ABSTRACT

A laminite deposit consisting of calcareous, dolomitic, and anhydritic layers interleaved with black organic material floors interior parts of the Middle Devonian Winnipegosis salt basin of western Canada.

Before compaction had affected it, part of the anhydritic laminite was diagenetically altered to coarsely crystalline calcite that enclosed the anhydrite and organic matter. This calcitized rock preserves features of the original sediment, and reveals that the laminar anhydrite had originally grown as anhydrite nodules in an organic substrate. In all essential respects, the crystal fabrics of the calcitized anhydrite nodules resemble those of Recent sabkha anhydrites, and the mode of growth was apparently identical.

Enterolithic structure resulted from buckling of sheets of coalesced nodules, and was caused by continued growth of the nodules within confining laminae.

Observed varieties of anhydrite fabric can be explained as products of compaction. The agency of pore-water pressure is important to the generation of flow fabrics. The range of anhydrite compaction-fabrics in general can be related to two matrix properties: compressibility and drainage.

The original sediment, before invasion by anhydrite or calcitization, resembled blue-green algal mat. Volumetrically, the organic material comprised a large fraction of the sediment even after anhydrite-nodule growth, though later compaction reduced it to black partings where it was not calcitized.

Stratigraphically, the presence of algal mat and nodular anhydrite on the basin floor implies nearly complete loss of water from the basin after Winnipegosis reef formation, and before halite accumulation supervened.