Synsedimentary Tectonics

Abstract

Two minor normal faults that were obviously active during sedimentation were photographed by the author, and figured in Gygi and Persoz (1986, Figs. 2 and 8). One of these faults is in the carbonate Pichoux Formation on the eastern wall of Combe des Geais north of the village of Grandval, east of Moutier in Canton Bern. The fault is shown in Fig. 8 of the paper mentioned, but only part of it is visible almost exactly in the center of the photograph. The fault is clear-cut, although it evolved in probably unlithified calcareous mud without evidence of early lithification. The calcareous mud, which was laid down south of the fault (to the right in the photograph), was sedimented in the form of a conspicuous wedge. The wedge-like configuration of the mud deposited directly adjacent to the active fault is evidence that the relief created by the fault was continuously equalized by sedimentation of calcareous mud. The vertical displacement along this synsedimentary fault in the Pichoux Formation is approximately 2 m. Another synsedimentary fault figured by Gygi and Persoz (1986, Fig. 2) was temporarily visible in the upper part of the Effingen Member on the western wall of the quarry of Jakobsberg east of Auenstein in Canton Aargau. The maximal vertical displacement by this fault, which is shown here in Fig. 4.12, is approximately 30 cm. It is evident from Fig. 4.12 that movement along the clear-cut fault going on in unconsolidated mud was gradual and very slow. The maximal displacement that is visible at the base of the photograph was reached after an estimated, compacted sediment thickness of 5 m was laid down until the activity of the fault died down. The sedimentation rate of the Effingen Member probably never exceeded 5 m of deposit (now compacted) per 10,000 years (see above). The small synsedimentary fault shown in Fig. 4.12 was, therefore, gradually active during a time span of at least 10,000 years. A shallow-rooted growth fault could have been the cause of the figured synsedimentary displacements near both Grandval and Auenstein.