Mesozoic tectonostratigraphy of the Eastern Alps (Northern Calcareous Alps, Austria): a radiolarian perspective / Mezozojska tektonostratigrafija Vzhodnih Alp (Severne Apneniške Alpe, Avstrija): radiolarijska perspektiva

Abstract

The topic of the field trip is the Mesozoic geodynamic evolution in the Western Tethys realm well recorded in deep-water settings, especially in the radiolarian-bearing sedimentary rocks and radiolarites in the Eastern Alps (Northern Calcareous Alps). The well preserved Mesozoic sedimentary successions deposited in the Northern Calcareous Alps reflect two different Wilson cycles with its mountain building processes:

Evolution of the Neo-Tethys Ocean to the south/southeast: The Middle Triassic oceanic break-up (Late Anisian) was followed by the Middle Triassic to Middle Jurassic passive margin evolution and later by Middle to early Late Jurassic thrusting related to ophiolite obduction and subsequent latest Jurassic to Early Cretaceous mountain uplift of the Neo-Tethys orogen to the south of the todays Northern Calcareous Alps.

Evolution of the Alpine Atlantic Ocean (named Penninic Ocean in the Eastern Alps) to the north/northwest: The Late Early to Middle Jurassic oceanic break-up was followed by the Middle Jurassic to Late Cretaceous passive margin evolution and Late Cretaceous to Palaeogene subduction of the Penninic realm, Palaeogene collision and subsequent Neogene mountain uplift with its gravitational collapse (Lateral Tectonic Extrusion) of the Alpine orogen s.str.

For another orogenesis in the “Mid-Cretaceous” (Aptian-Cenomanian), i.e. between these two well recognizable Wilson cycles, the geodynamic background has not been well explored or explained yet. This “Mid-Cretaceous” orogenesis draws a veil over the older Mesozoic plate configuration and has generated controversial discussion about the geodynamic evolution and palaeogeography in Triassic to Early Cretaceous times. However, this orogenesis is not connected to the Neo-Tethys or the Alpine Atlantic Wilson cycle.

The field trip will focus on Triassic to Early Cretaceous deep-water, radiolarian-bearing sedimentary rocks deposited during the geodynamic history of the Neo-Tethys in different basins: rift-basins, shelf areas to continental slope, oceanic domains, and trench-like foreland basins. Special emphasis will be on the Jurassic to Early Cretaceous history, i.e. the geodynamic evolution before the “Mid-Cretaceous” tectonic motions and the influence of the evolution of two oceanic domains on the depositional environment above a drowned Triassic shelf (Apulian or wider Adria plate) between the Neo-Tethys Ocean to the south/southeast and the Alpine Atlantic Ocean to the north/northwest.

The geodynamically triggered interplay between carbonate production, siliciclastic/volcanic input and deposition of siliceous rocks/radiolarites in combination with the asynchrony of basin formation frequently allows the calibration of radiolarians with e.g., ammonoids, conodonts, calpionellids and other organisms. Following the Middle Triassic (Late Anisian) Neo-Tethys oceanic break-up and the demise of shallow-water carbonate production, deposition of Middle Triassic (Late Anisian to Ladinian) radiolarian-bearing, mainly carbonate deep-water sediments is widespread all over the shelf. Deposition of radiolarites in the Eastern Alps is limited to the outer shelf/continental slope and the Neo-Tethys oceanic domain to the south/southeast. Widespread shallow-water carbonate production started again in the latest Middle Triassic (Late Ladinian) and lasted until the end of the Triassic, interrupted only by short-lasting siliciclastic intervals (“Mid-Carnian” turnover, Lunz event). In the Late Triassic huge carbonate platforms were formed. Deposition of Late Triassic open-marine and radiolarian-bearing sediments is therefore limited mainly to the outer shelf region and radiolarites were deposited only on the Neo-Tethys ocean floor.

