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Precambrian Research 196– 197 (2012) 106– 112

Contents lists available at SciVerse ScienceDirect

Precambrian Research

journa l h omepa g e: www.elsev ier .com/ locate /precamres

he Bou-Azzer glaciation: Evidence for an Ediacaran glaciation on the Westfrican Craton (Anti-Atlas, Morocco)

. Vernheta,∗, N. Youbia,c,d, E.H. Chellaib, M. Villeneuvee, A. El Archi f

University of Antilles and Guyane (UAG), Campus Fouillole, Laboratory L.a.R.G.E., 97159 Pointe-à-Pitre Cedex, French West Indies (FWI), FranceCadi Ayyad University, Faculty of Sciences Semlalia, Geology Department, P.O. Box 2390, Marrakech 40000, MoroccoCentre National pour la Recherche Scientifique et Technique, Angle avenues des FAR et Allal El Fassi, Madinat Al Irfane B.P. 8027 Nations Unies, 10102 Rabat, MoroccoCentro de Geologia, Universidade de Lisboa (CeGUL), Faculdade de Ciências, Departmento de Geologia, Lisboa, PortugalUniversity of Provence (Aix-Marseille 1), Laboratory G.S.R.C., 3, Place Victor Hugo, 13331 Marseille Cedex 3, FranceChouaib Doukkali University, Faculty of Sciences, Geology Department, P.O. Box 20, El Jadida 24000, Morocco

r t i c l e i n f o

rticle history:eceived 25 May 2011eceived in revised form 27 October 2011ccepted 18 November 2011vailable online xxx

a b s t r a c t

This work reports for the first time evidence for an Ediacaran glaciation in Morocco on the West AfricanCraton that we have named the Bou-Azzer glaciation. It is represented by glacially eroded surfaces andsedimentary features observed in Precambrian outcrops in two inliers of the Anti-Atlas Belt. The com-monly accepted stratigraphic framework constrains the glacially eroded surfaces to post-date a regionallydefined D2 deformational age known to be between 605 and 595 Ma. A minimum age for the glaciation

eywords:laciationnti-Atlasdiacaranorrelation

is less certain but likely pre-dates the end of Ouarzazate Group deposition at 560 Ma. This age range per-mits two possible correlations to glacial events known from elsewhere: the Bou-Azzer glaciation couldbe equivalent to the Gaskiers glaciation (∼580 Ma), which has not been recognised previously in the WestAfrican Craton, or it could be a late Ediacaran glaciation (∼560 Ma), which has been suspected in differentplaces on the Craton.

orocco

. Introduction

Three major glaciations mark the end of the Neoproterozoic.wo are Cryogenian in age, one at 723–735 Ma (Sturtian glacia-ion) and the other at 663–635 Ma (Marinoan glaciation) (Hoffmannt al., 2004; Hoffman et al., 2009; Condon et al., 2005; Corsettind Kaufman, 2005; Halverson et al., 2005; Allen, 2007; Eyles,008; Hoffman and Li, 2009); these underpin the “Snowball Earth”ypothesis (Hoffman et al., 1998). The third well-documentedeoproterozoic glaciation (Lu et al., 1985; Frimmel and Fölling,004; Xiao et al., 2004; Bingen et al., 2005; Alvarenga et al., 2007;homentovsky, 2008; Xu et al., 2009) is the Gaskiers glaciation and

s Ediacaran in age (580 Ma, Bowring et al., 2003, 2007; Halversont al., 2005). The occurrence of complex metazoans occurred soonfter the Gaskiers glaciation. Whilst the Sturtian and Marinoanlaciations are recognised worldwide (Hoffman and Li, 2009),eposits related to the Gaskiers glaciation are more restricted andypically do not have an associated cap carbonate. These character-

stics have led Halverson et al. (2005) to propose that the Gaskierslaciation is more like the non-global Paleozoic and Quaternarylaciations than the global Cryogenian glaciations.

∗ Corresponding author.E-mail addresses: vernhet.elodie@voila.fr, evernhet@univ-ag.fr (E. Vernhet).

301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2011.11.009

© 2011 Elsevier B.V. All rights reserved.

