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Fluvial-shallow marine-glacio¯uvial depositional environments of theOrdovician System in Jordan

B.S. Amireha,*, W. Schneiderb, A.M. Abeda

aDepartment of Geology, University of Jordan, Amman, JordanbInstitut fuÈr Geowissenscaften, TU Braunschweig, Postfach 3329, Braunschweig, Germany

Abstract

The Ordovician System, cropping out in southern and west-central Jordan, consists entirely of a 750 m thick clastic sequence that can be

subdivided into six formations. The lower Disi Formation starts conformably above the Late Cambrian Umm Ishrin Formation. According to

Cruziana furcifera occurring in the upper third of the Disi Formation, an Early Ordovician age is con®rmed. The Disi Formation, consisting

mainly of downstream accretion (DA) ¯uvial architectural element, was deposited in a proximal braidplain ¯owing N±NE from the

southerly-located Arabian±Nubian Shield towards the Tethys Seaway. The braidplain depositional environment evolved into a braid-

plain-dominated delta through the middle and upper parts of the Disi Formation and the lower part of the overlying Um Saham Formation.

The delta was replaced by siliciclastic tidal ¯ats, that in turn evolved into an upper to lower shoreface environment through the upper part of

the Um Saham Formation. The depositional environment attained the maximum bathymetric depth during the deposition of the lower and

central parts of the third unit, the Hiswa Formation, where offshore graptolite-rich mudstone with intercalated hummocky cross-strati®ed

tempestites were deposited. The Tethys Seaway regressed back through the upper part of the Hiswa Formation promoting a resumption of the

lower±upper shoreface sedimentation. Oscillation between the lower to upper shoreface depositional environment characterized the entire

fourth unit, the Dubaydib Formation, as well as the Tubeiylliat Sandstone Member of the ®fth unit, the Mudawwara Formation. The

depositional history of the Ordovician sequence was terminated by a glacio¯uvial regime that ®nally was gradually replaced by a shoreface

depositional environment throughout the last unit, the Ammar Formation. q 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction

An Ordovician sequence consisting invariably of sand-

stone and silt±mudstone sediments crops out in southern

Jordan and the area along the eastern margin of Wadi

Araba (Fig. 1). The Ordovician System in Jordan has been

studied by many authors, including Qunnel (1951), Bender

(1968), Lloyd (1968), Selley (1970), Masri (1988), Powell

(1989) and Khalil (1994). Some of these studies has been

summarized and correlated with Ordovician outcrops

throughout the Middle East countries by Alsharhan and

Nairn (1997). Most of the above studies involved lithostrati-

graphic subdivision of the Ordovician System into various

units, but without giving details of their depositional envir-

onments. Amireh (1993) conducted a sedimentological

investigation aimed at distinguishing between the Ordovi-

cian Disi Sandstone and the similar overlying Early Cretac-

eous Kurnub Sandstone, and consequently extended the

known occurrence of the Ordovician System 60 km north-

ward of the limit recorded in the previous geologic maps.

Makhlouf (1992, 1998) determined the depositional envir-

onment of random parts of the Ordovician System.

Therefore, it appears that a comprehensive sedimentolo-

gical study of the Ordovician System has not been under-

taken up until now. Thus, the present work documents the

detailed sedimentology of the Ordovician System based on a

systematic facies analysis and aims to determine the devel-

opment of depositional environments during the Ordovician

Period.

2. Geologic setting

In southern Jordan, the clastic Ordovician System starts

conformably above the Cambrian Umm Ishrin Formation,

and is overlain, also conformably, by the Silurian Batra

Mudstone Member of the Ordovician±Silurian Mudawwara

Formation. The contact with the underlying Cambrian sedi-

ments is problematic. It is based mainly on the color change

from brown, characteristic of the Cambrian Umm Ishrin

Sandstone, to white which is diagnostic of the Disi Sand-

stone (Bender, 1968).

Very likely, this lithologic contact does not represent the

Journal of Asian Earth Sciences 19 (2001) 45±60

1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.

PII: S1367-9120(00)00010-9

www.elsevier.nl/locate/jseaes

* Corresponding author. Fax: 1962-6-5348932.

E-mail address: [email protected] (B.S. Amireh).

actual boundary between the Cambrian and the Ordovician

Systems, since this study proves that the ®rst reliable indi-

cator of the Early Ordovician, Cruziana furcifera, is found

130 m above the base of the formation. Therefore, the age of

the lower 130 m of the Disi Formation may well be older

than Early Ordovician, that is Late Cambrian.

The Ordovician System attains an outcrop thickness of

750±800 m in southern Jordan and can be divided into six

formations (Powell, 1989), in ascending stratigraphic order:

Disi Formation, Umm Sahm Formation, Hiswa Formation,

Dubaydib Formation, Mudawwara Formation and Ammar

Formation. Excluding the lower Disi Formation, these

formations are exposed only in southern Jordan.

Along the eastern margin of Wadi Araba (Fig. 1), only the

B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±6046

Fig. 1. Outcrops of the Ordovician System in Jordan and location of the studied pro®les.

Disi Formation crops out and appears in a strip-like pattern

trending NNW, and extending from southwestern Jordan to

Wadi Nummeirah in central Jordan (Amireh, 1993). It rests

conformably above the Cambrian sandstones but is uncon-

formably overlain by the Early Cretaceous Kurnub Sand-

stone. The other Ordovician formations and the younger

Paleozoic formations were stripped away in central and

northern Jordan during the Hercynian and the Late Juras-

sic±Early Cretaceous tectonic events which affected Jordan

and the adjoining areas (Saint-Marc, 1978). On the other

hand, these Paleozoic formations were not deposited in

southern Jordan since the Tethys Sea did not reach this

part of Jordan during that time (Bender, 1968).

