Christopher Yip's Honours Thesis Abstract

Thesis Title: 
Sedimentology and Geochemistry of a Regressive Surface in the Chemical Sediments of the Paleoproterozoic Gunflint Formation

The 1878 ± 1Ma Gunflint Formation is a chemical-sedimentary unit deposited in the Animike Basin; it shows a sequence of transgressive-regressive cycles.  Wolff (1917) and Broderick (1923) divided the Gunflint into several individual members; lower cherty, lower slaty, upper cherty and upper slaty.  These members were then grouped into two different sequences; the upper and lower sequences.  The first and most extensive transgressive-regressive cycle is made up of the lower cherty member, while overlying transgressive-regressive cycles are made up of the lower slaty member, the lower cherty member and the upper slaty member.

Two separate outcrops found near Mink Mountain UTM: 329,520 E/5,338,163N and off the Magnetic Rock Trail in Minnesota662034E/5329885N were examined and sampled.  These two outcrops show a complete section through the peak lower regressive-transgressive sequence.  Detailed stratigraphic columns were constructed from the logs takes through these sequences, which were divided into three main units; 1) a grainstone unit forms the bottom unit and lies directly below 2) stromatolites which are capped off by a 3) oncolithic unit.  At the outcrop behind Mink Mountain the grainstone directly below the stromatolites is brecciated and shows injection of jasper and hematite throughout.  Microscopically the grainstone unit is composed of angular to rounded grains of chert.  The cement is predominantly chert with some blocky quartz found forming at grain boundaries.  The stromatolite unit is present above the pre-lithified grainstone unit and contains distinct stratiform and columnar stromatolites.  The top unit is an oncolith-rich grainstone.  The grains have a nucleus composed of either microquartz-rich chert or blocky quartz.  The cement of the unit is composed of a combination of a chalcedony-rich chert and blocky quartz.  Samples were crushed and digested for ICP-AES analysis for major oxides and trace elements.  Several samples were collected up through this section and sent to the OGS lab in Sudbury for ICP-MS analysis of rare earth elements.  The results were normalized to Taylor and McLennan (1985) Post Archean Australian Shale values and plotted.  All the samples taken from the grainstone layer and three samples taken from the stromatolites show a characteristic europium anomaly and a distinct positive cerium anomaly.  The Ce anomaly is indicative of an oxidized environment where Ce (IV) was being precipitated and scavenged by the sediments.  This requires oxygen production in the near-shore, and precipitation of Ce from sea-water that had not been previously exposed to significant oxygen.