Kranspoort
vehicle track
Karibib Fm
Keilberg Mb cap dolomite
Ghaub Fm diamictite
Ombaatjie Fm (base b8 )
Gruis Fm
Rasthof Fm cap carbonate
Chuos Fm diamictite
Basement complex
0 1.0 2.0km
Kranspoort salient
N
P1338
P1339P8519
P6520
P8522
P6521P1337
P1337
P5520
P1335
P1333
P1334
P8515
P8514P1336
P5507
P8505
P7016
P7507-P7510
P5517
P9514P1330
B1311
BP07
BP06
P1332
P6521
P6004
P1413
P1414
P1419
P1419
E1407
P1420P1421
P1414 P5519
measured sectionmeasured section with isotope data
CO1
Fig. 5 (below). Photomap of the shelf break on Kranspoort farm, showing the truncation of the Ombaatjie Fm by the upper Cryo-genian (sub-Ghaub Fm) glacial surface, which is overlain on the shelf by the Keilberg Mb (’cap’ dolomite) of the Maieberg Fm.
1
P1607P2013
2
P6539P6540
3
P2014
4
P8006 P2521G1029
5
P8002P2533
6
P7001P7005P7008P7017P6538P5004
P7004
P7018
8
P6535
9
P6537
P8007
10
P6536P8008P8009
11
P7003P6507P5003
P7002
13
P6506
Nar
acha
ams
Rast
hof
Fran
ni-a
usKa
ribi
b Gh
aub
WEST
-100
100
-200
-300
-400
-500
-600
200
300
0
A B
E N
A B
S
U B
G R
O U
PT
S U
M E
B S
U B
G R
O U
P
P7009
1516
P7014P9010
P5503P5501
P4027P7013 P7010
P9009
P7011
P6510
P6508
P6513P1426
P9520P8013
P8513P6526P6512
P6519
P8518P6520
P7503P6514
EAST
7 12 14
1718
19
2021 22 23
2425
26
27 28
29
30
31
32
33
34
35
-700
-800
-900
P8519P6515
P8500
Keilberg Rasthof
Om
baatjie
Maieberg
Gruis
A B
E N
A B
S U
B G
R O
U P
T S U M
E B S U
B G
R O
U P
-100
100
-200
-300
-400
-500
-600
200
300
400
0
-700
-800
-900
-1400
-1300
-1200
-1100
-1000
-1500
P9519
-1700
( m )
-1600
200
300
400
500
600
100
( m )
P1339
P1338
P1337P8512P6511
-1000
-1100
-1200
-1300
E1408P7515
P1425P1424P5502
P6516P1422
P1423P8520
J1405
E1407
P1419
P1415P1336
P1414P8515
P1413
P1332
41
42
43
44
-1900
-1800
C35
C39
2744
2666
Fransfontein
Malta 7
Cypress 64
Schoenau 60
Danube 59
Duurwater 66
Panorama 314
Kranspoort 475
Bothashof 476
Gemsbok-oord 477
Gros Tutara 55
Fransfontein
Aub 683
Naachaams 5
0 5 10
km
Kalahari GroupMulden GroupTsumeb SubgroupAbenab SubgroupOrosirian complex
12 3 4 5
34
611 14
16108
19913 20
212224
26
273031
Maieberg Formation Keilberg Member (cap dolostone) Ghaub Formationdolostone rhythmite, debrite
marly limestone rhythmite
as above with sea�oor cements
swaley laminated dolopelarenite
marly dolostone turbidite
geoplumb tubestone stromatolite massive carbonate diamictite
strati�ed diamictite with IRD
sorted rudite/arenite with IRD
Narachaams MemberGruis Formation Rasthof Formation
Franni-aus Member
Chuos Formation
OROSIRIAN (2.05-1.80 Ga)BASEMENT COMPLEX
oolite-clast debrite, rhythmite
dolostone turbidite, rhythmite
limestone turbidite, rhythmite
quartz siltstone, dolostone beds
allodapic dolostone, debrite
quartz arenite
dolostone microbialaminite
dolostone grainstone
dolostone stromatolite sublittoral microbialaminite
boulder diamictite, jaspilite
gneissic granodiorite,
porphyritic syenogranite
373542
38
49
292317
15o00’E
20o15’S
4036
Narachaams Garettes Fransfontein Malta Duurwater Panorama Kranspoort Bothashof Danube
columnar thrombolite
dolostone ribbonite
dolostone rhythmite
dolostone grainstone
dolostone ribbonite
limestone rhythmite, debritedolostone rhythmite, debrite
T SUMEB S UBGROUP A BENAB S UBGROUP (S LOPE F ACIES)
Ombaatjie Formation
dolostone rhythmite
dolostone microbialaminite
dolostone grainstone
dolostone stromatolite
dolostone ribbonite
marly dolostone ribbonite
limestone rhythmite
quartz siltstone, dolomite turbidites
arkose, conglomerate, debrite
A BENAB S UBGROUP (S HELF F ACIES)
6264
52
60
47
59
63
4132 39
57565551
43444546
dolarenite solution breccia with void-f illing f ibrous isopachous dolosparite cement
Fig. 4. Fence diagram for the Abenab and lower Tsumeb subgroups on Fransfontein Ridge. Section numbers 1 to 64 are keyed to the inset location map.
