Brettle Et Al 2001

Journal of the Geological Society, London, Vol. 159, 2002, pp. 379–391. Printed in Great Britain.

379

Identifying cryptic tidal influences within deltaic successions: an example from the

Marsdenian (Namurian) interval of the Pennine Basin, UK

M.J. BRETTLE1, D. MCILROY

1, T. ELLIOTT

1, S .J. DAVIES

2& C.N. WATERS

3

1Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, UK

(e-mail: [email protected])2Department of Geology, Bennett Building, University of Leicester, University Road, Leicester LE1 7RH, UK

3British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

Abstract: Research literature abounds on the depositional processes and products associated with macro-tidal

regimes, whereas there is little available literature on sediments deposited in micro-tidal regimes. This paper

presents new field-based sedimentological interpretations of the Marsdenian (Namurian, Carboniferous)

interval of the Pennine Basin, a basin-fill that is classically regarded as the archetypal fluvial-dominated delta

system. This paper reinterprets discrete lithostratigraphic units, and suggests they were deposited under the

influence of weak tidal currents. We highlight three lithofacies that contain tidally influenced deposits within

the Marsdenian interval of the Pennine Basin; a cross-bedded sandstone with mud drapes and reactivation

surfaces, a heterolithic ripple-laminated sandstone with muddy drapes and silty mudstone interlaminations,

and a rhythmic–parallel-bedded sandstone with mudstone–siltstone interlaminations. Evidence for cryptic

tidal signatures in tractionally transported and reworked sediments is qualitative, and largely dependent on the

sedimentologist’s view of what constitutes a diagnostic number of tidal indicators (i.e. mud-drape couplets,

reactivation surfaces). In areas away from either tractional deposition or reworking, sediments deposited from

suspension are more likely to preserve indicators of tidal processes. This paper focuses on a lithofacies

interpreted as a tidally influenced sand-rich delta-front mouthbar deposited from a buoyant effluent plume.

Time-series analysis of laminae thickness variations in this facies implies that these variations are rhythmic.

We review how the interaction of tidal currents and buoyant plume processes modifies depositional products.

This model implies that the rhythmic variation observed in the Marsdenian interval is attributed to the

modulation of plume deposition by tidal currents with a semi-diurnal and diurnal tidal periodicity.

Keywords: Marsdenian, Pennine Basin, deltaic sedimentation, tidal currents.

The characteristics of ancient macro-tidal successions have been

well documented in the sedimentological literature (Ginsburg

1975; Visser 1980; Allen 1981; Siegenthaler 1982; Demowbray

& Visser 1984; Kvale et al. 1989; Williams 1989; Allen 1991;

Nio & Yang 1991; Read 1992; Martino & Sanderson 1993;

Miller & Eriksson 1997; Adkins & Eriksson 1998; Greb &

Archer 1998; Fenies et al. 1999), whereas investigations of

micro-tidal environments are mainly restricted to present-day

examples (e.g. Pejrup 1986, 1988; Allen 1991; Hughes et al.

1998; Makaske & Augustinus 1998; Fenies et al. 1999;

Johannesson et al. 2000; Hossain et al. 2001). The identification

of tidal signatures, either macro- or micro-tidal, in shallow-water

marine systems is significant, as they constrain sedimentary

successions within a palaeogeographical and environmental zone.

This is especially true when sequence stratigraphic concepts are

applied, where there is a potential for the identification of incised

valley fills that often contain tidally influenced facies.

The delta systems of the Marsdenian interval form part of the

clastic infill of the northern Pennine Basin (Fig. 1), in which

sedimentary provenance is inferred to have been dominantly from

the north or NE (Drewery et al. 1987). The isolation of the

Pennine Basin from oceanic water masses is suggested to have

resulted in the absence of tidal currents from the Namurian

interval of northern England (Collinson 1988). The inference that

tidal currents were not important has led to current interpretations

of Namurian deltas being based on facies models generated from

river-dominated deltas. Regionally correlatable ammonoid-

bearing marine bands account for a thickness of ,5% of the

stratigraphy, and are generally thought to represent the only

intervals of marine influence during deposition of the otherwise

fluvially dominated Namurian deltaic systems (Holdsworth &

Collinson 1988). This paper, however, describes tidally influenced

facies that occur within sandstone delta cycles (Fig. 2), suggest-

ing that the basin remained linked to oceanic waters for a greater

period than that suggested solely by marine bands. Evidence for a

tidal influence has been suggested in the Westphalian (Broadhurst

et al. 1980; Broadhurst 1988) and Namurian (Aitkenhead & Riley

1996; Archer & Kvale 1997) intervals. In this paper we present

facies, time-series and stratigraphic data suggesting that tidal

currents influenced deposition during the Marsdenian interval,

and present models explaining the processes that may be

responsible for deposition within such micro-tidal regimes.

