The Transition Between Sheet-Like Lobe and Basin-Plain Turbidites in the Hecho Basin (South-Central...

Journal of Sedimentary Research, 2005, v. 75, 798819

DOI: 10.2110/jsr.2005.064

THE TRANSITION BETWEEN SHEET-LIKE LOBE AND BASIN-PLAIN TURBIDITES IN THEHECHO BASIN (SOUTH-CENTRAL PYRENEES, SPAIN)

EDUARD REMACHA,1 LUIS P. FERNANDEZ,2 AND EUDALD MAESTRO11Departament de Geologia, Universitat Auto`noma de Barcelona, 08193, Bellaterra, Spain

2Departamento de Geologa, Universidad de Oviedo, J. Arias de Velasco, s/n 33005 Oviedo, Spain

e-mail: [email protected]

ABSTRACT: Genetic facies analysis based on bed-by-bed correlations fromsheet-like lobes to the basin plain in some Hecho turbidite systemsdemonstrates that at least 50% of the flows building the sheet-like lobeskept moving downcurrent to the closely related basin plain and underwentflow transformations, interpreted to have resulted from interaction withtopography at the basin margin(s), that gave rise to specific facies. Thesefacies form a new facies tract that replaces the fine-grained group ofturbidite facies (very fine sand to mud) and characterizes the basin-plainbeds.

Beds in the sheet-like lobes evolve downcurrent in a way that ispredictable by the existing turbidite facies tract models, whereas 36% ofbasin-plain beds, which account for 78% of the basin-plain volume, do not.The latter have deposits from high-density turbidity currents at their basesand typically complete basin-floor coverage. The new facies tract developedwhen the flows obliquely encountered the southern foreland-margin ramp.At the ramp, the lower, sand-laden and high-density part of the larger flowswas deflected, evolving downcurrent along the ramp trend. The upper part ofthe flow, more dilute and thicker, was reflected from the foreland margin asa train of declining undular bores (moving hydraulic jumps). Subsequentreflections generated against the flanking margins in the closed basin led toponding, which resulted in an overall sheet-like stacking pattern acrosssheet-like lobes and the basin plain and is the diagnostic feature of the distalelement in the lower, sand-rich stages of the turbidite systems. Calcilutiteson top of beds, interpreted until now as hemipelagites, show field evidence ofhaving a turbiditic origin (hemiturbidites), thus forming the facies cappingthe new facies tract.

INTRODUCTION

Diagnostic criteria currently used to recognize and interpret basin-plainvs. lobe facies associations in outcrop and core stem largely from theturbidite fan model of Mutti and Ricci Lucchi (1972, 1975), Mutti (1977),and Mutti and Johns (1979). Mutti and Johns (1979) proposed a twofoldorigin for basin-plain beds (we will use the term bed in the sense ofCampbell 1967): (1) thin beds (thickness , 10 cm) would be most likelyassociated with the final stages of waning flows that, following the Bouma(1962) sequence, would have deposited their coarse load mainlyupcurrent; and (2) thick beds, coarser and more common than might beexpected, would be probably related to exceptional flows reaching thebasin plain after bypass of a lobe region. As a consequence, the detailedspatial relationships between lobes and basin-plain elements (we use theterm element in the sense of Mutti and Normark 1991) are assumed to becomplex (Pilkey 1987). In the Hecho basin (south-central Pyrenees, Spain;Fig. 1A, B), these relationships and the facies change from lobes to basinplain can be analyzed by means of high-resolution correlations (seeRemacha and Fernandez 2003) and a genetic (process-oriented) approachin the sense of Mutti (1992).

In the sandy (lower) stages of growth of the Hecho turbidite systems(types I and II turbidites of Mutti 1985), the inner depositional elements,i.e., channel-lobe transition merging into sheet-like lobes (we use the termsheet-like lobe following Normark et al. 1993), form a longitudinalcontinuum (Mutti et al. 1972; Mutti 1985; Mutti et al. 1985; Mutti et al.1988) and have a remarkable sheet-like character (Remacha et al. 1998b;Mutti et al. 1999). Additionally, high-resolution correlations (seeRemacha and Fernandez 2003) show that (1) sheet-like lobes mergedowncurrent into the basin plain without significant facies breaks, (2)basin-plain beds are deposited from the flows building the upcurrentlobes, and (3) the sheet-like character is maintained across both elementsas the general basinward wedging out of beds is balanced by thethickening of some beds.

In the Hecho basin, turbidite systems lap out against basin margins,suggesting that flows are topographically controlled. A growing body ofevidence from laboratory experiments has shown that flows change theirproperties after encountering topographic obstacles (Pantin and Leeder1987; Simpson 1987; Kneller et al. 1991; Edwards 1993; Edwards et al. 1994;Haughton 1994; Kneller 1995; Kneller et al. 1997; and references therein). Infield studies, such changes have been interpreted from the occurrence ofmultiple paleocurrent directions commonly associated with abrupt reversalsin grading within single beds (Ricci Lucchi and Valmori 1980; Marjanac1985; Pickering and Hiscott 1985; Hiscott et al. 1986; Marjanac 1990;Kneller et al. 1991; Edwards et al. 1994; Haughton 1994; Kneller andMcCaffrey 1999; amongst others). In the Hecho basin, little attentionhas been paid to the topographic control on facies (Rupke 1976; Remachaet al. 1998a; Remacha et al. 1998b; Remacha and Fernandez 2003).

In this paper, we review the significance of the basin-plain element inthe Hecho basin. To do this, we first determined the downcurrent extentof the beds constituting the sheet-like lobes and, hence, the dependence ofbasin-plain facies on the flows that constructed sheet-like lobes (seeRemacha and Fernandez 2003). Second, we reviewed and geneticallyclassified the basin-plain facies. Third, we developed interpretations offlow behavior that may have led to the change between the two faciesassociations. Finally, we developed a hypothesis for the genesis of thebasin-plain element on the basis of our detailed work and the regionalgeological setting.

To do the study, we selected a representative outcrop belt (Fig. 1) fromthe lower part of the Banaston-2 composite sequence (BanastonAllogroup; Remacha et al. 1998b). The study area extends for nearly30 km down depositional dip, from what have been interpreted as puresheet-like lobe facies association in the east (Aragon valley, north ofJaca), to pure basin-plain facies association in the west (Veral valley,south of Anso), respectively (see Mutti et al. 1972). The results presentedhere are consistent with observations for other Hecho basin turbiditesystems.

Data and interpretations presented here help the understanding offacies and sand-thickness distribution over large portions (tens of

Copyright E 2005, SEPM (Society for Sedimentary Geology) 1527-1404/05/075-798/$03.00

FIG. 1.A) Highly simplified geological map of the Pyrenees showing the outcrop belt of the Eocene strata of the south-central Pyrenees and the main structuralelements discussed in text: Bi5 Binies thrust, Bo5 Boltana anticline (lateral ramp), Co5 Cotiella thrust, Ga5Gavarnie thrust, Lk5 Lakora thrust, La5 Larra thrust,OB 5 OrozBetelu thrust. B) Simplified geological map of the south-central Pyrenees showing the outcrop belt of the lowermiddle Eocene strata and the BanastonAllogroup turbidites, and also the location of studied area. OB 5 OrozBetelu Massif. C) Simplified geological map of the studied area showing the stratigraphy of theBanaston Allogroup and the location of sections. Notice that sections are aligned roughly parallel to the paleocurrent directions of sole marks.

THE TRANSITION BETWEEN SHEET-LIKE LOBES AND BASIN PLAIN 799J S R

kilometers) of the depositional reaches (sheet-like lobe and basin-plainelements) of turbidite systems in confined basins, and provide anexplanation for these features in terms of sediment dispersal anddepositional mechanisms.

GEOLOGICAL SETTING AND SYNDEPOSITIONAL TECTONICS

Mutti et al. (1988) and Remacha et al. (1998b) identified six majorEocene turbidite wedges (allogroups), which stack to form the southward-migrating basin fill of the south-central Pyrenees foreland basin (HechoGroup after Mutti et al. 1972). Turbidite wedges are largely eroded in thenorth and onlap onto the foreland margin in the south, where they arepartially overlain by deltaic and terrestrial sediments of the Pyreneanmolasse stage (Bartonian to Miocene; Fig. 1A, B). Most of the Hechoturbidite systems fit into the model of Mutti (1985) and consist of twomain stages of growth (Fig. 2A). The lower stage of growth accounts forthe bulk of turbidite sand deposition all over the basin in front of large-scale submarine erosional features (types I and II turbidites of Mutti 1985).The upper stage comprises muddy slope-delta wedges (type III turbidites ofMutti 1985) and subordinate foreland carbonate-ramp toes. The upperstages are fully developed within the canyons in the southeast and thin tothe west within the foredeep, where they may be replaced by majorcarbonate megaturbidites. Eight major basin-wide megaturbidites arefound (Labaume 1983; Labaume et al. 1987; Teixell 1992) extending alongmost of the depositional zone (from channel-lobe transition to the end ofthe basin plain), although they are best developed in the basin-plain area,where they are thicker (up to 250 m) and display a complete developmentof the five sedimentary divisions defined by Labaume et al. (1987).

