Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 801

laterally adjacent sand-dominated successions. Some taxa, such as certain arthro-
pods and bivalves, exhibit a strong preference for gravel substrates and, in the
absence of significant clastic gravel in a succession, utilize bioclastic beds as an
alternative. Coarse shell material is typically characterized by numerous voids,
and thus many organisms live interstitially in shell hash beds. Commonly, these
organisms modify their host medium minimally or not at all and thus their ichno-
logical signature is absent, leading to the mistaken assumption of decreased bio-
diversity in ancient coarse-grained successions. Other organisms modify the
shells for use as protective domiciles. Shells within shell-rich substrates are com-
monly modified by boring organisms, allowing for development of Entobia -or
Trypanites -type assemblages in dominantly siliciclastic successions.
Abundant shell debris in sandstone successions may act as a baffle or barrier
to infaunal organisms. In the case of shell debris disseminated throughout the
sand, organisms may respond by burrowing around obstructions or, alternatively,
moving the obstruction to allow continued passage ( Goldring, 1995; Miller and
Curran, 2001 ). If the shell debris is small enough to be easily manipulated, the
organism may incorporate it into the burrow wall, either purposely, such as
the annelid Diopatra cuprea, or accidentally, as the animal shifts its burrow
upward in response to sedimentation ( Fig. 11 ). Other organisms may be less
adaptable and, in the presence of sufficient disseminated shells and shell debris,
may be completely precluded from colonization. This issue is exacerbated where
FIGURE 11 Teichichnus within the “A-Marker” Member of the lower Charlie Lake Formation
(Late Triassic), British Columbia, Canada, at 1075.2 m in Well b-086-E/94-G-09. The main matrix
is quartz-dominated fine-grained sand. The larger white grains are crinoid ossicles incorporated into
the burrow wall and into burrow laminae as the organism shifted its burrow upward.