Controls on the development of sedimentary sequences in continental basins: examples from the Cenozoic of Iberia

Abstract

The first goal of this thesis is to provide accurate and complete age models for key basins of Iberia. The Ebro, Almazán, and As Pontes basins were chosen because of their geological importance and because long sections with overlapping ages were available. All the studied records encompass a long time span (>7-Ma), which may be used for research in a wide range of time-scales. In particular, we focused in assessing the impact in the stratigraphy of the very long-term Milankovitch cycles (> 1Ma) and their relationship in the generation of sedimentary sequences at basin scales. Disentangling such relationship may shed light into the tectonic or climatic origin of observed sedimentary sequences. In parallel, the development of age models allows for a precise quantification of sedimentation rates.

The results of this PhD project provide insights on the expression of the Milankovitch cycles in continental basins and also on the relationship between accommodation and sediment supply in the development of sedimentary sequences. Our data together with results from other case studies allows for a discussion about the pathways in which Milankovitch cycles affected the continental sedimentary records through a mass-balance approach. In order to undertake these subjects, three independent studies have been carried out

In the Ebro Basin, it has been shown that the location of the lacustrine depocenters was conditioned by the migration of the subsidence distribution due to the thrust-belt structuring and the subsequent foreland evolution. Despite tectonics, the superimposed signal of climate was found to be the main cause of lake level variation. Times of orbital eccentricity maxima are associated to lake level highstands, indicating a positive water balance. Intervals with high-amplitude precession cycles, due to eccentricity modulation, are seen as the drivers of water recharge.

Oppositely to the Ebro Basin, the As Pontes Basin is a small basin that lacked a central lake system during the studied interval. The occurrence of peatlands in the distal parts, made the system very sensitive to shifts in the terrigenous input. In As Pontes, it has been shown that climatic stability at times of eccentricity minima appears associated to peat development and burial. This is likely more linked to resilient vegetation cover and equilibrium geomorphologic profiles, which reduced the sediment supply. The cyclic arrangement in the depocenters was shaped by changes in the sediment supply, which may increase, and dilute the accumulation of organic matter, rather than by variable lake level. Despite there is not enough resolution as to test for precession or obliquity, the stacking pattern shows no evidence of these higher frequency orbital cycles. It suggests a control of eccentricity on the expression of orbital forcing in a clastic-dominated system.

The analysis carried out in the Almazán Basin illustrates how the distribution of the depositional elements, their internal architecture, and the sedimentation rates are consequence of variations in the AS/SS ratio (Accommodation/Sediment supply). This makes the AS/SS ratio an excellent marker for the mass balance variations, the distribution of the total volume of sediment along a

basin, allowing for a quantification of the response of a basin to extrinsic changes, which are mainly climate and tectonics. Although if a single depositional element is analysed at short time scales, the ratio also may include the autogenic variations.

Aquesta tesi te com a principal objectiu datar conques Cenozoiques clau en l’evolució geològica de la Península Ibèrica mitjançant magneto- i astrocronologies. La tesis està constituïda per una recopilació d’articles. La unió dels diferents treballs realitzats dóna lloc a uns resultats i discussió a on s’aborden les diferents camins mitjançant els quals els registres sedimentaris poden expressar litològicament els cicles de Milankovitch, claus en la generació de seqüències sedimentaries.

Tres conques de la península Ibérica, Ebre, Almazán i As Pontes, han estat estudiades. Per separat, els resultats mostren que les seqüències sedimentaries a les parts centrals de la conca de l’Ebre estan relacionades amb el cicle orbital de 2.4 milions d’anys. A on els màxims d’excentricitat estan relacionats amb la expansió dels sistemes lacustres. A la conca d’As Pontes els canvis de la excentricitat orbital també s’han identificat com a causants de les seqüències. Em aquest cas, els mínims d’excentricitat de 400 mil anys i de 2,4 Milions d’anys semblen ser els precursors dels nivells de lignits. A la conca d’Almazán, degut a la deformació produïda per estructures, no hi trobem l’empremta dels cicles orbitals. El que sí s’observa es una relació entre les taxes de sedimentació i l’arquitectura i la distribució dels elements deposicionals. Els resultats finals indiquen que a banda de un règim climàtic concret la morfologia i característiques de les conques sedimentaries son elements claus en quant a l’expressió dels cicles de Milankovitch.