Géologie Méditerranéenne Tome XXI, n° 3-4, 1994, pp. 59-63.

Stratigraphical position and significance of reef faciès at the Northern margin of the Dinaric carbonate platform during the Late Jurassic and Cretaceous in Croatia and Bosnia

LDRAGICEVIC* LVEUC *•

1 -Introduction

The evolution of the Dinaric carbonate platform as a regionally recognizable unit started in the Late Triassic. In the wide region of the Dinarides this corresponds with the establishment of stable geological conditions for an extensive period of shallow marine sedimenta¬ tion. The same conditions prevailed during the Jurassic and the Cretaceous until the middle Eocene. Occa¬ sionally, some long» or shorter periods of nondeposi tion occurred which resulted in the existence of land areas and, in most localities, with the occurence of bauxite deposits. The carbonate platform lies on the basement which has the characteristics of a continental crust

2 -General setting

The northern margin of the carbonate platform underwent considerable changes during its lengthy geo¬ logical history. These are particularly related to the changes of its geographical orientation ; the more intensive tectonic regime in the north in comparison with its central part ; lithofacies diversity, and especial¬ ly heterogeneity of lithogenically important groups of organisms (reef building organisms) which in some periods played a crucial role in producing enormous amouts of bioclastic material and in spatial and tempo¬ ral arrangement of the main lithofacies.

3 -Stratigraphy

3-1. Late jurassic

By analysing the afore-mentioned factors it was pos¬ sible to distinguish three periods for which the most important changes on the northern platform margin were observed (fig. 1) : in the Late Jurassic (Kim-meridgian -Tithonian), Early Cretaceous (Barremian -Aptian) and the Late Cretaceous (Santonian -Maastrichtian). This resulted in regionally recognizable lithofacies or groups of lithofacies in the formation of which the biological potential of reef building orga¬ nisms and the tectonic dynamics of the platform margin played the crucial part It has to be emphasized that during the whole period of time analysed, the northern margin represents the boundary area between a shallow marine region of the carbonate shelf and the transition to open deep marine region of Tethys (HSÛ, 1971 ; DER-COURT et al., 1993), and it therefore represents a passi¬ ve margin.

During the Upper Jurassic more intensive vertical movements on the northern platform margin contribu¬ ted to an explosive developement of hydrozoan, bryo-zoan, coral, and mollusc biocenosis. They built reefs, most probably of the barrier type on the very edge of the platform facing the open sea (i.e. platform margin between Dm = Dalmatia and Bo s Bosnia in FOUR-CADE et al., 1993). Steady, vertical dynamics together with biological potential made the reef existence pos¬ sible during a long period in spite of intensive marine abrasion. The relicts of the Malmian reefs, as well as