Clastic rock types in detail: Sandstone
Sandstone is the indurated equivalent of unconsolidated sand containing clasts between 2 and 1/16 mm. They make up between 10% and 20% of sedimentary rocks. Interesting to sedimentologists for the information they contain about the provenance of sediment and depositional environments. For the practical person, they are important as fluid reservoirs (aquifers and petroleum reservoir rock). Indeed, sandstones hold over 1/2 of the worlds oil reserves.
Sandstone classification:There are MANY different classification schemes for sandstones, but the two defining parameters are:
- the relative abundance of:
- Framework (sand grains)
- matrix (mud)
- the composition of sand framework grains.
- grain size and distribution
Framework composition:Framework grains can consist of any rock or mineral, but for practical purposes, we reduce these to the common ones:
- Crystalline quartz: Generally abundant because of its resistance to weathering.
- Feldspar: Both K-spar and plagioclase are more susceptible to chemical and physical weathering, so they are less abundant in sediments, even if they are common in source rock. Their presence implies minimal chemical weathering or a nearby sediment source. This can be due to:
- Environmental conditions that minimize chemical weathering such as aridity or extreme cold.
- High topographic relief in the source region, so that it is eroded before it is chemically transformed.
- Rock (lithic) fragments: Fragments of whole rock containing more than one mineral grain. These provide information on sediment provenance.
Matrix: Fine grained matrix (<30μm), can be detrital or products of in situ feldspar diagenesis. Depending on the percentage of matrix, a sandstone can be a:
- Arenite: [Latin arena=sand]. 0% - 15% matrix.
- Wacke: (A.K.A. "graywacke.") 15% - 75% matrix. I.e. significant matrix by volume.
Depending on the predominant framework composition, arenites are broken down into:
- Feldspathic graywacke
- Lithic graywacke
- 1 mm = 0Φ
- 1/2 mm = 1Φ
- 1/4 mm = 2Φ
- 1/8 mm = 3Φ
- 2 mm = -1Φ
- a simple non-cumulative histogram (left)
- a non-cumulative size frequency curve fitted to the same data.
- Skewness: Degree of asymmetry. The positive skew above demonstrates that coarser sediments (negative Φ) are better sorted than fine grains.
- Kurtosis: The "peakedness" of the distribution.
- The degree to which the distribution deviates form a normal distribution (which appears as a straight line)
- The degree of sorting (curve steepens with improved sorting)
- Transport agents have different viscosities and different energies that sort grain sizes differently
- Wind transport - fined grained and well sorted
- Ice transport - coarse to fine grains, poorly sorted
- River transport (high gradient) - coarse grains deposited
- River transport (low gradient) - finer grains deposited
- Beach transport - well sorted, unimodal size
- Density currents (debris flows) - bimodal or polymodal
- Larger size --> more grain-to-grain impacts.
- Softer minerals are more easily rounded
- Transport in more turbulent flow (higher Re) yields more frequent collisions, thus:
- Wind and beach transport: Low viscosity fluid, high Re, high turbulence --> more impacts of higher intensity
- Ice transport: low Re and high viscosity --> fewer and less intense impacts
- River transport is variable and intermediate.
- Imbrication: Grains can line up in the direction of current flow
- Packing: Mutual spatial relationships between grains. Can be described as:
- Proximity: how many grains a clast touches
- Density: cumulative number of grains intercepted by a random line through the volume
- Well sorted sandstones have higher porosity - fewer relatively small grains to fill empty spaces (see example at right)
- Finer grained sandstones have higher porosity (because finer sands are rare.)
- sorting (better sorted = higher permeability.)
- mean grain size (larger grains = wider pore throats.)
Φ notation:To minimize inconvenience, sedimentologists use Φ notation to record grain size, where Φ = -log2 grain diameter in mm. Thus:
Grain size distribution:Described by variations on the theme of size distribution histograms. Above are
These graphically display:
Often, we use a cumulative size frequency histogram or curve (left) instead. When plotted on probability paper (right), we can easily see:
Implications of sorting:
Sedimentologists seek to identify agents of transport and depositional environment through quantitative analysis of grain texture:
Shape and rounding:
Shape: is a product of mineralogic characteristics - crystal habit, cleavage, etc.
Roundness: is a product of clast size, mineralogy, transport agent.
Fabric: The orientation and packing of grains.
Porosity: % empty space.
P = (bulk volume - grain volume) / bulk volume
Permeability: The "flowability" of fluids within a volume of rock and is expressed in Darcy units (cm/s). Permeability is STRONGLY constrained by geometry. The governing parameter is pore-throat diameter, or the size of the minimum space between grains. As such, permeability is affected both by:
Much of the foregoing can only be assessed meaningfully in thin section. Maturity , however, is a convenient general description that encompasses proportion of matrix, sorting, and rounding and can be approximated in the field:
- Immature: >5% clay (sorting and rounding not considered), often high in feldspars and rock fragments.
- Submature: <5% clay, moderately sorted, slightly more rounding than immature, feldspars and rocks fragments still common.
- Mature: <5% clay, well sorted, sub-angular to sub-round, few if any feldspars or rock fragments.
- Supermature: <5% clay, very well sorted, round to sub-round, few if any feldspars or rock fragments.
With all of this in hand, we can describe the major sandstone types.
- Typically deposited in stable cratonic environments such as eolian, beach and shelf.
- They are generally well lithified and well cemented
- Texturally mature to supermature.
- in cold or very arid environments where chemical processes are inhibited, or
- in warmer, humid environments where marked relief or local uplift allows rapid erosion of feldspar before they can be decomposed.
A good nearby example is the Newark Supergroup sandstone lining the locks at the C & O canal at Great Falls.
- Submature to mature
- Characteristic of river deposits that interfinger with conglomerates or
- in cold or very arid environments where chemical processes are inhibited, or floodplain mudstones, or of turbidites
- Often occur at the base of recent uplift where rock fragments are plentiful
- Often in the wedges of subduction zones and arcs, or in collision zones
- Characteristic of submarine fan, slope, and abyssal deposits