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Plate 134 Microfacies of a Late Jurassic BahamianType Platform: Sulzfluh (Graubünden, Switzerland)
The plate displays characteristic microfacies types of the platform limestones and microfacies criteria useful for
paleoenvironmental interpretation. Mudstones are represented by fenestral mudstones (-> 4, platform interior).
Peloidal and bioclastic wackestones occur in protected platform interior settings and open-marine parts of the
platform. Packstones, grainstones and rudstones are most common in open platform environments with good
water circulation. They comprise at least five types of skeletal grainstones, grainstones with aggregate grains
and/or coated and micritized grains (-> 1), and ooid grainstones forming platform margin shoals (-> 7). Small
reefs were formed within the sand flats. Frame-building organisms are corals (-> 2), chaetetid coralline sponges
and encrusters (foraminifera, various microproblematica, e.g. Bacinella and Lithocodium ). Most sediments were
deposited at very shallow depths, as indicated by the abundance of dasyclad algae (-> 1), microborers and
meteoric-vadose cement (-> 7), infillings of crystal silt in sheet cracks (-> 8) and fossil molds. Facies-diagnostic
fossils are calcimicrobes (-> 9), dasyclad green algae (-> 5) and larger foraminifera (-> 6).
Characteristic microfacies types
1 Poorly sorted intraclastic grainstone (intrasparite). Open-marine part of the platform. Larger black grains are aggregate
grains, smaller grains are peloids. Many grains are reworked cyanobacterial (C) and algal fragments. CL: encrusted
dasyclad alga ( Clypeina ). Open platform interior. Facies Zone FZ 7 (see Fig. 14.1). The sample corresponds to Standard
Microfacies Type SMF 11.
2 Coral framestone. Patch reef. Meter-sized coral patch reefs occur on the open platform. Corals and encrusting organisms
( Bacinella, arrows) form a rigid framework. Bacinella acted as stabilizers for the framework (see Sect. 10.2.6.2).
3 Peloid grainstone (pelsparite) with oncoids. Open platform . Reworking is indicated by poor sorting. Reworking may
have taken place by bioturbation, tidal currents, or stirring of the sediment by storms.
4 Laminoid fenestral mudstone (dismicrite). Restricted platform interior. The fabric characterizes a tidal flat facies. The
thin irregular fenestrae, some of which resemble plant roots, point to supratidal muds. Facies Zone FZ 8. Standard
Microfacies Type SMF 21-F ENESTRAL .
Facies-diagnostic criteria
5
Calcareous green algae: Clypeina (see Pl. 62/5, 6). The distribution of algae is facies-controlled, allowing the platform to
be differentiated internally (see Fig. 10.19). Restricted lagoonal environments of the platform are characterized by wacke-
stones with abundant Clypeina . The occurrence of Clypeina in grainstones of the open platform indicates transport of
lagoonal grains to the open platform parts.
6
Benthic foraminifera. Larger foraminifera are strongly substrate-controlled and limited to distinct parts of platforms.
Example: Kurnubia (Textulariina; see Pl. 69/3), characterizing high-energy open parts of platforms.
7
Ooids forming oolite shoals are common in platform-margin position but also occur in high-energy areas on the platform.
Example: Grainstone consisting of tangentially structured micritized ooids. The irregular shape of some ooids results
from biogenic encrustations, indicating only weak reworking of the grains. The grains are surrounded by symmetric
circumgranular calcite cements, confirming meteoric-phreatic overprint. Platform margin. SMF 15-C.
8
Early diagenetic sheet crack within grainstones, outlined with scalenohedral calcite. The remaining cavity is filled with
crystal silt (CS), indicating meteoric-vadose diagenesis possibly caused by subaerial exposure. Platform interior.
9
Skeletal cyanobacteria. Calcification intensity and abundance of cyanobacteria vary in different parts of inner platform
environments. Example: 'Cayeuxia' , characterized by parallel arrangement of tubes subsequent to their bifurcation. Compare
Pl. 53/1-3.
Fig. 15.15. Microfacies of a
Jurassic Bahamian-type plat-
form. Generalized facies zones
of the Late Jurassic Sulzfluh
platform. The sketch is based
on paleontological and micro-
facies data in the Sulzfluh-
Weißfluh area near the Swiss-
Austrian boundary. Lithofacies
and biofacies are similar to fa-
cies types of protected and
open interior parts of the Ho-
locene Bahama platform (com-
pare Pl. 3). Not to scale. Modi-
fied from Flügel (1979).
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