Development of a large fluvio-deltaic system during the Late Miocene evolution of the East Carpathian foreland basin

Abstract

During the Middle Sarmatian (Tortonian in the international timescale), large parts of the East Carpathian foreland basin were invaded by the Paratethys reaching its local maximum. The coastline subsequently retreated to the south, and deposits of a large fluvio-deltaic system accumulated in the foreland basin. We have studied these deposits, which belong to the so-called Balta Formation (BF – hereafter) in order to depict the palaeogeographic evolution of the fore- Carpathian area in the Late Miocene and earliest Pliocene. The BF is one of the many poorly understood deposits of the Miocene epicontinental basins of the East European Platform (EEP – hereafter). We have reviewed Ukrainian, Moldavian, Romanian and former Soviet literature on the stratigraphy and lithology of the BF, and have conducted our own lithofacial, lithostratigraphic and geomorphological analyses, using more than 200 outcrops and boreholes.

As a result of our study, the boundaries of the BF were revised and extended (Go to Search and Discovery to view Fig. 1). The BF occupies the uppermost parts of the modern uplands of the south-western borderland of EEP and fringes the Carpathian Orogen. Towards the east and west, the BF goes over into other sandy deposits of a coastal environment; in southward direction and downwards it goes over into thick offshore muds. The BF dips in S-SE direction with a thickness varying generally from 50 to 250 m. Its age interval is Middle Sarmathian - Meotian (11.5-5.8 Ma BP) with the age of the uppermost deposits becoming younger in S-SE direction.

Characteristic lithofacies and lithofacies successions of the BF were identified, described and interpreted. They suggest a fluvio-deltaic nature for the BF and make it possible to differentiate deltaic and alluvial deposits. The deltaic interpretation was primarily supported by scattered distribution of marine and fresh-water fauna in BF, including terrestrial vertebrates. The BF is characterized by a highly fragmented internal structure where sand bodies of different scales (10s m – 10s km) are incised in one another and frequently separated by muds. The majority of these bodies were interpreted to be deltaic (from minor distributary channels to whole deltas). We identified strong domination of fluvial processes in deltaic environment, associated with reduced basin salinity and corresponding homopycnal flow at the river mouths. Using related delta facies models we can assume that there were several rivers with relatively high discharge entering a shallow, low energy basin, forming numerous small terminal-distributary-channels characterized by frequent bifurcations and avulsions. The identified alluvial deposits are suggestive of flat-land meandering rivers.

The large fluvio-deltaic system was supplied by a vast drainage system, with a general S-SE sediment transport direction. This drainage basin was probably composed of three major provenances: Ukrainian Shield (to the north-east), territories that was recently abandoned by the Paratethys (to the north-west) and the Carpathian Orogen (to the west). The development of the BF was influenced by a multistage basin subsidence (with probable small inversions) and progradation of deltas with the successive outpace of progradation. The end of BF development was marked by the Pontian transgression and followed by isolation of the Black Sea basin and rearrangement of the former drainage system. It is assumed that BF beginning/ending reflected two powerful tectonic impulses came from the collision zone of the Carpathian Orogen with EEP and other active Alpine tectonic structures.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015