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as missing in the Shannon Sandstone to confi rm a tide-
infl uenced estuary or delta interpretation. Because of
the common lack of distributaries we interpret the
middle and distal reaches of these sandbodies to be
subaqueous. The outcrops of the Shannon, Kremmling
and Airport sands have a more 'distal' muddy charac-
ter and they can contain mixed-energy facies, though
they also show limited thicknesses of tidal cross strata
in their uppermost parts. The best overall appreciation
of these sandbodies can be gained by viewing the dis-
tal Berry Gulch and Wise Gulch sands of N. Colorado
(Hampson et al.
2008a
), together with their proximal
equivalents in the Haystack Mountains of S Wyoming
(Mellere and Steel
2000
) . Hampson et al. (
2008a
, their
Fig. 19A) proposed a large-scale reconstruction sug-
gesting southward-defl ected currents as the deltas built
out into the seaway. Figure
17.19
, based on facies as
well as paleocurrent patterns (Uroza
2008
) document
this defl ection, with sediment dispersal shift from east-
wards to southwards for the O'Brien Springs Sandstone.
Martinsen (
2003c
) noted that syn-sedimentary tecton-
ics also had signifi cant infl uence on coastline physiog-
raphy and bathymetry through much of the Upper
Cretaceous and that deltas may have been confi ned and
protected from waves at times by structurally-generated
embayments. The role played by the counter-clockwise
basin circulation and southward-sweeping currents ver-
sus the effects of syn-sedimentary structure at lowstand
(Martinsen
2003b
) in creating the southward-elongated
character of many of the sandbodies is still unclear.
2008a
; Hampson
2010
) tend to agree that the tidal
signals in the sandbodies, the rare storm-wave signals
(though there are some associated wave-generated
sandbodies) and the common medium-grained charac-
ter of the sands all suggest that the sandbodies are
derived from rivers in the west and northwest, and that
the strong currents defl ecting the sandbodies to the
south (Fig.
17.19
) are likely to have been tidal currents,
rather than purely wave-driven longshore currents or
other seaway currents. Modeling by Slingerland et al.
(1996) suggested that the steady-state surface circula-
tion pattern of the WIS was a counter-clockwise gyre,
with infl owing river currents from the west defl ecting
to the south and in the east defl ecting to the north, due
to Coriolis acceleration. Slingerland and Keen (
1999
)
further developed this model by simulating an 8-day
storm superimposed on the steady-state gyre, and com-
puted the net sediment transport on the western margin
of the seaway to be dominantly southerly. However,
the modeling emphasized mainly storm and wind-
driven effects on the seaway, probably most relevant
during sea-level highstands and wide shelf conditions.
It is likely that waves would have been dampened and
tidal currents accentuated in the counter-clockwise
gyre during sea-level lowstand periods, as discussed
below. Their regressive character indicates that most of
these tidal sandbodies were deltaic, though estuaries
are likely in the transgressive phases. Those farthest out
in the seaway may have become suffi ciently reworked
so as to become transgressive shelf sand ridges (Snedden
and Dalrymple 1999).
Figure
17.20
shows the southward-skewed Yangtze
tide-dominated deltas, proposed by Hampson and
others (
2008a
) as the best recent analog for the distal low-
stand deltas on the western coast of the WIS. However,
note that the scale of the defl ected system is quite differ-
ent in Figs.
17.19
and
17.20
, and the cause of the coast-
parallel currents need not be the same in both cases
17.3.3 Interpretation of the Tidal
Sandbodies in Setting 1
Following the long-lasting WIS shelf-sands debate
(see Suter and Clifton
1999
for summary) the sand-
bodies of Setting 1, when seen in their proximal as
well as distal expression, have most of the attributes of
falling-stage to lowstand deltaic shorelines. The bodies
have muddy to rippled prodelta and lower delta-front
reaches with a restricted ichnofauna, and upper delta-
front dunefi elds that display both landward and seaward
paleocurrents (Fig.
17.19
), though usually dominated
by the latter. The persistent basinal location of the
sandstones (with respect to their westerly highstand-
equivalent shorelines) is also consistent with this sequence
stratigraphic interpretation. Further, the most recent
researchers who brought new data to this debate
(Mellere and Steel
2000
; Uroza
2008
; Hampson et al.
17.3.4 Causes of Tidal Amplifi cation in
Setting 1: Lowstand Narrowing
and Northward Constriction
of the Seaway
Individual tidal sandbodies of Setting 1 occupy a maxi-
mum regressive position in their host sequence, i.e.,
they occupy the outermost 100+ km of regressive shore-
line transits that were 100s of km wide. In addition
they tend to occur progressively farther basinwards, as