Ediacaran sediments worldwide also record an important �13Cnegative anomaly (the Shuram-Wonoka anomaly) that has beenattributed by some workers to be the result of changes in oceanchemistry conditions associated with the Gaskiers glaciation, andthat this anomaly may be correlative worldwide (e.g. Calver,2000). Other workers, however, do not consider the anomaly to bedirectly related to the Gaskiers glaciation (Halverson et al., 2005;Le Guerroué et al., 2006) and still others attribute it to a global dia-genetic phenomenon (e.g. Derry, 2010). Regardless of the origin ofthe anomaly and its link to the Gaskiers glaciation, an additionallate Ediacaran glaciation (around 560 Ma) has been suspected onthe West African Craton (but not in Morocco) by different authors(Bertrand-Sarfati et al., 1995; Caby and Fabre, 1981; Deynoux et al.,2006).

The sedimentary evidence presented in this article establishesthe occurrence of an Ediacaran glacial event in Morocco and on theWest African Craton. However, the precise timing of this glaciationremains to be established.

2. Geological setting

The Anti-Atlas Belt is located in southern Morocco in NorthAfrica (Fig. 1A) and trends SW-NE from the Atlantic Ocean coastto the Algerian border. It is situated along the northern part ofthe West African Craton (that was part of the Paleozoic Gondwana

E. Vernhet et al. / Precambrian Research 196– 197 (2012) 106– 112 107

Fig. 1. (A) Regional context and location of the studied area. (B) Revised stratigraphy used in this study. (C) Regional correlations of the Tazegzaout and the Ingoum-suds et al. (

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ections showing the glacially eroded surfaces described herein. Ages from Maloof

upercontinent) and is bordered to the north by the Hercynianouth Atlas Fault and to the south by the Paleozoic Tindouf BasinFig. 1A). The Anti-Atlas Belt resulted from Hercynian deformationnd uplift that gave rise to several inliers in which Proterozoicnd Paleozoic rocks crop out. The inliers comprise a Panafricanelt resting upon a Birimian (circa 2000 Ma) basement (Leblancnd Lancelot, 1980). The first Panafrican tectonic event (D1) corre-ponds to the closure of an oceanic domain at ca. 660 Ma (Thomast al., 2004). A second event (D2) is marked by strong deformationf volcanic, volcaniclastic and conglomeratic deposits (includinghe potential glacial deposits described within the Tiddiline Forma-ion) and ended around 605–595 Ma (Leblanc and Lancelot, 1980;homas et al., 2004; Maloof et al., 2005). The third (and final) eventD3) corresponds to the deposition of the volcanic, volcaniclasticnd conglomeratic rocks of the Ouarzazate Group during an exten-ional tectonic regime. This group is overlain by the Adoudounianormation, which is considered to be of Late Ediacaran/Early Cam-rian in age (ca. 542 Ma, Maloof et al., 2005).

Because many of the conglomerate, volcanic and volcaniclas-

ic units have limited lateral continuity, regional correlations haveeen problematic. Thus, we propose using three main criteriao categorise the various units of previously proposed strati-raphic frameworks (e.g. Maloof et al., 2005) whilst retaining

2005).

nomenclature that has been used traditionally (Fig. 1B): (1) stratashowing a D2 schistosity belong to the Tiddiline Formation andpre-date the 605–595 Ma deformational event; (2) undeformedstrata of interbedded conglomerates and volcaniclastic rocks definethe Ouarzazate Group and were deposited during the D3 exten-sional tectonic regime between 605–595 Ma and 560 Ma; and (3)the Adoudounian Formation, overlying the Ouarzazate Group, ismarked by basal fluvial conglomerates dated at between 560and 542 Ma that overlain by early Cambrian, passive marginlimestones.

This work focuses on the inferred glacigenic erosional featuresat the top of the Tiddiline Formation in the Irghem inlier and thedeposits observed in the Ouarzazate Group in the Bou-Azzer inlier(Fig. 1C).

3. Glacial evidence

3.1. Erosional features

Striated surfaces have been observed in two inliers of theAnti-Atlas belt over 200 km apart from each other. These surfacesshow features commonly observed on glacially eroded substrates(Deynoux, 1980; Le Heron et al., 2005) and were first observed in