The application of sequence stratigraphy in studying deltaic

systems, namely, the identification of sand-rich incised valleys

and correlative interfluve areas, is well documented in the Upper

Carboniferous units of northern England (Maynard 1992; Church

1994; Church & Gawthorpe 1994; Hampson et al. 1996, 1997;

Wignall & Maynard 1996). In addition to the identification of

tidal deposits, this paper places tidal deposits within a sequence

stratigraphic context. We compare Marsdenian tidal deposits with

existing tidally influenced valley-fill models (Allen 1991;

Dalrymple et al. 1992; Allen & Posamentier 1993; Zaitlin et al.

1994) and discuss the wider implications for the analysis of

deltaic successions influenced by micro-tidal regimes.

The data presented in this paper come from 42 borehole

records (mainly written descriptions dated between 1900 and the

present), five public-access wireline well data records (from

petroleum exploration wells, dated from 1950 to the 1980s) and

from more than 90 field localities (Fig. 1), from which logged

sections, scaled field-sketches and facies analysis were con-

structed (Fig. 3).

Pennine Basin geometry and stratigraphy

The geometry of the Pennine Basin during the Marsdenian

interval was inherited from Dinantian rifting, formed by back-arc

extension during the northward-directed Rheno-Hercynian sub-

duction (Gawthorpe 1987; Leeder & McMahon 1988). Extension

produced a series of structurally high ‘blocks’ and basinal

depocentres (Lee 1988). These were part-filled by turbidite-

fronted deltaic systems during early Namurian to Kinderscoutian

time, with sediment supplied from the decaying Caledonian–

Appalachian Mountains (Collinson et al. 1977). The Marsdenian

interval marks the onset of a period of predominantly shallow-

water, mouthbar deposition. After Marsdenian time the younger

Yeadonian and Westphalian intervals record eventual infill of the

Pennine Basin and the development of coal-forming delta-top

swamp conditions (Guion & Fielding 1988; Guion et al. 1995;

Waters et al. 1996). Two mouthbar-dominated cyclothems are the

focus of this paper: the lithostratigraphic units that we will show

to be equivalent to the Readycon Dean Flags and Midgley–

Helmshore Grit (Brettle 2001). Both cyclothems possess region-

ally extensive regressive basal surfaces, with overlying tidally

influenced fluvial deposits and tidally influenced mouthbar

facies.

Tidally influenced deposits within the Marsdenianinterval

Previous research suggests that fluvial processes dominated

deposition in the late Namurian Pennine Basin, and that links

with open oceans were tenuous except at marine band intervals

(Collinson 1988). This paper describes three lithofacies that

suggest tidal processes operated in addition to fluvial processes

during the Marsdenian interval: (1) a cross-bedded sandstone

with mud drapes and reactivation surfaces; (2) a heterolithic

ripple-laminated sandstone with muddy drapes and silty mud-

stone interlaminations; (3) a rhythmic–parallel-bedded sandstone

with mudstone–siltstone interlaminations.

Lithofacies 1: cross-bedded sandstone with mud drapesand reactivation surfaces

This lithofacies comprises coarse- to fine-grained sandstone with

disseminated granules and pebbles, and thin muddy drapes. This

lithofacies is similar to cross-bedded sandstone lithofacies within

which it is often interbedded, but is distinguished by the presence

of mud drapes on toeset and foreset surfaces, and a greater

abundance of reactivation surfaces within bed-sets (Fig. 4a). The

drapes comprise micaceous–muddy laminae that often, but not

always, occur in pairs, up to 5 mm apart. When traced, mud

drapes in some cross-bed sets occur in bundles that are

rhythmically spaced (Fig. 4b).

Interpretation: tidally influenced cross-bedded sandstone

Cross-bedded sandstone is ubiquitous within the Namurian

interval of the Pennine Basin, and is often very micaceous. This

facies distinguishes itself by the presence of paired muddy

drapes, bundles of mud drapes, and reactivation surfaces suggest-

ing modulation of flow regime; potentially by either tidal

processes (Nio & Yang 1991) or rapid changes in river dis-

charge.

Either allocyclic variations in fluvial discharge or the mouth-

ward progradation of the fluvial system may explain the inter-

bedded relationship of this facies with the cross-bedded

sandstone. If tidal processes were responsible for the formation

of muddy drapes and reactivation surfaces, then the inertia

associated with the basinward flow of a body of fluvial water

Fig. 1. Map illustrating the northern Pennine Basin, Marsdenian

exposure and the localities named in the text. Stratigraphic position of

the Marsdenian interval within the Carboniferous period with marine

band nomenclature is taken from Riley et al. (1993).

