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In-Depth Information
cross-lamination (up and down-channel dipping)
(Fig.
14.7c, d
), (3) thickening-then-thinning upward
successions of sandstone-mudstone couplets with
bidirectional cross-lamination (up and down-slope
oriented) (Fig.
14.7e
), and (4) resedimented bioclastic
(coral and shelly debris from the shelf or foraminiferal
sand) and oolitic limestone-mudstone successions with
bidirectional cross-lamination (Fig.
14.7f
). The water
depths in which these successions are reported to have
been deposited are unclear, although the authors mainly
claim the upper slope as the depositional environment.
Alternative hypotheses for the origin of many of the
features described in these papers could be contour
currents or hyperpycnal currents (see Recognition
Criteria, below) (Mulder et al.
2002
; Mulder et al.
2001
), although the abundance of bi-directional paleo-
current indicators does point to a frequently changing
current regime. Additionally, because all of the deposits
they report are quite thick (Fig.
14.7
), if they are indeed
tidal in origin, their preservation would have necessi-
tated a very high sediment supply combined with local
accommodation space. Although this issue is not
addressed in the papers reviewed above, nor indeed in
any of the papers presenting internal tidal deposits,
I do review some of these controls in more detail in the
section on Preservation Potential, below.
Current reworked foraminiferal and volcaniclastic
sand has also been recognized in Cretaceous cores on
the Ontong-Java Plateau in what were interpreted as
very deep-water settings (2,200-3,000 m water depth)
(Klein
1975
) . Klein (
1975
) interpreted wavy, lenticu-
lar, and fl aser bedding with at least two paleocurrent
orientations, and an apparent cyclicity, as probably of
tidal origin. Similar sedimentary structures have been
found in the modern in deep-water foraminiferal sand
(see Modern Examples of Deep-water Tidal Deposits,
above) (Heezen and Hollister
1964
; Heezen and
Rawson
1977
; Lonsdale and Malfait
1974
) .
Shanmugam (
2003
) reexamined outcrops from the
Peira Cava outlier of the Eocene Annot Formation,
from which the fi rst vertical turbidite facies models
were produced (Bouma
1962
) . His interpretation was
that the tops of many turbidite beds had been reworked
by internal tides, generating sedimentary structures
such as planar laminae, ripple and sigmoidal (dune-
scale) cross-stratifi cation, and mud couplets. Planar
laminae, ripples, and dunes, however, are commonly
also formed by turbidity currents, a fact that has been
well documented in fl ume experiments (Fedele and
García
2009
; Kneller
1995
). True mud couplets (
sensu
Visser
1980
), are more diffi cult to explain with turbidity
currents, however, as they do imply slack-water condi-
tions between strong currents, in this case of opposed
direction (Fig.
14.8
). While refl ection of turbidity
currents, especially within enclosed basins (such as the
Peira Cava outlier), may be one possibility to account
for this (Kneller et al.
1991
), the conditions to set up
fl ows that can refl ect and yet develop slack-water con-
ditions between passage of the primary fl ow and its
refl ection seem less likely to occur than do deep-water
tidal currents. Therefore, it is not unreasonable to think
there may have been an infl uence of internal tides on at
least part of the Annot Formation.
Shanmugam et al. (
2009
) interpreted some parts of
a cored interval within a submarine canyon in the
Pliocene of the deep-water Krishna-Godavari Basin
(689-920 m, modern water depths (Shanmugam et al.
2009
)) as internal-tidal in origin. They distinguish
between sandy tidalites and muddy tidalites. Their
interpretation of sandy tidalites is based on the
presence of very rhythmic lamination, mud couplets,
lenticular and wavy bedding, parallel and ripple
laminae, mud-draped ripples, and thick-thin bundles
that they interpret as spring-neap cycles, respectively.
Their interpretation of muddy tidalites is based mainly
on rhythmic bedding and mud couplets. From the core
photographs it is unclear how rhythmic these deposits
are, or if any characteristic cyclicity is evident in the
deposits. Other workers interpret similar features as
the product of hyperpycnal currents (river-fl ood
generated turbidity currents) (Mas et al.
2010
;
Nakajima
2006
) .
May et al. (
1983
) interpreted the fi nal fi ll of the
upper-slope, Eocene Torrey submarine canyon,
exposed along Black's Beach in La Jolla, California,
as exhibiting a tidal infl uence due to multiple (mainly
up and down canyon) directions of ripple foresets.
The Cretaceous Wheeler Gorge Conglomerate has
been interpreted as the deposit of a channel-levee com-
plex, with axial facies represented by the conglomer-
ate, and levee or interchannel facies represented by
thinly interlaminated very fi ne-grained sandstones and
siltstones, termed zebra-striped intervals, which both
underlie and overlie the conglomeratic interval (Nelson
et al.
1977
; Walker
1985
) (Fig.
14.9
). The author
re-examined the lower zebra-striped interval in detail,
carefully logging and measuring the sandstone and
siltstone intervals and plotting the resulting data
