Mesozoic basins on the Adriatic continental margin – a cross-section through the Dinarides in Montenegro / Mezozojski bazeni na kontinentalnem robu Jadranske plošče – presek čez Dinaride v Črni gori

Abstract

The Dinarides, together with the Albanides and Hellenides, preserve stratigraphic successions derived from the eastern margin of the Adriatic microplate and remnants of ophiolites obducted from the Maliac-Vardar branch of the Neotethys Ocean. The main stages in the Mesozoic geodynamic history are: 1) rifting leading to opening of the Maliac Ocean in the Late Anisian, 2) onset of an east-dipping intra-oceanic subduction in the Early-Middle Jurassic and sea-floor spreading in a supra-subduction setting (Vardar Ocean), 3) formation of ophiolitic mélanges in trench-like basins, westward obduction of young supra-subduction ophiolites in the Middle-Late Jurassic and accumulation of flysch-type deposits in foreland basins in the latest Jurassic to Early Cretaceous, 4) subaerial exposure of the newly formed nappes followed by middle to Late Cretaceous transgression, and 5) continental collision in the Maastrichtian and Paleogene. On the continental margin, the Middle Triassic to Early Jurassic extension created a complex horst-and-graben geometry that is apparent in the stratigraphic record. The present day NW-SE striking tectonic units are in rough accordance with the Mesozoic paleogeography. Hence, the inferred configuration for the most complete SW to NE transect through Montenegro and Serbia is as follows: The Dalmatian Carbonate Platform, the Budva Basin, the High Karst Carbonate Platform, the Bosnian Basin, the Durmitor High, the Lim Basin, the Drina-Ivanjica High, and the deep-marine distal continental-margin domain.

We present a short description of the stratigraphy for these tectonic/paleogeographic units and discuss their possible connection with other units of the Dinarides and Hellenides. The field guide focuses on deep-water deposits, in which radiolarians are the crucial tool for dating. We describe the complete Mesozoic succession of the Budva Zone, the Middle Triassic pelagic episode of the High Karst Zone, the Upper Triassic and Jurassic pelagic rocks of the Lim Zone and two localities with radiolarites associated with ophiolites.

The largest part of the guide is devoted to the Budva Zone, a deeply rifted trough in the continuation of the Pindos Basin. The Budva Zone with its external location in the Dinaric orogen was a site of continuous pelagic sedimentation from the Middle Triassic to the end Cretaceous. Radiolarites characterize the Middle Triassic, Hettangian–Sinemurian, Aalenian to Tithonian, and Hauterivian–Barremian to lower Turonian; pelagic limestones prevail in the Upper Triassic, Berriasian–Valanginian and upper Turonian to Maastrichtian. Calcareous turbidites from the adjacent High Karst Carbonate Platform are interstratified in all units and completely replace radiolarites in the Pliensbachian.

Pelagic sequences also occur in the High Karst Zone, but are confined to the Middle Triassic syn- and early post-rift deposits. A 20 m thick unit of Middle Triassic nodular limestone and radiolarite within shallow-water carbonates is a typical example.

More internally, the western Ćehotina Subzone of the Lim Zone records pelagic sedimentation from the Middle Triassic to early Cretaceous, when synorogenic mixed carbonate-siliciclastic deposition began. This zone has been less investigated than the Budva Zone. A 100 m thick Norian to Rhaetian succession of limestone with chert nodules is dated with conodonts. A Callovian-early Oxfordian age of lime-free cherts is determined with radiolarians. The Mihajlovići Subzone that may have been part of the Drina-Ivanjica paleogeographic unit shows Triassic shallow-water carbonates and a Jurassic deepening upward sequence ending with Oxfordian radiolarites.

The last two field-trip stops show upper Bathonian-lower Callovian radiolarites in an ophiolitic mélange and upper Anisian radiolarites in direct contact with basalt. These ages, obtained in the south-westernmost ophiolite remnants of the Dinarides, agree with previously documented ophiolite ages in the wider region.

In comparison with the Southern Alps and the Apennines, pelagic deposits of the Dinarides are characterized by an earlier onset and considerably higher proportions of silica with respect to carbonate throughout the Mesozoic. The Dinaric basins were connected with the central Neotethys, where the high fertility of surface waters enabled radiolarite formation since the oceanisation (Anisian or earlier) until the early Late Cretaceous, when planktonic foraminifera and calcareous nannoplankton began to dominate worldwide.