3. Methods and terminology

Thirty-three pro®les of the Ordovician sequence across

southern and central Jordan (Fig. 1) have been studied

sedimentologically. Fig. 2 displays the variation of litho-

facies, architectural elements, trace fossil content and

interpretation of the depositional environment through

the six Ordovician formations compiled from these 33

sections.

The terminology of lithofacies and ¯uvial architectural

elements follow those of Miall (1985, 1988, 1996), whereas

the geometry of tidal sand bodies is based on the terminol-

ogy of Nio and Yang (1991). Some of the architectural

elements and lithofacies of tidal and marine sand bodies

and their codes are proposed in this study. Tables 1 and 2

summarize the lithofacies and architectural terms. The

Cruziana-ichnofacies analysis is based upon that of Seila-

cher (1990, 1992, 1994).

4. Formations description and interpretation

The lithofacies association and ichnofaunal content, as

well as the architecture of complex three-dimensional sand-

stone bodies, have been described and are utilized to inter-

pret the depositional paleoenvironment of the following six

Ordovician formations.

4.1. Disi Formation

The Disi Sandstone attains a maximum thickness of 300±

320 m (Fig. 2). The formation is characterized by a spher-

oidal weathering morphology, the absence of horizontal

bedding, and a snow-white color for the fresh surfaces. A

B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60 47

Table 1

Fluvial lithofacies identi®ed in the Ordovician System, Jordan, from Miall (1996), and proposed (this study) tidal and marine lithofacies

Facies code Lithofacies Sedimentary structures

Gmm Matrix-supported, massive gravel Weak grading

Gcm Clast-supported massive gravel Pseudoplastic debris ¯ow

Gt Gravel, strati®ed Trough cross-beds

Gp Gravel, strati®ed Planar cross-beds

St Sand, medium to very coarse, may be pebbly Solitary or grouped trough cross-beds

Sp Sand, medium to very coarse, may be pebbly Solitary or grouped planar cross-beds

Sr Sand, very ®ne to coarse Ripple marks of all types

Sh Sand, very ®ne to very coarse, may be pebbly Horizontal lamination, parting lineation

Sm Sand, ®ne to coarse Massive or faint lamination

Ss Sand, very ®ne to coarse, may be pebbly Broad, shallow scours

Sl Sand, very ®ne to coarse, may be pebbly Low-angle (,158) cross-beds

Spo Sand, ®ne to coarse Overturned planar cross-beds

Sto Sand, ®ne to coarse Overturned trough cross-beds

Sf Fine sand with mud Flaser-rippled strati®cation

Sw Fine sand with mud Wavy-rippled strati®cation

Hcs Fine sand and mud Hummocky cross-strati®cation

Scs Fine sand and mud Swaley cross-strati®cation

Fr Sand, silt, mud Ripple- to climbing ripple cross-lamination

Fm Sand, silt, mud Massive

Fl Sand, silt, mud Fine lamination, very small ripples

Table 2

Architectural elements in ¯uvial deposits, from Miall (1985, 1996) and

from tidal sandbodies (modi®ed from Nio and Yang, 1991)

Symbol Element Principal lithofacies assemblage

CH Channel Any combination

GB Gravel bars and bedforms Gm, Gp, Gt

SB Sandy bedforms St, Sp, Sh, Sl, Sr, Se, Ss

SG Sediment gravity ¯ow Sm, Sh

DA Downstream accretion

macroform

St, Sp, Sh, Sl, Sr, Se, Ss

LA Lateral accretion macroform St, Sp, Sh, Sl, Se, Ss; minor Gm,

Gt, Gp

LS Laminated sand sheet Sh, Sl; minor Sp, Sr

FF Overbank ®ne sediments Fl, Fm

MF Mixed tidal ¯ats Sf, Sw, St, Sh, Fr, Fl

SF Sandy tidal ¯at St, Sf, Sw, Sl, Sr

SW Sandwaves St, Sp, Sh, Sl, Sr

TB Tidal bar St, Sr, Fr, Ss, Sf, Sw

T Tempestite Sh, Fl, Hcs, Scs, Sw

TCH Tidal to subtidal channel Sm, St, Sp, Sh

remarkable feature of the lithology of the formation is the

ubiquitous presence of scattered well-rounded quartz

pebbles, that are restricted to the bases of a large-scale

trough cross-bedded sandstone facies, and attain a maxi-

mum diameter of 12 cm.

It can be stated here that the famous Nabatean City of

Petra (Fig. 1) was carved in the white-colored, lower part of

the Disi Formation, as well as in the reddish pink-colored,

lower Cambrian Umm Ishrin Sandstone.

4.2. Lithofacies and architectural elements

The lower third portion of the Disi Formation is generally

made up of several, up to 15 m thick sandstone bodies [Fig.

3(A)] composed predominantly of coarse-grained, trough

cross-strati®ed sandstone (St) showing northward transport

directions [Fig. 4(A)]. The individual sandstone bodies have

an erosional base and are usually composed of several

®ning-upward sequences that start with a basal conglomer-

ate and grade upward into a coarse±medium-grained sand-

stone, and might be terminated by silt±mudstone.

