Page 284 - Carbonate Facies in Geologic History

The Schwabian Alb 271

bivalves: Pecten, Trigonia, Mytilus; brachiopods: Rhynchonella, Terebratula; and
crinoids and echinoids: Cidaris and Stomechinus.
Another characteristic backreef microfacies is the well-bedded, light-colored
onkoidal micrite ("mummy beds"). In this sediment algal laminae encrust frag-
ments of large fragmented fossils such as corals, crinoids, gastropods, brachio-
pods and mollusks. Characteristic large foraminifera occur in the micrite in be-
tween the nodules: Pseudocyclammina, N autiloculina, Cristellaria, miliolids, and
textularids.
The algae preferentially are backreef, not reefoid forms. The calcareous algae
include Codiaceans in the form of small tufts (up to 1 cm across): Cayeuxia,
Marinella and Lithocodium. The blue-green encrusting forms are bean and biscuit
shaped, with irregular protuberances or concentric arcuate crusts impregnated
with Girvanella tubules. The algal-foraminifera association is interpreted to indi-
cate very shallow, warm, clear water of variable salinity. Circulation must have
been capable of moving the onkoids, but insufficient to winnow the mud. The
onkoids were formed with durable, calcified crusts. The main algal bed (Haupt-
mumienkalk) is extensive over the whole Jura Mountains.

The Schwabian Alb

Beyond the folded Swiss Jura, across the Rhine and north of the Danube, Jurassic
rocks are exposed in a broad limestone plateau trending eastward across southern
Germany, and constituting the Schwabian and Franconian Alb. The Upper Ju-
rassic (MaIm) outcrop here is 40 km broad and extends for several hundred km,
continuing the broad reef girdle exposed in the Jura Mountains of France and
Switzerland. These outcrops constitute the classic area for ammonite zonation
worked out by Oppel in the early 19th century. Recently, facies study has been
carried out by Gwinner (1971) and Aldinger (1968) in Schwabia, and Barthel
(1969) in Franconia. The reefs exposed here are interesting because of the facies
progression they display. Older buildups are sponge-algal micrite mounds depos-
ited in downslope positions and constituting a Type I shelf margin. During the
course of Late Jurassic time, corals succeeded the sponges, reefs built up into
wave base, and shallower water facies appeared (Type II shelf margin). These
facies are diachronic and migrate eastward as well as southward.
Generally the "normal" section of the MaIm in Schwabia is composed of
alternating marls and evenly and rhythmically bedded limestones. The sedimen-
tary texture is wackestone with ill-sorted bioclasts of mollusks, brachiopods,
serpulids, sponges, and a few onkoids. The megafauna is principally ammonites
and thin-shelled mollusks; foraminifera, spicules and many coccoliths also occur
in the micrite matrix. Beginning in the higher Oxfordian layers, a wide belt of
micritic mounds occur. These mounds are rich in siliceous sponges with much
encrusting algae and tubular foraminifera. Such beds are time equivalents of the
coral reefs of the Jura but represent somewhat deeper marine waters, a sag in the
reef girdle. The mounds, termed the Massenkalk facies, developed as irregular
masses scattered across the belt, and later (in MaIm Delta and Epsilon time) as a
wide sheet of massive limestone. The belt was possibly 100 km wide by late