Environmental Engineering Reference
In-Depth Information
other grain type ( Ekdale et al., 1984 ). However, some fundamental differences
in the siliciclastic and carbonate components of the mixed systems discussed
above have resulted in unique ichnological signatures.
One of these fundamental differences is the grain size. In many of the mixed
systems discussed herein (e.g., Montney Formation, Moenkopi Formation,
Liard Formation, modern Craig Bay intertidal), siliciclastic sand/sandstone suc-
cessions occur interstratified with variably abundant bioclastic detritus. The
grain sizes within the bioclastic component are highly variable but are generally
significantly coarser than in the siliciclastic component. The presence of shell
debris within a clastic succession introduces complexities into the infaunal hab-
itat that are clearly reflected in trace-fossil assemblages. Some of these differ-
ences are preservational, with trace fossils being inherently more difficult to
recognize in coarser bioclastic intervals than in fine-grained siliciclastic inter-
vals. Other differences occur at the faunal level. Examples include those where
some taxa are excluded by abundant admixed bioclastic detritus or by reduced
ecospace availability at the sediment/water interface.
Moreover, diagenetic processes commonly operate at different scales and
temporal intervals in siliciclastics and carbonates, leading to fundamental differ-
ences in preservation and in substrate consistency. In mixed systems, quintessen-
tial carbonate ichnotextures co-occur with prototypical siliciclastic ichnotextures.
In numerous examples discussed in this chapter (i.e., the Chase Group, the Schei
Point Group, and the Baldonnel Formation), mixed siliciclastic/carbonate depo-
sition resulted in sedimentary successions characterized by numerous firm-
grounds and co-occurring hardgrounds.
ACKNOWLEDGMENTS
Research on the ichnology of mixed siliciclastic/carbonate successions was supported by
NSERC discovery Grant 371662 to the senior author. We gratefully acknowledge Richard
Bromley and Dirk Knaust for inviting us to contribute to this volume. We also thank Carlton
Brett, Francisco Rodr´guez-Tovar, Richard Bromley, and Dirk Knaust for reviewing this
chapter and greatly improving it with their many helpful suggestions.
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