Fold and thrust structures in the Kintradwell ‘Boulder Beds’, Moray Firth

Abstract

Synopsis The Kimmeridgian shales, sands and ‘boulder-beds’ exposed at Kintradwell Point in Sutherland display a complex series of ‘soft-sediment’ deformation features, which can be assigned to four categories, on the basis of both geometry and temporal relationship. Small, closely-spaced, syn-depositional normal faults, throwing down to the S/SE. Slump folds with intensely curvilinear hinges, produced by differential movement within a wet-sediment pile slumping to the south, obliquely away from the Helmsdale Fault. ‘Thrust’ faults together with related low-angle slides and extensional faults. These gravity-driven structures moved perpendicular to and away from the Helmsdale Fault. They represent a section through the leading edge of a large, detached gravity-slide. Isolated, down-to-the-SE, brittle normal faults, tens of metres in length. These offset slump-fold and slide structures. Structures belonging to category 4 may be the product of tectonic strain related to the opening of the Moray Firth Basin. The remainder are all thought to be local, gravity-driven structures, generated on a late Jurassic palaeoslope dipping away from the Helmsdale Fault. Many geometric analogues for the Kintradwell structures are found not only in slump sheets documented elsewhere but also in the internal parts of contractional orogenic belts. Familiarity with the analysis of such ordered structures from within orogenic belts such as the Caledonides of the Scottish Highlands, facilitates an understanding of the section at Kintradwell. Sequences similar to that at Kintradwell have been discovered during offshore hydrocarbon exploration in the North Sea. The observations presented in this paper have implications for the analysis of cores from such sequences.

The structural and diagenetic evolution of injected sandstones: examples from the Kimmeridgian of NE Scotland

“…Surrounding beds of host sandstone are also calcite cemented. Synsedimentary folds in the host rocks (Roberts 1989) have not deformed the dyke and fold axes can be traced across the dyke without shear movement, illustrating the pure opening mode of the composite dyke (Fig. 2).…”

“…2). It was intruded into a succession of thinly bedded sandstones and carbonaceous silt and mudstones (Bailey & Weir 1932;Roberts 1989). The source for the sandstone dyke is not exposed.…”