Wright and Marriott 2007

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The dangers of taking mud for granted: Lessons from Lower Old RedSandstone dryland river systems of South Wales

V. Paul Wright a,b,⁎, Susan B. Marriott c

a School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE, United Kingdom

b BG Group, 100 Thames Valley Park Drive, Reading RG6 1PT, United Kingdomc School of Geography and Environmental Management, University of the West of England, Bristol, BS16 1QY, United Kingdom

Abstract

Mudrocks are a prominent feature of many ancient dryland successions but they are not always a product of the settling out of suspension load. From studies of the late Silurian–early Devonian Old Red Sandstone mudrocks of South Wales it has beenshown that many were not overbank sediments deposited from suspension on floodplains, but were emplaced as sand- and silt-sized aggregates transported as bed load and deposited in sinuous channels and as braid-bar complexes on multi-stagefloodplains in dryland river systems. Using the Old Red Sandstone examples criteria are provided for the recognition of similar deposits in the sedimentary record. One important aspect of these mudrocks is that they can represent multiple recycling eventsand can constitute condensed deposits that may be characteristic of closed alluvial basins with periodically limited sediment supply.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Mud aggregates; Dryland rivers; Old Red Sandstone; Vertisols

1. Introduction

The formation of secondary mud-grade matrices,through compaction of mud clasts or authigenic mineralformation is well established in studies of siliciclastic

sediments. However, such effects are generally restrictedin terms of the proportion of mud added to the succes-sion. In certain continental settings, mud-grade materialswere not deposited directly as suspension settle-out muds but were originally deposited as traction-load sand- and

silt-grade aggregates of mud-sized particles, that throughlater compaction, were restructured into a mudstone tex-ture. Such mudstones are an important component of some ancient dryland deposits. The aim of this paper is to provide criteria for recognition of such deposits in some

continental clastic successions and to draw upon our experience of examples from the Siluro–Devonian OldRed Sandstone of South Wales to highlight their impor-tance in assessing palaeoenvironments.

2. Clay aggregates in Quaternary soils

Sand and silt-sized, clay-silt grade aggregates (“clay pellets”) are a major feature of some modern fluvialsystems (Rust and Nanson, 1989; Maroulis and Nanson,1996; Gibling et al., 1998). These examples come from

Sedimentary Geology 195 (2007) 91–100www.elsevier.com/locate/sedgeo

⁎ Corresponding author. School of Earth, Ocean and PlanetarySciences, Cardiff University, Cardiff, CF10 3YE, United Kingdom.

E-mail address: [email protected] (V.P. Wright).

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the flood-dominated drylands of central Australia, but similar pelleted sediments are known from non-dry-land systems (e.g. the Red River, Manitoba, Canada,(Brooks, 2003)). They have also been recognized fromclay and mud ‘grains’ held together by salts in aeolian

deposits (Rust and Nanson, 1989). In the case of centralAustralia the pellets are produced in floodplain soilswith a high smectite content, typically associated withVertisol-type soils (although see Retallack, 2005, p.673). Such soils develop a fine, blocky to pelleted(crumb) structure in the upper parts of their profiles(McGarry, 1996; Mermut et al., 1996). Just a few wet-ting and drying cycles are enough to form small aggre-gates (Maroulis and Nanson, 1996), which have lowdensities and are readily entrained during flood events.Following transport, these sand- and silt-grade aggre-

gates are deposited and reworked as in-channel bedforms such as extensive braid bars. They also accumu-late on lateral accretionary bars or benches associatedwith anastomosing rivers (Gibling et al., 1998). Theintegrity of the pellets is quickly lost even during shal-low burial due to compaction (Rust and Nanson, 1989;

Maroulis and Nanson, 1996, their fig. 12). However,Rust and Nanson (1989) noted that these aggregatesmay be preserved if isolated in a framework of quartzgrains that would not undergo much compaction andalso recognized macroscale bedforms that indicated

transport of the aggregates as bed load. Retallack (2005) provides a general discussion about the preservation of clay aggregates.

