Sedimentología y geoquímica de los depósitos laminados microbianos y facies asociadas del mioceno del sector central de la cuenca del ebro

Resumen

The lower and middle Miocene sedimentary record of the central part of the Ebro Basin, in the Sierra de Alcubierre, is mainly represented by carbonate and sulphate deposits with less abundant siliciclastics. Stromatolites are distributed throughout the studied record. The interest of these stromatolites is based on their close and diverse relationship with the associated facies, their varied external and internal morphologies, and their lamina arrangement. This Ph.D. thesis, entitled “Sedimentology and Geochemistry of the Microbial Laminated Deposits and Associated Facies from the Miocene of the Central Sector of the Ebro Basin”, presents a detailed study of the stromatolites and their environmental and temporal significance. This work proposes a new approach to the understanding of stromatolites at different scales, from the external morphology (depositional, climatic, and hydrological connotations) to the simple laminae (depositional, climatic, and temporal meanings). This study includes: 1. Detailed sedimentological analysis of the stromatolites and all facies associated with them, which helps to interpret the depositional conditions in which the stromatolites were formed. The depositional conditions refer mainly to depth, sediment supply and water energy, and other parameters derived from those. 2. Geochemical study through stable isotope analyses of carbon and oxygen performed on different carbonate facies and, indeed, in the stromatolites, at different orders of lamina arrangement, which allows to know the factors that controlled the textural variations of the laminae and infer their temporal meaning. 3. Cyclicity analysis based on the lamina thickness and luminance to detect the lamination cyclic patterns and their possible relation to climate-related agents. 4. A comparison with other fossil and recent lacustrine and fluvial stromatolites. Materials and Methods The sedimentological study of the stromatolites and associated facies begun by gathering information from the field. A total of 183 sites were visited, where 1 to 5 stromatolite specimens and associated facies were taken in each of them; 36 stratigraphic sections (1:10 y 1:20), approximately 0,3 to 3 m thick, were made. The stromatolite specimens were used in the laboratory to obtain polished sections (112), thin sections (192), and powder samples. Polished sections helped in defining the structure (laminae shape and thickness, and internal morphologies, i.e., internal growth forms). Thin sections observed in optical microscope and small samples (1x1x1 cm) observed in SEM allowed to make the textural characterization of the types of laminae and the identification of the lamination arrangement. Powder samples were used for X-Ray Diffraction (mineralogical identification) and stable isotope analyses (δ13C and δ18O). The isotopic analyses were made at three scales (facies, composite lamina, and simple lamina). Spectral analysis was applied to time series of data of stromatolites (thickness and luminance) obtained from polished sections at the scale of simple lamina to detect cyclicity patterns. Finally, a comparative analysis has been done with other fluvial and lacustrine stromatolites to evaluate the interpretations made in this study. Main results and interpretations Stromatolite sedimentology: The observations in the field and polished sections show several external morphologies of the stromatolites in the study area. Thin planar stromatolites have greater lateral extension than height (mm – 10 cm) with smooth, flat to undulatory, laterally continuous laminae. Stratiform stromatolites are 10 to 30 cm high and 10 to 30 m in lateral extension, with complex internal growth forms as domes, columns, and undulatory laminated to flat-laminated forms. Domed stromatolites are 10 to 30 cm high and similar length, with internal growth forms similar to the stratiform stromatolites. These two types of stromatolites are laterally related on the outcrops. Considering the relationships between the stromatolites (external morphologies and internal growth forms) and the other lacustrine facies associated with them, the different situations in which stromatolites developed reflect mainly the relative changes in water depth, sediment supply and hydrodynamic conditions. Thin planar stromatolites that alternate with laminated limestones with parallel lamination and ripples represent shallow calm water conditions, where the thin planar stromatolites were prone to subaerial exposure. Indeed, these stromatolites represent first colonization after exposure or erosional processes. Domed and stratiform stromatolites developed with higher water level. Vertical and lateral associations of these stromatolites with laminated carbonate facies with hummocky-cross stratification and with parallel lamination suggest variable energy conditions, with better development of stromatolites during still conditions. Changes in lamina shape. Internal stromatolite morphologies based on lamina shape, lamina lateral continuity, and growth forms indicate water depth variations and changes in water energy. The upward change from slightly undulatory at the base to increasingly convex laminae forming domes and columns represent increasing water depth. Laminae are laterally continuous between columns and domes in still conditions, but if the energy levels are higher the laminae are discontinuous, then unlinked columns and breakage of the previously formed stromatolite occur. The upward change from convex to gently convex to slightly undulate laminae upward would develop as a result of the water level decrease. Types of laminae and lamina arrangement: Different types of laminae have been distinguished based on textural characteristics, mainly variations of crystal size, porosity and color. There are four types of simple lamina: light dense micrite laminae, light porous