The Banaston Allogroup formed between 47.8 and 41.8 Ma (middleLutetian; magnetostratigraphic and biostratigraphic dating by O. Oms,personal communication). It consists of four unconformity-boundedturbidite systems, named, from base to top, Banaston-1, -2, -3, and -4(Fig. 1C; Remacha et al. 1998b). The second system, or Banaston-2composite depositional sequence, is the principal subject of study in thiswork. It unconformably overlies the Mt-5 megaturbidite (also known asRoncal-Fiscal megaturbidite; nomenclature of megaturbidites afterLabaume 1983; Labaume et al. 1987; Teixell 1992) or the foreland-margin carbonates and is capped by the Mt-6 megaturbidite. The fourBanaston systems have very similar facies, spatial distribution of turbiditeelements, and stacking patterns, being fed through a single, structurallycontrolled funnel in the southeast of the south-central Pyrenees (Fig. 2B).

The structural framework of the Hecho basin during Banaston timescan be related to three thrust systems (Labaume 1983; Teixell 1992, 1996),named LakoraCotiella, LarraBoltana, and GavarnieBinies (Figs. 1A,B, 2B). Once the LakoraCotiella unit had been emplaced, the onset ofthrusting of the LarraBoltana cover system and, to a lesser extent, theinitial thrusting of the Binies cover thrust (an extension of the basement-involved Gavarnie thrust; Teixell 1996) led to a reorganization of theforedeep as summarized in Figure 2B. Synsedimentary thrusting of theLarraBoltana and Binies thrusts is demonstrated by the progressiveunconformities locally exposed in the Boltana and Binies ramp anticlines(see Remacha et al. 1998b, their figs. 7 and 8). At the western end of theBanaston turbidites outcrop belt, beyond the study area, mapping of theBanaston megaturbidites and the Mt-8 megaturbidite (Payros et al. 1994;Payros et al. 1999; see also Faci-Paricio et al. 1997 and references therein)has revealed an onlap pattern onto the OrozBetelu Massif (Figs. 1A, B,2B), showing that this massif constituted a submarine high forming thewestern closure of the deep-water Banaston basin. The OrozBetelubasement-involved thrust seems to represent the western prolongationof the LarraBoltana system (Camara and Klimowitz 1985; see alsoFaci-Paricio et al. 1997), connecting the poorly developed submarineorogen in the north (the Lakora and LarraBoltana thrusts) with thesouthern foreland margin (Fig. 2B). Therefore, the Banaston basin was

a structurally confined and relatively small basin, extending longitudi-nally for some 150 km. The basin width (northsouth) is more difficult tocalculate because of generalized erosion in the north but can be estimatedto be about 2545 km west of the Boltana anticline.

CORRELATION FRAMEWORK

We have followed the genetic facies approach of Mutti (1992), which isbased on high-resolution correlation patterns. In the depositionalelements of some turbidite systems, the highest (bed-by-bed) correlationlevel can be reached (see examples in Pickering et al. 1995). This can beattained only through a hierarchical procedure by splitting time-equivalent packages of beds bounded by precise time lines (markerturbidite beds, each deposited by a single flow) into smaller groups. Weenvisage three main correlation orders, termed first-, second- and third-order correlations (see Remacha and Fernandez 2003 for a detaileddiscussion).

First-order correlation is based on the two major megaturbidites (Mt-5and Mt-6) that bound the Banaston-2 sandy stage. These distinctive andmappable megaturbidites (Fig. 1C), which can each be up to 250 m thick,and which extend for almost the complete foredeep (see above), are themost outstanding markers for stratigraphic correlations on both regionaland local scales (Fig. 3). Selected sections between the megaturbiditeswere measured at maximum detail (bed by bed, separating all thedivisions forming each bed), forming the basis for second- and third-ordercorrelations. Second-order correlation relies on matching outstandinglythick beds between sections. Such beds, considered by some geologists asminor megaturbidites, also have a distinctive suite of facies (see below).The second-order correlation of the interval between Mt-5 and Mt-6(Figs. 3, 4) allows discrete packages of time-equivalent beds to becorrelated at the meter scale. Third-order correlation was done bycomparing the number, vertical arrangement, and facies features of thesingle beds within second-order packages and, further, on testing thecoherence of the downcurrent evolution of facies (textural and structuraldivisions) in single beds, thus providing the bed-by-bed correlationframework. Problematic correlations due to the presence of coveredintervals, small-scale faults, and local wedging out of beds were not takeninto account for calculations. A complete bed-by-bed correlation for thebest-exposed interval of the Banaston-2 resulted in 64% of the lobe bedscorrelating across the four studied sections, whereas 17.6% of bedscorrelated partially and 18.4% of beds did not correlate, and is the basisfor our analysis (see the simplified cross section in Fig. 4). This crosssection is roughly parallel to the paleoflow direction as determined fromsole-mark paleocurrents (see Fig. 1C) and summarizes the downcurrentevolution of beds between sheet-like lobes to the east (Jaca section) andthe basin plain to the west (Anso section) through transitional interveningsections (Estarrun and Aragues sections).

LITHOLOGICAL FEATURES OF BEDS IN SHEET-LIKE LOBE AND BASIN-PLAIN

ELEMENTS

The turbidite beds in the selected interval of Figure 4 can consist of upto five basic lithological divisions. From base to top of a bed, these are:

N Graded sandstones and/or graded coarse siltstones (clean-sandstonedivision).

N Medium to fine siltstones (clean-siltstone division).N Poorly sorted (muddy), graded, very fine sandstones to siltstones

(dirty sandstonesiltstone division).

N Homogeneous shales (shale division).N Marlstones or limestones (calcilutite division).

On the basis of Markov chain analysis, individual beds in the lobesand the basin plain show variable vertical trends in these lithologies

800 E. REMACHA ET AL. J S R

(Fig. 5). In summary, beds in sheet-like lobes are mainly bipartite,comprising a clean-sandstone division overlain by a shale division.Clean siltstone and dirty sandstonesiltstone divisions are scarce, beingpresent only in the thinnest beds and in some of the thickest beds,respectively. Finally, calcilutite divisions rarely appear on top of thelobe beds. Toward the basin plain, clean siltstone, dirty sandstonesiltstone, and calcilutite divisions become more common. Basin-plainbeds typically consist of a clean-sandstone division, a dirty-sandstonesiltstone division, a shale division and, finally, a calcilutite division.

The thinnest basin-plain beds tend to lack a dirty-sandstonesiltstone division and contain a clean-siltstone division, either overlyinga basal clean-sandstone division or forming the base of the bed. Also,calcilutite divisions can form single beds or directly overlie a basalclean sandstone or siltstone division. The relative importance ofseveral lithological divisions varies from lobes to the basin plain (Fig. 6;Table 1).

Beds at outcrop scale are always tabular. Their bases are flat, although,in some of the thickest lobe beds, tabular scours (Mutti and Normark

FIG. 2.A) Highly idealized sketch showing the arrangement of the Hecho turbidite systems according to Muttis (1985) model. To broadly illustrate its application tothe Banaston systems, the distribution of transfer-zone, lobe, and basin-plain elements as well the approximate position of the Boltana anticline have been added(compare with Figs. 2B and 3). B) Paleogeographic sketch map of the Banaston-2 turbidite basin (compare with Fig. 1A, B). The configuration of the basin for the othersystems of the Banaston Allogroup is very similar to this one. Structures older than Banaston times: Co 5 Cotiella thrust, Lk 5 Lakora thrust. Active structures duringBanaston times: Bi 5 Binies thrust, LaBo 5 LarraBoltana thrust, O-B 5 OrozBetelu thrust.


1987) or, more rarely, irregular scours are present. Contacts betweenlithological divisions are flat (although tops of clean-sandstone divisionscan be rippled, especially in the basin-plain beds), and sharp orgradational. Sharp contacts appear where clean sandstone is overlain

by shale or calcilutite. In these cases, significant intra-bed bypass ofthe intermediate grain-size populations, which are found fartherdowncurrent, took place. Also, where clean sandstone or clean siltstoneis overlain by dirty sandstonesiltstone, an intra-bed depositional

FIG. 3.A) Cross section of the Banaston Allogroup between the Boltana anticline (Janovas section) and the Anso area showing the Banaston-1, -2, -3, and -4composite depositional sequences (compare with Fig. 2A). B) Stratigraphic cross section of the Banaston-2 composite depositional sequence in the study area from sheet-like lobes (Jaca section, right) to basin plain (Anso section, left) showing the first-order and second-order correlation framework (reprinted from Remacha and Fernandez2003; with permission from Elsevier). Black bars at both sides mark the interval detailed in Figure 4.


break, which may be erosional but without a marked grain-size break, ispresent.

Beds range in thickness between 0.5 and 520 cm (Table 1). Mean bedthickness increases from the lobe to the basin-plain element (Table 1;Fig. 7), although the mean thickness of the clean-sandstone divisiondisplays the opposite trend. Moreover, the ratio of thick (. 10 cm) tothin (, 10 cm) beds increases from the lobe (1:3.4) to the basin plain(1:1.9), but clean-sandstone thickness per meter of measured sectiondecreases from 0.67 to 0.27 along the same trend. This is because thickbeds of the basin-plain element range widely in terms of clean-sandstone

thickness and typically are mainly built of a thick interval of dirty sandsiltstone plus shale (Table 1; Fig. 6).

