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

Comparison with other Spiculites 85
The facies displays a high ratio of spicules and very low amount of embedding matrix thus
inplying a reduced sedimentation rate. Accordingly, automicrites are scarce and only occur in the
interior of some of the rigid sponge skeletons so that the sediment resembles the grey limestone
facies from alpine basinal settings. In contrast to these sediments lacking diverse accessorial
faunas, the ridge of the deep sea volcano “Vesterisbanken” is characterized by more complex
biogenic mats with sponges, bryozoans, serpulids and hydrozoans. They formed on sands and
coarse volcanic gravel that provided larger surfaces also for encrusting filter-feeding organisms.
The sponge taxa encompasses several groups of Lyssacinosida (Rossellidae), non-rigid
Demospongiae (Geodiidae) and a few poeciloclerid Demosponges. A closer look onto the
spiculite mats show that fine-grained, microbial sediment is trapped within the meshwork of
sponge spicules and branched bryozoan colonies. Furthermore, the mats from Vesterisbanken are
also characterized by spicules of dead predecessors, by microbially induced carbonate
precipitation and by small interior cavities (Pl. 22) where larvae settlement of cryptic sponges
occur. Consequently, the mats could be seen as modern counterparts to the Liassic alpine
spiculite at Steinplatte with both displaying similarities in their formation and taphonomic
structure.