Page 54 - Sedimentology and Stratigraphy

Volcaniclastic Sedimentary Rocks 41

Peat is heterogeneous because it is made up of differ- 3.6.3 Oil shales and tar sands
ent types of vegetation, and of the various different
components (wood, leaves, seeds, etc.) of the plants. Mudrocks that contain a high proportion of organic
Moreover, the vegetation forming the peat may vary material that can be driven off as a liquid or gas by
with time, depending on the predominance of either heating are called oil shales. The organic material is
tree communities or herbaceous plants, and this will usually the remains of algae that have broken down
be reflected as layers in the beds of coal. A nomencla- during diagenesis to form kerogen, long-chain hydro-
ture for the description of different lithotypes of coal carbons that form petroleum (natural oil and gas)
has therefore been developed as follows: when they are heated. Oil shales are therefore impor-
Vitrain: bright, shiny black coal that usually breaks tant source rocks of the hydrocarbons that ultimately
cubically and mostly consists of woody tissue. form concentrations of oil and gas. The environments
Durain: black or grey in colour, dull and rough coal in which they are formed must be anaerobic to pre-
that usually contains a lot of spore and detrital plant vent oxidisation of the organic material; suitable con-
material. ditions are found in lakes and certain restricted
Fusain: black, fibrous with a silky lustre, friable and shallow-marine environments (Eugster 1985). Oil
soft coal that represents fossil charcoal. shales are black and the presence of hydrocar-
Clarain: banded, layered coal that consists of alter- bons may be detected by the smell of the rock and
nations of the other three types. the fact that it will make a brown, oily stain on other
Sapropelic coal has a conchoidal fracture and may materials.
have a dull black lustre (called cannel coal)oris Tar sands or oil sands are clastic sediments that
black/brown in colour (known as boghead coal). are saturated with hydrocarbons and they are the
Microscopic examination of these lithotypes reveals exposed equivalents of subsurface oil reservoirs
that a number of different particle types can be recog- (18.7.4). The oil in tar sands is usually very viscous
nised: these are called macerals, and are the organic (bitumen), and may be almost solid, because the
equivalent of minerals in rocks. Macerals are exam- lighter components of the hydrocarbons that are pres-
ined by looking at the coal as polished surfaces in ent at depth are lost by biodegradation near the sur-
reflected light under a thin layer of oil. Three main face. The presence of the oil in the pores of the
groups of maceral are recognised: vitrinite, the origin sediment prevents the formation of any cement, so
of which is mainly cell walls of woody tissue and tar sands remain unlithified, held together only by the
leaves, liptinite, which mainly comes from spores, bitumen that gives them a black or very dark brown
cuticles and resins, and inertinite, which is burnt, colour.
oxidised or degraded plant material.
A further analysis that can be made is the reflec-
tance of the different particles, which can be assessed 3.7 VOLCANICLASTIC SEDIMENTARY
by measuring the amount of light reflected from the ROCKS
polished surface. Liptinites generally have low reflec-
tance, and inertinites have high reflectance, but vitri- Volcanic eruptions are the most obvious and spectacu-
nite, which is by far the most common maceral in lar examples of the formation of both igneous and sedi-
most coals, shows different reflectance depending on mentary rocks on the Earth’s surface. During eruption
the coal rank. Vitrinite reflectance therefore can be volcanoes produce a range of materials that include
used as a measure of the rank of the coal, and because molten lava flowing from fissures in the volcano and
coal rank increases with the temperature to which the particulate material that is ejected from the vent to form
material has been heated, vitrinite reflectance is a mea- volcaniclastic deposits (Cas & Wright 1987). The
sure of the burial temperature of the bed. This is an location of volcanoes is related to the plate tectonic
analytical technique in basin analysis (24.8) that pro- setting, mainly in the vicinity of plate margins and
vides a measure of how deep a bed has been buried. other areas of high heat flow in the crust. The pre-
The coalification of carbonaceous matter into mac- sence of beds formed by volcanic processes can be an
erals and coal lithotypes takes place as a series of post- important indicator of the tectonic setting in which
depositional bacteriological, chemical and physical the sedimentary succession formed. Lavas are found
processes that are considered further in section 18.7.2. close to the site of the eruption, but ash may be spread