Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 638

the producing organisms take advantage of chemosymbionts utilizing the
gradient between reducing pore water and oxygenated bottom water pumped
into the burrow via the connection to the sea floor. During the long periods
between the deposition of successive turbidites, commonly hundreds or
thousands of years, the slowly deposited background sediment is colonized
by graphoglyptids. There are also tracemakers independent of this succession,
such as echinoids producing Scolicia , and the multilayer colonizers sensu
Uchman (1995b) which overprint the pre-existing ichnofabric to varying
degrees. Such a scenario of sequential colonization, elaborated on the basis
of Paleogene deep-marine turbidite successions, is also applicable to older
units, including at least part of the Upper Paleozoic ( Mikul ´ˇ et al., 2002 ).
Certain types of ichnofabrics can characterize different parts of a deposi-
tional system. For instance, Knaust (2009) recognized seven types of ichnofab-
rics in a Campanian deep-sea fan in the Norwegian Sea and related them to
amalgamated channels, lobate sand sheets, proximal and distal overbank, fan
fringe, and hemipelagic deposits.
8. ICHNOLOGY OF CARBONATE TURBIDITES
Trace-fossil preservation and diversity differs in siliciclastic and carbonate
(marly) Cretaceous-Paleogene flysch deposits ( Giannetti and McCann,
2010; Savrda, 2012; Uchman, 1999 ). Generally, trace fossils in marly turbi-
dites are more often of post-depositional than of pre-depositional origin. Gra-
phoglyptids are relatively rare, although there are exceptions (e.g., Monaco
and Checconi, 2010 ), and this is probably the result of preservation conditions
and benthic food level. For the preservation of pre-depositional trace fossils,
particularly graphoglyptids, subtle erosion is necessary, which scours such
burrow systems before casting. Late Jurassic and younger marls consist mostly
of microfossil remnants (e.g., Tucker, 1974 ), which behave hydraulically like
silt or even clay particles. Therefore, turbidity currents carrying calcareous
mud cannot cause erosion similar to that of muddy siliciclastic suspension
currents. Thus, marly turbidites mask a shallow-tier ichnofauna. On the other
hand, the marly turbidites often display features of eutrophic environments, in
which graphoglyptids are less abundant than in oligotrophic environments
( Uchman, 2003, 2007b ). Only few trace fossils apparently are characteristic
of either siliciclastic or marly sediment, respectively. Most probably, Cladich-
nus fischeri is restricted to marly sediments. Nereites irregularis is very com-
mon in marly turbidites; however, it also occurs in carbonate-free sediments,
such as in the variegated shales of the Upper Paleocene-Lower Eocene in the
Carpathians ( Leszczy ´ ski and Uchman, 1993 ). Filling of some late colonizer
burrows in calcarenitic tempestites consist of calcilutite. This is interpreted as
an effect of the long settling time of carbonate from suspension, which can
take months ( Miller et al., 2004 ).