Applying Miall's (1988, 1996) architectural elements,

downstream accretion macroforms [DA; Fig. 3(A)] domi-

nate over lateral accretion macroforms (LA) and sandy

bedforms (SB). The individual DA element extends more

than several hundred meters in the northward transport

direction [Fig. 3(C)] and several tens of meters across this

downstream direction. The internal structures of the macro-

forms consist dominantly of large-scale trough cross-

bedding (St), normal-scale trough cross-bedding [0.1±1 m;

St; Fig. 3(B)], and less common planar tabular cross-

bedding (Sp) and overturned cross-bedding with various

degrees of contortion and distortion (Sto, Spo). The troughs

can be considered three-dimensional megaripples that

reveal curved crests which can be traced over several

tens of meters and which exhibit regularly asymmetric

onlap structures. An Sc lithofacies is less common and

occurs as erosive, poorly structured thin channel-®lls

(CH) associated with gravel lag deposits. Throughout the

Disi Formation, these major DA elements are eroded by

subordinate channels (CH) of a small width:depth-ratio,

which are ®lled by all types of relevant ¯uvial lithofacies


Fig. 3. (A) A downstream accretion architectural element (DA) truncating an underlying, wedging-out, laminated sandstone element (LS) in the Disi

Formation. (B) Sh intercalated within St lithofacies in the Disi Formation. (C) A panorama for the Disi Formation illustrating the dominant DA element

extending for several hundred meters, and the intercalated LS containing Cruziana trace fossils. The outcrop face strikes northward from right to left. Scale bar

is 5 m.

Fig. 2. Variation of the lithofacies, architectural elements, trace fossil content and interpretation of the depositional environment in the compiled 33

Ordovician sections. X indicates position of the base of the Mudawwara Formation by Makhlouf (1992). Cr � Cruziana; Cr.fur. � Cruzina furcifera;

Sk � Skolithos; Di � Diplocraterion; Gr � graptolite; Cr.ac. � Cruziana acacensis; Br � brachiopd; Cr. p. � Cruziana petraea. For symbols of

lithofacies and architectural elements, see Tables 1 and 2, respectively.

including Sm, St, Sp, Sh and even overturned cross-strati-

®ed units (Sto, Spo).

The DA elements may be intercalated with laminated

sand sheet (LS) element comprising decimeter-thick struc-

tureless (Sm) or laminated sandstone beds [Sh; Fig. 3(B)].

Centimeter to decimeter thick mudstones or pelite beds (Fl),

that occasionally contain poorly-de®ned trace fossils, are

associated with the Sh lithofacies in the lower part of the

Disi Formation. All of these thin intercalations show a short

lateral extension due to erosion by the overlying DA macro-

forms [Fig. 3(C)].

The middle and upper parts of the Disi Formation display

the same lithofacies and architectural elements encountered

in the lower third, but remarkably, some pelite layers

contain well-de®ned trace fossils, among which is Cruziana

furcifera [Bender and Huckriede, 1963; Fig. 5(A)], that

indicates the Early Ordovician age. Other types of ichno-

fossils that can be de®ned include Gyrochorte zigzag [Seila-

cher, 1994; Fig. 5(C)], Cruzina sp. [Rusophycus form; Fig.

5(B)] and cf. Scolicia [Fig. 5(D)]. These Cruzina-bearing

pelites are also truncated by the overlying DA (¯uvial)

elements.

4.3. Interpretation

Based on the absence of body fossils, the presence of

®ning-upward sequences starting with gravel lags and termi-

nated by overbank ®nes, and the prevalence of a large-scale

trough and planar cross-bedding having unidirectional

paleocurrent trends [Fig. 4(A)], the lower third of the Disi

Formation was deposited in a ¯uvial depositional paleoen-

vironment (Allen, 1974; Walker and Cant, 1984). Further-

more, the sheet-like morphology, the scarcity of thick

conglomerate beds and the dominance of DA macroform

DA elements over LA macroform LA elements, SB and

channel architectural elements (CH) favor a distal braid-

plain setting (Turner, 1980; Miall 1985). The major litho-

facies (St and three-dimensional megaripples) were

deposited by sandwaves or dunes migrating along shallow

broad channels of high width:depth ratio (sand ¯ats) under

conditions equivalent to the upper part of the lower ¯ow

regime (Harms, 1975). The entire DA macroform element

was constructed by the downstream accretion of the large

mid-channel sand ¯ats in these shallow uncon®ned channels

of low sinuosity (Miall, 1985). On the other hand, the inter-

calated thin Sm and Sh lithofacies constituting the LS

macroform were deposited between the sand ¯ats during

sheet or storm (¯ash) ¯oods that reached the braidplain

under upper ¯ow regime, plane bed conditions (Rust,

1978). The intercalated pelites making up the overbank

®ne sediments (FF) architectural element can be interpreted

as overbank ®nes or abandoned channel ®lls formed by

vertical aggradation under decreasing water level conditions

(low ¯ow regime; Reineck and Singh, 1986). The inferred

braided river ¯owed northwards [Fig. 4(A)], from the

Arabian±Nubian Shield towards the Tethys Seaway that

was located in northern to northwestern Jordan in the Late

Cambrian Period (Amireh et al., 1994a). The large thickness

of the DA and SB elements in the lower part of the Disi

Formation indicates a high transport energy and sediment

load. The dominant DA elements in the formation may be

similar to the sandwaves of the lower South Saskatchewan

River (Cant and Walker, 1978) and of the Brahmaputra

(Bristow, 1993).


Fig. 4. Rose diagrams showing the paleocurrent directions of: (A) St of the

DA and LA elements in the Disi Formation. (B) St of the DA and LA

elements in the Umm Sahm Formation. (C) wave ripple crests in the Tubei-

liyat Sandstone Member.

The dispersed quartz pebbles throughout the Disi Forma-

tion, which are concentrated at the bottom of the St facies,

were deposited at the base of the shallow channels by the

downcurrent migrating sandwaves during high energy

conditions. These clasts are more common in the southern

parts of the study area which is close to the source rock, the

Arabian±Nubian Shield, whereas they decrease in number

and size in the northern parts of the study area.

