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35. NEOGENE EVOLUTION OF CANARY ISLAND VOLCANISM INFERRED FROM ASHLAYERS AND VOLCANICLASTIC SANDSTONES OF DSDP SITE 397 (LEG 47A)

Hans-Ulrich Schmincke, Institut Mineralogic der Ruhr-Universitàt, D 463 Bochum, Federal Republic of Germanyand

Ulrich von Rad, Bundesanstalt für Geowissenschaften und Rohstoffe (Federal Geological Survey),D 3000 Hannover, Federal Republic of Germany

ABSTRACTMiocene to Pleistocene volcaniclastic sediments were analyzed

from Holes 397 and 397A, northwest of Cape Bojador (NorthwestAfrica) and south of the Canary Islands. These sediments are of twotypes: (1) ash-fall deposits of silicic and alkalic (trachytic-phono-litic-rhyolitic) composition, and (2) deposits of submarine volcani-clastic mass flows (debris flows, turbidity currents) of basaltic com-position.

Air-fall ash layers (mostly 0.5 to 5 cm thick) occur at about 19m.y.B.P. but most are found in sediments ranging from about 14 to0.3 m.y. They consist of colorless to brown tricuspate and pum-ice shards, feldspar, and mafic phenocrysts, and small felsic rockfragments. Refractive indexes of about 1.52 to 1.54 suggest rhyoliticto phonolitic compositions. Glass is replaced by palagonite, clayminerals, zeolites etc. in rocks older than about 14 m.y., while theyounger shards probably all have been hydrated and chemicallychanged to some degree. Some ash layers are completely zeolitized(phillipsite, clinoptilolite).

Two middle Miocene volcaniclastic sandstones, a lower 7.8-me-ter-thick turbidite (V-3) and an upper 4.5-meter-thick debris flow(V-l), are indistinctly graded. The rocks of flow V-3 are fine to me-dium sand-sized hyaloclastites with dense to moderately vesicular,palagonitized "sideromelane" shards, minor igneous rock frag-ments (microgabbro, basalt, tachylite), and small amounts of non-volcanic rock fragments, detrital quartz, and biogenic debris. Thematrix is dominated by clay minerals (smectite) with minor admix-tures of carbonate and zeolite (analcime). V-4 is a 20 to 30 meterthick early Miocene hyaloclastite-rich sandstone.

The upper unit (V-l) is very poorly sorted and coarse grained. Itconsists chiefly of rounded to angular, generally non-vesiculartachylite to fully crystallized basalt and trachyandesitic rock frag-ments. Admixtures are trachyte, microgabbro, highly vesicular ba-saltic pumice shards (now replaced by clay and carbonate), andminor non-volcanic clastic and biogenic debris. The partly recrystal-lized matrix consists of brown clay and, less commonly, carbonate.Clinopyroxene (titanaugite) is common as phenocrysts in the rockfragments and as single crystals; clinopyroxene shows only minorreplacement. Olivine, next in abundance, is always replaced by lay-ered silicates or carbonate. Plagioclase is the most common ground-mass constituent of the rock fragments and is mostly albitized orreplaced by clay minerals.

Mineral composition (Ti-augite!) and paragenesis show that al-kali basalts and related derivative rock types typical of the volcanicrocks of the Canary Islands are the dominant or only source rockfor the igneous components. Good rounding, oxidation tex-tures in olivine, and a high ratio of crystalline relative to tachyliticand glassy basaltic fragments indicate that most or all volcanicfragments in the sandstone of the upper flow (V-l) are of subaer-ial, epiclastic derivation. Age and mineralogical composition ofrock fragments strongly suggest derivation from the eastern (Fu-erteventura) rather than central (Gran Canaria) Canary Islands.The older (ca. 19 m.y.B.P.) ash fall might have been supplied by

703

H.-U. SCHMINCKE, U. VON RAD

trachytic volcanoes from Fuerteventura or Lanzarote; those around14 to 9 and 4 m.y.B.P. may be from Gran Canaria; while theyounger ash-fall deposits may be due to explosive eruptions on thewestern Canary Islands, most likely Tenerife.

The stratigraphic distribution of the different types of volcani-clastics reflects the emergence of the Canary Islands in general andthat of Fuerteventura in particular. Volcanic activity is most intenseduring the shield-building stage; the mass flow deposits are thoughtto be the result of (a) the submarine shield stage (unit V-3: moder-ate to shallow water depth, possibly with subaerial detritus fromuplifted emergent plutonic and Cretaceous sedimentary rocks), and(b) the subaerial shield-building stage of Fuerteventura betweenabout 16 to 17 m.y.B.P. (unit V-l: subaerial erosional detritus andtachylite from lavas advancing into the sea). Volcanic episodes fol-lowing the shield-building stages were much less voluminous onall of the Canary Islands, although development of highly differen-tiated magmas resulted in abundant explosive activity producingnumerous ash layers particularly during the middle to late Miocene(14 to 9 m.y.), early Pliocene (4 m.y.), and late Pliocene to Pleisto-cene (3.3 to 0.3 m.y.B.P.).

INTRODUCTION

During Leg 47A (March/April 1976), two holes(397 and 397A) were drilled on the uppermost conti-nental rise, 100 to 150 km south of the Canary Islandsand 100 km northwest of Cape Bojador (northwest Af-rica) at 26°50.7'N, 15° 10.8'W, and 2900 meters waterdepth (Figure 1). At Site 397, 1300 meters of Neogenesediments underlain by 153 meters of Lower Creta-ceous rocks (Figure 2) were penetrated. About 15 ash-fall layers were recognized in the lower Miocene toPleistocene sediments, and several submarine volcani-clastic debris flow deposits were encountered in middleMiocene rocks. It is highly suggestive that these vol-canogenic sedimentary rocks are related to volcanic ac-tivity on the nearby Canary Islands. Moreover, suchdistinct layers could possibly reflect well-known anddated volcanic events on the Canaries and perhapseven reflect different stages in the volcanic evolution ofa single island or the entire island group.

