Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 476

regularly interbedded subfacies. The most distinctive subfacies comprises mod-
erately to intensely bioturbated (BI

2-5), interstratified sandy mudstones with
largely unburrowed, dark, fissile mudstone drapes and thin sandstone stringers
( Fig. 10 ). This heterolithic subfacies is characterized by wavy bedding with
oscillation ripples and rare current and combined flow ripples ( MacEachern
and Pemberton, 1994; Pattison, 1992 ). The trace fossils are more uniformly dis-
tributed than elsewhere in the estuary, are typically moderately diverse, and are
characterized by a predominance of structures inferred to reflect deposit-
feeding and dwelling behaviors. The trace-fossil suites are dominated by
Planolites , Teichichnus , and Schaubcylindrichnus freyi , with subordinate num-
bers of Palaeophycus , Siphonichnus , Lockeia , Chondrites , Thalassinoides , and
diminutive Rosselia . Uncommon components include Ophiomorpha , Diplocra-
terion , Cylindrichnus , Rhizocorallium , Phycosiphon , and diminutive Astero-
soma . The reduced size of many biogenic structures, coupled with the
presence of synaeresis cracks, supports the interpretation of persistently fluctu-
ating salinities and brackish-water conditions.
The estuary-mouth complex (outer estuary) comprises sand derived from
alongshore transport of sediment along the barrier margin and from tidal
exchange through the tidal inlet. On the estuary side of the barrier, most sand
deposition reflects washover events, flood-tidal delta accumulation, and tidal-
inlet deposits ( Pattison and Walker, 1994 ). These sandstones are typically the
most marine influenced in the estuary, and unless sedimentation rates are high,
they tend to be the most intensely bioturbated facies within the IVF. Succes-
sions generally display interstratified muddy sandstones and horizontal
parallel-laminated, planar cross-stratified, current-ripple laminated, and
trough-cross-stratified sandstones ( Fig. 10 C). Mudstone interlaminae are com-
mon, with moderate numbers of mudstone interbeds that are locally siderite
cemented. The bioturbation intensities are generally higher than in the other
associated estuarine deposits (BI

¼

1-4), although burrowed zones are sporad-
ically distributed, due to episodic deposition. The suites tend to be diverse but
are dominated by Planolites , Ophiomorpha , Teichichnus , and Palaeophycus .
Subordinate components comprise Arenicolites , Schaubcylindrichnus freyi ,
Rosselia , Diplocraterion , Thalassinoides , Siphonichnus , and fugichnia. Very
minor elements include Asterosoma , Skolithos , Cylindrichnus , Chondrites , Ber-
gaueria , Macaronichnus , and Lockeia . The trace fossils tend to be compara-
tively robust, which is attributed to higher and more uniform salinities.

¼

4.2 Mixed(?) Estuary, Montney Formation (Triassic),
Alberta, Canada
The Montney Formation was deposited on the west coast of the Pangaean super-
continent during the Early Triassic (basal Induan to end Olenekian). Within the
Western Canada Sedimentary Basin, the Paleozoic/Mesozoic transition records
a regional lowstand with concomitant subaerial exposure and development of