Structure and organization of submarine basaltic flows: sheet flow ...

Int J Earth Sci (Geol Rundsch) (2012) 101:2201–2214 DOI 10.1007/s00531-012-0783-2

ORIGINAL PAPER

Structure and organization of submarine basaltic flows: sheet flow transformation into pillow lavas in shallow submarine environments M. Carracedo Sańchez • F. Sarrionandia T. Juteau • J. I. Gil Ibarguchi

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Received: 27 October 2011 / Accepted: 18 April 2012 / Published online: 25 May 2012 Ó Springer-Verlag 2012

Abstract Distal pillows occur associated with a sheet flow and megapillows in the menãkoz outcrops of the Basque–Cantabrian Basin (N Spain). Basaltic volcanic rocks are interbedded with Turonian sediments and depict typical features of shallow submarine emissions. An exceptional basaltic flow displays four types of morphology: (1) sheet lava with columnar jointing, (2) welded columnar breccia, (3) megapillows, and (4) pillow lavas with sparse megapillows. The field data from menãkoz combined with experimental and field data from the literature for similar volcanic facies can be integrated into a new propagation model for the transition from sheet flows to pillow lavas in underwater environments. At near vent high emission rates, lava flows develop a thin crust immediately after its emplacement and break at the front under the magma pressure allowing for the massive propagation of lava as a sheet flow. Increased cooling promotes thickening of the lava outer crust far from the vent while continuous supply of fresh magma increases the pressure onto the thick crust until its rupture. The lava emitted in small volumes from the flow front promotes the formation of megapillows and pillow lavas that are later on covered by the advancing sheet flow. The lava flow freezes M. Carracedo Sańchez J. I. Gil Ibarguchi (&) Departamento de Mineralogıá y Petrologıá, Facultad de Ciencia y Tecnologıá, Universidad del Paı´s Vasco UPV/EHU, Sarriena s/n, 48940 Leioa, Spain e-mail: [email protected] F. Sarrionandia Departamento de Geodina´mica, Facultad de Farmacia, Universidad del Paı´s Vasco UPV/EHU, 01006 Vitoria, Spain T. Juteau Professor Emeritus, Universite´ de Bretagne Occidentale, Brest, France

progressively toward more distal parts, gradually increasing its viscosity until it stops. The crust temporarily holds the residual melt pressure increasing the volume of the flow distal section by inflation. Finally, the internal magma pressure breaks the crust and liberates lava at moderate-tolow flow rates producing pillows, while lava drainage inside the inflated sheet flow produces lava tunnels and gravitational collapse of the roofs by hydrostatic pressure to form breccias nurtured by columnar lava fragments. Keywords Pillow lava Sheet flow Welded breccia Subaqueous volcanism

Introduction Sheet flow and pillow-lava formation in underwater environments depends above all on the flow rate of emitted lavas. Experimental data and direct observations indicate that moderate flow rates in basaltic lava emissions promote the formation of pillow lavas, that is, tubular shaped morphologies, while higher flow rates generate sheetshaped flows, ponded lava (or lava pillars) and sheet flows with massive structure or columnar jointing (Fink and Griffiths 1990; Griffiths and Fink 1992a, b; Juteau 1993; Embley and Chadwick 1994; Chadwick and Embley 1994; Gregg and Fink 1995). Griffiths and Fink (1992b) suggest that, assuming a lava viscosity of 102 Pa s, the flow types require the following extrusion rates: pillows, \1 m3 s-1; lobate sheet flows, 1–100 m3 s-1; ropy sheet flows, 100– 3,000 m3 s-1; and jumbled sheet flows, [3,000 m3 s-1. However, these estimations are based on a direct extrapolation from wax experiments, and their validity for lava must be further tested in future studies. Deep submarine dives have revealed that structures such as pillow lavas,

