Page 154 - Sedimentology and Stratigraphy

Alluvial Fans 141

formed by migration of a bar or dune bedform, but these sensitive indicators in continental facies of palaeocli-
features may be migrating obliquely to the main chan- mate are palaeosols (9.7).
nel flow. Palaeoflow directions determined from cross-
beds in braided river bar deposits can show a variance of
around 608 either side of the mean channel flow. The 9.5 ALLUVIAL FANS
sinuous character of a meandering river channel will
also result in flow indications that will range at least 908 Alluvial fans are cones of detritus that form at a
either side of the overall flow direction of the river. Large break in slope at the edge of an alluvial plain. They
numbers of measurements from cross-bedding are are formed by deposition from a flow of water and
therefore required to obtain a mean value that will sediment coming from an erosional realm adjacent to
approximate to the overall flow direction in the channel. the basin. The term alluvial fan has been used in
It is also important to distinguish between channel and geological and geographical literature to describe a
overbank facies, because flow directions in the latter will wide variety of deposits with an approximately con-
often be perpendicular to the channel. ical shape, including deltas and large distributary
river systems. Some authors (e.g. Blair & McPherson
1994) restrict usage of the term to deposits that are
9.4.3 Fluvial deposits and palaeogeography unchannelised (i.e. not river deposits) and occur on
relatively steep slopes, greater than 18. However,
Within ancient fluvial deposits, the recognition of lower angle cones of detritus deposited by rivers at a
different fluvial depositional styles (e.g. braided and basin margin are also generally considered to be allu-
meandering channel fill) along with changes in grain vial fans (see Harvey et al. 2005).
size of deposit can be used to reconstruct the palaeo- The ‘classic’ modern alluvial fans described from
geography and provide evidence of changes through places such as Death Valley in California, USA (Blair
time. It may be expected that a conglomerate depos- & McPherson 1994: Fig. 9.18) occur in arid and semi-
ited by a pebbly braided river will have, down palaeo- arid environments. However, alluvial fans also form
flow, equivalent age sandstone beds deposited in a today in much wetter settings (see Harvey et al. 2005),
sandy braided river, and that this may in turn pass and alluvial fan deposits occurring in the stratigraphic
down palaeoflow to finer grained deposits with the record may have been deposited in a wide range of
characteristics of deposition by a meandering river climatic regimes. Larger scale deposits of sediment
(Fig. 9.1). In additional to these spatial variations in such as cones of glacial outwash deposited by braided
the fluvial deposits, a change from braided river rivers have also been considered to be alluvial fans (or
deposits up through the succession (and therefore ‘humid’ fans – Boothroyd & Nummedal 1978) and
through time) to meandering river deposits may indi- even larger deposits formed by the lateral migration
cate a decrease in the gradient of the river and/or a of a river to produce a cone of detritus have also been
reduction in the discharge in the river system. considered to be types of alluvial fans (or megafans –
Rivers vary in size from small streams only metres Wells & Dorr 1987; Horton & DeCelles 2001).
in width and tens of centimetres deep to rivers tens of Scree cones formed primarily of rock fall and rock
kilometres wide and tens of metres deep. This range in avalanche are commonly associated with alluvial fan
channel size over several orders of magnitude is also deposits at the basin margin. Sediment bodies that
seen in channel-fill deposits in fluvial successions and consist of a mixture of talus deposits (4.1) and deb-
the dimensions of deposits can be used to infer the size ris-flow deposits (4.5.1) are sometimes called collu-
of the river, and hence the size of the drainage basin vial fans: these features are common in subpolar
from which it was supplied. Provenance studies on regions where gravity processes are augmented by
fluvial sediments provide more details of the drainage wet mass flows of debris (Fig. 9.19).
basin area, indicating the types of bedrock that were
exposed at the time of deposition and helping to build
up a palaeogeographical picture. Information about 9.5.1 Morphology of alluvial fans
the palaeoclimate can also be determined if ephemeral
and perennial flow conditions can be established from Alluvial fans form where there is a distinct break in
the character of the fluvial deposits, but the most topography between the high ground of the drainage