The central and upper parts of the Disi Formation

were probably deposited within a ¯uvial dominated

braidplain delta (Elliott, 1986a; Miall, 1994). This inter-

pretation is based upon the Cruziana ichnofacies that

represents mixed ¯ats deposited in the low energy inter-

lobe positions of the delta, where the Cruziana-produ-

cing trilobites could dwell (Elliott, 1986a). However, in

these parts of the Disi Formation, the ¯uvial architec-

tural elements DA, SB, LA and CH were still present,

but partly as submarine extensions below the high tide level

in the form of sandwaves and tidal bars (TB; Nio and Yang,

1991).

4.4. Umm Sahm Formation

The Umm Sahm Formation crops out in the Sahl as

Suwwan area, east of Qa el Disi (Fig. 1) and consisits domi-

nantly of cross-bedded sandstone. It attains a thickness of

250 m at Jabal el Ghuzlan [Fig. 6(A)] and starts conform-

ably above the Disi Sandstone. Also, the top of the forma-

tion is conformable with the overlying Hiswa Formation.

The Umm Sahm Formation is distinguished from the under-

lying formation through horizontal bedding, dark brown to

black weathering colors, a smaller thickness of the sand-

stone bodies, better sorting and a distinct silici®cation by

syntaxial quartz overgrowth (Amireh et al., 1994b).


The Umm Sahm Formation consists mainly of medium-

grained, moderately to well sorted St and a less common Sp

lithofacies. Both types of sandstone show a broad spectrum

of northward transport directions [Fig. 4(B)]. Other less


Fig. 5. (A) Cruzina furcifera, indicating an Early Ordovician age, found in the upper 130 m of the Disi Formation. (B) Cruziana rusophycus in the Disi

Formation. (C) Gyrochorte zigzag in the Disi Formation. (D) cf. Solicia in the Disi Formation.

abundant lithofacies include laminated sandstone (Sh) and

pelites (Fl), that become abundant in the upper part of the

Umm Sahm Formation. The major sandstone lithofacies

constitute DA and SB ¯uvial architectural elements with

interbedded TB. Internal strati®cation in the TB macroform

includes horizontal bedding varying in thickness from a few

decimeters up to 1.5 m, ¯aser±wavy±lenticulal strati®ca-

tion, ripple cross-lamination, and oscillational and interfer-

ence ripples (l , 12 cm). No ichnofauna are found in the

lower part of the formation (Fig. 2). On the other hand, the

upper part of the formation is characterized by the appear-

ance of Planolites sp. and Cruziana sp. [Fig. 6(A,B)] in ®ne


Fig. 6. (A) Complete section of the Umm Sahm Formation, overlain by silt±mudstone facies of the Hiswah Formation (contact is indicated by arrow). In front

of Amireh, is an upper bedding plane of a sandstone bed riddled with Planolites. (B) Planolites sp. appearing in Fig. 6(A). (C) Alternating Sw and Sh with

interbedded silt±mudstone beds in the Hiswah Formation. (D) Small Cruziana in the Hiswah Formation. (E) Hummocky cross-strati®cation (Hcs) with an

interbedded Sh in the Hiswah Formation.

sand±siltstone layers. Skolithos is found only in one unit,

located about 180 m above the base of the formation (Fig.

2). The upper surfaces of these preserved ichnofacies suffer

from erosion by overlying tidal channels. However, a

considerable part is still preserved, thus they display a

remarkable lateral extension and may be regarded as marker

beds.

The upper part of the Umm Sahm Formation displays

®ning±upward sequences consisting of DA or SB elements

overlain by LS or FF elements including the Cruziana and

Skolithos-bearing pelites. Ten such cycles are counted in the

study area (Fig. 2).

4.6. Interpretation

The depositional environment of the lower part of the

Umm Sahm Formation (lower 120 m; Fig. 2), lacking the

ichnofauna but characterized by well-developed horizontal

bedding, is interpreted as a ¯uvial-dominated braidplain

delta (Elliott, 1986a) which has a continuation of that postu-

lated for the underlying Disi Sandstone. Sp dominating over

St facies indicates a slight decrease of transport velocity

(Harms et al., 1982) in comparison with underlying Disi

Formation. DA and SB elements were deposited above

high tide whereas TB element represents deposition below

the mean high tide.

The upper part (130 m), containing marine trace fossils

and interference wave±ripple cross-strati®cation, represents

a decrease of deltaic in¯uence, and instead indicates a

setting where intertidal sand ¯ats and mixed ¯ats domi-

nated. The latter were commonly eroded by tidal channels

of low sinuosity. Throughout the uppermost part of the

Umm Sahm Formation, the depositional paleoenvironment

became more marine, where lower foreshore to upper shore-

face conditions dominated, giving rise to parallel and ripple-

laminated horizontal beds of sandstone and silt±mudstone.

This interpretation is based on the appearance of Skolithos

and Diplocraterion, the abundance of sand±mud deposits

that are heavily bioturbated and ripple cross-laminated, and

the truncation of these deposits by shallower tidal channels

(Graham, 1982; Reinson, 1984; Elliott, 1986b).

4.7. Hiswah Formation

The Hiswah Formation crops out in Wadi Hiswa and Sahl

as Sawwan (Fig. 1). It begins conformably above the Umm

Sahm Sandstone, with a remarkable shaley unit easily

distinguished from the underlying competent formation

[Fig. 6(A)]. The top of the Hiswah Formation is also

conformable with the overlying Dubaydib Sandstone.

The Hiswah Formation attains a thickness of 60±70 m. Its

immediate base is characterized by a thin layer of intrafor-

mational conglomerate, followed by a red to violet colored,

thin bedded ®ne sand- to siltstone unit a few decimeters

thick.


