M. Hoyos

Vertebrate taphonomy in circum-lake environments: three cases in the Guadix-Baza Basin (Granada, Spain)

Abstract This work analyses the origin and taphonomy of three Plio-Pleistocene mammal assemblages from the Guadix-Baza basin (Granada, Spain): Huelago; Huescar-1; and Cullar de Baza-1. Similarities and differences in the taphonomic features are evaluated with regard to marginal lacustrine sedimentary processes involved in the accumulation of these mammal assemblages. Selective preservation of skeletal-part and taxa in relation to facies have been explored to determine their suitability for palaeoecological analysis. The mortality in Cullar de Baza-1 and Huelago-1 corresponds to a catastrophic pattern, but other taphonomic features of the bones indicate that accumulation processes were diachronic as occurred in Huescar-1. This could be explained by selective-predation and/or seasonal accumulation of bones. Hydraulic sorting and transportation greatly controlled the size and shape of accumulated skeletal elements, and the different preservational circumstances of each environment largely constrained the taxonomic composition of the fossil bone assemblages. Autochthonous fauna only occurs in Huescar-1 corresponding to subaquatic environments. The absence of carnivores among the identifiable remains from Huelago-1 has no plausible taphonomic or sedimentological explanation. Nevertheless, after taking into account the taphonomic factors, the differences in taxonomic composition between these three mammal assemblages reflect differences in the surrounding habitats. The great diversity of ruminant artiodactyls, in particular browsers, is indicative of wooded or bushy areas in the surroundings of Huelago-1 although not in the immediate area of the lake margin. This diversity coincides with that observed in other Villafranchian localities and represents the greatest diversity of artiodactyls known from the Neogene and Quaternary record of Western Europe. In turn, the high diversity of sub-aquatic forms and a smaller variety of artiodactyl species in Huescar-1, reflects an open, sparsely forested environment susceptible to seasonal drought. A similar sparse forested environment has been deduced for Cullar de Baza-1. The climate and environmental conditions in Huescar-1 and Cullar de Baza-1 are consistent with a climatic deterioration at the beginning of the Middle Pleistocene (‘Glacial Pleistocene’).

Extensional tectonics in the central Iberian Peninsula during the Variscan to Alpine transition

Abstract The passage from the Variscan cycle to the Early Alpine framework in the central part of the Iberian Peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes. 1. (1) A first Variscan compressional stage (VI, Middle Devonian to Early Carboniferous) dominated by compressional conditions leading to the building-up of the orogenic edifice. The stress regime was relevant to what might be called “Variscan-type” compression (E-W-oriented). This stage was characterized by major Himalayan-type tectonics with frontal nappes, thrusts, overturned folds, lateral transcurrent ramps, and localized anatectic magmatism. Minor synorogenic extension and plutonism was also recorded during this stage in the Tormes Granitic Dome. 2. (2) A second Variscan stage (V2, Early to Middle Carboniferous) was characterized by increasing extensional conditions leading to widespread plutonism (adamellites, granodiorites). Wanning compressional conditions were restricted to the eastern and southern realms of central Iberia (the eastern part of the Spanish Central System, and the Toledo Mountains). 3. (3) A third stage, here defined as Late Variscan (LV), developed from Middle Carboniferous to Early Permian, as a result of N-S late-orogenic extension. This episode is relevant to detachment tectonics and the gravitational collapse of the Variscan orogenic edifice under combined simple/ pure-shear conditions. Plutonism (granites and leucogranites) was still of major importance. Early Permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the Spanish Central System. 4. (4) A fourth stage, here defined as Early Alpine (EA, Early Permian to Triassic) marks the onset of the Alpine framework. This stage was characterized by what might be called an “Early Alpine-type” regional stress regime i.e. E-W extension and N-S compression, within a simple-shear model, and resulted in the configuration of the Iberian Peninsula into two contrasted realms: a western inherited Variscan block, and an eastern Alpine block subjected to post-orogenic extension. Elements developed during this event include N-S high-angle normal faults, NW-SE and NE-SW conjugate strike-slip faulting, and asymmetric rifting involving listric low-angle detachments.

