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

Plate 20
Steinplatte / Plattenkogel –
Special features of the Triassic-Jurassic boundary section at Plattenkogel hill.
(A): Assumed Triassic-Jurassic boundary section shown by the corroded surface of a lumachelle
layer from the clastic sequence (horizon 3a in Figure 20) and the cover of detrital spiculite
limestone facies. After corrosion and ferromanganese impregnation of borings and
interparticular cavities, the lumachelle was encrusted by biofilms (arrows).
(B): Detail of the facies change in (A) with a focus on micro-borings (arrow) and the margin of
a dark microbial crust (mc).
(C): Part of a stromatactis cavity in the spiculite facies displaying at its bottom infiltrated
sediments with syndiagenetically formed autigenic quartz.
(D): Detail of (C). Quartz crystals are scattered over an initial cement crust, pointing to elevated
salinities during spiculite formation.
Arzo / Italy –
Liassic spiculite facies (Broccatello).
Samples from the collection of Prof. Dr. Joachim Reitner (GZG – Göttingen).
(E): Skeleton of a non-rigid, axinellid demosponge. Monaxon oxeas and a few tetractins are
embedded in light grey automicrite.
(F): Spicules of an axinellid demosponge. Monaxons are arranged in bundles and show
plumose structures (ps). In the left center a dichotriaen sponge spicule (arrow) points to a
species of Tetractinellida (arrow). At the right bottom the solid skeleton of a coralline
demosponge (chaetetid type, Neuropora)
(G): Skeleton of a coralline Calcarea (pharetronid type).
(H): Same sponge type as in (C).