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blue dots limestone, all others dolomite
blue dots limestone, all others dolomite
Dolomitization truncates the earliest Ediacaran δ13Ccarb negative excursionPaul Hof fman (UVic/Harvard), Galen Halverson (McGill), Marcus Kunzmann (McGill), Eric Bellefroid (Yale), Ben John-son (UVic), Dan Schrag (Harvard). paulfho�[email protected]
landward progradation
Islay
Bitter Springs
Maieberg
Shuram
SPICE
Cambrian
Ma
EdiacaranCryogenianTonian
-10
-5
0
5
10
o /oo
VPD
B
800 750 700 650 600 550 500
Sturtian
glaciation
Gaskiers glaciation
Marin
oan g
laciation
δ13Ccarb
U-Pbages
Keele
peritidal cycles
Ombaatj ie Fm
~400 m
transgressive tract
maximum �ood
highstand tract
glacial surface(discontinuous lodgement tillite)
cap dolostone
exposure surface
peritidal cycles
E landshoek Fm
Maieberg Fm
Keilberg Mb
G
highstand limestone
0.4 km
P O S T G L A C I A L C A P - C A R B O N A T E S E Q U E N C E
S E A W A T E R
T ~ 45oC?S = 1
cement
G
Ghaub ice grounding-line wedgeGhaub ice grounding-line wedge
I C E S H E E T
ice grounding-zone wedge
S E A G L A C I E R
B R I N ET ~ -4.4oCS ~ 2
falling-stand wedge
>0.5 km
~5 kmO T A V I G R O U P C A R B O N A T E P L A T F O R M ( N A M I B I A )
transgressive cap dolostone
0.1 km
S Y N D E G L A C I A L C A P D O L O S T O N E
ice-stream trough dashed
Note paleodepthof ice grounding-zone wedge.
B R I N E
mixed layer ‘Glacial Lake Harland’ M E LT WAT E R
lodgement tillite
2
1
3isostatic adjustment
isostatic adjustment
partial isostatic adjustment
0 50km
E T O S H A
N A T I O N A L P A R K
E T E N D E K A
P L A T E A U
K A M A N J A B I N L I E R
14o00'E 15o00'E
19o00
'S20
o00
'S
Kamanjab
Khorixas
CENOZOIC
EARLY CRETACEOUS
CARBONIFEROUS-TRIASSICKaroo Supergroup
Kalahari Group
Etendeka volcanicsma�c intrusives
Etjo sandstone Tsumeb Subgroup
Abenab Subgroup
Otavi Group
Mulden GroupDamara Supergroup
Kuiseb Formation
~500 Ma granite
NEOPROTEROZOIC
CAMBRIAN
Ombombo Subgroup
Nosib Group Nabis Formation
PALEOPROTEROZOIC metavolcanic, meta- sedimentary & meta- plutonic complex(1.93 to 1.98 Ga)
Nosib and Otavi GroupsUgab Subgroup and Naauwpoort Formation
ungraded track
graded road
paved road
platform
foreslope
rainfall >150 mm
/yr
l imestone dolostone
TST
MFZ
‘cap’ dolostone(dolopelarenite)
Stratigraphic thicknesses(250-400 m in total) normalized by member. δ13C (o/oo VPDB)
HST dolostone limestone
M A
I E
B E
R G
F
O R
M A
T I
O N
cap
-car
bona
te s
eque
nce
M3
M2
M1
14678
glacial sur face635 Ma
E D
I A
C A
R A
N
Platform sections
-5 -4 -3 -2 -1 0-6
-5 -4 -3 -2 -1 0-6 1
1
Inne
r Pla
tfor
m
Out
er P
latf
orm
TST
MFZ
Stratigraphic thicknesses(250-400 m in total) normalized by member. δ13C (o/oo VPDB)
HST
M A
I E
B E
R G
F
O R
M A
T I
O N
cap
-car
bona
te s
eque
nce
M3
M2
M1glacial sur face
635 Ma
E D
I A
C A
R A
N
-5 -4 -3 -2 -1 0-6
-5 -4 -3 -2 -1 0-6 1
1
25
Platform sections
-10 -8 -6 -4 -2-12-14
1
0
-1
-2
-3
-4
-5
-6
δ18O (o/oo) VPDB
M1
δ13C (o/oo) VPDB
dolomitized sections
Foreslope sections
-10 -8 -6 -4 -2-12
1
0
-1
-2
-3
-4
δ18O (o/oo) VPDB
δ13C (o/oo) VPDB
-5
-6
M29e9w101213
18o
20o
18o
20o
22o
24o
12o 14o 16o 18o
26o
28o
20o
100 km
Kaoko orogen
10 cm
10 cm
25 cm
50 cm
50 cm
25 cm
10 cm
Witputs
Sub-basin
Zaris
Sub-basin
NaukluftAllochthon
Tsumeb
Kunen e R. A N G O L A
Kalahari
Desert
Omaheke
Desert
K a l a h a r i c r a t o n
C o n g o c r a t o n
Okavango R.
Rehobo
th Inlier
Waterb
erg
Walvis Bay
Swakop- mund
AngraPequena (Luderitz Bay)
“
Gariep orogen Orange R.
