A depositional model describes how and when certain geological layers were formed in geological time and their spacial and temporal distribution in the subsurface.
Deposits are sediments, which were accumulated and formed by natural physical and chemical processes in various depositional environments, such as for instance: Rivers, lakes, swamps, deltas, the sea, bioherms, desserts, glaciers etc. Depositional models may also be described for deposits such as salt, gypsum, phosphate, carbonates, limestones etc., which are primarily formed by chemical precipitation processes with or without biological interaction. Deposits may also consist primarily of organic remains such as peat, brown coal, tar-oil etc.
By using the principle of actualism and knowledge about present day natural depositional environments, the geological depositional environments where geological sediments accumulated can be interpreted, and the geometry and composition of the geological layers be described. This is done by sedimentologist, stratigraphers and palaeontologists by investigating the geological layers and study the numerous physical structures, that the depositional processes have left in the sedimentary rocks, such as bedding, lamination, cross-bedding, wave ripples, mud cracks, root structures, fossils etc. Biological activity from e.g. bivalves, snails, worms etc. also leaves traces and marks in the rocks (so-called trace fossils related to movements, eating and dwelling activities). The recognition and interpretation of such traces (ichnology – the science of biological traces, bioturbation) contributes together with the content of fossils (e.g. spores, pollen, foraminifers, ammonites etc.), to the interpretation and definition of a depositional model.
Reconstruction of depositional models
Based on the vertical and horizontal relationship between the various depositional environments/models it is possible to map and describe the geological development of the deposits in a given area. As an example, rivers transport erosion material from mountains to the sea as sand, silt and clay, where the sand is deposited at the coast in deltas, while silt and clay are deposited in deeper water, where conditions are calm.
A simple example of a depositional model could for instance be marine clay overlain by marine costal sand, which is succeeded by fluvial sand. The vertical succession of these layers in a well or in a rock face can be understood and described as the vertical succession formed by a delta building out in the sea. Based on several well-sections or rock exposures, a series of maps may be constructed showing where the river, coast and delta were placed at different geological times; such maps are termed paleo-geographic maps.
Depositional models can be constructed by using different types of data, e.g. data collected from natural profiles in mountainous areas, coastal cliffs, open mining pits, boreholes or seismic surveys, which displays the subsurface layers.
The purpose of constructing geological depositional models and maps displaying their distribution is, among others, to show and predict the occurrence of sandstones in the subsurface, which may contain valuable raw materials such as building materials, drinking water, warm geothermal water, oil, gas or other geological resources.