36
P5506
P5516
F430
P5507
P5505
P7504
P7505
P7506P7511
P7510F431
P7517P8505
38
45
46
47
48
P9516
P9517
P9518 P1416
P9514
4950 51
52 53 54
55 56
57
58
59
60
37
39 40
61 62 63
64
b8
b7
b6
b5
b4
b3
b2
b1
P600 P6528-9 P8502-4 P8506-7
Datum = base of Tsumeb Subgroup (younger Cryogenian glacial surface). Conservative vertical o�set of 500 m between the slope and shelf sections.
60 km
H U A B R I D G E
Cryogenian glacial topography across a carbonate shelf break, Otavi Group, Fransfontein Ridge, Kunene Region, Namibia
S H E L F
S L O P E
K R A N S P O O R T T R O U G H
D U U R W A T E R M O R A I N E
Paul Hof fman (UVic/Harvard), Eric Bellefroid (Yale), Blake Hodgin (Harvard), Ben Johnson (UVic), Marcus Kunzmann (McGill), Pierre Sansjofre (Brest), Justin Strauss (Dartmouth) and Dan Schrag (Harvard). Correspondence: paulfho�[email protected]
A Neoproterozoic carbonate shelf break is exposed in oblique cross-section on Fransfontein Ridge, northern Namibia, where a new fence diagram has been constructed from 64 measured sections. The Otavi Group is a <2.4-km-thick carbonate platform succession that was deposited between 770 and 590 Ma on the southwest promontory of the Congo craton. It is exposed in an arcuate 600-km-long fold belt. The carbonate platform was glaciated twice during the Cryogenian, and regionally extensive erosion surfaces beneath the respective glacial deposits (Chuos and Ghaub formations) divide the carbonate succession into three subgroups (S1 to S3). Subsidence of the platform occurred in two stages, the �rst in response to north-south crustal stretching and the second to post-rift cooling. The rift-to-drift transition occurs in the middle of S2 (Abenab Subgroup), during the nonglacial interlude. After the transition, a south-facing shelf break developed, dividing a shallow-water platform in the north from a distally-tapered foreslope and basin in the south. We studied shelf-break development on Fransfontein Ridge through mapping, closely-spaced measured sections and stable isotope chemostratigraphy. To our surprise, the development and location of the shelf break are not structurally controlled: no signi�cant growth fault intersects our transect. Instead, the dominant control on S2 stratigraphy and sedimentation was <1.8 km of local paleotopography on the older glacial surface, which is underlain by crystalline basement in this area. The location of the shelf break is dictated by >0.5 km of relief on the younger glacial surface, which ramps across the 0.3-km-thick upper S2 carbonate platform (Ombaatjie Fm) on farm Kranspoort 475 with a cut-o� angle of 8 degrees in the line of section.Preservation of <1.8 km of local paleorelief on the older glacial surface, inferred from stratigraphic onlap relations, is consistent with a nonglacial medial Cryogenian epoch (S2) of short duration.
ABSTRACT
A
Congo craton
Kal.
West
Africa
Otavi
-10 -5 0 5 10
-10 -5 0 5 10
δ13Ccarbo/oo (VPDB)
OmbomboSubgroup
Ugab Sbgp
AbenabSubgroup
TsumebSubgroup
Huttenberg
Elandshoek
Maieberg
Ombaatjie
RasthofGruis
Makalani Ridge
HuabRidge
Paleoproterozoic metamorphic-plutonic complex (Kamanjab inlier)
746 + 2 Ma- 759 + 1 Ma-
"
Devede
Nosib
Karibib
Naauw-poort
shallow-water carbonate
deep-water carbonate
glacigenic diamictite
cannibalized clastics (syn-rift)
bimodal alkaline volcanics
feldspathic sandstone (paleoflow)
Keilberg
0 (km) 150
0 (km)
-0.5
-1.5
O T A V I P L A T F O R MSouth
Chuos
Ghaub
Ugab
(635 Ma)
F O
R E
S L
O P
E
-1.0
~590 Ma
Ombo
mbo
Aben
abTs
umeb
Fig. 2. Abenab and lower Tsumeb subgroup shelf break (red box) in a restoration of the Otavi platform in the Kunene Region, Namibia. Composite inorganic carbon isotope record from the platform is shown (right).
