Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 768

3.3 Factors Affecting the Expression of Trace Fossils
The expression of trace fossils in deposits characterized by the Cruziana
Ichnofacies may be influenced by some of the same factors described in Section
2.3 for the Zoophycos Ichnofacies. Sandy chalk successions typically are het-
erogeneous in terms of texture and composition, for example, in abundance
of sand or glauconite. Hence, the visibility of trace fossils, particularly that
of relatively robust Thalassinoides , is commonly enhanced by bed-junction
preservation. The expression of trace fossils also may be enhanced in surface
exposures by differential cementation and weathering of burrow fills. Notably,
sandier chalk substrates generally were more cohesive than finer-grained
pelagic oozes and, hence, trace fossils produced therein were less susceptible
to mechanical compaction and other soft-sediment deformational mechanisms.

4. ICHNOFACIES

4.1 Environments and Substrates
Trace-fossil assemblages representing the firmground Glossifungites Ichno-
facies occur locally in deep-sea chalks (e.g., Bromley and Allouc, 1992 ; see
Savrda et al., 2001 ) but are most common in shelf-sea chalk successions
( Bromley, 1967, 1975; Kennedy and Garrison, 1975 ). Chalk firmgrounds devel-
oped in two ways: (1) erosional exhumation of semiconsolidated carbonate
ooze in response to significant sea-level changes, and (2) incipient synsedimen-
tary cementation of carbonate ooze associated with periods of increased
bottom-current intensity and sediment bypass.
Firmgrounds formed by the first mechanism, referred to here as type 1
firmgrounds, have sequence-stratigraphic significance. They locally occur at
the tops of shelf-sea chalk or marl successions that were truncated by coplanar
sequence boundaries/transgressive surfaces. Subaerial erosion during sea-level
lowstand and subsequent transgressive erosion resulted in the exhumation and
marine inundation of dewatered calcareous ooze. This mechanism is likely
responsible for the isolated firmground ichnofabrics developed at formational
boundaries in Cretaceous Gulf coastal plain successions, for example, at the
top of the Austin Chalk in Texas ( Dawson and Reaser, 1980; F¨rsich et al.,
1981 ), bases of the Saratoga and Annona chalks in Arkansas ( Bottjer,
1985 ), and the top of the Prairie Bluff Chalk in Alabama ( Savrda, 1991 ;
Fig. 8 A). Comparable firmground ichnofabrics also have been recognized
beneath scour surfaces that were cut into Tertiary deep-sea chalk in slope
and rise settings ( Fig. 8 B). Firmground development at these depths is linked
to extreme sediment starvation and increased impact of contour currents, both
of which were deep-water responses to phases of rapid sea-level rise ( Savrda
et al., 2001 ).
Firmgrounds formed by the second mechanism, or type 2 firmgrounds, mark
omission surfaces within relatively shallow-shelf sea chalk successions.

GLOSSIFUNGITES