Sandy braidplain deposition with minor pedogenesis in the labyrinthodontidae beds (middle buntsandstein) of the northeastern holy cross mountains (Poland)

Abstract

The Labyrinthodontidae Beds in the Holy Cross Mountains (Poland) occur in the middle part of the Middle Buntsandstein in the northern marginal Triassic belt around the basement core. The predominantly sandy sequence is laid down in channels and flood-plains of a mainly moderately- to weakly-braided river system with generally moderately- to widely-spaced streams enclosing moderately wide to extensive overbank plains. The main watercourses are surrounded by a levee belt of proximal crevasse splays which laterally pass into shallow little channels and sheets of distal crevasse splays that intersect the floodplain. The diversification of bedding plane features in both main stream and crevasse-splay watercourse sands testifies to a wide spectrum of depositional, erosional, deformational and biogenic processes accompanying and punctuating the general sedimentary mechanisms of channel bar migration and sand sheet accumulation. Bröckelbank carbonate breccias within the substratum sequences document formation of mainly submature palaeosols in emerged parts of streams and overbank plains during occasionally longer periods of subaerial exposition. Secondary-erosional removal of overbank fines is the main mechanism to trigger the origin of multistorey channel deposits by superimposition of successive stream sands, whereas primary-sedimentary restriction of formation is the control of the original distribution of palaeosols prior to their complete secondary destruction. The inverse relationship between widespread accumulation of suspension fines and rare origin of palaeosols is the result of persistent inundation of the overbank area by extensive floods giving rise to long-term drowning of large parts of the topstratum flats. The evolution of fluvial style is restricted to the range of a narrow spectrum within the general facies model, with the two end members of a continuous sequence of patterns being moderately-braided streams with partially well-drained floodplains and weakly-braided rivers with poorly-drained overbank areas which correspond to the lower and upper part of a fluvial megacycle, respectively. Laterally and longitudinally, both types of channel network coexist forming occasionally a zoned braidplain, but in most cases, the second type of alluvial pattern is generally very subordinate in comparison to the dominant river type and is mainly present in some reaches of the stream belt. The evolutionary trend of decreasing braiding of the channel system and increasing width of floodplains leads to passage from substratum-dominated cyclothems in the lower part of the sequence to progressively increasing importance of overbank sedimentation towards the top of the succession, initiating the transition to the mainly silty-clayey topstratum deposition in the following members. The Labyrinthodontidae Beds in the Holy Cross Mountains are assigned to the medial zone II within the sedimentary model of the whole Mid-European Triassic Basin, representing an almost entirely alluvial plain with some pedogenesis in temporarily abandoned and emerged parts of the channel and floodplain fabric. The Labyrinthodontidae Beds have a key position in the overall evolution of fluvial depositional environment in the Buntsandstein of the Holy Cross Mountains by being the transition from the gravelly braidplain complex of the Czerwona Gora and Stryczowice Beds to the overbank system of the Hieroglyphic Beds and the Pseudooolithic Beds. The particular significance of the Bröckelbank carbonate breccias is the documentation of a change from arid to semi-arid climate during course of the sedimentological evolution which comprises a fluvial magnacycle owing to tectonic stability of the source area that is undergoing progressive denudation after initial uplift and creation of relief. The possibility of correlation with the evolution in the Eifel underlines the importance of the Labyrinthodontidae Beds for linking of the depositional history of several parts of the Mid-European Triassic Basin.