Correlation between fracture surface morphology and orientation of cross-fold joints in Eocene chalks around Beer Sheva, Israel

Three fracture surface morphology styles occur on cross-fold joint surfaces in the Lower Eocene chalks. These are horizontal coarse straight axial plumes (with barb widths that vary from 0.5 cm to several centimeters), circular undulatory markings and transitional variation between the two. Occasionally, the three types occur in close proximity in the same outcrop. About 56% of the 225 fracture markings examined in four outcrops are circular and occur on fracture surfaces of set 328° (N32°W) and 29% are axial plumes that occur on surfaces of 309° and 344° joint sets. Transitional patterns make up 12% and about 3% are other combinations. A fourth surface morphology style occurs exclusively on joint surfaces of Middle Eocene chalks. This type is defined as horizontal delicate straight axial plume (with barb width below 0.5 cm). This type is associated with single-layer joint sets 031° and 357°. Hence, certain styles of fracture markings show affinities with particular joint sets. It is suggested that whereas the circular markings develop when the minimum principal stress σ_x is horizontal and normal to the joint surface and horizontal σ_y = vertical σ_z, the axial plumes represent the same conditions with respect to σ_x but with σ_y > σ_z.