In Jurassic times, after the demise/drowning of the Late Triassic carbonate platform, calcareous siliceous sediments were again deposited widely. Rifting in the Alpine Atlantic realm to the north/northwest started in the Early Jurassic with oceanic break-up occurring from the Early/Middle Jurassic boundary onwards. The opening of the Alpine Atlantic to the north/northwest and, contemporaneously, the onset of convergence in the Neo-Tethys to the south/southeast worked in concert with radiolarite deposition culminating in the Middle Jurassic. Radiolarites were deposited practically all over the drowned continent except the areas of the Adriatic Carbonate Platform. Obduction of Neo-Tethys derived ophiolites since the Middle Jurassic led to the formation of a thin-skinned orogen with the formation of trench-like foreland basins in front of the advancing ophiolites. In these basins sedimentary mélanges with a radiolaritic-argillaceous matrix were deposited until the early Late Jurassic. Kimmeridgian-Tithonian shallow-water carbonate production on upper surfaces of the nappes restricted radiolarite deposition to remaining deep-water basins. In the frame of mountain uplift from the latest Jurassic (Tithonian) onwards the palaeotopography becomes overprinted by unroofing. Remaining deep-water foreland basins were successively filled in the Early Cretaceous by the erosional products of the uplifted Middle-Late Jurassic Neotethyan orogen.

During this field trip in one of the most classical areas of the world, the central Northern Calcareous Alps with its world-wide known touristic highlights, we will visit locations documenting the interplay between siliciclastic input, volcanic activity, carbonate production, various tectonic motions and deposition of radiolarian-bearing siliceous rocks to radiolarites.

Key words: Western Tethys realm, Triassic, Jurassic, Radiolarites, Palaeogeography

IZVLEČEK

Ekskurzija je posvečena mezozojski geodinamični evoluciji zahodne Tetide. Ta je dobro zabeležena v globokomorskih okoljih, še posebej v radiolaritih in drugih radiolarijskih sedimentnih kamninah v Vzhodnih Alpah, katerih del so Severne Apneniške Alpe. Dobro ohranjena mezozojska sedimentna zaporedja v Severnih Apneniških Alpah odražajo dva različna Wilsonova cikla z gorotvornimi procesi.

Prvi cikel se nanaša na razvoj oceana Neotetida na jugu do jugovzhodu. Oceanskemu razpadu v srednjem triasu (zgornjem aniziju) je sledil razvoj pasivnega roba do srednje jure in pozneje, v srednji in zgornji juri, narivanje, povezano z obdukcijo ofiolitov. Na koncu jure in v spodnji kredi se je dvigal Neotetidin orogen, lociran južno od današnjih Severnih Apneniških Alp.

Drugi cikel je povezan z razvojem oceana Alpski Atlantik (imenovanega Peninski ocean v Vzhodnih Alpah) na severu do severozahodu. Oceanskemu razpadu proti koncu spodnje jure in v srednji juri je sledil razvoj pasivnega roba od srednje jure do zgornje krede in subdukcija Peninika v zgornji kredi in paleogenu. Sledila je kolizija v paleogenu, v neogenu pa nadaljnje dviganje orogena z gravitacijskim kolapsom (lateralnim tektonskim iztiskanjem) Alpskega orogena sensu stricto.

Obstajajo še dokazi za orogenezo v “srednji kredi” (aptij-cenomanij) med tema dvema dobro prepoznavnima Wilsonovima cikloma, vendar geodinamično ozadje te orogeneze še ni dobro raziskano ali pojasnjeno. “Srednjekredna” orogeneza zakriva starejšo mezozojsko konfiguracijo plošč, kar je vzrok za kontroverzno razpravo o geodinamičnem razvoju in paleogeografiji od triasa do spodnje krede. Ta orogeneza ni bila povezana z Wilsonovim ciklom Neotetide ali Alpskega Atlantika.

Fokus ekskurzije je na radiolarijskih globokomorskih sedimentnih zaporedjih na robu Neotetide od triasa do spodnje krede. Zaporedja so bila odložena v različnih okoljih: v riftnih bazenih, na šelfu in kontinentalnem pobočju, v oceanu in v predgornih bazenih. Poseben poudarek bo na evoluciji v juri in spodnji kredi oziroma na geodinamičnem razvoju pred “srednjekrednimi” tektonskimi premiki. Poudarjen bo vpliv razvoja dveh oceanov na sedimentacijsko okolje, ki se je diferenciralo, ko se je potopil triasni šelf (Apulijska ali širša Jadranska plošča) med Neotetido na jugu/jugovzhodu in poznejšim Alpskim Atlantikom na severu/severozahodu.

