Differential severity of Permian–Triassic environmental changes on Tethyan shallow-water carbonate platforms

Global and Planetary Change 55 (2007) 209 – 235 www.elsevier.com/locate/gloplacha

Differential severity of Permian–Triassic environmental changes on Tethyan shallow-water carbonate platforms Oliver Weidlich a,⁎, Michaela Bernecker b b

a Royal Holloway, University of London, Egham, Surrey, TW20 0EX, United Kingdom Institute of Paleontology, University Erlangen, Loewenichstrasse 28, D-91054 Erlangen, Germany

Received 10 June 2005; accepted 30 June 2006 Available online 26 September 2006

Abstract Significantly different Guadalupian–Late Triassic patterns are observed in the evolution of attached and isolated carbonate platforms of the southwestern Tethys (Oman Mountains). Close to the rim of the attached Arabian platform, carbonates of the Saiq and Mahil Formations reveal an almost complete Permian–Triassic sedimentary record. Guadalupian–Changhsingian 3rd order sequences consist of fossiliferous transgressive systems tracts and monotonous highstand systems tracts with mud/wackestone and coral bafflestone. The youngest Changhsingian beds are bioturbated floatstone with crinoids, sponges and bryozoans. All sediments indicate a healthy, tropical carbonate production. Above, a unique facies change begins with a pyrite-encrusted omission surface. Greenish mudstone rich in authigenic pyrite infills the relief of the unconformity and is overlain by clastic sediment and by laminated, microbialite-bearing carbonate. Unfossiliferous sediments and seafloor cements indicate a change in carbonate production towards abiotic processes. Prevailing anoxic conditions were interrupted by seven oxic event beds, as indicated either by low-diversity and small-sized ichnotaxa or by shell beds with low-diversity bivalve and crinoid assemblages. By comparison with published data, the described sedimentary sequence can be assigned to the Changhsingian–earliest Griesbachian. Beginning probably with the Anisian, bioturbated Griesbachian–Dienerian recovery period and the unconformity below to the latest grainsupported sediment textures mark the return to biogenic tropical carbonate production under oxic conditions. The Middle–Late Triassic carbonate platform consists of stacked high-frequency shallowing upward cycles. By contrast, carbonate production of Neo-Tethyan isolated platforms was discontinuous and interrupted by a large gap. Guadalupian deposits of the Al Jil Formation consist of bioclastic limestone typical of a tropical carbonate production. The uppermost bed, an impoverished bioclastic packstone capped by an unconformity, marks the onset of platform drowning which resulted from the end-Guadalupian mass extinction. Above, a polymict breccia witnessed rift pulses of the Neo-Tethys. The overlying pelagic mud- and packstone contains radiolarians and rare foraminifera of Lopingian age, and overlying microbialites. In the Carnian, tropical shallow-water carbonate production restarted with a low-relief platform and culminated in a Norian–Rhaetian reef-rimmed platform. Stacked Lofer cycles dominated the inner platform of Jebel Kawr (Misfah Formation). We here propose a differential onset and severity of the Late Permian mass extinctions for carbonate platforms. On the Arabian Plate, tropical carbonate production collapsed after the end-Lopingian mass extinction and was replaced by microbialites and sea-floor cements during the earliest Triassic. After approximately six million years, tropical shallow-water carbonate production resumed in the Middle Triassic. Neo-Tethyan isolated platforms drowned shortly after the end-Guadalupian mass extinction and did not recover before the Late Triassic. Absence of shallow-water limestone

⁎ Corresponding author. Present address: Earth Sciences Department, Sultan Qaboos University, P.O.Box 36, Postal Code: Al-Khodh 123, Muscat Sultanate of Oman. Tel.: +968 24141405; fax: +968 24413415. E-mail address: [email protected] (O. Weidlich). 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.06.014

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suggests that carbonate production of isolated platforms ceased for about 30 million years, a period exceeding the recovery of most marine ecosystems. © 2006 Elsevier B.V. All rights reserved. Keywords: carbonate platform; Permian; Triassic; mass extinctions; recovery; Tethys

1. Introduction The Paleozoic came to an end with the biggest mass extinction of the Phanerozoic. It is now widely accepted that two events, one at the end of the Guadalupian (Middle Permian) and one at the end of the Lopingian (Late Permian), caused the devastation of marine and terrestrial ecosystems (Stanley and Yang, 1994; Jin et al., 1994). The recovery period of marine ecosystems following the mass extinction was extraordinarily prolonged and for metazoan reefs took until the early Middle Triassic (Flügel, 2002; Flügel and Kiessling, 2002; Weidlich, 2002a,b). The impact of the Permian mass extinctions on biodiversity and ecosystems has been already analysed in detail (Erwin et al., 2002 and further references herein). However, the influence of the endPermian mass extinctions on carbonate platforms has rarely been reviewed (e.g., South China, Lehrmann et al., 1998, 2001, 2003; North Italy, Noé, 1987; Twitchett and Wignall, 1996). Sediments of carbonate platforms are high-resolution archives of environmental change. Marine carbonate precipitation represents a continuum of processes with (quasi-)abiotic (spontaneous mineralization), (non-enzymatic) biologically-induced and (enzymatic) biologicallycontrolled precipitation as end members (Webb, 2001; Schlager, 2003). Gradational boundaries especially between biologically-induced and controlled carbonate precipitation exist. Based on the dominance of ongoing seafloor processes, a tropical shallow-water factory with emphasis on biologically-induced and abiotic precipitation, a cool-water factory with emphasis on biological precipitation, and a mud-mound factory with emphasis on abiotic precipitation were differentiated (Schlager, 2003). Environmental perturbations may cause a change in carbonate production systems of platforms from a dominant tropical factory to a mud-mound carbonate factory, or may even provoke drowning (Schlager, 1999, 2003). Consequently, a switch from a tropical carbonate factory towards a mud-mound factory has been proposed during the aftermath of mass extinctions (Schlager, 2003). Sediments of the mud-mound factory deposited during the aftermath of mass extinctions are similar in appearance to

Early Paleozoic, or even Late Neoproterozoic, carbonates due to the scarcity of marine benthos and, therefore, have been described as “anachronistic” (Lehrmann et al., 2001). The combination of microbialites and sediment structures, including “flaser-bedded ribbon rock” (the result of interlayering of grainstone lenses with continuous lime mudstone drapes sensu Lehrmann et al., 2001), flat pebble conglomerate (Wignall and Twitchett, 1999) and wrinkle structures (Pruss et al., 2004) are testimonies of unusual marine environments after mass extinctions. We here compare the Middle Permian–Late Triassic development of the attached carbonate platform rimming the Arabian Shield with isolated platforms of the NeoTethys. The focus is on the Permian Saiq and Triassic Mahil Formations representing the attached carbonate platform and on the Permian Al Jil and Triassic Misfah Formations from the isolated platforms of the Neo-Tethys. Based on biostratigraphic data and integrated facies analyses, our main objectives are (i) to recognize largescale changes in the mode of carbonate precipitation and assign these changes to carbonate factories, (ii) to redefine the Permian–Triassic boundary in this area, and (iii) finally to interpret the impact of both mass extinctions on the long-term evolution of carbonate platforms. 2. Geological setting and study areas 2.1. Setting Permian–Triassic shelf deposits crop out along the eastern rim of the Arabian Shield in Saudi Arabia (e.g., Al-Laboun, 1986), in the Haushi–Huqf area of Central Oman (Dubreuilh et al., 1992), and in the Oman Mountains. Tectonic nappes of the Oman Mountains containing relics of Middle Permian–Late Triassic carbonate platforms include the Arabian Plate (Saiq and Mahil Formations) and Hawasina Basin (Neo-Tethys: Bai'id, Al Jil and Misfah Formations), see Fig. 1.1. According to palinspastic reconstructions, the isolated Permian Ba'id and the Triassic Kawr platforms of the Hawasina basin were situated north to northeast of the Arabian Plate in the Neo-Tethys, (Béchennec et al., 1990), see Fig. 1.2.

