Geology Reference
In-Depth Information
Different outcrop or map scales simply will
detect patches of different sizes but in compara-
tive proportion. Thus, in the illustrated case here,
it is believed that both systems were similarly
patterned in space and therefore the fi xed-point
vector as the bridge between space and time would
hold at all scales and between scales. This would
remove the need to compare landscapes and suc-
cessions of comparable sizes. In this study, facies
were at best several metres thick and a small
outcrop of few hundred square-metres could be
plausibly interpreted as accommodating a compar-
able number of facies (bearing in mind that the
assignment of facies always remains somewhat
subjective) as the moderately large modern study
area of about 70 km
2
. Diffi culties may arise when
estimating facies of vastly varying thickness, as
would be expected to be caused by variable depos-
ition rates. While the assumption of the fractal
nature of the landscape may still hold, care would
be needed to assure sampling at appropriate
scales - which would have to be evaluated using
spatial statistics.
Transition weightings between facies depend
strongly on environmental factors and a complex
interplay exists between ecology and sedimento-
logy. When deriving the temporal facies models
from the spatial adjacencies, the ecology of facies-
determining organisms (coral, algae, seagrass)
guided the choice of which transitions were pos-
sible. In particular, the dynamics of the corals,
as producers of bioherms, biostromes or rubble
in the recent Arabian Gulf and the Miocene
Leitha Limestone, strongly infl uenced the sedi-
mentology by producing well-defi ned biofacies
that were directly equivalent to sedimentary
facies. Algae and seagrass interacted with the
sedimentary environment in a more passive
way, presumably primarily as binders and baf-
fl ers. However, simply their presence and cover
of unconsolidated sediments would to a certain
extent protect the latter from erosion, thus giv-
ing them sedimentological relevance. Their clear
ecological preferences and distribution on the
banks made them useful indicators for facies on
the satellite images and proxies in the models.
Dense algae clearly was found primarily on the
grainstone-dominated platform edges and as sec-
ondary cover on dead coral frameworks, while
dense and sparse seagrass was found primarily
on the fi ner-grained bank interior (Purser, 1973;
Schlager, 2005).
An obvious key environmental factor with the
potential of shaping facies distribution is the
depth of the water column through which light,
thermal and wave energy transit to the benthic
biota and sediments. The importance of small-
scale sea-level oscillations and their infl uence
on the distribution of facies in shallow water is a
matter of active debate (Lehrmann & Goldhammer,
1999; Rankey, 2004; Burgess, 2006). Directional
changes in the facies transition likelihoods were
introduced to simulate sea-level, and via the
fi xed probability vector the expected distribu-
tion of facies in a new environment was obtained.
Such thought-experiments have the potential of
increasing our understanding of both lateral as
well as vertical facies succession, however, they
are only valid if the facies can indeed realistic-
ally transit into each other. In this study area,
a relatively mild depth gradient was observed
(Fig. 15d) with the potential of facies moving
relatively unhindered up and down the bank and
expanding or shrinking at each other's expense
in response to a small change in sea-level. Other
than eustatic, such changes could be caused
by gentle subsidence and subsequent fi lling of
accommodation space. If seen from this perspec-
tive, the model could be used as a quantitative
analogue to Ginsburg-type autocyclic sedi-
mentation (Ginsburg, 1971; Hardie & Shinn, 1986;
Schlager, 2005).
As can be expected in an offshore bank set-
ting, shallow and nearshore facies become more
and more limited in area in the rising sea-level
scenario, and in the falling sea-level scenario
the shallow infratidal and the deeper facies
became restricted. This disadvantage of deeper
facies was caused by the model not consider-
ing seafl oor > 8 m. Interestingly, this depth is
the boundary between typical high light/shal-
low and low light/deeper biota and sedimentary
environments in the recent Arabian Gulf. Yet
by masking, an artifi cial lower boundary was
created that need not exist. Facies occurring in
deeper water could simply move downslope
and therefore not require as markedly increased
refl ection (translated into decrease of facies
extent) at the lower boundary as in the model.
However, since the bathymetric information
was derived from the same satellite image as the
information about facies, it was limited to the
same depth and speculation was restricted to
what lay beyond. In particular, slope geometry
will have to be considered when parameteriz-
ing such models, since it will defi ne how facies
can migrate up or down while tracking sea-level.
Any cycles generated from such a Markov model