asymmetrical soft-sediment deformation structures triggered by rapid sedimentation in turbiditic...

I

FACIES 44 283-294 : Pf. 46-47 7 Figs. -- ] Erlangen 2001

Asymmetrical Soft-sediment Deformation Structures Triggered by Rapid Sedimentation in Turbiditic Deposits

(Late Miocene, Guadix Basin, Southern Spain)

Massimo Moretti, Bari, JesQs Miguel Soria and Pedro Alfaro, Alicante and Nicola Walsh, Bari

KEYWORDS: TU RBIDITIC DEPOSITS LIQUEFACFI()N ANI)K)R FLUII)IZATION - ASY M MI~TI ~,ICAL S()Iq'-SEDIM ENT I)EFOR MA'FION STRUCTURES- GUADIX BASIN (SOUTHERN SPAIN)- LATE MI()CENE

Summary.

Soft-sediment deformation structures in Tortonian turbiditic deposits of the Guadix Basin (southern Spain) have been described. The most common structures arc asymmetrical pillow structures and elongated sets of load- casts. The structures are metric in scale and have been interpreted as the result of liquefaction and/or fluidization processes triggered by the rapid sedimentation of single high concentration turbidites.

Final morphology of soft-sediment deformation structures is related to two main driving force systems: unstable density gradient and lateral shear stress. Thc latter is probably induced by the downslopc component of the sediment weight. The asymmetry of dcformational structures (in horizontal and vertical cross-scction) allows a clarification of the relationship between morphology of deformation and direction of lateral shear stress: this relationship secms ambiguous and confused in the literature. The interpretations both of dcforination mechanism and trigger agent have been supported with: - field analyses; - calculations on the liquefaction processes induced by rapid sedimentation; - qualitative models in laboratory.

1 INTRODUCTION

Soft-sediment deformation structures document the physical processes which take placc in a dcpositional environment when sediments are unconsolidatcd: generally, these structures have a diagnostic value in the interpretation of hydrodynamic conditions, the paleoslope orientation of a sedimentary environment and the presence of syndepositional seismic and/or tectonic activity (Mills, 1983; Van Loon and Brodzikowski, 1987; Van Loon, 1992; Maltman, 1994).

Deformation can occur in unconsolidated sediments when they arc in a liquid-like state brought about by liquefaction and/or fluidization processes (liquidization processes, sensu Allen, 1977). The main potential trigger agents for liquidization are overloading, unequal loading, wave-induced cyclical and/or impulsive stresses, carth-

quakes and sudden changes in groundwater levels (Owcn, 1987; 1996).

Some kinds of soft-sediment dcformation structures (sand- on-mud load cast structures, ball-and-pillow structures, convolute lamination, water cscapc structures, etc.) arc very common in the turbiditic successions (Sanders, 1956: Sullwold, 1959; Dzulynski and Walton, 1965; Ricci Lucchi, 1968; Sanders, 1965; I,cppard, 1978: Hein. 1982; Guy, 1992; Stromberg and Bluck, 1998) and they are basically intcrpre:.ed as indicators of liqucfaction and/or fluidization processes induced by rapid sedimentation (Lowc, 1975; Allen, 1982; Strombcrg and Bluck, 1998).

With reference to the diagnostic significance ol Ioad- struclurcs, they arc good "way-up" indicators since they have a vertical devel.opment which is produced by the cffcct of gravitational instability (Raylcigh-Tay'lor) in liquefied sediments (Allen, 1982: Sclkci', 1993); on the contrary, the dominantly vertical movements of sediment and consequent ran- dom orientation of the lamination generally prevent the use of load-structures as pahicoslopc and/or palcocurrent indicators (Collinson and Thompson, 1989). Nevertheless, asymmetrical fcatnrcs associated with h)ad-structures have bcen previ- ously described in the literature in sand-in-mud systems (see for example: "flow casts" of Shrock, 1948; Prentice. 19"$6; "'flow soll.r of Sorauf, 1965; "load-casted ripple mat'k.r of Dzulynski and Walton, 1965: "'asymmetrical foldinA, lami- #zae'" of Williams, 1969; "'recumbe#,4 _[lame structures" of Dasgupta, 1998): where load-structures lean from vcrtica!, a subordinate horizontally directed force could bc supposed (Allen. 1982). The nature of this lateral force hats bccn inter- pretcd as: fluid drag and/or downslopc cornponcnt of the scdiment weight (Sanders, 1965: Brcnchlcy and Ncwall, 1977), unequal loading related with irregularities such as ripples (Ten Ilaaf, 1956: Dzuh'nski and Walton, 1965), unequal loading rclated to channelizcd erosional surfaccs (Dasgupta, 1998). A clear distinction among diffcrcnt tangential shear stresses acting at the same time as vertical gravitational instability does not exist in literature (see general reviews on this topic in: Potter and Pcttijohn, 1977; Mills, 1983; Collinson, 1994; l)asgupta. 1998).

