Abstract

More than 300 individual occurrences of discontinuity surfaces were examined in well cores of limestone of the Beaverhill Lake Group in the Swan Hills area of central Alberta. Planar to bumpy surfaces with truncated shells, organic borings, and associated pyrite, glauconite, and intraclasts grade morphologically into bedding planes. Irregular reentrants, up to 15 cm deep and infilled with sediments lithologically similar to the overlying rocks, can be interpreted as burrows and/or solution cavities. Many of the discontinuity surfaces are present within burrowed carbonate mudstone of the Waterways Formation, which contains a brachiopod-gastropod-echinoderm-ostracod fauna typical of normal-marine, subtidal environments. Although only few Waterways surfaces can be correlated in that they form the upper and lower boundaries of the House Mountain-Deer Mountain reef complex (Swan Hills Formation).

The presence of discontinuity surfaces within subtidal, sub-wave-base limestone beds suggests that they are products of submarine lithification and erosion (mechanical, chemical, and biologic). These processes must have acted periodically to form hard clean areas of tens to hundreds of square miles of sea floor. Change in the normal circulation pattern of currents possibly triggered these processes and led to formation of discontinuity surfaces. A discontinuity surface at the upper contact of the reef complex indicates that rapid submergence, rather than emergence, probably terminated reef growth in the House Mountain-Deer Mountain area. The discontinuity surface just below the reef complex shows that reef growth was initiated from a hard surface.

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