Taitao ophiolite: a ridge eollision ophiolite in the toreare of southern Chile (460 S)
Eric Nelson Colorado School 01 Mines, Golden CO 80401, U.S.A. Randall Forsythe Unlverslly 01 North Carolina, Charlotte, NC 28223, U.SA John Diemer
Mike Allen U.S. Geologlcal Survey, Tucson, AZ. (presenl address: PRC Envlronmental Managemenl, Inc., 1099 18'h Slreel, SuKe 1960, Denver, Colcrado 80202, U.SA) Octavio Urbina Departamento de Geologla, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
ABSTRAeT
The Taita:> ophiolite, exposed at 46°S on the Pacific coast of Chile, consists of ultramafic rocks, gabbro, sheeted dikes, and volcanic and clastic sedimentary rocks. In June-July 1990 the Taitao ophiolite and surrounding rock units were studied using helicopter and zodiac support from the RN Polar Duke. Stratigraphy and structure of the ophiolite and structural relations with surrounding geologic units were refined from previous seant reconnaissance. In coastal exposures of the volcanic-sedimentary unit of the ophiolite the 8ahfa San Andrés Formation is detined as a 2,000-3,000 m thick sequence of volcanic, volcaniclastic, and sedimentary strata that records the initia! rifting and volcanism and progressive deepening ofthe basin. Two sheeted dike unitsare distinguishedwith perpendiculardike trends. The main, NW-trending steeply-dipping unit is ca. 6 km thick (normal to dikes) and contains basaltic and andesitic dikes; the northem, NE-trending steeply-dipping unit is basaltb and directly underlies and intrudes the Bahra San Andrés Formation. The strongly layered, but gene rally unfoliated gabbro unit is ca. 4 km thick (normal to verticallayers). It is interpreted to have formed in a short-Iived magma chamber in an active riftirig environment. The thin «1 km) ullramatic leclonite unit contains high-temp&rature flow fabrics overprinted bt brittle shear fabrics and is faulted against the 4 Ma old Cabo Raper tonalite pluton. Andesitic and rhyolitic volcanic rock3 are present in fault blocks and may represent evolution of the ophiolite magmatic system, or younger syn or post-obduction magmatism unconformable on the ophiolite. If the sedimentarylvolcanic cover is not Jnconformable on the igneous portions of the ophiolite, comparison with other ophiolites and the progressively deepening nature of the sedimentary sequence suggest that the ophiolite formed in a forearc rift aboye the subducting spreading ridge, rather than on the ridge prior to ridge collision.
Key worris : tOOao ophlol¡te, Pllocene, Tres Montes Penlnsula, Triple Junction, Southern Ch/le.
RESUMEN
Ofiolita Taitao: ofiolita de dorsal de colisión en el antearco del sur de Chile (46°S)_ La ofiolita Taitao, expuesta en la costa pacrfica de Chile a los 46°S, está formada por rocas ultramáficas, gabros, enjambres de diques, rocas volcánicas y elásticas sedimentarias. En Junio-Julio de 1990, la ofiolita y unidades geológicas circundantes fueron estudiadas utilizando helicóptero y botes inflables a partir del RN Polar Duke. Estos trabajos permitieron precisar su estratigratra intema y relaciones estructurales con unidades circundantes. La sección volcanosedimentaria superior (Formación Bahra San Andrés) incluye 2.000-3.000 m de niveles volcánicos, volcanoclásticos y sedimentarios que registran el volcanismo y 'rifting' inicial y una progresiva profundización de la cuenca donde se generó la ofiolita. La unidad de enjambres de diques comprende dos familias perpendiculares de diques subverticales: la principal, andesftica y basáltica, orientada en dirección noroeste alcanza hasta 6 km de potencia norma! a los diques, mientras que la secundaria, orientada en :lirección noreste, es basáltica e infrayace directamente o intruye a la Formación Bahfa San Andrés. La unidad
Rev/sta Geológica de Chile, Vol. 20, No. 2, p. 137-165, 11 F/gs., 3 plates, o..cetrbe, 1993. 138 TAITAO OPHIOLlTE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°S) de gabros, bandeada, y, en general, no foliada, alcanza hasta 4 km de espesor, medido perpendiculannente al bandeamiento subhorizontal y parece haberse fonnado en una enmera cámara magmática asociada a un ambiente de 'rifting' activo. El delgado «1 km) nivel de tectonitas ultramáficas aflora en contacto por falla con las tonalitas de 4 Ma del plutón Cabo Rapery presenta fábricas de flujo de alta temperatura a las que se superponen fábricas de cizalla frágil. Lavas andesrticas y riolrticas aparecen en bloques limitados por fallas, pudiendo representar productos evolucionados del sistema magmático ofiolCti::o, o bien, un magmatismo tardlo, sin o postobducción. Si los niveles volcánicos y sedimentarios superiores de la cfiolita (Fonnación Bahra San Andrés) son concordantes con el resto del complejo, la profundización progresiva registrada en la sedimentación más la comparación con otros complejos ofiolíticos, sugiere que la ofiolita Taitao se fonnó, no en una dorsal en ambiente oceánico franco, sino en el borde continental, en un 'rift' de antearco sobre una dorsal en proceso de ser subductada.
Palabras claves: O:lolita Taltao, Plioceno, Penlnsula Tres Montes, Punto trple, Sur de Chile.
