Detrital zircon ages of Neoproterozoic sequences of the Moroccan Anti-Atlas belt 

]acobo Abati a,. , Abdel Mohsine Aghzer b, 1 , Axel Gerdes c,2 , Nasser Ennih b 

• Departamento de Petrologfa y Geoquimica and Instituto de Geologia Econ6mica, Universidad Complutense/Consejo Superior de Investigaciones Cientificas, 28040 Madrid, Spain 
b Departament Ge%gie. Faculte des Sciences, Universite Chouaib Doukkali, EIJadida. Morocco 
c Institutftir Geowissenschaften. Minera/ogie, Goethe-University Frankfurt (GUF),Altenhoferallee 1. D-60438 Frankfurt am Main, Gennany 

Keywords: 
Anti-Atlas belt 
Morocco 
Detrital zircon 
U-Pb 

1. Introduction 

ABSTRACT 

Detrital zircon dating from Neoproterozoic success ions in the Sirwa inlier oftheAnti-Atlas belt in Morocco 
confirms that the maximum depositionai age of the main stratigraphic groups is significantly younger 
than has been previously proposed in lithostratigraphic correlations. This can probably be extended 
to the whole Anti-Atlas according to other recent data from the Saghro inlier. Although the relative 
stratigraphic position of the different units remains valid as published previously, a crucial implication 
of the new ages is that the sequences believed to be contemporaneous with oceanic crust and island 
arc formation during the rifting and break-up of the northern margin of the West African Craton (WAC), 
and believed to be involved in the first phases of the Pan-African orogeny, are actually late to post­
orogenic. The age of the main deformation associated with the collision of the oceanic- and arc-derived 
terranes to the WAC, allegedly affecting the sediments of the Saghro Group, has been estimated at around 
663-640 Ma. However, the youngest zircon populations of sediments of the Saghro and Bou Salda Groups, 
obtained in this study, cluster around 620-61 0 Ma, constraining the maximum age of deposition. This age 
of sedimentation is indistinguishable from the age of intrusive high-K cale-alkaline plutons of the Assarag 
Sui te, sugges ting a very rapid cycle of magmatism, relief formation, erosion and sedimentation in an active 
geodynamic scenario. Moreover, the proportion of the 61 0 Ma detrital zircons becomes less with respect 
to the Paleoproterozoic zircons at higher stratigraphic levels, suggesting that the source of young zircons 
was progressively eroded and more extensive cratonic areas, that probably underlie the Neoproterozoic 
rocks, were exposed. We interpret these data in terms of the development of a ca. 610 Ma magmatic 
arc, built upon WAC basement, and its progressive dismantling. This arc can be correlated with the 
voluminous late Neoproterozoic (ca. 640-570 Ma) arc magmatism characteristic of the north Gondwana 
margin and the peri-Gondwanan terranes. The diamictite beds that appear in the Imghi Formation of the 
Saghro Group have been correlated with the Sturtianglacial period ca. 700 Ma. However, zircons from one 
sample of these diamictites indicate that this correlation cannot be longer maintained, and instead they 
should be correlated with the Marinoan glacial period ca. 630-610 Ma, with a widespread distribution 
of glaciogenic deposits in West Africa. In addition, around 375 U-Pb concordant analyses obtained from 
Paleoproterozoic zircons from six samples represent a statistically significant population of this area of 
the WAC basement, which can be a useful database for comparison with the detrital zircon populations of 
the peri-Gondwanan terranes of Europe and North America, as the WAC margins were one of the major 
sediment suppliers for these terranes. 

Determining the age of statistically meaningful JXJpulations of 
detrital zircons within clastic sedimentary rocks has proven to be 

a powerful method to obtain relevant information on the nature 
of the crustal elements in the source region. This information 
has helped to resolve paleogeographic positions and geodynamic 
realms of terranes and paleocontinents by comparing the ages of 
the sediment provenance areas with the zircon signature of the 
stable cratonic areas of old continents or with the age of character­
istic tectono-metamorphic events. Recently, detrital zircon spectra 
have been widely used in the paleogeographic reconstruction of 
the Neoproterozoic margin of north Gondwana and the complex 
ensemble ofterranes which originated around this margin (Fig. 1 ); 
the peri-Gondwanan terranes (e.g. Fernandez-Suarez et al., 2002, 
2003; Avigad et al., 2003; Murphy et al., 2004; Linnemann et al., 



C~ 
oon1\an active 

, 

... , •• 1 •• :··)······ 

3.1-3.4 
(Ga) 

Ca. 570 Ma 
0.55-0.85 

1.1 -1.5 
1.6-2.5 \ 1.65-1.85 

2.6-3.1 \2.45-2.7 I 
(Ga) / (Ga) ~ ... --s ,. --" --- / 

. 0, craton ~ , .;.~'O 
\

.. & Saharan I '10.~~ 

. le "' , . ~~' 13,0 
" ...... 1.0-1 .57 - \ 'i~ <:,' .... (~'O\:~~~ 

/ 1.55-1.75 , Ji g, I 

A / . 19225 , <FE ~ ', mazoman . - . , c: 

,cr,\ton 2.6-3.2 I &' ~ 
(Ga) I 

"r-- - g ~ " ,I!! South '" " ~ ( 
.... Pole, ID "..\.. V , 

, J,g, O\~ I , c (l,) ,. 

\ 12 10 \ , " r.... \ ' 
,~ " ' , ~ ' .... __ .... ; 

Fig. 1. Simplified paJeogeography of Gondwana and related major peri-Gondwanan terranes at ca. 570Ma (modified from Linnemann et ai., 2007 and Dfez Femandez et ai., 
in press ). Numbers in the cratonic areas summarize their main zircon age spectra for provenance constraints. 