The lower part of the formation (0±20 m) consists of

alternating thin beds of siltstone and laminated (Sh),

wave±ripple, cross-strati®ed (Sw) and ¯aser±ripple strati-

®ed (Sf) ®ne sandstone with mud±claystone (Fl; Fig. 2). The

latter is gray and partly dark gray and reddish gray. Other

internal strati®cations include interference and linguoid

oscillational ripples and rare small-scale planar tabular

cross-bedding (Sp). On the other hand, this part of the

formation is also distinguished by the abundance of Cruzi-

ana [Fig. 6(D)] and Skolithos ichnofauna.

The middle part of the formation (20±38 m; Fig. 2)

consists of tempestite architectural element (T) composed

of a graptolite and brachiopod-bearing dark gray to green

mudstone with few intercalated siltstone thin beds. The T

architectural element is characterized by the ®rst appearance

of hummocky cross-strati®cation [Hcs; Fig. 6(E)]. The grap-

tolite, identi®ed as Didymograptus bi®dus [Fig. 7(B)], indi-

cates a Lanvirn age (Bender and Huckriede, 1963). The

brachiopods found are identi®ed as Elkaniidae [?Broeg-

geria] (Carls, personal communication, 1986).

The upper part (38±70 m) of the Hiswah Formation

consists of sand ¯at architectural element, composed of

parallel laminated (Sh) and wave±ripple-laminated (Sw)

®ne-grained sandstone with interbedded silt±mudstone

[Fig. 6(C)]. Similar to the lower part of the formation,

Cruziana and Skolithos ichnofauna are present in this

upper part. Among the former, Cruziana cf. acacensis is

identi®ed and characterized by current orientation [Fig.

7(A)]. This ichnofossil was hitherto found only in the

Lower Silurian of North Africa (Seilacher, 1992). Other

trace fossils present in this part of the Hiswah Sandstone

Formation are Scolicia and ?Arthrophycus alleghannensis.

4.9. Interpretation

The lower part of the Hiswah Formation was deposited in

the upper shoreface zone as indicated from the marine

ichnofauna and the various types of wave ripples, ¯aser±

wavy strati®cation as well as the parallel- and ripple cross-

laminated ®ne sandstone (Galloway and Hobday, 1983;

Stewart et al., 1991). The depositional environment then

became deeper, reaching the lower shoreface to offshore,

as indicated from the appearance of graptolites and tempes-

tite architectural element (T; Levell, 1980). The latter repre-

sents storm conditions that frequently offset the deep and

otherwise quiet, reducing H2S-bearing depositional condi-

tions of the dark-colored mudstones hosting the graptolites

(Galloway and Hobday, 1983; Boggs, 1987). Finally, the

depositional environment of the upper sand ¯at element

regressed back to the shallower upper shoreface as

concluded from the appearance of Cruziana and Skolithos

and the disappearance of tempestites (T element and Hcs

lithofacies) and the graptolites.


4.10. Dubaydib Formation

The Dubaydib Formation crops out in Sahl al Khreim

(Fig. 1), consists mainly of ®ne-grained well sorted sand-

stone and attains a thickness of 120 m. The base of the

formation is delineated by the appearance of huge popula-

tions of vertical burrows of Skolithos [Fig. 7(C)], whereas

the top is considered to be below the ®rst bed of a decimeter-

thick silici®ed sandstone facies that gives a characteristic

landscape of mesas and cuestas to the overlying Mudaw-

wara Formation [Fig. 7(D)]. This upper contact of the

Dubaydib Formation contrasts with former publications

(e.g. Powell, 1989; Makhlouf, 1992) which used the green

pelites that occur 30 m higher in the lithostratigraphic

section to mark this transition (position X on the log of

the overlying formation, Fig. 2). The Dubaydib Formation

can be subdivided into three parts.


The lower part of the Dubaydib Formation (0±40 m; Fig.

2) consists of horizontally bedded, poorly sorted silty sand-

stone (Sh) penetrated by large populations of Skolithos. The

thickness of the horizontal bed varies from 0.1 to 0.4 m.

Horizontal lamination (Sh) alternates with ¯aser±wavy

ripple strati®cation (Sf±Sw) throughout this part of the

formation. Ripples on bed surfaces are abundant, which

are mainly symmetrical with straight crests or less

commonly linguoid and sinuous-crested. These facies

together make up a sand ¯at architectural element (Fig. 2).

The middle part of the formation (40±95 m) consists of

three sets of channel-®lls (TCH I±III) elements [Figs. 2 and

8(A)] intercalated within horizontally bedded sandstone. The

channel-®lls are of low sinuosity and directed northwards, but

they exhibit a wide lateral paleocurrent variation. A tempestite

architectural element (T) characterized by hummocky±

swaley cross-strati®cation [Hcs±Scs; Fig. 8(B)], rich in

Skolithos occurs between the ®rst and second channels. The

channel-®lls contain a variety of internal facies including

Sm, St, Sp and Sh. The second channel (TCH II) is overlain

by lateraly extensive sandstone beds, that are, in turn,

followed by a 10±15 m thick green sandy pelite.

The upper part of the formation (95±120) overlying the

third channel-®ll (TCH III) consists of a horizontally bedded


Fig. 7. (A) Current-oriented Cruziana acacensis in the Hiswah Formation. (B) Didymograptus bi®dus graptolite in the Hiswah Formation. (C) Scolithos sp.

characterizing the Dubaydib Formation, note the bifurcation and the annulated nature. (D) Mesa morphology consisting of ¯at-topped hills (arrows) diagnostic

of the Mudawwara Formation.

®ne-grained sandstone facies (Sh) enriched with Skolithos,

comparable with the SF element of the lower part.

Other trace fossils present in the formation include: Ruso-

phycus, Cruziana petraea [Fig. 8(D)], Cruziana alamade-

nensis, Diplocraterion, and ?Planolites.

