Pre-, syn- and postdepositional clay coatings on grains of clastic deposits

Clay coats on skeleton grains in clastic deposits may be of allogenic origin (detritical coats) or authigenic origin (diagenetic coats). Three types of allogenic coats are distinguished: predepositional - inherited grain rims, syndepositional - rotational plasters on skeleton grains, and postdepositional - coats resulting from mechanical clay infiltration and those formed during pedological processes. The aim of this paper is a review of literature on allogenic coats on skeleton grains, with emphasis on identification of features which may be helpful in distinguishing coats of different origin. Analyses of thin sections carried out by means of polarizing and scanning electron microscopes are suitable techniques to study microstructure of the coats. Basing on them it is possible to find distinct differences between allogenic and autigenic coats. The allogenic coats are often variable in thickness and sometimes laminated. They are usually polymineral and contain organic matter and iron compounds. The amount of coats within a layer may change significally. Some coats form geopetal structures. Distinguishing between detritical clay coats and authigenic clay coats becomes difficult in the case of rocks subjected to advanced burial diagenesis and resulting recrystalization of clay minerals. The correct interpretation of origin of the coatings on the basis of microstructural analysis is essential to further sedimentological interpratations, as it provides more data on conditions of sediment transport and deposition as well as early and late diagenetic processes. Results of microstructural analyses of glacial deposits performed by different researchers in the last years led to discovery of new mechanisms responsible for formation of detritical coats. Thus the questions of the origin of some coatings still remain open

Sedimentary record of 2010 and 2011 Warta River seasonal floods in the region of Poznań, Poland

The Warta River near Poznań (West Poland) represents a meandering lowland river modified by hydro-engineering projects. Recently, two large floods occurred during the summer of 2010 and the winter of 2011. Rapid response surveys were conducted to document erosion and sedimentary records of the floods (spatial extent, grain size, composition, grain microtextures). Follow-up studies, which were conducted during the two years after the floods, assessed postdepositional changes in the flood deposits. A thick sand layer formed that ranged from an average of 10–15 cm (summer) to 30–35 cm (winter), building natural levees, side bars and crevasse splays. The sand layers consisted of fine- and medium-grained sands that were well-sorted, fine skewed and sourced from the river channel. The longer but smaller winter flood produced thicker and coarser deposits at different sites compared with the summer flood. An organic-rich mud layer and algal mats, which were short-existing, were also detected on the floodplain. The study proved that the flood record on engineered rivers may be only in the sand bodies near the river channel and their preservation is mainly controlled by their thickness. A single site may not record all floods, as particular events tend to leave deposits in various places