Tectono sedimentary evolution of the Umm Ghaddah Formation...

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Journal of Asian Earth Sciences 33 (2008) 194–218

Tectono sedimentary evolution of the Umm Ghaddah Formation(late Ediacaran-early Cambrian) in Jordan

Belal S. Amireh a,*, Mazen N. Amaireh b, Abdulkader M. Abed a

a Department of Geology, University of Jordan, Amman, Jordanb Department of Mining Engineering, Tafila Technical University, Tafila, Jordan

Received 21 December 2006; received in revised form 29 October 2007; accepted 9 November 2007

Abstract

The terrestrial Umm Ghaddah Formation of late Ediacaran-early Cambrian age was deposited in NE–SW elongated intracontinentalrift system basins and sub-basins bounded by active listric half-graben faults. Basin fill consists of conglomerate facies association A,deposited in a fault-controlled transverse alluvial fan system that drained northwestward and graded laterally into sandstone facies asso-ciation B, deposited by a braided river system flowing northeastward axial to the rift basin. The alluvial fan facies association was depos-ited by rock falls and non-cohesive debris flows of sediment gravity flow origin, and by sheetflood processes.

The Umm Ghaddah Formation is dominated by a large-scale fining upward succession interpreted to reflect a gradual cessation of thePan African Orogeny. Within this large-scale trend there are also minor fining and coarsening upward cycles that are attributed torepeated minor tectonic pulses and autocyclic shifting of the system.

The distribution pattern of the Umm Ghaddah Formation and the underlying Ediacaran Sarmuj Conglomerates, Hiyala Volcanic-lastics and Aheimir Volcanics in Jordan and adjacent countries in isolated extensional half-grabens and grabens formed during the exten-sional collapse phase of Arabia associated with the Najd Fault System seems to be unrelated to the present day Wadi Araba-Dead Seatransform fault system.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Jordan; Late Ediacaran-early Cambrian; Half-graben bounded by listric faults; Marginal alluvial fan; Dead sea transform

1. Introduction

The clastic Umm Ghaddah Formation has been recentlydocumented by Amireh and Abed (2000), and interpretedas an intracontinental rift basin-infill sequence, or a syn-riftfill. It is exposed at three localities in central Jordan andhas been identified in a series of wells (Fig. 1). The forma-tion consists dominantly of conglomerate (Fig. 2a) andminor sandstone facies. It overlies unconformably eitherthe Aheimir Volcanic Suite (Table 1 and Fig. 2b) of553 ± 11 Ma (Jarrar, 1992) or directly the Sarmuj Con-glomerate (Table 1 and Fig. 2c) of 595–600 Ma (Jarraret al., 1991) and is conformably overlain by the Saleb

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* Corresponding author. Tel.: +962 777498301; fax: +962 6 5348932.E-mail address: [email protected] (B.S. Amireh).

Arkosic Sandstone Formation (Table 1 and Fig. 2d) ofearly Cambrian age (Bender, 1968; Amireh et al., 1994).

Since the Umm Ghaddah Formation is stratigraphicallyconfined between the Aheimir Volcanics or the SarmujConglomerate of Ediacaran age and the Saleb Arkosicsandstone that is dated by Amireh et al. (1994) to be ofearly Cambrian age according to presence of Cruzianaaegyptica ichnofossil, the Umm Ghaddah Formation canbe constrained to a late Ediacaran-early Cambrian age fol-lowing the International Stratigraphic Chart of 2004(Gradstein et al., 2004).

The unconformity surface between the Neoproterozoicbasement complex and the Cambrian sediments could beeither an erosional peneplain surface (Fig. 2c) or a pro-nounced paleorelief (Fig. 2b) resulting from graben andhorst structures produced by rifting through the exten-sional phase of the Pan African Orogeny that affected

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Fig. 1. Map of Jordan showing locations of the outcrop sections and wells of Umm Ghaddah Formation. 1 = Wadi Abu Khusheiba (WAK); 2 = WadiUmm Ghaddah (WUG); 3 = Wadi el Mahraka (WM); HM-2 = Hammar-2 well; AJ-1 = Ajlun-1 well; NH-1 = Northern Highlands-1 well; WS-3 = WadiSirhan-3 well; WG-2 = Wadi Ghadaf-2 well, F-1 = Fuluq-1 well; JF-1 = Al Jafer-1 well. Position of Dana, Safi and Timna, as well as the seismic profile Aand B passing through the JF-1 well is indicated.

B.S. Amireh et al. / Journal of Asian Earth Sciences 33 (2008) 194–218 195

Jordan and the entire north African and Middle Eastregion (Stern et al., 1984; Bentor, 1985; Husseini, 1989,2000; Jarrar et al., 1991, 1992). This unconformity surfaceseems to be not relevant to the late Neoproterozoic glacia-tion events that gave rise to the ‘‘Snowball Earth’’ andaffected regions located even at low latitudes, as will be dis-cussed below.

The tectono sedimentary evolution of the late Ediaca-ran-early Cambrian Umm Ghaddah Formation is

explained here, in the context of an extensional half-grabenmodel. Amireh and Abed (2000) gave no detailed faciesanalysis of the Umm Ghaddah Formation, nor did theydecipher the type, geometry and orientation of the invokedrift basins.

The present work analyses the facies of the Umm Ghad-dah Formation in order to determine the depositionalenvironment, and to compare the formation withpossible modern analogues. It also aims at revealing the

Fig. 2. (a) Photograph of Facies A2, Umm Ghaddah Formation showing the cobble-boulder-grade of the conglomerate composed mainly of rhyoliteclasts (rhyolite petromict conglomerate) (hammer 30 cm long). (b) Paleorelief expressed as an irregular line between the Ediacaran Aheimir Volcanic Suite(AVS) and the Umm Ghaddah Formation (UGF) in Wadi Abu Khusheiba (width of the field view is 500 m). The other line expresses the upper limit of theUGF. (c) Even erosional contact between the Ediacaran Sarmuj Conglomerate (SC) and the overlying Umm Ghaddah Formation (UGF) in Wadi elMahraka. (d) Outcrop of the Umm Ghaddah Formation (UGF) in the type locality of Wadi Umm Ghaddah, overlain by the bedded sandstone of theoverlying Saleb Arkosic Sandstone Formation (SAF), which in turn is overlain by Quaternary deposits (Qt). Note the conformable contact between bothformations (line below arrow) (for scale, see the central bush that has a height of about 1.4 m).

196 B.S. Amireh et al. / Journal of Asian Earth Sciences 33 (2008) 194–218

configuration and regional and tectonic evolution of theEdiacaran rift basins. Is this late Precambrian rift relatedto the present day Dead Sea transform fault as proposedby many authors (Bender, 1968, 1974, 1982; Weissbrodand Karcz, 1988) or not is a question attempted to beanswered by the present paper.

2. Methods and terminology

The Umm Ghaddah Formation has been studied in sevensections cropping out at the following three localities: WadiUmm Ghaddah (the type locality at 30�26.0 0N and35�22.36 0E), Wadi Abu Khusheiba (30�16.347 0Nand 35�18.375 0E) and Wadi el Mahraka (31�06.446 0N and35�31.689 0E) (Fig. 1). According to the limited number ofoutcrops, six wells have also been investigated, which are:Ajlun-1 well (AJ-1), Hammar-2 well (HM-2), NorthernHighlands-1 well (NH-1), Wadi Sirhan-3 well (WS-3), WadiGhadaf-2 well (WG-2) and Fuluq-1 well (F-1) (Fig. 1).

The term facies is used to convey grain size, texturalparameters including type of framework support, orienta-tion fabric, grading, roundness, sorting and type of strati-

fication, as well as sometimes the internal structures andgeometry of clastic bodies.

Scaled field photographs were used to determine thesorting, roundness and sphericity by visual estimationusing Powerss’ (1953) chart, and percent of matrix.

Grain size analysis (based on one phi interval) was con-ducted on the finer than coarse pebble fraction that consti-tutes the matrix between the framework clasts. The weightof sieved samples ranges from 1 to 1.5 kg. Atterberg cylin-ders were used to determine the clay fraction of these sam-ples. The grain size scale of Blair and McPherson (1999)modified from Udden-Wentworth scale is used, since it isuseful for the coarse gravels of alluvial fans.

