Page 287 - Carbonate Facies in Geologic History

274 Reef Trends and Basin Deposits in Late Jurassic Facies

encrusting coral (M icrosolena) which is easily mistaken for them; more than
20 coral genera are recognized here. The core contains large coral stalks up to
11/2 m in diameter with many Lithophaga borings. Some of the corals are badly
broken, the delicate form Enallhelia making up much detritus. More than
100 species of mollusks have been noted from these beds. Especially large forms
occur such as Trichites, limn ids, pectinids, Praeconia, and pleurotomarids. Much
interstitial peloidal micrite is present in the core facies as well as coarser bioclastic
debris, and encrusting blue-green algae appear to be important sediment binders.
The flanking debris contains bioclasts and lithoclasts in grainstone and packstone
texture and many large displaced colonies of Thecosmilia-like fasciculate coral.
Some of these beds are very coarse-grained. Much solution and dolomitization of
these beds is evident, apparently owing to their permeability. Unlike the older
reefs at Kelheim, few ooids or onkoids are seen in the interreef beds and the
thick-shelled Diceras is lacking. Dasycladacean algae are absent as are coralline
red algae. From both core and flank facies, Cliona and Entobia sponge borings are
ubiquitous, infesting both sessile organisms and in-drifted shells of ammonites.
These differences in biological character may be caused by the younger reefs'
location which was somewhat removed from the open sea.

Summary of MaIm Reef Belt

1. Along the whole belt, reef cores are in patches surrounded by wide halos of
bioclastic reef flank deposits.
2. The reef belt is very wide (tens of kilometers) and its seaward slope very
gentle (1-2 degrees).
3. There is a progressive regression toward the Alpine-Tethyan trough up-
ward in the section-a buildout toward the south and east from the Vosges block
and Vindelician ridge, roots ofthe Hercynian orogenic belt.
4. Early development was of Type I downslope sponge reefs with encrusting
alga-foraminifera, probably in water tens of meters deep. These existed below
wave base rising as much as 100 m above the basin floor. Such buildups were
followed by a period of shallowing and by coral-hydrozoan colonization and
organic encrustation. Large mollusks (Diceras) were common as the buildups
grew; in general the buildups grew into wave base and extensive debris derived
from biota accumulated on top. Such construction developed a Type II shelf
margm.
5. In several places along the reef belt the organic accumulations surrounded,
protected, and helped restrict basins deep enough for deposition of unusually fine
carbonate mud which preserved a fauna derived from both the land and open sea.

The Solnhofen Facies

A series of world-famous, fossil-bearing deposits of platy to thin-bedded, very
fine-grained Late Jurassic limestones occur in southern Germany in Franconia
and Schwabia. These were formed in deep lagoonal basins lying behind the outer