Lava Formation

The Lava Formation is a Mesozoic geologic formation in Lithuania and Kaliningrad, being either the sister or the same unit as the Ciechocinek Formation. It represents the outcrop of Lower Toarcian layers in the Baltic Syncline and in the Lithuanian-Polish Syneclise (C8-borehole in Gdańsk Bay). It is known by the presence of Miospores and Pollen, as well Plant remains. The formation contains grey, greenish, and dark grey silt and clay with interealatians and lenses of fine-grained sand, pyritic concretions and plant remains (carbonised wood fragments). The Jotvingiai Group Toarcian deposits represent deposits laid down in fresh water and brackish basins, possibly lagoons or coastal plain lakes. The Bartoszyce IG 1 of the Ciechocinek Formation shows how at the initial phase of the Toarcian there was a regional transgression in the Baltic Syncline, indicated by greenish-grey mudstones, heteroliths and fine-grained sandstones with abundant plant fossils and plant roots, what indicates a local delta progradation between the Lava and Ciechocinek Fms. Then a great accumulation of miospores (+2500 specimens) indicates a local concentration, likely due to a rapidly decelerating fluvial flow in a delta-fringing lagoon forming a “hydrodynamic trap”, with the wave and currents stopping the miospores to spread to the basin. Latter a marsh system developed with numerous palaeosol levels, being overlayed by brackish-marine embayment deposits that return to lagoon-marsh facies with numerous plant roots (Radicites sp) and palaeosol levels in the uppermost section, ending the succession. Overall the facies show that the local Ciechocinek-Lava system was a sedimentary basin shallow and isolated, surrounded by a flat coastal/delta plain with marshes, delivering abundant spores and Phytoclasts, indicators of proximal landmasses with high availability of wood and other plant material. This climate at the time of deposition was strongly seasonal, probably with monsoonal periods. Due to the abundant presence of deltaic sediments on the upper part, it is considered to be related to the retry of the sea level. The Lava Formation was deposited on a mostly continental setting, with its upper part, dominated by argillaceous sediments, corresponding to the Ciechocinek Formation. There is a great amount of kaolinite content, being present laterally in the basin, decreasing and lifting space to increasing smectite to the south-west of the formation. On the other hand, there is a great amount of coarsest sediments, which consist mostly of sands.