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Fig. 1. Geological map and palinspastic reconstruction of Permian–Triassic carbonate-bearing strata. (1) Simplified geological map and study areas of shallow-water carbonates in the Oman Mountains. (2) Palinspastic reconstruction from the Arabian Plate and the Neo-Tethys (Hawasina Basin) during Permian–Triassic times (modified after Béchennec et al., 1990).

Shallow-water tropical carbonate deposition started on the Arabian Plate after the Permo-Carboniferous glaciation with the Saiq Formation. The succession of the Saiq

and Mahil Formations represents a transgressive–regressive sequence without larger gaps in sedimentation (Fig. 2). The oldest limestones of the Saiq Formation

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Fig. 2. Permian–Triassic overview showing time scale (Gradstein et al., 2005), distribution of formations (the boundary between the Saiq and Mahil Formations is uncertain and therefore stippled) and internal architecture of carbonate platforms. Two significant changes of carbonate platform evolution are associated with both mass extinctions. The record of Tethyan isolated carbonate platforms shows a gap of about 30 million years, while carbonate sedimentation prevailed almost continuously on the Arabian Plate. Note the change in internal platform architecture from (1) Guadalupian– Lopingian 3rd order sequences to (2) Early Triassic non-cyclic mixed carbonate–siliciclastic sediments and finally to (3) Middle–Late Triassic high frequency cycles. Signatures: black = shallow-water carbonates, gray = uncertain shallow-water carbonates, stripes = gaps in carbonate platform evolution.

have a Wordian age (Neoschwagerina schuberti Zone) and carbonate deposition persisted until the Changhsingian (Fig. 2) (Glennie et al., 1974; Montenat et al., 1976; Le Métour et al., 1994, 1995). The Saih Hatat, situated close to the rim of platform, was affected by repeated tectonic pulses during Guadalupian and Early Triassic time, which caused rapid facies changes and the deposition of mafic volcanics (Le Métour, 1987; Le Métour et al., 1986; Béchennec et al., 1993). Unusually

thick deposits of latest Permian–earliest Triassic sedimentary sequences accumulated locally in small halfgraben structures. The position of the Permian–Triassic Boundary (PTB) has not been fixed due to the absence of diagnostic conodonts. Stable isotopes have failed to provide unequivocal environmental signals because of early diagenetic dolomitization, and later metamorphism during obduction of the Semail Ophiolite. Triassic shallowwater sediments of the Mahil Formation begin with

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unfossiliferous mixed carbonate–siliciclastic deposits and switch to stacked shallowing upward cycles capped by subaerial exposure horizons. The nappes of the Hawasina Complex document the evolution of the Neo-Tethys. Complete isolated carbonate platforms have not been preserved and palinspastic reconstructions are controversial. However, there is strong evidence that a Permian isolated platform existed close to the rim of the Arabian Plate and a Triassic isolated platform occurred in a more distal position with respect to the shelf margin of Gondwana (e.g., Pillevuit et al., 1997). Relics of Middle Permian and Middle– Late Triassic shallow-water carbonates, so-called “Oman Exotics”, have been preserved as breccias or enormous mega-blocks attaining a size of several kilometers. Guadalupian shallow-water facies has been found in the Ba'id area (e.g., Blendinger, 1988) and near Nakhl, respectively. The presence of Lopingian–Early Triassic shallow-water carbonates remains questionable (cephalopod limestones of the so-called Wasit-block (e.g., Krystyn et al., 2003) are not indicative of a shallowwater platform environment). Shallow-water platform relics exposed at Jibal Kawr, Misht, Misfah, and Ghul (Kawr Group: Misfah Formation: Beurrier et al., 1986; Minoux and Janjou, 1986) are all Late Triassic in age. Upon a volcanic base, platform rim reefs and cyclic platform carbonates with a Carnian to Rhaetian age crop out at Jebel Kawr (Bernecker, 1996). The sedimentary record of the Neo-Tethys is punctuated by a large gap (Fig. 2). 2.2. Study areas Our interpretations are based on measured sections from the Arabian Plate and Neo-Tethys (Figs. 1 and 2).

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(3) Section Aday 3 (23°35′00″′N, 58°31′10″E) comprises the Middle Triassic–?Late Triassic interval of the study area. The measured log attains a thickness of 75 m. (4) Section Aday 4, (23°35′30″N, 58°31′20″E) immediately south of Aday circle, comprises Late Triassic sediments of the Mahil Formation. The section has a thickness of 15 m. 2.2.2. Neo-Tethys Relics of a Permian isolated platform were measured in a hillock in the northeastern part of the village Nakhl (23°23′30″N, 57°50′00″E). Guadalupian–Early Triassic sediments of the Al Jil Formation of section Nakhl attain a thickness of 30 m. The Late Triassic Misfah Formation is exposed in the Western Oman Mountains mainly at Jibal Misfah, Misht and Kawr in the area SW of Jebel Akhdar north of the road between Bahla and Ibri (Beurrier et al., 1986; Minoux and Janjou, 1986). Two sections were studied at Jebel Kawr: (1) Section Sint (23°09′50″N, 57°03′10″E) is situated at the northern rim of Jebel Kawr, northwest of the village of Sint and comprises the Carnian to Rhaetian of the study area. The section attains a thickness of 185 m. (2) Section Ala (23°05′30″N, 57°07′30″E), situated in Wadi Ala at the southeastern side of Jebel Kawr near the village of Ala, comprises the Carnian (?), Norian and Rhaetian. The measured log has a thickness of 180 m. 3. Carbonate platform evolution of the Arabian Plate 3.1. Guadalupian to Lopingian

2.2.1. Arabian Plate Along the road from Muscat to Quriyat, the Saiq and Mahil Formations were investigated in the vicinity of the capital area on the eastern side of Wadi Aday. The following sections were measured: (1) Section Aday 1 (23°33′00″N, 58°32′10″E), situated at a prominent hairpin 4.5 km south of Aday circle, comprises the Guadalupian–Lopingian of the Saiq Formation. The measured log attains a thickness of 230 m. (2) Sections Aday 2A and 2B (23°34′00″N, 58°31′ 30″E), approximately 2 km south of Aday circle, comprise the Lopingian–Middle Triassic interval. The Saiq and Mahil Formations attain a thickness of 200 m.

3.1.1. Stratigraphy A Wordian to Wuchiapingian age has been assigned to the Saiq Formation based on biostratigraphic data from Jebel Akhdar (Montenat et al., 1976). Revising these data, a Wordian to Changsinghian (Late Lopingian) age is now considered (Dawson et al., 1993). In the Saih Hatat, early diagenetic dolomitization and metamorphism complicate the establishment of a detailed biostratigraphic framework. Despite these difficulties, new findings of fusulinids and smaller foraminifera from samples of section Aday 1 confirm a Guadalupian–Lopingian age for the rim of the carbonate platform. A Guadalupian (Wordian– Capitanian) age is indicated by the presence of Neoschwagerina sp. having a large fusiform test, thick walls and well developed and regularly spaced septula. Verbeekina

