Facies sequences of the middle-upper jurassic carbonate platform (Amran Group) in the Sana'a region, Republic of Yemen

Marine and Petroleum Geology, Vol. 14, No. 6, pp. 643-660, 1997 ;i:= 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0264-8172(97)00030--5

0264-8172/97 $17.00+0.00

ELSEVIER

Facies sequences of the Middle-Upper Jurassic carbonate platform (Amran Group) in the Sana'a region, Republic of Yemen Khalid A. AI-Thour* Geology Department, P.O. Box 2027, Sana'a University, Sana'a, Republic of Yemen

Received 30 January 1996; revised 20 February 1997; accepted 26 April 1997 Rocks of the Amran Group in the Sana'a region of Yemen disconformably overlie the Kohlan Formation and unconformably underlie the Tawilah Group. Sea-level rises have produced landward migration of facies belts and the development of deeper water facies over shallow ones. The geometry of the depositional environments identified has enabled the passage of relatively short-lived transgressive-regressive cycles of sedimentation to be recognized. Three facies associations are introduced: (1) Carbonate platform facies, (2) Carbonate-marl alternation facies, and (3) Shallow water coral and stromatoporoid build-up facies. These facies are widely distributed and the whole sequence reflects deposition on a broad platform upon which shoals separated platform carbonates from basin sedimentation and an open marine environment. The repetition and interfingering of both fining- and shallowing-upward cycles within the study areas suggest that deposition occurred within the same basin with slightly different conditions in different places. The main factors controlling their deposition are sea-level changes and tectonics. © 1997 Elsevier Science Ltd. Keywords: facies; Jurassic; Amran Group; Yemen

The Jurassic Amran Group is extensively exposed in the western part of Yemen, to the west and north-west of Sana'a, the capital of the Republic of Yemen (Figure 1). The area investigated is covered mainly by carbonate rocks of the Amran Group (Figure 2) which disconformably overlies the Kohlan Formation and unconformably underlies the Tawilah Group. The Jurassic sequence in this area varies between 410 and 520m in thickness (F~qure 3). Through correlation between the different sections of each traverse, lithostratigraphic logs of the five traverses were drawn up (Figure 3). Correlation between traverses is based on the specific sedimentary characteristics of the key horizons, as well as fossils (e.g. algae, foraminifera, corals, stromatoporoids and palynomorphs), in different traverses (Simmons and A1-Thour, 1994). The carbonate sediments of the Amran Group are composed of outcropscale sedimentary cycles of various types and scales (A1Thour, 1992). The middle to late Jurassic was a time of gradual transgression of a shallow sea from the east, north and south into western Yemen. During this interval, the Amran Group carbonate platform went through steady subsidence resulting in a continuous transgression. This subsidence occurred against a background of sealevel fluctuations. These could be of a eustatic nature but this is difficult to prove in the absence of refined biostratigraphic correlations in Yemen. In this paper an attempt is made to clarify the different types of facies, predict a depositional model and discuss

the palaeogeography of the middle-upper Jurassic sediments in the Sana'a region. Facies associations from the Amran Group are described and conclusions are drawn concerning depositional environments and controls on the formation of the sedimentary cycles.

Regional tectonic framework The sedimentary cover of the Arabian platform can be divided into two large complexes. The lower complex comprises deposits from the Precambrian to the Carboniferous. These units can be traced with slight changes from Arabia to Zagros and Anatolia. Similar sequences are recorded in lran, showing that lran was a part of Gondwanaland and, together with Arabia, belonged to the passive margin of the Palaeozoic Tethys. The upper complex includes sediments from the Permian to the lower Tertiary (Beydoun, 1988, 1991), Mesozoic sedimentation was associated with the opening of young oceanic basins that originated when several microcontinents split off from the Gondwanian margin. The source area of clastic material during the Mesozoic was the Arabian Nubian shield, which was surrounded by a fringe of lagoons and tidal plains. These clastic facies gave way to more open marine environments to the north and north-east, replaced at times by conditions of restricted and evaporitic basins. All major transgressions invaded from the Tethys Ocean (Beydoun, 1991). Some structural elements controlled Mesozoic facies distributions and were associated with breaking up of the shelf region into a pattern of basins and swells. These elements were either ancient and had been established

*Tel: + 9 6 7 I 2(10522; fax: +967 1 200564 or 416 295.