2004 ; Samson et al., 2005; Gerdes and Zeh, 2006 ). A crucial part 
of the interpretations in this kind of studies relies on the compar­
ison between the zircon age signature of the sedimentary rocks 
and the age signatures of the involved cratonic areas. However, 
the database of radiometric ages of the main sources of zircons 
is often not as large as would be desirable for permitting a sta­
tistically meaningful comparison. For example, the West African 
Craton (WAC) covers a huge extension, and the available age data of 
magmatic and tectono-metamorphic events is probably still insuf­
ficient to distinguish between the different parts of the craton. In 
contrast, the detrital zircon studies are much more abundant in 
the peri-Gondwanan terranes than in the sedimentary rock units 
of Africa deposited upon the WAC basement. For that reason, one 
of the aims of the present study is to date the detrital zircons of 
the main Neoproterozoic Groups of the Anti-Atlas belt, which were 
dejXJsited overlying one of the Pan-African sutures of the northern 
part of the WAC. The zircon signatures of the six samples analyzed 
will be used to place time constraints on the Neoproterozoic strati­
graphic succession, to test their provenance from the northern part 
of the WAC and to obtain a more robust database, especially for the 
Paleoproterozoic zircons, that might be used for comparison with 
peri-Gondwanan terranes of Europe and North America. Possible 
sources of zircons and their ages are shown in Fig. 1. Regarding 
the tectono-magmatic evolution of the Anti-Atlas belt, the ages 
obtained for Neoproterozoic zircons have been used to correlate 

the dejXJsition of the sediments within the development of the 
long-lived peri-Gondwanan arc activity. Finally, one sample of a 
diamictite bed in the Saghro Group allowed us to correlate it with 
a widespread Neoproterozoic glacial period. 

2. Geological setting 

The West African Craton (WAC) is composed of three Archean 
and Paleoproterozoic metamorphic and magmatic shields sepa­
rated by two cratonic sedimentary basins: the Reguibat Shield 
and the Anti-Atlas belt to the north and the Man shield to 
the south (Fig. 2; Ennih and Liegeois, 2008 ). The basement of 
the WAC was built through several major orogenic cycles: the 
Paleoarchean-Leonian cycle (different episodes between 3.5 and 
3.0 Ga) related to continental accretion and volcano-sedimentary 
activity whose chronology remains uncertain (e.g. Rocci et al., 1991 ; 
Potrel et al., 1996; Kroner et al., 2001 ; Thieblemont et al., 2004), the 
Liberiancycle(2.95-2.75 Ga; Hurleyetai., 1971 ; Auvray et ai., 1992; 
Potrel et al., 1998; Key et al., 2008 ), the Eburnian-Birimian cycle 
(2.2-1.75 Ga; Abouchami et al., 1990; Liegeois et al., 1991; Boher et 
ai., 1992; Hirdes et ai., 1992; Ait Malek et ai., 1998; Schofield et ai., 
2006 ) and the Pan-African orogenic event (760-660 Ma; Leblanc 
and Lancelot, 1980; Saquaque et al., 1989; Hefferan et al., 2000 ; 
Thomas et al., 2002; Samson et al., 2004 ). One of the main char-



• Pan-African belts N 

j\ D Ncoprolcrozic and 
Pa lcozoic basins 

ITl Archcan and 
l..±..........: Pa icoprolcrozoic 

Meseta 
domain 

RS: Rcguibat Shield TS: Tuarcg Shield 
MS: Man Shield NS: Nigeria Sh ield 

1000 km -

20° 

Late Neoproterozoinc to Cambrian cover cover 

Pan-African ophioli tes, island-arc complex, 
volcano-sedimentary series and granitoids 

Paleoproterozoic in liers 

Fig.2. (al The Anti-Atlas belt at the northern edge of the West African Cratoll. (b) Geological sketch map of the Anti-Atlas belt in southern Morocco. 

acteristics of the WAC is that no Mesoproterozoic events or rocks 
are known, suggesting a quiescent period between 1.7 and 1.0 Ca 
(e.g. Ennih and Liegeois, 2008 ). Therefore, the lack of zircons of that 
age can be considered to be a typical feature of sediments sourced 
from the WAC. The exposed parts of the craton outcrop in two main 
uplifts: the Reguibat Shield in the north (Mauritania, Morocco and 
Algeria) and the Leo Shield in the south (Fig. 2a). 

The Anti-Atlas belt of southern Morocco is located on the north­
ern edge of WAC (Fig. 2 ). It is separated in the north from the 
High-Atlas and the Meseta domain by the South Atlas Fault (Fig. 2b) 
and is subdivided into three geographic domains: the western 
Anti-Atlas composed by Kerdous, Bas Ora, Ifni and Ighrem inliers; 
the central Anti-Atlas including the Sirwa, Zenaga and Bou-Azzer 
Inliers; and the eastern Anti-Atlas made up of Saghro and Ougnat 
inliers (Ennih and Liegeois, 2008 ; Fig. 2b). The Anti-Atlas con­
sists essentially of: (1) basement rocks of Paleoproterozoic age (ca. 
2 Ga) (Fig. 2b); (2) Neoproterozoic sequences with lowermost units 
involved in the Pan-African orogeny and (3) unconformably over­
lying Paleozoic rocks (Tata and Taroudant Groups; Thomas et aL, 
2004). 

2.1. Paleoproterozoic basement 

The Paleoproterozoic basement (2030-2200 Ma) (Ait Malek et 
aL, 1998; Thomas etaL, 2002; Walsh et aI., 2002) forms the northern 
margin of the WAC (Fig. 2a). It outcrops in several inliers (Zenaga, 
Ighrem, Kerdous, Ifni, Tagragra d'Akka, etc.; Fig. 2a) and mostly 
consisting of phyllites, schistes, gneisses and migmatites which 

are intruded by calc-alkaline plutonic rocks. These basement rocks 
record low- to medium-grade metamorphic event attributed to the 
Eburnian-Birimian orogeny. The latest magmatic Paleoproterozoic 
event identified in the Anti-Atlas has been dated at ca. 1760 Ma 
(Gasquet et ai., 2005). 

2.2. Neoproterozoic sequences 

These sequences are composed of units involved in the Pan­
African orogeny and the uncomfortably overlying post-collisional 
volcano-sedimentary rocks (Ouarzazate Group of Thomas et aL , 
2002 ; Fig. 2b). The Neoproterozoic rocks underlying the Ouarza­
zate Group are subdivided into lower units (Taghdout, Bou-Azzer 
and Iriri Groups; Fig. 3), affected by the main Pan-African orogenic 
events, and the upper sequences (Saghro and Bou Said a Groups) 
only affected by the latest stage of Pan-African orogeny. The lower 
Neoproterozoic units are presumed to have been deposited over a 
long period of time involving a sequence of geodynamic settings 
starting with a rifting phase and the creation of ocean basins, and 
subsequent subduction-arc complex formation, and finally to arc­
continent collision (Thomas et al. , 2002). 