Furthermore, the actual morphology of asymmetrical sc, ft-

Addresses: Dr. M. Moretti (m.moretti(a;gco.uniba.it) anti Dr. N. Walsh, Dip. di Geologia c Geofisica_ University ol13ari, via E. ()rabona 4, 1-70125 Bari, Italy; Dr. J.M. Soria and Dr. P. Allam, Dpto. de Ciencias de la Tierra, University of Alicante. Apdo. 99 Alicante, Spain

284

Fig. 1.1. Schematic geologic map of the Guadix Basin; 1.2. Areal distribution of Miocene-Pleistocene depositional sequences of the Guadix Basin.

sediment deformation structures is unclear since two-dimensional exposures described in the field show alignment of the structures either parallel and perpendicular to the supposed direction of the tangential shear stress (Owen, 1985). This confusion was not solved by the experimental studies: Anketell and Dzulynski (1968) and Anketell, Cegla and Dzulynski (1970) have shown experimentally that asymmetrical features in load-structures form where a horizontally-directed shear stress acts during a deformation process that is dominantly driven by vertical stresses and this simultaneous action leads to inclined transverse forms; nevertheless, comparing the geometries and the shear stress direction (see fig. 4 in Anketell and Dzulynski, 1968; fig. 9 in Anketcll et al., 1970), the relationship between asymmetry of the soft- sediment deformation structures (in plane and vertical view) and the direction of dominant lateral shear stress remains confused.

This study focuses on sand-in-sand load-structures with markedly elongated morphologies in the Tortonian turbiditic deposits of the Guadix Basin (Betic Cordillera, southern Spain) to obtain information about: �9 the mechanism of deformation; �9 the driving force systems; ~ the trigger mechanism; �9 the 3-D orientation and the plausible diagnostic value of this kind of soft-sediment deformation structures.

2 GEOLOGICAL SErlTING

The studied deposits crop out in the northern margin of the Guadix Basin. This is an intramontane basin located in the central sector of the Betic Cordillera (southern Spain), lying partially on the contact between the External Zones and the Internal Zones (Fig. 1.A). The north-western half of

its basement is made up of Mesozoic and Tertiary sedimentary rocks from the External Zones and the south-eastern half consists of Paleozoic and Triassic rocks from the Metamorphic complexes of the Internal Zones. The deposits of the Guadix Basin have been separated into six depositional sequences (sensu Mitchum, Vail and Thompson, 1977) which are bounded by major unconformities related to tectonic and/or eustatic events (Fern~indez, Soria and Viseras, 1996; Soria, Viseras and Fernandez, 1998). In the northern margin of the Guadix Basin, all these depositional sequences are represented (Fig. 1.B). The two oldest sequences (DS-I and DS-II, Tortonian) correspond to infilling that took place during the phase of marine sedimentation, the third oldest (DS-III, late Tortonian) corresponds to the marine-continental transition while the remaining three (DS-IV, DS-V and DS-VI, late Tortonian to Pleistocene) accumulated exclu- sively in continental settings.

The deposits showing soft-sediment deformation structures belong to the earliest depositional sequence of the Guadix basin (DS-I). The main outcrops of DS-I are located near Alictin de Ortega locality (Fig. 1.B): this depositional sequence consists of three facies associations (Sofia, 1993; 1994 - Fig. 2) which correspond to three different depositional systems (sensu Fisher and McGowen, 1967).

The eastern part of DS-I is the shelf depositional system which is characterised by sands displaying metre-scale sets of cross-stratification attributed to sand-waves.

