Ruth Bjorkman's Honours Thesis Abstract

Thesis Title: 
The Mapping and Petrography of an Archean Maar Deposit

An outcrop present along Hwy 11/17 east of Thunder Bay contains a sequence of stacked clastic rocks with dike intrusions.  The clastic units are dominated by surge flows and ash fallout deposits, with less common debris flows and fluvial reworking.  The presence of surge deposits and pyroclastic falls in association with one another show the sequence was deposited proximal to the source.  The ash beds are on average 3 cm in thickness and grade from medium-grained sand to very fine-grained sandy tops.  These normal graded beds are likely ash fallout deposits, and are frequent throughout the study outcrop.  The thin, very explosive and wet surge deposits likely formed from a maar volcano landform.  Deposition of the clastic units likely took place in a subaerial environment with subaqueous areas, such as a flooded plain or swamp.

Hornblendite dikes cut through the sequence and also occur as veinlets and sill-like intrusions infiltrating into the clastic material.  The intrusions incorporate the clastic material and often there is a dissipative zoning of sediment in the hornblendite intrusions.  This shows the sediment at the time of intrusion was wet.

The outcrop was formed in a volcanic environment within the Shebandowan Greenstone Belt of the Wawa Subprovince.