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• 1 media/IMG_8728.JPG media/IMG_8728.JPG 2021-05-17T16:57:10+00:00 Shale, sandstone, and dolomite 84 image_header 1416 2022-02-16T21:59:21+00:00
  Standing at the escarpment, looking at a grey, beige, or red rock you may wonder how you can tell if what is in front of you is shale, sandstone, or limestone/dolostone. Here we will learn how to differentiate some of the most common rock types in Niagara, your friends will be amazed! This is intended as a primer on the subject and in no way approximates what you should be learning in class, lab, and from reading if you are an Earth Sciences student.
  
  Here is the TL/DR to differentiate shale, sandstone, and limestone/dolostone. Note that telling them apart is of limited use if you don’t know why they are there and of what components they are made. You can find that context after these tips on differentiating them.
  
  These descriptors are for fresh surfaces at hand sample scale which is often aided by wetting part of the fresh surface:
  • Shale is primarily made of clasts that are not individually discernable to the naked eye, the individual pieces making up the rock are smaller than a grain of salt.
    • • Shale breaks up in a blade-like habit, leaving edges that can be sharp. These edges are a result of bedding in the sediment, in Niagara they are usually horizontal.
  • Sandstone has visible grains the size of a grain of salt and slightly larger. Shales are more likely to be dark in colour than are sandstones, but that is not diagnostic.
    • • Sandstone has a surface texture like sandpaper (shocking!) and you will likely be able to rub off small sand particles.
  • Limestone is usually grey and dolostone is usually grey perhaps a little tan. Limestone and dolostone are carbonates, often carbonates have small white shapes in them as cylinders, lines, or arcs; these are fossils.
  • Crystals in limestone, if present and visible, are likely clear-white calcite (CaCO₃), and may accumulate in vacancies (holes) called vugs.
  • Dolostone is difficult to tell apart from limestone, but in dolostone if you see crystals they are more likely to have a pinkish hue and looking closely you might find that those crystals have a curved surface, this is the mineral dolomite (CaMg(CO₃)₂). While calcite is often translucent and sometimes transparent, dolomite is not. Dolostone may also have more vugs and more complex mineralization in those vugs that might include siderite (FeCO₃) and/or gypsum (CaSO₄-2H₂O).
  • Note that calcite in limestone reacts vigorously with acid whereas dolomite reacts slowly and may need to be scratched to produce a reaction.

  Here is an example of limestone reacting with acid:
  

  A note to students perusing this site: This information provides a nice background, but it is not peer-reviewed and is not a published text, sources such as those are found at the bottom of the page. Bearing that in mind, this site should not be a source you cite in your work, but the sources listed on it are provided so that you can start your research there.
   

  And now on to the enlightening details:

  
  Both shale and sandstone are clastic sedimentary rocks, which are rocks composed of fragments eroded from other rocks. In sandstone these fragments are individual grains of quartz and likely some feldspar. In the case of shale the sediment is even smaller fragments called silt (0.0039 to 0.0625 mm) and clay (smaller than 0.0039 mm)^[1]. Clays are so small that they are comparable in size to a virus or a 'large' smoke particle. Size ranges used in the sedimentology field of geology are summarized in the simplified diagram at the right. Bear in mind that while clay is described as both a mineralogical group and size class, it is generally understood in geology that even when referring to the size class clay, that the particles being discussed are in fact most likely clays in the mineralogical sense.
  
  Weathered surfaces of shale and sandstone can look similar. These two rock types can grade (transition) from one to the other with no well-defined boundary. Colour is a poor indicator other than noting that clays often get deposited in low-oxygen (anoxic) environments which tends to make the resulting shales more likely to be dark and/or have a green hue attributable to more organic matter. Organic matter (dead organisms) can persist in the precursor muds because in a low oxygen environment there are fewer living organism to eat and decompose tissue. Fundamentally the difference is the size of the clasts which compose the rocks. Remember that sandstones have larger clast size than shale, if most of the rock volume is made of individually discernable clasts then you have sandstone. 
  
  It is easy to imagine clasts being broken off from larger rocks, but where to the clays come from? They are mostly re-arranged components of feldspars, removed by chemical erosion, which is related to why there is often more quartz in sandstone than feldspar (passive enrichment). Quartz is perfectly happy to remain quartz at the Earth’ s surface, largely ignoring the elements other than physical abrasion. The 1.88 billion-year-old La Cloche Mountains north of Georgian Bay/Lake Huron are a testament to this. Feldspars deteriorate slowly, but faster than quartz in comparable conditions, and faster still in warm and wet conditions.