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sp. is also an abundant taxon. Assignment to species level is inhibited by poor preservation. A Lopingian age is inferred from the presence of Dagmarita chanakchiensis. The exact position of the Guadalupian–Lopingian boundary cannot be determined at present. The LAD of Guadalupian fusulinids is within a transgressive systems tract of the Saiq Formation (at 125 m, Fig. 3), and the occurrence of D. chanakchiensis is within the overlying transgressive systems tract (at 210 m, Fig. 3). The intermittent thick-bedded highstand systems tract lacks biostratigraphic data. Considering that the Guadalupian–Lopingian boundary interval represents a pronounced 2nd order lowstand of sea level (Weidlich and Bernecker, 2003), it is reasonable to assume that the onset of the transgression coincides with the base of Lopingian. The Guadalupian– Lopingian boundary most probably coincides with the sequence boundary. 3.1.2. Facies types Textures of Guadalupian to Lopingian shallow-water platform carbonates of Section Aday 1 are well preserved despite early diagenetic dolomitization. Dark colors of beds are dominant, light gray and brownish colors occur less frequently. Colors are not facies diagnostic because changes are not related to textural or compositional changes of the deposits. The sediments of the Saiq Formation were deposited under fully marine to restricted conditions. Only a few beds are capped by subaerial exposure horizons and exhibit weak evidence of karst dissolution. Common erosive features at the base of limestone beds are the result of storms and indicate shallow water conditions. Facies types are grouped within 3rd order depositional sequences which can be recognized in the field (Figs. 2 and 3). Complete sequences consist of thin-bedded (0.05–0.7 m thick) transgressive systems tracts with grain-supported textures and thick-bedded (0.75–2.00 m) highstand systems tracts with mud-rich sediments representing increasingly restricted conditions (Weidlich and Bernecker, 2003). Breccias occur in the upper part of the Saiq Formation and indicate the proximity of the study area to the rim of the Arabian Plate. Rifting of the Neo-Tethys controlled this tectonic instability. 3.1.2.1. Mixed carbonate–siliciclastic wackestone to floatstone. This facies type with a characteristic brown color of weathered surfaces has a bed thickness of 5– 20 cm. The dominant grain size of components is sand and gravel. Percentage of quartz grains and lithoclasts is highly variable, whereas a muddy matrix is a common phenomenon of all samples. The sediments are restricted

to the lowermost 15 m of the Saiq Formation (Fig. 3) and mark the onset of transgression. Fragments of the tabulate coral Multithecopora sp. indicate a marine depositional environment. 3.1.2.2. Bioclastic wackestone to grainstone. The bed thickness of this abundant facies type varies between 15 and 40 cm. Beds consisting of different layers with respect to packing density of components, grain size and composition of grains (Fig. 5.1,2) are amalgamated tempestites. Diversity of grains is high, including smaller foraminifera (e.g., Globivalvulina, Paraglobivalvulina, Hemigordius, Dagmarita), fusulinids (Neoschwagerina, Verbeekina), dasycladacean algae (Mizzia, Goniolinopsis), cortoids, aggregate grains and peloids. Composition and packing density point to a productive platform environment during deposition of the transgressive systems tract. 3.1.2.3. Non-skeletal packstone to rudstone. These sediments, with a grain-supported fabric, have a bed thickness of 20–50 cm. Ooids, peloids and aggregate grains make up a high percentage of the sediment, while skeletal grains are rare to absent. Ooids form a prominent horizon within the highstand systems tract in the upper part of the Saiq Formation (Fig. 3), whereas peloids and aggregate grains occur throughout the Saiq Formation. 3.1.2.4. Microbial bindstone. Bed thickness of the laminated bindstone varies between 0.1 and 2 m. The microbialites occur in two different environmental settings. Thin bindstones (0.1–0.5 m) yielding recrystallized smaller forams indicate reduced sedimentation rates and normal marine conditions with respect to salinity (Fig. 5.3). This type of microbialite occurs within transgressive systems tracts. Thick microbialites (0.5– 2.0 m) are associated with mudstones devoid of calcified metazoans. The restricted environment inhibited the development of a diverse benthos. Thick-bedded microbialites are part of the highstand systems tracts of the upper part of the Saiq Formation (Fig. 3). 3.1.2.5. Brachiopod floatstone. Brachiopod floatstone with an approximate thickness of 50 cm is represented by in-situ accumulations with most specimens in life position or by tempestites consisting of disarticulated valves (Fig. 5.4). Although brachiopods subordinately occur in the bioclastic packstone to grainstone, monospecific associations are typical. Weathered rock surfaces with specimens in situ consist of associations dominated by the productid taxon Araxilevis minor,

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Fig. 3. Overview and details of section Aday 1, Saiq Formation (Arabian Plate), Saih Hatat. See Fig. 2.1 for an overview of the lower part of section. The fusulinids Neoschwagerina sp. and Verbeekina sp. indicate a Guadalupian age, Dagmarita chanakchiensis indicates a Lopingian age. A lack of biostratigraphic data prevents an exact placing of the Guadalupian–Lopingian boundary; its possible range is indicated by a stippled line.

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while disarticulated specimens of polished slabs are indeterminable. Productid brachiopods are often associated with fenestellid bryozoans. Both types are restricted to the transgressive systems tract of 3rd order sequences. In situ assemblages make up a significant part of the last transgressive systems tract below the PTB. 3.1.2.6. Bivalve floatstone. Alatoconchid bivalves accumulated in 50–70 cm thick beds. End members of sediment types are monospecific associations with bivalves in life position or tempestites with densely packed shell debris. Penetration of bioclasts by microand macroborers indicates repeated reworking of the sediment during abundantly occurring storm events. Both types of sediments are restricted to the transgressive systems tract in the lower part of the Saiq Formation. 3.1.2.7. Sponge/bryozoan/crinoid floatstone. This facies type with a thickness of 20–80 cm consists of crinoids, sphinctozoid sponges and bryozoans (Fig. 5.6–8). Sediments are either dominated by a specific taxonomic group, e.g., crinoid packstone or fenestellid bryozoan floatstone, or consist of several taxonomic groups. The number of tempestites significantly decreased compared with bioclastic wacke- to grainstone. Sometimes, bioclasts were subsequently incrusted by bryozoans. Despite the presence of reefbuilders, these associations formed only level-bottom communities and obviously lost their reefbuilding ability. This heterogenous facies type forms a large percentage of the transgressive systems tract below the Permian–Triassic boundary. 3.1.2.8. Coral bafflestone to floatstone. Rugose and tabulate corals form biostromes and small bioherms with a maximum thickness of 2.5 m. Stacked bioherms and biostromes attain a total thickness of 30 m in the lower part of the Saiq Formation. Frequent reefbuilders are the dendroid rugose coral Waagenophyllum sp. and the tabulate coral Multithecopora sp. In addition, solitary and cerioid taxa were found (Weidlich, 1999). The maximum diameter of individual waagenophylliid coral colonies reaches 1 m. Occasionally, the coral colonies are capped by exposure horizons. The baffle- to floatstone facies is typical of the highstand systems tract of the lower part of the Saiq Formation (Figs. 4 and 5.2,5). 3.1.2.9. Mudstone to skeletal wackestone. This monotonous facies forms beds with a maximum thickness of 2 m. Rare bioclasts are recrystallized and cannot be determined. Indistinct lamination may point to local microbial activity. Exposure horizons are present but difficult to trace. This facies type forms large parts of the

highstand systems tract of the upper part of the Saiq Formation. 3.1.3. Interpretation The presence of chloroforam and chlorosponge carbonates is indicated by the presence of dasycladacean algae, fusulinid foraminifera, alatoconchid bivalves, sphinctozoan sponges, aggregate grains and ooids (Beauchamp and Desrochers, 1997). This carbonatesecreting assemblage requests warm and sunlit waters high in oxygen and low in nutrients and, therefore, is an example of the tropical carbonate factory sensu Schlager (1993). Compositional variation in the biota is modulated by changes in water energy, salinity and water depth (sea level change). For example, the change from bioclastic wacke–packstone with fusulinids and dasycladacean algae to sponge/bryozoan/crinoid floatstone at the Guadalupian–Lopingian boundary is interpreted to result from a rise in sea level. 3.2. Permian–Triassic boundary and Early Triassic 3.2.1. Stratigraphy Boundary sections have not been described yet from shallow-water platform settings of the Arabian Plate. Despite the absence of the diagnostic conodont Hindeodus parvus, the Permian–Triassic boundary is inferred in section Aday 2A based on an integrated paleoecologic and sedimentologic approach (Fig. 4), including (1) the sudden disappearance of Changsinghian fauna prior to the PTB (2) the increase of pyrite immediately after the PTB (3) the dominance of anoxic facies after the PTB and (4) ichnotaxa, if present, with small body size and low-diversity (see also Wignall and Twitchett, 1996; Isozaki, 1997; Jin et al., 2000; Wignall and Twitchett, 2002; Erwin et al., 2002; Weidlich et al., 2003; Twitchett and Barras, 2004; Baud et al., 2005; Pruss et al., 2005). According to published lithologic descriptions, the top of the Saiq Formation has been described as a sequence of black sparitic limestone and yellow shales and the base of the Mahil Formation as gray or pink bedded dolomite (Le Métour et al., 1986). Applying this concept, the Saiq Formation extends into the Early Triassic (Fig. 2) and does not terminate with the Lopingian. 3.2.2. Facies types Close to the rim of the Arabian Plate, the Lopingian– Early Triassic was a time of intense tectonic activity

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Fig. 4. Measured Lopingian–Rhaetian sections Aday 2–4 of the Saiq and Mahil Formations (Arabian Plate), Saih Hatat. See Fig. 2.2 and 2.3 for a field impression of sections Aday 2 and 4. The Lopingian age is based on the presence of fenestellid bryozoans and the sphinctozoan Guadalupia sp. which became extinct at the end of Permian. The Permian–Triassic boundary has been set above the sudden disappearance of latest Permian calcified metazoans and below the onset of the anachronistic Induan facies.

