Sedimentological
criteria
to
differentiate
submarine
channel
levee
subenvironments: exhumed examples from the Rosario Fm. (Upper Cretaceous)
of Baja California, Mexico, and the Laingsburg Fm. (Permian), Karoo Basin, S.
Africa.
Ian A. Kanea* and David M. Hodgsonb
aSchool
of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
bDepartment
of Earth and Ocean Sciences, 4 Brownlow Street, University of
Liverpool, Liverpool L69 3GP, UK.
*Corresponding author: i.a.kane@leeds.ac.uk
Abstract
Two scales of levee confinement are commonly recognised from submarine channellevee systems on the seafloor and in the subsurface. Large-scale external levees bound
the entire system whilst smaller-scale internal levees bound individual thalweg
channels within the channel belt. Although thin beds are commonly identified in core
and well logs, their origin, and consequently their stratigraphic significance is
currently poorly understood. This knowledge gap stems, in part, from the lack of
unambiguously identified outcrop analogues of channel-levees, and in particular the
lack of identifiable internal and external levees. Here we report from two exhumed
channel-levee systems where both scales of confinement can be recognised: the
Rosario Fm. of Baja California, and the Laingsburg Fm. of South Africa. A suite of
characteristic sedimentary features are recognised from internal and external levees
respectively: internal levees are characterised by structures indicative of complexity in
the waxing-waning style of overspill, interactions with topography and flow
magnitude variability; in contrast, external levees are characterised by structures
indicative of simple surge-like waning flows, relatively uniform flow directions,
laterally extensive beds, and a lack of erosive events. Using these observations,
together with published literature, we propose a simple nomenclatural scheme for
levee sub-environments, and criteria to differentiate between levee sub-environments
in core or outcrop.
1
Keywords: submarine channel, channel-levee, architecture, deep marine,
turbidite.
1. Introduction
Submarine channel levees are elongate sedimentary deposits which form by
deposition at the margins of submarine channels, and act to partially confine the
sediment gravity flows within them (e.g., Buffington, 1952; Menard, 1955; Normark,
1970; Komar 1973; Hay, 1983; Hiscott et al., 1997; Piper & Deptuck, 1997; Stow et
al., 1998; Piper et al., 1999; Skene et al., 2002; Deptuck et al., 2003, 2007). In crosssection, they are wedge shaped and thin away from the channel (e.g., Skene et al.,
2002; Kane et al., 2007, 2010a). Large-scale slope and basin floor channel-levee
systems are partially to entirely bound by ‘external’ levees, whilst individual channels
within the channel belt may be bound by ‘internal’ levees. Differentiating between
these different scale levees, and the sub-environments within them, is an important
issue in hydrocarbon exploration as both are superficially similar but have different
reservoir properties and stratigraphic (and sequence stratigraphic) implications.
Commonly, submarine levees that confine related channel belts are identified
in seismic reflection and sidescan sonar datasets (e.g., Skene et al., 2002, 2005;
Deptuck et al., 2003; 2007). The robust interpretation of external submarine levee
deposits at outcrop, however, is more difficult and requires excellent exposure of finegrained and thin-bedded deposits that are demonstrably related to overspill from
submarine channel-belts (e.g., Walker, 1975; Winn & Dott, 1979; Morris & BusbySpera, 1990; Hickson & Lowe, 2002; Beaubouef, 2004; Schwarz & Arnott, 2007;
Khan & Arnott, this volume). Criteria for the identification of external levees include
using palaeo-horizontal datums to demonstrate a constructional wedge shape
geometry, a fining and thinning of beds away from an adjacent channel-belt, a
divergence of palaeocurrents from the related channel-belt, and sedimentary processes
dominated by traction (Walker, 1985; Hesse and Dalton, 1995; Hiscott et al., 1997;
Hickson & Lowe, 2002; Dutton et al. 2003; Kane et al., 2007, 2009, in press; Crane &
Lowe, 2008; Hubbard et al., 2008). Internal levees (sometimes referred to as terrace
deposits – see discussion below) have been identified in several geophysical surveys
as a dim and tabular seismic facies (Hübscher et al., 1997; Torres et al., 1997; Piper et
al., 1999; Piper & Normark, 2001; Deptuck et al., 2003, 2007; Babonneau et al.,
2004). However, despite forming a significant proportion of the fill of channel belts
2
(and canyon fills), only a few studies have reported the occurrence of internal levees
at outcrop (Schwarz & Arnott, 2007; Kane et al., 2009).
Here, the sedimentology and stratigraphy of external and internal levee
deposits from two exhumed deep-water systems, the Cretaceous Rosario Formation in
Baja California, Mexico, and from the Permian Laingsburg Formation, Karoo Basin,
South Africa, are described. The Laingsburg Fm. is fine-grained with a narrow grainsize range, whereas the Rosario Fm. is coarse-grained with a wide grain-size range.
Despite these differences, the deposits share many sedimentological and stratigraphic
characteristics that permit criteria for the recognition of internal and external levees,
and their sub-environments, to be developed and tested elsewhere.
2. Morphological terminology
Two scales of submarine channel-levee systems are commonly recognised within
channel-levee complexes (following nomenclature of Flood et al. 1991). Individual
thalweg channels or channel complexes (sensu Samuel et al., 2003) without flanking
levees, or ‘underfit’ channel-levee systems (Deptuck et al., 2003) with ‘inner levees’
(Hübscher et al., 1997) or ‘confined levees’ (Piper et al., 1999), may meander within a
wider channel belt in part bounded by ‘high levees’ (Piper et al., 1999) or ‘master
bounding levees’ (Posamentier, 2003). Here, we propose the terms ‘internal levee’
and ‘external levee’ to avoid confusion in the literature concerning the use of ‘inner’
and ‘outer’ levees for either two different scales of levees, or, channel-facing/channelopposing parts of the same levee. Large scale ‘external levees’ are generally
envisaged to form during the regressive phase of a base level cycle during degradation
and formation of the channel belt, whilst smaller scale ‘internal levees’ and associated
underfit channels form (or are preserved) during the transgressive phase of a base
level cycle as the channel belt aggrades (Hübscher et al., 1997; Deptuck et al., 2003).
