Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 789

burrows (cf. Beaconites ) do occur ( Fig. 3 ). Calcareous siltstone beds, which
commonly occur interbedded with both the crinoidal grainstone and brachiopod
rudstone beds, contain a low-diversity trace-fossil assemblage of dominantly
horizontal deposit feeders ( Asterosoma and Planolites ) as well as scattered cri-
noid and echinoid skeletal elements, brachiopods ( Aulacothyroides sp. and Dis-
cinisca sp.), and decapod crustaceans ( Chimaerastacus sp.).
Interfingering layers of clastic sandstone and carbonate beds characterize
the fringes of the biostromes ( Zonneveld, 2001 ). Clastic sandstone layers con-
tain an abundance of fossils that preferred loose or shifting substrates, such as
lingulide brachiopods ( Lingularia sp.) and a variety of bivalves and gastropods.
Where abundant shell debris is present in the sand, the traces are limited to hor-
izontal deposit-feeding and dwelling traces ( Asterosoma , Palaeophycus , Phy-
cosiphon, and Planolites ; Fig. 3 ). Lateral to the biostromes, in areas devoid
of significant shell debris, vertically oriented dwelling traces of infaunal filter
feeders (i.e., Cylindrichnus , Lingulichnus , Rosselia, and Siphonichnus ) occur in
roughly equal numbers with those of infaunal deposit feeders (i.e., Asterosoma ,
Phycosiphon , and Planolites ) and carnivores/scavengers (i.e., Palaeophycus
and Thalassinoides ; Fig. 3 ; Zonneveld, 2001 ).
The Liard biostromes represent “islands” or patches of stable substrate that
formed within shifting quartz-dominated sandy shoreface successions and
record interaction between characteristic carbonate and siliciclastic trace-fossil
assemblages. The compositions of trace-fossil assemblages in these successions
are clearly influenced by the nature and distribution of the different sediment
types as well as food availability at the interface of siliciclastic and

carbonate-dominated systems.

2.4 Baldonnel Formation (Late Triassic), British Columbia
The Carnian Baldonnel Formation in northeastern British Columbia, Canada, is
a carbonate-dominated succession with abundant admixed quartz-dominated
sand ( Martindale et al., 2010; Zonneveld and Orchard, 2002; Zonneveld
et al., 2004, 2007 ). Although this unit contains diverse and locally abundant
trace fossils, these traces are commonly subtly displayed and are difficult to dif-
ferentiate from host lithologies (not an uncommon problem in carbonate-
dominated successions). Similar to the pattern observed in the Liard Formation,
trace fossils exhibit a unique relationship between in-situ carbonate production
and trace-fossil assemblage composition.
Small coralliferous patch reefs occur in the lower Baldonnel Formation
( Martindale et al., 2010; Zonneveld et al., 2007 ). These patch reefs occur within
a succession of calcareous-quartzose sandstone. Trace fossils within the sand-
stone successions include Cylindrichnus , Diplocraterion , Palaeophycus , Pla-
nolites , Psilonichnus , Rhizocorallium , Rosselia , Siphonichnus , Skolithos ,
Scolicia , Teichichnus , and Thalassinoides. On the fringes of individual patch
reefs, bioclastic grainstone/rudstone interfingers with the sandstone. In these