Martín García, Rebeca

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First Name
Last Name
Martín García
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Mineralogía y Petrología
Petrología y Geoquímica
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Now showing 1 - 10 of 24
  • Publication
    Do magnesian clays play a role in dolomite formation in alkaline environments? An example from Castañar Cave, Cáceres (Spain)
    (Sociedad Geológica de España, 2015) Martín Pérez, Andrea; Alonso-Zarza, Ana María; La Iglesia, Á.; Martín García, Rebeca
    The speleothems of Castañar Cave are made of a complex mineralogical association including aragonite, dolomite, huntite, hydromagnesite and a small proportion of Mg-rich clays whose mineralogy is difficult to determine. These clays appear as thin mats composed by fibres that are commonly associated to dolomite. The composition of cave waters and the textural relationship between the minerals indicate that in these alkaline environments there are several possible evaporation/concentration stages (with waters with pH > 8) followed by other stages characterized by freshwater entrances. All this result in complex precipitation sequences in which Ca and Mg are always available, and then it is silica content the main factor driving Mg-clay formation.The high Mg/Ca ratios drive initial aragonite precipitation, which contributes to an additional increase in this ratio enabling precipitation of huntite and Mg-clays (if silica is available). The following stage is dolomite formation either by replacement of metastable huntite or by direct precipitation on the Mg-clays that act as templates.
  • Publication
    Discriminating between tectonism and climate signatures in palustrine deposits: Lessons from the Miocene of the Teruel Graben, NE Spain
    (Elsevier Science B.V., Amsterdam, 2012) Alonso-Zarza, Ana María; Meléndez Hevia, Alfonso; Martín García, Rebeca; Herrero Fernández, María Josefa; Martín Pérez, Andrea
    The Upper Miocene (Vallesian–Turolian) Unit II of the Teruel Graben comprises at its top a 25 m-thick sequence of palustrine deposits. Deposition of the entire unit commenced some 9 to 7 Ma ago in a halfgraben basin. Here, via a recent quarry, we examine in detail the lateral and vertical distribution of Unit II's palustrine facies and their features to determine the palaeogeography and main controls on deposit formation. Our findings suggest the deposits formed at a low-gradient lake margin with different energy levels. These energy levels controlled the type of primary deposit within the lake; wackestone to packstone sediments formed in low-energy conditions, whereas cross-bedded rudstones to floatstones formed under higher energy conditions, by erosion and redeposition of prior lacustrine deposits. Pedogenic and diagenetic modifications of the primary sediments took place during sedimentary discontinuities (SD) when the lacustrine sediments were subaerially exposed. These processes serve to explain the formation of eight different palustrine limestones: limestones with root traces, mottled limestones, brecciated limestones, flat pebble breccias, granular limestones, micro-karstified limestones with laminar calcretes, carbonate mounds and clayey limestones with laminar calcretes. Based on the features and thicknesses of the modified sediments five different morphological stages (I to V) of palustrine carbonates are defined. Stage I is characterized by incipient mottling and brecciation. Stage II shows mottling and strong brecciation that lead to the formation of intraclast breccias, in which the fragments are mostly “in situ”. In Stage III, the primary fabric is totally changed; intraclasts have moved and may have lost their initial morphology. This Stage III may also be characterized by the formation of micro-karst. Stage IV is typified by the presence of coated grains and thin root mats. The chronological data available suggest that the formation of Stage III (lacustrine deposition+palustrine modification) would require about 40,000 yr. Facies and the SD record changes across short horizontal distances, and thus reflect the topography of prior sedimentation/modification events. Small (50 cm) highs with micro-karst have their SD counterparts in lower areas of the lake, in which the SD is indicated by desiccation and mottling. The topographic differences of the micro-karst were filled by intraclastic rudstones sourced by the adjacent carbonate flats. The example examined not only clearly sketches the morphology of ancient palustrine systems or wetlands, it also provides evidence that recycling of previous carbonate deposits played an important role as a sediment source, apart from biogenic or physical–chemical production processes. Our geochemical data indicate LMC (Low Magnesian Calcite) as the main component and Fe contents lower than 1%, except for the mottled areas that are richer in FeO. Stable isotope compositions provide δ18O values close to −6.5‰ VPDB, and more varied δ13C (−3.39 to −6.97‰ PDB). Oxygen and carbon values reveal no covariation and clear trends are lacking. The homogeneity of δ18O values reflects the intense effects of meteoric waters. The deposition of these palustrine limestones took place under suitable semi-arid to sub-humid climates. Climate could also have a role in determining subaerial exposure periods. However, its imprint is not easy to detect neither in the geochemical signals nor in the vertical arrangement of the facies. This could be attributed to climate changes probably occurring over shorter periods than those that can be recorded in this type of sediment, such as the astronomical precession cycles, and suggests the unsuitability of palustrine carbonates for detailed palaeoclimate analyses. Tectonism controlled the location of the main lacustrine depocentre close to the basin's main fault. The activity of this normal fault during the sedimentation of Unit II determined long- and short-term sedimentary sequences. Such sequences are the response to small-scale subsidence pulses followed by the infill of the created accommodation space by shallow lacustrine deposits, which underwent early pedogenic and diagenetic processes after subaerial exposure.