In this paper, we will present some petrographicdata and tentative interpretations on the volcaniclasticdebris flow deposits and brief information on the ashfall layers. Altogether, 36 samples were selected forsmear-slide, coarse fraction, thin-section, and X-raydiffraction (XRD) analysis (Table 1). We are presentlypreparing a more detailed account comparing all ashlayers from the nearby DSDP Sites 368 and 396 (Leg41; see also Rothe and Koch, 1978) with those fromSite 397 and with the record from the Canary Islands.

STRATIGRAPHIC AND REGIONAL SETTINGFigure 2 shows the stratigraphic setting of the ob-

served ash-fall layers and volcaniclastic sandstones. Ex-cept for two 19.3 m.y.-old altered vitric ash layers (Sec-tion 397A-23-3, early Miocene), all ash-fall layers oc-cur in the late middle Miocene (14 m.y.B.P.) to Pleisto-cene (0.3 m.y.B.P.) section of Hole 397.' Most ashes

The age control of these volcanic events is interpolated and tiedinto the nannofossil biostratigraphy (Cepek and Wind, this volume)and the equivalent absolute age scale by Martini (1976, Table 1).

are discrete layers, a few mm to cm thick, whereastraces (63 µm fraction) of disseminatedvolcanic material have been found in the coarse frac-tion of many upper Neogene samples (see Site Report,this volume; Diester-Haass, this volume).

Burrowing organisms have caused extensive mixingof air-fall ashes with the overlying sediments, as shownby X-radiographs (F. McCoy, personal communica-tion). Four to five volcaniclastic debris flow units ("li-thofacies F-4B," see Site Report) have been identifiedin the lower middle Miocene (16.5 to 17.6 m.y.B.P.)section. They have been numbered from V-l a to V-4(Figure 2). V-l a is actually a pebbly mudstone con-taining only about 5 per cent of volcanic rock frag-ments ("lithofacies F-4A," see Site Report). Two ofthe debris flows (V-l and V-3) are several meters thick(Table 1, Plates 1 through 3). Because of their greaterdensity (GRAPE density: 2.1 to 2.5) and sonic velocity(V-l: 2.8 to 3 km/sec; V-3: 2.5 km/sec), they formstrong seismic reflectors and obstacles to drilling. Espe-cially reflector R-8 (at about 810 m sub-bottom) ap-pears to be caused by volcaniclastic debris flow V-3(Figure 2; see also Wissmann, this volume). Beforedrilling this site, the reflectors R-7 and R-8 werethought to represent Eocene cherts ("Horizon A")and/or Oligocene or older Paleogene unconformities.Reflector R-7 (and R-8?) now can tentatively be tracednorthward to the early Neogene (Grunau et al., 1975,fig. 6) volcanic aprons of the eastern Canary Islands.These volcanic aprons extend several tens of km sea-ward from the submarine volcanic cone of Gran Ca-naria and Fuerteventura into sediments of middle Mio-cene age, and are underlain by older sediments (seefrontispiece; Figure 1, and Wissmann, this volume).Apparently, the volcaniclastic debris flows or turbiditycurrents have traveled for distances of 100 to 200km down the island slopes and the South Canary Is-land Channel (Figure 1). They deposited their sedi-ment load within the deepest part of that channel,where it is bordered by the foot of the Cape Bojadorcontinental slope. Wissman (personal communication)has traced the extent of these volcaniclastic debris flows

704

NEOGENE EVOLUTION OF CANARY ISLAND VOLCANISM

29'

50 100

km

17°W 14° 13°

Figure 1. Location sketch of DSDP Site 397 with highly generalized geology ofTenerife, Gran Canaria, Fuerteventura, andLanzarote after Fuster et al (1968), Grunau et al. (1975), Schmincke (1976), and Mitchell-Thome (1976). Explanation ofsymbols: 1 = "young" (9 m.y.B.P.) Miocene volcanic rocks and pyroclastics; 4 = "basal complex" (upperPaleogene to lower mafic to ultramafic plutonic rocks, syenites, dike-in-dike complex, submarine volcanics etc ofthe Betancuria Massif); 5 = Cretaceous hemipelagic and terrigenous deep-sea sediments; 6 = major Quaternary eruptioncenters; 7 = young submarine volcano; 8 - continental or island shelf with shelf break; 9 - base-of-continental-slope bound-ary; 10 = "volcanic mass" or "socle", partly overlain by young sediments and/'or pyroclastics; 11 = seaward boundary of"volcanic apron," over- and underlain by sediments; 12 - middle Miocene paleobathymetry, indicated by depth contours ofseismic reflector R-7 ("D2", see Figure 2) in seconds two-way travel time below sea level, to show submarine relief of SouthCanary Island Channel during deposition of volcaniclastic debris flows V-1 to V-4; 13 = inferred paleocurrent direction ofvolcaniclastic debris flows. Source for 7, 9-12: seismic reflection profiles interpreted by G. Wissmann (this volume).

by the correlation of reflection profiles. They can bemapped as a relatively narrow (5-25 km) tongue over alength of at least 220 km and an area of about 2000 km2

(see Site Chapter; Arthur et al., this volume). If we as-sume an average thickness of 20 to 30 meters for all

volcaniclastic debris flows between reflectors R-7 andR-8, we arrive at a total volume of 40 to 60 km3. This isless than 1 per cent of the volume of the Gran Canariasubaerial shield (10,000 km3) which was produced in lessthan 1 m.y. (MacDougall and Schmincke, 1977). The

705


submarinevolcaniclasticdebris flow(or turb.current)4: MUDSTONE

PEBBLY MUDSTONE(DEBRIS FLOWCGL)

T.D.= 1435 m

Figure 2. Generalized lithostratigraphic diagram of DSDP Site 397 (Holes 397 and 397A) with acousto-, bio-, and magneto-stratigraphy, approximate age, location, and lithology of volcaniclastic layers. The genetic interpretation (magmatic com-position, mode of deposition, and possible source areas) is tentative and discussed in the text. Besides air fall ashes, there arecurrent transported volcaniclastic (tuffaeeous) sandstones containing volcanic and nonvolcanic components (V-la to V-4).

706


significance of the age of the different volcanic events,the mode of deposition, and possible source areas (Fig-ure 2) are discussed later in this report.

No volcanogenic sediments were found in the LowerCretaceous lithostratigrapic Unit 5.

