Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 793

due to minor erosional events in comparison to the more major events indicated
by the Thalassinoides -burrowed surfaces ( Chaplin, 1996, 2010 ).
Glossifungites -demarcated discontinuity surfaces occur at numerous hori-
zons in the Early Permian Chase Group ( Chaplin, 1996, 2010 ). Despite the fre-
quency with which they occur, they are invariably indicators of stratigraphically
significant surfaces. The Thalassinoides suites typically penetrate claystone or
shale substrates and are infilled with bioclastic wackestone, packstone, or grain-
stone. They typically have scratched walls and in several examples exhibit
secondary ferruginization. The monotypic trace-fossil assemblages that charac-
terize these transgressive surfaces are anomalous and likely reflect environmen-
tal or substrate preferences by the tracemaking organisms (likely arthropods).
The upper few centimeters were likely removed by erosion, and shallow-tier
trace-fossil communities are not represented in the preserved surface, thus low-
ering the apparent ichnodiversity ( Gingras et al., 2001 ). Most likely, other
deeply penetrating taxa were either incapable of penetrating the firm clay in
which these traces occur or intolerant of the active wave-dominated setting
in which this suite was emplaced.
The Rhizocorallium suite, which caps regressive surfaces and separates the
underlying marine units from the overlying continental units, does not exhibit
ornamentation although they are unlined and show detailed preservation of
spreiten ( Fig. 5 E; Chaplin, 1996, 2010 ). Thus, it is believed that the substrate
was not fully compacted, as the trace constructor was clearly able to adjust itself
within it. Moreover, the higher diversity of this assemblage indicates environ-
mental conditions more conducive to infaunal occupation than occurred at the
transgressive boundaries.
Other successions, similar to the Chase Group, characterized by substrate-
controlled trace-fossil concentrations at lithotype contacts are provided by Brett
et al. (2010). These include the Cambrian Wheeler Shale, the Ordovician Kope
Formation, and the Jurassic Blue Lias Formation.
3.2 Schei Point Group (Late Triassic), Sverdrup Basin
The Carnian-Norian Schei Point Group in the Sverdrup Basin, Nunavut, Northern
Canada, is a dominantly siliciclastic succession that contains several carbonate
intervals. These include the Gore Point Member of the Roche Point Formation
and the Eden Bay Member of the Hoyle Bay Formation, as well as several
horizons within the lowermost Barrow Formation ( Embry, 1991 ; Fig. 6 A). Major
lithostratigraphic boundaries are commonly characterized by substrate-controlled
trace-fossil assemblages. A low-diversity Glossifungites assemblage consisting
of Diplocraterion , Rhizocorallium ,and Skolithos characterizes the Hoyle Bay/
Barrow formational contact ( Fig. 6 A and B). Coplanar Glossifungites-Trypanites
assemblages characterize the top of the Gore Point Member ( Fig. 6 A and C).
Formation of the Trypanites surface on the upper contact of the Gore Point
Formation occurred first. Diminutive borings ( Trypanites isp.) as well as large