External levees are a dominantly depositional body forming a constructional
wedge of sediment that thins perpendicularly away from a channel belt (Fig. 1). The
external levee forms during the evolution of a genetically related channel-belt (or
slope valley, channel fairway) by flows that partially spill out of their confinement.
External levees can confine adjacent channel belts to form levee-confined systems
(Fig. 1). However, the bases of many external levees are (palaeo-)topographically
higher than their related and entrenched channel belts (Fig. 1). The external levee is
bounded by the basal levee surface that is parallel to the regional slope gradient, and a
3
steeper top levee surface that dips away from, and more steeply into, the channel belt
(Fig. 1). The levee crest is the highest point of the external levee, and runs parallel to
the course of the channel belt, separating the external levees into outer external levees
and inner external levees (Fig. 1). The levee crest may either be a constructional
crest, where aggradation of the channel and levee were contemporaneous and
stratigraphically connected, or a cut crest, where the crest is a remnant after later
erosion and/or failure and remobilisation, and the channel-fill is in part younger than
the levee (Fig. 1). The type of levee crest determines the genetic relationship between
the levee deposits and channel belt-fill. The levee crest trajectory can be mapped in
some datasets and may migrate toward or away from the channel system. External
levees may be much less sinuous than the levees of an individual channel-levee
system as they do not follow one particular channel but may be the product of
overspill from one or more channels or channel-levee systems meandering within the
wider channel belt (Deptuck et al., 2003; Posamentier, 2003).
Internal levees are constructional features fed by flows that partially spilled out of
channelized confinement, but were largely unable to escape the confinement of the
channel belt (Fig. 1). The flows which build internal levees may interact with the
main confining surface, i.e., the external levees, and/or the channel belt erosion
surface, and are liable to erosion by the migration or avulsions of channel thalwegs,
and the overbank passage of large flows not confined by the internal levees. As a
consequence of lateral migration, internal levees may be better preserved on inner
bends (Schwarz and Arnott, 2007). Internal levees form only when confinement has
been established, through the construction of external levees and/or the degradation
and entrenchment of the composite erosion surface of the channel belt, or within
canyon confinement (Deptuck et al., 2003). Internal levees may form distinct wedges
of sediment where enough space is available; where space is limited, i.e., where
overspill from underfit channels interacts with external levees or erosional
confinement, overspill deposits may appear superficially similar to terrace deposits,
which are widely identified in the subsurface (e.g., Damuth et al., 1988; Nakajima et
al., 1998; Babonneau et al. 2004). These apparently horizontal deposits commonly
overlie low relief erosion surfaces (terrace surfaces) (Babonneau et al., 2004). If the
larger container fills through net aggradation, then the internal and external levee
deposits may merge and become difficult to distinguish (e.g., Deptuck et al., 2003).
4
The term ‘inner levee’ is used to describe that part of the levee between the
channel thalweg and the levee crest; ‘outer levee’ describes that part of the levee
between the levee crest and the termination of the levee against the adjacent slope or
confining topography (in the case of internal levees). Levee sub-environments are
termed ‘channel-proximal levee’ or ‘channel-distal levee’ in relation to their
perpendicular distance from the channel thalweg (Fig. 1); these environments are
gradational and the terms are purely qualitative.
3. Identification of sub-environments in exhumed levee deposits
3.1 Cretaceous Rosario Formation
The Rosario Formation is the youngest unit of a belt of Upper Cretaceous sedimentary
rocks which crop out discontinuously along the Pacific coastal margin of southern
California and Baja California (Beal, 1948; Gastil et al., 1975) (Fig. 2). The Rosario
Formation consists of non-marine, shallow-marine, and deep-marine sediments
dominantly sourced from volcanic and plutonic rocks of the Upper Jurassic to Early
Cretaceous former arc complex to the east (Gastil et al., 1975), but also including
sedimentary and metasedimentary rocks (Morris & Busby-Spera, 1990), and
deposited into the Peninsular Ranges fore-arc strike-slip basin to the west (Busby et
al., 1998). In the area of Canyon San Fernando, the Rosario Formation consists of a
submarine canyon fill incised into slope mudstones and overlain by a genetically
related channel-levee complex (Morris & Busby-Spera, 1988; 1990; Dykstra &
Kneller, 2007) (Fig. 2). The canyon and channel complex are together approximately
1 km thick and the fill was deposited over ca 1.6 Myr, a time-scale consistent with a
third order sea-level cycle (Dykstra & Kneller, 2007). The Canyon San Fernando
system is comprised of a series of coarse-grained channel belt fills, which were
initially canyon confined. The canyon trended obliquely to the regional slope
gradient, potentially due to underlying fault control (Dykstra and Kneller, 2007).
Consequently, as the canyon-fill aggraded, a large external levee developed on the
downslope side, maintaining the slope oblique trajectory, whilst the system was
bound by the slope on the upslope side (Morris & Busby-Spera, 1990; Dykstra &
Kneller, 2007, 2009; Kane et al., 2007). During canyon confinement and later external
levee confinement, multiple channel thalwegs formed and built associated internal
levees (Kane et al., 2009). The internal levee at Playa Esqueleto, in Canyon San
Fernando, is identified by its lateral bed thickness decay and lithofacies variation, and
5
relationship to coarse-grained thalwegs (Kane et al., 2009). The channel-belt fills are
dominated by thin-bedded turbidites also of a putative levee origin (although other
origins are possible for these sections – which have received less attention),
punctuated by coarse grained thalwegs and widespread debris flow facies. The
external levee which bounds one side of the entire system in later times is a laterally
extensive (~2.5 km wide and at least 100 m thick) wedge shaped body of thin-bedded
turbidites that decrease in thickness away from the channel belt, and have predictable
facies relationships recording waning flow away from the channel (Kane et al., 2007).