GENETIC FACIES ANALYSIS

The third-order correlation framework enables us to perform a geneticfacies analysis, i.e., to understand how sediment gravity flows evolved inboth space and time from the sheet-like lobe to the basin-plain elements.Following Mutti (1992, p. 4953), the facies tract in a bed records thedownstream facies changes produced by transformations of a flow during

FIG. 4.Simplified version of a detailed bed-by-bed correlation of the selected interval of the Banaston-2 composite depositional sequence from sheet-like lobes (Jacasection, right) to basin plain (Anso section, left). Note the overall thickening of beds toward basin plain. See Figures 1C and 3B for location of sections and forstratigraphic location of the selected interval, respectively. Bars at both sides mark the stretch detailed in Figure 12. Bed marked with asterisk is depicted in Figure 11C.


its motion. The lateral and vertical changes in depositional divisionswithin beds record the flow evolution in space and time.

On the basis of the textural and structural features of, and the vertical andlateral relationships among, the lithological divisions that form the turbiditebeds of the studied interval, two main groups of facies have been distin-guished in the transition from sheet-like lobes to basin plain. The featuresof these facies are based on field observations. The equivalence

between these facies and the lithological divisions above is described inTable 2.

GROUP 1: FACIES FROM PRIMARY (DEFLECTED) FLOWS

Table 3 summarizes the main features of Group 1 facies. Although thetable is largely self-explanatory, some comments help to understand these

FIG. 5. Lithological structuring of thin(, 10 cm) and thick (. 10 cm) beds fromsheet-like lobes (Jaca section) to basin plain(Anso section). Markov chain analysis hasbeen done following the method of Powers andEasterling (1982). Numbers denote residuals[(observed transitions fitted transitions)/sqrfitted transitions], of which the positive onesare shown. x2 testing is statistically significantat the 99.9986% level, well beyond the 95%needed to reject the null hypothesis ofrandomness (n 5 number of beds involved incalculations). The commonest bed types aregiven by the preferred transitions (highestresiduals shown by bold lines); compare withthe real examples of beds displayed inFigure 12.


facies. They form part of the facies tract of Mutti (1992), refined by Muttiet al. (1999), and therefore are designated following his nomenclature.From the sheet-like lobes to basin plain, only the coarse-grained and thefine-grained facies groups are present (Fig. 8A, B). The very coarse-grained facies (boulders, cobbles, and pebbles) and the first deposits ofthe coarse facies (small pebbles to coarse sand) are restricted to thetransfer and channel-lobe transition elements (see Remacha et al. 1998b)and therefore are not discussed here. Facies of Group 1 are the onlyconstituents of all but the thickest lobe beds, and of the thinnest basin-plain beds. In other cases, they form the lower part of beds (see below;Fig. 8). Also, these facies are arranged forming simple fining trends bothupwards and downcurrent.

F5 consists of poorly sorted to well-sorted, graded sandstones. Coarse-tail grading (Middleton 1967) may be clear or subtle. In the study area, F5always forms the basal division of beds. The base of F5 deposits is usuallyflat, although in some beds it locally displays impact features (Mutti andNormark 1987) and the deposit contains rip-up clasts. F5 is interpreted torecord deposition from high-concentration flows (see Mutti 1992, Knellerand Branney 1995, and references therein; cf. S3 from Lowe 1982; see alsodiscussion in Kneller and Buckee 2000).

F7 deposits are well-sorted, fine- to medium-grained sandstones withparallel-stratified, millimeter-thick, inversely graded or ungraded layers,which thin and fine upward through the deposit. F7 deposits can overliethe base of the bed or evolve from an underlying F5. Upward, the layerscommonly become indistinct and pass into F8 deposits (see below) or areoverlain abruptly by secondary-flow facies. These layers have beeninterpreted as traction carpets (Lowe 1982) or as longitudinallysegregated coarse material (Hiscott 1994) possibly due to reworking ofnewly deposited sediment upcurrent (Mutti et al. 1999). Data from thisproject do not allow selecting amongst these interpretations or evenconsideration of others (see Leeder 1999, p. 223).

F8 consists of well-sorted, graded sandstones, apparently withdistribution grading and, according to Mutti (1992), is the strictequivalent of Boumas Ta division. F8 deposits can gradually overlieF7 or form the bases of beds. Mutti (1992) interpreted this facies as a latesuspension-sedimentation stage from a gravitationally reconcentrated(Fisher 1983) sandy high-density turbidity current.

F9 encompasses Boumas Tb through Te divisions (Mutti 1992) butalso includes calcilutites (formerly referred to as hemipelagites; seebelow). It is the most common facies in the transition between lobes andbasin plain. F9 deposits were deposited by traction-plus-fallout processesfrom low-density turbidity currents. This facies has been discussedextensively elsewhere (see review in Pickering et al. 1989; see also Komar

FIG. 6. Lateral trend of the bulk lithological composition of sections fromsheet-like lobes (Jaca section, right) to basin plain (Anso section, left). Symbols asin Figure 5.

TABLE 1.Main lithological parameters from sheet-like lobes to basin plain.

Section Jaca Estarrun Aragues Anso

No. of turbidite beds (*) 367 (5901) 431 287 277No. of thick (. 10 cm) beds 83 123 92 96No. of thin (, 10 cm) beds 284 308 195 181

Bed thickness range (cm) 0.5310 1520 0.5301 0.5353

Mean bed thickness (cm) All beds 8.387 10.788 13.609 16.887thick (. 10 cm) beds 22.276 25.105 32.140 38.973thin (, 10 cm) beds 4.328 5.071 4.922 5.172

Mean clean-sandstone thickness (cm) All beds 5.585 5.184 5.046 4.563thick (. 10 cm) beds 15.131 13.669 13.431 11.895thin (, 10 cm) beds 2.795 1.795 1.021 0.674

Mean clean-siltstone thickness (cm) All beds 0.05 0.192 0.265 0.536thick (. 10 cm) beds 0.054 0.284 0.282 0.610thin (, 10 cm) beds 0.049 0.155 0.255 0.496

Mean dirty sand+siltstone thickness (cm) All beds 0.142 0.492 1.870 3.837thick (. 10 cm) beds 0.627 1.711 5.726 10.607thin (, 10 cm) beds , 0 0.005 0.031 0.075

Mean shale thickness (cm) All beds 2.552 4.690 5.491 6.455thick (. 10 cm) beds 6.371 8.767 11.061 13.729thin (, 10 cm) beds 1.436 3.062 2.834 2.769

Mean calcilutite thickness (cm) All beds 0.046 0.032 0.839 1.474thick (. 10 cm) beds 0.093 0.027 0.984 2.609thin (, 10 cm) beds 0.033 0.034 0.781 1.159Beds/meter 11.923 9.269 7.348 5.922Sand ratio 0.67 0.48 0.37 0.27

(1) In Jaca section there is a fairly thick covered interval, hence fewer turbidite beds (367). Total number of beds in Jaca section (590) has been extrapolated froma smaller interval for which 73% of Jaca beds are in the Estarrun section.

(*) Calcilutite intervals with a transitional base have been incorporated into the respective underlying beds.


1985) and further discussion is outside the scope of this work. However,some comments about the Tc divisions are in order.

Tc divisions consist of cross-laminated very fine sandstone to coarsesiltstone, commonly showing convolute bedding, and lack mudstonedrapes that separate sets of ripples. Tc ripples are small (wavelength is lessthan 10 cm and usually less than 7 cm), asymmetric, 2D to 3D (linguoid)bedforms that migrated in the same direction as paleocurrents indicatedby sole marks (Figs. 4, 9). Also, Tc divisions form a part of a strict Boumasequence, and unless overlain by Group 2 facies (see below) they underlierelatively thin (usually less than 9.0 cm) Td plus Te divisions.

Interpretation of the Group 1 Facies

Paleocurrent directions indicated by sole marks (Fig. 1C) and bothdowncurrent and vertical facies evolution suggest that facies of this groupwere deposited from axially evolving, simple waning flows. Also, the sole-mark paleocurrent pattern following the trend of the southern forelandmargin (Fig. 1C) suggests that these axially evolving flows were deflectedby the basin topography (cf. Kneller et al. 1991; Kneller and McCaffrey1999). Because no other major modifications of the flows depositing thesefacies seem to have taken place, facies of Group 1 are interpreted as thedeposits from primary (deflected) flows.

GROUP 2: FACIES FROM SECONDARY (REFLECTED) FLOWS

We define a new group of fine-grained facies that do not fit Muttis(1992) general facies tract (Fig. 8). The new group involves the samegrain-size populations as F9 and consists of four main facies: Fm-1 toFm-4. The volume represented by these facies increases from 18% in thelobe, where they are only present in , 2% of beds, which are the thickest,to up to 78% in the basin plain, where they become widespread,appearing in at least 36% of beds (81% of thick beds) in the Anso sectionand being a diagnostic feature. Other key points of this group of facies,compared to the group of primary (deflected) flow facies, are: (1)paleocurrent directions diverge from those of the underlying primary(deflected) flow facies, and also among the facies of this group in the sameturbidite bed (Figs. 4, 9); and (2) facies of this group in the same bed forma complex, overall graded sequence with abrupt grading reversals(sawtooth grading of McCaffrey and Kneller 2001) that evolves intoa thick mudstone cap (Fig. 8A, C), distinctively thicker than that of F9facies (Td plus Te; Table 3). The main features of these facies aresummarized in Table 3. Additional comments are given below to bettercharacterize them.

