Cover folding in the Sonmartel Chain (Jura Neuchatelois)

Sonmartel forms part of the anticlinal Cornu-Foulets Chain of the Neuchâtel High Jura. The strata exposed in the district include much of the Malm (Argovien to Purbeckien), the Lower Cretaceous (Berriasien to Hauterivien) and some Tertiary rocks (Siderolithique, Helvetian Miocene). An outline account is given of the Mesozoic formations, the distribution of marker bands being shown on the map (Pl. 1). The structure of the area is discussed in terms of its constituent parts. The folding is concentric but due to erosion-breaching following the earliest movements, subsequent deformation of the Pre- and Post-Kimeridgien rocks differed. The former are gently diapiric, while the folding of the latter is, in part, a gravity effect. Faults normal to the fold axes are present everywhere. They originated as tension cracks but later permitted transcurrent movements. Tear faults are best developed in regions where the trend of the fold axes changes. Thrusts are restricted to the Grand Sonmartel area. The joint systems are thought to result from stresses due to bed-over-bed sliding during the folding. The different types of deformation combine to produce a rhomboidal pattern of dissected cross-folds. Each of these strain-phenomena show that the region has been subjected to stresses of two different orientations, with a third also affecting the vicinity of Grand Sonmartel. These owe their origins to relative movements of basement blocks beneath the area — thrusting during the Post-Stampian phase, the thrusts re-activated as sinistral shears during the Post-Pontian phases when they were joined by conjugate dextral shears. On this basis the Alpine Push theory for the origin of the Folded Jura must be rejected. The Wegmann hypothesis of basement shears accounts best for the structures observed.