Prepared by T. Weiland for GEOL 1121

Sedimentary rocks - rocks formed by the accumulation and consolidation of sediments (surficial processes).

I. Sediments - material that is deposited by wind, water or ice; material that is precipitated from seawater; or deposits of organic origin.

A. Major Components - (clastics, biogenic and chemical precipitates). *This is the basis for the classification of sedimentary rocks. The name of the rock indicates how the rock formed. It attempts to delineate the conditions of deposition and the source of the sediments.
1. Clastic - particles derived from the breakdown of pre-existing rocks (sedimentary, metamorphic or igneous). Clastic sediments make up >75% of all sediments on the earth's surface. Breakdown can be physical, chemical or biogenic; however, dissolution does not occur.
2. Chemical Precipitates - minerals that are precipitated from concentrated solutions. Examples - halite from brines, calcite from seawater, etc.
3. Biogenic - included with chemical sediments (called chemical organic) in the book; however, they deserve a separate mention. *Sediments that are produced through the life activities of plants and animals. Examples - CaCO3 - calcite produced by aquatic plants and animals forming coral reefs, quartz (silica-rich) deposits formed by the accumulation of the test of microscopic plankton animals.
B. Characteristics of Sediments - features that can be used to decipher the geologic history.
1. Composition - depends on the mineralogy of the source rock, duration and intensity of weathering (climate and relief) and the rate of burial.
*Most abundant components of sediments are quartz, feldspars, iron oxides, clays, calcite and rock fragments. (Low temp. minerals and fine grain size result in greater stability.)
2. Size and sorting - depend on the mineralogy of the source rock, distance traveled, and the method of transport water, wind or ice). **Used in conjunction with composition to determine rock name.
a. Size Classification - Wentworth Scale



Clastic Rx. Name






Conglomerate or Breccia












Siltstone (mudstone)



Mudstone or Shale

*Finer grain size often reflects a greater distance of transport. Example - continental margin deposits.
b. Sorting - range of particle size. This is controlled by the transporting agent (wind only transports fines - well sorted deposits, glaciers moves everything - poorly sorted deposits, and water is variable depending on amount of energy).
3. Roundness - the degree of abrasion of a clastic particle as shown by the amount of sharp edges on the grain. This is also controlled by the method of transport. Examples -wind transport is highly abrasive resulting in highly rounded grains; whereas, ice transport is more passive, depositing very angular clasts.
4. Color - highly variable - dependent on the original mineral color, weathered mineral color, percent of organic material, etc.

II. Sedimentary Environments - the physical, chemical and biologic conditions that exist at the location where the sediment is deposited. The different environments on the earth (deserts, ocean floor, beach, etc) are characterized by different types of sedimentary deposits. By characterizing the modern sediments, we can infer how and where the sedimentary rocks of the past, thereby extracting the geologic history from the rocks.

