hrvatski

Platformni facijesi od gornjega titona do gornjeg alba u zapadnoj Istri i prijelaz u tempestitne, klinoformne i rudistne biolititne facijese donjega cenomana u južnoj Istri (ekskurzija B)

The Upper Tithonian peritidal limestones near Rovinj, which have been well known for centuries uder the name Pietra d'Istria or Orsera as the building stone used for the construction of many buildings and palaces in Venice, Ravenna and Loret, are characterized by shallowing-upward cycles. This unit represents the first part of the second transgressive-regressive megase-quence of Jurassic-Cretaceous shallow-water carbonates of western Istria. Above the underlying bauxites it begins with oscillating transgression characterised by thin beds of mudstones, clayey marls, clays and black-pebble breccia. Shallowing-upward cycles are composed of three members. 1)- a thin, laterally variable bed of black-pebble breccia with carbonate, clayey or marly matrix. This member was formed by the redeposition of material originating from marsh deposits enriched in organic matter, which was eroded and transported dur-ing a rise in the relative sea-level. 2)- thick (100 to 200 cm) stylolitised mudstone with rare bioclasts of Clypeina jurassica FAVRE, Salpingoporella annulata CAROZZI, Campbelliella striata (CAROZZI). In some cycles its upper part is characterised by vertical bioturbations, fenestral fabric, desiccation cracks and erosional surfaces. This member was deposited in low-energy subtidal. 3)- is characterised by variable lithology and structural fabric, predominantly represented by vadose fabrics, intraclasts with pisoid coatings, and in places by brackish to fresh wather (?) stromatolites. The upper bedding surfaces are sharp, irregular with desiccation cracks and/or erosional features. This member was formed in an intertidal and/or vadose zone. The lithostratigraphic unit Fantazija, i. e. Berriasian dolomites, represents a part of the second megasequence of Jurassic-Cretaceous platform carbonates in western Istria. It is composed of numerous shallowing-upward cycles consisting of dark macrocrystalline late-diagenetical dolomites (originating from subtidal-intertidal limestones) and light supratidal early-diagenetical dolomites. Many structures and textures typical of supratidal dolomitization can be found: stromatolites, tepee-structures, fenestral fabric, load casts and microcasts, desiccation cracks, shrinkage cracks, "sand-watch" structures, supratidal breccia, tide channels, erosional surfaces, etc. The lithostratigraphic unit Istarski žuti (building-stone known under the name of Istrian Yellows) is part of the Lower Aptian massive limestones forming the uppermost part of the second megasequence in the predominant part of Istria. This unit is characterised by rhythmical alternations of mudstone and bacinella-oncolites (i.e. oncoid floatstones. In the uppermost part there are indications of a relative sea-level fall and the beginning of a regression phase culminating in the general Upper Aptian emersion. A further relative sea-level fall resulted in clearly visible emersion with palaeokarstified relics of Istarski žuti limestone and a clay bed. This bed represents a stratigraphic gap lasting from the Upper Aptian to the beginning of the Upper Albian. The beginning of the Upper Albian deposition is characterised by oscillating trans-gression, and deposition of peritidal limestones and high-energy conglomerates. The following 15 m of succession is composed of similar peritidal and fore-shore limestones with two additional weak emersions. In the area today belonging to Istria, emersion commenced in different periods, and is characterised by variable duration. This is explained by the relative sea-level fall and different local influences of synsedimentary tectonics. Upper Albian shallow marine limestones are characterised by cyclical alternations of pelletal packstone/grainstones (locally with small-scale current ripples), laminated pelletal wackestone/packstones with sporadic stromatolite laminae, and stromatolites. By the end of the Albian only intraclast-peloidal packstones and grainstones were deposited in shoreface environments. Synsedimentary tectonics is manifested by slumps, sliding, creeping and small-scale faulting, probably formed during the tectonic activity caused by platform tilting. Tectonical disturbances, together with probable eustatic changes, led to a rise in relative sea-level. Therefore, Lower Cenomanian deposits are characterised by depo-sition predominantly in the offshore-transition and the lower shoreface environments, pre-dominantly close to the fair-weather wave base (hummocky cross-bedding tempestites. The are overlying by 180 cm thick bed composed of "nodular" fine-grained and kompletly bioturbated peloid-orbitolinid grainstones with basal lag containing large rudist shells and debris. In a further suc-cession there is a few-metre-thick carbonate sandstone body composed of fine-grained car-bonate biodetritus, overlaid by clinoforms composed of rudist debris. This clinoform limestones presents a typical coarsening-upward sequence, formed by progradation of rudist debris and colonies toward the South-East.

Karbonatni facijesi, periplimni ciklusi oplićavanja naviše, emerzije, sinsedimentacijska tektonika, slampovi, tempestiti, gornji titon, donja kreda, cenoman, Jadranska karbonatna platforma, Istra, Hrvatska

nije evidentirano