Incompetent Strata Research Articles

Influence of rock mass rating and in situ stress on stability of roof rock in bord and pillar development panels

The re-orientation of bord and pillar layout is dictated by the direction of the horizontal stress. However, the competence of strata expressed in terms of rock mass rating (RMR) and the magnitude of in situ stress have a persuasive influence in deciding the gain out of re-orientation of galleries. However, a few queries like: a) Is it essential to re-orient all bord and pillar workings vis-a-vis the direction of in situ stress?; b) What if the strata are competent and the workings are subjected to low magnitude of horizontal stress?; c) What happens to a bord and pillar workings affected by high horizontal stress and having incompetent strata? have remained unaddressed. This paper, therefore, addresses such queries through a new approach of classification of the bord and pillar development workings using numerical modelling. This classification system will aid field engineers in deciding the need for re-orienting galleries vis-a-vis the RMR and in situ stress.

The origin of anomalous fold patterns in the N.W. part of the South Wales Coalfield

Tectonic structures exposed in the Middle Coal Measures on the Glyn Glâs Opencast Coal Site are described. These are upright mesofolds with a NE-SW trend; meso and minor recumbent folds; and upright meso and minor N-S folds. All are buckle folds formed at a high structural level. The absence of such structures in the underlying and overlying strata is attributed to the greater competence of these beds. Although they form a deformation pattern similar to that of repeated folding in many fold belts, polyphase deformation is not regarded as being the cause of the structures. After comparison with structures elsewhere in the South Wales Coalfield they are regarded as being the result of main phase Variscan deformation in incompetent strata, the associated anomalous structures being due to localised accommodation in relation to penecontemporaneous sinistral tear faults.

The geometry of folded and thrusted rocks in the Rocky Mountain Foothills near Grande Cache, Alberta

Lower Cretaceous clastic rocks make up the Inner Foothills of the Rocky Mountains along the Smoky River near Grande Cache, Alberta. Shortening of these rocks is accomplished by folding and thrusting and averages 33% over the area studied.Folds in the area are of the chevron variety. They are caused by shortening of a multilayered sequence of alternating competent and incompetent strata, where the thickness of the competent layers is fairly constant. The folds are generally cylindrical and maintain their shapes over distances of up to 2 km along trend. However, at their tapering ends they are conical.The majority of the faults are southwest-dipping thrusts, displaying ramps that cut up stratigraphic section and flats that are parallel to bedding. The best exposed fault in the area is the Mason Thrust. Displacements along this fault increase from 150 m in the northwest to 1500 m in the southeastern part of the area. A prominent anticline below the Mason Thrust diminishes gradually towards the southeast. A conical fold with a half apical angle of 7° describes this geometry. Shortening by this fold decreases from about 30% in the northwest to 10% in the southeastern part of the area. This indicates that a decrease in shortening by folding is compensated by an increase in thrust fault displacement. It is an example of fault-to-fold displacement transfer.

Mesozoic volcanics of western sicily

The Mesozoic lavas and minor intrusions in the thrust sheets of western Sicily have the following characteristics: 1. The lavas in the Triassic Mufara Formation in the north were broken into fragments which rotated independently within the incompetent strata that enclose them. This behavior is characteristic of igneous rocks found within the more internal (northerly) thrust units. 2. The Jurassic lavas in the more external (southerly) units have consistent directions which agree with those of the Ammonitico Rosso limestones in the same zone and lie about 30° clockwise from those of coeval autochthonous formations in Tunisia.Schult's presumed Cretaceous directions from Custonaci on the north coast (similar to those found in the Cretaceous Scaglia Rossa at Terrasini to the east byChannel et al., 1980) are rotated still more (140°) with respect to those of the autochthonous Iblean platform of SE Sicily. These differences are believed to reflect rotation of the thrust sheets during tectonic transport in Cenozoic times, the internal units being the most strongly rotated. 3. All the igneous rocks are highly altered: generally the original mineralogy cannot be completely determined. Relative abundances of some of the less mobile elements (Ti, Sr, Y) suggest that they are intraplate basalts.