Mount Cap formation

The Mount Cap formation is a geological unit exposed in the Mackenzie Mountains, northern Canada. It was deposited in a shallow shelf setting in the late Early Cambrian, and contains an array of Burgess Shale-type microfossils that have been recovered by acid maceration.

The formation is one to three hundred metres thick, and comprises shales, silt- and sand-stones with a high glauconite content. It has been exposed to remarkably little metamorphic activity given its great age; it is dated to the Bonnia–Olenellus Trilobite Zone. This zone lies within the Lower Cambrian Waucoban stage in North America, which is equivalent to the Caerfai in Wales, and thus the Comley of England, and has yet to be formally ratified. Nevertheless, this makes it just younger than the earliest trilobites, and thus the earliest known Burgess Shale-type deposit, though this is disputable when considering the age of Chengjiang County fauna. Its organic-walled fauna, known as the "Little Bear biota", includes both non-mineralized and originally-mineralized taxa, including hyolith and trilobite fragments, anomalocaridid claws, arthropod carapaces and brachiopods.