Geodinamična evolucija in medsebojni vplivi med produkcijo karbonatov, siliciklastičnim ali vulkanskim vnosom in odlaganjem kremenični sedimentov/radiolaritov v kombinaciji z asinhronim oblikovanjem bazenov omogočajo, da se v določenih obdobjih radiolariji pojavljajo skupaj z drugimi organizmi, npr. amonoidi, konodonti in kalpionelidami. Po razpadu Neotetide v srednjem triasu (zgornjem aniziju) in prenehanju produkcije karbonatov v plitvi vodi so bili po celotnem šelfu razširjeni srednjetriasni (zgornjeanizijski do ladinijski) radiolarijski, predvsem karbonatni globokomorski sedimenti. Odlaganje radiolaritov je bilo v Vzhodnih Alpah omejeno na zunanji šelf in kontinentalno pobočje ter na oceansko območje Neotetide na jugu/jugovzhodu. Razširjena produkcija karbonatov v plitvi vodi se je ponovno vzpostavila na koncu srednjega triasa (v zgornjem ladiniju) in je trajala do konca triasa. Prekinjena je bila le s kratkotrajnimi siliciklastičnimi intervali (»srednjekarnijski« obrat, dogodek Lunz). V zgornjem triasu so nastale obsežne karbonatne platforme. Odlaganje zgornjetriasnih globokomorskih sedimentov in sedimentov, ki vsebujejo radiolarije, je bilo torej omejeno predvsem na območja zunanjega šelfa, radiolariti pa so se odlagali zgolj na oceanskem dnu Neotetide.

V juri, po potopitvi zgornjetriasne karbonatne platforme, so se s kremenico bogati karbonatni sedimenti ponovno odlagali na širšem območju. V spodnji juri se je začel tudi rifting na severu/severozahodu, ki je na meji med spodnjo in srednjo juro privedel do oceanizacije Alpskega Atlantika. Odpiranje Alpskega Atlantika na severu/severozahodu in sočasni začetek konvergence v Neotetidi na jugu/jugovzhodu sta hkrati delovala na poglabljanje bazenov na kontinentalnem robu, tako da je odlaganje radiolaritov v srednji juri doseglo višek. Radiolariti so se odlagali tako rekoč po celotnem potopljenem območju razen na Jadranski karbonatni platformi. Obdukcija ofiolitov z območja Neotitide od srednje jure dalje je privedla do oblikovanja tankoslojnega orogena in nastanka jarkom podobnih predgornih bazenov pred napredujočimi ofioliti. V teh bazenih so se do začetka zgornje jure odlagali melanži z radiolaritno-glinastim vezivom. V kimmeridgiju in tithoniju se je na novo nastalih pokrovih vzpostavila plitvovodna karbonatna produkcija, radiolariti pa so ostali omejeni na preostale globokovodne bazene. Zaradi dvigovanja orogena od zgornje jure (od tithonija) naprej in posledično erozije se je paleotopografija popolnoma spremenila. Preostali globokomorski predgorni bazeni so bili v spodnji kredi drug za drugim zapolnjeni z materialom, erodiranim z dvignjenega srednje do zgornjejurskega orogena Neotetide.

Ekskurzija je speljana po enem najbolj klasičnih območij sveta, osrednjih Severnih Apneniških Alpah, s svetovno znanimi turističnimi znamenitostmi. Obiskali bomo lokacije s sedimentnimi zaporedji, iz katerih lahko razberemo medsebojno povezanost med vnosom siliciklastitov, vulkansko dejavnostjo, produkcijo karbonatov, različnimi tektonskimi dogajanji ter odlaganjem radiolaritov in drugih kremeničnih kamnin z radiolariji.

Ključne besede: Zahodna Tetida, trias, jura, radiolariti, paleogeografija