Cairo Universityscholar.cu.edu.eg/?q=ezzeldinkhalaf/files/10.1007... · Late Neoproterozoic sedimentary basins formed in the NED of the Egyptian Eastern Desert (Fig. 1). Most of these

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Bulletin of VolcanologyOfficial Journal of the InternationalAssociation of Volcanology andChemistry of the Earth`s Interior(IAVCEI) ISSN 0258-8900Volume 75Number 2 Bull Volcanol (2013) 75:1-31DOI 10.1007/s00445-013-0693-6

Syn-eruptive/inter-eruptive relationships inLate Neoproterozoic volcano-sedimentarydeposits of the Hamid area, North EasternDesert, Egypt

Ezz El Din Abdel Hakim Khalaf

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RESEARCH ARTICLE

Syn-eruptive/inter-eruptive relationships in LateNeoproterozoic volcano-sedimentary deposits of the Hamidarea, North Eastern Desert, Egypt

Ezz El Din Abdel Hakim Khalaf

Received: 2 June 2012 /Accepted: 21 January 2013 /Published online: 7 February 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Nonmarine volcano-sedimentary successions inthe Late Neoproterozoic Hamid Basin were studied in orderto examine the distinctive characteristics of accumulationduring syn-eruptive and inter-eruptive periods in a depo-center associated with active volcanism and both extension-al and dextral strike-slip tectonics. In particular, the syn-riftfill in this area comprises a wide range of compositions andtransport and depositional processes in which lava flowscoexist with pyroclastic and epiclastic deposits in the sameaccumulation space. Seven different accumulation unitswere identified in the syn-rift fill: (1) polymictic alluvialfan units, (2) fluvial braid plain units, (3) lacustrine units, (4)coherent volcanic bodies/shallow intrusion units, (5) pyro-clastic fall units, (6) phreatomagmatic volcanic units, and(7) pyroclastic density current units. These deposits areorganized into several stratal packages with contrastinggeometries. Analysis of these units and the relationshipsbetween them provided insights into the evolution of thesyn-rift sedimentary environments and permitted identifica-tion of different stages of effusive activity, explosive activ-ity, and relative quiescence, determining syn-eruptive andinter-eruptive rock units. These units provide importantclues to the distribution of, and temporal changes in, accom-modation space, and hence the configuration and structuralevolution of the Hamid Basin. Two accumulation stageswere defined. The underfilled stage occurs when the mate-rial supplied to the depocenter during the eruptive events isnot enough to level the existing topography, allowing the

development of high-gradient alluvial systems during thenext inter-eruptive period. The overfilled stage occurs whenextensive pyroclastic density current deposits choke theaccumulation space during syn-eruptive periods, causinglow-gradient sedimentary systems to develop during thesubsequent inter-eruptive periods. The Hamid Basin is thusinterpreted to have been hybrid in nature, influenced by thedynamic changes of the basin–margin faults, which wereeither normal or strike-slip.

Keywords Volcanic rifts . Syn-eruptive . Inter-eruptive .

Syn-rift . Hamid Basin

Introduction

The stratigraphic record of volcanic regions comprises bothprimary eruptive products generated by volcanism and thesyn- and post-eruptive volcaniclastic deposits that resultfrom the immediate or subsequent reworking of this materialby surface processes, including weathering and erosion(Manville et al. 2009; Kwon et al. 2011). Smith (1991)sub-divided volcaniclastic sequences into: (1) “syn-erup-tive” units, formed coevally with volcanic activity throughthe immediate reworking of pyroclastic material, andextending through the period where the landscape is stillresponding to the hydrological and sedimentary-yield con-sequences of the eruption; and (2) inter-eruptive sequences,where normal “background” sedimentary processes occurwithout a direct volcanic influence. “Syn-eruptive periods”thus produce rapid aggradation in proximal and medialsettings and comprise the period of landscape response tothe volcanic perturbation (Smith 1991; Orton 1996;Manville et al. 2009; Procter et al. 2009).

Editorial responsibility: V. Manville

E. E. D. A. H. Khalaf (*)Geology Department, Faculty of Science, Cairo University,Giza, Egypte-mail: [email protected]

Bull Volcanol (2013) 75:693DOI 10.1007/s00445-013-0693-6

Author's personal copy

The characteristic physiography inherent to a rift land-scape promotes development of a wide diversity of accumu-lation systems (Gawthorpe and Leeder 2000). Structure isdominated by normal faulting, allowing the existence ofrelatively small, highly compartmentalized depocenters,with abrupt changes in slope steepness during most of theirhistory (Morley et al. 1999). This situation produces amultiplicity of coexisting processes in the same accumula-tion space, generating numerous variations in the sedimen-tary systems inside the depocenters (Young et al. 2003;Jackson et al. 2005). The profuse volcanic activity common-ly associated with the development of extensional environ-ments (e.g., Ziegler and Cloething 2004; Aguirre-Díaz et al.2008) leaves a strong imprint on the sedimentary systems.Such interactions can be observed in the rock record of thesyn-rift megasequence with a variable pattern recording thealternation of periods of active or inactive volcanism. Theexistence of such different periods has been addressed as amajor control over the sedimentary sequences that composeother basin types (e.g., Smith 1987, 1991). Therefore, identi-fying the facies variations in the volcanic edifices can help toanalyze the stratigraphy of this kind of extensionalenvironment.

The Hamid area (600 km2) is located between 26°58′ and27°10′ N latitude and 32°50′ and 33°02′ E longitude(Fig. 1). The Wadi Hamid area, known for its Pb-miningactivity, lies west of Gabal Dokhan and is occupied by theNeoproterozoic Dokhan-Hammamat volcano-sedimentarysuccessions (Fig. 1). These successions are unconformablyoverlain from the west by Phanerozoic sandstone of theNubia facies. Publications on the North Eastern Desert(NED), including the study area, are limited comparedwith published works on the central and southern partsof the Eastern Desert (e.g., Dardir and Abu Zied 1972;Ghanem et al. 1973; Khalaf 1999; Mohamed et al.2000). Previous works focused on larger scale aspectsinvolving geochemistry, geotectonic setting, and limitedradiogenic dating of the volcano-sedimentary successionsin Egypt (Willis et al. 1988; Khalaf 1995; Wilde andYoussef 2002; Breitkreuz et al. 2010). Comparativelylittle is known about the internal lithofacies subdivisionsof the eruptive sequence; volcanological and sedimento-logical facies analysis and the implications for the erup-tion style, transport, and depositional processes. Newfield observations and a stratigraphy for the internalsubdivision of the Neoproterozoic volcano-sedimentarysuccessions in the Hamid Basin, based on regional map-ping, extensive detailed stratigraphic logging, and petro-graphic analysis are presented here. The purpose of thiscontribution is to identify and understand the interrela-tionship between sedimentary and volcanic processes,focusing on the syn- or inter-eruptive character of thedifferent units (Smith et al. 1999).

Geological setting

Based on field relations and radiogenic ages, Stern andHedge (1985) subdivided the Egyptian Eastern Desert intonorthern (NED), central (CED) and southern (SED) parts(Fig. 1). The oldest rocks are concentrated in the southernpart, while the youngest units comprising the DokhanVolcanics, the Hammamat sediments, young granites, anddyke swarms are common in the northern part. The NED–CED boundary is generally represented by a N60° E trend-ing thrust (dipping NW) or dextral strike-slip fault (Sternand Hedge 1985; Stern and Gottfried 1986; El-Gaby et al.1990; Greiling et al. 1994) (Fig. 1). A number of small-scaleLate Neoproterozoic sedimentary basins formed in the NEDof the Egyptian Eastern Desert (Fig. 1). Most of thesebasins, including the Hamid Basin, underwent rifting aftercollision between East and West Gondwana and were laterfilled by kilometer-thick successions of non-marine silici-clastic fluvio-lacustrine sediments (Hammamat Group) to-gether with abundant andesitic and rhyolitic DokhanVolcanic deposits. This collision ended at 615–600 Ma andextensional collapse occurred within the 600–575-Ma timespan, followed by transpressional tectonism along majorshear zones until 530 Ma (Greiling et al. 1994).

The Dokhan Volcanics and Hammamat Group

The Dokhan Volcanics and the Hammamat Group have beenrecognized in the Eastern Desert for many years (El Ramly1972; Stern and Gottfried 1986; Khalaf 1995, 1999; AbdelRahman 1996; El Gaby et al. 2002; Mohamed et al. 2000; ElSayed et al. 2004). The Dokhan Volcanics typically includebasaltic andesite, andesite, dacite, and rhyolite that someconsider to be a bimodal suite (Stern and Gottfried 1986;Khalaf 1995; Mohamed et al. 2000) although this conclu-sion has been challenged (Eliwa et al. 2006). They consti-tute an almost unmetamorphosed succession that varies inthickness from basin to basin, ranging from a few tens ofmeters to 1,300 m (El Ramly 1972; Akaad and Noweir1980). However, their undeformed character, temporal, andspatial association with post tectonic A-type granite, andhigh Zr/Y ratio suggest that their emplacement followedthe cessation of subduction in the Eastern Desert in anextensional, within-plate setting (Khalaf 1995; Johnson etal. 2011). Recent SHRIMP zircon dating gave weighted U–Pb ages of 593±13 and 602±9 Ma for two andesites fromthe Gabal Dokhan volcanics (Wilde and Yossef 2000).

The Hammamat Group typically comprises greenish-graysiltstone, lithic sandstone, and polymict conglomerate con-taining pebble-sized clasts of quartz, foliated granite, purpleDokhan-type andesite, felsic volcanic rock, basalt, quartzporphyry, and undeformed pink granites. It ranges from c.4,000 m (Abd El-Wahed 2009) to c. 7,500 m thick (Fritz and

693, Page 2 of 31 Bull Volcanol (2013) 75:693


Fig. 1 Simplified geologic map showing the distribution of theDokhan volcanics and Hammamat Group in the North Eastern Desert,Egypt (Data were collected from several resources including AbdelRahman 1996; Hassan and Hashad 1990; Grothous et al. 1979; Wilde

and Youssef 2002). The approximate boundary between North EasternDesert (NED),Central Eastern Desert (CED), and South Eastern Desert(SED) according to Greiling et al. (1994)

Bull Volcanol (2013) 75:693 Page 3 of 31, 693


Messner 1999). Relative proportions of the DokhanVolcanics and the Hammamat Group vary from basin tobasin. Some contain only volcanic or sedimentary rocks;other basins contain both. Because of their varied distribu-tion and different relationships in basins where both typesoccur, the stratigraphy of the Dokhan Volcanics and theHammamat Group is debated. Some workers consider thatthe Hammamat Group underlies the Dokhan Volcanics (e.g.,Stern and Hedge 1985; Willis et al. 1988); others believethat the Hammamat overlies the Dokhan (e.g., El Ramly1972; Akaad and Noweir 1980; Ries et al. 1983; Hassan andHashad 1990); yet others infer that the two interfinger andare essentially contemporaneous (Ressetar and Monrad1983; Stern et al. 1984; Khalaf 2004; Eliwa et al. 2010).The later conclusion is consistent with the overlap of wholerock Rb–Sr ages; 610–560 Ma for the Dokhan Volcanics,600–585 Ma for the Hammamat Group, and 600–550 Mafor the Younger granites (Willis et al. 1988; Beyth et al.1994; Jarrar et al. 2003).

