A Coarse Grained Turbidite System with Morphotectonic Control (Middle Albian, Ondarroa, Northern Iberia)

Serlirnentology (1994) 41, 383407

A coarse grained turbidite system with morphotectonic control (Middle Albian, Ondarroa, northern Iberia)

L. M. AGIRREZABALA and J. GARCIA-MONDEJAR Departamento de Estratigrajia y Paleontologia, Universidad del Pais Vasco, Ap. 644, 48080 Bilbao, Spain

ABSTRACT The Middle Albian Ondarroa turbidite system is a coarse grained, deep water unit which outcrops in the north-eastern part of the Basque-Cantabrian region, west of the Pyrenees. It is about 18 km long and 7 km wide, and shows an unusual ‘L’ shape resulting from both a direct morphotectonic confinement and the presence of nearby shallow water carbonate buildups. Eight main facies have been distinguished within this turbidite system: (1) clast-supported conglomerates; (2) mud-supported conglomerates; (3) slump deposits; (4) normally graded pebbly sandstones; (5) cross stratified sandstones; (6)interbedded graded sandstones and mudstones; (7) interbedded non-graded sandstones and mudstones; and (8) mudstones. Inner system, middle system, outer system and basin plain divisions have been distinguished. The inner turbidite system is characterized by stacked channel fill conglomerates and lesser sandy turbidites and mudstones. The middle system consists of sandy and conglomeratic fining upwards sequences, normally several metres thick. The outer system has alternating non-channellized sandstones and mudstones, without any predictable vertical arrangement. The basin plain is characterized by mudstone-siltstone laminations and lesser, randomly occurring thin bedded sandy turbidites. Three main channel fills make up the inner turbidite system. Although all of them can be compared with the valley channel fills of the modern Mississippi Fan, and thus their bases can be interpreted as sequence boundaries, only the lowermost and the uppermost channel bases are documented as allocyclic boundaries. The Ondarroa turbidite system was deposited in an immature passive margin subjected to transtensional movements. It filled a composite pull apart depression with coarse clastics derived from a narrow platform to the north of the present outcrops which was invaded by fan deltas. A major pattern of sinistral strike-slip faulting linked to the opening of the Bay of Biscay is invoked to explain the Ondarroa turbidite system appearance and its tectonic confinement.

INTRODUCTION The Ondarroa turbidite system outcrops in the north- work, the characteristic ‘L’ shape of the system was east of the Basque-Cantabrian Basin of northern reported and attributed to the topographic control Iberia. It belongs to the Deba Formation (Garcia- exerted by synsedimentary extensional tectonics, and Mondejar, 1982), a unit related to the Black Flysch inner, middle and outer system divisions were Group (Souquet et al., 1985) in the Pyrenean domain inferred and succinctly described. (Fig. 1). The presence of turbiditic conglomerates and The aim of this paper is to describe new infor- sandstones in the cliffs surrounding Ondarroa had mation and to develop the later work into a more long been reported (e.g. Voort, 1963; FeuillCe, 1967; advanced sedimentary model by (1) establishing the Aguilar, 1975; Engeser et al., 1984). Nevertheless, it changed geometries and dimensions of the turbidite was not until the work of Agirrezabala et al. (1989) system, (2) describing and interpreting its constituent that an individual deep water turbidite system was facies, (3) characterizing its main divisions sedimen- identified and its main features described. In that tologically, (4) constructing a depositional model and

383 384 L. M. Agirrezahala and J. Garcia-Mondkjar

Tou Io u s e 0

Fig. 1. (A) Map of outcrops of the Black Flysch along the Pyrenees and location of the study area. (B) Geological map of the Ondarroa tui-bidite system (units 3, 4 and 5) and encasing rocks (units I, 2 and 6). Legend: (1) Urgonian limestones; (2) mudstolies and marls with limestone megabreccias (black triangles); (3) conglomerates, sandstones and mudstones; (4) sandstones and mudstones: (5) mudstones; (6) mudstones and sandstones. Letters a-h refer to the measured sections of Figs 7, 9, 12 and 13, and the location of Fig. 8 is also shown.

(5) reconstructing the palaeogeography and dis- deposits with up to 15 km of composite vertical cussing the sedimentary controls on its origin and section, folded and thrusted during the Alpine or- development. ogeny. The origin and development of this basin were linked to the Mesozoic extensional tectonics that opened the North Atlantic (Wiedmann et al., 1983; GEOLOGICAL SETTING Garcia-Mondeiar et al.. 1986; Garcia-Mondejar, The Basque-Cantabrian Basin in northern Iberia 1989). During the Aptian-Albian interval, Iberia consists of a series of Mesozoic and Palaeogene drifted to the south-east with respect to the Ondarroa turbidite system, Spain 385

. _.. -

. 0: '-. - . 0. . \- ' Yadwlxr

2-0.5 0 5 10 km 0 0.5-0.062 - - Fig. 2. Palinspastic restoration of the dispersal pattern of the Ondarroa turbidite system based on maximum grain size variation and palaeocurrent measurements (small arrows). The inner, middle and outer system divisions are represented for the maximum areal extent of the turbidite system

European plate as a result of the opening of the Bay and a thick cover of vegetation, which hampers of Biscay (Le Pichon et al., 1971; Montadert et al., correlation and mapping of its constituent units in 1979). This motion was most probably accomplished detail. The best outcrops are in the north, just to the by means of a system of NW-SE orientated, strike- east of Ondarroa (Fig. l), where sea cliffs between the slip faults (Le Pichon et al., 1971; Choukroune & Saturraran beach and the Artibai River reveal a Mattauer, 1978; Rat et al., 1983; Boillot, 1986; 0.75 km long stretch of excellent exposures of rocks. Garcia-Mondejar, 1989). These faults, and other subordinate faults orientated NE-SW, created both areas of relatively slow subsidence (probable horsts in STRATIGRAPHY the basement) and areas of very fast subsidence (probable grabens in the basement). In the slow The Ondarroa turbidite conglomerates, sandstones subsiding blocks, mainly rudist limestones grew and mudstones were formed in a small, narrow and vertically in shallow waters, whereas in the fast elongated basin, surrounded by topographically subsiding blocks mark and a diverse suite of ter- higher areas with shallow water lutites or carbonates. rigenous deposits filled relatively deep (up to a few The present day spatial distribution of facies and hundred metres) intraplatform, elongated basins. facies associations has allowed us to distinguish The Ondarroa turbidite system was formed in one inner, middle and outer turbidite system divisions of these tectonically produced troughs. It crops out along with the basin plain (Figs 2 and 3). Both the (Fig. 1) in an area with strong Alpine deformation geological map of the system and surrounding areas 386 L. M. Agirrezabala and J. Garcia-Mondijar

NNE SSW I WNW ESE

Inner I Middle I Outer I

-.-...... _ ...... ,..-eps/ ’..._..

5 10 km I I I I

Fig. 3. Schematic longitudinal compound cross section of the Ondarroa turbidite system, showing system divisions and variation in thickness of the unit at the 1:25 000 scale (Fig. 1) and the map of the preserved part of the underlying unit and its laterally reconstructed clastic dispersal system (Fig. 2) reveal a equivalent units have an ammonite fauna character- clear ‘L’ shape. The inner and middle systems have a istic of the Middle Albian (the early Middle Albian NNE-SSW orientation, while the outer system and dentatus Zone) (Agirrezabala et a/., 1992): Meta- adjacent basin plain have a WNW-ESE orientation. hamites cf. sablieri (d’orbigny), Anagaudryceras sp., Taken as a whole, the system is on average 15-16 km Kossmatella (Kossmatella) romana Wiedmann, Puzo- long and 6-7 km wide, and it occupies an area of sia (Anupuzosia) sp., Lyelliceras sp., Neophlycticeras about 100 km2. At the end of its sedimentary history sp. and Hamites sp. the turbidite system was less confined and There is also a thick lutitic unit on top of the approached a radial or true fan form; it then reached Ondarroa turbidite system. It is rather sandy and 18 km in length, 14 km in width and about 140 km2 contains scattered siliciclastic turbidites only a few in surface area. The thickness of the complete sedi- centimetres thick. Ammonites from this unit suggest mentary body is highly variable (Fig. 3), ranging a Late Albian age: Mortoniceras (Mortonicrras) sp., from 295 m at the more proximal outcrops to about Kossmatella muehlenbecki (Fallot), Kossmatella 1OOOm at the distal end. A discussion about the (Kossmatella) romana Wiedmann, Puzosia cf. quenst- possible characterization of the Ondarroa system as a edti (Parona & Bonarelli), Hysteroceras sp. and deep water fan is included in the later section on Puzosia sp. ‘Disperal pattern’. Laterally equivalent to the turbidite system there The Ondarroa turbidite system lies on a thick unit are lutites and rudist limestones. The lutites are of black, slightly calcareous lutites which contains a attributed to either dilute turbidity currents or discontinuous interval of limestone megabreccias continuous vertical settling. The limestones were (Fig. 1). These megabreccias have olistoliths tens of probably deposited in shallow waters as suggested by metres long. They were deposited on a muddy talus their palaeontological content: rudists, corals, orbito- slope, except near Altzola, where the megabreccias linids, the ostreid-like bivalves Chondrodontu sp. and rest on shallow water rudist limestones. The lower calcareous algae. Dating fossils in the limestones boundary of the Ondarroa system is strongly discon- include Hensonina lenticuluris (Henson), Orbitolina formable, at least in its northernmost, most proximal (Mesorhitolina) texana (Roemer) and Orbitolina part, where a minimum of 55m of the underlying (Mesorbitolinu) subconcavu Leymerie, which suggest lutites were lost by erosion before or during emplace- an Early-Middle Albian age. At the limestone/lutite ment of the first turbiditic conglomerates. The upper transition there are several limestone megabreccias Ondarroa turbidite system, Spain 387 deposited through rock fall processes at the foot of lar and laterally discontinuous, and in places hard synsedimentary reliefs. to distinguish. Most of the conglomerates show The Ondarroa turbidite system was assigned to poor sorting and contain scarce, medium to very the Middle Albian by Engeser et al. (1984) and coarse sandy matrix. They have been classified by Agirrezabala et al. (1989). Indirect dating, taking into taking into account their internal grading, or their account the age of the enclosing units (e.g. Gomez de lack of it. Three different subfacies have been distin- Llarena, 1958; Rat, 1959; Ramirez del Pozo, 1971), guished: non-graded, inverse to normally graded and points to the same age. Ammonites in the turbidite normally graded. system deposits (Oxytropidoceras? sp. and Puzosia sp.) support these assignments to the Middle Albian. Subfacies la: non-graded On the other hand, dating of the scarce limestone clasts resedimented along with the quartz conglom- This subfacies appears chaotic internally, without erates of the proximal part of the turbidite system stratification or vertical gradation (Fig. 4A). Clasts suggests a time range from Aptian to Middle Albian are poorly sorted and they commonly lack parallel (Engeser et al., 1984; Agirrezabala et al., 1989). alignment of their long axes. Clast-supported conglomerates made up of boulders and cobbles are up to 3.5 m thick and typically fill erosional scours. FACIES ANALYSIS The very base of each bed may show crude inverse grading. This subfacies holds the biggest The main components of the turbidite system are extraformational clasts of the section: limestone siliciclastic conglomerates, sandstones and mud- blocks up to 3.1 m long. stones. The conglomerates consist of well rounded The non-graded, disorganized character of these clasts of quartzite, vein quartz and siliciclastic beds allows comparison with debris flow deposits, conglomerates, up to 0.5 m in diameter. The quartz- but the absence of interstitial mud instead suggests a ite and quartz lithologies probably derived from non-cohesive mechanism of transport (Ineson, 1989). Permian or Triassic outcrops, and the clasts of Most probably, these conglomerates were deposited previous conglomerates probably had their origin in from high density turbidity currents, or very poorly Carboniferous outcrops of the Pyrenean realm. Some cohesive debris flows and they were rapidly deposited shallow water Urgonian limestone blocks up to 3.1 m by frictional freezing (e.g. Pickering et al., 1986). long are included in the uppermost part of the conglomerate succession. In the same way, siderite, Subjacies 1b: inverse to normally graded sandstone and mudstone intraclasts up to 9m long appear within the coarsest grained conglomeratic This subfacies is equivalent to R,-R, facies of Lowe units. Sandstones, on the other hand, are quartz (1982) and A,, facies of Pickering et al. (1986). arenites, litharenites, greywackes (Badillo-Larrieta Although an entire bed of conglomerate may be & Garcia-Garmilla, 1989) and subarkoses (this inversely graded, it is common that an inversely study); they occasionally contain abundant commin- graded zone up to 1 m thick is vertically followed by uted plant debris and sometimes appear intensely a normally graded zone, sometimes with a non- bioturbated. graded interval intervening (Fig. 4B). Beds of cobble Eight facies and three subfacies have been recog- conglomerates up to 2.5 m thick are commonly nized in the turbidite system. In the following sec- lenticular, have scoured bottoms and show well tions they will be described and then compared with developed imbrication. other facies schemes (those of Mutti & Ricci Lucchi, According to Lowe (1982), the presence of both 1972, 1975; Lowe, 1982; Pickering et al., 1986), inverse grading in the lower part of a bed and well in order to attribute each facies to one or more developed imbrication suggests a transport mech- processes or mechanisms of deposition. anism dominated by dispersive pressure, followed by rapid deposition through frictional freezing of the traction carpet at the base of a high concentration Facies 1: clast-supported conglomerates turbulent flow. The overlying normally graded con- Clast-supported boulder, cobble and pebble conglomerates are thought to represent rapid deposition glomerates form amalgamated sequences with from suspension in a highly concentrated turbidity multiple scoured surfaces. Beds are normally lenticu- current. 388 L. M. Agirrezabalu and J. Garcia-Mondkjur

