LATE Westphalian and early Stephanian sediments of the Dobrudzha Coalfield, NE Bulgaria

Abstract

The Bolsovian to Cantabrian sediments in the Dobrudzha Coal Field are about 800 m thick. They were deposited in a depression bounded to the east, south and west by active synsedimentary faults. Most of the succession is coal-bearing clay- and siltstones, including occasional lacustrine deposits with fresh water faunas (molluscs and ostracods). Basin-wide sandstone intervals are associated with two periods of erosion and non-sequence. There are also localized sandstone bodies with vitrinized plant remains associated with the basin-margin faults. Conglomerates are only thin and rarely developed, and pebbles are only rarely found in the sandstones. There are only three basin-wide coals (m₅ at the middle Makdonka Formation, n₁ at the base of the Krupen formation and p₃ in the middle Gurkovo Formation) representing times of diminished influx of clastic sediment, and thus of reduced movement along the basin-margin faults. Based on the depth of the erosive down-cutting, it is estimated that the elevation of the basin was at least 180 m above sea-level. The area surrounding the basin is supposed to be an alluvial plain covered with vegetation and swamps. The dense vegetation cover suggests periods of high precipitation, whereas dry conditions with active weathering are most probable during deposition of the Velkovo Formation. Volcanic activity occurred near to the depression, perhaps related to the boundary faults. In the stratigraphically highest part of the succession (the Kabalak Member), volcaniclastics are the main component of the detritic sediments. The basin eventually inverted to become a horst around which Permian sediments were deposited.