ASH LAYERSAbout 15 ash layers have been recognized so far in

the entire section, ranging in age from 19.3 to 0.3 m.y.B.P. (Table 1). Their maximum thickness is 20 to 25cm (Sections 397-73-1 and 397-77-2), but most layersare less than 5 cm, and some less than 1 cm thick. Themedian grain size of all ashes is less than 1 mm andmost are between 0.05 and 0.15 mm. The significantdifferences among the ash layers as to type of shard,degree of alteration, and proportion of non-volcanicconstituents are discussed below (see Table 1).

Felsic Glass ShardsTwo major types of shards of felsic composition

were found in the ash layers: composite (pumice) shardsand simple (tricuspate and bladed) shards (Plate 5,Figure 1). Both types occur in all ash layers; pumiceshards are generally more common in the larger grainsize (medium to coarse ash size). Nevertheless, the ashlayer of Sample 397-22-7, CC is dominated by pumiceshards. Very small pumice shards (5 cm) occur not atthe base, but near the middle part of the flow (Plate 2,Figure 3). The basal third of the flow is suprisingly ho-mogeneous and less coarse-grained, and only the low-ermost 10 cm is distinctly graded (Plate 2, Figure 4).

The massive unit "V-3" (Cores 397-84 and 397-85)is a 7.8-meter-thick hyaloclastic turbidite with a strik-ingly different texture (Table 1). It is much finergrained (sandstone, no pebbles >2 mm) and muchbetter sorted than V-l (Plate 3, Figure 4).

Consolidated ashes (mainly 0.063 to 2 mm) arecalled "tuffs," and consolidated lapilli (>2 mm) arecalled "lapillistones" (Schmincke, 1974). Because thevolcanic fragments in the volcaniclastic debris flows(e.g., V-l) are generally "epiclastic," i.e., fragmentedby erosion and not by pyroclastic and hydroclasticprocesses, they may be called tuffaceous (or volcani-clastic) sandstones. Such redeposited volcaniclasticrocks generally contain various amounts of non-vol-canic components.

Some of the "hyaoloclastites" of flow V-3 are vitrictuffs, containing mainly volcanic debris (altered glass)of hydroclastic and pyroclastic origin.

A mixed epiclastic-volcaniclastic sandstone ("hyalo-clastite V-4") was discovered at 960 to 990 meters(397A-6-7, early Miocene). This poorly sorted sand-stone is about 20 to 30(?) meters thick and contains 30per cent quartz and 25 per cent altered palagonite in a

707


TABLE 1Station Data, Texture, and Petrography of Investigated Volcanic Ashes and Volcaniclastic Sediments

'B

1 in

cSa

m(I

nt«

397-3-2, 80397-10-5,80397-14-2,80397-15-2,80

397-19-5, 29397-22 CC

397-34-2, 132

397-37-4, 130

397-56-3, 44397-57-3, 80-84

397-57-4, 140397-59 CC397-61-4, 109-111397-73-1,95-97

397-76-4, 27-29

397-77-2, 133-136397-77-2, 145-147397-78-4, 39-43

397-79-2, 78-80397-79-3, 21-23397-79-3, 91-93397-79-3, 141-143397-79-4, 37-40

397-79-4, 92-94397-80-1,20-27397-80-1, 107-116

397-81-1,4-6

397-84-2, 40-42397-84-3, 74-77397-84-4, 2-5397-85-3. 77-80

397-bit (ang. piece)397A-2-1, 74-76

397A-2-2, 24-29

397 A-6 CC

397A-23-3, 30-33

0

C? !C oO T3

2 • "

O iα> Oò ε' ?00

co

m

P

m

PPPvp

vpvpvp

m

mm

Pm

m

P

m

vp

m

Texture

εN

c

at

-α

e

c

W

150-300

60-300

60-200500-2000500-2000100-300500-3000

500-2000500-3000500-3000

200-1000

100-300200-50050-2000500-1000

100-4001000-2000

50-100

30-2000

50-80

εBα>

C

ε3

B'8

s

1

5

1

55

1015

85

>20

1.5

0.51.042

0.54

0.2

2

0.35

§ Λcr ^

| | ?1 | g

+ fs

• fs

+ cpx+ fs

+ fs

• fs

15.1 cpx,fs,ol5.1 cpx,fs,ol0.6 cpx,fs,ol2.4 cpx,fs,ol1.4 cpx,fs,ol

3.5 cpx,fs,ol2.4 cpx,fs,ol1.0 cpx,fs,ol

• cpx,ol,fs

• cpx,fs• cpx

0.00.4

+ cpx.fs• cpx,fs,ol

•pi

Volcanic Components

"S g

C £ ^

T3 rM j£

. ,

C 3

1 | I•β •a èC D O

P > Z

5.89.6

11.111.59.6

13.011.5

1.0

•

#6.6 39.2

16.4 17.8

•

•

•

•α

S3

"S


TABLE 1 - Continued

Nonvolcanic Comp.

'plu

toni

c)?

[met

am.,

:nts

a ε

% •~

>>•o

ù 2

mqt

mqt

mqt

mqt

mqt

mqt

o

+ sil+ sil+ sil

• fm,mo

• fm,by

+ fm,pe,li+ fm,pe+ fm.ech+ fm,ech• fm,li,ech

+ fm,mo+ fm,pe+ fm,pe

• f m• f m• fm,np+ fm,mo,by

• fm,mo,by

+ fm

cem

ent

+><

1Ew

30.937.932.134.338.7

38.837.831.4

39.043.5

Matrix/CementAlteration Products

mor

. etc

.)Is

(mon

iin

era

εJau

+ +

++

•

•

•

•

••

•

•

•

•

•

•

•

•

•

•

*

•

•

•

c

cem

a2,


clayey matrix (see Arthur and von Rad, this volume). Itis probably the cause for the impedance change, produc-ing reflector R-9 ("rise-D,"; see Site Chapter).