2.2.1. Pre- to syn-Pan-African evolution 
The nomenclature and descriptions used in this section are 

taken mainly from Thomas et al. (2002, 2004). The break-up and 
rifting of the northern margin of the WAC led to the formation 
of a passive margin sequence (Taghdout and Lkest Groups) which 
consists essentially of stromatolite-bearing carbonates at the bot-



~ Syn-Ouarzazate Group 
~ intrusive rocks 
~ Assarag Sui te (High-K 
~ ca lc-alkaline gra nitoids) 

D Younger cover rocks 

I:;:,:::,: :;:'.:::; ::~ Ta ta Group 

1:0: :G: =1 Ouarzazate Group 

II1II Bou Salda Grou p 

h:?'·::{::j Saghro G roup 
_ 

Bou Azze r, Iriri and 
Taghdoul Groups 

_ Zenaga Inlier 

Thrust 

Fault 

AAM F: Anti-Atlas 
Major fault 

Fig. 3. Simplified geological map of the Sirwd inlier (modified after Thomas et ai., 2002 ) with the location of the samples. More detailed maps are show n in Figs. 5 and 6. 

tom and quartzites at the top. In the Zenaga inlier, the Taghdout 
Group includes basaltic rocks underlying the limestones (Fig. Sa), 
and doleritic sills of continental tholeiitic signature (Ifzwane Suite) 
interbedded in the lower part of the sequence (Fig. Sa). The 
para-autochtonous passive margin sequence recorded a low-grade 
tectono-metamorphic event related to the Pan-African collision. 

The rifting culminated with the creation of an oceanic basin 
between the northern edge of the WAC and an unknown terrane. 
The relics of the oceanic crust created (Bou-Azzer Group) are pre­
served in the Bou-Azzer inlier (Bou-Azzer ophiolite) and Sirwa 
window (Khzama Complex) as highly sheared allochthonous ophi­
olite complexes. Dating of plagiogranite intrusions in the Sirwa 
inlier indicate formation of oceanic crust at 760Ma (U-Pb zircon 
age; Samson et al., 2004 ), and given the absence of reliable radio­
metric data on the passive margin sequence, 760 Ma is considered 
as the minimum age of its formation and this rifting stage may be 
correlated with the eariyNeoproterozoic break-up of the supercon­
tinent Rodinia. After the creation of the ocean basin, a subduction 
phase was initiated with island-arc formation (the Iriri Group in 
the Sirwa inlier; Thomas et al. , 2002). In the Sirwa inlier, the 
cale-alkaline rocks of the island-arc comprise a lowermost gneis­
sic unit (Tachakoucht andesitic gneisses) (Huch, 1988; Bassias et 
al., 1988) overthrusted by an upper orthogneiss migmatitic unit 
(Iriri migmatite). The ophiolites and the associated arc-complex are 
affected by syn-collisional medium to high-grade metamorphism 
(Thomas et ai., 2002). 

The cores and overgrowths of zircon grains from the Iriri 
migmatite yielded two U-Pb SHRIMP ages at 743 ± 14 and 

663 ± 13 Ma respectively (Thomas et al., 2002). The older age 
obtained from the cores is interpreted as the age of crystallization 
and emplacement of the protolith to the migmatite in an island­
arc complex, whereas the younger age is thought to represent the 
ophiolite obduction and arc accretion onto the northern edge of 
the WAC (Thomas et al., 2002). In the Bou Azzer inlier, U-Pb zircon 
dating of the gneissic and metagabbroic rocks of the Tazigzaout 
complex yielded similar protolith ages of753 ± 2 and 743 ± 14 Ma 
respectively (D'Lemos et al., 2006). Leucogranites post-dating the 
gneissic and mylonitic fabrics of these rocks yielded an age of 
700 Ma (D'Lemos et al., 2006 ). This second magmatic phase is inter­
preted as a continuous arc-building event by Gasquet et al. (2008). 
In the Bou-Azzer inlier, the age of the main phase of Pan-African col­
lision is constrained by syn-tectonic cale-alkaline intrusions which 
yielded U-Pb zircon ages of 654 and 640 Ma (lnglis et ai., 2004 ). 

2.2.2. Late- to post-collisional Neoproterozoic evolution 
The Pan-African collision was followed by the deposition of the 

Saghro Group. This group is defined in the Sirwa window by Thomas 
et aL (2002) as a thick pile of flysch sediments, volcanic rocks 
with cale-alkaline composition, voleanoclastic and clastic rocks, 
deformed and metamorphosed under greenschist-facies condi­
tions (Fig. 3). These authors have defined six lithostratigraphic 
formations showing a lower flysch succession with the occurrence 
of glacial diamictites in one of the lower formations (Imghi For­
mation) and an upper succession with clastic dejXJsits. Numerous 
geodynamic scenarios have been proposed for the Saghro Group. 
Most of them considered the group to be the pre-Pan-African dis-



tal equivalent of the platform margin sequence (Taghdout Group) 
(Saquaque et ai., 1992; Ouguir et ai., 1996; Thomas et ai., 2002 ). 
However, in the Saghro inlier, recentU-Pb dating of detrital zircons 
from the Kellat Mgouna subgroup yielded an age of deposition of 
this succession between 630 and 610 Ma (Gasquet et al., 2008 ). 

The Bou Salda Group overlies with angular unconformity the 
Paleoproterozoic basement and the Neoproterozoic Tagdout, Bou­
Azzer, Iriri and Saghro Groups. The group is recognized in the Sirwa 
window (Thomas et aL, 2002 ) (Fig. 3 ), the Kerdous and Bas Ora 
inliers (Anzi and Tafrawt series) and in the Bou-Azzer inlier (Tid­
diline Formation). The Bou Salda Group is a volcano-sedimentary 
sequence which was folded during the latest Pan-African tectonic 
phase with local development of incipient foliation (O'Lemos et al., 
2006) and a mylonitic fabric in the N'kob area (Thomas et al., 2002 ). 
The sequence show flysch and molasse features and was deposited 
in narrow fault-bounded troughs. In the Sirwa window, the age of 
the Bou Salda Group is indirectly obtained by the dating of two rhy­
olitic units (Tadmant and Tamriwine rhyolites) which intrude the 
Saghro Group and are overlain by the Ouarzazate Group, yielding 
U- Pb zircon ages of 605 ± 9 and 606 ± 6 Ma (Thomas et aI., 2002 ). 