Fm-1

Fm-1 refers to rippled very fine sandstone to coarse siltstone. Bedformsare slightly asymmetrical to symmetrical (cf. Marjanac 1990; McCaffreyand Kneller 2001). In a few cases (some 10 beds in the studied interval),the plan shape of the bedforms is visible and shows well-defined laterallycontinuous, sinuous crests that may bifurcate (Fig. 10). Ripplewavelength, averaging 4060 cm (Fig. 10) and reaching a maximum of2 m, is the most diagnostic feature of this facies when compared withprimary ripples (Boumas Tc). Ripple height ranges between 1 and 12 cm,and crest-sinuosity wavelength is up to 2 m (Fig. 10A). Rare examples ofrounded, isolated, hummock-like bedforms have also been detected.Cross lamination is rarely affected by convolution and displays climbingpatterns that evolve upward from stoss-erosional to stoss-depositionaland finally sinusoidal, showing silty drapes that thicken into the troughs.Very thin laminae of mud can be interleaved toward the top of the silty

FIG. 7. Lateral trend of lithologicalparameters from sheet-like lobes (Jaca section,left) to basin plain (Anso section, right)expressed as fractional variation. See Table 1for the corresponding numerical values. Rep-rinted from Remacha and Fernandez (2003),with permission from Elsevier.

TABLE 2.Equivalence between lithological divisions and facies.

Lithological divisions Facies

Clean sandstone F5, F7, F8, F9lam (Tb), F9rip (Tc), Fm-1, Fm-2Clean siltstone F9shear (Td), F9grad (coarser part of Te), Fm-3Dirty sandstone/siltstone Homogeneized intervals consisting of Fm-2+Fm-

3+Fm-4Shale F9shale (finer part of Te), Fm-4Calcilutite Hemiturbidite


drapes within troughs. This facies commonly displays an erosional baseand evolves upward, either sharply or transitionally, into a thin, relativelypoorly sorted mudstone division (Fm-4 facies; see below) forming Fm-1Fm-4 couplets (Fig. 8C).

Fm-1 intervals overlie a primary facies and may consist of up to two orthree Fm-1Fm-4 stacked couplets, although usually there is only one.When several couplets are stacked, the mudstone (Fm-4) caps of the lowerones are preserved only locally, leading to amalgamation of Fm-1

TABLE 3.Main facies features.

Facies (no. of bedscontaining each facies)

Mean Thickness (cm):(Std. Dev.); Range

Mean MaximumGrain Size (Range) Texture

PrimarySedimentaryStructures

OtherFeatures

InterpretedSedimentary Processes

Group 1: facies from primary (deflected) flows

F5 (13) 19.4 (22.0); 1.070.0 M/C (Granule toM/F)

Poorly sorted.Ungraded tocoarse-tail grading

None Common impactfeatures and rip-upmudstone clasts.Frequent fluid-escape structures

En massesedimentation.Hindered settling

F7 (22) 6.1 (4.5); 2.220.5 M (C to F) Inversely gradedlayers forming anoverall fining-upward division

mm-thickhorizontal layers

Commonlyindistinct layers

Frictional freezing oftraction carpets (?).Longitudinallysegregated grains (?)

F8 (Boumas Ta division)(239)

10.3 (8.5); , 157.0 F/VF (M to VF) Well sorted.Distributiongrading

None Tabular scours andrip-up mudstoneclasts. Rare fluid-escape structures

Grain-by-grainsuspensionsedimentation

F9lam (Boumas Tbdivision) (26)

3 (1.8); 1.07.0 VF Rarely F Well sorted Submillimetricparallel laminae

Rare fluid-escapestructures

Upper-regime tractionplus fallout

F9rip (Boumas Tc division)(1066)

3.7 (3.5); , 129.5 VF to siltExceptionally F

Well sorted Ripple-drift cross-lamination

Commonconvolution.Linguoid, cm-spaced ripples

Lower-regime tractionplus fallout

F9shear (Boumas Tddivision) (241)

0.8 (0.7); , 15.0 Silt and mud Well sorted silt- andclay laminae.Overall fining-upward trend

Parallel lamination Shear sorting in theviscous sublayer

F9grad (Coarser part ofBoumas Te division)(144)

0.9 (0.7); , 13.5 Silt to mud Well sorted. Normalgrading

None Fallout

F9shale (Finer part ofBoumas Te division)(1039)

3.7 (2.6); , 121.0 Mud to clay Homogeneous None Common upwardenrichment incarbonate

Fallout

Group 2: facies from secondary (reflected) flows

Fm-1 (38) * 3.6 (2.8); , 111.0 VF to silt Well sorted. Overallfining-upwardtrend

Ripple-drift crosslamination tosinusoidallamination

Long-wavelength(up to 2 m) ripples.Sinuous crestswhich sometimesbifurcate

Traction plus fallout.Strongest undularbores

Fm-2 (14) * 1.7 (1.5); , 110.0 VF/silt to silt Well sorted. Normalgrading

None Shearingdeformationalfeatures

Fallout. Intermediateundular bores

Fm-3 (14) * **; , 13.0 Silt and mud Well sorted silt-and clay laminae.Overall fining-upward trend

Parallel lamination Shearingdeformationalfeatures

Fallout; shear sortingin the viscoussublayer(?). Weakundular bores

Fm-4 (125) * 20 (36.5) *; , 1300(?)* Mud and clay Normal gradingto homogeneous.Poor sorting ofbasal divisions

None Shearingdeformationalfeatures. Commonupward enrichmentin carbonate

Collapse of dampedprimary flow. Falloutfrom the weakestundular bores

Hemiturbidite (Finest partof Te/Fm-4 divisions)(257)

2.7 (2.1); , 110 Mud and lime mud.Planktonic organisms

Homogeneous,sometimes w/a graded or graded-laminated lowerpart

None Common downwardenrichment in clay.Moderatelybioturbated

Fallout ofhydraulically sortedcarbonate particles

* Referred to entire intervals of this facies which may be composed of several stacked divisions (see text).** Not calculated because of inaccurate measures of thickness.


divisions (Fig. 8C). Upward through the deposit, Fm-1 units thin andfine, whereas Fm-4 units thicken slightly. Ripple laminae directly belowthe erosional bases of couplets can show drag features (folded or evenoverturned laminae).

Paleocurrents from Fm-1 diverge from those of the underlying primaryripples and sole marks at angles up to 120u (Figs. 4, 9). Fm-1 paleocurrentvalues from the same section are not constant, spreading over about 210u.The degree of spread is, at least partially, attributable to the long-wavelength sinuosity of bedform crests not fully exposed in the smalleroutcrops.

All these features allow distinction between Fm-1 and Tc (primaryflow) ripples (see above). However, discriminating between Fm-1 ripplesand Tc ripples must be undertaken with caution because their respectivecharacteristics overlap to some extent, and also because the Fm-1paleocurrents are not constant. Consequently, we have taken a conserva-tive attitude and classified Fm-1 ripples as only those bedforms clearlydisplaying the features described above. Doubtful cases have beenincluded in the primary-flow-ripples (Tc) category.

Bedforms in Fm-1 seem to differ from those developed in very fine sandunder purely unidirectional flows (see Yalin 1972, Baas 1994, andreferences therein) and are more similar to bedforms generated bycombined flows (cf. Arnott and Southard 1990; Banerjee 1996). However,an increase in both ripple spacing and symmetry has been reportedexperimentally as concentration of fine-grained sediment increases in thedriving unidirectional flow (Kuenen 1967; Bradley 1986). For very finesand, Kuenen reported the formation of low, nearly symmetrical ripplesspaced as much as 25 cm and consisting of alternating laminae of siltysand and silty clay. These bedforms resemble the Fm-1 ripples, but thelatter show a tendency to bifurcate and sometimes have rounded forms,and their spacing is much greater. In addition, they lack clay-rich laminae(except in their upper part; see above). On the other hand, the formationof internal waves on the upper surfaces of turbidity currents has beendocumented in both natural and experimental flows (Wright et al. 1988;Kneller et al. 1997). Moreover, Kneller et al. (1997) have demonstratedthe ability of internal waves to interact with the unidirectional flow at thebed (see below) with the resulting combined flow being, thus, able to formcombined-flow bedforms.

Fm-2, Fm-3, and Fm-4

In most cases, the Fm-1 facies interval or a primary flow facies unit isoverlain by a thick (up to 4.5 m, averaging 34.6 cm) interval that can bedescribed in terms of two units (Figs. 8D, 11A). They are: (1) a gradedmuddy sandstonesiltstone overlain by (2) a graded to homogeneousmudstone cap.

The graded muddy sandstonesiltstone unit is formed of poorly sorted,muddy sandstonesiltstone grading upward into muddy siltstone witha dappled appearance produced by darker intraclasts (Fig. 11B). Thelatter consist of (a) small pseudonodules (usually up to 12 cm) composedof relatively clean, very fine sandstone or siltstone (Fm-2; see below) or ofsilt-laminated mudstones (Fm-3; see below), and (b) pieces of recumb-ently folded silt-laminated mudstones (Fm-3) displaying clearly vergentfolds. Intraclasts become smaller and scarcer upward until they disappearat the top of the lower unit. The maximum grain size of this unit is veryfine sand, always finer than that of the underlying facies in the same bed.

The graded to homogeneous mudstone cap is thicker than Boumas Te(F9) primary subfacies (Table 3). It may consist of one to several gradeddivisions separated by subtle but sharp grain-size breaks. Commonly, thisunit is in gradational contact with the underlying one, but in some cases,mainly above an Fm-1 or primary flow facies unit, its base is sharp.Toward the top it almost always grades into a calcilutite division.

