Geology Reference
In-Depth Information
describe drowning effects in different parts of platforms
and slopes and to distinguish unconformities caused
by drowning from those caused by subaerial exposure.
A discussion of discontinuity surfaces, including sub-
marine condensation surfaces that may be associated
with the drowning of platforms, can be found in Sect.
5.2.
•
Increase in open-marine biota, associated with shal-
low-marine constituents.
•
Increase in 'deep-water' non-photic organisms, e.g.
sponges.
•
A shift of the benthos/plankton ratio towards the
plankton.
Many carbonate platforms suffered subaerial expo-
sure prior to drowning by renewed flooding. Exposure
phases are indicated by vadose-meteoric cement types,
pedogenic structures and paleokarst (e.g. Bernecker et
al. 1999; see also Sect. 15.1 and Sect. 15.2).
Indications of drowning
•
Successions of shallow-marine neritic sediments
rapidly passing upward into deep-marine deposits.
•
Vertical replacement of shallow-water grain assem-
blages by deeper-water grain assemblages.
•
Hardgrounds with crusts of oxide, phosphate and
glauconite divide neritic and deep marine deposits,
and indicate a period of non-deposition.
15.6.2.2 Case Study: Platform Drowning
Reflected by Microfacies
•
Burrowed discontinuities.
•
Reefs: Increasing reworking of platform-margin reef
material leading to the destruction of protecting rims.
The loss of protecting elevated margins results in
stronger reworking of upper slope sediments and
platform sediments. This may be reflected by an in-
crease in the abundance of reef- and platform-de-
rived material in slope sediments.
The Jbel Bou Dahar in the Atlas Mountains of Mo-
rocco represents an Early Jurassic carbonate platform
that was drowned at the beginning of the Toarcian. The
pre-drowning phase (Late Sinemurian to Late Pliens-
bachian), drowning phase (Late Pliensbachian to Early
Toarcian) and post-drowning phase (Early Toarcian to
Aalenian) were studied by Blomeier and Reijmer (1999)
with regard to facies changes reflected by qualitative
and quantitative microfacies criteria. Pl. 133 and Fig.
15.14 summarize the development of the platform over
time.
Methods:
The thin sections were studied with re-
spect to sediment composition and fossils. Quantita-
tive analyses were carried out with point-counting, us-
ing 200 points. A total of 25 categories were differenti-
ated, and subsequently grouped into 8 point-count
groups comprising matrix (micrite and cement), shal-
low-marine biota (living on the platform top and the
platform margin), suspension feeders and grazers (echi-
noderms and brachiopods characteristic of the drown-
ing phase), open-marine biota (marine organisms char-
acterizing deep-marine environments; e.g. ammonites,
siliceous sponges, filaments), encrusted grains (includ-
ing zoogenic oncoids, micro-oncoids and aggregate
grains), non-skeletal grains (ooids and litho- and ex-
traclasts), peloids, and terrigenous input (mainly detri-
tal quartz).
Every phase is characterized by an association of
characteristic microfacies types:
•
Reefs: Significant changes in dominant growth
forms of reef-building organisms, e.g. from massive
to low-laminar, or massive to high-dendroid growth
forms, indicate adaption to changes in water energy
and depth.
•
Abrupt vertical changes in dominating microfacies
types.
•
Grainstones and packstones with abundant shallow-
marine biota, intercalated in slope- and basinal sedi-
ments.
•
Grainstones and rudstones with abundant platform-
derived rounded lithoclasts, intercalated in slope
sediments (see Sect. 15.7).
•
Increase in microborings and the frequency of cor-
toids (see Sect. 15.7.1).
•
Increase in biogenically encrusted grains, indicat-
ing longer periods of ceased sedimentation.
•
Decrease in thin-section diversity. Environmental
stress related to drowning may contribute to the de-
velopment of monomict biotic associations.
•
Mineralized microbial crusts.
•
Fe-, Mn- and P-enrichments are common in crusts
of drowning and post-drowning carbonates and can
be related to a primary higher nutrient content in the
water column (Föllmi et al. 1994; Drzwiecki and
Simo 1997) reflecting changes from oligotrophic to
eutrophic conditions.
Pre-drowning phase
During this phase, the
platform interior
was charac-
terized by restricted-marine lagoonal environments with
limestone/marl alternations separated by distinct ex-
posure surfaces, and protected by scattered buildups
and cemented debris at the platform margin. Common
•
Increase in suspension-feeders and grazers, e.g. echi-
noderms and gastropods.