  Shales

  
  Shale is a type of mudstone that has the quality of being fissile and/or having laminations. Fissile means the rock breaks apart readily and produces plate-like pieces as it breaks along planar surfaces. Laminations are millimeter-scale layers in the rock which relates to the rock's fissility. Laminations correspond to a period of consistent deposition, which could be as short as a season or even a singular storm event. The bladed  or platey appearance of broken shale is largely a result of original bedding and of clays adopting a ‘ flat’ horizontal orientation as they are deposited, like tossing a ream of paper on the floor. As the name mudstone implies, today’s mud composed of silt and clay is perhaps tomorrow’ s mudstone (tomorrow meaning thousands of years from now).
  
  Shales of the type found in the Queenston shale and Cabot Head shale which are below and above the Whirlpool sandstone respectively formed from that mud which had been deposited when the sea level was higher, and the finest particles were settling to the bottom in that calm water below the sea surface and below the bottoms of waves. We may note here that lagoons between a land mass and a reef also provide important calm-water environments in which shales may be deposited, and in this type of environment the shale of the Rochester Formation is thought to have been deposited.

  Clays are important so let’ s take a brief look at what they are:

  In mineralogy, clays refers to a set of minerals that have a specific type of molecular structure which is sheet-like, they are a mineral class called phyllosilicates. It is no surprise they share part of their name with phyllo pastry since phyllo is Greek for leaf; it would be fair to say that clays have a form like microscopic leaves.
  
  On a molecular level clays are made of alternating sheets of molecules. Each sheet is just a few atoms thick, either in the form of a sheet of interlocked pyramids with four sides (including the base) called tetrahedrons, or of octahedrons which have eight triangular faces. Octahedrons might be thought of as two five-sided pyramids (including the bases) with their bases stuck together, leaving eight faces exposed. Both shapes host an atom in the middle, balanced by electromagnetic charge with atoms at the points. Within the sheets, each tetrahedra or octahedra shares the atoms at its points (vertices) with the neighbouring tetrahedron or octahedron.
  
  
  
  Visualizing octahedrons can be tricky at first if you have not seen one, below is a piece of fluorite that has formed as an octahedron. You can rotate the image by clicking on it and dragging. Note that crystals such as this do not take a shape by chance, this is a direct result of fluorite’ s composition and atomic crystal structure.
  
  
  Clays are composed of alternating sheets of tetrahedra and octahedra in some combination forming sets of sheets like a sandwich. Between these sets of sheets, there are places for positively charged atoms (called cations) to reside. Most clays have 2:1 structure with a sandwich of tetrahedron-octahedron-tetrahedron. The 2:1 clays are classed based on composition as illites, smectites (montmorillonites), and chlorites. Kaolinite is a clay with 1:1 structure produced from the weathering of feldspars, it has a layer with one tetrahedron sheet and one octahedron sheet and is one of the most common clays.
  
  The varying sets of atoms occupying the inside of the tetrahedra, octahedra, and inter-layer space distinguish the clay type and contribute to unique properties of those clays such colour, hardness, and their affinity for water molecules. Intermolecular forces (attraction between the molecules and sheets) cause the sheets of clays to want to stick together, but these forces are not very strong, part of why shales are fairly easy to break apart.

  Sandstones

  
  The sand that makes up a sandstone is usually pieces of quartz with lesser feldspar and then some other minerals in much smaller quantities. These clasts were eroded from larger rocks which were eroded from whole rock formations. Clasts in sandstone are 0.0625 mm to 2 mm in size, portions of sandstone made up of particles smaller than that (silt and clay) are called matrix.
  
  Geologists look at the roundness and angularity of clasts to determine how far (in relative terms) the sediment has travelled. As the particles are transported by water or wind they bump in to each other and over time and distance the clasts become more round and less angular. Additionally, chemical breakdown contributes to the erosion of the clasts.  
  
  Quartz  is more resistant to weathering than feldspars and thus the most well-travelled sands (geologists would say the more mature sand) have more quartz. Maturity of sediment is determined by the composition (i.e., less feldspar, greater proportion of quartz), the clast shape (roundness and sphericity), and by sorting (statistically examining the distribution of clast sizes). Feldspars may persist longer in cool and dry environments where they are subject to less chemical dissolution. Beach and offshore sand is typically more mature than sand in terrestrial environments like channels and alluvial fans, but massive flow deposits (underwater avalanches) may transport less mature sand further out in sea.  
  
  Sandstones are cemented together, providing a bonding force in the rock. In less well-sorted sandstones this may be accomplished by some clays and silt included in the sediment. Cement can enter the rock after deposition via fluids; this cement may be carbonate, iron oxide, or another mineral that can occupy space in the sediment. The post-deposition changes to sediment leading to lithification (becoming rock) are called diagenesis.
  