In some beds, these two units, especially the lower one, pass laterallyand vertically into an organized deposit formed of three facies, namelyFm-2, Fm-3, and Fm-4. The transition always takes place throughincreasing deformation (see below) of the Fm-2 to Fm-4 intervals, whichprogressively become dismembered and homogenized (Fig. 11B, C), untilonly the intraclasts bear witness of the original fabric (Fig. 11B).

On the basis of these exceptionally preserved examples, the originalfeatures of the Fm-2 to Fm-4 facies can be recognized and theirrelationships reconstructed (Fig. 8A, C).

Fm-2 consists of a thin (less than 10 cm thick) division of relativelywell-sorted, very fine sandstonecoarse siltstone, grading upward intomedium siltstone (Fig. 11C). Distribution grading seems to be present onthe basis of field observations. Fm-3 is a very thin (up to a few centimetersthick), graded, parallel-laminated division consisting of one to severalcouplets of silt laminae and mud laminae (Fig. 11C) and closely resemblesthe Bouma Td division. Fm-4 is a poorly sorted, silty mudstone,sometimes with scattered very fine sand grains, grading upward intohomogeneous claystone that, in some cases, forms the whole division. Nolaboratory analyses have been made to detect the internal fabric. Thethickness of this facies ranges from less than a centimeter to 3 m,although, locally, some of the thicker examples are actually composed ofseveral stacked Fm-4 divisions (Fig. 11D). The thickness and compositionof this facies varies depending on position within the deposit. As a rule,the higher this facies lies, the thicker and finer-grained it is.

These three facies stack to form an elemental unit, a fining-upwardsequence composed of up to three divisions from Fm-2 to Fm-4 (Fig. 8C).In turn, several elemental units stack to form an overall graded interval.The lower elemental units are sandier and dominated by Fm-2 and Fm-3divisions (see also Fig. 11C). Usually no more than two stacked elementalunits of this type are found. In successively higher elemental units, (a)Fm-2 fines and thins (Fig. 11B) until it disappears, (b) Fm-3 decreases inthe number of siltstonemudstone couplets, and (c) Fm-4 quicklythickens and fines by losing the silt- and sand-size particles. As a result,the intermediate elemental units consist of Fm-3 and -4 divisions, and theupper ones form a stack of Fm-4 divisions that typically are hard tosubdivide (Fig. 11D). Finally, the uppermost division of homogeneousFm-4 is always the thickest and merges upward into a calcilutite division,which represents the end of the reconstructed sequence.

The basal division of an elemental unit, especially where it is Fm-2 orFm-3, may have a loaded and sheared base where it overlies Fm-4 in thetop of the previous elemental unit. There is a complete spectrum of loadstructures from load casts attached to the parental layer to detachedpseudonodules in the thicker, underlying Fm-4 divisions (Fig. 11B).Shearing affects both the load structures and the host sediment toa variable extent, fading downwards and exhibiting both folding andthrusting of the laminated Fm-3 division below the Fm-4 division of theelemental unit (Fig. 11C, D). Shear direction may not agree withpaleocurrents from sole marks (i.e., primary-flow direction) and/or fromFm-1 and are even reversely directed (to the south; Figs. 4, 9, 11C).

r

FIG. 8.A) Facies tracts of both primary flows (cf. Mutti 1992, his Figs. 26, 27, and 32) and reflected flows. B) Markov chain analysis of vertical facies transitions.Analysis has been done following the method of Powers and Easterling (1982). Numbers denote residuals [(observed transitions fitted transitions)/sqr fitted transitions]of which the positive ones are shown. x2 testing is statistically significant at the 99.9986% level. Calculations have been performed on the whole set of beds from the fourstudied sections (Jaca, Estarrun, Aragues, and Anso). C) Idealized sequence of facies in a basin-plain bed deposited from a large-volume flow. D) Usual appearance of thesequence of facies in a basin-plain bed deposited from a large-volume flow (see text for details; compare with Fig. 11A).


FIG. 9. Simplified map of the Banaston outcrop belt between Jaca and Anso showing the pattern of paleocurrents from flutes, primary, and reflected ripples(Boumas Tc and Fm-1, respectively) and from shear directions (see also Figs. 1C and 4; see text for details). Paleocurrents have been taken in the four main sectionsdiscussed in the text plus an additional outcrop located very close to the wedging out of the Banaston-2 against the foreland margin, and correspond to the intervaldepicted in Figure 3B.

FIG. 10.Secondary (reflected flow) ripples, Fm-1. A) Field aspect of bedding surface. Notice the large spacing of bedforms and both sinuosity and bifurcation (arrow)of crests. Hammer for scale is 33 cm long. Santaliestra Allogroup at Urdues (see Fig. 1C for location). B) Close-up view of nearly symmetrical secondary ripples.Wavelength is 5060 cm (each segment of the meter stick is 20 cm long). Anso section.


Moreover, shear directions may diverge amongst groups of elementalunits.

Fm-2 formed by direct suspension sedimentation from a turbulentsuspension of relatively high sediment concentration. Absence of tractionsuggests lower flow power or higher suspended-load fallout rates than forFm-1. Under weaker, low-density, turbulent suspensions, Fm-3 mayform, analogously to Bouma Td divisions, by shear sorting of silt and clayparticles by burst-and-sweep cycles at the viscous sublayer (Hesse andChough 1980), although other interpretations have been postulated (Stowand Bowen 1980). In contrast, the rather poorly sorted, gradedmudstones and/or the apparently homogeneous mudstones of Fm-4 arerelated to collapse of a muddy flow with a sufficiently high suspended-sediment concentration to damp turbulence severely (cf. McCave andJones 1988). However, the uppermost divisions of this facies within a bed(Fig. 11D) are, wholly or partly, more likely related to deposition froma progressively more dilute remnant flow.

Interpretation of the Group 2 Facies

Any interpretation of this group of facies must account for the featuresdescribed above: (1) the paleocurrent divergence with respect to theprimary facies and within this group of facies (Figs. 9, 11C); (2) thevertical arrangement in a complex, overall graded sequence with abruptgrading reversals; and (3) the occurrence of these facies forming the upperpart of the basin-plain beds, whereas lobe beds almost always consist onlyof primary flow facies. In other words, simple waning flows depositing inthe lobe evolved downcurrent into flows that first deposited as simplewaning flows (lower part of the basin-plain beds) and then as complexwaning flows with energy pulses (upper part of the basin-plain beds).

Fm-1Fm-4 Couplets.Fm-1 bedforms, as discussed above, probablyresult from traction plus fallout under combined-flow conditions. Theformation of internal waves on the upper surfaces of flood-derived,sustained turbidity currents has been documented (Wright et al. 1988), ordeduced from facies features (Mutti et al. 1999), in shallow-water settings,in front of delta or fan-delta systems, a scenario quite different from thedeep-water environment discussed here (see below). Also, this interpre-tation fails to explain why the inferred combined-flow conditions mostlydevelop, or are at least detected, in the basin-plain element and notupcurrent, in the lobe region. Indeed, strikingly, the complex waningbehavior of the flow revealed by the whole vertical sequence of the faciesof Group 2 is first evident in the basin-plain beds. If this behavior hadbeen inherent to the flows, it would be expected to be present also, andmainly, in the lobe beds (see Kneller and McCaffrey 2003). The complexvertical sequence (sawtooth grading) has been attributed to flowreflection from basin margins (McCaffrey and Kneller 2001), a possibilitysupported by the paleocurrent divergence, which is considered the bestindicator of flow reflection from basin margins (Kneller et al. 1991). Allof that suggests that Fm-1 ripples could be the result of reflected flows.Ripple formation by transverse internal waves generated by reflectedflows has been suggested by several authors (Pickering and Hiscott 1985,Kneller et al. 1991, Edwards et al. 1994, and references therein). Althoughthe absence of any significant grain-size break between the primary-flowdeposits and the reflected-flow deposits argues against flow reflection,recent laboratory data show that reflected flows have a velocity on thesame order as the original flow and thus have the potential to transportsediment deposited by the forward flow (Kneller et al. 1997). This abilitywould be enhanced by the potential bulking through erosion of thereflecting flow as it goes back down the ramp of the obstacle, a possibilitydemonstrated in experiments with saline water (Garca and Parker 1993).Moreover, Kneller et al. (1997, see their Fig. 9) have demonstrated fromlaboratory data that weaker reflected flows (undular internal bores ofSimpson 1987; type-A bores sensu Edwards 1993; Edwards et al. 1994)

display orbital fluid motions with a period in the range of a few seconds,and that these internal waves may affect the bed in contrast with thosegenerated during the passage of a normal turbidity current, which donot. Therefore, near-bottom combined-flow conditions may exist whenturbidity currents undergo reflection. Furthermore, the profile of velocityvs. time during the passage of a train of solitary waves has a sinusoidalform with stepwise reversals and a mean shear velocity declining withtime (Kneller et al. 1997, see their Fig. 9). This pattern agrees with thevertical arrangement of the stacked couplets of Fm-1 plus mudstone (Fm-4), which fit well in a waning pulsing succession of sedimentation events.During the passage of a wave, traction progressively wanes while falloutrapidly increases to become dominant during the inter-bore quiescentperiods when the thin mudstone (Fm-4) drapes are most probablyproduced (see below; Edwards 1993; Edwards et al. 1994; Kneller et al.1997). Features of the remaining facies of this group suggest that Fm-1records the passage of the strongest bores recorded.

