Fluvial-shallow marine-glacio¯uvial depositional environments of the
Ordovician System in Jordan
B.S. Amireh
a,*, W. Schneider
b, A.M. Abed
a 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 furciferaoccurring 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.q2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
An Ordovician sequence consisting invariably of sand-stone and silt±mudsand-stone 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
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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±60
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
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
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-cross-bedding [0.1±1 m; St; Fig. 3(B)], and less common planar tabular cross-bedding (Sp) and overturned cross-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 B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
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 containingCruzianatrace 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
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 isCruziana furcifera [Bender and Huckriede, 1963; Fig. 5(A)], that indicates the Early Ordovician age. Other types of ichno-fossils that can be de®ned includeGyrochorte zigzag [Seila-cher, 1994; Fig. 5(C)],Cruzinasp. [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).
B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
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 Formaforma-tion. 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).
4.5. Lithofacies and architectural elements
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 B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
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-laminastrati®ca-tion, 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 ofPlanolitessp. andCruzianasp. [Fig. 6(A,B)] in ®ne B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
sand±siltstone layers. Skolithosis 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 theCruzianaand 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 ofSkolithos andDiplocraterion, 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.
4.8. Lithofacies and architectural elements
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)] andSkolithosichnofauna.
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 asDidymograptus 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 areScoliciaand ?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 ofCruzianaandSkolithos 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.
4.11. Lithofacies and architectural elements
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 ofSkolithos. 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 Skolithosoccurs 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 B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
®ne-grained sandstone facies (Sh) enriched withSkolithos, 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. B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
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.
4.15. Lithofacies and architectural elements
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).
4.16. Interpretation
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.
4.18. Lithofacies and architectural elements
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).
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 beddslump-ing 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 isLingula-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.
4.19. Interpretation
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).
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 B.S. Amireh et al. / Journal of Asian Earth Sciences 19 (2001) 45±60
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;
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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