Page 277 - Carbonate Facies in Geologic History

264 Reef Trends and Basin Deposits in Late Jurassic Facies

water deeper, more saline, and more anaerobic than normal neritic. These basins lay
directly behind reefs and organic buildups which faced the Alpine trough and stretch
completely across Europe. A variety of Standard microfacies 3 (Plate XXVIII).

Middle Eastern Shelf Facies

J 10. Algal-bioclastic coarse grainstone. This sediment is particularly common in the Middle
East Arab zone. The worn and rounded particles are up to several mm in size. Some
are coated. There is a large amount of micritization, coarse crystallization and solution
of particles. Cement consists of both even, drusy, grain-lining types and of blocky
mosaic calcite. Organisms consist of badly altered pieces of red? algae, foraminifera,
gastropods, and occasional bivalves or brachiopods. The sediment represents accumula-
tion in current-washed shoals or bars, probably in a somewhat hypersaline environment.
Little oolite is present. Variety of Standard microfacies 11 (Plate VII A).
J 11. DasYcladacean grainstone-packstones. The Late Jurassic dasycladacean Clypeina juras-
sic is well known. It occurs in diversely textured sediments and in various stages of
disarticulation. More robust forms such as Actinoporella and Salpingoporella are also
common. These are especially typical of the Middle East grainstones (Arab Formation).
Dasycladacean algae at the present time thrive in extremely shallow water (only a few
meters deep) but will tolerate considerable salinity variation-from brackish water to
salinities of more than 60 per mil. Standard microfacies 18 (Plate VIA).

Alpine Basin-Slope Microfacies

J 12. Argillaceous and silty dark lime mudstones and wackestones with scattered benthonic
as well as pelagic bioc1asts. Sponge spicules are common. Mollusks, echinoids, brachio-
pod and crinoid fragments occur. Arm plates of pelagic crinoids such as Saccocoma and
ophiuroids are abundant in some layers. Bedding is thin and planar with rhythmic
marl intercalations. Such slope limestones are transitional from shelf margins into
basins. For example, Effinger marls and Birmenstorfer beds of the Central Swiss Jura.
Standard microfacies 1 or 3.
J 13. Dark micropelletoid micritic limestone. Composed of calcitic silt and fine sand-sized
calcareous particles with lime mud or spar matrix. Thin-graded laminations of a few em
represent distal parts of turbidites. The sand-size layers occur within a dominantly
micrite matrix. Such limestones are thin, planar and regularly bedded. The Quintner-
kalk of the Helvetic Alpine nappes is an example. In the Middle East, such black
limestones are interbedded with evaporites and deposited in basinal environment. Stan-
dard microfacies 1.
J 14. Light-colored pelagic (cherty) pure lime mudstone. This sediment is common in the
Austroalpine nappes and Southern Alps, variously Aptychuskalk, Oberalm, Maiolica,
Biancone. It is thin and rhythmic-bedded micritic limestone with very thin marl part-
ings. Some beds may be spiculitic but basically the limestone is composed of calcareous
nannoplankton rich in calpionellids and coccoliths. The fauna is pelagic: ammonite
aptychi, Nannoconus, Globochaete, tintinnids and radiolarians. The key-hole brachio-
pod, Pygope, occurs in some of these beds. In some sections thicker beds of fine-grained
allodapic (turbidites) limestones and even locally coarse slump breccias are interbedded,
such as the Barmstein Limestone of the Salzburg area. Very early diagenetic or deposi-
tional solution of aragonitic material at its compensation depth is indicated for some
beds and the possibility exists for their formation in troughs of several thousand meters
(Garrison and Fischer, 1969). Standard microfacies 3 with some beds of Standard
microfacies 4 (Plate III).