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during rifting of the Neo-Tethys. In the study area, breccias with a high fitting of the clasts and slumping structures documented rift pulses. Synsedimentary faults locally dissected the Lopingian carbonate platform leading to drastic lateral facies changes.

Lopingian fossil-bearing carbonates of the Saiq Formation are capped by an unconformity. The maximum relief of the unconformity attains 10–15 cm. The transgressive nature of underlying sediments, the smooth topography of the surface lacking sharp rillenkarst

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features and absence of subaerial dissolution below the unconformity strongly support a submarine origin of the unconformity. Above, the following facies types (Fig. 6) occur in sections Aday 2A and 2B (Fig. 4), which differ significantly in their composition from Lopingian carbonates: 3.2.2.1. Pyrite-rich siliciclastic mudstone and pyritic hardgrounds. The unconformity is partly covered by pyrite crusts with a maximum thickness of 1.5 cm. Pyrite also stains the underlying 50 cm of Lopingian bioclastic sediment and lines the walls of open Thalassinoides burrows. Troughs of the unconformity were filled with unfossiliferous pyrite-rich siliciclastic mudstone (Fig. 6.1,6.2). A second pyrite hardground locally incrusted the siliciclastic mudstone. The combination of pyritic hardgrounds and pyrite-bearing sediments is unique throughout the Phanerozoic shallow-water evolution of the Arabian Plate and confirms an abrupt collapse of tropical carbonate production. 3.2.2.2. Vari-colored siliciclastic mudstone and siltstone. This siliciclastic sediment has a maximum thickness of 16 m, lateral variation of thickness occurs because of synsedimentary block faulting of the underlying Lopinging carbonates. The sediment has mostly a yellow, orange, red or brown color and is unfossiliferous. Two conodont samples (4 kg/sample) provided no fauna. Lamination of sediment resulted from changes in grain size and color (Fig. 6.4). The lack of ichnofossils and skeletal metazoans is unusual and indicates unfavourable conditions of the seawater. 3.2.2.3. Bioclastic packstone to floatstone. This bioclastic deposit with a thickness of about 1 m consists of a variety of textural sediment types. End members are crinoid packstone, bivalve floatstone, mudstone and lithoclastic floatstone. Crinoid ossicles are small and disarticulated. Erosion and lateral pinch-out of individual beds indicate a high-energy environment. This facies type represents a unique bed after the PTB (Fig. 4) and is restricted to the lower part of the Early Triassic section.

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The high percentage of skeletal grains is clear evidence of oxygenation and rapid recovery after the PTB. 3.2.2.4. Laminated lime mud- and grainstone. Characteristic dark- and light-colored intercalations of dolomitized mud- and grainstone occur throughout the middle part of section Aday 2A. Locally, the sediments are intensely brecciated. Angularity of clasts and high fitting of clasts indicate a tectonic origin of the deposits without significant lateral transport. Although mudstone prevails, other textures, including peloidal wackestone and fine-grained packstone, were observed. The small components of the packstone are too recrystallized for determination. Form and dimension of peloids indicate fecal pellets (Fig. 6.5), although no internal structure has been preserved. Abundant sediment structures are laminations and flaser bedding (Fig. 6.6). This facies resembles the so-called “ribbon rock” of Early Triassic age from the Great Bank of Guizhou (Lehrmann et al., 2001). 3.2.2.5. Bioturbated lime mudstone. This facies type forms 6 thin intervals with a thickness of 20–60 cm which occur throughout the carbonate dominated part of the Early Triassic section. Commonly, laminated dark and light-colored sediment is bioturbated by a Planolites-like ichnotaxon. Variations in density and size of burrow structures suggest that ichnotaxa were not the same in every horizon but differed. The presence of trace fossils clearly indicates an oxygenated seafloor environment for a limited period of time. 3.2.2.6. Microbial bindstone. Microbial bindstone is rare and forms one horizon with flat domes. The microbialites have a laminated internal structure. 3.2.2.7. Black sparitic limestone and yellow mudstone. Intercalations of these characteristic deposits form the upper part of section Aday 2B and the base of section Aday 3 (Fig. 4). Most of the yellow mudstone is finely laminated except of a few bioturbated horizons (Fig. 6.7). Slumps and debris flow deposits are

Fig. 5. Guadalupian–Lopingian facies of the Saiq Formation (Arabian Plate), Saih Hatat. (1) Guadalupian wackestone to grainstone, transgressive systems tract. Components comprise bioclasts, peloids and fusulinids (Neoschwagerina sp., Verbeekina sp.). Erosional surfaces separating the layers and textural changes of the sediments point to amalgamated tempestites. (2) Guadalupian coral floatstone and bioclastic packstone, highstand systems tract. Base of slab is a tempestite with reworked tabulate coral colonies (Multithecopora omanensis), top is a bioclastic packstone. (3)–(4) Lateral variation of a Guadalupian firm- to hardground, transgressive systems tract. (3) Microbial bindstone and wackestone containing abundant smaller foraminifera. (4) Brachiopod floatstone; note the presence of Thalassinoides burrows at the top. (5) Guadalupian coral bafflestone, highstand systems tract; monotonous dendroid coral facies (Waagenophyllum sp.). (6)–(8) Lopingian sponge/bryozoan/crinoid floatstone, transgressive systems tract. (6) Bioturbated bryozoan floatstone; the close-up shows a cross section of bryozoan; (7) Bryozoan-crinoid floatstone; note close-up of a bryozoan fragment; (8) Crinoid floatstone. All scales are 0.5 cm.

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associated with the dark calcite. The black sparitic limestone consists of centimetre-scale calcite crystals with pronounced twin lamellae. These precipitates resemble Olenekian seafloor cements of the Union Wash Formation of the southwestern U.S. (Woods et al., 1999). 3.2.3. Interpretation The change from Guadalupian–Lopingian to Induan–Olenekian facies indicates (1) the collapse of carbonate production by phototrophic and heterotrophic metazons (tropical factory sensu Schlager, 2001) at the PTB, (2) a drop in carbonate production and an increase in clastic sediments during the aftermath of mass extinction and (3) the switch to abiotic and microbial carbonate production (mud-mound factory sensu Schlager, 2001). 3.3. Middle to Late Triassic 3.3.1. Stratigraphy Macro- or microfossils of biostratigraphic significance have been obliterated by recrystallization and the age of this part of the Mahil Formation remains enigmatic. Considering that the maximum time span of the anachronistic facies following the end-Lopingian mass extinction covers the entire Early Triassic on global scale (e.g., Lehrmann et al., 2001: Great Bank of Guizhou, southern China; Schubert and Bottjer, 1995: western U.S.; Baud et al., 2005: western Taurus, Turkey), a similar maximum age is inferred for the anachronistic sediments of the study area. Consequently, an early Anisian age is inferred for the unconformity capping the anachronistic facies, in the absence of more precise biostratigraphic data. 3.3.2. Facies types A drastic facies change is evident in the lower part of section Aday 2B above an unconformity (Fig. 4). The sediment of the unconformity itself contains angular clasts embedded in structureless reddish terra-rossa matrix. These criteria indicate subaerial exposure under arid conditions. The overlying rocks encompass lightgray and pink dolostones, a facies corresponding with published descriptions of the Mahil Formation (Le Métour et al., 1986). The sediments were deposited in a subtidal to intertidal environment (Glennie et al., 1974). From a sequence stratigraphic point of view, stacked shallowing-upward high-frequency cycles prevail, which are often capped by subaerial exposure horizons (Weidlich and Bernecker, 2003). Despite intense dolomitization, relics of the primary textures have been preserved in the Mahil Formation. The following facies