An important period of widespread magmatism characterized 
the late to JXJst- Pan-African evolution. It is marked by the emplace­
ment of a high-K, calc-alkaline (HKCA) granitoids (AI Ansari and 
Sagon, 1997; Errami, 2001; Ennih and Liegeois, 2001; Thomas et 
aL, 2002) into the Saghro Group. SHRIMP U-Pb zircon dates show 
that this plutonic event occurred between 615 and 579 Ma (Ait 
Malek et aI., 1998 ; De Wall et aI., 2001; Thomas et ai., 2002; lnglis 
et aL, 2004). The Ouarzazate Group unconformably overlies the 
Paleoproterozoic, the Neoproterozoic sequences previously men­
tioned and the HKCAgranitoids. It consists of a thick (up to 2000 m) 
and extensive post-orogenic volcano-sedimentary sequence that 
cover most of the Anti-Atlas inliers (Fig. 3). U-Pb zircon ages of 
acid volcanic rocks, granites and granodiorites of the Ouarzazate 
Group show that this magmatic event ranged in age from 577 to 
552Ma (Thomas et aI., 2002; Walsh et aI. , 2002; Gasquet et aI., 
2005). 

3. Description of the samples and U-Pb results 

What follows is a brief description and the laser U-Pb results of 
six samples collected from the Taghdout, Saghro, Bou Salda (Sirwa 
inlier) and Ouarzazate Groups (Zenaga inlier), following a bottom to 
top order and encompassing the major units of the Neoproterozoic 
sequences of the Anti-Atlas. After filtering the discordant results, a 
variable number of analyses for each sample (between 67 and 126) 
have been considered valid. The schematic column of Fig. 4 shows 
the stratigraphic position of the samples. 

3.1. Taghdout Group 

This group consists of a volcano-sedimentary para­
autochtonous unit corresponding to a passive margin shallow­
water sequence, interpreted as the oldest rocks overlying the WAC 
(Thomas et ai., 2002 ; 740-8007 Ma). 

Sample AA1 (30037'4.4"N/7°16151.4"W): this is an undeformed 
pale yellow quartzite of the Mimount Formation collected from the 
type locality of Taghdout village (Fig. Sa), with abundant shallow­
water sedimentary structures, including cross-bedding and ripples. 
All the zircons from this rock are very similar, with a pale pink 
colour and variably eroded prismatic faces. From the 131 zircon 
cores analyzed, we selected the fraction with <5% of discordance in 
order to avoid, as far as possible, disturbances in the U-Pb system. 
The zircon age spectra (Figs. 6 and 7) show a major Paleoprotero­
zoic population, a few Archean zircon grains and no Neoproterozoic 
zircons. The 207/206 ages range from 1809 to 2936 Ma, with major 

peaks at 2086 and 2200 Ma and a number of other minor peaks 
(1818, 1950,2005, ... ) and a scarce Archean signal (only five grains) 
(Fig. 7). Theyoungestzirconfoundgave anageof1809 ± 1 5 Ma (98% 
concordance) and the youngest JXJpulation age is 1818 Ma, giving 
a maximum age of sedimentation. The oldest zircon is 2936 ± 1 5 
(99% concordance). To facilitate the comparison between samples, 
we have grouped the 207/206 ages in age intervals representing 
the main orogenic events of the West African Craton, with the per­
centages of each group represented in circular diagrams (Fig. 7). 
The most important population, 81% of the zircons, can be related 
to the Eburnian-Birimian orogeny. 2% of the zircon population has 
ages matching the Liberian orogeny, and the remaining 17% are in 
the interval 2.75-2.25 Ga. The last zircon age group, present in four 
of the six samples, is a relevant finding of this study that will be 
discussed below. 

3.2. Saghro Group 

The succession of this group mainly consists of sedimentary 
rocks with subordinate volcanics. In the Sirwa inlier, the Saghro 
Group was subdivided by Thomas et aL (2002) into six forma­
tions which are from bottom to top: the Tittalt, Agchtim, Tizoula, 
Imghi, Azarwas and Tafiat Formations. The two upper formations 
correspond to coarse-grained clastic rocks, while the lowermost 
succession is a volcanic turbidite and flysch-like sequence (Thomas 
et al., 2002, 2004). The following lithological descriptions of the 
Imghi and Azarwas Formations, where the samples AA3, AA5 and 
AA6 were collected, are in part after De Kock et al. (2000) and 
Thomas et al. (2002, 2004). 

3.2.1. Imghi Formation (diamictite: AA3 and arkose: AAS) 
This formation (Fig. 5b) is comJXJsed by thick beds of 

greywacke and turbidite displaying WSW-ESE paleocurrent direc­
tions (Thomas et al., 2002 ). It is characterized by the occurrence 
of several folded and faulted diamictite beds of <1-30 m in thick­
ness. The top of the sequence comprise subarkosic sandstone and 
quartz-pebble conglomerate with interbedded cherts. The diamic­
tites are overlain towards the south by a succession of graded 
conglomerate with interbedded shale. Stratigraphic way-up struc­
tures suggest that this succession is younging to the north (Gresse et 
al., 2000). Bedding is well developed and dips steeply to the north. 
The shales are affected by an incipient cleavage which is gener­
ally parallel to bedding. The massive diamictites consist of pebbles 
of granite, schists, gneisses and Mimount quartzite in medium- to 
coarse-grained quartzo-feldspatic matrix. The clasts and the matrix 
are generally poorly sorted and well rounded, corresponding to 
second- or third-cycle deposits and show parallel slip fractures 
(striations). They have been interpreted as resedimented glacial­
derived clasts which were transported into the basin by turbidity 
currents (Gresse et al., 2000 ). 

Sample MS (30"44'49.8'N/7"27'54.3'W): it is a light-grey, fine 
to medium-grained immature feldspathic sandstone (arkose). It 
consists predominantly of quartz, plagioclase, and microcline, with 
minor amounts of muscovite and biotite. A mesoscopic lamination 
defined by lamination-parallel micas probably represents primary 
stratification. From 124 zircon grains analyzed, only 64 are <5% 
discordant. They gave a mixture of Paleoproterozoic and Neo­
proterozoic dates, with a significant proportion of young zircons 
(80-20%; Figs. 7 and 8a, b). The youngest zircon found gave a date 
of 588 ± 1 0 Ma (105% concordant) and the youngest JXJpulation age 
is 620 Ma. The oldest zircon is 2610 ± 14 (96% concordant). 

