EPS 101 Spring 2003– Adrian Brearley - Lectures 17

Sedimentary Rocks II

Key concepts

Diagenesis – chemical and physical processes that occur to transform a loose sediment into a solid rock. Results from burial of sediment and resultant increase in temperature.

Lithification – the processes of changing a sediment into a solid sedimentary rock.

Porosity – percentage of the volume of a rock occupied by open pores (spaces) between mineral grains.

Cementation – chemical diagenetic change involving the precipitation of minerals in pores of sediments that cements the rock together. Precipitation occurs from pore waters.

Compaction – physical diagenetic change caused by the squeezing together of grains as sediments are buried.

Sedimentary Structures

Bedding (stratification) – parallel layers of different grain sizes or compositions that indicate successive depositional surfaces formed during sedimentation.

Cross-bedding – sets of bedded material deposited by wind or water that are inclined at angles of up to 35° to horizontal.

Graded bedding – progression from coarse-grained material at the base of a sedimentary layer to fine-grained at the top. Normally each bed is a few cms to several meters in thickness.

Ripples – Very small dunes of sand or silt whose long dimension is at right angles to current. Formed in wind swept dunes, underwater sandbars in rivers and under waves on beaches.

Bioturbation structures – tube-like structures produced by burrowing organisms that corss cut and disturb sedimentary layers.

Bioclastic particles – fragments of previously deposited and broken up bioclastic calcium carbonate sediments.

Classification of clastic sedimentary rocks

Classification by grain size- clastic sedimentary rocks

Coarse – gravels (sediment) and conglomerate (sedimentary rock).

Medium – sands and sandstone The particles are considered to be suspended in the fluid and are carried long with the flow.

Fine – silt and siltstone, mud, mudstone and shale, clay and claystone.

Gravel – particles larger than 2 mm in diameter includes pebbles, cobbles and boulders.

Conglomerate – sedimentary rock formed from gravels.

Sand – medium sized particles – 0.062-2 mm in diameter. Moved by moderate currents.

Sandstone – sedimentary rock formed from sand.

Quartz arenite – very quartz-rich sandstone, resulting from extensive weathering before and during transportation.

Arkose – sandstone containing more than 25% feldspar – derived from rapidly eroding granitic and metamorphic terrains.

Graywacke – sandstone consisting of a heterogeneous mixture of rock fragments and angular grains of sand in a clay matrix.

Silt – clastic sediments where most of the sediment is 0.0039-0.062 mm in diameter.

Siltstone – sedimentary rock formed from silt – similar in character to a mudstone.

Mud – fine-grained clastic sediments with a particle diamter less than 0.062 mm. Deposited by rivers and tides.

Mudstone and shales – sedimentary rocks formed from muds. Muds are black and show poor or no bedding. Shales contain silt and clay and break readily along bedding planes.

Clay – most abundant component of fine-grained sediment. Less than 0.0039 mm in size. Consists largely of clay minerals.

Claystone – sedimentary rock made exclusively of clay-size particles.

Classification of chemical and biochemical sediments

Classification by chemical composition

Limestone – biochemical sediment consisting of lithified carbonate sands and muds. Consists largely of calcium carbonate (calcite or aragonite).

Dolostone – carbonate sediment consisting of calcium magnesium carbonate – dolomite CaMg(CO₃)₂. Formed by diagenetic alteration of limestone and other carbonate sediments.

Marine evaporites – chemical sediments formed by evaporation of seawater and the precipitation of salts. Consist of the mineral sequence calcite, gypsum and halite

Non marine evaporites – environments in shallow waters of continental shores. Sedimentation controlled by relatively gentle currents. Clastic or chemical sediments.

Chert – Biochemical sediment consisting of microcrystalline quartz, precipitated biochemically as the shells of ocean dwelling organisms

Organic matter – diagenesis of organic matter in sedimentary rocks produces coal, an organic sedimentary rock and oil and gas.