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 Setie Belts
consist of an internal metamorphosed part and an external sedimentar y 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 auter 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 sheet s form
complexes up to 15 m and more thiekness due to amalgamation by multilateral coalescence
and multivertical stacking of individual genetical units. In the lower part of
the series, the number of ehannels is rather high suggesting a non-hierarchical channel
pattern where high- and low-sinuosity rivers coexisto The middle portion refleets
a smaller number of very large channels thus indicating an amelioration of
the hierarchical pattern. The upper part consists of variegated sabkha pIain mudstones
and evaporites where channel deposits are almost absent. The floodplain facies
is divided into proximal and distal parto The proximal floodplain facies comprises
interbedded sandstones and mudstones containing sorne layers of nodular pedogenie 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 sequen ces 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 overhank plain between the large maín
streams. The evaporitic complex at the top of the Triassic originates in en arid coastal
intertidal belt and supratidal sahkha 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.