Geology Reference
In-Depth Information
2. Tidal deposits of
Setting 2
, occurring frequently in
time and space within the entire
transgressive
parts
of Campanian sequences. They occur most com-
monly as thin (m-scale) transgressive caps to regres-
sive strata, but sometimes accumulate to greater
thicknesses, especially where incised valleys or other
high-accommodation areas developed and became
infi lled. An increase in the thickness and extent of pre-
served tidal deposits during transgression is a well-
established phenomenon (Cattaneo and Steel
2003
).
nature of these tidal deposits implies signifi cant cur-
rent energy, but there are also thin mudstones and
organic-matter concentrations. The key criterion is
the presence of thick (>5 m), stacked, well-ordered
sets of planar or trough cross stratifi cation (Figs.
17.5
and
17.6
) (see also Willis
2005
; Dalrymple
2010
),
commonly in 'blocky' or upward-coarsening and thick-
ening regressive successions. Individual sets of cross
strata that are sited seaward of the distributary mouth
commonly have a sigmoidal geometry, with evidence
of landward-directed (fl ood tide) currents (Fig.
17.5
)
in addition to seaward-directed paleocurrents. Willis
and Gabel (
2003
) documented upward-coarsening,
delta-front bedsets in Lower Sego Sandstone that are
relatively steeply inclined (5-15°) and 6-12 m thick.
The commomly sharp erosional base of the Sego
bedsets was interpreted in terms of tidal current scour
within a tidal-channel mouth-bar system (Willis and
Gabel
2001,
2003
) .
Thin muddy or organic drapes are common in most
tidal cross-stratal foresets, because of the frequency of
slack-water periods in tidal settings (Fig.
17.6
). Other
reliable criteria are double mud drapes on foresets and
bottomsets (Fig.
17.7
) indicative of two slack-water
periods bounding the weaker of the fl ood or ebb tidal
current intervals (Fenies et al.
1999
) and thick-to-thin
foreset bundling along the transport length of the dune
set (Fig.
17.8
), suggesting spring-neap tidal bundling
(Dalrymple and Choi
2007
) .
In addition to the tidal signals within single dunes,
it is a characteristic feature of WIS upper delta-front
sandbodies that individual dunes combine to form
compound dunes (see also Dalrymple
2010
), within
which both ebb and fl ood tidal currents can be recorded
(Dalrymple and Rhodes
1995
) (Fig.
17.9
). Although
compound dunes occur also in some fl uvial systems
(Collinson
1970
), the tidal cases tend to show evidence
of bi-directional paleocurrents, as well as being associ-
ated with tide-infl uenced mouth bars and transgressive
estuarine channels and bars (as in the Chimney Rock
Sandstone of N. Utah; Plink-Bjorklund
2008
) .
ManyWIS tide-dominated, delta-front sandbodies
additionally show a characteristic occurrence of more
extensive (>100 m) but very thin (mm to cm) mudstone
beds, that occur between the bedsets and sets of cross-
stratifi ed and ripple-laminated sandstone, (see also
Willis
2005
), seen well in the O'Brien Springs Member
of Hackstack Mountains Formation (Fig.
17.10
).
Such extensive mud drapes and layers may sometimes
17.2
Criteria for Recognizing Tidal
Deposits in the Western Interior
Seaway
General criteria for recognizing tidal signals in recent
and ancient strata are relatively well known (De Raaf
and Boersma
1971
; Nio and Yang
1991
; Dalrymple
1992
,
2010
; Fenies et al.
1999
), and have also been
well used in the WIS (e.g., Rahmani
1988
; Shanley
et al.
1992
). However, we choose here to look at tidal
criteria within the context of sub-environments within
coastal depositional systems. Following the tidal envi-
ronmental subdivisions of Dalrymple and Choi (
2007
)
we summarize criteria from the key environments
(Fig.
17.4
). Tidal signals in regressive and transgres-
sive strata are not greatly different, though tidal bars
and compound dunes tend to be somewhat more thickly
developed in estuaries, and some facies successions
(e.g., compound dunes fi ning upwards into tidal fl at
and supratidal muds) are more common in estuaries,
whereas in deltas compound dunes tend to cap upward-
thickening parasequences.
17.2.1 Tidal Criteria: Campanian Delta-
Front and Distributary-Mouth Bars
Many of the most sand-rich Campanian tidal deposits
are interpreted to occur within the subaqueous distri-
butary mouth bars and delta-front reaches (Fig.
17.4b
)
of tide-infl uenced or tide-dominated deltas, e.g., in the
Morapos Sandstone of N. Colorado (Hampson et al.
2008a
), Blair Sandstone (Devlin et al.
1993
; Martinsen
2003c
), O'Brien Springs, Seminoe and Hatfi eld 1
sandstones of S. Wyoming (Mellere and Steel
1995b
;
Martinsen
2003c
) and Sego Sandstone in N. Utah
(Willis and Gabel
2001
). The grain size and sand-rich
