Discontinuity surfaces in limestones

Discontinuity surfaces (referred to in the literature also as surfaces, pits, and zones of corrosion and as emersion surfaces) indicate non-deposition and breaks in the sequence. These structures have been described from many different areas in Europe and North America in Iimestones ranging from Ordavieian to early Tertiary. Morphology and stratigraphic occurrence and significance of selected discontinuity surfaces are briefly discussed. The origin of these surfaces has been ascribed to either subaerial exposure or exclusively submarine agencies. Many discontinuity surfaces show clear evidence of having been lithified prior to the deposition of the overlying beds. Submarine lithification of carbonate sediments at or close to the surface by calcite or aragonite cement is not known, and no theoretical explanation can at present be given for its occurrence in a comparable scale at depths less than 1 so m below sea bottom. In tropical and subtropical latitudes the subaerially exposed earbona te sediments often show a considerable degree of lithification. This and the stratigraphic evidence, particularly the connection of certain discontinuity surfaces with undoubted land surfaces, make it highly probable that the surfaces which can be shown to have been lithified before the deposition of overlying beds have been subaerially exposed and represent emersion surfaces. Discontinuity surfaces occur in areas with an extensive leve! bottom of the ancient sea. Their areal distribution in a palaeogeographic sense indicates mostly islands and peninsulae. The extremely low altitude of these land areas and the probable ciaseness of the base leve! of karst erosion to the land surface have prevented the formation of karst phenomena. There scarcely exists any soil upon comparable Recent areas. The discontinuity surfaces evidently obtained their final shape during and after the subsequent submersion by possible corrosion in the intertidal zone, scouring and polishing effect of calcarenitic grains in suspension, boring and enerusting organisms, and by other agencies. A small-seale Recent discontinuity surface is briefly described. Breaks in a carbonate sequence without any lithologic evidence of a break may be due to submarine non-deposition, erosion, or both. It is suggested that such breaks may have been formed also by subaerial exposure in a cool elimate and without lithification of the emerged sediment. Distinct surfaces with or without a stain can be formed also in exclusively submarine en viranment. Same of them indicate breaks and hence represent discontinuity surfaces. The need for detailed descriptions of the morphology of discontinuity surfaces is stressed.