Igneous Components

Minerals

ClinopyroxeneClinopyroxene is ubiquitous in all volcaniclastic

rocks (Plate 3, Figures 1 and 3; Plate 4, Figures 1 and3). It occurs as single crystals and as pheoncrysts inrock particles, generally in amounts less than 1 per centby volume. Clinopyroxene is up to 2 mm long andprobably of titanaugite composition, judging from itscolor and common twinning and zoning (Plate 4, Fig-ure 2). Titanaugite is by far the freshest of all pyro-genic minerals, although in many rocks it is partiallyaltered. Replacement minerals are carbonate and, lesscommonly, layered silicate (smectite?). Titanaugite isalso common as a groundmass phase in the morecoarsely crystallized rock fragments, most of which arequite fresh (Plate 4, Figure 1).

Olivine

Olivine occurs as carbonate and, less commonly, lay-ered silicate (smectite?) pseudomorphs (Plate 5, Figure2) in all volcaniclastic sandstones, both as single crys-tals and as phenocrysts in rock particles. Because it isalways completely replaced, exact estimates of its vol-ume percentage are impossible; it may be slightly moreabundant than clinopyroxene. In some microgabbros(Plate 3, Figure 1), olivine is replaced by serpentine,apparently a pre-depositional alteration that is stable inthe present environment. Moreover, much of the oli-vine occurring as phenocrysts in rock fragments hadbeen previously altered to iddingsite along their mar-gins, probably during cooling and subaerial weather-ing.

Feldspar

Plagioclase is the most abundant feldspar phase. Itoccurs both as phenocrysts (Plate 4, Figure 2) andgroundmass crystals (Plate 4, Figures 1 through 4) but,in contrast to pyroxene and olivine, rarely as singlecrystals. This indicates that liquidus phases in the ba-salts, the dominating rock type, were chiefly olivineand olivine plus clinopyroxene.

Plagioclase is generally albitized and partly replacedby smectite(?), chlorite(?), or carbonate. Fresh plagio-clase occurs in a few rock fragments, especially in non-vesicular tachylite.

Feldspar microlites in more felsic rocks, especiallytrachyte, are probably anorthoclase and perhaps sani-dine in some rocks (Plate 4, Figure 4). K-feldspar alsooccurs in the groundmass of some well-crystallized al-kalic microgabbros.

Fe/Ti OxidesTitanomagnetite (?) is a ubiquitous groundmass

phase in all basaltic rock fragments except sideromel-

ane shards (Plate 4, Figure 1). As a phenocryst phase,it occurs only in the more differentiated rock (Plate 5,Figure 2).

Titanomagnetite(?) is a ubiquitous groundmassphase in all basaltic rock fragments except sideromel-ane shards (Plate 4, Figure 1). As a phenocryst phase,it occurs only in the more differentiated rock (Plate 5,Figure 2).

Other phases include minor amounts of amphibole,mica, and apatite (Table 1).

Igneous Rock Fragments

Alkalic MicrogabbrosThe term "microgabbro" is used loosely here for

rocks which (a) are holocrystalline; (b) are relativelycoarse-grained, though mineral components are gener-ally less than 2 mm in diameter; (c) have an isotropicfabric such as subophitic intergrowth suggesting growthduring near-static conditions; and (d) in some cases,have a decidedly alkalic composition as indicated bybiotite or amphibole (Plate 3, Figure Id). Fragmentsof these "microgabbros" can be derived from the cen-ter of lava flows several meters thick; more likely, theyoriginated from subvolcanic intrusive bodies, becausethick flows are not common among the shield-buildinglavas in the Canary Island. Several thin-sections con-tain one or more such fragments, although their totalvolume is less than 5 volume per cent of all volcanicrocks (Table 1).

Crystalline BasaltWe use the term "crystalline basalt" here for rocks

which are (a) holocrystalline; (b) fine-to-mediumgrained; and (c) commonly have a slightly flow-alignedfabric (c in Plate 3, Figures 1 through 3). Mineralog-ically, apart from phenocrysts, they consist predomi-nantly of plagioclase and clinopyroxene microlites, withminor titanomagnetite (Plate 4, Figure 1). There are allgradations from microgabbros to fully crystallized ba-salts. Basalts, as used here, encompass the more maficvarieties such as ankaramites and picrites, and the moreevolved types, hawaiites and mugearites (Plate 4, Fig-ure 3).

TachyliteWhen basaltic and intermediate lava is rapidly

cooled (not quenched), it forms an oxide-studdedopaque rock called "tachylite" which sometimes con-tains microlites and phenocrysts. We distinguish twovarieties of tachylite in the rocks studied: type 1 is ve-sicular and appears to have formed as a primary pyro-clastic particle judging from its shape (b[ in Plate 3,Figures 2 and 3; Plate 4, Figure 6). Type 2 is generallyless vesicular and angular, and may have formed bybreakage during erosion or granulation by contact withseawater (b2 in Plate 3, Figures 2 and 3; Plate 4, Fig-ure 5). In general, tachylite fragments are larger in sizethan associated fragments of the rock types discussedabove. This may be controlled to a minor degree by

710


density differences causing larger particles of the lessdense and more vesicular tachylite to be hydraulicallyequivalent to smaller fragments of the denser crystal-line basalt; it may, however, also be due to shortertransport distances. While the microgabbros may breakup more easily during erosion than the "tougher" ba-salt, they also may have come from a more distantsource. The presence of vesicular tachylite, and tachy-lite showing little evidence of rounding (in contrast toall crystalline basalt and microgabbro fragments), sug-gests strongly that some or most of the tachylite wasgenerated not far from shore, perhaps in the littoralzone.

There are all gradations between tachylite and crys-talline basalt, and transitional varieties are more abun-dant than between crystalline basalt and microgabbro orbetween tachylite and sideromelane (Plate 3).

Rock Fragments of Intermediate and FelsicComposition

Some rock fragments have only small amounts ofmafics and oxides and a few consist almost exclusivelyof feldspar microlites; both rock types commonly havea pronounced trachytic texture. We interpret theserocks as "trachyandesite" ("intermediate" benmore-ite) and "trachyte," respectively (Plate 4, Figure 4).The volume of these rocks is always less than 5 andgenerally less than 1 per cent by volume, although inthe tachylitic and glassy components exact estimates ofchemical composition cannot be made. No highly dif-ferentiated rocks that could be called phonolite or rhy-olite were found.