The Fm-2RFm-4 Sequence.The Fm-2RFm-4 sequence is a continu-ation of the sequence formed by the Fm-1Fm-4 couplets. The overallcomposite sequence indicates a waning, pulsing flow that conforms to thepattern of behavior of a train of bores decaying in time formed byreflected flows and separated by quiescent periods. In addition, evidenceof flow reversals given by the shear directions (Fig. 11C), albeit moresubtle than in Fm-1, are present in Fm-2 to Fm-4. We speculate thatduring quiescent periods the rear parts of the residual primary flow suffera severe loss of energy, carried away by the passing bores (Edwards et al.1994; Haughton 1994) and then collapse rather abruptly from a relativelyhigh-density, nonturbulent flow (most Fm-4 divisions; see above).

Sedimentary Record of Bores and Synsedimentary Deformation.Summarizing, the sequence formed of the facies of Group 2 is interpretedas recording deposition from a waning, pulsing flow that may haveresulted from the passage of decaying undular bores (moving hydraulicjumps; see Simpson 1997; type A bores, Edwards 1993, Edwards et al.1994; see also Kneller et al. 1997) generated by flow reflection from thebasin margins. Complex traction plus fallout, under combined-flowconditions (Fm-1), was succeeded by fallout alone (Fm-2 to Fm-4) assuccessive internal waves of progressively lower power swept across thebasin. During the quiescent periods between waves, the residual forwardflow, with its energy severely damped, collapsed to form Fm-4 divisions.

The abundant deformation that affects the intervals formed of Fm-1 toFm-4 facies is interpreted to be syndepositional to immediately post-depositional, because of its occurrence as discrete horizons withinturbidite beds, with a downward decrease of deformation (see Fig. 11C),the plastic rheology displayed, and the close relationships between thedeformed and the parental materials. The flat depositional topography,deduced from the correlation (Fig. 4; see also below), precludes an originby slumping or creeping and points to shearing by an overriding flow.This would account for the deformed cross-laminae in the Fm-1 divisions(cf. Rust 1968) and the folds and thrusts observed in some Fm-3 divisions(Fig. 11C). Shear features principally underlie Fm-1 facies, suggestingthat shear on the bottom was predominantly produced by the strongestbores. Also, it is interpreted that the combination of liquification andplastic deformation accounts for the overall soft-sediment deformationprocesses that led to the progressive obliteration of the original sequencesof facies Fm-2 to Fm-4, and finally resulted in the graded muddysandstonesiltstone and the graded to homogeneous mudstone capcontaining relics of the constituent facies (Fm-1 and Fm-2 pseudo-nodules). Because of the complex interaction among these processes, theirdetailed relationships have not been fully deciphered and further studiesare necessary. In light of the preliminary results, two main processesassociated with the repetitive passage of waves are envisaged: (1)vibration liquification (or liquefaction; see Nichols 1995) triggered by


cyclic wave loading and (2) shear liquification. Subordinate seepageliquification (or fluidization) and plastic behavior features also wouldaccount for obliteration of the primary features. This process wasparticularly effective where relatively thick newly deposited Fm-4divisions were present, suggesting strongly that inter-bore deposits weremetastable during wave passage. That points to a rapid deposition ofmost of the Fm-2 to Fm-4 facies, which displayed a thixotropic behaviorduring vibration.

CALCILUTITES: HEMIPELAGITES OR HEMITURBIDITES?

Calcilutites (Table 1; hemiturbidite facies in Table 3) are moderatelybioturbated, medium-gray marlslimestones, weathering to a distinctivewhitish color (Fig. 11E), which commonly lightens upward, suggesting anincrease in carbonate content. Their qualitative mineralogical composi-tion is similar to that of the sediments of the southern-forelandmargincarbonate ramp (Burgui Marls Formation of Camara and Klimowitz

1985). The biogenic content is scattered and variable from bed to bed; itconsists mainly of planktonic foraminifera and coccoliths, and rarebenthic foraminifera (Mutti et al. 1972).

Calcilutite divisions are , 10 cm thick and most commonly transi-tionally overlie either Boumas Te (F9) (Fig. 11E) or the uppermost Fm-4division of a bed. The transition is usually gradual through a significantportion of the terrigenous mudstone division (more than half of thethickness in the case of Te), although there is a steeper gradient of thecarbonate content at the top, marking the base of the calcilutite. Both inthe basin plain and laterally toward the southern foreland margin,calcilutite divisions also directly overlie Boumas Tc (primary ripples),thus forming the mudstone division, or constitute individual beds. In thelatter case the coarsest biogenic particles are concentrated near the base,forming a graded or parallel-laminated lower interval (Td division). Also,there is a relationship between calcilutites and the underlying facies withinthe same bed. For the dataset of the four studied sections, 45% of bedswith secondary-flow facies have a calcilutite cap (mean thickness 3 cm),

FIG. 12.Detailed stratigraphic cross section from sheet-like lobes (Jaca section, right) to basin plain (Anso section, left) showing calcilutite beds as the end membersof downcurrent evolution of beds (e.g., interval between Jaca section beds # 8 and 9). Note that lobe beds tend to consist of a sandstone division sharply overlain bya mud division, whereas in basin-plain beds sandstone division gradually pass into the mudstone division via a siltstone interval (compare with Fig. 5). Also note theoverall thickening of thicker lobe beds and the wedging out of the thinner lobe beds toward the basin plain (e.g., interval between Jaca section beds # 4 and 6). SeeFigure 4 for stratigraphic location.

r

FIG. 11.A) Thick bed displaying secondary-flow facies (overturned section, stratigraphic top toward lower left corner). Arrows mark the base and top of the bed.From base to top, bars lie at the limits between primary flow facies, an Fm-1Fm-2 interval, the graded muddy sandstonesiltstone, and the mudstone cap. B) Typicalfield aspect of a graded muddy sandstonesiltstone interval. The unit between the lower and middle bars, which overlies the primary facies interval, is a transitional casebetween the sandier part of the graded muddy sandstonesiltstone unit and the original, Fm-2-dominated interval; notice the large pseudonodules (bold arrows). Abovethat, the interval between the middle and upper bars is formed of Fm-2 and Fm-3 facies showing well developed load casts, locally detached and forming pseudonodules(white arrow on the right). Finally, the interval above the upper bar is a thick graded muddy sandstonesiltstone unit showing deformed fragments of the immediatelyunderlying interval that draw tight recumbent folds facing to the left (white arrows on the left). Notice the presence of small pseudonodules elsewhere (e.g., black arrowsin the upper part). Diameter of coin is 2.1 cm. C) Exceptionally well-preserved example of two elemental units, each composed of an Fm-2Fm-3 couplet. Noticesynsedimentary folding and thrusting of the uppermost Fm-3 laminae in the upper elemental unit marking a rightward (southward) shearing from overriding flow. Theoverlying interval in the same bed, out of the field of the photo, is a pervasively deformed, graded muddy sandstonesiltstone interval. Coin is 2.5 cm. See Figure 4 forlocation of this bed. D) Thick mudstone cap formed by stacked Fm-4 elemental units, which are barely distinguishable. Notice the syndepositional deformation revealedby the truncation surface and angular relationships of the lowermost elemental units (lower arrow) and by the recumbently folded upper elemental unit (upper arrow)marking a drag towards the right (northwards). Curvilinear, fan-shaped features at the right correspond to plumose pattern of a joint. Scale in centimeters and inches. E)Calcilutite-rich, thin bed interval showing the gradational relationships between Te mudstone (light gray) and the calcilutite (whitish) divisions within each bed. Sandstoneand siltstone divisions of beds are dark to medium gray. Scale in centimeters and inches. The five photographs are from Anso section.


while only 15% of beds lacking secondary-flow facies have a calcilutitecap, which is also thinner (mean thickness 2.5 cm).

Calcilutites appear locally in the lobe and become widespread in thebasin plain. In correlated beds, they evolve from thin marls in the lobeand lobebasin-plain transition to thicker marlslimestones in the basinplain, where they can form the whole bed following the pinch-out of theunderlying terrigenous divisions (Fig. 12). Furthermore, the downcurrentthinning of Boumas Te (F9) division is accompanied by a downcurrentthickening of the calcilutite. Laterally toward the southern forelandmargin, calcilutite divisions become amalgamated as the terrigenousportions of beds onlap onto the topographic highs, which are draped bycondensed calcilutite intervals.

Since the work of Mutti et al. (1972), calcilutites have been interpreted,not only in the Hecho basin, as hemipelagites, i.e., the non-turbiditicsediments in the basin, and considered as one of the diagnostic features ofthe basin-plain facies association (e.g., Mutti and Ricci Lucchi 1972,1975; Rupke 1976; Mutti 1977, 1979; Mutti and Johns 1979; Nilsen 1984).Notwithstanding, a turbiditic origin has also been suggested (e.g., Kuenen1964; Van der Lingen 1969; Hesse 1975; Rupke 1975; Stanley andMaldonado 1981; pertinent papers in Stow and Piper 1984; Stow andWetzel 1990), giving rise to the term hemiturbidites (Stow and Wetzel1990). We also point to a turbiditic origin for the calcilutites of thestudied interval, although this controversy cannot completely be resolvedon the basis of field observations.