The slope depositional system of DS-I, which contains the soft-sediment deformation structures described in this work, crops out to the west of the shelf depositional system. It is composed of thick interbedded sandy and shaley beds. The sandy beds show three main turbiditic facies which result mainly from resedimentation of the shelf sands: the first consists of turbidites with clear Bouma sequences (Ta-

285

Fig. 2. Synthetic stratigraphic sec- lion of DS-I and synthetic sedimen- larv cohm~ns of maic~ depositic, nal systems. Soft-sediment deformation structures are rcstriclcd to the central portion of the slope depositional says- ten1. Palcocurrcnl illC:_tSlliClll,2nts (rose diagrams) carried out on the turbidilic facies of the slope dcpositional system arc shown (A - flute casts: I3 - groove casls). The third l])Se diagram ((7') shows the align- merit of the longest axis of the pillow structures in plane ~icw. Note IhaI first two rose diagrams have a iadius of 50e),i while the third has a radiu,; of I()(1<~..

d and Ta-c); the second is represented by normally graded and occasionally amalgamated sandy beds with convolute lamination and large-scale pillows (fluidized-Iiquefied flows s e n s u Middleton and Hampton. 1973) while the third is characterised by massive beds with abundant groove marks at their base (grain flow, s e n s u Middleton and l lampton, 1973). The paleocurrent indicators (groove casts and flute casts) measured in the whole of turbiditic facies document flows coming mainly from E-N E with a certain dispersion of the paleocurrents pattern (Fig. 2). These data arc in accordance with the presence of a shelf m the same direction (and with the presence of a pelagic depositional system to the W- NW, see below) and with the occurrence of downslopc- directed (parallel and oblique to the regional slope) turbiditic flOWS.

Finally, the western part of DS-I is represented by the pelagic depositional system, which is characteriscd by marls with abundant planktonic organisms and intcrbeddcd diatomites and distal turbidites (Td).

3 SOFt-SEDIMENT DEFORMATION STRUCTURES 3.1 Morphological features

Soft-sediment deformation structures crop out in seven beds of the central part of the slope depositional system (Fig. 2). They are represented mainly by coarse-grained sand-in-fine-grained sand pillow structures (P1.46) that vary from 0.2 to 2 m in length and from 0.3 to 1.5 m in height. The

dimension of pilh)w structures seems to bc in relation with the gram-size of the invoh.'ed sediments: in particular, larger pillow structures seem to characterize the systems formed by very coarser sand-in-fine sand. Sediments surrounding :he pillows are mainly fine sand and silty sand lacking primary lamination and locally showing a slurried and disturbed texture. The presence of a clear contrast of grain size and ~:he

lack of continuity of sedimentary features of the pillow structures in the surrounding sediments allow us to exclude the possibility that these structures could bc nodules of concretionary origin (which arc very widespread in many ancient turbiditic deposits - see McBride e t al . , 1995).

The exposures allow the three-dimensional morphology of the structures to be examined. In plane \,icw, the pillow structures are elliptical (Pl. 46/1-2) with an about const',mt ratio bclween the longest and shortest diameter (1)m ix,' Drain : 1.3): furthermore, lonoest diameters of the pillows arc aligned in the same direction: dip direction measure- ments showing a clear unimodal pattern of elongation in every deformed beds (N240 ~ - F'ig. 2.C) with little dispersion of data.

In vertical cross-section (parallel to the longest diameter), the pillow structures show a clear asymmetry (Fig. 3): the "front part" (located in the south-western side of the structures - l in Fig. 3) is inclined towards NE (N60~ while the "rear part" (located in the north-eastern side of Ihe structures - 2 in Fig. 3) is vertical or inclined toward NE (N60~ Regular pillow strnctures are less abundant and are always associated with asymmetrical pillow structures: a!so

286

Fig. 3. Sketches of the asymmetrical features of the pillows in vertical section (parallel to the long axes). 3.1. A pillow with front (1) and rear part (2) which have different inclination but are both N60 ~ oriented; 3.2. A pillow with N60 ~ oriented front (1) and subvertical rear part (2).

in these structures it is possible to distinguish the two sides with same orientation.

