engleski

Lacustrine coarse-grained delta-front to prodelta succession (Early Miocene, Mt. Požeška, North Croatian Basin)

The Lower Miocene sediments unconformably overlay pre-Miocene basement of Mt. Požeška. They are probably Ottnangian in age. Mt. Požeška belongs to the North Croatian Basin which represents a south-western part of the Pannonian Basin System. The basin began to develop by continental rifting during the Ottnangian. The sedimentary evolution of the basin started with fresh-water deposition. The first phase was characterized by deposition in braided alluvial fans strongly influenced by syn-sedimentary tectonics. During the second sedimentation phase, a deep and hydrologically open fresh-water lake formed due to tectonic subsidence. A sequence of fresh-water sediments was studied on the north-western part of the Mt. Požeška. The thickness of the deposits is 77 m. The succession is characterized by a fining-upward tendency, i.e. a general transition from unfossiliferous coarse-grained to fine-grained sediments with fresh-water fauna. The sediments are classified into five facies: 1. Cross-bedded chaotic conglomerates. The sediments of this facies occur in the lower part of the succession. The thickness of cross-beds varies from 15 to 400 cm in amalgamated units. Cross-beds are steep (ca. 300) and planar. The lower and upper bedding planes are irregular to planar. The conglomerates are clast- to matrix-supported. The clasts are mostly angular and of pebble to cobble size, while fragments up to 50 cm in diameter are rare. The sediments are very poorly sorted and their internal organization is chaotic. The matrix is composed of coarse-grained sandstone to siltstone. The matrix content is very variable, and increases towards the upper parts of some beds showing normal grading. Cross-beds indicate progradation to the NW. Chaotic internal structure, matrix support, very poor sorting and irregular to planar lower bedding planes indicate deposition from debris flows. The occurrence of normal grading in some beds indicates turbidity currents. The angular clasts and fragments up to 50 cm in diameter imply short transport and deposition very close to the source area, whereas steep cross-bedding may represent foresets formed by coarse-grained delta-front progradation on the bottom of a steep slope. 2. Horizontally bedded matrix-supported conglomerates. These sediments occur in the middle and upper part of the succession. The beds are 10-220 cm thick and the lower bedding planes are flat. The rounded pebbles and cobbles are scattered in the silty matrix. The sorting is poor. The flat lower bedding planes, silty matrix support, scattered pebbles and poor sorting indicate sedimentation from debris flows. This facies probably represents deposition in the proximal prodelta environment. 3. Horizontally bedded clast-supported conglomerates. These conglomerates are found in the middle and upper part of the succession. Their bed thickness varies from 5 to 140 cm. The lower bedding planes are erosive. The pebble to cobble sized fragments, up to 12 cm in diameter, are subangular to rounded. Some beds show inverse grading in their lower parts, while normal grading occurs in the upper parts of most beds. The matrix is sandstone, and the sorting is poor. The erosive lower boundaries, grain size, clast support and normal grading indicate deposition from gravelly high-density turbidity currents. The inverse grading indicates grain interaction in a highly concentrated flow. The deposition was probably in the prodelta zone. 4. Horizontally bedded sandstones. The sandstones occur in the middle and upper part of the measured column. The sandstone beds are mostly few mm to 15 cm thick. The bed thickness decreases upwards. The lower bedding planes are slightly erosive or flat. The sandstones are coarse-grained, sometimes pebbly, to fine-grained. The thicker beds show normal grading. The erosive lower bedding planes and normal grading may be a consequence of deposition from sandy high-density turbidity currents. The thicker beds suggest sedimentation in the proximal prodelta, whereas thin beds indicate more distal prodelta. 5. Horizontally bedded siltstones. The siltstones are found in the middle and upper part of the succession, where they prevail. Their units are 10-500 cm thick. They are almost massive, but horizontal lamination occurs in the upper units. The sediments are rich in terrestrial flora content in some units. Tropical spores are dominant: Echinatisporites sp.div., Polypodiaceoisporites sp.div., Osmundacidites nanus gracilis, and Cicatricosisporites mecsekensis. Subtropical spores Leiotriletes sp.div. and temperate pollen Caryapollenites simplex are also abundant. Freshwater planktonic organism Heliospermopsis hungaricus are found in some samples. The ostracods Potamocypris, Heterocypris and Cypridopsis occur in the siltstones. The siltstones were deposited from suspension in the deeper part of the lake. The association with coarse-grained sediments indicates a prodelta environment. In conclusion, the studied sediments were deposited in a lacustrine coarse-grained delta-front and prodelta environment showing general deepening of the environment. The cross-beds, angular clasts and fragments up to 50 cm in diameter indicate short transport and deposition on the bottom of steep slope very close to the source area, i.e. probably in the vicinity of a fault scarp at the active basin margin. Based on the sporomorphs, deposition occurred in a warm subtropical climate.

coarse-grained delta; Early Miocene; Mt. Požeška; Pannonian Basin System

U radu se opisuje razvoj donjomiocenske krupnozrnate delte na Požeškoj gori.