Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 770

Omission surfaces characterized by the Glossifungites Ichnofacies occur only
locally in North American chalk successions, for example, in the Austin Chalk
of Texas ( Dawson and Reaser, 1990 ). However, they are particularly common
in relatively pure Cretaceous chalk successions in north-western Europe
( Bromley, 1967, 1975; Kennedy and Garrison, 1975 ; Table 1 ) wherein they
define decimeter- to meter-scale scour cycles and are commonly associated
with nodular chalk or flints ( Figs. 8 C and 9 ).

4.2 Ichnocoenoses and Ichnofabrics
In both type 1 and type 2 chalk firmgrounds, crustaceans were the preeminent
tracemakers. Type 1 firmgrounds are dominated by vertically extensive, sharp-
walled Thalassinoides ( Bottjer, 1985; Savrda, 1991; Savrda et al., 2001 ;
Fig. 8 A), some of which exhibit well-defined wall ornamentation (e.g., scratch
traces, the Spongeliomorpha of Frey and Bromley, 1985 ), but they locally also
include Rhizocorallium ( Dawson and Reaser, 1980 ).
Ichnocoenoses characterizing type 2 firmgrounds also are dominated by
sharp-walled Thalassinoides burrow networks but may include other related
arthropod traces such as Pseudobilobites , Spongeliomorpha , and “Glossifun-
gites ”(

Rhizocorallium )( Bromley, 1967, 1975; Bromley and Allouc, 1992;
Kennedy and Garrison, 1975 ; Table 1 ). Thalassinoides burrow morphologies
varydepending on the extent of substrate cementation at the time of emplacement.
Thalassinoides formed during earlier stages of omission—that is, before the onset
of syndepositional cementation—are relatively undeformed, or idiomorphic. In
contrast, burrow systems formed contemporaneously with incipient cementation
and localized growth of hard nodules exhibit more distorted or stenomorphic con-
figurations ( Fig. 9 ), reflecting evolving heterogeneity of the available burrowing
space ( Bromley, 1967, 1975 ).
Regardless of the mechanism responsible for formation of firm substrates,
the carbonate sediments that comprise them were originally soft oozes. Hence,
firmground Thalassinoides invariably overprint pre-erosion or preomission
softground ichnocoenoses representing the Zoophycos or possibly the Cruziana
ichnofacies. Ichnofabrics of omission surfaces may be particularly complex.
In some cases, ichnofabrics record periodical filling and reexcavation of
Thalassinoides burrows ( Bromley, 1967 ), reflecting the amalgamation of omis-
sion surfaces. Moreover, ichnofabrics formed in type 2 firmgrounds may be
overprinted by a postlithification omission suite of borings of the Trypanites
Ichnofacies—if extended omission subsequently led to complete sea-floor
cementation—and/or by a postomission softground trace-fossil suite ( Fig. 9 ).
The latter suite reflects the resumption of normal sedimentation, filling of irreg-
ular firmground burrows with soft ooze, and exploitation of a soft burrow fill by
reestablished softground bioturbators. Burrows emplaced within these burrow
fills are distorted or xenomorphic ( Bromley, 1967 ) as a result of the irregular
geometry of the soft-sediment burrowing space.

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