Page 286 - Carbonate Facies in Geologic History

Franconian Reefs 273

Higher in the section (MaIm Zeta, upper Kimmeridgian or Tithonian) the
sponge-algal mounds are overlain and colonized by reef corals like those of the
Swiss Jura Mountains. A backreef facies of cross-bedded bioclastic oolitic sedi-
ment developed to the east and north, at this later time (Gwinner, 1962). The
particles are of echinoid, bivalve, bryozoan, coral and calcareous sponge frag-
ments and lithoclasts. Micritized rinds occur on the particles. The matrix may be
micritic or sparry. The texture is generally packstone but some grainstone is
common. A fauna of thick-shelled mollusks is present (Nerinea, and Diceras),
along with algae, associated with the corals. Also, as mounds increased in relief,
the reefs grew into wave base and the basins between became filled with "cement
marls" and in places by thin platy beds with a pelagic and nektonic fauna, but
with no benthos. This Solnhofen type deposit is known at Nusplingen in Schwa-
bia. The considerable submarine relief which developed at this time is indicated
by widespread, though relatively uncommon, fine-graded beds, graded breccias
and glide and slump masses. These masses are mixtures of reef debris and broken
pieces of lime mud from flank beds.

Franconian Reefs

The plateau continues north of the Danube, farther east in the German land of
Franconia. The lower MaIm consists there of sponge mounds and bedded lime-
stones; higher in the MaIm occur abundant, very fossiliferous coral reefs in scat-
tered knolls having individual relief of a few tens of meters. The reef patches vary
from 25 to 70 square km. Locally, 1 or 2 km of flank beds are seen around an
individual core. They are well exposed in the valleys of the Altmiihl and Danube
Rivers (Fig.IX-7) and have been paleontologically studied in various places
(Bausch, 1963; Bausch and Zeiss, 1966; Barthel et a1., 1971). The reefs extend
through the highest Jurassic (upper Tithonian) and are regressive southward
toward the Tethyan trough (Barthel, 1969). The highest Jurassic reef facies is
i.nterbedded with black lime mudstone resembling the Helvetic Quintnerkalk, in
wells which penetrate the molasse near Munich. Farther south, age-equivalent
beds in the Austroalpine nappes of the Northern Calcareous Alps are in an en-
tirely different geosynclinal facies.
The composition of the reef knolls varies somewhat according to stratigraphic
position within the latest Jurassic. The older sponge-algal knolls at Kelheim, near
Regensburg on the Danube, are of lower Kimmeridgian age, and according to
Bausch and Zeiss (1966) are capped by the encrusting coral Microsolena and by
spongiomorph hydrozoan framework (18% of volume) with minor (less than 2%)
megafauna consisting of crinoids, snails, brachiopods, solitary corals, and
sponges, also occurring in the central core. Reef detritus constitutes 80% of the
bulk volume of the accumulation and consists of crinoids, snails, solitary corals,
and large bivalves, including Diceras. About half of the debris is within the coral-
hydrozoan framework and half is reef-flank debris dipping steeply off the core.
Other beds at the southwest edge of Franconian outcrops near Neuburg a.d.
Donau, display somewhat different reef facies. Here the core of the Laisacker reef
(of lower Tithonian age) rarely contains spongiomorph hydrozoans but much