Handout #3

A. Continental Environments -fresh water and subaerial conditions.
1. Alluvial Fans - fan shaped deposits of gravel, sand and mud which form at the base of mountain ranges. Deposition occurs as flash floods and intermittent streams sweep the loose debris down steep gradients. At the base of the mountains, the water does not have enough energy to transport the sediment over flat areas.
Characteristics - coarse-grained, poorly sorted, very heterogeneous mixture of mostly rock fragments, sand and mud. (Form coarse-grained unsorted conglomerates).
2. Eolian Deposits - dunes constructed of wind-blown sand. Commonly found in desert areas.
Characteristics - well-sorted, quartz-rich (more resistant), well-rounded, sandstone is commonly formed which often displays cross-bedding.
3. Fluvial Deposits - form in association with river systems. The great rivers of the world are the major transporters of erosional material to the ocean basins.
Characteristics - highly variable, dependent on the amount of energy available for sediment transport. Recognition of fluvial deposits is generally dependent of stratigraphic relationships.
*Channel deposits are characterized by cross-cutting relationships with adjacent deposits, occasional cross-bedding and coarser-grained deposits than the flood plain or sand bars.
*Sand bars tend to have better sorted, lens-shaped deposits of sand which form in the inside of meanders, where the water velocity is lowest.
*Floodplains are characterized by fine-grained deposits of mud that often have a high percentage of organics (fertile).
4. Lake Deposits - Lakes are generally characterized by low energy. The larger the lake, the greater the degree of sorting and the finer-grained the sediments deposited in the center of the lake.
*Characteristics - fine-grained siltstone and shale, which are well stratified (layered) commonly, form in the central portion, whereas some well-sorted sandstone is also formed along the margins. Exception -playa lakes - lakes in arid areas which undergo extensive evaporation forming halite, gypsum and anhydrite to form. (White Sands N.M., Great Salt Lake, Death Valley).
5. Glacial Deposits - sediments deposited by the glaciers and their associated meltwater. The glaciers pluck and scrape rock material as they move along.
*Characteristics - generally less sorted and rounded than other deposits except possibly alluvial fans. Meltwaters carry sediments farther, resulting in a higher degree of sorting. Primary glacial deposits often contain a wide range of size and compositional varieties.
B. Shoreline Deposits - transitional or mixed environments.
1. Deltaic Deposits - include deposits of mud and sand that accumulate at rivermouths (where the rivers terminate at the coast). Example - Mississippi River Delta - 400 million tons of sediment are deposited here each year.
*Characteristics - often difficult to recognize due to the variety of sediments that occur in these environments. They are generally fine-grained, contain both marine and non-marine fossils, and sometimes have mud cracks.
2. Beach Deposits - include shoreline deposits such as lagoons, inlets, beaches, etc.
*Characteristics - usually quartz-rich, well rounded, well sorted mixtures of mud and sand or just sand. Cross-bedding is common in beach deposits. Abundant marine fossils.
3. Tidal Flat Deposits - areas covered by water at high tide and exposed at low tide.
*Characteristics - Predominantly mud deposits with abundant marine fossils. Mud cracks, ripple marks and burrows are also common.
C. Marine Environments - the deeper and more restricted the water, the finer grained and better sorted the deposits.
1. Shallow Marine Deposits - includes from the shoreline to the edge of the continental shelf. Deposits depend on the sediment supply, climate, wave energy, circulation and temperature.
*Characteristics - sand, calcium carbonate, and mud are the major constituents. Well-sorted sands found at areas of high wave energy and mud found in the deeper more restricted areas. Marine fossils are common.
2. Organic Reefs - ridge- or mound-like structures built of sedentary calcareous organisms (esp. corals). These deposits form only in shallow, warm, clear water where there is minimal sediment input.
*Characteristics - in situ remains of calcareous organisms.
3. Deep-marine Deposits - includes the ocean floor beyond the continental shelf (deep-sea floor and abyssal plains). Because these areas are so removed from any sedimentary sources, only biogenic and extremely fine-grained sediments are deposited.
*Characteristics - fine-grained well-sorted deposits of shale, mudstone, chert (accumulation of siliceous organisms). *Turbidites - sediment deposited by bottom-flowing density currents laden with coarser-grained sediment. These deposits are characterized by graded-bedding.

III. Diagenesis - all changes that the sediment undergoes after its initial deposition (exclusive of weathering and metamorphism.

Lithification - process where a sediment is converted into a rock.

A. Compaction - rearrangement of particles due to increasing weight of overlying materials (results in a reduction of pore space and volume).
B. Cementation - precipitation of crystalline materials between particles. The most common cementing agents include silica (SiO2), calcite (CaCO3) and iron oxide (Fe2O3). These cements are usually derived from circulating pore fluids that become saturated with these minerals.
C. Mineralogic Changes - diagenesis often changes the mineralogy of the sediment. This includes the breakdown of less stable grains such as feldspars to form a clay matrix, the dissolution of grains or matrix such as shell material, or the transformation of organics to hydrocarbons.

IV. Sedimentary Rock Types - the various rock types reflect different methods of transport, various source rocks, and different environments of deposition.

A. Clastic Rocks - those composed of particles derived from pre-existing rocks. These rocks are classified according to grain size.
1. Conglomerate - cemented, rounded, gravel-sized particles.
2. Breccia - cemented, angular, gravel sized particles.
3. Sandstone - cemented, sand-sized particles which often contain a high percentage of quartz due to their high maturity. (Exception - South Florida beaches)
4. Siltstone - cemented, silt-sized particles.
5. Claystone - cemented clay-sized particles. *If the rock is composed of two or more grain sizes, the name is based on the most abundant size.
6. Shale - fine-grained rock composed of silt and clay which tends to part along the bedding planes. It is the most abundant sedimentary rock.
B. Chemical Precipitates - formed by the precipitation of minerals from saturated solutions.
1. Limestone - CaCO3 (only a minor fraction of all limestone appears to have formed this way).
2. Dolomite - MgCO3 - can form by the direct precipitation of seawater or by the diagenesis of limestone by Mg-rich pore fluids.
3. Chert - can form by the direct precipitation of seawater, but most commonly forms by biogenic processes.
4. Evaporites - precipitates formed by evaporation of seawater. Includes halite, gypsum and anhydrite.
C. Biogenic Accumulates
1. Limestone - can form by the accumulation of any calcareous remains.
coquina -loosely cemented shell material coral - accumulations of the hard exoskeleton secreted by coral polyps.
chalk - soft, porous limestone made of microscopic marine organisms (commonly deep-marine also.)
2. Chert - siliceous deposits of diatoms (microscopic plants with siliceous coverings (deep-sea deposits).
3. Coal - organic deposits that form in swamps and are later deeply buried until converted into a carbonaceous rock.