M. J. BRETTLE ET AL .380

could have suppressed any tidal signature and allowed erosion of

sedimentary evidence for tidally influenced deposits.

Lithofacies 2: heterolithic ripple-laminated sandstonewith muddy drapes and silty mudstoneinterlaminations

Within parts of the Helmshore Grit, lithofacies comprising thin

parallel-laminated micaceous siltstone to fine-grained sandstone

interlaminated with grey to black micaceous silty mudstone are

common. These often overlie distal bayhead delta mouthbar

facies, and commonly have a sharp erosive base. Sedimentary

structures are delicate, and comprise interlaminated fine-grained

sandstone with a muddy siltstone matrix. Asymmetric starved

ripple cross-laminations with common flow reversals and sym-

metrical wave-ripples characterize these siltstones, whereas re-

activation surfaces are common in the sand-rich portions (Fig. 5).

Proportions of sandstone to siltstone can vary, and mica is

abundant in both sandstone- and siltstone-rich components of

this lithofacies. The ichnofauna includes Planolites isp., Curvo-

lithus isp., Rosselia isp. and rare Chondrites isp. Similar

ichnofaunal assemblages have been described throughout the

Silesian deposits of the Pennine Basin (Eager et al. 1985), and

have been attributed to brackish environments. Fragments of

carbonaceous debris are locally common.

Interpretation: tidally reworked bayhead mouthbardeposits

The layer-parallel, interlaminated fabric, high mica content and

small size of the cross-laminations imply that this facies was

deposited dominantly directly from suspension, but was subject

to frequent low-velocity currents and reworking. The variable

proportions of fine sandstone to siltstone may be dependent on

the proximity of the point of deposition to the locus of sediment

transport or local fluctuations in flow velocity. This facies

represents deposition in a shallow-water mouthbar front, where

the substrate is affected by wave or current reworking in the

shallow water depths. Current-ripple laminations with bimodal

flow indicators represent periods of flow reversal during deposi-

tion. Subtle laterally discontinuous reactivation surfaces are

subparallel to bedding, and are commonly truncated and over-

lain by ripple structures. Bayhead mouthbars form thin and

laterally extensive sheets in shallow water, where sediment entry

points (either mouthbar feeder channels or crevasse splays) are

often close together (Van Heerden & Roberts 1988). The

interaction of sediment input from several close entry points

may provide conditions where flow reversals are generated.

Alternatively, flow reversals may have been caused by ebb and

flood tidal influences affecting the mouthbar front during

deposition. The ichnofauna suggests a brackish water column

(Eager et al. 1985), corroborating the interpretation of a tidal–

marine influenced depositional environment. The input of fluvial

waters into either brackish or marine waters suggests that

flocculation may have been a significant mechanism responsible

for the deposition of the clay-sized suspended fraction in such

environments.

Tidal winnowing of mouthbar sediment

Whereas sediments deposited by plumes have a high preservation

potential, those in the submerged mouthbar prodelta are con-

Fig. 2. Schematic cross-section through

study area with current lithostratigraphic

units, facies associations and stratigraphic

position of Figs 4, 5, 7, 8, 9 and 10.

MARSDENIAN CRYPTIC TIDAL SIGNATURES 381

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M . J. BRETTLE ET AL .382

stantly reworked. This implies that although flow reversals are

common, cyclic laminated successions are rarely preserved. Tidal

processes constantly winnow the substrate surface in the sub-

merged mouthbar front, and resuspend mud and silt into the

water. Once suspended, landward movement of the water into

shallower areas during the flood cycle forces the flow to

accelerate the water column (Fig. 6) (Wiseman et al. 1986). The

subsequent ebb cycle allows suspended sediment in the shallower

water to return to deeper parts of the system, where it begins to

settle through the water column. The reworking of the sand

bedload, in association with the settling of mud from suspension

during tidal slacks, deposits an interlaminated sand–mud lithol-

ogy.

Lithofacies 3: rhythmic–parallel-bedded sandstonewith mudstone–siltstone interlaminations

This lithofacies comprises fine- to medium-grained sandstone

with a mudstone matrix, and interlaminations of mudstone and

siltstone. Lamination and bedding surfaces are subparallel,

whereas beds are massive, laterally persistent across exposures

and range between 0.05 and 0.1 m in thickness (Figs. 7 and 8).

Rare flute and tool marks are seen on bedding planes. Trace

fossils present in this lithofacies include Olivellites isp. and

Pelecypodichnus isp.

