Geology Reference
In-Depth Information
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Fig. 15.14.
Microfacies reflecting drowning history of a carbonate platform. Case study
Jbel Bou
Dahar, High Atlas (Early
Jurassic).
Modified from Blomeier and Reijmer (1999).
A:
Changes in the sediment composition within different stages of drowning and in different platform sections.
The changes
in the diagnostic criteria are derived from quantitative microfacies analysis. Note the distinct differences in shallow-marine
biota, frequency of suspension feeders and grazers (mainly echinoderms and brachiopods), open-marine organisms (pelagic
filaments, ammonites, siliceous sponge spicules) and the frequency of lithoclasts (intra- and extraclasts).
B:
Platform morphology and distribution of microfacies types within different phases of pre-drowning, drowning and post-
drowning.
SMF: Standard Microfacies Types (see Box 14.1 for definitions), MF: local microfacies types. Arrows mark the
main directions of sediment transport.
C:
Sea-level variations and drowning stages.
1a: Pre-drowning phase and sea-level highstand. Carbonate production takes
place in the flooded platform interior. 1b: Pre-drowning phase and sea-level lowstand. Subaerial exposure of the platform
top, the carbonate production has shifted to the platform margin and upper slope. 2: Renewed flooding of the platform top at
the beginning of the drowning phase. 3: Gradual transition to deep-marine sedimentation during the post-drowning phase.
microfacies types include wackestones (pelmicrites and
biopelmicrites corresponding to SMF 22; Pl. 133/1),
mudstones (micrites, dismicrites; SMF 21), fenestral
microbial mats (SMF 19), and intercalated grain- to
packstones (pelsparites; SMF 17). The
upper and mid-
slope
were dominated by thick-bedded, coarse-grained,
poorly sorted limestones, deposited through debris
flows during sea-level lowstands. Microfacies types are
wackestones/floatstones and grainstones/rudstones
dominated by shallow-marine bioclasts, peloids and
encrusted grains (Pl. 133/2). Sea-level highstand de-
posits occurring at the toe-of-slope consist of medium-
bedded marls and turbiditic limestones (grain- to pack-
stone with abundant, highly diverse shallow-marine
skeletal grains and peloids; SMF 4) marls and hemipe-
lagic limestones (wackestones characterized by bio-
turbated biopelmicrites with tiny grains; SMF 9; Pl.
133/3).
posited on the platform slope. The drowning succes-
sion of the
mid-slope
consists of thin- to medium-bed-
ded, red-brownish carbonates and marls, unconform-
ably onlapping at a low angle the thick limestone beds
of the pre-drowning phase. The most common micro-
facies are packstones and wackestones (biomicrites and
biopelmicrites; Pl. 133/5).
The composition of these limestones differs drasti-
cally from that of the carbonates of the pre-drowning
phase. Non-skeletal grains, especially lithoclasts, domi-
nate the grain spectra (Pl. 133/6). In contrast to the pre-
drowning phase, encrusted grains are almost completely
missing. Intercalations of brachiopod and oyster co-
quinas are common. Platform interior and slope suc-
cessions exhibit cyclic variations in sediment compo-
sition, possibly reflecting small-scale sea-level fluctua-
tions.
Post-drowning
The transition to the post-drowning phase is con-
formable and gradual. During this phase, thick-bedded
marls and thin-bedded limestones (wackestones and
mudstones; biopelmicrites and micrites; SMF 3; Pl.
133/7) were deposited elsewhere on the platform, fol-
lowed by the deposition of calcisiltites (SMF 2; Pl. 133/
8). Common skeletal grains are derived from suspen-
sion feeders and grazers, and open-marine biota. Shal-
low-marine biota are almost completely absent. The
facies types indicate uniform, deep-marine and quiet-
water sedimentation below the storm-wave base.
Drowning phase
This phase is characterized by a condensed section
and a fundamental environmental change along the
entire platform expressed by a distinct facies change.
The base of the drowning succession on the
platform
top
is marked by the abrupt occurrence of red-brown-
ish carbonate beds, unconformable overlying sediments
of the pre-drowning phase, and showing totally differ-
ent microfacies.
The microfacies differs in the large quantities of frag-
ments of suspension feeders and grazers (mainly echi-
noderms) and recrystallized bioclasts of shallow-ma-
rine platform biota (Pl. 133/4). These particles were
deposited in three pulses with decreasing absolute
amounts.
Densely packed skeletal grainstones represent the
switch to high-energy environments within the plat-
form interior, causing erosion of former pre-drowning
lagoonal sediments. The erosional products were rede-
15.6.3 Case Studies: Platform and Ramp
Carbonates
Many carbonate platforms exhibit recurring microfacies
patterns that reflect the hydrodynamic conditions in
specific parts of the platforms and the varying compo-
