Porifera-microbialites of the Lower Liassic (Northern Calcareous ...

The Schnöll Formation (Spiculite Facies) 53
5. The Schnöll Formation (Spiculite Facies)
5.1. The Schnöll Formation at Adnet (Rot-Grau-Schnöll Quarry)
The section of the Hettangian Schnöll Formation is composed of thin to thick bedded limestone
in reddish or grey colors. A lot of the beds are separated by surfaces of pressure solution,
omission, or erosion. In the latter case, the beds are often covered with ferromanganese crusts of
varying thickness. All layers are characterized by varying amounts of siliceous sponges. The
interior of the former sponge bodies respectively interstices between sponge spicules are
predominantly closed by microbial carbonates. Sponge remains are mostly embedded in
biodetrital micrite, that is sometimes also closely interwoven with microbialites. A poor
accessory fauna is found in the Schnöll facies, comprising (mainly allochthonous or
parautochthonous?) tests of radiolarian and foraminifers, fragments of small gastropods,
brachiopods, bivalves and ammonite shells. Encrusting organisms are only presented by a few
forams and “microproblematics”.
The quarry floor and the lowest part of the NE profile (Rot-Grau-Schnöll Qarry) are formed by
grey spongy limestone (Fig. 15, horizon no. “2”) with high content of radiolarians. The sediment
is mainly formed by microbial carbonates that enclose isolated spicules of non-rigid sponges and
small stromatactis cavities (Pl. 10A). The facies is similar to that, found in several examined
samples from the nearby Langmoos Quarry. The next 60 cm of the profile (Fig. 15, horizon
no. “3”) shows undulated beds with very high abundance of complete sponges or partly fractured
or collapsed individuals. Quarry walls display mainly round or elongated cross sections of tube-
to cup-shaped species that were apparently all dropped and partly aligned by water currents.
Their densely packed skeletal remains form 5-10 cm thick bulbous layers, recognisable in the
field as light grey blotches in a red to pink matrix (Fig. 28A, Pl. 9D, 10B). They are intercalated
by thin and wavy reddish-brown sparse biomicrites and are attributed to the Upper Langmoos
Member (sensu Böhm et al. 1999). The top of the Langmoos Member at the Rot-Grau-Schnöll
Quarry is capped by an erosional surface (Fig. 15, horizon no. “4”) with reworked sediment
clasts as well as fragments of sponges and ammonites that were successively impregnated and
covered by ferromanganese precipitates (Fig. 28B, Pl. 10D). The stratigraphic allocation of the
following unit (Fig. 15, horizon no. “4-5”) is uncertain but it presumably belongs to the Guggen
Member. It is not possible to correlate the unit and its ferromanganese crusts with the whole
section of sponge-rich layers in the SW profile, thus the unit is assumed to represent a section
that corresponds with solely the “sponge layer” in the SW profile (Fig. 15, horizon no. “7”) that
is clearly attributed to the Guggen Member by ammonites (Böhm et al. 1999). In the first 90 cm
above horizon no. “4” (Fig. 15) sponges still form the main part of the sediments with highest
densities of sponges occurring at the base of some of the layers, close to pressure solution seams