Fig. 4. (A) Non-graded, clast-supported conglomerates (facies la) with cobbles and boulders; arrow marks the base of a bed. (B) Inverse to normally graded conglomerates (facies lb); arrow marks the base of the bed. (C) Normally graded conglomerates (facies lc) showing well developed clast imbrication. (D) Disorganized, mud-supported pebble conglomerates (facies 2); base to the left. Ondarrou turhidite system, Spin 389

Subjucies Ic: normally graded Facies 4: normally graded pebbly sandstones This consists of clast-supported cobble and pebble These are characterized by well developed, normally conglomerates in beds up to 2.5 m thick, with graded, clast-supported pebble conglomerates pass- scoured, planar bases and well developed imbrication ing gradationally upwards to very coarse, coarse and (Fig. 4C). Normal grading may be present in the medium grained sandstones, in beds 0.5-2 m thick entire bed or only in its upper part, the lower part of (Fig. 5B). The underlying beds are eroded (with relief the bed being non-graded in the latter case. The clasts or smoothed) at outcrop scale, and load structures or of this facies have better sorting and are finer grained flute casts occur. Amalgamation of beds is very than the clasts in the non-graded or inverse to common. The upper sandy parts of beds may show normally graded facies. In places, a bed a few centi- very coarse sand- and granule-rich flat laminae with metres thick of coarse to very coarse sandstone normal andlor inverse grading (facies S, and S, of drapes these conglomerate beds; this bed may be Lowe, 1982). cross stratified. This facies is similar to the A,, facies of Pickering Conglomeratic beds showing continuous normal et al. (1986), and it is considered to represent depo- grading result from the rapid settling of gravel in a sition of high density turbidity currents. Well devel- high density turbidity current (Lowe, 1982). Never- oped pebble size normal grading reflects rapid grain theless, the beds that show grading only in their by grain deposition from suspension without signifi- upper parts may be interpreted in two ways (Ineson, cant traction transport, and the upper sandy part 1989): (a) very rapid sedimentation from high density reveals both traction and grain flow processes (see turbidity currents, with insufficient time for the devel- Lowe, 1982). opment of grading due to a sharp decrease in flow competence, or (b) rigid plugging of a sandy debris flow followed by shearing or flow transformation, Facies 5: cross stratified sandstones with concomitant grading. These consist of coarse sandstone to granule conglomerate beds with tabular and convex up top Facies 2: mud-supported conglomerates shapes and internal high to low angle trough cross stratification. Cross stratified sets are single, typically These consist of structureless mud-supported con- 15-25 cm, but up to 70 cm, thick. Cross laminae dip glomerates in beds commonly 0.5-2m, but up to steeply (up to 30") and they show asymptotic or 5.5 m, thick. The beds are lenticular to tabular and slightly tangential lower contacts (Fig. 5C). show little erosion into the underlying sedimentary This facies compares with facies B, of Mut'ti & layers. Clast abundance and size are very variable, Ricci Lucchi (1972, 1975), and facies Bz.z of and the more common lithologies of boulder and Pickering et al. (1986). Bypassing turbidity currents megablock (up to 3 m long) intraclasts are siderite, and/or bottom currents are interpreted to have sandstone and mudstone (Fig. 4D). reworked sandy bottoms by tractional processes, This facies is similar to A, and A, facies of resulting in bedforms and cross stratification. Pickering et al. (1986), which result from cohesive debris flows. Intergranular friction and cohesion would have been responsible for the freezing of the Facies 6 interbedded graded sandstones and flow and sedimentation. mudstones The average sandstone to mudstone ratio in this Facies 3: slump deposits facies is about 1:l. These consist of sandstone beds Coherently to semi-coherently folded contorted beds characterized by normally graded coarse sand to of turbiditic sandstones and mudstones form hor- granule gravel at their bases (Ta division of Bouma izons up to 1.5 m thick (Fig. 5A). The bases of these sequence), overlain by medium to fine grained sand- horizons are commonly shear planes, and their tops stones with parallel lamination (Fig. 6A), locally are irregular. Lateral continuity of individual hor- passing up to ripple lamination (Tb and Tc Bouma izons at outcrop may reach up to 70m. There are divisions, respectively). Beds are 20-70 cm thick, and deposits with intermediate characteristics between show flat bases with sole casts of only a few centi- slump deposits and mud-supported conglomerates metres relief. Amalgamation of beds is common, and (facies 2). lateral changes in bed thickness at outcrop scale may 390 L. M. Agirrezahala and J. Gurcia-Mondijar

Fig. 5. (A) Slumped sandstones (facies 3); base to the left. (B) Pebbly sandstone bed (facies 4); arrow points to the base of the bed. (C) Cross stratified sandstones (facies 5) with asymptotic laminae (dashed line); arrow points to the base of the bed. Ondarroa furhidite system, Spain 39 1

Facies 6 is similar to facies C of Mutti & Ricci Lucchi (1972, 1975), and the facies S,, Tb and Tc of Lowe (1982). Well developed normal grading at the base of beds reflects sedimentation from the suspension created by a high density turbidity current. When this current undergoes deceleration, traction deposition from low density turbidity currents occurs (Tb and Tc Bounia divisions), and subsequently mud particles in suspension are deposited.

Facies 7: interbedded non-graded sandstones and mudstones These consist of medium to very fine grained sandstone and lesser siltstone without basal normal grading, alternating with mudstone. Sandstone beds mainly show Tb and Tbc Bouina divisions and, more rarely, Tbcde and Tcde divisions. The beds are up to I m thick, although they commonly range from 5 to 50 cm in thickness. They are tabular (very rarely lenticular) at outcrop scale, and show flat and sharp bases with occasional load structures, and gra- dational tops. Well developed ripple drift lamination in fine grained sandstone to siltstone beds up to 11 cm thick may be present in Tc Bouma divisions (Fig. 6B). The sandstone to mudstone ratio ranges from 1:I to 1:3. Facies 7 is similar to facies D of Mutti & Ricci Lucchi (1972, 1975), and facies C2.3 and D2., of Pickering et al. (1986). It is considered to be the deposit of low density turbidity currents.

Facies 8: mudstones These are dark coloured, in places bioturbated, commonly fissile clay to silt grade mudstones. They may contain scattered ammonites, resedimented orbitolinids and vegetal fragments. There are two types of mudstones: (a) uniform, structureless silt to clay grade mudstone, and (b) laminated alternations of mudstone and siltstone (Fig. 6C). The laminated mudstones and siltstones comprise alternating beds 0.5-2 cni thick, and many of the primary sedimentary Fig. 6. (A) Normally graded, granule sandstone (Ta) inter- structures appear to have been destroyed by biotur- bedded with mudstones (facies 6); arrow points to base. (B) bation. Ripple laminated sandstones (Tc) interbedded with mudstones (facies 7). (C) Laminated mudstones (dark) and Uniform, structureless mudstones are interpreted siltstones (light) (facies S), showing bioturbation. to be deposited from low density, low velocity turbidity currents. Some of them may represent end- members of slump debris, flow turbidity current occur. Mudstones gradationally overlie the sand- transformation events (Stanley & Maldonado, 1981). stones. They consist of silty quartz and black or dark Laminated siltstones and mudstones, on the other grey argillaceous particles, and show little evidence of hand, were probably deposited from low density bioturbation. turbidity currents. 392 L. M. Agirrezahala and J. Garcia-Mondkjar

TURBIDITE SYSTEM DIVISIONS

Many deep water turbidite systems with a point Minor channels 8 non-channellized lc.6 (73) source can be subdivided into three main depo- t sandstones sitional settings or subenvironments: inner, middle and outer, respectively, according to proximal, inter- MAIN CHANNEL mediate and distal turbidity currents, deposits and Debris flows & sequences (Normark, 1970, 1978; Mutti & Ricci slumps Lucchi, 1972, 1975; Walker, 1978). The Ondarroa turbidite system can be subdivided into three similar palaeosubenvironments (Figs 2 and 3), by means of .- Minor channels the spatial distribution and interpretation of facies I 6,lc,lb associations and vertical sequences of facies. The non-channellized sandstones (4,7,5) inner system is characterized by coarse grained conglomerates filling large channels, interpreted as products of high concentration turbidity currents. The middle system is characterized by fine to medium grained conglomerates and sandstones arranged in MAIN CHANNEL lb,lc,2 (la) fining and thinning upwards sequences, interpreted as products of high to low concentration turbidity currents in small channels. The outer system is charac- 1c (7) terized by alternating plane parallel sandstones and mudstones, interpreted as products of relatively low \MAIN CHANNEL la,lc (7) concentration turbidity currents expanding out of channel mouths (Figs 2 and 3). Alpine structural complications and poor exposure have prevented a complete establishment of mutual relationships among the different facies, as well as detailed Fig. 7. Schematic section of the inner turbidite system at mapping of significant sedimentary bodies such as locality 'a' (see Fig. 1B). Large scale sequences, facies channel and lobe sandstones. For this reason, each associations and facies are indicated (less abundant within turbidite system division has been characterized with parentheses). data from only incomplete vertical sections (b-h in Fig. 1 B), except for the inner system in the northernmost part, where a complete vertical section has been and 2), with minimum depths of 55 and 25 m for the studied (a in Fig. IB). lower and upper surfaces, respectively (Fig. 8). The width (perpendicular to the palaeocurrent direction) of each erosional surface has not been possible to Inner turbidite system measure directly, but the available outcrops suggest This system is characterized by deep erosional chan- that they range between 0.75 and 3 km. The erosional nels (main channels) filled with conglomerates. At margins of the surfaces are relatively steep (30-35" Saturraran, close to Ondarroa (a in Fig. I), three for the western margin of the lower one, for facies associations have been distinguished (Fig. 7): instance), and the upper surface is asymmetric, with (a) conglomerates filling main channels; (b) sand- its eastern margin steeper than its western one. stones filling minor channels, and non-channellized The erosional surfaces at the base of the conglom- sandstones; and (c) debris flow and slump deposits. erate units are considered true turbiditic channels (in the sense of Mutti & Normark, 1987), rather than mega-scours, despite their unknown downpalaeo- Main channel association current lengths. This is because: (1) both the width Three main conglomerate units make up to the bulk and the depth of each surface are comparable with of the vertical succession. These conglomerates fill those of turbiditic channels from some small scale, large and deep erosional surfaces incised in mud- modern submarine fans such as the Crati Fan (Ricci stones and mud-supported conglomerates (Packs 8 Lucchi et al., 1985), or the Navy Fan (Normark & Ondurroa turbidite system, Spin 393