Despite the poor preservation potential of such ag-gregates in Holocene floodplain sediments, their former presence has been recorded or inferred from a variety of ancient deposits (Rust and Nanson, 1989; Ìkes, 1993;Talbot et al., 1994; Gierlowski-Kordesch and Rust,1994; Marriott and Wright, 1996; Gierlowski-Kordesch,1998; Tanner, 2000; Gierlowski-Kordesch and Gibling,2002; Marriott and Wright, 2004; Müller et al., 2004;

Marriott et al., 2004). Müller et al. (2004) have recordedwell-preserved mud aggregate pellets, both in Vertisol profiles and as reworked material, from the Upper Tri-assic dryland alluvial successions of the Lunde Forma-tion of the northern North Sea. Retallack (2005) hasquestioned the interpretations of some of these

Fig. 1. Lower Old Red Sandstone outcrops in South Wales, UK. Localities from which examples are shown in Figs. 4–

8, FWW–

Freshwater West;FWE – Freshwater East; LL – Llansteffan.

92 V.P. Wright, S.B. Marriott / Sedimentary Geology 195 (2007) 91 – 100



examples, favouring an origin for the aggregates for theTriassic examples documented by Talbot et al. (1994)and Müller et al. (2004) not from Vertisols but fromreworked kandic soils (deeply weathered tropical soils);

this seems to ignore the fact that both examples comefrom arid dryland successions with prominent Vertisol palaeosols, and in the case of the latter, the aggregatesare found in situ in Vertisol profiles.

3. Significance of mudrocks formed as aggregated

bed load deposits

The occurrence of tabular mud-grade sediment bodiesin alluvial successions is usually interpreted as the pro-duct of suspension load fallout in overbank (floodplain)settings. Interbeds of mud-grade material in channel deposits are commonly inferred to be slack water deposits

indicating an irregular flow regime. In both cases, deposi-tion of mud aggregates transported as bed load, followed by compaction, could produce the same results.

One consequence is that it is possible to create non-

stratified mudrocks that, by virtue of lacking stratifica-tion, resemble pedogenically-destratified suspensiondeposits. Müller et al. (2004) have named this effect “ pseudo-pedodestratification”. In these cases, bed loaddeposits could be mistaken for weakly developed palaeo-sols. Such materials are pedoliths, that is redeposited soils(Retallack, 2005), although it would be difficult to es-tablish that all such aggregates were formed in soil pro-files. Bed load aggregate deposits can be further affected by pedogenesis leading to sediment recycling (Marriott and Wright, 1996), whereby aggregates are removed fromthe upper horizons of soils, transported and deposited as bed load on a floodplain, to be incorporated into a newsoil

Table 1Stratigraphy of the Lower Old Red Sandstone of south west Wales, UK

93V.P. Wright, S.B. Marriott / Sedimentary Geology 195 (2007) 91 – 100



profile. This type of recycling will be an especiallysignificant process in systems starved of sediment such asthe Channel Country of central Australia, where floodplain aggradation rates are as low as 0.04 mm/year (Gibling et al., 1998). Sediment starvation also limits the

input of coarser grade sediment, resulting in relatively thin packages of multi-cycle mudrocks. This means that themudrock is not the product of a single phase of floodplainaggradation but hides a long and complex history of recycling. It represents a type of condensed facies that may be characteristic of basin-fills of drylands with lowrelief hinterlands, especially inland drainage basins wherefine sediments are effectively trapped in the basin (e.g. theChannel Country cratonic basin, Australia (Gibling et al.,1998)) and continental rift settings (e.g. the HartfordBasin, USA (Gierlowski-Kordesch and Gibling, 2002)).

