The Wishart Formation of the Labrador trough (early Proterozoic)

End_Page 992------------------------------

is a high-energy, terrigenous shelf deposit. Chert is a minor but widespread component of the Wishart and occurs in three main forms: (1) interstitial cements consisting of a mosaic of fine quartz crystals; (2) thin layers and lenses of very fine-grained chert lutite with finely disseminated impurities; and (3) peloids to flat pebbles of fine-grained chert lutite. The clasts of chert lutite are clearly intraformational; they are closely associated with and texturally identical to the in-situ chert layers. Chert cements are also found in intraclasts. All of the chert types are restricted to a few thin intervals which form good marker beds, and none show any signs of replacement. These observations indicate the cherts are primary siliceous deposits. Based on their textures and the sedim ntary structures in which they are found, the cements must have formed as rigid precipitates; the lutites as mud-like, slack-water accumulations; and the peloids to pebbles as rip-up clasts. The silica was probably hydrothermal in origin because the stratigraphic distribution of the chert is independent of facies and there is no evidence of basin restriction or evaporative conditions. The Fleming Formation, which lies directly beneath the Wishart, may have been the source of the postulated silica-rich hydrothermal waters. The Fleming consists mostly of brecciated and silicified rocks and an abundance of crystalline, void-filling quartz. The Wishart cherts offer proof that siliceous sediments and cements can accumulate rapidly in terrigenous marine environments. They provide an example of one mechanism for making the similar chert peloids which are abundant in early Proterozoic iron-formations.

End_of_Article - Last_Page 993------------