4.12. Interpretation

According to the physical and biogenic structures

described above, the depositional paleoenvironment of the

sand ¯ats of the lower part of the Dubaydib Formation is

interpreted as a lower foreshore that deepened to the upper

shoreface (Fig. 2). The middle part of the formation repre-

sents deepening of the depositional environment, where the

®rst set of subtidal channels (CH I) eroding the sand ¯ats

was deposited. The tempestite element (T) deposited

between the ®rst and second subtidal channel system repre-

sents the highest bathymetric depth recorded in the forma-

tion, where it reached the storm±wave base in the lower

shoreface (Swift, 1984; Walker, 1984). The sea level fell

during the last phase of the formation, where the second and

third sets of subtidal channels (CH II±III) and the sand ¯at

elements of the upper part of the formation were deposited

in the lower foreshore to the upper shoreface.

4.13. Mudawwara Formation

The Mudawwara Formation crops out in southeastern

Jordan adjacent to the Hijaz railway in the Mudawwara

area (Fig. 1). Again, the formation forms broad ¯at-

topped mesas and slightly inclined cuestas (ªSchichtstu-

fenlandschaftenº) due to differential weathering of alter-

nating beds of soft shale and competent sandstone [Fig.

7(D)].

The formation is divided by the NRA mapping project

(Powell, 1989) into three members, from bottom to top:

Tubeiliyat Sandstone, Batra Mudstone and Ratiya Sand-

stone. The basal part of the Batra Mudstone Member

contains graptolites indicative of the early Silurian, such

as Diplograptus modestus modestus and Glyptograptus

?tenuis (Rushton cited in Powell, 1989). Therefore, the

latter two members will be not included in this study.


Fig. 8. (A) Channel-®ll occurring in the middle part of the Dubaydib Formation. (B) Swaley cross-strati®cation (Scs) in Dubaydib Formation. (C) The green

massive siltstone facies (paleoloess; PL) of the Ammar Formation overlain by the ¯uvial massive sandstone channel-®ll (CHF) facies. (D) Cruziana petraea in

the Dubaydib Formation. Bar � 1 cm.

4.14. Tubeiliyat SandstoneMember

The Tubeiliyat Sandstone Member attains a thickness of

170 m. In contrast to Andrews (1991) and Makhlouf (1992),

who regarded the base of this member as the varicolored

(green, gray, mauve and pink) pelites (position X, Fig. 2),

we consider it to be 30 m below, where the ®rst cuesta or

mesa starts, overlying the Skolithos-ichnofacies of the

Dubaydib Formation. On the other hand, the top of the

member is considered as the base of the graptolite-rich

mudstones of the overlying Batra Mudstone Member.


The Tubeiliyat Sandstone Member consists mainly of

alternating friable, poorly sorted clayey±silty sandstone

facies making up a tempestite architectural element (T)

with indurated (due to quartz cementation), well sorted,

horizontally bedded ®ne sandstone facies (Sh) constituting

a sand ¯at architectural element in the form of coarsening±

upward cycles. About 20 cycles are present in the member

(Fig. 2). The thickness of the sand ¯at element attains 3±6 m,

whereas that of the T element ranges from 0.4 to 0.8 m.

The internal strati®cation of the sandstone facies of the T

element is characterized by horizontal bedding (Sh), oscil-

lation wave ripples (l , 0.12 m), interference and ladder±

back ripples having crests that may show several maxima,

and hummocky cross-strati®cation (Hcs). The trend of the

waves was mainly N±S and less commonly NW±SE [Fig.

4(C)]. Trough cross-strati®cation (St) within ¯at channels

constituting TCH architectural element is found in the

middle and upper parts of the Tubeiliyat Sandstone Member

(Fig. 2). On the other hand, the clayey±silty sandstone

facies of the sand ¯at element is characterized by small-

scale ¯aser±wavy ripple strati®cation (Sf±Sw) and parallel

lamination (Sh). Excluding the hummocky±cross-strati®ed

units, both architectural elements are invariably penetrated

by Skolithos, but in less abundance than in the underlying

Dubaydib Formation. Cruziana petraea (Seilacher, 1992,

1994) occurs directly above the varicolored siltstones and

indicates a Late Ordovician age. Moreover, brachiopods are

found in the upper part of the Tubeiliyat Sandstone Member

(Bender and Huckriede, 1963).


The widely distributed horizontal bedding, the presence

of oscillation ripples, ripple cross-strati®cation, Skolithos

and Cruziana ichnofossils, the frequent occurrence of

brachiopods, and the occurrence of subtidal channel and

tempestite architectural elements indicate a shoreface

depositional paleoenvironment (Levell, 1980; Graham,

1982; Walker, 1984). The thick, bioturbated (Skolithos

and Cruziana-rich) clayey±silty sandstone facies of sand

¯at element was deposited in the upper shoreface during

fair weather conditions. On the other hand, the tempestite

architectural element (T) consisting of the thin beds of ®ne-

grained sandstone characterized by hummocky cross-strati-

®cation may indicate storm conditions (tempestites), that

carried and deposited sand below the wave base within

the lower shoreface (Levell, 1980; Soegaard and Eriksson,

1985). Therefore, the depositional environment of the

Tubeiliyat Sandstone Member ¯uctuated several times

between the lower and the upper shoreface. The St lithofa-

cies within the ¯at channels encountered in the bedded sand-

stone facies represents the distal reaches of tidal ¯ats, or

more probably, subtidal channels.