3. Geologic setting and stratigraphy

The Precambrian–Cambrian boundary has beenrecently determined according to the appearance of thetrace fossil Phycodes pedum which is preserved as a seriesof branched, hypichnial ridges on the lower surface of asandstone bed (Brasier et al., 1994). Such trace fossil isnot identified neither in the sandstones of the Umm

Table 1Stratigraphic position of the Umm Ghaddah, underlying Sarmuj and overlying Saleb Arkosic Sandstone Formations and correlation with equivalents inadjacent countries


Ghaddah Formation (facies B) nor in the conformablyoverlying Saleb Arkosic Sandstone Formation. On theother hand, the reliable marker trace fossil identified indic-ative of the early Cambrian is C. aegyptica in the SalebArkosic Sandstone, located 100 m above the basementcomplex (Amireh et al., 1994). Thus it is very difficult todetermine the exact Precambrian–Cambrian boundary, isit at the contact between the two formations, or more prob-ably somewhere below this contact. Therefore, as statedabove, the Umm Ghaddah Formation is considered of lateEdiacaran-early Cambrian age.

This stratigraphic position of the Umm Ghaddah For-mation is portrayed in Table 1. The underlying Neoprote-rozoic crystalline basement, Aqaba Complex, consistsmainly of granitoids and sporadic occurrences of maficand ultramafic rocks that range in age from 800 to610 Ma (Lenz et al., 1972; Bender, 1974; Jarrar, 1985;

Rashdan, 1988; Jarrar et al., 1993), and locally of younger(Ediacaran) alkaline rhyolite lava flows (Aheimir VolcanicSuite, McCourt, 1988) and equivalent Feinan Granites(Jarrar et al., 1991). At specific localities, anchi-metamor-phic Sarmuj Conglomerate sequence and a slate-pyroclasticsuccession of the Hiyala Volcaniclastic Formation make upthe uppermost part of the Precambrian basement(McCourt, 1988; Jarrar et al., 1991). These igneous andmetamorphic rocks constitute the northernmost flank ofthe Arabian–Nubian Shield.

The oldest rocks in the basement complex of Jordan arehigh-to medium grade gneisses and metasediments (Jarrar,1985), which were intruded during the Pan African Orog-eny by voluminous calc-alkaline granitoids (Jarrar et al.,1991). The evolution of the Pan African Orogeny wasterminated by bimodal magmatic activity and post-oro-genic deposition of a rift-related magmato-sedimentary


intermontane succession (Jarrar et al., 1991). The Precam-brian was terminated by an isostatic uplift of the juvenilecrust. The uplift gave rise to an unconformity in form ofa profound erosional surface (peneplain), or a much lesssignificant irregular relief separating the Neoproterozoicbasement from the Cambrian sandstone (Bender, 1968;Abed, 1985; Ibrahem and Rashdan, 1988).

Although it is expected that a glaciation event couldhave occurred in the study area at the final uplifting stagesof the Pan African Orogeny that caused relief elevationwhich in turn may have triggered the Marinoan ice sheetas well as local mountain glaciers (Deynoux et al., 2006),no sort of any evidence of glaciation has been found nei-ther in the Umm Ghaddah Formation nor in the underly-ing or overlying sequences.

Therefore, this unconformity separating the Neoprote-rozoic basement from the Cambrian sedimentary sequencewas probably not related to the erosional phase accompa-nying the Neoproterozoic Marinoan/Varanger galciationevent. This glacial event, the so-called ‘‘Snowball Earth’’event, that extended to low latitudes (down to 15–24�)and terminated at 635 Ma (Allen, 2006), or any other youn-ger glaciation event like the Squantum-Gaskiers (dated582 Ma) did not affect the study area during the Ediacaranto early Cambrian because:

(1) No evidence of any sort of glacially related deposits,such as glacial diamictites, striated substrates, striated, fac-eted and shaped clasts, lacustrine varves with dropstones,post-glacial glacioeustatic transgressive sequence, or capcarbonates has been reported neither in the rocks underdiscussion, that is Umm Ghaddah Formation, nor in theunderlying Sramuj Conglomerate–Hiyala Volcaniclastics,or in the overlying Saleb Arkosic Sandstone.

(2) All these Precambrian glacial events are much olderthan the invoked unconformity separating the latest Neo-proterozoic basement from the early Cambrian deposits.

(3) The paleoposition of the Arabia was at low latitude(about 15� south) as can be seen in Fig. 10 which probablydid not allow sea level glaciation at the time of depositionof the Umm Ghaddah Formation. Although, as mentionedabove, the older Marinoan/Varanger Neoproterozoic gla-ciation event affected low latitude regions (15–24�), the factof having no single glacial evidence pertained to UmmGhaddah Formation, could lead to the conclusion thatsuch low latitudes were not influenced by other youngerglaciation events, if they exist, at the close up of the Neo-proterozoic Era.

From other point of view, Fig. 10 also shows the succes-sive positions of Arabia from 600 Ma till the Devonian. Inthe late Ordovician, the study area, as part of northwestArabia was located at about 55�S (Konert et al., 2001),where glaciation affected NW Arabia. The first and thirdauthors of the present paper were the first to record thislate Ordovician glaciation event in southern Jordan (Abedet al., 1993).

The subsequent Phanerozoic sedimentation took placeon the stable shelf of Gondwana that flanked the southern

(Baltic) margin of the Paleo-Tethys Ocean (Amireh et al.,1994). Sedimentation began with continental conditions,where braided rivers dominated through the Cambriandraining the Arabian–Nubian Shield and gave rise to theSaleb Arkosic Sandstone and Umm Ishrin Sandstone For-mations (Table 1). Jordan and the entire Middle Eastregion were influenced by a major transgression of thePaleo-Tethys seaway in the late-Early to early-MiddleCambrian (Amireh et al., 1994) that was responsible fordeposition of the regional Burj Limestone Formation(Table 1).

The Ordovician Period, similarly started by continentalbraided river sedimentation, but was gradually replaced bymarine conditions, that covered Jordan and the adjoiningcountries and persisted throughout the late Ordovicianand the subsequent Silurian Period (Bender, 1968; Khalil,1994; Droste, 1997; Amireh et al., 2001). No late Paleozoicdeposits have been found in Jordan, which were probablydeposited but later truncated by the ‘‘Hercynian’’ uncon-formity (Andrews, 1991; Konert et al., 2001).

4. Facies description and interpretation

The Umm Ghaddah Formation outcrops have beenstudied at the three localities mentioned above (Fig. 1).Due to the inaccessibility to the Wadi Umm Ghaddahand Wadi el Mahraka areas, only one exposure of theUmm Ghaddah Formation has been studied in each local-ity (Fig. 3). Five outcrop sections have been measured inWadi Abu Khusheiba (Fig. 4). The type locality of theUmm Ghaddah Formation is considered to be at WadiUmm Ghaddah (Amireh and Abed, 2000), where it attainsa maximum thickness of 60 m (Fig. 3). This outcrop formsa NE-extending belt that is located between the exposedAheimir rhyolites and the Upper Cretaceous and Tertiarycarbonates.

The analysis of lithofacies and architecture of the UmmGhaddah Formation at the outcrops has revealed alluvialfan and braided river facies associations. The former con-stitutes the entire Umm Ghaddah Formation at WadiUmm Ghaddah and Wadi Abu Khusheiba, whereas thebraided river facies association is restricted to the Wadiel Mahraka (Fig. 5).

4.1. Facies association A: alluvial fan

Facies association A consists of four facies made upmainly of reddish brown clast-supported conglomerate.These conglomerates are composed almost entirely of rhy-olitic rock fragments with granule to sandy matrix of thesame composition, thus can be called rhyolite petromictconglomerate. The facies association displays the principalcharacteristics of an alluvial fan deposit as defined by Bull(1972), Blair and McPherson (1994), Blair (1999a,b,c) andWent (2005).

This alluvial fan depositional environment is interpretedaccording to a thick sequence of a conglomerate having

Fig. 3. Log of the Umm Ghaddah Formation in Wadi Umm Ghaddah (type locality) illustrating the variations in grain size, sedimentary structures andfacies distribution, position of samples (UG1–4) and facies interpretation. Lower arrow indicates direction of pebble imbrication, whereas central andupper arrows indicate orientation of channel axis.



clasts ranging in size from pebbles up to fine blocks, absenceof horizontal bedding, absence of body or trace fossils, verypoor sorting, the occurrence close to the Aheimir volcanicsource rocks, the red color indicating the general oxidizingconditions prevailing in alluvial fans, and scarcity of sedi-mentary structures (Bull, 1972; Nilsen, 1985). In the fan site,adjacent to the Aheimir volcanic source rocks, the dominantdepositional processes appear to have been sediment gravityflow processes including rock fall, unconfined debris flowsand sheetfloods. A contemporaneous braided river systemdeveloped away from the fan site, at the center of a half-gra-ben basin, forming facies association B. Within faciesassociation A, there is a change in facies corresponding todown-fan transition from proximal to distal fan.

