Misbehaving
Corals: Increased Cover with Depth in Chagos Archipelago Lagoons and the
Importance of Non-Reef Habitats
Riegl, Bernhard1, Sam Purkis1,
Charles Sheppard2 (1) Nova Southeastern University, Dania Beach, FL
(2) University of Warwick, Warwick, United Kingdom
The protected hydrodynamic regime and
higher temperature/salinity on the inside of large atolls and deeper off-reef
areas in general should translate into fine or muddy sedimentation and decrease
in prevalence of reef-builders. Wave energy, oxygenation, and regular oceanic
salinity presumably concentrate framebuilding on the atoll rim or the bioherm,
also he locus of the coarsest sediments. While generally true, neither all
atolls nor carbonate platforms behave uniquely that way. In the Chagos, we
found deep lagoons (>40m Peros Banhos, >30m Solomon) covered by extensive
and dense biostromal coral frameworks, so-called coral carpets, growing in a
reticulate fashion with spaces filled by Halimeda sediment. In Solomon, the
sediment in the coral framework was finer than in shallow-water but in Peros
Banhos coral cover and coarseness of sediment increased downslope of lagoonal
patch reefs to reach a maximum between 20 and 40m with 80% coral cover and 20%
Halimeda cover. The density of coral frameworks and Halimeda was likely a
function of flushing which is strong in both atolls. Biostromal coral
frameworks in deeper water are more widely distributed in the recent and fossil
Indo-Pacific and Tethyan realm than usually acknowledged, covering many thousand
square kilometers. Examples from the Paratethyan Miocene are used as
illustration. While coral biostromes can form framestones, they also produce
expansive float- and rudstones which have frequently been misinterpreted as
reefs or as deposited in the immediate vicinity of a reef, which is not always
the case.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California