INTRODUCTION
Since mid-Tertiary the Pacific coast of southern during ridge collision. A second model suggests that most Chile has been the site of convergence and the ophiolite formed in a forearc rift from magmas collision of an actively spreading oceanic ridge (Chile rising from the spreading ridge, as it began to subduct Rise) with the continental margin of the South below the leading edge of the continen!. American plate. The present point of ridge collision The Taitao peninsula is very isolated and access (46.5°S) forms the triple junction between the Nazca, is inhibited by dense vegetation and high wave energy Antarctic, and S,::>uth American plates, and lies only a along a rugged, cliffy coast. Thus, the peninsula few kilometers from the Península de Taitao (text remains largely unexplored. Previous workfocussed Fig. 1). A number of late Cenozoic features on this on the innercoast ofthe peninsula, wherethe ophiolite peninsula are related to ridge collision over is poorly exposed and outcrops are fractured or approximately the last 5 million years (see Forsythe sheared along a fault zone; only about 10 field sites et al., 1986; Nelson and Forsythe, 1989): a- generally had been visited in the well-exposed portions of the silicic, epizonal ntrusions, some dated as Pliocene ophiolite. Thus, little was known about the types and (Forsythe and Nelson, 1985; Mpodozis et al., 1985); distribution of rock units. The 1990 expedition used b- marine strata containing Oligocene and Miocene helicopter and zodiac support from the RIV Polar fossils exposed along the inner shores of the Penín• Duke, and studies focussed on the outer coasts in sula de Taitao and Península Tres Montes, and more Bahía San Andrés and Estero Cono, and on the clearly exposed along the shores of the islands in the mountainous interior; over 80 new sites were studied. Golfo de Tres Montes (DeVries and Stott, 1984; Of these sites, 10 were in the ultramafic unit, 21 in the Forsythe et al., 1985; Stott and Webb, 1989); and c gabbro unit (and its transition zone with the ultramafic the Taitao ophi06te, including serpentinized ultramafic unit), 20 in the sheeted dikes, and over30 in the upper rocks, gabbro, diabase dikes, pillow lavas, volcanic volcanic-sedimentary member. Despite these and breccias, and clastic strata, exposed in the interior of other new observations pertaining to the non-ophiolitic the Península de Taitao and along the outer coast plutonic rocks and overall structural relationships, (text-Fig. 2; Forsythe et al., 1986). many details of unit distribution and contact The primary objective of this paper is to present relationships remain obscure due to vegetation cover the results of significant additional field work on the and to the paucity of field sites, given the area Taitao ophiolite. These findings help to clarify its rela covered by the ophiolite and related rocks (ca. 125 tion to surroundhg units, the history of ridge collision, knf). On a geologic map of the ophiolite (text-Fig. 2) and its mode of origin and emplacement. Previous most contacts are approximately located or inferred. studies have led to several views regarding the The proportion of inland exposure is best in the modes of igneous origin and structural emplacement ultramafic and gabbro unit (nearly 90% exposed), of the ophiolite. -:::>ne model suggests that the Taitao less in the transitional zone and sheeted dike unit ophiolite formed offshore, on a late Miocene segment (roughly 10-25% exposed), and low in the volcanic of the Chile Rise and was subsequently obducted and sedimentary unit «1 % exposed). The proportion E. Ne/son, R. Forsythe, J. Diemer, M. Al/en and O. Urb/na 139
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ULTRAMAFIC ROCKS Four modes of occurrence of ultramafic rocks are are locally intruded by thin «10 cm) diabase dikes; b recognized in th;¡ Taitao ophiolite: a- ultramafic and unfoliated, layered, cumulate ultramafic rocks are mafic tectonites are well exposed in lenticular bodies present between the tectonite ultramafb rocks and (1/2-1 km long and 200-300 m wide; Plate 1, Fig. 1) the gabbro unit (Plate 1, Fig. 2); these rocks inelude along the southwestern margin of the ophiolite; the Iherzolite and rare dunite(?); c- gene rally strongly foliated and transposed tectonites include equidimensional blocks (up to 300 m in diameter) of harzburgite and gabbroic rocks; the ultramafic rocks massive ultramafic rock are present within the gabbro are partially to totally serpentinized and commonly unit, and are cut by a stockwork of gabbro dikes (text display mesh structure; some contain large (1-2 cm Fig. 2; Plate 1, Fig. 3); d- unfoliated, layered, cumulate long) porphyroclasts of pyroxene, and some contain ultramafic rocks are enelosed as layera within the a relatively high proportion of magnetite; the gabbroic lower portio n of the gabbro unit; these rocks are rocks locally have been rodingitized and contain predominantly Iherzolite, although one clinopyroxinite magnetite and hydrogrossular; these ultramafic bodies (websterite) layer (3 cm thick) was observed. GABBROUNIT The base (nct necessarily paleohorizontal) of the layering). Layering in the gabbros exhibi:s a number gabbro unit is fairly sharp where observed, although offeatures including isomodallayering, modally graded dikes ot very coarse-grained gabbro íntrude the layering, undulatory orcuspate (Plate 2, Fig. 2) layering adjacent cumula:e ultramafic rocks. The approximate and angular unconformities (truncated la}'ering; Plate thickness (measured normal to subvertical 1, Fig. 5) . Grading is both modal (compositional) and compositionalla'{ering) of the gabbro unit is 4-5 km. textural in layers tens of cm thick; coarse to very Near the base ofthe unit, the gabbro contains at least coarse grain size and high pyrox-ene/olivine ratios at one major layer of ultramafic rock (type 4 aboye) as the 'base' grade upward to finer grain siz.e and lower large as 3 km long and 100 m thick. pyroxene/olivine ratios and a sharp conta,::t at the 'top' Gabbroic rocks include olivine gabbro, troctolite, (Plate 1, Fig. 6). Although the unconformities clearly norite, gabbronorite, olivine gabbronorite, and minor indicate currentflow orsyndepositional slumping within anorthosite. Gabbros