Middle Devonian (Givetian) supratidal sabkha anhydrites from the Moesian Platform (Northeastern Bulgaria)

Abstract

The Middle Devonian (Givetian) anhydrites from deep wells in the Moesian Platform (Northeastern Bulgaria) are commonly distinguished into nodular and contorted structures. The latter includes a great variety of non-bedded (nodular, nodular-mosaic, mosaic and mosaic to massive), bedded (bedded nodular, bedded nodular-mosaic and bedded-mosaic to massive) and distorted (distorted nodular-mosaic, distorted mosaic, distorted bedded-mosaic, and “ropy bedded”) types. Microscopically, the anhydrite types are characterized by felted, subfelted and microcrystalline (aphanitic) microfabrics. The associated carbonate lithofacies and the synsedimentary/early diagenetic anhydrite features suggest that these sediments were formed in an arid peritidal setting with well distinguished subtidal (ostracod mudstones and wackestones, paleosiphonoclad wackestones and packstones and ostracod-peloidal packstones and grainstones), intertidal (microbial mats) and supratidal (sabkha anhydrites) zones. The described nodular and contorted anhydrites are interpreted as originated by similar mechanisms as those operating in recent sabkha settings in the Arabian Gulf. Most probably, they are the result of replacement/displacement intrasediment sulphate growth within the vadose and upper phreatic zones of the supratidal sabkha. The various nodular and contorted types are regarded as formed from incipient nodular structure and represent different stages of anhydrite nodule enlargement and growth deformation due to additional sulphate accumulation in the supratidal zone.