4.17. Ammar Formation

In southeast Jordan (Wadi Hiswa, Batn el Ghul, Sahl el

Batra and Muddawwara areas, Fig. 1), the upper part of the

Tubeiliyat Sandstone Member was incised by glacial

valleys that were later ®lled with glacio¯uvial sediments

(Abed et al., 1993). The latter have designated these sedi-

ments as the Ammar Formation and constrained its age

between the Ashgillian to early Llandoverian. The Ammar

Formation is restricted to NS-striking paleovalleys/paleode-

pressions distributed in a narrow belt of about 4 km width

and 70 km length (Abed et al., 1993). These ªchannelsº cut

the adjacent well-bedded Tubeiliyat Sandstone Member and

wedge out laterally over a length of about 3 km without

revealing their bases, whereas the tops are sharply overlain

by the mudstones of the Batra Mudstone Member.


The Ammar Formation starts in most localities, such as

Barga, Kharawi and Al Hatiya, with a channel lag conglom-

erate facies (Gmm) ®lling an erosional surface truncating

the Tubeiliyat Sandstone Member and constituting a gravel

bedform architectural element. It ranges in thickness

between 0.2 and 1 m. The pebbles and cobbles of this facies

are composed mainly of exotic quartz and quartzite clasts,

and less commonly sandstone, mudstone lithoclasts and

granite as well as rare monomineralic microcline clasts.

The quartzitic and granitic clasts are remarkably faceted

and striated, have crushed margins and ¯atiron-shapes, indi-

cating clearly a glacial origin (Pettijohn, 1975; Edwards,

1986).

The overlying part of the Ammar Formation consists of a

30 m thick sediment gravity ¯ow architectural element

comprised of gray-greenish, massive-structureless, well

sorted sandy±siltstone (Sm) lithofacies. No sedimentary

structure, neither macro, micro, ichnofossil or rootlets

have been found. On the other hand, the light and heavy

minerals and clay mineral content of this element are iden-

tical with that of the Tubeiliyat Sandstone Member (Abed et

al., 1993).

This lower sediment gravity ¯ow architectural element is

overlain unconformably by channel architectural element

(CH) composed mainly of massive Gcm, Sm lithofacies

and rarely of Sh lithofacies [Fig. 8(C)]. Many pebbles of

quartzite, granite, metamorphic rocks and less commonly


sandstone±siltstone occur as lag deposits at the base of these

channels. Most of these pebbles are striated and/or faceted.

At some places, these sandstone channel-®lls exhibit slump-

ing structures and steeply inclined, irregular bedding along

with water escape structures. This massive sandstone facies

grades upward into the cross-strati®ed sandstone facies (St,

Sto) and less common Sp and Spo facies displaying a north-

ward transport direction.

The upper part of the Ammar Formation consists of a

sand ¯at architectural element composed of thin, uniformly

bedded sandstone facies (Sh) with subordinate St and Sp

lithofacies. The upper bedding planes of this facies exhibit

oscillation ripples (l , 0.15), undeterminable grazing trace

fossils, and at least two forms of brachiopods. One type of

the latter is Lingula-like, the other is a large scale-ripped

form. The latter exhibits a similarity with the brachipod

faunal assemblage of Spain that appeared immediately

after the Late Ordovician glaciation, among which is Menta-

cella cantabrica (Villas and Cocks, 1996) that inhabited

foreshore and shoreface paleoenvironments.


The Ammar Formation has been interpreted by Abed et

al. (1993) to be of glacial origin. Glaciers advancing north

and northeastward from Arabia during the Late Ordovician

North African±Arabian glacial event (McClure, 1978;

Vaslet, 1990) incised deep paleovalleys in the underlying

marine Tubeiliyat Sandstone Member. These depressions

were later ®lled by transported tillites that were further

reworked and deposited by ¯uvial processes, giving rise to

a glacio¯uvial sequence exhibiting little evidence of the

glacial origin.

The basal architectural element (gravel bedform) of the

Ammar Formation, containing faceted and striated pebbles

and cobbles, represents a lodgment till transported from

Arabia, reworked by a braided stream and ®nally deposited

at the ¯oor of the paleovalleys as channel lag-deposits.

The lower sediment gravity ¯ow architectural element of

the Ammar Formation, composed of the enigmatic sandy

silty facies and devoid of any physical or biogenic struc-

tures, may be interpreted as paleoloess deposits blown from

the adjacent Tubeiliyat Sandstone Member. The glacial

paleovalleys incised this marine formation. An alternative

interpretation, proposed by Abed et al. (1993), is that it

represents a rock ¯our of the underlying Tubeiliyat Sand-

stone Member which was dumped quickly into the glacial

paleovalleys as indicated by the identical light, heavy and

clay minerals in both the Tubeiliyat Sandstone Member and

this facies.

The middle part of the Ammar Formation, consisting of

CH architectural element [Fig. 8(C)], was deposited by a

braided river ¯owing northward within a glacial valley. This

interpretation is based upon the truncation of the massive-

structureless sandstone, the channel geometry, the absence

of body fossils and marine ichnofossils, and the dominance

of unimodal trough cross-bedded sandstone facies (Rust and

Gibling, 1990; Brown and Plint, 1994). The massive

conglomerate±sandstone facies (Gcm and Sm) may repre-

sent deposition under critical ¯ow conditions of ¯ash ¯oods.

The upper part of the Ammar Formation, consisting of

well-bedded, ®ne-grained sandstone, ripple cross-strati®ed

and exhibiting trace fossils and brachiopods (sand ¯at archi-

tectural element), represents the change of the previous

continental system (glacial and glacio¯uvial) into a shallow

marine regime, particularly foreshore to upper shoreface

(Walker, 1984) in advance of the early Silurian transgres-

sion that gave rise to the overlying fully marine Batra

Mudstone Member.

5. Depositional model

The Ordovician depositional events of the study area,

located on the stable shelf of the Gondwana side of the

Tethys Seaway, were in¯uenced by the preceding Cambrian

depositional history. The latter was characterized in Jordan

by several ¯uctuations from braided rivers to shallow

marine depositional environments (Amireh et al., 1994a).

