Intrabasinal thrust-tectonic versus climate control on rhythmicities in the Eocene South Pyrenean Tremp-Graus foreland basin: inferences from forward modelling

• Published in: Tectonophysics 1995-09, Vol.249 (1), p.93-107
• Main Authors: Peper, Tim, de Boer, Poppe L.
• Format: Article
• Language: English
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/0040-1951(95)00009-C

Small-scale (10 m) sedimentary cycles of alternating fine-grained and coarse-grained sediments occur in 40–60-m-thick fan-delta sequences in Eocene deposits in the Tremp-Graus foreland basin. These deposits are investigated by means of numerical forward modelling of short-term (10–100 kyr scale) changes in sediment flux due to orbitally induced climate changes and related fluctuations in precipitation and vegetation (De Boer et al., 1991), and—as an alternative control—intrabasinal fault activity. We show that pulsating intrabasinal uplifting related to deep-seated thrusting may serve as an alternative to climate-induced fluctuations in sediment flux, provided that thrusting is cyclic. In the models, faulting between the source area and the site of the rhythmic succession causes temporal intrabasinal uplift, leading to contemporary erosion of the uplifted area and to a nearly instantaneous redistribution of sediment in a thin veneer across a wide area. This reduces the effect of flexural subsidence due to fault activity, so that major lateral facies shifts through time are absent, while significant changes in sediment flux do occur. The models also provide new insights into the dominant geodynamic mechanisms during deposition of the Eocene sequence. To explain the observed sediment thickness in the studied sequence, either a sea-level rise, accompanied by relatively high transportation coefficients (larger maximum erosion rates—millimetres per year), or orogenic wedge growth, accompanied by smaller transportation coefficients (lower maximum erosion rates—tenths of millimetres per year) has to be adopted. In the case of the eustasy scenario, it is predicted that the basin thins to the north. In case of the orogenic wedge growth scenario, a northward thickening of the basin fill is predicted, unless a subsurface triangular fault zone is adopted.