Sideromelane Shards

We use the term sideromelane for formerly glassyshards which are now completely changed into palago-nite, as in the hyaloclastites (Plate 4, Figures 5 and 6),or layered silicates (Plate 5, Figure 3), and rarely car-bonate as in the volcaniclastic sandstones. Amongsome 15,000 points counted, only 2 glass fragmentswere found that were completely isotropic. They prob-ably consist of hydrated and chemically changed glass.From the shape of the shards, a generally low degreeof vesiculation, and their association with more crystal-line varieties, we assume that they once were sidero-melane, i.e., transparent glass of basaltic composition.We distinguish two varieties: non-vesicular angularshards with no or rare vesicles, as dominating in hy-aloclastite (e.g., Sample 397-84-4, 2-5 cm), or more ve-sicular varieties which might be termed "scoria" (e.g.,Sample 397-85-3, 77-80 cm). The rare glass fragmentsin the volcaniclastic sandstones (Table 1; Plate 4, Fig-ure 6; Plate 5, Figure 3) are extremely vesicular (roundvesicles) and might be called "pumice" (of basalticcomposition).

Modal Composition

Modal compositions were determined in 10 thin-sec-tions by counting from 1100 to 2300 points in each sec-tion (Table 1; Figure 3). Compositions are only approx-

imate for some components, however, because thereare many difficulties in making exact identifications ofsome types of clasts. For example, glass is probablyunder-represented in the counts, because it is alwayscompletely replaced, mostly by brown clay or rarely bycarbonate. Likewise, olivine is always completely re-placed and clinopyroxene and plagioclase are partlypseudomorphed by phyllosilicates and/or carbonate.For these reasons, matrix (and cement) are maximumvalues and their volume contains probably about 10 to15 per cent of the replaced components listed above.Minerals, chiefly clinopyroxene, are only counted asseparate components when they occur as single crys-tals. When they occur as phenocrystic or groundmassphase in rock fragments they are counted with the spe-cific type of rock fragment.

Three main rock types can be distinguished frommodal compositions; most thin-sections are from themassive volcaniclastic debris flow V-l (Cores 397-79and 397-80, Figure 2). Within this unit, significant var-iations exist (Table 1), most of which are showngraphically in Figure 3 and in the photomicrographs ofPlates 3 and 4. Shards, mostly highly vesicular, occurthroughout the unit in fairly equal amounts except inthe basal specimen where they are practically absent.Crystals are abundant in the fine-to-medium grainedsandstone from the top of debris flow V-l, where theymake up 15 modal per cent. In most other specimens,however, they comprise less than 3 per cent and arerare in the basal part of debris flow V-l. Tachylite,crystalline basalt, and microgabbro are fairly evenlydistributed throughout most of the unit, except near thebase where crystalline basalt and microgabbro arestrongly concentrated relative to tachylite (Figure 3a).Non-volcanic fragments are slightly enriched in the ba-sal layer of V-l and are abundant at the top (Figure3d). Most of these changes in modal composition areclearly controlled by textural variations which, in turn,are related to the position within the debris flow. Thecoarse-grained base (median >2 mm) contains rela-tively abundant dense rock fragments, while singlecrystals and detrital non-volcanic mineral grains occurin the smaller size grades and therefore, are, enrichedin the fine-grained top of the unit. This distribution isconsistent with the depositional model suggested by theshipboard party (mass flows encompassing distal debrisor grain flows and/or turbidity currents) and which isdiscussed in more detail by Arthur and von Rad (thisvolume).

The second main class of rocks differs from theabove chiefly in the much higher content of palagonit-ized sideromelane shards (Table 1; Figure 3); sincethose shards are generally dense to moderately vesicu-lar (Plate 4, Figure 5), we call these rocks hyaloclas-tites. The hyaloclastites occur mainly in the more fine-grained, massive debris flows or turbidites V-3 and V-4(Cores 397-84 and 397-85; Sample 397A-6-3, CC; Figure2), show significant variations. Single crystals are par-ticularly rare in both hyaloclastites (Figure 3c), whilemicrogabbros are relatively abundant (Figure 3 b). Non-

711


SIDEROMELANE SHARDS SIDEROMELANE SHARDS

O1&8I VOLCANICLASTIC• 2-7J SANDSTONES (V-1)

HYALOCLASTITES (V-3)

CRYST. CRYST,BASALT BASALT

"MICROGABBRO" "MICROGABBRO"

SIDEROMELANE SHARDS

TACHYLITE+

CRYST. BASALT+

"MICROGABBRO"

CRYST. BASALT "MICROGABBRO" CRYSTALSNONVOLCANICPARTICLES

Figure 3. Petrography (modal composition from point count analysis) of volcaniclastic sandstones (samples 1 through 8; V-1)and palagonite tuffs (altered hyaloclastites, samples 9 and 10; V-3). Sample numbers (see Table 1): 1 = 397-79-2, 78-80 cm;2 = 397-79-3, 21-23 cm; 3 = 397-79-3, 91-93 cm; 4 = 397-79-3, 141-143 cm;5 = 397-79-4, 37-40 cm; 6 397-79-4, 92-94cm; 7 = 397-80-1, 20-27 cm; 8 = 397-80-1, 107-116 cm; 9 = 397-84-4, 2-5 cm; 10 = 397-85-3, 77-80 cm.

volcanic components are rare in Core 397-84 but areunusually common (sandstone fragments) in Core 397-85 (Plate 3, Figure 4) which also contains much morevesicular shards than Core 397-84.

A third class of rocks, not studied in detail, containsa few scattered volcanic rock fragments in the "muddy"matrix of a marly conglomeratic sandstone (e.g., Sam-ple 397-78-4, 39-43 cm = V-1 a). A clayey very finequartz sandstone alternating with layers of foraminif-eral sand (?contourite) contains about 15 per cent of al-tered volcanic rock and glass fragments (Sample 397 A-2-2, 24-29 cm).

Non-Volcanic ComponentsThere are two main types of non-volcanic compo-

nents. The most common is single grains of quartz,feldspar, and rock fragments such as quartzite. Possi-bly, the high quartz and quartzite content indicatesderivation from Cretaceous quartz sandstones on Fu-erteventura rather than from the African mainland (seebelow).

