The Mesta Basin in southwest Bulgaria is a graben that contains a Paleogene-age siliciclastic and volcaniclastic succession deposited in alluvial and fluvial settings. A sedimentological analysis has shed light on conglomerate provenance, and the links between deposition and tectonic setting. Petrographical and chemical analysis of conglomerate clasts and matrix from the Dobrinishka, Gradinishka, Osikovo (or Osenovo) and Zlataritsa formations reveal both local, and more distal source provenance ages. The basal conglomerates are subdivided into three types, a lower and upper polymictic and a middle granitic conglomerate type. Petrographical and chemical analysis reveals granite, gneiss and amphibolite clasts that were sourced from the Sidironero–Mesta Unit of the Middle Allochthon of the Rhodope Metamorphic Complex, and the Rila–Rhodope Batholith to the east of the basin. Cathodoluminescence analysis of quartz sand grains reveals an increased input of red-and violet-luminescent volcanic grains. Volcanic quartz in the oldest conglomerates indicates a hitherto unknown early (pre-Oligocene) phase of volcanic activity in the vicinity of the Mesta Basin. The conglomerates were deposited in association with movement on the Ribnovo low-angle normal fault during the Late Eocene, creating subsidence and the development of considerable accommodation space. The establishment of a fluvial environment followed subsequent Oligocene-age volcanic activity.
- Mesta Basin