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lava lobes, megapillows, sheet flows, and lava lakes are common features of ocean volcanism (Juteau and Maury 2008 and references therein). Thus, in present-day deep ocean floors, specially those at mid-ocean ridges where intermediate-to-high accretion rates are recorded, lava lakes, sheet flows, and pillow lavas alternate through time and space conditioned mainly by the flow rates of products emitted from the volcanic focus (Juteau 1993; Kennish and Lutz 1998; Umino et al. 2000). Nonetheless, it appears also that lava lakes and sheet flows might turn into pillow lavas when the magma at moderate flow rates (\1 m3 s-1, according to Griffiths and Fink 1992b) is channelized into tubes during late stages of their course, that is, in their final longitudinal sections (Francheteau et al. 1979; Crane and Ballard 1980; Juteau 1993; Embley and Chadwick 1994; Chadwick and Embley 1994; Kennish and Lutz 1998). Recent observations at the East Pacific Rise (98460 N– 98560 S) have confirmed this assumption and have shown that lava flows erupted there during the years 2005 and 2006 were characterized by inflation and sheet morphologies in the flow interiors, and pillow forms at the terminal flow fronts (Soule et al. 2007). Yet, the transition between these structures, that is, from sheet flows to pillow lavas, is difficult to directly observe in such deep water environments. Conversely, many ancient underwater volcanic sequences formed by basic lavas allow the in situ observation of structures similar to those found presently on the ocean floors. Most such volcanic sequences display pillow lavas on top of sheet flows or vice versa, which suggests that sheet flows or lava lakes might disappear when the magmatic flow rate decreased in the vents, thus favouring the formation of pillow lavas (Dimroth et al. 1978; Wells et al. 1979; Cuevas et al. 1981; Baragar 1984; Rossy 1988; Carracedo et al. 1999; Furnes et al. 2001). In other cases, sheet flows and pillow lavas appear within the same volcanic flow, even in contact with each other, but represent discrete emplacement units. Moreover, on a two-dimensional view, the sheet flows do not appear to act as distribution feeders to associated pillow lavas. Bear and Cas (2007), for instance, have suggested that Miocene sheet structures, megapillows, and pillow lavas that outcrop in Maori Bay (New Zealand) were emplaced as multiple lobes of a single lava flow from just one sustained eruption. In this case, it has been assumed that the magma discharge rate at the vent site was high although the actual magma supply rate in distal sections of the flow could vary considerably. Higher magma supply rates would result in either thick and massive or thin sheet structures, whereas megapillows and normal pillow types would form at the same time as the sheet structures but restricted to parts of the distal flow subjected to lower magma supply rates. Finally, the sections formed by sheet flows in some ancient

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Int J Earth Sci (Geol Rundsch) (2012) 101:2201–2214

volcanic sequences have been interpreted as proximal or close to the emission focus, while those mainly formed by pillow lavas and pillow breccias would correspond to distal positions (Dimroth et al. 1978; Wells et al. 1979; Waters and Wallace 1992). Also, a number of researchers have suggested that sheet flows might in some cases represent former feeder channels by which molten lava was conveyed from the vent so as to form pillow structures at the flow front (Dimroth et al. 1978; Van Andel and Ballard 1979; Walker 1992; Gibson et al. 1999). Two main types of pillow lava are currently acknowledged by most authors. One type, formed under moderate flow rates, generally appears directly connected to feeder dikes at vent sites (Moore 1975; Yamagishi 1985; Juteau 1993; Chadwick and Embley 1994). The other type is that rooted into large tubes (megapillows, 1–15 m in diameter) or sheet flows, far away from the vent sites (Baragar 1984; Umino et al. 2000; Goto and McPhie 2004; Soule et al. 2007). The transition from lava sheet flows or megapillows to pillow lava has been reported thus far only from the islands of eastern Hudson Bay (Baragar 1984) and from the Stanley Peninsula, NW Tasmania (Goto and McPhie 2004). In those localities, Baragar (1984) and Goto and McPhie (2004) found indisputable evidence that megapillows and sheet lavas acted as master feeder channels by which molten lava was transferred to the advancing pillows. Here, we describe a Cretaceous volcanic facies association from the Menãkoz beach (Basque–Cantabrian Basin, Northern Spain) that depicts a first-class example of distal pillows associated with sheet flows and megapillows (Fig. 1). There, submarine mafic lavas display well-preserved primary surface shapes and internal structures that differ in a number of aspects from those observed in the flows of Stanley Peninsula and expand the inventory of features characteristic of this type of volcanic products. Our contribution is thus intended to provide a better understanding of the morphology, structure, and propagation mechanisms of subaqueous mafic lavas in recent or ancient volcanic successions, particularly where the transitional structures are not present.