RUACANA
OPUWA
KAMANJAB
KHORIXAS
OUTJO
14o 16o 18o17o15o
20o
19o
18o18o
19o
20o
14o
15o 16o 17o 18o
0 50 100km
O w a m b o B a s i n
Kamanjab InlierEtendeka Plateau
Epupa Inlier
K a o k o B e l t
D a m a r a B e l t
Karoo, Etendeka and Kalahari group covers
Ediacaran-Cambrian orogenic belts and granites
Mulden Group foreland basin clastics (Ediacaran)
Otavi Group carbonate platform (770 - 590 Ma)
Orosirian basement and Nosib Group clastic cover
L E G E N D
Fransfontein Ridge
TSUMEB
OTAVI
shelf break
Fig. 1. Location of Fransfontein Ridge (red box) and approximate shelf break (yellow line) in the upper Otavi Group.
1
23
4 5
6
89 10 11 13
Nar
acha
ams
Rast
hof
Fran
ni-a
usKa
ribi
b
-100
700
-200
-300
-400
-500
-600
800
900
0
A B
E N
A B
S
U B
G R
O U
PT
S U
M E
B S
U B
G R
O U
P
1516
7 12 14
1718
19
2021 22 23
2425
26
27 28
29
30
31
32
33
34
35
-700
-800
-900
Rasthof O
mb
aatjieM
aieberg G
ruis
A B
E N
A B
S U
B G
R O
U P
T S U M
E B S U
B G
R O
U P
-100
100
-200
-300
-400
-500
-600
200
300
400
0
-700
-800
-900
-1400
-1300
-1200
-1100
-1000
-1500
-1700
-1600
200
300
400
500
600
100
( m )
-1000
-1100
-1200
-1300
41
42
43
44
(m)
-1800
36
38
45
46
47
48
4950 51
52 53 54
55 56
57
58
59
60
37
39 40
61 62 63
64
H U A B R I D G E
S H E L F
S L O P E
K R A N S P O O R T T R O U G H
D U U R W A T E R M O R A I N E
Older Cryogenian glacial erosion surface (sub-Chuos Fm)
Younger Cryogenian glacial erosion surface (sub-Ghaub Fm)
Narachaams Garettes Fransfontein Malta Duurwater Panorama Kranspoort Bothashof Danube
60 kmWEST EAST
Fig. 3. Cryogenian glacial erosion surfaces beneath the Chuos (magenta) and Ghaub (blue) formations.
Fransfontein Ridge
Kunene Region
Namibia
-200
-300
-100
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
1 5δ C (VPDB)
-5 7 93-3-7 -1
δ C (VPDB)1 5-5 7 93-3-7 -1
100
200
300
0
m
DanubeBothashofKranspoortSchoenauKranspoortPanoramaPanoramaDuurwaterDuurwaterDuurwaterMaltaFransfonteinP8502/4P8505P9514P1332P1337P8518P6517P8513P8512P8013P7503P9500
datum between sections
correlation lines
Trezona CIE
basement-clast diamictite (iron-formation)
clast-supported boulder conglomerate
feldspathic/subfeldspathic quartz sandstone
quartz siltstone, �ne-grained sandstone
shale, marly shale
plutonic/high-grade metamorphic complex
strati�ed carbonate diamictite
non-strati�ed carbonate diamictite
G ha ub gla
c ial e
ros io
n sur fa ce
C hu os gla
cial e
ros ion su r face
1
2
3
4
5
TsumebSubgroup
K amanjabbasement
13 24 27 29 31 35 40 43 46 47 50 64
littoral microbialaminite (tepees/breccia)
ooid/intraclast dolarenite
columnar or mounded stromatolite
sublittoral microbialaminite/thrombolite
�aggy dololutite ribbons or rhythmite
�aggy dololutite with coarse debrite
marly dololutite ribbons or rhythmite
limestone ribbons or rhythmite
�aggy micropeloidal dolarenite
peloidal dolarenite (tube stromatolite)
Carbonate lithotopes
Abenab Subgroup δ13Ccarb data
Ghaub Fm
Franni-aus Mb
Narachaams Mb
Rasthof Fm
Keilberg Mb
Ombaatjie Fm
Gruis Fm
Rasthof Fm
Keilberg Mb
Tsumeb Sbgp A
benab Subgroup
Chuos Fm
Abe
nab
Subg
roup
T
sum
eb S
ubgr
oup
Fig. 6. Carbon isotope data from the Abenab Subgroup on Fransfontein Ridge and chemostratigraphic correlations across the shelf break.
NB: 20x vertical exaggeration
300 m
3.0 km
Sections with Abenab Subgroup high-resolution isotope data
Sections with Tsumeb Subgroup high-resolution isotope data
Field work in Namibia was supported by CIFAR (Canadian Institute for Advanced Research)
in agreement with the Geological Survey of Namibia. Stable isotope measurements were
performed in the Laboratory for Geochemical Oceanography at Harvard University.
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