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Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 647 during the Palaeozoic and subsequently reactivated, or were new and related to Mesozoic tectonic activity. From the beginning of the Mesozoic to the Turonian, sedimentation on the shelf was largely controlled by northnortheast directed tectonic structures (Beydoun, 1988, 1991; Kerr and Holden, 1997), The transverse Mesozoic structures of the Arabian shelf were created by movement on Precambrian faults reactivated in the late Palaeozoic early Mesozoic. Kazmin et al. (1986) suggest that this reactivation was connected with the opening of an ocean southeast of Arabia, with the separation of India from Afro-Arabia. The ocean is believed to have formed in Jurassic times, following continental rifting. The north-east orientated structures of the Arabian shelf are parallel to this zone of major continental break-up. During Triassic to middle Jurassic times, Yemen was part of the Afro-Arabian plate of western Gondwanaland. In the Toarcian Bathonian, the sea did not extend as far as the studied areas, and western Yemen was a region of subsidence in which continental deposits of the Kohlan Formation accumulated. In the Bathonian, part of the marginal area between western and eastern Gondwanaland was subjected to a marine transgression. In the late Callovian, the sea transgressed, providing a passage between the Arabian Sea and African Sea, including Somalia and Ethiopia (Ministry of Oil and Mineral Resources, pers. commun.). During the late Jurassic, sea-floor spreading was well established in the Gulf of Somalia. The north south spreading direction would have induced a north-south extensional direction in south-east Sudan, Ethiopia, Somalia and Yemen; combined with the slow net eastward drift of the Afro-Arabian plate (Ministry of Oil and Mineral Resources, pers. commun.). The complex interaction of the Gulf of Somalia spreading ridges with differential spreading rates and offset by a series of right lateral transform faults of varying magnitude of relative displacement are thought to be signs of the presence of a late Jurassic mantle hot spot/plume under the Ogaden and Yemen regions (Ministry of Oil and Mineral Resources, pers, commun.). This produced a triple junction in the Yemen and Somalia section of the Afro-Arabian plate as a result of the rapid subsidence of the Marib-AI-Jawf basin in the central part of the Yemen (the Empty Quarter) (Figure 1), controlled by NW SE trending extensional faults (A1-Thour, 1988). The sediments of the lower part of the Amran Group (AI-Khothally Formation, Figure 3) were deposited during the earlier part of the basin development, prior to any significant rifting. Downwarping of the basin resulted in a deepening of environments throughout the basin, leading to the deposition of the middle part of the Amran Group (Raydah Formation) formed in response to continued transgression. In contrast, the uppermost part of the Amran Group (Wadi Al-Ahjur Formation) is characterized by much more significant microfacies variations suggesting more significant depositional gradients.

ations caused largely by relative sea-level changes. Simmons and A1-Thour (1994) have classified these sediments into ten biozones (Figure 4). The Amran Group can also be divided into three formations (A1-Thour, 1992) (Fiqure 3), on the basis of regional variations in thickness, facies and stratigraphy, which from the base to the top are:

A l-Khothally Formation The AI-Khothally Formation was first described by AIThour (1992). The type locality is 75 km to the northwest of Sana'a, at latitude 15'45' and longitude 43:45' near Kohlan Afar (Figure 2). The age is late CallovianOxfordian (Zones no. 1 5 of Simmons and Al-Thour, 1994), on the basis of microfossils, and the thickness is about 200m. It consists of sandy, oolitic, oncolitic, peloidal, partly dolomitic, massive, thick limestone. Hardground surfaces, gutter casts, bioturbation and burrows are well represented. The base of the formation either rests disconformably on the Kohlan Formation or rests unconformably on Precambrian basement. The upper surface passes gradationally up into the Raydah Formation.

Raydah Formation The Raydah Formation was described from the Raydah area by A1-Thour (1992), and is 56 km to the northwest of Sana'a. The type locality is at Hamdah (latitude 15'~51' longitude 43"57/) (Figure 2). The age is early Kimmeridgian (Zones no. 6--8 of Simmons and A1-Thour, 1994) and the thickness is about 400m. It shows several lithological facies. It starts with massive, thick, cherty, fossiliferous, bituminous grainstone, packstone, wackestone and mudstone. These pass up into marly fossiliferous limestone and at the top massive mudstones with echinoids, stromatoporoids and corals occur. This formation is very rich in fossils (e.g. bivalves, gastropods, brachiopods, echinoids, foraminifera, sponges, stromatoporoids, algae and corals). Its lower surface is gradational from the Al-Khothally Formation and the formation passes gradually up into the Wadi A1-Ahjur Formation.

