Page 289 - Carbonate Facies in Geologic History

276 Reef Trends and Basin Deposits in Late Jurassic Facies

reef barrier, and separated from each other by reef knolls. These Solnhofen depos-
its were quarried in earlier times for lithographic stone and more recently for
flagstones. The depositional basins of these strata actually stretch all along the
reef girdle, lying behind its front and being generally of Kimmeridgian-Tithonian
Jurassic age. They are known from Montsech in the Pyrenees, Cerin in the French
Jura, Nusplingen in Schwabia, and most importantly, in the valley of the AltmUhl
River, in the Franconian village of Solnhofen and from Eichstatt. Here the some-
what irregular distribution of the basins has been illustrated by Freyberg in 1968
(Fig.IX-7). The swales lie roughly between north-south trending reef knolls and
platforms. The platy basinal limestones are not very fossiliferous but what fauna
is present is strikingly preserved with many soft parts moulded as delicate impres-
sions, for example insect wings and bird feathers. The extensive collections in
museums in Eichstatt, Munich, Frankfurt, Stuttgart, and Berlin result from
200 years of painstaking hand quarry operations.
Many persons have studied the fossil collections and recently sedimentological
studies have added greatly to our understanding of these fascinating strata.
Earlier some paleontologists considered the Solnhofen limestones to be tidal flat
deposits but now the consensus favors an origin in quiet, somewhat deep local
basins. Controversy also continues over the origin of the extraordinarily fine
micrite sediment. The description of the deposits and their fossils given below
follows modern sedimentological interpretations in reviews of the problems by
Barthel (1970), Van Straaten (1971), and De Buisonje (1972).

Lithology

1. Thelimestone is very fine-grained micrite, with calcite mosaic of 3-4 microns.
Electron scanning microscope studies have recently shown that the micrite con-
tains abundant coccolith plates (FlUgel and Franz, 1967).
2. It occurs in thin planar beds from a few mm to 30 cm thick.
3. The layers of pure micritic limestone (Flinze) are locally interbedded with
marls (Faule).
4. The limestone are about 97-98% CaC0 3 , the 2% residue, being Fe and
silica. The interbedded marls are actually 85-91 % CaC0 3 and contain clay min-
erals.
5. The rhythmic bedding resembles that of certain deep-water and slope lime-
stones found in basins and geosynclines. Perhaps the limestone beds represent
kinds of turbidity or density current deposits, rapidly deposited following peri-
odic storms (Plate XXVII).
6. Marl layers seem more common over older high areas; limestone beds
thicken into the intervening basins.
7. Individual limestone beds are traceable from quarry to quarry over as
much as 8km.
8. The strata lack the sedimentary structures found in recent tidal flats; no
algal stromatolites, mud cracks, tidal channel or washout deposits, fenestral fab-
ric, or flat pebble conglomerates occur. Ripple mark is extremely uncommon.