types were observed from sections Aday 2B–Aday 4 (Fig. 4): 3.3.2.1. Lime mudstone. Lime mudstone is the dominant lithology throughout the Mahil Formation and also forms its base. It is the typical facies of the lower part of the shallowing-upward cycles. Thick-bedded mudstone units, which may attain maximum thicknesses of 1.5 m, commonly consist of intensely bioturbated and therefore structureless units interrupted by laminated units with a thickness of a few centimetres. The following sub-types can be differentiated: (1) bioturbated mudstone (Fig. 7.1b, 2b, 5a–b) (2) laminated mudstone (Fig. 7.2a) and (3) mudstone with chicken wire vugs (Fig. 7.3). A low-energy environment in a protected, partly evaporitic, lagoonal environment is likely. Bioturbation is the most important feature of the mudstone which clearly marks the return of long-lasting oxic conditions at the seafloor. 3.3.2.2. Non-skeletal grainstone. Grain-supported sediments frequently dominating the upper part of cycles can be recognized despite complete dolomitization (Fig. 7.1a, 7.4). Grain-support texture and erosion at the base (Fig. 7.2c) indicate a high-energy environment. The non-skeletal nature of the bulk of the grains is not easy to detect. However, good sorting of grains, grain size between 1 and 2 mm, circular shape of individual grains strongly suggests ooids as precursor components. 3.3.2.3. Bioclastic wackestone to packstone. This facies type is heterogeneous with respect to texture and composition. Most common are indeterminable bioclasts, the sediments are either bioturbated (Fig. 7.7) or laminated (Fig. 7.8). The debris of calcified metazoans is typical of a biotically controlled carbonate production. 3.3.2.4. Mollusk floatstone. Gastropods, bivalves and smaller foraminifera in a micritic groundmass are characteristic of this facies type and suggests normal marine conditions. 3.3.2.5. Microbial bindstone. This facies type is the least abundant of the Mahil Formation. Wrinkly laminae and layers with either mud-rich or grain-support textures point to the existence of microbes which trapped sediment (Fig. 7.6). In contrast to the abundant microbial bindstones of the Lofer cycles, birdseye structures, vadose silt, desiccation cracks and sheet crack breccias

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Fig. 6. Induan–Olenekian mixed carbonate–siliciclastic sediments of the Saiq Formation (Arabian Plate), Saih Hatat. (1) Pyrite-rich siliciclastic mudstone, the first sediment filling the relief of the Permian–Triassic unconformity. (2) Photomicrograph showing the unfossiliferous nature of the pyrite-rich siliciclastic mudstone. (3) Polished slab of slightly bioturbated lime mudstone. (4) Photomicrograph of laminated lime mud and grainstone. Bedding resulted from alternations of mud- and grain-support textures. (5) Photomicrograph of laminated peloidal wackestone. The structureless fecal pellets cannot be determined. The bioclast is probably a microgastropod (lower part). (6) Photomicrograph of flaser-bedded lime mud- and grainstone. Note indistinct ripples covered by lime mud. (7) Weathered bedding plane with bioturbated lime mudstone. The ichnotaxon is probably Planolites. All scales 0.5 cm.

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Fig. 7. Middle–Late Triassic carbonates of the Mahil Formation (Arabian Plate), Saih Hatat. (1) Polished slab of bioturbated grainstone (a) and mudstone (b). (2) Polished slab of horizontally bedded mudstone (a), bioturbated mudstone (b), and massive grainstone (c). Note erosive base cutting deeply into the bioturbated mudstone. (3) Polished slab and photomicrograph of evaporitic mudstone with chicken wire vugs. (4) Photomicrographs of non-skeletal grainstone. (5) Polished slab and photomicrograph of bioturbated dark-grey mudstone (a) and light-grey mudstone (b). Note tiering of trace fossils. (6) Photomicrograph of microbial bindstone. Light-grey areas are packstone. (7) Photomicrograph of indistinctly bioturbated bioclastic wackestone. (8) Photomicrograph of bioclastic packstone separated by thin mudstone layers. All scales 0.5 cm.

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are absent. This facies is believed to represent a very shallow subtidal environment. 3.3.2.6. Pedogenic sediments. High-frequency cycles are often capped by subaerial exposure horizons which can be recognized in the field by their reddish color. Depending on the dominant processes abundant lithotypes are breccias, rhizoliths, and monotonous mud- to siltstone. 3.3.2.7. Siliciclastics. Siliciclastic sediments with a thickness between 0.5 and 4.0 m occur in the upper part of the Mahil Formation (Fig. 4). The sediment is commonly a well-sorted siltstone and sandstone. Association with subaerial exposure horizons suggest a terrestrial genesis for most of the sandstone beds. 3.3.3. Interpretation Abundant grain-supported sediments with non-skeletal and skeletal grains indicate the return to tropical carbonate factory conditions. The frequent occurrence of bioturbation and totally bioturbated beds are unequivocal evidence of widespread temporal and spatial oxic conditions. 4. Evolution of isolated platforms of the Neo-Tethys 4.1. Guadalupian to Early Triassic 4.1.1. Stratigraphy In the village of Nakhl, an overturned section of the Al Jil Formation is exposed with a Guadalupian– Lopingian sequence documenting the development of an isolated platform of the Neo-Tethys (Fig. 8). The time framework of section has been established on the basis of fusulinids, smaller foraminifera and radiolarians. A Wordian to Capitanian (Middle-Late Guadalupian) age has been confirmed by the presence of the fusulinids Neoschwagerina sp. and Verbeekina sp. (Fig. 9.3, 9.5). Two independent lines of evidence indicate a Lopingian age for the upper part of the section, including the lagenid foraminifer Rectostipulina quadrata (Fig. 8.3) with an elongate test and a two-layered wall (Vachard et al., 2002) and the radiolarian Entarctinia sp. with a simple single-layered shell (Sashida et al., 2000; He et al., 2005). The radiolarian species is indeterminable because of poor preservation. By comparison with published data the microbialites could be assigned a Lopingian age despite the absence of age-diagnostic conodonts. Consequently, the end-Guadalupian boundary falls together with the LAD of the fusulinid-bearing beds. From base to top, the section consist of three units,

