Page 280 - Carbonate Facies in Geologic History

Swiss Jura Mountains 267

Swiss Jura Mountains

Figure IX-3 is a northwest-southeast trending section across the Swiss Jura some
kilometers west of Basel. It is located along the northeast corner of the map of
Fig.IX-4, roughly through Tiergarten and Moutier. A question exists about the
correlation of the Effinger marl-whether it is a fill-in of a starved basin and just
younger than the lower reef-lined shelf margin (Bolliger and Burri, 1970) or
whether it is a facies equivalent of the reef as interpreted by Ziegler (1962). This is
not of consequence to the facies interpretation.
Three main facies provinces are seen, regressive to the southeast.
1. To the northwest, a shallow shelf with tidal flat and fresh-water pond
sedimentation.
2. A northeast-southwest trending belt with corals and bioclastic reef debris,
including abundant oolite deposits.
3. To the southeast slope limestones and marly sediment with open marine
faunas including ammonites. Equivalent beds in the frontal Alps-both in the
Helvetic nappes and the autochthonous section over the Aar massif are found in
the Quintnerkalk, a black somewhat argillaceous (?) limestone with a pelagic
fauna.
Owing to the above stratigraphic studies, and those of Gygi (1969), one may
precisely correlate upper Oxfordian strata across these facies belts in the Jura
Mountains from south to north. These strata, diagrammed on Fig.IX-3, are dis-
cussed below in detail because they offer an excellent example of the range of
facies across a shelf margin of Mesozoic age and because microfacies and paleon-
tology are particularly well-known.
The basinal reef-equivalent strata (Birmenstorf beds) are brown, spiculitic,
pelleted mudstones and wackestone with chert concretions formed from decay of
hexactinellid sponges. Typical fossils, in addition, are rhynchonellid and terebra-
tulid brachiopods, large bivalves such as Lima, echinoderms, gastropods, belem-
nites, and perisphinctid ammonites. The beds become more argillaceous farther
from the reef area. Northward, toward the reef edge, the argillaceous off-reef
strata pass into microbioclastic wackestones, in places containing some minor
coarser reef debris. No real reef talus occurs. The fauna is much like that of the
basinal slope beds; cidarid echinoids and shallow water bivalves occur in addition
to the cephalopod and sponge biota.
The 25 m of basinal-slope beds thicken northward to a bank of coral-bearing
nodular limestone termed Liesburg beds. These strata are transitional from un-
derlying sandy argillaceous beds and are extremely fossiliferous with many plate-
shaped coral colonies of Thamnasteria (as much as half the rock), crinoids, cidarid
echinoid, pectinid bivalves, terebratulid brachiopods, and lithistid sponges. The
fossils are commonly silicified. Overlying the Liesburg unit is massive coral-
bearing limestone. The lower corals of this bank are plate-shaped Thamnasteria in
a micrite matrix. Higher up this genus is replaced by roundish coral heads and
large branching types of corals such as Latomaeandra .. only a few recognizable
echinoderms, brachiopods and bivalves are associated. Although no lithoclastic
reef talus is seen, much of the upper reef limestone is formed of coarse bioclastic
packstone. The coral buildups appear to consist of a series of individual patches