Environmental Engineering Reference
In-Depth Information
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A
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FIGURE 7 Cruziana Ichnofacies in typically sandy and shelly shallow-shelf chalks. (A) Sche-
matic representation of common trace fossils, including Cylindrichnus (Cy), Thalassinoides
(Th), and Chondrites (Ch). Associated robust shells or phosphatic molds thereof commonly contain
Entobia (En) and other borings. Intervals characterized by the Cruziana Ichnofacies may be inter-
bedded with finer-grained chalk characterized by the Zoophycos Ichnofacies (base). (B) Cruziana
Ichnofacies in sandy, glauconitic chalk of the Maastrichtian Prairie Bluff Chalk, Alabama. The
chalk formed in a shallow-shelf setting during a period of sediment starvation. The ichnofabric
is dominated by robust Thalassinoides (Th), Cylindrichnus (Cy?), and Chondrites (Ch). The Chon-
drites tracemaker preferentially exploited coarser, glauconitic fills of the larger burrows. Note the
Entobia -bored (En) oyster and phosphatic molds (circled).
structures that may have been produced in these substrates normally are not
preserved owing to heavy overprinting by deeper-tier trace fossils.
Trace-fossil assemblages within sandy chalk successions, including the
Maastrichtian Prairie Bluff Chalk exposed in the U.S. Gulf coastal plain, mainly
comprise relatively simple deposit-feeding structures but also may include
structures produced by sessile suspension feeders. Virtually all assemblages
include Thalassinoides , most contain Planolites , Cylindrichnus , and Chondrites
( Fig. 7 ), and some locally include Skolithos , Teichichnus , and Trichichnus
( Table 1 ). Notably, Zoophycos is conspicuously absent. Shells and phosphate
nodules within these deposits commonly contain borings produced by sponges
( Entobia ; Fig. 7 ) and bivalves ( Gastrochaenolites ).
Comparatively little is known about trace-fossil tiering in these sandier sub-
strates. Where present, Chondrites typically cross-cuts all other ichnotaxa and
commonly is concentrated in the fills of larger Thalassinoides ( Kennedy, 1975 ;
Fig. 7 B). This indicates that Chondrites was deeply emplaced. However, given
the environmental dynamics associated with these shallower-water deposits, it
is not clear whether the occupation of substrates by the Chondrites tracemaker
and other organisms were contemporaneous. The observed cross-cutting rela-
tionships may reflect the superimposition of two or more temporally disjunct
ichnocoenoses rather than true tiering.
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