Gravelly to sandy braidplain deposition in the czerwona góra beds and stryczowice beds (middle buntsandstein) of the northeastern holy cross mountains (poland)

Abstract

The Czerwona Góra Beds and Stryczowice Beds in the northeastern part of the Holy Cross Mountains in Poland represent the installation of an extensive open high-energy braidplain following the precursory buildout of a chain of restricted alluvial fans in the preceding Zwola Beds which initiate the Buntsandstein deposition. The conglomerates and sandstones originate in channels of braided rivers as gravelly longitudinal bars, sandy transverse bars and sand sheets on the floor of the watercourses and on moderately-elevated platforms within the streams. Topstratum deposition particularly including suspension settling is very restricted due to effective primary suppression of origin as a consequence of persistence of vigorous floods. Resulting from short periodicity and large extension of high-water surges, the conglomerates and sandstones which successively accumulate on the bottom of the streams coalesce by vertical stacking and lateral amalgamation to thick and extensive complexes of multistorey substratum deposits. Indications of minor aeolian influence are only present in the basal Zwola Beds, with some reworked ventifacts testifying to almost complete subsequent destruction of rarely formed aeolian features in very limited spots of the river complex. The petrographical composition and textural maturity of the gravel is largely independent from the depositional processes. The laterally changing composition of the pebbles and cobbles in the Czerwona Góra Beds with almost exclusive presence of siliceous clasts around Czerwona Góra and occurrence of considerable amounts of carbonate clasts around Stryczowice testifies to a diversified regional geology in the provenance area and to delivery of the detritus in several discrete flows which are directed to different areas and which are laterally overlapping. Preferential downcutting of the carbonate source rocks below the base level of erosion and absence of significant rejuvenation of the relief results in upwards diminishing and finally vanishing of the exclusively extraformational carbonate material, with merging of both separate lobes to one large flow no longer exhibiting compositional differences. The submature textural composition of the Zwola Beds underlines the first-cycle nature of the detritus which is laid down on an alluvial-fan complex immediately neighbouring the source area after only short distances of fluvial transport and suffering from inhibition of improvement of sorting and rounding due to incorporation into bed-load saturated slurry-type flows. In the fluvial braidplain of the Czerwona Góra Beds, the amelioration of textural maturity of both siliceous and carbonate gravel testifies to longer distances of cumulative transport in high-energy streams with particular effectivity of rounding and sorting processes during multiphase aggradation with partially repeated movement of the clasts prior to their ultimate accretion. The condensation of the successions to polygenetic complexes is mainly the result of effective primary-sedimentary inhibition of origin of overbank and waning-flow watercourse fines due to persistent high-energy overspilling. The evolution of fluvial style comprises only minor variations within the general frame of a highly- to moderately-braided river system with narrowly- to moderately-spaced channels and includes the passage from a conglomerate-dominated lower part into a sandstone-dominated upper part, with the textural changes in bed-load material taking place regardless of its petrographical composition. The depositional history of the braidplain is a continuous unidirectional sequence of successively decreasing braiding of the river system throughout the fluvial megacycle. Persistent inundation of the alluvial plain independent of the energy level of the floods and restricted phases of short-term emergence of smaller patches of channels and overbank flats keeps the environment unsuitable to even totally hostile for the origin of non-fluvial sediments, and the absence of longer periods of subaerial exposure even in the advanced stage results in complete inhibition of formation of aeolian sands, with the absence of gravel in the upper part preventing recognition of minor influences that are ineffective to leave products in the depositional record. The comparison with the Eifel and the evaluation of presence vs. absence of dune sands leads to recognition of two main types of coarse braidplains in arid climate, with high-energy floods of wide extension and short periodicity giving rise to very restricted temporal and spatial emergence thereby suppressing accumulation of wind-blown sand in the Holy Cross Mountains, contrasting with low-energy floods of short extension and high periodicity which result in abundant periods of prolonged emergence of broad topstratum flats and considerable parts of the channels favouring formation of aeolian sands in the Eifel. This diversification highlights the important role of the Holy Cross Mountains as a valuable counterpart to the Eifel for thoroughly assessing the range of the spectrum of fluvial styles and their influences on distribution of associated environments in coarse fluvial systems.