Sample M3 (30"44'58.0"N/7"27'50.9'W): sampled in a higher 
stratigraphical level of the Imghi Formation than AA5, it is an 
unsorted, matrix-supported and poorly stratified conglomerate 
level (diamictite) with a fine-grained quartzo-feldspathic matrix 
and numerous clastic fragments. The clasts show a gradation in 



Ouarzazate Group Basal Epiclastic conglomerates AA7 

Agalf Member 

'" 
Bou Salda Group Lmakhzen member! AA4 

Q) 
u Ighil Member c: 
Q) 
:> 
c-
Q) 

en Tafiat Formation u 
·N Azarwas Formation AA6 
0 Saghro Group ~ 

U--AA3 Q) Anti-Atlas Supergroup Imghi Formation --0 It.AA5 ~ 

Tittalt Formation a. 
0 
Q) 

z 
Bou Azzer and Iriri Groups: Ophiolites 
(Khazama and N'Kob ophiolites) and Iriri Migmatites 

Mimount Formation 
Taghdout Group Tirsal Formation 

AA1 

Agouniy Formation 

Fig.4. Stratigraphic sketch column of the Sirwd Window showing the location of the samples of this study. 

size from fine rock fragments to coarse cobbles (up to 20cm). The 
most typical clasts are rounded quartzite pebbles, and less frequent 
are fragments of igneous and metamorphic rocks, with occasional 
angular faces. 126 concordant zircon analyses show essentially a 
sub-equal mixture of Paieoproterozoic and Neoproterozoic ages 
(Figs. 7, 8e, d ), and only two Archean grains. The only remark­
able difference with the previous sample (both from the Imghi 
Formation) is a significantly lower proportion of young zircons: 
50-50% compared to 80-20% in the MS arkose. The probability 
plots of Fig. 7 show that that the Archean and Paleoproterorozoic 
zircon populations of this sample are similar to the Ml quartzite 
(and also similar to the other samples), except for the absence of 
Liberian (2.95-2.75 Ca) zircons. The distribution of Paleoprotero­
zoic zircons is 43% for the Eburnian interval (2.2-1.78 Ca) and 5% 
for the 2.75-2.25 Ca. The youngest Paleoproterozoic zircon has an 
age of 1789 ± 22 Ma (97% concordance) and the youngest popula­
tion age is 1798 Ma. The oldest zircon was dated at 2722 ± 14 (1 00% 
concordance). 

The Neoproterozoic JXJpulations have been represented in more 
detail in the concordia and probability plots of Fig. 8c, d. In the prob­
ability plots the 206/238 ratios have been used, because at this age 
range they are much more precise than the 207/206 ratios. The 
younger Neoproterozoic zircons show a marked peak at 610 Ma. 
The age of the youngest zircon is 591 ± 12 Ma (101 % concordance), 
and the oldest Neoproterozoic zircon was dated at 714±12Ma 
(101% concordance). The probability plot shows another minor 
peak at 635 Ma which extends until 644 Ma, and after that a sig­
nificantgap is observed between 644 and 714 Ma (one zircon) with 
only two ages at 661 and 662 ± 12 Ma. The age distribution is 50% 
of zircons of 0.55-0.65 Ga, and 2% of 0.66-0.77 Ga. 

3.2.2. AzarwQS Formation (whitish arkosic sandstone: M6) 
This formation comprises three members, from bottom to top: 

Wissadene, Amajjar and Tougmast Members (Thomas et al., 2002). 
The Wissadene Member consists of sandstone, arkose and con­
glomerate. The Amajjar Member contains arkosic sandstone, shale 
and siltite. The Tougmast Member is the most extensive; the main 
body consists of arkosic greywacke, cross-bedded arkosic sand­
stone, whitish arkose and interbedded siltstone with subordinate 
shale and conglomerate and some basic and acid volcanic rocks. 

Sample M6 (30039134.0''N/7°22117.0"W): whitish arkosic sand­
stone collected west of Adrar Tougmast (Fig. 6a). The zircon age 
population in the collected sample is quite different with respect 
the other samples of the Saghro Croup (arkose MS and diamictite 
AA3), because is restricted to two age groups and the proportions 
of each group are very different. The presence of Ediacaran zir­
cons (0.65-0.55 Ca) is much lower, only seven grains from 107 
(6.5%), with 206/238 ages of 600 ± 12 (two grains), 607 ± 11 and 
621 ± 13 Ma (Fig. 8e, 0. The remaining 93% zircons are in the 
Eburnian-Birimian interval (2.2-1.78 Ca). 

33. Bou Salda Group (M4: conglomerate) 

Although this formation is considered as a volcano-sedimentary 
sequence occupying a transitional stratigraphic JXJsition between 
the underlying Saghro Croup and the overlying Ouarzazate Croup, 
it presents some similarities with the Azarwas Formation. Their 
occurrence is limited to narrow fault-bounded troughs. It is consid­
ered by Thomas et al. (2002) as a syn-collisional molasse consisting 
of arc-derived volcano-clastic rocks deposited in a fore-arc basin. 
The Bou Salda Croup is mainly composed by two members which 





N 

A0 
/ A'-:7( 

K - /\ /\ 

A A A 
/\ i\ /\ 

r-:-:l Boutazarte Granite 
L........'J (Ammassine suite) 
oc--n Arg Gahhro 
~ (Assarag suite) 

C Younger cover rocks 

k:::::;I Sandstones, Tuff 
~ Conglomerates 

'" _ Dacites 
~ 
~ r=:=:J Andesites 
t:: ~ Rbyolite and trachytes 
" o t· 0·:1 Epiclastic rocks 

Saghro Group 
roo:o,,::'-01 Arkosic sandstone, 
lO.',:O.", sandstone, conglomerate 

~ Thrust 
___ Fault 

D Younger rocks 

h~\1 Ouarzazate group 

Taghdout, Bou Azzer, 
SF! Saghro and Bou Salda 

groups. Assarag suite 

• Paleoproterozoic basement 

o 25 km 

Fig. 6. (a) Geological map of southern part of the Sirwa inlier, modified from the 1 :50.000 geological map ofTaghdout sheet (Geological Survey of Morocco, 2000). (b) 
Geological sketch map of the Zenaga inlier. 



