Naslov
Upper Cretaceous Microfacies of the Pučišća Formation (Campanian)of the Island of Brač, Croatia.

Autori
Cvetko Tešović, Blanka ; Gušić, Ivan

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
1. hrvatski geološki kongres, Opatija, Abstracts / Vlahović, Igor ; Velić, Ivo ; Šparica, Marko - Zagreb : Hrvatski geološki institut, 1995, 25-25

Skup
1. hrvatski geološki kongres

Mjesto i datum
Opatija, Hrvatska, 18.10.1995. - 21.10.1995

Vrsta sudjelovanja
Poster

Vrsta recenzije
Nije recenziran

Ključne riječi
Campanian; microfacies types; large benthic foraminifera; Pučišća Formation; Island of Brač; Croatia

Sažetak
Brač, the largest central Dalmatian island, is mainly built of Upper Cretaceous and only partially of Paleogene deposits. The Pučišća Formation is one of six lithostratigraphic units of the island of Brač which include limestone deposits ranging in age from the Upper Cenomanian to Maastrichtian. The Pučišća Formation (Campanian in age) can be subdivided into three superpositional-lateral units (GUŠIĆ & JELASKA 1990): the Brač Marbles, the Rasotica, and the Lovrečina unit. In the Brač "Marbles" two microfacsies types can be found: 1. "Proximal" type, bioclastic packston, SMF-5, SFB-4.This microfacies was deposited on the outer side of the reef, i.e. in the fore-reef environment. Proximal type is composed of grain supported varieties that were deposited near rudist buildups in agitated water, above the normal wave base. 2. "Distal" type, skeletal mudstone - wackestone, SMF-4 , SFB-3. Distal type has mud-supported lithologies with scattered bioclasts or skeletal grains, and was deposited at a larger distance from the buldups on the open intraplatform slope, below the wave base. The Rasotica and Lovrečina units are characterized by rich association of "large" benthic imperforate foraminifera which occur in different varieties of mud-supported skeletal limestones. These varieties are indicative of their deposition in the enviroment of back-reef platform. In that environment, five microfacies types can be found. This division is based on criteria of sedimentary structures and composition of fossil assemblages. These five types are correlated with the standard microfacies types (SMF) according to FLUGEL (1972) and standard facies belts (SFB) according to WILSON (1975): 1. MF-1 Bioclastic packstone ("biocalcarenite"), SMF-5, SFB-6. This MF-type originated by destruction of patch-reef like rudist buildups which grew in high energy environment and the material was deposited in immediate proximity. That MF-type is similar to the so-called proximal type of the Brač Marble deposits (the latter, however, were deposited on the outer side of the "reef", i.e. in the fore-reef environment). That MF-type is therefore the back-reef equivalent of the Brač Marble proximal variety. The foraminiferal assemblage does not unequivocvally prove the exact synchoneity of the two facies, but they cannot be stratigraphically very different either. MF-2 Foraminiferal-peloidal packstone, SMF-9, SFB-6 - SFB-7. This microfacies was deposited in protected shallow subtidal environment, as indicated by abundant carbonate mud, numerous pellets, peloids, micritic lithoclasts (micrite grains), and numerous benthic imperforate foraminifera. 3. MF-3 Bioclastic grainstone, SMF-11, SFB-6. This MF-type was also deposited in shallow subtidal environment, but on open shelf with agitated water where the detritus transported by waves and/or tidal currents was accumulated. Internal sediment indicates sand bar environment, sporadically emerged and affected by vadose diagenetic processes. 4. MF-4 Foraminiferal-peloidal wackestone, SMF-9, SFB-7. High percentage of carbonate mud, pellets, and peloids indicates that that MF-type was deposited in an environment similar to MF-2: shallow, restricted carbonate platform with low water energy and steady accumulation regime. Similar to MF-2, that environment was also well suited for larger imperforate foraminifera because it offers constant, stable conditions (normal salinity without abrupt changes in temperature, trophism and other parameters). 5. MF-5 Foraminiferal mudstone, SMF-19, SFB-8. Foraminiferal mudstone is characteristic of shallow subtidal to intertidal environments within restricted carbonate platform. Lower intertidal environment and fresh water (meteoric) influence is indicated both by fenestral fabric and geopetal infillings of cavities and specific, low diversity biotic association (miliolids, discorbids, ostracodes). The benthic imperforate foraminiferal assemblage indicates stable conditions in the depositional enviroment exemplified by permanent, shallow, restricted subtidal belt where deposition of mud-supported limestone varieties prevailed (except MF-3). Larger foraminifera, i.e. those with complex innner morphology, are known to be K-strategists, often supported by endosymbiotic relationship with unicellular algae. These foraminifera are characterized by slow growth and late maturity, resulting in such reproduction rate and offspring quantity that enables them to retain optimal population density in stable environmental conditions.

Izvorni jezik
Engleski

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