Subsequently, the Ordovician System evolved from braided

rivers to a braidplain-dominated delta, then to foreshore±

shoreface and further to offshore depositional environments.

Finally, the depositional history terminated by glacial to

glacio¯uvial conditions that were gradually replaced by

foreshore±shoreface sedimentation in preparation for the

regional Silurian transgression that affected Jordan and the

entire region (Andrews, 1991). The depositional model of

the Ordovician System is portrayed as a series of six-block

diagrams (Fig. 9).

The lower 140 m of the ®rst Ordovician formation, the

Disi Sandstone, consisting of sandstone sheets (DA archi-

tectural element) with few intercalated beds of siltstone±

mudstone (LS, FF) devoid of identi®able trace fossils,

were deposited by a braided river ¯owing northward from

the Arabian±Nubian Massif towards the north±northwest-

located Tethys Seaway [Fig. 4(A,B)]. The DA macroform

was constructed by downstream accretion of mid-channel

sand ¯ats whereas the LS and FF architectural elements

represent vertical aggradation of sands and ®nes after

decline of storm ¯oods. This initial event of the Ordovician

Period sedimentation is shown in Fig. 9(A).

The middle part of the Disi Formation witnessed the ®rst

marine in¯uence where Cruziana ichnofacies were depos-

ited in the form of thin silt±mudstone intercalations within

the DA ¯uvial sandstone bodies. The Cruziana-producing

trilobites dwelled in mixed ¯ats (MF) located in the low

energy interlobes of a braidplain-dominated delta. On the

other hand, these deltaic tidal bars and sandwaves elements

probably represent a submarine continuation of adjacent

continental sandy bodies [DA; Fig. 9(B)]. The marine in¯u-

ence increased slightly in the upper part of the Disi Forma-

tion where more Cruziana ichnofacies are encountered and


horizontal bedding is well developed in the braidplain-

dominated deltaic deposits [Fig. 9(B)].

The braidplain-dominated deltaic depositional envir-

onment of the upper part of the Disi Formation

persisted through the lower 120 m of the overlying

Umm Sahm Formation [Fig. 9(B)]. Afterwards, the

depositional environment underwent a gradual marine

inundation through the remaining 130 m of the Umm

Sahm Formation where sand and mixed sand±mud tidal

¯ats replaced the preceding braidplain-dominated delta.

These were followed by upper to lower shoreface ®ne

sand and silt±mudstone deposits containing Skolithos

and Diplocraterion and dissected by several tidal chan-

nels [Fig. 9(C)].

The marine depositional environment continued deepen-

ing below the lower shoreface, where it achieved a maxi-

mum water depth during deposition of the offshore

graptolite- and brachiopod-bearing mudstone with the inter-

calated tempestite sandstone beds (T architectural element)

of the Hiswah Formation [Fig. 9(D)]. Afterwards, and

during deposition of the upper part of this formation (sand

¯at architectural element), the offshore depositional


Fig. 9. Depositional Model of the Ordovician System in Jordan. (A) Depositional environment of the lower part of the Disi Formation. (B) Depositional

environment of the middle±upper parts of the Disi Formation and the lower part of the Umm Sahm Formation. (C) Depositional environment of the middle and

upper parts of the Umm Sahm Formation. (D) Depositional environment of the Hiswah Formation. (E) Depositional environment of the Dubaydib Formation

and the Tubeiliyat Sandstone Member of the Mudawwara Formation. (F) Depositional environment of the glacio¯uvial Ammar Formation. S � Skolithos;

D � Diplocraterion; G � graptolite; Br � brachiopod; T � tempestite; W � wind blown sediments.

environment changed back to the shallow lower shoreface

and, further, to the upper shoreface [Fig. 9(D)].

Oscillation between the lower and upper shoreface

continued through deposition of the Dubaydib Formation

and the Tubeiliyat Sandstone Member of the Mudawwara

Formation [Fig. 9(E)]. The lower shoreface depositional

environment was characterized by sedimentation within

subtidal channels (CH architectural element) with interca-

lated hummocky cross-strati®ed sandstone beds of a

tempestite origin (T architectural element), whereas the

upper shoreface depositional environment was dominated

by sand ¯at architectural elements. The tempestite architec-

tural element represents deposition in the lowermost shore-

face (Levell, 1980; Reineck and Singh, 1986).

The Ordovician Period closed with a glacial/glacio¯uvial

event where the Ammar Formation terminated the Ordovi-

cian System. Glaciers advancing north and northeastward

from northern Arabia during the Late Ordovician glacial

event incised the marine Tubeiliyat Sandstone Member,

forming paleovalleys that were later ®lled with reworked

tillites by a braided river ¯owing northward towards the

Tethys Seaway. A channel lag conglomerate facies with

diagnostic striated and faceted pebbles and cobbles was

deposited at the base of these glacial paleovalleys [Fig.

9(F)]. Aeolian deposits blown from the neighboring Tubei-

liyat Sandstone Member gave rise to the green massive silt±

mudstone facies above the conglomerates [Fig. 9(F)]. A

braided river system persisted throughout the middle inter-

val of the Ammar Formation. Subsequently, the sea gradu-

ally returned to the region and foreshore to upper shoreface

clastics were deposited, forming the upper part of the

Ammar Formation. Ultimately, the Tethys inundated the

entire study area during the regional early Silurian trans-

gression and the offshore Batra Mudstone Member of the

Mudawwara Formation was deposited.

Acknowledgements

Thanks are due to Professor A. Seilacher for help in the

identi®cation of the ichnofossils. Professors K. Burke and

P. Eriksson are greatly acknowledged for the comments that

improved the paper. The ®nancial support of the research by

the Deutsche Forschungsgemeinschaft and the University of

Jordan is greatly appreciated.

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