Basin Research - Volume 3, Issue 3, 1991

Three end‐member models of half‐graben development (detachment fault, domino‐style, and fault growth) evolve differently through time and produce different basin‐filling patterns. The detachment fault model incorporates a basin‐bounding fault that soles into a subhorizontal detachment fault; the change in the rate of increase in the volume of the basin during uniform fault displacement is zero. Younger strata consistently pinch out against older synrift strata rather than pre‐rift rocks. Both basin‐bounding faults and the intervening fault blocks rotate during extension in the domino fault block model; a consequence of this rotation is that the change in the rate of increase of the volume of the basin is negative during uniform extension. Basin fill commonly forms a fanning wedge during fluvial sedimentation, whereas lacustrine strata tend to pinch out against older synrift strata. In the fault growth models, basins grow both wider and longer through time as the basin‐bounding faults lengthen and displacement accumulates; the change in the rate of increase in basin volume is positive. Fluvial strata progressively onlap pre‐rift rocks of the hanging wall block, whereas lacustrine strata pinch out against older fluvial strata at the centre of the basin but onlap pre‐rift rocks along the lateral edges. These fundamental differences may be useful in discriminating among the three end‐member models. The transition from fluvial to lacustrine deposition and hanging wall onlap relationships observed in numerous continental extensional basins are best explained by the fault growth models.

The stratigraphy of the Eocene‐Miocene peripheral foreland basin in Switzerland consists of basal deposits of Nummulitic Limestones and Globigerina Marls representing a phase of deepening, followed by two shallowing‐up megacycles culminating in fully continental sedimentation.

The onset of sedimentation was diachronous and took place on an unconformity surface with increasing stratigraphic gap to the north and west. In the Ultrahelvetic units, which were derived from the south and have a provenance between the Helvetic shelf and the Penninic ocean, the stratigraphic gap is minimal. This restricts the initiation of erosion of the southern European margin due to emersion to post‐Maastrichtian and pre‐late Palaeocene. This coincides with the final closing of the Valais trough and may therefore be interpreted as the point at which continental flexure s. s. started. In the autochthon, the subcrop map of the unconformity surface shows that the regional pattern of subcropping units is oblique to both neo‐Alpine tectonic structures and Helvetic (Mesozoic) passive margin structures. There are local zones of disruption to the broad regional pattern suggesting that the basal unconformity was corrugated. Both the paliaspastic restoration of the autochthon relative to the thrust front during the Palaeocene, and the regional pattern of erosion indicate that the basal unconformity may be due to erosion of a flexural forebulge.

Following deposition of the shallow water Nummulitic Limestones and the deeper water Globigerina Marls, clastic sediments were shed from the orogenic wedge in the south. These turbidites, the Taveyannaz Sandstones, filled both ponded basins at the contemporaneous thrust front and the frontal trench or foredeep. Evidently, early thrusts drove at a shallow level into the embryonic basin as ‘front‐runners’, whereas most shortening and uplift continued to take place within the main part of the orogenic wedge further to the south. Eventually, the frontal palaeohighs, together with the turbidite basins, were buried by the northward emplacement of surface mud‐slides, and sediment depocentres were translated northwards onto the foreland.

The most likely cause of the underfilled ‘Flysch’ stage is the rapid advance of a submarine thrust wedge over the flexed European plate which resulted in (i) low sediment fluxes and (ii) high subsidence rates associated with the rapid migration of the load and depocentre. Later, as the rate of advance slowed and the wedge became subaerially exposed, the basin rapidly filled with coarse‐grained detritus representing the ‘Molasse’ stage.

By calculating mass accumulation rates for foreland basin sediments, the changing capacity of the basin can be monitored through time. It has often been assumed that there was a direct link between foreland basin sedimentation and tectonic deformation and lithospheric loading in the adjacent orogenic belt. The results of this study suggest that tectonic deformation is most likely associated with the changing capacity of the basin and the rate at which sediments accumulate within it, However, there appears to be no relation between tectonic deformation and the lithology of sediment which accumulates in the foreland basin. Instead, eustatic sea‐level fluctuations appear to have significant control, through their impact on water depth, on the lithology of sediments accumulating in the foreland basin. These relations are evidenced by mass accumulation rates calculated for foreland basin strata in north‐west Alberta and north‐east British Columbia, Canada.