ABSTRACT

Massive pure calcilutite mounds in the Tully Limestone contain the distinctive carbonate spar-filled, smooth-bottomed, unsheltered cavities known as stromatactis, for which this report offers a possible inorganic origin.

Contrasting with continual uniform compaction observed in more argillaceous sediments, compaction in the Tully mounds apparently was nonuniform, local, and occurred only during initial unmixing of water from an originally partly water-supported accumulation of pure lime mud. Sediment collapse in local areas of loosely packed lime mud formed cavities bounded along the top by irregular surfaces that separated more tightly packed coherent mud above, from loosely packed incoherent mud below. The collapsed mud flowed until supported, either in place or through a network of connected voids, and left water-filled smooth-bottomed cavities that are called stromatactis when filled with spar.

Shapes of loosely packed zones, positions of larger grains and so forth, contributed to differences apparent in stromatactis. Some differences in size, associated structures, and spar mineralogy are related to stage of dewatering of the mound in which the cavity was preserved. Many small cavities were filled early by various forms of calcite spar. Large connected cavities containing pebbles of more coherent mud and fossil fragments served as channels for later water movement; these and even later desiccation cracks joining them were filled subsequently by ferroan dolomite spar.

This hypothesis of stromatactis formation may explain the occurrence of similar features in other pure calcilutite mounds, and obviates the need for postulating either total obliteration of rigid skeletons or decay of soft organisms as a cause where no definite evidence exists.