exhibit cumulate layering (Plate the magma chamber, it is unclear if the graded layers 2, Fig. 1) in the lower and middle portions of the unit were formed by a sedimentary or crjstallization and generally become more massive upward (here process. The Q3bbro unit is intruded locally by diabase 'upward' is toward the sheeted dike unit and normal to dikes TRANSITION ZONE The zone be:ween the gabbro and sheeted dike zone contains a mixture of gabbro, diabase, basaltic units is transitional and contains a number of features rock, and serpentinite. Gabbro in this zone is coarse distinct from either unit (text-Fig. 2). The map width of to very coarse-grained, commonly a.tered to a this zone, up to 3 km, may not be representative of greenschist mineral assemblage, and is weaklyfoliated initial thickness beca use of the likelihood of faulting in locally. Diabase is present in two modes: a- it forms this interval (sea Structure section). The transition thin «15 cm thick), irregular dikes that are usually E. Ne/son. R. Forsythe. J. Dlemer. M. ARen and O. Utblna 143 relatively low dipping (28-44°), as well as thicker, were observed, buttheir relationship with surrounding steeply-dipping tabular dikes; b- it is present in irregu gabbro and diabase is unknown. Tha transition zone lar bodies al outcrop scale with non-planar, di11use also contains thin (cm's wide) shear zones which contacts; these textures are interpreted to have 10rmed contain either serpentine andlor 1ragments 01 the during magma mingling. Rare outcrops 01 serpentinite surrounding gabbroic or diabasic material. SHEETED DIKE UNIT Sheeted dikes are exposed in two regions: a main textures, some are andesitic or even more silicic. body well 81Cposed along the Pac~ic coast and on Dikes in the northern body strike NNE and dip ridges in the interior 01 the peninsula northeast 01 the moderately to the southeast, include light green. to transition zene, and a northern body poorly exposed grey aphanitic rocks, 1eldspar a,dlor pyroxene along the inner, southern shore 01 Estero Cono (text porphyries, and are more highly metamorphosed Fig. 2). The main body is approximately 6 km 'thick' than the majority 01 dikes in the main body. In the (measured approximately perpendicularto the dikes). southeasternmost outcrop 01 Estero Cono dikes have Individual di VOLCANIC-SEDIMENTARY UNIT Interbedded sedimentary and volcanic rocks are consideredto be correlative. The Barrientos sequence, exposed in two areas on the Península·de Taitao: a described in Mpodozis et al. (1985), was not studied along the NW coast 01 Bahía Barrientos, and b- along in detail during the 1990 expedition. the SW coast 01 Bahía San Andrés (text-Fig. 2). The The Bahía San Andrés sequence consists 01 northwestern shore of Bahía Barrientos north of about 3,000 m of volcanic and sedimentary rocks well 46°45'5 exhibits a narrow «112 km wide) topographic exposed in seacliffs along the southern shores of platform 150-200 m above sea level. Boulders 01 Bahía San Andrés to Caleta Pascuas (point A-G, text gabbro 10u,d on this platform are interpreted as Fig. 3). A 4-5 km-long semicontinuoJs coastal profile glacial dri1t, as no gabbro was 10und in outcrop. Rare (text-Fig. 4) was constructed1romobservations made outcrops of pillow lavas and pillow breccias were 1rom zodiac and helicopter at a distance 01 10und on thg platform, and outcrops 01 volcanic and approximately 100 m, and on landings in selected sedimentar! rocks are present along the shore. The areas which permitted more detailed observations. volcanic-sedimentary sequence in Bahía Barrientos The Bahía San Andrés sequence, because 01 its (here in10rmally termed the Barrientos sequence) is relationship (discussed below) to sheeted dikes in the poorly exposed, locally strongly sheared, ánd in most northern dike body, is interpretad as being the localities does not appear to extend inland more than stratigraphic cover 01 the ophiolite. This section is the a 1ew 100 m. It consists 01 basaltic, andesitic and best and most complete exposure 01 this part 01 the rhyolitic pilbw lavas, pillow breccias, banded 110ws, ophiolite, and is described here as the type section 01 and sills interbedded with shale (Iocally black), siltstone, the Bahía San Andrés Formation. The base is placed sandstone and conglomerate. Bedding orientations in a poorly-exposed transition zone between the are variablE:, but in general are not parallel to those in northern sheeted dike complex and the dominantly Bahía San Andrés (text-Figs. 7d, 7e). Because of volcanic and volcaniclastic breccia units containing 1undamental differences wi.th the Bahía San Andrés dikes approximately 1 km 1rom the southeastern end Formation (::Iescribed below), the Barrientos sequen.ce 01 Estero Cono (point A, text-Fig. 3). We estimate that and the Bahía San Andrés Formation are not as much as 1-2 km of dikes and volcanic breccia líe 144 TAITAO OPHIOLlTE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°5) Text-FIG. 3. Aerial photomosaIc olthe southern shore 01 Bahla San Andrés. Prolile llne A-G shows portion 01 shore represented by proliies In Ilgure 4. Localllles 1-4 show locallon lor Iogs 1-4 shown In text-flgure 6: 1 Is lhe eastern headland 01 Caleta Monona; 21s the western IIp 01 Isla Cono; 3 Is Punta Monona; 4 Is Ihe western shore olthe unnamed bay ca. 1 km west 01 Punta Monona. beneath the eastern end of the profile in text-Fig. 4. near the top of the section. The breccias are matrix The Bahía San Andrés Formation is divided into supported, poorly-sorted, gene rally unstratified, and three members, described below: A lower, dominantly contain angular clasts of pillows up to a meter in volcanic member; a middle, dominantly sedimentary diameter. No well-stratified sedimentary deposits were member; and an upper, mixed volcanic and observed in the lower member. Whereas dikes that sedimentary rrember. The middle, sedimentary intrude the lowermost lavas tend to have planar member was accessible in Caleta Monona, on Isla margins, dikes that intrude the middle and upper Cono, and at the unnamed bay 1 km to the west of volcanic and agglomerate beds have very irregular, Punta Monona, and therefore, was studied in greater bulbous margins. This suggests that the upper dikes detail than the