Fluvial architecture of the buntsandstein-facies redbeds in the middle to upper triassic (Ladinian-Norian) of the southeastern edge of the Iberian Meseta (Southern Spain)

Abstract

In the southeastern edge of the Iberian Meseta in Southern Spain, fluvial continental Buntsandstein-facies red beds of Middle to Upper Triassic (Ladinian — Norian) age unconformably overlie the folded and eroded Hercynian basement. The Betic Belts consist of an internal metamorphosed part and an external sedimentary zone containing the Triassic deposits, with the latter seam being in turn divided into the Prebetic Zone with continental to shallow marine facies and the Subbetic Zone with pelagic facies. The Buntsandstein-facies red bed series is vertically split into three main facies associations: alluvial fans and pebbly braided rivers (conglomerates and sandstones), low-sinuosity proximal and distal sandy braided rivers (sandstones and mudstones) and coastal evaporitic sabkha (mudstones, marls and gypsum). Alluvial-fan and pebbly braided river sediments occur at the base of the sequence and cover the palaeorelief of the pre-Triassic morphology. The inner fan zone is characterized by debris-flows or mud-flows, the mid fan zone is dominated by sheet floods, and the outer fan zone is governed by stream flood and stream flow passing into pebbly braided rivers in front of the fans. With transition from restricted alluvial fans to an open braidplain, the pebbly rivers soon evolve both vertically and horizontally into sandy stream networks consisting of channels and floodplains. The channel facies comprises sheet-type and ribbon-type sandstone layers. The sheets form complexes up to 15 m and more thickness due to amalgamation by multilateral coalescence and multivertical stacking of individual genetical units. In the lower part of the series, the number of channels is rather high suggesting a non-hierarchical channel pattern where high- and low-sinuosity rivers coexist. The middle portion reflects a smaller number of very large channels thus indicating an amelioration of the hierarchical pattern. The upper part consists of variegated sabkha plain mudstones and evaporites where channel deposits are almost absent. The floodplain facies is divided into proximal and distal part. The proximal floodplain facies comprises interbedded sandstones and mudstones containing some layers of nodular pedogenic carbonates and originates by overbank sheet-flood, levee overtopping and crevasse-splay sedimentation. The distal floodplain facies is built up of mud with minor layers of silt and fine sand with intercalations of micritic lacustrine limestones and originates in overbank lakes and ponds. Depositional sequences within the sandy braided river series comprise major sequences that are produced by migration of alluvial subenvironments during course of their aggradation, and minor sequences that are related to alternating high- and low-water stages with changing channel abandonment and stream neoformation. The fluvial architecture is highlighted by two types of depositional settings: proximal and distal facies. The proximal facies is characterized by predominantly straight channels without or with only poorly-developed levees and being infilled with sediments under rather high-energy conditions. The distal facies is characterized by low-sinuosity channels surrounded by better developed levees and being infilled under lower energy conditions than the proximal equivalents. With passage from the proximal to the distal facies, the floodplain sediments consist of increasingly more backswamp, levee and crevasse-splay deposits as well as of sediments of small meandering channels operating in the overbank plain between the large main streams. The evaporitic complex at the top of the Triassic originates in an arid coastal intertidal belt and supratidal sabkha seam. Within the sandy braided river complexes, the different magnitudes and effectivities of currents in large channels, small watercourses and floodplain reaches are underlined by partially divergent, bimodal or even bipolar palaeocurrent directions. Copper mineralizations of mixed syngenetic and epigenetic type in the terrestrial red beds are associated with plant debris in sediments of inactive to abandoned secondary channels and comprise azurite, chrysocolla and malachite. The Triassic palaeogeographical setting represents a large bay between the European and African plates. Fluvio-lacustrine red beds of Buntsandstein facies surrounded the bay and graded laterally into coastal and shallow marine carbonates.