A good example of this lithofacies is seen at Kebroyd Bridge

(SE04452120), where an exposure with subhorizontal bedding

reveals cyclical variation in bed thickness on a centimetre and

Fig. 4. Lithofacies 1: cross-bedded sandstone with mud

drapes and reactivation surfaces. (a) Trough cross-bedding

with bed set comprising double micaceous mudstone

drapes and reactivation surface (section oblique to foreset

dip). Similar lithologies occur in isolated examples

throughout the lower Marsdenian interval, suggesting a

cryptic tidal (ebb-dominated) influence. Slab from

Midgley Grit, Moselden Heights Quarry (SE044163). (b)

The planar cross-bedded set in the lower half of the image

has three zones with a number of muddy laminae. These

could be mud drapes deposited from suspension during

tidal slack water. ‘Mud-couplets’ as described by Visser

(1980) are not observed, probably because the ebb regime

dominates in such fluvial systems. The ebb cycle has a

high potential to erode mud drapes formed during slack

water and sandstones formed during subordinate current

stages (i.e. the weak asymmetrical tide of Nio & Yang

(1991)). Taken from Fosters Delph Quarry (SE022273)

(locality 29, Fig. 3).


metre scale (Fig. 7). A repeated pairing, of 0.01 m scale, of

thin–thick laminae is clear (Fig. 8), and can be seen on the

laminae thickness bar charts (Figs. 9, and 10b and c). On this

scale, bed thickness varies from 0.02 to 0.1 m on a c. 25–28 bed

cycle, and individual beds commonly possess a micaceous silty

lamination (1–5 mm thickness) on their upper surface. The base

of this lithofacies commonly lies either with a sharp contact or

erosively on the silty interdistributary bay or offshore facies. The

upper surface is either truncated by shoaling mouthbar–distribu-

tary channel facies, or overlain by a flooding surface and

offshore marine–interdistributary facies.

Interpretation: tidally influenced sand-rich mouthbar

The dominance of layer-parallel bedding suggests this facies was

deposited from suspension. Variations in sediment grain size are

evident from the rapid spatial and temporal variations in the

proportion of mudstone to siltstone. This facies is typical of

sediments deposited in a mouthbar environment, where suspen-

sion-deposited sands are interbedded with rare tractionally trans-

ported sand (compare flute and tool marks). The cyclical laminae

thickness variation seen at Kebroyd Bridge (SE04452120) sug-

gests a rhythmic fluctuation in the rate of suspension deposition.

It seems likely that either increased fluvial discharge or the

progradation of a fluvial channel over its associated mouthbar is

responsible for the presence of the large-scale internal scour

surfaces, rather than the subtle cyclic or paired laminae. These

broad scour features are overlain by cross-laminated sandstone,

implying high-flow discharge and the input of bedload-trans-

ported sand. Similar facies have been described in the Carboni-

ferous sequences of the Pennines and the Appalachian Basin, and

have been interpreted as deposited by tidally influenced currents

(Broadhurst 1988; Read 1992; Aitkenhead & Riley 1996).

To determine whether the laminae are truly cyclical, Fourier

time-series analysis is used to ascertain whether the succession

contains periodic components. Fourier time-series analysis was

run on sections 1 and 3 by A. Archer, using the same Fourier

analysis program as was used to analyse Kinderscoutian sections

(Fig. 10b, c and e) (Archer & Kvale 1997). The short length of

the input data string, along with the apparently ‘noisy’ nature of

the data, implies that the output of the Fourier transform is not

as refined as that of the Kinderscoutian sections. This is because

periodicities greater than 10 laminae cannot be resolved in the

sections detailed here, owing to the short length of the dataset.

Both datasets have similar ranges of harmonic output when

compared with the Fourier transform results from laminae in the

Kinderscoutian deposits, and the results of the analysis from

sections 1 and 3 share broadly similar peaks and troughs (Fig.

10e). The harmonic wave output of sections 1 and 3 falls into

two frequency groups, of 1.9 2.6 and 3.6 4.4, suggesting an

output equating approximately to two and four laminae.

Although periodicities greater than 10 laminae cannot be

resolved because of the short length of the dataset, periodicities

between the thickest laminae in sections 1 and 3 (43 and 48

laminae, respectively) may be invoked (Fig. 10b and c).

Processes occurring during deposition from an effluentplume and the influence of tides on a plume

We need to consider the processes that operate within the

mouthbar if we are to understand how tidal currents may

influence deposition from an effluent plume. Tidally influenced

Fig. 5. Lithofacies 2: (a) heterolithic

ripple-laminated sandstone with muddy

drapes and silty mudstone interlaminations.

(b) Slab of rock from Warland Wood

Quarry (SD947202; locality 6, Fig. 3)

revealing reverse ripple lamination, and a

back-flow ripple, that appears to have

grown up the lee slope of an older ripple.