3.2 Rosario Formation: External levee
3.2.1. Bed thickness distributions
Within the outer external levee, sandstone beds are laterally extensive over distances
of up to 2 km away from the channel-belt. Individual beds show a relatively
consistent tapering away from the crest (well-described by a power law, Kane et al.,
2007), with no examples of abrupt lateral thinning or pinch-out recorded in the outer
external levee. On the inside of the cut-crest, i.e., the inner external levee, in the
vicinity of a zone of deformation close to the channel belt margin, beds onlap slide
blocks and slide surfaces (Fig. 3B & C), commonly pinching out (depositionally or
erosionally) over a few metres (Fig. 3B & C). Sandstone bed thickness varies from
exceptionally thick (3-4 m) at the base of, or as a precursor to, the external levee
succession, to 0.5 m thick beds at the base of the well organised levee succession, to a
mean bed thickness of approximately 50 mm in channel proximal outer external levee
(Fig. 4A). Kane et al. (2007) examined vertical trends in bed thickness and reported
thinning upwards trends from the inner external levee, close to the cut crest, and
thickening upwards within the channel proximal outer external levee (Fig. 4C). These
trends are superimposed by numerous smaller scale thickening- and thinning-upward
trends. Defining thickening or thinning cycles remains somewhat ambiguous for these
sections, in the absence of any strong stratigraphic subdivision. Despite over 100 m of
levee stratigraphy being well exposed, the broad trend throughout the measured
sections may also be unrepresentative of the overall growth history of the external
levee, as the uppermost portion has been removed by subaerial erosion (marked by a
palaeosol and overlain by the Palaeocene fluvial Sepultura Formation). Perhaps the
most distinctive trend to be drawn from these sections is the general decrease in the
6
standard variation of bed thickness with stratigraphic height which is a common
feature of all the measured sections (Fig. 4B).
3.2.3. Sedimentary structures and their distribution
In the outer external levee sedimentary structures indicative of deposition from and
tractional reworking by waning turbidity currents demonstrate a channel-proximal to
channel-distal evolution from parallel lamination, to climbing ripple lamination, to
ripple lamination. Starved ripple trains persist for tens of metres in the channel-distal
outer external levee. Close to the cut crest of the external levee, overturned ripples are
found. Low down in the inner and channel-proximal outer external levee stratigraphy,
climbing ripple lamination is relatively common, but on the whole the external levee
is dominated by upper-stage parallel lamination and ripple cross-lamination.
Convolute lamination is rare, with the notable exception of thick (0.5-3 m) sandstone
beds found at the base of the external levee succession.
In the inner external levee, palaeocurrents are variably aligned with those
obtained from thalweg deposits. Outside of the crest, in the external levee,
palaeocurrents vary from approximately crest aligned in channel-proximal areas to
more scattered in the channel-distal areas (Kane et al., 2007). Closer analysis of
overbank flow behaviour by examining grain fabric suggests the progressive rotation
of palaeoflow from at first strongly divergent to the channel, to later being more
closely aligned (although still divergent) (Kane et al. 2010b). In broad terms,
overbank flow over external levees was relatively consistently aligned at first down
the levee backslope and then down the regional slope; spatial and temporal variability
of palaeocurrents is progressive rather than abrupt (see internal levee section).
Erosional structures at the bases of beds are rare, with the exception of a few
small flutes (noted from ~ 1000 m of centimetre-scale logging). Larger-scale erosion
and amalgamation has only been observed in sections which have undergone
considerable deformation, which are confined to a zone of deformation marking the
channel-belt margin (see below).
The spectacular deformation of the Canyon San Fernando system inner
external levee (inside the cut crest) has been documented by Morris and Busby-Spera
(1990) and Morris and Normark (2000). Large slide blocks above rotational surfaces
indicate that the external levee probably had a relief of at least 100m (Dykstra and
Kneller, 2007). Strata within slide blocks are commonly coherent suggesting
7
relatively short transport distances (Fig. 3). Within the deformed zone, multiple
episodes of deformation and healing are indicated by rotated onlap surfaces and
ponding of debris flow and hemipelagic sediment within the deformation topography.
In the outer-levee of the external levee, deformation is uncommon and restricted to a
few sub-metre scale slump structures (Kane et al. 2007).
3.3. Rosario Formation: Internal levee
3.3.1. Bed thickness distributions
In general beds extend from channelised facies into internal levee deposits, potentially
over distances of > 1 km (Kane et al., 2009). In contrast to the outer external levee,
some beds may thin or be cut out over relatively short distances. Beds which thin over
short distances (metres) may simply pinch-out, or may be relatively large-scale low
amplitude long wavelength bedforms (Fig. 5). Outcrops of this facies are within the
range of 1 to 10 metres thick and the determination of bed thickness trends is
therefore hindered. Bed thickness exhibits much more stratigraphic variability
between beds than the outer external levee (Fig. 6). The inner internal levee of the
Playa Esqueleto sections interfingers with coarser-grained channel belt fill and is
often erosionally truncated in its upper parts, suggesting that preservation of any bed
thickness trends may be incomplete. Some small scale cycles of thinning or
thickening upwards have been identified but have not been studied in detail (Fig. 5).
Lateral accretion surfaces within conglomerate bodies indicate channel migration
dominantly towards the east, suggesting that these deposits (to their west) may be
inner-bend internal levee deposits (e.g. Schwarz and Arnott, 2007). However, these
conglomerate bodies are interpreted as relatively small-scale gravel bedforms, rather
than the large meander bends envisaged by Schwarz and Arnott (2007) and described
by Deptuck et al. (2003).
3.3.2. Sedimentary structures and their distribution
Internal levee deposits of the Pelican Point outcrops documented by Dykstra and
Kneller (2009) feature laterally extensive parallel-laminated, ripple and climbingripple cross-laminated beds. Internal levees of the Playa Esqueleto outcrops (Kane et
al., 2009) are dominated by ripple cross-lamination and upper-stage lamination.