.c 
nI .c e 
Cl. 
Q) 

> :;::; 
nI 
Q) 

0:: 

P PROTEROZOIC ARCHEAN 
A ~------,----------,--------------~----,---~ 
L. NEO- MESO- PALEO- NEO- MESO-

500 

0% c:: ____ 9~ 
30lo;'~----~ 

00.55 - 0.65 
. 1.78-2.2 

00.65 - 0.55 
00.77-0.66 
. 2.2-1 .78 
.2.75-2.25 

00.65 - 0.55 
.2.2-1.78 

19% 1% !!!!!!.r.;;:-- 00.65 - 0.55 
, 70% 00.77-0.66 

2% 

81% 

. 2.2-1 .78 

.2.75-2.25 

00.65 - 0.55 
[;l 0.77-0.66 
. 2.2-1.78 
.2.75-2.25 

17% 

2% 

• 2.2-1 .78 (Eburnian-Birimian) 
• 2.75-2.25 (10) 
. 2.95-2 .75 (Liberian) 

95-105% conc.J 

'" ::2: AA 7: Basal Conglomerate 
o 
co 
o 
N 

(Ouarzazate) N=67 

Top 

AA4: Quartzitic cobble 
from conglomerate 
(Bou Salda) N=81 

AA6: 
Arkosic sandstone 

(Fm. Azarwas) N=107 

AA5: 
Arkose (Fm. Imghi) 

N=89 

AA3: Diamictite 
(Fm. Imghi) N=126 

AA1 : Taghdout 
quartzite, N=116 

Bottom 

1000 1500 2000 2500 3000 

207/206 U-Pb age (Ma) 

Fig. 7. Probability plots of detrital-zircon U-Pb age populations from the Neoproterozoic formations of the Anti-Atlas belt samples anaJyzed in this study. Plots are arranged 
with the oldest stratigraphic unit sampled at the bottom and the youngest unit at the top. 



0.109 

0.107 

0.105 

0.103 

0.101 

0.099 

0.097 

e l--------------:;,.-----, 

AA6: Arkosic 
sandstone 

(Fm. Azarwas) 

Neoproterozic zircons with 
less than 5% discordance 

0.095 L----L_~_~~_~_~~_~ __ ___' 

0.76 

0.13 

0.12 

0.11 

0.10 

0.09 

O.BO 0.84 O.BB 

AA5: Arkose 
(Fm.lmghi) 

___ ·r L'U-X 

0.92 0.96 

Neoproterozic zircons with 
less than 5% discordance 

O . OB LO'-~-~-~-~-~-~-~--.J 

0.6 

0.1 

0.115 

0.7 

O.B 1.0 1.2 1.4 

AA3: Diamictite 
(Fm.lmghi) 

Neoproterozic zircons with 
less than 5% discordance 

O.B 0.9 1.0 1.1 

3rIJr-----------------~ 

~ 
E 
::> 
z 

2 

500 550 600 

\ 
650 700 

206/238 age (Ma) 

AA6 
N=7 

750 BOO 

14 r@}f------------------~ 

~ 
Q) 

.Q 

E 
::> 
z 

12 

10 

B 

6 

4 

2 

500 

j 

550 

v 

I~ 
600 650 700 

206/238 age (Ma) 

AA5 
N=71 

750 BOO 

1B ~f----------------------, 

16 

14 

12 

t 10 
.c 
E B 
::> 
Z 6 

4 

2 

500 550 600 

AA3 
N=65 

650 700 750 

206/238 age (Ma) 

BOO 

Fig.8. U-Pb Concordia diagrams and detailed relative probability plots depicting the Neoproterozoic zircon populations forsampJes MS (a, b), AA3 (c, d) andAA6 (e, f). They 
are arranged from bottom to top according to their stratigraphic position. 



10 
0.102 9 

AA7: Conglomerate AA7 AA7 
(Ouarzazate) 8 N=66 ::c 0.098 (I) 

7 iii -
0.094 6 <. 

(I) 
~ 

" 206 Pb '" 5 .Q ~ 

E 0 
'''u 0.090 4 er 

:::I '" z 3 2: 
0.086 ~ 

2 

Neoproterozic zircons with 
less than 5% discordance 

500 550 600 650 700 750 800 

0.62 0.66 0.70 0.74 0.78 0.82 0.86 2061238 age (Ma) 
207Pb(135U 

0.116 a 3jili} 
AA4: Quartzitic cobble 

0.112 (Bou Salda) A AA4 ::c 
N=6 (I) 

iii 
2 f- -0.108 

<. 
~ 

(I) 

'" " .Q ~ 

E 0 
0.104 :::I er 

'" Z 2: r- ::+ 
0.100 I" 

'< 

0.096 \) \ 
500 550 600 650 700 750 800 

0.092 2061238 age (Ma) 

0.74 0.78 0.82 0.86 0.90 0.94 0.98 1.02 

Fig. 9. U-Pb concordia diagrams and detailed relative probability plots depicting the Neoproterozoic zircon populations for samples M4 (a, b) and AA7 (c, d). They are 
arranged from bottom to top according to their stratigraphic position. 

suggests that it could be reworked sedimentary rocks from this 
Group. 

3.4. Ouarzazate Group 

SampieAA7(30032IS0.S"Nj7°07117.4"W): epiciastic conglomer­
ate of the Ouarzazate Group just above the discordant contact with 
the materials of the Saghro Group (Fig. 6b). 97% of the zircon JXJpu­
lation is the 550-650 Ma interval, and the remaining 3% (two grains) 
have 207/206 ages around 21 00 Ma. The maximum of the young zir­
con population is 571 Ma which, given the volcaniclastic nature of 
the conglomerate, is probably very close to the deposition age. The 
youngest zircon found was dated at 537 ± 12 Ma (96% concordance) 
and the youngest population age is 557 Ma. The oldest zircon gave 
an age of 2121 ± 17 (101 % concordance). 

4. Discussion 

4.1. Pasive margin sequence 

The age of the platform margin sequence covering the Pale­
oproterozoic basement (Taghdout and Lkest Groups) is not well 

constrained. The dikes crosscutting the basal part of the series are 
dated by an old Rb-Se whole rock at 787 ± 1 0 Ma ((ahen et ai., 
1984a,b). The other constrains are indirect: they are below the 
Bou Azzer ophiolitic complex with an obduction age of 660 Ma, 
and a minimum age of 788 ± 9 is indicated by a Rb-Sr age on clay 
fractions from Taghdout metasediments (Clauer, 1976). The new 
zircon data obtained in this study from the Taghdout quartzite 
(AA1) do not narrow very much the previous time constraints; the 
youngest zircon is 1809 Ma, thus not precluding the possibility that 
the sequence could be Paleoproterozoic. 