The Hammamat Group is texturally immature, this hasbeen interpreted as reflecting rapid uplift, erosion, transport,and deposition in alluvial fans and braided streams within aseries of more or less isolated inter-montane basins(Grothaus et al. 1979; Ries et al. 1983; Khalaf 2004).Wilde and Youssef (2002) favor deposition in a major flu-vial system of continental proportions that linked the vari-ous basins, and was possibly linked to similar successions inSinai and Jordan. Other workers infer that the group wasdeposited in isolated, foreland, strike-slip pull-apart, andfault-bounded intermontane basins (Grothaus et al. 1979;Fritz et al. 1996; Abdeen and Greiling 2005; Shalaby et al.2006). The range of inferred structural controls includesthrusting, normal faulting, strike-slip faulting, N–S toNW–SE extension, and magmatic doming. Both theHammamat and the Dokhan units were affected by rapidhinterland uplift at about 595–588 Ma (Fritz et al. 1996;Loizenbauer et al. 2001) and subsequently intruded by the585-Ma younger granite (Andresen et al. 2009). Several ofthese molasse basins occur adjacent to the metamorphic corecomplexes in the CED and their formation is generallyattributed to the exhumation of these core complexes duringthe final stage of Pan-African evolution (650–550 Ma)(Greiling et al. 1994; Shalaby et al. 2006).

Granitic rocks

Granitic rocks predominate in the Eastern Desert and Sinaiand belong to two main stages in the geotectonic develop-ment of the Egyptian Shield. The older stage (900–650 Ma)comprises calc-alkaline syn-tectonic diorite, tonalite,trondhjemite, and granodiorite intrusions. These rocks oc-cupy the southeastern and the eastern part of the study areathat are traversed by numerous amounts of dyke swarms

(Fig. 2). They underlie the Hammamat conglomeratesbeneath an erosional unconformity surface. The youngerstage (590–520 Ma), comprises late- to post-tectonicgranodiorite, granite, and alkali granite (Jackson et al.1984; Stern and Hedge 1985; Stern and Gottfried 1986).These granites intrude Older granites, Dokhan Volcanics,and Hammamat Group in the form of dykes, off shoots,and veins.

The Hamid volcano-sedimentary successions (HVSS)

The Hamid Basin is situated in the NED (Fig. 1),where acomplete NE–SW-trending Dokhan Volcanics-HammamatGroup succession is preserved directly overlying the base-ment, which comprises gneissose granites and low-grademetasediments and metavolcanics (Abdel Rahman 1996;Mohamed and El-Sayed 2008). These volcano-sedimentarydeposits are mainly composed of lava flows, pyroclastics,and fluvial siliciclastic sedimentary rocks and are intrudedby younger granites and subvolcanic intrusions of sill- anddyke-type. These volcano-sedimentary successions are con-sidered part of the Egyptian Pan-African belts in the NED.U–Pb zircon dating of the Dokhan Volcanics has yielded anage of 616±4 and 615±5.4 Ma (Ediacaran age) (Breitkreuzet al. 2010) and a 585 Ma age has been reported for thedeposition of the Hammamat Group (Wilde and Youssef2000).

The Hamid Basin is c. 15 km in length and c. 9 km inmaximum width (Fig. 2) that records the recurrence ofregional extensional and strike-slip tectonics after the oro-genic events that formed the Pan-African Belts (Stern 1994).It is a pull-apart basin bounded by NE- or ENE-trendingdextral strike slip faults along the northeastern and south-western margins and by NW-trending normal faults alongthe northwestern and southeastern margins (Fig. 2). TheNE–SW trend is one of the predominant trends crossingthe Precambrian terrains of the NED. One of the major shearzones belonging to this trend is the Qena–Safaga shear zonethat separates the NED from the CED (Fig. 1) and occupiesa major structural discontinuity along which the basementrocks are different. This shear zone is characterized by anabsence of ophiolitic ultramafics, mélange, and Banded IronFormation-bearing metavolcanics (El-Gaby et al. 1990;Greiling et al. 1994).

The Hamid Basin is divided into southwestern and north-western subbasins, by intrabasinal strike-slip faults thattrend NE (Fig. 2). Some of these faults are responsible forintense deformation, marked by the steepening to nearly 90°and a high density of minor faults in broad fault zones, butthese structures rarely juxtapose different units of the HVSS.The Hammamat Group, the lowermost stratigraphic unit inthe basin, consists of polymictic conglomerate and fluvio-lacustrine deposits that show marked variations in stratal



patterns along the basin-bounding faults. The polymicticconglomerate therefore provides an opportunity to

investigate the initial basin-forming processes and tectonicsthat cause the variations in stratal patterns and architecture.

Fig. 2 Geological map of the Hamid area, North Eastern Desert, Egypt (modified after Khamis 1995)



The Hammamat Group that interfingers with mafic vol-canics and pyroclastics of the Dokhan Volcanics crop outmostly along the southwestern margins of the southwesternblocks in the northeastern subbasin (Fig. 2). These sedi-ments strike 50° to 55° and dip at an average of 40° to theNW (Fig. 2). In the southwestern subbasin, transverse foldsare recognized at different stratigraphic levels near the NE-trending border faults (Fig. 2). Their axes generally trend 50to 140, are roughly perpendicular to the northeastern borderfaults and plunged 40–60° toward the NW. In the north-western subbasin, the felsic Dokhan Volcanics and inter-bedded felsic pyroclastics with distinctive layeringconstitute the uppermost stratigraphic unit of the HamidBasin. At the Wadi Hamid Pb mine location (Fig. 2),these volcanics dip at 20° to 25° towards the east,marking an angular unconformity between the strata inthe northeastern and northwestern subbasins (Fig. 3). Inthe northwestern part of the mapped area, the NW–SEfaults form normal faults constituting a graben system

enclosing the Nubia Sandstone on its downthrown side(Khamis,1995). The presence of normal faults boundingthe north sub-basin to the north (Fig. 4) suggests anoriginal half-graben geometry for the basin, confirmedby the northerly onlap of the younger units. Beddingmeasurements and lithological similarities suggest thatthe rock sequence in the northwestern subbasin wasdeformed and folded into asymmetrical syncline foldwith an axis trending about 55°. The northern limb ofthe syncline dips southeast at 33°–35° and forms a thickstratified sequence of alternating felsic Dokhan volcanicsand volcanogenic sediments (Fig. 3). However, the basin-bounding faults are not folded, indicating that these intra-basinal folds were produced by syndepositional fault ac-tivity rather than post-depositional compression. Despitethe intrabasinal faults, the strata of the Hamid Basin canbe correlated by the conglomerate and fluvio-lacustrinedeposits at the base, the lava flow and pyroclastics inthe middle, and the felsic volcanics at the top.

Fig. 3 Synthetic S-N geological cross section (a–b) showing the distribution of the various facies in the Neoproterozoic Hamid volcano-sedimentary successions, see Fig. 2 for location and Table 1 and Fig.6 for detailed description



Methodology

Study of the HVSS was carried out through detailed geo-logical mapping and measuring of stratigraphic and sedi-mentary sections. For the purposes of this contribution, theuse of the term “volcaniclastics” is restricted to the compo-sition of the fragments that make up a certain deposit, inkeeping with its original definition (Fisher 1961). A numberof lithofacies and lithofacies associations were determined(Table 1). The facies associations include alluvial fan, flu-vial, lacustrine, pyroclastic density current, and volcaniclas-tic mass flow deposits. Compositional characteristics ofvolcanic and pyroclastic rocks were studied through theanalysis of thin sections. Petrographic analyses of the di-verse clastic rocks helped constrain the nature of their prov-enance and their relationship with the volcanic facies.Discrete depositional units, which will be referred to as“accumulation units,” were defined based on the identifica-tion of distinct bounding surfaces, in conjunction with thelithofacies associations that compose them.

Depositional systems in volcanic environments tend toreflect variations in the frequency and intensity of volcanicactivity (Runkel 1990; Waresback and Turbeville 1990). Thegreat volume of material released during a volcanic eruptionhas a clear effect on the sedimentary systems (Manville et al.2009). To distinguish inter-eruptive from syn-eruptive sedi-mentary units, analysis of the volcaniclastic deposits focused

on three concepts: (a) composition (Haughton 1993; Riggs etal. 1997), (b) mechanisms of transport and deposition (Waltonand Palmer 1988; Bahk and Chough 1996), and (c) aggrada-tional versus degradational behavior of the accumulation units(Smith 1987; Bahk and Chough 1996; Riggs et al. 1997). It isimportant to highlight that these three concepts were original-ly developed and applied to volcanic successions with a wideareal distribution, situated in non-compartmentalised basins(i.e., a foreland basin; Smith 1987, 1991). In contrast, in theHamid area the volcanic activity took place entirely in narrowextensional depocenters. Therefore, the previous models can-not be applied unless they are adapted to the specific geolog-ical setting of the study area.

