Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 772

4.3 Factors Affecting the Expression of Trace Fossils
Firmground ichnofabrics typically are rather well expressed. Thalassinoides
are robust, their burrow boundaries are sharp and their burrow fills commonly
contrast markedly with the host sediments. Enhanced visibility of burrows
related to bed-junction preservation occurs in both types of firmgrounds but
is particularly obvious in type 1 firmgrounds, which invariably are overlain
by sediments that differ appreciably in texture and/or composition from the host
chalk ( Fig. 8 A). In type 2 firmgrounds, Thalassinoides may be further accen-
tuated by the formation of authigenic glauconitic, phosphatic, or ferruginous
crusts along the burrow walls, particularly if burrowing was followed by
substrate lithification ( Bromley, 1967, 1975; Kennedy and Garrison, 1975 ).
Diagenetic preservation of burrow fills by silica replacement at or beneath
omission surfaces, manifested in flint nodules, is a common feature in the Mid-
dle and Upper Chalk of the United Kingdom ( Fig. 8 C and D). This special mode
of preservation reflects the precipitation of silica, derived from intrastratal dis-
solution of biogenic components such as diatoms and sponge spicules, in sub-
surface redox zones that were stabilized at shallow substrate depths in response
to extended periods of non-deposition ( Madsen and Stemmerik, 2010 ). Flint
nodules are preferentially nucleated in or around burrow fills, particularly that
of Thalassinoides , and commonly avail three-dimensional views of the trace
fossils they incorporate ( Bromley and Ekdale, 1984b ; Fig. 8 D). Although silici-
fication is associated with omission and in some cases may have selectively pre-
served firmground Thalassinoides ( Bromley, 1967; Kennedy and Garrison,
1975 ), conditions necessary for silica precipitation commonly developed at sub-
strate depths below active firmground bioturbation ( Fig. 9 ). In the latter case,
flint-nodule growth preferentially preserved components of softground ichno-
fabrics in chalk subjacent to omission surfaces ( Bromley and Ekdale, 1984b;
Kennedy and Garrison, 1975 ).
Other diagenetic processes may have had adverse impacts on the expression
of firmground ichnofabrics. As exemplified by some European chalk omission
surfaces, particularly those associated with nodular chalks, chemical compac-
tion, and the formation of solution seams around harder nodules locally have
produced flaser-bedded chalk fabrics within which burrows may be distorted
or obscured ( Kennedy and Garrison, 1975 ).

5. ICHNOFACIES

5.1 Environments and Substrates
The Trypanites Ichnofacies develops in lithified carbonate substrates—in
rockgrounds, formed by the exhumation of once deeply buried limestones
(e.g., Bromley and Asgaard, 1993; Malfait and Van Andel, 1980 ), and in true
hardgrounds, formed by synsedimentary lithification of carbonate ooze. Hard-
grounds, like the type 2 firmgrounds from which they developed, result from

TRYPANITES