Structural analysis of soil microrelief in palaeosols of the Lower Jurassic “Laterite Derivative Facies” (Mishhor and Ardon formations) Makhtesh Ramon, Israel

Abstract

A sequence of palaeosols developed on exposed overbank sediments of successive alluvial cycles of the Lower Jurassic “Laterite Derivative Facies” (LDF) at Makhtesh Ramon, contains well-developed, penecontemporaneous soil microrelief structures. The folded structures commonly show overthrusting, and vary in wave length and amplitude. They are characterized by slickensided surfaces, cuspate in cross section with the curviplanar deformed surfaces oriented concave up. The palaeosol stratigraphy is well established over the entire Makhtesh Ramon area and this afforded the opportunity of obtaining controlled structural measurements on the folded surfaces and their associated slickensided striae as a means of providing data for reconstruction of the fold geometry. Stereographic projection of the poles to the LDF fold surfaces, on two separate palaeosol horizons from several localities, together with the slickensided striae, were plotted on the lower hemisphere of a Schmidt equal-area net. The LDF folds were seen to be symmetrical and asymmetrical structures with a gently plunging fold axis of 4–10°. Maximum and minimum dip values of the cuspate curviplanar surface (SP 1) were well emphasized by the plots (20° and 30° for symmetrical structures) and data from the two horizons obtained from several localities showed remarkable uniformity. A second slickensided, curviplanar surface of deformation (SP 2) shows the same range of dip values as SP 1 and intersects the SP 1 surface normal to the SP 2 fold axis. The internal arrangement of symmetry elements within the LDF folds remains approximately constant regardless of the large variation between the gross morphological parameters such as amplitude and wavelength, and this appears to be a reflection of the response of a soil with inherent anisotropies, to stress. The geometry of the LDF structure resembles that of linear or wavy gilgai. The SP 1 surface therefore is most likely the result of upthrusting on the edges of drainage rills affected by differential wetting. The SP 2 surface bears a constant geometric relationship with SP 1 and is therefore considered to have formed penecontemporaneously during the upthrusting of the SP 1 blocks. A rectangular surface fracture pattern parallel to and normal to topographic slope, is considered to be an alternative means of initiating the upward growth of the soil microrelief features.