Internally, unlike pillow structures described in the literature, which display concentric deformation of laminae (see Macar, 1948; 1951; Pettijohn and Potter, 1964; Dzulynski and Walton, 1965; Ricci Lucchi, 1968; Howard and Lohrengel, 1969; Rascoe, 1975; Weaver, 1976; Johnson, 1977; Weaver and Jeffcoat, 1978; Montenat, 1980) or homogenised textures (Moretti and Tropeano, 1996; Moretti, 1997), the pillow structures in the Gaudix Basin show some unusual features: large pillow structures are internally normal-graded with coarser grains and clay chips at the base and fine/medium-grained sands on top (P1.46/3, Figs. 4.1, 4.2 and 4.3); in a cross section parallel to the longest axis (P1.46/ 3, Figs. 4.1 and 4.2), the pillow structures with larger dimensions display an asymmetrical zonation of the normal grading: the intervals with different grain size are gently deformed and generally inclined (toward SW) in an opposite direction compared with the dip direction of the margins of the pillow structures (front and rear part). In vertical section, parallel to the shortest axis, the normal grading is perfectly symmetrical (Fig. 4.3).

Somewhat, the small pillow structures show an upper fine-grained sandy interval with parallel lamination and deformation of internal laminae is visible along the borders of the pillow structures (Fig. 4.4).

Moreover, unusual elongated sets of linked load-casts with constant planar elongation (N240 ~ occur (P1.47/1). They arc often isolated in very fine silty sands and consist of 3-4 linked load-casts reaching a maximum length of 2 m (P1.47/2). Between the single load-casts which form the sets, restricted zones in which the underlying fine sands are projected upward are well-visible, suggesting vertical fluid- escape morphologies (P1.47/1). Locally, this vertical fluid- escape morphology is more clear and the escape zones irregularly cut adjacent pillow structures (P1.47/3).

Summarising, unlike the paleocurrent directions of the whole of turbiditic facies are relatively disperse, the seven beds with pillow structures and sets of load-casts show an about perfect unimodal pattern of elongation in plane view with constant direction (N240 ~ and constant asymmetrical features in the cross-section parallel to the longest axis (see Fig. 2).

3.2 Mechanism of deformation

All the field evidence shows that deformation occurred when the sediments were unconsolidated and water-saturated. Deformation affected an initial unstable density gradient system in which the upper and denser bed is represented by the sedimented coarse/medium grained sands of the pillow structures and the underlying bed is represented by the fine-grained sands (fine sandy substrate) which en- velop the asymmetrical pillows. Deformation mechanism was characterised by viscous-fluid behaviour (sensu Owen, 1987) in sediments lacking in shear strength. Liquefaction/ fluidization processes are indicated by: - the absence of primary sedimentary structures in the underlying fine sandy bed that envelops the pillow structures; - the deformation within and on the borders of the pillow structures; - the presence of fluid-escape morphologies.

The presence of asymmetrical features in the described load-structures (elongation in plane view and asymmetry in the vertical cross-section parallel to the longest axis) needs the action of a driving force system which can be defined as tangential shear stress (sensu Owen, 1987) and which acted on liquefied sediments in association with an unstable density gradient (that implies vertical movements of the units with different bulk density).

Making reference to the literature interpretations, it is possible to exclude the action of unequal loading (related to

P l a t e 46

Fig. 1.

Fig. 2.

Fig. 3.

Morphologic features of the soft-sediment deformation structures of the Guadix Basin (Spain).

An unusual "pillow structures field" highlighted by selective erosion. Planar form of pillows is elliptical and longer diameters are oriented N240 ~ Detail of the elongated (elliptical) morphology of the pillow structures in plane view. Note the constant planar alignment. Asymmetrical features of a large pillow-structure (parallel to the longest diameter). We have removed and upturned this pillow to observe the 3-D morphology and the internal texture in cross-section (see the actual orientation in Fig. 4.2)

P l a t e 46 287

288

Fig. 4. Internal texture of the pillow structures. 4. I. Scheme of the removed pillow structure shown in PI. 46/3. Note the nornlal ~.rading within the pillow and the unusual

) o " - " " zonation ~ fdifferent ~.rm n-slze classes which is reclined and defomled. 4.2. Scheme of the original position of the pillow-structure. Note the inclination of front (south-westena side of the structure) and rear part (north- eastern side of the structure). 4.3. Normal grading within a large pillow structure which is cut along a section perpendicular to that of PI. 46/3 and Fig. 4.1.4.4. Detail of the deformation of the laminae within a small- scale pillow structure.

the presence of ripples or Iocalised channelized erosional surfaces) since large-scale depositional or erosional irregularities are absent. The nature of the tangential shear stress could be related m this case to two possible driving force systems: 1 ) current drag: 2) downslope component of the sediment weight. The actual origin of this tangential shear stress could only be interpreted since the final morphology is similar whatever the two driving force systems have acted.