current vector deduced from the deposits which fill them, are notably steeper (up to 30-35") than the margins of some large present day scours such as those on the Navy Fan (Normark et ul., 1979);(3) the fining and thinning upward organization of the coarse grained conglomeratic units filling the erosional surfaces, and their internal subdivision into smaller scale sequences of the same type, point more to a persistent channel rather than to mega-scour deposits; (4) the low dispersion unipolar pattern of palaeocurrents (Fig. 2) is quite characteristic of uni- directional confinement of the turbiditic flows; (5) __-_-_ scours of similar size are rare in modern submarine L' I _-__-~_ turbidite systems; and (6) huge amounts of sandy Fig. 8. Cross section representing the vertical superposition turbidites are inferred to have bypassed the erosional and minimum widths and depths of the three main channels surfaces to form the c. 1000 m thick outer system. of the inner system (for location see Fig. IB). The lower and upper channel fills show fining and thinning upward sequences, which onlap the channel margins (Figs 7, 9 and IOA). Conglomerates are the Piper, 1985); (2) the margins of the concave-up predominant lithology (facies 1, clast-supported and, erosional surfaces, which are parallel to the palaeo- less abundantly, facies 2, mud-supported). Sand-

4ir

> (cm1

Fig. 9. Detailed section of the inner turbidite system at locality 'a' (same as in Fig. 7, location in Fig. JB). Modal and maximum clast size values are represented, as well as facies. Note well developed thinning and fining upward sequences between 235 and 285 m. 394 L. M. Agirrezahala and J. Garcia-Mondijur

Fig. 10. (A) Very thick (50 m), coarse conglomerate channel fill deposits corresponding to the upper main channel of the inner turbidity system (see Fig. 7). White arrow in foreground points to the base of the channel and person in background (encircled) gives the scale. (B) Small scale thinning and fining upward sequences of the channel fill deposits shown in (A). Person (bottom right, encircled) for scale. stones are the next most abundant lithology, with sandstones (facies 5), and finally to mudstones (facies pebbles and cross stratification (facies 4 and 5, respec- 8). These subordinate sequences contain 8-10 beds, tively). Mudstones (facies 8) make up the rest of the and they are attributed to the filling of channels succession. As the upper channel fill demonstrates subordinate to the main channel with different kinds (Figs 9, 10B and 1 1), major sequences contain subor- of migrating bars (facies 5). dinate fining and thinning upward sequences less than 10 m thick. Minor chunnel and non-channellized sandstone The lower part of each of these sequences generally assocution consists of very thick bedded conglomerates (facies Ib, Ic and 2), passing upward into thick bedded This association gradationally overlies the main pebbly sandstones (facies 4), then into cross bedded channel association (Fig. 7). It is made up mainly of Ondarroa turbidite system, Spain 395 WNW Mean palaeocurrent el

lfrl Mud-supported conglomerate

Mud DClast-supported conglomerate @ Shallow water limestone boulder (granule-pebble) (up to 3m) 0Sand Clast-supported COnglOmerate Mudstone intraclast (up to 9m) (cobble-boulder) Fig. 11. Outcrop sketch showing the lower part of the upper main channel fill in the inner turbidite system (same outcrop as in Fig. IOA; for locality, see Fig. IB). Numbers refer to facies. Note the presence of small scale thinning and fining upward sequences and the inclusion of limestone blocks in the lower part of the fill. planar sandstone beds with or without the Ta Bouma and 9, 210-235 m). It is made up of mud matrix-rich interval, alternating with mudstones (facies 6 and 7, conglomerates (facies 2), slump deposits (facies 3), respectively) and lenticular beds of pebble conglom- several metres thick calcareous lutites with biotur- erate (facies lc and Ib) and pebbly sandstone (facies bation (Chondrit~sand other burrows; facies 8), and 4). Less abundant Packs are cross stratified sand- less abundant thin to medium bedded, thoroughly stones (facies 5) and mud-supported conglomerates ripple laminated sandstones (facies 7). No special (facies 2). arrangement in vertical sequences has been observed Fining upward and thinning upward sequences in this facies association, but in places it interdigitates 1-7 m thick are common in this facies association laterally with thick to very thick conglomerate beds (Fig. 9). From the base upwards each sequence (facies Ic, lb and la), pebbly sandstones (facies 4) normally has conglomerates (facies lc or Ib), pebbly and graded sandstones (facies 6). sandstones (facies 4) and/or alternating sandstones This facies association records deposition on a and mudstones (facies 6, 7 and 5). These sequences muddy slope with frequent mass flow and slumping are attributed to the vertical filling of channels and processes. Few turbidity currents (and diluted, if interchannel deposition. present) would have originated on this slope, which, according to the carbonate enrichment of its lutites and the presence of significant bioturbation and Debris flow and slump association some ammonites, suggests a relatively slow rate of When present, this association rests conformably on sedimentation. top of deposits of the minor channel and non- To summarize, the major vertical arrangement of channellized sandstone association, and is overlain the inner system deposits consists of three fining and by deposits of the main channel association (Figs 7 thinning upward sequences up to several tens of 396 L. M. Agirrezahalu and J. Gurcia-Mundkjur

Sand) 1 Gravel

Fig. 12. Measured partial sections representative of the middle turbidite system deposits (b, c, d refer to locations in Fig. 1 B). These deposits are organized into thinning and fining upward sequences several metres thick. metres thick (65, 170 and 50 m in thickness, from corresponding facies in the inner system. Middle bottom to top; Fig. 7). The sequences have conglom- system deposits are arranged in vertically stacked erates filling main channels (main channel facies fining and thinning upward sequences, normally association), overlain by conglomerates filling minor 34m thick, but ranging from I to 10m thick channels and planar sandstone beds (minor channel (columnar sections b, c and d of Fig. 12). These and non-channellized sandstone association), and. in sequences have ratios of conglomerate plus sandstone the 170 m thick sequence, debris flow and slump to mudstone ranging from 2.5:1 to 5.1. They have deposits (debris flow and slump association) at the channellized bases with relief of up to 1.5 m at top. outcrop scale, and commonly consist of &6 beds (but up to 10 beds in one case). The fill of the channels is made up of: (a) clast-supported pebble Middle turbidite system conglomerates, characterized by either normal grad- In the middle turbidite system, both bed thickness ing (facies Ic) or no grading (facies la); (b) pebbly and clast size are smaller, on average, than in the sandstones (facies 4); and (c) lesser mud-supported Ondurrou turhidite system, Spain 397 conglomerates (facies 2). Maximum clast size reaches and may contain mud-supported conglomerates 19 cm in quartzite extraclasts and 40 cm in sandstone (facies 2). Parallel to the grain size decrease from intraclasts. Sandstones alternating with mudstones proximal to distal parts in the outer system, the rest on top of the lower conglomeratic facies. These sandstone (plus conglomerate) to mudstone ratio sandstones show flat bottoms and tops with Ta changes from 3:l to 1:2. Distal sandstones of the Bouma sequences (facies 6), flat bottoms and rippled outer system show higher mica and plant remain tops with Tbc, Tb and Tc Bouma sequences (facies contents than their proximal counterparts. 7), or cross stratification (facies 5); they always Outer system deposits are rarely organized in appear associated with mudstones (facies 8). vertical sequences and, when they are, the sequences The fining and thinning upward sequences of the coarsen and thicken upwards in intervals 8-1 5 m middle system are interpreted as channel fills. Chan- thick. These sequences are attributed to sandy lobe nel geometries have not been documented in these aggradations and progradations, in the sense of deposits, but their fill characteristics and their down- Shanmugam & Moiola (1 988). Their scarcity suggests current location from larger turbiditic channels of the a lack of points of supply of turbidity currents at the inner system suggest they are in fact channel fills. The channel mouths persistent enough to favour the thicknesses of the fining upward sequences suggest formation of lobes. The more distal outer system (fan that the channels were shallow (0.5-2 m deep), and fringe, section h in Fig. 13) is characterized by a they either migrated laterally or jumped abruptly, sandstone to mudstone ratio of about 1.2, the leaving channel margin or interchannel deposits, total absence of facies 6 and abundance of thin respectively. The progressive fining and thinning of beds (normally less than 30 cm thick). These fan conglomerates and thinning of sequences towards the fringe deposits intertongue laterally with basin plain south-west indicate shallower and smaller channels deposits. downslope, perhaps due to a more ramifying channel Basin plain deposits mainly consist of parallel pattern. The small number of beds per sequence and laminated mudstones and siltstones (facies 8), with the absence of fining and thinning characteristics in variable degrees of bioturbation and varve-like the thickest interchannel interval (19-30 m in section aspect (Fig. 6c). They hold scattered thin and very b of Fig. 12) suggest an autocyclical origin for these thin beds of fine to very fine grained sandstones, sequences, e.g. lateral migration of channels. frequently with intense ripple lamination (facies 7).

Outer turbidite system and basin plain DISPERSAL PATTERN In contrast with the inner and middle parts of the turbidite system, the outer system has no channel- The term ‘deep water fan’ ‘normally suggests a lized deposits and it shows well developed and fan-shaped deposit that results from long-term active laterally continuous plane parallel sandstone beds turbidite-type deposition, related to a point source, (sections e, f, g and h in Fig. 13). Medium to fine and taking place in a large unconfined basin of grained, plane parallel sandstone beds up to 1 m low relief and with gentle gradients’ (Bouma et ul., thick, with Tb, Tbc and Tbcde Bouma sequences, are 1985a, p. 9). Both the models proposed for ancient the dominant deposits (facies 7). The tops of the beds and modern deep water fans consist of fan-shaped are frequently ill defined and rippled; sometimes the bodies of sediments deposited in unconfined basins beds appear somewhat slumped, or they show load (Normark, 1970, 1978; Mutt1 & Ricci Lucchi, 1972, structures at their bases. Coarse to medium grained 1975; Walker, 1978). The great variability of both sandstones in beds 20-50 cm thick with slightly ancient and modern deep water fans (e.g. Bouma rt irregular bottoms and plane tops, Ta or, more rarely, al., 1985b; Weimer & Link, 1991) has caused stan- Tab and Tac Bouma sequences, are also abundant dard deep water fan models to be re-examined, and (facies 6). the idea of a unique deep water fan model is no Close to the middle-outer system transition (sec- longer widely accepted (e.g. Pickering et al., 1989). In tion e in Fig. 13), facies 6 is more common than facies this light, we discuss whether the Ondarroa turbidite 7, and it is associated with normally graded pebble system can be interpreted as a true deep water fan. conglomerates (facies Ic) and pebbly sandstones Characteristics in the Ondarroa system which depart (facies 4). These latter facies are up to 1 m thick, have from those considered typical of a ‘true’ fan are: (a) a slightly erosional bottoms (less than 10 cm in relief), long and narrow form for its major part; (b) a 398 L. M. Agirrezabala und J. Garcia-Mondkjur