Key words: Dinarides, Neotethys, radiolarites, continental margin, ophiolitic mélange

IZVLEČEK

V Dinaridih, Albanidih in Helenidih so ohranjena stratigrafska zaporedja vzhodnega roba Jadranske mikroplošče in ostanki ofiolitov, narinjenih na kontinent iz oceana Maliak-Vardar, ki je bil del Neotetide. Glavne stopnje v mezozojski geodinamični evoluciji tega ozemlja so bile: 1) rifting, ki je v zgornjem aniziju privedel do odprtja oceana Maliak, 2) v spodnji do srednji juri začetek intraoceanske subdukcije in raztezanje oceanskega dna v suprasubdukcijskem okolju Vardarskega oceana, 3) v srednji do zgornji juri formacija ofiolitnega melanža v jarkom podobnih bazenih in obdukcija mladih suprasubdukcijskih ofiolitov proti zahodu ter na koncu jure in v spodnji kredi akumulacija flišnih sedimentov v predgornih bazenih, 4) emerzija novo nastalih pokrovov in nato transgresija v srednji do zgornji kredi, 5) kolizija kontinentov v maastrichtiju in paleogenu. Kontinentalni rob se je med ekstenzijo od srednjega triasa do spodnje jure diferenciral na horste in grabne, kar se odraža v stratigrafskem zapisu. Današnje NW-SE usmerjene tektonske enote v Dinaridih se v grobem ujemajo z mezozojsko paleogeografijo, iz česar sklepamo, da je bila konfiguracija kontinentalnega roba v prečnem preseku čez Črno goro in Srbijo naslednja: Dalmatinska karbonatna platforma, Budvanski bazen, karbonatna platforma Visokega Krasa, Bosanski bazen, Durmitorski prag, Limski bazen, prag Drina-Ivanjica in globokomorski distalni kontinentalni rob.

V članku je najprej na kratko opisan stratigrafski razvoj tektonskih oziroma paleogeografskih enot tega preseka in domnevna povezava z drugimi enotami v Dinaridih in Helenidih. V nadaljevanju so opisane ogledne točke ekskurzije s poudarkom na globokomorskih sedimentnih kamninah, ker so za določanje starosti teh kamnin radiolariji najpomembnejši in pogosto edini fosili. Podrobno predstavljamo celotno mezozojsko zaporedje Budvanske cone, srednjetriasno pelagično epizodo v coni Visokega Krasa, zgornjetriasne in jurske pelagične kamnine Limske cone in dve lokaliteti z radiolariti v ofiolitih.

Največji del vodnika je posvečen Budvanski coni v Zunanjih Dinaridih. V mezozoiku je bila ta cona globokomorski jarek v nadaljevanju bazena Pindos s kontinuirano pelagično sedimentacijo od srednjega triasa do konca krede. Radiolariti so značilni za obdobje srednjega triasa, hettangija in sinemurija, aalenija do tithonija ter hauterivija-barremija do spodnjega turonija. Pelagični apnenci prevladujejo v zgornjem triasu, berriasiju in valanginiju ter od zgornjega turonija do maastrichtija. Karbonatni turbiditi, prinešeni s sosednje karbonatne platforme Visokega Krasa, so interstratificirani v vseh formacijah, v pliensbachiju pa prevladujejo in popolnoma izpodrinejo pelagične sedimente.

Pelagična zaporedja v coni Visokega Krasa so omejena na sinriftne in zgodnje postriftne sedimente. Kot tipičen primer predstavljamo 20 m debelo zaporedje srednjetriasnih gomoljastih apnencev in radiolaritov znotraj plitvovodnih karbonatov.

V bolj interni Limski coni je za podcono Ćehotina značilna pelagična sedimentacija od srednjega triasa do začetka krede, ko so se začeli odlagati sinorogeni mešani karbonatno-siliciklastični sedimenti. Stratigrafsko zaporedje te podcone do sedaj ni bilo podrobneje proučeno. V članku je prvič datiran 100 m debel profil apnencev z gomolji roženca, ki smo ga s konodonti uvrstili v norij in retij. Z radiolariji smo dokazali callovijsko do spodnjeoksfordijsko starost plastovitih rožencev brez karbonata. V podconi Mihajlovići je stratigrafski razvoj podoben kot v enoti Drina-Ivanjica. Triasnim plitvovodnim karbonatom sledijo jurski apnenci, ki kažejo na postopno poglabljanje sedimentacijskega okolja. Zaporedje se konča z oxfordijskimi radiolariti.

V Črni gori so ohranjeni najbolj jugozahodno ležeči ostanki ofiolitov v Dinaridih. Zadnji dve točki prikazujeta bathonijske do spodnjecallovijske radiolarite v ofiolitnem melanžu in zgornjeanizijske radiolarite v kontaktu z bazaltom. Te starosti se ujemajo z do sedaj znanimi datacijami v ofiolitih širše regije.

V primerjavi z Južnimi Alpami in Apenini je za pelagične sedimente Dinaridov značilno, da so se začeli odlagati prej in da so skozi ves mezozoik vsebovali znatno višji delež kremenice glede na karbonat. Dinarski bazeni so bili povezani s centralno Neotetido, kjer je visoka produktivnost površinskih voda omogočala nastanek radiolaritov od oceanizacije (v aniziju ali še prej) do sredine zgornje krede, ko so po vsem svetu začeli prevladovati foraminifere in kalcitni nanoplankton.

Ključne besede: Dinaridi, Neotetida, radiolariti, kontinentalni rob, ofiolitni melanž