Accumulation units of the HVVS

The studied volcano-sedimentary successions exhibit a largelateral and vertical variation in lithology and thickness,younging along a south-north trend (Figs. 2 and 3). Thesesuccessions show variations in lithology and degree of de-formation. They begin in the south with the older granitesand lava flows intercalated with volcanogenic sedimentsand end in the north with the pyroclastic felsic rocks(Fig. 3). Chlorite, epidote, sericite, and quartz mineralassemblages show that the volcanics have undergonegreenschist facies metamorphism. Based on grain size,

Fig. 4 a. Panorama view of pyroclastic density current deposit (FA7)near the Pb mine showing stretched ignimbritic rocks (Inw/Iw) withramp structure, overlying bedded conglomerates (Cb), and underlyingby laminated vitric tuffs (Tv). b. Field sketch of the facies architecture

in Fig. 4A. Note the younger granite (YG) and subvolcanic rhyolites aswell as normal faults cross-cutting these deposits. Older granites (OG)lie at the base of these deposits to the left of the panorama



Tab

le1

Descriptio

nandinterpretatio

nof

thefacies

intheNeoproterozoicHVSsuccession

,Ham

idarea,North

Eastern

Desert,Egy

pt

Faciesandfacies

code

Characteristics

Interpretatio

nFaciesassociation

Disorganized

polymictic

cong

lomerate

(Cm)

Disorganized,

massive,ill-sorted,

clast-supp

ortedpebb

le–bo

ulder

cong

lomeratewith

poorly

sorted,subrou

nded

tosubang

ular,clastsize

5–50

-cm.po

lymictic

(granites,vo

lcanics);granule-rich

ormud

dysand

matrix;

laterally

discon

tinuo

usandrare

amalgamated

with

shallow

scou

rs;

rare

parallelalignm

entof

elon

gateclasts;rare

norm

alor

inversegradingat

base

andtopof

beds.

Debrisflow

,hy

percon

centratedflow

,or

high

-magnitude

floo

dflow

sAllu

vial

fan

Massive

Sandstone

(Sm)

Faintly

laminated,fining

upward,

crossbedd

edstructurehasbeen

observed

insomeplaces.Mod

eratelyto

poorly

sorted,qu

artz-feldspar-rich

arenites,

occasion

ally

volcanic

clasts-richwith

size

upto

6cm

.

Sandy

braidedrivers

Fluvial

braidplain

Gravely

sand

ston

e(Sg)

Poo

rlysorted,faintly

laminated

orno

structure,arko

seto

lithicarenite,

occasion

ally

volcanic

clasts-richin

somevarieties.

Fluvial

channelfill

Fluvial

braidplain

Lam

inated

mud

ston

e(M

l)Parallellamination,

with

nodesiccationcracks,silt-mud

intercalation

Deeplacustrine

depo

sits

Lacustrine

Lavaflow

(LF)

Grayish-purple,massive,aphy

ric,with

rubb

ly,vesicularbasesandtops.

Colum

narjointsarecommon

.These

individu

alsheetscontainplagioclase-

hornblende

setin

pilotaxitic

tofeltedmicolites,abun

dant

opaque

and

epidote-rich

matrix.

Sub

aerial

lava

flow

sCoh

erentvo

lcanic

bodies

Beddedcoarse

tuffs(Tcb)

Mod

eratelyto

poorly

sorted,mantle

bedd

ing,

norm

alandinversegrading,

vitric

tuffs,crystalsandlithics-rich,

occasion

ally

interbeddedwith

volcaniclastic

cong

lomerate(Cv)

andhy

aloclastic

rocks(H

y)

Pyroclastic

fallforTcb

and

phreatom

agmatic

depo

sitsforCvand

Hy

Pyroclastic

fallandph

reatom

agmatic

depo

sits

Lith

ophy

sae-rich

ignimbrite

(Il)

Mod

eratelywelded,

indu

ratedandsilicifiedlitho

physa-rich.These

nodu

les

arequ

artz-filled,circulartostar-shapedinternalcavitiesexhibitin

gaxiolitic,

spherulitic,andpectinatetexture.

Weldedpy

roclastic

flow

depo

sits

(uncon

finedplainfacies)

Pyroclastic

density

currentdepo

sits;spherulites

with

largecentralcavities,in-filled

with

quartz

during

diagenesis(cf.Holzhey

1999

,20

01)

Massive

breccias

(Bm)

Poo

rlysorted,clast-supp

orted,

nobedd

ingandgraded

bedd

ing,

lithics-rich

(granitesandvo

lcanics)

Co-ignimbriticbreccias

lagdepo

sits

Pyroclastic

density

currentdepo

sits

Lam

inated

vitric

tuffs(Tv)

Pervasive

planar

tolow-ang

letrun

catin

glaminationwith

sigm

oidalripp

les/

dunes.Quartz-feldspar-richwith

vitricclastsandoccasion

ally

accretionary

lapilli.Soft-sedimentdeform

ationhasbeen

observed

insomeplaces.

Phreatomagmatic

fallof

surgedepo

sits

Base-surgeof

pyroclastic

density

currentdepo

sits;

subaerialor

very

shallow

water

erup

tionand

depo

sitio

n

Bedded

polymictic

cong

lomerate

(Cb)

Indistinctly

bedd

edor

low-ang

lecross-stratifiedwith

coarse

tovery

coarse

sand

matrix,

which

iseither

mud

dyor

not;shallow

todeep

scou

rsat

base;

weakim

brication;

indistinct

norm

alandinversegradingin

theup

perand

lower

parts,clast-supp

orted,

ormatrix-supp

ortedpebb

le-cob

ble

cong

lomerate,angu

larto

sub-roun

ded,

clastsize

10-30cm

,po

lymictic

(granites,vo

lcanics,pelites).

High-concentrationfloo

dflow

onalluvialfans;o

raccretionof

coalescing

bars

with

inbraidedchannels

Allu

vial

fan

Non

weldedto

welded

ignimbrite

(Inw

/Iw)

Fiamme-rich,incipiently

todenselywelded,

parataxitic

oreutaxitic

texture,

porphy

ritic,lithic-rich

atbase

topu

miceandcrystals-richat

top.

No

litho

physae

orno

duleshasbeen

observed.

Non

-weldedto

weldedpy

roclastic

flow

depo

sits(uncon

finedplainfacies)

Hot

pyroclastic

density

currentdepo

sits

Sub

volcanic

dykes/sills

(Sd/

SS)

Con

cordantto

discordant

tabu

larrhyo

litic

bodies,po

rphy

ritic,qu

artz-K

feldspar-rich

Intrusivecoherent

subv

olcanicbo

dies

Coh

erenthy

paby

ssal

volcanic

bodies



Fig. 5 Generalized stratigraphic column of the Hamid area



sedimentary structures, and bedding features, combinedwith thin section studies, 14 sedimentary facies and sevenkinds of accumulation units or facies associations (FA) weredefined in the HVSS (Table 1, Figs. 5 and 6) representingpolymictic alluvial fan units (FA1), fluvial braid plain units(FA2), lacustrine units (FA3), coherent volcanic bodies/shal-low intrusion units (FA4), pyroclastic fall units (FA5),phreatomagmatic volcanic units (FA6), and pyroclastic den-sity current units (PDC, FA7). The first six facies associa-tions form the lower part of the HVSS succession wherethere is intermixing of volcanic rocks with sediments,whereas the next seven facies form the upper part of theHVSS succession (Fig. 5). These successions have a totalthickness of 600 m (Figs. 2 and 6).

FA1: polymictic alluvial fan units

Description These units are formed by epiclastic depositswhich fill incised depressions of up to 30 m deep and 300 mwide intercalated either with pyroclastic density current

Fig. 6 Five profiles showing the various stratigraphic successions in Hamid area from the south to the north. Note that the numbers refer to thedifferent evolutionary phases characterizing the Hamid Basin

Fig. 7 Deposit features of facies associations (FA1 to 3). a Alluvial(FA1), fluvial (FA2), and lacustrine units (FA3) affected by a syn-sedimentary faults and overlain by deposits of pyroclastic densitycurrent (PDC). b Disorganized pebble-boulder conglomerates (Cm)and intervening massive sandstone lens (Sm) of FA1 overlying theolder granites with an erosional unconformity. Note sharp and con-cave-up base and diffuse and convex-up top of the conglomerates. cCrudely stratified pebble- to boulder conglomerates (Cb) and interven-ing massive sandstone layers (Sm). Note the roundness of the clasts(arrows). d General view of bedded conglomerates (Cb). Note thegradational contact between the bedded conglomerates and the overly-ing eroded, red-oxidized massive ignimbritic rocks (Inw/Iw) near thePb mine. e Outsized jointed and rounded volcanic clast in beddedconglomerates (Cb). f Medium-to thick-bedded massive sandstones(Sm) are generally laterally persistent over a few tens of meters. Thelower contact is mostly sharp with the underlying massive conglom-erates (Cm). g Plane polarized light photomicrograph showing poorlysorted massive sandstone (Sm) composed of quartz (Qz), feldspar (fsp),and volcanic fragments (VF). h Cross polarized light photomicrographdisplaying coarse-grained gravely sandstone containing subangularfeldspar (fsp),quartz (Qz), rock fragments (RF), and chert clasts (Ch)set in recrystallized groundmass of quartz and feldspar

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units or the fluvial braid plain units (Figs. 3 and 7a). FA1crops out mostly along the southwestern margins of thesouthwestern subbasin (Fig. 3). The basal part of FA1 un-conformably overlies on the older granitoids (Fig. 7b). Thecomposition of the deposits is typically polymictic, with a

predominance of volcanic, plutonic, and metamorphic lith-oclasts. They also contain quartz and feldspar crystal frag-ments and sedimentary lithoclasts in a smaller proportion.FA1 is mostly composed of disorganized (Cm) to crudelystratified (Cb) conglomerates (Table 1).



The disorganized conglomerates (Cm) occur as lensoidalbodies and show convex bases and planar tops, whoseinternal lithofacies are poorly sorted and clast-supportedwith a polymodal matrix. Their upper surfaces are generallyirregular due to protruding clasts and are always gradationalwith the overlying sandstone-mudstone beds (Fig. 6, profileI). Sandstone beds (Sm), intercalated within the conglom-erates, are less than 0.5 m thick and have wedge or lenticulargeometries. They drape and fill the irregular space betweenthe protruding clasts of the underlying conglomerates(Fig. 7b). The crudely stratified conglomerates (Cb) formlobate bodies, composed of poorly sorted, matrix-supportedfine to medium conglomerates that show planar bases andconcave tops (Fig. 7c). They overlie vitric tuffs (Tv) andoccasionally appear as an intercalation in the ignimbritesuccession in profile V (Fig. 6) near the Pb mine locality.Their upper contact with the ignimbrite beds is alwaysdiffuse or gradational (Fig. 7d). Some domains of Cb dis-play rounded outsized clasts up to 1 m in diameter (Fig. 7e).