Fig. 13. Measured partial sections representative of outer turbidite system deposits (e, f, g, h refer to locations in Fig. 1 B). These deposits mainly consist of interbedded plane parallel sandstones and mudstones without any recognizable vertical organization. longitudinal dispersal pattern and distribution of system instead of turbidite fan because of the discrep- subenvironments; and (c) an essentially confined ancies with the ‘classical’ characteristics of fans depositional setting. However, certain of its features reported above, although it is well known that many can be considered characteristic of what is normally modern and ancient deep water turbidite systems, considered a fan: (a) it is a system related to a point described as fans, are also partially confined andlor source; (b) it was radial in form in at least the elongated (e.g. Barnes & Normark, 1985; Stow, latter stages of its evolution; and (c) it can be 1985). subdivided into proximal, intermediate and distal The geometry of the sedimentary body, facies parts based on facies and sequence analysis. We have analysis, clastic dispersal patterns, major sequential preferred in this paper to employ the term turbidite arrangement and local palaeocurrent vectors have Ondarroa turbidite system, Spain 399

Fig. 14. Palaeogeographical reconstruction of the Ondarroa turbidite system for the first stages of its development, when it was morphotectonically controlled by Urgonian limestone buildups and the Markina-Elgoibar Fault (southern escarpment). all been used to define a model for the Ondarroa correspond to palaeocurrents directed towards the turbidite system. Most of the palaeocurrents come ESE (Fig. 2). from the more proximal inner system (Fig. 2), Both palaeocurrent data and the areal distribution where 67 measures have been taken from flute, of grain size shown in Fig. 2 demonstrate filling of a groove, prod and crescent casts, parting lineations longitudinal trough (Fig. 14). In the channellized part and channel axes. Similar to the data obtained by of the turbidite system (inner and middle system) the Voort (1963), the provenance of turbiditic currents turbidite currents flowed towards the SSW, and has been established to be septentrional (NNE, further downwards in the outer system and basin V=23"). There is a low dispersion (r=0.94), which plain, they flowed towards the ESE. This means that is interpreted to be a consequence of the great a turn in direction of approximately 90" occurred confinement of the palaeocurrents in a trough or near the middle-outer system transition. A minor canyon. Measures from the outer system strongly local supply of materials also occurred during the differ from those from the inner system, and they first stages of the system development (Fig. 14), as 400 L. M. Agirrrzahala and J. Garcia-Mondkjar

Fig. 15. Palaeogeographical reconstruction of the Ondarroa turbidite system for the last stages of its development. Note the burial of many of the limestone buildups of previous stages by turbidites. recorded by limestone megabreccias deposited by inner system formed in an intraplatform trough some rock fall processes at the foot of steep submarine 100-200 m deep in its more proximal part. Moreover, cliffs. These were shed from local carbonate highs the sharp increase in subsidence from proximal to adjacent to the turbiditic trough. distal parts of the turbidite system (Fig. 3) indicates The ichnofauna present in materials from the inner the creation and maintenance of a high bathymetric turbidite system (Rhizocorallium, Chondrites and var- gradient from the inner to outer system during the ied burrows) is not determinative enough to make time of turbidite system construction. approximative estimations about the palaeobathym- In the final stages of system development, probably etry of the trough. However, taking into account the during the period of activity of the main upper general palaeogeogrdphy and by comparison with channel, the turbidite system filled the graben, and other examples in the Basque-Cantabrian Basin, in lapped onto horst blocks until it acquired a more which carbonate platform clinoforms give minimum radial shape (Fig. 15). Sandy turbidites of these final depths for intraplatform troughs, we suggest that the stages of the system, which rest directly on top of Ondarroa turhidite system, Spain 40 1

Fig. 16. Quartzite cobble with a hemispherical massive coral growing on top of it (black arrowheads point to the top of the coral and the key is 4 cm long). adjacent karstified carbonate highs, indicate that a Agirrezabala & Garcia-Mondejar, in press). On the generalized drowning event followed a previous other hand, the conglomerates filling the upper main period of exposure of these highs. channel of the Ondarroa section (inner system) con- According to the fxies descriptions of previous tain some quartzite clasts up to 20 cm in diameter sections, the Ondarroa example is a turbidite system with half-dome hermatypic corals primarily attached with an inner braided conglomeratic channel part to them (Fig. 16). This is clear evidence of the (inner system), a middle region with conglomerates existence of gravel on a marine platform with enough and sandstones filling smaller multiple braided residence time to be colonized by corals. This gravel channels (middle system), and a distal region with must have arrived intermittently to the platform by sandstone-mudstone turbidite couplets rarely organ- no other transport mechanism than fluvial/alluvial ized in thickening and coarsening upward sequences currents (fan or braid deltas), similar to some present (outer system). These characteristics are similar to day gravels in the Red Sea (e.g. Hayward, 1985). those of the suprafan model described by Walker The three large scale fining and thinning upward (1978). sequences present in the inner turbidite system (Fig. 7) can be attributed either to autocyclical processes (sedimentary dynamics of the turbiditic channels) or MAJOR DEPOSITIONAL CONTROLS to allocyclical processes (local tectonics and/or relative sea level changes). Using the autocyclical hypoth- The size and shape of cobble and boulder gravel esis, the sequences can be attributed to lateral clasts in the inner system suggest proximity to the migration of the major turbiditic channels. Two lines source area, which was perhaps located only a few of evidence support the allocyclical hypothesis, at kilometres from the present outcrops. It probably least with respect to the first and third sequences. One was an emergent Palaeozoic massif (Biscay Massif of is the sudden appearance of the lower channel con- Voort, 1963), which supplied very coarse grains to a glomerates on top of mudstones, without any kind of narrow platform. Although no direct evidence of this intermediate facies suggestive of lateral migration of platform has been found to the north of Ondarroa, the channel toward that site (i.e. overbank or channel presently below sea level, the large amount of coarse margin deposits). The other evidence is the correla- clasts (up to boulder size) suggests a marine platform tion between the terrigenous deposits on top of the fed by fan deltas similar to those described for the karstified carbonate highs adjacent to the Ondarroa Lower Albian in nearby outcrops (Monte grande fan trough, and the conglomeratic deposits filling up the delta, Robles et al., 1988; and Otoio fan delta, upper major channel. In this latter case, karst dates in 402 L. M. Agirrezabala and J. Garcia-MondCjar adjacent blocks and further away (e.g. Agirrezabala limestone megabreccias at the foot of carbonate & Garcia-Mondejar, 1989; Agirrezabala et al., 1992) highs, following the same NNE-SSW orientation; indicate that limestone karstification was produced and (3) the characteristic sharp and rectilinear mar- simultaneously, at least in its last stage of develop- gins of the carbonate highs with the same orientation. ment, with the channel formation and bypassing The faults responsible for the graben could have episodes. This suggests a relative sea level fall in the controlled the location of the main turbiditic chan- origin of the upper major channel. Extending this nels of the inner system in every episode. Separate allocyclical hypothesis to all three major channels, movements of the eastern and western bounding the development of minor channels and non- faults could have determined the displacement of the channellized sandstones above the conglomeratic main channel towards the area of the downfaulted channel fill deposits may have resulted from a pro- block closer to the moving fault (Fig. 8). A fault gressive inactivation and retrogradation of the chan- control of this kind on the location and stacking of nels as the relative sea level proceeded to rise again turbiditic channels is presently operating the La Jolla after the major fall. Fan (Bachman & Graham, 1985). On the other hand, Weimer (1990) has shown the Mississippi Fan to the sharp change of 90” in direction of the Ondarroa be made up of 17 seismic sequences, each with a turbiditic dispersal system (Fig. 2) can be attributed series of channel, levee and associated overbank to the presence of a major fault active during the deposits, among other deposits. The formation of Middle Albian, which is coincident with the strike- submarine canyons in the upper slope and outer slip Alpine Markina-Elgoibar Fault. This fault shelf, which was concomitant with the appearance of prevented the turbidite system from expanding south- mass transport complexes in the fan, is interpreted as westwards and obliged it to turn south-eastwards and a result of retrogressive slumping during a lowering expand in this direction, by means of creation of a of sea level. Channels are interpreted to be filled with longitudinal submarine relief with a local slope dip- coarse grained sediments, and channel-levee systems ping NNE. So, the turbiditic currents run mainly are attributed to the lowest positions reached by a sea level experiencing Pliocene-Pleistocene eustatic cycles. Thin layers of hemipelagic sediment, finally, are considered the meagre results of deposition N during highstands in sea level. t It seems very probable that in the Ondarroa example the three major channels (or channel valleys if we adapt Weimer’s terminology for these 1-6 km wide erosional features in the Mississippi Fan) resulted from relative sea level falls, similar to those inferred for the Mississippi Fan (Weimer, 1990). Whether these relative falls correlate with the eustatic falls reported by Haq et al. (1988) remains problem- atic, owing to the lack of precise dating. Ammonite information indicates only that at least part of the Ondarroa turbiditic episode is correlative with third order sequence 1.4 from Haq et al. (1988). Therefore, the boundaries of this eustatic sequence (at 100.5 and 99 Ma, respectively) might be represented in the studied succession by some of its three channel valleys, although this remains purely speculative for the moment. The location on the slope of the main turbiditic channels as a whole was most probably controlled by Fig. 17. Schematic illustration of the basement faulting NNE-SSW orientated synsedimentary faults. This is inferred to be responsible for the origin and development of supported by the following facts: (1) the vertical the turbiditic trough. Sinistral strike-slip action along two stacking of turbiditic channels in a relatively narrow major faults slightly divergent towards the east created the zone (6-7 km wide); (2) the presence of rock fall ‘L’-shaped composite trough. Ondarroa turbidite system, Spain 403

TURBlDlTE SYSTEM

Inner Middle Outer \ Major basement fault

/ Secondary basement fault 50 Km

Fig. 18. Geodynamic model of the block faulting shown in Fig. 17 included in the Basque Arc. This forms a part of, and supports, the general sinistral strike-slip model proposed for the Basque-Cantabrian Basin by Garcia-Mondejar (1989).

towards the ESE in the external part of the lower (or fault system) could have been a precursor of the middle and outer system areas, parallel to the local Alpine Lekeitio Fault (Soler et al., 1981); and (b) in slope of fault origin (Figs 2, 14 and 15). the south, a precursor of the Alpine Markina- The onset of the Ondarroa turbidite system in a Elgoibar Fault, as described in a previous paragraph. trough surrounded by relative carbonate highs, According to all characteristics discussed, the ‘L’- together with the rectilinear boundaries of this shaped trough was in fact a small, complex pull-apart trough and its characteristic ‘L’ shape, suggests the basin, with local geodynamics comparable, perhaps, action of extensional movements which produced to that of certain transtensional basins from the block faulting in the basement, with strong differ- present California continental margin, e.g. San Pedro ences in subsidence from block to block (Fig. 17). (Gorsline, 1978; Howell et al., 1980), or Tanner Basin The extension took place. along NW-SE and NE-SW (Gorsline, 1987). A scheme of this latter basin, which directions, which generally coincide with the main combines data from different figures reported in Teng directions of extension of the NW-SE sinistral strike- (1985) for the Upper Pliocene to Holocene, is shown slip pattern proposed for the Albian of the entire in Fig. 19. It is worth noting that, similar to the Basque-Cantabrian Basin by Garcia-Mondejar Ondarroa trough, the Tanner Basin shows a clear ‘L’ (1989). Figure 18 shows the model represented in Fig. shape in its northernmost part, and it is controlled by 17 superimposed on this general pattern of strike-slip the presence of a major fault line downslope. The (Garcia-Mondejar, 1989, fig. 8). The main strike-slip inner and middle fan of the turbidite system of this faults operating the area would have been (Figs 17 basin, which is sourced in northern Nidever Bank and 18): (a) in the north, the Fault or fault system (Teng, 1985), is practically restricted to the first part separating the turbiditic setting from the platform, of the ‘L’, whereas the outer fan extends, changing and this latter from the continental reliefs; this fault direction, further downwards. The size of the present 404 L. M. Agirrezahala and J. Garcia-Mondhjar

