Sedimentology upper cretaceous central part of Vardar zone.

[V. Gajić]

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Geo-nauke - Petrologija / Geoscience Petrology

Područje istraţivanja obuhvaćeno doktorskom disertacijom nalazi se u centralnim delovima Vardarske zone. Karakteriše se sloţenom geološkom građom koju grade mezozojske tvorevine trijaske, jurske i kredne starosti, kao i kvartarni sedimenti. U trijasu je istraţivano područje pokrivalo more, sve do granice sa Šumadijom. Tokom jure sloţenim geotektonskim procesima stvaraju se ofiolitske stene rasprostranjene u juţnim i delom u centralnim delovima istraţivanog područja. U donjoj kredi se pokretima dinarske faze izdiglo kopno, koje je egzistiralo skoro u celoj donjoj kredi područja istraţivanja. U alb-cenomanu pojedini delovi istraţivanog područja se spuštaju i zahvata ih morska transgresija. Dalja kompresija, tokom gornje krede, dovodi do tektonskih deformacija u ovom području i otvaranja basena u kojima se odvijala karbonatna i karbonatno-terigena sedimentacija. Predmet istraţivanja ove doktorske disertacije predstavljaju sedimentne stene taloţene tokom cele gornje krede i njihova evolucije kroz vreme i prostor centralnog dela Vardarske zone. Rezultati dobijeni ispitivanjima petroloških, biostratigrafskih i genetskih karakteristika sedimentnih stena omogućila su raščlanjavanje gornje krede. Izdvojene su četiri litostratigrafske jedinice: 1. jedinica bazalnih gruboklastita, 2. jedinica šelfnih klastično-karbonatnih stena, 3. jedinica krečnjaka i laporaca sa roţnacima kontinentalne padine i 4. jedinica klastično-karbonatnih turbidita. Gornjokredne jedinice po svojim karakteristikama ukazuju na kontinuitet u sedimentaciji, bez naglih promena u reţimu taloţenja. Na plitkovodni karakter basena najviše upućuje postepeni prelaz priobalskih, plitkovodnih (šelf) i basenskih karbonatnih ili karbonatno-klastičnih stena (kontinentalna padina). Prvu izdvojenu jedinicu bazalnih stena čine najstarije sedimentne stene gornje krede (alb-cenoman). Postoje dva tipa razvića bazalnih gruboklastita. Prvi tip se moţe definisati kao konglomerati i konglomeratični krečnjaci izgrađeni od valutaka ofiolitskih stena, dok se konglomerati i konglomeratični krečnjaci drugog tipa razvića karakterišu fragmentima koji vode poreklo od karbonatnih stena. Nakon brze sedimentacije, usledila je mirnija faza koja je dovela do pojave konglomerata sitnijeg zrna i konglomeratičnih krečnjaka sa proslojcima laporaca i glinaca. Diferenciranjem morskog dna se pored dubljih morskih delova, obrazuju i podvodni grebeni na kojima se razvijaju sprudne i subsprudne stene. Takvim tvorevinama odgovaraju analizirani crveni krečnjaci u Drenovcu. Tokom cenomana se kontinuirano na bazalne stene taloţe stene koje pripadaju litostratigrafskoj jedinici šelfnih sitnozrnih karbonatno-klastičnih stena. Izgrađena je od alevrolita, kalcitskih alevrolita, glinovitih alevrolita, peskovitih alevrolita i peskovito-glinovitih alevrolita. Oni čine postepene prelaze ka kalcitskim peščarima, kalkarenitima i kalkruditima, koji