Credit River Valley sandstone - DP/EC 11five EarthCache

Introduction:

The Whirlpool Sandstone (Lower Silurian) has been extensively quarried as building stone in the Milton-Inglewood area, Ontario. It has been used under the trade name of “Credit Valley Sandstone" in many Government and University buildings in Southern Ontario.

The economics of the industry and the depletion of the easily accessible reserves require that only shallowly (3-5m) buried sandstone be extracted which shows good splittability 'reed' of the quarryman) and does not require sawing.

The sedimentological analysis of the Whirlpool Sandstone was designed to determine the environments of deposition of this unit, but also to explain and expand the practical experience of the quarrymen to try and predict which areas have the best potential for additional extraction of suitable rocks. It was found that good reed is presented not only by plane beds, but also by large scale shallow cross-beds and swales. Sandstone with these types of sedimentary features is usually formed in large streams, coastal bars and shelf sheets.

These sandstones may be locally reworked by small sand dunes or dissected by channels generating medium scale, steep cross-beds kurls) forming unworkable, waste stone. The rock with good reed may grade laterally and vertically to ripple cross-laminated 'Feathery reed' of the quarryman) sandstone which requires sawing to be used in constructions.

The regional sedimentological analysis of the Whirlpool Sandstone indicates that no economic large deposit of sandstone with good reed can be found in the Milton-Hamilton area where cross-bedded intervals prevail.

The best area for extracting stone is the Limehouse-New Smithson quarry region, although small kurls may interrupt the continuity of the extractable zones.

Some good prospects can also be present in the Limehouse-Milton region where parts of the shallowly buried secondary escarpment of the Whirlpool remains unexplored.

(Source: Martini,I. P. and Salas, C. (1983). Depositional Characteristics of the Whirlpool Sandstone, Lower Silurian, Ontario. Ontario Geological Survey Open File Report 5363, 124p.)

What is a “sandstone”?

Sandstone is a sedimentary rock composed of sand-size 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. It is often mined for use as a construction material or as a raw material used in manufacturing. In the subsurface, sandstone often serves as an aquifer for groundwater or as a reservoir for oil and natural gas.

(Source: https://geology.com/rocks/sandstone.shtml )

(Trough) Cross Bedding? What is it?

You will have seen that in the introductory paragraph, I have highlighted the terms referring to this specific geological aspect of the Whirlpool Sandstone.

The report cited in point one further states that: “This facies comprises primarily trough cross-beds, rare tabular cross-beds and occasional sigmoidal units. The trough cross-beds can be further subdivided into units with steep foresets, and units with shallow foreset dips. The second type has usually plane laminated foresets with good splittability.”

So…what does this all mean?

Cross bedding or stratification:

Is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. Cross stratification forms under laminar flow conditions, either in water (rivers, deltas, beaches) or in air (sand dunes). They can form with uni-directional flow (eg. rivers), or bi-directional flow (eg.waves on beaches, tidal flows, winds that change directions. The original depositional layering is tilted, such tilting not being the result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, which are known as cross strata. Cross-bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes; it indicates that the depositional environment contained a flowing medium (typically water or wind). (Partly sourced from: https://en.wikipedia.org/wiki/Cross-bedding)

Within this category of bedding, there are two (2) different types:

1. Planar Cross bedding – Tabular cross-stratification is produced by the migration of straight-crested, asymmetrical ripples or sand waves. Trough cross-stratification is generated by the migration of linguoid ripples or dunes. The term ‘cross-lamination’ is applied to cross-stratification formed by the migration of ripples; ‘cross-bedding’ is used for cross strata formed by the migration of large-scale forms such as dunes, sand waves, or bars. The term ‘cross set’ is used to define the cross-stratification preserved between any upper and lower bounding surface. Where the original bedform which produced the cross set is preserved and forms the upper bounding surface to the set, the term ‘form set’ is used. A number of cross sets preserved within a single bed are called a ‘coset’.
2. Trough Cross Bedding – A family of primary sedimentary structures formed by the migration of the slip-faces of rippled bedforms or of bars. It is characterized by inclined laminations (foresets) bounded by planar surfaces (planar or tabular cross-stratification), or by scoop-shaped surfaces (trough cross-stratification). The foresets dip at the angle of repose of the sediment on the ripple slip-face and are oriented in the direction of migration of the ripple, i.e. they have asymptotic bottoms!

(Source: Adapted from - https://www.encyclopedia.com/earth-and-environment/geology-and-oceanography/geology-and-oceanography/sedimentary-structures#1O13crossstratification )

Image sources:

Left: H. E. REINECK and I. B. SINGH, Depositional Sedimentary Environments. With Reference to Terrigenous Clastics (Springer Study Edition. Corrected Reprint of the I^st Edition). XVI, 439 S., 579 Abb., 23 Tab. Berlin–Heidelberg–New York 1975. Springer-Verlag.

Top Right: http://www.seddepseq.co.uk/SEDIMENTOLOGY/Sedimentology_Features/Unidirectonal%20Flow%20Bedforms_files/Image68.jpg

Bottom right: https://i.pinimg.com/originals/6d/30/11/6d3011dfae622900307e551c3406b886.jpg

So now that you know how to distinguish these two tyes of cross-bedding, get on down to Toronto City Center and have a go at this EarthCache:

You need to focus on the south and east sides of the pillar that serves as the base/pedestal for the lamp.

Here you need to tell me:

1. What is the grain size of the sandstone? (Choose between Fine, Medium and Coarse)
2. At the base you have a foreset. Identify whether this is a “Tabular” or “Trough” cross bed.
3. How would you describe the colour of the sandstone? Red, white, grey, brown or purple?

Send me the answers through my profile page and you can log. I will validate the answers and correct you if I need to.

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