Wadi A 1-Ahjur Formation The Wadi AI-Ahjur Formation was first described by ElAnbaawy (1984). The type locality of the Wadi Al-Ahjur Formation is 74 km to the west of Sana'a. It is located between longitude 4335' and latitude 1515' (Figure 2). The age is late Kimmeridgian-Tithonian (Zones no. 9 and 10 of Simmons and AI-Thour, 1994) and the thickness is about 300 m. It consists of several carbonate intervals which are mudstone, wackestone, packstone and grainstone intercalated with sandy dolomitic marly shale, fossiliferous and partly silty limestone. A palaeokarst surface including a Jurassic calcrete has been recognized at the top of the formation. The formation passes gradually up from the Raydah Formation and is unconformably overlain by the Cretaceous Tawilah Group.

Stratigraphy Palaeoecological investigations of the Amran Group (A1Thour, 1992) have revealed a range of benthic associations which principally record variations of oxygen levels, substrates and environment stability. Amran Group deposition was marked by widespread facies vari-

Facies associations The logged stratigraphic sections of the Amran Group carbonate sediments comprise three facies associations. Their organization is characterized by interfingering relationships, indicating that these sediments were

648

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deposited, in varying palaeoenvironmental conditions. Facies abundance with respect to stratigraphic sections are shown in Figure 5. The facies are grouped on the basis of shared sedimentological and palaeontological features, and named after the dominant environment in which they were interpreted to have been deposited. The three facies association, distinguished within the Amran Group, are: (F1) Carbonate platform facies (F2) Carbonate-marl alternations facies (F3) Shallow water coral and stromatoporoid buildup facies F1 dominates in all formations of the Amran Group. F2 is well developed in the Raydah and Wadi A1-Ahjur Formations whilst F3 can be recognized in the upper part of the Raydah Formation, but its distribution is limited.

Carbonate platform facies (F1) These shallow-water sediments rich in peloids and ooids are characterised by relatively condensed sequences with

mixed algal limestone of different microfacies and the presence of trace fossils Thalassinoides and possibly Rhizocorallium. The associated invertebrate macrofauna is both abundant and diverse, dominated by bivalves (Chlamys); gastropods (Globularia, Eunerinea, Trochalia, Nerinella) and echinoids (Cidaris, Pseudocidaris, Acrocidaris, Balanocidaris, Botryopneustes, Clypeus). Sponge occurrences are associated with relatively small numbers of bivalves, but a richness ofbrachiopods, indicating slightly deeper water facies (Hallam, 1975). Calcareous sponges and bryozoans are common in shallow water deposits. The upper unit of the A1-Khothally and Wadi Al-Ahjur Formations exhibits a repetition of coarsening-upward units with ooids and skeletal debris indicating regressive events. The presence of well developed ooids suggests depths of less than 3 m (Hallam, 1975). The deepest water sediments were probably laid down in water no deeper than 10-20 m and tend to be uniformly fine grained. Shallow-water carbonate sand bodies, oolitic, shelly packstone-grainstone have been reactivated in response

Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 649

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Figure 5 Stratigraphic sections showing facies associations and abundance within the different rock units of the Amran Group

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Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour

to storm conditions. Generally, storm deposits have been recognized to dominate many carbonate platforms (Aigner, 1985) and are found to be valuable tools in facies and palaeogeographic analysis. The Amran Group sediments show the development of storm events in the lower unit of the A1-Khothally Formation where some shells have been driven by strong currents into a muddy facies with clear development of gutter casts. In the upper unit of the AI-Khothally Formation graded bedding has been recognized indicating shallow-marine environments and/or storm events (Einsele, 1992). In the upper part of the Raydah Formation, storms are indicated by the deposition of silty mudstone within the marly shaly limestones. In the Wadi Al-Ahjur Formation, the variations of microfacies as well as the development of a calcrete and palaeokarst surface indicate emergence and sea-level fall. The rare and/or absence of ammonites may relate to variations in salinity on the shallow carbonate platform. Faecal pellets are generally formed within protected areas in association with fine-grained lime sediment. They may have been produced by gastropods which ingest muddy sediments and produce pellets. This facies is distributed in all rock units of the Amran Group.

matoporoid build-ups are developed locally, but they never form true reefs. The dominant corals are Stylina, Amopniastrea and Isastrocenia lobata which occur locally as coral and stromatoporoid build-ups. This facies can be recognized in the upper part of the Raydah Formation, but its distribution is limited.

Depositional models Based on the analysis of the outcrops in the studied areas, preliminary predictive depositional models are based on several key observations concerning the depositional regime and several assumptions about overall depositional patterns. A depositional model of the Al-Khothally and Raydah Formations (Figure 6) represents a carbonate platform which shows a marginal hydrodynamic shoal where ooids thrived and from which currents distributed bioclasts landward and seaward, with the development of protected basins. A back-platform hydrodynamic build-up consisting of stromatoporoids, corals and bryozoans in more protected conditions.

