Geology and ground models - Practical Engineering Geology - page 52

the distribution of excess electrical charges, they tend to link end-to-
face and form open structures with very low density and high water
content. This open structure is broken down, partly by bioturbation
but also by the weight of overlying sediment. The clays develop a
laminar structure and water is squeezed out. Eventually, once the
clay is buried by about 2 km, theoretically there may be dry contact
between clay crystals with the development of strong, covalent bonds
(shared ions) (Osipov, 1975). There would also be transformation of
clay, say from smectite to illite. During burial and self-weight compac-
tion and consolidation, water may be expelled dramatically, with the
formation of mud volcanoes on the sea floor. Mud volcanoes also

occur on land, with the mud apparently sourced by erosion of mud-
stones at great depth, although the mechanism is uncertain. The LUSI
mud volcano in East Java has had eruption rates of up to 180,000m 3
per day, continuing over several years (Davies et al., 2011).

3.3.4.2 Sedimentary environments
The sketch in Figure 3.13 illustrates a number of sedimentary environ-
ments. The source rock, nature of weathering and erosion and especially
the method by which the sediment is transported and

finally deposited
result in the wide range of sediments and sedimentary rocks encountered.

3.3.4.2.1 ONSHORE
Sediments deposited on land generally include colluvium (landslide
deposits and slope wash) and glacial deposits, which are often poorly

glacial moraine

poorly sorted wadi deposits in
ephemeral stream

rock fall
deposits

alluvial sands and gravel

desert sands

deltaic sands in subsiding
basin

welded tuff

salt cemented,
sabkha

sea

beach
deposits
at coast

salt deposits in
inland lake (hot
climates)

deep ocean muds

offshore sand and muds,
fossiliferous, sometimes
calcareous

submarine landslide/turbidity
current − massive greywackes

Figure 3.13 Examples of sedimentary environments.