The rocks beneath our feet record nearly 400 million years of Earth history. The oldest rocks, referred to as 'Wrangellia' by geologists, include limestone and volcanic rocks that formed below a deep ocean. These rocks are cut by granite that was injected in a molten state and later cooled and solidified. During the 'age of dinosaurs', the rocks of Wrangellia were buried by sediments that transformed into sedimentary rocks of the 'Nanaimo Group', now the foundation of Nanaimo. Finally, a partial blanket of glacial till, sand, and gravel was left behind by glaciers during the ice age.
During the latter part of the Cretaceous Period (from around 85 to 65 million years ago), clastic sediments were deposited in a long trough-shaped basin extending from south of Duncan to north of Campbell River. The Nanaimo Group has a total thickness of close to 5000 m. It includes a series of alternating layers of conglomerate, sandstone, siltstone and mudstone. Within these same sedimentary rocks we can find coal deposits, composed of organic materials.
A likely explanation for the varied nature of Nanaimo Group sedimentation is that there were quite significant variations in the rate of coarse sediment input into the basin. During times when uplift, erosion and sedimentation rates were low, there would have been very slow accumulation of mudstone in the deeper parts of the basin, with coarse material being deposited only near to shore. Following periods of rapid uplift, the sedimentation rates would have been high, high volumes of coarse sediments would have been input, and with the periodic collapse of fan deposits these materials would have been deposited well out into the basin. In this scenario the fine-grained parts of the Nanaimo Group would have been deposited during periods of slow erosion and deposition, while the coarse-grained parts would have been deposited during periods of rapid erosion and deposition.
Mudstone, a type of mudrock is a fine grained sedimentary rock whose original constituents were clays or muds. Individual grains can be too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the appearance of parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due either to original texture or to the disruption of layering by burrowing organisms in the sediment prior to compaction. Mud rocks, such as mudstone and shale comprise some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.
Conglomerate is made up of rounded pebbles cemented together. The pebbles have been rounded by the action of moving water. After the pebbles have been deposited they are compacted by the sediments that pile up on top of them. Over very long periods of time the pebbles become cemented together by minerals. Silica, calcite, and iron oxides are the most common cementing minerals.
Sandstone is a sedimentary rock composed of sand-sized grains of mineral, rock or organic material. It also contains a cementing material that binds the sand grains together and may contain a matrix of silt- or clay-size particles that occupy the spaces between the sand grains. Sandstone is one of the most common types of sedimentary rock and is found in sedimentary basins throughout the world.
Siltstone is a clastic sedimentary rock. As its name implies, it is primarily composed of silt sized particles. Siltstones differ significantly from sandstones due to their smaller pores and higher propensity for containing a significant clay fraction.
Parking for this earthcache is readily available at the Maffeo Sutton Park parking lot. Monday to Friday the first 3 hours are free, the weekends are free to park. Sometimes it is limited when special events are taking place. Please do not post answers in your log - after sending the answers by email you can go ahead and log. We'll contact you if there's a problem.
Questions:
1) At the coordinates you will see a wall with several sedimentary layers - which can you easily identify?
2) There are many color variations - in your opinion what may have caused some of them?
RESOURCES
http://www.rocksandminerals4u.com/sedimentary.html
http://web.viu.ca/earle/mal-cut/nanaimo-group.pdf
http://web.viu.ca/geoscape/nanaimo%20group.htm
http://web.viu.ca/geoscape/Geology.htm
http://web.viu.ca/geoscape/images/map_sedimentary.jpg
http://en.wikipedia.org/wiki/Mudstone
http://www.rocksandminerals4u.com/conglomerate.html
http://geology.com/rocks/sandstone.shtml
http://en.wikipedia.org/wiki/Siltstone
The Geology of Southern Vancouver Island - C.J. Yorath and H.W. Nasmith
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