  • Publication
    Unravelling calcrete environmental controls in volcanic islands, Gran Canaria Island, Spain
    (Elsevier, 2020) Alonso-Zarza, Ana María; Rodríguez Berriguete, Álvaro; Martín Pérez, Andrea; Martín García, Rebeca; Menéndez, Inmaculada; Mangas, José
    Volcanic islands are unique contexts to analyze the controls on calcrete development in settings with varied relief, climate and vegetation, and lacking carbonate host rocks. This paper discusses the formation of Quaternary calcrete profiles in Gran Canaria volcanic Island, under direct influence of the SAL (Saharian Air Layer). Calcrete profiles are multi-storey and composed of prismatic, massive, laminar, pisolithic, ooid-bearing, sandy mudstones with rhizoliths and brecciated horizons. Host rocks include basalts, siliceous mudstones, volcanic sands and gravels and bioclastic-rich sands. Carbonate is mostly calcite but some dolomite occurs in massive horizons. The main microfabrics include laminated micrite, fine crystalline dolomite, desiccated micrite, micrite-clay groundmass, oriented Mg-rich clays, peloids, spherulites and coated grains. δ13C varies from −3.26 to −9.18‰ VPDB and δ18O from +0.86 to −3.24‰ VPDB. The 87Sr/86Sr ratios are between 0.707504 and 0.708860, indicating that calcium was mostly supplied from the CaCO3-rich aeolian dust coming from the Sahara Desert. Calcrete profiles are mostly pedogenic with roots and microorganisms enabling carbonate precipitation within the soils. The correlation trend of δ13C and δ18O points to a climate control on precipitation, with the heavier isotope values in the horizons containing dolomite. Sedimentation rates interplayed with calcrete formation processes and erosion to outline the profiles/horizons. Amalgamated laminar horizons formed when the rate of calcrete formation was higher than sedimentation and erosion, whereas areas or periods with relatively higher sedimentation allowed the separation of calcrete horizons. The direction of the winds supplying aeolian dust, and the varied relief, vegetation and climate favored better development of calcretes in the relatively arid and lowland areas of the eastern side of the island. Our study shows that calcretes contain the record of the paleoenvironmental controls that operated in volcanic islands with no previous carbonate substrates.
  • Publication
    Origen y diagénesis de los espeleotemas de Cueva Masero, Monumento Natural Cuevas de Fuentes de León, Badajoz
    (Sociedad Geológica de España, 2018) Alonso-Zarza, Ana María; Martín García, Rebeca; Martín Pérez, Andrea; Olmeda Zafrilla, Paula; Fernández Amo, Francisco J.
    Las cuevas que forman el Monumento Natural Cuevas de Fuentes de León se desarrollaron en una sucesión cámbrica detrítico-carbonática, que incluye algunas dolomías. Cueva Masero es una de las de mayor tamaño y contiene una amplia variedad de espeleotemas compuestos de calcita, aragonito y en menor proporción hidromagnesita formando el moonmilk. Las texturas de los espeleotemas son: 1) primarias, como las fibras de aragonito, las placas de calcita y el moonmilk; 2) diagenéticas, como los mosaicos y los megacristales de calcita o 3) de los dos tipos como la micrita y las columnas. La formación de aragonito e hidromagnesita en la cueva está condicionada por la presencia de Mg en las aguas de infiltración, por disolución de las dolomías. Los cambios en la composición del agua controlados climáticamente y la composición aragonítica inicial de algunos espeleotemas han dado lugar a procesos diagenéticos como transformación aragonito-calcita, micritización, cementación y recristalización. Nuestro estudio revela el interés de este Monumento Natural debido a la variedad y complejidad de sus espeleotemas.
