Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 795

3.3 Baldonnel Formation (Late Triassic), British Columbia
The Baldonnel Formation (Middle to Late Carnian) in northeastern British
Columbia, Canada, is a predominantly carbonate succession with a high propor-
tion of quartz-dominated sand admixed into most facies and with subordinate
clastic sandstone beds interfingering with bioclastic beds in many areas
( Zonneveld and Orchard, 2002; Zonneveld et al., 2004 ). Parasequence bound-
aries (marine flooding surfaces) in the Baldonnel Formation are commonly
characterized by abrupt bounding surfaces demarcated by substrate-controlled
ichnofacies ( Fig. 7 ; Zonneveld and Orchard, 2002; Zonneveld et al., 2004 ).
These surfaces commonly exhibit a low diversity, although trace-fossil densi-
ties are typically quite high ( Figs. 8 and 9 ). Diplocraterion , Rhizocorallium ,
Thalassinoides , and small, unlined tubes characterize Glossifungites surfaces
in the Baldonnel Formation. Trypanites , flask-shaped traces (cf. Gastrochaeno-
lites ) and tube-shaped borings, characterize Trypanites assemblages ( Fig. 8 ).
The Baldonnel Formation records an interval of regional marine transgression
during the Middle and Late Carnian ( Zonneveld and Orchard, 2002 ). The unit is
highly diachronous, withwestern (basinward) occurrences older than eastern (land-
ward) occurrences. These surfaces are invariably characterized by lags of coarse-
clastic detritus including quartz sand, chert granules and pebbles, and phosphatic
ooids and granules ( Fig. 9 B). Bioclastic detritus is also common at these surfaces.
The presence of abundant phosphatic clasts concentrated at these horizons results in
sharp increases in gamma radiation at these contacts.
Abundant substrate-controlled trace-fossil assemblages in the mixed silici-
clastic/carbonate Baldonnel succession are consistent with erosional exhumation
of previously buried strata prior to or during marine transgressions. Coplanar
Glossifungites-Trypanites demarcated discontinuity surfaces record complex
scenarios that must include initial deposition, burial, erosional exhumation, fol-
lowed by burrowing by firmground-preferring organisms, then shallow burial and
cementation, and finally boring by hardground-preferring organisms.
3.4 Green River and Wasatch Formations (Eocene), Wyoming
Glossifungites -demarcated discontinuity surfaces are common where fluvial sys-
tems incise into underlying layers within Eocene (Wasatchian-Bridgerian) strata
in the Green River Basin ofWyoming, USA ( Fig. 10 ). Although these surfaces are
present in purely siliciclastic successions—where fluvial channel sandstone units
occur incised into subjacent paleosols—they are particularly abundant in basin-
margin settings where calcareous mudstone and bioclastic packstone/grainstone
units of the lacustrine Green River Formation occur interbedded with clastic sand-
stone units of the fluvial Wasatch Formation ( Fig. 10 A-E; Zonneveld et al., 2003,
2006 ). The bioclastic grainstone and packstone lacustrine beds are dominated by
gastropods and ostracods with subordinate unionid bivalves. Stromatolitic and
oolitic horizons occur interspersed throughout the succession as well.