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notably (1) partly dolomitized limestone with mud- and grain-supported textures (0–21 m of section), (2) dolomitized conglomerate (22–23 m of section) and (3) laminated mudstone (23–30 m of section). 4.1.2. Facies types of unit 1 The bioclastic facies of unit 1 (0–21 m) resembles Guadalupian dolomitized limestones of the section Aday 1, Arabian Plate (see above), with respect to faunal content and diversity of facies types. Therefore, the facies types have not been described in detail herein. The sequence stratigraphic interpretation is unclear because of the condensed nature of section. At the top, unit 1 is capped by an unconformity. The nature of the unconformity is under discussion, absence of evidence for karst features, however, suggests a submarine origin. The following facies types constitute unit 1: 4.1.2.1. Mudstone to skeletal wackestone. Monotonous beds with a mud-supported texture have a thickness up to 1 m. Biota comprise gastropods and smaller foraminifera. Black pebbles, numerous unconformities and small karst cavities (Fig. 9.1) indicate very shallow water and repeated subaerial exposure. 4.1.2.2. Microbial bindstone. Microbialites with a thickness of about 50 cm consist of low-relief mats and yield abundant smaller foraminifera (Hemigordius) and gastropods. The gastropods trapped in the bindstone presumably grazed on the microbial mats and, probably, prevented the development of a columnar growth form. Small karst cavities indicate subaerial exposure (Fig. 9.2). 4.1.2.3. Bioclastic wackestone to packstone, floatstone. Bed thickness varies considerably between 0.1 and 2.0 m (Fig. 8). Skeletal grains comprise smaller foraminifera, fusulinids, calcareous algae, brachiopods, mollusks and recrystallized bioclasts. Sediment textures vary considerably within the dimension of the thin section (Fig. 9.3) and indicate rapid changes of hydrodynamic conditions. Bioturbation points to a well-oxygenated seafloor. Fusulinid floatstone with a grainstone groundmass is abundant. A bioclastic packstone yielding the calcareous alga Gymnocodium bellerophontis forms the uppermost bed of unit 1. This bed is a special case as the impoverished biotic content indicates an increase in water depth. Numerous specimens of the smaller foraminifera Globivalvulina vonderschmitti indicate a Guadalupian age (Fig. 8.1). 4.1.2.4. Coral floatstone. Thick-bedded units commonly also consist of coral floatstone. The rugose coral Praewentzelella irregulare and the tabulate coral

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Multithecopora sp. are reworked (Fig. 9.5). The thickbedded units commonly exhibit a coarsening-upward trend from bioclastic pack- to grainstone (Fig. 9.6) at the base, to coral floatstone (Fig. 9.5) to bioclastic cortoid floatstone at the top (Fig. 9.4).

4.1.3. Facies types of unit 2 Above an unconformity, a conglomerate occurs with a minimum thickness of two meter. Clasts with a maximum size of 90 cm consist of reworked platform facies types of unit 1. The presence of Neoschwagerina sp.

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(Fig. 8.2) indicates that the lithoclasts have a Middle– Late Guadalupian age. Most of the clasts and the matrix of the conglomerate suffered from dolomitization. Most likely, the conglomerate represents tectonic pulses during the opening of the Neo-Tethys (e.g., Béchennec et al., 1990). 4.1.4. Facies types of unit 3 Unit 3 of section Nakhl is characterized by a dramatic change in facies towards finely laminated monotonous carbonate which prevails throughout this part of section. Petrography shows that the lamination is caused by mudstone and fine-grained packstone intercalations (Fig. 8.3). The absence of bioturbation indicates oxygen-poor conditions at the sea floor. The radiolarian taxon Entarctinia sp. which occasionally occurs in the mudstone indicates a deep-marine depositional environment. Samples processed for conodonts yielded no results. The fine-grained packstone units consist of indeterminable bioclasts. However, peloids and the smaller foraminifera R. quadrata indicate a limited export of shallow-water platform carbonates and, thus, a mixture of sediment types (Fig. 8). Unit 3 is terminated by three horizons yielding microbialites (Fig. 8.4) with a variable morphology, comprising flat laminites, small domal structures and small columnar microbialites. 4.1.5. Interpretation The transition from unit 1 to unit 3 represents a deepening-upward sequence which is terminated by platform drowning. The uppermost bioclastic packstone of unit 1 marks the onset of drowning prior to tectonic movements. Rare shallow-water grains in the packstone layers of unit 3 indicate insignificant shallow-water carbonate production during the Lopingian.

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1993; Pillevuit et al., 1997). It forms the base of the Misfah Formation and is regarded as the lowermost unit of this Neo-Tethyan isolated carbonate platform. The Misfah Formation, exposed at Jibal Misfah, Misht and Kawr (Beurrier et al., 1986, ‘Oman Exotics’ sensu Glennie et al., 1974) consists of bedded shallow water platform carbonates of Late Triassic age. At Jabal Kawr, the occurrence of Poikyloporella duplicata, Clypeina besici (both calcareous algae), and Aulotortus praegaschei (foraminifer) at the base of Sint section points to a Carnian age (Fig. 10.1). The top of Wadi Ala section yields Triassina hantkeni (smaller foraminifera) indicative of a Rhaetian age (Fig. 10.2). The termination of Jebel Kawr shallow-water platform sedimentation is indicated by platform drowning during the Lower Jurassic (Fatah Formation, Section Ma'Wa: Pillevuit, 1993). Summarizing these data, a Carnian to Rhaetian age is backed by our own biostratigraphic determinations and a Ladinian to Early Jurassic age has been assumed in the literature (Pillevuit, 1993) and cannot be ruled out. 4.2.2. Facies types of unit 1 The base of Jebel Kawr represents the onset of shallow-water carbonate deposition in the Carnian. Carbonate beds with volcaniclastic components indicate the presence of environmental perturbations caused by volcanic activity. Bedded wackestone facies with peloids and bioclasts (porostromates, dasycladacean algae and foraminifera) dominate in the interior, oolitic grainstone at the margin.

4.2. Middle to Late Triassic

4.2.2.1. Mudstone, partly bioturbated. This monotonous facies type forms beds with a thickness of 20– 50 cm. The mudstone contains only a few components (pellets, bioclasts, intraclasts) and contains locally porostromate algae. Distinct mottling due to bioturbation can partly be observed (Fig. 11.1).

4.2.1. Stratigraphy The Subayb Formation of Jebel Misfah, comprises a succession from mafic volcanics to dark nodular limestones and is dated as Ladinian–Carnian (Pillevuit,

4.2.2.2. Bioclastic and peloidal wackestone to grainstone. Beds of this facies type are 20–40 cm thick and contain a varying amount of bioclasts and peloids. (Fig. 11.2) The microflora and -fauna consisting of

Fig. 8. Measured section showing Neo-Tethyan Guadalupian isolated carbonate platform development (unit 1 in text), phase tectonic activity (unit 2) and Lopingian drowning followed by deep-water microbialite formation (unit 3), Al Jil Formation, Jebel Nakhl. (1)–(3) Thin-section overviews and photomicrographs. (1) Pre-drowning Guadalupian shallow-water bioclastic packstone with abundant peloids and the smaller foraminifer Globivalvulina sp. (2) Top of the mostly dolomitized conglomerate. Note packstone lithoclast yielding a Guadalupian foraminiferal assemblage of Neoschwagerina sp. and Globivalvulina sp. (3) Syn-drowning intercalations of mudstone and fine-grained packstone. The upper photomicrograph shows the transition from packstone with peloids and bioclasts to radiolarian wackestone with Entarctinia sp. The packstone yields abundant shallow-water peloids and the Lopingian foraminifer Rectostipulina quadrata. (4) Post-drowning phase with microbialite and pelagic lime mudstone. Scale of all photomicrographs consists of 0.1 mm units.

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Fig. 9. Guadalupian isolated platform shallow-water facies, Al Jil Formation, Jebel Nakhl. (1) Photomicrograph of mudstone and gastropod skeletal wackestone. Black pebbles, unconformities and small karst cavities indicate subaerial exposure. Note geopetal infill in a gastropod. (2) Laminated microbialite with gastropods and foraminifera. (3) Bioturbated pack/grainstone with Neoschwagerina sp. (4)–(6) Coarsening-upward cycle of a bank. (4) Top of bank is a diverse bioclastic cortoid floatstone with bioclastric packstone matrix. (5) Center of bank is a coral floatstone (Praewentzelella sp.) with a pack/grainstone matrix. Note Guadalupian fusulinids including Verbeekina sp. (see also close-up) and Neoschwagerina sp. (6) Base of bank is a bioclastic pack- to grainstone. All scales are 0.5 cm.