  • Publication
    Reply to the comment on “Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess” by A.M. Alonso-Zarza, Z. Zhao, C.H. Song, J.J. Li, J. Zhang, A. Martín-Pérez, R. Martín-García, X.X. Wang, Y. Zhang and M.H. Zhang
    (Elsevier, 2010) Alonso-Zarza, Ana María; Zhao, Z.; Song, C. H.; Zhang, J.; Martínez Pérez, Álvaro; Martín García, Rebeca; Wang, X. X.; Zhang, Yaofeng; Zhang, M. H.; Meléndez Hevia, Alfonso
    Guo's et al. comments on our paper (Z.T. Guo, J.Y, Ge, G.Q. Xiao, Q.Z. Hao, H.B. Wu, T. Zhan, L. Liu, L. Qin, F.M. Zeng, B.Y. Yuan, Comment on “Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess” by A.M. Alonso-Zarza, Z. Zhao, C.H. Song, J.J. Li, J. Zhang, A. Martín-Pérez, R. Martín-García, X.X. Wang, Y. Zhang and M.H. Zhang [Sedimentary Geology 222 (2009) 42–51], Sedimentary Geology, 2010-this issue) mostly stress their previous data and their model of configuration and evolution of the study area; it is not a real discussion of the sedimentological features we describe. In this reply we will discuss some of the key features of the basin configuration, correlations and sedimentology of the Tinshui basin. Our work has followed the common procedures used in stratigraphy and sedimentology and so we can confirm our interpretation on basin configuration and correlations. In all cases we have taken into account previous papers, including those of Guo's group. In addition the sedimentological model we proposed is new due to the lack of previous sedimentological studies, including facies analysis and petrography, in the studied area. Our model of a continental alluvial-lacustrine basin fits well with other well-known examples over the world and explains clearly the lateral facies transitions across the basin. It is not the aim of this reply to discuss all the previous papers by Guo's group, but to reply to their main comments on our paper. © 2010
  • Publication
    Gypsum speleothems in lava tubes from Lanzarote (Canary Islands). Ion sources and pathways
    (Elsevier, 2019-04-01) Huerta, Pedro; Martín Pérez, Andrea; Martín García, Rebeca; Rodríguez Berriguete, Álvaro; La Iglesia, Á.; Alonso-Zarza, Ana María
    Lava tubes from Lanzarote Island in the Canary Archipelago commonly show white speleothems that stand out from the black basaltic rock. Mineralogical analyses of the speleothems from El Covón and Chifletera lava tubes show that gypsum is the dominant mineral with minor amounts of halite. Speleothems composed of microcrystalline gypsum (up to 150 μm long) are: coatings, globules, or extensive white powder accumulations covering the tube floor. Those composed of macrocrystalline gypsum with millimetric-size tabular and lenticular crystals are: crusts and stalactites. Uranium series dating of speleothems show ages ranging from 6217 ± 1644 yr to 40,039 ± 4748 yr. δ34S and the δ18O of gypsum speleothems (δ34S is 20.97‰ V-CDT and δ18O is 9.78‰ V-SMOW) is similar to that of sulphate dissolved in seawater. 87Sr/86Sr from speleothems (0.708665–0.708976) suggests that the main source of Ca is seawater, but additional Ca contributions from aeolian dust have reduced the Sr isotope values. These data support the idea that gypsum precipitates in the lava tube by evaporation of marine spray or solutions derived from marine spray. Two probable vias for ions input into the lava tube are considered: 1) sea spray circulating through the lava tube; 2) low-frequency rain infiltration leaching the marine spray salts precipitated at the surface. The constant supply of ions from sea spray, air currents in the cave, and the fast, but partial, evaporation due to the high relative humidity in the lava tube favours accumulation of major amounts of gypsum and subordinately halite. Scarcity of precipitation in the western Canary Islands prevents dissolution of gypsum speleothems.
  • Publication
    Clay minerals associations in palaeoweathering profiles from Central Spain: genesis and implications
    (, 2012-03) Doval, Mercedes; Martín García, Rebeca; La Iglesia Fernández, Ángel; Alonso-Zarza, Ana María
    This study examines part of the thick palaeoweathering mantle that formed on the northern area of the Spanish Central System. The study of a compound profile indicates that despite weathering processes, the primary structure of the metamorphic rocks is preserved, and is only partially lost in some intervals of the upper part of the compound profile. Macro/micromorphology, mineralogy and geochemical changes within the profiles revealed two weathering paths. In the first path, Fe-chlorite weathered to chlorite-smectite mixed-layer/smectite/kaolinite+ iron oxides. In the second path, biotite and/or muscovite weathered to kaolinite + iron oxides. The profiles show a progressive decrease, from base to top, in mica and mixed-layers and an increase in smectite and kaolinite. Thus, the profiles only comprise the lower or intermediate zones of the weathering mantle. The weathering occurred under humid climates; the lower zones of the profiles were poorly drained, whereas the topmost zones were better drained and more oxidizing. The results obtained indicate that detailed mineralogical studies are very useful to reconstruct the characteristics of the weathering mantles, and as palaeogeographic and palaeoclimatic indicators.
