Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Fig. 1: Reconstruction of Darwin’s coral reef specimen exhibit and its likely relationship to his generalized reef
transect for Cocos (Keeling) atoll (published in simplified form in DARWIN 1842). We infer that caption “2 and 3”
refers to these two specimens of Millepora platyphylla HEMPRICH & EHRENBERG 1834.
How does Darwin’s work at Cocos (Keeling) relate to ancient reefs? Around that time, Darwin made an
intriguing and important sketch (CUL DAR 41.83, STODDART 1995) of how he envisaged a subsiding atoll’s
growth through geological time. Although he never published a fair copy of this, his long footnote (1842,
pp. 116-118) is conceptually related to his sketch (“I may take this opportunity of briefly considering the
appearances, which would probably be presented by a vertical and deep section across a coral formation
…. This is a subject worthy of attention, as a means of comparison with ancient coral strata.”).
152
x
B
A
Fig. 2: Darwin’s sketched reef model
(CUL DAR 41.83 in STODDART 1995)
envisages reef breccia and detrital
material etc. prograding seawards (i.e.
to left) over in situ corals, and this
process repeating itself as successive
subsidence creates new
accommodation space for the corals to
start again. A and B were parts of a
single diagram by Darwin, here
separated into atoll rim (A) with inset
(B) of seaward reef. Symbol ‘x’
indicates Darwin’s imagined outcrop
profile of an upraised or ancient reef.