Geological setting The Basque–Cantabrian mountain range may be regarded as the western continuation of the Pyrenees chain, a [400km-long alignment that separates the Iberian Peninsula from mainland Europe (Fig. 1). The formation of this chain reflects a succession of geological events that occurred along the northern edge of the Iberian plate since Early Paleozoic times. Post-Paleozoic events were essentially related to the lateral drift of the Iberian plate and the collision with the European plate (Fig. 2a). During the


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Fig. 1 Sketch map of the Pyrenees showing the areal extent of Cretaceous magmatic rocks along the North-Pyrenean Zone (adapted from Azambre et al. 1992). The westernmost volcanic area enclosed

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in the circle of picture b corresponds to the Cretaceous igneous rocks of the Biscay syncline

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Fig. 2 a Geotectonic sketch of the Gulf of Biscay during Cretaceous times showing the main fractures that controlled the magmatism (based on Boess and Hoppe 1986). b Proposed ages for the Basque–

Cantabrian basin magmatism: (1) Rat (1959), (2) Ciry et al. (1967), (3) Jerez (1968), (4) Lamolda et al. (1983), (5) Castanãres et al. (1997), (6) Montigny et al. (1986). Time scale from IUGS (2009)

Mesozoic, the Pyrenean domain suffered intermittent extensional episodes that promoted crustal thinning, volcanism and, in due time, the development and infill of associated sedimentary basins. This was the result of a southeastern drift of Iberia with respect to Europe that involved sinistral displacement and anti-clockwise rotation

of the Iberian plate (Fig. 2a). During the Cretaceous, the Basque–Cantabrian realm underwent intense subsidence as reflected in the up to 17,000-m thick accumulated sedimentary pile. The climax of the subsidence was reached during Albian times (112–99.6 Ma) when a two-sided basin developed in a WNW-ESE trending deep sea trough

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(Robles et al. 1989; Pujalte et al. 1993). From the Albian up to the Santonian (83.5 Ma), marine sedimentation took place coeval with submarine volcanism in particular during upper Albian and Santonian times (Fig. 2b). Convergence and oblique collision of the Iberian and European plates was related to the Alpine orogeny and took place during late Cretaceous to Miocene. This accounts for the positive inversion and deformation of the sedimentary sequence (Barnolas and Pujalte 2004). Along the Alpine syncline of Biscay (Basque–Cantabrian Range), there occurs a [ 1,000-m thick volcanic sequence interbedded within Cretaceous sediments (Fig. 3a). The volcanic sequence shows typical characteristics of eruptions under shallow submarine conditions including pillow lavas, sheet flows with marked columnar joints, dikes, synvolcanic sills, and a wide range of pyroclastic and autoclastic (hyaloclastite, autobreccia, peperite, and talus debris) deposits and syn-eruptive reworked facies (Cuevas et al. 1981; Rossy 1988; Carracedo et al. 1999; Castanãres et al. 2001). This volcanism has a sodic alkaline character and is mainly represented by variably vesicular basalts with minor amounts of trachyte to trachyandesite. The volcanic rocks underwent hydrothermal transformations under sea water at conditions of ca. 200 °C and \1 kbar (Rossy 1988). Their primary minerals, plagioclase ± clinopyroxene ± olivine ± amphibole in basalts, and alkali feldspar ± pyroxene ± amphibole in trachytes, were transformed into secondary albite, calcite, chlorite,

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pumpellyite, and prehnite. On the basis of associated benthic foraminifera biofacies, Castanãres et al. (2001) have suggested that this volcanism was developed in bathyal to middle bathyal environments. The sites with the highest rate of volcanic emission and thickest volcanic deposits corresponded to an average depth of 900 m while sites of thinner volcanic units accumulated at minimum depths of 1,100 m. This magmatism represents the westernmost expression of the North-Pyrenees Cretaceous alkaline province (Fig. 1) and attest to extensional activity through the area between 110 and 85 My (Cabanis and Le Fur Balouet 1990; Rossy 1998; cf. Fig. 2b).

Working terminology In order to better understand the descriptions and interpretations in the present study, a brief summary on the terminology adopted for the volcanic facies and facies associations at the Menãkoz beach is presented below. Sheet flow The term sheet flow is used for lava flow units composed exclusively of massive lavas with planar flat surfaces. Sheet flows may form broad, laterally extensive blankets of lava. The term sheet does not presuppose the flow mechanics or the thickness of the flow, only its flat shape

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Fig. 3 a Geotectonic sketch of the Biscay syncline where the Upper Cretaceous igneous rocks studied outcrop. b Detail of the geology around Menãkoz (based on EVE 1995). The circle shows the location of the basaltic lava flow