predstavljaju završnu fazu plitkovodnih facija i najvećim delom su erodovani. U sitnozrnim sedimentima Ješića i na Rajcu razvijena je breţuljkasta kosa slojevitost koja u višim delovima prelazi u planarnu kosu slojevitost malih uglova. Ove teksture nastaju kada je taloţenje povezano sa olujama, a tokom mirnih vremenskih perioda sa plimatskim strujama. Na osnovu litoloških, biostratigrafskih i genetskih osobina dve najstarije gornjokredne izdvojene jedinice, one se moraju posmatrati kao celina. U vreme njihovog formiranja fizičko-hemijski uslovi sredine bili su promenljivi, što se odraţava kroz redosled taloţenja materijala. U gornjem turonu, usled dalje trangresije produbljavanja basena bilo je još izraţenije. Sedimentacija se obavljala u basenu sa smanjenom energijom vode i pojavom pelaških organizama: radiolarija i foraminifera (Globotruncana), a od makrofaune amonita i inoceramusa. Stvarale su se pelaške sedimentne stene, predstavljene laporcima, laporovitim krečnjacima, glinovitim krečnjacima i krečnjacima, svi sa konkrecionim roţnacima. Jedinica krečnjaka i laporaca sa roţnacima zauzima najveći prostor na ispitivanom terenu centralnog dela Vardarske zone. Sedimentacija se odvijala na kontinentalnoj padini bez prekida do kraja mastrihta. Karbonatne stene mikritskog do biomikritskog sastava predstavljaju autohtone stene ove jedinice. U vreme visokog nivoa mora (trangresije) koja je dovodila do jaku eroziju obale, u basen se povremeno turbiditnim tokovima donosila velika količina materijala. Nastajale su turbiditne stene (grubljeg i finijeg zrna) koje su se smenjivale i mešale sa autohtone karbonatnim stenama basena. Prisustvo turbidita (kalkrudita i kalkarenita) sa karakterističnim teksturama iz Boumine sekvence potvrđuje povremeni prinos plitkovodnog materijala turbiditnim tokovima niz kontinentalnu padinu. Kao neodvojivi deo jedinice pojavljuju se konkrecioni roţnaci, koji nastaju obaranjem silicije iz rastvora i metasomatskim zamenjivanjem karbonata u ljušturama fosila. Proces nastanka konkrecionih roţnaca objašnjava se niskim vrednostima pH i nepovoljnim uslovima za opstajanje silicije u rastvorenom obliku. Gornjokredni basen u centralnom delu Vardarske zone, tokom senona, širio se ka Šumadiji, dok je u njegovim zapadnim i jugozapadnim delovima došlo do izdizanja kopna. Tektonsko izdizanje planinskih masiva omogućilo je jače fizičko i hemijsko raspadanje stena i taloţenje debelih naslaga turbidita na kraju kampana i u mastrihtu. Iz turbiditnih tokova preteţno klastičnog detritusa, nastalih iz više izvora sa šelfa i margine padine, taloţile su se stene u obliku spojenih lepeza koje su formirale široku zonu u obliku zastornih turbidita (apron turbidite). Oni čine najmlađu izdvojenu litostratigrafsku jedinicu u gornjokrednim sedimentnim stenama područja istraţivanja. Najveće rasprostranjenje imaju pločasti sivi srednjozrni peščari bogati liskunima, alevroliti i ređe konglomerati, dok su u manjoj meri su zastupljeni laporci i krečnjaci. Kampan-mastrihtski turbiditi na području istraţivanja deo su ljiškog i šumadijskog fliša.