  • Publication
    Loss of primary texture and geochemical signatures in speleothems due to diagenesis: Evidences from Castañar Cave, Spain
    (Elsevier, 2009-11-15) Martín García, Rebeca; Alonso-Zarza, Ana María; Martín Pérez, Andrea
    Geochemical signals from speleothems are commonly used in the investigation of palaeoenvironments. In most cases, however, little attention is paid to whether or not these signals are primary or altered by diagenesis. The speleothems of the Castañar Cave (Cáceres, Spain), which are initially formed of calcite or aragonite, have undergone a variety of meteoric diagenetic processes such as micritization and neomorphism (inversion), that collectively modify their primary features (textures, mineralogy, geochemical signals). The mean δ13C and δ18O values of the aragonites in the cave are −8.66 and −4.64 respectively, whereas the primary calcites have mean δ13C and δ18O values of −9.99 and −5.77, respectively. Following the diagenetic process of micritization, the aragonite isotopic signals averaged −7.63 δ13C and −4.74 δ18O and the calcite micrite signals −9.53 δ13C and −5.21 δ18O. Where inversion took place, some secondary calcites after the aragonite show preserved aragonite, whereas others do not. The secondary calcites without aragonite relics show isotopic values slightly higher than those of the primary calcite due to the inheritance of the aragonite signal. Where aragonite relics are preserved, the isotopic signatures are very similar to those of the aragonite micrite. In addition, the stable isotopic values and Sr and Mg contents of the speleothems became also modified by micritization and/or inversion. These diagenetic processes were driven by the changes in composition of the cave waters over time and space, but also, in the case of aragonite, by its initial unstable mineralogy. The present results highlight how important diagenesis is in caves and how the initial features of cave minerals may be lost. These changes alter the geochemical signals shown by speleothems, which may have an impact on the interpretation of the results obtained in palaeoenvironmental studies.
  • Publication
    Features and Origin of Red Clays in Castafiar Cave: A Touch of Colour
    (Springer Verlag, 2010) Martín Pérez, Andrea; Martín García, Rebeca; Alonso-Zarza, Ana María; Herrero Fernández, María Josefa; Andreo, Bartolomé; Carrasco, Francisco; Durán, Juan José; LaMoreaux, James W.
    In Castaiiar Cave (Caceres, Spain), coatings of red clays cover the walls of the chambers, coexisting with diverse speleothems of aragonite, calcite, huntite and dolomite. The mineralogy of the clays is mainly illite, chlorite, kaolinite, smectite, quartz and Fe oxides and hydroxides such as goethite. They can be transported into the cave by infiltration waters or form by in situ alteration of the host rock: layers of dolomite rich in Fe and magnesite interbedded with greywackes and shales. Present-day hydrological conditions in the cave and conditions during the formation of speleothems have determined that the clays have not been transported by any flooding or seepage, but mostly staying in situ, and not included into carbonate crystal forms . Thus, most of the well-preserved speleothems are white and not stained, conforming an interesting chromatic contrast with the red clays that represent an additional attraction in this show cave.
  • Publication
    Diagenesis of continental carbonate country rocks underlying surficial travertine spring deposits
    (Elsevier, 2017-05-12) Rodríguez Berriguete, Álvaro; Alonso-Zarza, Ana María; Martín García, Rebeca
    Diagenesis of Miocene-Pliocene continental rocks in extensional Teruel Basin related to deposition of a travertine at surface has been studied. Most of the diagenetic processes described here are in relation tothe travertine deposition, so they are not widespread along the basin. Due to their high induration degreethese rocks may be misinterpreted with travertine/tufa facies, so there is a need of clear criteria for their recognition as they can supply important hydrological and paleoenvironmental information. The diagenetic processes, that deeply modified the substrate, include dissolution, recrystallization and cementation and lead to a high induration of the Miocene-Pliocene rocks. These processes were driven by groundwater flow leading to travertine deposition at surface and appeared to be closely related to the fluvial incision of Alfambra River and to extensional fracturation during Middle Pleistocene times probably under humid conditions. The cementation sequences and the organization of fractures and vadose micrite point to alternating vadose-phreatic conditions and syndiagenetic movements of the faults. Isotopic signal show lighter d18O than that of the unaltered Miocene-Pliocene rocks and similar d13C, suggesting lighter oxygen signal in waters during interglacial Pleistocene periods, and a common origin for carbon from marine Mesozoic rocks from the substrate of the basin. All of that suggest the strong control of tectonics, and climate at different timescales in diagenesis of the rocks serving as substrate for surficial travertine deposition.