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dasycladacean algae (C. besici, Poikiloporella duplicata) and foraminifera (e.g., A. praegaschei) are age diagnostic for the Carnian. 4.2.2.3. Volcaniclastic wackestone. This medium thick-bedded (ca. 30 cm) carbonate beds are located in the basal part of the sequence. The reddish to yellow mudto wackestone contains elongated mm-size volcanic extraclasts (Fig. 11.2). These extraclasts are indicators of ongoing volcanic eruptions in the Hawasina basin which may have hampered carbonate production on the isolated platform at the beginning of its development. 4.2.2.4. Oolitic grainstone. The medium sorted oolitic grainstone occurs in the sequence prior to the reef growth. The normal marine ooids with a partly micritized tangential microfabric probably formed high energy shoals. 4.2.2.5. Crinoid floatstone to rudstone. This bioclastic float- to rudstone is composed of accumulations of crinoid ossicles. The biofabric varies from loose and disperse packing to grain-supported fabric. The crinoid stems are broken in ossicles, but show no signs of significant lateral transportation or erosion. Some crinoids exhibit syntaxial cement rims. Only a broad Carnian to Norian age can be assigned to this facies types, as long as fossils of biostratigraphic relevance are absent. 4.2.3. Facies types of unit 2 The Misfah Formation of Jebel Kawr consists of shallow-water carbonates up to 800 m thick, represents the main part of the isolated Kawr platform and comprises platform rim as well as bedded inner platform facies (Fig. 10). The inner platform is characterized by stacked high-frequency cycles comparable to the Lofer cyclothems with subtidal to intertidal carbonate sequences, terminated by subaerial exposure horizons (e.g., Fischer, 1964; Goldhammer et al., 1990; Enos and Samankassou, 1998; Haas, 2004). 4.2.3.1. Coral bafflestone with solenoporacean algae and/or chaetetid sponge. Dendroid coral colonies form bioherms up to 20 m thick together with large solenoporacean algae (diameter 2–5 cm) of globular and dendroid growth form. Frequent coral taxa are Retiophyllia norica, Cyclophyllia cyclica and Margarosmilia charlyana. The sediment between the colonies is a wackestone yielding bioclasts and small reef foraminifera. The reef organisms represent a typical Norian–Rhaetian fauna. Corals (R. norica) and chaetetid sponges (Blastochaetetes dolomiticus, Bauneia annosciai) occur also as isolated colonies in biostromal beds with a thickness of 30–50 cm.

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4.2.3.2. Coral and sponge boundstone. Cerioid and thamnasteroid coral colonies are the dominant reef builders. Frequent taxa are Gablonzeria profunda, Pamiroseris rectilamellosa, Seriastrea multiphylla. Sponges are less abundant (Cryptocoelia siziliana, Cryptocoelia tenuparietalis, Weltheria sp.) and build clusters within the reef framework. This boundstone is also constructed by different encrusting organisms like Spongiostromata crusts, the microproblematicum Microtubus communis, foraminifera (e.g., Alpinophragmium perforatum) and small sponges (Uvanella norica). These boundstones are of Norian age, inferred from their reef fauna assemblages (Bernecker, 2005). 4.2.3.3. Coral floatstone to rudstone. The float- and rudstones are bedded with a maximum thickness of 1– 2 m. They yield angular to subangular reef derived bioclasts up to 10 cm in diameter. This facies type occur at the top of the reef facies and is representative of the end of reef development. 4.2.3.4. Bioturbated mudstone to wackestone. The beds of this facies type have a thickness between 40 and 70 cm. The bioturbation is clearly visible, with burrows of about 1 cm in diameter. The burrow structure differs in color and fabric from the surrounding sediment (mudstone). The burrow filling is a wackestone containing recrystallized bioclasts, peloids and fecal pellets of decapod crustaceans (Favreina). 4.2.3.5. Bioclastic wackestone to grainstone. The bed thickness of this facies varies between 20 and 40 cm (Fig. 12.6). The components are recrystallized bioclasts and peloids. Bioturbation is rare, but if it occurs, it is obvious by a change in texture. The surrounding sediment is a wackestone and the burrow infill a grainstone. 4.2.3.6. Megalodont floatstone. The well-bedded floatstone has bed thicknesses between 80 and 150 cm. Large bivalves up to 60 cm in diameter occur in cluster or beds, the largest forms being common in the Norian. The sediment between the megalodont clams (e.g., Megalodon, Neomegalodon) and other taxa of large bivalves is a mud- to wackestone (Fig. 12.1). Most of the abundant molds of the bivalves are articulated and still in life position. The megalodontids lived on shallow muddy substrates in lagoonal low-energy environments. 4.2.3.7. Gastropod and bivalve floatstone. Gastropods and bivalves accumulated in 40–60 cm thick beds and the shells are commonly aligned (Fig. 12.2–4).

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Single beds consist of several layers which differ with respect to packing densities and shape of the shells (Fig. 12.3).

4.2.3.8. Laminated bindstone. The laminated bindstone forms beds varying from 5 to 20 cm. The facies type is called loferites (Fischer, 1964) and consists of

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Fig. 11. Carnian carbonates, low-relief platform, Jebel Kawr, Misfah Formation. All scales of photomicrographs are 0.5 cm. See Fig. 10 for position of samples. (1) Bioturbation of mud- and wackestone, (2) Bioclastic grainstone with peloids, characterized by varying packing densities. Note evidence of subaerial exposure, (3) Exposed base of bank is an intercalation of bioturbated mud- and wackestone units indicating low- to moderate water energy.

microbialites with a locally developed fenestral fabric. An alternation of wavy laminae is characteristic. The microbial mats are often associated with birdseyes and desiccation cracks, indicating a supra- to intertidal environment. The laminated sediment is partly reworked at the top of the bed (Fig. 12.5). 4.2.3.9. Irregular disconformity surfaces. The reddish or greenish argillaceous micrite, carbonate silt or marl contain scattered fenestral pores with geopetal fills and mm- to cm-sized intraclasts of microbialite. Red coated grains and aggregates with Fe-oxide staining and ce-

ment are probably of pedogenic origin. The subaerial conditions prevailed prior to and contemporaneous with the onset of the tidal-flat deposition. 4.2.4. Interpretation Correlation of measured sections of the Misfah Formation from Jebel Kawr indicate that tropical carbonate production commenced on a low-relief platform for the following reasons: (1) The lack of depositional relief is indicated by the absence of talus breccias close to the rim of low-relief platform. (2) Oolites and crinoid floatstone, interpreted as comprising the rim of the low-

Fig. 10. Measured sections showing Neo-Tethyan Carnian to Rhaetian isolated platform development, Misfah Formation, Jebel Kawr. (1) Section Sint is situated at the margin of the isolated platform and exhibits the development from low-relief platform to high-relief reef rimmed platform. The maximum flooding surface below the coral bioherm of section Sint correlates with the mfs above the coral and chaetetid sponge biostromes of section Ala. The low-relief platform margin at the base is dated as Carnian by dasycladacean algae (Clypeina besici, Poikiloporella duplicata) and foraminifers (Aulotortus praegaschei). The coral reef communities (e.g., Retiophyllia norica) are characteristic for Norian–Rhaetian age. (2) Section Ala represents the cyclic inner platform sedimentation with scattered coral and chaetetid sponge biostromes. The top of the section is dated as Rhaetian by the foraminifer Triassina hantkeni.

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Fig. 12. Carnian–Rhaetian inner platform facies of isolated high-relief carbonate platform. All scales of photomicrographs are 0.5 cm. (1) Megalodont floatstone in a packstone matrix. (2) Gastropod-bivalve floatstone; disarticulation and alignment of shells point to storm deposits. (3) Intercalation from base to top comprising (a) bioclastic wackestone, (b) gastropod floatstone and (c) bivalve floatstone with packstone matrix, (4) Bioturbated bivalve floatstone, (5) Subaerially exposed laminated microbialite, (6) Bioturbated wackestone.

