Geology Reference
In-Depth Information
of carbonate induced by the life cycle and the decay of
these organisms. The environmental interpretation of
these SMF Types should consider the classification of
benthic microbial carbonates described in Sect. 9.1.5
and summarized in Fig. 9.1.
SMF 23 is a poorly constrained type. The lack of
fossils in micritic limestones or dolomites may have
many reasons, one of which could be an elevated salin-
ity. SMF 23 should only be considered if pseudomorphs
after gypsum, anhydrite or halite crystals, or the strati-
graphic position of the micritic beds support the inter-
pretation. SMF 24 represents a common characteristic
of tidal deposition but can also originate from storms
in shoreface settings.
Coastal and inland sabkhas are characterized by
SMF 25, subaerially exposed areas in various climate
zones by SMF 26. Both types reflect non-marine con-
ditions.
microfacies types' are sometimes caused by climatic
controls determining biogeographical distribution pat-
terns of shelf organisms. One example is the congruent
microfacies types of Late Triassic platform and reef
carbonates in different parts of the Tethys. Other rea-
sons for time-dependent microfacies types are the com-
bined effects of specific oceanographic conditions and
evolutionary stages, exemplified by the worldwide dis-
tribution of nodular deep-shelf carbonates in the Juras-
sic, and the Late Cretaceous pelagic chalk facies trig-
gered by evolutionary bursts of the calcareous plank-
ton.
Lists of the common stratigraphic microfacies types
for the Devonian, Late Paleozoic, Late Triassic, Juras-
sic and Cretaceous can be found in Wilson (1975).
14.3.4 Common Microfacies Types of
Carbonate Ramps
Depositional variability and Standard Microfacies
The depositional texture and composition of bed-
ded limestones can change within a scale of some mil-
limeters and a few centimeters. These changes are re-
flected by different microfacies that are of local impor-
tance only. However, some of these small-scale micro-
facies types indicate substantial changes that are re-
flected by Standard Microfacies Types. Examples are
shown in Pl. 127.
The SMF Types, describing sediments of tropical
rimmed shelves, can only be sometimes found in car-
bonate shelves formed in differing latitudinal and cli-
matic settings and developing different geometries. The
following sections compare the common microfacies
criteria of carbonate ramps with those of rimmed plat-
forms and summarize the common and recurrent
microfacies types of ramp carbonates.
Many microfacies criteria of platform carbonates can
also be seen in thin sections of limestones formed in
ramp settings. However, there exist differences con-
cerning the distribution and type of the prevailing ma-
trix, grains and fabrics. The occurrence of these crite-
ria on inner, mid- and outer ramps is controlled by off-
shore and onshore transport by storms, the growth of
mud mounds and reefs in different parts of the ramps,
and the bathymetric dependence of benthic organisms
that contribute to carbonate production.
SMF Types do not cover the whole spectrum of micro-
facies
Some samples characterized by conspicuous micro-
facies criteria can not be attributed to an SMF Type of
the rimmed platform model, owing to the lack of ap-
propriate SMF Types. Examples are burrowed lime
mudstones (e.g. Pl. 25/4) common in deep shelf and
deep-shelf margin environments, and intraclast pack-
stones and wackestones occurring in restricted platform
interior, in toe-of-slope settings and common in dis-
tally steepened ramps. Nodular limestones (Pl. 21 and
Pl. 139) represent another example. Some of these
microfacies types are included in 'Stratigraphic Micro-
facies Types'.
14.3.4.1 Microfacies Criteria of Carbonate
Ramps
The fine-grained
matrix
is micrite and calcisiltite.
Mi-
crite
is common in deeper outer ramps and in protected
areas of inner ramps. Both allomicrites and automicrites
contribute to carbonate production. Allomicrites are
characterized by poorly sorted matrix exhibiting very
fine peloids within a matrix, with small bioclastic de-
bris probably caused by bioerosion, variations in pack-
ing and mixed microfossil faunas. Automicrite matrix
is a common constituent of mud mounds. Clues to in-
place carbonate production in deeper ramps (Jeffery
14.3.3 Stratigraphic Microfacies Types
Geologists working in different parts of the world know
the amazing similarities of the facies of carbonate rocks
of similar age. Limestones of specific long-range time
intervals are characterized by recurrent microfacies
types exhibiting a wide geographic distribution and a
limitation to particular major facies belts. The signifi-
cant global coincidences of some of these 'stratigraphic
