub For-
mation glacial surface. The Ghaub Formation
itself is poorly developed in the Vrede domes (0-
5.5 m), but the lithologically distinctive basal
Ediacaran cap dolostone, the Keilberg Member,
is present in every section and ranges from 6.0
to 19.3 m in thickness. The Keilberg cap dolo-
stone rests disconformably on the Chuos diamic-
tite at the apex of the wedge, locally with car-
bonate-clast diamictite of the Ghaub Formation
between them. The wedge of Chuos diamictite
does not fill a palaeovalley incised into the un-
derlying Ugab Subgroup. It appears to represent
a moraine-like buildup that was draped by the
Rasthof carbonate, fully buried by the argilla-
ceous Narachaams Member, and erosionally de-
capitated during the Ghaub glaciation.
Acknowledgements
Field work was carried out under a re-
search agreement with the Geological Survey of
Namibia. Travel and operating costs were partly
funded by the Earth Systems Evolution Program
of the Canadian Institute for Advanced Research
(CIFAR). ADM was supported by a grant from
Hamilton College. GPH and KGL were support-
ed by a Canadian National Science and Engi-
neering Research Council (NSERC) Discovery
grant to GPH.
References
Alves, A. Janassi, V.A. Neto, M.C.C. Heaman,
L. & Simonetti, A. 2013. U-Pb geo-
chronology of the granite magmatism in the
Embu Terrane: Implications for the evolution
of the Central Ribeira Belt, SE Brazil. Pre-
cambrian Research, 230, 1-12.
Chemale, Jr. F. Mallmann, G. Bitencourt, M.F.
& Kawashita, K. 2012. Time constraints on
magmatism along the Major Gercino Shear
Zone, southern Brazil: Implic-ations for West
Gondwana reconstruction. Gondwana Re-
search, 22, 184-199.
Domack, E.W. & Hoffman, P.F. 2011. An ice
grounding-line wedge from the Ghaub glacia-
tion (635 Ma) on the distal foreslope of the
Otavi carbonate platform, Namibia, and its
bearing on the Snowball Earth hypothesis.
Bulletin of the Geological Society of America,
123, 1448-1477.
Dowdeswell, J.A. Ottesen, D. & Rise, L. 2006.
Flow switching and large-scale deposition by
ice streams draining former ice sheets. Geolo-
gy, 34, 313-316.
Dowdeswell, J.A. Whittington, J.A. Jennings,
A.E. Andrews, J.T. Mackensen, A. & Marien-
field, P. 2000. An origin for laminated glaci-
marine sediments through sea-ice build-up
and suppressed iceberg rafting. Sediment-
ology, 47, 557-576.
Faleiros, F.M. Campanha, G.A.C. Martins, L.
Vlach, S.R.F. & Vasconcelos, P.M. 2011.
Ediacaran high-pressure collision meta-
morphism and tectonics of the southern Ribei-
ra Belt (SE Brazil): Evidence for terrane ac-
cetion and dispersion during Gondwana as-
sembly. Precambrian Research, 189, 263-
291.
Frets, D.C. 1969. Geology and structure of the
Huab – Welwitschia Area, South West Africa.
Bulletin of the Precambrian Research Unit,
University of Cape Town, 5, 235 p.
Goscombe, B. Gray, D. Armstrong, R. Foster,
D.A. & Vogl, J. 2005. Event geochronology
of the Pan-African Kaoko Belt. Precambrian
Research, 140, 103.e1-103.e41.
Hedberg, R.M. 1979. Stratigraphy of the
Owamboland Basin, South West Africa. Bul-
letin of the Precambrian Research Unit, Uni-
versity of Cape Town, 24, 325 p.
Heilbron, M. Tupinambá, M. Valeriano, C.M.
Armstrong, R. Siva, L.G.E. Melo, R.S. Simo-
netti, A. Soares, A.C.P. & Machado, N. 2013.
The Sierra da Bolívia complex: The record of
a new Neoproterozoic arc-related unit at Ri-
beira belt. Precambrian Research, 238, 158-
175.
Hofmann, H.J. 1969. Attributes of stromatolites.
Papers of the geological Survey of Canada,
69-39, 58 p.
Hoffmann, K.-H. & Prave, A.R. 1996. A prelim-
inary note on a revised subdivision and re-
gional correlation of the Otavi Group based
on glacigenic diamictites and associated cap
dolostones. Communications of the Geol-
ogical Survey of Namibia, 11, 77-82.
Hoffman, P.F. 2011. Strange bedfellows: glacial
diamictite and cap carbonate from the Mari-
noan (635 Ma) glaciation in Namibia. Sedi-
mentology, 58, 57-119.
Hoffman, P.F. & Halverson, G.P. 2008. Otavi
Group of the western Northern Platform, the
Eastern Kaoko Zone and the western Northern
Margin Zone. In: Miller, R.McG. (Ed.) The
Geology of Namibia, Volume 2. Section 13
pp. 69-136. Geological Survey of Namibia,
Windhoek.