The quartzite only contains Archean and Paleoproterozoic zir­
cons, whose main characteristics are discussed below. The lack 
of zircons with more than 3.0 Ga (Leonian orogeny) indicate that 
the provenance area of the sediments was relatively far away of 
the principal potential source of Meso- and Paleoarchean zircons, 
located in the Man and Nigerian shields in the south part of the 
WAC (Fig. 2 ; Rocci et al., 1991; Kroner et al., 2001; Thh~blemont et 
al., 2004). The only known source of zircons of that age in the north­
ern part of the WAC is restricted to small parts of the Amsaga area 
in the Reguibat shield (Auvray et al., 1992; Potrel et al., 1996; Key 
et al., 2008 ). Therefore, the source area of the quartzites is inferred 
to be the proximal, authocthonous basement of the Anti-Atlas belt, 



N 
active arc 

'" U-~ 

0.55 - 0.65 
1.8-2.2 
2.2-2.7 Ga? 

1.8-2.2 S 
2.2-2.7 ? 
2.7-2.9 Ga 

~':\j 
~~-------------------~~~~ 

~Bousalda 

[: :::::~.: ::J Saghro Group 

• Ophiolit ic complex (Bou Azzer) 

r-::::::l Pasive margin sequence 
~ (Taghdout and Lkset Groups) 

~ Peri-Gondwnan type basement? 
L....::::::J (previously detached from WAC?) 

r-+l West African 
~ craton basement 

• 
Cale-alkaline magmas, 
HKCA granitoids 

• Oceanic crust 

WAC 

Ca. 610 Ma 

0.6 Ga 
~ Possible sources or zircons 

Fig. 10. Geological sketch section showing the possible source areas of the zircons for the main lithostratigraphic groups of the Anti-Atlas according to the data presented 
in this study. 

formed essentially by Paieoproterozoic components (Fig. 10). As a 
more general conclusion, an interesting feature of the zircon sig­
nature of sediments whose source area is the northern part of the 
WAC should be the absence of more than 3.0 Ga zircons. The old­
est groups of Archean zircons is very scarce and only have been 
found in the Taghdout quartzite (AA1), where they only represent 
2% of the total zircon population. Their ages vary from 2936 to 
2746 Ma, and can be ascribed to the Liberian orogeny. The follow­
ing younger group of zircons, between 2.75 and 2.25 Ga, represent 
a significant group with a proportion 17% (see circular diagrams in 
Fig. 7 ). It is the most intriguing population, because the Archean 
and Paleoproterozoic evolution of the WAC does not register any 
tectonothermal event in that range of ages (typical zircon age 
ranges of the WAC are shown in Fig. 1 ). The youngest granitoids 
related with the Neoarchean development of the Reguibat shield 
are 2726 ± 7 Ma (Potrel et ai., 1998; Key et ai., 2008 ), and the fol­
lowing registered events are the early Eburnian ca. 2250 Ma. Thus, 
a gap on zircon ages between ca. 2700 and 2050 Ma is assumed 
to be characteristic of rocks coming from the WAC (Nance et al., 
200S ). The presence of these concordant zircon ages in the platform 
sequence located directly over the WAC suggest that some impor­
tant magmatic or orogenic event within this age interval should be 
registered in some part of the north WAC basement that have not 
been discovered yet. It is interesting to note that small JX)pulations 
of zircons between 2.75 and 2.25 Ga are present in the sediments 
of some peri-Gondwanan terranes of NW Iberia whose source area 
is assumed to be the WAC (e.g. Diez Fernandez et al., in press ). 
Within the zircons of this group, the main age peaks are located 
at 2450 and 2514Ma. One possible location for these basement 
rocks could be the Eglab Massif an the north of the Reguibat shield 
in Algeria, where some Sr and Nd model ages suggest the pres­
ence of rocks about 2.4-2.5 Ga (Peucat et al., 2005), although the 
ages have not been confirmed by U-Pb in zircon. The third group 
of zircons is the main population of zircons (S3%, Fig. 7) coming 
from the "old" basement (2.2-1.7S Ga). This group is related with 
the important Eburnian-Birimian tectono-magmatic activity reg­
istered elsewhere in the WAC, especially in the north part of the 
Reguibat shield and in the basement of the Anti-Atlas belt. 

4.2. Saghro, Bou Salda and Ouarzazate Groups 

Neoarchean and Paleoproterozoic zircons. The first age group 
found is between 2.75 and 2.25 Ga. It is lacking in samples AA6 and 
AA7, represents a small proportion in samples AA3 and AA5 (1 % and 
5%), and only reaches a significant proportion, 17% and 16%, in sam­
ples AAl and AA4 respectively (Fig. 7 ). A JX)ssible source area for 
these zircons could be the northern terrane that should have been 

accreted to the WAC margin immediately after the obduction of the 
ohiolites (Gasquet et al., 200S ; Fig. 10). This terrane would be fur­
ther to the north of the Saghro arc, and at present is hidden by the 
south Atlas major fault (Fig. 2), but it would presumably be equiv­
alent to the basement of the present peri-Gondwnanan Meseta 
terranes to the north of the Atlas chain (Ennih and Liegeois, 200S ). 
The last group corresponds to the Eburnian cycle (2.2-1.7S Ga), and 
their proportion with respect to the total population varies between 
93% (sample AA6, sandstone from Azarwas Formation) and 3% in 
the younger sample (AA7, Ouarzazate Group conglomerate). 

Neoproterozoic zircons. The proportion of Neoprotero­
zoicjPaleoproterozoic zircons varies considerably from the 
bottom to the top of the sedimentary sequence, showing the 
fluctuations in the source area, depending on the nature of the 
exhumed rocks that were being uplifted and eroded: the cratonic 
basement or the rocks related with the Pan-African orogeny. The 
oldest passive margin Taghdout quartzite (AA1), located below 
the suture, does not have any Neoproterozoic zircons, and going 
to the top of the sequences above the suture, the proJX)rtion of 
Neoproterozoic zircons in each sample is SO% (AA5), 52% (AA3), 
7% (AA6), 5% (AM) and 97% (AA7) (Figs. 4 and 7). The youngest 
zircon JX)pulation in Saghro and Bou Salda Groups is 610-620 Ma, 
whereas in the discordantly overlying Ouarzazate Group is 557 Ma. 
In the case of the zircons from the Imghi Formation (arkose AA5 
and diamictite AA3), the differences in the zircon population could 
be related with a proximal supply of sediments for the arkose and 
a more distal source for the diamictite, due to longer transport 
in ice blocks favouring a higher proJX)rtion of zircons from the 
Eburnian basement. 

