Geology Reference
In-Depth Information
Fig. 14.12
(
a
) Photomosaic and (
b
) interpretation of the tidalite
section in the Cajiloa submarine canyon. The section is under-
lain by a slump (MTD 1) which created a local topographic low
on the seafl oor. This low was a critical factor in the deposition
and preservation of the tidalites, as it created accommodation
space for the tidalites to fi ll, and a location sheltered from the
strongest turbidity currents. The slump is overlain by a 4-8 m
thick interval of interbedded turbidites and tidalites, and is over-
lain by another slump (MTD 2). The locations of the detailed
tidalites sections in Fig.
14.13
are shown
incomplete records are laid down or preserved (e.g.
neap cycles too weak to move sediment or spring tides
strong enough to erode, removing laminae), spectral
analysis will not necessarily show a good correlation
to any typical tidal cycle. Cowan et al. (
1998
) , for
example, showed that semi-diurnally driven sedimen-
tation in a macrotidal deep-water estuary in Alaska left
a record of thick-thin couplets that varied in number
3-18 per tropical month (the lunar orbital period deter-
mined by the moon crossing over the Earth's equator),
with an average of only 7.4 couplets per cycle, showing
that even in a very well constrained setting variability
in deposition and preservation can be extreme
(Dalrymple et al.
2003
) .
Due to the paucity of study of internal tide deposits,
it is diffi cult to nail down distinct recognition criteria.
Zhenzhong et al. (
1998
) suggest that in channelized
settings, bidirectional current indicators oriented both
down-channel and up-channel may provide good evi-
dence of internal tidal deposits. In certain cases, how-
ever, turbidity currents are known to be able to produce
refl ections off of obstacles (channel-bends, slumps,
fault-scarps) which may send a bore up-channel and
can move, erode, rework, and deposit sediment in
that direction (Morris and Alexander
2003
) ; in fi eld
observations bi-directional current indicators in
deep-water deposits have therefore most commonly
been attributed to turbidity current refl ections or
defl ections. Many of these interpretations are based
on a divergence between sole structures in a bed and
ripples higher in the same, apparently normally-
graded bed (Kneller and McCaffrey
1999
) . In such a
case where the bed appears continuously graded, and
the difference in current directions is via indicators
formed during very different energy conditions (e.g.
sole structures = erosional; ripples = depositional), it
stands to reason the same current is likely responsi-
ble for development of both current indicators, sug-
gesting refl ection. If, however, bi-directional current
indicators consist of sedimentary structures formed
under similar energy conditions, or they appear to be
part of different event-beds, especially if separated
by a drape of mud or hydraulically equivalent grains,
then it stands to reason they were formed by different