Fig. 19. A modern ‘L’-shaped turbidite fan conditioned by the strike-slip action of major basement faults, in the continental platform of California (Tanner Basin; simplified from Gorsline, 1987).

day Californian example is approximately double the built, which provided rock fall megdbreccias laterally size of the Albian Basque-Cantabrian example, and to the turbiditic trough. the latter is far more coarse grained. 2 The inner system consists of three stacked valley The morphotectonically controlled Albian Ondar- channel fills of conglomerates, sandy turbidites and roa turbidite system compares well with similar con- mudstones. The middle system consists of sandy and temporaneous systems described by Souquet et al. conglomeratic braided channel fills, and sandy and (1985) in the neighbouring Pyrenean depositional muddy overbank deposits. The outer system consists setting. A major sinistral strike-slip action with a of plane parallel sandy turbidites and mudstones, complex pattern of block faulting is also put forward related to lobes. The basin plain, finally, is made up by these authors to account for the geometry of of mudstones, siltstone laminae and thin bedded depositional systems. Together with the Pyrenean sandy turbidites. systems, the Ondarroa coarse grained turbidite sys- 3 Eight main facies have been distinguished within tem can be considered an example of a confined the turbidite system: (I) clast-supported conglomer- deep-water fan formed on a rifted margin of a craton, ates, (2) mud-supported conglomerates, (3) slump that was subjected to strike-slip movements. deposits, (4) normally graded pebbly sandstones, (5) cross stratified sandstones, (6) interbedded graded sandstones and mudstones, (7) interbedded CONCLUSIONS non-graded sandstones and mudstones and (8) mudstones. 1 The Ondarroa turbidite system is a Middle Albian 4 The provenance of turbidites was from the NNE conglomeratic turbidite system with a characteristic for the inner system, whereas the outer system shows ‘L’ shape, with minimum dimensions of 15-18 km in a dispersion pattern towards the ESE. Turbiditic length, 6-14 km in width and 295-1000 m in thick- currents flowed longitudinally along an ‘L’-shaped ness. It was confined by horst blocks, on which the trough, some 100-200m deep in its proximal part contemporaneous shallow water carbonates were and much deeper distally. The turbidite system Ondarroa turhidite system. Spain 405 widened with time and it had characteristics similar entre cab0 0g0ii0 e ltziar (Aptiense-Albiense superior), to the suprafan model of Walker (1978). Inner system Region Vasco-Cantabrica nor-oriental. In: Symposium, XII Congreso Nucional de St.dimerzto!ogiu (Ed. by S. quartzite cobbles and boulders, occasionally with Robles, J. Garcia-Mondtjar & A. Garrote), pp. 11-20. attached half-dome corals, suggest the existence of a UPV & EVE, Bilbao. probably narrow marine platform invaded by high AGIRREZABALA,L.M. & GARCLA-MOND~JAR,J. (in press) La gradient fan or braid deltas. serie de fan-delta albiense de Otoio (Lekeitio, Bizkaia). 5 At least two of the three valley channel fills of the Facies sedimentarias y consideraciones paleogeograficas. Kobie. inner system (the lowermost and uppermost ones) AGIRREZABALA,L.M., MARTIN~L,R. & GARC~A-MOND~JAR, had an allocyclical origin, and the uppermost fill J. (1992) Fauna de Ammonites del trinsito Complejo coincided with a relative sea level fall (correlative Urgoniano-Flysch Negro entre Gernika y Deba (Albiense palaeokarst in adjacent carbonate highs). Similar to medio y superior. Region Vasco-Cantabrica Septentri- what is occurring in the modern Mississippi Fan, onal). Treballs Mus. geol. Barcelona, 2, 143-169. AGUILAR,M.J. (1 975) Sedimentologia y paleogeografia del these two valley channels could be considered as Albense de la Cuenca Cantabrica. Est. Geol., XXXI, sequence boundaries. 1-213. 6 The ‘L’-shaped turbiditic trough is considered a BACHMAN,S.B. & GRAHAM,S.A. (1985) La Jolla Fan, small pull-apart basin, created by NW-SE and Pacific Ocean. In: Submarine Fans and Related Turbidite NE-SW extensional movements within an area Systems (Ed, by A. H. Bouma, W. R. Normark & N. E. Barnes), pp. 65-70. Springer-Verlag, New York. bounded by major faults. These faults were the BADILLO-LARRIETA,J.M. & GARC~A-GARMILLA,F. (1989) presently offshore Lekeitio Fault and the Elgoibar- Petrology of the terrigenous Middle-Upper Albian series Markina Fault; sinistral strike-slip action along both (Basque-coast, Northern Spain). In: X Re@onal Medng explains the appearance of the trough, and this action on Sedimentology (I.A.S.) (Ed. by M. KezmCr). pp. 5-6. can be integrated in the major sinistral strike-slip Hungarian Geological Institute, Budapest. pattern proposed for the entire Basque-Cantabrian BARNES,N.E. & NORMARK,W.R. (1985) Diagnostic parameters for comparing modern submarine fans and ancient region by Garcia-Mondejar (1989). Modern com- turbidite systems. In: Submarine Funs and Relared Turbid- parable transtensional settings with ‘L’-shaped fans ite Systems (Ed. by A. H. Bouma, W. R. Normark & are in the California continental platform (e.g. San N. E. Barnes), pp. 13-14. Springer-Verlag, New York. Pedro and Tanner basins), and other Albian BOILLOT,G. (1986) Comparison between the Galician and examples with morphotectonic control were also Aquitaine Margins. Tectonophysics, 129, 243-255. BOUMA,A.H., NORMARK,W.R. & BARNES,N.E. (1985a) present in the Pyrenean domain. COMFAN: Needs and Initial Results. In: Submarine Fans and Reluted Turbidite Systems (Ed. by A. H. Bouma, W. R. Normark & N. E. Barnes), pp. 7-11. Springer- ACKNOWLEDGMENTS Verlag, New York. BOUMA,A.H., NORMARK,W.R. & BARNES,N.E. (eds) (1985b) Submarine Fans and Related Turbidite Systems. Thanks are given to Dr Ricard Martinez for his Springer-Verlag. New York. contribution in an ammonite classification and CHOUKROUNE,P. & MATTAUEK,M. (1978) Tectonique dating. Constructive reviews of the manuscript by des plaques et Pyrtntes: sur le fonctionnement de la F. Ricci Lucchi, P. Souquet and C. Busby are faille transformante nord-pyreneenne; comparaisons acknowledged. Partial funds for this work were pro- avec des modeles actuels. Bull. Soc. gPol. France, 20, 689-700. vided by Universidad del Pais Vasco, Projects UPV ENGESER,T., REITNER,J., SCHWENTKE,W. & WIEDMANN,J. 121.310-E148/91 and UPV 121.310-EA060/92, and (1984) Die Kretazisch-alttertiare Tektogenese des Basko- Basque Government, Project PGV 12 1.31 0-€30 18/92. Kantabrischen Beckens (Nordspanien). Z. dt. geol. Begoiia Bernedo typed the manuscript. gesell., 135, 243-268. FEUILLBE,P. (I 967) Le Cenomanien des Pyrenees basques aux Asturies. Essai d’analyse strdtigraphique. A4C.m. Soc. REFERENCES gPol. Frunce, 46. AGIRREZABALA,L.M., BADILLO,J.M. & GARC~A-MONDEJAR,GARCiA-MONDhJAR, J. (1982) Aptiense- Albiense. In: El J. (1989) El sistema de abanico turbiditico en “L” de Cretucico de EspuFiu. Universidad Complutense Madrid. Ondarroa (Albiense Medio, Euskal Herria). Caracteriz- pp. 63-84. acion, facies y controles sedimentarios. In: Symposium, GARCLA-MOND~JAK,J. (1 989) Strike-slip subsidence of the Xii Congreso Nacional de Sedimentologiu (Ed. by S. Basque-Cantabrian Basin of Northern Spain and its Robles, J. Garcia-Mondejar & A. Garrote), pp. 167-175. Relationship to Aptian-Albian opening of Bay of Biscay. UPV & EVE, Bilbao. In: Extensionul Tectonics and Stratigraphy of the AGIRREZABALA, L.M. & GARCiA-MONDkJAR, J. (1 989) North Atlantic Margins. (Ed. by A. J. Tankard & H. R. Evolucion tectosedimentaria de la plataforma urgoniana Balkwill), Mem. Am. Ass. petrol. Geol., 46, 395409. 406 L. M. Agirrezabalu and J. Garcia-Mondejar