Fig. 4. Five logs of the Umm Ghaddah Formation in Wadi Abu Khusheiba (tsedimentary structures and facies distribution, sample position (Abk1–4) and

Facies A1 and A2 represent proximal alluvial fan facies,deposited by sediment-gravity flow processes includingmainly non-cohesive debris-flows (sub-aerial, high viscositydebris flow of Gloppen and Steel, 1981) of facies A2 andless common rock falls of facies A1. On the other hand,fluid gravity processes (stream flow) gave rise to mid to dis-tal alluvial fan sheetflood deposits of facies A3 and incised-channel flood deposits of facies A4.

Pebbles, cobbles, boulders and blocks were generated inupland drainage areas by collapse of bedrock cliffs and col-luvial slope failure in response to fracturing and weatheringunder the influence of gravity (Hadley, 1964; Harp andKeefer, 1989). These destabilized materials were then trans-ported downward from the drainage basin towards the

wo logs in (A) and three logs in (B)) illustrating the variation in grain size,facies interpretation.

Fig. 4 (continued)


alluvial fan site mainly by sediment-gravity flow processesand partly by fluid gravity processes of catastrophic stormsor intense thunderstorms (Middleton and Hampton, 1976;Blair and McPherson, 1994; Blair, 1999b). The absence ofstratification, and any structure or fabric indicating fluidflow processes in facies A1 and A2 suggest that these facieswere not deposited by fluid-gravity flows, but by sedimentgravity flow processes. On the other hand, stratificationand orientation of clasts suggest that water flow processes,particularly sheetflood flows were responsible on deposi-tion of facies A3 and A4 (Nilsen, 1985).

4.1.1. Facies A1

This facies consists of massive, clast-supported (lessthan 10% sandy matrix), very poorly sorted, sandy, coarsecobble to fine block conglomerate (Fig. 6a and c). Theclasts are randomly oriented, subrounded to rounded.This facies occurs in the lowermost 25 m interval of theUmm Ghaddah Formation in the type locality (Fig. 3)and the lowermost part at Wadi Abu Khusheiba (profileno. 1; Fig. 4A). It is intercalated with the other facies(Figs. 3 and 4A and B). No type of stratification or inter-nal sedimentary structures is visible in this facies (Figs. 2aand 6a). Clasts are up to 5.5 m in size (Fig. 6c) and themean size of the 10 maximum clasts within each unit var-ies from 70 to 150 cm (Figs. 3 and 4A and B). The clastsare invariably subrounded to rounded and vary in shapefrom discoidal to oblate.

Grain size analysis of the matrix [

Fig. 5. Log of the Umm Ghaddah Formation in Wadi el Mhraka, illustrating the variation in lithofacies (Sp, Sh and St), sedimentary structures andpaleocurrent directions of facies association B, and interbedded facies association A. The two arrows indicate plaeocurrent direction of lithofacies St,whereas the rose diagram at lower right represents the total Sp cross-bedding measurements. M.C.S. = maximum clast size in facies association A.


The presence of a sandy, slightly silty matrix in thisfacies can be attributed to subsequent infilling of originalinterclast pores with sand and silt percolated downward(cf. Nemec and Kazanci, 1999).

The unusually well rounded clasts may indicate strongabrasion action due to clast-to-clast collision during a rel-

atively long distance of transport through the fan catch-ment area to the fan site (Nilsen, 1985). An alternativeexplanation could be mechanical weathering, particularlyexfoliation in the rhyolitic source rocks as compared tosuch processes prevailing at the present time in severallocalities in Jordan.

Fig. 6. (a) Facies A1 at the lower part of the Umm Ghaddah Formation (UGF) in Wadi Abu Khusheiba (profile no. 4 in Fig. 4B). Note the unconformitysurface between the Umm Ghaddah Formation and the Ediacaran Aheimir volcanics (AV) indicated by the arrow. (b) Facies A1 intercalated within faciesA4 and facies A2 in Wadi Umm Ghaddah. (c) An oversized block of facies A1 floating in facies A2, Wadi Umm Ghaddah. Note the rhyolite banding thatappears as an erroneous sedimentary structure. (d) Well-developed imbrication fabric of cobble-boulder clasts of facies A3, Wadi Umm Ghaddah. Notewater bottle (15 cm long) for scale.

Table 2Grain size analysis of the finer than coarse pebble fraction

Feature UG1 UG2 UG3 UG4 ABK1 ABK2 ABK3 ABK4 Fac. B

Facies A2 A3 A2 A3 A2 A2 A3 A3 SpMode �2 to (�1) �1 to 0 �2 to (�1) �1 to 0 �1 to 0 �2 to (�1) �2 to (�1) �2 to (�1) 1 to 2Sort 1.8 1.47 1.78 1.78 1.55 1.62 1.72 1.32 1.71G–S–M 42–54–4 44–54–2 58–40–2 33–65–2 44–54–2 57–41–2 51–48–1 55–44–1 11–87–2GradeM pebble 1.2 2.8 13.8 3.6 8.6 13.7 10.2 0.6 0F pebble 9.6 14.9 19.8 18.6 8.5 19.1 18.6 18.6 1.9Granule 31.9 26.6 24.3 10.1 27.2 24.0 22.5 35.7 9.1Vc sand 23.6 28.9 21.5 29.5 27.5 19.1 19.6 21.7 17.8C sand 12.5 15.5 9.7 19.4 16.1 12.2 15.5 12.9 17.3M sand 8.7 5.5 4.4 9.1 6.5 5.6 8.3 6.1 27.4F sand 5.2 2.6 2.8 4.7 2.9 2.7 2.8 2.5 20.5Vf sand 3.8 1.4 1.8 2.5 1.3 1.4 1.2 1.0 3.8Silt 4.0 1.8 2.0 2.5 1.3 2.2 1.2 1.0 2.2Clay 0 0 0 0 0 0 0 0 0


4.1.2. Facies A2

This facies is clast-supported with variable content ofsandy matrix ranging from 10% to 25%. It is ungraded toinversely graded, and disorganized (Fig. 2a). Clasts are ran-domly oriented and rounded to well rounded, discoidal tooblate-shaped, very poorly sorted, sandy, pebbly, cobble to

fine boulder-grade (Fig. 2a). This facies is the most abun-dant one in the type locality (Fig. 3), and three profilesof Wadi Abu Khusheiba (Fig. 4A and B). Similar to theprevious facies, there are no internal sedimentary structuresor external stratification in the beds, but it is characterizedby having smaller-sized clasts and containing more sandy


matrix. A few channels have been found filled with slightlyoriented clasts (facies A3) (Figs. 3 and 4). Mean grain sizeof 10 maximum clasts of units of this facies, varies from 7to 30 cm (Figs. 3 and 4). The matrix consists of poorlysorted silt to medium pebbles (SI = 1.55–1.78) (samplesUG1, UG3, ABK1 and ABK2, Table 2). The granule, fineand medium pebbles account for 44–58% (40–54% sand,2% silt).

According to the coarse grain size (boulder to fineblocks), absence of stratification, rarity of sedimentarystructures, random orientation of clasts and the ratherpoor sorting, facies A2 is attributed as a cohesionless debrisflow deposit (Bull, 1972; Walker, 1975; Heward, 1978;Wells and Harvey, 1987; Blair, 1999c; Jarrett and Costa,1986; Blair, 1987a; Blair and McPherson, 1994). Non-cohe-sive sediment-gravity flow is generated by high-water dis-charge down a steep channel containing abundantsediments (Church and Desloges, 1984).

This type of flow has been recorded in the recent Roar-ing River alluvial fan of the Rock National Park, ColoradoMountain (Blair, 1987a) and in the present alluvial fans ofHowgill Fells, northwest England (Wells and Harvey,1987).

Debris flows are generally related to a thick weatheringcover or other metastable accumulation of debris, and tendto be triggered by heavy rainfall (Nemec and Kazanci,1999). Moreover, these debris flows were common in thepre-vegetation times, such as the Precambrian (Erikssonet al., 1999; Went, 2005).

Both facies A1 and A2 of rock fall and non-cohesivesediment gravity flow processes were most likely depositedin the proximal part of the alluvial fan (Hooke, 1967; Bull,1977; Gloppen and Steel, 1981; Nilsen, 1982). These pro-cesses are usually associated with each other as in manymodern alluvial and colluvial fans (Nemec and Kazanci,1999; Turner and Makhlouf, 2002). On the other hand,the finer, stratified and oriented and better sorted faciesA3 and A4 might represent mid to distal fan facies (Hooke,1967; Bull, 1977; Gloppen and Steel, 1981; Nilsen, 1982).