The mode of occurrence of calcilutites and their internal structuringwhen forming a whole bed (cf. Fauge`res et al. 1984) strongly suggesta turbiditic origin. Also, bioturbation is less intense than in truehemipelagites. The downcurrent and lateral evolution of calcilutitessuggest that the flows, as they dropped their terrigenous load, began todeposit carbonate sediments, which can finally form entire beds inthemselves. This means that calcilutites may constitute the distal endmembers in both the primary and the secondary facies tracts (endsubfacies of F9 and of Fm-4; Fig. 8). We suggest that these hemiturbiditesare the result of hydraulic sorting of the finer and lighter carbonateparticles toward the upper part of the flow. The lower settling velocities ofthe carbonate particles, when compared to the terrigenous muds, mostlikely result from a decreased ability of the former to flocculate (see Piper1978; Stow et al. 1984). Thus, the fine-grained carbonate sediment wouldremain in suspension, settling slowly until the flow ceased. Hydraulicsorting of carbonate particles affected not only the mud-size population,as we propose here, but also the coarser (sand grade) population, asdemonstrated by Fontana et al. (1989), who invoked both density andshape of the carbonate particles as the factors controlling their hydraulicsorting.

The qualitative mineralogical composition suggests that the sedimentsof the foreland-margin carbonate ramp in the south were the main sourceof the calcilutite sediment. Carbonate sediment was most probablyincorporated into the flows by bulking through erosion in two locations:first from the foreland margin adjacent to the shelf, the canyon, and theinner depositional elements and, later, when the turbidity currentsobliquely encountered the foreland margin ramp and underwent re-flection, flowing up and then down the ramp (see below). Carbonateenrichment of flows during reflection is supported by the relativeabundance and thickness of calcilutite divisions in beds with secondaryfacies vs. beds lacking secondary facies.

RELATIONSHIPS BETWEEN SHEET-LIKE LOBE AND BASIN-PLAIN ELEMENTS

The bed-by-bed correlation, simplified in the cross section of Figure 4,demonstrates that the sheet-like lobe and basin-plain elements aregenetically related, because all of the beds in the basin-plain element(Anso section) are found in the upcurrent sheet-like lobes (Jaca section)(see Remacha and Fernandez 2003). However, only 50% of the lobe beds,

principally the thickest and coarsest-grained beds, extend downcurrent tothe basin plain (see also Fig. 7). Statistically, lobe beds thicker than10 cm, containing a clean sandstone division thicker than 6 cm, havea 95% probability of reaching the basin plain. Conversely, most of thethin lobe beds thin downcurrent and wedge out before reaching the basin-plain element; the rate of loss is about 2.3 percent of thin beds perkilometer. Also, 50% of the beds that reach the basin plain thickenthrough the lobe-basin plain transition, this being accomplished mainlyby means of the homogenized Fm-2 to Fm-4 intervals (i.e., the dirtysandstonesiltstone and shale divisions; see Table 1; Figs. 4, 7, 12).

The most striking feature in the cross section in Figure 4 is the even andparallel correlation pattern that extends across both elements. Thispattern results from the downcurrent thickening of the thickest beds,which compensates for the downcurrent thinning to wedging-out trend ofthe thinnest beds (see also Fig. 12). This highest-frequency mechanism,related to flow reflection, provides a possible explanation of thecompensation process.

Lateral Facies Evolution

Primary facies divisions are in accordance with Muttis (1992) faciestract (Fig. 8A, B). Thick (. 10 cm) beds in lobes are characterized bya high-concentration flow facies interval (F5 to F8 divisions) overlain bydilute-flow deposits (F9 interval). The latter typically consists of a Tcdivision sharply overlain by a Te division (Fig. 13). Toward the basinplain, beds progressively lose the basal high-concentration flow depositsand consist mainly of F9 beds (Tbe to Tce) capped by a hemiturbiditeinterval, or of top-missing F9 beds (Tbc to Tcd) capped by a secondaryfacies interval (Fig. 13). Naming beds after their basal facies, the F5 (i.e.,the beds whose basal part is formed of an F5 division), F7, and F8 bedstend to maintain or slightly decrease in frequency toward the basin plain.In contrast, F9 (Tc) beds show a marked decline in frequency toward thebasin plain, accompanied by a concomitant increase in F9 (Td) beds(Fig. 14). Hemiturbidite beds in the basin plain are more frequent in theAragues Section, possibly as a consequence of the marginal position ofthis section with respect to the basinward, but more axial, Anso Section.Bed-by-bed correlation shows that lobe beds with high-density turbidity-current facies (F5 to F8 beds) are more prone to reach the basin plain,while F9 lobe beds deposited from low-density currents tend to wedge outbefore reaching the basin plain (see also Remacha and Fernandez 2003).

Reflected facies, as they become more common toward the basin plain,vary in character (see Fig. 13); Fm-2 to Fm-4 deposits acquire moreimportance with respect to Fm-1 divisions. The latter also tend topreferentially overlie dense primary-flow facies (F5 through F8), i.e., Fm-1 deposits tend to appear in the thicker beds. Moreover, bed-by-bedcorrelation shows that the ability of flows to undergo reflection uponreaching the basin plain may be linked to flow volume and momentum.The larger flows depositing F5, F7, most of F8 and the thicker (. 12 cmthick) Tce beds in the lobes underwent reflection in the basin plain, whilethe smaller and more dilute flows depositing thinner Tce and Tde lobebeds did not, provided that they reached the basin plain.

DISCUSSION

Processes in Topographically Driven Modified Flows

Considering the evolution of flows, two groups can be distinguished:simple waning flows, which evolve downcurrent strictly following Muttis(1992) facies tract, and composite (pulsing) waning flows, which do not(Fig. 8A, C). Simple waning flows are the smaller and more dilute flowsthat have an incomplete basin coverage and do not display any evidenceof reflection processes. This type of flow is recorded mainly in sheet-likelobes essentially by F9 beds, which thin downcurrent and wedge outeither in the transition to basin plain or within the latter element.


Composite waning flows are the large-volume and high-density flows.They have a high to complete basin coverage and the beds that theydeposited are the main component (volumetrically) of the basin-plainelement. In the sheet-like lobes they behaved as simple waning flows,being recorded by beds that on average are 20 cm thick and contain36.5% of the dense-flow facies (F5 to F8). Composite waning flowsrecord a spatial change after encountering a topographic obstacle. Thisobstacle is represented by the southern foreland-margin ramp, assuggested by the distribution of Fm-1 paleocurrents, which mainlyspread radially away from it (Fig. 9). After encountering the southernbasin margin, downdip of the sheet-like lobes, the flows experienceddecoupling of a lower higher-density part and an upper lower-density part(see Kneller and McCaffrey 1999). The lower part was deflected to flowparallel to the basin margin, as deduced from the sole-mark paleocurrents

parallel to the ramp trend (Figs. 9, 15), and continued evolving witha simple waning behavior. The upper lower-density part of the flows wasreflected on the ramp, changed their properties to composite waning, andgave rise to the modified-flow facies. Mainly on the basis of publishedexperimental data (see references above), we can gain insight into theseprocesses.

At the foreland margin, a substantial part of the mud-rich, low-densityturbulent suspension forming most of the thickness of the forward flowwas forced to travel obliquely up the ramp and then to be reflected downthe ramp (Fig. 15). During this process, some entrainment of fine-grainedsediment from the distal carbonate ramp (Burgui Marls) could have takenplace. At the ramp toe, this process led to the formation of a bulge ofdenser fluid that was fed primarily by the flow coming back from theramp but also by the residual forward flow approaching the foreland

FIG. 13.Markov chain analysis of verticalfacies transitions for the four discussedsections. Analysis has been done following themethod of Powers and Easterling (1982).Numbers denote residuals [(observedtransitions fitted transitions)/sqr fittedtransitions] of which the positive ones areshown. x2 testing is statistically significant atthe 99.9986% level. The preferred transitionsdisplay the highest residuals and are shownwith bold lines.


margin (Edwards et al. 1994). After a time, the bulge propagated awayfrom the ramp as a bore (gravity current, or moving internal hydraulicjump, undercutting the residual forward flow) advancing on the newlydeposited sediments, and a new bulge started to be formed (Edwards et al.1994). With time a series of bores with an overall dispersive behavior wasproduced (Kneller et al. 1997).

The paleocurrents directed to the south (Fig. 11C; see also Figs. 4 and9) suggest that the flow reflected from the southern foreland margin,upon encountering the northern margin (poorly developed submarineorogenic wedge formed of the LarraBoltana plus Lakora thrusts), wasalso reflected from there by means of weaker bores. Successive reflectionsby both margins and also from the western closure of the basin (structural

rise related at least to the OrozBetelu thrust) led to the completeblocking of the currents, i.e., to ponding.

The suite of secondary facies allows us to deduce that the strongestrecorded bores are of undular type (type A bores; see above). Fm-1records the stronger (leading) recorded bores, reflected mainly from thesouthern margin. Fm-2Fm-3 would result from the later intermediatebores, from leading bores associated with less powerful flows, or fromsecond-generation, north-derived reflections, as shearing below someFm-2 divisions suggests (Fig. 11C). Finally, Fm-3, when forming the baseof an elemental unit and the uppermost Fm-4 divisions within a bed(Fig. 11D), would represent the weakest bores, deriving from anyconfining margin. In spite of the usual synsedimentary deformation and

FIG. 14.Pie diagrams showing thecomposition of each studied section in terms ofbeds as named after their basal facies (see textfor details). Notice the downcurrent evolutionfrom lobe (Jaca section) to basin plain (Ansosection) elements. F9 beds have been split intosubcategories according to the basal subfacies(cf. Table 3).

