An ancient analogue of carbonate beach complex: the Upper Triassic limestones of Tran Formation, southwestern Bulgaria

The study of two stratigraphic sections of the Tran Formation (upper Carnian–Norian) reveals the presence of a regressive carbonate beach succession. The interpretation of lower shoreface, upper shoreface, foreshore, and backshore deposits is based on field data, microfacies analysis and comparison with Holocene and ancient analogues. The various lower shoreface deposits include automicritic boundstones/cementstones which attest to a temporary establishment of the M factory most likely as an aftermath of the global Carnian Pluvial Episode. The upper shoreface grainstones show trough cross-bedding reflecting deposition by longshore currents. Grainstones/packstones of the foreshore subenvironment have several diagnostic features including low-angle plane lamination, keystone voids, eroded beachrock clasts, and well-sorted textures with marine phreatic and vadose cements. Their deposition was controlled by supply of granular material from the shoreface, incessant wave activity and daily tides, and early lithification protecting them from erosion and remobilization. The backshore strata comprise storm washover deposits (packstones, wackestones), mudstones deposited in marginal ponds, and agglutinated stromatolites (formed by trapping and binding in a tide-influenced setting). Some of the foreshore and backshore sediments were pedogenically modified, resulting in the formation of secondary matrix, root development, desiccation cracking, and grainification. Specific products of the early vadose alteration are laminoid fenestrae with crystal silt and diagenetic grainstones, packstones, and wackestones. The good preservation of the regressive beach succession was favored by late highstand progradation of the shoreline in a low accommodation setting.