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deeper-tier structures normally are the last to disappear before bioturbation
ceases altogether (e.g.,
Fig. 6
D). Indeed, ichnofabrics characterized only by
Chondrites
commonly appear immediately below or above laminated, carbona-
ceous chalks or marls (
Bromley and Ekdale, 1984a; Ekdale, 1985
). Notably, in
chalk-marl successions that accumulated under fluctuating levels of benthic
oxygenation, various oxygen-related ichnocoenoses typically overprint one
another or laminated intervals with which they may be intercalated (
Figs. 3
B
and
6
E). Recognition of stacked oxygen-related ichnocoenoses in these succes-
sions provides a means of deciphering basin oxygenation histories (
Erba and
Premoli-Silva, 1994; Savrda, 1998a,b; Savrda and Bottjer, 1989
;
Fig. 6
E).
2.4 Factors Affecting the Expression of Trace Fossils
The quality of the expression of ichnofabrics and trace fossils in pelagic carbon-
ates, or in any of the carbonate facies described herein, mainly is controlled by
(1) the level of textural and compositional contrast between biogenic structures
and background sediment, and (2) the extent to which structures have been
deformed in depositional and diagenetic regimes. The trace-fossil contrast is
governed by factors such as stratigraphic heterogeneity, tracemaker behavior,
preferential diagenesis, and weathering. In some relatively pure chalks, the nat-
ural trace-fossil contrast is extremely low, and detailed ichnofabric observations
may not be possible without employing contrast-enhancing techniques (oil
application, high-contrast photography, image analysis, etc.;
Bromley, 1981;
Bromley and Richter, 1999
). The natural contrast commonly is higher in more
heterolithic successions containing alternating light and dark beds (e.g., chalks
and marly chalks;
Figs. 3 and 5
); at bed transitions, the structures typically are
filled with or comprise sediments that are darker or lighter than the sediments
in which they were emplaced (
Lauridsen et al., 2011; Locklair and Savrda,
1998a
). This type of bed-junction preservation (
Simpson, 1957
) is biased toward
(1) open burrows that are maintained as dwelling or feeding structures but are
susceptible to passive filling from above (e.g.,
Thalassinoides
) and (2) structures
produced by deep-dwelling reverse conveyors that actively transport surface or
near-surface sediments into the transition layer (e.g.,
Zoophycos
;
Kotake, 1989
).
Natural enhancement of a subtle primary textural or compositional contrast
also may result from early diagenetic processes (
Berger et al., 1979
). The trace-
fossil visibility in chalkmay be accentuated by preferential precipitation of early
diagenetic minerals in or around biogenic structures (i.e., the diagenetic preser-
vation of
Simpson, 1957
). For example, in some European shelf-sea chalk suc-
cessions containing omission surfaces (
Glossifungites
Ichnofacies), elements of
Zoophycos
Ichnofacies ichnofabrics are more readily apparent in diagenetic flint
nodules than in unmineralized host chalks (
Bromley and Ekdale, 1984b
). Weath-
ering commonly has the opposite effect; leaching, oxidation, and formation of
superficial crusts diminish the natural trace-fossil contrast in surface exposures
(
Dawson and Reaser, 1990; Frey and Bromley, 1985
).
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