  One of the best things about sedimentology is that you can learn to recognize structures from sediment deposition millions of years ago. Geologists study sandstone bedforms, the patterns of sediment resultant from the type and rate of flow of water or air that moves the sediment. Below is an example of cross-stratification from the Mississaugi Formation, a quartzite that is about 2 billion years old. Quartzite is sandstone that has been metamorphosed, in this case to a low degree. After 2 billion years we can still examine the cross-stratification from the Whitefish Falls, ON area and determine that the stream in which these sands were deposited had flowed from what is today the north-west.
  
  This begs the question, if you haven’ t taken a class in sedimentology (or it has been a while), what is a cross-stratification? Cross-stratification is patterns in layers of sediment produced by certain flow parameters (flow regimes) that results in three-dimensional arrangement particular to the flow regime. In outcrop we can usually see one or two views of these 3-D patterns, and usually not the easiest to recognize! When ripples or dunes form on the bed surface in flowing water and air, material accumulates in the lee of the ripple/dune which is the downstream side. Thinking of a sand dune, the lee is the steeper side where you might hide from the wind (and get covered in sand) whereas the stoss side is the longer rampart up which the wind travels. Under certain conditions the pattern in cross-section of the ripples or dunes is sinusoidal as in the picture above and drawing below.
  
  
  
  Standing at the escarpment, perhaps looking at the Whirlpool Formation, you may be able to pick out bedforms that were recorded in the sediment hundreds of millions of years ago. Those forms may have been created in just hours or minutes, it is truly a remarkable property of sedimentary rock to preserve such moments.
  
  
  Classification of sandstones can be done in many ways, but commonly the Dott classification is employed. This system uses the mineral content (quartz-feldspar) versus rock fragment proportion in a triangle compared to the proportion of the rock that is matrix. Matrix refers to the proportion of a sandstone’ s clasts that are silt-sized or smaller, which is convenient because it follows the shale-sandstone differentiation and because silt is smaller than the naked eye can reliably discern as individual clasts. Below is a basic diagram derived from Dott’ s method^[2]. Note that the vertical direction in the triangle corresponds to compositional maturity (greater with higher proportion of quartz) and the near-horizontal line approaching 0% matrix at the left corresponds to textural maturity (greater with more sorting which in this case corresponds to higher proportion of sand-sized clasts).
  
  
  Using the Dott classification, we have sandstone classifications of arenites which are over 85% sand-size clasts and rock fragments, and wackes which are 25 to < 85% sand-size clasts and rock fragments. Within those two clast-size derived classifications, we use the proportions of quartz, feldspar, and rock fragments to add a descriptive prefix in front of wacke or arenite. This descriptor may be difficult to establish in the field.

  Why are sandstones near-shore and shales marine?

  When sediment is carried toward lakes and seas in streams (rivers always head toward a lake or sea) the size of the sediment that can be carried depends on how fast the water is moving. This is because of the interplay between the sediment particles (like sand or smaller clay particles) and the power exerted by the water molecules. Imagine standing in a slow-moving stream up to your waist; there is a constant force pushing you downstream, but perhaps not enough to knock you off balance. Now imagine standing to the same depth in a fast-moving stream, it may not only knock you off balance, but it might then carry you downstream before you can right yourself. The process in moving sediment is similar. If the stream is moving fast enough, it will carry larger particles downstream.
  
  Now consider the inverse: A stream that has been flowing very quickly has lots of sand and pebbles moving along with it, but then it enters an area where the stream widens and so the water slows. As the water slows, the largest sediment drops to the bottom and stays there or moves along the bottom much more slowly.
  
  If that stream entered the sea, then having strapped on your scuba gear, as you walk on the sea floor from the stream’ s mouth toward the sea the sediment beneath your feet would on average get smaller and smaller. Perhaps at first there are nice smooth stones like you would find at the shore. Further out there are fewer stones, they are smaller, and mostly you are walking on sand. Still further, the sand grades slowly into what you might describe as mud. This is one way we can tell in what kind of environment the rocks of the escarpment were deposited.
  
  Sandstones, at least those with the characteristics of the Whirlpool sandstone, are rocks that formed from sand deposited at the edge of a sea, sometimes under water and sometimes being eroded above the water as tides and sea level fluctuated.  When sand was eroded, small channels were cut. When erosion and deposition were approximately in balance, braided streams formed as one channel filled in and another was cut nearby. Channels would subsequently be filled in with more sand when the water level rose again, or a storm reworked sediment. Often, you can find the braided streams and other features like ripple marks in sandstone.
  
