Page 279 - Carbonate Facies in Geologic History

266 Reef Trends and Basin Deposits in Late Jurassic Facies

c) Nodules formed by segregation of lime early in diagenetic history by solution-
compaction and pressure solution under some overburden and under water of variable
depth, less than 1000 m (H.Zanld, personal communication).
d) Nodular fabric resulting from mass movement of partly lithified sediment off high
areas of sea floor, nodules being in part true clastic particles.
Such limestones are widespread in the Mediterranean from Morocco to Greece, being
characteristic of strata from Lias to Malm age. They have been found also in the western
Atlantic basin just east of the Bahamas. Most geologists consider these peculiar sedi-
ments as having formed slowly at depths below wave base but in an oxidizing environ-
ment. Bernoulli (1972) concludes that these deposits formed on tops and slopes of
submarine swells at variable depths on the southern part of the northern continental
margin of the Tethys. They were underlain probably by continental crust and hence
were deposited in depths shallower than a few thousand meters.

The Reef Girdle of Central Europe

An area of thick and massive carbonates encompassing the whole of the Upper
Jurassic (600-900 m) trends east to west from the Caucasus, southern Poland,
southern Germany, and curves south through the Jura Mountains of Switzerland
and France (Fig. IX-1). In southern Germany the old names Maim and Weiss Jura
are used for this section. The trend is a wide reef belt but does not include all the
coral reefs in the European Late Jurassic strata. Thousands of coral biostromes
and patch reefs exist behind (north and west) of this fringe, mainly in Oxfordian
strata. They are well known in Britain and have also been described in the Y onne
Valley of central France (Rutten, 1956). In the Swiss Jura Mountains, a classical
area for detailed study of Jurassic reefs, the old stage named Rauradan designates
the reefy rock units, and the Argovian the off-reef basinal facies.
The reef facies extends from the Jura across the essentially unfolded strata of
the Schwabian and Franconian Alb or plateau north of the Alpine belt. A regional
paleotectonic cause for this reef girdle is partly clear. The reefy carbonates are
apparently shelf-margin deposits facing into the pre-orogenic Tethyan trough. In
Switzerland, transitional facies can be seen in outcrops where the east-west reef
trend changes into deeper water carbonates and shales to the south. In Switzer-
land the reef belt may have been localized by the Late Paleozoic (Hercynian)
ridges and massifs, some of which stood as islands on the Late Jurassic shelf. Reefs
were probably developed along subsiding margins of these structural trends.
The outcrops of the reef belt are wide-as much as 100 km, chiefly because
reef development is regressive, moving from northwest to south and east; in the
latter directions, reefs rise in the section from upper Oxfordian through Tithonian
at the top of the Jurrasic. Jura reefs of Portlandian-Kimmeridgian age may be
seen near Belley and Yenne in France, 80 km east of Lyon, and are known in the
Basel region of northeast Switzerland in Oxfordian beds through work of Ziegler
(1962), and Bolliger and Burri (1970). In the Schwabian plateau, sponge reefs of
upper Oxfordian to mid-Kimmeridgian (MaIm Beta to Epsilon) are capped by
coral reefs of late Kimmeridgian (Maim Zeta) age. In Franconia, farther east
along the plateau near Neuburg an der Donau, Eichstiitt, Kelheim, and Regens-
burg coral reefs of very youngest Jurassic (Tithonian) are developed atop sponge-
bearing beds of early Kimmeridgian age.