Magnetostratigraphy of the Miocene continental deposits of the Montes de Castejón (central Ebro basin, Spain): geochronological and paleoenvironmental implications

A detailed magnetostratigraphic study has been carried out in the early to middle Miocene distal alluvial and lacustrine sediments of the Montes de Castejón (central Ebro Basin). The study was based on the analysis of 196 magnetostratigraphic sites sampled along a stratigraphic interval of about 240 meters. Local magnetostratigraphy yielded a sequence of 12 magnetozones (6 normal and 6 reverse) which could be correlated with the Geomagnetic Polarity Time Scale (GPTS) interval C5Cr to C5AD (between 17 and 14.3 Ma.). The sampled sedimentary sequences include the boundary between two tectosedimentary units (TSU, T5 and T6) already defined in the Ebro Basin. The magnetostratigraphy of the Montes de Castejón allows to date the T5/T6 TSU boundary at 16.14 Ma, within chron C5Cn.1n. This magnetostratigraphy also allows us to analyse in detail as well as to discuss the variations in sedimentation rates through space and time between different lacustrine environments: Outer carbonate lacustrine fringes and distal alluvial plains (Montes de Castejón sections) show higher sedimentation rates than offshore lacustrine areas (San Caprasio section, 50 km east of Montes de Castejón)

Temporal aspects of genetic stratigraphic units in continental sedimentary basins: Examples from the Ebro basin, Spain

The utility of genetic stratigraphy lies with the fundamental relationship between genetic stratigraphic units and correlative intervals of geologic time. While high-resolution studies may reveal varying degrees of diachroneity that are associated with their bounding surfaces, genetic stratigraphic units are generally viewed as chronostratigraphic in nature, representing specific periods of basin fill. Despite the vast literature on basin analysis, an evaluation of the temporal character of the boundaries between genetic stratigraphic units, particularly in cases where they are conformable, has not yet been undertaken. The Cenozoic Ebro basin is a foreland basin on the north-eastern Iberian Peninsula that provides a case for assessing the temporal character of the boundaries between genetic stratigraphic units. These units are termed “tecto-sedimentary units” following the method of tecto-sedimentary analysis that is used in some continental basins. In the Ebro basin tecto-sedimentary analysis is based on direct field observations. In this work, magnetostratigraphic data from four tecto-sedimentary units (units T4 to T7) that span rocks of lower and middle Miocene age (ca. 1000 m thick) in the central and western areas of the basin are analysed. The study area contains alluvial, fluvial and lacustrine deposits that were sourced from Pyrenean and Iberian areas, whose catchments were structured during the collision of Iberia and Eurasia. New magnetostratigraphic data from this study and previously published magnetostratigraphic data enable us to determine the ages of these tecto-sedimentary unit boundaries throughout a 200-km-long, east–west transect that extends from the basin centre to the southwestern margin. The results indicate that the diachrony of the three boundaries between the Miocene tecto-sedimentary units through the central Ebro basin is less than 0.3 Ma where they are conformable. This low degree of diachroneity may be attributed to the effects of allogenic, largely tectonic processes that operate in the catchment areas and methodological inaccuracies. These results provide empirical support to the idea that genetic stratigraphic units are bounded by surfaces that exhibit low amounts of diachroneity where they are conformities

Lacustrine stromatolites: Useful structures for environmental interpretation – an example from the Miocene Ebro Basin

The significance of stromatolites as depositional environmental indicators and the underlying causes of lamination in the lacustrine realm are poorly understood. Stromatolites in a ca 600 m thick Miocene succession in the Ebro Basin are good candidates to shed light on these issues because they are intimately related to other lacustrine carbonate and sulphate facies, grew under variable environmental conditions and show distinct lamination patterns. These stromatolites are associated with wave-related, clastic-carbonate laminated limestones. Both facies consist of calcite and variable amounts of dolomite. Thin planar stromatolites (up to 10 cm thick and less than 6 m long) occurred in very shallow water. These stromatolites represented first biological colonization after: (i) subaerial exposure in the palustrine environment (i.e. at the beginning of deepening cycles); or (ii) erosion due to surge action, then coating very irregular surfaces on laminated limestones (i.e. through shallowing or deepening cycles). Sometimes they are associated with evaporative pumping. Stratiform stromatolites (10 to 30 cm high and tens of metres long) and domed stromatolites (10 to 30 cm high and long) developed in deeper settings, between the surge periods that produced hummocky cross-stratification and horizontal lamination offshore. Changes in stromatolite lamina shape, and thus in the growth forms through time, can be attributed to changes in water depth, whereas variations in lamina continuity are linked to water energy and sediment supply. Growth of the stromatolites resulted from in situ calcite precipitation and capture of minor amounts of fine-grained carbonate particles. Based on texture, four types of simple laminae are distinguished. The simple micrite and microsparite laminae can be grouped into light and dark composite laminae, which represent, respectively, high and low Precipitation/Evaporation ratio periods. Different lamination patterns provide new ideas for the interpretation of microbial laminations as a function of variations in climate-dependent parameters (primarily the Precipitation/Evaporation ratio) over variable timescales

Testing the record of climate-related cyclicity in microbial lamination: An example from Miocene oncolites in the Ebro Basin, Spain

