Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 797

FIGURE 8 Outcrop photographs of substrate-controlled trace-fossil assemblages in the medial
Baldonnel Formation (Late Triassic) at Brown Hill (outcrop reference section for the Baldonnel
Formation), Williston Lake, British Columbia. This Glossifungites -demarcated discontinuity
surface is overwhelmingly dominated by Rhizocorallium isp., but other tube structures (cf. Plano-
lites ) also occur.

Where these bioclastic beds are overlain by siliciclastic claystone or silty
to very fine-grained sandstone beds, the contact is typically sharp but appears
conformable and trace fossils have not been observed crossing the facies
boundary. Where the contact consists of medium-grained (or coarser)
cross-stratified sandstone that incises into calcareous mudstone or bioclastic
packstone/grainstone, low-diversity Glossifungites assemblages occur
( Fig. 10 C and D). Traces identified within these assemblages include Cambor-
ygma , Lunulichnus , Planolites , Skolithos ,andcf. Thalassinoides ( Zonneveld
et al., 2003 ). Although these surfaces also occur subjacent to laterally
restricted fluvial channels, they are particularly abundant subjacent to region-
ally correlatable erosional surfaces characterized by amalgamated fluvial-
channel successions ( Zonneveld et al., 2003 ). Although these strata were
deposited within a continent-interior

foreland basin at great distance

( 1,000 km) from any possible marine influences, these substrate-controlled

trace-fossil assemblages are similar to Glossifungites assemblages that occur
within marginal and fully marine settings. It should be noted that important
differences do occur between fluvial and marine/marginal-marine Glossifun-
gites surfaces, including the nature of the traces that characterize the individ-
ual surfaces. Marine/marginal-marine Thalassinoides tend to be even in
thickness, have smooth walls and branch regularly, whereas fluvial forms
are less even, have rougher walls and branch irregularly. U-shaped traces
( Diplocraterion and Rhizocorallium ) are absent in fluvial successions but
are common below marine-influenced Glossifungites surfaces.

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