Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 802

early diagenesis produces cemented horizons or patches in the substrate at
shallow depths (i.e., intertidal and shallow subtidal beachrock formation).
Buried shell layers prevent some taxa from achieving optimum burrow depths
( Frey et al., 1978; Goldring, 1995; Lunz, 1937 ). In areas with shell or consolidated
mud layers buried by a sand veneer, the shrimp Callichirusmajor restricts its burrow
to shallow depths ( 30 cm), or it may be absent completely from these undesirable

locations ( Frey et al., 1978; Lunz, 1937 ).Thisphenomenonisexemplifiedinthe
rock record by the distribution of trace fossils proximal to the biostromes in theMid-
dle Triassic Liard Formation, discussed above ( Fig. 3 ), where distinct differences
occur between quartzose sandstonebedswith rare scattered shell debris and laterally
equivalent shell-rich sandstone beds deposited adjacent to brachiopod-dominated
biostromes. The presence of abundant shell debris has a clear effect on the compo-
sition of trace-fossil assemblages with fewer vertical dwelling and suspension-
feeding structures occurring in biostrome-proximal settings.

4.3 Infaunal Exclusion by Living Taxa
(with Dendraster excentricus as an Example)
The intertidal succession at Craig Bay, Vancouver Island, Canada, is character-
ized by a laterally extensive (

1100 m), long wave length (40-80 m), low
amplitude (0.2-0.5 m), shore-parallel ridge, and runnel system ( Hale and
McCann, 1982 ). In the middle and lower intertidal zones, the runnels are char-
acterized by dense populations of the Western Sand Dollar ( Dendraster excen-
tricus ). These flattened echinoids live on the sediment surface and within the
upper decimeter of the sand-dominated substrate. Preliminary field observa-
tions have indicated that intervals with exceptionally abundant sand dollars
are devoid of burrowing bivalves, whereas the ridge areas have few sand dollars
but numerous infaunal bivalves. Similar to Craig Bay, studies elsewhere have
shown that the presence and activities of Dendraster has an exclusionary effect
on the presence of other infaunal taxa ( Brenchley, 1981 ).
Although the total bivalve faunas are moderately diverse, individual ridge
systems were only observed to contain two to three taxa. The taxonomic com-
position changes gradually from the upper intertidal to the lower intertidal zone.
Taxa observed include the Bent-nosed Macoma ( Macoma nasuta ), the Baltic
Macoma ( M. balthica ), the White Sand Macoma ( M. secta ), the Fat Gaper
( Tresus capax ), and the Common Pacific Littleneck ( Protothaca staminea ).
Lower intertidal successions are characterized by T. capax , which lives infaun-
ally at considerable depth (15-70 cm below the surface). This large veneroid
has an elongated fused siphon that, due to its size, cannot be fully retracted into
its shell. Present within the upper 15 cm of the substrate from the lower to

middle intertidal zone is the smaller veneroid P. staminea (also with a fused
siphon). The macomas all have broad environmental ranges, extending from
the upper intertidal to the subtidal zone. M. balthica and M. secta live deep
within the substrate (commonly 25 cm), whereas M. nasuta lives much

>