Gravel bars in braided-river channels of the basal buntsandstein-facies conglomerates in the cuenca province (Southeastern Iberian ranges, Central Spain)

Abstract

The Buntsandstein facies of the Cuenca Province in the Iberian Ranges in Central Spain consists of three parts: a lower conglomerate unit of braided river origin, a middle siltstone and mudstone unit of floodplain to ephemeral lake formation with associated calcrete pedogenesis, and an upper unit of conglomerates to sandstones of braided stream to meandering river origin. The boundary to the Muschelkalk facies is an omission surface at the top of the Buntsandstein facies representing a long period of subaerial exposure and lack of sedimentation. The Buntsandstein facies lies unconformably on the Palaeozoic basement, and its lower unit is divided into four levels: Basal Breccias, Lower Conglomerates, Upper Conglomerates and Upper Sandstones. The Basal Breccias represent scree-type gravity slope deposits and high-viscosity mass-flow sediments which are closely controlled by the palaeorelief. The upper portions are occasionally modified by running water in form of sheet floods or incipient channelized flows, with current action, however, being often only slight to even negligible as revealed by the absence of imbrication. The conglomerates are low-viscosity stream deposits. The Lower Conglomerates originate as medial or longitudinal bars in broad shallow channels of braided river systems with marked high and low discharge stages. The complex grain-size distribution is governed by various processes: considerable discharge fluctuations, segregation of sand and gravel at the bar top during transport and rapid loss of stream power by infiltration of water into the open framework gravel during waning floods. Sandy matrix was infilled into the gravel fabric at a later stage. Vertical accretion of longitudinal gravel bars ceased during low-water stages when the top of the bar emerged. Some lateral accretion took place when the bars were fully developed. During intermediate flows, small transverse bars migrate in the channels. The Upper Conglomerates originate mainly as medial and transverse bars in similar braided river systems than the lower rudite series, but with better defined, deeper and narrower channels. The clasts often display a chaotic arrangement inside the foresets due to rapid deposition by grain-flow during avalanching. Fining-upwards sequences represent low to moderate relief transverse bars migrating along the deepest parts of the channels. Coarsening-upwards sequences are interpreted as prograding medial or slightly asymmetric bars with well-developed coarse bar heads and finer-grained bar tails and a morphology comprising platform and supraplatform. Emergence of the medial bars during low-water stages divides the active channels where the smaller transverse bars migrate. Associated minor sandstones represent waning-flow drapes of bar tops, tails or flanks, slough channel deposits or chute channel sediments. Rootlet horizons in some abandoned channel infillings record occasionally longer interruptions of sedimentation with temporally and spatially restricted accretion of some left watercourses. The Upper Sandstones mainly originate as transverse bars in again shallower braided channels in the distal part of the system. The upwards decrease of bed thickness and foreset angle as well as the increase of sandstone levels give evidence of a progressive deceleration of energy in the channels which was largely caused by loss of water by evaporation and infiltration in the proximal parts of the alluvial fans. The general evolution of the depositional system comprises the passage from screes and local pediments with debris flows via coalescent alluvial fans to an incipient alluvial plain. Transport direction also changes from dominant transverse to the axis of the basin (alluvial fans) to longitudinal (alluvial plain).