Dedolomites associated with karstification. An example of early dedolomitization in lacustrine sequences from the Tertiary Madrid basin, central Spain

A variety of calcite fabrics formed by dedolomitization of middle to upper Miocene continental sediments of the Madrid basin, central Spain. The dedolomites are associated with other carbonates that show abundant diagenetic features. The diagenetic carbonates (Diagenetic Carbonate Zone, DCZ) overlies and grades laterally into lacustrine dolomite (Lower Dolomite Unit, LDU) and evaporites (Evaporitic Unit, EU), and in turn is capped by a paleokarst surface.The main dedolomite fabrics include sutured calcites and radial-fibrous calcites, the latter consisting of pseudospherulite mosaics and fibrous crusts. Other subordinate dedolomite fabrics consist of micro to mesocrystalline mosaics of rhombic, occasionally zoned calcites as well as reworked pseudospherulite crystals. These fabrics are interpreted to have been formed from extensive calcitization of dolostones and associated evaporite facies, the process being triggered and fostered by input of meteoric waters throughout a karstic system. The origin of the radial- fibrous calcites is discussed more specifically in view of their similarities to other calcite fabrics (e.g. Microcodium) often described in meteoric diagenetic environments.Geochemical evidence indicates that dedolomitization occurred in the shallow subsurface (<40m depth) and was achieved by oxidizing meteoric-derived ground waters. The diagenetic system behaved as an open system for nearly all trace elements analyzed; but in the shallower zone (vadose zone) the system is partially close respect to strontium. Vertical trends of the stable isotopes also support the proposed paleokarst model characterized by an irregular shallow water table and a narrow vadose zone.The results obtained from the geochemical simulation support that dedolomitization could develop by two combined hydrographical pattems: authigenic recharge through limestones and allogenic recharge. The dissolution/precipitation rates calculated for the main minerals involved in the process are consistent with petrographic- and geochemical evidence.

Slickenside and fault surface kinematic indicators on active normal faults of the Alpine Betic Cordilleras, Granada, southern Spain

Abstract Twenty four mesoscopic slickenside kinematic indicators are described here for the Pliocene-Quaternary active normal faults of the Alpine Betic Cordilleras, Granada, southern Spain. The indicators are classified into twelve groups depending on the down- or up-slope asymmetric orientation of features such as concavities, damaged versus sharp grain-borders or step-edges, V-shaped markings, fractures, and trailed material. Seventeen of the indicators were not previously described in the literature. The new kinematic indicators include two types of asymmetric grains, a carrot-shaped marking, six substructures of congruous and incongruous steps, trailed fault material, a drop-shaped figure, a V-shaped marking, three types of synthetic hybrid fractures with congruous steps, and two varieties of pluck holes. We think that some of these kinematic indicators provide information on the amount of displacement, aseismic versus seismic slip rates, cataclastic flow, frictional wear, or surface polishing.

Cenozoic intra-plate volcanism related to extensional tectonics at Calatrava, central Iberia

The Calatrava Volcanic Province is characterized by an intra-plate alkaline undersaturated magmatic association of melilitites, nephelinites, alkali olivine basalts and leucitites that were extruded in the central Iberian Peninsula (Spain) during Late Miocene–Quaternary time. This volcanism can be related to the following two-stage scenario: (1) an initial mantle-diapir stage, during which magma generation would have occurred at the lithosphere/asthenosphere boundary triggering minor accumulation processes at the crust/mantle interface, and consequent weakening/extension of the upper crust; and, (2) a final indentation stage related to disruption effects caused by the tectonic welding of the Prebetic Arc onto the Iberian foreland.

