Geology Reference
In-Depth Information
such that deposition began immediately, precluding
erosion of the bed (Kneller and Branney
1995
) . In the
Cajiloa submarine canyon, Cretaceous Rosario
Formation (see Ancient Examples, above), the tidalites
were in a position both lateral to the main channel axis
(Fig.
14.11
), and were sheltered by seafl oor topogra-
phy from an underlying slump, and were subsequently
buried by an overlying slump (Figs.
14.12
and
14.15
).
In the Salir Formation, Turkey, Hayward (
1984
) found
an inverse relationship between proximity to the active
channel and the abundance of bottom-current reworked
chalk beds (currents which he interpreted as tidal in
origin), also suggesting that being lateral to the active
channel can aid preservation.
Zhenzhong et al. (
1998
) found internal tidal deposits
in an open-slope setting, where the potential for
erosional removal by turbidity currents is lower, and if
the slope was aggradational (Dykstra and Kneller
2007
), then preservation potential might have been
quite high. This example from the Yankou Formation
(see Ancient Examples, above) is in fact the thickest
deposit (30 m) yet documented. On some continental
slopes mass-failure is the rule rather than the excep-
tion, however, and many tidal records may thus be
removed by slumping (Posamentier and Walker
2006
) .
Additionally, some very energetic internal tidal sys-
tems may actually cause net erosion rather than leave
any depositional record (see discussion of Heezen and
Rawson (
1977
) in Modern Examples, above, and dis-
cussion of Cacchione et al. (
2002
) in Morphological
Impact of Tides in Deep-Water Settings, below).
Another major factor in the preservation potential
of tidalites must be sedimentation rates. If sediment
availability is low, then the internal tides may rework
sediment a bit, but not much can accumulate, and pres-
ervation of anything that does accumulate becomes
less likely. If sediment availability is high, then the
converse may be true. The case of good sediment avail-
ability is exemplifi ed by the Cajiloa submarine canyon
(see Ancient Examples, above). In that case, a slump
created a local seafl oor low (Figs.
14.12
and
14.15
).
Into this (and presumably on the seafl oor high sur-
rounding the low), a turbidite was deposited which
mantled the base of the seafl oor low but did not fi ll it
(Fig.
14.15b
). Above that turbidite is the fi rst appear-
ance of tidalites in the succession (Figs.
14.12
and
14.15c
). Therefore, what probably happened was that
tidal currents running through the submarine canyon
reworked the turbidite from the seafl oor above the
14.7
Preservation Potential
While it is obvious from current velocity records in
modern submarine canyons that internal tidal currents
are strong enough in some cases to move fairly coarse
sediment in traction or even suspension on diurnal or
semi-diurnal intervals, preservation of this sediment as
deposits in the rock record is less likely, due to a high
probability of sediment reworking by later, more pow-
erful turbidity currents in the canyon. Deep-water
tidalites are therefore much more likely to be preserved
where current strength is a bit lower, and the power of
intervening turbidity currents is reduced, such as in
abandoned meander bends (Damuth et al.
1988
) , on
the back-sides of levees (both master-bounding and
internal,
sensu
(Kane et al.
2007
) ), within topographic
lows such as those created by structural deformation
on the surface of submarine landslides (Figs.
14.12
and
14.15
), including submarine landslides on the
inside of levees (Dykstra
2005
) , and within plunge-
pools along submarine canyons (Gamberi and Marani
2007
). Additionally, preservation potential probably
increases during the waning phase of any given
energy cycle that might be helping to externally con-
trol turbidity currents entering the deep-water (e.g.
sea-level rise, decrease in sediment fl ux due to climate
change, a decrease in tectonic activity onshore, etc.)
(Fig.
14.16
).
Both Zhenzhong and Eriksson (
1991
) and May
et al. (
1983
) reported on tidalites preserved in the upper
part of submarine canyon fi lls, stratigraphically well
above the coarse-grained turbidites that comprised the
lower fi ll. In the Ordovician Bays Formation, the
tidalites are overlain by a thick slump consisting of
outer shelf material, which in this case may have
plugged the submarine canyon and aided in fi nal pres-
ervation (Zhenzhong and Eriksson
1991
) . They inter-
preted the internal tidal interval as that of a highstand
of sea-level, which in this case appears to have cut off
the energetic turbidity-current system which in the
lower part of the canyon fi ll overwhelmed any poten-
tial internal tidal signal. In other cases, however, inter-
nal tidal deposits can be below (Wheeler Gorge, see
Ancient Examples, above) or interbedded with coarse
channel-fi ll facies (Shanmugam
2003
; Shanmugam
et al.
2009
), yet were apparently sheltered from ero-
sional removal. This could occur if the subsequent tur-
bidity currents were over-capacity when they arrived
