Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 776

(2.5-4 cm) laminated to cross-laminated packstone to grainstone (divisions
T b and T c ) with an assemblage of discrete trace fossils ( Thalassinoides , Teich-
ichnus , Phycodes , and Ancorichnus ?); and (3) an upper heavily bioturbated
wackestone (divisions T d and T e )with Planolites , Chondrites ,or Taenidium
( Fig. 10 B). Interbedded background limestones contain poorly preserved bur-
rows, including Zoophycos , Planolites , and/or Thalassinoides , none of which
penetrate into subjacent turbidites. Based on these observations, Miller et al.
(2004) concluded that the ichnofabrics of these calciturbidites reflect the work
of distinct short-lived associations of endobenthic organisms contemporaneous
with and immediately following event deposition. Notably, the four ichnotaxa
identified in these turbidites ( Ancorichnus ?, Thalassinoides , Teichichnus ,
and Phycodes ) are apparently intimately associated with one another as com-
pound structures ( Fig. 10 B) and may represent the work of crustacean trace-
makers, possibly ones that were transported by associated turbidity currents.
Very similar compound structures attributed to allochthonous crustaceans
recently have been documented in siliciclastic storm beds in siliceous shelf
deposits ( Savrda et al., 2010 ).
So, to what ichnofacies can calciturbidite trace-fossil suites be assigned? In
the general absence of pre-depositional traces, assignment to the Nereites Ich-
nofacies is difficult to justify. Given the evidence that post-depositional traces
in many calciturbidites reflect unique event-related benthic communities,
assignment to a distinct ichnofacies may be favorable. Bromley and Asgaard
(1991) and Bromley (1996) argued that suites of post-depositional structures
that result from the work of transient opportunistic or pioneer communities
in turbidites and storm beds warrant consideration as the Arenicolites Ichnofa-
cies. Assignment of calciturbidites to this ichnofacies, after some modification
of its diagnostic description to account for a greater variety of traces and asso-
ciated behaviors, may be most appropriate.
7. SUMMARY
Throughout the post-Triassic, but particularly during the Cretaceous, chalk and
related carbonates accumulated in a significantly broad array of deep- to
shallow-marine depositional regimes. When integrated with sedimentological
observations, trace-fossil assemblages can be employed to discriminate these
regimes and the depositional processes that operated therein, for instance, slow
pelagic deposition versus mass-transport mechanisms. Broad ichnological dif-
ferences between ichnofacies (e.g., Zoophycos versus Cruziana ichnofacies), as
well as within ichnofacies (e.g., Zoophycos Ichnofacies assemblages with or
without Thalassinoides ), can help recognize general paleobathymetric gradi-
ents. Moreover, ichnological variations at the bedding scale (e.g., alternation
of low versus high diversity Zoophycos Ichnofacies assemblages or overprint-
ing of Zoophycos Ichnofacies suites by Glossifungites and Trypanites ichnofa-
cies associations) can be used to decipher short-term changes in benthic