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Communication Dans Un Congrès Année : 2011

Permian-Triassic extinction and recovery: results from the Muscat -Gutech field meeting

Résumé

The Permian–Triassic transition has been surveyed in the Oman Mountains and new detailed sections have been presented (Baud and Bernecker, 2010), from autochthonous shallow-water units (Saih Hatat and Al Jabal al-Akhdar) to slope deposits in the Jabal Sumeini area (Wadi Maqam units), from distal tilted block (Wadi Wasit) to oceanic deep-water deposits (Buday’ah). Middle Permian Transgression At the dawn of the Wordian (Middle Permian), the "Fusulinid Sea" transgressed over most of Oman with the exception of Jabal Ja'alan and the Huqf-Dhofar High. This transgression enabled the establishment of a vast carbonate platform in Al Jabal al-Akhdar, a 700 m-thick succession of cyclic shallow-marine carbonate, the Saiq Formation (Middle and Late Permian, basal Triassic (Baud et al., 2001a, b, 2005; Richoz et al., 2005; Richoz, 2006). A similar succession occurs in Saih Hatat (Le Métour, 1988; Weidlich and Bernecker, 2003; Chauvet, 2007), in the Musandam (Bih Formation, Maurer et al., 2009), as well as in the Interior Oman and in the Haushi area (Khuff Formation, Angiolini et al. 1998, 2003). Clearly, for us, this transgression was the result of the break-up of the Neo-Tethyan rift and the associated thermal subsidence. Following the peak of the thermal subsidence in the Wordian–Capitanian, a stable carbonate platform became established on the Arabian Peninsula. The Saiq, Khuff and Hagil formations show a strong regressive tendency at the end of the Guadalupian (Middle Permian), with restricted environment facies and a reduced biophase, mainly associated with a global fall in sea level at this time and climate changes (Isozaki, 2009). During the Lopingian (Late Permian), the subsidence as recorded in the Saiq mega-cycle B (up to 300 m of shallowing upward cycles) was still well active. The most striking effect of the climax of the Neo-Tethyan extension was the formation of a continental slope (Sumeini) and a basin (Hawasina) that constituted with the adjacent Arabian Platform, the southern continental passive margin of the Neo-Tethys Ocean. Furthermore early-rifted blocks detached from the edge of the Arabian Shield formed isolated proximal platforms along the continental slope (later they were incorporated in the Hawasina Nappes). The continental margin slope deposits are clearly identified (with slumps and intraformational breccia) in the northwestern part of the Oman Mountains (Jabal Sumeini), where they form the basal part of the Maqam Formation dated as Roadian (Middle Permian). The distal isolated platform identified as nappes in Baid and Jabal Qamar areas by Béchennec (1988), Béchennec et al. (1992), Pillevuit (1993) and Pillevuit et al. (1997) are mainly made of Middle–Late Permian open-shelf carbonates. The Jabal Qamar unit includes a fragment of the pre-Middle Permian Basement (Rann, Ayim and Asfar formations, Pillevuit, 1993) overlain in unconformity by the late Early to early Middle Permian shallow-marine carbonate Qamar Formation with its quartz-sandstone basal member. The Baid unit is truncated at the base and is made of about 100 m of the Middle–Late Permian (Capitanian–Wuchiapingian) shallowmarine carbonate (Baid Formation, Béchennec, 1988; Pillevuit, 1993; Pillevuit et al., 1997; Baud et al., 2001b). The distal paleogeographic position of these Permian tilted blocks in regard with the Arabian Platform is documented by: (1) the differences in terms of facies (open marine with ammonoids) with those restricted to the others parts of the Oman Mountains (Al Jabal al-Akhdar, Saih Hatat and Musandam); and (2) the presence of reworked boulders originating from these isolated platforms in the calcirudites of the proximal units of the basinal Hawasina Nappes. Basinal facies of the Middle Permian are present in the Hawasina Nappes at the base of numerous tectonics units, made up of formations from the Hamrat Duru Group. These successions generally start with thick volcanic sequences (Al Jil and Buday’ah formations). These volcanic rocks are either of MORB type or alkali basalt-related (Maury et al., 2003; Lapierre et al., 2004). The volcanic succession is filled and overlain by red ammonoid limestones dated as Middle Permian (Capitanian) followed by radiolarian chert and shales newly dated as Lopingian in Buday'ah. In the Wadi Wasit area, the volcanic series is capped by red cephalopods-bearing carbonate, dated Middle Permian (Wordian, Blendinger et al., 1992; Pillevuit et al., 1997; Baud et al., 2001b), by shales and breccia with reworked blocks of Middle Permian to basal Triassic platform carbonate (Béchennec et al., 1992b; Pillevuit, 1993; Pillevuit et al., 1997; Krystyn et al., 2003; Weidlich and Bernecker, 2007). Near Nahkl the volcanic series includes blocks of Middle Permian shallow-marine carbonate