Sedimentary Rocks

As rocks are exposed to the elements at the surface of the earth, those elements (Wind, Water/ice, even air itself) start breaking down the rock into smaller and smaller pieces. Those pieces or sediments are normally transported and accumulate in the lower parts of the earth surface (valleys, river deltas, ocean floor).

Through successive layering as new sediments are deposited, those below become compacted more and more and eventually become lithified (turned to rock).

All rocks of all classifications will erode and form sediments at the earth’s surface. Since sedimentary rocks have for the most part only been physically broken, they will usually retain some indication of the source rock – which as stated in the Igneous section – is important to mining exploration. Sometimes you may need a microscope to see it but the source rock is there.

So to start classifying sedimentary rocks we should look at grain size. The grains are the individual pieces that make up the rock and can be as large as a house or as small as the tiniest piece of mud.

Classification: Size matters

How small are the parts?

Grain size and history

The grain size as well as the shape or angularity of the individual grains that make up a sedimentary rock can tell you allot about its history. Typically the larger the grains the closer to the source rock it is. Big rocks move slower by water and wind plain and simple. The really fine grained stuff (think mud) is usually along way from its source rock – lakes, oceans.

If the grains are well rounded it usually indicates a moderate amount of repetitive erosion likely due to water. The rocks on a beach are constantly rolled back and forth as waves crash, as rocks high in a mountain get small enough to be carried in the bed of a river – the further they go the more rounded they become.

Angular pieces of rock indicate something more violent and the breaking of the rock – likely before being transported far. Ice will crack/fracture rocks, they fall due to gravity, unless water starts carrying them and wearing down the sharp bits, they will stay close to their source.

How fast is the water?

The faster the water, the bigger the pieces it can move (Except when ice moves the biggest pieces). Large pieces indicate either you are close to the source with the fastest flowing water or there was a massive flow event, like a tsunami. The smaller the particles will travel the farthest downstream until tiny clay minerals will finally sink in very calm waters (lakes/deep ocean).

Source Rocks

The source of the sedimentary rock is of great importance to exploration.

Typically mafic volcanic subduction zones seem to be a great place to start looking for precious minerals like gold and platinum. So looking for sedimentary rocks indicative of that environment would be a good spot to start.

So if you have banded iron formations, which indicate old rocks in an ocean environment, with mafic pyroclastic Tuffs which indicate volcanic activity at an oceanic subduction zone you would be on the right track although you wouldn’t find any fossils.

However if you were finding bio-chemical limestones (shallow ocean) and evaporites you probably wouldn’t look for gold, but would have fun looking for fossils.


Size and origins
Clastic beach conglomerate - Tsunami deposited


Clastic rocks are formed from the typical weathering of all rocks. The process of big pieces becoming smaller and smaller. They then deposit somewhere (rivers, lakes, oceans) and lithify into rocks as the become buried by fresh layers of sediments deposited above them.

Clastic rocks can further be broken into subgroups based on grain sizes.

Conglomerates (Picture above – conglomerate on top)

Large, easily identifiable rocks of various types held together by smaller sand/mud sized grains.

Sandstone (Picture above Sandstone layers bottom)

As with sandpaper in rock form – usually more uniform and texture.


Primarily made up of clay minerals with individual grains that cannot be seen by eye.

Given the ages of rocks in Northwest Ontario, the vast majority of sedimentary rocks are now metamorphosed to some extent but can usually be recognized for the sedimentary rocks they were.

This is essential to understanding what type of environments they formed and were deposited in and as such their geological history is essential in exploration for precious minerals like gold and platinum.

Banded Iron formation - Beardmore


Minerals dissolved in water become supersaturated (to much for the water to hold) and precipitate (become solid) and sink to the bottom – forming layers.

The most common are the evaporites. In desert areas where salt water with all its dissolved minerals (salts) dry out and deposit their mineral load, then flood and dry out, etc. etc.. These are evaporites and include Halite, sylvite, barite and gypsum. Nova Scotia has huge deposits of salts and Gypsum – in its past it was a desert much like the Persian Gulf today.

A more common form of rock in NW Ontario are Banded Iron Formations – which cannot form in todays oxygen rich atmosphere.

In the past (Hundreds of millions/Billion of years) when there was no oxygen in earth’s atmosphere, Iron (Fe) would dissolve in water and would flow downstream. When the iron became supersaturated it would come out of solution and form Iron minerals including Hematite and Magnetite in alternating bands as they settled out. These Banded Iron Formations are common throughout NW Ontario and are targets for active exploration and mining in some areas, not only for their iron but for their trapping ability for gold (Musselwhite Mine).

Today Iron does not dissolve in water, it oxidizes into rust.

The above Picture is of an Iron formation in Beardmore. Metamorphism has distorted the banding extensively.

Limestone from Balsam Lake Ontario - with Crinoid fossils


These rocks are the direct result of biological process’s. Although very rare in NW Ontario, these are some of the more interesting rock for one single reason: Fossils.

Limestones are the most common form of bio-chemical rock as they are formed by the secretions/skeletons and exoskeletons of marine “critters”. Of course this is a primary source of fossils, given what they are made of.

Shells, trilobites, all sorts of marine life is preserved in limestones around the world. Typically the remnant of ancient reefs with abundant marine life they tend to be rich in fossils.
The picture above is of crinoid rich (white circles/stem) limestone from Balsam Lake in Southern Ontario. The Niagara escarpment is the most famous Ontario limestone “reef”, This ancient reef, now exposed well above sea level is rich in fossils and provided the word with Niagara Falls. Building stones throughout the Niagara region can usually be identified by the presence of the fossils.

Coal is also a bio-chemical rock of some use. Formed by plant life dying, becoming buried and breaking down into carbon. The purity of the coal is determined by the pressure/temperature at which it forms with purer forms of coal being less “dirty” to burn.

Oil, although not a bio-chemical rock is similar to coal although it is the remnants of dead marine life (algae and zoo plankton) are buried underneath sedimentary rocks.

Cherts, formed by the accumulation of silica rich skeletons of oceanic diatoms. Chert is very hard and has been used by ancient peoples for tools like scrapers, spear heads and even drills.

Pyrosclastic flow /ash from Chile


The so called other sedimentary rocks are comprised of catastrophic, instantaneous events (in geological time).

These would be what we would call natural disasters. From huge landslides, to violent volcanic eruptions to meteorite impacts, whenever something catastrophic happens there is a breaking, propelling and deposition of material. This material then forms sedimentary rocks outside the standard types. Usually noted by sharp, angular fragments fused together by smaller powder materials – and not well sorted grain sizes. These would be called breccia’s.

Pyroclastic flows or Tuff’s are very common around NW Ontario and are the remnants of ancient volcanic eruptions and in particular the ash/lava/rock fragments that come with violent eruptions of certain types of volcanoes found around subduction zones (think Mt. Saint Helens).  This would indicate at sometime in NW Ontario’s geological past, there were active subduction zones.

Another common (well sort of) are impact breccias, which as the name suggests would be formed by an meteor impact. Both by the fracturing underneath the impact as well as the material ejected forming a sedimentary layer around the impact site. Most notable occurrences are Sudbury – the whole nickel thing is the result of a meteor impact.  On Lake superior we also have a smaller impact site on the Slate islands.

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