Paleoecological and sedimentological analysis of Neogene carbonate platforms in the Western Mediterranean

Abstract

[spa] La presente tesis consiste en una recopilación de seis artículos que tratan sobre la interpretación de las condiciones paleoambientales y de los procesos sedimentarios de la rampa del Tortoniense inferior de Menorca (Islas Baleares, España), la rampa del Mioceno inferior del Lazio-Abruzzo (Italia central) y la formación del Plio-Pleistoceno conocida como "Calcarenite di Gravina" cerca de Matera (Sudeste de Italia). Cada sistema carbonatado es analizado y detalladamente descrito. Además, se han establecido nuevos criterios y métodos interpretativos, aplicables a otros sistemas carbonatados neógenos. La producción de carbonatos es, en su mayor parte, de origen biogénico o, al menos, está controlada biológicamente. Por tanto, factores ambientales como la disponibilidad de nutrientes, salinidad, hidrodinamismo, niveles de oxígeno, turbidez, tipo de substrato y disponibilidad del mismo, tasa de sedimentación, batimetría, luz y temperatura desempeñan un papel fundamental en el control de las factorías de sedimento. Las condiciones ambientales están sometidas a controles locales y regionales, que pueden cambiar con el tiempo, tal y como queda reflejado en los diferentes tipos de rocas calizas. Los métodos usados en el estudio de estas plataformas incluyen trabajo de campo y de gabinete. En el campo se ha llevado a cabo la elaboración de mapas, el levantamiento de columnas estratigráficas y la recogida de muestras. Éstas han sido estudiadas en lámina delgada y las microfacies se han descrito de acuerdo a las características texturales y composicionales observadas al microscopio. Además, se ha desarrollado un nuevo método para caracterizar las condiciones paleobatimétricas en base a la variabilidad de la forma del género de macroforaminífero Amphistegina, tal y como se explica en el artículo: "Test shape variability of Amphistegina d'Orbigny, 1826 as a paleobathymetric proxy: application to two Miocene examples". La luz es esencial para los organismos que, como los macroforaminíferos, albergan algas simbiontes. En consecuencia, el crecimiento de tales organismos se restringe a la zona fótica. Entre los macroforaminíferos, los grandes rotálidos presentan cambios en la forma de su caparazón a medida que aumenta la profundidad, siendo más delgados y planos en ambientes más hondos. Esta variabilidad morfológica se pone claramente de manifiesto en el género Amphistegina. En aguas oligotróficas del Indo-Pacífico, dicha variación se puede expresar matemáticamente mediante la función Zo = 2,592 T/D-2,293, donde Zo representa la profundidad y T/D la ratio entre el grosor de la concha respecto a su diámetro. La variabilidad en la forma de la Amphistegina presenta una alta correlación con la extinción de la luz a lo largo de la columna de agua. La penetración de la luz depende de la transparencia del agua, la cual disminuye a medida que la productividad biológica aumenta. Así, Zo debe ser corregida para ambientes mesotróficos, con una menor penetración de la luz. En ambientes mesotróficos, la ecuación puede expresarse como Zm = 1,037 T/D-2,2093, mientras que en situaciones de transición entre escenarios mesotróficos y oligotróficos, la expresión es Zom = 2,046 T/D-2,293. Zo, Zm y Zom pueden usarse como indicadores batimétricos cuantitativos, además de ayudar en la identificación de los lugares de producción y por tanto en la caracterización de los procesos de transporte de sedimentos.

