Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 785

preferential colonization of one lithotype over another. Finally, consistency or
compactional differences in different lithology types may result in ichnotaxo-
nomic segregation as some taxa prefer loose media within which to burrow and
others prefer firm or cemented substrates.
Mixed systems occur throughout the geological record; however, they are
more common in specific intervals. Arguably, the most notable among these
include the lowermost Mesozoic. Examples are derived from western Canada
and the western United States. In both of these regions, the Permian was dom-
inated by carbonate deposition. The demise of many of the carbonate-producing
taxa coincides in many areas with a shift from carbonate dominance to clastic
dominance at the Permian/Triassic boundary. Although syndepositional diage-
netic controls have also likely played an important role, the prevalence of bio-
clastic packstone and grainstone beds in preextinction successions and their
absence in similar deposits in postextinction settings were in part a function
of the demise of many shell-secreting organisms (i.e., many brachiopod and
echinoderm taxa) rather than a shift in environmental setting. Survival and reco-
lonization by survivor taxa as well as the origination and environmental expan-
sion of new forms produced mixed clastic/carbonate successions in many Early
and Middle Triassic successions worldwide, of which some examples are
included in this section (e.g., Montney Formation, Alberta; Liard Formation,
British Columbia; Moenkopi Formation, Nevada and Utah).
2.1 Montney Formation (Early Triassic), Alberta
Shallow- and marginal-marine systems dominated by siliciclastic siltstone and
sandstone and bioclastic packstone and grainstone characterize Early Triassic
successions in west-central Alberta, Canada ( Moslow and Zonneveld, 2005;
Orchard and Zonneveld, 2009 ). These units comprise the Vega and Phroso
members of the Sulphur Mountain Formation in the Foothills and Front Ranges
outcrop belt and the Montney Formation in the subsurface. Although siltstone
and sandstone throughout the entire Montney Formation are dominated by
quartz, dolomite (both syndepositional and late diagenetic) is also abundant
and, indeed, commonly occurs in nearly equal abundance with quartz. In the
middle part of this succession, a bioclastic grainstone/packstone succession
known as the Coquinal Dolomite Middle Member (Mackenzie Dolomite Lentil
in outcrop) is present. Stratigraphic and sedimentological evidence indicates
that the middle part of the Montney Formation records deposition in a proximal
ramp setting and includes units interpreted as shoreface, foreshore, and inter-
tidal flat ( Moslow and Zonneveld, 2005; Orchard and Zonneveld, 2009 ).
Shoreface units exhibit gradation from very fine-grained and fine-grained
sandstone at their bases to bioclastic sandstone and sandy bioclastic pack-
stone/grainstone at their tops ( Moslow and Zonneveld, 2005 ). Bioclastic
units consist of whole and fragmentary bivalves, gastropods, and lingulide
brachiopods. Bioturbation within the shoreface sandstone units consists of