processes of soft-sediment clast formation in the intertidal zone

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Sedimentary Geology 18

Processes of soft-sediment clast formation in the intertidal zone

Jasper Knight *

Department of Geography, University of Exeter, Tremough Campus, Penryn, TR10 9EZ, UK

Received 21 April 2005; received in revised form 21 July 2005; accepted 1 September 2005

Abstract

Muddy soft-sediment clasts found on the sandy beach at Formby Point, north-west England, are formed by wave erosion of late

Holocene intertidal sediments that are exposed during summertime ridge and runnel development. Break-up processes of the

intertidal sediments are strongly controlled by pre-existing bedding and surface desiccation cracks. Erosion of the intertidal

sediments and formation of soft-sediment clasts contributes to the provision of fines into this dominantly sandy environment, but

loss of the archaeologically significant Holocene intertidal sediments is a potentially important management issue along this coast.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Formby Point; Intertidal sediments; Coastal erosion; Ridge and runnel; Sediment intraclast

1. Introduction

Soft-sediment clasts, also termed mud clasts or mud

balls, are discrete blocks of sediment that have been

eroded and transported intact from their original site of

deposition, and deposited elsewhere (Haas, 1927;

Picard and High, 1973). The preservation of soft-sedi-

ment clasts in the geological record has implications for

clast provenance and palaeogeography, and direction

and strength of palaeocurrents (Little, 1982; Diffendal,

1984; Hall and Fritz, 1984). Soft-sediment clasts that

are found in modern fluvial, marine, coastal and glacial

environments can also provide information on sediment

source area, erosional processes, and the mode, rate and

direction of clast transport (Knight, 1999). Despite

these wide environmental implications, however, the

processes and controls on the formation, transport and

0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sedgeo.2005.09.004

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E-mail address: [email protected].

emplacement of soft-sediment clasts remain poorly

known.

In coastal environments, three-dimensional sediment

bodies and coastal landforms can be considered as geo-

logically transient features with low preservation poten-

tial. An exception is where blocks of coastal sediment are

preserved as allochthonous intraclasts, providing valu-

able information on past environments and processes for

which there may be otherwise little or no record. When

eroded, these soft-sediment clasts can also contribute

substantially to local sediment budgets, provide organic

and/or fine grained sediments into environments which

might not otherwise have these sediment types available,

and release heavy minerals and/or contaminants into

ecologically sensitive locations. Despite these implica-

tions, however, few studies have considered more than

the occasional presence of soft-sediment clasts in the

coastal zone (e.g. Stanley, 1969; Hall and Fritz, 1984;

Kale and Awasthi, 1993; Tanner, 1996).

This paper has three main aims: (1) to describe the

morphology and processes of soft-sediment clast for-

mation from the coast of north-west England at For-

1 (2005) 207–214

Fig. 1. Location of the Formby study area in north-west England.

J. Knight / Sedimentary Geology 181 (2005) 207–214208

mby, Lancashire; (2) to evaluate the processes of and

controls on soft-sediment clast formation at this loca-

tion; and (3) to discuss the implications of soft-sedi-

ment clast formation for coastal dynamics, preservation

of the sediment beds from which the clasts are derived,

and for coastal conservation.

Fig. 2. Cross-section through the eroded seaward edge of the clay unit showi

Trowel for scale is 29 cm long.

2. Coastal setting of the study area

The Lancashire coast of north-west England, near

the town of Formby (53835V N, 3805V W), is located

along the mesotidal Irish Sea and adjacent to the Ains-

dale Dunes National Nature Reserve (Fig. 1). Coastal

ng the termination of V-shaped desiccation cracks at sediment laminae.

J. Knight / Sedimentary Geology 181 (2005) 207–214 209

landforms and late Holocene evolution of this coastline

have been described extensively (e.g. Steers, 1946;

Tooley, 1976, Atkinson and Houston, 1993; Pye and

Neal, 1993; Pye et al., 1995). This coastline mainly

comprises well-developed sand dune ridges (b25 m

high) and associated sand flats that are present up to

4 km inland (Steers, 1946; Pye and Neal, 1993). Dunes

at Formby Point, dated by radiocarbon and lumines-

cence methods to 3 kyr BP (Pye et al., 1995), have been

in an erosional regime for the past century (Pye, 1991).

