Page 75 - Carbonate Sedimentology and Sequence Stratigraphy

66 WOLFGANG SCHLAGER

difference seaward and landward of the reef belt decreases, cies 3/4, the slope, and facies 5/6, the rim. All these belts are
wave energy levels become similar, too. Under these cir- characteristic of rimmed platforms in open-ocean settings.
cumstances, the barrier reef may be replaced by a zone of In summary, actualism and the standard facies belts re-
isolated, radially symmetric shelf atolls, on which standard main key concepts for the interpretation of ancient epeiric
facies belts still can be recognised (Fig. 4.13). In extreme sit- carbonates. The actualistic interpretation should rely on
uations, reefs and shelf atolls may spread over large areas three sources of insight: recent open-ocean carbonate sys-
of the epeiric sea and the facies belt 2 may gradually pass tems, recent carbonate epeiric seas and sediment dynamics
into facies 7, i.e. a lagoon-like inland sea with patch reefs. in recent siliciclastic epeiric seas.
This configuration, not uncommon in epeiric seas, lacks fa-

A)
Fig. 4.13.— Holocene-Pleistocene
shelf atoll in the Gulf of Carpentaria, a
recent epeiric sea. A) Bathymetric map
based on multibeam sonar survey. Flat
top at 27 m depth and lack of reefs built
to sea level indicate incipient drowning
of the reefs. The atoll was thriving in the
early Holocene (and intermittently in the
Pleistocene) when sea level was lower.
B) Seismic profile (Sparker) of shelf atoll
in A. Tentative interpretation in terms
of standard facies belts (in red, added
by the author) refers to early-Holocene
climax of reef growth and is based on
rock samples and microtopography
shown in A. After Harris et al. (2004),
modified. Copyright 2004, reprinted with
permission from Elsevier.

B)
0
S N
20 2 3/4 5 7 5 3/4 2

40 m