Convolute lamination is particularly common in internal levees, and specifically
within parts of levees inferred to have been deposited close to the point of overspill.
8
Commonly, internal levee palaeocurrent indicators suggest multiple flow
directions, with as many as six apparent reversals recorded in one bed (albeit from 2D
exposure) (Kane et al. 2009).
Small flutes (< 30 mm in length) are found on the bases of beds. Larger flutes
(100-300 mm in length), or scours up to half a metre across, or larger, are less
common but are found within the internal levees of Canyon San Fernando (Fig. 5).
More intense erosion leading to sandstone amalgamation is uncommon, but has been
observed within the internal levee of Pelican Point (Fig. 5).
Intra-bed erosion surfaces and multiple fining upwards trends within
individual beds are common in internal levees throughout the Canyon San Fernando
sections (Fig. 5). In the Playa Esqueleto sections these are common and may be
associated with palaeocurrent changes across the intra-bed erosion surface (Kane et
al., 2009). Scouring also occurs at the tops of sandy parts of individual turbidites, the
erosion surfaces are then draped by Td and Te siltstones and claystones,
demonstrating that the erosional event may have occurred during the same event.
Other areas where minor sandstone amalgamation occurs are generally restricted to
inner internal levees, including at the margins of erosional features, such as channel
cut banks, and where minor slumping has occurred.
In the internal levee, deformation is less common than in the external levee,
and where present tends to be less pervasive in nature. In the Playa Esqueleto outer
internal levee section, several beds are slightly deformed and overlie a slide surfaces
indicating transport away from the interpreted thalweg (Kane et al. 2009). Elsewhere
in the Canyon San Fernando outcrops, which are potentially of internal levee origin
(although this is not as well constrained as at Playa Esqueleto), small (metre-scale)
syn-sedimentary slumps are found in thin-bedded facies in close association with
channel thalwegs (Fig. 5).
3.4. Permian Laingsburg Formation, Karoo Basin, South Africa
The Karoo Basin, South Africa, is interpreted as a thermal sag basin (Permian) that
transformed to a retroarc foreland basin (Triassic) (Tankard et al., 2009) (Fig. 7). In
the Laingsburg depocenter the 1.3 km-thick Permian Ecca Group comprises a basal,
claystone-prone succession (van der Merwe et al., 2009), overlain by a sand-prone
basin-floor to slope succession (Laingsburg and Fort Brown Formations respectively;
9
Grecula et al. 2003; Sixsmith et al. 2004; Flint et al. this volume) (Fig. 8). Mapping
and correlating the deep-water stratigraphy in the Laingsburg area is aided by the
regional exposure of hemipelagic claystone units which maintain constant thicknesses
(15-50 m-thick) where not eroded (Fig. 7C, 8). These claystones separate fine-grained
sandstone-prone units (Units B-F) that can vary abruptly in thickness and lithofacies.
Most of the sandstone-prone units contain one or two internal claystones (1-5 m-thick;
Fig. 8) Beneath several of the sandstone-prone units, and within the regionally
extensive hemipelagic claystones, are thin (1-10m-thick) and laterally extensive
sharp-based and sharp-topped fine-grained sandstones that are informally referred to
as ‘interfans’ (Fig. 8). These are interpreted as intraslope lobes based on their facies
and geometry (Figueiredo et al. 2010; Flint et al. this volume).
Large-scale erosion surfaces (kms wide, >80 m deep) in the sandstone-prone
units are marked by abrupt lateral lithofacies and architectural changes. Within the
erosion surfaces is a complicated stratigraphy with smaller-scale erosion surfaces of
different magnitudes (5-50 m deep), which are adjacent to laterally extensive thinbedded and fine-grained successions. This relationship is seen repeatedly at several
stratigraphic levels (Units C, D, E, and F; Fig. 8) (Figueiredo et al. 2010; Flint et al.,
this volume). The areas of complicated stratigraphy are interpreted as submarine
channel belts that underwent repeated episodes of cutting-and-filling of small-scale
channel features (Grecula et al. 2003; Figueiredo et al. 2010; Flint et al., this volume).
The thin-bedded and fine-grained successions are interpreted as external levees where
the deposits form a demonstrably constructional wedge (Figueireido et al. 2009; Flint
and Hodgson, this volume). To document this wedge geometry required the
identification of a palaeo-horizontal lower datum, and the measurement of multiple
sedimentary sections. In most cases this horizontal datum is an interfan (Fig. 8).
Thin-bedded deposits within the fills of the Laingsburg Formation channel
belts are interpreted as either channel-margin deposits or internal levee deposits.
Channel margin deposits change thickness and lithofacies over short distances (<50
m) and onlap onto erosion surfaces. However, significant portions of channel-belt-fills
comprise thin-bedded siltstones and very-fine-grained sandstones that only gradually
change thickness or lithofacies over 100’s of metres. These deposits are adjacent to
vertically stacked channel complexes and lie within the composite channel-belt
erosion surface. These are interpreted as internal levee deposits as they are not
10
confined by channel-complex scale erosion surfaces, but are (largely) confined by the
channel belt erosion surface.
3.5 Laingsburg Formation: external levee
3.5.1. Bed thickness distributions
Typically, the constructional wedges adjacent to channel-belts in the Laingsburg
system are up to 80 m-thick and 10 km wide. They comprise thin-bedded and
siltstone-prone deposits, a low proportion of sandstone, (<10%), fine away and thin at
a rate of ~5-10 m/km away from the channel belt (Fig. 9). Channel proximal outer
external levee environments are characterised by the thickest and most sand-prone
beds (0.2-0.5 m thick fine-grained sandstone beds; Fig. 10). Commonly, lenticular
(<200 m wide and <0.5m thick) fine-grained sandstone beds with steep climbingripple lamination, locally stoss-side preserved, are present (Fig. 11D). Siltstone and
very-fine-grained beds are apparently tabular over 100’s metres (Fig. 11A). Typically,
channel distal outer external levee sub-environments only have thin (<0.1 m)
sandstone beds at their base, but dominantly comprise thin-bedded tabular (at outcropscale) siltstones (<5% sandstone) (Fig. 12A).
