ABSTRACT

Chert sharpstone breccias are preserved in some stratigraphic sections of the Kananaskis Formation (Pennsylvanian) of the Rocky Mountain Group in the Banff area, Alberta. The breccias incorporate, or occur immediately above, or above and below, an organic chert whose fauna dates the formation (McGugan & Rapson, 1960). In other sections a mature chert conglomerate is developed and contains numerous sharpstone fragments. Similar sharpstone fragments are mixed with slump breccias of laminated dolomitic sandstone.

Petrographic observation indicates the following sequence of events:

Chert Breccias. (1) Preservation of the microfauna in a silty carbonate mud. (2) Replacement of the calcareous organisms by silica and polymerization of silica around or within the organic debris to form a gel "nodule" or "lens." (3) Syneresis or contraction of the latter to give a brecciated surface to the "nodule" or "lens." (4) Crystallization of the silica and cementation of the primary cryptocrystalline breccia by microcrystalline silica, the whole still in an unconsolidated silty carbonate mud. (5) Selective replacement of the microcrystalline silica by dolomite with consequent isolation of the chert splinters. (6) Further replacement of microcrystalline and cryptocrystalline silica by dolomite, with slight displacement of the fragments and the introduction of detrital ma erials. The stages can be traced outward from the center of the organic chert nodules. Brecciation must have taken place "in situ" before complete consolidation of either matrix or nodule.

Breccio-conglomerates. The sharpstone fragments were removed from some areas of formation, transported without rounding and mixed with (a) a pre-existing mature "gravel," (b) a penecontemporaneous slump breccia.

A third type of silicification cements both round and sharpstones and apparently replaces the dolomite matrix.

The geochemical and environmental conditions necessary for such phenomena are discussed. The environment suggested is an evaporitic one cut by local distributaries and modified by a retreating sea. A simile is drawn with the present-day environment along the southeast coast of South Australia (Alderman & Skinner, 1957).