4.1.3. Facies A3

Facies A3 consists of clast-supported conglomerate witha sandy matrix varying from 10% to 30%. Beds areungraded, poorly sorted and consist of sandy, pebbly, fineto coarse cobble conglomerate (Fig. 7a). Clasts are paralleloriented, subrounded to rounded and discoidal to oblate.This facies occurs at 15 m above the base of the formationand in the upper 15 m in section Wadi Umm Ghaddah(Fig. 3), and constitutes a small proportion of profiles no.1 and 2, and most of profiles no. 3, 4 and 5 in Wadi AbuKhusheiba (Fig. 4). It is crudely stratified (Fig. 7b). InWadi Umm Ghaddah it shows clast imbrication, revealinga NNW dispersal direction (Figs. 3 and 6d and 7c). Thisfacies has sharp but non-erosional lower boundaries. Meansize of 10 maximum clasts is 7 cm (Figs. 3 and 4).

The matrix ranges from silt to medium pebble-grade(samples UG2, UG4, ABK3 and ABK4, Table 2) and is

poorly sorted (SI = 1.32–1.78). The granules, fine and med-ium pebbles account for 33–55% (44–65% sand, 1–25%silt).

The facies A3 was most likely formed by sheetfloods inthe mid to distal alluvial fan. This interpretation is sup-ported by the presence of stratified gravels and sands, theclasts parallel to bedding planes, the sheet-like geometry,the moderate sorting of clast-supported pebbles and cob-bles, and absence of cross-bedding (Gloppen and Steel,1981; Nilsen, 1985; Blair and McPherson, 1994; Nicholsand Hirst, 1998; Blair, 1999a,b; Nemec and Kazanci,1999; Went, 2005).

Sheetfloods are unconfined (unchannelized) water flowsmoving down a slope. When the non-cohesive sediment-gravity flow reached the mid to distal fan site, it becamecompletely unconfined and laterally expanded outwardforming a sheetflood (Blair, 1987a). Sheetfloods are surgesof sediment-laden water spread out from the end of thestream channel on a fan (Bull, 1972), or supercritical stand-ing waves of expanding sheetflood on the fan (Blair,1999b). Such supercritical (upper flow regime) flow condi-tions resulted from high fan slope, and high sediment andwater discharge (Blair, 1999b). Transportation of gravelin the sheetfloods occur generally as a bedload by tractiveforces under supercritical flow conditions (Blair, 1987a),whereas imbrication may indicate, particularly, rolling(Blair, 1987a).

The lower percentage of fine pebbles and the more sandcontent of facies A3 than facies A2 may indicate that thefacies A3 was more distal in the fan system than facies A2.

Therefore, debris flow processes dominating the proxi-mal fan were replaced by stream flow processes that dom-inate the distal fan (Smith, 1970; Wells and Harvey, 1987).

4.1.4. Facies A4

Facies A4 is a lenticular to channel-shaped conglomer-ate (Figs. 7d and 8a). It is clast-supported with 20–40%sandy matrix. Such high content of sandy matrix in clast-supported conglomerate is recorded by Blair (1987a). Thisfacies is largely ungraded, internally stratified, moderatelysorted and consists of sandy, granular to pebble-grade con-glomerate. The clasts are subrounded to rounded and dis-coidal, oblate to prolate in shape. Three units of thisfacies are encountered within the lower, middle and upperparts of the formation in Wadi Umm Ghaddah (Fig. 3);one unit in the central part of profile no. 4, and anotherunit in the upper part of profile no. 2 in Wadi Abu Khushe-iba (Fig. 4). The lenses or channels vary in width from 10 to18 m, and up to several decimeters in thickness (Fig. 8a).The bases of these lenses and channel fills are either convexdownward (Fig. 8a) or irregular, whereas their tops couldbe flat (Figs. 7d and 8a) or also irregular. Axes of theselenses or channels are dominantly NNW-ward.

There is a shale layer (Fm) (10–15 cm thick) in Sections2, 4 and 5 in Wadi Abu Khusheiba (Fig. 4), interbeddedwithin facies A2 and A3 (Fig. 7b). X-ray diffraction analy-sis of this shale shows that it consists of kaolinite and illite.

Fig. 7. (a) Facies A3 characterized by 30% sandy matrix, Wadi Umm Ghaddah. (b) Horizontal stratification of facies A3 with a shale layer (Fm, behindthe hammer) interbedded within facies A2 (below) and facies A3 (above), Wadi Abu Khusheiba. (c) Imbrication of cobble clasts of facies A3 indicating afan paleoflow towards the NNW (arrow), Wadi Umm Ghaddah. Note the pebble-size conglomerate of facies A3 below this imbricated unit. (d) Channelinfill of facies A4 truncating facies A2 and characterized by a downward convex base and a flat top. Direction of channel axis is towards the NNW, WadiUmm Ghaddah. The channel infill is overlain by facies A3.


The lower 25 m of the formation in Wadi Umm Ghad-dah is characterized by the large clasts of facies A1,whereas the upper 35 m exhibit a gradual decrease in clastsize, showing an overall fining upward tendency (Fig. 3).On the other hand, in Wadi Abu Khusheiba, five outcropsare characterized by several coarsening and fining upwardunits (Fig. 4).

The granule- to pebble-conglomerate and pebbly coarsesandstone channel fills and lenticles with their faint internalstratification and faint lamination may be interpreted asincised channel fills that represent an extension of thedrainage-basin feeder channel onto the fan (Blair andMcPherson, 1994), or a cut-and-fill structure by channelsentrenched only a short distance downslope from the fanapex (Bull, 1972). Another interpretation could be deposi-tion in incised shallow channels in the sheetflood depositsof distal fan during waning flood discharge or recessionalflood clear-water flows (Gloppen and Steel, 1981; Nilsen,1985; Blair, 1987a; Blair and McPherson, 1994; Nicholsand Hirst, 1998; Nemec and Kazanci, 1999). The channelfills would indicate NNW-ward runoff from the fan apexto the fan margin (Figs. 3 and 4).

The stratification characterizing this facies, visible dueto segregation of pebble-rich, granule-rich and sand-rich

horizons, similarly to facies A3, is attributed to upper flowregime deposition by a shallow supercritical flow (Blair,1987a).

The thin shale unit (Fm) interbedded within facies A3and A2 in Wadi Abu Khusheiba (Figs. 4 and 7b) mightbe interpreted as a mud deposit at the end of distal lobe,where mud was deposited in a small topographic depres-sion where ponding occurred (Bull, 1972; Blair, 1987a).Or, it can be interpreted as having formed during a periodwhen no deposition was occurring on the fan because ofreduced supply, allowing the axial fluvial sediments tobuild up over the fan (Gloppen and Steel, 1981; Nicholsand Hirst, 1998).

4.2. Facies association B: braided river

In Wadi el Mahraka, the Umm Ghaddah Formationunconformably overlies the Ediacaran Sarmuj Conglomer-ate (Fig. 2c) attaining a thickness of 30 m (Fig. 5). The for-mation consists of facies association B, but with fourintercalations of beds of facies A2 that are located withinits upper half (Fig. 8b). Facies association B shows threesandstone lithofacies: planar tabular cross-bedded (Sp;Fig. 8c), trough cross-bedded (St; Fig. 9), and horizontally

Fig. 8. (a) Channel infill of facies A4 characterized by a downward convex base and a flat top, and having an axis orientation towards the NNW, WadiAbu Khusheiba. The channel is incised in facies A2 and is overlain by a shale layer (Fm) that in turn is followed by faciesA3. (b) Horizontal layers of faciesassociation B that extends across the entire width of the outcrop, with interbedded units of facies A2 (arrows indicate two beds), Wadi el Mahraka. (c)Planar tabular cross-bedding (Sp) and horizontal stratification (Sh) of facies association B, Wadi el Mahraka. (d) Facie A2 overlying facies A4 andinterbedded with stratified sandy conglomerate of facies A3.


laminated sandstone (Sh; Fig. 8c). Sp is the major lithofa-cies having unit thicknesses of 20–90 cm. The Sp and St arecomposed of pebbly, medium-grained, poorly sorted(Table 2) sandstone, whereas the Sh is made of fine-grained, well sorted sandstone.

Grain size analysis of the dominant lithofacies, Sp,reveals fine pebble to silt-grade (Table 2), with a mode atmedium sand, and is poorly sorted (SI = 1.71). The finepebbles and granules account for 11%, sand 87% and silt2%. No clays have been detected. Microscopic analysisshows that the sand grains are subrounded to rounded,and of moderate to high sphericity.

The three lithofacies constitute sandstone sheets (down-stream accretion macroforms, DA, of Miall, 1996) of 1–5 m thickness that extend laterally across the entire widthof the outcrop (Fig. 8b). These sandstone sheets usuallystart with a planar to slightly irregular base that mighttruncate the underlying sheet. Some of these sheets havea basal gravel lag and may exhibit a fining upwardtendency.

The paleocurrent direction based on three-dimensionalplanar foreset azimuths and axes of the trough foresets is

NEE and NE, respectively (Fig. 5). Therefore, the dispersaldirection of the transporting medium, to be interpretedbelow as a braided river, was consistently towards theNEE during deposition of facies association B of theUmm Ghaddah Formation (Fig. 5).