In our opinion, the most probable driving lorce is the presence of a slope gradient. This interpretation is based on the observation that the paleocurrents, nteasured in the whole of the turbiditic facies, show a relatively great dispersion of data (the mean values are about N250-260~ On the contrary, the soft-sediment deformation structures show a constant pattern of elongation in plane view (N240 ~ and asymmetrical features in vertical cross-section. Further- more, the morphology of the soft-sediment delormation structures of Alictm de Ortega is consistent with a deformation wlaich acts with a constant magnitude during and after the sedimentation of the turbidite (see stages of deformation in the next paragraph): probably this Iact could demonstrate that the tangential shear stress exerted directly by the lurbiditic flow has a negligible effect. Nevertheless, the action of the tangential current drag exerted di,'ectly by the turbiditic flow can not be completely discarded on the basis of the available data and it is impossible to exclude also that both the driving force systems acted contemporaneously in some stages of the deformation.

3.3 Trigger agent and stages of deformation

Liquefaction and/or fluidization processes can be gener- ated by many t~igger agents: some are typical of the sedimentary environment (wave action, overloading, unequal loading, etc.) while others are of external origin (earth- quakes and tsunamis). The whole of above-explained data allows us to assume the rapid sedimentation of coarse- medium sands on a fine-sandy substrate as more probable trigger agent for deformation (in agreement with Dzulynski and Simpson. 1966). This process could induce an interstitial overpressure and a drastic decrease in shear strength m the fine sandy substrate (see next paragraph) which reaches the liquefaction state when this overpressure equals the total tension (shear strength "r = 0).

In Fig. 5 is shown an interpretation of the stages of deformation according to the observed final morphologic features of the soft-sedilnent deformation structures. �9 the initial stage (I in Fig. 5.1) is represented by the presence of a fine-grained sandy or silty sand subsirate which can be related with low-density turbiditic currents (or with distal deposition of high-density turbiditic flows - the complete obliteration of the primary sedimentary structures do not allow in-depth sedimenticological analyses).

�9 the sedimentation of a high density turbidite induces a drastic loss of shear resistance in the fine-sandy substrate (II in Fig. 5.1). The loss of shear allows the del'ormatioi1

P l a t e

Fig. 1.

Fig. 2. Fig. 3.

47 Morphok)gic features of the elongated load-casts (Guadix Basin. Spain).

Lateral view of a set of load-casts. Note the presence of restricted zones with vertical fluid escape morphologies between adjacent load-casts. Frontal view of an elongated set of load-casts with constant N240 ~ orientation. hTegular fluid escape zone between adjacent pillow structures: lamination in the underlying fine-grained sands is lk)lded upward (white arrow).

P l a t e 47 2,~9

290

Fig. 5. Supposed mechanism of dclknmation and stages of deformation: 5.1. Tile rapid sedimentation of a turbiditic bed induces liquefaction in the underlying fine sandy suhstrate (1 and [1). Initial stages of deformati~m (I[I) are dominated by the elTect o1: the gravitational inslahility (p). After the beginning ot" the restoration o1" the grain contacts (related to the sedimentation fFom tile liquefied state), the main driving force svqem is represented by lhe lateral shear stress (z - IV) . During time. tile lateral shear stress acts on less thickness o| sediment ( V/. This process induces the asymmetr 3 in cross-section. 5.2. Schematic 3-D development of pillow structures and nets of load-casts.

