Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 633

Uchman, 1996b ). The ratio between r-selected and K-selected ichnotaxa can
vary from formation to formation in one basin in response to environmental
changes such as oxygenation, sedimentation rate, and grain size ( Tunis and
Uchman, 1996a; Uchman, 1991, 1992, 2004b ). Commonly, trace-fossil assem-
blages rich in graphoglyptids occur in thin- to medium-bedded turbidites char-
acterized by greenish to grayish shales, formed under moderately oligotrophic
conditions ( Miller, 1991b; Tunis and Uchman, 1996b ). However, Olivero et al.
(2010) noted abundant graphoglyptids in levees of a channel formed in Eocene
deeper shelf settings of Tierra del Fuego in Argentina, while Monaco et al.
(2010) recorded occurrences in the levees of deep-sea channels and in turbidites
along the margin of a submarine high in the Oligocene-Miocene turbidites of
the Apennines in Italy. All these settings, especially channel areas, are expected
to be rich in organic matter transported from the land and shelf, but most
of it being refractory while derived from a terrestrial source (e.g., Baudin
et al., 2010 ).
Not all turbidity currents bring enough oxygen allowing colonization of the
sea floor by infauna in an area experiencing a general oxygen deficiency. Prob-
ably, many turbidites were not colonized because of this reason ( Leszczy ´ ski,
1991; Uchman, 1991, 1992; Wetzel and Uchman, 2012 ). The ichnodiversity is
low if oxygenation drops below a threshold value. However, when slowly accu-
mulating sediment is very well oxygenated and if only little organic matter is
available for the burrowing organisms, diversity can decrease ( Leszczy´ski and
Uchman, 1993; Wetzel and Uchman, 2012 ). This is well demonstrated by the
relationship between ichnodiversity and sediment color: the highest diversity
occurs in gray sediments, and the lowest in red or black deposits ( Leszczy´ski,
1993b ). The diversity can also be lowered by too frequent disturbances by
turbidity currents ( Leszczy´ski, 2003 ). Taking into account observations
of the impact of modern physical disturbances on soft-sediment benthos
(e.g., Hall, 1994 ), an influence of this factor is possible.
7. ICHNOFABRICS
Turbidites, like other event beds, are normally colonized from the top. Turbidity
currents and other gravity-induced flows can transport tracemakers from shal-
lower to deeper water. These tracemakers can survive transport and produce bur-
rows ( Crimes, 1977; F¨ llmi and Grimm, 1990 ), but this mechanismappears to be
overestimated in the interpretation of the ichnological record (see Section 5).
In detail, turbiditic/pelagic or turbiditic/hemipelagic couplets consist of
rapidly deposited turbidites and slowly accumulated background sediment.
Basically, two intervals can be distinguished in the couplets, a spotty and an
elite layer ( Uchman, 1999 ). These correspond roughly to the mixed layer and
the transitional layer, respectively, in modern deep-sea sediments
( Berger et al., 1979; Bromley, 1990, 1996; Ekdale and Berger, 1978; Ekdale
et al., 1984 , and references therein). The characteristic spotty layer is entirely