The Espanola Formation is part of the Huronian succession (2.1 to 2.5 b.y. old) of the north shore of Lake Huron. It is unique among Huronian formations in its high carbonate content. In the Quirke Lake region the formation may be divided into three members which, in ascending sequence, are limestone, siltstone, and dolostone. Southward thickening of the formation is due to increased thickness of the siltstone member and to the presence of an additional sandstone member at the top of the formation. Fining-upward cycles in the sandstone unit suggest a fluvial origin for that member. Paleocurrents in the same unit indicate a southerly sediment transport.
Structures in the Espanola Formation include breccias of both sedimentary and tectonic origin, cross-bedding, ripple marks, graded bedding, desiccation cracks, and a variety of injection structures. Breccias formerly described as “intraformational” occurred later than some faulting and clastic dike intrusion but before penetrative deformation. Most of the breccias were the result of downward intrusion into early formed fissures in carbonate-rich units. Development of intrusive breccia in the Espanola Formation is spatially related to areas in which there is evidence of folding before deposition of the overlying Gowganda Formation. Microprobe analyses of dolomite in rusty weathering dolostone confirm the presence of ferruginous dolomite. The limestone and dolostone members are considered to be shallow marine deposits whereas the intervening siltstone member may have been deposited in deeper waters by turbidity currents. The Espanola Formation is interpreted as the product of diachronous deposition by facies migration, involving a marine transgression (following withdrawal of the glaciers responsible for deposition of the underlying Bruce Formation). During the regression which followed, a prograding fluvial regime was established, the distal facies of which is represented by the sandstone member of the Espanola Formation. Absence of this member in more northerly areas may be attributed to contemporaneous erosion in these areas, possibly related in part to isostatic recovery following loss of the Bruce ice sheet.