108 E. Vernhet et al. / Precambrian Research 196– 197 (2012) 106– 112

Fig. 2. Field photographs of the glacially eroded surface in the Tazegzaout section in the Bou-Azzer inlier (A–E) and in the Ingoum-Sud section in the Irghem inlier (F–I). (A)General view of the glazed, undulated, fluted surface (arrows). Undulations are due to alternating berms and furrows. (B) Close-up view of a berm showing parallel striationson its surface. Small faults perpendicular to striation cut the berm. (C) Striated surface of a berm with crescentic structure (in the circle). Black arrow gives the N-NW directionof ice-sheet displacement, (D) view of the striated surface along strike. The surface is cut by small-scale faults perpendicular to the striae. (E) Close-up of the surface cutby small-scale faults showing that the striae are not due to penetrative schistosity or preferential clast orientation during ignimbrite flow. (F) View of the eroded surface(dashed line) at the base of the overlying, non-cleaved (ns: non schistose) Adoudounian Formation and on top of the underlying foliated/schistose Tiddiline Formation. (G)Contact between the deformed (schistose) Tiddiline Formation (lower part of the photograph) and the non-schistose Adoudounian Formation conglomerates (upper part ofthe photograph), (H) crescentic structures on the striated surface; arrow shows the ice-sheet displacement direction N-NE, (I) striated (arrows) quartzite pebble.

tllobsflI(T(fitfl

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he Tazegzaout section in the Bou-Azzer inlier (Fig. 1C). A particu-arly well-developed surface is located between two volcaniclasticayers of the Ouarzazate Group and is characterised by locally devel-ped, brown-stained glazing and large-scale undulations definedy a succession of parallel berms and furrows (Fig. 2A). Small-scaletriations, consistently orientated North-Northwest, overprint theuted surface and are parallel to the berms and furrows (Fig. 2B).

n places the berms are marked by lunate, crescentic structuresas much as 5 cm in relief) that overprint the striations (Fig. 2C).he striated fluted surface can be traced for more than 100 mFig. 2D). It is note worthy that the striations and glazing are sur-cial phenomenon (Fig. 2E) and, therefore, cannot be attributedo a preferential orientation of elements during ignimbriteow.

In the Ingoum-sud section (Irghem inlier), another surface is

bservable on top of the deformed, matrix-supported Tiddilinem conglomerates (Fig. 2F and G). There, the evidence for glacialrosion is marked by rare crescent structures and deep, long stri-tions and grooves (Fig. 2H); the striations have low-angular,

divergent directions. In addition, striated quartzite pebbles arepresent (Fig. 2I), albeit rare. Striated surfaces have been alsoobserved in the Anrouy section (Irghem inlier).

3.2. Mud-supported breccia unit

Mud-supported breccia deposits have been observed in all vis-ited sections in Bou-Azzer inlier and in the Anrouy section inIrghem inlier (Fig. 1C). They are thick bedded and very poorly sorted(Fig. 3A). Matrix between the clasts consists of very fine to fine sandand mud (Fig. 3B). Clasts 1–10 cm in diameter are abundant and out-sized clasts (Fig. 3C) can be as large as 1 m in diameter. The clastsare sub-angular to sub-rounded and consist of both igneous andmetamorphic lithologies, all of which can be linked to local prove-

nances. The deposits have planar bases, tabular geometries and arelaterally continuous for hundreds of metres. No internal organisa-tion has been observed (Fig. 3C). This facies overlies directly thevolcaniclastic and volcanic rocks of the Ouarzazate group.

E. Vernhet et al. / Precambrian Research 196– 197 (2012) 106– 112 109

Fig. 3. Illustration of the mud-supported breccias unit. (A) Contact between the Adoudounian Formation fluvial conglomerates (on top of the dashed line) and a OuarzazateGroup mud-supported breccia (down), Anrouy section, Irghem inlier. Hammer for scale in the circle. (B) Close-up of the mud-supported breccia, Tazzegzaout section, Bou-A large qA t chan

3

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zzer inlier. (C) Contact between the Adoudounian Formation fluvial facies with

nrouy section, Irghem inlier. This important facies change implicates an importan

.3. Stratified breccia unit

This facies is clast-supported, polymictic breccia, shows normalrading, thin bedding and crops out in lateral continuity over sev-ral hundreds of metres (Fig. 4A). Grain size grades from gravels toands (Fig. 4B), but some clasts can reach 20 cm in diameter, rarely0 cm, and exceptionally up to 1 m (Fig. 4C and D). Matrix betweenhe clasts is fine- to medium-grained sand. Breccias grade upward

nto sandstone beds (Fig. 4B) that are typically many cm thick and

ith planar bases. Rare channelised structures (20 cm deep) haveeen observed but no current-related sedimentary structures haveeen identified. The thickness of the stratified breccia unit ranges

uartzite pebbles (top) and the Ouarzazate Group mud-supported breccias (low),ge in depositional conditions.

from 40 to 80 m and overlies the mud-supported breccia unit inall visited sections in the Bou-Azzer inlier. This facies has not beenobserved in the Irghem inlier.