generally inaccessible lower and upper intruded into unconsolidated deposits and were members_ probably feeders to that particular stratigraphic level. A tabular breccia, observed 1,200 m along the LOWER VOLCANIC MEMBER profile, does not offsetthe stratigraphic section, and is thus interpreted as a clastic dike ratherthan a tectonic The lowervalcanic member, exposed frompoint A breccia. The dike is vertical in orientation, and is to the west approximately 1.5 km, is ca. 1 ,000 m thick. therefore discordant to, and youngerthan, the suite of Dikes and beds have fairly consistent orientations dikes in the lower section, which are normal to bedding. throughoutthe exposures (text-Fig. 7e). This member is composed of a mixture of coarse volcanic DEPOSITIONAL ENVIRONMENT agglomerate, vesicular and/or amygdaloidal pillow lava, and columnar to radially-jointed dikes (or irregu The preponderance of pillow lava in the lower lar small intrusions). Pillows range in vertical dimension member indicates that the volcanic environment was from 10-15 cm 10 over 2-3 m. Two unpillowed flows dominantly submarine. The two blocky, unpillowed that may represent subaerial eruptions are present flows seen near the top of this member may indicate E. Ne/son, R. Forsythe, J. Olemer, M. Allen and O. UrlJlna 14S A Lower Volcanic Member 20% leeder dikes B m 50 40 % pillow lavas 40% pillow breccia o B e •. • s •• - ~'O;'::;~". ,,:.>-,-."':.-.I~~~;., ~ ...... ' teo --~-.~ e D Cebos flows tilloral shelf sands and conglomerate o D E lurbidiles lilloral shelf sands wave·reworked o E upper pillow lava • few dikes F mixed wiln Ihinly bedded lurbldiles o F G m 50 o Text-FIG. 4. Geologlc profUes constructed by zodlac reconnalssance Irom polnt A-G shown In text-llgure 3. TopoQlaPhy 01 the coast Is dlagrammallc. Black areas Indicale talus cover; aoss-halched areas Indlcale sea caves. ... O> .00 2pOO tlLLZUü t ~IOO Covered inlerval ILOO • ~ [[] 8001 Ir I I,')OO~ l .p ~ Turbidiles 7oo-llfq'sa4 -:.a~~1 1,800 ._-~-. Sandslone wilh coarse·grained ñpples ,. l:':·~~A&] ~~ 600-t":-.; 't VI ~ :;1 ~ 'lOO Channelized sandslone ~ o '00 ~ 1.6 00 ":I: i5 :i; ro > ..J Conglomerales LOG S ~ 40O I ~ ~ > ".oo~ :Il :; 6 > Tuffaceous sandslone CI ::>, g m LO e I ~ O 1,400 I O ro ro 111 , Volcaniclaslic breccia ¡¡; 'O" ~IJ, , 1 '\Jv E. Ne/son, R. Forsythe, J. Dleme;, M. A//en and O. Uro/na 151 UPPER VOLCANIC ANO SEOIMENTARV MEMBER the lower pillow sections, where pillows are commonly weathered along fractures rather than bedding The remaining 1,000 m of the Bahía San Andrés surfaces. In Caleta Pascuas, thin «0.3 m) vol Formation to the west of site 4 were observed at a caniclastic sandstones were observed in the pillow distance, 1rol"1 zodiac and helicopters. This upper lava sequence. member is cJmposed of, at least, four cycles of interbedded pillow lavas and gray to grayish-green OEPOSITIONAL ENVIRONMENT graded beds IElss than 30 cm thick The graded beds are laterally persistent for meters to tens of meters. The upper volcanic and sedimentary member is a The colors, erosional contacts, and thicknesses of marine unit. Few dikes and no evidence of explosive these sedimentary beds appear very similar to the volcanism were observed, suggesting off-axis fa4 deposits ·Jf the uppermost middle sedimentary volcanism. The sedimentary strata are interpreted as member. distal turbidites. Therefore, the upper volcanic and Volcanic r:>cks in the upper member appearto be sedimentary member formed in a deepening basinal composed of :>illow lavas and few dikes; no volcanic environment, more distal from detritai sources than breccia was observed. Pillowforms are well exhumed, the middle sedimentary member. suggesting less metamorphism than that observed in PLUTONIC ANO OTHER ROCK UNITS A number of plutonic bodies has be en mapped in Just south of the peninsula, this seque1ce is probab/y the region of the Península Taitao and Golfo Tres fau~ed against diabase and a~ered, vesicular, pillow Montes (see Forsythe and Nelson, 1985; Mpodozis et basa~, interpreted to be near the tcp of the main al., 1985; Forsythe et al., 1986). In addition, at least sheeted dike unit. The relationships of the Cabo two other plutons were mapped or sampled during the Gallegos andesite to the Bahía San Andrés Formation 1990 expedition. Along the north shore of Estero and the Barrientos sequence are unknown. The Cono psammitic and pelitic metasedimentary rocks, andesitic rocks (and rhyolitic rocks in the Barrientos correlated wilh the late Paleozoic basement of the sequence) may represent a progressive change Archipiélago de los Chonos (Chonos Archipelago) to upward from more basaltic volcanism in the Bahía the north (Davidson et al., 1987), are intruded by San Andrés Formation,orthey may rest unconformably tonalite (here termed the Cono intrusion) with equi on the Bahía San Andrés Formation. granular to porphyritic textures and sulfide miner alization. A second pluton, informally termed the STRUCTURE Central pluton, was mapped near the center of the main sheeteddike unit (text-Fig. 2). This felsic, tonalitic The Taitao ophiolite has be en tiltad and faulted pluton is por¡::hyritic, contains large xenoliths or roof against the surrounding basement roci F1 foliotion in ultromofic Poles to sheeted dikes tectonite bodies o northern body o moin body a N b N \~ o \~ \~ 'o \ ~ \ ~ ~ \ • \..>. • o +.~ \~ + \ '::. o 1 o ( o " \ <" \ ~ o " o \ <2- o \~ " " o \ • N= 12 N=65 O" Poles lO compositionol Bedding olong N E coost loye"ring in gObbro Bahía Borrientos N N e d o o o " o o o o o .