In tectono-stratigraphic reconstructions of the Anti-Atlas belt, 
the deformation affecting the Saghro Group was classically 
attributed to the Pan-African collision, and according to this view, 
the age of this group should be older than 660-700 Ma (Thomas 
et al., 2004 ). However, recent U-Pb zircon dating of the turbidites 
from the Saghro inlier place this group in the 630-610Ma age 
range (Gasquet et al., 200S ). Our new data presented here from 
the Sirwa inlier are in the same age range. Thus, one important 
conclusion of this study is the confirmation that the deposition 
of the Saghro Group actually post-dates the accretion of the ca. 
750Ma ophiolites and arc derived terranes (Iriri arc) to the WAC 
margin, which probably occurred during the second phase of the 
Pan-African deformation (ca. 660Ma; 02 of O'Lemos et al., 2006). 
The deposition age younger than 610 Ma demands a reassessment 
of the origin and stratigraphic JX)sition of the Saghro Group. The 
deformation described on the sediments of the Saghro Group can 
no longer be related with the main phase of Pan-African deforma­
tion, instead it should be ascribed to a post-610Ma deformational 



event related with the late to post-collisional evolution. This defor­
mation may be at the origin of the exhumation of the Pan-African 
metamorphic section. 

Considering that the Saghro Group rest directly over the Anti­
Atlas suture, another interesting feature of the zircon spectra is the 
scarcity of 800-650 Ma zircons (at least from the Imghi Formation 
to the top of the sequence; Fig. 7), which implies that the source 
of the sediments was in some way isolated from the suture area. 
It is possible that a topographic barrier, e.g. accretionary prism, 
restricted sediment transport from the active orogen to the fore­
land basin (Fig. 10 ). The data presented in this study indicate that 
the source of the sediments was a terrane with north-WAC base­
ment and a younger component of620-s80 magmatic rocks, with 
a major peak around 610 Ma. On the other hand, the high-K cale­
alkaline granites intrusive in the Saghro Group range from 615 to 
580 Ma (Ait Malek et ai., 1998; De Wall et ai., 2001; Levresse et ai., 
2001; Thomas et al., 2002; Inglis et al., 2004 ), being indistinguish­
able from the age of sedimentation of the wall rocks, with a lower 
limit marked by the age of the discordant overlying sequence, the 
Ouarzazate Group, that show intrusive igneous rocks of ca. 580 Ma 
(Toufghrane Suite; Mifdal and Peucat, 1985; Thomas et al., 2002 ). 
Thus, the age of deposition and the intrusion of granitoids seems 
closer than the resolution limit of the radiometric dating, suggest­
ing a highly active geodynamic realm where magmatism, relief 
formation, erosion and sedimentation occurs at a high velocities. 
This apparent synchronicity of the sedimentation and their subse­
quent metamorphism and intrusion by plutonic bodies is a typical 
feature of magmatic arc settings (Abati et al., 2003 ), and therefore 
we proJXJse that the Saghro Group come from the erosion of a ter­
rane located further to the north of the Anti-Atlas suture, probably 
a magmatic arc built uJXJn WAC basement. The arc should have 
been incipiently detached from its margin, in a way that most of 
the detrital material came from the arc itself and their basement, 
and permitting only limited communication with the suture area 
(Fig. 10). This JXJssible arc activity of ca. 61 0 Ma could be correlated 
with the long-lived arc surrounding the northern Gondwana mar­
gin (Fig. 1 ), characterized by abundant cale-alkaline voleanic rocks 
and cogenetic plutons, sedimentation and deformation associated 
with the opening and closing of arc-related basins that gave rise 
to the peri-Gondwanan terranes and finally to the opening of the 
Rheic Ocean (e.g. Murphy and Nance, 1989; Keppie et al., 1996; 
Murphy et al., 2004; Nance et al., 2008 ). The different proJXJrtions 
of sediments coming from the arc with respect to the basement can 
be interpreted in terms of the birth, development and erosive dis­
mantling of the arc. The strong signal of ca. 650-550 Ma (50% of the 
zircons) appearing in sample AA3, reaching a maximum of 70% in 
sample AAs (Fig. 7 )would be related with the formation of the arc. 
The subsequent decreasing of this group of zircons going to higher 
stratigraphicallevels (7% and 4% in samples AA6 and AA4) would 
be associated with the progressive dismantling of the arc. 

According to the former interpretation of the age of the Saghro 
Group, the diamictite beds that appear in the Imghi Formation have 
been correlated with the Sturtianglacial period ca. 700 Ma (Thomas 
et al., 2002, 2004 ). However, the zircons from one sample of these 
diamictites (sample AA3) indicate that the correlation cannot be 
maintained longer, and instead they should be correlated with the 
Marinoanglacial period ca. 630-610 Ma (Kennedyet al., 1998 ), with 
a widespread distribution of glaciogenic deposits in West Africa 
(Deynoux et ai., 2006). 

5. Conclusions 

1. The lack of zircons older than 3.0 Ga indicates that the source 
of the Neoproterozoic sedimentary sequences of the Anti-Atlas 
belt was the northern part of the WAC autochthonous basement 
(Reguibat shield). 

2. The group of Neoarchean-Paleoproteroic zircons (2.75-2.25) 
suggests the existence of a tectonothermal event(s) in the north­
ern part of the WAC with a peak at ca. 2.5-2.4 Ga, which have not 
been described yet. 

3. The major zircon source of the sediments was the igneous and 
metamorphic Eburnian rocks located in the Reguibat shield 
(except for the Ouarzazate Group). 

4. The depositional age of the Saghro Group is younger than 
620-610Ma and hence the deformation that affects the sedi­
ments should be late- to post-collisional. 

S. The sedimentary sequences of the Saghro Group probably 
reflects the development and subsequent erosive dismantling 
of a magmatic arc located to the north of the Pan-African suture 
and built upon north WAC basement. 

6. This arc can be correlated with the long-lived arc-system 
surrounding the northern Gondwana margin during the 
Neoproterozoic-Lower Cambrian. 

7. The diamictite beds appearing in the Sagrho Group are related 
to the Marinoan glacial period. 

Acknowledgments 

We would like to express our thanks to Bob Thomas, an 
anonymous reviewer and the editor P. Cawood for helpful and con­
structive reviews. Field work was funded by project A-7287-06 
from the Agencia Espafiola de Cooperacion Internacional (AECI), 
and analytical work was funded by project CM-UCM-910129 from 
the Comunidad de Madrid. 

Appendix A. Supplementary data 

Supplementary Frei and Gerdes, 2009; Gerdes and Zeh, 2009; 
Jackson et al., 2004; Janousek et al., 2006; data associated 
with this article can be found , in the online version, at 
doi:l 0.1 016/j .precamres.201 0.05.018 . 

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