Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 762

A

B

Ch

Diffuse burrow
mottling

Pl

Zo

Ch

Pl

1 cm

C

Pl?

Zo

Te ?

2 cm

1 cm

FIGURE 4 Examples of ichnofabrics in deep-sea chalks and marls, Eocene, ODP core, site 1073,
Leg 174A, New Jersey margin. (A) Core section showing both diffuse burrow mottling in relatively
carbonate-poor mud (top) and distinct trace fossils in chalk (Pl; Planolites , Zo; Zoophycos , Ch;
Chondrites ). (B) Close-up of the chalk ichnofabric in A. (C) Ill-defined structures in clay-rich ooze.
Burrow mottling may include Planolites (Pl?) and Teichichnus (Te?).

(e.g., in abyssal settings below the lysocline), substrates become less cohesive.
As a result of lower sediment shear strengths, even transition-layer structures
may be significantly influenced by compaction. Consequently, in modern
deep-sea marly oozes and in some Cretaceous and Tertiary marls, ichnofabrics
are characterized by lower diversity assemblages of ill defined, typically hori-
zontal structures (e.g., Fig. 4 C) that cautiously may be ascribed to Planolites ,
Chondrites ,or Thalassinoides ( Ekdale, 1980; Ekdale and Berger, 1978; Ekdale
and Bromley, 1982, 1984b; Frey and Bromley, 1985; Wetzel, 1991 ).
Ichnocoenoses representing the Zoophycos Ichnofacies also vary as a func-
tion of benthic redox conditions ( Figs. 3 B and 6 ). While modern carbonate ooze
and most Cretaceous and Tertiary chalks accumulated in oxygenated settings,
some pelagic carbonates accumulated beneath dysoxic or anoxic waters. These
include some deep-sea carbonates—for example, those deposited during Creta-
ceous oceanic anoxic events ( Ekdale, 1985 ) or the K/T boundary interval
( Droser and Bottjer, 1991 )—and chalks and marls deposited in stratified epeiric
basins, such as the U.S. Western Interior ( Savrda, 1998a,b ; see Fig. 6 ) and the