according to the presence of two driving force systems (unstable density gradient and lateral shear stress). Prob- ably, m the initial stages of the defln'mation, the unstable density gradient lepresents the most important driving force system: the first terms of the high-density turbidite are represented bv coarse-grained sands which could have a larger bulk density compared with the bulk density of the free-sandy substrate: this unstable density gradient becomes larger if the fine-sandy substrate reaches a complete liquefaction state (since the bulk density decreases during Iique- faction): therefore, during and soon alter sedimentation ot the turbidite, the interface between coarse-medium and underlying fine sands becomes the place where the gravita- lional instability is greatest. The initial morphology of the pillows and sets of load-casts could be consistent with a simple unstable density gradiem system with the upper term of lower kinematic viscosity (see Fig. 18 in Dzulynski and Simpson, 1966: systems A in fig. I of Anketell e ta l . . 1970): the deformathm driven by tile presence of a simple unstable density gradient implies basically a partial gravitational readjustment (with variable degree and style of deformation depending on the actual bulk density; contrast, thickness oF involved beds, dynamic viscosity, etc. see Anketell et al.,

1970, Selker. 1993).

�9 the stage of partial gravitational readjustment is completed before the gradual restoration of granular framework begins (between t I and t2 in Fig. 5.1 ). This process is related with the sedimentation from a liquefied state (Allen and Banks, 1972, Allen, 1982: Allen, 1985: Owen, 1987; Collinson and

Thompson, 1989). The re-establishment of grain packing begins at the base of the liquefied fine sandy substrate and moves upward as a "'front of reconsolidation" (stages IV and V in Fig. 9.A). During these stages, the main driving force system is represented by the lateral shear stress (the gravitational read.justment leans to decrease during tilne). This stress acts for a hmger period on the upper part of deformed units (IV and V in Fig. 5.1): in this way, the asymmetry in cross-section records the succession of the deformation stages which are connected with: - the action of two driving force systems (unstable density gradienl and lateral shear stress) in the first stages of delk)r-

mation. - the rise of the base of liquefaction ("front of recon-

solidation"). - and the constant action of the lateral shear stress.

The eh)ngation in phme view of pillow structures and sets of hmd-ca.,,ts (Plates 46 aim 47 and Fig. 5.2) probably demostrates the constant action of the lateral shear stress which induces a downslope alignment of the load-struc-

tLires.

4 THE RAPII) SEDIMENTATION AS TRIGGER AGENT FOR LIQUEFACTION

4.1 The sand-in-sand systems

The observations carried out on the soft-sediment defter- marion structures of the Guadix basin show that they are related to liqnefaction and/or fluidization in unconsolidated

291

Fig. 6. Schematic sketches for systems of rapid sedimentation- induced liquefaction. 6.1. Simple sketch of a sand-in-sand system. 6.2. A system made up of a thick sandy bed with a permeability banter on top.

sediments. The deformation related to liquefaction induced in f ine-grained sands by the rapid sedimentation of coarse- grained sands could be roughly schematised by considering the shear strength ('[) of tile fine-sandy substrate in relation to his thickness H and bulk density Tsat I (Fig. 6.1), this is gives us (applying Coulomb-Terzaghi law - Terzaghi, 19471 -> z = (cyt - u) tg~: [1] 1:= (u i - 'yw) H tg~ ='Y'l H t g ~ with 7sat I = saturated bulk density and yw = water density:

Y'l = ysati - ) ,w; ~) = friction angle of the fine sands.

As regards the loss of shear strength of the substrate, this is induced by the rapid sedimentation of a turbiditic bed of thickness h (and 7sat2= bulk density of the sedimented turbidite). Loading on fine-grained sandy substrate pro- duces an excess pore presstire which could be equal to the vertical tension exerted by the turbidite. If tile process is very rapid and the fine-sands do not dissipate tile excess pore pressure quickly, the shear strength of the substrate can be drastically redt.ced. Considering the condition of absence of shear strength in the substratum (complete liquefaction which occurs at depth H) as due to tile instantaneous sedimentation o f a turbidite with thickness h (in agreement with the field observations which show liquefaction effects):

[2] z = (7'i H tg0 -y': h) = 0 It is possible to calculate:

[3] h = (Y'i H t g 0 ) / Y ' e Therelbre, for example, a sandy bed (with 7" l-- 8 KNm :~

and 4) = 40 ~ - see average values of density and porosity in Hamilton, 1976) of thickness H = 0.5 ill reaches the state of complete liquefaction ('c = 01 after the instantaneous sedimentation of a bed of thickness h = 0.6-0.7 in (and y ' : = 7 KNm-3). if the underlying substrate dissipates a percentage of the excess pore presstlre (after a time At and in a variable degree depending on its permeahili ty and discharge conditions - Terzaghi, 1947) by filtration, the depth ofl ique faction in the fine sandy substrate decreases. Nevertheless also considering very small depths of liquefaction, there arc the conditions for tile decrease of shear strength in the underlying fine-sandy substrate: lhat is sufficient to induce deformation in relation with the presence of driving force systems

(unstable density gradient, tangential shear stress, unequal loading, etc.).

