Microfacies and diagenesis of lower and middle cretaceous carbonate rocks of NW-Yugoslavia (Slovenia, Trnovo area)

Abstract

A profile of about 1400 m thickness near the village of Trnovo in the Trnovo forest in SW-Slovenia was studied for the microfacies and diagenetic development of the carbonate rocks. Because of the detailed biostratigraphic subdivision being just in improvement, the interval from the problematic Jurassic/Cretaceous to the Tertiary boundary (Berriasian-Cenomanian, fault, Senonian) was subdivided into ten zones and several subzones. These subdivisions are of practical use and can be found in profiles studied in other areas of SW-Slovenia and Istria as well. The data are based on field observations, thin sections and acetate peels. The following development can be recognized: (1) During the Berriasian(?)-Hauterivian a shallow-marine to intertidal restricted environment was established on the coherent Dinaric Carbonate Platform. This is indicated by low faunal diversity and numbers as well as the common occurrence of marine and meteoric vadose cements, birdseyes and algal-layers.Clypeina jurassica, Bankia striata, Favreina salevensis, and trocholinid foraminifera are chacteristic of the lowermost part (Uppermost Jurassic?, Berriasian-Valanginian) of this section. (2) In the Barremian-Aptian a lagoonal environment with patch reefs was established, characterized by normal marine conditions. High faunal and floral diversity and large numbers occur (hydrozoans, corals,Palorbitolina lenticularis, Salpingoporella dinarica). During this time period the platform was divided into smaller tectonic units with different facies development. (3) An open marine ingression from the Slovenian Basin occurred in the Albian, indicated by dolomitic limestones rich in sponge spicules and silica nodules and layers. (4) In the Cenomanian, mainly bioclastic sands (rich inOrbitolina conica) and micritic limestones (withPithonella) were deposited in alternation with micrites rich in ostracods and limestones containing birdseyes, vugs, and microsparite. This indicates a strong interfingering of different facies on a gently dipping ramp, probably due to sea level fluctuations. (5) The Turonian is missing in the section analyzed due to a fault. (6) In the Senonian bioclastic sands rich in fragments ofSabinia sp. were deposited around rudist mud mounds formed in inner shelf areas under normal marine conditions. Eocene flysch overlies the sequence and influences it diagenetically by silica authigenesis in the upper decimeters of the underlying limestones.