~ o" + 1 o N= 3 N=18 Poles to sedimentory bedding Poleocurrent azimuths Ccleta Pascuas o eost coast N • west coosl N e f + o o · ;te • N=25 Text-FIG. 7.... : Equal area, Iower hemisphere, slereographlc plols 01 orienlallon dala; a- uKramallc leclonlle unl\. Dashed llne shows orlenlallon 01 Iens-shaped bodles 01 uKramallc laclonlle along lhe base 01 lhe ophlollle; b-po les lo dlkes In lhe maln and northern sheeled dlke unll; e-poles lo colf1lOsKlonallayerlng In \he gabbro unll; d-poles lo beddlng along lhe NE coasl 01 3ahla Bamenlos. Shad6dareashowsdlslributlonolpoleslromSanAndrésFonnatlon(lhlsllguree): ..poleslobecldInglromlheBahlaSanAndrés FormEllon (daIS Irom Estero Cono; squares lrom Calela Pasasas); ,- hlstogram (10 degree Inlervals) 01 dlredlonal paleocurrenl data la lhe mlddle sedlmenla¡y member 01 lhe Bahla San Andrés Formatlon. E. Ne/son, R. Forsythe, J. Diemer, M. Al/en and O. Urb/na 153 Late Paleozoie emplacement of the ophiolite. Also, although a few B metamorphk: basement steep (>70°) lineations were measured, most linea tions in these ultramafic rocks plunge between 5-30°, suggesting a component of horizontal ductile flow, The orientation of compositional layering and pillow lava/breeeia +clastic strata cs. 4 km gabbro dikes within the gabbro unit is veryconsistent; strikes range from 055-125° and average ca. 095°, andes~ i c and dips are steep (>50°) to vertical (text-Fig. 7c). agglomerate Thus, the strike of gabbro layerin';:l is essentially perpendicular to that of foliation in the ultramafic rocks. Gabbros in the transition zone locally contain a foliation and thin «20 cm thick;, serpentinized sheeted dike complex ductile and brittle-ductile shear zones which dip 30- cs. 6 km 60° east to southeast when the main sheeted dike unit is rotated to vertical. Thus, these foliations and shear zones strike perpendicularto cikes in the main grani1i c pluton sheeted dike unit. Assuming the dikes formed parallel to a paleo-spreading ridge, these relatively low-dipping shear features may have formed along a transform diabase"gabbro fault, possibly by transtensional or transpressional trans;tion zone deformation. The orientation of sheeted dikes within the main body is very consistent; strikes ran!;e from 80-155° layered gabbro + and average ca. 126°, and most dips are steeply ultramatic lenses northeast to vertical (text-Fig. 7b), -Iowever, dikes (peritod~e) 08. 4 km within the pillow lavas in Caleta Pascuas dip to the southwest between 60-80°. This reversal of dip could have been caused by rotation of nonnal fault blocks at the spreading ridge environment, or by faulting or cumulate pendotlte folding during obduction emplacement. tect enlte pendotl!e The orientation of sheeted dikes within the northern granitic plul on body is perpendicularto those in the main body, with A strikes averaging about 040°, and dips averaging Texl-FIG. 8. Cross seclion oflhe TaHao ophlolile; x= laun. See leXl 70°, mostly to the southeast (text-Fig. 7c). The orien ligure 2 lor localion 01 saclion line. tation of bedding in the Bahía San Andrés Formation is very regular along the southwest coast of Bahía San Andrés from Estero Cono tothe eastern shore of other litholo ~ies, Within about 10m of the basal Caleta Pascuas, averaging 25-26°N'N (text-Fig, 7e). contact, the tectonites are completely serpentinized However, on the western shore of Caleta Pascuas and locally contain a phyllitic, anastomosing cleavage the orientation of bedding is roughly perpendicular typical of sleared serpentinite. Strongly-foliated, and dips are to the NE (text-Fig. 7e). The divergence lenticularblocks (ca. 3x5 min map dimension) ofthe of these latter bedding orientations (only two were Cabo Raper granodiorite are enclosed within obtained) may be related to fault block rotation or to serpentinite 'Nithin this basal shear zone, and foliation folding, although the strike of dikes coes not change and elonga1ion of the bodies are parallel to the significantly. The internal structure of the narrowfault contact. The basal contact trends approximately 334°, block along the NE coast of Bahía Barrientos is whereas the dominant strike of ductile foliation and unknown, but is probably complex, as rocks are compositional layering in the ultramafic bodies is commonly sheared orfractured, and t:edding attitudes about NS (text-Fig. 7a), a relationship suggesting a are highly irregular and discordant to those in the con-ponent cf left lateral shear during or after obduction Bahía San Andrés Formation (text-Fig, 7d). 154 TAlTAO OPHIOLITE: A RIDGE COLLISION OPHIOLlTE IN THE FOREARC OF SoUTHERN CHILE (46°S) At least, four megascopic faults were mapped or exposed on a topographic bench southeast 01 the inferred in the Taitao regian (text-Fig. 2). A10ng the gabbro and main sheeted dike units. The shores 01 base of the ophiolite, as described aboye, shear Bahía Barrientos are composed 01 linear segments, fabrics and blocks of foliated granodiorite in the basal and expose many highlyfracturedoutcrops. Also, two serpentinite indicate the presence of a shear zone, hot springs were located along this shore. The 1ault here termed the basal fault. Along with clasts of the zone also contains an isolated block01 Eocene marine granodiorite, c1asts of slate were observed in float siltstone (Forsythe et al., 1985). The juxtaposition 01 near this contacto However, due to heavy vegetation pillow lavas againstthe gabbro and main sheeted dike covering the actual fault contact was not observed. units suggests components 01 right slip andlor down Sen se of shear and amount of displacement alongthe to the south slip i1 the Bahía Barrientos sequence is, basal fault are unknown, but are probably large, as in part, correlative with the Bahía San Andrés this fault represents the surface along which the Formation. However, Kaeding st al. (1990) proposed ophiolite was obducted, andlor later displaced. a left lateral offset on the basis of geochemical The northeastern margin of the ophiolite is correlation between the Cabo Raper pluton and the presumed bour.ded by a fault (here termed the Cono pluton exposed on the south shore 01 Bahía Barrientos 1ault), which is hidden below Estero Cono and the low (text-Fig. 2). valleyto the southeast. Basement rocks and a granitic Afault, heretermedthe Yunquesfault, was mapped pluton, exposed on the northeast shore 01 the fjord, along an EW topographic lineament between Estero are both hydrothermally altered and contain sulfide Lobos and the Pacific coast. Other evidence for this mineralization, suggesting hydrothermal circulation 1ault, includes the concentration 01 hydrothermal along the fault. No evidence was observed to indicate alteration in the transition zone, and the presence of sense or amount 01 displacement along the Cono slates of unknown affinity (probably basement) along 1ault. the north side ofthe Iineament. The sense and amount The southeastern margin 01 the ophiolite is als~ of displacement along this fault are unknown; the bounded by a 1ault