Interpretation The disorganized conglomerates (Cm) sug-gest deposition from debris flows, whereas the crudelystratified conglomerates (Cb) and laterally discontinuousbeds suggest turbulent flows or hyperconcentrated floodflows (Nemec and Steel 1984; Sohn et al. 1999; Orton2002) of a channel-filling nature, indicating origin in abraided river system (Todd 1989; Brierley et al. 1993).Poor development of internal erosion surfaces suggests thatthe channels were rapidly filled with sediment during floods(Karcz 1972). The similarity in clast composition and sizebetween these conglomerates (Cm & Cb) suggests eitherthat the source region of the coarse debris remained un-changed, or that there was active intrabasinal erosion andresedimentation. The sandstone interbeds were probablydeposited by sand-rich flood flows that accompanied thedebris flows or by hyperconcentrated flood flows duringthe waning stage of a mass-flow event (Nemec and Steel1984; Pierson and Scott 1985; Smith 1986). The overallcharacteristics of the deposits suggest a mass-flow-dominated alluvial fan environment along the fault-controlled basin margins (Blair 1999; Benvenuti 2003; Sohnand Son 2004; Kim et al. 2009). The association of thesefacies is indicative of alluvial processes (sensu Blair andMcPherson 1994) which developed as the passive infill of asteeply incised landscape generated by previous erosiveevents (Fig. 7a). Similar Hammamat-type conglomeratescropping out in the CED and NED have also been interpretedas alluvial fan deposits (Grothaus et al. 1979). The abundanceof basement-derived clasts (i.e., plutonic and volcanic rocks)coincides with the compositions expected for deposits thatoriginated during periods of reduced explosive volcanic ac-tivity (Haughton 1993; Riggs et al. 1997). The developmentof degradational surfaces that were subsequently filled by

polymictic and ash-poor lithofacies suggests an inter-eruptive origin for this lithofacies association.

FA2: fluvial braid plain units

Description Interpreted fluvial deposits occur in the form oflaterally continuous beds and lenses in the southeastern blockthroughout profiles I, II, and V, representing periods of streamand river reworking and re-establishment (Fig. 6). Thesedeposits are composed of reddish brown to red massive sand-stones (Sm) and gravelly sandstones (Sg). They are common-ly underlain and overlain by horizontally stratified mudstonebeds (MI) with gradational contact, showing an upward-finingtrend (Fig. 6, profile I and Fig. 7a). The massive sandstonelayers (Sm) are 0.1–0.5 m thick and consist of moderately topoorly sorted, fine to very coarse sand, crudely stratified, low-angle cross-stratified, or massive (Fig. 7f), and composedalmost exclusively of andesitic or rhyolitic detritus; crystalsof quartz, feldspar, and ferromagnesian minerals in addition topumice and chert fragments (Fig. 7g). The matrix is fine sandcemented by iron oxides and clay minerals. The Sm faciesthus resembles arenites and lithic arenites. The gravelly sand-stones (Sg) are characterized by alternations of gravelly andsandy layers. The gravelly layers consist of granule- to pebble-sized clasts, forming discontinuous (3–4 m long) stringers orstreaks. Elongate clasts are generally aligned parallel to bed-ding planes. The sandstone layers are 0.1–0.5 m thick andconsist of mineralogically immature, moderately sorted, me-dium to very coarse sand. Composition varies from arkose tolithic arkose depending on local provenance and mainly com-prises sub-rounded to subangular quartz and feldspar withoccasional volcanic lithic and chert clasts (Fig. 7h).

Interpretation This facies association is thought to representthe deposits of an aggrading sandy braided-stream wherepoorly channelized, shallow stream flow conditions promotedthe development of low-relief bars (Kwon et al. 2011). Theindistinct alternation of gravelly and sandy layers may haveresulted from flow fluctuation or intermittent gravel supplyduring deposition (Rhee and Chough 1993). Similar facieswere found in ancient arc-adjacent alluvial plains induced byexplosive volcanism (Smith 1987; Palmer 1997).

FA3: lacustrine units

Description These units occur dominantly at the top ofprofile I (∼25 m thick). The thickness of FA3 increases upsection with a corresponding decrease in the thickness of thesandstone beds in the amalgamated facies sequence. Theyshow a very restricted stratigraphic position in the studiedarea, lying almost invariably on top of fluvial sandstone



units and below lava flows (Figs. 6 and 7a). They arecharacterized by a tabular geometry with planar bases andslightly wavy tops (Fig. 7a). Their most conspicuous struc-ture is an even planar or wavy rhythmical lamination, 1–3 cm thick, of mudstone and fine silt-sized debris interca-lated with laminae of fine silt and probably clay-sized sedi-ments. Desiccation and mud cracks have not been observed.

Interpretation These units correspond in part to Miall’s(1977) Fl and Fm (laminated and massive silt/mud) facies.This facies association may be interpreted as low-energy deeplacustrine basinal siliciclastic turbidites (cf. Postma 1986). Itmost probably represents the distant (outer) alluvial fandeposits (Walker 1984). The absence of desiccation cracksin mudstone beds suggests that the floodplains were alwayswater-saturated (Elliott 1974). The lack of any primary pyro-clastic deposits in the FA2 and FA3 successions implies theabsence of a simultaneous explosive event. Hence, these unitsclearly constitute a series of inter-eruptive deposits occurringduring the evolution of the syn-rift volcanic environment.

FA4: coherent volcanic bodies/shallow intrusion units

This association comprises all effusive or intrusive sub-volcanic rocks piercing the effusive phase of the volcano(Fig. 8a). Intermediate to felsic lava flows and shallowintrusions are the most common. The lava flows have amassive coherent core with a relatively thin carapace ofblocks-formed breccias, with rubbly, vesicular bases andtops (Fig. 8b). They form irregular tabular rock bodies from3.0 to 20 m thick with conspicuously convex tops andrelatively planar bases (Fig. 8c). Lava flows show lowpaleo-slope gradients (15–23°) with the styles of blockyand “'a'a” lava flows. They are greenish black to pale purplein color, columnar-jointed, and porphyritic, including pla-gioclase phenocrysts and less abundant amphibole and cli-nopyroxene that are semi-aligned in the matrix (Fig. 8d).This matrix shows a hyalophylitic, sometimes trachytictexture, composed of plagioclase microlites and an orephase.

Sub-volcanic intrusions were observed in all the profileswith the thicknesses of individual bodies ranging from 1.0to 10 m. They occur as cryptodome or NE–SW-trendingnecks and plugs with irregular sheets (Fig. 2) that cut entiresuccession within the study area and the neighboring gran-itoids. The most conspicuous feature is the color variationwhich ranges from grey to red and deep purple. The sub-volcanic bodies exhibit blocky jointing and are commonlyporphyritic, massive or flow-banded and contain quartz andK-feldspar phenocrysts (1.0–4.0 mm in size), as well as fewand small-sized plagioclase crystals in an abundant glassymatrix (Fig. 8e).

Interpretation The volcanic facies, represented by interme-diate rocks are interpreted as viscous, slow-moving blockylava flows (Mueller 1991) associated with lava domes andcoulées (Orton 1996). The massive to brecciated unitsdisplay the attributes of a coherent flow in which auto-brecciation processes were prevalent and produced top andbasal breccias during flow advance (Bonnichsen andKauffmann 1987). The quartz–feldspar subvolcanic faciesis original an assemblage of coherent viscous lavas, lobes,and sills/dykes of phenocryst-rich, highly viscous silicicmagma and its emplacement into the whole HVSS indi-cates that these rocks represent the youngest facies and thelast intrusive event of the silicic volcanism. The facies canbe characterized as subvolcanic, near-vent, and non-explosive. Its limited volume is attributed to its higherviscosity and yield strength in comparison to the aphyr-ic/porphyritic lava flows. Their presence is of specialsignificance as they possibly represent feeder dykes, im-plying that a volcanic edifice had been partly or complete-ly constructed on the Hamid basement.

FA5: pyroclastic fall units

Description Pyroclastic fall deposits are only found in pro-files II, III, and V as single beds, up to 2.5 m thick, and in asimilar stratigraphic position (Fig. 6).The fall units showcharacteristics indicative of atmospheric suspension settlingincluding mantle bedding (Fig. 8f), normal and inversegrading, and the absence of tractional features such astrough- and ripple-cross laminations. They exhibit grainsizes of medium to coarse ash and are composed ofpoorly-sorted bedded coarse tuffs (Tcb) formed of glassshards, pumice, and crystals (Fig. 8g). Lithic-rich horizonswith dense clasts of andesitic compositions and older volca-niclastic fragments have also been observed.

Interpretation FA5 is interpreted to be a distal plinian ashfall deposit, based on its composition and characteristicssuch as mantle bedding, normal grading, and the presenceof juvenile fragments. This facies association was formed ina sustained eruption column that experienced variability ineruption intensity over time (Cas and Wright 1987; Careyand Sigurdsson 1989; Bursik et al. 1992). The occurrence ofthe lithic clasts, predominantly andesitic lavas presumablysourced from the original edifice, suggests a shallow frag-mentation level. The presence of lithic clast-rich horizons atthe base of graded intervals likely reflect periods of ventwidening, increased magma discharge rate and/or conduitwall rock instability (Bear et al. 2009). The fall depositstypically drape surfaces interpreted as syn-eruptive low-gradient floodplains of a meandering river system (cf.Carey and Sigurdsson 1989; Bursik et al. 1992).



Fig. 8 Deposit features of facies associations (FA4 to 5). a Subvol-canic dykes crosscut the lava flows and volcaniclastic rocks. b Sche-matic log of a typical lava flow. c Coherent lava flows with convex-upper surface and planar-base. Note autobrecciated structure withvesicles-rich top. d Crossed polarized light photomicrograph showingglomeroporphyritic plagioclase crystals (Plag) embedded in fine-

grained matrix with flow texture. e Rod perthitic K-feldspar (fsp) andquartz (Qz) phenocrysts set in a glassy matrix. f Primary pyroclasticfall units mantling underlying topography with typical bedding struc-ture. g Crossed polarized light photomicrograph of bedded coarse tuffs(Tcb) consisting of feldspar (fsp), quartz (Qz), and volcanic fragments(VF) set in a devitrified fiamme-rich matrix



FA6: phreatomagmatic volcanic units

Description These units are composed of volcaniclastic ma-terial reworked by epiclastic processes (Cas andWright 1987).They form widespread continuous sequences in the studyarea, particularly in the upper and lower part of profiles IIand V, where they reach thicknesses of up to 10 m (Fig. 6).Their beds are intercalated with lava flows and other pyro-clastic deposits at various stratigraphic levels and extendlaterally for several meters, showing pinch-and-swelling orlenticular geometries. Lensoidal bodies with concave basesand planar tops are also found. They are dominated by volca-niclastic conglomerates (Cv) and hyaloclastic rocks (Hy).Individual “Cv” units are composed of medium sand- togranule-size altered volcanic lithics, crystals with hairlinecracks, pumice fragments, and variably altered shards im-mersed in a sandy-silty matrix (Fig. 9a). These deposits arepredominantly interbedded with fine-grained, moderate-towell-sorted greenish tuffaceous sandstones. The hyaloclasticsinclude pumice, vitrophyric (obsidian), and crystal fragmentswith a matrix of hyaline and perlite that shows devitrificationstructures and includes very fine-grained fragments of thesame phenocrysts. Despite recrystallization, it is apparent inplaces that a curved, stringy sericite–quartz mesh mimicsperlite concentric cracking, consistent with an original glassycharacter for the groundmass (Fig. 9b).

