Ryszard Gradziński

Vegetation-controlled modern anastomosing system of the upper Narew River (NE Poland) and its sediments

Abstract The anastomosing system of the upper Narew River consists of a network of interconnected channels. The channels are relatively deep (width/depth ratio 2–10), straight to sinuous, and they lack natural levees. They are characterised by a low water slope and very low stream power. The river is distinctly bedload-dominated and the transport of suspended clastic fines is minimal. Channel deposits consist almost exclusively of medium- to coarse-grained sand. Laterally extensive interchannel areas are flat and covered with peat-forming vegetation. These stable wetlands are flooded for many weeks during high water stages. Except for the channels, the valley fill consists of peat layer reaching 4 m in thickness. The rate of vertical aggradation of the peat deposit is estimated at 1–1.5 mm/year. The radiocarbon dating indicates that the peat layer is predominantly late Holocene in age. The impact of vegetation on the system is overwhelming. Vegetation produces an erosion-resistant peat layer, stabilizes channel banks and slows down the water flow. Vegetation also stimulates aggradation of bedload material on the channel bottom, and contributes to avulsion by blocking the channels. The channel network owes its origin to repeated though infrequent avulsion. Avulsion in the studied system is a small-scale, gradational and slow process. New channels evolve very slowly because of unfavourable hydrologic conditions and the presence of a resistant peat substratum. A new channel eventually intercepts only a part of the flow, while the old channel is still active, though to a limited extent. Although newly formed channels might subsequently be abandoned, long-lasting ones predominate within the system.

Dinosaur- and mammal-bearing aeolian and associated deposits of the Upper Cretaceous in the Gobi Desert (Mongolia)

Abstract The Barun Goyot Formation (? Campanian-Maastrichtian) consists of continental red-bed type sandstones. The formation was interpreted in terms of intertonguing and/or alternating dune deposits and sediments of intermittent lakes and streams. The mega cross-stratified sandstone units which show considerable lateral continuity and little variation of foresets dip were described as buried transverse dunes. They are rather poor in animal fossils. Most dinosaurs and mammals occur in water-deposited interdune sediments. This is due to ecological differences between bare dunes and the interdune depression which had a vegetation cover.

Trace fossils from interdune deposits—an example from the lower triassic aeolian Tumlin Sandstone, central Poland

Abstract The Tumlin Sandstone, up to 105 m thick, is the deposit of a midcontinent, extensive (at least 1500 km2) dune field. It consists of dune deposits and of subordinate intercalations of interdune deposits. The latter are represented by horizontally bedded sandstone accumulated in frequently changing conditions: on dry, damp, and wet surface. Dry-surface deposits predominate. Subordinate but relatively common are damp-surface deposits with characteristic adhesion structures, and wet-surface deposits represented by very thin mudstone layers and horizons of wave ripples, which originated in ephemeral ponds. The interdune deposits contain numerous trace fossils whose occurrence is almost entirely connected with wet-surface and damp-surface deposits. The dune deposits are devoid of trace fossils. The following forms have been recognized: ?Arenicolites isp., Cruziana problematica, Diplocraterion isp., Gordia marina, Planolites montanus, Planolites isp., Palaeophycus ?tubularis, tetrapod footprints, radial structures, shallow double furrows, and various oval depressions. In places, intensive but shallow bioturbation of sediment is observable. The trace fossil assemblage is similar to the Scoyenia ichnofacies but lacks Scoyenia and Ancorichnus. The assemblage is more comparable to the mixed Arenicolites-Cruziana ichnofacies sensu Bromley and Asgaard (1979). The invertebrate burrows show an r- selected strategy in sediment colonization. Around the Palaeozoic/Mesozoic transition the highly diversified ichnofauna compared to that of the Permian interdune deposits shows increased colonisation of interdunes by an invertebrate, mainly arthropod, fauna.

Sedimentary history of a Mississippian to Pennsylvanian coal-bearing succession: an example from the Upper Silesia Coal Basin, Poland

The coal-bearing succession of the Upper Silesia Coal Basin consists of deposits filling a flexural foredeep basin. Accumulation initially compensated for regional and differentiated subsidence, after which the general depositional surface remained nearly flat. The deposition of the coal-bearing succession started at the end of Mississippian times (Pendleian Subage) and continued with hiatuses through almost the whole of Pennsylvanian times, and stopped in the Westphalian D Subage. The up to 8500 m thick coal-bearing succession traditionally has been divided into four main units called ‘Series’, and all of them are subdivided into subsidiary units known as ‘Beds’. The occurrence of the intervals containing marine faunas within the lower ‘Paralic’ part of the coal-bearing succession resulted from eustatic ingressions. The higher ‘Limnic’ part of the succession was laid down in fluvial systems, while the lower part was formed mostly in a fluvial and, to a lesser extent, complex coastal system. Sedimentation of the coal-bearing succession was controlled by both autogenic and allogenic factors.

Deep blowout depressions in the aeolian Tumlin Sandstone (Lower Triassic) of the Holy Cross Mountains, central Poland

Abstract The Tumlin Sandstone, up to 105 m thick, is the deposit of an extensive dune field governed by unidirectional aeolian sand transport. The Tumlin Sandstone accumulated as a result of regular migration and climbing of slightly crescentic dunes. This process was occasionally and locally interrupted by the formation of blowout depressions. The largest blowout depressions were at least 200–300 m wide and reached at least 30 m beneath the generalized depositional surface of the dune field. They dissected several generations of older dune sediments, separated by flat first-order bounding surfaces. The slopes of some blowouts exceeded 30°. The blowouts were filled with blown sand shortly after their formation. The sediments filling the blowouts resemble large, wide trough sets of cross-strata, and the azimuths of dip of the constituent cross-strata are generally the same as in the whole formation.