Alexander Pucci Honours Thesis Abstract

The 2.8 Ga Archean Steep Rock Group of north-western Ontario, Canada, contains one of the oldest well-preserved carbonate ramps in the world.  The Mosher Carbonate is host to a number of unique ancient depositional textures.  The purpose of this thesis is to perform a thorough analysis of the depositional textures of carbonate precipitates in the Steep Rock group’s Mosher carbonate.  These textures include: columnar stromatolites, microbial mat and fenestrate microbialites, sheet cracks, and crystal fans.  Columnar stromatolites are found in walled and unwalled varieties.  Microbial mat, microbial supports, and carbonate-filled void spaces in differing ratios make up fenestrate microbialites.  The most common form of fenestrate microbialites at Steep Rock are tented microbialites and “net-like” microbialites.  These are a result of a lesser influence from microbial supports and void spaces and domination by horizontal filmy laminae.  Sheet crack structures are common and have shown up to 4 stages of successive growth in some samples.  Crystal fans at Steep Rock form distinctive giant domes, alternating in deposition with fenestrate microbialites.  Fans grow in all directions, though downwards is uncommon and lateral growth is blocked early on by other fans.  Crystals grow preferentially from the tops of microbial supports, and in some cases from a darkened surface that may represent dissolution.