Carbonate-marl ahernations facies (F2)

The Al-Khothally Formation

The carbonate-marl alternations are characterized by variable marly shaly limestone sequences, rich in fauna and intercalated with thin, massive limestone beds. The thickness of the marly shaly limestones are on average three times thicker than the massive limestone beds. This facies shows abundant and diverse invertebrate macrofauna, dominated by bivalves (Chlamys, Pholadomya, Plagiostoma); brachiopods (Ornithella, Kutchithyris), gastropods (Globularia, Ampullospina, Eunerinea, Trochalia, Nerinella) and echinoids (Cidaris, Pseudocidaris,

The carbonate sequence in the A1-Khothally Formation starts, in the late Callovian, with a transgression of the sea indicated by the deposition of skeletal packstone with common quartz sand. This formation comprises a thick sequence of carbonate deposits showing both fining- and coarsening-upward cycles at different scales (e.g. m and din). The lower unit of the AI-Khothally Formation (Figure 3) represents a relative sea-level rise as indicated by the appearance of abundant marine fossils including foraminifera (Kurnubia palastiniensis), algae, echinoderms, sponge spicules, ostracods, and molluscan debris. The following lithotypes occur: peloidal packstone, peloidal wackestone, oncolitic wackestone, skeletal wackestone, skeletal packstone and marly and shaly fossiliferous limestone. The sediments are organised in fining-upward sequences in large scale, but smaller-scale coarseningupward cycles (about 0.30-0.40m thick) are well developed showing the domination of hardground surfaces capping the top of each cycle. The coarse cycle tops are dominated by carbonate beds with burrows, intense bioturbation, fractures and characterized by sandy mudstone, sandy peloidal wackestone and packstone microfacies; and not dominated by shaly beds (Figure 7). The sediments of this unit were deposited in a low energy, shallow marine environment. The presence of burrows indicates an oxygenated sediment-water interface in an environment protected from constant wave winnowing and reworking. Hardgrounds indicate early diagenetic events associated with an interruption in deposition and synsedimentary cementation (Kennedy and Garrison, 1975). They are encrusted by thin shelled oysters and sands indicating a brief sedimentary break. Hardgrounds occur as an integral part of ooid-bearing ancient platform margin sequences (Halley et al., 1983) or as the termination of shallowing upward sequences (Purser, 1969; James, 1979; Moore, 1989). If the rate of sediment accumulation is low enough to allow partial to complete cementation, the rock is likely to be periodically uncovered on the sea floor by currents and thus subjected to intensive boring. Iron hydroxide crusts are common on shelf hardgrounds. The greenish shaly limestone

Acrocidaris, Balanocidaris, Botryopneustes, Clypeus). This facies is well distributed in the Raydah and Wadi AI-Ahjur Formations, while it is very limited in the AlKhothally Formation. Einsele and Ricken (1991) and Arthur and Dean (1991) suggest that there are three basic processes which can produce limestone marl alternations. These are dependent on variations in carbonate production, periodic carbonate dissolution, and periodic terrigenous dilution. Each process represents an end member of alternations with simultaneous oscillation of several inputs. Diester-Haas, 1991 states that, terrigenous dilution cycles are indicated by the increase of terrigenous influx in relation to variation in climate, vegetation and/or sealevel change, and may contain intercalations of distal carbonate and siliciclastic tempestites. With increasing terrigenous influx, limestone marl successions are replaced by clay-marl sequences displaying thicker bedding couplets but thin marl beds. Shallowing of the sea favours reworking, omission, channeling and interfingering and replacement of the bedded limestone by bioclastic carbonate arenites and reefs. The marly limestone alternations of most of the Amran sequence are rich in benthonic fossils and were evidently deposited in quite shallow water. Therefore, carbonate dissolution can be ruled out.