In any 1-D section there is an overall fining- and thinning-upward to the
stratigraphy (Fig. 8), with fine-grained sandstone beds concentrated toward the base
of the levee successions in channel-proximal external levee areas (Fig. 9). However,
this is not a simple or uniform trend in decrease grain-size and bed thickness. There is
a smaller-scale packaging of strata (~5-10m), with thickening- and thinning upward
packages, and more monotonously stacked thin-bedded siltstone units (Fig. 11A,
12C).
3.5.2. Sedimentary structures and their distribution
In the fine grained sandy parts of the levee, ripple cross-lamination and climbingripple cross-lamination are common. Parallel-laminated sandstone is less common. In
general the sedimentary structures indicate high energy flow conditions. Large-scale
bedforms are present in the upper parts of the Unit D channel distal outer external
levee, and suggest flow at up to 90° to the channel belt orientation (Fig. 12B). In
channel-proximal areas palaeoflow data typically record flow parallel to the
orientation of the channel belt at the base of the levee, and more oblique toward the
levee top surface. Channel distal palaeoflow measurements typically show a trend
11
from sub-parallel to highly oblique to the channel belt. Typically, bed bases are sharp
with small grooves. Well developed flutes are observed where fine-grained turbidites
lie immediately on claystone (e.g., base C1). Most external levee crests identified in
the Laingsburg Fm. are demonstrably cut crests, and inner external levee deposits are
primarily remnants of outer external levee settings (Fig. 10). The inner external levee
environments are prone to remobilisation with slide scars and contorted, often steeply
dipping strata commonly found on the channel belt side of the cut crest.
Constructional crests, separating the inner external levee and the outer external levee
have not been unambiguously identified, however, a candidate for this relationship
may be present in the eastern margin of the Unit D channel belt at the C/D ridge (Fig.
9). In channel-distal external levee settings small syn-sedimentary slumps have been
identified (Fig. 12D).
3.6 Laingsburg Formation: internal levee
3.6.1. Bed thickness distributions
In the Laingsburg system, successions interpreted as internal levees are up to 80 mthick, and overlie low relief but laterally extensive erosion surfaces Typically, internal
levee deposits occur in the aggradational phase of the channel belt evolution, i.e. in
the younger stratigraphy.
Internal levees comprise thin- and very-fine-grained sandstone and siltstone beds that
gradually change thickness and lithofacies over 100’s of metres away from the
adjacent channel complex-fill. This contrasts to thin-bedded channel-margin deposits
that change thickness and facies over 10’s metres and onlap onto erosion surface.
Pringle et al. (in press) describe a rare example in Unit C where a constructional crest
built during channel aggradation such that sandstone beds pinched out onto a
constructional form. Overall, internal levees are more sand-prone than the external
levee. Generally, in any 1-D section a weak fining- and thinning-upward trend is
present, but this is less apparent than in external levee deposits, and no small-scale
packaging has been identified.
3.6.2. Sedimentary structures and their distribution
Interbedded cm-thick siltstone beds and fine- to very-fine-grained sandstone beds
form the internal levee lithofacies association. Typically, the sandstone beds contain
climbing-ripple-lamination (10-20°) with local ripple stoss preservation, and parallel
12
lamination. Commonly, climbing-ripple lamination recorded from sandstone beds low
in the internal levee stratigraphy indicates strong variability in palaeoflow between
beds, and evidence for changes in flow direction during a single depositional event.
Overall, the dominant palaeoflow direction is obliquely away from the adjacent
channel complex. Erosional and deformational structures are rare in internal levee
settings apart from grooves on bed bases.
4. Discussion
Once a levee interpretation has been applied to a particular stratigraphic unit, it is
useful to have criteria with which to differentiate levee sub-environments, for
example, internal vs. external levees, or inner external levee vs. outer external levee.
This is of particular importance in core studies where the distinction has significant
implications for palaeoenvironmental reconstructions and predictions of sand body
distribution, geometry and connectivity. In comparing and contrasting the
observations from the Rosario and Laingsburg systems some criteria to aid
identification in other systems are discussed.
4.1. Sedimentological character of external and internal levees
Studies of the external and internal levees Rosario and Laingsburg reveal a number of
shared characteristics which are indicative of levee environment and subenvironment, which are generally independent of grain-size (Table 1). The
sedimentological differences between internal and external levees can be accounted
for by a simple model of overbank flow between external levees (confined overbank
flow), and overbank flow over the top of external levees (unconfined overbank flow)
(Fig. 14). Confined overbank flow from underfit channels (sensu Deptuck et al., 2003)
between external levees will be subject to reflection and deflection from channel belt
topography, and interaction of overspill from different parts of the (potentially
sinuous) channel. Not only will this affect local flow trajectories (and palaeocurrent
records), but it will impart an unsteadiness on flow over any given point on the levee,
potentially leading to intra-bed erosion surfaces and/or multiple fining up units within
individual event beds. During the later stages of channel-belt-filling depositional
processes in the internal and external levees may be similar. In Unit C at the C/D
ridge in Laingsburg, for example, ultimately the internal levee built above the external
levees (Fig. 13). This would mean that the internal levees grew and extended beyond
13
the original confinement. A similar style of evolution, has been recognised by Mayall
and Stewart (2000).