and is overlain by pelagic limestone (Weidlich, 2007). In the Rustaq area the volcanic succession is also capped by a condensed carbonate sequence (Hallstatt facies type) dated as Middle Permian (Wordian, Blendinger et al., 1992; Pillevuit et al., 1997; Baud et al. 2001b; Richoz et al., 2005). Different types of deep-water black limestones are also identified in the basinal units of the Batain Plain (southeastern part of the Oman Mountains), the "Qarari Limestone" with a base dated as Roadian (Middle Permian, Immenhauser et al., 1998) and the top as Changhsingian. From the Permian/Triassic Transition to the Basal Triassic (Figure 1) At the end of the Permian (top of KS3 sequence of Koehrer et al., 2010), regressive conditions up locally to emersion (?) are recorded as well on the Arabian carbonate platform (Al Jabal al- Akhdar, Saih Hatat and Musandam). On the slope of the continental margin, we observe a shallowing in the Sumeini unit deposits. Shallow tidal influenced carbonate platform is the main component of the Induan dolomitized deposit in the Al Jabal al-Akhdar (Units C1 to C4 of the Saiq Formation in Baud and Bernecker, 2010, correlated with Khuff sequences KS2–KS1) that is now dated by conodonts. During the Dienerian, part of the margin was affected by a renewed extensional regime, tilting and drowning resulting in erosive deposition and accumulation of carbonate breccia (Unit C2 of the Saiq Formation) followed in the Al Jabal al-Akhdar by high-energy, partly oolitic dolomitized shallow-water deposits, Dienerian in age (Unit C3) and renewed breccias (Unit C4). The Saiq-Mahil transition (correlated with the Khuff-Sudair transition) is probably of late Induan age (chemostratigraphical correlations, Richoz, 2006). On the slope of the continental margin, a continuous carbonate deposition and shale has been recently precisely dated from Changsingian to Spathian. Overlying the Wuchiapingian– Changhsingian, deep-water chert and dolostone (upper Member B of the Maqam Formation), we note the deposition of upper Changsingian shallowing siliceous strongly bioturbated lime mudstones. A major facies change occurs with the Griesbachian papery, laminated calcimicrobial mudstone overlying the boundary clay (base of C1c Member of the Maqam Formation). The calcarenite, calcirudite turbidites and avalanches with shallow water upper Permian lime clasts start in the Dienerian (instability period). The incredible thickness of the Smithian deposits (platy limestones, shales and megabreccia up to 900 m of thickness, middle and upper Member C of the Maqam Formation) indicate high carbonate productivity on the platform and a very active subsidence at the base of slope (Watts, 1985; Baud et al., 2001b; Richoz et al., 2005; Richoz, 2006). On the Baid Exotic block, after karstification of part of the tilted Permian carbonate platform and the Dienerian drowning event, the Dienerian-Smithian deep-water red ammonoid limestone is filling fissures and cavities (Hallstatt breccia) and is deposited over the Permian limestones (Tozer and Calon, 1990; Pillevuit, 1993; Pillevuit et al., 1997; Baud et al., 2001a; Richoz, 2006; Wood and Baud, 2008). In the proximal deep-water basin (Wadi Wasit units) the Lopingian allodapic limestones are partly eroded by a submarine avalanche breccia (Dienerian) containing Permian to basal Triassic mega-blocks. One of these blocks with a unique Permian–basal Triassic record has been analyzed in Krystyn et al. (2003). Upper Dienerian–Smithian deep-water platy limestone overlay the Lower Dienerian mega-block breccia. In the distal Hawasina basin (Buday’ah), the Upper Permian radiolarian chert deposits are overlain by Changhsingian siliceous shales and calcareous shales followed by Griebachian laminated platy limestones and shales and Dienerian–Smithian papery limestones. Conclusions The Neo-Tethys Ocean opened with the northward drifting of the Iran/Mega-Lhasa microcontinent followed a rifting extensional phase in the Roadian–Wordian. Thermal subsidence, with the development of the continental margin, is well recorded in the Wordian–Capitanian carbonate succession and continued during the Lopingian. Tectonic instability of the margin, with block tilting, platform drowning and (fault) breccia deposits started at the dawn of the Triassic with the main climax during the Dienerian and the Smithian.
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hal-03277358 , version 1 (03-07-2021)

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  • HAL Id : hal-03277358 , version 1

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Aymon Baud, Michaela Bernecker, Leopold Krystyn, Sylvain Richoz, Oliver Weidlich, et al.. Permian-Triassic extinction and recovery: results from the Muscat -Gutech field meeting. Third Arabian Workshop: The Permo-Triassic sequence of the Arabian Plate (IGCP 572), Nov 2011, Kuwait city, Kuwait. pp.11717. ⟨hal-03277358⟩
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