INTRABASINAL THRUST-TECTONIC VERSUS CLIMATE CONTROL ON RHYTHMICITIES IN THE EOCENE-SOUTH PYRENEAN TREMP-GRAUS FORELAND BASIN - INFERENCES FROM FORWARD MODELING

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 (1 0-100 kyr scale) changes in sediment flux due to orbitally induced cli mate changes and related fluctuations in precipitation and vegetation (De Beer et al., 1991), and-as an alternative control-intrabasinal fau lt activity. We show that pulsating intrabasinal uplifting related to deep-seated thrusting may serve as an alternative to climate-induced f luctuations in sediment flux, provided that thrusting is cyclic. In th e models, faulting between the source area and the site of the rhythmi c succession causes temporal intrabasinal uplift, leading to contempor ary erosion of the uplifted area and to a nearly instantaneous redistr ibution 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 chan ges in sediment flux do occur. The models also provide new insights in to the dominant geodynamic mechanisms during deposition of the Eocene sequence. To explain the observed sediment thickness in the studied se quence, either a sea-level rise, accompanied by relatively high transp ortation coefficients (larger maximum erosion rates-millimetres per ye ar), or orogenic wedge growth, accompanied by smaller transportation c oefficients (lower maximum erosion rates-tenths of millimetres per yea r) has to be adopted. In the case of the eustasy scenario, it is predi cted 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.