Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 597

FIGURE 3 Trace fossils as indicators of sediment bypass (tubular turbidites) in slope strata.
(A) Photograph and (B) line-drawing of a slope channel filled with fine-grained mass-transport
deposits (MTD) in the Late Cretaceous Cerro Toro Formation, Chile ( Hubbard et al., 2008 ). Are-
nicolites ( Ar ) subtend from the margin of the channel into background slope strata. (C) Close-up of
Arenicolites from area outlined by small box in (A). Despite the fact that the fill of the channel is
devoid of sandstone, the trace fossils were passively filled by sand from bypassing turbidity currents.
ichnofacies is represented by high-diversity suites corresponding to a predomi-
nance of deposit-feeding strategies and subordinate grazing behaviors.
Trace-fossil suites from units deposited in moderate to low-energy slope
environments are commonly dominated by similar deposit-feeding ethologies,
leading to their assignment to the archetypal Cruziana Ichnofacies or to distal
expressions of the Cruziana Ichnofacies (cf. MacEachern and Bann, 2008;
MacEachern et al., 2007a ). Slope environments include areas of relative quies-
cence, at least periodically, particularly on the upper to middle slope (e.g., Bal-
lance et al., 1984; Savrda et al., 2001 ), within channels (e.g., Eyles et al., 1992;
Phillips et al., 2011 ), interchannel areas (e.g., Buck and Bottjer, 1985 ), areas
influenced by low-energy bottom currents (e.g., Eyles et al., 1992; Wetzel
et al., 2008 ), confining margins of intraslope topographic depressions (e.g.,
Phillips et al., 2011 ), and intraslope minibasins (e.g., Shultz and Hubbard, 2005 ).
Occurrences of suites attributable to the Cruziana Ichnofacies are somewhat
impoverished compared to those typifying the offshore and distal lower shore-
face. Trace-fossil diversities are typically less than 6 recognizable ichnogenera
in most deposits. The most commonly documented trace fossils include
Chondrites , Palaeophycus , Planolites , Taenidium , Teichichnus , Thalassinoides ,
and Zoophycos . Asterosoma , Helminthoida , Ophiomorpha , Phycodes , Phyco-
siphon , Rhizocorallium , and Schaubcylindrichnus are locally important, for