Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 800

Downward- and upward-conveyors actively transport particles through their
guts resulting in both passive and advective transport of sediment particles
( Bouchet et al., 2009 ). This may result in a transport distance of a few millime-
ters to several centimeters. Biodiffusors move sediment particles in a random
manner over short distances (generally millimeters), resulting in diffusive trans-
port that is most easily recognized as the partial to complete disruption of phys-
ical bedforms. Gallery diffusors dig open framework tube or tunnel systems that
promote the transport of material from the sediment-water interface to deep
within the burrow. Sediment transport distances are commonly in the order
of decimeters. Regenerators are also gallery-digging species that promote both
biodiffusive sediment admixtures, with large amounts of sediment transported
out of the burrow and into the overlying water column, as well as passive down-
ward transport of surface sediment to the bottom of the burrow after burrow
abandonment.
With the exception of biodiffusors, all these behaviors can result in a strati-
graphic concentration of certain particle types or grain sizes. In shell-rich inter-
tidal and shoreface-sandstone systems, there is commonly a grain-size
dichotomy between the two mineralogical end-members. In several examples
from the North Sea, lugworms ( Arenicola marina ) were observed to play an
active sediment unmixing role producing distinctly graded bedding
( Bromley, 1996; Sch¨fer, 1962; Trewin and Welsh, 1976; Van Straaten,
1952, 1956 ). Grains of all sizes are drawn down the vertical shaft, but the larger
fractions are too large to be eaten and accumulate at the head level of the worms
( Bromley, 1996 ). Smaller grains adhere better to the mucus on the proboscis
than do larger ones, and thus the fecal strings deposited on the sediment surface
are generally finer grained. Lugworms rapidly turn over the upper few decime-
ters of the sediment ( Cad´e, 1976; Swinbanks, 1981 ) and thus can rapidly seg-
regate coarse bioclastic detritus from finer-clastic detritus in mixed systems.
Under the right circumstances, excavation of deep, open-framework burrows
may provide conduits for surface material to be transported from the sediment/water
interface to deeper horizons. Mudbanks on the coast of Florida and on the Caicos
platform are penetrated by numerous, deep Callianassa networks ( Tedesco and
Wanless, 1991 ). These banks are affected by numerous storms, and consequently
the decapod burrow networks are frequently infilled with bioclastic detritus from
the sea floor ( Tedesco andWanless, 1991;Wanless et al., 1988 ). Repetitive domicile
excavation and subsequent storm infilling of burrow networks result in gradual
replacement of the original mud substrate with bioclastic packstone and grainstone.
4.2 Influence on Infaunal Taxa
A major influence of mixed siliciclastic/carbonate sediment on trace-fossil com-
munities is the function of grain size and sortingon infaunal distribution. Sufficient
bioclastic material may promote colonization by shell-gravel dwellers ( Kidwell,
1986 ) and result in trace-fossil assemblages distinct from those that occur in