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

The Schnöll Formation (Spiculite Facies) 67
the world show special paleoenvironmental conditions that provide uncollapsed individuals of
this group. Brückner (2003) and Brückner et al. (2003) recently described complete fossil
Lyssacinosida from Upper Cretaceous limestone and Oligocene mudstone, where the
preservation was apparently caused by sedimentary conditions, that are similar to Adnet’s
Schnöll facies but are characterized by a higher degree of pyritization. A few aspects of the
taphonomy of non-rigid sponges have already been reported by Reitner & Neuweiler (1995),
Flajs et al. (1996) and Vigener (1996) from Paleozoic mud mounds. Concerning Liassic faunas,
studies on Moroccan carbonates by Dresnay et al. (1978) and Neuweiler et al. (2001a) have
shown that lithified sponges were important benthic components in the colonization of the early
central Atlantic Ocean. In addition, siliceous sponge faunas from the southern margin of the
Tethys are known from Lower Jurassic limestone of the Trento Platform (Southern Alps, Italy).
However, these are dominated by Hexactinosa, lithistid, and coralline demosponges rather than
by Lyssacinosida (e.g. Krautter 1996). Outcrops comparable to the Adnet facies and
paleoenvironment are those of Arzo near the Lake Lugano in southern Switzerland. The
Rhaetian reef limestone at Arzo exhibit red Hettangian and Sinemurian limestones
(Wiedenmayer 1963) with numerous sponge remains (Neuweiler and Bernoulli 2005). In
contrast to the hexactinellid-dominated communities in Adnet, the sponge community of Arzo is
dominated by soft demosponges like hadromerids and Halichondrida, coralline demosponges
(“Chaetetidae”, Neuropora), and first occurences of modern pharetronids (Calcarea).
Hexactinellid spicules are rare.
5.1.1.4. Sponge-related Stromatactis Cavities
Stromatactis voids respectively fenestral cavities seem to be a common feature of autochthonous
spiculites that are dominated by non-rigid sponges (sensu Krause et al. 2004, Neuweiler and
Bernoulli 2005), but at the Rot-Grau-Schnöll Quarry of Adnet stromatactis is scarcely present.
Large, cm-sized cavities that form a network of connected voids in different layers were only
detected in the strongly condensed sequence of the SW profile. The bottom of the larger cavities
(Pl. 10H) is frequently filled with reworked material, often micropeloidal automicrites, or
infiltrated by fine sediments. This suggests that these stromatactis cavities formed very early in
the upper subsurface of the sediment, hence the developing cavity network represents an open
system that was still connected with the bottom water regime. Their formation is postulated as
being the result of heterogeneous sediment compaction due to high contents of organic mucus
that becomes partly mixed with the embedding sediment. Because such a process seems to be
related to low or very heterogenous sedimentation rates, at Adnet it is rarely present and only