Abstract Basinal sediments in the stratigraphic record can be recognized using a biofacies model based on the distribution of shelly fauna and bioturbation in modern basins. Stagnation in modern basins is produced by density stratification which insulates bottom water from atmospheric oxygen, and by sills which prevent lateral exchange at depth with oxygenated ocean water. Such basins exhibit a tripartite layering in their water columns: (1) a mixed surface layer, well-oxygenated, approximately 50 m deep, (2) a stratified layer (pycnocline) in which oxygen decreases rapidly with depth, approximately 100 m in thickness, (3) a stagnant zone in which oxygen is absent, extending from the 150 m depth down to the basin floor. The major changes in faunal composition take place within the pycnocline, as dissolved oxygen drops below 2 ml/l. Marginally oxygenated waters (dysaerobic) contain communities with lower diversity, generally smaller body sizes, a greater dominance by infauna, and fewer calcified species than communities in well-oxygenated (aerobic) conditions. A complete lack of oxygen (anaerobic) eliminates metazoan animals. Basinal water conditions are reflected by corresponding biofacies: Aerobic—shelly fauna, infaunal bioturbation Dysaerobic—shelly fauna lacking, bioturbation by infauna persists Anaerobic—shelly fauna lacking, bioturbation also lacking; sediments laminated Recognition of these biofacies in the stratigraphic record should permit the reconstruction of basin contours and hydrography; paleoslope direction and dip angle and absolute water depths should be calculable from the biofacies distribution. Cyclic alternations of normal marine and euxinic facies can be explained as repeated lateral shifts of the basinal environments into a shallow shelf. An ancient basinal deposit, the Upper Devonian Middlesex Shale of New York, was sampled laterally in order to test the model; because the shale is thought to represent a time plane, facies changes should depict ‘'instantaneous” paleoenvironments in the basin in Late Devonian time. A west-to-east trend was established from laminated unfossiliferous shale through bioturbate non-fossiliferous mudstone to bioturbate and sparsely fossiliferous mudstone, indicating a transect from anaerobic through dysaerobic to nearly aerobic conditions. Basin depth therefore increased westward; the width of outcrop of dysaerobic facies is a function of the bathymetric gradient, calculated as 1:400 (0°09') for the Middlesex basin. The eastward limit of anaerobic facies marks the ancient pycnocline base at 150 m depth, and provides a bathymetric tie-point on the basin slope. West of this point the basin floor was continuously anoxic, while to the east dysaerobic and aerobic conditions alternated. Alternations are explained by transgression of the pycnocline up onto the eastern shelf, bringing basinal environments into areas which were normally oxygenated. Repeated shifts produced the alternating euxinic and fossiliferous sediments in the Upper Devonian marine sequence. Comparative examples of euxinic facies from carbonate basins show similar patterns. The Mississippian Rancheria Formation of New Mexico and the Permian Cutoff Formation of West Texas both overstep erosion surfaces cut into fossiliferous shelf carbonates, indicating a shift of basinal conditions shelfward. From published descriptions, both the Rancheria and Cutoff are here interpreted as representing dysaerobic conditions in waters 50-150 m deep. A further example, the shelf carbonates of the Pennsylvanian Paradox Basin, differs from the present model as the euxinic rocks are associated with an evaporite basin. The euxinic carbonates (probably dysaerobic facies) may have resulted from salinity rather than oxygen fluctuations, but the concept of deep basinal water periodically invading the shelf to produce cyclicity is similar to the Middlesex Shale example. Transgressions of the pycnocline may have accompanied actual sea level shifts or have occurred because of a change in mixing depth or sill depth. The latter situation would produce euxinic conditions across a shelf without increasing water depth there. Regressions of the pycnocline would return the shelf to aerobic conditions while the majority of the basin remained anaerobic.

Abstract An integrated group of conceptual models for evaporite deposition is presented for a shallow salina-type basin, supplied both with marine and non-marine water, and with a water level separated from and drawn down below world sea-level. Knowledge of, and terminology for, modern limnology is used in these models in order to build a new and more comprehensive link between the hydrography and hydrochemistry of brines and the depositional and stratigraphical record anticipated in such basins. In these modelled basins it is assumed that (as in saline lakes): (1) the evaporite deposition reflects the stratification-mixing cycles in the brine column; (2) the evaporative crystallization of salts culminates during the mixing periods; and (3) the evaporite facies are linked to the position of water zones separated by a horizontal pycnocline. The models are prepared especially for interpretation of subaqueous coarsely crystalline gypsum (selenite) deposition in perennial-to-ephemeral saline pans, and also for fine-grained gypsum deposition (clastic, microbialite and pedogenic) on specific, flat, semi-emerged shoals (majanna-type shoals), commonly flooded by wind-driven brine sheets, in between them. Coarse, crystalline selenites appear to form below the pycnocline and the crystallization of particular well-developed grass-like selenite beds is thought to be connected with the pycnocline highstands. Such beds are both marker beds and ideal datum surfaces. The models presented are used for sedimentological interpretation of the various gypsum facies of the Badenian (Middle Miocene) evaporite basin in the northern Carpathian Foredeep. The observed architecture of these facies suggests that the margin of the basin was occupied by a system of variable saline pans (dominated by selenite deposition) and evaporite shoals (dominated by gypsum microbialite deposition); some of these shoals were majanna-type. Many features of the studied facies can be clearly explained by the models presented, particularly by the rapid and variable fluctuations in water and pycnocline levels incomparable in time scale and size with any global sea-level changes. All these features together consistently suggest that the Badenian gypsum basin was a salina-type basin with a water level below global sea-level, although salts in the basin are/were basically marine in origin.