Interpretation The absence of tractional structures or diffusivestratification with gradational contacts suggests that Cv bodiesresult from the rapid, progressive aggradation of hyperconcen-trated sheet flows (Smith 1986; Kralj 2011), implying analluvial context (sensu Blair and McPherson 1994). An alter-native interpretation of Cv is peperite. The presence of juvenileclasts within a sedimentary host, secondary alteration of thefriable fragments, and jig-saw fit textures that were mainlydefined by development of hairline cracks in crystals (Fig. 9a)support peperites (Doyle 2000; Erkül et al. 2006). The prove-nance of this FA6, which mainly comprises pyroclastic andeffusive volcanic clasts, indicates high affinity with the volca-nic landscape, as it is almost unrelated to the country rocks(i.e., older granites). The absence of major erosive surfaces anddegradational cycles is consistent with a high aggradationalrate. All of these characteristics are typical of syn-eruptivedeposition (e.g., Smith 1991). Relict perlitic fractures are com-monly present in ancient, altered, formerly glassy volcanicrocks (Allen 1988). These fractures are accentuated by recrys-tallization and devitrification of secondary microcrystallinequartz in the cracks during hydrothermal stage.

FA 7: pyroclastic density current units (PDC)

The upper part of the HVSS is composed of a series of PDCdeposits (FA7) and volcanogenic sediments. These units are

most abundant in the northwestern subbasin (Fig. 2) wherethey cover a vast area and host at their contacts with theyounger granites the famous Hamid Pb mine (Fig. 2). FA7 iscomposed of four lithofacies (Table 1) that grade laterallyand vertically into each other: (1) massive volcanic breccias(Bm); (2) laminated vitric tuffs (Tv); (3) lithophysae-richignimbrite (Il); and (4) nonwelded to welded ignimbrite(Inw/Iw). Facies Il and Inw/Iw drape the lower and uppersuccessions in the study area, respectively.

Massive volcanic breccia facies (Bm)

Description Bm is only observed in the northern region,near the Hamid Pb mine (Fig. 6, profile IV). It occurs in5-m-thick sheets with planar bases and eroded tops thatshow no signs of grading or sorting. The pink color andcommon vesicular texture is characteristic. The principalcomponents are angular to subangular, poorly sorted,pebble-boulder-size lithic clasts, predominantly volcanic inorigin (Fig. 9c). Beds commonly exhibit scour filling tovarious lithic clast trains including immature lensoid rhyo-lite boulders, andesitic, and rarely granitic fragments. Thematrix of the deposits is commonly composed of lithic andpumice fragments, crystals, and glass shards, showing sig-nificant alteration to clay minerals. The fragments do notshow any alignment within the matrix or jigsaw-fit texture.

Interpretation The massive breccias are interpreted to be co-ignimbrite lag breccias that occur within pyroclastic flowdeposits (Sparks and Wilson 1990; Druitt et al. 1999). Co-ignimbrite lag breccias are defined by Druitt and Sparks(1982) and Walker (1985) as thick (1–20 m), coarse-grained(up to meter size blocks), stratified, poorly sorted, variablyfines-depleted, lithic and dense juvenile clast-rich depositsformed in the deflation zone of a collapsing eruption column(1–8 km from vent) at the onset of a caldera collapse or ateruption of stratovolcanoes. Such breccias thus represent aproximal ignimbrite facies, formed by near vent depositionand segregation of dense lithic clasts during pyroclastic flowemplacement (Druitt and Sparks 1982). The texture of thesepyroclastic beds indicates their density current origin which issupported by the common scour filling, poorly sorted clasttrains, and no internal stratification (Németh et al. 2012). ThePDC is therefore interpreted to have been relatively small,dense, and affected strongly by the substrate topography(Schumacher and Schmincke 1990; Hughes and Druitt 1998).

Laminated vitric tuffs facies (Tv)

Description This lithofacies appears in sequences of differ-ent thickness, up to 40 m and crops out in profiles III, IV, andV near the base of the Pb mine in the north (Figs. 3 and 6). It



exhibits a sharp erosional contact with the underlying gran-ites (Fig. 9d), but other contacts are poorly exposed becauseof weathering. It appears to be have an undulatory uppersurface that is discordant with the bedding in the overlyingignimbrites (Fig. 9e). There is also a discordant surfacewithin this facies, comprising a relatively steep (35–45°),sharp, planar surface that truncates the underlying units andis draped by succeeding deposits. The deposits of these faciesinfill topographic lows in U-shaped gullies (Fig. 10a). Inoutcrop, the vitric tuffs are light brown to light purple incolor and are characterized by planar to low-angle lamination(Fig. 9f). Well-developed antidune structures indicate thedirection of transport. At least two different dune beddingtypes are observed; (1) type d (Cole 1991; Fig. 10b) or type II(chute and pool; Schmincke et al. 1973); and (2) type b (Cole1991; Fig. 10c) or type III (Schmincke et al. 1973) antidunes.These dune structures commonly have wavelengths of 1–3 mwith amplitudes of 9–13 cm. In some areas, syn-depositionalstructures (such as cross-stratification, Fig. 11a) are com-mon. Soft-sediment plastic deformation structures, includingconvoluted and contorted beds with slumping have also beenobserved (Fig. 11b).

Under the microscope, the Tv facies is lithic-poor, poorlysorted and matrix-rich. It has a framework of feldspars,quartz, pumice, and occasional vitric fragments, and a rareaccretionary lapilli. Rare accretionary lapilli are spheroidal,less commonly discoidal, and 1 mm to 4 cm in size (Fig. 9g).These lapilli have been subjected to diagenetic and metamor-phic changes that make the determination of their originalmorphology difficult. However, well-preserved devitrifiedshards containing quartz-filled vesicles occur in the core ofsome accretionary lapilli. The matrix surrounding them con-sists of fine-grained quartz and sericite that are probablyproducts of devitrified and recrystallized fine vitric ash.

Interpretation Tv deposits that display structures indicativeof lateral transport (i.e., presence of ripples, dunes, low-angle cross bedding, etc.) have been interpreted as beingdeposited by base surges (Bull and Cas 2000). Additionalfeatures consistent with a base-surge origin recognized inthis study include: the U-shaped gully (Fig. 10a), the dis-cordant surfaces (Fig. 9e), poor sorting, and the abundanceof ash matrix in the tractional deposited units. Types “b” and“d” dunes of Cole (1991) were identified within the Tv beds.Type “b” dunes are asymmetrical and built from planar bedswith progressively steepening layers. Sand-wave crests al-ways migrate in the downstream direction, therefore they areprogressive. Similar structures are termed “type III” dunestructures by Schmincke et al. (1973). Type “d” sand wavescomprise steeply dipping stoss-side layers sigmoidal inshape. They migrate in the upstream direction and may beconsidered as regressive. They are similar to the “type II”dune or “chute and pool” structure of Schmincke et al.

(1973). Occurrence of both progressive and regressive typeswithin the same deposit can be explained by a pulsatorynature and change in the flow regime of the base surges(Cole 1991). Ash-dominated Tv base surge facies displaylateral/vertical change in bedding style, i.e., from planar towavy or from planar to dune beds. Units with similar facieschanges and climbing dunes are interpreted to occur due to adecrease in suspended-load transport rate and/or an increasein bedload transport rate and a decreasing of the surgeenergy (Sohn and Chough 1989). Their occurrence withinmore massive parts of Tv suggests a relatively high particleconcentration within the depositional boundary layer of theproximal-medial pyroclastic density current (cf. Druitt1992). Most probably, the PDC of the Tv facies was denseand nonturbulent causing it to be ponded in topographiclows, resulting in the abruptly pinching-out deposit geome-try (Jeong et al. 2008).

Lithophysae-rich ignimbrite facies (Il)

Description The lithophysae-rich ignimbrite facies (Il)overlies lava flows intercalated with volcaniclastic rockswith no intermediate paleosol deposit. It occurs in profilesII and III, intercalated with lava and volcaniclastic rocks(Fig. 6) along gradational contacts. Deposits are massiveand poorly sorted with average grain sizes ranging fromcoarse ash to bombs and blocks with diameters up to10 cm. The rocks of this facies form massive or flow-banded sheets, up to 30 m thick, composed of pumice lapilliand lithic fragments supported in a matrix of devitrifiedvitric ash and shattered crystals. Welding compaction canbe recognized by the presence of large nodules (1–10 cm indiameter) hosted in the more deeply weathered rock mass.

Fig. 9 Deposit features of facies associations (FA6 to 7). a Crossedpolarized light photomicrograph showing volcaniclastic conglomerate(Cv) comprising quartz and feldspar crystals with hairline cracks aswell as volcanic and pumice fragments immersed in a sandy–siltymatrix. b Crossed polarized light photomicrograph showing remnantsof relict concentric perlitic fractures with complete devitrification ofmicrocrystalline quartz in the cracks (Qz). Note the decomposition ofmost propably mafic phenocrysts into epidote-opaque-quartz products.c Close-up of the fabric in the massive breccias (Bm) with polyhedralblocky clasts in fine-grained matrix. Note the vesicles-rich top in thejuvenile clasts. d Laminated vitric tuffs (Tv) overlie younger granite.Note Pb–Zn mineralizations filling fractures. e An angular unconfor-mity formed by rapid draping and burial of volcanotectonically dis-turbed vitric tuffs (dip to the right) by rheomorphic ignimbrites (dipvertically ∼90°). Note the welding zones in ignimbritic rocks, whichrange from incipiently at the base welded to densely welded at the top.f Planar to low-angle lamination in vitric tuffs. g Crossed polarizedlight photomicrograph showing a strongly recrystallized quartz (Qz),and decomposed pumice clasts present in a completely devitrified fine-grained matrix. Note remnants of multiple-rim in a recrystallized (Qz)accretionary lapilli (AL), originally composed of layers of alternatingfine and very fine ash

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Most of the nodules are silicified, indurated, flattened, andelongated by deformation. Some nodules show spheroidalconcentric fractures resembling large-scale perlitic texture(macro-perlite, Fig. 11c). Each nodule contains a quartz-

filled, circular and star-shaped internal cavity. This cavitydisplays fine axiolites (acicular crystals) growing inwardfrom their walls that defines a pectinate texture (Fig. 11d,McArthur et al. 1998).



