engleski

Sea Level Curves from Eocene Carbonate Deposits from Istria (Croatia): Causes, Implications and Consequences

Four geological columns from northern-central part of Istria have served as a case study for palaeoecological interpretation. Three main criteria were used: establishing of the first and the last appearance for some larger foraminifera used as environmental indicators ; composition of predominant foraminiferal community in the samples ; establishing the age of the Upper Creatceous basement and of the transgressive Eocene deposits. All these parameters served for the construction of sea level curves. Four hypothetical situations regarding the ages of underlying (Cretaceous) and overlying (Eocene) deposits were considered: (1) Upper Cretaceous deposits of different age overlain by Eocene deposits of different age ; (2) Upper Cretaceous deposits of different age overlain by Eocene deposits of the same age ; (3) Upper Cretaceous deposits of the same age overlain by Eocene deposits of the same age ; and (4) Upper Cretaceous deposits of the same age overlain by Eocene deposits of different age. These differences are important insofar as they define, in a way, the morphology and hypsometrycal differences of the Cretaceous paleorelief, and the onset of the Eocene transgression. In the Eocene, two phases of the colonization of the larger foraminifera can be distinguished: first we observe initial colonizing by not numerous species, followed by an increased diversification in a stabilized environment. The real situation at the investigated localities approximatively corresponds to the hypothetical model (3): Middle-to-Upper Cenomanian deposits are overlain by Middle Eocene deposits. Emersion surfaces on those localities are represented by different facies, caused both by different paleoenvironments and synsedimentary tectonics. Main criteria used for the construction of the sea level curves were the distinguishing of palaeoecological conditions indicative of different paleodepths. Appearance of the different foraminiferal taxa or their domination inside the community is controlled by the sea level oscillations. In the construction of sea level curves, test structure, test shape, and hydrodynamic conditions of the environment were taken into account. Foraminifera with porcelaneous test (suborder Miliolina: e.g. miliolids and alveolinids) are indicators of a shallower environment. Lamellar-perforate forms with hyaline test are represented by nummulitids and orthofragminids, which indicate a somewhat deeper environment. Uneven relative sea level oscillations at the different localities, and the approximate contemporaneous colonisation of forms with different test microstructure are the consequences of local diachronism caused by different morphology of the Cretaceous paleorelief, in relation to the onset of the Eocene transgression. The constructed sea level curves show a general transgressive trend, with numerous oscillations at all investigated localities. The consequences of different transgressive trends are visible in the field, and can be distinguished by repeated alternations of heterogenous foraminiferal communities. In detail, those trends may be described as transgressive-regressive, where the total intensity of the transgression, when represented in a coordinate system, is at least for the first positive decimal place larger than zero, in the relation to the total intensity of the regression. This situation conclusively resulted in deepening.

Larger Foraminifera; Predominant foraminiferal community; Onset of the Eocene transgression; Colonization; Sea level curves; Transgressive trend; Deepening

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