Fluvial systems with long-term aggradation rates aslow as those of the Channel Country should producefloodplain lithosomes with well-developed soils and palaeosols. This is a consequence of the fact that thedegree of pedogenesis a unit of floodplain sediment undergoes is the inverse of the deposition rate (Marriott and Wright, 1993). However, the recycling processescreate unstable surfaces preventing major soil develop-ment. Many modern and ancient dryland successionscontain calcic and petrocalcic soils and palaeosols (cali-che or calcrete) (e.g. Machette, 1985; Blodgett, 1988;

Müller et al., 2004). The time-dependent stages of development found in these units have been used todevelop simple estimates of sedimentation rates (Leeder,1975; Wright and Marriott, 1996). The absence or weakdevelopment of these pedogenic features would nor-

mally be used to indicate relatively short residence timesin the solum and hence high sedimentation rates. It would be critical in such cases to establish whether thelack of strongly developed features represents a short period of soil development, or whether recycling hasoccurred. The ability to recognize former aggregatedeposits would be essential for this purpose.

The availability of mud aggregates for entrainment and transport as bed load in alluvial systems may result in a high concentration of mud layers in fluvial channelsandstones, and hence permeability baffles or barriers to

later fluid flow.

4. Lower Old Red Sandstone mudrocks

Thethick, Upper Silurian (Pridolian)–Lower Devonian(Emsian) Lower Old Red Sandstone successions of SouthWales (Fig. 1. Table 1), which are up to 4.5 km thick, arecomposed mainly of alluvial sediments. These comprise of 80% mudrocks, with relatively thin sandbodies and tuff beds that act as regionally correlative markers (Allen andWilliams, 1982). Earlier studies had revealed not only the

Fig. 2. Two main associations of mudrocks in the Lower Old Red Sandstone that contain petrographic evidence of former mud aggregates.




presence of well-developed Vertisol palaeosols (Allen,1986), but also that reworking of the floodplains was arelatively common process, resulting in distinctiveintraformational (calcrete-clast) conglomerates (Allenand Williams, 1979; Marriott and Wright, 2004). Marriott

and Wright (1993, in press) were able to document detailed histories that indicate polyphase development of floodplains in the dryland systems of the Old RedSandstone. Using evidence from soils that had been buried and later exhumed and reactivated, they demon-strated that, for selected stratigraphic units, the sequences built up from multiple episodes of aggradation anderosion. With abundant Vertisol palaeosols and evidence,from the presence of intraformational conglomerates andreactivated soils, of periods of soil erosion, the discoveryof deposits composed largely of clay-silt aggregates was

in some ways to be expected (Marriott and Wright, 1996).The initial examples discovered represented sinuouschannels but subsequent studies (Marriott and Wright,2004; Marriott et al., 2004) revealed that mudrocksoriginating as aggregated sediments were also depositedas sheet-like bodies, probably as braid-bar complexesduring major floods. Their studies showed that a signifi-cant proportion of the mudrock volume in the Lower Old Red Sandstone was likely to have been deposited as bed load, mud aggregates. Additional reworking of floodplain surface sediments (i.e. pedogenic aggregates)

occurs during dust storms in the dry season in theChannel Country (M. Gibling 2002, personal commu-nication) and elsewhere in the Australian region (Hesseand McTainsh, 2003) and may have been an important process of redistribution in the Lower Old Red

Sandstone. The importance of bed load transport of aggregates in the Siluro–Devonian mudrocks is testa-ment to the ability of the pellets to survive at least sometransport, but might also reflect the likely absence of avegetative cover at this time favouring the presence of areadily erodible active surface soil layer (Marriott andWright, in press).

The types of associations of mudrocks deposited asaggregated sediments are summarized in Fig. 2.

5. Criteria for the recognition of former mud

aggregated deposits

Rust and Nanson (1989), Gierlowski-Kordesch(1998) and Gierlowski-Kordesch and Gibling (2002)have provided criteria by which mud aggregates may berecognized at the microscale and have given valuableexamples from fluvial successions of Carboniferous,Triassic and Jurassic age. In addition, it has been observedthat aggregates may be preserved by early cementation, particularly by calcite. Ripple forms in mudrocks contain-ing early calcite cement were found in the Jurassic East

Fig. 3. Features suggesting the role of aggregates in mudrock deposition based on the Lower Old Red Sandstone, South Wales.




