Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 686

change in substrate consistency may be documented by continuously changing
trace-fossil morphologies and their excellent and detailed preservation, even of
meiobenthic trace fossils less than 1 mm in shortest dimension ( Knaust, 2007a,
2010b ).
Another contribution comes from the microbial mats and biofilms, which
are typical in such environments. They cover the sediment surface (including
fresh animal traces) and prevent subsequent destruction, creating a slightly
reducing microenvironment and in some cases even preventing the oxidization
of the tracemakers, in which case they are preserved together with the traces.
Rapid coverage by a new layer of fine-grained sediment (obrution) together
with early-diagenetic cementation further increase the chance to preserve the
traces in an optimal way.
Specific carbonate particles in combination with early-diagenetic cemen-
tation and alteration are another aspect of carbonates. A diverse worm and
crustacean fauna contributes with major sediment-feeding organisms to biotur-
bate and rework the sediment, in many places completely, and results in a con-
siderable production of fecal pellets. Such accumulations of microcoprolites are
subject to early lithification and may turn into carbonate rocks. Fecal pellets are
regarded as trace fossils and provide valuable information about the paleoenvi-
ronment (e.g., Knaust, 2008 ). Moreover, broken shells, ooids, and lithoclasts are
commonly affected by microboring organisms that partly or completely destroy
the structure of the grains while generating peloids ( Wisshak, 2012 ).
2.3 Early Diagenesis
Taphonomic processes are important for trace-fossil preservation in siliciclas-
tics, especially the casting of traces due to contrasting lithotypes such as sand
and mud, whereas grain-size contrasts (e.g., arenite or rudite on carbonate mud)
typically play only a minor role in the preservation in carbonates. However, the
taphonomic history of trace fossils in carbonates differs from that in siliciclas-
tics by the importance of diagenetic processes that lead to preferential preser-
vation and trace-fossil selection ( Narbonne, 1984 ). The reason for this is
sea-floor lithification, and therefore less compaction than in siliciclastics,
and a better preservation of sedimentary and biogenic structures. In other cir-
cumstances, carbonate cementation and recrystallization are able to alter trace
fossils in carbonates, which can lead to a poor level of preservation.
Diagenetic processes on carbonate platforms and ramps play an important
role in the transition of sediments to rocks. The different stages of substrate con-
sistency and the timing of the inherent processes are often reflected by signif-
icant trace-fossil assemblages (including borings in hardgrounds) and thus can
be utilized in the recognition of the kind of substrate, duration of sedimentation,
omission, discontinuity surfaces, etc. ( Bromley, 1975; Ekdale and Bromley,
2001; Goldring and Ka´mierczak, 1974; H¨ lder and Hollmann, 1969; Kobluk