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and relatively large aspect ratio (widths–tens of m to a few km, thicknesses-1–100 m) (Ballard et al. 1979; Dimroth et al. 1978; Kennish and Lutz 1998; Juteau and Maury 1999; Batiza and White 2000). Massive sheet flows commonly have a relatively flat, smooth, pavement-like surface with a microrelief of less than a few centimeters. Less often, they exhibit a variety of surface textures such as lobate, ropy, lineated, and jumbled, depending on the local flow conditions when the upper surface crust formed (Kennish and Lutz 1998). Generally, massive units show flow layering or joints parallel or perpendicular to the roof and the base. Also, columnar joints and irregular polygonal jointing are common in the massive sheet flows (Dimroth et al. 1978; Juteau and Maury 1999; Kennish and Lutz 1998). Pillow lava A pillow is a lava flow tube that is approximately as wide as it is thick. The roughly elliptical pillow-like shapes that characterize two-dimensional transverse exposures of this type of lava are in fact cross-sections through interconnected lava tubes and lobes (Moore 1975; Yamagishi 1985; Walker 1992; McPhie et al. 1993; Juteau and Maury 1999). Most pillow lava structures found in present-day ocean floors and ophiolites are less than 1 m in diameter (Walker 1992; Juteau and Maury 1999; Umino et al. 2000). Here, we follow the pillow lava terminology of Dimroth et al. (1978) who takes into account the diameter of the transversal section of the lava tubes. Thus, ‘‘very small’’ and ‘‘normal-size’’ pillows have diameters of \30 and 50–150 cm, respectively, whereas ‘‘megapillows’’ are [150 cm in cross-section. Where the megapillows have been welded, the flow may appear massive and their presence may be just signaled by some vestiges, as remnants of tube shapes, cooling cracks (circular and radial joints), and vesicle fronts.

The Menãkoz beach volcanic group: classification of the observed structures In the southwestern part of the Menãkoz beach (N of Bilbao), where the whole series appears inverted due to the Alpine orogeny (EVE 1995; Fig. 3b), basaltic and trachybasaltic flows, and basic volcanoclastic deposits (mudstone to breccia) outcrop interbedded concordantly with Turonian marine sediments (carbonatic detrital flysch; EVE 1995). A dominantly detritic sequence outcrops on top of the volcanic levels. This sequence is made of marly layers and sandy beds with pyrite nodules and scarce microconglomeratic levels. The section including the volcanic flows consists of lutites, marls, calcarenites, and

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pyroclastic rocks (breccias and basic tuffs cemented by calcite). Underlying the volcanic section outcrops a sequence of limestone strata alternating with marl or marly limestones containing abundant remnants of inoceramids and equinodermata (Cuevas et al. 1982). The lowermost and thickest (ca. 20 m) flow there offers excellent outcrops along the cliff and over the intertidal zone (Fig. 4). Outcrops at the cliff show mainly pillow lava structures while the intertidal outcrops allow the recognition of a greater variety of structures. This study is focussed on the intertidal exposures as they illustrate better the morphology, emplacement, and thermal contraction history of the volcanic products erupted there. The variety of structures recognized in the intertidal exposures has been categorized as follows: (1) the pillow lava section, (2) the transitional section, and (3) the sheet lava flow with prismatic polygonal jointing (Fig. 4). These structures are described in detail below. The pillow lava section A ca. 170-m long and 20-m thick section of lava flow with pillow lava structures occurs close to the cliff where the best outcrops are displayed (Fig. 5a). It is partially eroded toward the beach side and extends along ca. 30 m in the sea direction while it penetrates, albeit very covered, about 140 m inland. The pillow lava section includes a pile of approximately cylindrical lava tubes of up to 20 m in length and 2.20 m in diameter. The tubes present rough or smooth surfaces (Fig. 5b, c) and are either delimited by a thin layer of autoclastic materials or molded each other. The lower pillows show flat bottoms and semicircular shaped transversal cross-sections (Fig. 5d). The basal contact is complex in detail, involving intricate interpenetration between lava and sediments, together with sharp, planar unmixed contact; blocky and globular peperite occur where the lava burrows into unconsolidated marly sediments (Fig. 6). Transversal cross-sections of the tubes show a concentric and/or radial inner structure defined by one or more of the following elements (Fig. 5b, c): (1) concentric and/or radial joints, (2) \1–8 mm in diameter vesicles/amygdales arranged in concentric surfaces, (3) radially oriented, pipe-type (cylindrical) vesicles/amygdales up to 7 cm long and 1.5 cm in diameter, (4) central geodes, generated by the coalescence of amygdales or formed after the tube drainage, (5) crystallinity degree, which increases toward the core with occasional vitreous crust. The megapillows correspond to the largest tubes with diameters of 1.50–2.20 m, while the rest of the tubes form normal (diameter B1.50 m) or small (diameter B30 cm) pillows. The megapillows show frequently a well-developed columnar radial jointing with fractures that delimit pyramidal shaped sections of pentagonal, hexagonal or