In contrast, unconfined overbank flow is largely controlled by gravity forces
as the flow overspills, and potentially the development of a broad overbank wedge of
fluid moving directly down the regional slope gradient (Peakall et al., 2000; Kane et
al., 2010b). The deposits of unconfined overbank flow may therefore, in lieu of any
topographic interactions on the levee (e.g., Hiscott et al., 1997), record uniform to
temporally and spatially rotational palaeocurrents, and sedimentary structures
characteristic of simple waning flows. In addition, external levee crests tend to have a
lower degree of sinuosity downchannel (than internal levees), as they are formed by
overspill from multiple thalwegs active through time. In comparison, internal levee
crests may be relatively sinuous as they bound individual channels (e.g., Deptuck et
al., 2003; Posamentier, 2003). Accordingly, overspill from external levees may have a
relatively consistent direction of spill, whereas overspill from internal levees has
highly variable trajectories dependent on the local crest orientation.
The gravitational collapse of flows overtopping external levees may lead to a
drop in flow capacity (sensu Hiscott, 1994) inducing deposition across the grain-size
spectrum, and initially the deposition of poorly sorted units (i.e., Bouma’s (1962) Ta),
particularly in lower parts of external levee stratigraphy. Continued overspill results in
tractional reworking of these deposits into Tb and Tc divisions, consequently Ta beds
are poorly represented in the external levee, and tend to be restricted to thicker beds
lower down in the stratigraphy when flows were less confined, e.g., at the base of the
Rosario external levee (Fig. 4). Flow over the external levee is therefore initially
strongly depositional, and wanes through time; this character of the flow explains the
lack of erosional structures both at the bases of beds and within beds, and the
prevalence of climbing ripple lamination, with local stoss-preserved bedforms. Softsediment deformation in the outer external levee is of a relatively small-scale and
local. In contrast, the inner external levee is commonly characterised by extensive
slide and slump deposits that lie inside cut crests.
In both the Rosario and Laingsburg systems there is a progressive fining and
thinning in the external levee from the channel-proximal to channel-distal positions.
This supports the view that most sediment in each overspilling flow is deposited
closer to the crest, and that over time the levee builds a wedge. One difference is that
sandstone beds in the Laingsburg external levee are more lenticular than the Rosario
14
system. Where lenticular sandstone beds are observed in the Laingsburg system they
tend to occur in packages with each bed shingled or offset. More work is required, but
these may mark constructional crests, and/or silt-prone sediment waves that are
commonly identified on the modern seabed on the outside outer bends of external
levee (e.g., Normark et al., 1980; Normark et al., 1983; Migeon et al., 2001; Wynn
and Stow, 2002; Nakajima and Satoh, 2001; Lewis and Pantin, 2002; Heiniö and
Davies, 2009). Sediment waves have potentially been identified within putative levee
deposits of the outcropping Cerro Toro Formation (Chile) (Beaubouef, 2004; Crane
and Lowe, 2008).
In summary, the distribution of erosional features, the vertical grading
character, and the palaeocurrent complexity within individual event beds is
significantly different between internal and external levees despite both settings
sharing characteristics of rapid deposition.
4.2. Levee Stacking patterns
Large scale thinning-upwards cycles within external levees have been recognised
from the Laingsburg Fm. (Figuereido et al. 2010), and a complex thinning then
thickening upward pattern is recognised from the Rosario (Kane et al., 2007). Also,
smaller-scale packages (1-10 m-thick) of thinning and thickening upwards trends are
recognised in the external levees of both systems. The recognition of smaller-scale
trends remains somewhat ambiguous for both systems. Perhaps the most distinctive
trend to be drawn from external levee sections within the Rosario and the Laingsburg
is the general decrease in the standard variation of bed thickness with stratigraphic
height, i.e., bed thickness becomes less variable and more organised with stratigraphic
height. In both the Rosario and Laingsburg Fms., internal levees tend to be less
organised; although a general thinning upwards trend may be recognised from some
internal levees, these weakly defined trends can be interrupted by anomalously thick
sandstones giving an overall unorganised appearance.
Thinning upwards of beds within levee sequences has widely been reported
(e.g. Walker, 1985; Normark et al., 1997; Beaubouef, 2004), whereas thickening
upwards trends have been reported elsewhere (Cronin et al. 2000; Maejima et al.,
2006; Kane et al. 2007), but are typically attributed to submarine fans, or other
progradational systems (e.g. Mutti & Ricci Lucchi, 1972; Ricci Lucchi, 1975: Hiscott,
15
1981; Mutti, 1984; Walker, 1984; Waldron, 1987; Shanmugam & Moiola, 1988;
Pickering et al, 1989).
Because levees are built by deposition from overspill of partially channelised
turbidity currents, overbank deposition is controlled primarily by the relationship
between levee height and flow thickness, determining the volume of overspill. Thus
stratigraphic variability of turbidite thickness is explained by some change in either,
or both, of those factors, which may be autogenic or allogenic (Hübscher et al., 1997;
Deptuck et al. 2003). Variation in the ratio of intra-channel and extra-channel
deposition can be an autocyclic stratigraphic response. For example, in bypass
dominated systems, thalweg aggradation may be retarded with respect to the levee,
hence as levee relief increases, flows become more confined and, given a relatively
narrow range of flow sizes, the volume of overbank flow and deposit thickness
decreases with stratigraphic height. Alternatively, if the thalweg channel migrates,
one external levee will become progressively closer to the point of overspill, whereas
the other will become most distant. This may lead to asymmetry in the volume and
number of flows the levee experiences, and may be recorded by asymmetric levee
relief and stacking patterns, with pronounced thinning-upward at the increasingly
distal external (and vice versa). However, the same stratigraphic response of the levee
may occur by allocyclic flow magnitude variation, in the above case through
decreasing flow magnitude. In both the autocyclic and allocyclic cases the
stratigraphic response of the levee may be one of thinning upwards, even if the overall
system response may be one of progradation (autocyclic bypassing case) or
retrogradation (allocyclic decreasing flow magnitude case), with different
connotations for a sequence stratigraphic interpretation.