or series of faults (here termed the position near the transition zone suggests that the Barrientos fault system) in1erred by a number of Iines fault origin may be related to detachment at this level of evidence. Along the northwestern shore of Bahía of the ophiolite during ocean ridge evolution, although Barrientos south 01 Estero Lobos, pillow lavas are the presence of the slate suggests later movement. DISCUSSION OPHIOLlTE'S TECTONIC AFFINITY short-lived magma chamber in an active rifting environment. This contrasts with ophiolites, such as Although field work during the 1990 expedition the Oman ophiolite, in which nearly all of the layered added a large number of field sites to existing gabbros are foliated by magmaticflow incontinuously knowledge of the Taitao ophiolite and associated replenished magma chambers (Nicolas, 1989). rocks, many details concerning the nature and Although two sheeted dike units were recognized, relationships of the various units remain unclear. their trends are mutually perpendicular. Exposure is High-temperature deformation fabrics in the thin «1 insufficient in the interiorof the peninsula to determine km) ultramafic tectonite unit formed during if the two dike units are correlative, but have been aesthenospheric flow, possibly prior to formation of rotated by faulting or folding, or if they evolved the magmatic ¡::ortion of the ophiolite. The gene rally separately. In the lattercase, one ofthe dike units may low-angle ductile Iineations may have formed by have formed parallel to the paleo-ridge axis, whereas channelling of mantle flow along the ridge axis (Nicolas, the other, may have formed in a transform fault 1989, p. 77). These fabrics are overprinted by brittle environment. Alternatively, the axis of spreading could shear fabrics formed during obduction emplacement have rotated in time. andlor subsequent faulting. The strongly layered, but The clastic nature of the Bahía San Andrés gene rally unfoliated gabbro unit probably formed in a Formation c1early indicates that the ophiolite formed E. N/Jlson, R. Forsyth/J, J. DI/Jm/Jr, M. Al/en and O. Urolna 155 in proximity to a continental source. The basin initially Lors. Typieal LOT features inelude: a clastic filled rapidly with on-axis submarine volcanic flows sedimentary cover, contaet with metamorphic conti and breccias and built up to, and possibly aboye, sea nental erust, laek of ehromite pods in the ultramafie level. Subsequently, the basin subsided progressively section, and rodingized gabbro dikes in the ultramafie while clastic, wave-dominated shelf and then section. A typical feature of Hors is a thick, well submarine fan strata'were deposited during lulls in developed, but undeformed gabbro unit. On the basis volcanic activ ty. The basin widened and deepened of these comparisons, and on the basis of the present and deposits :>ecame increasingly finer, more distal state of knowledge of field relations in the Taitao and further re"oved from the effects of wave action. region, the Taitao ophiolite is inter¡:reted to have Whereas, the volcanic deposits in the lower parts of formed in a rift basin within the foreare, aboye the the Bahía San Andrés Formation are cut by extensive magmatie system of the subdueting spreading ridge swarms of dikes and, apparently, represent on-axis (suprasubduetion setting). volcanism, th~ uppermost voleanie flows have few dikes and are likely products of off-axis volcanism. OPHIOLITE FORMATION ANO EVOLUTION OF THE Nicolas (1989) discusses twofundamental ophiolite CHILE MARGIN TRIPLE JUNCTION types: harzburgite ophiolite types (HOT) and Iherzolite ophiolite types (LOT). HOrs tend to form in well AII KI Ar age dates (Mpodozis et a~, 1985) for the developed, rapid (>2emlyr) spreading systems igneous units ofthe Taitao and Tres Mo::>ntes Peninsu whereas LOrs tend to form in incipient rifts or lar regions (text-Fig. 9) were obtained from felsic transtensionéU settings. The Taitao ophiolite seems to intrusions and volcanic units eollected trom the shores have affinities to both types, although more so to of Bahía Barrientos and Golfo de Tres Montes. No • • • • l. Distributed intrusions '. l·I . • I and a. soe iat ed- voleanies I I I Ophiolitie • ., pillo", lavas and I I a'luog ene voleanie breeeias I Bahía Barrienlos • 8 > liistogram 01 isolopie oges. u ..c 6 Irom 011 near-treneh suites ..u" 4 ~ Ma Text-FIG. 9. Radiometrlc ages 01 near-trench rnagrnatlsm and periods 01 r1dge colllslon under the Penlnsula de Taltao reglon. Ages are Irom Mpodozis el al. (1985). Perlods 01 rldge colllslon are estimated lrom the plate klnematlcs shown In tex1-lIgure 10. Clrde =whole roc)(; vertical bar =blollte; horizontal bar =hornblende. 156 TAITAO OPHIOLITE: A RIOGE COLLISION OPHIOLlTE IN THE FOREARC OF SoUTHERN CHILE (46°S) age dates have been obtained 1rom the lower plutonic and Tres Montes and Taitao Fracture Zones collided sections 01 the ophiolite or 1rom the two more recently with the margin at 6 Ma and 3 Ma, respectively. For identi1ied plutons (Cono andcentral pluton).ln addttion, each colliding ridge segment asthenosphericwindows the Bahía San Andrés sections have not been dated, opened, as these segments were consumed under although 10ramini1era 01 Plio-Pleistocene age have the South American plate. Due to the oblique angle of been reported from the middle sedimentary member trans10rm convergence the tracks 01 the successively (Forsythe et al., 1985). colliding ridge segments (and their respective These KJAr ages demonstrate that the volcanic asthenospheric windows) have small regions 01 untts have ranges in ages similarto the 1elsic intrusions. overlap. The Taitao peninsula is situated in a regio n 11 the volcanic rocks dated 1rom Bahía Barrientos which had an asthensphericwindowpass underneath represent the upper strata10rm part 01 the ophiolite, at 6 Ma and at 3 Ma. Thus, the range 01 ages 10r the then the ophiolite has a prolonged history extending intrusive units 1rom 3-6 Ma is consistent with the plate 1rom 6-3 Ma. This minimum 01 3 million years 01 kinematic data. volcanism can be understood most easily in terms 01 Regarding the formation 01 the ophiolite, the the kinematics 01 ridge collision. situation is slightly more problematic. 