GARC~A-MONDEJAR,J., PUJALTE, V. & ROBLES,S. (1986) NORMARK,W.R. (1970) Growth patterns of deep-sea fans. Caracteristicas sedimentologicas, secuenciales y tec- Bull. Am. Ass. petrol. Geol., 54, 2170-2195. toestratigraficas del Triasico de Cantabria y norte de NORMARK,W.R. (1978) Fan valleys, channels, and depo- Palencia. Cuad. geol. Ibhrica, 10, 151-172. sitional lobes on modern submarine fans: characters for G~MEZDE LLARENA,J. (1958) Datos paleontologicos del recognition of sandy turbidite environments. Bull. Am. Flysch litoral de Guipuzcoa: el Vraconiense de Septarias Ass. petrol. Geol., 62, 912-931. de Motrico. Notas y Comun. Inst. geol. Min. EspaEa, 50, NORMARK,W.R. & PIPER,D.J.W. (1985) Navy Fan, Pacific 5-21. Ocean. In: Submarine Fans and Related Turbidite Systems GORSLINE,D.S. (1978) Anatomy of margin basins. J. sedim. (Ed. by A. H. Bouma, W. R. Normark & N. E. Barnes), Petrol., 48, 1055-1068. pp. 87-94. Springer-Verlag, New York. GORSLINE,D.S. (1987) Deposition in active margins basins: NORMARK,W.R., PIPER,D.J.W. & HESS,G.R. (1979) Dis- with examples from the California continental border- tributary channels, sand lobes, and mesotopography of land. In: Deposition in Active Margin Basins (Ed. by D. s. Navy submarine fan, California Borderland, with appfi- Gorsline), Pacijic Section Soc. econ Paleont. Miner.. 54, cations to ancient fan sediments. Sedimentology, 26, 749- 33-5 1. 774. HAQ, B.U., HARDENBOL,J. & VAIL,P.R. (1988) Mesozoic PICKERING,K.T., HISCOTT,R.N. & HEIN,F.J. (Eds) (1 989) and Cenozoic chronostratigraphy and eustatic cycles. In: Deep Marine Environments. Unwin Hyman, London. Sea-Level Changes: An Integrated Approach (Ed. by C. K. PICKERING,K., STOW,D. WATSON,M. & HISCOTT,R. (1986) Wilgus, C. A. Ross, H. Posamentier & C. G. St.C. Deep-water facies, processes and models: a review and Kendall), Spec. Publ. Soc. econ. Paleon. Miner., 42, classification scheme for modern and ancient sediments. 7 1LI 08. Earth-Sci. Rev., 23, 75-174. HAYWARD,A.D. (1985) Coastal alluvial fans (fan deltas) of RAM~REZDEL Pozo, J. (1971) Bioestratigrafia y microfacies the Gulf of Aqaba (Gulf of Eilat), Red Sea. Sediment. del Jurasico y Cretacico del Norte de Espafia (region Geol., 43, 241-260. Cantabrica). Mem. Inst. geol. Min. Esparla, 78. HOWELL,D.G., CROUCH,J.K., GREENE,H.G., MCCULLOCH, RAT, P. (1959) Les pays cretaces basco-cantabriques D.S. & VEDDER,J.G. (1980) Basin development along the (Espagne) . These Publications de l’universite de late Mesozoic and Cainozoic California margin: a plate Dijon. tectonic margin of subduction, oblique subduction and RAT,P., AMIOT,M., FEUILLEE,P., FLOQUET,M., MATHEY, transform tectonics. In: Sedimentation in Oblique-Slip B., PASCAL,A. & SALOMON,J. (1983) Vue sur le CrCtact Mobile Zones (Ed. by P. F. Ballance & H. G. Reading), Basco-Cantabrique et Nord-Iberique. Une marge et son Special Publ. int. Ass. Sedimentol.. 4, 43-62. arriere-pays, ses environnements sedimentaires. MPm. INESON,J.R. (1989) Coarse-grained submarine fan and slope gCol. Universite de Dijon, 9. apron deposits in a Cretaceous back-arc basin, Antarc- RICCI LUCCHI,F., COLELLA,A,, GABBIANELLI,G. and tica. Sedimentology, 36, 793-8 19. NORMARK,W.R. (1985) Crati Fan, Mediterranean. In: LE PICHON,X., BONNIN,J., FRANCHETEAU,J. & SIBUET,J.C. Submarine Fans and Related Turbidite Systems (Ed. by A. (1971) Une hypothese d’evolution tectonique du Golfe H. Bouma, W. R. Normark & N. E. Barnes), pp. 51-57. de Gascogne. In: Histoire Structurale du Golfe de Springer-Verlag, New York. Gascogne, Vol. 2, (Ed. by J. Debyser, X. Le Pichon & ROBLES,s., GARCfA-MONDEJAR, J. & PUJALTE, v. (1988) A L. Montadert), pp. VI(I l.l)-VI(l144). Technip. Paris. retreating fan-delta system in the Albian of Biscay, north- LOWE,D.R. (1982) Sediment gravity flow: 11. Depositional ern Spain: facies analysis and paleotectonic implications. models with special reference to the deposits of high- In: Fan Deltas: Sedimentology and Tectonic Settings, (Ed. density turbidity currents. J. sedim. Petrol., 52, 279-297. by W. Nemec & R. J. Steel), pp. 197-211. Blackie, MONTADERT,L., ROBERTS,D.G., DE CHARPAL,0. & Glasgow. GUENNOC,P. (1979) Rifting and subsidence of the North- SHANMUGAM,G. & MOIOLA,R.J. (1988) Submarine fans: ern continental margin of the Bay of Biscay. Initial characteristics, models, classification and reservoir poten- Reports of the Deep Seu Drilling Project, XLVIII, tial. Earth-Sci. Rev., 24, 383428. pp. 1025-1059. US Government Printing Office, SOLER,R., LOPEZ-VILCHEZ,J. & RIAZA,C. (1981) Petroleum Washington. Geology of the Bay of Biscay. In: Petroleum Geology of MUTTI,E. & NORMARK,W.R. (1987) Comparing examples the Continental Shelfof North- West Europe (Ed. by L. V. of modern and ancient turbidite systems: problems and Illing & G. D. Hobson), pp. 474-482. Institute of Petro- concepts. In: Marine Clastic Sedimentology (Ed. by J. K. leum, London. Leggett & G. G. Zuffa), pp. 1-37. Graham and Trotman, SOUQUET,P., DEBROAS,E.J., BOIRIE,J.M., PONS,P., FIXARI, London. G., Roux, J.C., DOL, J., THIEULOY,J.P., BONNEMAISON, MUTTI,E. & RICCI LUCCHI,F. (1972) Turbidites of the M., MANIVIT,H. & PEYBERNES,B. (1985) Le Groupe du Northern Apennines: introduction to facies analysis. Flysch Noir (Albo-Cenomanien) dans les Pyrenees. Bull. (English translation by T. H. Nilsen, 1978). Int. geol. Centres Rech. Exp1or.-Produd. Elf-Aquitaine, 9, 183-252. Rev., 20, 125-166. STANLEY,D.J. & MALDONADO,A. (1981) Depositional MUTTI,E. & RICCILUCCHI, F. (1975) Turbidite facies and models for fine-grained sediments in western Hellenic facies associations. In: Exumples of Turbidite Furies and Trench, eastern Mediterranean. Sedimentology. 28, 273- Associations from Selected Formations of the Northern 290. Apennines (Ed. by E. Mutti et al.), Field Trip Guidebook STOW, D.A.V. (1985) Deep-sea clastics: where are we A-1 1, 9th Int. Ass. Sedimentologists Congress, Nice. and where are we going? In: Sedimentology: Recent Ondarroa turbidite system, Spain 407

Developments and Applied Aspects (Ed. by P. J. Brenchley WEIMER,P. (1 990) Sequence stratigraphy, facies geometries & B .J. P. Williams), Spec. Publ. geol. Soc. London, 18, and depositional history of the Mississippi fan, Gulf of 67-93. Mexico. Bull. Am. Ass. petrol. Geol.. 14, 425453. TENG,L.S.Y. (1985) Seismic stratigraphy study ofthe Cali- WEIMER,P. & LINK,M.H. (Eds) (1991) Seismic Facies and fornia continental Borderland basins: structure. stratigra- Sedimentary Processes of Submarine Fans and Turbidite phy and sedimentation. PhD thesis, University of Systems. Springer-Verlag, New York. Southern California, Los Angeles. WIEDMANN,J.A., REITNER,J., ENGESER,T. & SCHWENTKE, VOORT,H.B. (1963) Zum flysch problem in den West- W. (1983) Plattenktonik, Fazies und Subsidenzgeschichte pyrenaen. Geol. Rdsch., 53, 220-233. des basko-kantabrischen kontinentalrandes wahrend WALKER,R.G. (1978) Deep-water sandstone facies and kreide und Alttertiar. Zitteliana, 10, 207-244. ancient submarine fans: models for exploration for stratigraphic traps. Bull. Am. Ass. petrol. Geol., 62, 932- 966.

(Manuscript received 15 June 1992; revision accepted 27 July 1993) Sedirnentology (1995) 42, 523-530

DISCUSSION A coarse-grained turbidite system with morphotectonic control (Middle Albian, Ondarroa, northern Iberia)

THOMAS PLETSCH* and MARTIN MESCHEDE Institut und Museum fur Geologie/Palaontologie der Universitdt Tubingen, Sigwartstr. 10, 0-72076 Tubingen, Germany

The Albian ‘Black Flysch’ of the Deba Syncline in calculated the areal extent of the OTS (p. 3861. the Basco-Cantabrian Basin (BCB) of northern However, the amount and direction of shortening Spain offers the opportunity to study the ge- are not documented. The main structural feature ometry of depositional systems in a transtensional of the Deba Basin is a NW-SE-trending N-vergent setting. In this respect, the detailed sedimento- foldbelt which deviates in the Ondarroa area to logical study of the ‘Ondarroa turbidite system’ run E-W (Fig. 1). Apparent shortening in the (OTS) by Agirrezabala & Garcia-Mondejar (1994) Ondarroa area perpendicular to the fold trend is is a welcome contribution to the knowledge of at least 20-30%. Additionally, the Arno Anticline coarse-grained, siliciclastic turbidite systems and is thrust towards the north. This anticline is their tectonic and sea-level controls. Previous dissected by NE-SW- and NW-SE-trending studies have already discussed the role of the normal, reverse and tear faults. According to turbidite system which has its inner part or feeder Agirrezabala & Garcia-Mondejar’s fig. 2, apparent in the Ondarroa-SaturrarBn area (e.g. Voort, 1964; shortening between Ondarroa and Berriatu, where FeuilliBe, 1967). Aguilar-Tom& (1975) gave a most of the inner OTS crops out, would only be detailed account on the petrography and several about 5% in a N-S compressional regime (Pinet sedimentological features of his ‘SaturrarBn et al., 1987). Between Ondarroa and Elgoibar, flysch’ (p. 126, our quotes). Wiedmann et al. apparent shortening deduced from the same map (1983, p. 228) interpreted the OTS in the is about 10%. We believe that these amounts of Ondarroa area as the inner part of a deep-sea fan. shortening underestimate the deformation of the More recently, the OTS was placed in a trans- study area. A restoration which uses the proposed tensional framework by several authors (Engeser values of shortening would result in about et al., 1984, fig. 7; Reitner, 1984; Meschede, 1987, double the size of the OTS of that calculated by fig. 3; Rat, 1988, fig. 7). Data presented in Agirrezabala & Garcia-MondBjar. This would Agirrezabala et al. (1989) and Agirrezabala & allow for a better comparison of the OTS with the Garcia-Mondejar (1994) support most of these Recent turbidite system of the Tanner Basin of findings. California cited by the authors. Based on stratigraphic, sedimentological and (2) Agirrezabala & Garcia-Mondejar claim that tectonic investigations in the Biscay Synclino- the OTS was confined in its upper reaches by rium and its surroundings (Meschede, 1986, intermittent activity of synsedimentary faults 1987) and on mapping (1:lO 000)of the Ondarroa (p.402). Later, they attribute these faults to con- area (T. Pletsch, unpubl. obs.) we criticize some of temporary extensional movements in the BCB the conclusions reached by Agirrezabala & Garcia- (p. 403). The presence of Keuper clays, evapor- MondBjar regarding geometry, size and internal ites and volcanics immediately adjacent to the facies of the OTS, although we generally agree SaturrarBn outcrops goes unmentioned in the with their sedimentological and stratigraphic text as well as in the outcrop map. The intimate description and the overall interpretation. relationship between diapirism and subsidence (1) Based on a palinspastic restoration, has been broadly discussed for the BCB by Agirrezabala & Garcia-MondBjar (1994, their fig. 2) many authors (e.g. Brinkmann & Logters, 1968; von Stackelberg, 1967; Boss, 1984; Schwentke & *Present address: Geologisches Institut, Abt. Mikro- Wiedmann, 1985),but is ignored by Agirrezabala palaontologie, Univ. Kiel, Olshausensk. 40-60, & Garcia-MondBjar. We acknowledge that in the D-24118Kiel, Germany. case of the OTS, an influence of diapirism on @) 1995 International Association of Sedimentologists 523 524 T. Pletsch and M. Meschede

11medium-fine grained Black fault + anticline mj coarse grained Flysch EZFl Urgonian limestones reversel wrench/ thrust fault tear fault mj Keuper clays, evaporites, volcanics - - Fig. 1. Geological map of the study area with main structural elements (simplified after Pletsch, 19901. Only anticlines are depicted. Diagrams display stratal measurements for the coastal strips from the western map limit to Ondarroa, Saturraran area, Mutriku area to Deba and in front of the thrust fault, respectively. N=number of measurements. All measurements projected on the lower-half Schmidt net.

the basin morphology is hard to prove, but In most localities where these blocks are indi- this explanation seems just as sensible for the cated on the outcrop map (their fig. I), we found confinement of the inner system toward the strongly deformed quartzitic and dolomitic East and for the westward-migrating channels as lithologies, sometimes showing mylonitic texture, the hypothetical activity of synsedimentary and associated with mineralizations along the faults. thrust front mentioned in point (1). These (3) Agirrezabala & Garcia-MondBjar mention findings have already been documented by limestone megabreccias (p. 386) containing olis- Euroestudios S. A. (1976). An alignment of this toliths with individual blocks measuring tens of facies in NNE-SSW direction (p. 402) has metres in calcareous lutites at the base of the OTS neither been observed nor is it apparent from which are interpreted as rock-fall breccias. This Agirrezabala & Garcia-Mondbjar’s fig. 1. Rather, facies is taken as evidence for the steep morphol- the ‘blocks’ follow the thrust trend and other ogy of the sysnsedimentary faults (pp. 387, 400). contacts with the lower limestone member, where