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Fig. 4 Aerial view of the main basaltic flow in the Menãkoz beach showing the cliff and intertidal zone outcrops. Dashed lines delimit the upper and basal contacts of the flow with Turonian sedimentary rocks. The three main sections of the basaltic flow from proximal to

distal facies are shown: (1) the sheet lava flow with prismatic polygonal jointing, (2) the transitional section, and (3) the pillow lava section

trapezoidal base up to 20 cm in diameter (Fig. 5e, f). The pyramidal fragments are radially arranged becoming narrower toward the center of the tubes where they finally converge. The intersection of these joints with the external surface, and also with the concentric joints, produces polygonal joints similar to tortoise shell joints or bread crust types (Fig. 5e, f).

gravitational collapse of tunnel roofs. These megapillows are similar to the welded megapillows described from the Rouyn-Noranda area in Que´bec, for instance (Dimroth et al. 1978). Toward the sea, they change laterally into columnar lavas while toward the beach they branch into lobes, tubes, and protuberances that conform the pillow lava section.

The transitional section

The sheet lava flow section

This zone, 28-m long and 20-m thick, is located between the cliff and beach outcrops of the pillow lava section described above and the sheet lava flow with columnar jointing section that is described below. The main characteristic of this section is the existence of megapillows with diameters up to 5 m in size. Lava tubes there can appear either individualized by a thin coat of fine hyaloclastitic material or practically welded each other. In the later case, the flow acquires a massive appearance but the concentric and radial joints, sometimes with a pyramidal habit, enable an easy identification of the flow and of the cooling tubular units (Fig. 5e, f). Small channels in some tubes are possibly related to drainage cavities that led to the

This section of the lava flow is more than 30 m long and 20 m thick being the farthest from the coast accessible only during down tide (Fig. 4). The rocks of this section exhibit a conspicuous, organ tube-shaped, columnar jointing developed perpendicularly to the flow plane. The prisms or columns are up to 7 m in length and 50 cm in diameter and show pentagonal or hexagonal bases and constant dimensions throughout. Additionally, the lava locally shows ca. 15 cm regularly spaced platy joints, parallel to the roof and the base of the lava flow (Fig. 7a, b). Some lava blocks detached from the flow occur dispersed in the surrounding area (Fig. 4). The blocks are composed by volcanic breccias with chaotic inner

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Fig. 5 Outcrops at the Menãkoz beach showing the morphology and structures in pillow lava and transitional sections along a single basaltic flow: a Panoramic view of the pillow lava section at the creek limited by dashed lines; the base of the flow to the right is characterized by its flat shape. Note the group of people at the bottom of the creek. b Pillow lava tube partially filled by welded columnar breccias after drainage. The edge of the drainage cavity is delimited by the dashed line. c Cross-section of pillow lava depicting well-

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developed radial-columnar joints. White spots are of calcite amygdales while the white arrows indicate the location of varioles. d Detail of the flat-shaped basal contact of a megapillow with marine sediments. e Cross-section of a megapillow tube close to the lava sheet section. The white arrow indicates a characteristic tortoise shell jointing clearly visible on the megapillow external surface. f Crosssection of a megapillow tube close to the pillow lava section; note the well-developed columnar-pyramidal and tortoise shell jointings

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Fig. 6 Globular and blocky peperites at the base of the Menãkoz basaltic flow: a interaction of the pillow lavas with non-consolidated wet sediments, standing out the intricate contact of the lava and marly

sediments. b Details of typical textures observed in the Menãkoz peperites: jigsaw-fit texture by quench fragmentation, chilled margins at the lava contacts and crenulated borders

Fig. 7 Field aspects of various types of morphology and structures along a single flow of the Menãkoz beach within the sheet lava flow section. a and b Boulders from the massive flow section showing well-developed columnar and horizontal joint systems that

individualize centimeter size polygonal fragments. A slight column folding is appreciable in boulder (a). c and d Breccia welded due to roof collapse; some column fragments were slightly folded during the formation of this particular type of deposit

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structures. They include monomictic and centimeter-tometer heterometric clasts without any clear structure (Fig. 7c). The largest blocks ([5 m3 in volume) include columns up to 4 m in size, parallel, and welded to each other apparently without any cement involved. The columns are also strongly welded to angular fragments (cm to dm in size) that in most cases preserve their original prismatic morphology (Fig. 7c, d). All these features support the facies relation between the breccias and the prismatic section of the flow for which reason we propose the term welded columnar breccia for this type of formation.