11 the ophiolite As pointed out by Cande and Leslie (1986) and wascreated between 3 and 6 Ma, bytypical spreading Cande et al. (1387), the relative motion 01 the Nazca, processes at rates comparable to the offshore Chile Antarctic and South American plates is such that the Rise spreading center, at least 300 km 01 crust would trans10rms (orfracture zones) are riding obliquely into have formed in a 3 m.y. interval. Given the small size the continental margin (text-Fig. 10). Segments of the 01 the Taitao ophiolite (sheeted dike complex is ca. 6 Chile Rise between the Esmeralda and Tres Montes, km wide), the ophiolite would have 10rmed due to a NalCO 9C~ SQut~~t1'/'J' Amenco Antarctica m 2c "" Text·FIG. 10. The shaded area represents the reglon whlch has been underrlddenbythe two Chile Rlse spreadlng segments that were sltuated between Ihe TaHao, Tres Monles, and Esmeralda Fraclure Zones. An area underlhe Penlnsula de TaHao was underrldden by bolh rldge segmenls, al roughly 5·4 Ma, and 3.5-2.5 Ma, due lo the sllghl obllquHy 01 lhe Nazca-SoulhAmerlcan andAnlarctlca Soul;, American plate motlons. E. Ne/son, R. FOisythe, J. O/emer, M. Al/en and O. Uro/na 157 few 100,000 years of magmatic activity. Thus, an water to subaerial sediments. atypical model must be evoked to explain the ages. Case 2 (text-Fig. 11 a). The ophiolite¡ includes the Two models are under present consideration, given volcanic rocks of Bahía Barrientos and Bahía San the available age and regional tectonic and Andrés sequences. The ages ofvolcanic units reflect stratigraphic data (text-Fig. 11). The two cases are the duration of 'spreading'. However, the environment presented below, along with arguments to support for spreading is that of a forearc rift that evolves in two each case. stages. The first stage is during the collision at 6 Ma, Case 1 (text-Fig. 11 b). Only the lower plutonic and the second during the collision at 3 Ma. The first portions of the complex are truly part of an ophiolitic phase would have had morecontinenta contamination, complex. The volcanic units were brought up through and magmas potentially would have evolved within fissures withil the ophiolite post-obduction or were subaerial to submarine environments during the early extruded by nearby volcanic centers at a later time. In collapse and rifting of the continental margino The this scenario, the plutonic units could be greater than second phase, potentially, could have involved more 6 Ma in age, and not likely formed during the Chile extensive differentiation as well as remelting of Rise collision. Strataform units of the ophiolite formed previously formed ophiolitic units. During this phase, prior to ridge collision would have been deposited in the rift was reactivated during the second interval of deep water, and then unconformably overlain by ridge collision, and magmas workedtheir waythrough volcanic depcsits and shallow water sediments during previously formed units. The sediments interbedded and post-obduction. The section may have been with the volcanic units would have evolved from intruded byfe~ic intrusion and unconformably overlain shallow to deep and proximal to distal facies. by felsic volcanic flows interbedded with shallow Of these two scenarios, which are :liagramatically a- Case 2 b- Case 1 Obduclion <1.0 Ma - R~;~I~I Phase 11 stratigraphic or.lap Rilting 01 near· trench calc· alkaline magna'ism Phase I Rifting generation d normal (inlraoceanic) ophiolile belore ridgo oollision (>5 Ma) Texl-FIG. 11. Dlagrammatlc sectlons lor Iwo models 01 ophiolna lormation and basin sedlmenlation; a- (on left) 15 Ihe pralarrad model lor oplliome generation. Here, wilhin an Inciplenl lore arc rHI Ihal Innially opens up over Ihe 5-4 Ma collision zone, magmas ascend Inl.:>, and are conlamlnaled by, lore arc crust. In a subsequenl second 2·3 Ma phase, Ihe rift Is readivaled. The basin preserves an upward flnlng sequence, and 15 uplifled during Ihe Plelstocene; Ihe second model, b- (on the right), has only one oceanlc pt-ase 01 magmatlsm, whlch was produced >4·5 Ma ollshore, on the segment 01 the Chile Risa that was s-tuated between Ihe Tres Montes and Esmeralda Fraclure Zones. Afler obductlon, a second phase 01 volcanlsm would be created during Ihe 2·3 Ma colllslonal period, bul In Ihls model thls Is near·trench 'blow torch' magmatism. 158 TAITAo OPHIOLlTE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°S) portrayed in text-figure 11, the first fits some of the of the exposed section. OVerall, the sedimentary observations and the second, others. Cross-cutting or facies described earlier in this paper can be viewed as multiple intrusive relationships are observed within an upward fining saetion from shallow to deep water the lower gabbros. The gabbros and sheeted dikes facies. There have be en no unconformities described are themselves cut by a late-stage felsic pluton. The within the Bahra San Andrés section, which includes ophiolite, as mentioned earlier, is most similar to the in its lower parts abundant screens of subparallel 'LOT' types, which are believed to typify ophiolites of dikes. The volcanic units along Bahra Barrientos continental rift environments. The northern dike could, however, conceivably rest unconformably on complex contains screens of hornfels that were the lowerunits ofthe ophiolite, butsuch an unconformity originally metamorphic basement or preexisting has not been identified todate. These observations plutonic rocks. The lower sedimentary units within the are best interpreted in light of the second case for a strataform parts of the ophiolite contain shallow to reactivated (ortwo phase) forearc rift structure floored subaerialfacies, and include debris flows with abundant bythe Taitao ophiolite. The extension and magmatism clasts of base ment material mixed with volean ic clasts. within this rift persisted for ca. 3 m.y. during two ridge The Bahía San Andrés Formation includes a lower collision events. and upper volcanic-dominated member separated by Uplift of the ophiolite may only have been initiated a central sediment-dominated member. The lower in the Pleistocene, asthe continental marginjust north volcanic member has abundant screens of feeder of the Taitao ophiolite was subject to the collision of dikes, while the upper member appears to have the segment of the Chile Rise, north 01 the Taitao volcanic flows fed by a lateral source out of the plane Fracture Zone. ACKNOWLEDGEMENTS Funds for this research were supplied by U.S. Polar Programs, U.S. National Science Foundation. National Science Foundation grants 8618725 and R. Palroa, with Aeromet Helicopters, did a spaetacular 9106638. We thankJ. C. Parra from Servicio Nacional job piloting. The Empresa Nacional del Petróleo de Geología y Minería (SERNAGEOMIN) for his heip in (ENAP) kindiy supplied helicopterfuel. M. E. Baek, Jr. arranging field logistics. Field logistics were supplied (Western Washington University) and B. Nevins made bythe RIV Polar Duke, operated bythe Department of thoughtful suggestions in reviews. REFERENCES Allen, P.A. 1985. 