[eng] The aim of this thesis is to interpret the paleoenvironmental conditions and sedimentary processes occurring in the lower Tortonian ramp of Menorca (Balearic Islands, Spain), the lower Miocene ramp of the Latium-Abruzzi (Central Italy) and PlioPleistocene Calcarenite di Gravina Formation (Southeastern Italy). Each carbonate system is described and analyzed and new criteria/interpretative methods, applicable to other Neogene carbonate systems, are established. Carbonate production is mostly biogenic or biologically controlled. Therefore, environmental factors such as nutrient availability, salinity, water energy, oxygen, turbidity, type of substrate and availability, sedimentation rates, bathymetry, light and temperature play a major role controlling the sediment factories. Environmental conditions are modified by local and regional controls and may change through time, which is recorded in the different types of limestones. This thesis documents the paleoenvironmental conditions and sedimentary processes occurring in the lower Tortonian distally steepened ramp of Menorca (Balearic Islands, Spain), the lower Miocene homoclinal ramp of Latium-Abruzzi (Central Italy) and the Plio-Pleistocene, mixed bio-lithoclastic system of the Calcarenite di Gravina (Matera, Southeastern Italy). Methods used to study each platform include field and laboratory work. Field work consists of mapping, logging stratigraphic sections and sampling. Samples have been studied in thin section and microfacies have been described according to textural and compositional features. Moreover, a new method to infer paleobathymetry is developed based on the mathematical modelling of the test shape variability of the large benthic foraminifer (LBF) Amphistegina, related to depth and/or light conditions. Information from microfacies description is integrated with the ecological meaning of the skeletal components that form the bioclastic fraction. Comparative analysis of the carbonate-producing scenarios allows to determine the common elements as well as those that are particular of each system. Sediment production in the lower Tortonian ramp of Menorca occurred in euphotic shallow water conditions (seagrass meadows) but mostly in the oligophotic zone (middle ramp/upper ramp slope) and corresponded to red algae rhodoliths and LBF. In the Latium-Abruzzi, major carbonate production took place in the aphotic zone, corresponding to the photoindependent heterozoan factory. However, minor production occurred in the euphotic (seagrass meadows and coral carpets) and the oligophotic (red algae rhodoliths) zones. In the mixed litho-bioclastic system of Calcarenite di Gravina, in the earlier transgressive stages, carbonate sediments mostly resulted from the erosion of the Cretaceous. Progressively, as sea level rose, rodalgal factory started to produce and seagrass meadows widespread and bioclastic production increased. The interaction between the type of sediment production and the prevailing hydraulic energy determines the depositional profile. Thus, in Menorca, moderate to strong hydrodynamism induced a selective, basinwards transport of the finer sediments, which along with the gravel-sized rhodoliths of the oligophotic factory accumulated on the slope edge. Once the equilibrium profile was exceeded, steep (15º20º) clinobeds prograded basinwards and formed the distally steepened profile. The low-angle, homoclinal profile of the Latium-Abruzzi ramp results from low hydraulic energy levels, major carbonate production in the deeper aphotic zone, very minor transport from inner and middle ramp settings and the subhorizontal physiography of the Cretaceous basement. The Calcarenite di Gravina formation developed under high hydraulic energy and continued currents regime, within a transgressive context. Waves eroded the Cretaceous basement and the resulting lithoclastic sediments, along with the seaweed-derived bioclasts, prograded downdip forming the infralittoral prisms. Longshore currents controlled along strike continuity of the prisms. Where basement continuity was interrupted a lithoclastic fan deposited from dumping sediments carried by shore-parallel currents. Contour-parallel bottom currents started to flow towards the Southeast when bathymetry increased. It is inferred from bioclastic-rich sedimentary bodies (mounded drift at the toe of the Matera horst). Transgression enhanced bioclastic production (increased available area for seaweeds to thrive). Coeval to the sea level rise, sediment transport direction shifted northwards to the Viglione strait and sediments coalesced forming an apron. Part of the material was reworked by westerndirected contour-parallel bottom currents, forming medium-scale tractive forms. Bioclastic components in Menorca and Latium-Abruzzi platforms include LBF, tropical to subtropical red algae taxa and zooxanthellate coral genus Porites, which points to warm temperatures. Despite the occurrence of hermatypic corals, the absence of reefal structures and the occurrence of seagrass meadows are consistent with mesotrophic conditions. Nevertheless, components indicate nutrient levels somewhat higher in the Latium-Abruzzi ramp.Components of the Calcarenite di Gravina are similar to those occurring in the present-day Mediterranean area, indicating temperate climate. Moreover, biota related to low-oxygen levels and/or increased organic matter is moderate to scarce and higher abundances might be related to organic matter production in seagrass meadows. The occurrence of segrasses is strongly documented in this thesis. Micropaleontological, paleontological and sedimentological evidences that confirm previous interpretations based on textural and sedimentological criteria. New interpretations are mostly based on the occurrence of epiphytic foraminiferal taxa and their ecological meaning, according to Langer’s morphotypes.