The intertidal sand flat fronting the wide beach (Fig. 1)

is affected by the summertime development of ridges

and runnels (Parker, 1975). North of Formby Point, this

process episodically exposes older sediments otherwise

buried by beach sand, including organic-rich, laminated

intertidal sands, and estuarine silts and muds (Parker,

1975; Pye and Neal, 1993). This exposed sedimentary

unit (N0.7 m thick), informally termed the Scrobicu-

laria Clay, comprises planar-laminated sand to clay

beds, the upper surface of which is non-erosional and

planar with a slight seaward dip (28–58). The clay unit

crops out (a few hundred metres long, 8–12 m wide)

along the upper intertidal zone of the beach. In detail,

the clay unit comprises alternating sandy and silty

layers individually 2–8 cm thick (Fig. 2). Bulk samples

give a median sediment grain diameter of between

7.64–10.39 Am and modal diameter of 17.60–18.22

Am (n =3). The uppermost part of the exposed clay

unit is characterised by the presence of Scrobicularia

plana shells that are preserved vertically in their growth

position, usually with both valves intact (Fig. 3). Some-

Fig. 3. Intact shells of Scrobicularia plana, preserved in growth pos

times the shells are missing, leaving oval-shaped holes

that pit the upper surface of the clay unit, discussed

later.

Radiocarbon dates from shells still attached to the

sediment surface range from 310 to 5960 BP, suggest-

ing that some shells are in situ whilst others have later

bored into the sediments from above (Pye and Neal,

1993). The presence of the preserved non-erosional

surface of the Scrobicularia Clay unit suggests rapid

burial by wind-blown sand (forming the adjacent

dHillhouse CoastT; Tooley, 1976; Pye et al., 1995) ratherthan erosion during sea-level transgression which was

largely completed before this time (Tooley, 1982).

The Scrobicularia Clay unit is important because (1)

it provides evidence for mid- to late-Holocene sedimen-

tation in intertidal and estuarine environments prior to

adjacent sand dune formation; and (2) it contains im-

portant archaeological evidence for human occupation

during this time period. Animal and human footprints

and associated materials are developed in the Scrobi-

cularia Clay unit (and related sediments) at a number of

places in north-west England (Huddart et al., 1999).

These record the close relationship between human

occupation of the intertidal zone during the late Meso-

lithic and early Bronze Age, and contemporary envi-

ronmental and faunal changes at this time (Roberts et

al., 1996; Huddart et al., 1999).

The intact surface of the Scrobicularia Clay is erod-

ed due to wave undercutting when the unit becomes

exposed in the emergent runnels during the early sum-

mer. Soft-sediment clasts are formed as the clay unit is

ition on the clay unit surface. Trowel for scale is 29 cm long.

Fig. 4. Detached soft-sediment clasts within a U-shaped embayment in the clay unit. Note the steeply eroded cliffed edge of the clay unit. Beach

sand still covers the clay unit in the extreme top left of the photo. Trowel for scale (top left) is 29 cm long.


broken up. The morphology and characteristics of these

clasts are now examined in detail.

3. Description of soft-sediment clasts

The eroded seaward edge of the Scrobicularia Clay

unit is characterised by dheadlandsT that extend 2–3 m

in a seaward direction. These headlands are 1.5–2.0 m

wide, and located between the headlands (2.5 m apart)

are soft-sediment clasts that have broken off from the

main sediment body (Fig. 4). The clasts are detached

Fig. 5. Detail of empty Scrobicularia holes and crack pattern across t

along two different and distinct lines of weakness. (1)

Empty, oval-shaped Scrobicularia shell holes are locat-

ed randomly across the sediment upper surface (Fig. 5).

Many of these shell holes are also located at the inter-

sections of surface cracks. For example, in a typical 1

m2 sample area, 33 (55%) Scrobicularia holes are

found located within individual cracks or at the inter-

sections of cracks, compared to 27 (45%) holes which

are not located in cracks (n =60). This is despite cracks

occupying only ~4% of the surface area and Scrobicu-

laria holes only ~1.8%. These cracks usually link

he clay unit surface. The visible part of the pencil is 9 cm long.

