Geology Reference
In-Depth Information
Table 14.1
Important features of turbidites, hyperpycnites, contourites, and tidalites are compared (Modifi ed from Mulder
et al.
2002
)
Turbidite sequence
(Bouma-like)
Hyperpycnal turbidite
sequence (hyperpycnite)
Bed type
Contourite sequence
Tidalite sequence
Flow type
Turbulent surge
Turbidity current
Contour current
Internal tides
Flow behaviour
Unsteady, mainly
waning; unidirectional
Mainly steady, waxing then
waning; unidirectional
Almost completely
steady, waxing then
waning; unidirectional
Waxing, waning, reversing,
repeat; sometimes abrupt
changes in direction,
sometimes gradual
Flow regime
Turbulent
Turbulent
Turbulent
Laminar to turbulent
Flow duration
and time for
deposition
Minutes to days
Hours to months
1,000s to 10,000s years
Hours in each direction
Base contact
Erosive to sharp
Gradational
Gradational
Gradational to erosive
Top contact
Gradational
Gradational
Gradational
Gradational
Intrabed contact
Occurs sometimes
between facies
Frequent, erosive to sharp
None
Gradational to erosive,
depending on when formed
during tidal cycle (spring or
neap)
Grading
Clear, normal
Clear, normal then inverse
Crude, normal then
inverse
Clear to crude; normal or
inverse to normal
Bioturbation
Absent to intense
Absent to intense
Thorough and intense
Absent to slight
Ichnofacies
Few
Few
Many
Few
Structures
Well-developed
parallel and cross-
bedding, convolutes
Well-developed parallel and
cross-bedding, climbing
ripples frequent
Crude and sparse
parallel and cross-
bedding, frequent
mottles and lenses
Parallel and wavy lamination,
ripple-scale cross-bedding,
fl aser bedding, mud drapes
and mud couplets common
Fauna/fl ora
Allochthonous, mainly
marine
Allochthonous, mainly
continental, frequent plant
and wood fragments
Mainly autochthonous
Unknown - probably
mainly autochthonous
14.6
Stratigraphic Successions
the spring tide part of the cycle, and thus be inverse
to normally graded (Zhenzhong et al.
1998
) . In non-
channelized successions, however, the fl ood and ebb
internal tides (adopting terminology from surface
tides) may not follow the same paths such that unidi-
rectional currents may dominate, although in the case
of the Yankou Formation in an unchannelized open
slope setting bidirectional current indicators were
apparently common (Zhenzhong et al.
1998
) .
In the Cajiloa submarine canyon succession (see
Ancient Examples, above), the successions vary, some
consist of a series of stacked thickening and thinning
intervals, 5-40 cm thick, comprised of plane-parallel
to wavy laminae (Fig.
14.13a
), while others consist of
laterally accreted bundles of ripple cross-laminae
(Fig.
14.13b
). The latter tend to be organized into suc-
cessions 5-20 cm thick, with smaller-scale ripples at
the bottom and top of a succession, and larger-scale
ripples in the middle.
Zhenzhong et al. (
1998
) point out that due to the neap-
spring-neap cycles inverse-to-normal grading is to be
expected in vertical successions. This symmetrical
grading may also apply to relative thicknesses such
that couplets of sandstone-mudstone for example may
be relatively thicker during the spring part of the
cycle (Fig.
14.7
). They also point out that due to energy
concentration in channelized environments (Hotchkiss
and Wunsch
1982
), the neap-spring part of the cycle
may frequently be absent, preserving only asymmetri-
cally graded cycles from the spring to the neap tides.
Any such cycle will also naturally be modulated by
sediment availability; as in any system, if only mud is
available, the succession will look different than if
only sand and no mud is available, or if only carbonate
grains are available. In non-channelized settings, verti-
cal successions are more likely to preserve the neap to