The research area covered by doctoral thesis is situated in the central part of the Vardar zone. It is characterized by a complex geological structure built by Mesozoic (Triassic, Jurassic and Cretaceous) and Quaternary sediments. During Triassic age investigated area was covered by sea to the Šumadija area border east. During the Jurassic complex geotectonic processes created ophiolitic rocks, widespreaded in the southern and partly in the central parts of the study area. In the Lower Cretaceous, tectonic movements of Dinar phase uplift the mainland, and these conditions existed almost throughout the whole Lower Cretaceous. In the Alb-Cenomanian, some parts of the study area were lowered and they became under the influence of sea transgression. Further compression during the Upper Cretaceous lead to tectonic deformation in this area causing basin opening, and sedimentation of carbonate and carbonate-terigenous began. Subject of this dissertation are sedimentary rocks deposited during the whole Upper Cretaceous and their evolution through time in the central part of the Vardar zone. Results obtained from petrologic investigation, biostratigraphic and genetic characteristics of sedimentary rocks allowed division of the Upper Cretaceous. Four lithostratigraphic units are separated: 1. Unit of basal coarsegrain clastites, 2. Unit of clastic-carbonate rocks from shelf, 3. Unit of limestone and marl with cherts of the continental slope, and 4. Unit of clasticcarbonate turbidites. Upper Cretaceous units by their characteristics indicate continuity in sedimentation, with no abrupt changes in the regime of deposition. Gradual transition of coastal, shallow-water (shelf) and basin carbonate or carbonateclastic rocks (continental slope) indicates shallow character of the basin. The first determined unit was formed from basal sedimentary rocks of AlbCenomanian age with two distinct types of basal coarse-grain clastic rocks. The first type can be defined as conglomerate and conglomeratic limestones made of ophiolite pebbles, while conglomerates and conglomeratic limestone of second type are characterized by fragments originating from carbonate rocks. After fast sedimentation, a calmer phase followed, which led to the occurrences of conglomerates with smaller grains and conglomeratic limestones with layers of marl and shale. By differentiation of the sea bottom, close to the deeper parts of the sea, (submarine cliffs) reef and subreef rocks were formed. Such formations correspond to analyzed red limestones in Drenovac quarry. During the Cenomanian age the lithostratigraphic unit of shelf with finegrained carbonate-clastic rocks was deposited continuously on basal rocks. It consists of siltstones, calcites siltstones, clayey siltstones, sandy siltstones and sandy-clayey siltstones. They make a gradual transition to calcite sandstones, calcarenite and calcrudites, which represents the final phase of shallow-water facies and they are mostly eroded. In fine-grained sediments on Ješić and Rajac locality, hummocky cross bedding, which in upper parts are exceeding in planar cross bedding with small angles are developed. This type of texture occurs when the deposition is associated with storms or during the quiet periods with high tide currents. On the basis of lithological, biostratigraphic and genetic characteristics of the two oldest Cretaceous units, they must be considered as a one unit. At the time of their forming, physical and chemical environmental conditions were variable, which reflects the order of deposition. In the Upper Turonian, due to further transgression, deepening of the basin was even more evident. Sedimentation was with reduced water energy and appearance of pelagic organisms: radiolarians, foraminifera (Globotruncana) and macrofauna with ammonites and Inoceramus. Pelagic sedimentary rocks were created, represented by marls, marly limestones, clayey limestones and limestones, all with concretion cherts. Unit with limestones and marls with cherts occupies the largest area in the central part of the investigated area of the Vardar Zone. Sedimentation took place on the continental slope without interruption until the end of Maastrichtian age. Carbonate rocks of micritic to biomicritic composition are autochthonous rocks of this unit. At the time of high sea level (transgression) which led to the occasional strong coastal erosion, turbiditic flows brought large amounts of material in the basin. Turbiditic rocks, made of coarser and finer grains, alternated and mixed with the autochthonous carbonate rocks from the basin. Presence of the turbidites (calcrudites and limestones) with characteristic textures from Bouma sequence confirms the occasional bringing of shallow material with turbiditic flows down the continental slope. Concretion cherts appear as an integral part of the unit, which were created by settling of silica from solution and metasomatic replacing of carbonates in the shells of fossils. The origin of concretion cherts is explained with low pH values and unfavorable conditions for the persistence silica in (solution) form. Upper Cretaceous basin in the central part of the Vardar Zone, during the Senonian age was spreading to Šumadija region, while, in its western and southwestern parts, mainland uplifting occurred. Tectonic uplift of mountain massifs allowed stronger physical and chemical disintegration of rocks and deposition of thick layers of turbidites at the end of the Campanian and Maastrichtian age. From turbidite flows, predominantly with clastic detritus formed from several shelf and margin slope sources, rocks were deposited in the form of a merged fan, which further formed a wide zone of apron turbidites. They represent the youngest defined lithostratigraphic units in the Upper Cretaceous sedimentary rocks in the investigated area. The largest distribution have flattened gray medium coarse sandstones, rich in mica, siltstones and rarely conglomerates, marls and limestones. CampanianMaastrichtian turbidites in the research area are the part of the Ljig and Šumadija flysh.

Vardarska zona, gornja kreda, transgresija, litostratigrafske jedinice, sedimentologija, turbiditi

Vardar Zone, Lithostratigraphic Units, Upper Cretaceous, transgression, sedimentology, turbidites

55:551.21/.24.03/.35/.762/.763 552:552.54 (497.11) (043.3)

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