FIG. 15.Paleogeographic sketch map showing the dispersal pattern of both axial and reflected parts of large-volume flows (compare with Figs. 1B, C, and 9; see textfor details). Bi 5 Binies thrust, LkCo 5 LakoraCotiella thrust, LaBo 5 LarraBoltana thrust.


the multi-source reflections, the number of Fm-1 and Fm-2 divisionssuggests that few (24) stronger (recorded) undular bores were generated(cf. Edwards 1993; Kneller et al. 1997). Furthermore, the increasingthickness of the quiescent-period deposits (Fm-4) suggests that thestronger waves were progressively more spaced. This agrees with theexperimental results of Kneller et al. (1997).

Facies-Tract Variation

Topographically driven changes of flow properties suggest a new faciestract relative to Muttis (1992) (Fig. 8). This variation concerns the fine-grained facies group. It depends strictly on (1) high flow volume and (2)a lower, sand-laden, high-density part, relatively well segregated froma thicker upper, muddy, low-density part. This facies tract overliesa sharply truncated standard Muttis (1992) facies tract up to Bouma Tc(F9) and combines facies from Fm-1 to Fm-4. It is composed of a pulsingsequence comprising the stacking of divisions Fm-1, Fm-1Fm-4, Fm-2Fm-3Fm-4, Fm-3Fm-4, and Fm-4 (Fig. 8C) until the completeexhaustion of the flow by means of the final sedimentation ofa hemiturbidite division cap. Because of synsedimentary deformation,the deposit overlying Fm-1Fm-4 becomes reorganized as a muddysandstonesiltstone grading into a mudstone cap, in which the muddysandstonesiltstone distinctively contains small soft-sediment deforma-tion structures as floating intraclasts (Figs. 8D, 11A, B).

Analogs to these facies and to some of the associated deformeddeposits have been described in other basins from the Paleozoic to theQuaternary and interpreted as the product of flow-reflection processes(e.g., Ricci Lucchi and Valmori 1980; Pickering and Hiscott 1985;Pickering et al. 1989; Marjanac 1990; Porebski et al. 1991; Haughton1994; Edwards et al. 1994; Kneller and McCaffrey 1999; McCaffrey andKneller 2001; Roveri et al. 2002). In some of these cases, a closeresemblance to the facies described here has been pointed out (Roveriet al. 2002). However, it is important to emphasize that the ideal faciestract as well as possible local variations would depend not only on flowfeatures but also on the degree of evolution of the flow at the point whereit meets a topographic obstacle, among other factors (see Simpson 1987;Edwards 1993). In summary, the study of flow-reflection processes andthe resulting facies tract must be considered case by case. In our study,flows met the ramps obliquely after a distance varying between some 60and 90 km from the transfer zone. The sediment load calculated for someof the most outstanding flows extending the entire length of the basin(between Boltana and near Pamplona) also allows them to be classified asmud-rich flows (about 3540% sand load).

Frequency of Large-Volume Flows and Maintenance of the Flat Floor

Within the depositional elements of sandy stages, the sheet-like lobesdisplay the highest preservation potential of beds. Upcurrent, in thechannel-lobe transition, this preservation potential is lower because oferosional processes. Downcurrent from sheet-like lobes, the thin bedsrelated to small-volume flows tend to wedge out (see above). Asa consequence, the ratio of thick beds to thin beds in the sheet-like lobesrecords the frequency of large-volume flows in the sandy stage, i.e.,frequency of flows that may change their properties and build the basinplain. This ratio is about 1:4, shifting to 1:2 in the proximal parts of basinplain.

Therefore, given the sheet geometry of the system and the continuitybetween sheet-like lobes and basin plain, the main effect of depositionfrom reflected flows on the floor morphology is to compensate whatevertopographic lows are created by the stacking of a discrete number ofsuccessive thin beds. Moreover, the complete blocking of the currents bythe three confining margins (ponding) led to the regulation of the flat-topped growing pattern, extending even as far back as the sheet-likelobes. In the latter element, the upper parts of the muddy divisions (partof the Te and the hemiturbidite) of the thick beds (. 10 cm) containinghigh-density facies may therefore be the final result of ponding thatcontributed substantially to the overall sheet-like character across thedepositional zone.

CONCLUSIONS

Basin-plain and lobe elements are closely related because all of thebasin-plain beds are found in sheet-like lobes. At least 50% of the flowsbuilding the sheet-like lobes reach the basin-plain element. These are thelarger turbidity currents, which have a lower high-density part. Incontrast, the smaller, low-concentration currents, after having themaximum preservation potential in the sheet-like lobes, wedge outdowncurrent within the transition to basin plain or within the latterelement, having a loss ratio of about 2.3 percent of thin beds perkilometer.

Beds in sheet-like lobes follow Muttis facies tract, i.e., correspond tosimple waning flows, whereas 36% of the beds in the basin plain do not.These basin-plain beds account for 78% of the volume of the element,forming the bulk of the distal basin environment, and correspond tocomposite waning flows. This flow behavior is due to the topographicallydriven changes of primary-flow properties and provides the diagnosticfeatures of the basin plain.

TABLE 4.Diagnostic criteria of lobe vs basin-plain beds.

Lobe Basin Plain

Facies in thick beds F5-F7-F8-F9 (Tb-c/e or Te). Reflected flow (Fm)facies may be present atop exceptionally thick beds.

Rare F5 and F7. F8-F9 and F9 (Tb-e). F9 restricted toTb-c. Reflected flow (Fm) facies overlie F5 to Tc divisions.

Facies in thin beds F9 (from Tb-e to Te) F9 (from Tb-e to Te) Some beds display Fm facies atopTc divisions.

Top of sandstone divisions Sharp. Commonly flat, or with thin, poorlydeveloped ripples. Small (wavelength, 10 cm)2D to 3D (linguoid) current ripples

Gradational. Always rippled. Large (wavelength , 2 m),sinuous to 3D (hummocky-like), slightly asymmetricalto symmetrical combined-flow ripples

Siltstone divisions Absent. May be present in ripple troughs Always presentShale divisions Usually thin (high net-to-gross ratio). Thick for

exceptionally thick bedsUsually thick (low net-to-gross ratio)

Calcilutite divisions Rare to absent CommonMiscellaneous Locally frequent amalgamation of sandstone

divisions by scouring (tabular erosional featuresand local impact features; Mutti and Normark1987). Scour marks.

Rare amalgamation of sandstone divisions. Tool marks

Criteria in italics are from the present authors: cf. Table 1 and Fig. 8 (reprinted from Remacha and Fernandez 2003; with permission from Elsevier).


At the southern foreland margin, the large, axially evolving flows wereforced to change their properties as follows: a lower sand-laden high-density part underwent deflection, flowing downcurrent parallel to theforeland margin. This part deposited high-density facies following Muttisfacies tract. The upper part, more dilute and thicker, was reflected fromthe foreland-margin ramp, generating a train of undular bores that laterunderwent multiple flow reflections by the flanking margins, i.e., thesouthern foreland margin, the northern margin in the poorly developedsubmarine orogen and the western closure of the basin. As a result, thecomplete blocking of the currents (ponding) prevented flow-out fartherthan the western boundary of the south-central Pyrenees turbidite basin,indicating that no connection between the Hecho and the Bay of Biscaydeep-water turbidite basins existed.

The passage of bores is recorded by means of four main facies typesthat form a pulsing facies sequence combining different facies: Fm-1,Fm-1Fm-4, Fm-2Fm-3Fm-4, Fm-3Fm-4, and Fm-4, each of whichmay be repeated. The passage of bores may have produced vibrationliquification by cyclic wave loading. As a consequence, syndepositional toimmediately postdepositional soft-sediment deformation destroyed theoriginal appearance of the deposit overlying the first relatively thick(greater than a few centimeters) Fm-4 division. As a result, an overallgraded muddy sandstone containing distinctive small intraclasts of Fm-2and Fm-3 divisions (pseudonodules and fragments of very thin laminae)merges into a thick mudstone cap. In the muddy sandstone, the floatingintraclasts become smaller upward until disappearing.

Calcilutite divisions reported in previous literature as true hemipela-gites are here envisaged as the product of hydraulic sorting of carbonateparticles forming the residual sediment load, which settles at the end ofthe event. Therefore, calcilutites have a turbidite origin (hemiturbidites).

The bed-by-bed correlation has shown an overall sheet-like stackingpattern extending between sheet-like lobe and basin-plain elements.Reflection processes have been envisaged as the factor responsible forbalancing any topographic low produced by small-volume flows. Asa result, reflection processes related to the relatively frequent large flowscontrol the sheet-like aggradational pattern in both elements studied. Thefrequency is 1 thick bed to 4 thin beds in the sheet-like lobes and 1:2 in thebasin plain.

The relationships between sheet-like lobes and basin plain presentedhere and the new set of facies described permit an update of the classicaldiagnostic criteria for recognition of lobe vs. basin plain (Table 4; see alsoRemacha and Fernandez 2003; cf. Mutti and Ricci Lucchi 1972, 1975;Mutti 1977; Mutti and Johns 1979).

ACKNOWLEDGMENTS

This research was supported by the Direccion General de InvestigacionCientfica y Tecnica of the Spanish government (Project PB94-1312-C02). Wethank A. Gardiner, T.A Hickson, B. Kneller, and D. Mohrig for theirthorough review of this manuscript, which led to much better organizationand exposition of data and interpretations. Editorial work by J.B. Southard isalso highly appreciated.

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