The other type of clast is made of various types ofbiogenic debris. These are often replaced and recrystal-lized so their volumetric abundance is underestimated.

QuartzEpiclastic quartz grains are quite common in the

volcaniclastic sandstones of debris flows V-1 (Plate 3,Figure 3) and V-3 (20 to 30% in Samples 397-85-3, 77cm and 397A-6-3, CC; see Plate 3, Figure 4). Smallsilt-sized grains are angular, whereas the larger grainsare well-rounded and commonly polygranular, gradinginto quartzite.

Sedimentary Rock FragmentsSedimentary rock fragments are common in debris

flow V-1 and include quartzose limestone (Sample397-79-3, 91 cm), foraminiferal micrite (Sample 397A-1, 74 cm), marlstone (Plate 2, Figure 3), and silici-fied(?) clay stone fragments which are sometimes diffi-cult to distinguish from altered fine-grained volcanicrocks (Table 1). Glauconite, and oncoids, which are

712


slightly broken, healed by secondary calcite, and over-grown by calcareous algae and bryozoans, testify to ashallow-water photic zone source environment of thosecomponents (Plate 5, Figure 7). Those sedimentaryrock fragments and fossils were either derived from thelittoral zone, or picked up by the volcanic debris flowon its way to the South Canary Island Channel.

Crystalline Rock FragmentsCrystalline rock fragments occur only in debris flows

V-l and V-la and include polycrystalline quartz grainswith feldspar and amphibole (up to 5 mm) and "meta-quartzite" with elongated quartz grains and crenulatedcrystal boundaries (Plate 5, Figure 6). The ultimatesource of those grains is enigmatic: most of the poly-crystalline quartz is probably of plutonic origin,whereas the metamorphic rock fragments might be de-rived from the Cretaceous terrigenous sediments under-lying the Betancuria Massif of Fuerteventura (Rothe,1968; Schmincke, 1976). According to Bernoulli et al.(in press), the Lower Cretaceous flysch-type sedimentswere deposited in a distal deep-sea fan or rise environ-ment extending off northwest Africa prior to the earlyNeogene volcanic uplift of the Canary Archipelago.

FossilsPlanktonic foraminifers are common (e.g., Plate 4,

Figure 5; Sample 397-80-1, 29 cm), benthic foramini-fers rare (Samples 397-85-3, 77 cm and 397A-79-2, 78cm). Bryozoans (Samples 397-78-4, 39 cm, and 397-85-3, 77 cm), echinoid fragments, and calcareous algae("Lithothamnium"; Plate 5, Figure 8) indicate a shal-low-water source environment for these components,possibly an island shelf.

Alteration and Secondary Minerals (matrix/cement)

The main components of the volcanic rock particlesare glass, and the mineral phases clinopyroxene, oli-vine, and plagioclase. Of these, glass and olivine are al-ways completely altered, mostly to phyllosilicates(probably smectite) and carbonate. In general, carbon-ate (mostly calcite), which partly or completely re-places phyllosilicates, is a later diagenetic phase. Weare unable to determine why clinopyroxene and plagio-clase are remarkably stable, although the feldspar ismore easily altered than clinopyroxene. Such differ-ences are typical of moderately altered sequences ofbasaltic submarine series (see, e.g., La Palma pillowcomplex: Schmincke and Staudigel, 1976). Alterationwas apparently dominated by pore solution chemistryand not be elevated temperatures, as this would haveled to complete albitization of the plagioclase and for-mation of quartz, epidote, and other phases (Schminckeand Staudigel, 1976). Measured in-situ temperatures ofabout 40° C at 750 to 800 meters (see Site Report, thisvolume) suggest that alteration may be more advancedthan in rocks of similar age in low heat-flow areas.

There are three main types of secondary minerals:"layered silicates" (clayey matrix) carbonate, and zeo-lites.

Clayey to Marly Matrix

By far the most common secondary minerals in bothrock types are "layered silicates" which occur in sev-eral varieties, distinguished by color, birefringence, andhabit. Most common are light brown "smectites" andsome illite(?) which make up much of the "matrix"and are also found as pseudomorphs of olivine, glass,etc. They generally occur as vermicular aggregates. Atthe other end of a wide color spectrum are deep greento blue-green varieities which are not common and oc-cur mainly in vesicles of sideromelane shards (Plate 5,Figures 3 and 4). They encompass glauconite-celado-nite and perhaps some chlorite. Serpentine was foundonly as rare pseudomorphs of olivine, chiefly in the mi-crogabbros; very likely it formed prior to erosion andsedimentation.

The matrix of the volcaniclastic debris flow V-l(e.g., Sample 397-79-4, 92 cm) consists of yellowishgreen, isotropic to cryptocrystalline clay minerals (Plate3, Figure 3). This phyllosiliçate might be a diageneti-cally altered smectite (montmorillonite, XRD determi-nation by H. Rösch) of volcanogenic origin, althoughthis problem should be further investigated (see alsoRiech, this volume; Chamley and d'Argoud, this vol-ume). The hemipelagic foraminiferal marlstone under-lying flow V-l (Sample 397-80-1, 107 cm) is distinctlydifferent from the brownish cryptocrystalline, clayey(?smectite) matrix (Plate 2, Figure 4). This also suggeststhat a large part of the green clay matrix is derived fromfine-grained altered ash material.

The clayey to marly matrix (e.g., in debris flow V-3;Sample (397-84-4, 2 cm) contains some foraminifers,coccoliths, and discoasters (Plate 4, Figure 5). In thiscase, the clay fraction might be identical to, or mixedwith the underlying hemipelagic marl.

Carbonate CementCarbonate occurs in all rocks, although mostly in

small amounts (


specimens, although usually in small amounts. Largeamounts of fibroradiated to bladed zeolite crystals (20-30 µm long), probably phillipsite, are the latest cementand fill open cavities in the volcaniclastic sandstones offlow V-l (e.g., Samples 397-79-3, 31 cm; and 397-79-3,91 cm; Plate 5, Figure 5). In the hyaloclastite of flowV-3, zeolites make up a few per cent. In one specimen, amacrocrystalline zeolite cement was determined by XRDas analcime (H. Rösch; Table 1). Smaller amountsof phillipsite and rare chabazite(?), not detected byXRD might be present. In several rocks, textural evi-dence suggests that zeolites formed from layered sili-cates, while in most volcaniclastic sandstones zeoliteforms rims around cavities, but is later replaced bycarbonate.