This NEE to NE fluvial dispersal deviates from theregional northward fluvial paleoflow direction prevailingin the Paleozoic as well as in the Mesozoic sequencesin Jordan, where detritus were shed persistently fromthe south-located Precambrian basement of the ArabianShield to the north-located Paleo- or Neo-Tethys,respectively (Amireh et al., 1994, 2001; Amireh, 1997,1999; Amireh and Abed, 1999). This deviation can beattributed to the restriction of braided river systemwithin the NE confined rift basins of the Umm Ghad-dah Formation in comparison with extensive widebraidplains flowing northward in the Paleozoic andMesozoic Eras.

From other point of view, the fluvial dispersal in theCambrian Amudei Shelomo Sandstone Formation ofTimna region located westward of study area (Fig. 1) isrecorded by Karcz et al. (1971) to be westward. The

Fig. 9. (a) Two sets of trough cross-bedding (St) with interbedded thin unit of Sh, Wadi el Mahraka. (b) Small-scale coarsening upward unit (60 cm thick),Wadi Abu Khusheiba. (c) Sandy matrix of facies A2 which is interbedded within facies association B in Wadi el Mahraka. (d) A relict of the AbuKhusheiba sub-basin filled by facies association A2 of Umm Ghaddah Formation (UGF) within Aheimir Volcanic Suite (AVS). The line delineates theboundary between UGF and AVS.


deviation between this paleoflow and that of Umm Ghad-dah Formation can be interpreted as follows. The AmudeiShelomo Sandstone Formation is located below the mar-ine Lower Cambrian Hakhlil Formation that is equivalentto the Burj Formation in study area. Therefore, the Amu-dei Shelomo Sandstone Formation is equivalent to theSaleb Arkosic Formation. It is recorded by Amirehet al. (1994) the latter is exposed in the Dana–Safi areasouth of the Dead Sea (Fig. 1) and characterized by arather unusual high thickness and a northwestward paleo-flow. They attributed these characteristics to a synsedi-mentary subsidence that caused divergence of theprograding braidplain from the usual northward flow toa northwestern direction. Probably, this divergence ofpaleoflow continued westward until reaching Timnaregion, where braided streams were flowing to the west(Karcz et al., 1971). Timna region was probably locatedimmediate to Dana–Safi area in the early Cambrian tak-ing into consideration the offset of study area relative toTimna region due to Miocene-present day sinistral lateraldislocation of the Arabian plate along the Dead Seatransform fault. Another explanation can be proposedbased on Bender (1974, 1982) assumption that the south-ern part of Wadi Arab was elevated eastern of the pro-posed Ediacaran rift more than western of this rift, thus

detritus were shed from the elevated Precambrian base-ment westward towards Timna region.

Interbedded facies A2 (0.2–1.5 m thick) consists of cob-ble to fine boulder rhyolitic clasts in sandy matrix (Fig. 9c)of rhyolitic arkosic arenite type (Amireh and Abed, 2000).The mean size of the maximum 10 clasts ranges from 8 to12 cm (Fig. 5).

Facies association B is interpreted as sand deposits of aproximal braided river flowing normal to the marginal allu-vial fan bodies, as will be discussed in the section of tectonosedimentary evolution.

A fluvial depositional environment is interpretedbecause of the absence of body fossils and horizontalbedding, whereas the dominance of sandstone sheetsexhibiting a unidirectional paleocurrent flow (Fig. 5)and an erosional base, and the absence of silt or mud-stone deposits may indicate the braided rather than themeandering nature of the river (Rust and Jones, 1987;Brown and Plint, 1994). This interpretation of a braidedriver in a continental setting, inferred from facies associ-ation A, is consistent with the absence of meandering riv-ers in the Precambrian pre-vegetational time. This isbecause such vegetations would have otherwise promotedstabilization and trapping of sediments required formeandering rivers (Galloway and Hobday, 1983;

Fig. 10. Successive paleopositions of Arabia between the Ediacaran andthe Devonian Periods after Konert et al. (2001). Note that Arabia reacheda southerly position responsible for glaciation in the late Ordovician.

Fig. 11. Tectonic development of Arabia during the Ediacaran Period after Hresponsible for the following NW-trending Najd Fault System, and the NE-tr


McCormick and Grotzinger, 1993; Tirsgaard and Øxne-vad, 1998). Generally, in the Precambrian time, sandyto pebbly braidplain deposits were associated with allu-vial fan deposits (Eriksson et al., 1999).

The major lithofacies Sp was produced by downstreammigration of fields of straight-crested bedforms (sand-waves, transverse and linguoid bars) under lower flowregime conditions (Smith, 1970; Miall, 1992). Moreover,these cross-bed sets may indicate the presence of largemacroforms or ‘sand flats’, that were formed in channelsup to 8.5 m deep (Eriksson et al., 1999). The less com-mon lithofacies St represents dunes migrating along thedeeper part of the channel under the upper part of thelower flow regime. The laminated sandstone beds Sh weredeposited at the bar tops during sheetfloods under upperflow regime conditions (Miall, 1996). The sandstonesheets were constructed by channel aggradation of fieldsof sandwaves, transverse and longitudinal bars and largedunes.

Upon comparing the sheet architecture and the internalstratification features of the sandstone bodies of faciesassociation B with the modern fluvial styles, both the PlatteRiver, Nebraska (Crowley, 1983) and the South Saskatch-ewan River, Canada (Walker and Cant, 1984) could beanalogous. This conclusion of the Platte type of the depos-iting river has been also recorded in similar early Protero-zoic deposits consisting of St with interbedded Shlithofacies by Sweet (1988).

usseini (2000). Fracture zones formed during the Amar Collision (620 Ma)ending Oman, Dibba and Jordan Valley rifts.


5. Facies distribution in Jordan and tentative regional

correlation

The facies associations A and B of the Umm GhaddahFormation are exposed only in three localities in central Jor-dan, which are Wadi Abu Khusheiba, Wadi Umm Ghaddahand Wadi el Mahraka (Fig. 1). A petrographic analysis ofsubsurface samples of four wells (Ajlun-1 well, Hammar-2well, Northern Highlands-1 well and Wadi Sirhan-3 well;Fig. 1) also reveals the two facies associations (Amirehand Abed, 2000). It is apparent that the Umm GhaddahFormation occurs in certain areas within or close to theexposed Ediacaran Aheimir rhyolitic extrusion, and as farnorth away as in the wells Ajlun-1 and Northern High-lands-1, and Wadi Sirhan-3 eastwards (Fig. 1). These threewells are also located farther eastern away from the presentday Dead Sea transform fault. The underlying Sarmuj Con-glomerate is similar in origin to the Umm Ghaddah Forma-tion, and can be tentatively correlated with equivalentdeposits in adjoining countries (Table 1 and Fig. 13).

It is to be emphasized here that the following correlationattempts of the Sarmuj Conglomerate and the Umm Ghad-dah Formation are tentative ones since they concern verydistant deposits, although they depend on striking faciessimilarities, but the deposits could be indirectly dated bydifferent techniques of radiometric dating having differentuncertainties, thus the deposits may be heterochronousand the correlation could be questionable. A similar casehas been recently reported by Deynoux et al. (2006) forNeoproterozoic to Cambrian glacial deposits in forelandbasins of West Africa.

The Sarmuj Conglomerate can be correlated with theElat conglomerate in the Negev region, and the Hamma-mat Formation further westward in the Eastern Desert ofEgypt (Table 1 and Fig. 13). Both are composed of a vol-cano conglomerate sequence with interbedded coarse andfine clastics (Willis et al., 1988). Age of the Sarmuj Con-glomerate has been constrained by Jarrar et al. (1991)between 595 and 600 Ma, which is exactly the same radio-metric age of the Hammamat Formation (Willis et al.,1988) (Fig. 13).

The Sarmuj Conglomerate can be correlated with theDerik volcaniclastic Formation in SE Turkey (Cater andTunbridge, 1992) (Fig. 13) and the Shammar Group inthe south (NW Saudi Arabia) (Fig. 13), which consists ofa volcanic series with interbedded coarse clastics (Al-Shan-ti, 1993).

In South Yemen and Oman, the Sarmuj equivalentdeposits of Ghabar and Huqf Groups, respectively, startwith a coarse clastic sequence but are overlain by carbonateand evaporite deposits (AlSharhan and Nairn, 1997; Dros-te, 1997; Table 1 and Fig. 13). The coeval sequence of theHormuz Complex consists entirely of evaporite sedimentsin South Iraq and Iran (AlSharhan and Nairn, 1997;Fig. 13).

