Microfabrics of a recumbent fold in cross-bedded sandstones

Abstract

Plates 1-2)Summary. A thin-section study of the orientation of long-axes of quartz grains was conducted for a similar type fold of overturned cross-bedding in Carboniferous sandstones. Preferred orientation of detrital grains of the fold shows that (a) apparent long-axes of grains are parallel to the axial plane in a plane normal to the fold axis, {b) long-axes of grains in the axial plane are oriented in a high angle to the fold axis, and (c) grains are aligned parallel to the fold axis in a plane perpendicular to the other two directions. From the dimensional orientation of quartz grains in this sample, it is concluded that the overturned fold was made by a shear mechanism immediately following sedimentation in which the original quartz grains were reorientated by small-scale slippage along the shear planes.

Deformed cross-stratified deposits in the Early Pleistocene tidally-dominated Catanzaro strait-fill succession, Calabrian Arc (Southern Italy): Triggering mechanisms and environmental significance

“…Two main genetic mechanisms have been invoked for the Type I and Type II deformation of cross-stratified deposits (see a complete review in Wells et al, 1993): (i) sliding of sediment down the frontal surface of an advancing cross-stratified sand body (Rice, 1939;Rust, 1968;Puga-Bernbabèu et al, 2010); and (ii) current drag action on the underlying liquidized cross-stratified sets (Robson, 1956;Allen and Banks, 1972;Owen, 1987Owen, , 1996. The trigger mechanism for the complete liquefaction or decrease of shear strength in the crossstratified units is under debate too being be interpreted as result of: (a) earthquakes (Selley, 1969;Allen and Banks, 1972;Mazumder et al, 2006); (b) wave action (Dalrymple, 1979); (c) overloading/sand-laden flood (Coleman, 1969;Hendry and Stauffer, 1975;Yagishita and Morris, 1979); (d) flow regime changes as in the dune/plane-bed transition (Røe and Hermansen, 2006); and (e) sudden changes in the groundwater level (Williams, 1970).…”