Shallow water coral and stromatoporoid build-up .facies (F3) This facies consists of low-energy shallow carbonate platform deposits as shown by the presence of corals. Stro-

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association of the middle part of this unit indicates a very quiet water or basin type of environment. The upper unit (Figure 3) represents a relative sealevel fall as indicated by the domination of high energy microfacies such as sandy grainstone, peloidal grainstone, peloidal packstone, grainstone-packstone and their association with algae, corals, stromatoporoids, foraminifera, ostracods, bryozoan debris and sponge spicules. The shallowing-upward sequence of the upper unit displays less hardground surfaces than the lower unit, but graded bedding is well developed. Coarsening-upward cycles are developed within 28 m of the upper unit (Figure 8). The lower part comprises a skeletal wackestone microfacies including common stromatoporoid debris and rare algae and foraminifera• Burrows can be recognized. This microfacies is followed by peloidal grainstone.including coral debris, ostracods and algae. The middle part is characterized by oncolites that are associated with algae and stromatoporoids, bryozoa and ostracods. Graded bedding is well developed showing small scale coarsening- and fining-upward portions. The upper part starts with a skeletal wackestone including stromatoporoids, foraminifera and algae. This is followed by peloidal to sandy grainstone including sponge spicules, molluscan shells, foraminifera and algae. Graded bedding is a characteristic feature of this unit. It is found in beds of variable thickness, the average thickness of such beds being about 0.80 m. It is associated with moderate to high energy microfacies, and characterized by a fining-upward sequence of bioclasts, oncoids and pellets that either rest upon a sharp erosional base to produce normal graded bedding or show variation in texture, starting with coarsening-upward and ending with fining-upward, thereby producing composite graded bedding. Graded bedding demonstrates reworking followed by the redeposition of sediment, under conditions of progressively falling current or wave action.

There are several pieces of evidence for the effects of storm deposition in the Al-Khothally Formation. (1) The presence of densely packed concentrations of skeletal material, overlying (2) sharp erosional bases with (3) gutter casts. (4) The presence of sandy grainstone indicate storm deposits. (5) Small-scale fining-upward cycles also occur in the form of alternating marly limestones with fragmented fossils, sharp erosional bases and gutter casts, and shaly limestones with bioturbation and intact fossils. These fining- and coarsening-upward cycles are interpreted to be the product of episodic erosion followed by rapid redeposition of sediments and quiet conditions during which shaly limestone was deposited. The association of benthonic foraminifera and algae (Kurnubia sp., Trocholina spp., Salpingoporella annulata 'Trinocladus perplexus'), miliolids, echinoderms and stromatoporoids indicates that these sediments were all deposited in a shallow marine environment. The sediments of this unit indicate deposition in a moderate to high energy shallow marine environment. The graded bedding, grain size, and the rounding of the bioclastic grains indicates these sediments were subjected to transport in a wave-dominated environment. The predominance of corals and calcareous green algae indicate deposition within the photic zone. Stromatoporoid buildups are developed locally, but they never form true reefs. The presence of burrows and a fairly diverse fauna indicate marine conditions and an oxygenated sediment. Well rounded stromatoporoid fragments in packstone are evidence of prolonged abrasion of bioclastic debris in a relatively high energy regime. Beds of wackestone were deposited in the lee of stromatoporoid build-ups or in relatively low energy inter-shoal areas (Fiqure 6). The cyclicity within the A1-Khothally Formation can be interpreted as being due to variations of relative sealevel. The cycles represent a shallowing-upward of facies following a relative rise in sea-level. The shallowing-up

652

Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 6

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probably results from sediment prograding seaward in respond to high shallow water carbonate production.

Raydah Formation The early Kimmeridgian Raydah Formation shows the continuation of the same pattern of deposition with the development of large-scale fining-upward cycles (Figure 3). A wide range of fossils occurs including algae, sponge spicules, foraminifera, ostracods and bivalves. Chert nodules are found as well as an intercalation of the different medium to high energy microfacies with marly shaly fossiliferous limestone. The presence of quartz silt indicates the involvement of currents which might have been developed by storms. The main differences from the AI-Khothally Formation are firstly that the marly fossiliferous limestone beds are on average three times thicker than the massive skeletal wackestone, skeletal packstone and oncolitic packstone beds (Figure5). Therefore, the marly limestone alteration with the rest of the microfacies is a striking characteristic feature of the Raydah Formation. Secondly, within the large-scale cycles, either continuous transgression or shoals have allowed the growth of coral

and stromatoporoids build-ups. Burrows can be recognized, and coquinas occur that are highly ferruginous due to intensive sea floor alteration. The cycles show a range from coarse to fine grained microfacies associated with different fauna. Fining-upward cycles can be recognized including a succession of sharp erosional bases associated with high energy sediments overlain by marly shaly fossiliferous limestone. This feature has been repeatedly developed in all traverses and sometimes at the same stratigraphic level, for example the upper part of biozone seven, particularly in AI-Khothally, Hamdah and Wadi Nager traverses (Figure 3). Nodular limestones are common in the Raydah Formation. In general, nodularity within limestone has been recorded by different workers from a range of carbonate environments (Fttrsich, 1973; Jenkyns, 1974; Wilson, 1975; Aigner, 1985). In the Raydah Formation, it is more likely that a combination of both burrowing and burial diagenesis has caused the development of nodular limestone. The presence of microstylolites, microstylolite seams, clay seams and gradational top and bottom boundaries of nodules show evidence for pressure solu-

