Objectives: 

Sedimentary Rocks

Sedimentary are formed from either debree of other rocks (clastic sedimentary rocks) or by chemical precipitation from a body of water.  Processes of erosion and deterioration, such as mechanical and chemical erosion, weathering by wind water and ice, deposition in a body of water and compaction and cementation are important in the formation of sedimentary rocks.  Sediments are deposited in a wide variety of different environments, including oceanic, lacustrine (lake) and alluvial (stream) environments.  In alluvial environments, larger particles can stay in suspension if the stream water is turbulent and in a high energy state.  As the energy state and turbulence decreases, the large particles are the first to drop out of suspension.  As the water continues to migrate to lower energy conditions downstream, finer and finer particles are deposited, with clay-sized particles being transported the furthest. 

The most common constituents of sedimentary rocks include quartz, calcite, clay, other minerals, and rock fragments.  Quartz is the most abundant of these minerals in sedimentary rocks, particularly in clastic sediments.  Since it is so hard, it resists degradation by mechanical erosion, and it is also very stable in the surface environment and therefore resists chemical decomposition.  Calcite is the major component of limestone, where it occurs as a direct precipitate from seawater or in the form of shells of shelled organisms.  Calcite is also a very common cementing agent in sandstones and shales.  Clay minerals form by the decomposition of feldspars an other silicate minerals concentrated in the earth’s crust, particularly in igneous rocks.  The fine-grain size of the clay particles means they can be transported great distances, to quiet water environments where they can be deposited.

Sedimentary rock textures fall into three categories:  1) clastic textures (containing fragments of other rocks), and 2) crystalline textures (grown from solutions), and 3) organic.

Clastic Sedimentary Rocks

Clastic sedimentary rocks are are made from fragments of other rocks (Figure 7-1) and are generally classified according to grain size of the grains or fragments (Figure 7-2).  Degree of rounding and sorting provides clues about the history of the clastic sedimentary rock.  The more rounded the grains of the rock, the longer it has been exposed to abrasion.  Rounded particles usually indicates transport by wind or water, whereas angular particles usually indicates transport by ice or directly by gravity movement.  Degree of sorting is another indicator of the history of the rock.  Prolonged washing by water and currents causes particles of similar size and density to become concentrated together, which are said to be well-sorted sediments.  Glacial environments tend to deposit particles chaotically, mixing different sizes together, and are said to generate poorly-sorted sediments.  The sediments finally become lithified and turned into rock when a cementing agent, such as calcite, quartz, iron or chert, binds the particles together.

Fig. 7-1
A. Conglomerate	B. Unsorted Sandstone	C. Sort Sandstone
pending permission to use	pending permission to use	pending permission to use

Crystalline Sedimentary Rocks

When minerals are precipitated from seawater or lakes and develop a network interlocking crystals they form what are called crystalline sedimentary rocks.  The most well known example is limestone, which forms by precipitation of calcite from seawater.  Crystalline sedimentary rocks are also classified according to size of the grains (Figure 7-2).  Some types are microcrystalline, ie, the grains cannot be distinguished without the aid of a microscope.  The microcrystalline varieties are often characterized by the presence of banding.  Other varieties of limestone include oolitic limestone (containing small spheres) and skeletal limestone.  Oolitic textures form by the accretion of calcium carbonate to a tiny shell fragment during wave action on a beach.  Skeletal textured limestone forms when shelled organisms die and their hard calcite shells sink to the ocean floor and become lithified.  Other types of chemically precipitated sedimentary rocks include rock gypsum and rock salt, which form “evaporites”,  banded iron deposits, phosphorous deposits and chalcedony.

pending permission to use	Figure 7-2 A.  Size classification of clastic sedimentary rocks (from Hamblin & Howard, 1971).
pending permission to use	Figure 7-2 B.  Size classification of crystalline sedimentary rocks (Hamblin & Howard, 1971).

 

Organic Sedimentary Rocks & Other Substance

Organic sedimentary rocks include substances such as peat and coal, both of which are formed from plant remains which accumulate in a swamp environment.  Peat is only partially disintegrated plant material, while coal is plant material which is completely altered to carbon.  The three main varieties (in increasing order of competence) are lignite (brown coal), bituminous coal (soft, black coal), and anthracite (hard, black coal).   Petroleum might also be classified in the same group with these other organic substances.  Most petroleum deposits are hosted in sedimentary rocks and their sources are typically organic rich shales.  During compaction and lithification of the shales, the oil is liberated and may migrate through and be deposited in permeable formations such as sandstones.

LowTemperature Sediment-Hosted Deposits (LTS)

Another type of sediment-hosted deposit are low temperature copper and lead deposits hosted in clastic sedimentary rocks such as sandstone, siltstone, shale and dolomite.  Locally these deposits have significant silver and cobalt content.  The deposits are more or less stratiform, and have relatively less deformation than many deposit types (Figures 7-3 and 7-4).  Evidence suggests the host rocks were not metamorphosed to any significant extent.  Other evidence suggests the mineralization may have been formed during or just after diagenesis.  The deposits consist of disseminated and hosted in red bed and evaporite sequences, siltstones or dolomite.  LTS deposits often contain red bed units which are interbedded with organic-rich siltstone, shale or sandstone.  The organic rich unit typically serves as the host rock for this type of mineralization.

Ores precipitate in the rocks as a result of complex oxidation and reduction reactions.  The deposits are associated with alternating beds of oxidized and reduced mineral assemblages. The salty or briney fluids appear to have been involved with the transport of the metals in solution.  The high organic content of the host rocks served to fix the metals from the solutions and cause precipitation.   The mineral assemblages show a regular change over time.  For example pyrite becomes replaced by chalcopyrite, which is later replaced by bornite and magnetite and hematite. 

Figure 7-3.  Mufulira copper deposit, Zambia Copper Belt, example of LTS deposit.

Figure 7-4.  Kuperschiefer LTS hosted copper, lead and zinc deposit (after Rentzsch, 1974).

Mississippi Valley Type Deposits (MVT)

Mississippi Valley Type (MVT) deposits are similar to LTS deposits in that they are hosted in sedimentary rock sequences and deposited by low temperature, briney fluids.  In contrast, they are always hosted in carbonate rocks and have simple mineralogy consisting almost entirely of galena and sphalerite, with locally abundant fluorite and barite. MVT do however have evidence for multiple mineralizing events forming a complex history.  They typically occur as coarse- to fine-grained euhedral sulfides in open space fillings in limestones and dolomites affected by karsting and dolomitization processes.  The geometry of MVT’s ranges from veins to mantos (parallel, subhorizontal layers).