This flow reversal, along with the

abundance of mud drapes, suggests a tidal

influence to this facies.


plume processes are not as well documented as models of tidally

influenced duneform deposition (e.g. Visser 1980; Allen 1981).

Variations in the amount of fluvial effluent entering the basin

modulate the amount of sediment transported into the mouthbar

(Fig. 11a). In basins with marine waters a ‘saline wedge’ forms

during periods of low effluent velocity, when density difference

between plume and basinal waters allows underflow of the saline

wedge (Nemec 1995). During periods of low discharge the saline

wedge is a significant feature, propagating 15–20 km upstream

from the outlet source, and being displaced from the channel

only during periods of very high discharge (Wright & Coleman

1974). Within the distributary channel the relatively static nature

of the saline wedge inhibits seaward bedload transport, and the

mixing of fluvial and marine waters creates turbulence and forces

the bedload into suspension (Nemec 1995). On the interface

between fluvial and marine waters, waves of turbulence (internal

waves) form, and these propagate mouthward. The increased

turbulence entrains bedload within the water column and carries

it to the mouthbar, where it is deposited (Ewing 1950; Wright &

Coleman 1974). If the internal waves within the plume are stable

features, the passage of alternating bodies of turbulent and non-

turbulent water may also have the potential to deposit rhythmi-

cally laminated sediment. As the plume passes over the shoaling

mouthbar and the saline wedge, it is compressed, undergoes a

hydraulic jump and becomes non-turbulent. The resultant ‘buoy-

ant plume’ creates a strong density layering, which is elongated

by the outward-flowing effluent, and carries sand-sized clasts into

the distal mouthbar (Wright 1977).

Tidal-current modulation influences the position of the saline

wedge within the outlet channel and therefore inertial force of

the plume (Fig. 11b). Flood tides suppress outflow as fluvial

water is banked up within the channel. In the Mississippi, periods

of high tide are shown to correlate with increased episodes of

crevasse splaying in the upstream fluvial system. This suggests

that tidal fluctuations can hold up fluvial outflow and lead to

banking up of water within the channel (Andorfer 1973). De-

creasing fluvial outflow forces vertical mixing within the chan-

nel, and allows the saline wedge to migrate upstream (Fig. 11a;

low discharge). This increases outflow turbulence, depositing

sediment in proximal mouthbar, and creating a headward shift in

grain-size distribution. During the ebb tide (Fig. 11b; ebb tide),

the inertia of the outflowing plume is enhanced, creating a

mouthward shift in grain-size distribution. At periods of slack

water, increased mixing of the saline wedge with fluvial waters

from the distributary channel decreases effluent outflow rates,

accelerating flocculation of clay particles and their deposition

from suspension. The hypothetical grain-size distribution of a

tidally modulated low-discharge plume (Fig. 11b) has the

potential to generate the repeated thin–thick sandstone couplets

Fig. 6. Processes responsible for the development of lithofacies 2. The

ebb (a, e) and flood (c) tidal currents entrain bedload, and resuspend

clay-grade material, whereas the shallowing bay area amplifies the

velocity during the flood tide, further entraining and resuspending

sediment from the bay floor. Deposition of clay-grade material occurs

during both high (b) and low (d) slacks. This may result in a gross

decrease in clay-grade material in the distal area, if tidal range is

sufficiently high and slack water durations are short. Modified from

Wiseman et al. (1986) and Nemec (1995).

Fig. 7. Lithofacies 3: rhythmic–parallel-

bedded sandstone with mudstone–siltstone

interlaminations. Photomosaic of Kebroyd

Bridge locality (SE045213; locality 33, Fig.

3), showing unconformable base of ‘sharp

based’ mouthbar. Broad internal erosive

scours should be noted within the tidally

influenced sand-rich mouthbar, suggesting

erosion by fluvial processes during high

distributary discharge (asterisks denote

position of measured sections 2 and 3; see

Figs. 9 and 10).


with intercalated silty mudstone laminae seen in the Marsdenian

field examples (Figs. 8–10).

The grain size of sediment in the fluvial channel also controls

laminae thickness characteristics in the sediment. In the case of

the Marsdenian mouthbar facies, the sediment has a bimodal

grain size, comprising very fine- to medium-grained sandstone

and siltstone or silty mudstone, which is commonly rich in mica

flakes. Whereas the muddier component of this sediment is

carried in suspension, the coarser, sand-rich portion is carried by

either tractional or mixed saltation–suspension processes. By

inference, both mud and sand components are transported during

periods of higher flow regime (i.e. the ebb tide, and to a lesser

extent the flood tide), with the sand-rich component being

rapidly deposited during periods of waning flow. The mud and

silt fractions are deposited during periods of either no or very

low flow regime (i.e. during tidal slack water).

