Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 758

periodical non-deposition and/or winnowing resulted in partial or complete
syndepositional cementation at the sea floor. Different trace-fossil suites,
including those representing the Nereites and Arenicolites ichnofacies, devel-
oped locally in oozes that were rapidly deposited by turbidity currents or other
gravity-flow processes in deeper basinal settings. The goal of this chapter is to
review and exemplify ichnofabrics, ichnocoenoses, and ichnofacies expressed
in chalk and related carbonates from the complete spectrum of deep-sea and
shallower settings.

2. ICHNOFACIES

2.1 Environments and Substrates
Ichnocoenoses representing the Zoophycos Ichnofacies predominate in chalks,
marly chalks, and marls that accumulated in relatively deep, quiet settings; that
is, environments that were below storm-wave base and unaffected by turbidity
currents. Such ichnocoenoses are dominant in modern deep-sea oozes and in
Cretaceous and Tertiary pelagic carbonate successions recovered in DSDP
and ODP cores, but they also are well represented in Cretaceous outer-shelf
and epeiric basin chalk successions exposed in North America and Europe
( Table 1 ). Despite their collective grouping into the Zoophycos Ichnofacies, ich-
nocoenoses and ichnofabrics do vary, reflecting differences in benthic condi-
tions as well as in taphonomic influences. Before reviewing this variability,
it is helpful to address general traits that are commonly shared among deposits
characterized by the Zoophycos Ichnofacies.

ZOOPHYCOS

2.2 Common Ichnofabric Traits
Pelagic deposits assigned to the Zoophycos Ichnofacies exhibit three common
characteristics. First, with some notable exceptions related to episodes of
benthic oxygen-deficiency, these deposits are thoroughly bioturbated. Second,
ichnofabrics in these deposits typically comprise two general components:
(1) a homogeneous or burrow-mottled background fabric that is overprinted
by (2) a low-diversity assemblage of relatively distinct endogenic structures
dominated by simple to moderately complex feeding traces, including planar
to gently inclined spreiten. Third, cross-cutting relationships among distinct
trace fossils are commonly systematic; that is, certain recurring ichnotaxa reg-
ularly cross-cut or overprint others.
Thorough bioturbation requires little explanation. Associated deposits accu-
mulated very slowly in quiet settings in which there was ample time for benthic
organisms to thoroughly rework sediments many times over.
The two-component aspect of Zoophycos Ichnofacies fabrics can be
explained by the general burrow stratigraphy that has been recognized in mod-
ern deep-sea pelagic oozes ( Berger et al., 1979; Ekdale and Berger, 1978;