Other phases of diagenetic origin include pyrite andTi-phases (possibly anatase or leucoxene) which occurin concentric bands inside palagonitized sideromelaneshards (Plate 5, Figure 2).

DISCUSSION

Origin and Deposition of Clastic Volcanic DebrisThree processes may lead to the deposition of clastic

volcanic debris on the sea floor: (1) eruption and pro-duction in situ; (2) transportation of volcanic debriserupted at the sea floor from a distant location (mid-ocean ridge, fracture zone, seamount) to the presentsite of deposition by currents; and (3) eruption on landwith (a) direct fallout and sedimentation in the sea or(b) erosion, subsequent reworking, and downslopetransport onto the sea floor.

There is no evidence for model 1, as the volcaniccomponents are always interbedded or intermixed withnon-volcanic detritus. In support of model 2, subma-rine volcanoes or the submarine stage of a volcanic is-land are likely sources for the hyaloclastites. Most ofthe volcanic material, however, is probably related tothe nearby Canary Islands, i.e., model 3-a can be ap-plied for the ashes and model 3-b for the "debrisflows."

Before we concentrate on a specific island, we maysubdivide the rock types into diiferent groups accordingto their possible occurrence during various evolutionarystages of an oceanic island.

There are four major clastic processes by which vol-canic material from a seamount or volcanic island isgenerated and can be supplied to bottom sediments:

a) Shard and pillow formation during the subma-rine stage. We tentatively interpret the samples takenfrom the drill bit of Hole 397 as representing a rela-tively deep water stage (because of the non-vesiculatednature of most shards); and Samples 397-84-4, 2-5 cmand 397-85-3, 77-80 cm from debris flow V-3 as repre-senting a shallow-water stage (more vesicular shards)in the construction of a seamount. The latter two sam-ples probably indicate partial emergence of an islandcontaining alkalic gabbroic rocks.

b) Fragmentation of lava flows entering the seafrom land. This process produces large volumes oftachylitic to finely crystalline rock fragments.

c) Erosion of volcanic and plutonic rocks depositedand cooled on land and transported into the sea by riv-ers. The evidence that these components are derivedfrom a volcanic island is many fold. Alkalic compositionas described above is typical of volcanic rocks from is-lands in general and the Canary Islands in particular.Shield-building basalts are usually by far the most vo-luminuous rock types on the Canary Islands. Theyclosely resemble the crystalline basalt fragments in thevolcaniclastic sandstones (V-l), with olivine and ti-tanaugite as main liquidus phases, and clinopyroxeneand plagioclase dominating the groundmass mineral-ogy. Iddingsitization and, in a few specimens, formerhigh-temperature oxidation of olivine crystals are fur-ther evidence that these rocks crystallized in a sub aer-ial environment. Finally, the great variety of rock typesranging from alkalic microgabbros through basalts totrachytes is generally not developed in a submarinevolcanic rock association, but is fairly typical of oceanicislands, especially the Canaries (see review of the geol-ogy of the Canary Islands by Schmincke, 1976).

d) A fourth process would be highly explosive erup-tions, principally of more differentiated magma on anisland. This process, responsible for the ash layers, in-volved vertical projection in high eruption columns,,lateral eolian transport, and sedimentation by passivefallout through the water column.

Source Area of Volcaniclastic Debris FlowsBased on our conclusion that the Canary Islands are

the most likely source area for most or all of the vol-canic components found in Holes 397 and 397A, wenext may ask which of the islands is the most likelysource area, if individual islands can be singled out.Judging from the proximity to the drilling site, GranCanaria would have been the most likely candidate, asoriginally proposed by the shipboard party. However,detailed data on the geochronology, geology, petrogra-phy, and chemistry of the early subaerial history ofGran Canaria (Schmincke, 1976; MacDougall andSchmincke, 1977; Lietz and Schmincke, 1975) indicatethe following: (a) the island did not rise above sealevel prior to about 14 m.y.B.P., (b) there are noknown Miocene alkali gabbros, and (c) the differenti-ated lavas and ignimbrites overlying the shield basaltsare dominantly rhyolites and later nepheline phono-lites. On the other hand, the reported ages of shield ba-salts of the island of Fuerteventura (Figure 2) spanthe range from 11 to 20 m.y.B.P. (see Arthur et al., thisvolume). This age range would fit well with the ageof Cores 397-78 to 397-85 (17 to 15.5 m.y.B.P.). Ifthese debris flows directly reflect the major phase ofshield building, this would strengthen the conclusion ofSchmincke (1976) and MacDougall and Schmincke(1977) that shield-building stages on the Canary Is-lands are very short-term events (subaerial shieldphase on Gran Canaria: 13.7 to 13.5 m.y.B.P.) andthat some of the previously published age determina-tions are in error. Nevertheless, there are also olderplutonics, ranging from ultramafics to syenites, exposedon Fuerteventura as well as trachytes older than the

714


Miocene shield basalts (Fuster et al., 1968). Ages forthese older rocks range from middle Miocene toOligocene (maximum age: 38.6 m.y.B.P.; Abdel-Monem et al., 1971), 20 m.y. old. Grunau et al. (1975)suggest that the bulk of plutonic and dike-in-dike rocksare not much older than early Miocene. Thus, the ageand (to a lesser degree) composition of volcanic com-ponents indicate that Fuerteventura may have been themain source area for the volcanic sandstones en-countered at Site 397 (Cores 397-78 to 397-85).

CONCLUSIONS: RECONSTRUCTION OF THEEARLY EVOLUTION OF THE CANARY

ISLANDS1. Starting with the oldest investigated pyroclastic

sample, there was apparently a large trachytic explo-sive eruption about 19 m.y. ago (early Miocene)which produced glass shards, pumice, rock fragments,and feldspar crystals, all of trachytic composition (Fig-ure 2). The source is unknown, but could have beenFuerteventura or Lanzarote judging from age data ofsimilar rock types on these islands (Abdel-Monem etal., 1971).

