Gonzalo Pardo

Latest Oligocene–Late Miocene lacustrine systems of the north-central part of the Ebro Basin (Spain): sedimentary facies model and palaeogeographic synthesis

Abstract In the central part of the Ebro Basin (Spain), north of the Ebro River, three allostratigraphic or tectosedimentary units differentiated within the latest Oligocene–Late Miocene (upper Agenian–Vallesian) interval are sedimentologically characterized. These deposits represent carbonate and sulphate lacustrine environments fed by Pyrenean alluvial systems (Luna system and Huesca system). In each unit, several facies associations or lithofacies were mapped. These interfinger through time: gypsum lithofacies occupy the southernmost part of the study area and are surrounded by carbonate and/or detrital lithofacies to the north. Each lithofacies consists of different carbonate, sulphate and/or detrital facies, arranged in simple sequences, decimetres to metres thick. The four types of sequences characterized reflect a complex shallowing evolution involving carbonate or sulphate contexts. Thus, each lithofacies represents the sedimentation in a particular lacustrine or alluvial subenvironment. A single lacustrine system is proposed for the central part of the basin. Its evolution followed cycles of water level variations, which caused kilometre-scale migrations of the lake shoreline. In this model, two distinct situations or lakes alternated through time. (a) One represents high levels associated with a single body of dilute carbonate water, where massive carbonate facies formed influenced by organism proliferation. These lakes had wide palustrine vegetated margins, in which bioturbated facies developed. (b) The other represents low levels that correspond to a playa-lake model. Laminated sulphate deposits formed in the lake waters, while nodular gypsum originated in the surrounding saline mud flats and within the exposed, previously deposited carbonate sediments. Transitions from one situation to the other, including minor water level fluctuations within carbonate sedimentation conditions, were marked by the development of laminated and stromatolitic facies. Lake level variations were caused by climatic cycles and gave rise to the simple sequences and to the lithofacies lateral relationships recognized in this study. Correlation among tectosedimentary units in the studied Pyrenean domain and in the Iberian domain of the Ebro Basin shows that for units N1 and N2 the facies that resulted from dilute carbonate waters are rare in the lacustrine areas of the Iberian margin. This indicates that a strong hydrological contrast existed between both regions. On this basis, a theoretical asymmetrical facies model is proposed for the single lacustrine basin that extended over these regions.

Stable-isotope characterization of the Miocene lacustrine systems of Los Monegros (Ebro Basin, Spain): palaeogeographic and palaeoclimatic implications

The lacustrine carbonate facies from three allostratigraphic units (N1, N2 and N3) of Miocene age in the Los Monegros region (Ebro Basin) are characterized isotopically. The dominant facies are marls, laminated limestones, stromatolitic limestones, massive limestones and bioturbated limestones. These are associated with lacustrine sulphate deposits and distal alluvial facies. Palaeohydrological reconstructions of the study area are supported by the isotopic and the mineralogical composition of the carbonates. Massive and bioturbated limestones occupy a very similar compositional domain and have the lowest isotopic compositions of all the facies (−9‰ < δ18OPDB < −4.5‰ and −6.4‰ < δ13CPDB < −0.4‰), reflecting a short residence time of the water and variable, commonly high influence of biogenic CO2. Laminated limestones and stromatolites define a comparatively enriched domain with δ18O values ranging from −6 to 0‰ and δ13C values ranging from −3.5 to 0‰ These values indicate stronger evaporation and enhanced 12C assimilation due to intense biological activity during periods of longer water residence time. Marls have isotopic values intermediate between these two domains and correspond to periods of lacustrine dilution. Dolomite-bearing samples, mainly laminated and stromatolitic facies, show a Δ18ODOL-CAL = 7.5‰ with respect to calcite from the same facies, while δ13C values show very little enrichment (up to 1‰). The positive correlation (r = 0.85) between 18O and dolomite contents suggests that dolomite resulted from the progressive evaporative concentration of a single water mass and not from the mixing of waters of different compositions. The dolomite is thought to be primary or, at most, the product of very early diagenetic processes. The isotopic composition of the facies and their spatial and temporal variations depend on the depositional environment and were constrained by changing regional paleogeography. Isotopic trends from units N1 to N3 (Upper Agenian-Vallesian period) display a regular depletion in heavy isotopes that indicates a climatic change towards wetter conditions.