In the Cerro Toro Formation, Beaubouef (2004) relates thinning upwards
trends within a levee sequence to the gradual decrease in both flow magnitude and
frequency within the channel, both allocyclic controls. Conversely, thinning upwards
trends within the Amazon channel-levee are interpreted to arise autocyclicly through
increasing levee height in a bypass dominated system (Hiscott et al., 1997; Normark
et al., 1997). The thickening upwards trends of the Rosario levee could be attributed
to more rapid aggradation of the channel belt, or to increasing flow magnitudes
through time, or both. The relationship of stacking patterns to autocyclic or allocyclic
trends, and the relationship of these patterns to intra-channel (or intra-channel belt)
16
processes and deposits remains ambiguous and clearly is a direction for further
research efforts.
Finally, in both systems the thickest and most sand-prone part of external
levee successions are typically confined to the lower few metres where 0.2-1.0 m
thick beds with climbing ripple lamination, cross-bedding, and scouring are observed.
This observation calls into question the timing of formation of a constructional wedge
fed by overspilling flows. Figureido et al. (2010) and Flint et al. (this volume)
interpret thick-bedded sands at the base of the external levee succession as remnants
of precursor or frontal lobes (compare to Fig. 4). This interpretation implies that
external levees are a composite of different depositional environments and
sedimentary processes. An additional factor that may locally complicate bed thickness
trends is the development of crevasse splays at various levels within the levee
sequence.
5. Conclusions
A simple nomenclature is proposed to help the definition and comparison of different
external and internal levee sub-environments and morphologies (Fig. 1). The
sedimentological differences between internal and external levees can be accounted
for by a simple model of overbank flow between external levees (confined overbank
flow), and overbank flow over the top of the external levees (unconfined overbank
flow) (Fig. 14). Confined overbank flow between external levees will be subject to
reflection and deflection from the inner external levees, and to the effect of overspill
from different parts of a potentially sinuous channel. Not only will this affect
palaeocurrent records, but will impart flow unsteadiness over any given point on the
levee, potentially leading to intra-bed erosion surfaces and/or multiple fining up units
within individual event beds.
In contrast, unconfined overbank flow is largely controlled by gravity forces
as the flow overspills, and potentially the development of a broad overbank wedge of
fluid moving directly down the regional slope gradient. The deposits of unconfined
overbank flow may therefore, in lieu of any topographic interactions on the levee,
record uniform to progressively swinging palaeocurrents, and sedimentary structures
characteristic of simple waning flows, i.e., partial Bouma sequences (Bouma, 1962).
17
Outer external levees are characterised by: laterally extensive beds which taper, fine,
and change facies gradually away from the channel; large-scale thinning or thickening
upwards trends superimposed by smaller-scale thickness trends; bed thickness
becomes less variable and better organised with stratigraphic height; sedimentary
structures indicating waning flow conditions of relatively simple surges;
palaeocurrents which suggest progressive flow evolution; very few erosional
structures; large-scale deformation at the channel-belt margin, smaller (metre-scale)
features in channel-distal areas (Fig. 15).
Internal levees which form inside of the cut- or constructional external levee crest are
characterised by: variable lateral extent of beds, locally with sudden pinching out;
poorly-defined vertical bed thickness trends but with some small scale trends of
thickening or thinning; sedimentary structures indicative of complex unsteady
overspill, multiple fining sequences and erosion surfaces within individual beds;
palaeocurrents are often complicated and suggestive of overspill from different points
of the channel and recombination of older and younger overspill; erosional structures
are much more common, sometimes leading to bed amalgamation; deformation is
restricted to small slumps and slides (Fig. 15).
Stratigraphically, internal levees are found in the most aggradational parts or phases
of channel-belt filling. In part, this is because during the early stage of channel-belt
evolution component channels migrate horizontally, so preservation potential is low,
but the later stages of channel-belt-fill are aggradational. In contrast, the bases of
external levees can comprise remnants of precursor lobes indicating that they form, in
part, during system degradation and development of the channel-belt.
An important area of further research is the controls on vertical bed thickness trends,
which have been well documented in many channel-levee systems but not definitively
linked to the depositional regime, in terms of allocyclic forcing (sediment flux) or
autocyclic system responses (levee relief to flow height, channel migration etc).
6. Acknowledgements
IK thanks Prof Ben Kneller and Dr Mason Dykstra for introducing the Rosario
Formation during his PhD research. DH thanks colleagues in the Stratigraphy Group,
18
University of Liverpool for field discussions and data collection. This paper was
written whilst IK was in receipt of postdoctoral funding from Statoil, who are
gratefully thanked.
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Figure 1. A) Simplified channel-levee morphology and proposed nomenclature for
levee subenvironments. Note that the boundary between channel-proximal and
channel-distal environments is gradational. B) Proposed nomenclature of internal
levees.
Figure 2. A) Location of the Rosario Formation study area, within Canyon San
Fernando near El Rosario, Baja California, Mexico. B) Detailed map of the Canyon
San Fernando area, with previous studies highlighted. C) Stratigraphy of the of the
Rosario Embayment (modified after Morris and Busby-Spera, 1990). D) Simplified
palaeogeography of the Canyon San Fernando system of the Rosario Formation
(modified after Kane et al., 2009).
Fig. 3. External levee of the Rosario Formation. A) Overview photograph: the person
in the foreground is walking over the deformed section facing into the channel belt
(on the right of the photo). In the midground is a section of older underlying channel
belt and internal levee facies of the Playa Esqueleto outcrops. Forming the horizon is
an extensive exposure of the external levee from channel-proximal (right) to channel
distal (left). B) Deformed external levee facies, note person for scale. C) Close up of
deformed levee facies showing slumps and slide blocks. D) Undeformed channelproximal external levee facies. This section is capped unconformably by a Palaeocene
fluvial conglomerate.
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Fig. 4 A) Simplified schematic composite log through canyon fill facies into the
overlying external levee confined system. B) Vertical standard variation of bed
thickness from two characteristic logs; Log D is in the channel proximal external
levee whilst Log C is closer to the levee crest, this is reflected in the organisation of
bed thickness, which increases away from the channel. C) Schematic cross section
through the external levee with representative sandstone thickness trends
superimposed (Logs A-D), modified from Kane et al. (2007).
