Early Proterozoic ( 1 . 9 Ga ) Thrombolites of the Rocknest Formation , Northwest Territories , Canada

Thrombolitic fabrics of the Rocknest Formation, Northwest Territories, Canada (Hoffman, 1975; Grotzinger and Hoffman, 1983; Grotzinger, 1986a, 1989b) were disputed by Kennard and James (1986) and Kennard (1989) and classified as cryptomicrobial boundstones. Under close re-examination, these rocks reveal mesoscopic and microscopic fabrics which are indeed thrombolitic when considered in the context of the recent tripartate microbialite classification scheme developed by Kennard and James (1986) and Kennard (1989). Thrombolitic fabrics from the Rocknest Formation demonstrate conclusively that, in cases of pervasive dolomitization, palimpsest fabrics can continue to provide evidence for the depositional environment of microbial communities. These thrombolites are significantly different from younger Proterozoic thrombolites (Aitken and Narbonne, 1989) and their Phanerozoic counterpoints (Kennard and James, 1986; Kennard, 1989), containing far less detrital sediment and greater amounts of marine cements, which suggests rethinking of models pertaining to the origin of thrombolites, as well as how microbial communities interact with their environment in the formation of characteristic diagenetic fabrics. Additionally, Rocknest thrombolites indicate that development of diagnostic, clotted fabrics may result from microbially-induced "inorganic" calcification of microbial communities, rather than by in situ calcification of microbial sheaths (Kennard and James, 1986; Kennard, 1989). This suggests a stronger reliance on physical and chemical aspects of depositional environments in the formation of thrombolitic fabrics than previously suggested. Thesis Supervisor: John P. Grotzinger Title: Assistant Professor of Geology