4.2 The s y s t e m s wi th p e r m e a b i l i t y barr i er s

The above-described deformation mechanisrn is very different from the sand-on-nlud systems described in literature: deformation in tile latter systems occurs often on account of the presence of permeabili ty barriers (Lowe, 19751. "File excess pore pressure induced in systems with permeabili ty barrier could be schematised through the Terzaghi 's theory of one-dimensional consolidation. The simple model in Fig. 6.2 is represented by a sandy bed (lq = thickness, E = deformabili ty modulus, Ks = permeabili ty coeff.) covered by a muddy bed (L = thickness, K = permeability coeff. - with K~>>K). The sedimentation of a turbidite on this simple system, could be expressed by the appli- cation of a unifol'mly distributed tension cy. This vertical stress induces an excess pore pressure (u) in the sands that, at the time to = 0. is equal to ~. Supposing u unifornily distributed within the bed of sands of thickness H, the dissipation of u occurs by filtration through the inuddy bed. The height of the water which passes through the ban'ier permeabili ty represenled by the muddy bed is equal to the "'displacement" in the sandy bed (if H>>L. the "'displacement'" in the muddy bed is negligible).

This relation cc)uld be expressed in this way (considering A -- unit area of filtration and Tw = water density): [a] Adh/Adt = ku/u ==> dh/dt = ku/y,,L but it is also:

dh/dt = doc H/dr E and taking into account that:

(y~=ci-u and that d(Y/dt=0 it results:

ku/T,,L = du H/dr E ==> du/u = k E dr/T,, L H and integrating between 0 and t and

between UO and ill:

[hi k E t/}'~,, L H = log uo/u, [el k E t/T,, L H = - In ut/n,

Therefore, for example, a system made tip of a sandy bed (with E = 1 (P KPa ; H = 0.5 m) covered by a muddy bed (with k = 10 -'~ ill/s: L = ().1 ill) is sut!iect instantaneously to a tension r = 2.1 KPa {which is equal to the tension induced by the rapid sedimentation o1:0.3 nl of sands with u 7 KNm- ~): it is possible to calculate the t i rnets , which corresponds to the time needed for tile dissipation of the 50% of the excess interstitial pressure f= 1 KPa): [d] ts0 = (T,, L H In 2)/k E = 3.45 104 s = 1(1 hours.

Tile calculations on mass sedimentation processes in sys tems with pern /eab i l i ty barr iers ( represen ted by interbedded mud-shale/sands which are very widespread for example in deha and turbiditic deposits) show that solne hours are needed to halve the excess pore pressure induced by some tens of cenlimetres of instantaneously sedimented sands: during this time. the soft-sediments have a very low shear strength and can be deformed under any driving force system I Moretti. 19971. Probably, regularly laminated load- cast and ball-and-pil low structures in turbidites could be formed in this way, througl] slow overloading processes

292

Fig. 7. Laboralory experiments. 7.1. Sketch of the experimental conditions: 7.2. Ini- tial system, driving force systems and final morphologies of the deformation, 7.3 and 7.4. Photo and sketch of final soft-sediment detom~ation structures: note that the borders of the load structures arc inclined in an opposite direction con> pared with the initial angle of the slope

when consolidation effects are low and also in response to small irregularities of the water-sediment interface (which represent negligible systems of unequal loading in ordinary conditions of shear strength in the involved sediments).

5 QUALITATIVE MODEL OF THE OBSERVED DEFORMATION

The calculations carried out show that liquefaction and drastic decrease in shear strength induced by rapid sedimentation seem ordinary processes both in systems with permeability battlers and in sand-in-sand systems. In our interpretation, after liquefaction started by rapid sedimentation, deformation is driven by two driving force systems (unstable density gradient and tangential shear stress related to the downslope component of the sediment weight): a simple qualitative analogic model has been built to verify that. with the presence of these two driving force systems, the final morphologies are similar to those ones observed in the field.