micrite to microsparite laminae, dark dense micrite laminae, and fibrous sparite laminae. The combination of the simple micrite laminae forms composite laminae. Dark composite laminae are formed of dark dense laminae that include intercalated thinner light porous laminae, or successive dark dense laminae. Light composite laminae are formed of thick light porous laminae with intercalated thin dark dense simple lamina, or an alternation of porous and light dense laminae. Three lamination patterns have been distinguished from the combination of simple and composite laminae: simple alternating lamination (alternating dark dense and light porous simple laminae), cyclothemic lamination (succession of cycles, each consisting of a light dense lamina, followed by a light porous simple lamina, and a dark dense simple lamina), and alternating composite lamination (alternating dark composite laminae and light, either simple or composite, laminae). δ13C and δ18O analysis through facies successions: Isotopic analyses along with the sedimentological analyses of several stromatolite-containing facies successions allow to interpret them in terms of water level variation, i.e., as representing deepening or shallowing cycles. The isotopic analyses reveal higher δ13C and δ18O values in the stromatolites and laminated limestones than in the other carbonated facies (marls, and massive bioclastic and bioturbated limestones), indicating their development during periods of little lake water renewal that led to more saline conditions. Environmental parameters and lamina significance: δ13C and δ18O data from stromatolites are used to assess the potential of ancient lacustrine stromatolite lamination as an archive for high-resolution paleoclimatic changes. The significant correlation between δ13C and δ18O of the laminae suggest that, in the context of the studied closed lake system, the precipitation/evaporation (P/E) ratio was the main factor controlling the isotopic composition of precipitates. Light laminae (simple or composite) have low isotopic values, while dark laminae (simple or composite) have higher isotopic values. This behavior is present at three orders of lamination, and allows to assume an annual duration of each pair of light-dark laminae that is related to seasonal changes in the P/E ratio. The third-order cycles, recorded by the simple laminae, possibly formed by seasonal P/E changes. The second-order cycles, represented by the dark or light composite laminae reflect several years of lower or higher P/E ratio, respectively. The first-order cycles, formed by the succession of a light composite lamina and a dark composite lamina, reveal lake level shallowing evolution during several years due to the decrease of the P/E. The isotopic evolution of the studied stromatolites through the stratigraphic units supports a general trend toward less saline conditions from units T5 and T6 to unit T7. Climate-related agents affecting stromatolite lamina arrangement: Assuming that each light-dark lamina formed in one year, the spectral analysis of time series based on stromatolite lamina thickness and luminance reflects significant periodicities around 2.5, 3.7, 5, 7, 10, and 22 years. The 2.5 year may correspond to the Quasi Biennial Oscillation (QBO), the biennial component of El Niño-Southern Oscillation (ENSO) or the North Atlantic Oscillation (NAO) variability. The 3-5 and 5-7 year bands could be linked to ENSO or NAO variability. The 8-11 and 22-23 year bands are related to the 11-year Schwabe and 22-year Hale sunspot cycles. Overall, stromatolite growth is controlled by NAO, ENSO, and solar activity. The cyclicity study also allows to estimate growth rates of the stromatolites, with a mean value of 0.362 mm/yr and lifespan between 120 and 500 years of the studied stromatolite specimens. There is a correlation between the cyclicity orders based on the stable isotope analysis and the periods and climatic signatures. Third-order cycles are identified with the light-dark simple lamina couplets (annual cycles) used in the spectral analyses. Second-order cycles (each dark and each light composite laminae) can be correlated with the cycles of lamina couplets with periods around 3.5, 5, 6, and 8 years, which are related to NAO/ENSO-like variability. First order cycles (each pair of consecutive light and dark composite laminae) are identified with 9-12 couplets, and are related to NAO/ENSO-like or 11-years Schwabe solar cycles. This study also reinforces the evidence of the presence of ENSO and NAO precursors during the Miocene due to the inter-annual, decadal, and even multi-decadal climatic variability revealed by the persistent significant periods in the time series analysis in the studied stromatolites. Comparison with other lacustrine and fluvial stromatolites: The external morphology and internal growth forms detected in the Sierra de Alcubierre lacustrine stromatolites are similar to other lacustrine stromatolites and differ from fluvial stromatolite morphologies. The lacustrine stromatolites morphologies compared in this study coincide in the interpretations that their morphologies are mainly influenced by extrinsic factors, such as lake level fluctuations, water energy or detrital sediment supply. Textural characteristics of the stromatolite lamination from the lacustrine stromatolites of Sierra de Alcubierre have some similarities with the lamination of some lacustrine stromatolites from the Green River Formation. The light and dark laminae alternation is also characterised by micrite and microsparite forming the light laminae while micrite form dark laminae. The main similarities with fluvial stromatolites are related to some textural attributes, where simple porous and dense laminae are detected organised in composite laminae. δ13C and δ18O analyses indicate cyclic variations in fluvial and lacustrine stromatolites that coincide with textural variations. These variations are related to warm and/or dry periods and cold and/or humid periods.