Environmental Engineering Reference
In-Depth Information
and bone beds, as partially decomposed sediments such as the peat-lignite-coal sequence
or as completely decomposed organic precipitates in carbonate and bio-silicate rocks.
Dissolved minerals are precipitated in most environments but especially in the
terrestrial warm arid zone, in tropical tidal zones and in deep oceans. Evaporation leaves
dissolved minerals which recrystallize from shallow lakes, or fluids surfacing through
capillary action to form surface concretions, and tidal lagoons and mud flats (
sabkhas
and
salinas
). The most common
evaporites
and their total solids percentage in sea water
are
halite
(NaCl, 78 per cent),
potash salts
(K
2
SO
4
, 18 per cent),
gypsum/anhydrite
(CaSO
4
/CaSO
4
.2H
2
O, 3·6 per cent) and
calcite
(CaCO
3
, 0·3 per cent). Fractional
recrystallization occurs in the sequence carbonates → anhydrite/gypsum → halite →
potash and is common today in tropical epicontinental seas such as the Persian Gulf,
intermontane basins (Great Salt Lake, Utah, and Salar de Atacama, Chile) and continental
basins (Lake Eyre, Australia). They were extensive in Permian tropical Pangaea, when
the Zechstein Sea and Delaware basins generated the large evaporite deposits of modern
northern Europe and the south-western United States. Capillary action due to high
evaporation in arid environments also draws dissolved minerals to the surface, forming
crystalline
duricrusts
of calcrete (Ca-rich), silicrete (Si-rich) or ferricrete (Fe-rich)
(Figure 12.16). Deep marine precipitation is explained below.
DIAGENESIS AND LITHIFICATION
Diagenesis describes alterations which sediment may experience - either
syn
genetically,
as facies accumulate, or at any time thereafter - up to the point of any eventual
metamorphism, weathering or erosion.
Lithification
refers only to those diagenetic
mechanical and chemical processes which convert unconsolidated and invariably wet
sediment to hard 'dry' rock, primarily by autocompaction and dewatering. This
distinguishes between diagenetic changes, such as the development of
load casts
and
bioturbation structures
, which alter the character of rock without substantially altering
its strength; and wholesale decomposition, solution, leaching, replacement of soft body
parts and the conversion of less stable aragonite skeletons to calcite.
Overburden pressure from accumulating sediment piles and overlying ice or water
bodies stimulates consolidation, by reducing the void ratio to a level determined by
sediment texture and pressure and expelling pore fluids. Mass shrinkage, which can
exceed 50 per cent, establishes orthogonal joint or
discontinuity
systems in which
bedding planes form the principal horizontal set. Dewatering (dehydration) may also
create load casts
