Triassic–Jurassic intraplatform basins in the Dinarides: Myth or reality? The Neo-Tethys geodynamic history along a cross-section through the inner to outer Dinarides (Serbia–Montenegro)

The topic of the field trip will be the Triassic–Jurassic depositional history of the Dinarides with special focus on the relations between areas with shallow-water carbonate production (ramps, plaforms) and related deepwater sedimentary successions. To understand the Mesozoic (mainly Triassic–Jurassic) geodynamic evolution of the Western Tethys Realm this is essential, with the Dinarides as a very important and crucial orogen in that region. In fact, the Dinarides are the only mountain range in the Western Tethys Realm which allows insights in the Mesozoic evolution along a relative complete east-west cross-section below the Middle to early Late Jurassic obducted ophiolites, which derive from the Neo-Tethys (or Vardar Ocean in different nomenclature) to the east. Meanwhile it is in general accepted that all ophiolites have their provenance east of today’s Dinarides and are not part of small independent and long persistent (~70-80 Ma) oceanic basins between microcontinents (terranes). The Triassic(–Jurassic) central shelf arrangement, that is the area between the tectonic units of the Drina–Ivanjica unit in the east and the Pre-Karst/High Karst units to the west is supposed to consist of a series of shallow-water carbonate production areas (“platforms”) with intermediate long-lasting deep-water basins between these platforms. This scenario of long persisting deep-water basins between “platforms” has also to be restricted to the time span Middle to Late Triassic (Late Pelsonian to Rhaetian; ~40 million years). Only in some areas shallow-water carbonate production prevailed until the Early Jurassic, but carbonate platforms could not be formed in the Early Jurassic after the mass extinction at the Triassic/Jurassic boundary. This area comprises today the East Bosnian–Durmitor megaunit, a unit which experienced during the history of geological studies various interpretations and modifications. We will discuss the Middle Triassic to Jurassic various deep-water successions in the light of supposed deep-water (intraplatform) basins between timeequivalent carbonate platforms in relation to facts regarding: A) Carbonate production under tropical shallow-water conditions forming carbonate platforms, B) Carbonate platform models with respect to sea-level fluctuations, C) Carbonate production during times of environmental crises and perturbations, D) Facies models of carbonate platforms, and E) The principles of carbonate sequence stratigraphy and highstand shedding. The field trip will focus on the various deep-water sedimentary rocks during the Triassic–Jurassic geodynamic history deposited in different basins: rift-basins, shelf areas, oceanic domains, trench-like basins, foreland basins. To understand the complex Triassic to Jurassic sedimentological and geodynamic evolution we will visit and study: • The Middle Triassic to Middle Jurassic passive margin evolution: all depositional realms from the continental slope to the central shelf. • The Middle Triassic continental break-up: Anisian demise of shallow-water carbonate production (ramp geometry), horst-and-graben formation (breakup unconformity), and deposition of deep-marine sediments elsewhere (drowning succession). • Restart of shallow-water carbonate production in the Middle and Late Triassic: the sedimentary sequences in the deep-water depositional realm as mirror of platform progradation (sequence stratigraphy, highstand shedding, and carbonate production potential). • Reasons for the demise of the Triassic shallow-water platforms and the expression of these events in deep-water settings. • Condensed Early–Middle Jurassic deep-water sedimentary rocks. • Active continental margin evolution: Middle to Late Jurassic trench-like basin formation and nappe stacking in front of obducting ophiolites, large-scale mass movements (radiolaritic/argillaceous matrix), mélange formation. The field trip area, that are the main parts of the East Bosnian–Durmitor megaunit, provides well preserved insights in the whole Mesozoic geological/sedimentological/ geodynamic evolution of the Dinarides as part of the mountain ranges in the Western Tethys Realm with an identical history and tectonostratigraphy. The sedimentary sequences of the East Bosnian–Durmitor megaunit, that is the lowermost tectonic unit in that part of the cross-section allows a detailed reconstruction of the latest Permian to Middle/early Late Jurassic history of the central shelf area. In Middle Jurassic times in the area of the East Bosnian–Durmitor megaunit new trench-like deep-water basins were formed in the frame of ophiolite obduction, and in front of the west-ward propagating nappe stack. These newly formed deep-water basins in front of advancing nappes received material from the nappe stack bulldozed by the west-direct obducting ophiolites onto the foreland of the wider Adriatic plate, with the Dinarides as part of it. These trench-like foreland basins were later incorporated into the nappe stack of the Dinarides forming today various mélanges, originally mainly sedimentary mélanges, that are chaotic basin-fills with a general coarsening-upward trend. These mélanges in the various basins formed in sequence from east to west contain as reworked material (from cm-sized components to km-sized blocks in a deep-water radiolaritic/ argillaceous matrix) all facies zones from the Triassic to Middle Jurassic outer to central passive margin configuration to the oceanic realm, but predominantly from the continental slope and outer (deep-water) shelf environment. We will visit sequences originally deposited on the continental slope, the outer shelf region (Hallstatt facies), and the basinal sequences near to the reef rim, the reef-basin transition, and the open lagoonal area. Furthermore, in cases complete sequences are also preserved in far-travelled nappes bulldozed in front of the obducting ophiolites onto the foreland. These nappes rest above the various sedimentary mélanges and below the obducted ophiolites with their ophiolitic mélanges at the base. The whole story is very well visible and preserved in our field trip area, with some special highlights of rather practically nowhere in the Western Tethys Realm preserved sequences, but well exposed in northern Montenegro. The question “Are there Triassic–Jurassic intraplatform basins in the Dinarides?” will be answered during the field trip by the various sedimentary rocks and the depositional history through time and space. The history from deposition in a rift setting to a passive continental margin evolution and finally to an active margin setting, in combination with the rules of facies and sedimentology, the principles of carbonate production, basin formation and evolution, beside other conformities withnatural laws, is preserved in the different sedimentary succession like in a book. We will try to read that book, and try to understand why scientists in different times had read this book in a different way and gave us a different translation, i.e. interpretation. The Mihajlovici section from East Bosnian–Durmitor megaunit at northern Montenegro is the type-locality of the newly defined Early Jurassic (?Middle/Late Hettangian to Late Pliensbachian) Mihajlovici Formation.