0 1995 International Association of Sedimentologists, Sedimentology, 42, 523-530 A coarse-grained turbidite system, Iberia 525 percolating fluids preferentially precipitated to Agirrezabala & Garcia-Mondejar’s fig. 2. Coarse- form weathering-resistant mineralizations. With- grained facies which we found hard to distinguish out supplementary field data for the study area from those at Ondarroa are shown to crop out that indicate a previous sedimentary emplace- some 3 km to the SE of Berriatu on their fig. 1but ment of these blocks, they cannot be used to trace only less than 1.5 km on their fig. 2. Our mapping a former facies transition. shows that the medium- to fine-grained facies (4) Agirrezabala & Garcia-Mondejar deduce an associations which crop out along the coast L-shape of the OTS from the present-day facies between Saturraran and Mutriku cannot be corre- distribution (p. 385) and from the internal dis- lated with the Ondarroa feeder channels but form persal pattern (p. 386). Although the authors men- their cover (Fig. 1). Only the short and strongly tion strong Alpine deformation (p. 385) for the deformed Mutriku harbour section can be taken as Ondarroa area, they interpret rectilinear contacts equivalent to the OTS. To the east of Mutriku, between the Urgonian limestones and the silici- correlation is even worse due to a fault that runs clastic ‘Black Flysch’ as primary sedimentary fea- parallel to the south-eastern outcrop limit of the tures (p. 402). We interpret these supposed abrupt Triassic materials mentioned above (Fig. 1).Most faces transitions as faults. Their relation to pri- good outcrops of the ‘Black Flysch’ along the mary facies boundaries is not apparent. Further- coast between Saturraran and Deba are thought more, the authors omit any proof of their most to be unrelated to the OTS. We question the important argument for the L-shape of the OTS relevance of present-day outcrop limits to the depositional system, except for two faint arrows geometry of a depositional system, notably in an (their fig. 2). It is not clear from which sedimen- area of Alpine deformation. We believe that tary structure they infer the 90”bend of palaeocur- neither the outcrop geometry nor the dispersal rent directions. Groove cast measurements by us pattern as presented in Agirrezabala & Garcia- (T. Pletsch, unpubl. data) taken 2.5 km SSE of Mondejar (1994) show convincing evidence for an Berriatu (Middle-Outer OTS according to fig. 2) L-shape of the OTS. indicate NNE-SSW directions with low scatter. This means that in the western part of the OTS, palaeocurrents most probably pointed south- ACKNOWLEDGMENTS west, as would be expected. Although this does not rule out a bend of palaeocurrent towards the Mutrikuko Udala and Gipuzkoaku Foru Aldundia east in other parts of the OTS, we interpret it as an supplied unpublished well reports and 1:5000 indication of ‘normal’ radial dispersal. It is inter- maps. Jimmy Nebelsick corrected the English text. esting to note that evidence for a continuation of Field work was funded by the German Research the OTS towards the west has been neglected on Council (DFG).

L. M. AGIRREZABALA and J. GARCIA-MONDEJAR Departamento de Estratigrafia y Paleontologi’a, Facultad de Ciencias, Universidad del Pais Vasco, Apartado 644,48080 Bilbao, Spain

Pletsch and Meschede comment on our work facies analysis made. Below, we take the issues (Agirrezabala & Garcia-Mondhjar, 1994), criticiz- for discussion in turn, making reference to our ing our conclusions regarding geometry, size and geological map (Fig. 2). internal facies of the Ondarroa Turbidite System (OTS). Although some authors had previously reported the presence, petrology and age of deep- PALINSPASTIC RESTORATION water conglomerates in the Ondarroa-Saturraran coastal area, suggesting a possible inner fan origin Pletsch and Meschede suggest we underestimate for them, only in Agirrezabala et al. (1989) and the tectonic deformation and they think that Agirrezabala & Garcia-Mondejar (1994) was a our palinspastic map (Agirrezabala & Garcia- complete turbidite system defined and detailed MondBjar, 1994, fig. 2) shows a too small

L. M. AGIRREZABALA and J. GARCIA-MONDEJAR Departamento de Estratigrafia y Paleontologi’a, Facultad de Ciencias, Universidad del Pais Vasco, Apartado 644,48080 Bilbao, Spain

0 1995 International Association of Sedimentologists, Sedimentology, 42, 523-530 526 L. M. Agirrezabala and J. Garcia-Mondkjar restoration. They also propose, without support- In summary, we maintain that both our short- ing data, that the OTS should be twice the size ening estimates (2145%) and our palinspastic we calculated. Our restoration, however, was restoration are correct. Consequently, we consider made from geological sections based on surface that the estimate of the OTS made in our work is data, considering N-S shortenings of 21% and also correct. NE-SW shortenings of 25%, which we reaffirm. Pletsch and Meschede make two calculations of the aparent shortening from our fig. 2. They DIAPIRISIM deduce a shortening of 5% between Ondarroa Upper Triassic (Keuper) sedimentary and igneous and Berriatu and a shortening of 10% between rocks near Mutriku are well documented (e.g. Ondarroa and Elgoibar. We do not believe these ENPENSA, 1964). They occupy the small blank values can be taken as indicators of the maxi- area (shown without symbol by error) to the west mum shortening, since (a) the Ondarroa-Berriatu of Mutriku in fig. 1, and are clearly identified in direction is 62" oblique with respect to the our previous publications about the area (e.g. direction of maximum shortening of the area Badillo-Larrieta et al., 1988; Agirrezabala & (N-S) and (b) the shortening between Ondarroa Garcia-Mondkjar, 1989; Badillo et al., 1989). and Elgoibar was heterogeneous, as we discuss The Keuper rocks outcrop in two thin strips, later. Our restoration was made taking into each generally less than 0.5 wide following account the boundaries of the turbiditic system, km the traces of two faults, NE-SW and E-W orien- and the locations of the villages inside were tated, respectively. Both faults were active during added only approximately, for orientation the Alpine phase of compression, but we have purposes. found no stratigraphic, sedimentological or The basis of the palinspastic restoration of the palaeotectonic feature in the Albian of the area OTS (Agirrezabala & Garcia-MondBjar, 1994, fig. that can give us an indication about possible 2) was not explained before to avoid lengthening synsedimentary diapiric action of the Keuper. A the paper. The restorations are based on surface NE-SW-orientated fault also present at Mutriku, data taken as we made the 125 map of the 000 in contrast, shows signs of Middle Albian area. Two sectors were considered in fig. 1 of synsedimentary action (see point 4) along with Agirrezabala & Garcia-Mondkjar (1994), character- other similarly orientated faults. Our Alpine ized by materials with major differences in their interpretation in the area of Mutriku agrees with respective types of deformation: (a) SW of other studies (e.g. EVE, 1989). Ondarroa-Mutriku, and (b) SW of Mutriku-Itziar. In (a), to the SW of the Ondarroa-Mutriku line, where units 3, 4 and 5 of the OTS outcrop (fig. I), MEGABRECCIAS a N-S average shortening of 21% was calculated restoring folds and faults. In the N25"E direction We reaffirm the presence of Urgonian lime- (axis of the OTS), it was calculated that the rocks stone megabreccias and distinguish two types experienced a shortening of 19-5%, as tested by (Agirrezabala & Garcia-Mondkjar, 1994, pp. 386, comparing the distance between the northern- 387, 399 and 400). The first type are those most conglomerate outcrops close to Ondarroa included in the unit underlying the OTS, and and the Markina-Elgoibar fault line on both the consist of individual limestone olistoliths up to geological and the restored maps (figs. 1 and 2). In 158 m long; they are interpreted as the products of (b), to the SW of the Mutriku-Itziar line, where gravitational resedimentation on a slope apron units 1, 4 and 5 outcrop (fig. I), a shortening of preceding the turbiditic trough. They outcrop 25% in a NE-SW direction was calculated restor- discontinuously in a band from Milloi (NW of ing folds and faults. To establish the width of the Berriatu) to the west of Mutriku, and in another outer OTS (fig. 2) corresponding to the maximum outcrop to the north of Markina (fig. 1).In the area areal extension reached by the turbidites (during to the NW of Berriatu a local succession up to the later stages of the formation of the system], the 150 m thick shows megabreccia units up to 20 m palinspastic restoration was calculated assuming thick alternating with marls, with limestone olis- the same tectonic shortening of 25% in a NE-SW toliths up to 158m long. To the east of Berriatu, direction. In this case we have taken as references limestone olistoliths up to 7m long have been the northernmost outcrops in the vicinity of Itziar discovered, and in one outcrop 2 km to the ENE of and the Markina-Elgoibar fault line to the south of Berriatu, the olistoliths appear nearly completely the system. silicified because of the action of mineralizing

0 1995 International Association of Sedirnentologists, Sedirnentology, 42, 523-530 A coarse-grained turbidite system, Iberia 527 fluids related to the Alpine movements of a model conceived for the unit was based firstly on reverse fault which affects the succession. To the characteristics of a very detailed map, and the north of Markina the succession shows mega- then on the elaboration of a clastic dispersal breccia units, up to 30m thick, and limestone system. This, in its turn, was based primarily on olistoliths up to 18 m long (fig. 1). the grain-size changes and secondarily on palae- The second type of megabreccias are those occurents measured from sedimentary structures. composed of limestone blocks of smaller size (up Pletsch and Meschede write: ‘the authors omit to 1-5m long), which outcrop patchily along the any proof of their most important argument for margins of the OTS (pp. 399400). Only these the L-shape of the OTS depositional system, megabreccias are considered to have formed except for two faint arrows’. We will answer this through rock-fall processes (pp.386-387), comment with appropriate data from the paper although in fig. 1 they appear with the same and with two methodological assumptions. The symbol as type I. However, the legend to fig. 1 map shown in fig. 1, although simplified for the suggests a diachronism between both types of paper, was elaborated very carefully at 125000 megabreccias. Megabreccias laterally adjacent to for sedimentological interpretation of the distin- the lowermost part of the OTS have been guished units. Thus, the lateral facies changes of observed only in two outcrops: one located the main clastic lithologies of the turbidite system 5.5 km to the south of Ondarroa (to the north of (OTS), shown with an indented line in fig. 1, Jaungoikomendi mountain) and the other located reflect exactly the following: 5.3 km to the west of the same village (Milloi). In 1 the main conglomeratic body of rock is elon- both locations the megabreccias appear adjacent gated NE-SW, which is in agreement with the to platform limestones in normal stratigraphic average value of the palaeocurrent measurements position (figs 1 and 14). The megabreccias to the made in the Ondarroa area (see fig. 2); south of Ondarroa are aligned approximately 2 the conglomerates change very rapidly to NNE-SSW, following the boundary of the plat- mudstones toward the NW, practically without form limestones. To the north of this outcrop, intermediate sandstone facies except for their where the boundary with platform limestones lowermost and uppermost parts in the south- becomes tectonized (Arno thrusting), there are westernmost area; abundant mineralizations along the limestone 3 the conglomerates also change very rapidly to front; nowhere in our paper have we reported the mudstones without intermediate sandstone facies presence of megabreccias along this tectonized to the SE of Ondarroa, except for their uppermost boundary, as Pletsch and Meschede suggest. part, again with intermediate sandstone facies up to the village of Itziar; 4 the conglomerates laterally change to sand- SHAPE OF THE SYSTEM stones towards both the SW and the SE in their southernmost outcrop and in all their thickness. The present boundary between the platform lime- These sandstones, in their turn, are rapidly stones and the inner and outer OTS is, for its most replaced by mudstones toward the SW (east of part, a reverse fault (fig. 1). We have never con- Markina), but very slowly by mudstones toward sidered this boundary as a primary sedimentary the SE (Madarixa) (fig. 1). feature. However, the quick disappearance of the From these observations and from the presence reported fault to both the SE and the SSW (in the of rock-fall megabreccias, we deduce a palaeohigh later case megabreccias attributed to rock-fall pro- to the east of the conglomeratic trough; this struc- cesses succeed laterally the limestones), leads us ture was probably bounded by an active basement to the conclusion that the limestones of the fault and a steep slope or submarine talus to the upthrusted block could have been at the platform NW, making up the eastern margin of the trough. margin (see also EVE, 1989). Occasionally, some turbiditic flows developed at Accounting for small Alpine displacements on the beginning and at the end of the conglomeratic faults, demonstrated by careful1 examinations of phase of deposition expanded to the west (figs 14 facies continuity, grain-size concordances and and 15). They left a few tens of metres of thick- small boundary offsets across the faults, we con- ness of deposits, which in no way can be com- cluded before, and reaffirm now, that the palins- pared with the more than 400 m recorded in the pastic restoration applied (see point 1) is enough adjacent conglomeratic trough. This demonstrates to obtain the approximate primary geometry of the small importance of the westerly directed the studied stratigraphic unit. The sedimentary turbidite flows. We also deduce a rapid loss of $3 1995 International Association of Sedimentologists, Sedimentology, 42, 523-530 528 L. M. Agirrezabala and J. Garcia-Mondkjar

