Geology Reference
In-Depth Information
Ecology:
Bivalves live predominantly marine as endo-
benthic suspension feeders on soft bottoms and as epi-
benthos on the sediment. Some bivalves are borers.
disarticulation, plan-view and side-view orientation,
fragmentation, abrasion, bioerosion and encrustation,
sorting, species diversity. Many of these criteria can
only be seen in large thin sections if shell accumula-
tions should be genetically interpreted. Shell concen-
trations (lumachelles, coquinas) can be formed by vari-
ous processes (Kidwell 1991). Event concentrations
originate from short-term events, e.g. storm deposits
(Pl. 102/3, 5), composite concentrations are caused by
multiple reworking and transport), condensed concen-
trations are formed by multiple reworking and trans-
port of long duration associated with low sedimenta-
tion rates, and lag concentrations as a result of multiple
reworking associated with the winnowing of fine-
grained sediment and corrosion.
Shell beds should be studied in detail because they
may be used as markers for recognizing the develop-
ment of sequences and cyclothems (Carter et al. 1991).
Transgressive system tracts and nearshore highstand
system tracts contain either worn and transported bi-
valve shells, or in-situ faunas diagnostic of intertidal
and shallow subtidal conditions. In contrast, the maxi-
mum flooding surface is characterized by an offshore
fauna.
Bivalve limestones
Many limestones yield high numbers of bivalve
shells contributing to the formation of bioclastic wacke-
stones, packstones, floatstones, and rudstones. Bivalve
limestones originate in a variety of marine depositional
settings, from the beach across the shelf and the shelf
margin to the slope. To interpret these limestones basic
concepts developed for the discussion of shell beds must
be taken into account.
Shell beds
: Biofabric and taphonomic signatures of
shell accumulations provide information on sedimen-
tary processes, such as fair weather and storm waves,
storm flows, and long-term currents as well as rates of
sedimentation. Fürsich and Oschmann (1993) distin-
guished nine types of skeletal concentrations. Essen-
tial criteria are sedimentary structures (e.g. cross bed-
ding, planar lamination), erosive base, grading, intra-
formational pebbles, bioturbation, matrix, packing of
skeletal elements (bioclast-support or matrix-support),
Fig. 10.40.
Megalodon
limestones. The fine-grained bioclastic wackestone contains abundant molds of bivalves, most of
which are double-valved and still in life position. The megalodontids lived on shallow muddy substrates in lagoonal low-
energy environments. The well-bedded
Megalodon
limestones are a part of cyclic platform sequences in Oman as well as in
other parts of the Triassic Tethys recorded by alternations of bivalve limestones and microbial/algal carbonates. Late Triassic
(Mishrif Formation, Norian): Wadi Ala, Jabal Kawr, Oman Mountains. Coin for scale. Courtesy of M. Bernecker (Erlangen).