Berlin and Shuttle Meadow Formations (Gierlowski-Kordesch and Rust, 1994; Gierlowski-Kordesch, 1998).Müller et al. (2004), from the Upper Triassic Lunde

Formation, show clear associations between aggregatesand calcite crystallaria giving a boxwork texture.

Criteria for distinguishing well-preserved in situ

pedogenic aggregates from reworked ones that arevisible in cut cores have been presented by Müller et al.

(2004). Here we focus on criteria developed from stud-ies of Old Red Sandstone units where the aggregates arenot clearly seen by the naked eye in the field. Con-clusive evidence that the mudrocks were deposited asaggregates comes from petrographic examination but there are macroscopic criteria that aid in identifyinglikely units for further investigation. None of the macro-scopic criteria are by themselves diagnostic of former mud aggregate deposition. These criteria (shown dia-grammatically in Fig. 3) are:

•

Lack of lamination—

fine scale lamination, includingcross-lamination or fine grading is missing fromthese mudrocks. This could be the result of compaction or pedogenic processes such as self-mulchingthat is regarded as a significant process affectingVertisols, but in these mudrocks vertic features arenot ubiquitous (Fig. 4). Bioturbation could be an-other possible explanation but although burrows arecommon in many Old Red Sandstone mudrocks, theyare usually found in units that also display someremanent lamination (Marriott and Wright, 2004) andare rarely seen in the mudrocks that provide petro-

graphic evidence of former aggregates.• Contacts—many mudrocks displaying petrographic

evidence of former aggregates have erosive, evenchannelised contacts with underlying lithologies

Fig. 4. Massive red mudrock with minor pedogenic features from theFreshwater West Formation, Freshwater West (Fig. 1; Table 1).Hammer for scale is 400 mm.

Fig. 5. Channelised contact between highly pedified mudrock (lower part) and low-angle cross-bedded mudrock (also subsequently pedified) fromthe Red Marls Group (correlated with Freshwater West Formation) Llansteffan (Fig. 1; Table 1). Walking stick for scale is 500 mm.




(Fig. 5), suggesting that they may not have beenemplaced simply as suspension load sediments. Thisis not to imply that suspension load material couldnot be deposited on an erosive surface, but that theconsistent occurrence of mudrocks above erosivesurfaces may point to the mudrock having been emplaced by currents with the hydraulic capacity to

erode fine-grained sediments.• Associations—in some cases these mudrocks imme-

diately overlie intraformational conglomerates composed of calcrete clasts (Fig. 6), with no evidence of

other sediment types or grading. Whereas some larger (gravel-grade) calcrete clasts are present within thesemudrocks, they are relatively uncommon. The generalrestriction of larger clasts to the bases of the mudrocksis not characteristic of cohesive debris flow deposits(debrites), but is more likely to have occurred if thecalcrete conglomerates represent the bases of deposits

of sand-sized mud aggregates. Another commonassociation is that of structureless mudrocks overlyingtruncated, pedified mudrocks displaying characteris-tics of Vertisols. A discontinuous layer of calcrete

Fig. 6. Low-angled layers of mudrock and intraformational conglomerate containing calcrete clasts and mud matrix from the Red Marls Group(correlated with Freshwater West Formation) Llansteffan (Fig. 1; Table 1).

