Geology Reference
In-Depth Information
Box 14.6.
Common microfacies types of Paleozoic and Mesozoic ramp carbonates.
The list starts with microfacies types
of the deeper outer ramp zones, and continues with microfacies types of mid-ramps and shallow inner ramp zones.
Microfacies types of the bioclastic and oolitic carbonate sand shoals and banks formed in inner and mid-ramp settings
are listed at the end of the box. Numbers in brackets indicate corresponding SMF types.
Outer ramp
Thin- to medium-bedded, fine-grained, often burrowed
limestones and marls. Laminated marls alternate with lime
mudstones. Skeletal grains commonly well preserved, not
worn. Mudstones, wackestones, packstones. Some tempes-
tite beds (grainstones). Mud mounds (see Sect. 16.1.2).
RMF 1:
Calcisiltite and mudstone with peloids, very fine
skeletal debris, sponge spicules, sometimes fine-laminated
(SMF 1).
RMF 2:
Argillaceous burrowed mudstone and wackestone;
rare agglutinated foraminifera, ostracods, echinoderms.
RMF 3:
Burrowed bioclastic wackestone and packstone
with diverse, common to abundant fossils (bivalves, bra-
chiopods, echinoderms) and peloids. Skeletal grains not
worn; whole fossil preservation common (SMF 8).
RMF 4:
Peloidal wackestone and packstone (~ SMF 2).
RMF 5:
Pelagic mudstone with planktonic microfossils
and open-marine nektonic fossils (e.g. ammonites) (SMF 3).
RMF 6:
Graded, laminated and finely cross-bedded bio-
clastic and peloidal grainstone (tempestite).
Mid-ramp
Medium-bedded, fine-grained bioclastic limestones and
marls, often burrowed. Skeletal grains often worn. Echi-
noderms common. Mudstone, wackestone, packstone, some
grainstones. Distally steepened ramps exhibit slumps and
various signs of reworking and transport of fine-grained
ramp material. Distal steepening occurs in outer ramp set-
tings or near the mid-ramp-outer ramp boundary. Calcare-
ous sand shoals and sand banks (see right side). Various
kinds of reefs occur in mid-ramp and inner ramp settings
(e.g. coral patch reefs, coral-red algal reefs).
RMF 2 , RMF 3, RMF 5 and
RMF 7:
Bioclastic packstone with abundant echinoderms
and common bivalves and foraminifera. Skeletal grains
worn (SMF 10).
RMF 8:
Burrowed packstone and grainstone with various
skeletal grains, intraclasts, oncoids and peloids.
RMF 9:
Wackestone, packstone, floatstone with micritic
intraclasts and ramp-derived bioclasts; sometimes micro-
breccias (distally steepened ramps) (~SMF 5).
RMF 10:
Limestone conglomerates (distally steepened).
RMF 11:
Marls with intraclasts and limestone pebbles (dis-
tally steepened).
RMF 12:
Boundstones comprising coral and coral-crust
framestone, red algal framestone (SMF 7).
Inner ramp
Inner ramp sediments are bedded, microfacially differen-
tiated limestones and dolomites forming relatively thin se-
quences. Marls are of minor importance. The inner ramp
comprises open-marine environments with good water cir-
culation, protected environments with restricted water cir-
culation, sand shoal and bank environments characterized
by oolitic and bioclastic grainstones and packstones (see
below), lagoonal environments behind shoals or islands
and peritidal environments. The latter are separated from
other inner ramp parts as back-ramp environments (Fig. 14.3).
Common texture types of open and protected inner ramps
are bioclastic packstones and wackestones. Coral and coral-
ed algal boundstones (RMF 12) and other reef limestones
(bivalve reefs) are common in open inner ramps. Other
reefs, e.g. serpulid biostromes, occur in protected inner-
ramp settings.
Open inner ramp environments
RMF 13:
Bioclastic wackestone and packstone with abun-
dant larger foraminifera (e.g. orbitolinids) (SMF 18-F
OR
).
RMF 14:
Bioclastic packstone and wackestone with skel-
etal grains, various amounts of intraclasts and some ooids
(near-shoal).
RMF 15:
Bioclastic floatstone with various reef-derived
material (near-reef coral-, algal- or bivalve-debris) (~SMF 6).
Protected and low-energy inner ramp environments
A common microfacies type in these environments is
RMF 7 characterized by common to abundant echinoderm
fragments. Associate skeletal grains occurring in different
quantity are bivalve shells, gastropods, bryozoans, and
benthic foraminifera. Other microfacies types are pack-
stones and grainstones with larger foraminifera (RMF 13).
RMF 16:
Mudstone, wackestone or packstone with abun-
dant miliolid foraminifera (SMF 18-F
OR
).
RMF 17:
Bioclastic wackestone with dasyclad green al-
gae (SMF 18-D
ASY
).
RMF 18:
Bioclastic wackestone with ostracods.
Lagoonal environments
RMF 17 (low-diverse flora) and
RMF 19:
Non-burrowed lime mudstone.
RMF 20:
Bioclastic wackestone and packstone with cal-
careous algae and benthic foraminifera.
RMF 21:
Oncoid packstone and floatstone (SMF 22).
Peritidal zones
RMF 19 and RMF 21 and
RMF 22:
Fine-laminated dolomitic/lime mudstone (SMF
19).
RMF 23:
Fenestral bindstone (SMF 21).
RMF 24:
Intraclast mudstone and packstone.
RMF 25:
Laminated evaporite-carbonate bindstone (SMF 25).
Carbonate sand shoals and banks
Storm-dominated ramps are characterized by accumu-
lations of ooids, skeletal grains and peloids. These grains
occurring separately or associated, form grainstone and
packstone textures. Bedding and cross bedding are com-
mon. These sediments originate in mid-ramp and inner-
ramp settings. Bioclastic banks may be formed by storm-
induced but also by coast-parallel bottom currents.
RMF 26:
Medium- and coarse-grained bioclastic grain-
stone and packstone with various benthic skeletal grains.
RMF 27:
Bioclastic grainstone and packstone composed
of few dominant skeletal grains (e.g. predominantly echi-
noderms, or predominantly foraminifera).
RMF
28:
Bioclastic floatstone and rudstone exhibiting a
strongly disorganized fabric.
RMF 29:
Ooid grainstone with densely packed concentric
ooids (common in shoals and banks) (SMF 15-C).
RMF 30:
Shelly ooid grainstone and packstone (common
in banks).