Regarding the Umm Ghaddah Formation, it is notexposed in the Negev region, but is penetrated in many

subsurface wells, where it is assigned to the Zenifim Forma-tion of unnamed late Ediacaran Group (Weissbrod, 1969;Fig. 13).

In southeastern Sinai, the Taba Formation of late Edi-acaran age (El Kelany and Said, 1989) is equivalent tothe Umm Ghaddah Formation. In NE Desert of Egypt,no equivalent clastic deposits are recorded, but the volcanicflow of the Post-Hammamat Felsite is the time equivalentrock (Willis et al., 1988; Fig. 13).

The Umm Ghaddah Formation is correlatable with thelate Ediacaran-early Cambrian Jibalah Group in north-western Saudi Arabia (Fig. 13). This group is subdividedinto three formations (Table 1). Husseini (1989, 2000) con-sidered the group to be an extensional intracontinental riftbasin filling the Najd wrench rift. Both the conglomerateand the sandstone of the lower Rubtayn Formation ofthe Jibalah Group are well correlated with facies associa-tion A and facies association B of the Umm Ghaddah For-mation, respectively.

Further southeast, it seems that there is no clasticdeposit coeval to the Umm Ghaddah Formation, wherethe late Ediacaran-early Cambrian sequence is entirelycomposed of carbonates and evaporites, i.e., the HarmutFormation of the Ghabar Group of South Yemen andthe Bauah and Ara Formations of the Huqf Region ofOman (AlSharhan and Nairn, 1997; Husseini, 2000;Fig. 13). In southern Iraq and Iran the Ediacaran-earlyCambrian sequence is similarly constituted of an evaporiteand dolomitic sequence of the Hormuz Complex (AlShar-han and Nairn, 1997; Husseini, 2000; Fig. 13).

In NW Syria, no Ediacaran deposits are known, wherethe deepest formation penetrated is the Lower CambrianZabuk Formation in the Khanaser-1 well, located north-eastern Syria (Lababidi and Hamdan, 1985). In SE Turkey,however, the Sadan Formation (Fig. 13) conglomeratic redbeds considered as intracontinental rift sequence by Caterand Tunbridge (1992) might be equivalent to the UmmGhaddah Formation.

It is apparent from the surface and subsurface distribu-tion of the Umm Ghaddah Formation in Jordan (Fig. 12;Amireh and Abed, 2000) and the tentative correlationattempts with those in the adjacent countries (Table 1and Fig. 13) that the formation neither occurs everywhereabove the Ediacaran crystalline basement in Jordan nor itis restricted to the present day Wadi Araba-Dead Sea Rift.The same pattern of distribution is applied on the underly-ing Sarmuj Conglomerate.

6. Tectonic development

The study area is located at the northern flank of theArabian Shield within the Arabia (Fig. 11). This shieldwas formed by a series of island arc complexes and micro-continent collisions and accretions during the time intervalof about 900–750 Ma (Stoeser and Camp, 1985). At thistime, it was separated from the African fragments ofGondwana by the Mozambique Ocean (Hoffman, 1999).

Fig. 12. Paleogeographic–paleotectonic map showing the position of late Ediacaran-early Cambrian Wadi Abu Khusheiba–Wadi Umm Ghaddah half-graben basin and the Wadi el Mahraka half-graben basin, the Wadi Abu Khusheiba and Wadi Umm Ghaddah alluvia fans, the Ediacaran AheimirVolcanic Suite swell and the Sarmuj Conglomerate, and location of the present Dead Sea Transform fault. The line starting from A represents part of theseismic line passing through the Al Jafar-1 well which is located eastward of the margin of the figure.


Then it collided with these fragments of Gondwana at theperiod 750–650 Ma (Abdelsalam and Stern, 1996), closingthis ocean and becoming part of the northern part of theEast African Orogen (Hoffman, 1999). This tectonic activ-ity is known as the Pan African Orogeny (Kröner, 1984).

Subsequently, the western margin of the Arabia (Afif ter-rane) witnessed a collision with the Rayn micro-plate (cor-responding to central and eastern Arabia) giving rise to theN-trending ‘‘Amar Suture’’ between 640 and 620 Ma (Hus-seini, 2000). This Amar collision created an EW-directed

Fig. 13. Regional map illustrating the occurrences of the equivalent deposits to the Umm Ghaddah Formation and the Sarmuj Conglomerate in some ofthe Middle East countries.


compression that may have produced NW-trending frac-ture zone responsible for the following Najd Fault System,as well as the NE-trending fracture zone responsible forOman, Dibba and Jordan Valley rifts (Fig. 11).

The above early cratonic phase of accretionary andcollisional tectonic processes ceased in the Arabian Shieldat about 620 Ma. It was replaced by an extensional col-lapse phase accompanied by deposition in syn-rift basinsduring the time interval between 620 and 530 Ma(Schmidt et al., 1979; Fleck et al., 1980; Stern, 1985; Hus-seini, 1989, 2000; Abdelsalam and Stern, 1996). Thisextensional phase was dominated by intrusions of post-orogenic alkali rich granites and rhyolites (A-type granitesderived from the mantle). The extensional collapse of theArabian Shield culminated between 570 and 530 Ma inthe development of regionally extensive Najd Fault Sys-tem and its complimentary syn-rift, down block-faulted,extensional basins that make up the Najd Rift System(Husseini, 2000).

Husseini (2000) defined the Najd Rift System to consistof the Najd Fault System of the Arabian Shield (Fig. 11),the Oman, Punjab and Dibba Rifts, the Zagros Faultand the Sinai Triple Junction that consists of the branches:NW-trending Najd Fault System, the EW-trending Egyptrift, and the NE-trending Jordan Valley Rift that continuesto the Derik rift (Fig. 11). Fig. 11 shows the fracture zonesformed during the Amar Collision prior to the initiation ofthe Najd strike-slip system.

The Najd Fault System is about 1200 km long and 300km wide. Stern et al. (1984), Abed (1985) and Husseini(1989, 2000) suggested a connection between the develop-ment of Ediacaran syn-rift, extensional sedimentary basinsand the Najd Fault System. These basins have a trend inagreement with that of the Najd Fault System, and areassociated with abundant volcanics and dikes of the sametrend. For example, in Sinai and NE Egypt, basins anddike swarms (595–540 Ma) trend NE–SW and E–W (Sternet al., 1984; Stern, 1985). Also, the Shammar volcanics in


Saudi Arabia and the Dokhan volcanics in NE Egypt arecontemporaneous with the Najd Fault System (Sternet al., 1984).

In the study area, numerous NE trending dikes dissectthe Sarmuj Conglomerate, and the overlying Hiyala Vol-caniclastics (Jarrar et al., 1991), which are almost coevalwith the Najd Fault System. These dikes probably indi-cate the existence of the NE branch of the Sinai triplejunction in Jordan associated with syn-rift extensionalbasins. A seismic profile located south of Wadi AbuKhusheiba (in Al Jafer area, Fig. 1) (Jordan Hunt OilCompany, 1989) reveals the presence of a series ofasymmetrical half-grabens bounded by listric faults inthe Ediacaran and early Cambrian magmato-sedimentarysequence (Fig. 14).

The Ediacaran rifting is indicated in the study area bythe following evidence: (1) presence of a series of half-gra-bens in the Ediacaran and early Cambrian volcano-sedi-mentary succession revealed by seismic data; (2) theabundance of NE–SW trending sub-parallel bimodal dikeswarms corresponding to the post-orogenic extensionalphase of the Pan African Orogeny (Jarrar, 1992; Jarraret al., 1991; Beyth and Heimann, 1999); (3) the chemicalfeatures of the syn-rift volcanic sequences (alkali-rhyolitelava flows) similar to rift-related granites and rhyolitesincluding high abundance of silica, total alkalies, REE,low abundance of MgO, CaO, Eu and transitional ele-ments, the chondrite normalized trace element pattern,and the Nb vs. Y, Rb vs. Nb + Y and R1–R2 diagrams(Jarrar, 1992; Jarrar et al., 1993); (4) the chemical bimodal-ity of alkaline to peralkaline character of the extrusive igne-ous rocks and dykes characteristic of rifting (Stern et al.,1984; Willis et al., 1988; Jarrar et al., 1992, 1993); and (5)the presence of a volcano-sedimentary succession filling

Fig. 14. Seismic profile (A and B) in the southern part of the study area (Al Jahalf-grabens bounded by listric faults. For the location of profile (A and B), s

intracontinental or intermontane rift-related basins andsub-basins in Jordan and adjacent countries dated at600–550 Ma (Jarrar et al., 1991, 1993; Husseini, 1989,2000). The rift-related volcano-sedimentary succession con-sists of the Sarmuj Conglomerate, the conformable overly-ing Hiyala Volcaniclastics, the Aheimir Volcanics, and theclastic Umm Ghaddah Formation. This succession wasdeposited NE–SW-trending basins, as discussed below thatwere developed in Jordan, in response to the Najd FaultSystem (Fig. 12). This suggests the presence of a rift systemextending NE–SW in the study area in the Ediacaran Per-iod (Fig. 11).