Fig. 7. Thin mudrock layers in heterolithic unit from the Moor Cliffs Formation, Freshwater East ( Fig. 1; Table 1). Coarse sand and conglomeratelayers have mud matrix that is likely to have been formed from sand-sized mud aggregates.




clasts may be present at the contact but it is common tosee these mudrocks, with petrographic evidence of aggregates, simply capping a sharply truncatedVertisol palaeosol. By analogy with modern Vertisolsit is likely that mud aggregates would have been

released during the erosion of the upper parts of the profile. A further common association is that of thin(b0.2 m) mudrock layers within inclined heterolithicdeposits (Fig. 7). The mudrocks are typically inter- bedded with calcrete-clast conglomerates or siliciclas-tic sandstones. Again these do not appear to bedeposits of waning flows but have sharp contacts withunderlying coarser layers, and occur frequently near the bases of lateral accretion units suggesting that theyare more likely to have been emplaced as bed load. On petrographic examination, the former aggregated

nature of these mudrocks is revealed.• Stratification—some mudrocks display a range of stratification types (Ìkes, 1993; Marriott and Wright,1996). One common feature is the presence of con-centrations of coarse sand- to fine gravel-sized calcreteclasts as isolated lenses or defining trough-sets withinmetre-thick mudrocks. These units probably represent point bars of small sinuous channels (Marriott andWright, 1996). The presence of these types of bed formstructures suggests that the mudrocks were not emplaced from debris flows. Another characteristic of these lenses or trough-sets (Fig. 8) is that the calcrete

clasts may be clast-supported or matrix-supported. Theexplanation for the latter is that the mud-grade matrixwas deposited as sand-grade mud aggregates, later compacted back into a secondary mud matrix.

• Petrographic evidence—thin section examinationreveals that far from exhibiting either fine laminationor pedogenic microstructures, the mudrocks have a poorly-sorted, chaotic microstructure with sand-sizedgrains of quartz, calcrete fragments, irregularly

shaped mm-sized mud clasts and sand-sized rounded pellets of clay, clay-silt and silt. In some cases there isa pedogenic overprint on these textures, such ascalcite crystallaria, but most do not display suchfeatures. The pellets vary from being sharply definedto grading into a finely mottled groundmass, withinwhich it is difficult to impossible to see pellet out-lines. Pellet forms are commonly more clearly seen in patches where calcrete clasts and quartz grains createa local framework. Rust and Nanson (1989) have alsonoted that in the floodplain sediments of the Channel

Country the aggregates are more likely to be pre-served when isolated between other grains. RecentlyRetallack (2005) has provided a classification of pedolith types. In this classification reworked clayeysoils are termed sepic pedoliths but many of theaggregates from the Old Red Sandstone do not showclear soil sepic or argillan microfabrics, and this probably reflects their origins from weakly devel-oped upper soil horizons.

6. Conclusions

Mud-grade sediments transported as sand-sizedaggregates can be a significant sediment type in drylanddeposystems, particularly where the fine-grained sedi-ments are smectite-rich and wetting and drying cycles

Fig. 8. Mudrock overlying intraformational calcrete clast conglomerate lenses showing low-angle cross bedding from the Moor Cliffs Formation,Freshwater East (Fig. 1; Table 1).




favour Vertisol formation and fine, granular-surface soilstructures. The growing number of examples of ancient sediments containing significant volumes of redeposited pellet aggregates supports such a view. The correct differentiation of floodplain versus channel deposits

requires the accurate identification of mudrocks depos-ited by suspension versus those deposited from bedload, and criteria for such differentiation at outcropdeveloped from analysis of Old Red Sandstone alluvialdeposits in South Wales are presented. Some mudrocksmay be the products of multiple cycles of deposition, pedogenesis, erosion, transportation and redeposition,and are effectively condensed horizons yet, by virtue of representing unstable land surfaces, do not show evi-dence of prolonged pedogenesis.

Acknowledgements

The authors wish to thank Elizabeth Gierlowski-Kordesch and Peter Friend for their helpful commentson an earlier draft of this paper. Paul Revell drew thediagrams. We thank the reviewers, Steve Driese andGreg Retallack for their insightful comments. Fieldwork by SBM was funded by a grant from the Faculty of theBuilt Environment, UWE, Bristol.

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Wright and Marriott 2007