Windhoek
Okaha
ndja
Khom
as ac
cr.
prism
Hakos
marg
inSwa
kop
mic
roco
ntine
nt
B O
T S
W A
N A
SO
UT
H A
FR
IC
A
WKz
SKZ
Witvl
ei Rid
ge
KALAHARI CRATONWitlvei - Nama groups (NP - C cover)
Hakos zone: deformed margin
1.0 - 1.4 Ga basement: Namaqua Belt
Khomas zone: accretionary prism
Swakop zone: arc-bearing microcraton
Outjo belt: northern collision zoneDAMARA OROGEN (ca 555 Ma)
CONGO CRATON Neoproterozoic cover
1.8 - 2.0 Ga basement
KAOKO OROGEN (ca 590 Ma)
Central zone: deformed margin
Western zone: accreted forearc
CKz
WKz
Carboniferous - Cenozoic
SKz Southern zone: deep-sea fan
Eastern zone: foreland thrust beltEKz
Marmora zone: accreted terranes
Port Nolloth zone: deformed margin
GARIEP OROGEN (ca 540 Ma)
EK
z
CK
z
Dam
ara orogen
geosuture thrust (teeth on hangingwall)
133 Ma igneous rocks
L EGEND
Epupa inlier
Nam
ib Desert
Er
Me
Br
Abstract The Maieberg Formation (Otavi Group) of northern Namibia o�ers an unusually expanded carbonate isotope record of earliest Ediacaran time (Fig. 1) and is the type example of a global negative δ13C excursion, the Maieberg CIE (carbon isotope excursion). The formation presents a complete depositional sequence comprised of three members (Fig. 2): M1, a basal transgressive dolopelarenite (aka ‘cap’ dolostone), deposited above storm wave-base diachronously during initial syndeglacial regression and subsequent profound marine inundation; M2, a middle marly limestone rhythmite, deposited mainly below storm wave-base during maximum �ooding (post-GIA); and M3, a regressive dolostone member that �lled accommodation created by tectonic subsidence and erosion during the preceding Marinoan glaciation (Fig. 3). The formation is 250-400 m thick on the carbonate platform (Fig. 2), while isotopically correlative strata taper to 40 m distally on the foreslope. The negative CIE encompasses the entire formation, but its nadir at -5.5o/oo PDB coincides with M2 (Fig. 4). However, where M2 is locally dolomitized, the nadir is 3o/oo heavier than normal (Fig. 5), indistinguishable isotopically from M1 and M3. Moreover, the light ‘tail’ in the upper M1 also vanishes where M2 is dolomitized (Fig. 5), yet Mg:Ca data indicate that the light ‘tail’ is not correlated with calcite content. Carbonate δ18O and δ13C are positively correlated, both being heavier where M2 is dolomitized (Figs. 7 and 8). Isotopic enrichments in C and CAS (carbonate associated sulfate) in areas of the platform where M2 is dolomitized were previously ascribed to restriction and lateral gradients in platform waters (Hurtgen et al., EPSL 245: 551-570, 2006). However, M2 dolomitization is also associated with CIE truncation on the unrestricted foreslope (Fig. 6), casting doubt on this explanation. Isotopic alteration during prolonged burial diagenesis (Bold et al., this session) overcomes this problem, but requires enormous volumes of solute. Whatever the mechanism, pre-existing dolomite (the M1 ‘tail’) was evidently altered as well as limestone. Quantitatively variable expression of other CIE’s may be related to alteration during burial diagenesis, particularly inareas like Namibia, where widespread chemical regmagnetization suggests long-lived ground-water �ows driven by orogenic topography during the tectonic assembly of Gondwana.
dolo
mit
ized
Fig. 1. Carbonate δ13C record from 820 to 490 Ma (modif ied after Halverson et al., 2005) showing the basal Ediacaran Maieberg CIE (red box).
Fig. 2. Earliest Ediacaran Maieberg Formation (postglacial depositional sequence) in section 4 (see Fig. 10), upper Hoanib River near Ombaatjie, Kunene Region, Namibia.
Fig. 3. Model scenario for Marinoan deglaciation and the Maieberg Fm depositional sequence. Deglaciation is assumed to generate a global meltwater lid >1.0 km deep at an average rate of 15 Sv (i.e., in 2 kyr), consuming ~6 Wm2 of energy.
Fig. 4. Maieberg CIE in platformal sections where M2 (middle member) is limestone. Fig. 5. Maieberg CIE in platformal sections where M2 (middle member) is dolomite.For section locations, see Fig. 10.
Fig. 6. Maieberg CIE in foreslope sections where M2 is marly limestone (open circles) and where it is dolomite (closed circles). Section locations, see Fig. 10.
Fig. 7. Member-grouped δ13C/δ18O crossplot for the platformal sections. Fig. 8. Member- and site-grouped δ13C/δ18O crossplot for the platformal sections.
M2
M3
Stratigraphic thicknesses(12-60 m total for M1-2) normalized by member.
Platform sections
Fig. 9. Tectonic map of Namibia (SW Africa) and this study area (red box).
Outjo b
asin
Otavi platform
( Karoo glacial paleovalley)
Etendeka
platform
foreslope
projected shelf break (Otavi Group)
Fig. 10. Geological map with section numbers for isotope data (Figs. 4-8).
See mapFig. 10
See mapFig. 10
See mapFig. 10
M19e9w10111213
‘cap’ dolostone(dolopelarenite)
M2
M3
M1
annual rainfall (cm)
Kamanjab inlierglacialpaleovalleys
Rasthof
Trezona
-5.0 -4.0 -3.0 -2.0 -1.0 0.0 δ13Ccarb (o/oo VPDB)
1.0-6.0
9e9w10111213
M1
Foreslope sections l imestone; dolomitized
M22 1
3
4
56
78
9101112
13