In a transparent test tank of little dimensions (25 x 40 x 25 cm) we have reproduced a slope {with an angle variable from 2 ~ to 9 ~ ) made up of coarse-grained sands on fine- grained sands (Fig. 7.1). Sands were introduced in the test tank by settling in water.

L, ique faction has been induced by shaking the support of the test tank. After liquefaction, load-structures (Fig. 7.2.7.3 and 7.4) formed in accordance with the presence of driving force systems (unstable density gradient and lateral shear stress - Fig. 7.2): liquefaction acted also on the angle of the slope which drastically decreased alter deformation (in agreement with the results of Owen. 1987; 1996).

Resulting deformation showed morphologies characterised by asymmetrical features. In plane view, the morphology of the load-structures was elliptical, perfectly parallel to the initial slope, in a cross-section, parallel to the slope, vertical borders of the pillows were inclined in an opposite direction compared with the initial angle of the slope and rnain asymmetrical featm'es were the same ones observed in the field (Fig. 7.2, 7.3 and 7.4).

293

The carried out experiments arc based on the hypothesis

that the final morphology is function only of the driving force system and is independent from the trigger mechanism and this hypothesis is documented in many works (Owen, 1987; 1996; Moretti et al., 1999); nevertheless, in this case it was not possible both to model the syn-sedimentary deformation (since the experimental conditions allow us to

analyse only a post-depositional deformation) and to estab- lish the role of the shear stress exerted directly by a turbiditic flow. In other words, the qualitative experiments show only that the contemporaneous presence of an t, nstablc density gradient and slope is able to produce the asymmetrical morphologies (also in absence of the tangential stress exerted directly by the turbiditic flow) which we have observed in the field.

5 CONCLUDING REMARKS

The present study has been focused on some asymmetrical load-structures in the turbiditic deposits of the Guadix Basin. The analyses carried out on the asymmetrical soft- sediment deformation structures have shown that they arc

induced by single rapid and in mass sedimentation events. Elongated morphology is induced by the contemporaneous action of a vertical gravitational instability and a tangential

shear stress, this latter probably related with the presence of

a regional slope; this interpretation is consistent with: - the

supposed mechanism of deformation; - the absence in the soft-sediment deformation structures of the oblique orienta- tions which characterise the other turbiditic facies; - the experiments carried out in the laboratory.

Nevertheless, the direct action of the current drag exerted by the overlying turbiditic flow (or the simultaneous action of two lateral driving force systems) can not be completely discarded. Data coming fi'om new outcrops and, probably, new experimental studies are needed to solve this problem in a ultimate way.

The asymmetry of the pillow structures seems to record the deformation stages associated with two driving force systems which act during a different amount of time. In particular, the lateral shear stress acts continuously on the liquefied sediments, while the unstable density gradient acts till the partial gravitational readjustment is completed.

The shown data (resulting from field and laboratory analyses) in order to the orientation in plane and vertical view allow to clarify the relationship between the orientation of the tangential shear stress and the final morphology (H:the soft-sediment deformation structures. In particular, longest axis in plane view are aligned always parallel to the tangential shear stress; in vertical cross-section (parallel to the longest axis), the front and rear part (with same main orientation) are inclined in the opposite direction into re- spect the tangential shear stress.

The calculations carried out on the deformation processes induced by rapid sedimentation show that, both in systems with relatively high permeability (sand-in-sand systems) and in systems with barrier of permeability (sand- in-mud), the liquefaction seems an ordinary process.

Acknowledgements

Financial aid was provided by Research Project PB'-)7- 0808 DGESIC. 1--unding was provided also by MURST grants (IX cycle of Ph.l).), AS1 tars-96-47 project), MU RST 4()(}~ 1996 (Pieri found) and MURST 60% (Sabato found), We thank K.W. Tictzc, P. Picri, 1_. Sabato, M. Tropcano, G. Owen, P. Haughton, and S. Dzulynski for the useful discus- sions on soft-sediment deformation and R~r the very con- structive reviews and comments on the manuscript.

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Manuscript received October 15, 2000 Revised manuscript accepted March 22, 2001

asymmetrical soft-sediment deformation structures triggered by rapid sedimentation in turbiditic...

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