Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 605

to upper slope but could extend to lower slope positions, particularly where oce-
anic currents were sufficiently energetic to supply food and oxygen to the bed.
Such pervasive and uniformly bioturbated mudstones are consistent with set-
tings prone to low sedimentation rates largely unaffected by turbiditic events.

4.3.3 Ambient, Bioturbated Muddy Sandstone
Pervasively bioturbated (BI

5-6) muddy and locally glauconitic sandstones
are also common in the Nise Formation. Sand ranges from lower fine to upper
medium in grain size. Trace fossils are uniformly distributed and overprint one
another. The suite is dominated by Scolicia , Zoophycos , Asterosoma , Thalassi-
noides , Chondrites , Planolites , Phycosiphon , Palaeophycus , Helminthopsis ,
Cosmorhaphe , and Schaubcylindrichnus ( S. freyi ), with rare Ophiomorpha
( O. rudis ) as well as very rare Nereites and Spirophyton . The presence of robust
Scolicia , muddy Zoophycos , small Asterosoma , and possible Nereites marks
this as a sandy, high-diversity expression of the archetypal Zoophycos Ichnofa-
cies. The substrate consistency and grain size of the facies probably limited the
abundance of surface grazing structures, which lends the unit an appearance
more proximal than is likely warranted by its actual depositional position. Nev-
ertheless, the abundance of grazing structures in these sandy intervals is atypical
of shallow-marine sandy settings. Sandstone-dominated environments in more
proximal positions (e.g., inner shelf and distal shoreface) typically display
suites attributable to the archetypal Cruziana Ichnofacies, proximal expressions
of the Cruziana Ichnofacies and distal expressions of Skolithos Ichnofacies (cf.
MacEachern and Bann, 2008; MacEachern et al., 2007a, 2012 ).
Distal depositional positions are supported by the high abundance of grazing
and low-energy deposit-feeding traces, despite the coarser grain sizes within the
facies. Additionally, few of the biogenic structures present in the suite facilitate
thorough churning of the sediment; bioturbation intensities of BI

¼

5-6, there-
fore, indicate that the deposits accumulated through slow, gradual sedimenta-
tion (cf. Wetzel et al., 2008 ). This may be distinctive of ambient slope
settings, wherein oceanic currents (geostrophic, contour, etc.) are responsible
for slow transport of material as bedload and are also a source of food and oxy-
gen. Much of the mud admixed within the sand may have been introduced bio-
genically (e.g., fecal strings, fecal pellets, and mucous linings), rather than
through suspended sediment settling.

¼

containing isolated burrows that cross-cut it from higher levels. The ambient suite is diverse, and
includes Cosmorhaphe ( Cr ), Phycosiphon ( Ph ), Zoophycos ( Z ), Asterosoma ( As ), Chondrites
( Ch ), and Planolites ( P ). (C) Bioturbated muddy sandstone showing BI 5. The ichnological suite

includes Phycosiphon ( Ph ), Asterosoma ( As ), Spirophyton ( Sp ), Scolicia ( Sc ), and Zoophycos ( Z )
and is attributable to the Zoophycos Ichnofacies. (D) Bioturbated (BI ¼ 5) muddy sandstone with
Skolithos ( Sk ), Scolicia ( Sc ), Asterosoma ( As ), Thalassinoides ( Th ), Chondrites ( Ch ), and abundant
Phycosiphon ( Ph ) attributed to the Zoophycos Ichnofacies.

¼