Within plume deposits, thick–thin pairs of sandstone laminae

are formed by the ebb and flood cycles within a single semi-

diurnal tidal cycle, with the thicker sandstone laminae interpreted

to have been deposited during the ebb stage (Fig. 11b). During

the ebb period, flow regime increases as fluvial inertia and

outgoing tide both flow in a downstream direction. The resultant

higher flow regime increases the amount of entrained coarser

clastic sediment (typically very fine- to medium-grained sand-

Fig. 8. Example of lithofacies 3 at Kebroyd Bridge (SE045212). (a) Tidally influenced sand-rich mouthbar facies showing fine-grained sandstone

interlaminated with micaceous laminae and silty mudstone. (Note thinning of sand-rich laminae upwards, towards the centre of picture, followed by

marked thickening in the upper and lower parts.) (b) Close-up of a succession with thicker sandstone laminae. Beds occur in repeated thick–thin pairs. In

some examples the thin lamina forms a thin veneer less than 1 mm thick, bounded by a thin micaceous lamina.

Base of distributarychannelDistributary channel

sandstone

1020

3040

50L

amin

ae n

um

ber

1020

3040

50

Lamina thickness (mm)Lamina thickness (mm) Lamina thickness (mm)

60 60

0 010 1020 2030 3040 4050 50

Kebroyd Bridgesection 3



60 01020304050

1020

3040

Markerbed

Markerbed

‘Sharp-basedmouthbar(see Figure 7)

Chondritesintense

bioturbation

increaseintensity of

bioturbationincrease in degree of

current ripplelamination

* Fig. 9. Graphic plot of laminae thickness,

for three sections at Kebroyd Bridge (see

Fig. 7 for position of these sections).

Marker beds represent beds traced laterally

along the exposure; these reveal a thinning

in the middle of section 2 by six sandstone

laminae. This may represent removal by

erosion during periods of high discharge.

Asterisk denotes position of image in Fig.

8a.


stone), and creates a seaward shift in the distribution of fine- to

medium-grained sandstone. When the tide is at its lowest slack

water, the lack of flow allows deposition of mudstone, or more

commonly in the Marsdenian deposits, a micaceous veneer when

viewed on laminae planes (Fig. 11b; low slack). The flood tide

reworks underlying deposits, which are deposited as the tidal

flow wanes, creating a thin muddy sandstone lamina that is

almost always thinner than the sandstone lamina deposited by the

ebb current. At high-tide slack water a second mudstone or a

micaceous lamina is deposited, completing the thick–thin, or

semi-diurnal tidal cycle (Fig. 11b; high slack). Therefore, any set

of mouthbar deposits influenced by tidal processes may possess

sandstone laminae producing repeated periodicities of two (Fig.

11b; logs A and B).

On any point of the Earth’s surface, two gravitational maxima

pass in any 24 h period, creating two semi-diurnal tidal bulges of

different magnitudes that are generated independently of each

other. The stronger, or dominant, tide is created by the gravita-

tional pull of the Moon on the ocean surface, whereas the

weaker, or subordinate tide, is formed by centripetal forces as the

Earth rotates and pushes the ocean surface away from the Earth.

The dichotomy in tidal range during a 24 h period is known as

the diurnal inequality. When this is applied to the model of

tidally influenced plume deposition, diurnal inequality should

generate a periodicity of four, as it occurs over two semi-diurnal

periods. Diurnal inequality should produce a thin–thick cyclicity

in alternate semi-diurnal tidal units (Fig. 12a), as the dominant

tide produces thicker sandstone laminae pairs and the subordi-

nate tide produces thinner sandstone laminae pairs.

As the Moon rotates around the Earth, the Sun and the Moon

fall into alignment (or syzygy) or lie perpendicular (or in

quadrature), on a 14 day periodicity (Fig. 12b). When the Sun

and Moon are in alignment, the tidal bulge is amplified by up to

30%, increasing tidal ranges and creating stronger (or spring)

ebb and flood tides. When the Sun–Moon system is in quad-

rature relative to the Earth, the combined gravitational forces are

not as strong, and the resultant tidal bulge is smaller, creating

weaker (or neap) tides. Therefore, when 28 or more semi-diurnal

lamina number

0

0

20

10 20 30 40 50 60 70 80 90(a)

(c)

(b)lamina number

60

0

20

40

10 20 30 40 50

10 20 30 40 50

lamina number

pow

er

spect

ral

densi

typow

er

spect

ral

densi

ty

10

10

-10

20

5

0

0

0

0

0.1

0.1 0.5

0.2

0.2

4.4 3.62.6

2.4both =1.9 1.3

1.2

0.6

0.3

0.3 0.7 0.9

0.4

0.4 0.8 1.0

2.35-2.072.86-2.675.0-4.6

8.93

19.61

30.30

frequency

frequency

(Archer & Kvale, 1997)