7. Tectono sedimentary evolution

The late Ediacaran-early Cambrian Umm GhaddahFormation is interpreted as part of the volcano-sedimen-tary succession that represents the extensional phase ofthe Pan African Orogeny. It was deposited in intraconti-nental rift basins, i.e., half-grabens bounded by listric faults(Fig. 15) rather than the symmetrical, full-grabens,bounded by normal faults with a down-faulted valley floor(Illies, 1981). This interpretation is based on the subsurfacegeophysical data and comparison with modern analogousof ancient half-graben continental rifts, such as those inthe East African System (Gibbs, 1984; Rosendahl et al.,1986; Frostick and Reid, 1987).

Half-graben basins are generally characterized by: (1) alarge size and great depth (Neugebauer, 1983), and uncon-formable contact with underlying basement complexes; (2)in map view, alternating asymmetrical scoop-shaped seg-ments, each constituting a sub-basin and separated by bed-rock ridges called crossover or accommodation zones(Gibbs, 1984; Rosendahl et al., 1986; Frostick and Reid,

fer area) carried out by JOHC (1989) indicating the presence of a series ofee Figs. 1 and 12.

Fig. 15. Tectono sedimentary model for the late Ediacaran-early Cambrian Umm Ghaddah Formation based on those of Leeder and Gawthorpe (1987)and Frostick and Reid (1987).


1987; Dunkelman et al., 1988); (3) a long history of verticalsubsidence of the basin floor and uplift and non-depositionof the shoulders (DeRito et al., 1983); (4) a structural pat-tern as a combination of major listric bounding faults thatform the footwall uplands, and sole in low angle deep crus-tal detachments, and a roll-over zone at the opposite sideconstituting the hangingwall lowlands which is usually dis-rupted by smaller antithetic and synthetic faults that leadto development of small-scale horsts and grabens (Gibbs,1984; Rosendahl et al., 1986); (5) main boundary listricfaults that tend to be discontinuous and curved in plan,and reverse in polarity along rift valleys, and are separatedby transfer faults (Leeder, 1995); (6) vertical stacking ofbasinward stratigraphical units corresponding to majortectonic phases (Mckenzie, 1978) that consist of syn-riftsequence consisting of bimodal volcanics, coarse subaerialalluvial fan sediments, subaqueous lacustrine and/or sub-aerial fluvial deposits, and an overlying (post-rift) sequenceof thermal subsidence phase.

The precise configuration of the basins in which theUmm Ghaddah Formation was formed is difficult to deter-mine as there poorly exposed outcrops (Fig. 9d), and onlyone subsurface geophysical section is available. However, itis very likely that the basinal axis trends NE–SW (Figs. 12and 15), for the following criteria:

(1) A reliable paleoflow data obtained from the azimuthsof planar tabular cross-bedding in the axial rift fluvialsystem (facies association B) in Wadi el Mahraka

(Figs. 5 and 12) reveals a NE paleoflow that is paral-lel to the basin margin (Fig. 15; Leeder and Gaw-thorpe, 1987).

(2) The NE–SW trend of the exposed Aheimir Suite vol-canic belt (Fig. 12) that shed detritus to the UmmGhaddah Formation, and is interpreted as a syn-riftvolcanic sequence (Jarrar, 1992; Jarrar et al., 1992).

(3) The NE–SW distribution of the outcrop belt of faciesassociation A in Wadi Umm Ghaddah and WadiAbu Khusheiba. This orientation of the outcrops sug-gests the presence of a sub-basin that trends NE–SW-ward in Wadi Umm Ghaddah and Wadi AbuKhusheiba for 1 and 3 km, respectively (Fig. 12).

(4) The NE–SW trend of dikes cutting the Sarmuj Con-glomerate that directly underlie the Umm GhaddahFormation and has the same rift origin (Jarraret al., 1991).

(5) This NE–SW rifting direction is in full agreementwith the normal NW–SE regional crustal extensionthat affected the northwestern part of the ArabianShield (Abdelsalam and Stern, 1996; Husseini, 1989,2000).

A tectono sedimentary model for the half-grabens of theUmm Ghaddah Formation has been constructed (Fig. 15)based upon those of Leeder and Gawthorpe (1987) andFrostick and Reid (1987). Small-scale alluvial fans of ahigh-slope were developed at the margin of the mainbounding listric fault (footwall) transverse to the rift axis,


whereas larger, transverse alluvial fans of a lower-slopeformed over the roll-over zone of the hangingwall (Hooke,1972; Leeder and Gawthorpe, 1987). Facies association Awas deposited in these two sets of alluvial fans. The areabetween these transverse alluvial fans was occupied by anaxial braided river system flowing parallel to the strike ofthe rift and its bounding faults (Fig. 15) that gave rise tofacies association B. Similar thick alluvial fan and fluvialfacies associations forming rift-basin successions have beenrecorded by Hoffman (1991) for the Late Precambrian Belt-Purcel Supergroups of western North America, and byNyambe (1999) for the Ordovician-Jurassic Sinakumbe-Karoo basin, in the present mid-Zambezi Valley Basin,southern Zambia.

Sediments were transferred to these alluvial fans acrossthe fault scarps through transverse drainage systems evolv-ing on the footwall uplands and the hangingwall lowlands(e.g., Leeder and Gawthorpe, 1987; Frostick and Reid,1987; Leeder, 1995), and to the central part of the basinthrough the axial sediment flux (e.g., Blair, 1987b; Leeder,1995). Although the sediment flux into the central rift basinwas mainly parallel to the strike of the rift and its boundingfault, the ultimate source of this axial flow was the sum ofthe transverse components (mass wasting and fluvial ero-sion) gathered from the footwall and hangingwall uplandsource rocks (cf. Leeder, 1995).

The entire succession of the Umm Ghaddah Formationin Wadi Umm Ghaddah (facies association A, Fig. 3) rep-resents the transverse alluvial fan deposits accumulating atthe footwall margin of the listric half-graben basin(Fig. 15). This interpretation is based on the presence ofrock fall deposits (facies A1). On the other hand, profilesno. 1, 2 and 3 in Wadi Abu Khusheiba are also interpretedas deposits of transverse footwall alluvial fans of anothersegment or sub-basin of the half-graben rift zone(Fig. 15). Stratified, well-oriented pebbles and cobbles(profiles no. 4 and 5) may represent hangingwall alluvialfans located at the southwestern part of the Abu Khushei-ba sub-basin (Fig. 15). The stratified deposits alternativelyrepresent transverse footwall fans deposited in the distalfan by sheetflood processes.

A supply of coarse fraction was sufficient to accumulatefacies association A of a small steep alluvial fan at WadiUmm Ghaddah (Fig. 15) because numerous fault offsetsand transfer zones in the footwall may have caused a largerthan average alluvial fan system (cf. Crossley, 1984; Leederand Gawthorpe, 1987; Gawthorpe et al., 1990).

The adjacent Umm Ghaddah and Abu Khusheiba sub-basins (Fig. 12) may have become linked along a structuralstrike when the volcanic Aheimir crossover highs werebreached, enabling discharge to become axial along anNE–SW fault strike (cf. Leeder and Jackson, 1993; Jacksonand Leeder, 1994) as the case of the recent Rio Grand riftof Colorado and New Mexico (Kelley, 1979; Ingersollet al., 1990), and the Humbold River in northern Nevada(Blair, 1987b). The position of this axial stream was con-trolled by the encroaching transverse fans, periodic tilting,

emplacement of landslide debris flows and possible intra-basinal horst structures (Leeder, 1995).

Facies A1 was deposited adjacent to the fault scarp inWadi Umm Ghaddah and Wadi Abu Khusheiba. In WadiAbu Khusheiba, grain size of clasts decreases from profile 1to 5 (Fig. 4), suggesting that the alluvial fan progradedsouthward. Moreover, the imbrication of clasts in faciesA3 and channel orientation suggest westward flow fromthe main bounding fault of the half-graben sub-basin(Fig. 15). Thus, the alluvial fan extended northwestwardnormal to the fault scarp and encroached the NE axial flu-vial system located somewhere between the two profiles.This NE direction of the fluvial system is indicted fromthe paleocurrent data obtained from facies association Bin Wadi el Mahraka (Fig. 5).

Therefore, the Wadi Umm Ghaddah, Wadi Abu Khu-sheiba and Wadi el Mahraka half-graben sub-basins inwhich the Umm Ghaddah Formation was deposited hada NE–SW orientation (Figs. 12 and 15). A similar marginalalluvial fan and axial river system filling a half-grabenbasin is recorded by Blair (1987b) for the Jurassic-LowerCretaceous Todos Santos Formation, Chiapas, Mexicoand the Devonian Munster basin, Ireland (MacCarthy,1990).