Fig. 2. Geological map of the Ondarroa turbidite system (units 3, 4 and 5) and encasing rocks (units 1, 2 and 6) showing structural data. From this map the reduced version of fig. 1 in Agirrezabala & Garcia-MondBjar (1994) was obtained. Legend: (1) Urgonian limestones; (2) mudstones and marls with limestone megabreccias (black triangles); (3) conglomerates, sandstones and mudstones; (4) sandstones and mudstones; (5) mudstones; (6) mudstones and sandstones. competence of the turbidite flows at about 2 and the most important indicators of the shape of the 3 km to the NE of Markina (fig. l), and a main- dispersal system. tenance of this competence for many kilometres Finally, two methodological assumptions from that area toward the SE; this implies a 90" upon which we based our dispersal system turn in the direction of the palaeocurrents, interpretation were: necessarily caused by the presence of a steep 1 'Internal changes in bed thickness, sandstone/ slope dipping NE and coinciding with the major shale ratio, texture, sedimentary structures, and Markina-Elgoibar fault line. The geometry of the other characteristics define directional relation- system thus obtained and the high-density ships and serve to reconstruct the shape of the gravitational processes invoked in its origin are ancient fan' (Bouma et a:., 1985, p. 9);

0 1995 International Association of Sedimentologists, Sedimentology, 42, 523-530 A coarse-grained turbidite system, Iberia 529

2 ‘Scalars are those properties that are specified In summary, we reaffirm our interpretations by magnitude alone (. . .). Measurement of a scalar and clarify two details (Keuper symbol on the quantity at a point has no directional significance. map and information about the approximate However, when scalars are mapped across a location of the villages on the palinspastic map). region they define the directional derivative or We consider that the objections of Pletsch and gradient of a scalar function (. . .).’ ‘It also seems Meschede are based on a poor knowledge of the reasonable to expect a regular size decline of whole studied area, on an improper perception of granules and the fine gravel down the slope on the main geological features of the Ondarroa area submarine fans of turbidite origin’ (Potter & and on an inappropriate conception of the use of Pettijohn, 1977, pp. 3 and 283, respectively). facies parameters to elaborate a sedimentary model.

DATA RELIABILITY ACKNOWLEDGMENTS All the points brought up for discussion by Pletsch and Meschede question either the proper Partial funds for this work were provided by existence of, or the interpretation of, the basic Basque Government, Project PIGV 121.310- field data used by us. The criticisms of Pletsch B018/92, and Universidad del Pais Vasco-Euskal and Meschede are based on their map (Fig. 1) of Herriko Unibertsitatea, Project UPV 121.310- the Ondarroa area. We include for comparison our EA016/93. map of the study area (Fig. 2) from which the reduced version of fig. 1 in Agirrezabala & Garcia- Mond6jar (1994) was obtained. REFERENCES The first thing to emphasize when the two Agirrezabala, L.M., Badillo, J.M. and Garcia-MondBjar, maps are compared is that ours represents a J. (1989) El sistema de abanico turbiditico en ‘L’ de much larger area. We have much more detailed Ondarroa (Albiense medio, Euskal Herria). Caracter- regional information from which to obtain con- izacidn, facies y controles sedimentarios. In: Sympo- clusions. More importantly, Pletsch and sium, XII Congreso Nacional de Sedimentologia (Ed. Meschede’s map (Fig. 1) does not show any by S. Robles, J. Garcia-Mondbjar and A. Garrote), stratigraphic succession within the Black Flysch. pp. 167-175. UPV & EVE, Bilbao. Agirrezabala, L.M. and Garcia-MondBjar, J. (1989) The knowledge of this stratigraphy is crucial to Evolucion tectosedimentaria de la plataforma urgoni- criticize our work, since the stratigraphic succes- ana entre Cab0 Ogofio e Itziar (Aptiense-Albiense sion of the Black Flysch to the south of Deba, for superior, Regidn vasco-cantabrica oriental). In: Syrn- instance, with the lateral change of the upper- posium, XIZ Congreso EspaAol de Sedimentologia most part of the Ondarroa conglomerates to (Ed. by S. Robles, J. Garcia-MondBjar and A. Garrote), sandstones toward the east (Itziar), is a key point pp. 11-20, UPV & EVE, Bilbao. to demonstrate the L-shape of the turbiditic sys- Agirrezabala, L.M. and Garcia-MondBjar, J. (1994) A coarse grained turbidite system with morphotectonic tem. The two major NE-SW Alpine faults to the control (Middle Albian, Ondarroa, northern Iberia). south of Ondarroa, whose deduced palaeo- Sedimentology, 41, 38 3-40 7. tectonic action is of key importance to explain Aguilar Tomas, M.J. (1975) Sedimentologia y paleo- the location of the megabreccias and the geografia del Albense de la Cuenca Canthbrica. Estud. Ondarroa conglomerates, do not exist for the Geol., 31, 1-213. authors above. We have included in Fig. 2 field Badillo-Larrieta, J.M., Agirrezabala, L.M. and Garcia- data which justify the presence of these faults MondBjar, J. (1988) CaractBres generales de la suc- esidn albiense superior del Flysch Negro entre and the notable distortion that they cause in the Elantxobe y Deba (Bizkaia y Gipuzkoa). In: ZI Con- local geology. Equally, major mistakes in Fig. 1 greso Geologic0 de Espafia. (Vol. I), pp. 35-38. are the lack of representation of important out- Granada. crops of conglomerates and megabreccias, the Badillo, J.M., Agirrezabala, L.M. and Garcia-MondBjar,J. omission of a major reverse fault to the north of (1989) Sedimentos de talud terrigeno entre altos Berriatu (the one with related extrusion of carbonatados urgonianos en el sector de Cabo Ogofio- Keuper materials in Mutriku) and the erroneous Itziar (Albiense medio, Bizkaia y Gipuzkoa). In: XZZ Congreso Nacional de Sedimenfologia (Ed. by S. location of some of the major fold axes present Robles, J. Garcia-MondBjar and A. Garrote), pp. 93- in the area. Minor details with which we dis- 96. UPV & EVE, Bilbao. agree abound, and they are easily noticed when Boss, J. (1984) Der Diapir von Gernika (Nordspanien). the two maps are compared. Z. dt. geol. Ges., 135, 7-21. 8 1995 International Association of Sedimentologists, Sedirnentology, 42, 523-530 530 L. M. Agirrezabala and J. Garcia-Mondbjar

Bouma, A.H., Normark, W.R and Barnes, N.E. (1985) Lefort, J.P., Arrieta, A. and Riaza, C. (1987b) Crustal COMFAN: needs and initial results. In: Submarine thinning on the Aquitaine shelf, Bay of Biscay, from Fans and Related Turbidite Systems (Ed. by A. H. deep seismic data. Nature, 325, 513-516. Bouma, W. R. Normark and N. E. Barnes), pp. 7-11. Pletsch, T. (1990) Kartierung, Sedimentologie und Springer-Verlag, New York. Stratigraphie im “Schwarzen Flysch von Deva” bei Brinkmann, R. and Logters, H. (1968) Diapirs in West- Saturraran, Prov. Bizkaia/Gipuzkoa, N-Spanien. ern Pyrenees and Foreland, Spain. In: Diapirism and Unpubl. MSc thesis, University of Tiibingen. Diapirs (Ed. by J. Braunstein and G. D. O’Brien), Potter, P.E. & Pettijohn, F.J. (1977) Paleocurrents and Mem. Am. Ass. petrol. Geol., 8, 275-292. Basin Analysis, 2nd edn. Springer-Verlag,New York. Engeser, T., Reitner, J., Schwentke, W. and Wiedmann, Rat, P. (1988) The Basque-Cantabrian Basin between J. (1984) Die kretazisch-alttertiiire Tektogenese des the Iberian and European plates: some facts but Basko-Kantabrischen Beckens (Nordspanien). Z. dt. still many problems. Rev. SOC. Geol. Espaiia, 1, Geol. Ges., 135. 243-268. 327-348. ENPENSA (1964) Estudio geol6gico de la regi6n de Reitner, J. (1984) Mikrofazielle, palokologische und Deva. Notas y Comunicaciones del Inst. Geol. y Min. palaogeographische Analyse ausgewahlter Vorkom- de Espaiia, 76, 237-244. men jlachmariner Karbonate im Basko- Euroestudios, S.A. (1976) Hidrogeologia de la zona de Kan ta brisch en Strike-Slip Fa ult-Beckensystem Motrico. Unpublished hydrogeologcial well report (Nordspanien) an der Wende von der Unterkreide zur deposited in the Mutriku town hall. Oberkreide. PhD thesis, University of Tiibingen. EVE (1989) Mapa geologic0 del Pais Vasco. Ondarroa Schwentke, W. and Wiedmann, J. (1985) Oberkreide- [63-I]. Edited by Ente Vasco de la Energia. Entwicklung und Diapirismus im Estella-Becken Feuilliee, P. (1967) Le Cenomanien des Pyrenees (Nordspanien). Facies, 12, 1-74. Basques aux Asturies. Essai d’analyse strati- von Stackelberg, U. (1967) Der Diapir von Murguia graphique. Mkm. SOC.gkol. France (N.S.), 108,l-343. (Nordspanien). Beih. Geol. Jb., 66, 63-94. Meschede, M. (1986) Die Geochemie der Vulkanite und Voort, P. (1964) Zum Flyschproblem in den West- die sedimentare Entwicklung des Biscaya- pyrenaen. Geol. Rdsch., 53, 220-233. Synklinoriums in Nordspanien wahrend der Mittel- Wiedmann, J., Reitner, J., Engeser, T. and Schwentke, und Oberkreide. PhD thesis, University of Tiibingen. W. (1983) Plattentektonik, Fazies- und Subsidenzge- Meschede, M. (1987) The tectonic and sedimentary schichte des basko-kantabrischen Kontinentalrandes development of the Biscay sinclinorium in northern wahrend Kreide und Alttertitir. Zitteliana, 10, Spain. Geol. Rdsch., 76, 567-577. 207-244. Pinet, B., Montadert, L., Curnelle, R., Cazes, M., Marillier, F., Rolet, J., Tomassino, A., Galdeano, A,, Discussion received 21 July 1994; Reply received Patriat, Ph., Brunet, M. F., Olivet, J.L., Schaming, M., 14 November 1994

0 1995 International Association of Sedirnentologists, Sedimentology, 42, 523-530