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Hummockystratification: Bulletin, Vol. 66, No. 2, p. 138-157. signilicance 01 its variable bedding sequen ces: Mpodolis, C.; Hervé, M.; Nasi, C.; Solfia, J.; ForSfthe, R.; Ge%giC6.l Society of America, Bulletin, Vol. 93, No. 8, Nelson, E. 1985. El magmatismo Plioceno de Penrnsu p.663-680. la Tres Montes y su relación con la subducción del Duke, W.L. 1985. Hummocky cross-stratification, tropical Punto Triple de Chile Austral. Revista Geo/ógica de hurricanes, and intense winter storms. Sediment%gy, Chile, No. 25-26, p. 13-28. Vol. 32, N:l. 2, p. 167-194. Mutti, E. 1979. Turbiditésetconessous-marinsprolonds./n Ouke, W.L.; Leckie, DA 1986. Origin 01 hummocky cross Sedimentation detritique (fluviatile, littorale et marine) stratilication; Part 2, paleohydraulic analysis indicates (Homewood, P.; editor)./nstitutde Gé%gie, Uoiversité lormation by orbital ripples within the wave-Iormed de Fribourg, p. 353-419. Fribourg, Swillertand. flatbed liBid. /n Shell sands and sandstones (Knight, Nelson, C.H.; Nilsen, T.H. 1984. Modem and ancient deep R.J.; McLean, J.R.; editors). Canadian Society of sea-Ian sedimentation. Societ." of 'Economic Petro/eu,." Ge%gists, Memoir, No. 11, p. 339. Pa/eont%gists and Minera/ogists, Short COlXse, Vol. Forsythe, R.D.: Nelson, E.P. 1985. Geological manilestations 14,404 p. 01 ridge collision: evidence Irom the Gollo de Penas Nelson, E.P.; Forsythe, R.D. 1989. Fidge col ision at Taitao Basin, southern Chile. Tectonics, Vol. 4, No. 5, convergent margins: implications lor Archean ,nd post p.477-495. Archean crustal growth. /n Growth 01 the continental Forsythe, R.C.; Olsson, R.K.; Johnson, C.; Nelson, E.P. crust (Ashwad, L.D.; editor). Tectonoohysics, Vol. 161, 1985. Stratigraphic and micropaleontologic observations No. 3-4, p. 307-315. Irom the Gollo de Penas-Taitao Basin, Southern Chile. Nicolas, A. 1989. Structures 01 ophiolites and dynamics 01 Revista Geo/ógica de Chile, No. 25-26, p. 3-12. oceanic lithosphere. K/uwer Academ.'c Pub/isJ-ers, 367 Forsythe, R.C.; Nelson, E.P.; Kaeding, M.E.; Carr, M.J.; p. Dordrecht. Hervé, M.; Mpodozis, C.; Soffia, J.M.; Harambour, S. Nottvedt, A.; Kreisa, R.O. 1987. Modelforlhe combined flow 1986. Pliocene near-trench magmatism in southem origin 01 hummocky cross-stratificati·:ln. Ge%gy, Vol. Chile; a :lossible manilestation 01 ridge collision. 15, No. 4, p. 357-361. Ge%gy, 'vol. 14, No. 1, p. 23-27. Stott, L.D.; Webb, N. 1989. The Neog/obocuadrina Harms, J.C.; Southard, J.B.; Walker, R.G. 1982. Structures continuosa last appearance datum level in tt-e South and sequences in clastic rocks. Society of Economic Pacifico Micropa/eont%gy, Vol. 35, No. 1, p. 63-71. Pa/eont%gists and Minera/ogists Short Course, No. 9, Southard, J.B.; Lambie, J.M.; Federico, O.C.; Pie, H.T.; Chapter 3, p. 30-32. Weidman, C.R. 1990. 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Manuscript received: May 19, 1992; accepted: September 16, 1993. 160 TAITAO OPHIOLITE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°S) PLATE 1 Fealures 01 malle and ullramallc rocks In Ihe Tallao ophlolHe; p. 140 Figures Aertal vlew 011en5-5haped bodles 01 uHramallc lec:tonHe along Ihe base 01 lhe ophlollle. The Cabo Raper plulon undertle5 Ihe hllls lo Ihe IeH. 2 Comp05HlonaJ layerlng In unlollaled uHramallc rocks. 3 Ullramallc block wHhln gabbro unH cul by stockwork 01 gabbro dlkes. 4 Ducllle mesoscoplc lolds In ullramallc lec:tonHe unl\' 5 Angular unconlormlly or channelln composHlonallayerlng 01 gabbro unH. Nole. layertng 15 nearly vertical In thl5 outcrop. 6 Graded composlllonal layerlng In gabbro unH. Note. up (toward Ihe dlke unlt) 15 lo leH. E. NeJson, R. Forsythe, J. D/emer, M. A//en and O. Urb/na 161 PLATE 1 • ! 1 2 3 4 5 6 162 TAITAO OPHIOLlTE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°S) PLATE2 Features 01 gabbrolc rocks In the TaHao ophlolHe; p. 140 Figures Fine composHlonallayerlng In a boulder 01 gabbro (In oUlcrop, layerlng Is conslstently steeply-dlpplng). 2 Undulatory compositlonallayerlng In gabbro uni!. E. Ne/son, R. FJrsythB, J. Diemer, M. Al/en and O. Uro/na 163 PLATE2 1 :~,.) . .' !'.... . 2 164 TAITAO OPHIOLITE: A RIDGE COLLISION OPHIOLITE IN THE FOREARC OF SoUTHERN CHILE (46°S) PLATE3 Features 01 volcanlc-sedlmentary unH In the Taltao ophlollte; p. 147, 148 Figures Photomosalc 01 two 9 meter-thlck coarsenlng-upward sequences In lacles assoclaUon 2 01 mlddle sedlmentary member atlocallty 2 (text-Flg. 3). The uppersequence has large-scale cross beddlng and Is capped by pillow basal!. Note person lor scale and hellcopter tail at left. 2 Photomosalc 01 a cross-stratHled gravel unH wHh a horizontal base and a convex upward top In lacles assoclaUon 3 (Iog 3,17 m level, text-FIg. 6b). The cross beddlng dlps to the rlght (NW). The gravel bed Is slmilarto the coarse-gralned ripples 01 Leckle (1988). 3 Beddlng plana vlew 01 Rhlzocoralllum In lacles assoclaUon 3 (Iog 3, 28 m level, text-Fig. Gb). 4 Cross sectlon vlew 01 lour graded beds (Iog 4, 45 m level, text-Flg. Gc). The thlrd graded bed Is the thlckest and Is composed 01210 3 cm 01 fine sandstone gradlng up to 15 to 20 cm 01 horlzontally-stratnled sandstone whlch In lurn grades up to 5 cm 01 cross-Iamlnated sandstone. 5 Cross secUon vlew 01 eroslvely-based sandstone gradlng up Into cllmblng rlpple cross lamlnaUon Just aboye the scale (Iog 4,47 m level, text-Flg. Gc). 6 Horizontally-and cross-Iamlnated sandstone In lacles assoclation 4 wlth an angularvolcanlc bomb (Iog 4, 50 m level, text F/g. Gc).