Size-Frequency Distributions of Carbonate Build-Ups in the Maldives Archipelago
The Maldives archipelago is well poised for the study of
scales of accumulation of carbonates. It is one of the largest assemblages of
reefs and carbonate platforms in the world, lies far from continental siliciclastics and has monsoon climate with less windward-leeward asymmetry
than the trade-wind belts. Reef development has proceeded uninterruptedly since
the Oligocene with a constant theme of deposition: a stiff rim of reefs holding
a fill of loose sediment. This pattern of growth, the “bucket
structure”, repeats across scale. It is as characteristic for individual lagoonal patch reefs as for the morphology of the entire archipelago. The
Maldivian reef features were digitized from Landsat imagery covering eight
orders of magnitude in area and analyzed for their area-frequency distribution.
Trends were then sought that give an adequate mathematical representation of
the data. In contrast to the power
-law distributions reported for contiguous facies mosaics, we show the trends within the Maldivian dataset to vary with
scale. Reefs that comprise the left tail of the size
spectrum
, from 10^3 -
10^5.5 sq. m, are clearly not a
power
-law but close to an exponential
size-frequency distribution. Units in this size range correspond to the
smallest instances of lagoonal and atoll rim patch reefs that can be resolved
from Landsat. The central portion of the dataset (10^5.5 - 10^8 sq. m) is
represented by larger lagoonal and atoll rim reefs and follows a
power
-law.
Reef units exceeding 100 sq. km, the atolls comprising the archipelago itself,
are separated from the central portion by a discontinuity. The atolls also
follow a
power
-law distribution, albeit with a different scaling exponent. The
results of the study, that the area-frequency relationship of the Maldivian
reefs systematically changes with size, has important implications for the
notion of predictability in scale of reefal carbonate deposition. We frame the
results against the ecology of reef growth and hydrodynamic set-up of the
archipelago in an effort to append real-world process information to the
observed switches in trend with scale.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California