Sedimentation in extensional basins generally is con-trolled by interplay of: sediment input, faulting and blockrotation that creates tectonic slopes and topography, subsi-dence and tectonic changes. Furthermore, the study ofmodern alluvial fans reveals that sedimentation is con-trolled by both allocyclic variables, such as climate and tec-tonism, and autocyclic variables, such as channel diversionand bar migration that are related to the fan depositionalsystem (Hooke, 1972; Schumm, 1977). These variables willbe employed to interpret the cyclic sedimentation throughthe Umm Ghaddah Formation.

The Umm Ghaddah basinal sequence is characterizedby several minor coarsening and fining upward sequences,but there is an overall or a large-scale fining upward ten-dency (Figs. 3 and 4a and b). Such cyclicity may reflect afault-controlled tectonic movement through the time ofthe Umm Ghaddah Formation.

Some of the coarsening upward sequences result fromthe occurrence of proximal debris flow deposits above dis-tal sheetflood deposits as the upper part of section no. 1(Fig. 9b), central part of section no. 2, the entire profileno. 3 in Wadi Abu Khusheiba (Fig. 4a and b). Other coars-ening upward sequences are observed within the sheetflooddeposits as the case in the central and uppermost portion ofprofile no. 5 in Wadi Abu Khusheiba (Fig. 4a and b).

These coarsening upward sequences could be attributedto fan progradation (Ramos et al., 1986), and to tectonicuplift of the adjacent basement (Heward, 1978; Gloppenand Steel, 1981; Ramos et al., 1986) as the case in mostalluvial fan sequences (Blair, 1987b).

The fining upward trend is observed within the lowerpart of Wadi Umm Ghaddah, where it displays three ofsuch sequences (Fig. 3), and the upper portion also exhibits


another sequence. The central part of the Umm GhaddahFormation in Wadi Abu Khusheiba is characterized by afining upward trend in all the studied five sections(Fig. 4a and b). This fining upward cyclicity resulted froma vertical change in fan subenvironments from proximaldebris flow (facies A2) to distal sheetfloods (facies A3, 4a,b; Fig. 15; Heward, 1978), and due to gradual abandon-ment or retreat of fan lobes (Gloppen and Steel, 1981) cou-pled with tectonic quiescent intervals (lower fining upwardcycle at profile 2 in Wadi Abu Khusheiba).

The overall (large-scale) fining upward tendency of theUmm Ghaddah Formation in the type locality (Fig. 3)could be attributed to the gradual cessation of the tectonicuplift associated with an increase in active fault subsidence,and consequent transgression of fine-grained sediments tofill the basin (cf. Nyambe, 1999). Moreover, this overall fin-ing upward tendency reflects the fact that the source arearelief became subdued (Howard, 1966) following the highrelief of the source terrain at the beginning of the UmmGhaddah time. This high relief is indicated by the thicknessand coarse grain size of the terrigenous detritus of faciesassociation A (cf. Mack and Rasmussen, 1984).

Only in the Wadi el Mahraka outcrop, and in the sub-surface AJ-1 well (Amireh and Abed, 2000), the basin-mar-gin alluvial fan (facies A2) and the basin-axis fluvialdeposits (facies association B) are found vertically stacked(Fig. 8b), but without showing any trend of cyclicity. Thisstacking may indicate that periodically the basin-axisbraided river occupied the basin-margin and that at othertimes alluvial fan deposition took place there (Blair,1987b). During tectonic subsidence, fluvial system was ini-tiated at basin margin due to creation of a topographicdepression, rather than to a phase of tectonic quiescence(Blair, 1987b). Subsequently and aided by the lower rateof tectonic subsidence, the fan prograded and displacedthe fluvial environment basinward (Blair, 1987b). In thisdepression the sand flats of the Umm Ghaddah braidedriver aggraded vertically to give rise to the high thicknessof facies association B, that attained 900 m in NH-1 well(Amireh and Abed, 2000). This might also reflect a highrate of erosion in the uplifting hinterlands and a high axialsediment influx (Leeder, 1995).

8. Palaeogeography

Facies associations A and B of the Umm Ghaddah For-mation were deposited under terrestrial conditions in sev-eral isolated half-graben sub-basins (Figs. 12 and 15). Inthe Ediacaran-early Cambrian, terrestrial conditions dom-inated over the study area, central and northwest of theArabian Shield, whereas marine conditions affected thenortheastern and southeastern portions of Arabia, whichwere inundated from the Asiatic side of the Paleo-TethysOcean (Fig. 11; Husseini, 1989; AlSharhan and Nairn,1997).

Fig. 12 is a paleogeographic/paleotectonic map showingthe distribution, arrangement of the half-graben sub-basins

including the Wadi Umm Ghaddah–Abu Khusheibasub-basin and Wadi el Maharaka sub-basin, the AheimirVolcanic Suite swell, a series of half-grabens revealed byseismic study in the Al Jafer area that are filled withUmm Ghaddah and other Ediacaran clastics and alkalinevolcanics, and the present day Dead Sea transform fault.Since these volcano-clastic successions, besides the underly-ing sequence of the Sarmuj Conglomerate–Hiyala Volca-niclastics and the Aheimir Volcanics represent syn-riftdeposits, as interpreted above and as recorded by Jarraret al. (1991, 2003), respectively, it can be concluded thata rift system trending NE–SW existed in the study areain the Ediacaran Period. According to the available surfaceand subsurface data, it is not possible to determine thewidth of this rift system. This inferred rift could be incontinuation with the Derik rift in SE Turkey recordedby Husseini (2000), and supports the presence of the Sinaitriple junction proposed by the latter.

It is obvious from Fig. 12 that the direction of the Edi-acaran half-graben basins and consequently the Ediacaranrift is oblique to that of the present Dead Sea transformfault. Moreover, the sub-basins in the Al Jafer area arefar away from the location of the present day Dead Seafault. The same is applied to the subsurface basins of WadiSirhan (WS-3 well), Ajlun (AJ-1 well) and Northern High-lands (NH-1 well) that are also remote from the Dead Seatransform fault (Fig. 1).

Therefore, the Ediacaran rift system differ in trend fromthe present Dead Sea transform fault and extends to areasmuch eastern of it, suggesting that this late Precambrianrifting phase could not be related to the present day WadiAraba-Dead Sea rift as proposed by some authors.

9. Conclusions

Outcrop, subsurface and laboratory data reveal the rela-tionship between alluvial depositional systems and tectoniccontrol for the late Ediacaran-early Cambrian UmmGhaddah Formation in Jordan.

The Umm Ghaddah Formation was deposited in exten-sional fault-controlled basins and sub-basins analogous tointracontinental half-grabens with listric-fault margins.The basin-fill succession consists of marginal alluvial fansthat drained towards the northwest from the NE–SWtrending, major bounding listric fault, and encroachedupon an axial braided fluvial system flowing northeast par-allel to the margin of the half-graben basin. The alluvialfan facies association exhibits a characteristic developmentfrom proximal facies (A1 and A2) to distal facies (A3 andA4). Facies A1 and A2 were deposited by rock falls andnon-cohesive debris flows of the sediment gravity flow pro-cesses, respectively, whereas facies A3 and A4 representsheetflood deposits.

The Umm Ghaddah Formation was dominated by ver-tical subsidence and uplift expressed in various types ofcyclicity. One large-scale fining upward cycle is attributedto gradual waning of the final phase of the Pan African


Orogeny, whereas several smaller scale, coarsening and fin-ing upward cycles indicate minor tectonic pulses and auto-cyclic shifting of alluvial fan-building processes.

The Umm Ghaddah Formation is interpreted as a syn-rift depositional sequence which together with underlyingEdiacaran Aheimir Volcanics, and Hiyala Volcaniclas-tics–Sarmuj Conglomerates could prove the existence of arift system in the study area in the Ediacaran Period relatedto the Najd Fault System. The distribution of the surfaceand subsurface occurrences of the Umm Ghaddah Forma-tion in Jordan which is not restricted to the vicinity of thepresent day Wadi Araba-Dead Sea transform fault mayindicate that the late Precambrian rifting phase may benot related to the present day Dead Sea transform fault.

Acknowledgments

The research has been carried on during the sabbaticalleave offered to the principal author from the Universityof Jordan during the academic year 2005/2006. The Natu-ral Resources Authority is acknowledged for providing theauthors with the core and cutting samples from the sixwells. Thank to Miss Islam Aldabsheh for drawing Figs.10, 11, 14, 15 and setting Table 1.

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Tectono sedimentary evolution of the Umm Ghaddah Formation...

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