Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 803

shallower and, unlike other bivalve taxa, occurs with its shell emplaced horizon-
tally in the substrate. All Macoma taxa have separated siphons.
Diminutive, opheliid polychaetes ( Euzonus mucronata ) occur in large num-
bers but dominantly within the ridge systems. Larger carnivorous nephtyid
polychaetes (including Nephtys sp.) are less common but occur in both ridge
and runnel systems. Other taxa occur as well. Among these, solitary anemones
occur within all runnel systems occupied by sand dollars.
The lower to middle intertidal portion of the Craig Bay ridge and runnel
system shows clear segregation of both extant biota and thus the biogenic sedi-
mentary structures they created. Abundant Dendraster , to the exclusion of most
other organisms, dominate the runnel systems. During intervals of high tide or
falling tide, the Dendraster are commonly oriented vertically and are involved
in suspension-feeding, catching mobile prey and fragments of algae with their
spines, tube feet, and cilia ( Ricketts et al., 1985; Timko, 1976 ). During intervals
of low tide, the Dendraster typically either lay prone upon the surface or are shal-
lowly buried and feed upon detritus in/on the sediment ( Ricketts et al., 1985 ). At
these times, the Dendraster , particularly those on the fringes of the runnels,
may move about the surface leaving numerous horizontal meniscate structures.
Of note, in areas of particularly high abundance, suspension-feeding becomes
dominant and the proportion of deposit-feeding diminishes ( Fodrie et al.,
2007 ). Also present, albeit less common on the fringes of the runnels, are plug-
shaped tracesmadebyburrowinganemones ( Bergaueria or Conichnus ). The ridge
systems are dominated by bivalve dwelling and adjustment traces ( Lockeia and
multiple forms of Siphonichnus ). Diminutive horizontal ( Planolites ) and vertical
( Trichichnus ) feeding traces constructed by Euzonus occur dominantly within the
ridge systems. Open-framework dwelling traces of carnivorous polychaetes
( Palaeophycus ) occur in both the ridges and the runnels.
Mixed siliciclastic/carbonate intertidal flats exhibit inherently complex dis-
tributions of fossils and trace fossils. In the Craig Bay example, the distribution
of shell-dominated successions is controlled by the topography of the ridge and
runnel system. Dendraster aggregates in high abundance only in the lows
(i.e., in the areas where water remains the longest during low parts of the tidal
cycle). The physical environmental control on the distribution of Dendraster
results in biogenic control of the distribution of infaunal bioturbators. Thus,
dense accumulations of shells may strongly influence the presence of infaunal
bioturbators in portions of the intertidal flats.
5. SUMMARY
Mixed siliciclastic/carbonate successions are common in the rock record, argu-
ably as common as the end-member successions. However, in many cases, the
mineralogical mixing appears not to have affected trace-fossil assemblages.
Indeed, with few exceptions, organisms do not care whether the sediment they
burrow in is composed of carbonate grains, quartz grains, olivine grains, or any