Geology Reference
In-Depth Information
crofossils found in acid residues, e.g. conodonts or
silicified foraminifera.
16.3.2 Case Studies
•
Microfacies criteria and microfacies types of boul-
ders should be typified using large thin sections in
order to get an idea of microfacies variation and bi-
otic diversity.
The following case studies demonstrate the methods
of using microfacies for reconstructing depositional en-
vironments that are only recorded by clasts and pebbles.
•
Unlike common microfacies studies, grains and fos-
sils should be described in great detail, both quali-
tatively and quantitatively. These details facilitate a
comparison of boulders and the differentiation of
groups characterized by similar microfacies.
16.3.2.1 Case Study: Platform Facies Patterns
Derived from the Microfacies of Early
Carboniferous Conglomerates (Southern Spain)
•
Assignment of the microfacies seen in thin sections
of boulders to Standard Microfacies Types might as-
sist in recognizing rough facies zones, but could be
dangerous because of circular thinking. Use of the
zones distinguished in the Wilson model will lead
to platform environments and ooid sand bars will
be placed near platform margins. See Sect. 14.3.4
for the differences when using ramp models.
Herbig (1984) studied limestone boulders occurring in
the post-Bashkirian Marbella Formation outcropping
in the Betic Cordillera of southern Spain. The Marbella
Formation represents deep-water sediments, predomi-
nantly conglomerate lenses incised in shales. The lower
part of the formation is characterized by oligomict lime-
stone conglomerates whose clasts can be dated with
conodonts and benthic foraminifera as Late Viséan in
age. A thorough analysis of the boulders, based on thin
sections and acid residue data indicates a Late Viséan
age (zone 16 of the foraminiferal zonation) of the lime-
stones recorded by the boulders in the conglomerates.
This allows the use of the microfacies types observed
in the boulders to reconstruct a relatively short phase
of Early Carboniferous shelf sedimentation. Pl. 146
•
The original depositional areas on former shelves,
ramps or platforms may be derived from the esti-
mation of environmental conditions. Water energy
and circulation, and substrate are reflected by ma-
trix, grains and biota (see Chap. 12). These criteria
should be used in evaluating water depths, wave base
and storm wave base.
>>>
Fig. 16.14.
Reconstruction of the Late Viséan shelf derived from the microfacies analysis of limestone boulders
of the Marbella
Formation near Malaga and Granada, southern Spain. Modified from Herbig (1984).
The model shows a highly differentiated, shallow carbonate shelf embracing the Standard Facies Zones FZ 8 (restricted
platform interior), FZ 7 (open-marine platform interior), FZ 5/6 (platform-margin sand shoals and reefs) and FZ 4 (slope).
The spatial arrangement of the microfacies types is based on joint grain associations and joint biota, e.g. the close proximity
of microfacies b1 and b2 is indicated by the joint occurrence of ooids. Rare ooids in the oncoid rudstone of microfacies c1
point to a proximity of this microfacies type to microfacies b1 and b2.
The sketch displays common matrix and grain types as well as the distribution of facies-diagnostic fossils.
Pseudodonezella
is a dasyclad green alga (see Pl. 60/1).
Fasciella
is interpreted as green or red alga (see Pl. 56/3, 4). The name
Saccaminopsis
refers to a rock-building, generally single-chambered foraminifer. Corals are represented by single and colonial rugose corals
and heterocorals (see Pl. 83/2).
Microfacies types:
Slope
.
a2
- crinoid grainstone.
a3
- limestone breccia consisting of cm-sized clasts representing
various microfacies.
Ooid and lime sand bars and adjacent areas
.
b1
- ooid grainstone (Pl. 146/1).
b2
- oolitic dasyclad
grainstone (Pl. 146/2).
b3
- coated crinoid packstone (Pl. 146/3).
b4
- skeletal grainstone with diverse biota. Protected and/
or deeper parts of the lime sand areas.
c1
- oncoid rudstone (Pl. 146/4).
c2
- fine-grained foraminiferal grainstone (Pl. 146/
5).
c3
- skeletal packstone.
Luvward and central part of the open-marine shelf lagoon
.
e1
- grainstone with abundant
aggregate grains.
e4
- skeletal packstone and grainstone with crinoids and bryozoans.
e5
- crinoid packstone.
e6
- bioclastic
wackestone (Pl. 146/6).
Leeward part of the platform interior.
f1
- wackestone with micrite clasts.
f2
- grainstone with
aggregate grains and amalgamated peloids (Pl. 146/8).
f3
- grainstone with
Saccaminopsis
.
Platform interior with restricted
water circulation.
g1
- wackestone with
Saccaminopsis
.
g2
- fine-grained peloidal grainstone.
g2
- peloidal grainstone with
densely packed peloids (Pl. 146/8).
g3
- laminated fenestral mudstone/bindstone (Pl. 146/7).
g4
- quartz-bearing mudstone.
g5
- floatstone with variously-sized micrite clasts.
g6a
- quartz-bearing ooid grainstone with radial ooids (Pl. 146/9). Not
specifically indicated in the sketch is the position of organic buildups, represented by microfacies
d1
(
Pseudodonezella
bafflestone), widely distributed all over the shelf (see signature of
Pseudodonezella
), and microfacies
d2
(bindstone), com-
mon in the restricted Facies Zone 8.
