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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