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sition of the sediment. Distributional patterns of cal-
careous algae and benthic foraminifera as well as the
occurrence of particular grain types (e.g. ooids, oncoids)
are important facies-diagnostic criteria. In addition, ce-
ment types and other diagenetic features can be used
in recognizing subaerial exposure and sea-level falls.
The following three case studies on platforms of
different types and ages are based on the investigation
of geological sections or core sections and the inter-
pretation of microfacies data derived from thin-section
analyses.
15.6.3.1 A Late Jurassic BahamianType
Carbonate Platform from the Northern
Calcareous Alps
The example shown in Pl. 134 comes from Sulzfluh
limestone at the Austrian/Swiss border (Ott 1969;
Flügel 1979), a Late Jurassic (predominantly Tithonian)
non-attached platform
resting on tectonic highs within
a deep ocean. Sulzfluh limestone is a massive and thick-
bedded white limestone exposed in spectacular moun-
tain cliffs. The limestone is generally very poor in non-
Text continued on p. 768
Plate 133 Microfacies of Platform Drowning: Early Jurassic of the High Atlas (Morocco)
Drowning of carbonate platforms is reflected by drastic changes in microfacies and in dominant grain types. The
carbonates of the Jbel Bou Dahar platform in the High Atlas of Morocco documents environmental changes in
platform interior areas and various parts of the slope (Blomeier and Reijmer 1999). Profiles comprise all phases
of platform growth and demise. Bed-per-bed studies and microfacies data combined with the investigation of
large-scaled platform geometries and depositional features show that the ultimate drowning resulted from a
series of disadvantageous processes starting with (a) the destruction of former reef communities related to sub-
aerial exposure caused by high-frequency sea-level fluctuations at the end of the pre-drowning phase, followed
by (b) local environmental effects during the renewed flooding of the platform top, hampering the re-establish-
ment of a flourishing carbonate-producing community, (c) prevailing high-energy conditions during almost the
entire drowning phase, resulting in continuous transport of eroded platform-top material across the margins to
the slope, (d) competition between better adapted echinoderms and mollusks with the carbonate-producing or-
ganisms, and (e) last but not least regional/global changes in ocean circulation patterns, producing unfavorable
conditions for platform growth during the drowning phase.
The plate shows characteristic microfacies types of the pre-drowning, drowning, and post-drowning phases
(see Fig. 15.14A). The drowning phase might have lasted several hundred thousands of years.
Pre-drowning phase
1
Platform interior.
Laminated pelmicrite. Standard Microfacies Type SMF 22.
2
Upper slope.
Coarse, poorly sorted grainstone/rudstone. The recrystallized skeletal grains are encrusted by foraminifera
and bryozoans. Arrows point to composite grains. SMF 5.
3
Lower slope.
Fine-grained, well-sorted packstone. Grains are peloids, terrigenous quartz, sponge spicules, and thin-shelled
mollusks. White arrows point to burrowing.
Drowning phase
4
Platform top.
Bioclastic grainstone. Most grains are echinoderms and recrystallized shells. The black grain is an extraclast.
Characteristic for the drowning phase is the abundance of suspension feeders (echinoderms). Note the clear syntaxial
cement enclosing the echinoderm grains. These cements are common in grainstones deposited on the top of the platform
during the drowning phase. SMF 12.
5
Mid-slope
. Coarse-grained, poorly sorted rudstone consisting of large rounded extraclasts and smaller mud peloids. The
extraclasts are a characteristic criteria of the drowning phase. Note the isopachous cement crusts on the lithoclasts (ar-
rows). This cement type occurs exclusively within the platform interior and slope sediments of the pre-drowning and
drowning phases.
6
Mid-slope.
Densely packed wackestone (biopelmicrite). Grains are peloids, micritic extraclasts, some shells and echino-
derm fragments embedded within a heterogenous microspar matrix.
Post-drowning phase
7
Mid-slope.
Bioclastic wackestone (biomicrite). The microfacies is characterized by pelagic filaments associated with
phosphatized and bored echinoderm fragments. Note the absence of allochthonous shallow-marine biota. SMF 3.
8
Basin.
Densely packed, quartz-rich, well-sorted calcisiltite, deposited adjacent to the lower slope during the last post-
drowning phase. The arrows point to burrowing. SMF 2.
-> 1-8: Blomeier and Reijmer (1999).
