Kovadúsulásos és alumíniumdúsulásos mállási és üledékképződési folyamatok a kora-albai folyamán a Csehbányai-medencében

Abstract

The tectonically controlled uplift affecting various sectors of the Csehbanya Basin in early Aptian times seems to have been very inhomogeneous. As a result, individual, tectonically controlled blocks show very different denudation histories reflected by the basal layers of the Albian cover sequence (Tes Clay-Marl Formation) postdating the uplift-cumerosion event. At places where the Tes Fm is underlain by Jurasssic to Aptian formations these basal layers developed in the form of the clastic.Kepekő Member rich in chert fragments. At other places where the immediate bedrock of the Tes Fm is Late Triassic limestone or limestone with intercalated dolomitic layers, at the unconformity surface there are often bauxites or Al-rich clays belonging to the Alsopere Bauxite Formation. This suggests that the pre-Tes Fm. relief must have been highly variegated. Therefore both the depositional and the early diagenetic environments of the Alsopere Bauxite Fm and the clastic, cherty Kepekő Member could have been quite different. The differences may reflect different climates and/or different paleoenvironmental conditions. On the other hand since they both belong to one and the same major regional unconformity they have also some characters in common. These are: (1) the mostly local source material,(2) the predominantly oxidized nature of the sediment, (3) the abundance of “slack-water” microfacies, (4) sphericity, roundness and sorting of the coarse detrital material. However, unlike the bauxite, the material of the Kepekő Member is obviously of local origin. It is essentially an in situ dissolution residue without any signs of transport-related concentration/separation of even the chert fragments. Limestone-fragments in the bauxite on the other hand must have been added to the fine-grained sediment in the course of some transport/redeposition, to the distance of which we have no clue. The original source of the prebauxitic material is not known, either. Had it been of local origin it could not have been of the same age as the cherty Kepekő Member since the climatic and/or hydrologic conditions favourable for bauxitisation are substantially different from those favouring the accumulation of the silica-rich sediments, particularly because the latter show even some degree of silica mobilization and reprecipitation.