Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 766

In addition to the mechanical compaction discussed above, other deforma-
tional processes may act to obscure ichnofabric elements in deposits character-
ized by the Zoophycos Ichnofacies. What were once distinct trace fossils may be
smeared or truncated as a result of (1) downslope remobilization of carbonate
ooze (via slow creep or rapid slumping or flowage; Bromley and Ekdale, 1987 )
or (2) chemical compaction and formation of solution seams, stylolites
( Garrison and Kennedy, 1977; Kennedy and Garrison, 1975; Scholle, 1977 ),
or microstylolites (e.g., Ekdale and Bromley, 1988; Steinich, 1967 ).

3. ICHNOFACIES

3.1 Environments and Substrates
Ichnofabrics best ascribed to the Cruziana Ichnofacies are characteristic of
sandy, fossiliferous, commonly glauconitic, and locally phosphatic chalk and
marl that accumulated in relatively shallow-shelf settings. Such facies are well
represented inCretaceous shelf-sea chalk successions, including those of Europe
( Kennedy, 1975 ) and North America, such as the Annona and Saratoga chalks of
Arkansas ( Bottjer, 1985, 1986 ), the Prairie Bluff Chalk of Alabama ( Frey and
Bromley, 1985 ), and the Austin Chalk of Texas ( Dawson and Reaser, 1985,
1990 ; see Table 1 ). Abundance of siliciclastic sand, concentrations of molluscan
skeletal debris, and the localized occurrence of shallow channels and scour sur-
faces indicate deposition in periodically energetic settings between fair-weather
and storm-wave bases. The prevalence of glauconite and the common occurrence
of encrusted and bored shells and phosphatic molds indicate relative sediment
starvation and stratigraphic condensation.

CRUZIANA

3.2 Ichnocoenoses and Ichnofabrics
Chalks characterized by the Cruziana Ichnofacies are thoroughly bioturbated,
except rare vestiges of laminated or cross-laminated sand or calcarenite. As
with the Zoophycos Ichnofacies, any surface structures and shallow endogenic

reflect combined redox-dilution cycles linked to orbitally driven wet/dry climate cycles. The
photograph includes the transition from the limestone-dominated Fort Hays Member to the more
heterolithic Smoky Hill Member of the Niobrara Formation. (B) Ichnofabric in relatively pure chalk
deposited under well-oxygenated bottom waters. Thalassinoides are cut by Teichichnus (Te),
Planolites (Pl), Zoophycos (not visible in the photograph), and Chondrites (Ch). (C) Marly
chalk-reflecting moderate paleo-oxygenation. The trace-fossil assemblage includes Teichichnus
(Te), Planolites (Pl), Zoophycos (Zo), and Chondrites (not visible in the photograph), but lacks
Thalassinoides . (D) Ichnofabrics reflecting the progressive decrease in trace-fossil diversity and
burrow size associated with a phase of bottom-water deoxygenation. Ch, Te, Th, and Zo are defined
as above. (E) Ichnofabric formed in response to increased benthic oxygenation. Note the penetration
of lighter Chondrites (Ch) into previously unburrowed laminated calcareous shale.