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�man@gmail.com

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