Abstract
In this thesis a tectono-sedimentary forward model has been presented, devised to simulate sediment erosion and deposition in a coupled drainage basin - foreland system, as well as accumulating a three-dimensional stratigraphy.
The aim of the research was to investigate which features recorded in the foreland basin architecture are diagnostic of
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the balance and interplay between two main external forcings: repeated tectonic activity and eustatic sea-level variation. Special attention has been paid to differences in stacking patterns of fluvial depositional systems and the character of the sequence-bounding unconformities.
The sediment yield generated by fluvial bedrock erosion in the orogenic drainage basins has the typically asymmetrical shape of a response curve (Chapter 2). Yield gradually increases during tectonic activity, and declines exponentially during tectonic quiescence. Syn-tectonic yields are increasing, but they are not sufficient to completely fill the accommodation space created in the adjacent foreland basin due to flexural response upon active tectonic thrust loading. As a result and counter-intuitively, deposition of alluvial fan gravels in the foreland basin retreats during tectonic activity, whereas progradation is characteristic of phases of tectonic quiescence and reduced flexural subsidence. A history of pulsating tectonic activity is reflected in the alluvial architecture of the basin as a succession of coarsening-up, prograding gravel sheets that laterally connect during quiescence, and alternate with basin-wide onlap of fine-grained sediments marking renewed tectonic activity.
The experiments of Chapter 3 show that there are two contrasting types of sequence boundaries developed in the alluvial stratigraphy when a eustatic sea-level variation is superposed upon the alternation of tectonic activity and subsidence:
A) During intervals of tectonic activity, eustatic fall and rise of sea level form prograding, shallowing up sequences, which are bounded by Type-2 unconformities and subsequent flooding surfaces. The syn-tectonic, high flexural subsidence rates prohibit the sea level to drop below the delta break in slope, safeguarding the stratigraphy from severe incision.
B) During intervals of tectonic quiescence, Type-1 unconformities are formed, because eustatic falls now drop below the delta break in slope, as they are no longer compensated by the subsidence component in relative sea level. Because multiple eustatic sea-level cycles may occur during a quiescence interval, the resulting Type-1 unconformity at the base of the delta-top sheet sandstone can be a composite and therefore poor time marker.
The suites of amalgamating, axial channel belts that characterize this delta top sheet sandstone have a preference for the depression between the basin-margin alluvial fans and the conical delta surface that was formed during a previous tectonic phase.
Similar suites of amalgamating axial channel belts are created when the foreland basin is detached from its substratum by a hinterland-dipping sole thrust and transformed into a thrust-sheet top basin (Chapter 4). In the light of these model results the Eocene Castissent Formation in the Pyrenean Tremp Basin, previously interpreted as a incised valley system, is explained as a phase of moderate tectonic reduction of the accommodation space. This mechanism explains the continuous marine influence on the delta plain simultaneous with a forced regression and increased sandstone interconnectedness at the cost of fine-grained intervals.
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