Interpretation The characteristics of the nodules suggestthat these are lithophysae preserved in a weathered ignim-brite. Lithophysae are large spherulites that develop centralgas cavities wherever the external pressure is low enough toallow dissolved gas to expand a cavity. During diagenesis,aqueous solutions of silica are carried to these cavities anddeposited as agate, quartz, and sometimes common opal orjasper. This process of silicification was interpreted as beingcaused by vapour-phase alteration (sensu Cas and Wright1987; Streck and Grunder 1995) and/or deuteric alteration inthe syn-volcanic stages. Lithophysae, filled with chalcedonyand quartz, are quite common in the periphery of bothsubaqueous rhyolite lobes (cf. Kano et al. 1991) and sub-aerial lava domes (cf. Holzhey 1999, 2001). High tempera-ture crystallization domains include lithophysae, spherulite,and pectinate textures (McArthur et al. 1998). The massive,poorly sorted character suggests deposition from the mainbody of a high particle concentration pyroclastic flow with

fluid escape-dominated flow-boundary zones (Branney andKokelaar 2002). The units of pyroclastic accumulation con-stitute the record of explosive volcanic activity during theevolution of the lower succession.

Fig. 11 Deposit features of facies associations (7). a Small-scale crossstratification in vitric tuffs. b Vitric tuffs show penecontemporaneousslumping and folding reflecting soft sediment deformation. c Planepolarized light photomicrograph showing circular and half moonshapes for lithophysae quartz-filled nodules (Qz). Note the flow fold-ing of the devitrified fiamme (arrow). d Plane polarized light photo-micrograph demonstrating pectinate texture defined by fine axiolitesgrowing inwards from the walls of a juvenile pyroclast containing aquartz-filled vesicle (Qz). Note the reddish oxidation of the feldspar(fsp). e, f Scanning electron micro-images of juvenile clasts are highlyvesicular with sub-spherical to smoothly irregular-shaped bubbles. gPlane polarized light photomicrograph exhibiting vesicles-rich glassyvolcanic fragment (VF) enclosed in welded ignimbritic rocks. h Planepolarized light photomicrograph showing flow banding and folding inwelded ignimbritic rocks

Fig. 10 Deposit features offacies associations (7).a U-shaped erosive gullyinfilled by deposits of the surgefacies (vitric tuffs). b Chute andpool/type II (Schmincke et al.1973) or type d (Cole 1991)dune structure. Note theflattened (?) tectonicallycompacted sigmoidal ripple/dune (arrowed). c Type III(Schmincke et al. 1973) or typeb (Cole 1991) dune structure

b







Nonwelded to welded ignimbrite facies (Inw/Iw)

Description This facies is topographically and stratigraphi-cally higher than facies Tv, occurring in the upper succes-sion of HVSS, making it probably the youngest primaryvolcanic deposit in the studied succession. Its deposits drapethe pre-eruption topography, thickening in valleys anddepressions and occur as single units or a series of stackedbeds. Their lower bounding surfaces are flat or reflect thepaleosurface, their tops are mostly eroded. In fact, this lowercontact and facies Tv is marked by an unconformity andlocal occurrence of reworked beds (Fig. 4). Beds are alwaysinternally massive, poorly sorted and lack internal stratifi-cation. Color, varying from creamy beige to reddish purple,reflects changes in the degree of welding and alteration.Thicknesses of single units can vary from 2.0 to 70 m withan average of 50 m. The most prominent feature is theoccurrence of well-developed ramp structures (Fig. 4), con-sisting of sets of flow laminations defined by the alignmentof juvenile fragments and variations in color and degree ofvesicularity. The main flow directions are E–W, and SW toWSW. Most of the hydrothermal alteration in these rocktypes occurs as fracture and cavity fillings. Based on petro-graphic characteristics, the degree of welding has beendivided into four categories that vary from incipientlywelded of rank II (Peterson 1979; Streck and Grunder1995) through partially and moderately welded of rank IIIand IV (Wilson and Hildreth 2003; Quane and Russel 2005)to the densely welded of rank V (Peterson 1979; Branneyand Kokekaar 1992; Wilson and Hildreth 2003) (Figs. 9eand 12). Clear boundaries between subunits are not identi-fied, but highly welded rheomorphic stratified lithofacies aredeveloped throughout. Welding facies are completely gra-dational and the entire deposit shows a gradual decrease incompaction features upwards. The welding is dependentupon compaction of the tuff components or soon afterdeposition and the temperature of particles and gases upondeposition (cf. Ross and Smith 1961).

The basal facies of the studied ignimbrite is representedby a 15–30-cm incipiently welded vitrophyre (Fig. 12),comprising 20–30-cm-long vitrophyric sections of massive,perlitic, and largely devitrified glass. Strongly flattenedfiamme textures are locally preserved in a more alteredmatrix. The basal boundary is accompanied by typicallythin, 30 cm long, low-angle domains of glassy vitrophyrepenetrating the devitrified welded facies. A 5–10-m-thick,dense, moderately to densely welded eutaxitic deposit over-lies the basal vitrophyre (Fig. 12). The juvenile fragments in

these zones consist of well-preserved, millimeter to centi-meter long, dense or pumiceous lapilli, embedded in amatrix of recrystallized ash and glass shards. These particlesshow blocky shapes with abundant irregular, spherical tosub-spherical vesicles (Fig. 11e). Individual pumice frag-ments exhibit very intense vesiculation and can be up to12 cm long in pumice concentration zones (Fig. 11f).Lithoclasts and alkali feldspars behaved as solids in a ductilematrix of deformed and compacted fiamme and finer-grained components. Lithoclasts are rimmed by rotationalductitle shear marks and pressure shadows with asymmetricσ-type tails of glass shards trailing away from lithic clasts(Fig. 12) (Schmincke and Swanson 1967), whereas fiammeare preferentially flattened. Fiamme are blocky or lenticularin shape and their edges are cuspate or ragged. The majorityof fiamme shows bi-cuspate and tricuspate, plate-like orcrescent-shaped morphologies. Major lithic components ofthese welded zones are andesitic lava clasts, up to 30 cm indiameter (Figs. 11g and 12). Micro-scale rheomorphic flowfolds (harmonic, disharmonic, sigmoidal, and isoclinal areobserved (Fig. 11h).

The top of the cooling units consist of partially erodedpoorly welded and weakly compacted porous ignimbrite(∼5 m thick) (Fig. 12). Clasts are randomly oriented, de-formed, and rotation and shear features are absent. Highlywelded and flattened ignimbritic fiamme consist mainly ofmicrocrystalline alkali feldspars and quartz, which wereprobably formed during devitrification and post-emplacement vapor phase crystallization. These ignimbriticdeposits are commonly normally graded in the basal partand inversely graded in the topmost part for both lithiclapilli and pumice (Fig. 12).

Interpretation The overall characteristics of this facies sug-gest rapid deposition from a pyroclastic density currentwithout tractional grain segregation induced by turbulence(e.g., Hughes and Druitt 1998; Branney and Kokelaar 2002;Boyce and Gertisser 2012). The characteristic red color isinterpreted as due to vapour phase alteration that led to high-temperature oxidation during devitrification of the matrixand pumice fragments, transforming them into a secondarymineral assemblages. Flow banding and folding are primarytextures, characteristic of rheomorphic rhyolites, and reflectlaminar and folding flowage (Cas and Wright 1987; Scutteret al. 1998).

This facies is mostly massive and locally shows crudeinternal layering, basal normal grading and near-top inversegrading of lithics, either erosional or non-erosional lowersurfaces, and flat-lying to imbricated grain fabrics. Basalnormal grading suggests the influence of action turbulence.Inverse grading at the top of the unit suggests constrainedparticle motion and rapid deposition from highly concen-trated and laminar flow with minimal turbulent or granular

�Fig. 12 Schematic stratigraphic column of the upper ignimbritic cool-ing unit and its major welding facies. Left: stratigraphic column at Pbmine locality; the width of the log corresponds to the average grainsize, in millimeter. Photomicrographs are in plane polarized light



shear close to the lower flow boundary (Branney andKokelaar 2002). The coexistence of these apparently con-tradictory depositional features in the same deposits is inter-preted as the result of nonuniformity and spatial-temporalvariations of the properties of the pyroclastic density cur-rent, such as the degree of turbulence, particle concentra-tion, and suspended-load fallout rate (Branney and Kokelaar2002; Vazquez and Ort 2006). The change in abundance ofclast types from lithics-dominated to pumice-dominatedsuggests changing clast supply conditions at the vent, pos-sibly reflecting tapping of progressively deeper levels of themagma feeder system. Scarcity of basement-derived acci-dental components beneath these deposits is interpreted tobe due to shallow-level fragmentation of magma followedby immediate generation of pyroclastic density currentsfrom shallow-level blasts at the onset of eruption (Buesch1992). The welding process involves sintering, compaction,and flattening of pyroclasts in response to sufficient loadstress under high temperature emplacement rates ( 500–650 °C, Cas and Wright 1987; Quane and Russell 2005).

Discussion

Syn-eruptive/inter-eruptive process interaction

In the previous sections, the eruptive and syn-eruptive/inter-eruptive characteristics of the Hamid rock units were de-scribed. In order to establish the main interactions betweenthe accumulation processes, some key relationships amongthem will be considered. Two distinctive cases will beanalyzed in detail: (1) polymictic alluvial (FA1) and fluvialunits (FA2 and FA3) passively infilling incisions on oldergranites and coherent lava flows rock units; and (2) ignim-britic and pyroclastic surge rock units (FA7) deposited ontop of large lava flows, fall deposits, and phreatomagmaticvolcanic rock units (FA4, FA5, and FA6) that are intrudedby subvolcanic intrusions.

