Fernando García-García

Sedimentology of geomorphologically controlled Quaternary tufas in a valley in southern Spain

Four vertical facies sequences characterizing different environments have been identified in relation to two phases of tufa deposition (probably Pleistocene and Holocene) within a valley located in the central sector of the Betic Cordillera (southern Spain): (1) alluvial-fluvial (non-tufa) deposits, (2) fluvial tufa, (3) fluvio-lacustrine facies, and (4) spring with waterfall tufa. The first three created a stepped axial depositional system and the spring with waterfall tufa formed a transverse system. During the first phase of tufa construction, the vertical evolution in the axial system from alluvial-fluvial (non-tufa) to fluvio-lacustrine tufa deposits is interpreted as the consequence of geomorphological control. The evolution through time from a high-gradient and stepped fluvial system to a low-gradient fluvio-lacustrine system occurred upstream from where the progradation of the transverse, perched spring tufa system became narrower and finally dammed the valley. The tufa barrage caused an increase in the vertical accumulation within the axial system and a consequent upstream decrease of accommodation space and slope of the longitudinal profile. After a period of fluvial incision, tufa formed again during the Holocene in a high-gradient and stepped fluvial system.

Thick brachiopod shell concentrations from prodelta and siliciclastic ramp in a Tortonian Atlantic–Mediterranean strait (Miocene, Guadix Basin, southern Spain)

Carbonate production by brachiopods in shallow-water habitats is generally expected to be not sufficiently high and temporally persistent to allow them to form very thick and densely packed shell concentrations. The formation of thick brachiopod concentrations requires long-term persistence of populations with high density of individuals, and such circumstances are assumed to be rare especially during the Cenozoic. However, here we show that the large-sized brachiopod Terebratulaterebratula, the most common species in benthic assemblages with epifaunal bivalves and irregular echinoids, formed several decameter- to meter-thick, densely packed concentrations in shallow siliciclastic, high-energy environments, in a seaway connecting the Atlantic Ocean with the Mediterranean Sea during the Latest Tortonian (Late Miocene, Guadix Basin, southern Spain). This brachiopod formed (1) meter-scale, thick, parautochthonous concentrations in a prodelta setting and (2) thin, mainly allochthonous, tide- and storm-reworked concentrations in megaripples and dunes. The abundance of brachiopods at the spatial scale of the Guadix Basin seems to be mainly related to intermediate levels of sedimentation rate and current velocity because abundance and thickness of shell concentrations decline both (1) in onshore direction towards delta foresets with high sedimentation rate generated by debris flows and (2) in offshore direction with increasing levels of tide- and storm-induced substrate instability. Although brachiopods in dune and megaripple deposits are more fragmented, disarticulated, and sorted, and have a higher pedicle/brachial valve ratio than in prodelta deposits, taphonomic damage is still relatively high in prodelta deposits. Terebratula terebratula thus formed thick concentrations in spite of that disintegration processes were relatively intense along the whole depositional gradient. Therefore, population dynamic of this species was probably characterized by production maxima that were comparable to some Cenozoic molluscs in terms of their productivity potential to form thick shell concentrations in shallow subtidal environments. We suggest that temporal changes in brachiopod carbonate production have a significant spatial and phylogenetic component because multiple large-sized species of the family Terebratulidae, which underwent radiation during the Cenozoic, attained high abundances and formed shell concentrations in temperate regions.

Reworked marine sandstone concretions: a record of high-frequency shallow burial to exhumation cycles

Concretions, with abundant calcite-dolomite cement-replacement textures originally hosted in shallow-marine sandstones, were reworked into Lower Cretaceous fluvio-deltaic conglomerates and shoreface sandstones (External Zones, Betic Cordillera). A cycle of host sand deposition, early diagenetic concretion formation and concretion reworking is documented: (1) Well-sorted shoreface sandstone deposited. (2) Spherical to ovoid, non-ferroan calcite-cemented concretions formed below flooding surfaces at shallow-burial depths during early eodiagenesis. Non-ferroan calcite cements were precipitated from the bicarbonate derived from seawater and from dissolution of marine bioclasts, as shown by isotope analyses. (3) Concretions were reworked and exposed on the seafloor in a high-energy setting as indicated by the presence of numerous bivalve borings (Entobia ichnofacies), laminated micritic microbial crusts around the concretions, and epilithobionts (oysters, barnacles and corals) on the concretion surface. Concretions also appear as erosional remnants on the floor of channels which were incised into the shoreline sandstone when sea-level fell. (4) The fluvio–deltaic channels were filled with sediment during flooding in the late lowstand of sea-level. (5) The concretions are partly dolomitized, and the presence of siderite, pyrite and barite in the outer part of the concretions precipitated before the dolomite, suggests that the latter formed during shallow burial.

From the Geomorphic Process to Basin Architecture: Anatomy of the Infill of an Alluvial-Lacustrine System in Southern Spain

The heavy rains in the winter of 2009-2010 in Spain activated all the sediment-feeder systems of a small reservoir built in the 1970s north of the Sierra de Almijara (province of Granada). The intensity of this activity has aided in recognizing a series of geomorphic features allowing the re-interpretation of previous data on the subsoil obtained from Ground Penetrating Radar (GPR) and sedimentological analysis involving the study of a series of shallow trenches dug in various parts of the basin. The unequal influence of three feeder systems in the process of silting up this small artificial lake has been noted. The main contributor is the longitudinal axial drainage system, which is building up a huge delta whose progradation and aggradation dynamics are strongly influenced by the obstruction of a transverse delta fan on a highly erodible source area comprising Tertiary detrital sediments. Far less important in the construction of the stratigraphic architecture are the transverse fans lying against the Palaeozoic to Triassic metamorphic basement. The most notable geomorphological change over the last 35 years has been in the longitudinal axial system. Over a period of only eight years (1977-1984), it morphed from a coarse-grained braided fluvial system with a single channel in which longitudinal bars and thalwegs constantly changed position with every flood to a multichannel anastomosed system characterized by interwoven narrow, sinuous channels. Many of these channels are only covered during floods, and they are separated by heavily vegetated islands that act as traps for the overbank deposits. The difference in soil uses in the different parts of the basin also plays an important role, causing the progradation rate of the deltas to range widely, from 0 all the way to 100 m yr. This study reveals that in countries of the Mediterranean region torrential rains, the scanty vegetation cover, and the alteration of the longitudinal profile of dammed rivers interact causing extreme siltation of reservoirs. Under these situations it is extremely important to study the dynamics of mass movement in the parts of the basin with different lithology to carry out effective plans of management of the reservoir and to minimize the possibility of environmental and economic negative impacts.