Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 655

youngest burrows within a specific horizon can cut through previously
produced ones, whereas all those that have been produced later can intersect
the oldest bioturbation structures. Thus, tiering can be deduced by the analysis
of the cross-cutting relationships and the number of tiers can be established.
A cross-cutting relationship can result from the vertical migration of one endo-
faunal community keeping pace with the sedimentary surface during accumu-
lation ( Bromley and Ekdale, 1986; Miller, 1991; Werner and Wetzel, 1982 )or
from the juxtaposition of burrows of replacing communities while environmen-
tal conditions change (e.g., Miller et al., 2004; Savary et al., 2004; Wetzel and
Uchman, 2001 ).
Using the tiering concept for environmental analysis, steady-state condi-
tions are often invoked. However, this is not always likely, as illustrated by
the following example. Holocene deep-sea sediments off NW Africa accumu-
lated at a rate of 10 cm/ka and the tiers may be as deep as 50 cm ( Werner and
Wetzel, 1982 : 280). In this case, steady-state ichnofabric formation would need
stable conditions for more than 5,000 years, a requirement seldom met if the
stable-isotope record in Quaternary deep-sea sediments is analyzed (e.g.,
Seibold and Berger, 1996 ). Consequently, it is necessary to evaluate whether
the burrowing organisms and the related trace fossils of the deeper tiers are asso-
ciated with actual conditions or whether they represent responses to past con-
ditions provided by the host sediment. Therefore, the conditions during the
formation of deep-tier traces have to be considered by compensating for their
penetration depth and estimating the lifetime intervals of their producers
( L¨wemark, 2007; Wetzel, 1981 ).
2.2 Bioturbational Structures
Two general types of bioturbational sedimentary structures can be distin-
guished. (1) Trace fossils exhibit sharp outlines and possess a characteristic
recurrent geometry that allows their classification in terms of ichnotaxonomy,
whereas (2) biodeformational structures have less distinct outlines and do not
display a recurrent geometry that would allow their classification; biodeforma-
tional structures are formed within soft to soupy or incoherent sediment (e.g.,
Wetzel and Uchman, 1998b; Lobza and Schieber, 1999; Leszczy ´ ski, 2004 call-
ing them “turbichnia”). They destroy pre-existing sedimentary structures
( Fig. 2 ). An intermediate class represents cryptobioturbation, constituted by
small-sized trace fossils in such an overwhelming abundance that it results in
a quasi-homogenization of the sediment (e.g., Pemberton et al., 2008 ).
In the fossil record, two main behavioral and hence often differently pre-
served groups of trace fossils are distinguished. Trace fossils that are produced
on or very close to the sediment surface become preserved only when they are
scoured and cast by depositional events; with respect to the event they are pre-
depositional ( Ksi˛˙kiewicz, 1954 ). Especially, the highly organized, often del-
icate graphoglyptids ( Fuchs, 1895 ) belong to this category. Post-depositional