Abstract

Prismatic crystal clusters occur in dark, organic-rich, bioturbated lime mudstone and wackestone of the Lower Carboniferous (Visean) Mount Head Formation in the southern Canadian Rocky Mountains. Clusters are typically rosettes and irregular aggregates composed of prisms of calcite, chert or silica. Rosettes and clusters are 3-10 centimetres in diameter, while individual elongate and distally widening prisms are 2-6 centimetres long. Prisms have diamond shaped cross sections normal to length. Terminations are slightly offset from the prism’s long axis. Prisms also have dense walls with granular calcite microcrystal interiors, internal cruciform blades parallel to the long axis, and internal ribs normal to length.

The structures observed in the prismatic crystal clusters are similar to those found in thinolite, a known calcium carbonate pseudomorph of the mineral ikaite (CaCO₃⁶H₂O). Thus, we propose that prismatic crystal clusters from the Mount Head Formation are pseudomorphs after ikaite. At the planet’s surface ikaite is a metastable carbonate mineral that requires temperatures lower than 7°C, typically between about 7°C and 0°C, to form and stabilize; at surface conditions threfore, it is a useful mineral thermometer. In the sedimentary geologic record, as a syndepositional precipitate it is a practical paleothermometer.

Significantly, in the Mount Head Formation prismatic crystal clusters occur as syndepositional, authigenic growths in carbonate rocks that accumulated below fair weather wave-base on a low gradient, low-latitude carbonate ramp. The occurrence of pseudomorphs after ikaite indicates that areas of this Carboniferous subequatorial ramp were influenced by cold to very cold bottom waters. Cold waters may have been introduced into this setting from polar or subpolar regions through oceanic currents, or through extensive upwelling of deep oceanic waters.