Environmental Engineering Reference
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A
B
O
2
Shelf-sea
chalk
Th
common
Deep-sea
chalks
Th rare or
absent
Ch
Zo
Te
Pl
Th
FIGURE 3
Schematic representation of typical ichnofabrics and recurring trace-fossil suites repre-
senting the
Zoophycos
Ichnofacies in chalks deposited in (A) well-oxygenated deep-sea
versus
shallow-shelf settings, and (B) variably oxygenated basins (Ch;
Chondrites
,Zo;
Zoophycos
,Te;
Teichichnus
,Pl;
Planolites
,Th;
Thalassinoides
). As shown in bothA and B, the expression of the trace
fossils is improved in heterogeneous successions at transitions between lighter and darker beds (bed-
junctionpreservation). As depicted inB, systematic changes in trace-fossil diversity, burrowdiameters,
and depths of burrow penetration with changes in benthic oxygenation result in the stacking of distinct
oxygen-related ichnocoenoses. Note in B that burrows representing one ichnocoenosis may overprint
laminae or ichnofabrics formed under previous conditions. Examples of deep-sea, shelf, and oxygen-
deficient epeiric basin chalk ichnofabrics are provided in
Figs. 4, 5 and 6
, respectively.
Tiering is particularly well manifested by cross-cutting relationships among
recurring ichnotaxa in outer-shelf and epeiric basin chalks (e.g.,
Ekdale and
Bromley, 1982, 1991; Frey and Bromley, 1985; Locklair and Savrda, 1998a,b
;
Fig. 3
).
Thalassinoides
typically is overprinted by all other ichnotaxa and, thus,
characterizes a shallow tier. In contrast,
Chondrites
is superimposed on all other
structures (e.g.,
Fig. 5
C), reflecting its prevalence in the deepest tier. Other recur-
ring ichnotaxa (
Phycosiphon
,
Planolites
,
Taenidium
,
Teichichnus
, and
Zoophy-
cos
) represent intermediate tiers.
The substrate composition and consistency may have an impact on ichno-
coenoses and ichnofabrics. As carbonate contents decrease substantially, either
due to increased influx of siliciclastic muds or enhanced carbonate dissolution
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