  The reason that sandstones and siltstones are layered in the escarpment is largely because the sea level changed, a process called eustacy. Formations like those of the Escarpment and the way they are layered is how we know about changes to sea level and climate long before any human was around to observe it first-hand. Isostasy is the balanced elevation of a given point on land which can also change and affect where sediment of a given type is deposited. Facies models are the typical series of facies (for our purposes, rock types) related to changes in sea level and land surface elevation. In Niagara we are familiar with isostatic rebound which is the uplift of land as a recovery after the burden of ice sheets, however the isostatic rebound currently and recently occurring in Niagara is related to the recent geologic past (< 15’ 000 years) and as such not related to the formation of our Paleozoic rocks.

  Dolostone & Limestone

  Dolostone is a product of altered limestone, except in unusual occurrences. The name dolostone is taken from the name of the mineral dolomite (Ca,Mg)CO₃ which constitutes the bulk of dolostone. Through diagenesis, limestone is transformed from a rock primarily composed of calcite and aragonite to one composed of dolomite.
  
  So, to understand dolostone one must first understand limestone. Limestone is a different type of rock from shale and sandstone as it involves the biochemical precipitation of a mineral that constitutes the majority its volume. That mineral is calcite or aragonite, two different arrangements (polymorphs) of calcium carbonate, CaCO₃. To confuse things, nature likes to include sediment such as clay and sand, and if there is a lot of clay and/or sand we call the rock argillaceous limestone. In formations like the Rochester there is much more clay and silt than carbonate so the rock is a shale even though there is some carbonate.
  
  The calcium carbonate in limestone is from the shells and skeletons of marine creatures such as plankton, brachiopods, gastropods, coral, and algae. Some of the creatures are microscopic (e.g., algae) and some not (e.g., brachiopods). The larger ones are often still visible to the naked eye hundreds of millions of years after they lived. If the limestone has many visible fossils we call it fossiliferous limestone. Truthfully, almost all limestone is fossiliferous, especially under microscopic inspection.
  
  Limestone is formed in marine environments, commonly as reefs. The Great Barrier Reef off the coast of Australia is a precursor to what could be limestone bedrock one day. Niagara was home to reefs during the Paleozoic, evidence of the landmass having then been nearer the equator and enjoying the associated warm climate. The reef system in Niagara was part of a greater network of reefs in the Michigan Basin.
  
  Most of the fossils in limestone formed where the limestone is deposited, making the fossils autochthonous and we call these orthochems. Sometimes skeletal matter and shells are broken up and transported (often down the marine shelf) and so they are allochthonous, we call these allochems. Classification of limestones can get a little involved, it is largely based on the fabric of the rock, particularly what the dominant material is (e.g., sparry limestone has many skeletal fragments versus micritic limestone with microcrystalline carbonate having little substantive form).
  
  Limestone forms in warm waters, typically tropical, and shallow enough (< 20 meters[1]) for light to penetrate sufficiently to support photosynthetic organisms. Much of the carbonate originally formed was aragonite rather than calcite due to thermodynamics which make aragonite easier to make than calcite. Aragonite is less stable than calcite and most aragonite is converted to calcite over time.
  
  The diagenetic process responsible for converting limestone which is full of calcite and aragonite into dolostone which is full of dolomite is not known with certainty. The process can be replicated in laboratories from fluid replicating sea water, but only at higher temperatures than what we observe in appropriate natural systems.
  
  There are several explanatory models, most of which involve the passive enrichment of magnesium (Mg) in an evaporative environment where removal of Ca²⁺ ions by precipitation of calcite and gypsum leaves a brine very rich in Mg. Active enrichment is thought to be possible by the addition of meteoric water (fresh water in the ground or on the surface) that has interacted with magnesium-rich volcanic rock. Another possibility is Mg²⁺ from dewatering clays and silt in mudstone, but this is not estimated to produce sufficient fluid to explain the volume of dolostone observed.
  
  The rock record indicates that eventually limestone is likely to transform into dolostone. The oldest carbonates on Earth are mostly dolostone and there is progressively a higher proportion of limestone in younger carbonates^[1]. Perhaps conditions to form dolostone were more favourable in the past and that is why it is not observed now.  Another factor is preservation as dolostone is more stable than limestone which is chemically weathered more readily. We also know that there is proportionately more carbonate from hot periods in Earth’ s history when there was more warm-ocean area.
   
   

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  [1] Prothero, D., & Schwab, F. (2014).  Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy  (Third edition.). W.H. Freeman and Company.

  [2] Dott, R. H. Jr. (1964). Wacke, graywacke and matrix – What approach to immature sandstone classification?. Journal of Sedimentary Petrology (3) 34 p.625-632