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tion (Wanless, 1979). Logan and Semeniuk (1976) have used the name stylonodular rock fabric to identify such structures. At certain horizons, marly shaly fossiliferous limestones are intercalated with silty mudstones indicating storm events. The storm-induced depositional mechanism is the reasonable and accepted interpretation for these sort of alternations (Einsele, 1992), but the paucity of distinctive sedimentary structures and the dominance of intensive bioturbation make this interpretation uncertain. The sharp bases and composition of the microfacies demonstrate that the marly shaly fossiliferous limestone are distal, deeper water sediments. The appearance of silty mudstones within the marly shaly fossiliferous lime-

stone also indicates storm events (Brett, 1983). Bioturbation followed by episodic post-event colonization suggests that these massive limestones are the most distal fine tails of storm flows (Aigner, 1985) in offshore deeper water of an intraplatform basin (Figure 6).

Wadi Al-Ahjur Formation The Wadi AI-Ahjur Formation, (late KimmeridgianTithonian,) was developed within a shallow, carbonate platform environment (Figure 9). The influx of terrigenous materials was moderate and the sediments are mixed carbonate-silicidastic near the shoreline. The formation comprises oolitic grainstone, dolomite

654

Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour Coastal mixed carbonate siliclastic facies belt

Terrestrial Kohlan Sandstone

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and marly shaly and partly silty limestone intercalations. In the Wadi A1-Ahjur, nodular mudstone and/or wackestone show continuous transitions to distinctly bedded limestone/marly limestone alternations. The gradational nodule boundaries with zones of microstylolites are typical of pressure-solution effects. This part of the Amran Group is characterised by a predominance of coarsening upward cycles. The predominance of shallowing upward cycles indicates frequent transgressiv~regressive events affecting the platform top. The Wadi A1-Ahjur Formation shows that carbonate deposition persisted through the early Tithonian. Following this a sequence of mixed carbonat~siliciclastics of the late Tithonian was deposited, characterized by alternations of shales or marls with sandy limestone microfacies. This represents the shallowest part of the Amran Group. The sediments of this formation were highly affected by both sea-level change and tectonic movement. Cyclicity within the lower part of the Wadi A1-Ahjur Formation shows the development of both coarsening- and fining-upward cycles, and indicate a similar pattern of deposition to those sediments of the A1-Khothally and Raydah Formations. At the top of the formation, a palaeokarst surface (Figure 10) is developed within mixed carbonate siliciclastics. An upward transition occurs from the muddy facies to a coarser sandy and dolomitic facies. No corresponding sequences have been identified in the A1-Khothally and Raydah Formations. This upward transition from fine grained facies to very coarse grained facies is associated with shallowing-upward cycles (Figure 10). The abrupt change from mudstone to conglomeratic sandstone marks a sharp unconformity between the Jurassic Amran Group and the Cretaceous Tawilah Group. This results from emergent conditions that led to the formation of a palaeokarst surface overlain by conglomerates of the basal Tawilah Group. This is followed by a rapid tran-

sition into fluviatile deposits of the A1-Ghiras Formation (A1-Subbary et al., 1993) The palaeokarst surface has fracture controlled, funnel-shaped cavities of various sizes locally with flat

