Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 635

siliciclastic deposits if the supply of pelagic carbonate particles is significant. In
oligotrophic, well-oxygenated environments, the spotty layer is lighter than the
underlying sediments. In eutrophic environments, the spotty layer is commonly
darker than the sediment underneath ( Wetzel and Uchman, 1998 ). Some trace
fossils of the spotty layer are preserved as pre-depositional forms on the soles
of overlying turbidites. They belong to the so-called background ichnofauna
( Leszczy´ski, 1993a ), which includes mainly graphoglyptids, which are hardly
recognizable in cross section.
The spotty layer is sometimes mistaken for purely pelagic or hemipelagic
sediment but in fact represents the mixed layer in modern deep-sea sediments,
which in the rock record is not only composed of pelagic material but, due to
bioturbation, also of some material derived from turbidity currents ( Uchman
1995a, 1999 ). Pelagic sediment is not only transported downward into the tur-
bidite layer, but turbidite particles are also conveyed upward to the sea floor
that is mainly covered with pelagic material. This can be deduced from the fact
that, in modern environments, differently colored sediment of the transitional
layer is conveyed to the sea floor by burrowers ( Ekdale et al., 1984 ).
The spotty layer cannot be considered as being entirely equivalent to the
mixed layer when fossilized, especially with respect to thickness. The formation
of the spotty layer in turbidites is probably a dynamic process. Initially, a thin
mixed layer is formed at the top of the sediment following turbidite deposition.
Trace fossils produced during this phase are hard to recognize due to lithologi-
cal contrast and do not have sharp outlines. At the same time, pelagic particles
accumulate. If the rate of biological reworking exceeds the rate of the accumu-
lation, the pelagic material becomes totally mixed by local, homogenizing pro-
cesses and non-local, far downward-reaching transport within burrows into
turbiditic sediment. The lower boundary of the spotty layer can be either dis-
tinct, because of the lithological contrast, or gradational due to intense biotur-
bation by producers of Scolicia or Ophiomorpha . The contrast between the
spotty layer and the turbiditic mud can be low if they are lithologically similar.
The most notable trace fossils occur below the spotty layer in the elite layer
(elite trace fossils sensu Ekdale and Bromley, 1991 ). In most marly turbidites,
deep-tier elite Chondrites , Planolites , Nereites , Phycosiphon , and Scolicia
occur ( Fig. 5 A and B) . To estimate the depth of burrowing, the distance to
the top of the elite layer should be known. Some burrows penetrate below
turbidites and they form the so-called exichnial elite layer ( Uchman, 1999 )
( Fig. 5 C).
Ichnofabric analysis is useful in reconstructing the complex colonization
history of a single layer in turbidites/hemipelagic sediments. It appears that
the cross-cutting relationships between trace fossils result not only from the
accretion of sediment and upward movement of tiers such as in (hemi)pelagic
sediments, but also from the sequential colonization of event layers over time
( Wetzel and Uchman, 2001 ). Newly deposited turbidites contain relatively
well-oxygenated pore water and a relatively high amount of food. Initially,