What is a fan delta and how do we recognize it

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What is a fan delta and how do we recognize it?
Introduction
Fan-delta concept and related problems
Geometry and related characteristics of the
feeder system
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Introduction
During the last two decades, since the term 'fan
delta' was introduced by Holmes (1965),
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Fan-delta concept and related problems
Existing definition
Suggested modifications
Towards a revised definition
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Existing definition
In the existing literature, fan delta has been
defined most commonly as 'an alluvial fan that
has prograded from an adjacent highland into a
standing body of water, either a lake or the
sea' although somewhat differently phrased, this
original definition is attributed to Holmes
(1965). The definition, however clear in itself,
relies heavily on the definition of an alluvial
fan. The latter, in turn, is normally defined in
broad, essentially geomorphic terms to encompass
at least some typical representatives of the
actual spectrum of fans encountered in nature
. Since the existing definitions commonly invoke,
either as the basis of definition or for sheer
illustration, the classical, 'semi-conical' or
steep dry-type fans, ideally related to tectonic
escarpments, some authors tend to restrict the
term 'alluvial fan' (and also 'fan delta') to
such cases only (e.g. McPherson et al., 1987,
this volume Wet type (stream-dominated) alluvial
fans, particularly those fed by glacial sources,
have been considered 'non-representative' of
alluvial-fan processes or deposits, and virtually
discarded as members of the alluvial-fan spectrum.
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The main arguments for such consideration
(summarized by McPherson et al., 1987) are as
follows (i) The sedimentary processes and facies
of wet-type fans often resemble those of
braidplains or braided rivers, hence are
difficult to distinguish from those in the rock
record. (ii) The gradients of wet-type fans are
usually much lower and dimensions larger than
those of 'typical' alluvial fans (iii) Some
wet-type fans are related to glacial sources
rather than to faulted mountain fronts, hence
'valuable paleotectonic and paleogeographic
information is lost' if wet fans are considered
as alluvial fans. It should be noted that there
are many other depo-sitional systems whose
products are similar and often appear difficult
to identify or to discriminate from one another
in the ancient record, especially when the
exposures are limited or inadequate even such
disparate systems as braided rivers or deltas and
deep-sea fans are cases in point .
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Suggested modifications

• In our opinion, there are at least three aspects
  of the original concept and definition of a fan
  delta that seem to require reconsideration and
  modification at thepresent stage. These are
  discussed briefly below.
• Fan deltas have often been confused with
  fan-shaped common (river) deltas, as correctly
  pointed out by McPherson et al. (1987, and this
  volume). In order to avoid such confusion, we
  suggest that the adjectives 'alluvial-fan' delta
  and 'river' delta be used, at least when the
  definitions are given and whenever any
  possibility of confusion appears in scientific
  reports.
• By analogy with river deltas, an alluvial-fan
  delta should be considered as the delta of an
  alluvial fan. In other words, an alluvial-fan
  delta should not be defined as 'an alluvial fan
  (that has prograded... etc.)', but as the actual
  delta developed by such fan. A river delta is the
  delta created and fed by an active drainage
  system in the form of a solitary river
  (single-river system), whereas an alluvial-fan
  delta, by analogy, is created and fed by an
  active alluvial-fan system.
• The definition of a fan delta should not imply
  that the alluvial fan necessarily has to
  prograde, all the way from a highland, to reach
  the standing body of water this might also imply
  that the resulting depositional succession must
  necessarily represent a pro-gradational
  regression.

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Towards a revised definition
As the conceptual basis for a working revised
definition, our proposal is as follows a fan
delta is a coastal prism of sediments delivered
by an alluvial-fan system and deposited, mainly
or entirely subaqueously, at the interface
between the active fan and a standing body of
water. Fan deltas represent interaction between
heavily sediment-laden alluvial-fan systems and
marine or lacustrine processes. A fan delta may
be marine or lacustrine. A fan-delta complex
consists of coalescent or vertically
stackedfan-delta lobes created by an alluvial fan
or fans . 'Fan complex' means coalescent or
stacked fan lobes. For convenience, the term
fan-delta system may be used to encompass a fan
delta and its subaerial feeder fan. A
stream-dominated fan delta is normally built in
such a way that the channels can be extended just
above the sea- or lake-level (flattening of the
fan slope and the development of a distinct
'delta plain'). whereas mass flow-dominated fans
usually prograde offshore without building such a
platform-like sub-aerial extension.
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Geometry and related characteristics of the
feeder system
Alluvial fans
Recognition of fan deltas
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Alluvial fans
As a geological term, '(alluvial) fan' includes
more than the classically simple, semi-conical or
triangular-shaped (in plan view) accumulations at
the mouths of valleys or ravines, ideally
associated with active tectonic scarps. Such
'ideal' fans (Fig. 1, A-C) are developed when an
incised trunk stream emerges from a region of
rugged relief on to a plain in which this stream
is free to change course, by channel swinging or
branching, through an arc of approximately 180.
Several aspects of fan geometry can beseverely
affected by active tectonics or pre-existing
basin-floor topography. If faulting or
downwarping persists and the subsidence is fairly
uniform, the fan may be thickest next to the
fault scarp and thus may attain a truly
semi-conical geometry but when the basin-floor
subsidence is relatively slow. many fans appear
to be thickest midway between their upper and
lower segments.
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Recognition of fan deltas
In our opinion, the main reason for this lack of
interest has been the widespread lack of belief
that fan deltas might be sufficiently distinct
systems to be recognizable in the stratigraphic
record. Rust states (p. 10) The term fan delta
has been applied to alluvial fans deposited
partly in standing water in a lake or the sea...
However, apart from minor reworking by waves,
there is little evidence that coarse-grained fans
formed in this way are sufficiently different
from those of completely subaerial origin... in
the ancient record they would be identifiable
only where the alluvial deposits were interbedded
with strata containing marine fossils'. We know
today that fan deltas are distinct depositional
systems, and that their facies characteristics
are specific and often almost unique. As a
principal criterion in fan-delta recognition,
there should be the evidence of an alluvial fan
as the feeder system of the delta. However, we
tend to consider the term 'alluvial fan' rather
broadly, as discussed earlier. Proximity to a
highland or u fault scarp is important, but not
critical. Documented examples of ancient wet-type
fans indicate fan radii (axial lengths) ranging
from a few kilometres to as much as 200-300 km
(see review by Schumm. 1977)moreover, there are
fans virtually unattached to a highland.