3.4. Interpretation

Striated and fluted surfaces have been commonly describedin Precambrian (Deynoux, 1980), Ordovician (Deynoux, 1980; Le

Heron et al., 2005) and Quaternary glaciogenic deposits (Larsenet al., 2010; Evans et al., 2006) and, are commonly interpreted asresulting from ice-sheet movement on a soft-sediment substratum(Le Heron et al., 2005). Berms correspond to the accumulation of

110 E. Vernhet et al. / Precambrian Research 196– 197 (2012) 106– 112

F is unis Outcr

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ig. 4. Stratified breccias in Bou-Azzer inlier. (A) Outcrop view, Afezza section. Thection. Clasts are angular, cm-sized and organised in normal-graded thin beds. (D)

ediments eroded from the furrows and laterally pushed during thece-sheet displacement. Finer particles in the moving ice sheet pol-sh the surface of the substratum to a glazed patina (Deynoux, 1980;e Heron et al., 2005). Pebbles in the ice sheet erode the surface andorm striae and grooves (Deynoux, 1980). It is this variety of pro-esses that we envisage generated the fluted and striated surfacesn the Bou-Azzer and Irghem inliers.

The mud-supported breccias lack current-related sedimentarytructures and its regional extend makes it unlikely to be simplelluvial fan deposits (as commonly proposed). The abundance ofub-angular clasts and poor sorting combined with the presencef a fine matrix is compatible with, but not conclusive of, an originelated to a sedimentary discharge at the front of or along the lateralargins of a retreating ice sheet or glacier.In contrast, the better sorting, smaller overall clast sizes (typ-

cally cm-sized), the absence of a muddy matrix and the normalrading of the stratified breccias indicate that those sediments

xperienced reworking and deposition by a decreasing energyurrent. The stratified breccias are organised into a successionf thin, normal-graded beds that indicate a repetitive sedimen-ary process of deposition. The clasts in both the stratified and

t consists of thin beds with rare out-sized clasts. (B and C) Close-up, Zagora roadop view, Tazegzaout section; out-sized clasts are abundant.

mud-supported breccias are indicative of derivation from the sameprovenance areas. Thus, we interpret the stratified breccias aspro-glacial deposits related to short term, high-frequency melt-ing episodes. Their lateral continuity and relatively non-erosivebases indicates that the depositional surface was relatively uni-form. In summary, we highlight the striated and fluted surfaces,the presence of rare striated clasts, and the lateral extent and con-tinuity of matrix-supported breccias as the main lines of evidencefor inferring a glacial origin.

The evolution of the depositional environment is proposed inFig. 5. Sedimentary features observed on the striated surfaces ofthe Tazegzaout section (Bou-Azzer inlier) suggest that ice sheet dis-placement occurred on soft sediments (e.g. Le Heron et al., 2005). Inthe Ingoum-sud section (Irghem inlier), sedimentary structures aremore poorly developed because the ice sheet moved on a hard sub-stratum (the deformed Tiddiline Formation conglomerates). Theoverall facies succession indicates a deglaciation sequence: (1) the

fluted surface records the in situ ice sheet advance, (2) the mud-supported breccias are glaciogenic deposits that may correspondto the marginal ice-sheet sediments deposited during ice-sheetretreat and (3) the stratified breccias are proglacial outwash. Fig. 5

E. Vernhet et al. / Precambrian Research 196– 197 (2012) 106– 112 111

Fig. 5. Paleoenvironmental reconstruction and time evolution of the Ediacaran Anti-Atlas Belt (Morocco). (A) D2 compressive event deformed the Tiddiline Formation.(B) Rifting and volcanism of the D3 extensional event. The glacially eroded surfaces formed on ignimbrites in the Tazegzaout section (Bou-Azzer inlier) and on deformedTiddiline Formation conglomerates in the Ingoum-sud (Irghem inlier). In the Anrouy section (Irghem inlier), only the mud-supported breccias provide evidence for theBou-Azzer glaciation. (C) The retreated ice sheet deposited moraines (mud-supported breccias), then glacio-fluvial deposits (stratified breccias). (D) Post glaciation ands an riseT

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tart of Adoudounian sedimentation marked by fluvial deposition. (E) Early Cambriazegzaout section (1), the Ingoum-Sud section (2) and the Anrouy section (3).

s our inferred palaeogeographic and palaeoenvironmental recon-truction.