Hoffman, P.F. Halverson, G.P. Domack, E.W.
Husson, J.M. Higgins, J.A. & Schrag, D.P.
2007. Are basal Ediacaran (635 Ma) post-
glacial "cap dolostones" diachronous? Earth
and Planetary Science Letters, 258, 114-131.
Hoffman, P.F. Hawkins, D.P. Isachsen, C.E. &
Bowring, S.A. 1996. Precise U-Pb zircon ages
for early Damaran magmatism in the Summas
Mountains and Welwitschia Inlier, Northern
Damara Belt, Namibia. Commun-ications of
the Geological Survey of Namibia, 11, 47-52.
Hoffman, P.F. & Macdonald, F.A. 2010. Sheet-
crack cements and early regression in Mari-
noan (635 Ma) cap dolostones: Regional
benchmarks of vanishing ice-sheets? Earth
and Planetary Science Letters, 300, 374-384.
Jung, S. Brandt, S. Nebel, O. Hellebrand, E.
Seth, B. & Jung, C. 2014. The P-T-t paths of
high-grade gneisses, Kaoko Belt, Namibia:
Constraints from mineral data, U-Pb allanite
and monazite and Sm-Nd/Lu-Hf garnet ages
and garnet ion probe data. Gondwana Re-
search, 25, 775-796.
Koppes, M. & Hallet, B. 2006. Erosion rates
during rapid deglaciation in Ice Bay, Alaska.
Journal of Geophysics Research, 111, F2023,
doi:10.1029/2005JF000349
Le Heron, D.P. Busfield, M.E. & Kamona, F.
2013. An interglacial on snowball Earth? Dy-
namic ice behaviour revealed in the Chuos
Formation, Namibia. Sedimentology, 60, 411-
427.
Maloof, A.C. 2000. Superposed folding at the
junction of the inland and coastal belts, Da-
mara Orogen, NW Namibia. Communic-
ations of the Geological Survey of Namibia,
12, 89-98.
Miller, R. McG. 1983. The Pan-African Damara
orogen of South West Africa/Namibia. In:
Miller, R. McG. (Ed.) Evolution of the Dama-
ra Orogen of South West Africa/ Namibia.
Special Publication of the Geological Society
of South Africa, 11, 431-515.
Miller, R. McG. 2008. The Geology of Namibia,
Vol. 2, Neoproterozoic to Lower Palaeozoic.
Geological Society of Namibia, Windhoek.
Narbonne, G.M. Xiao, S. Shields, G.A. & Geh-
ling, J.G. 2012. The Ediacaran Period. In:
Gradstein, F.M. Ogg, J.G. Schmitz, M.D. &
Ogg, G.M. (Eds) A Geologic Time Scale
2012. Elsevier, Amsterdam, pp. 413-435.
Oyhantçabal, P. Siegesmund, S. Wemmer, K.
Presnyakov, S. & Layer, P. 2009. Geochrono-
logical constraints on the evolution of the
southern Dom Feliciano Belt (Uruguay).
Journal of the Geological Society, London,
166, 1075-1084.
Pruss, S.B. Bosak, T. Macdonald, F.A. McLane,
M. & Hoffman, P.F. 2010. Microbial facies in
a Sturtian cap carbonate, the Rasthof For-
mation, Otavi Group, northern Namibia. Pre-
cambrian Research, 181, 187-108.
Rooney, A.D. Macdonald, F.A. Strauss, J.V.
Dudás, F.Ö. Hallmann, C. & Selby, D. 2014.
Re-Os geochronology and coupled Os-Sr iso-
tope constraints on the Sturtian snowball
Earth. Proceedings of the National Academy
of Sciences (USA), 111, 51-56.
Rooney, A.D. Strauss, J.V. Brandon, A.D. &
Macdonald, F.A. 2015. A Cryogenian chro-
nology: Two long-lasting synchronous Neo-
proterozoic glaciations. Geology, 43, 459-
462.
SACS (South African Committee for Strat-
igraphy) 1980. Damara Sequence. In: Kent,
L.E. (Compiler), Stratigraphy of South Africa,
Part 1. Handbook of the Geological Survey of
South Africa 8, 415-438.
Schreiber, U.M. 2006. Geological Map of Na-
mibia 1:250,000 Geological Series, Sheet
2014 – Fransfontein. Geological Survey of
Namibia, Windhoek.
Stanistreet, I.G. Kukla, P.A. & Henry, G. 1991.
Sedimentary basinal response to a Late Pre-
cambrian Wilson Cycle: the Damara Orogen
and Nama Foreland. Journal of African Earth
Sciences, 13, 141-156.
Tupinambá, M. Heilbron, M. Valeriano, C. Jr.
Dios, F.B. Machado, N. Silva, L.G.E. Cesar,
J. & Almeida, J.C.H. 2012. Juvenile contribu-
tion of the Neoproterozoic Rio Negro Mag-
matic Arc (Ribeira Belt, Brazil): Implications
for Western Gondwana amalgamation.
Gondwana Research, 21, 422-438.
