stromatolites and calcareous algae of münder formation

OVIDIU N. DRAGASTAN & DETLEV K. RICHTER
Fig.5 - Stromatolite bioherms Type 2: bulbous and with protuberances, with 3 growth phases. Accumulated in the lagoon between
coastal sabkha and oolites, serpulid banks to marlstone with serpulid biostromes, Thüste quarry, Collection Prof. Dr. Detlev K.
Richter, scale bar 2 cm.
The alga Brachydactylus reisi is represented by
nodular thalli with small protuberances crossed by
fascicles of tubes, dichotomic branched, minidigitated,
sometimes incompletely filled with calcite after gypsum
crystals. The Brachydactylus thalli were not affected by
gypsum especially to the top of this phase; some parts of
the thalli remained unchanged. It cannot be excluded that
the genera Chlorellopsis and Brachydactylus belong to the
group of halophilic algae.
There are small depressions between the knolls that
contain coprolites of gastropods (Pl. 12, Fig. 5), ostracods,
fragmented bivalve shells, caddisfly, and pupal larvae of
Trichoptera (Pl. 11, Figs. 5–6, Pl.12, Fig. 4).
Channels can be observed at the top of this phase or
between phases 2 and 3 (Pl.1, Fig. 1, Pl. 2, Fig. 1, Pl. 3,
Fig. 1). These channels are filled with mat - clasts, oncoids
and ostracod shells that indicate a “reworking” event,
which might have been followed by a hiatus in trapping
and binding of microbial mats. The phase ends with an
erosional surface that is crossed by channel–fills; on its
top a mm-thick pyritized lamina follows, bacterial in
origin, which is also an indicator for the presence of
anoxic conditions.
Growth phase 3 (5.0 cm-7.0 cm) starts with basal,
domal laterally-linked microbialites, which in some parts
(inwards) of the lagoon passed into planar structures and
wavy microbial laminae (Pl. 3 Fig.1, Pl. 7, Fig.1 to the top
of photo).
The domal structures contain up to 10 undulatory
laminae showing couplets between the thicker laminae.
These may be possibly formed by coccoids and thinner
laminae. The latter ones are produced by filamentous
cyanophyceans. This kind of couplet-laminae could be
an indicator for trapping and binding processes under
warm climate conditions, perhaps during two seasonal
periods.
144