Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 773

bottom-current winnowing and sediment bypass and thus are characteristic of
omission surfaces ( Figs. 8 E, F and 9 ). While such hardgrounds can form at con-
siderable water depths of up to

4 km for some Quaternary substrates
( Bromley and Allouc, 1992 ), most well-documented examples are those con-
tained in relatively shallow-shelf sea chalk successions of Late Cretaceous
age in north-western Europe ( Bromley, 1975; Kennedy and Garrison, 1975 ).

5.2 Ichnocoenoses and Ichnofabrics
Hardground Trypanites Ichnofacies assemblages, or postlithification omission
suites, comprise bioerosion structures ( Bromley, 1967, 1975 ). Owing to tapho-
nomic biases imposed by extended phases of omission and bioerosion
( Bromley, 1994 ), hardground ichnocoenoses developed in shelf-sea chalks typ-
ically are dominated by relatively deeply emplaced borings such as Entobia ,
Trypanites , and Gastrochaenolites produced by sponges, polychaetes, and
bivalves, respectively ( Bromley, 1975 ; Fig. 9 ). Notably, Gastrochaenolites is
generally rare or absent in deep-sea hardgrounds ( Ekdale et al., 1984b ).
Because the sediments that embody chalk hardgrounds originated as soft
ooze and subsequently passed through a firmground stage prior to lithification,
assemblages of borings invariably overprint earlier formed softground and firm-
ground ichnocoenoses ( Fig. 9 ). Indeed, borings not only penetrate downward
across omission surfaces but also commonly were emplaced at various orienta-
tions into the hardened, mineralized walls of open firmground burrows of the
prelithification suite ( Bromley, 1975 ).

5.3 Factors Affecting the Expression of Trace Fossils
The visibility of bioerosion structures in hardgrounds is generally good. Like
firmground burrows, borings have extremely sharp walls, which may be accen-
tuated by diagenetic mineralization ( Bromley, 1975 ; Fig. 8 F), and are com-
monly filled with sediment that contrasts with the host substrate.

6. TRACE FOSSILS IN REMOBILIZED SUBSTRATES
Most of the deposits described above are true pelagites or hemipelagites. How-
ever, chalk or limestone that formed via redeposition of pelagic ooze and other
carbonates by gravity flows, including turbidity currents, may be important
locally.

6.1 Ichnofabrics of Slump, Ooze-Flow, and Debris-Flow Deposits
Structures and fabrics indicative of mass transport of oozes by slumping,
gliding, and ooze or debris flow are represented at least locally in Cretaceous
and Danian chalk successions throughout northern Europe, reflecting