Case I: Polymictic alluvial and fluvial units overlyingpre-and syneruptive units

Inter-eruptive polymictic alluvial units overlie both pre-andsyn-eruptive units, i.e., the older granites and coherent lavaflows. This kind of relationship can be found in the lowerportions of the HVSS (Figs. 5 and 6). The contact betweeninter-eruptive and syn-eruptive units is either sharp or gra-dational, unconformable and developed over irregular topog-raphy. The inter-eruptive units are deposited in depressionswith variable degree of incision into tops of the syn-eruptiveunits. When the syn-eruptive and the inter-eruptive alluvialunits are in contact, they can be distinguished by their

compositional features. Development of independent depo-sitional systems along the southwestern and northeasternsubbasin margins (Fig. 2) suggests that accommodation cre-ation was strongly differential and the relief was varied alongthe basin margins in the initial stage of basin development.The earliest basin is thus interpreted to have comprisedmultiple, either isolated or partly interconnected subbasinswith independent conduits for sediment dispersal and multi-ple depocenters. The basin is envisaged to have beenbounded by incipiently developed NW-trending normal faultsegments with variable displacement (Fig. 7a). These faultsare therefore interpreted to have experienced considerablesubsequent displacement, creating the main accommodationspace during the early rifting of the Hamid Basin. Individualsubbasins were probably filled by alluvial, fluvial, and la-custrine deposits that were sourced from incipient footwallcatchments (localities I and II, Fig. 2). The basin is inter-preted to have been rifted rather rapidly to accumulate forman alluvial fan, fluvial, and lacustrine facies (Fig. 13a/b).

The inter-eruptive polymictic alluvial units originated aspassive infill after the erosional or degradational stage,indicating a hiatus in volcanic and pyroclastic supply rate.The development of a deeply incised topography occursafter the cessation of an eruptive event (Smith 1991;Manville et al. 2009). The great volume of material deliv-ered during the eruptive periods modified the sedimentarysystems, raising the local base level. The subsequent rees-tablishment of the original gradients causes the incision ofdeep, narrow valleys through the syn-eruptive units (Smith1987; Eroy et al. 2011) and the deposition of inter-eruptiveunits in an alluvial context, which constitute bypass zonesfor the sediment transported by stream flows (Blair andMcPherson 1994; Eriksson et al. 2006). The abrupt transi-tion from fluvial-braided plain (FA2) to lacustrine (FA3)deposits suggests rapid deepening of the basin due to sig-nificant displacement of the basin-bounding faults.Northwestward tilting and younging of strata (Fig. 2), anorthwestward shift of depositional systems, and the accu-mulation of relatively thick lacustrine deposits collectivelysuggest that the initial basin was deepening towards thenorthwest, developing a half-graben geometry (Fig. 13b).The thick accumulation of these deposits in the southern-western subbasin is attributed to development of a transferzone because of the overlap of normal faults segments. Thetransfer zone formed a locus for syntectonic drainage devel-opment and a conduit for sediment dispersal through thehangingwall high (e.g., Gawthorpe and Leeder 2000; Sohnand Son 2004), developing later into an intrabasinalanticline.

Accumulation of the thickest strata (FA4, FA5, and FA6)near the wadi El Mesdar (locality III, Fig. 2) indicate abruptmigration of the loci of main subsidence from the north-western and southeastern normal-fault margins toward the



Fig. 13 Cartoon illustrating the evolution of the Hamid Basin. a, bearly basin formation is predominantly controlled by normal faultsbetween incipiently developing strike-slip faults. c the role of strike-slip faults on the southwestern and northeastern margins increases after

deposition of the alluvial sediments. d, e the strike-slip faults continuedtogether with the northwestern normal faults. The overall basin floor isinclined toward the northwest. The depocenters developed near thebasin corners, resulting in thick accumulation of deposits here



northeastern and southwestern strike-slip fault margins(Fig. 13c). It is interpreted that the strike-slip faults playedan important role in the formation of accommodation spaceand basin configuration at this stage. Independent deposi-tional systems and multiple depocenters are interpreted tohave developed in different parts of the basin on the basis ofthe different stacking patterns. Abrupt cessation of deposi-tion of coarse- to medium-grained deposits (FA1 and FA2)suggests significant modification of the earlier footwallphysiography by lateral propagation of the normal faults inassociation with the strike-slip fault activity and consequentmigration or exhumation of the footwall-derived drainagesystems (Villamor et al. 2011). After the following pause ofbasin extension, coarse-grained volcaniclastic debris gradu-ally prograded into the lowland of the basin, resulting in thedeposition of FA5 and FA6 because of the onset of explo-sive eruptions (FA4). These deposits suggest that volcanismgenerated mostly small-volume, probably phreatomagmatic,and fine-grained volcanic plumes and currents that could notdisperse lapilli-size tephra over large areas (Fig. 13c). Thephreatomagmatic volcanic rock units originated due to re-mobilization of pyroclastic materials synchronous to theeruptive event.

Case II: ignimbritic and pyroclastic base surge unitsoverlying large lava flows and pyroclastic units

The sudden deposition of large amounts of material bypyroclastic density currents implies levelling of a pre-existing topography created by volcanic edifices and exten-sional faulting (Sulpizio et al. 2007; Sulpizio and Dellino2008). Vertical facies changes from lava flows and relatedpyroclastics to PDC deposits in the northwestern subbasinindicates accelerated basin subsidence near these margins.Maintenance of the depocenters or the loci of the greatestsubsidence near these margins suggests that the strike-slipfaults continued to play a significant role in basin extensionand subsidence (Fig. 13d). Units “Bm” and “Tv” record asudden input of volcaniclastic debris into the Hamid Basin.The fine grain size of unit “Tv,” mainly fine vitric tuff(Table 1), suggests that the eruption was also phreatomag-matic (Whohletz 1986; Heiken and Wohletz 1991). Therestricted occurrence and rapid pinch-out of this unit sug-gests that the magnitude of the eruption was small, and thePDC was dense and nonturbulent so it could be ponded intopographic lows (Schumacher and Schmincke 1990;Hughes and Druitt 1998) (Fig. 13d). Sudden input and thickaccumulation of the alluvial-fan conglomerates (Cb) abovethe vitric tuffs (Tv) was most likely caused by the develop-ment of larger drainage basins and rapid denudation of theuplifted footwall region associated with the activity of thebasin-bounding faults (Gawthorpe and Leeder 2000). Thelack of intervening lacustrine deposits in “Cb” suggests that

the sediment supply greatly exceeded the displacement rateof the faults. The basin was probably overfilled and theformer lake shoreline retreated away from the basin margin.The transition from unit “Tv” to unit “Inw/Iw” reflects achange in the eruption style from a small- or medium-sizedphreatomagmatic eruption to a large magmatic eruption. Thechange in style was probably caused by increases in the fluxof magma from a deep reservoir and the mass eruption rate asthe conduit or vent widened (Fig. 13e), which would havelimited the potential interaction of eruption magma withexternal water (Giordano 1998). Unit “Inw/Iw” indicates anignimbrite-forming eruption after a hiatus allowed for depo-sition of meter-thick, nonvolcaniclastic conglomerates (Cb)above unit “Tv” (locality V, Figs. 2 and 13e). The eruptionoccurred through many vent sites, producing a seeminglysingle but actually composite ignimbrite unit. It is proposedthat a composite ignimbrite with the characteristics of thePDC deposits in the northwestern subbasin near the Pb minecan be an exemplary product of syntectonic volcanism thatcan provide an insight into the interpretation of structural andstratigraphic evolution of a sedimentary basin.

The apparently coarsening-upward sequence from unit“Tv” to unit “Inw/Iw” without significant amounts of non-volcaniclastic sedimentary interbeds suggests that the wholesequence represents the product of a single eruption com-posed of a few eruptive phases (terminology of Fisher andSchmincke 1984). The eruption probably waxed in eruptionrate or eruption intensity and underwent a transition ineruption style from phreatomagmatic to magmatic (De Ritaet al. 1997; Giordano 1998). Volcanic eruptions from rhyo-litic calderas develop in rifted regime (de Silva 2008;Seghedi 2011) generate the most widespread and severevolcanic hazards known, including primary effects such asPDCs, ash fall, and lahars (Tanguy et al. 1998; Davi et al.2011). It is clear that small caldera volcanoes may haveformed in the volcanic field near the Pb mine and beenresponsible for the beginning of high sediment supply.

Accumulation models for the HVS successions

The bulk of the Hamid successions was deposited in a megahalf-graben bounded by basin margin faults and sedimentaccumulation took place under different fault-controlledsubsidence regimes with intervening tectonically static peri-ods. The Hamid area represents a small volcano tectonicbasin with multiple eruptive phases including explosive,effusive, and cryptodome-forming episodes separated bylong eruption repose periods allowing the accumulation ofvolcaniclastic aprons during inter-eruptive times. This erup-tion behavior is typical for long-lived, polygenetic compos-ite volcanoes (Fisher and Smith 1991; Carrasco-Nunez andRiggs 2008; Manville et al. 2009). The Hamid volcanicshowever were not erupted through such multiple magma



pulses as indicated by its relatively homogeneous faciescharacteristics suggesting a small isolated magma chamberresponsible for the formation of these volcanics. The differ-ent types of relationships between syn-eruptive and inter-eruptive units established in the Hamid area depend on thedistinct signature of the preceding syn-eruptive periods.Pyroclastic units are much thicker and better distributed incase II than in case I, indicating eruptive events of a differ-ent magnitude and volume. Accordingly, the low-volumeeruptive periods in case I produced thin lava flows, pyro-clastic deposits, and syn-eruptive volcaniclastic units thatfailed to fill the available accumulation space, occupyingrestricted areas along valleys and flanks of the previouspositive features in the landscape (i.e., volcanic edificesand uplifted faulted blocks). The deposition of ignimbritesand volcaniclastic units was enough to raise the local baselevels of the sedimentary environment, but not enough to fillthe whole depocenter (Lucchi et al. 2010; Muravchik et al.2011). As a consequence, degradation events developed anirregular topography on top of the previous syn-eruptiveunits of the lower succession. High-gradient polymicticalluvial units deposited in the topographic low. In contrast,extensive voluminous ignimbrites characterize the pyroclas-tic units in case II. Volcanic activity in extensional environ-ments typically generates caldera-like settings where thedeposition of large-volume ignimbrites is accommodatedthrough rapid large-scale subsidence (Petrinovic et al.2010). This combination of magma chamber emptying andextensional faulting is not only observed in purely orthogo-nal depocenters but also in transtensional ones (Moore andKokelaar 1998; Aguirre-Díaz et al. 2008; Petrinovic et al.2010). Stratigraphic analysis reveals a marked thicknesschange in case II along the extensional faults (Fig. 4), sug-gesting the addition of a volcanotectonic subsidence com-ponent to the accommodation of such large ignimbrites(Muravchik et al. 2008). Therefore, the widespread con-glomerate units (Cb) represent the readjustment of the hy-drological system to the extensive low gradient area left ontop of the welded ignimbrites and the modification of thedrainage network, causing a dramatic depocenter-scale ef-fect (Franzese et al. 2007).