. Discussion

.1. About the age of the glacially derived surfaces

The Gaskiers Formation at the eponymous type locality in New-oundland, Canada, has been considered to be late Precambrian inge (Myrow, 1995; McCall, 2006). Precise U–Pb zircon dating ofnterbedded multiple volcanic ash layers has shown it to be 580 Mand to have lasted less than 1.0 million years (Bowring et al., 2003).owever, age constraints for assumed Gaskiers-equivalent stratalsewhere are less well defined: the Hankalchough diamictite inW China is between 615 ± 6 and 542 Ma (Xu et al., 2009); thewna Group, Anglesey Island, Wales, is between 595 and 550 Ma

Kawai et al., 2008); the Squantum Member In Massachusetts, USA,s between 595 and 570 Ma (Thompson and Bowring, 2000); the Lasantanas Formation, Uruguay, is between 615 and 579 Ma (Gauchert al., 2008); the Cottons Breccia, Australia, is variably consideredo be ca. 600 Ma (Calver, 2000) or between 586 and 572 Ma (Direennd Jago, 2008); and the diamictites of the Ribeira Belt, SE Brazil,

re thought to be younger than 580 Ma (Campanha et al., 2008).hus, only the Gaskiers glaciation at the type locality is tightlyonstrained at 580 Ma, whereas other occurrences of presumablyquivalent strata have ages that vary by as much as 10 Ma or more.

of sea level and deposition of shallow-marine limestones. Inferred position of the

Following the stratigraphical framework proposed by Maloofet al. (2005) for the Bou-Azzer inlier, the inferred glaciallyscoured surfaces in the Ouarzazate Group would be between577 and 560 Ma in age (Fig. 1C). This is compatible with ourproposed regional stratigraphic framework in which the striatedsurface postdates the 605–595 Ma D2 event but predates the560 Ma Adoudounian basal conglomerates. In the Irghem inlier, theabsence of the Ouarzazate Group introduces a degree of uncertaintyto correlations, but the striated surface also falls between the agerange of 605–595 Ma and 560 Ma. Thus, Bou-Azzer glaciation maybe time-equivalent to Gaskiers glaciation.

However, a recent age obtained by Youbi (2011, pers. comm.)in the Bou-Azzer inlier further restricts the age of the OuarzazateGroup to between 565 and 542 Ma. If this age is correct, then theBou-Azzer glaciation is a late Ediacaran glaciation, distinct from theGaskiers glaciation. Hebert et al. (2010) show that several Ediacaranglaciations postdating the Gaskiers Glaciations have been identi-fied worldwide. This late Ediacaran glacial event (younger than560 Ma) has been suspected but not clearly demonstrated in severalplaces in the West Africa Craton (Deynoux et al., 2006): the Hog-gar “Serie pourprée” (Caby and Fabre, 1981) and Fersiga Formationof the Northeastern part of the Taoudeni basin (Bertrand-Sarfatiet al., 1995). According to the paleogeographical reconstruction of Li

et al. (2008), by ca. 560 Ma, the West African Craton may have beenaround 70◦S, thus glacial conditions at these latitudes would beplausible. Therefore, these erosional surfaces may result from localglaciers or small ice caps with a limited lateral extent, as implied by

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he locally derived character of the clasts in the inferred glacigenicocks in the Bou-Azzer and Irghem inliers.

. Conclusion

This work documents evidence for an Ediacaran glaciation inwo inliers of the Anti-Atlas Range (Morocco) on the West Africanraton. Glacially striated surfaces and associated sedimentary suc-essions record a deglaciation sequence that occurred between05–595 Ma and 560 Ma The data presented in this work supporthe suggestion of a widespread Ediacaran glaciation that is youngerhan the 580 Ma Gaskiers glaciation.

Future work on the abundant volcanic rocks in the Ouarzazateroup may provide absolute ages to better constrain the tim-

ng of Bou-Azzer glaciation. Such dating could help distinguishhe diachronism (or synchronism) of glacially eroded surfaces and

ay help determine if the Bou-Azzer glaciation was a regionallyxtended ice cap, or several local glaciers. Additionally, better sed-mentary characterisation of the numerous coarse detrital depositsf the Moroccan Neoproterozoic successions could constrain theepositional conditions of diamictites and enhance understandingf the transition periods between non-glacial and glacial condi-ions.

cknowledgment

The authors thank Evans Edinger for English improving.

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