Abstract

Many sections of carbonate rocks exhibit abrupt changes from limestone to dolomite both vertically and laterally. The distribution pattern may be controlled by either the availability of a dolomitizing water or the physical-chemical character of the rock. The Turner Valley Formation exposed at Moose Mountain, Alberta, Canada, has been dolomitized where the original sediment was deposited as a carbonate mud with varying amounts of floating sand-sized and larger particles. Partially dolomitized limestones and possibly some completely dolomitized rocks were deposited as carbonate sands with interparticle lime mud. Those limestones which received less than 15 per cent dolomite were deposited as mud-free carbonate sands. The selectivity is believed to be due to permeability differences after predolomitization diagenesis or the greater reactivity to dolomitizing water of fine-grained sediment.

Dolomite distribution is controlled by the distribution of lime mud. To understand the sedimentologic factors relating to the distribution of the lime mud, we have divided the carbonate rocks into two general categories depending on whether or not the sand-sized and larger particles show evidence of current transport. The mud-free carbonate sands in this study are considered to have been deposited from tidal currents. The sand-sized and larger particles of the mud-bearing dolomitic limestones and dolomites are interpreted as having been deposited essentially at their place of origin—noncurrent deposited. The ratio of sand-sized and larger particles to lime mud within the noncurrent-deposited sediments was controlled by the competing roles of particle production and lime-mud deposition.

The current-deposited crinoidal sands and the noncurrent-deposited muddy sediments are visualized as being deposited contemporaneously. The current-deposited sands were derived from the reworking of noncurrent-deposited sediments by northeast-southwest-trending tidal currents. The crinoids and blastoids grew in greatest abundance near the edge of the areas of current deposition. The reduction in their population and the reduction in current velocity away from the areas of current deposition resulted in the reduced number of sand-sized particles and the increased amount of mud deposition.

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