Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 757

The composition and texture of modern and ancient nannofossil-rich
carbonates vary considerably, reflecting the diversity of depositional regimes
in which they formed. While some deposits are nearly wholly composed of
coccoliths, nannofossil ooze may be variably admixed with coarser skeletal
material, siliciclastic sand or mud, biogenic silica, organic matter, or some com-
bination of these to form various hybrid deposits; for example, calcarenitic
chalk, sandy chalk, marly chalk and marl, diatomaceous chalk, etc. Diagenetic
mineral assemblages also vary; chalks locally may contain pyrite, glauconite,
phosphatic crusts and nodules, and chert or flint concretions, in addition to car-
bonate cements. Whether primary or diagenetic, compositional differences
reflect temporal and spatial changes in physical and chemical conditions.
Notably, in many chalk successions, particularly, but not exclusively, those
of Cretaceous age, temporal changes in the paleoenvironmental regime
are manifested by decimeter-scale variations in composition, texture, and/or
degree of lithification. This rhythmic bedding has been attributed to cyclic
changes in carbonate production, siliciclastic influx, benthic oxygenation,
bottom-current energy, and/or carbonate dissolution ( Arthur et al., 1986; Bottjer
et al., 1986 ). Such paleoceanographic cycles, in turn, are linked to climate
rhythms mediated by Earth's orbital perturbations—that is, Milankovitch
cycles—that in various ways impacted ocean circulation, sediment delivery,
and diagenetic regimes in settings characterized by carbonate ooze sedimenta-
tion (e.g., Fischer and Schwarzacher, 1984; Gale, 1995; Weedon, 1993 ).
Temporal and spatial variations in chalk depositional regimes are likewise
reflected by ichnological attributes. The ichnology of these deposits has been
well documented in studies of modern carbonate ooze as well as in studies
of Cretaceous and Tertiary shallow- to deep-water chalk and limestone succes-
sions revealed in outcrops, especially those exposed in northern Europe and
North America, and in cores recovered during the Deep Sea Drilling Project
(DSDP), the Ocean Drilling Program (ODP), and other ocean or continental
drilling ventures ( Table 1 ). These investigations indicate that oozes and equi-
valent sedimentary rocks are characterized by a wide array of ichnofabrics
and ichnocoenoses—representing as many as six different ichnofacies—that
are indicative of environmental parameters such as general bathymetry, benthic
oxygenation, substrate consistency, bottom-current regime, and depositional
process.
The carbonate deposits addressed herein may contain trace-fossil suites that
are representative of the Zoophycos , Cruziana , Glossifungites , Trypanites ,
Nereites , and, possibly, Arenicolites ichnofacies ( Table 1 ; Fig. 1 ). Zoophycos
Ichnofacies suites are most common; they predominate in modern carbonate
oozes and ancient deep-sea, outer-shelf, and epeiric basin chalks. Cruziana ,
Glossifungites , and Trypanites ichnofacies assemblages, representing soft, firm,
and hard substrates, respectively, are largely restricted to chalks and limestones
that formed in shallower shelf-sea settings in which the environmental energy
was at least episodically higher and, in the case of the latter two ichnofacies,