(this study)

Kebroyd Bridge Section 3Kebroyd Bridge Section 1

Key for v)

(e)

(d)

(Archer &Kvale, 1997)

(this study)

1.9- 2.63.6- 4.4

2.07- 2.864.6- 5.0

correspondingrange

43 lamina

48 lamina40

20

Fig. 10. Fourier analysis of lithofacies 3

laminae cyclicity. (a) Bar chart of thickness

of sand-rich mouthbar facies laminae, from

Aitkenhead & Riley (1996). Hag Farm

Borehole (Kinderscoutian), near Keighley,

Yorkshire, UK. (b, c) Bar charts showing

thickness of mouthbar facies laminae from

sections 1 and 3 of the Kebroyd Bridge

section (Fig. 9). Drawn to the same scale as

the bar chart of Aitkenhead & Riley (1996).

(d) Fourier analysis plot of data from (a) by

Archer & Kvale (1997). (e) Fourier analysis

plot of data from (b) and (c) by A. Archer

(University of Kansas), and corresponding

ranges between this dataset and that

displayed in (d).


units appear in a stacked succession of tidally influenced plume

deposits, and no hiatal or erosive surfaces are present, it may be

possible to resolve a cyclical motif associated with lunar

precession. When deposition is influenced by a spring–neap

cycle, periodicities of 56 laminae are expected (representing 28

semi-diurnal cycles deposited over 14 days). Deviations from the

expected cyclicity may occur in association with increased fluvial

discharge. In these circumstances, erosion of the substrate leads

to stripping of laminae and the generation of discontinuities

within the tidal mouthbar unit. Therefore, if a plume is tidally

influenced, cyclicity in sandstone laminae should be observed on

a semi-diurnal (two), a diurnal (four) and half-lunar monthly (56)

periodicities.

Discussion: the significance of identifying cryptic tidalinfluences and its significance in the Carboniferoussequence of the UK

The observation of systematic bed thickness variations has

suggested a tidal influence during mouthbar deposition, without

the necessity of identifying the presence of mudstone drapes.

Comparing results of the Fourier transform and the model for

tidally influenced plume deposition suggests that laminae peri-

odicities of two and four represent semi-diurnal tides and

unequal semi-diurnal tides over a diurnal period. The presence of

thick–thin–thick cycles over 43 and 48 laminae for sections 1

and 3 is suggestive of a spring–neap–spring cyclicity. Assuming

this represents a 14 day lunar cycle, the absence of a full set of

56 laminae may be due to reworking and erosion by fluvial

processes, which can be demonstrated by the presence of scour

features (Fig. 7). Conversely, the number of days in a lunar

month was marginally greater in the past (c. 30.5 � 1.5 during

Precambrian time compared with 28.5 at the present day

(Williams 1989), suggesting that more than 56 laminae would be

expected within the spring–neap–spring cycle. The critical

observation remains, however: that laminae systematically occur

in repeating sets of two (indicating semi-diurnal tides) and four

(indicating unequal semi-diurnal tides) with characteristic thick-

ness variations. The integration of Fourier time-series analysis

and the model for a tidally influenced mouthbar provides a useful

tool for the identification of cryptic tidal signatures within plume

deposits. Such deposits have a higher preservation potential than

sediments deposited within areas where higher flow regimes

could either rework or erode part of the succession.

The effect of tidal processes in settings dominated by macro-

tidal ranges is well documented in modern deltas (Wright &

Coleman 1973, 1974; Coleman & Wright 1975; Galloway 1975;

Coleman 1981; Wiseman et al. 1986; Allen 1991), whereas the

subtle influence of tidal processes on areas with lesser tidal

ranges is underrepresented in modern and ancient sedimentologi-

Fig. 11. Processes responsible for the development of a cyclic tidal signature in heterolithic mouthbar facies (after Nemec 1995). Only during the lowest

river discharges, when the saline wedge propagates headward allowing the lofting of sediment, do tidal currents enhance or suppress river outflow to the

extent that it influences the rate of deposition and grain size deposited.


cal literature. The presence or absence of a tidal regime may

have significant connotations for interpretation of sedimentary

systems, regardless of the size of the tidal range. Tidal regimes

have a significant effect on the coastline geometry (Wright &

Coleman 1973, 1974; Coleman & Wright 1975), grain-size

distribution at river mouths, or the cleaning or winnowing of

clay-grade particles from a succession (Orton & Reading 1993).

This may be particularly true with respect to the degree of clay

redistribution that may occur in settings influenced by micro-tidal

regimes (i.e. Fig. 6).

