Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 783

basin configuration related to a changing sea level resulted in basin deflection of
shallow oceanic currents that dramatically affected the amount of siliciclastic
material delivered to depocenters ( Guertin et al., 1999; McNeill et al., 2004 ).
In other cases, the influx of siliciclastic material transported basinward during
intervals of rapidly dropping sea level inhibited carbonate accumulation
( Campbell, 2005; Schlager, 1991 ). In these systems, lowstand systems tracts
are dominated by siliciclastic sedimentation, whereas transgressive and high-
stand systems tracts are characterized by dominantly carbonate deposition
( Garc ´ a-Hidalgo et al., 2007; Southgate et al., 1993 ).
Mixed siliciclastic/carbonate systems also occur, albeit more rarely, in con-
tinental successions. In our experience, these most commonly occur in mixed
fluvial/lacustrine and marginal-lacustrine successions. The continental exam-
ples discussed herein are limited to examples of these environments.
Even though mixed siliciclastic/carbonate systems are common, arguably as
common as the pure end-members, ichnological analyses that have focused specif-
ically onmixed successions are sparse (i.e., Brett et al., 2010; Chaplin, 1996, 2010;
Zonneveld et al., 2001 ). Similar to purely carbonate or siliciclastic systems, the
trace-fossil distribution in mixed systems reflects physical and chemical parame-
ters that control the distribution of organisms. Ichnological studies of mixed sys-
tems reveal that these systems commonlypreserve the cominglingof typical clastic
and carbonate facies and fabrics. Trace-fossil associations (such as Glossifungites
assemblages), commonly considered characteristic of clastic successions, may
occur in carbonate-dominated mixed systems. Conversely, bored shells, cobbles,
or boulders andboredhardgroundsurfaceswithvariablydiverse Trypanites assem-
blages may be locally common in clastics-dominated mixed systems.
This chapter summarizes the current body of literature that deals with the
ichnology of mixed siliciclastic/carbonate systems. Although many works that
focus on depositional or taphonomic attributes of mixed siliciclastic/carbonate
successions make mention of trace fossils, in few of these, it is clear how the
mixed lithology nature of the succession influences the trace-fossil distribution.
As well, examples of mixed argillaceous calcareous mudstone successions are
common in the rock record. Although these are clearly excellent examples of
mixed siliciclastic/carbonate successions, there has been no documentation that
their ichnotaxonomic composition and diversity differ in any appreciable way
from either purely argillaceous or purely calcareous shale successions deposited
in identical paleoenvironments. Although we view this as an area in need of
further investigation, it is not further explored herein.
This study focuses solely on examples where admixture of clastic and carbonate
sediment produces a unique ichnological signature. These are discussed using case
studies that provide examples wherein sediment mixing produces unique relation-
ships within and between trace-fossil assemblages. The examples discussed herein,
all of which were chosen from Phanerozoic successions in North America ( Fig. 1 ),
represent one of two end-member circumstances. In the first, spatial variability/
lateral facies mixing resulted in unique relationships between trace-fossil assem-
blages. This includes examples wherein variability in clastic and carbonate