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Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 655 bottoms. These cavities are filled with Cretaceous sediments. The special criteria which are used to recognize palaeokarst are reviewed in Esteban and Klappa (1983) and Choquette and James (1987). The most obvious are the morphology of the surface; the nature of the materials which fill cavities, the presence of collapse breccias, soils and some diagenetic features of the wall rock (Vera et al., 1988; Wright et al., 1991). The palaeokarst surfaces at the top part of the A1-Ayein traverse (Figure 10) are characterised in plan view by a surface cut by two sets of vertical cracks at right angles that divide the surface into irregular squares. The overlying Tawilah Group partially covers the underlying palaeokarst and consists of five lithofacies: (1) breccia formed of reworked limestone forming a soil regolith; (2) conglomerate representing gravel-dominated braided-fluvial deposits; (3) sandstone representing storm dominated fluvio-lacustrine deposits; (4) interbedded sandy limestone, mudstone and clay and (5) ferruginous red sandstone showing graded bedding. Karst processes have been traditionally regarded as being related to near-surface meteoric (rain-sourced) water, but it is becoming increasingly clear that significant dissolution of carbonates can also take place in other settings, related to other fluids besides simply meteoricsourced groundwaters. Most meteoric paleokarsts have originated where shallow-marine limestones have become subaerially exposed by a fall in relative sea-level (Wright et al., 1991). This palaeokarst is interpreted as a product of emergence and meteoric diagenesis because of the terrestrial depositional environment of the overlying Cretaceous fluvial sediments (Tawilah Group) and the presence of dissolution cavities in the underlying sediments (Amran Group). The base level was not far below the surface of the emergent platform because of the shallow (tens of cm) depth of penetration of cavities in the underlying limestone. Direct evidence of vegetation cover can be found in the form of tree root moulds in the palaeokarst surface (Figure 10) but any soil present must have been subsequently removed by fluvial processes. The karst is regarded as immature because the subsurface karst is not well developed and there is an absence of large caves and thick speleothems. Therefore, a short period of time was associated with extensive emergence of a flat late Jurassic carbonate platform. Rapid burial by Cretaceous sandstones and conglomerates helped preserve the palaeokarst. The preserved fracture pattern of the Upper Jurassic platform carbonates indicates that the joints were either developed, or were in the process of forming at the time karstification occurred. Features on the karst surface demonstrate that the absence of rillenkarren means that the exposed Jurassic carbonates formed a widespread flat surface within the Thula area. The palaeokarst at Thula is related to a stratigraphic break which affected the Amran Group during the late Jurassic (Figure 10). It is interpreted to be related to local emergence resulting from both listric faulting and eustatic sea-level fall.

Evolution of the Amran Group platform The middle to late Jurassic was a time of gradual transgression of shallow sea from the east and south. During this period, the platform went through steady subsidence,

which occurred against a background of sea-level fluctuations. These could be of a eustatic nature but this is difficult to prove in the absence of refined biostratigraphic correlation. A quite stable pattern of sedimentation was established in the basin with abundant stromatoporoid and coral build-ups. Mixed carbonate-siliciclastic sediments were deposited at a later stage, indicating tectonic differentiation of the platform. The first transgression phase led to the deposition of the AI-Khothally Formation (Figure 11). The second phase produced a deeper water facies of the Raydah Formation. The relative sealevel changes together with increasing tectonic activity (Figure 11) led to the deposition of mixed carbonate siliciclastic sediments of the Wadi AI-Ahjur Formation, with the development of palaeokarst surfaces at the top part of the Al-Ayein traverse indicating eventual emergence. Biostratigraphic evidence shows that the development of the carbonate platform, in the studied areas, began in late Callovian time. During Oxfordian-Kimmeridgian times, some tectonic differentiation occurred and the carbonate platform was submerged with deposits of the Raydah Formation. This indicates a major relative rise of sea-level that may be related to a gradual subsidence of the shelf. However, the global picture at this time suggests that it is more likely that this reflects a eustatic rise of sealevel (Hallam, 1978, 1988). The sedimentation pattern changed sharply in the Tithonian, when a major sea-level fall occurred. Carbonate and siliciclastic deposits were established on the shelf and parts became emergent.

Palaeogeography (Figure 12) There are no carbonate outcrops of beds of early Jurassic age known in Yemen and none have been recognized in wells drilled in the central and south-eastern part of the country (Figure 12). The oldest recorded carbonate outcrops are of Bathonian age in Shuqra (Beydoun, 1964). In the studied areas, neither Bajocian nor Bathonian sediments have been recorded. Microfossil and palynomorph analysis confirm that deposition of the Amran Group started in the late Callovian. This may indicate that there was a break in deposition, within the studied areas, from early Jurassic to late Callovian, and that the Kohlan Formation could represent a considerable time span. At the beginning of the Mesozoic, north-east trending basins were formed in Yemen as well as in northern Somalia, which were filled with the fluviolacustrine shales and sandstone of the Kohlan Formation and its equivalent, the Adigrat Sandstone (Figure 12). Unpublished reports (Ministry of Oil and Mineral Resources, pers. commun.) suggest that the Kohlan Formation (taken as the lower unit of Amran Group) is of early-middle Jurassic age. The oldest exposure of the Amran Group is likely to be no older than late Callovian, suggesting a time break. These discrepancies between ages need to be resolved, and require further analysis of samples from different exposures as well as from wells. Following the deposition of the Kohlan Formation and Adigrat Sandstone, a major marine transgression resulted in the deposition of carbonate sediment over Yemen, Somalia, Oman and Saudi Arabia (Figure 13). This flooding of the Arabian platform primarily from the north-east, east and from the south as well, had already initiated carbonate deposition in Saudi Arabia, as indicated by the upper part of the Marrat and Dhruma For-

