Cyclicity and basin axis shift in a piggybank basin: towards modelling of the Eocene Tremp-Ager Basin, South Pyrenees, Spain

This paper is included in the Special Publication entitled 'Cenozoic foreland basins of Western Europe' edited by A. Mascle, C. Puigdefabregas, H.P. Luterbacher and M. Fernandez. A new database of the Eocene Montanyana delta in the Tremp-Ager piggyback basin of the South Pyrenees is exemplified and discussed. It consists of a grid of nine transverse and five longitudinal stratigraphic cross-sections and structural profiles, some examples of which are shown. Detailed facies maps and a large number of digitized sedimentary logs formed the basis for the stratigraphic correlation of the traverses with a resolution at the scale of 5 m, i.e. the size of minor architectural elements. The correlation was calibrated chronostratigraphically with biostratigraphic and recently published palaeomag data. The database so provides a detailed picture of the geometry, internal architecture and stacking patterns of the increments of basin infilling. The database allowed recognition and definition of eight megasequences, between 148 and 404 m thick, that are aperiodic, spanning time intervals between 400 and 1400 Ma. Although most of the megasequence boundaries can easily be related to third-order sea-level fluctuations, predominant structural control is indicated by their correlation with sharp reversals in the pattern of basin axis shift in transverse cross- sections of the basin, and with flank unconformities. Several modes of structural control by thrust sheet displacement are proposed. Sea-level fluctuation modified the megasequential architecture accounting, for instance, for extreme progradation as observed in the Castissent Sandstone. The megasequences are subdivided into a large number of basin-wide cycles, on an average 44 m thick, with a calculated average periodicity of 124 ka approximating the 100 ka of orbital forcing. Aggradational, amalgamated sheet, fan progradational and fluvial expansion cycles are distinguished and correlated with episodes of specific structural and sea-level control. The pattern of stacking of the cycles conforms to the structurally controlled megasequential basin-axis shift on which it is superposed. Climatic fluctuations appear to have played a prominent role only in generating minor subcycles observed in some parts of the basin fill. This basin analysis is meant to be a step towards 3D numerical tectono-sedimentary modelling of the Tremp-Ager piggyback basin.