The Bruce Formation is adiamictite succession located in the lower-most portion of the Paleoproterozoic Quirke Lake Group, of the Huronian Supergroup. The Formation is exposed on the north shore of Lake Huron, and covers an area east of Sault Ste. Marie to east of Sudbury, Ontario, Canada. This assemblage of rocks represents deposition on a passive margin after initial rifting and volcanism at approximately 2450 Ma (Fralick and Miall, 1989). The passive margin succession is composed of three glaciogenic cycles beginning with outwash sandstones, which are overlain by glacial diamictites and capped by marine deposits. The Bruce Formation is situated above the Mississagi Formation, which commonly shows evidence of extensive soft sediment deformation of its cross-stratified sandstones (Fig. A). These are also more rarely intruded by clastic dykes (Fig. B), orientated perpendicular to bedding and filled with diamictite. The upper contact with the Espanola Formation is sharp and conformable. The Espanola represents the oldest cap carbonate known in the worldwide geologic record and may be evidence of a snowball Earth event. The Bruce Formation was studied in detail in an attempt to understand the ambient conditions that were present at the time of its deposition. The study included measuring and describing 16 detailed sections in the field, plus petrological and geochemical analysis of samples collected from the field areas.
Glaciogenic deposits can be either terrestrial or marine and marine deposits can be further divided into three lithofacies zones: 1) the grounding ice line zone, 2) the floating ice-shelf zone, and 3) the proximal iceberg zone. Deposits in each of these environments have distinctive characteristics that, where present, can be used to identify the depositional setting. Most outcrop areas in the sections studied consisted of massive diamictite that contained considerable quantities of sand and only scattered pebbles and cobbles. The thickness of the assemblage of diamictites making up the Formation varied drastically from section to section,with three less than 10 meters thick and others hundreds of meters in thickness. Within these massive diamictites are areas with more distinctive features. Lenses and layers of moderately well-sorted sandstone are not uncommon. In other portions of the section mud layers and mud wisps highlight layering in the diamictite, and at one location a fine-grained succession within the diamictite contained a dropstone (Fig. C). If other dropstones exist in areas of the diamictite they are commonly impossible to recognize without layering to deform. However, where clay-rich layers and wisps are present in the diamictite they can be seen to bend under and be terminated by penetration of dropstones. Layers of pebbles and cobbles in clast support were more rarely present (Fig. D). These represent deflation lags produced by current activity removing the fine-grained material. Rarest of all were flat bottomed and convex-up mounds of pebbles and cobbles in clast-support approximately one meter across, although a larger deposit of this type may also have been present in one section. Similar features are produced in modern environments by uneven melting of icebergs causing flipping and dumping of melt-out debris on their surface. In Pleistocene successions diamictite deposited near the grounding ice line commonly contains subaqueous outwash sand layers and lenses. The diamictite deposited below floating ice shelves is generally massive but has features, such as mud-rich layers and sand-rich layers, which indicate current activity. Dropstones and deflation lag layers can also develop in this zone. The proximal iceberg zone can also be dominated by massive diamictite, which has a lower clast concentration, and also contains dropstones. The most conclusive evidence of the iceberg zone is the presence of mound shaped iceberg dumps, which are created when an iceberg overturns. This implies that large segments of the glaciogenic succession were deposited under floating ice with open water present in the basin. The open water is necessary to develop current activity. Although this data does not negate the development of a snowball earth at some point during this glacial period it does not support it.