Dedolomitization and other early diagenetic processes in Miocene lacustrine deposits, Ebro Basin (Spain)

Abstract A variety of meteoric diagenetic features reveal the development of a syngenetic karst on lacustrine deposits of the Ebro Basin. Diagenetic processes that operated on lacustrine laminated and stromatolitic carbonates include the following. (1) A first syndepositional stage with processes such as dolomitization, desiccation and related breccia formation and sulphate precipitation, either as lenticular gypsum crystals or nodules. This stage took place under progressive evaporation due to lake level fall, when the previous carbonate deposits became exposed as a supra-littoral fringe surrounding saline mud flats of adjacent sulphate depositional environments. (2) A second early diagenetic stage in which processes such as sulphate dissolution and collapse brecciation, dedolomitization, calcite spar cementation and silicification occurred as a result of meteoric water input that caused a progressive rise in lake level. Light isotopic compositions (δ 13 C and δ 18 O) of diagenetic calcites, versus heavier compositions in primary laminated and stromatolitic limestones, confirm a meteoric influence. The syngenetic karst is best developed at the boundary between two allostratigraphic units and coincided with one of the extensive stages of sulphate deposition at the end of the Early Miocene. The karst facies occurred in an area that was a low-relief barrier that separated two sites of sulphate deposition during low lake levels. This indicates that the karst development was controlled by topographic changes within the basin and record a shift from arid to wetter climatic conditions, as suggested by the overlying freshwater carbonate deposits. The presence of diagenetic features such as those described in the central Ebro Basin affecting saline lacustrine carbonates is relevant because they can be used as indicators of subaerial exposure periods in terrestrial environments and they also reveal important palaeogeographic and palaeoclimatic events of basinal extent.

Seasonal record from recent fluvial tufa deposits (Monasterio de Piedra, NE Spain): sedimentological and stable isotope data

Abstract Physical and hydrochemical parameters and sedimentation rates were monitored twice a year from August 1999 to March 2003 at the Monasterio de Piedra area (NE Spain). Different tufa facies related to distinct fluvial subenvironments were characterized and the isotopic composition of water was analysed seasonally. Sedimentary features (thickness, texture and structure) and stable isotope composition of the seasonal record on tablets were analysed. The seasonal intervals were identified from six-monthly thickness measurements on tablets. Sedimentation rates had a strong seasonal pattern with higher values in warm periods than in cool ones, although erosive events and sporadic, warmer-than-normal climate conditions altered it. Three main types of fluvial facies were studied in detail: dense, stromatolitic tufa; dense to porous, massive tufa; and spongy, moss- and alga-bearing, crudely laminated tufa. Textural features of deposits from warm and cool periods had a variable pattern. The sediment δ18O composition showed a rhythmic variation, with higher values in cool periods and lower in warm ones, caused by the fractionation due to seasonal temperature variations. The calculated temperatures for a theoretical equilibrium precipitation accord with the actual measured temperatures. The sediment δ13C composition had an irregular pattern, indicating that other parameters than temperature intervened in the fluvial system.

New magnetostratigraphic dating of the Palaeogene syntectonic sediments of the west-central Pyrenees: tectonostratigraphic implications

Abstract New magnetostratigraphic results from a 3300 m-thick section across the syntectonic fluvial sediments of the Campodarbe Formation (Upper Eocene–Oligocene) in the Ebro foreland basin (NE Spain) are presented. The new data allow the top of the Campodarbe Formation to be correlated to Chron 7r (Chattian), younger than previously stated (C10r), therefore shifting the age of significant palaeogeographical changes in the foreland basin. The deformation in the southern front produces the cannibalization in the piggyback basin of 1300 m of sediments spanning c. 3.7 Myr. Average accumulation rates are lower in the Ebro foreland basin than in the piggyback basin and decrease from 35 to 27 cm kyr−1 by the time the San Felices thrust sheet activity decelerates (at c. 28 Myr). Shifts of accumulation rates result from accommodation space changes, which occur locally and are linked to the activity of the San Felices thrust, while the sediment supply occurs at orogenic scale (source of sediments is c. 200 km to the NE). Finally, sequence boundaries previously considered isochronous in the continental record of the Cenozoic Pyrenean basins are revealed to be 1.8–1 Myr older in the piggyback basin than in the Ebro foreland basin.