Page 162 - Sedimentology and Stratigraphy

Further Reading 149

Some types of modern and fossil soils have been deposits are mainly alternating thin sandstone sheets
given particular names. For example, seatearths are and mudstones with palaeosols; small lenticular bodies
histisols, argillisols or spodosols that are common in of sandstone may represent crevasse splay deposition.
the coal measures of northwestern Europe and North Palaeocurrent data from within channel deposits
America (Percival 1986). They are characterised by a are unidirectional, with a wider spread about the
bed of organic matter underlain by a leached horizon mean in meandering river deposits; palaeocurrents
of white sandstone from which iron has been washed in overbank facies are highly variable.
out. Laterites are oxisols that are the product of Alluvial fan deposits are located near to the mar-
extensive weathering of bedrock to form a soil that gins of sedimentary basins and are limited in lateral
consists mainly of iron and aluminium oxides: exam- extent to a few kilometres from the margin. The facies
ples of laterites may be found in the stratigraphic are dominantly conglomerates, and may include
record as strongly reddened layers between basalt matrix-supported fabrics deposited by debris flows,
lava flows and provide evidence that the eruption well-stratified gravels and sands deposited by sheet-
was subaerial. Iron-rich oxisols that become cemen- flood processes and in channels that migrate laterally
ted are known as ferricretes and they are a type of across the fan surface. Alluvial and fluvial deposits
hardened soil profile called a duricrust. Duricrusts will interfinger with lacustrine and/or aeolian facies,
are highly resistant surfaces that develop over very depending on the palaeoclimate, and many (but not
long time periods (e.g. they are found associated with all) river systems feed into marine environments via
major unconformities; Retallack 2001); as well as coasts, estuaries and deltas. Other characteristics of
iron-rich forms there are records of silcretes, which fluvial and alluvial facies include an absence of ma-
are silica-rich. rine fauna, the presence of land plant fossils, trace
Identification of a palaeosol profile is probably the fossils and palaeosol profiles in alluvial plain deposits.
most reliable indicator of a terrestrial environment.
Characteristics of fluvial and alluvial fan deposits
Channels are not unique to the fluvial regime because
. lithologies – conglomerate, sandstone and mudstone
they also occur in deltas, tidal settings and deep ma-
. mineralogy – variable, often compositionally imma-
rine environments, and thin sheets of sandstone are
ture
also common to many other depositional settings.
. texture – very poor in debris flows to moderate in
However, sometimes the recognition of a palaeosol
river sands
can be made difficult by diagenetic alteration (18.2),
. bed geometry – sheets on fans, lens shaped river
which can destroy the original pedogenic features.
channel units
. sedimentary structures – cross-bedding and lamina-
tion in channel deposits
9.8 FLUVIAL AND ALLUVIAL FAN . palaeocurrents – indicate direction of flow and
DEPOSITION: SUMMARY
depositional slope
. fossils – fauna uncommon, plant fossils may be
Fluvial environments are characterised by flow and common in floodplain facies
deposition in river channels and associated overbank . colour – yellow, red and brown due to oxidising
sedimentation. In the stratigraphic record the channel conditions
fills are represented by lenticular to sheet-like bodies . facies associations – alluvial fan deposits may be asso-
with scoured bases and channel margins, although ciated with ephemeral lake and aeolian dunes, rivers
these margins are not always seen. The deposits of may be associated with lake, delta or estuarine facies
gravelly braided rivers are characterised by cross-
bedded conglomerate representing deposition on chan-
nel bars. Both sandy braided river and meandering river FURTHER READING
deposits typically consist of fining-upward successions
from a sharp scoured base through beds of trough and
Best, J.L. & Bristow, C.S. (Eds) (1993) Braided Rivers. Special
planar cross-bedded, laminated and cross-laminated Publication 75, Geological Society Publishing House, Bath.
sandstone. Lateral accretion surfaces characterise Blum, M., Marriott, S. & Leclair, S. (Eds) (2005) Fluvial Sedi-
meandering rivers that are also often associated with a mentology VII. Special Publication 35, International Asso-
relatively high proportion of overbank facies. Floodplain ciation of Sedimentologists. Blackwell Science, Oxford.