Nodular anhydrite growth controlled by pedogenic structures in evaporite lake formations

Several nodular gypsum beds exhibiting a remarkable vertical arrangement of individual nodules are present in the Miocene continental evaporite formations of the Calatayud Basin, northeastern Spain. The growth of the gypsum nodules, initially anhydrite, took place within magnesitic carbonate deposits which display incipient pedogenic features such as rhizoliths and clotted to peloidal textures. Simultaneous and after the pedogenic modification of the carbonate substratum, displacing nodules started to grow, their arrangement being closely conditioned by the development of vertical fissuring related to root penetration. The transformation of anhydrite to gypsum was probably realized early after little burial of the sulphate–carbonate deposits. The occurrence of vertically oriented gypsum nodules can be seen as a diagnostic feature for palustrine conditions developed in evaporitic lake settings, this peculiar morphology of the sulphate nodules having been strongly controlled by the internal structure of the palustrine palaeosols.

Morphologically ductile criteria for the sense of movement on slickensides from an extensional detachment fault in southern Spain

Abstract New criteria for the sense of movement on slickenside surfaces displaying evidence for mesoscopic ductility are described in the Neogene Jaloche extensional detachment fault in southern Spain. This detachment develops spectacular slickensides and fault gouges, and four of the criteria for the sense of movement observed along the highly polished slip surfaces are new to the literature. They are: (1) synthetic secondary fractures with hangingwall drag-effects in the form of roll-over microsynclines; (2) metre-scale oval-shaped asymmetric culmination and depression features; (3) microthrusts verging towards the direction of motion of the opposite block and dragging/overthrusting previous planar elements on the fault surface; and (4) flakes of fault-surface material trailed and plastered in the direction of the missing block. The geometry and microstructure of these features reveal that they deformed in a morphologically ductile manner by the coupled processes of cataclastic flow and shear localization, probably during aseismic sliding. During transient seismic pulses, deformation occurred along shiny/polished slickenside fault planes occasionally bearing jigsaw implosion breccias. Fluids under high pressure present initially may have promoted distributed cracking, whereas the process of dilatancy hardening may have served to facilitate cataclastic flow in the deformation history.

Meteoric calcitization of magnesite in Miocene lacustrine deposits (Calatayud basin, NE Spain)

Abstract Sedimentary magnesite deposits are commonly present in evaporite successions in the Tertiary Calatayud basin (NE Spain). Outcropping evaporite sequences, mainly composed of secondary gypsum after anhydrite–glauberite, show centimetre-thick layers of magnesite passing laterally into calcite pseudospar. Petrographic and stable isotope data indicate that calcite pseudospar formed via late-stage calcitization of magnesite under near-surface conditions. Calcitization occurred by the interaction between magnesite and Ca-enriched waters derived from the dissolution of gypsum and/or glauberite. The geochemical simulation of this process, petrographic evidence, and the correlation between the magnesite/calcite ratio and the δ18O values of both magnesite and calcite samples across the reaction front are in agreement with the existence of neoformed 18O-depleted magnesite during the waning stages of the calcitization process.

Hydration diapirism: a climate-related initiation of evaporite mounds in two continental Neogene basins of central Spain

Abstract While many halokinetic models consider that a thick overburden is necessary before diapirism develops, other studies show that salt might begin to move soon after deposition in response to different factors (e.g. regional extension). We describe two continental Neogene basins in central Spain, characterized by three Miocene sedimentary units, where unusually early initiation of evaporite mounds or ‘incipient pillows’ occurs, with no overburden on top of them. The arguments suggesting the early bulging of the evaporites of the Lower Unit, forming a palaeorelief before and during the deposition of the Intermediate Unit, rely on certain characteristics of the palaeokarsts developed on the roof of the Lower Unit, as well as in the nature, disposition, thickness, and synsedimentary processes of the Intermediate Unit. Folds, faults, tension gashes, and collapse structures develop within the evaporite mounds, and these deformation features might be related to their uplift history. The agent of initiation suggested in this paper for the evaporite mounds ‘hydration diapirism’, or a volume increase related to the hydration of anhydrite to gypsum has not been previously recognized in the literature. A series of sedimentological and palaeontological arguments further suggest that this chemical reaction was triggered by a climatic change at the boundary between the Lower and the Intermediate Units.