This work tests the relationship between quasi-periodic climate-linked parameters and cyclicity recorded through microbial lamination using spectral analysis of time series based on lamina thickness and luminance, as well as C and O stable isotope data. Three oncolites of the Ebro Basin were used: specimens C4-1 and C4-31 from a lacustrine bed dated at 13.8 My, and specimen VD-13 from a fluvial bed whose age is younger than 12 My. Lamination consists of alternating light and dark calcite laminae, which correspond to crystal size and porosity variations. Composite light and dark laminae can be seen by the naked eye. Spectral analysis based on the thickness and luminance measurements of simple light–dark lamina couplet successions revealed that the three specimens had recurrent periods of 2.5, 3 and 10–13. On the hypothesis that each simple light–dark lamina couplet represents a 1-year duration, the 2.5 and 10–13 periods can be related to the 2.5-year quasi-biennial oscillation and the 11-year Schwabe sunspot cycle, respectively. Other less frequent periods present around 5 and 7, together with period 3, could be related to the most typical modes of the North Atlantic Oscillation and/or El Niño Southern Oscillation. The periods at 19–23, present in specimen C4-31, can be correlated with the 22-year Hale sunspot cycle. The 10–13 and 19–23 periods were also detected using the δ13C and δ18O values of successive laminae in specimen C4-31. These data suggest that solar irradiance variations influenced the characteristics of the oncolite laminae, through temperature and precipitation. Although the studied oncolites do not present an optimal thickness lamination pattern for spectral analysis, their lamination is capable of registering periodic climatic variations, and these results provide further evidence that microbial laminations can serve as high-resolution records of climate and climate-linked cycles over different time scales

Magnetostratigraphy of the Miocene continental deposits of the Montes de Castejón (central Ebro basin, Spain): geochronological and paleoenvironmental implications

A detailed magnetostratigraphic study has been carried out in the early to middle Miocene distal alluvial and lacustrine sediments of the Montes de Castejón (central Ebro Basin). The study was based on the analysis of 196 magnetostratigraphic sites sampled along a stratigraphic interval of about 240 meters. Local magnetostratigraphy yielded a sequence of 12 magnetozones (6 normal and 6 reverse) which could be correlated with the Geomagnetic Polarity Time Scale (GPTS) interval C5Cr to C5AD (between 17 and 14.3 Ma.). The sampled sedimentary sequences include the boundary between two tectosedimentary units (TSU, T5 and T6) already defined in the Ebro Basin. The magnetostratigraphy of the Montes de Castejón allows to date the T5/T6 TSU boundary at 16.14 Ma, within chron C5Cn.1n. This magnetostratigraphy also allows us to analyse in detail as well as to discuss the variations in sedimentation rates through space and time between different lacustrine environments: Outer carbonate lacustrine fringes and distal alluvial plains (Montes de Castejón sections) show higher sedimentation rates than offshore lacustrine areas (San Caprasio section, 50 km east of Montes de Castejón)

Magnetostratigraphy of the Miocene continental deposits of the Montes de Castejón (central Ebro basin, Spain): geochronological and paleoenvironmental implications

A detailed magnetostratigraphic study has been carried out in the early to middle Miocene distal alluvial and lacustrine sediments of the Montes de Castejón (central Ebro Basin). The study was based on the analysis of 196 magnetostratigraphic sites sampled along a stratigraphic interval of about 240 meters. Local magnetostratigraphy yielded a sequence of 12 magnetozones (6 normal and 6 reverse) which could be correlated with the Geomagnetic Polarity Time Scale (GPTS) interval C5Cr to C5AD (between 17 and 14.3 Ma.). The sampled sedimentary sequences include the boundary between two tectosedimentary units (TSU, T5 and T6) already defined in the Ebro Basin. The magnetostratigraphy of the Montes de Castejón allows to date the T5/T6 TSU boundary at 16.14 Ma, within chron C5Cn.1n. This magnetostratigraphy also allows us to analyse in detail as well as to discuss the variations in sedimentation rates through space and time between different lacustrine environments: Outer carbonate lacustrine fringes and distal alluvial plains (Montes de Castejón sections) show higher sedimentation rates than offshore lacustrine areas (San Caprasio section, 50 km east of Montes de Castejón)

Dating the northern deposits of the Ebro foreland basin; implications for the kinematics of the SW Pyrenean front

Dating the sedimentary infill of the northern margin of the Ebro foreland basin informs about the orogen-basin evolution. A magnetostratigraphic section of ~5 km-thick combines new magnetostratigraphic results from the syntectonic alluvial ncastillo Fm (Upper Oligocene-Lower Miocene, 1 km-thick, the Fuencalderas section) with previous (3.3 km-thick) and new (0.7 km thick) magnetostratigraphic data from the underlying fluvial Campodarbe Fm (Luesia and San Marzal sections, Eocene-Oligocene). This composite section allows dating the entire basin infill in this sector, therefore bracketing the timing of the deformation in the southern margin of the Pyrenees after considering previous kinematic studies. Deformation recorded by the continental deposits spans from syn-Gavarnie nappe activity (Broto and Fiscal basement thrust sheets) from 31.3 to 24.55 Ma, Rupelian-Chattian (Oligocene) to the syn-Guarga thrust activity, from 24.55 to 21.2 Ma Chattian-Aquitanian (Oligocene-Miocene). The Accumulation rates vary from ca. 22 to ca. 39 cm/kyr between the genetic stratigraphic units in the Uncastillo Fm through the section. These rates are close to those of the underlying deposits of the Campodarbe Fm (average of ca. 36 cm/kyr). Deformation for the latest Pyrenean front (syn-Guarga thrust) is younger than previously assigned in ~ 5.4. ~1.5 and ~1.1 Myr of the beginning of the Punta Común thrust sheet (from 29.4 Ma to 24 Ma), Lower Riglos thrust sheet system (from 24 to 22.5 Ma) and the Upper Riglos thrust system (22.5 to 21.4 Ma) respectively. Other significant changes in accumulation rate and its derivative also indicate variations in the tectonic activity ca. 33 and 28 Ma. Tilt variations in the Uncastillo Fm also record the tectonic activity with 40º variations of the Punta Común thrust sheet at ca. 20º with the Upper Riglos thrust sheet