An interpretation of the Menãkoz volcanic facies association Four different types of morphology have been recognized in lateral transition along 200 m of a single lava flow within the volcanic outcrops of the Menãkoz beach (Fig. 4), these are: (1) sheet lavas with columnar jointing, (2) welded columnar breccias, (3) megapillows, and (4) pillow lavas with rare megapillows. Such a facies distribution is broadly comparable to the theoretical and experimental results reported for volcanic analogs (Fink and Griffiths 1990; Gregg and Fink 1995). In a series of laboratory simulations with polyethylene glycol (PEG) extruded at a constant rate beneath cold sucrose solution and maintained at a temperature below the wax freezing point, Fink and Griffiths (1990) produced five distinct flow morphologies by systematically varying the rates of cooling and effusion. In the experiments with the lowest effusion and highest cooling rates, small toes or so-called laboratory pillowed flows (pillow lavas) were produced. As the effusion rate increased and the cooling rate decreased, pillowed flows gave way to laboratory lobate flows (submarine lobate sheets), laboratory rifted flows (submarine lineated sheets), laboratory folded flows (submarine ropy sheets), and laboratory leveed flows (submarine jumbled sheets). It appears, therefore, that the formation of sheet flows requires of the highest effusion rate [1–3,000 m3 s-1, according to Griffiths and Fink (1992b)] and the lowest cooling rates while, on the contrary, the pillow lava formation requires the lowest effusion rate [\1 m3 s-1, according to Griffiths and Fink (1992b)] and the highest cooling rates (Fink and Griffiths 1990). Consequently, pillow lavas might develop in the front of a sheet flow through a decrease in the lava flow rate and an increase in the cooling rate. The formation of pillow lava structures from massive lava flows was also experimentally reproduced using PEG by Fink and Griffiths (1990). In this case, at very low extrusion rates of the PEG from a punctual source, the wax formed a dome-like mass with a solid skin and a liquid

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interior. Continuous injection of PEG into the dome increased their internal pressure, stretching the skin until it broke and the small amount of wax that escaped through the fractures formed pillow-like bulbous lobes that solidified quickly. On this basis, an explanation for the volcanic facies association found at the Menãkoz beach is proposed that involves a four stage lava flow propagation model as follows (Fig. 8): Stage # 1 (Fig. 8a) during this stage, a low viscous, high-temperature fluid sheet flow is directly emitted from the vent at a high flow rate or else it was generated by the coalescence of various smaller and fluid flows emitted from the vent at moderate flow rates (inflation). Stage # 2 (Fig. 8b) as the flow velocity decreases, the flow starts to cool and solidify massive basalts with polygonal joints, which is the result of the thermal contraction of the outer parts, while the inner zones remain molten. This stage is responsible for the development of the observed prismatic sheet lava flow section. Stage # 3 (Fig. 8c) rapid drainage of the lava results in the formation of megapillows, either welded or not, in the front of the flow. Drainage of the massive lava toward the megapillows produces tunnels along the lava flow. Partial collapse of the tunnel roofs, where the prismatic joints were completely developed, leads to the accumulation of heterometric prismatic columns inside the tunnels. The high temperature inside the tunnels allows warm piled fragments to weld each other and form the welded columnar breccias. The lava contained in the largest tubes is channelized into various smaller flows and cooling units at progressively more restrained flow rates, which results in the formation of the pillow lava section. Stage # 4 (Fig. 8d) collapse of the tunnels would result in unwelded talus breccias where the collapse has occurred. The development of such breccias at this stage is speculative as they are not seen/preserved in the example here. Nevertheless, the gravitational collapse of lava tunnel roofs is a common process in submarine environments (e.g., Francheteau et al. 1979, 1980).

On the structure and organization of submarine basaltic flows The lava distribution system proposed here for the Menãkoz outcrops is similar to those observed in subaerial pahoe-hoe type flows elsewhere (e.g., Jones 1968; Swanson 1973). It is also comparable to those frequently assigned to flows emitted under present-day and ancient subaquatic environments (Dimroth et al. 1978; Van Andel and Ballard 1979; Walker 1992; Juteau 1993; Embley and Chadwick 1994; Chadwick and Embley 1994; Kennish and Lutz

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Fig. 8 Four stage propagation model sketch for the Menãkoz basaltic flow based on field data: a Development of a sheet flow near the vent due to high emission rates. b During cooling, a thick crust with columnar jointing enveloped the sheet flow, while continuous magma supply progressively inflated the flow. c Rupture of the thick crust due to magma pressure and emission of small magma volumes from the front of lava advance; this would promote the formation of pillow lavas and the drainage of the sheet flow whereby some hot blocks would fall from the roof welding each together and generating the welded columnar breccia. d Finally, the flow stops and the sheet flow roof collapses due to water pressure thus promoting the formation of talus breccias

1998; Gibson et al. 1999; Soule et al. 2007) or to those identified as the submarine extension of littoral lava flows (Moore et al. 1971, 1973). Such propagation style of the lava flows is evident for instance in the basanitic flow outcrops of the Stanley Peninsula (NW Tasmania) as mentioned above. In this Australian locality, submarine sheet flows with columnar and platy joints emitted 0.5–4 m in diameter and 1–20 m long pillow flows from the base of the flow front (Goto and McPhie 2004). A similar situation was observed at the frontal parts of large, 10–20 m in cross-section and up to 90 m long, submarine lava tubes that exhibit radial-columnar joints (Goto and McPhie 2004).