656

Faciessequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour

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mations in the lower-middle Jurassic (A1-Thour, 1992). Palynomorph and microfossil analysis suggest that the transgression in Yemen is no older than Bathonian and in the studied areas would be no older than late Callovian. Saint-Marc (1991) showed that the Jurassic sediments in the northern part of the Arabian Peninsula are discordant upon the Triassic. This is interpreted as evidence of orogenic movements at the end of the Triassic which presumably led to the emergence of most of the central and southern parts of the region where marine deposits are absent for the lower and most of the middle Jurassic. In the middle-late Jurassic, the Arabian shield was largely covered by a transgressive sea which extended to the Yemen where it was in continuity with the East African Sea (Figure 13). This transgression caused the deposition of the Shuqra Formation in the south and south-east

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in Bathonian-Callovian times. As the sea continued to transgress, the sediments of the AI-Khothally Formation were deposited in the late Callovian-late Oxfordian. By the early Kimmeridgian, Yemen was wholly covered by a shallow sea that extended across to Somalia and Ethiopia as indicated by the sediments of the Raydah Formation (Figure 14). During the late Kimmeridgian-early Tithonian, the facies indicate that the sea covered most of Yemen. Subsidence led to deeper water deposits being laid down. Near the end of the Tithonian, the sea began to regress as indicated by the facies. Emergence in the region exerted an influence on facies and thickness, permitting deposition of clastics, particularly in the upper parts of AI-Ayein and Wadi A1-Ahjur traverses. Elsewhere during the late Jurassic, particularly the Kimmeridgian-Tithonian, epeirogenic oscillations in a

Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 657

YEMEN

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Figure 12 Facies map of the Lower Jurassic of Yemen, Somalia and Ethiopia

realm of arid climate led to the periodic interbedding of evaporites with shallow carbonate sediments in the central and south-eastern part of the country. This caused the deposition of the Safer Formation (A1-Thour, 1992). Correlative evaporites extend from Iraq across Eastern Arabia to Yemen, excluding Oman and Somalia. Late Jurassic (Tithonian) and early Cretaceous (BerriasianValanginian) tectonic movements led to differential vertical uplift of blocks within many parts of the region and resulted in the variable removal of the earlier part of the succession from many of the elevated highs (in Yemen, Central Arabia, Iraq and Syria), (Beydoun, 1988, 1991). Locally, this tectonism was accompanied by volcanic activity in the western part of Yemen (Saint-Marc, 1991). Clastics were transported radially out into the surrounding subsiding basins leading to interfingering of elastics with carbonates in the Naifa Formation (Beydoun, 1964, 1988; AI-Thour, 1992). In conclusion, the Arabian Peninsula represents a huge carbonate platform developed in Mesozoic time, with the deepest water sedimentation in north-east Iraq. Yemen represents the shallowest part of the platform as indicated by a comparison of the facies and their age correlation within the whole peninsula. A depositional model for the Arabian Peninsula has been constructed, in an attempt to clarify the depositional pattern of the carbonate sediments (Figure 15).

Conclusions The Jurassic Amran Group, extensively exposed in the western part of Yemen, has been interpreted in the light

of five major logged sections and facies analysis of the whole sequence. Three facies associations have been identified: 1. Shallow water carbonate platform; 2. Carbonate-marl alternations; 3. Shallow water coral and stromatoporoid build-ups. The facies variations reflect deposition on a broad platform upon which shoals separated platform carbonates from an intraplatform basin of sedimentation and an open marine environment. The lateral and vertical changes of facies are interpreted as due to relative sea-level changes created by associated tectonism and eustacy. The repetition and interfingering of both fining- and shallowing-upward cycles within the study areas suggest that deposition occurred within a period of fluctuating sealevel. The sediments of the Amran Group were deposited on a large carbonate platform that occupied much of the Arabian Peninsula. The Raydah Formation was deposited in a shallow basinal environment, deeper than the A1-Khothally Formation, while the Wadi AI-Ahjur Formation represents the shallowest portion of the sequence. The overlying Cretaceous Tawilah Group is composed of fluvial siliciclastic sediments.

Acknowledgements The author acknowledges the useful comments of Professor Dr Z. Beydoun on reading the final manuscript of this work.

658

Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI- Thour

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Facies sequences of the Middle-Upper Jurassic carbonate platform: K. A. AI-Thour 659 CARBONATE PLATFORM

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