Geopix UK - Sedimentation

Terrestrial Environments

Environments of terrestrial deposition are varied and variable. Some involve water, others do not. Where water is involved, it may change in amount and energy from season to season, or even from day to day. On so on.

There is also a lot of overlap:

a desert may have a river flowing through it, for example the River Nile across the Sahara
the deltaic environment includes the river and lake environments
fluvio-glacial deposits may interbed with glacial deposits.

Taken together, the rock type (lithology) and associated fauna and flora (fossils) can be assigned to a particular facies.

The main types of terrestrial environments include:

FLUVIATILE (rivers)

Rock types
sandstone (grit), pebbly sandstone, feldspathic sandstone (arkose)

Sedimentary structures
cross bedding;
asymmetrical ripples

Fossils
plants and freshwater molluscs

Other points
current bedding shows the direction of flow of the river and therefore indicates the position of the high land at the time

DELTAIC

Rock types
sandstone (grit), feldspathic sandstone (arkose); shale; coal

Sedimentary structures
cross bedding;
asymmetrical ripples;
channels and wash-outs

Fossils
plants and freshwater molluscs;
marine fauna when the sea floods across the delta (marine band)

Other points
cyclic conditions leads to the formation of cyclothems, a repeated succession of coal, shale and sandstone

LACUSTRINE (lakes)

Rock types
clays, silts, marl;
evaporite minerals in desert lakes

Sedimentary structures
lamination; varves in glacial lakes;
desiccation (mud) cracks and salt pseudomorphs in ephemeral desert lakes

Fossils
plants, freshwater mollucs and fish

Other points
lake deposits may include diatomite;
beach deposits may be preserved as terraces

DESERT

Rock types
millet seed sandstone, loess, marl;
wadi conglomerate

Sedimentary structures
dune bedding; asymmetrical ripples;
desiccation (mud) cracks

Fossils
none (except in lake deposits)