Abstract

The development of evaporite-solution breccias may take place by solution of the evaporite and production of large voids, followed by collapse of the nonevaporite rock. Study of solution breccias, however, in which all stages of breccia development are preserved indicate that during evaporite-solution brecciation gradual subsidence, evaporite flowage, and small-scale precipitation may occur simultaneously with gross evaporite removal; thus cavern formation and roof collapse may be relatively unimportant. As evaporite dissolves, overlying and interbedded nonevaporite strata subside and fragment; evaporite flows so that rock fragments are floating in it; a grain-supported mass gradually forms; and finally, an evaporite-free breccia.

The breccias studied in cores from the Upper Devonian Leduc Formation, Alberta, Canada, consist of angular dolomite clasts in a matrix of either dolomite or anhydrite. The clasts range from single dolomite crystals and rock fragments a few tens of microns in diameter to large cobbles. In some cores the clasts float within the anhydrite matrix; in others, the anhydrite is less abundant and the clasts are in grain–to–grain contact. In anhydrite-free breccia, the dolomite matrix is composed of fine clasts equivalent to those floating in the matrix of anhydrite-bearing breccia. Many clasts are broken, but the fragments have been held in relative position in the matrix and can be identified. The basal contact of the breccia is sharp. The breccia grades upward into highly fractured in situ rock.

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