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In this sense, the outcrop at the Menãkoz beach shares many similarities with those reported from the Stanley Peninsula, both in the lava propagation style and in the preserved flow and cooling structures of the lava flows. However, there are a number of differences worth mentioning for the proper interpretation of such volcanic outcrops. Firstly, the fact that in Menãkoz, there exists a gradual transition from sheet flows to megapillows and pillow lavas and, second, that the pillow lava section, made of megapillows ? pillow lavas, is more than 150 m long while in Stanley, the largest pillow lavas (not the megapillows) are just 20 m long. It is also noteworthy that in Stanley, the emission of pillow lavas was the first step in the


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Fig. 9 Proposed model for submarine basaltic flows propagation combining field data from the Menãkoz beach with those of Goto and McPhie (2004) in the Stanley peninsula (see text for details)

flow propagation, followed later on by the advancement of the lava onto the previously front-emitted pillows; the lava that flows there consists of an upper massive part and a lower pillow part. In the proposed model for Menãkoz, the lava that fed the pillows would have been emitted from a practically static front of a sheet flow (or lava lake), without any important contribution of fresh lava, with the sheet flow changing laterally into pillow lavas. The combined evidence from the volcanic facies described in Stanley and Menãkoz thus confirms the generation of pillow lavas both in the advancing front of sheet flows and of megapillows. Goto and McPhie (2004) have also proposed that, provided a continuous supply of magma, episodic processes of pressure build-up in the flow front, protrusion and subsequent cooling of the pillows, overriding of the massive lava and forward movement of the flow front would repeat continuously; therefore, sheet lavas can propagate by repeated processes of pillow formation and overriding by an upper tabular flow (Goto and McPhie 2004). In the absence of a significant fresh magma supply, the front of the flow could stop or stagnate and progressively solidify starting from its external parts. The rupture of the partially consolidated front could lead the lava to escape at a moderate flow rate and form pillow lavas as it happened in Menãkoz. However, there exist submarine sheet flows that are massive and do not show direct connection with pillowlike facies (e.g., Cuevas et al. 1981; Furnes et al. 2001; Castanãres et al. 2001). In the case of low-viscosity lavas and important flow rate, a rapid in-mass propagation in the

flow front could be possibly with the result of stretching and breaking of the thin crust and inhibition of the pillow lava formation. Therefore, the propagation mechanism of the submarine lavas observed in Stanley might not be extensive to all types of sheet lavas or, at least, to all the propagation stages of sheet flows. The field data gathered in the Menãkoz and Stanley outcrops, combined with the experimental results on analog models (e.g., Fink and Griffiths 1990; Gregg and Fink 2000), may be integrated into a new propagation model for sheet flows (Fig. 9). The experimental data suggest a consistent relationship between the flow morphology and the distance from the vent at which solid crust begins to form. This distance is, in turn, determined by the extrusion rate, rheology of the liquid wax (or magma analog), magnitude of the surface heat flux, and the amount of cooling required to solidify the flow surface. Flows in which the crust formed very slowly usually produce marginal levees that contain and channel the main portion of the current. Cooler flows, that is, those that result in a more rapid crust growth, tend to form regularly spaced surface folds, complete multi-armed rift structures with shear offsets, and globular lobate forms similar to the pillow lavas observed at ocean depths. Similar transitions between different modes of surface deformation have been also generated by keeping the ambient water temperature constant and decreasing the extrusion rate (Fink and Griffiths 1990). At the starting point of the proposed model (Fig. 9), a sheet flow advances over a uniform slope that, according to

