Geology Reference
In-Depth Information
Fig. 17.7
Repeated double mud drapes on 40 cm-high, cross-
stratal foresets record semi-diurnal tidal cycles. Ripped-up mud
clasts are also very common. Note the rippled cap of the large
set, as well as ripple-lamination within the large set. The latter
are frequently reverse-fl ow tidal ripples and not fl ow separation-
eddy ripples that would be confi ned to a lowermost foreset posi-
tion. Sego Sandstone, Jim Canyon, Colorado
Fig. 17.8
Alternations of
thicker and thinner foresets
(possible spring-neap tide
bundling) within a 40 cm
high, sigmoidal cross-set
recording migration of 2-D
dune, Sego Sandstone, Jim
Canyon, Colorado
assemblage, typical of areas near a river mouth where
salinity is lowered and sedimentation rates relatively
high. In the lower delta-front to prodelta reaches of
WIS cases in S Wyoming, bioturbated very fi ne sand-
stones and siltstones with
Teichichnus, Ophiomorpha,
Cylindrichnus
and
Rosselia
are recorded (Mellere and
Steel
2000
). In the more distal reaches of the same
deltas in N Colorado, Hampson et al. (
2008a
) addition-
ally recorded
Schaubcylindrichnus, Paleophycus and
Conichnus
traces.
