Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 640

up-current, similarly to flute casts. The orientation of some trace fossils, like
Paleodictyon meshes, can also indicate the paleocurrent direction ( Crimes
and Crossley, 1980 ). In addition, several physical taphonomic features of gra-
phoglyptids (stretching, smoothing, bending, tapering, thinning, and straighten-
ing) indicate that the burrow systems were exhumed at least partly by bottom
currents prior to turbidite deposition ( Monaco, 2008 ). The direction of the
currents can be deduced from such features.
10. CYCLICITY AND SEQUENCE STRATIGRAPHY ASPECTS
Milankovitch cyclicity controls many aspects of sedimentation, and this is espe-
cially evident in the pelagic environment. For turbidites, Heard et al. (2008) noted
cycles in the intensity of bioturbation of 41 ka and about 112 ka in very-thin- and
thin-bedded turbidites of the Eocene Ainsa Basin in the southern Pyrenees,
related to glacio-eustasy and related changes of bottom-water oxygenation.
The 600 m thick section of Eocene, mostly thin- to medium-bedded turbi-
dites in the Basque Basin, N Spain, displays changes in ichnodiversity, without
any clear relationship with turbidite facies trends. The changes appear to reflect
sea-level fluctuations affecting the basin-margin areas as evidenced by
sequence-stratigraphic correlations ( Rodr´guez-Tovar et al., 2010 ). In this
way, sea-level fluctuations influence the trophic conditions, temperature, and
oxygenation and thus the ichnofauna. Corresponding trends have also been
noted for the benthic foraminifers.
In the Late Paleocene turbidites of the Zumaia section, northern Spain,
the lowstand systems tract, and the early transgressive systems tract are
characterized by a reduced rate of sedimentation. These intervals display the
Zoophycos trace-fossil assemblages ( Zoophycos , Chondrites , Trichichnus ,
Palaeophycus ), while the highstand systems tract, characterized by a high rate
of carbonate accumulation, displays the Palaeophycus tubularis assemblage
( Palaeophycus tubularis , Chondrites , Ophiomorpha recta , Fucrculosus , Nere-
ites , Thalassinoides , Phycodes , Trichichnus ; Giannetti and McCann, 2010 ; see
also Giannetti, 2010 ).
The influence of cyclicity and eustasy on the ichnology of turbidites is a new
aspect that now becomes increasingly studied. Nevertheless, the results suggest
that not only sedimentological processes affect trace fossils, but that complex,
often multi-feedback mechanisms influence the atmosphere, hydrosphere, and
biosphere, and thus also sedimentology and ichnology.
11. EVOLUTIONARY ASPECTS
Tracemakers are, just like other organisms, subject to evolutionary changes.
The ichnological record of their changes and the impact on sediments is intrigu-
ing. This problem was outlined by Seilacher (1962) , but the first attempts to
elucidate the evolution of deep-sea trace-fossil communities were made later.