Title: A quantitative comparative study to investigate aggradation rate as a predictor of fluvial architecture: implications for fluvial sequence stratigraphy
1A quantitative comparative study to investigate
aggradation rate as a predictor of fluvial
architecture implications for fluvial sequence
stratigraphy
- Luca Colombera, Nigel P. Mountney, William D.
McCaffrey
Fluvial Eolian Research Group University of
Leeds
2Alluvial architecture models
Allen (1978)
LAB (Leeder-Allen-Bridge) models describing
large-scale fluvial architecture in terms of
channel belts distributed in floodplain
background. Fundamental implication with
supposed predictive value channel-body density
is inversely correlated to aggradation rate,
hence controlling channel-deposit connectedness
and geometries.
subsidence rate
Bridge Leeder (1979)
Allens (1978) model did not consider aggradation
rate as a predictor of sedimentary architecture.
Following models did.
3Fluvial sequence stratigraphy models
Wright Marriott (1993)
LAB model relationship between channel density
and aggradation incorporated in continental
Sequence Stratigraphy concepts, models and
practice.
- RSL-based systems tracts
- informal accommodation-based
- systems tracts
- accommodation-based settings
4Alluvial architecture models limitations
Bryant et al. (1995)
LAB models do not consider the complex manner in
which several controls may interplay in
determining variations in fluvial channel-body
proportions and geometries (e.g. role of
aggradation rate as a control on avulsion
frequency concurrent variation in channel
avulsion and mobility with changes in
aggradation).
Bristow Best (1993)
Some scale models contradict LAB model
predictions concerning aggradation rate and
channel density.
5Comparative study overview
SCOPE investigate relationships between
deposystem aggradation rates (and its temporal
variations) and large-scale fluvial sedimentary
architecture (and its temporal
variations). METHOD comparative study of
large-scale sedimentary architecture of several
fluvial successions for which constraints on
overall aggradation rate are available.
6FAKTS database
Relational database for the digitization of the
sedimentary and geomorphic architecture of
classified fluvial systems. Stored data include
types, geometries, spatial relationships and
hierarchical relationships of three order of
genetic units. Here, focus is on large-scale
depositional elements.
after Colombera et al. (2012)
7Data Entry
Depositional elements classified as
channel-complex or floodplain element, and
distinguished geometrically. Published summary
data also included.
8Database output
? GENETIC-UNIT PROPORTIONS
GENETIC-UNIT GEOMETRIES ?
? GENETIC-UNIT SPATIAL RELATIONSHIPS
? SYSTEM SPATIAL/TEMPORAL
EVOLUTION
9Database method limitations
- geometrical approach to the definition of
channel complexes (not representing
inter-avulsion channel belts) - common lack of 3D control for element
definition - source works having variable cut-offs of size of
smallest mappable units - necessity to include also summary data (e.g.
channel-complex W/T scatterplot) - common lack of control on down- and cross-system
variability - simplistic qualitative classification in
proximal-distal framework, or case-specific
quantification - subset attributes referring to average
conditions through time, even over different time
scales.
10Results channel-complex proportions
- Cases for which temporal evolution is tracked
- Omingonde Fm. (Holzförster et al., 1999)
- Chinji Fm.
- (McRae, 1990)
- Blackhawk Fm.
- (Hampson et al., 2012)
- Price River/North Horn Fm.
- (Olsen, 1995)
- No system displays a temporal evolution in
agreement with LAB model predictions - a weak positive relationships between mean
aggradation rate and channel-complex proportion
is observed across all case studies.
11Results channel-complex geometries
- Channel-complex maximum thickness considered
width distributions include real cross-stream
widths, uncorrected apparent widths and
incompletely observed widths - no clear trend is observed between the central
tendency or dispersion of channel-complex
thickness and the mean aggradation rates of the
stratigraphic volume - although a positive trend between
channel-complex median width and mean aggradation
rate is seen, this is not statistically
significant.
12Results channel-complex geometries
- Five out of six temporal changes show a positive
relationship between changes in channel-complex
thickness and changes in average aggradation rate
- the same temporal changes show a positive
relationship between changes in channel-complex
width and changes in average aggradation rate - evolution likely related to effect of
channel-body clustering.
13Results channel-complex geometries
Correction on empirical relationships linking
proportions with geometries to consider effect of
channel clustering
- Lack of any significant relationship between
aggradation rates and channel-complex normalized
geometrical parameters, when these two parameters
are considered together.
14Results channel-complex connectivity
- No particular relationship is seen between the
mean or maximum connected thickness and the mean
aggradation rate, when evaluated across different
systems - a positive relationship between variations in
mean connected thickness and mean aggradation
rate are observed within systems for which
evolution is tracked.
15Results channel-complex spacing
- A weak positive relationship is seen between
mean channel-complex spacing and mean aggradation
rate, but it is not statistically significant - negative relationships between variations in
mean channel-complex spacing and mean aggradation
rate are observed within systems for which
evolution is tracked.
16Discussion
- Lack of agreement on the univocal definition of
the concept of subaerial accommodation space - problems overlook of its three-dimensional
character, or the consideration of it as a pure
control on stratal organization - here accommodation as the volume within the
elevation difference between the long-term river
equilibrium profile and the topography
practically quantified as a vertical distance
rates of creation of accommodation inferred on
the basis of aggradation rates implication
difficulty in treating accommodation as a
variable that is independent of sediment supply.
17Discussion
Results do not support the use of aggradation
rate as a predictor of architectural style as
implied by the LAB models. Sequence stratigraphy
models and practice considering temporal changes
in channel proportions and geometry as indicative
of changes in the rate of creation of
accommodation (e.g. the use of accommodation-based
systems tracts) need to be re-evaluated. Evidence
is against the practicability of inferring low-
or high-accommodation settings from
channel-deposit proportions and geometries alone.
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18Conclusions future work
- Current work exposes the inadequacy of
established models and sequence stratigraphy
practice - necessity to substantiate results with more case
studies - necessity to focus this type of investigations
on architectural response to controls causing
changes in aggradation rate - necessity to evaluate architectural repsonses at
different time-scales. - All this requires a continuing effort in field
studies combining architectural characterization
with derivation of constraints on system boundary
conditions.
19Thank you for your attention!
- And thanks to our sponsors