Abstract
The Tortonian deposits in northwestern Bulgaria show a great variety of structures which are divided into three main classes. The first one comprises structures formed during the deposition of gypsum and includes laminae and layers resulting from chemical precipitation and/or mechanical replacement of particles. The second group, comprising structures produced after deposition but prior to complete consolidation of the sediments, consists of load cast, of ridges showing a roughly polygonal pattern and of syngenetic breccia. Structures which formed after the deposition and consolidation of gypsum sediments are in the third group. Genetic relationships between various structures are discussed. The areal distribution of structures follows a pattern which reflects primary paleogeographical conditions. For instance, laminated and stratified gypsum sediments are found in the areas situated near the margins of the sedimentary basin, while unstratified and massive gypsum deposits mostly are confined to regions further from the marginal zone. The observed deviations are related to local changes in sedimentation conditions.
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