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relief platform, lack the capability to create a high relief slope. (3) Comparisons with published studies show that new carbonate depositional systems always evolved from a ramp to a rimmed platform (Cenozoic: Bahama Bank, Betzler et al., 1999; Triassic: Great Bank of Guizhou, Lehrmann et al., 1998, 2001; Late Paleozoic: Capitan Reef Complex, southwestern US, Saller et al., 1999). The start of shallow-water sedimentation at Jebel Kawr is characterized by an initial phase of carbonate production, interrupted by volcanic episodes. The low-relief platform consists of a shallow subtidal to peritidal interior and oolite shoals at the margin. The change in the architectural style to a rimmed platform during the Carnian–Norian is evidenced by the presence of a reef. In the Norian vertical accumulation caused an increase of the platform height and the development of a relief along the margins, which progressively increased through the aggrading reef stage. The possibility that a reef rim existed later but was then removed by erosion is suggested by the (1) Sint reef and (2) breccia intervals with clasts of reef limestone 1–60 cm in diameter and (3) olistoliths of similar reef limestones of 5–10 m thickness in the surrounding areas. The reef clasts contain a diverse fauna with scleractinian corals, sponges and several different encrusting organisms forming a boundstone fabric. These boundstone clasts could have been derived from diverse platform margin reefs that were partly eroded from the margin and only preserved in the Sint reef. 5. Discussion 5.1. Rim of the attached Arabian platform Three phases are recognized in the evolution of the Permian–Triassic platform rim (Saih Hatat) of the Arabian Plate (Fig. 13): 5.1.1. Guadalupian–Lopingian platform growth Guadalupian–Lopingian carbonates comprise biotically controlled precipitates of the tropical carbonate factory (sensu Schlager, 2003) and chloroforam and chlorosponge carbonates (sensu Beauchamp and Desrochers, 1997). A diverse benthos of calcified metazoans, microborings in large bioclasts and bioturbation of sediment indicate a well oxygenated sea floor throughout this time span. The change from Guadalupian coral bioherms to Lopingian level-bottom communities enriched in bryozoans, sponges and crinoids was caused by the end-Guadalupian extinction event and a 2nd order rise of Lopingian sea level.

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5.1.2. Induan–Olenekian platform perturbations The end-Lopingian mass extinction caused a sudden collapse of the tropical carbonate factory. The Induan– Olenekian is characterized by extreme oceanic perturbations which significantly affected and sometimes hampered shallow-water carbonate production. End members of recorded shallow-marine perturbations are (1) carbonate dissolution/undersaturation during the formation of submarine hardgrounds, (2) biotically induced carbonate precipitation controlled by microbes and (3) abiotic precipitation of carbonate cements on the sea floor. Feature (2) and to a certain extent feature (3) correspond to the descriptions of the mud-mound carbonate factory of Schlager (2003), the predominant mode of carbonate production during the aftermath of mass extinctions according to this author. One shortlived oxic interval is extraordinary which allowed biotically controlled carbonate production by crinoids and bivalves. Possibly, it is contemporaneous with a Griesbachian shell bed of the Neo-Tethys (Krystyn et al., 2003; Twitchett et al., 2004). 5.1.3. Middle–Late Triassic platform growth After a gap of approximately 6 millions years, Middle–Late Triassic carbonates represent the return of biotically-controlled carbonate production of the tropical factory. The sea floor was again fully oxygenated, favoring the bioturbation of sediment. Fluctuations in the diversity of calcified metazoans and ichnotaxa were mainly controlled by salinity changes, as beds with normal marine salinity contain bivalves, gastropods, smaller foraminifera and calcareous algae. The evolution of Permian–Triassic isolated carbonate platforms of the Neo-Tethys consists of three phases: 5.2. Isolated platforms of the Neo-Tethys 5.2.1. Guadalupian platform stage Guadalupian carbonates are biotically controlled precipitates of the tropical carbonate factory and resemble with respect to biotic composition and evolution the attached platform of the Arabian Plate. However, the uppermost Guadalupian bed indicates a decrease in carbonate production and marks the onset of platform drowning. 5.2.2. Platform drowning Starting with the end of the Guadalupian, tropical carbonate production came to an end on isolated carbonate platforms of the Neo-Tethys. No published data exist that described significant carbonate production

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Fig. 13. Conceptual model predicting the differential severity of oceanic perturbations after the end-Permian mass extinctions. Note predicted decrease in severity of perturbations from the Neo-Tethys to the landward segment of the platform rimming the Arabian Plate. Oxic events are potential time lines, whereas unconformities turned out to be diachronous. The Griesbachian (Lower Induan) “oxic event” marked with an asterisk is based on the correlation of the only striking shell bed from the Saih Hatat (Arabian Plate) and the Neo-Tethys (Wadi Wasit block: Krystyn et al., 2003; Twitchett et al., 2004). Microbialites of the Upper Kartham member (Olenekian part of the Khuff Formation) have been reported from the Buraydah and Khuff sections, central Saudi Arabia (Vaslet et al., 2004). Lithology of central Saudi Arabia based on Le Nindre et al. (2003).

and biostratigraphic data indicate the dominance of pelagic carbonate production. Obviously, platform drowning resulted directly or indirectly from disturbances of the end-Guadalupian mass extinction. The absence of Neo-Tethyan isolated platform blocks of Lopingian–Olenekian age confirm the drowning hypothesis (cephalopod limestones of the so-called Wasitblock (e.g., Krystyn et al., 2003) are not indicative of an shallow-water platform environment). 5.2.3. Late Triassic platform stage After a gap of about 30 million years, tropical carbonate production started in the Carnian with a lowrelief platform and quickly developed into a rimmed isolated platform during the Late Triassic.

5.3. Conceptual model for perturbations Using a conceptual model, it is possible to predict the onset and duration of Induan–Olenekian perturbations (Fig. 13): Isolated platforms of the Neo-Tethys were affected most heavily. Consequences were less drastic for the attached platform of the Arabian Plate which did not drown but responded with the production of anachronistic carbonates. Based on existing biostratigraphic data, the model predicts a diachronous duration of sea-water perturbations on the shelf and a contemporaneous onset with the end-Permian crisis. The longest perturbation, with an approximate duration of 6 million years, occurred close to the rim. The time span significantly decreased on the landward part of the platform. Earliest Triassic

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microbialites reported by Vaslet et al. (2004) confirm a short-lived interval of anachronistic carbonate precipitation during the Induan. Even more intriguing is the complexity of oceanic perturbations. Anoxia and hypercapnia (CO2-poisoning) are theories explaining the prolonged recovery phase (see Erwin et al., 2002; Wignall and Twitchett, 2002 for details and further references herein). Our data provide evidence for both models. On one side, carbonate corrosion along submarine unconformities indicates supersaturation of sea water with respect to CO2 and, probably, hypercapnia. On the other side, increase in pyrite, preservation of tiny lamination and absence of bioturbation suggest longlasting oxygen deficiency. Marine anoxia was interrupted by seven oxic events as evidenced by an unusual shell bed and six bioturbation horizons. At least the shell bed seem to be contemporaneous, as a similar facies has been described from the Neo-Tethys (Wadi Wasit block: Krystyn et al., 2003; Twitchett et al., 2004). 6. Conclusions The significantly different evolution of Guadalupian– Late Triassic carbonate platforms from the Arabian plate and the Neo-Tethys is interpreted in terms of the endPermian mass extinctions and Early Triassic recovery. Tropical carbonate production of the platform rimming the Arabian Shield ceased after the end-Lopingian crisis. Oceanic perturbations, including O2-deficiency and CO2-supersaturation, were responsible for the production and deposition of benthic anachronistic carbonates. The maximum time span of perturbation is 6 million years close to the rim and significantly shorter in landward facies belts close to the Arabian Shield. In the oceanic realm of the Neo-Tethys, isolated platforms were hit heavily at the end of Guadalupian. Drowning and a period of about 30 million years devoid of isolated platforms document the devastative and long-lasting impact of the double mass extinctions and the recovery period. Beginning with the Late Triassic, isolated carbonate platforms rebuilt and flourished again. Tropical shallow-water carbonate production started with a low-relief platform stage in the Carnian and an isolated rimmed platform developed during the Late Triassic. From the perspective of carbonate platforms, recovery in the Neo-Tethys was extremely retarded, coming to an end after the recovery of metazoan reefs. Acknowledgements This study was financed by the German Science Foundation (project We1804/8-1,2). We thank the Min-

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