Fig. 5. Internal levee of the Rosario Formation. A) scour at the base of levee turbidite
(arrowed). B) Lenticular sandstone (arrowed) which generated depositional
topogography and is truncated on its downstream end (right). Note the overlying
trains of starved ripples picked out by iron concretions. C) Slump fold with ponded
sandstones filling the slump topography (immediately beneath the hammer handle).
Note the overlying conglomeratic thalweg deposit which truncates the internal levee
facies. The close spatial relationship of these facies is common in the channel belt fill
of the Canyon San Fernando channel-levee complex. D) opposing ripple crosslamination is common within the internal levee – as many as six apparent reversals
recorded in one bed. E) Small scale thinning and fining upward cycle. F) Scour
surface cutting through probable internal levee (scour surface cuts from left to right,
marked by arrows at its base).
Fig. 6. Measured sections through the internal levee at Playa Esqueleto. A & B are
both outer internal levee sections, with ‘A’ being relatively more channel-distal than
‘B’. C) Inner internal levee, in close association with conglomeratic channel thalwegs.
These sections correspond to Measured Sections 3, 4 and 6, respectively, of Kane et
al. (2009).
Figure 7: (A) Location map for the town of Laingsburg, Western province, South
Africa. (B) Simplified geological map around Laingsburg and Matjiesfontein. The
map
pattern
highlights
the
post-depositional
E-W
trending
folds.
The
lithostratigraphic units of interest, the Laingsburg and Fort Brown Formations, are
shaded. (C) Aerial photograph of the Baviaans syncline, with the location of
correlation panels in Figures 9 and 10.
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Figure 8: Schematic lithostratigraphy of the Vischkuil, Laingsburg, and Fort Brown
Formations, with broad depositional environments that show the overall
progradational pattern of the succession. The measured section is through Units B/C,
C and D, which are separated by hemipelagic claystone. In this location, at the SW
end of the Baviaans ridge, Unit C and D are thin-bedded and interpreted as external
levees. Submarine levees have been identified in Units E and F (Figueiredo et al.
2010).
Figure 9: (A) Correlation panel through Units C and D at the SE end of the Baviaans
ridge (see Fig. 7 for location). Note the fining- and thinning of Unit C and D to the
east, and the B/C interfan, which is used as a palaeohorizontal datum. Both units form
constructional wedges of thin-bedded sediments that are interpreted as external levee
succession. The cut crest (Unit C), and possible constructional crest (Unit D) are
labelled. Note the sandstone-prone C1 sub-unit, which is in part interpreted as a lobe.
Unit C is cut by the Unit D aged channel belt. The lower and upper claystones in C
are used as markers to identify the contact between the C channel belt and the external
levee to the NW. (B) The same correlation panel but without the horizontal scale
change.
Figure 10: Correlation panel through D in the central part of the Baviaans ridge where
the Unit D channel belt cuts into Unit C. The panel shows the cut crest, and the inner
external and outer external levee of Unit D. The crest is nearly twice as thick as at the
SE margin (Fig. 9). The steep inner external levee surface is interpreted to be due to
mass movement into the channel belt. Note that the external levee is largely above the
underfilled entrenched channel belt. Key same as Figure 9.
Figure 11: Representative photographs from proximal external levee settings. (A)
Sub-unit C2 on the northern limb of the Baviaans syncline. Metres-scale fining- or
thinning- upward packages can be identified in an overall fining- and thinning-upward
succession. Very-fine and fine-grained sandstone beds are lenticular, and climbing
ripple laminated, and comprise 10-20% of the succession. (B) Cross-bedded finegrained sandstone with scouring, typical of the lower part of Sub-unit C1. The basal
parts of proximal external levee successions are commonly the most sandstone-prone.
Compass for scale. (C) Large-scale tractional and accretionary bedforms composed of
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individual thin siltstone and very fine-grained sandstone beds. Camera lens for scale.
(D) Fine-grained sandstone in proximal levee setting with steep climbing ripple
lamination, and stoss side laminae preservation, indicating high rates of deposition.
Figure 12: Representative photographs from distal external levee settings. (A)
Coarsening- and thickening-upward then fining- and thinning-upward at the base of
the Unit D distal external levee at the NW part of the Baviaans ridge. This is the most
sandstone-prone part of the succession (<5%). (B) Large-scale asymmetric bedforms
in the upper part of the distal external levee indicating palaeoflow at a high angle (90
degrees) to the trend of the channel belt. Dilute flows are interpreted to be strongly
influence by the slope of the external levee. Bag for scale. (C) Aggradational thinbedded siltstones in the Unit D external levee. (D) Evidence of small-scale softsediment deformation. Note the change in bed thickness across the folds to the left.
Pen for scale.
Figure 13: Correlation panel of the eastern internal levee of Unit C in the C/D ridge
that overlies a widespread low relief erosion surface. Individual correlated beds
demonstratte their lateral continuity. The top of the aggradational channel complex
and the internal levees are palaeotopographically higher that the external levee, which
suggests that the internal and external levees merge. The internal levee is cut by the
Unit D erosion surface to the east. Inset photographs illustrate (A) a lack of clear bed
stacking, and (B) a sandstone bed with climbing ripple lamination.
Figure 14. A) Plan view of unconfined overspill vectors associated with relatively
consistent flow over the external levee. Palaeoflow indicators suggest slightly
opposing flow directions associated with deflection from the inner levee. Overspill
tends to take the form of simple surge like flows, depositing normally graded
turbidites with structures indicative of waning flow. B) overspill associated with
internal levees is much more complex due to the sinuosity of the levee, and other
channel-belt topography, combined with reflection and deflection from the inner
external levee. Inner levee turbidites record this with internal erosion surfaces,
multiple fining upwards events within event beds, and complex palaeocurrent
histories.
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Fig. 15. Summary sedimentological models of A) external levees and B) internal
levees. See text discussion.
32