Amplification of tidal range within an incised valley may

explain how one might expect a tidal current to be amplified

(Allen & Posamentier 1993); but it does not take into account

the complexity of depositional processes that occur within the

valley; specifically, the influence of tidal processes on mouthbar

plume deposits, especially when involving the mixing of saline

and non-saline waters in a fluvial regime that may be fluctuating

in discharge (Wright & Coleman 1974; Nemec 1995; Hughes et

al. 1998a). The use of mud drapes and reactivation surfaces as

evidence for tidal influences is well reported (Ginsburg 1975;

Visser 1980; Allen 1981; Siegenthaler 1982; Demowbray &

Visser 1984; Nio & Yang 1991), but such structures may be

absent in micro-tidal regimes or areas with low deposition rates.

Within mouthbar-dominated incised valley fills the examination

of successions deposited by plumes may reveal the presence of

tidal currents.

Tidal deposits have been recognized in Southern North Sea

facies (O’Mara et al. 1999), whereas they have not in onshore

outcrop (Collinson 1988). To ascertain that tidal currents influ-

enced the Namurian Pennine Basin is regionally significant, as it

confirms that selected onshore depositional systems form analo-

gues for Carboniferous Southern North Sea reservoirs (Hampson

et al. 1997, 1999).

Conclusions

The Namurian Pennine Basin has been interpreted by many

workers as an example of the quintessential fluvial-dominated

delta. It has been suggested by most previous studies that the

Pennine Basin was geographically restricted from oceanic waters

and that tidal reworking had no effect on the deposition of

Namurian deltas. This paper has identified a stratigraphic interval

within the Namurian interval of the Pennine Basin that contains

facies that are influenced by tidal processes. We have identified

facies within the Namurian interval that demonstrate tidal

processes operated, and that the Pennine Basin must therefore

have been connected to oceanic waters throughout deposition

and not only during marine band deposition. This new interpreta-

tion requires that existing palaeogeographies are reassessed and

implies that alternative depositional models should be sought for

the Namurian interval.

The identification of cryptic tidal facies similar to those

described here in other basins may be useful in aiding the

identification of tidally influenced systems and ascertaining the

true extent of palaeogeographical connectivity with oceanic

water masses.

These findings form part of the Ph.D. thesis of the principal author. Thanks

are due to J. Bagshaw for access to Fletcher Bank Quarry (Marshalls), A.

Archer (University of Kansas) for running Fourier analyses on the datasets

described in this paper, N. Riley and J. Macquaker for their critical

reviews and A. Nuttall for assistance in the field.

Appendix: localities with examples of lithofacies 1–3

Examples of lithofacies 1 are observed at in the Woodhouse

Flags at Fosters Delph Quarry (SE022273; locality 12, Figs 3

Fig. 12. Model representing the

depositional patterns expected from the

tidally influenced plume during a diurnal

and lunar 14 day cycle. Approximately

every 24 h, any point on the Earth’s surface

is influenced by two tides (a): a dominant

tide created by the pull of the Moon, and a

subordinate tide produced by centripetal

forces that push the ocean surface away

from the land. The differing tidal strengths

are expressed by thinner (subordinate tide)

or thicker (dominant tide) semi-diurnal

units. The Moon revolves around the Earth

every 28 days. During this period it either

lies in alignment with the Sun (syzygy) or

perpendicular to it (quadrature) (b). During

alignment the combined gravitational pull

of the Sun and Moon increases tidal bulges,

generating a spring tide. When the Sun and

Moon are at quadrature, the combined

gravitational pull is less, and a weaker or

neap tide occurs. Tidal ranges are greater

during the spring tide, and tidally

influenced mouthbar laminae occur in thin–

thick packages every 56 tidal laminae (or

28 semi-diurnal laminae), equivalent to a

14 day period.


and 4a), the Midgley Grit at Moselden Heights Quarry, Scam-

monden (SE043164; Fig. 4b) and the Midgley and Helmshore

Grit at Fletcher Bank Quarries, Ramsbottom (SD805164).

Examples of lithofacies 2 are observed in the Helmshore Grit

at the upper part of Whittle-le-Woods Quarry (SD584217),

Warland Wood Quarry (SD947202), Harper Clough Delph

(SD716317) and Fletcher Bank Quarries, Ramsbottom

(SD805164).

Examples of lithofacies 3 are observed in the equivalent of the

Alum Crag Grit at Kebroyd Bridge (SE044212; locality 16, Fig.

3) and in the equivalent of the Alum Crag Grit and the Readycon

Dean Flags of the Noah Dale Core, Rishworth (SE019217;

locality 17, Fig. 3).

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Received 30 May 2001; revised typescript accepted 14 February 2002.

Scientific editing by Joe Macquaker


Technology

Brettle Et Al 2001