Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 609

constituents of the channel fill can be broken up into axial deposits, which are
dominated by amalgamated sandstone, and margin facies, largely made up of
thinly interbedded sandstone and siltstone ( Fig. 8 ). Marginal facies can be sub-
divided into “proximal margin” (relative to the channel axis) and “distal mar-
gin” associations. Both margin facies consist of non-amalgamated sandstone
beds; however, sandstone beds are significantly thinner in the distal margin
deposits. The transition from channel axis to channel margin records the shift
from high- to low-energy turbulent sediment transport. The upper part of the
channel fill is characterized by abandonment deposits, which are manifest by
a thinning- and fining-upward stratal package formed through the waning of
turbidity current input. Axial abandonment facies consist of non-amalgamated,
thick-bedded turbidites. Laterally, toward the channel margins, these beds pass
gradationally into thinly interbedded turbidity-current deposits (distal abandon-
ment), which are distinguished from the distal-margin deposits by an increase in
organic detritus and trace-fossil diversity ( Fig. 8 ).
Trace-fossil suites in the channel fill record rare tocommongrazing traces (e.g.,
Neonereites ), uncommon to common deposit-feeding traces (e.g., Asterosoma ,
Gyrochorte ,and Teichichnus ), and common to abundant opportunistic strategies
(e.g., Diplocraterion , Ophiomorpha and Thalassinoides )( Figs. 8 and 9 ). Amal-
gamated channel-axis sandstones are generally devoid of bioturbation, with
palimpsest firmground suites consisting of Diplocraterion habichi and Thalassi-
noides confined to autogenic scour surfaces within the channel fill in the proximal
margin setting ( Fig. 9 B). Bioturbation indices are highly variable, ranging from
BI 0-5 across the various intrachannel facies ( Fig. 8 ). Trace-fossil diversity,

however, varies among the intrachannel facies, depending upon current energies
as well as temporal and spatial variations in erosion. In particular, a systematic
increase in trace-fossil diversity can be observed from the channel axis to the
channel margins. Similarly, trace-fossil diversities increase upward through
the channel-fill succession. As such, the highest diversity trace-fossil suites are
notable in distal abandonment deposits ( Fig. 9 C-F). Bypass drape-deposits, which
are associated with the highest energy flows that passed through the channels, are
characterized by the lowest diversity trace-fossil suites and tend to lack elements
reflecting opportunistic ethologies ( Fig. 9 G). Numerous intrachannel subenviron-
ments are characterized by similar appearing thin-bedded turbidites (cf. Bouma,
1962 ; Fig. 9 ). Ichnological analysis is more effective than the physical sedimen-
tology in differentiating these facies types in the Tres Pasos Formation.

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4.6 Levee Deposits, Nanaimo Group, Gulf
Islands, British Columbia
The Late Cretaceous Nanaimo Group of the Gulf Islands consists of a thick
( > 4000 m) succession of siliciclastic slope-fan deposits that accumulated
in the foreland basin setting of the Strait of Georgia during encroachment of
Vancouver Island to the mainland of British Columbia ( Mustard, 1994 ).