%0 Journal Article
%A Alonso-Zarza, Ana María
%A Meléndez Hevia, Alfonso
%A Martín García, Rebeca
%A Herrero Fernández, María Josefa
%A Martín Pérez, Andrea
%T Discriminating between tectonism and climate signatures in palustrine deposits:Lessons from the Miocene of the Teruel Graben, NE Spain
%D 2012
%@ 0012-8252
%U https://hdl.handle.net/20.500.14352/42555
%X The Upper Miocene (Vallesian–Turolian) Unit II of the Teruel Graben comprises at its top a 25 m-thick sequenceof palustrine deposits. Deposition of the entire unit commenced some 9 to 7 Ma ago in a halfgrabenbasin. Here, via a recent quarry, we examine in detail the lateral and vertical distribution of Unit II'spalustrine 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 sedimentsformed in low-energy conditions, whereas cross-bedded rudstones to floatstones formed under higherenergy conditions, by erosion and redeposition of prior lacustrine deposits. Pedogenic and diagenetic modificationsof the primary sediments took place during sedimentary discontinuities (SD) when the lacustrinesediments were subaerially exposed. These processes serve to explain the formation of eight different palustrinelimestones: limestones with root traces, mottled limestones, brecciated limestones, flat pebble breccias,granular limestones, micro-karstified limestones with laminar calcretes, carbonate mounds and clayey limestoneswith laminar calcretes. Based on the features and thicknesses of the modified sediments five differentmorphological stages (I to V) of palustrine carbonates are defined. Stage I is characterized by incipient mottlingand brecciation. Stage II shows mottling and strong brecciation that lead to the formation of intraclastbreccias, in which the fragments are mostly “in situ”. In Stage III, the primary fabric is totally changed; intraclastshave moved and may have lost their initial morphology. This Stage III may also be characterized by theformation of micro-karst. Stage IV is typified by the presence of coated grains and thin root mats. The chronologicaldata 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 priorsedimentation/modification events. Small (50 cm) highs with micro-karst have their SD counterparts inlower areas of the lake, in which the SD is indicated by desiccation and mottling. The topographic differencesof the micro-karst were filled by intraclastic rudstones sourced by the adjacent carbonate flats. The exampleexamined not only clearly sketches the morphology of ancient palustrine systems or wetlands, it also providesevidence 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 lowerthan 1%, except for the mottled areas that are richer in FeO. Stable isotope compositions provide δ18O valuesclose to −6.5‰ VPDB, and more varied δ13C (−3.39 to −6.97‰ PDB). Oxygen and carbon values reveal nocovariation and clear trends are lacking. The homogeneity of δ18O values reflects the intense effects of meteoricwaters.The deposition of these palustrine limestones took place under suitable semi-arid to sub-humid climates. Climatecould also have a role in determining subaerial exposure periods. However, its imprint is not easy to detectneither in the geochemical signals nor in the vertical arrangement of the facies. This could be attributedto climate changes probably occurring over shorter periods than those that can be recorded in this type ofsediment, such as the astronomical precession cycles, and suggests the unsuitability of palustrine carbonatesfor detailed palaeoclimate analyses.Tectonism controlled the location of the main lacustrine depocentre close to the basin's main fault. The activityof 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 thecreated accommodation space by shallow lacustrine deposits, which underwent early pedogenic and diageneticprocesses after subaerial exposure.
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