Abstract The Baltic Sea is a semi-enclosed intracontinental sea of approximately 418 500 km 2 , surrounded by Scandinavia and the lowlands of Central and Eastern Europe. Its catchment area is about four times larger than the area of the sea itself. The Baltic Sea has an average depth of around 50 m, and is connected to the Atlantic Ocean via the shallow and narrow Danish Straits. A characteristic feature of the Baltic Sea is thermohaline stratification, in particular the occurrence of a permanent halocline. The northern part of the Baltic Sea is situated within the Precambrian Baltic Shield, whereas the southern part lies on the East European Platform. A small SW part lies on the Palaeozoic West European Platform. During the Pleistocene period, Scandinavian ice sheets advanced on the area of the present Baltic Sea several times, resulting in damage to the bedrock and a deepening of the Baltic Basin. The present-day Baltic Sea had its beginnings in the retreat of the last Scandinavian ice sheet, which melted approximately 15.5–14.5 ka ago in the area of the southern part of today's Baltic Sea. The Quaternary cover of the seabed in the area of the whole Baltic Sea is formed from Pleistocene sediments of glacial, glaciofluvial and limnoglacial origin, as well as from sediments of Holocene marine accumulation. In the southern part of the Baltic, during the Holocene, the sea constantly encroached upon the land, destroying Pleistocene sediments. In the northern part, as a result of uplift of the land and regression of the sea, Pleistocene glacial and glaciofluvial deposits, as well as early Holocene Baltic sediments, are being eroded. In the Baltic Sea, sandy and gravelly sediments are present above the pycnocline, while muds have been deposited below the pycnocline.

Abstract The hydrography and brine flow patterns in the Middle Miocene (Badenian) evaporite basin of the northern Carpathian Foredeep (in Ukraine, Poland and the Czech Republic) are reconstructed based on studies of the peculiar, conformably oriented, bottom-grown gypsum crystals present in the selenite deposits along the basin margin. The crystal apices are turned in a similar horizontal direction that is interpreted as the product of consistent flow of the bottom brines during crystal formation. Similarly the regular millimetre-scale growth zoning in these crystals presumably reflects the annual stratification-mixing pattern in the brine column typical of monomictic basins. In the central, deeper parts of the basin deposition was dominated by Na-chloride, and the selenitic facies are lacking. These central areas are interpreted as being meromictic during the oriented selenite deposition. The permanent pycnocline separated a mixolimnion, at the surface, from an anoxic (euxinic) monimolimnion, at the bottom, where direct evaporative crystallization of gypsum was not possible. The mixolimnion, which extended far onto the shallow margin of the basin, showed only a seasonal (annual) pycnocline and monomictic hydrography. Oriented selenites grew just in this mixolimnetic marginal zone, under predominantly counterclockwise (cyclonic) flow.

ABSTRACT The Devonian Catskill Sea of the Appalachian region was stratified much of the time as recorded by the deposition in it of black shale and evaporites. Submarine topography which developed along the southeastern perimeter of the sea consisted of a basin margin and a gently sloping clinoform constructed by sediment progradation. Intersection of the surface of stratification (pycnocline) with the seafloor marked the basin margin-clinoform junction and it caused a separation of sedimentary processes. Shoreward of the intersection, on the basin margin, bottom flow driven by various current-producing phenomena, transported, deposited and reworked sediment. A complex mosaic of facies resulted, including a typical suite of lenticular sandstone, bioturbated shales and mudstones, most of which are abundantly fossiliferous. Basinward of the intersection, on the clinoform, density currents originating at or shoreward of the intersection moved down the clinoform onto the basin floor. Relatively simple facies accumulated, including turbidites and other, pelagic sediments, with very few fossils. At the intersection, internal waves moved along the pycnocline, shoaled, broke and reworked sediment, leaving a record of relatively thick sandstones, some with hummocky cross-bedding. This separation of process is likely to have been a hallmark of sedimentation in epicontinental seas where stratification can be inferred.