Fig. 6. Plan view of joined and non-orthogonal desiccation cracks. Trowel for scale is 29 cm long.


together, forming 4 to 6-sided polygons (b15 cm

across). The cracks (b5 mm deep, 4 mm wide) are

usually straight, are the same thickness throughout

their length, and form a V-shaped wedge in section

that terminates or changes in thickness and direction

at laminae boundaries (Fig. 6). The crack-defined poly-

gons can be classified as joined and non-orthogonal

(Allen, 1987a) and are present across the exposed

sediment surface. (2) Tension cracks are developed in

some places along the margins and seaward-facing

front of the clay unit where soft-sediment clasts are

Fig. 7. Detailed view of tension cracks (to the right of the pencil) developing

from the clay unit. Pencil for scale is 15 cm long.

being detached from the main sediment body (Fig. 7).

These cracks, developed parallel to the erosional front

of the clay unit, are sometimes infilled with or overlain

by beach sand.

The majority of soft-sediment clasts (N99%) are

found adjacent (within 4 m distance) to the seaward

edge of the clay unit. Only rarely are clasts found

further away, and these tend to be small (few cm

diameter), blade- or disc-shaped, and well-rounded.

The shape characteristics of soft-sediment clasts

found adjacent to the clay unit were evaluated by field

parallel to sediment block margins during the process of detachment


observations and measurements. Two groups of clasts

were identified. Attached clasts are defined as those

clasts that are located immediately adjacent to (b30

cm distance) that part of the sediment unit from which

they were derived. Detached clasts are defined as those

located more than 30 cm away from the sediment unit.

Clast characteristics were described using a modified

Zingg–Powers scheme based on axial dimensions of the

clast’s a–b, b–c and a–c planes, and degree of clast

angularity. Results are shown in Table 1. Detached clasts

are, on average, 25%–31% smaller than attached ones.

Their shape characteristics, however, are almost identi-

cal and are dominated (75%–80%) by discs with minor

equant and bladed clasts (Fig. 8). There are also marked

differences in angularity between attached and detached

clasts (Table 1), with a statistically significant correla-

tion coefficient of �0.629. No clasts were observed to

be armoured with beach sand or any other material.

4. Processes of soft-sediment clast formation

The formation of soft-sediment clasts is closely re-

lated to the development of surface cracks in the clay

unit which act as lines of weakness along which the

soft-sediment clasts can become dissociated. The sub-

aerial location of these cracks, crack morphology, and

morphology of the crack-defined polygons suggest they

were formed by desiccation following summertime ex-

posure of the clay unit in emergent runnels (Allen,

1987a; Weinberger, 2001). The close association of

surface cracks with the position of Scrobicularia shell

holes suggests that these holes serve as areas of crack

nucleation. If most desiccation cracks develop in an

Table 1

Morphological characteristics of attached and detached soft-sediment

clasts at Formby

Characteristic Attached clasts Detached clasts

Mean a–b plane length (cm)

(range, standard deviation)

19.6 (32-7, 5.39) 14.7 (22-9, 3.35)

Mean b–c plane length (cm)


16.5 (25-6, 3.76) 11.4 (17-6, 2.81)

Mean a–c plane length (cm)


7.0 (15-3, 2.33) 5.3 (9-2, 1.80)

n 30 30

Angularity n % n %

Angular 13 43.3 0 0

Subangular 11 36.6 2 6.6

Subrounded 6 20.0 12 40.0

Rounded 0 0 12 40.0

Well-rounded 0 0 4 13.3

Total 30 99.9 30 99.9

upward direction from laminae boundaries (Weinber-

ger, 2001), then these shell holes likely decrease the

thickness of the sediment layer, making desiccation

more likely to take place (Fig. 9). Supporting this

model, plumose fracture patterns along crack margins

were also sometimes observed. The presence of cracks

that are infilled with beach sand suggests that some (at

least) are perennial features that are reopened upon tidal

or seasonal exposure. Hysteresis effects also mean that,

once opened, cracks do not close when re-covered by

water (e.g. Moore, 1914; Plummer and Gostin, 1981).

The termination of cracks at laminae boundaries

(e.g. Fig. 2) is consistent with observations elsewhere

(e.g. Allen, 1987b) and largely controls the depth of

clast a–c planes. Clast upper and lower boundaries are

defined by coarser laminae (here, coarse sand) which

are less prone to shrinkage, less cohesive, and more

prone to undercutting by wave erosion. Equant soft-

sediment clasts are likely to form where sediment layers

are thick and massive (long a–c plane), and discs where

sediments are more finely laminated (short a–c plane).