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experimental data and direct observations at seamount limbs, must be \58 since the lava keeps the tabular morphology (Batiza 1989; Gregg and Fink 2000; Gregg and Smith 2003). At the high emission rates necessary to create sheet flows, a slow cooling rate and a high lava flow velocities near the vent could allow the lava flow to develop a thin crust just seconds after its emplacement and thus break under the melt pressure and allow for a massive propagation of the lava (Fig. 9a, b). Instead, at points far from the vent, the increased cooling would promote thickening of the outer crust that envelopes the lava while the continuous input of fresh magma would increase the pressure onto the thick crust until its rupture. The small volumes of lava emitted at the front of advance could thus promote the formation of pillow lavas that, later on, would be covered by the sheet flow due to the massive input of fresh lava fed from the vent. Repeated processes of pillow formation and mass advance of the lava would result in structures similar to those observed in Stanley (Fig. 9c–g). Finally, as the lava progressively froze toward more distal positions, its viscosity would increase gradually until it stops (Fig. 9g). At that stage, the thicker crust thus formed could temporarily bear the pressure exerted by residual melts and the inflation in the flow front would be limited by the internal magma pressure (Fig. 9h). Alternatively, the internal magma pressure could be high enough to break the crust and liberate moderate-to-low lava amounts [\1 m3 s-1, according to Griffiths and Fink (1992b)], creating pillows (Fig. 9i). Whatever the case, the pillow lavas certainly advanced by either episodic rupture of a near solid crust and emergence of hot lava or by continuous stretching of the outer crust at the tube front. In the end, if there were no fresh magma supply, the lava drainage inside the sheet flow would create lava tunnels leading to a gravitational collapse of the roofs (Fig. 9j) and, finally, total collapse of the tunnels due to the water pressure. This later process would result in breccias nurtured by columnar lava fragments (Fig. 9k). The formation of tunnels in sheet flows and lava lakes is quite common in present-day ocean floor eruptions. Likewise, there are tunnels with collapsed roofs in sheet flows and ponded flows attested by striking lava pillar structures (Francheteau et al. 1979; Bryan et al. 1994; Embley and Chadwick 1994; Chadwick and Embley 1994; Chadwick et al. 1999). Yet, there are few accounts on the characteristics of autoclastic formations that result from collapsing events. According to published descriptions, it should appear that, once the lava subsidence began due to drain out, the upper (original) sheet lava crust collapsed where it was no longer in physical contact with, and therefore supported by, the underlying molten lava. The original crust apparently broke up, foundered, and was assimilated

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within the underlying molten lava, since broken fragments of this crust are not observed within collapsed areas except around pillars and roof remnants where they clearly have fallen after the flow solidified (Chadwick et al. 1999). Obviously, were the lava had been drained out into pillows after the formation of a noticeable thick crust, including the development of columnar joints, the original crust fragments removed from the roof would no longer be assimilated but they could weld each other and be preserved into the tunnel sectors of the flow. In this respect, the welded columnar breccia and related structures from the Menãkoz beach confer to this outcrop a marked significance. It suggests that the occurrence of this type of welded breccias in recent or ancient submarine volcanic formations could be related to the collapse of the roofs in lava tunnels from sheet flows or lava lakes.

Conclusions A gradual and lateral transition between four different volcanic facies or morphologies has been recognized in the Menãkoz beach along a flow of alkali-basalt emitted under shallow marine conditions during Turonian times. The recognized morphologies are as follows: (1) sheet lavas with columnar jointing, (2) welded columnar breccias, (3) megapillows, and (4) pillow lavas with sparse megapillows. Based on the observed field relationships and taking into account previous experimental and field studies, we propose a new propagation model for submarine basaltic lavas that accounts for the development of the four mentioned facies along the same flow. The lava emitted in great volumes would promote initially the formation of sheet flows near the vent, with development of columnar jointing in the outer crust. Internal magma pressure could break the flow crust and moderate-to-low lava flow volumes be emitted allowing the formation of pillows from the advancing front of lava. Subsequent lava drainage from the sheet flow would create lava tunnels that led to the gravitational collapse of the roof and the generation of breccias nurtured by columnar lava fragments. Both the pillow lavas and the welded columnar breccia formed apparently at a waning stage of the eruption when the supply of fresh lava to the sheet flow was considerably reduced. The volcanic facies described from the Menãkoz beach illustrate the generation of pillow lavas in the advancing front of both sheet flows and megapillows and attest to the possibility of roof collapse in lava tunnels from sheet flows or ponded flows either in recent or ancient submarine formations. Acknowledgments Financial support by the Spanish Ministerio de Ciencia e Innovacioń (Grupo Consolidado project CGL2008-01130/ BTE) and the Universidad del Paı´s Vasco/EHU (project GIU09/61) is

Int J Earth Sci (Geol Rundsch) (2012) 101:2201–2214 acknowledged. Constructive reviews of the manuscript by J. Martı´ and D. Jerram are greatly acknowledged.

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