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

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sheltered by the adjacent sand drift that probably served as a small barrier against bottom
currents (Fig. 30).
Fig. 30. Successive stages of the
development of Liassic spiculites on
the lower slope of the Adnet reef
(Rot-Grau-Schnöll Quarry, Adnet).
(A): Colonization by tubular- or cupshaped
sponges took place first on
biodetrital sediments. Dislocation of
dead sponges occurred by local
water currents.
(B): Accumulation of sponge
remains in front of the sand drift
barrier. Sponges were episodically
embedded due to irregular
sedimentation rates and/or
occasional high-energy events.
Declining wave base caused the
erosion of the top of the sand drift
facies.
(C): First distinct hiatus is
characterized by bio- and lithoclasts
and several generations of
ferromanganese encrustations.
(D): Sedimentation became more
and more dominated by crinoidal
debris. The sequence of sponge-rich
biomicrites is several times
interrupted by sedimentary breaks
and ferromanganese crusts. Towards
the reef top, it is replaced by a
strongly condensed horizon,
especially by the conspicuous
“sponge layer”, that discordantly
overlies the sand drift facies.
Hexactinellid sponges like those of the Adnet fauna, are known from deep water environments
with low sedimentation rates (recent species in several hundreds of meters; e.g. Ijima 1926;
Schulze 1887, 1904). Nevertheless the real water depth of the Schnöll Formation, where sponge
settlement took place, is difficult to evaluate. The foraminifer- and radiolarian fauna does not