Tension cracks are caused by wave undercutting of

more competent (finer) layers on the seaward margin

of the clay unit. Wave undercutting is minimised,

through negative feedback, where this seaward edge

is protected by previously detached sediment clasts

(Fig. 4). Wave attack also ceases to be an important

process when the clay unit is covered over again by

beach sand in the autumn as the ridge/runnel system

breaks down.

Despite differences in size, the shape characteristics

of attached and detached clasts are almost identical

(Fig. 8), suggesting they are derived from the same

population. The size difference between the clast

groups reflects the time period over which clast erosion

has taken place (e.g. Karcz, 1969). Because all clasts

are still located near the source clay unit, erosion (and

decreased angularity) of clasts takes place mainly in

situ rather than during longshore transport. This con-

trasts with most other studies where coastal soft-sedi-

ment clast erosion is related to tidally driven movement

of the clasts up or across the shoreface (e.g. Stanley,

1969; Hall and Fritz, 1984; Kale and Awasthi, 1993;

Tanner, 1996). The fine sediments released during the

process of clast erosion are transported southwards

alongshore, contributing to muddy sediments that are

trapped within the runnels (Parker, 1975).

5. Discussion and implications

This study describes the processes associated with

the formation of soft-sediment clasts in the coastal

Fig. 8. Zingg plots of the axial proportions of soft-sediment clast that are (a) attached and (b) detached from the clay unit. N =30 in both cases.


zone. Important conclusions are that (1) pre-existing

fractures in the Scrobicularia Clay unit exert a domi-

nant control on clast size and shape, and (2) the clasts

Fig. 9. Sketch of a cross-section along a crack margin in the clay unit

showing the role of Scrobicularia shell holes as areas of desiccation

crack propagation, and the patterns of plumose fractures that are

developed outwards towards these shells holes (after Weinberger,

2001).

decrease in size and angularity largely in situ by loca-

lised wave-related attrition. This latter point contrasts

markedly with previous studies in which transport away

from the sediment source area causes clast erosion and

changes in clast shape (e.g. Haas, 1927; Knight, 1999;

Faimon and Nehyba, 2004). Despite the mesotidal

range and presence of wide intertidal zone along the

Formby coast (Fig. 1), the lack on longshore transport

may be due to the presence of the ridge and runnel

system which acts to dissipate incoming tidal and wave

energy (Kroon and Masselink, 2002).

The presence and dynamics of soft-sediment clasts

also demonstrate the importance of sediment reworking

in the coastal zone, in this case, the exhumation of sub-

beach material. As such, reworking of older sediments

by contemporary processes can (1) temporarily increase

sediment budgets, (2) add different grain size/shape/

mineral components into the coastal system, (3) release


nutrients, contaminants, heavy minerals, organics and

other materials, and (4) be used as tracers to evaluate

sediment transport and erosion rates and processes.

Allen (1987b) estimated that reworking of fine sedi-

ments in the Severn estuary (UK) amounted to 7%–

70% of total fluvial sediment input, suggesting that

such reworking of pre-existing sediment is a significant

component of coastal dynamics.

The exhumation of cohesive sub-beach sediments

also impacts on aspects of wave-energy dissipation and

wave runup, and therefore the effectiveness of beach

sediment as a buffer against coastal erosion. Similar

soft-sediment clasts to those described here can also be

sourced from other cohesive coastal sediments such as

machair, sand dunes, saltmarsh, beachrock, and intertid-

al and estuarine muds and peats. These environmentally

important sediment types are also sensitive to coastal

erosion and therefore require management strategies to

conserve biodiversity and, in the example of the Formby

coast, important archaeological evidence. Understand-

ing the processes of soft-sediment clast formation and

breakup is therefore a wide-ranging issue.

Infrequently, soft-sediment clasts are preserved in

the geological record as allochthonous intraclasts (e.g.

Little, 1982; Diffendal, 1984; Knight, 1999). In coastal

settings, burial by later beach sand may lead to mis-

leading interpretations of these intraclasts, such as

storm deposits, erosional lags, or post-depositional

cemented or pedogenic artefacts. Consideration of

clast source area, pre-depositional shape and palaeogeo-

graphy can help to more accurately interpret these

sedimentary features.

Acknowledgements

I thank the anonymous referees for their comments.

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processes of soft-sediment clast formation in the intertidal zone

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