As a result, two conceptual models of accumulation canbe envisaged to describe the examples considered above(Fig. 14). Case I is an underfilled model where the amountof syn-eruptive volcaniclastic material created is relativelysmall. Inter-eruptive processes consist of extensive erosion asa consequence of the readjustment of the sedimentary systemsto the new base levels and deposition. Case II is an example ofan overfilled model in which the volume of syn-eruptivepyroclastic material was so large that it covered the northernarea, completely choking the accumulation space of the depo-center and levelling the previous topography. The presence ofan extensive capping volcanic conglomerate sheet above the

topmost vitric tuffs (Tv) suggests that Hamid's low aspectratio volcanic edifice must have been partially covered by anactive braided river network, re-established after its forma-tion. Pyroclastic density current deposits (case II) in thecapping succession of the Hamid area suggests that its erup-tion reached a high magma discharge rate stage in its finalphase in an eruption that represents a unimodal eruption style,a common feature in long-lasting eruptions of compositevolcanoes (Karaoĝlu and Helvaci 2012).

Scenarios like those described above are expected tohappen several times during the lifespan of a volcanic rift.Their occurrence and frequency will depend on the rate,frequency, and duration of volcanic activity and its interac-tion with the evolving extensional structures. The profusevolcanic activity commonly associated with the develop-ment of extensional environments (e.g., Ziegler andCloething 2004; Aguirre-Díaz et al. 2008) leaves a strongimprint on the sedimentary systems. Such interactions canbe observed in the rock record of the syn-rift megasequence,which records the alternation of periods of active or inactivevolcanism. The existence of such different periods has beenaddressed as a major control over the sedimentary sequencesthat compose other basin types (e.g., Smith 1987, 1991).

Deformation of the strike-slip shear zones can be sub-divided into an early and late transpressive events duringorogen-parallel extension (Abd El-Wahed 2009). Duringearly transpression (660–645 Ma), older granite shear zoneswere deformed in a sinistral sense. During the late trans-pressive phase (645–560 Ma), an external set of NW–SEtrending strike-slip shear zones was deformed in a sinistralsense, followed by reactivation of NE–SW-oriented low-angle normal faults as well as intrusion of the DokhanVolcanics and younger granites. Many authors have previ-souly suggested that the Hammamat Group sedimentation inthe most basins in the CED was initiated in a fault-boundedhalf-graben under a transtensional regime, which laterevolved into a pull-apart basin under a transpressional re-gime developed at a prominent releasing bend in the sinistralNajd Fault System (Rice et al. 1983; Shalaby et al. 2006).This accommodated orogen-parallel extension on the flanksof structural high. It should be concluded that a combinationof extensional (transtensional regime) and strike-slip tecton-ic phases (transpressional regime) has occurred in theEastern Desert between 600 and 530 Ma (Stern 1985). Theformation of the Dokhan Volcanics and Hammamat Groupsmay confirm the association of these tectonics (Abd El-Wahed 2009).

Implications for Basin configuration

The HVSS provides not only information on a variety ofvolcaniclastic depositional processes in a nonmarine basinbut also important clues to the configuration and evolution



of the Hamid Basin. Several characteristics of the HVSS areworth reiterating in this respect, including (1) the occurrenceof the thickest fluvio-lacustrine units in the northeasternextremities, (2) the apparent ponding of PDC deposits inthe northwestern part of the mapped area, (3) the truncationof Tv unit by the overlying fluvial conglomerates in thesame area near the Pb mine, (4) the thickening of FA4,FA5, and FA6 toward the northeastern subbasin, and (5)the overall good preservation of the HVSS as a basin-widevolcaniclastic units with a fairly uniform thickness (Fig. 6).

All these features are interpreted to be related to thespatial distribution of, and temporal changes in, the primar-ily tectonically controlled accommodation space in the basin(Blair and Bilodeau 1988; Wilgus et al. 1988; Gawthorpe etal. 1997; Gawthorpe and Leeder 2000). Features 1 and 2indicate that the depocenter loci of greatest subsidence wereclose to the northeastern and northwestern border faults.This interpretation is in accordance with structural and sed-imentologic observations of the HVSS, which indicategreater subsidence at the corners of the basin (cf. Son et

al. 2000). Feature 3 indicates that there was insufficientaccommodation space for deposition of unit “Tv” in thenorthwestern subbasin after emplacement of unit “Inw/Iw”.However, feature 4 indicates that considerable accommoda-tion space remained unfilled in the northeastern area afteremplacement of PDC deposits, suggesting that: (1) therewas more pre-eruption accommodation space and (2) thatthe southwest-northeast-trending axes of the basin wereplunging gently toward the northwest. The floor of theHamid Basin has been divided into the NW and NE sectorswith a structural high or a horst in between (Fig. 15). Thisstructural high was sufficient to act as a topographic barrierto the dense and nonturbulant pyroclastic density current ofunit “Tv”.

In spite of the generally low preservation potential oftephra in fluvial settings (Palmer and Shawkey 1997;Königer and Stolhofen 2001), the HVSS is well preservedas a basin-wide stratigraphic unit with a relatively uniformthickness, although its constituent units thin, thicken, andare variably truncated by overlying fluvial conglomerates

Fig. 14 Schematic evolutionary models for Case I and Case II. a, dvolcanic and pyroclastic eruptions characterise syn-eruptive periods.The overall constant aggradation dominates the syn-eruptive stages.The magnitude of pyroclastic activity is greater in case II (d) than inCase I (a). During the subsequent inter-eruptive periods, the sedimen-tary systems readjust to the newly created conditions. In case I, the

sedimentary systems respond to the elevation in base level by erodingthe landscape and depositing the inter-eruptive sequence in the gener-ated depressions (c). In case II, the volume of material delivered by theprevious eruptions drapes the original landscape, flattening the surfaceand choking the existent sedimentary systems (f)



(Fig. 6). The almost complete preservation of the HVSSthroughout the basin necessitates rapid reestablishment ofthe fluvial system and the accumulation of background“inter-eruption” deposits without deep incision of the volca-niclastic “syn-eruption” deposits. This requirement can befulfilled by a high subsidence rate or rapid base-level rise inthe basin that matched or exceeded the eruption-inducedsedimentation rate throughout the basin (Smith 1991;Palmer 1997; Riggs et al. 1997; Königer and Stolhofen2001).

The overall stratigraphic pattern of the Hamid Basinsuggests a progressive increase in the extension rate towardthe top of the HVSS, eventually followed by intrusion andextrusion of the subvolcanic dykes/sills (Sd/Ss) orientatedNE–SW (Fig. 2), indicating a basement fracture control. ThePDC deposits, intercalated in the topmost part of the HVSS,is therefore interpreted to have accumulated under overallaggradational conditions, probably involving acceleratedcrustal extension and subsidence of the Hamid Basin shortlybefore the onset of the rift climax (Prosser 1993). This kindof association, in some case synchronous, between exten-sional faulting and volcanic activity appears to be commonin many extensional tectonic provinces (Janecke et al. 1997;Aranda-Gomez et al. 2003; Jeong et al. 2008).

Summary and conclusion

Late Neoproterozoic volcano-sedimentary successions inthe Hamid area, NED, Egypt, are characterized by complexfacies architecture in a complex depositional environment.

The stratigraphy of the HVSS can be divided into lower andupper parts, based on field and petrographical character-istics. The lower part is composed of alluvial sediments,coherent lava flows, and phreatomagmatic volcanic depos-its. The upper part includes pyroclastic density currentdeposits and feeder-dykes/sills that are more voluminousthan the lava flows and are mostly interpreted to haveoccupied small caldera. The distribution and short-termchanges of the accommodation space, and hence the struc-tural configuration of the Hamid Basin, can be inferred fromthe basin-scale architecture of the depositional units. Thebasin-wide preservation of these units and their fairly uni-form thickness, in spite of the active hydraulic remobiliza-tion of tephra in fluvial settings, suggests that the HVSSaccumulated in an overall aggradational setting that wascaused by accelerated extension of the Hamid Basin on themargin of the rift climax. The eruption of the Hamid vol-canics is thus interpreted to have been closely associatedwith basin-formation, providing a good example of syntec-tonic volcanism.

Facies analysis of the HVSS led to the definition of 14lithofacies types which have been grouped into seven lithof-acies associations. Their depositional sequence shows that anincrease in felsic volcanism occurred in the NW part of themapped area, represented by a high rate of volcaniclasticsediment supply. Genetic interpretation of the different accu-mulation units has made it possible to identify periods mainlyby sedimentary processes and periods dominated by volcanic-related processes. Syn-eruptive units (volcanic-related pro-cesses) can hence be distinguished from inter-eruptive units(sedimentary processes) within the syn-rift succession.

Fig. 15 Reconstruction of the paleogeography of the Hamid Basin. Approximate locations of the outcrop localities of the studied rocks areindicated. The relief of the basin margins and the horst is greatly exaggerated



Based on the relationship between the facies associationsin the study area, two accumulation stages were defined.The underfilled stage occurs when material supplied to thedepocenter during eruptive events is insufficient to level theexisting topography, allowing development of high-gradientalluvial systems during the next inter-eruptive period. Theoverfilled stage occurs when voluminous pyroclastic densitycurrent deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systemsto develop during the subsequent inter-eruptive periods.

Acknowledgments The author thanks Mr. H. Khamis for his assis-tance in the field. He would like to thank V. Manville for providingextremely helpful reviews of the manuscript. An anonymous reviewerimproved the quality of the paper and is gratefully acknowledged.

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Cairo Universityscholar.cu.edu.eg/?q=ezzeldinkhalaf/files/10.1007... · Late Neoproterozoic sedimentary basins formed in the NED of the Egyptian Eastern Desert (Fig. 1). Most of these