Western Tethyan Realm Research Articles

Nautiloids of the Permian-Triassic Khuff Formation, central Saudi Arabia

ABSTRACT Nautiloids are described for the first time from the outcrops of the lower part of the Midhnab Member and the lower part of the Khartam Member of the Khuff Formation in central Saudi Arabia. The nautiloids from the lower Midhnab Member, including Tirolonautilus gr. hoernesi, were found at two localities, and are associated with conical shaped cephalopodes (bactritids), bivalves, brachiopods, foraminifers, algae and ostracods. The nautiloids were recovered from the most marine horizon of the Khuff Formation. The nautiloid fauna confirms the Late Permian (Changhsingian) age assigned to the Midhnab Member based on the foraminiferal assemblage. In the lower part of the Khartam Member, a single specimen of Tirolonautilus feltgeni n. sp. is described here. It occurs in association with other cephalopods (bactritids), bivalves, foraminifers, and ostracods. The specimen confirms the Late Permian age (late Changhsingian) assigned to the Lower Khartam Member based on foraminiferal and ostracod assemblages. This marine fauna is located within the latest Permian maximum flooding event of central Saudi Arabia. The Khuff Formation nautiloids are compared to other fauna in the Peri-Tethys, particularly the southern Alps, where similar forms are described. Their similarity confirms a Late Permian marine exchange between the Arabian platform and the Western Tethyan realm.

Palynological investigations in the Carnian of the Cave del Predil area (Julian Alps, NE Italy)

The palynological analysis on a continuous shallow-marine succession of the western Tethys realm allows a better characterization of the Carnian (late Triassic) paleofloristic and paleoclimatic scenario. The classic succession of the “Raibl Group”, once the “Raibler Schichten”, in the Southern Alps (northern Italy) was investigated. Ammonoids and conodonts constrain the age of the fossil-rich terrigenous-carbonate sequence to the Julian and Tuvalian, the Julian/Tuvalian boundary lying in the upper part of the Tor Formation. In order to document the pollen biostratigraphy and the paleoecologic trends of this late Triassic interval, results of qualitative and quantitative spore and pollen analysis are presented. The new combination Converrucosisporites tumulosus (Leschik) Roghi is proposed. The recognition of three palynological assemblages facilitates realistic regional palynostratigraphical correlation of Carnian successions in the Southern Alps. Quantitative analysis of the pollen/spore assemblages is used to confirm humid environmental conditions during latest Julian and earliest Tuvalian times.

PALAEOENVIRONMENTAL INTERPPRETATION OF THE LATE TRIASSIC FRAELE FORMATION (ORTLES NAPPE, AUSTROALPINE DOMAIN, LOMBARDY)

The Fraele Formation crops out in the Ortles Nappe (upper Valtellina, Northern Italy), structurally part of the Central Austroalpine Domain. It consists of fine siliciclastics alternating with carbonates, mostly limestones,rare dolostones and marls. The formation differs lithologically from the underlying Norian Dolomia del Cristallo because of different paleonvironmental evolution.The change in environmental parameters was controlled mainly by a climatic change to more humid conditions.This favoured on one hand the mobilisation and trasport by rivers of siliciclastic material from the continent to the Tethys gulf,and on the other influenced the sea-water chemistry.Freshwater influxes lowered salinity and inhibited early dolomitisation. Input of low density freshwater resulted in the astablishment of a permanent water mass stratification which influenced the benthic life. This paleoenvironmental reconstruction fits with the sudden clastic input which occurred in several palaeogeographic domains of the western Tethys realm (Austroalpine, Southalpine, Apennine, Hungary, Poland, Slovakia) during the Late Norian.

Plate tectonics and intracratonic mountain ranges in Morocco - The mesozoic-cenozoic development of the Central High Atlas and the Middle Atlas

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a reactivation of an inherited pan-African or Hercynian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifting and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic — earliest Eocene), and continental convergence between Europe (Iberia) and Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the Central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic — earliest Middle Jurassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajocian — Bathonian), followed by continental redbeds and final uplift (late Mid Jurassic — late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coincided with the main orogenic movements of the Betico-Rifean arc.

Palaeozoic strike-slip rift pulses and palaeogeography in the circum-Mediterranean Tethyan realm

Five main rifting cycles, supported by different types of field evidence (including magmatism, sedimentation and tectonics), have been recognized in the circum-Mediterranean area (western Tethys realm) before the opening of the Jurassic Tethys proper. They occurred roughly in the late Cambrian-early Ordovician, early Silurian, late Devonian-Dinantian, early-mid Permian and mid-Triassic times. Each of these rifting cycles began with intracontinental, mainly shallow-marine conditions, evolving usually to basinal environments of quite different bathymetry (the Palaeozoic CCD has been proved to be much less deep than the post Liassic one). Minor suboceanic seaways cut across and fragmented the Gondwana-South European plate following the early Palaeozoic rifting cycles. The long mid Palaeozoic (Devono-Dinantian) rifting cycle migh possibly have resulted in a wider system of suboceanic sea-ways connecting a Palaeo-Tethys ocean to the east of Gondwana with the Rheic ocean to the west of it. The birth of Palaeo-Tethys, or of its westward extension, would have been directly linked with the gradual death of the Devono-Mississippian Rheic Ocean. A complex, large-scale, dextral transform system is inferred to have connected the two respectively expanding and contracting oceans. Fragmentation and mutual displacement of microblocks in the connection area (the circum-Mediterranea area and Southern Europe) resulted from strike-slip, oblique rift pulses with coeval, local contractional and extensional processes. A major change of the Gondwana rotational vector is believed to have produced the mid-late Carboniferous transpressional strike-slip Hercynian orogen. This is a good explanation of the peculiar thermal regime of the Hercynian diastrophism all over Europe. There is no support for the hypothesis of a very wide (more than 3000 km) Palaeozoic ocean separating Northern Europe from Southern Europe or Gondwanaland. This idea was mainly based on poor palaeomagnetic data and is heavily contradicted by palaeobiogeographical and geological data. Furthermore, it ignores the dramatic shortening of the Hercynian orogen all over Europe. The early Palaeozoic to early Mesozoic history of the circum-Mediterranean area involves the cyclic recurrence of oblique rifts, culminating in the early Jurassic strike-slip rift and subsequent limited opening of the Tethys proper. To avoid confusion in terminology, one should use prefixes referring to age, bathymetry-physiography and geotectonic setting to qualifying the general term “Tethys” (e.g., shallow Cretaceous Tethys, basinal intracontinental Devonian Tethys, oceanic Jurassic Tethys, etc.).

Permian age from radiolarites of the Hawasina nappes, Oman Mountains

The Hawasina napper of the Oman Mountains yielded Permian radiolarians from cherts stratigraphically overlying a thick volcanic basement (Al Jil Formation) at the base of the Hamrat Duru Group. This fauna represents the first Permian radiolarians and radiolarites in the central and western Tethyan realm. A Permain age for pelagic sequences within the Hawasina Complex of Oman has major significance for regional paleogeographic reconstruction. A clear differentiation between platform (reefal sediments) and basin (radiolarites) from the base of the Late Permian (255 Ma) is implied. It suggests a flexure of the platform during Permian time; the present data implies that a zone of rifting was already developed adjacent to the northeast Gondwana platform margin during the Late Permian. The Hamrat Duru Basin corresponds to an opening intracontinental rift area (sphenochasm) between Arabia and northeast Gondwana, a reentrant of the paleo-Tethys.