Lecture 6: Sandstones and Conglomerates

G342 Sedimentation and Stratigraphy

Lecture 6: Sandstones and Conglomerates

16 February 2006

Assoc. Prof. A. Jay Kaufman

Sandstones and Conglomerates

Because of their formation requirements (coarse detritus, strong current transport) and their physical properties (high porosity and permeability), the majority of sedimentary research has focused on sandstones and conglomerates. This is even though they represent, at most, 25% of the stratigraphic record. Below are sets of criteria and classifications that are handy in describing these lithologies

Conglomerate and Breccia

Lithified gravel and rubble are called conglomerate and breccia, respectively. Conglomerates are very special in the sedimentary record for two reasons. First, they are always associated with high energy systems. Second, they bring large chunks of material right to your feet. As such, they are very helpful in reconstructing provenance of the hinterland, and are often linked to nearby tectonic activity.

Orthoconglomeratesconsist primarily of framework grains and <15% matrix. Paraconglomerateshave a matrix of sand and finer clasts and are matrix-supported. The systematic orientation of clasts in a conglomerate is termed imbrication. In conglomerate classification, there main concern is composition, but there are also textural issues around grain-size distribution and roughness of grains.

Diamictite is another term for a paraconglomerate, and is often used to denote glacial rocks. Clasts that appear to pierce finely laminated sediments were likely transported by sediment-laden icebergs and are called dropstones.

In addition to a glacial origin for conglomerate and breccia, these coarse-grained deposits can also result from subaerial and subaqueous debris flows, subaqueous grain flows, turbidity currents, and bolide impacts.

Sandstone

Sandstone is the indurated equivalent of unconsolidated sand containing clasts between 2 and 1/16 mm. Sandstones hold over ½ of the worlds oil reserves

There are MANY different classification schemes for sandstones, but the two defining parameters are the percentage of matrix and the composition of sand framework grains.

Name / Grain-size (mm) / Φ / Composition / Other
Quartz arenite / 2 – 0.064 / -1 to 4 / >95% quartz / Surprisingly common
Arkose / 2 – 0.064 / -1 to 4 / >25% feldspar / Often associated w/ recycling of continental cratons
Lithic arenite / 2 – 0.064 / -1 to 4 / >25% lithic fragments / Often associated w/ recycling of arcs and mountain belts
Graywacke / 2 – 0.064* / -1 to 4* / 15-75% mud / Often associated w/ recycling of arcs and deep basins
Conglomerates / >2 mm / < -1 / varied / Round clasts
Breccias / >2 mm / < -1 / varied / Angular clasts†
Orthoconglomerate / >2 mm / < -1 / varied / Framework supported
Paraconglomerate / >2 mm* / < -1* / varied / Matrix supported

* actual mean and modal grain size varies with mud content

† there are many different origins to breccias – be careful in interpretation.

Quartz arenite

Quartz arenites are typically deposited in stable cratonic environments such as eolian, beach and shelf. They are generally well lithified and well cemented, and texturally mature to supermature. These sediments are most likely the product of multiple recycling of quartz grains (polycyclic). If they are first generation it suggests an intense weathering environment.

Feldspathic arenite

A medium to coarse grained lithic arenite with >25% feldspar is known as an arkose. These sandtonse form from the rapid disentigration of granite to form grus, which is rapidly deposited, most often in alluvial fan deposits. They originate either 1) in cold or very arid environments where chemical processes are inhibited, or 2) 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 Old Red Sandstone along barge canal at Great Falls.

Greywacke

Arenites are differentiated from wackes by the percent of matrix grains. Wackes contain >15% and arenites less.The proportion of rock fragments to feldspar in wackes is believe to be a reliable index of sandstone provenance, one supracrustal (i.e. volcanics and fine-grained sedimentary rock) and the other subcrustal (i.e. granite and gneiss)

Textural maturity of siliciclastic rocks

There are four stages of textural maturity based on 1) the proportion of clay clasts, 2) the sorting of the sand grains, and 3) the roundness of the sand grains. Sandstone is texturally immature if there is >5% clay clasts, regardless of sorting or rounding. These also tend to be rich in feldspar, and are common in alluvial fans, turbidites, and overbank deposits. Submature sandstones have <5% clay but are otherwise similar to immature sandstones. Mature and supermature sandstones also have <5% clay, but the sand framework is well rounded and sorted. These are common in fluvial channels, beaches, and aeolian dunes.

Grain textures are the chief factors that control the porosity and permeability of sediments in siliciclastic settings. These include 1) grain size distribution (mean, median, and sorting), 2) shape (sphericity), 3) packing, 4) composition, and 5) cementation. Porosity is the % of void space,

P = (bulk volume – grain volume)/bulk volume

Porosity is NOT constrained by geometry. The distribution of pores in a volume is independent of porosity. In general, better sorting results in better porosity, and finer grain sizes show higher porosity.

Permeability is 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 sorting and mean grain size. A heterogeneous fabric diminishes permeability by increasing the tortuosity of the pore system. Permeability is not flow rate, which varies as a function of viscosity, temperature, and pressure.

Packing is the mutual spatial relationships between grains. This includes packing proximity, or how many grains another grain touches, and packing density, or the cumulative number of grains intercepted by a random line through the volume. Rocks with identical grain-size distributions can have different porosity and permeability as a function of packing.

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