3.5 Biofacic,
39
The central part of the Groninger Plate is a sandy platform, inhabited by
L. cOf/chilega in varying densities. This facies was surrounded by A. marina dominated sandtlats. As on the Swinnplate, small-scale areas belonging to facies type I,
2 and 8 were found. Edge gully zones were found at the border to the northerly and
southerly situated tidal channels.
Each facies type also had characteristic features in the vertical direction. At the
Swinnplate, six out of the eight characteristic types were sampled with corers. A
summary of their characteristics is presented in Table 3.5.1. The stratigraphical
time span varied with the situation within the area of investigation. In general, an
increase of age from the edge to the centre of the plate could be observed, with the
time scale reaching back from a few tides up to several decades or even the order
of magnitude of centuries.
Successions can be shown by Lebensspuren of dominant species in different
sediment depths. Fig. 3.5.2. shows a relief of sediment in the central part of the
Swinnplate, including a sequence of 3 M. edulis colonizations at this place. The
occurrence of sub-articulated shells in the deeper layer indicates strong and fast
sedimentation after the death of the organisms and may be interpreted as proof for
locall y produced autochthonous shells. In the deepest horizon, tubes of
L. conchi/ega from earlier colonization phases can be observed.
Facies sequences of L. conchilega and M. edulis do not give a clear-cut indication that Lanice aggregates and Mytilus beds occurred in a temporal sequence and
excluded each other. Two possible processes producing such a pattern have to be
considered: (I) The patchy distribution of L. conchilega and M. edulis in the central part of tidal tlats, as observed by Hertweck (1995, 1998) includes free areas
between the Mytilus aggregates inhabited by Lanice. Horizontal shifts e.g. of the
mussels within the area may lead to a vertical succession of both facies in a sediment core although both species occurred contemporaneously at that site. (2) Settlement of Mytilus juveniles in dense Lanice aggregates may result in a qualitative
shift from one facies to the other (Hertweck 1995). The facies sequence observed
is the result of the settling behaviour of M. edulis which is dependent on hard substrate (in this case Lanice-tubes) and points to a causal biotic interaction.
3.5.3
Case Study Myti/us Bed
For the biofacies "Mytilus bed" the method of the relief cast was combined with
geochemical analyses (e"", N,Ol' Plot' pH, Eh) of the biogenic mud (Hertweck &
Liebezeit \996). The longer a Mytilus bed exists at one place, the deeper the mud
layer beyond the mussels and the higher the content of particles < 63 flm in the
mud (loc. cit.). A comparison of the mature (with large number of adult specimen)
Mytilus bed at the Swinnplate with an older receding bed in the area of the Hullplate showed that the latter one had a 15 cm thick persistent layer of mud. The elN
ratio indicated that here, besides erosion and bioturbation, a clear decomposition of
the organic material occurred, while in the area of the active Mytilus bed a minor
decomposition was recorded.
39
The central part of the Groninger Plate is a sandy platform, inhabited by
L. cOf/chilega in varying densities. This facies was surrounded by A. marina dominated sandtlats. As on the Swinnplate, small-scale areas belonging to facies type I,
2 and 8 were found. Edge gully zones were found at the border to the northerly and
southerly situated tidal channels.
Each facies type also had characteristic features in the vertical direction. At the
Swinnplate, six out of the eight characteristic types were sampled with corers. A
summary of their characteristics is presented in Table 3.5.1. The stratigraphical
time span varied with the situation within the area of investigation. In general, an
increase of age from the edge to the centre of the plate could be observed, with the
time scale reaching back from a few tides up to several decades or even the order
of magnitude of centuries.
Successions can be shown by Lebensspuren of dominant species in different
sediment depths. Fig. 3.5.2. shows a relief of sediment in the central part of the
Swinnplate, including a sequence of 3 M. edulis colonizations at this place. The
occurrence of sub-articulated shells in the deeper layer indicates strong and fast
sedimentation after the death of the organisms and may be interpreted as proof for
locall y produced autochthonous shells. In the deepest horizon, tubes of
L. conchi/ega from earlier colonization phases can be observed.
Facies sequences of L. conchilega and M. edulis do not give a clear-cut indication that Lanice aggregates and Mytilus beds occurred in a temporal sequence and
excluded each other. Two possible processes producing such a pattern have to be
considered: (I) The patchy distribution of L. conchilega and M. edulis in the central part of tidal tlats, as observed by Hertweck (1995, 1998) includes free areas
between the Mytilus aggregates inhabited by Lanice. Horizontal shifts e.g. of the
mussels within the area may lead to a vertical succession of both facies in a sediment core although both species occurred contemporaneously at that site. (2) Settlement of Mytilus juveniles in dense Lanice aggregates may result in a qualitative
shift from one facies to the other (Hertweck 1995). The facies sequence observed
is the result of the settling behaviour of M. edulis which is dependent on hard substrate (in this case Lanice-tubes) and points to a causal biotic interaction.
3.5.3
Case Study Myti/us Bed
For the biofacies "Mytilus bed" the method of the relief cast was combined with
geochemical analyses (e"", N,Ol' Plot' pH, Eh) of the biogenic mud (Hertweck &
Liebezeit \996). The longer a Mytilus bed exists at one place, the deeper the mud
layer beyond the mussels and the higher the content of particles < 63 flm in the
mud (loc. cit.). A comparison of the mature (with large number of adult specimen)
Mytilus bed at the Swinnplate with an older receding bed in the area of the Hullplate showed that the latter one had a 15 cm thick persistent layer of mud. The elN
ratio indicated that here, besides erosion and bioturbation, a clear decomposition of
the organic material occurred, while in the area of the active Mytilus bed a minor
decomposition was recorded.