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

The Schnöll Formation (Spiculite Facies) 69
there are also samples still showing complete specimens or at least sponge fragments with
spicules preserved in almost original configuration (Pl. 19) known from mesohyl, dermal, and
gastral sponge “tissues”. Although exact classifications are impossible due to the lack of
microscleres, the interpolation of numerous cross sections allows to differentiate several species
of globular to irregular bulbous shapes. Additionally one sample has shown circular transverse
sections (Pl. 19A) indicating also the presence of sponges with tubular or cup-shaped growth
forms. Former areas of sponge tissues show clotted micropeloidal fabrics (Pl. 19B, C) that are
typical features of in situ-formed organomicrites (Reitner 1993; Reitner and Neuweiler 1995;
Reitner and Schumann-Kindel 1997; Schmid 1996; Vigener 1996; Delecat et al. 2001).
Interspace between sponge-microbialites is closed by biodetrital matrix comprising fine shell
debris, radiolarian, small gastropods, and a few foraminifers (mainly miliolids and lagenids, e.g.
Nodosariidae, Lingulina sp., Ammodiscus sp., Involutina liassica, Ophtalmidium sp., a.o.).
Bioturbation is rare. The sediment has also infiltrated the sponge remains, thus there is no
distinct difference to the embedding matrix, hence corroded specimens are absent. The ratio
between automicrites and allomicrites varies and often they are closely intertwined. Although the
microbialites are well to distinguish by their microtexture from the allomicrites, the attempt to
discriminate them by fluorescence or EDX analyses failed.
Most remains of the collapsed sponge skeletons merge into spicular mats of several millimeters
up to a few centimeters in thickness (Pl. 18A, B). Stromatactis cavities particularly occur in
between the sponge-microbialite-horizons, where two kinds of different sizes are distinguishable.
The bigger ones are some millimeters up to 1cm in height and form horizontally arranged
networks that do not cut any components of the sedimentary matrix (Pl. 18A, B). The network
separates spicular layers displaying further cavities that are much smaller in size. Latter
microstromatactis cavities often occur in several levels inside the sponge remains (Pl. 18B, 19C).
Their distribution and irregular contour is determined by the sponge skeletons and the
automicrites. Nearly all stromatactis cavities display irregular roofs that are defined by
micropeloids as well as intraclasts, fossil debris, or sponge spicules. In contrast, bases of the
bigger cavities are rather flat, due to fine detrital sediments or peloidal micrites that were either
washed in or represent reworked material from cavity walls (Pl. 18A-C). Many cavities are
nearly filled by these infiltrated sediments (aborted stromatactis, Neuweiler et al. 2001b) that
sometimes fade smoothly into subjacent micrites so that a definite original base of these cavities
is not discernable. Remaining space of the cavities is closed first by radiaxial fibrous cements,
secondary by internal sediments or blocky sparites.