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 channel-

levee 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.

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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 cross-

section, 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 fine-

grained and thin-bedded deposits that are demonstrably related to overspill from

submarine channel-belts (e.g., Walker, 1975; Winn & Dott, 1979; Morris & Busby-

Spera, 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

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(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 grain-

size 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/channel-

opposing 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

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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).

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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

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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

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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

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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 climbing-

ripple 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.

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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 thin-

bedded 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 climbing-

ripple 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 outcrop-

scale) 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 climbing-

ripple 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

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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 m-

thick, 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 sub-

environment, 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. Soft-

sediment 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 channel-

proximal 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 cross-

lamination 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 fine-

grained 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 thin-

bedded siltstones in the Unit D external levee. (D) Evidence of small-scale soft-

sediment 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.

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