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2.3 Liard Formation (Middle Triassic), British Columbia
Brachiopod- and echinoderm-dominated biostromes are common within late
Ladinian strata of the Liard Formation in northeastern British Columbia, Canada
(
Zonneveld, 2001; Zonneveld et al., 1997
). These biostromes are composed pri-
marily of bioclastic material but occur within dominantly siliciclastic shoreface
successions. This material provided the stable substrate that the biostrome taxa
colonized. They formed through both allogenic and autogenic taphonomic feed-
back mechanisms (
sensu
Kidwell and Jablonski, 1983; Zonneveld, 2001
).
The base of all biostromes observed consists of a layer of abraded, disarticulated
bioclastic detritus interpreted to reflect storm-induced concentrations of shell
debris. The Liard biostromes are dominated byorganisms that preferred hard or sta-
ble substrates such as the terebratulide brachiopod
Aulacothyroides
sp., the articu-
late crinoid
Isocrinus
sp., the cidaroid echinoid
Miocidaris
sp., and an ostreid/
pseudo-ostreid of unknown affinity (
Zonneveld, 2001
). The architecture of these
biostromes is complex and differs between occurrences. In some examples,
brachiopod-dominated rudstone occurs interbeddedwith either calcareous siltstone
or medium- to coarse-grained crinoidal grainstone that in turn occurs interbedded
withquartzose sandstone. In other occurrences, the crinoidal beds are absent and the
brachiopod-dominated rudstone beds occur interbeddedwith quartzose sandstones.
Trace fossils in biostrome-center deposits are limited to borings such as rare
examples of the pedicle-attachment trace
Podichnus
(
Zonneveld, 2001
) and
diminutive, solitary pits (cf.
Rogerella
,
Fig. 3
). Rare patches of admixed calcar-
eous/quartzose sand occur in biostrome-center deposits, wherein low-diversity
trace-fossil assemblages of
Asterosoma
, and
Palaeophycus
, and
Planolites
have
been observed (
Fig. 3
). Where present, the crinoidal grainstone deposits are pri-
marily massive in appearance, but large, dominantly horizontal, meniscate
FIGURE 3
Distribution of trace fossils adjacent to brachiopod-echinoderm biostromes, Liard For-
mation (Middle Triassic), northeastern British Columbia, Canada. Trace-fossil distributions are
intended to reflect their distribution at a specific point in time. Within interfingering strata,
trace-fossil distributions may be quite complex, but at any point in time, trace distributions follow
the distribution illustrated herein. Individual biostromes are 10-100 s of meters across and decime-
ters to meters thick.
Adapted from
Zonneveld (2001)
.
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