2. The next younger rocks (about 17 to 17.6 m.y.B.P., lowermost middle Miocene) are hyaloclastites thatappear to be almost entirely of submarine origin, possi-bly recording the submarine stage of Fuerteventura'sshield basalt phase (Figure 2). Microgabbro fragmentsare present in the hyaloclastites from flow V-3, muchof which appear to be the result of shallow-water vol-canic activity. This indicates (as may the older trachytictuff) that part of Fuerteventura was aready an islandbefore its major basaltic shield-forming phase.

3. About 14.5 to 17 m.y. ago (middle Miocene) ba-saltic subaerial volcanism was fully active, judgingfrom the composition of clasts in debris flow V-l. Fromthe abundance of tachylite, we infer that many lavaflows entered the sea at this time. This voluminousnear-shore volcanic activity may have been one reasonfor the supply of particularly abundant detritus thatslumped off (?during volcanic earthquakes) to triggermassive debris/grain flows and turbidity currentsavalanching southward to the South Canary IslandChannel. This near-shore volcanic activity also explainsthe lack of similar rocks higher in the stratigraphic col-umn.

4. Later events (upper/middle Miocene-Quarter-nary: 14 to 0.3 m.y.B.P.) were mainly air-fall ashesfrom explosive eruptions of more differentiated volca-noes of the Canaries which were much less voluminousthan the volcanic output during the shield-buildingstage. Tentatively, three episodes can be distinguished:(a) about 14 to 10 m.y.B.P. (middle to late Miocene)strong explosive activity is recorded producing up to25-cm-thick ash layers of ?rhyolitic composition andcoinciding with "Magmatic Phase I" on Gran Canaria(Lietz and Schmincke, 1975). (b) After a long non-vol-canic interval (about 9 to 4.1 m.y.B.P.), a second vol-canic activity started about 4 m.y.B.P. (early Pliocene),possibly correlative with the second magmatic phase(Roque Nublo group) on Gran Canaira. (c) The

youngest volcanic ashes are 3.3 to 0.3 m.y. old, andmight be derived from Pliocene-Pleistocene trachytic tophonolitic eruptions on Tenerife (see also Arthur et al.,this volume).

An alternative interpretation of the succession ofvolcaniclastic layers at Site 397 would be episodic sedi-mentologic events acting on a fairly continuous processof island construction, growth, and destruction. Thiswould make many of our speculative interpretationsquestionable. We do, however, believe that the island'sgrowth is to some degree reflected in the texture andcomposition of the volcaniclastics as outlined above.

ACKNOWLEDGMENTSIn writing this paper, we relied heavily on the results of

the sedimentologists (M. Arthur, B. Lopatin, F. McCoy, M.Sarnthein, O. Weser) and of the paleontologists of Leg 47A(see Site Chapter, this volume). We are grateful to V. Riech(Hannover) for measuring refractive indexes, photographingthin-sections, and commenting on the zeolites; to H. Rösch(Hannover) for supplying several X-ray diffraction analyses;and to G. Wissman (Hannover) for geophysical information(Figure 1). F. McCoy (Lamont-Doherty Geological Observa-tory) and L. Diester-Haass (Kiel) kindly provided additionalsamples for our study. The present study was done while thesenior author held a grant of the Volkswagen-Stiftung at theUniversity of California, Santa Barbara. R. V. Fisher criticallyread the manuscript. Our work on DSDP material was sup-ported by the Deutsche Forschungsgemeinschaft.

REFERENCESAbdel-Monem, A., Watkins, N. D., and Gast, P. W., 1971.

Potassium-Argon ages, volcanic stratigraphy, and geo-magnetic polarity history of the Canary Islands: Lanzar-ote, Fuerteventura, Gran Canaria, and La Gomera, Am. J.Sci., v. 271, p. 490-521.

Bernoulli, D., Fustero, J. M., Hottinger, L., and Renz, O., inpress. Mesozoic and early Tertiary sediments and faunasfrom Fuerteventura (Canary Islands).

Fuster, J. M., Cendrero, A., Gastesi, P., Ibarrola, E., andRuiz, J. L., 1968. Geology and volcanology of the CanaryIslands, Fuerteventura. Inst. "Lucas Mallada," Madrid,Internat. Symposium Volcanology, Tenerife, 1968, Spec.Publ.

Grunau, H. R., Lehner, P., Cleintuar, M. R., Allenbach, P.,and Bakker, G., 1975. New radiometric ages and seismicdata from Fuerteventura (Canary Islands), Maio (CapeVerde Island), and Sao Tome (Gulf of Guinea). In Bor-radaile, G. J., et al. (Eds.). Progress in geodynamics: Am-sterdam/New York (North-Holland Publ. Comp.), p. 90-118.

Lietz, J. and Schmincke, H. U., 1975. Miocene-Pliocene sealevel changes and volcanic phases on Gran Canaria (Ca-nary Islands) in the light of new K-Ar ages, Palaeogeog-raphy, Palaeoclimatology, Palaeoecology, v. 18, p. 213-239.

MacDougall, I. and Schmincke, H. U., 1977. Geochronologyof Gran Canaria Canary Islands: age of shield buildingvolcanism and other magmatic phases, Bull. Volcanolo-gique, v. 40, p. 1-21.

Martini, E., 1976. Cretaceous to Recent calcareous nanno-plankton from the central Pacific Ocean. In Schlanger, S.O., Jackson, E. D., et al., Initial Reports of the Deep SeaDrilling Project, v. 33: Washington (U.S. GovernmentPrinting Office), p. 383-424.

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Mitchell-Thomé, R. C, 1976. Geology of the middle Atlanticislands: Stuttgart, Berlin (Borntraeger).

Rothe, P., 1968. Mesozoische Flysch-Ablagerungen auf derKanareninsel Fuerteventura, Geol. Rundschau, v. 58, p.314-322.

Rothe, P. and Koch R., 1978. Miocene volcanic glass fromDSDP Sites 368, 369, and 370. In Lancelot, Y., Seibold,E., et al., Initial Reports of the Deep Sea Drilling Project,v. 41: Washington (U.S. Government Printing Office), p.1061-1064.

Schmincke, H. U., 1974. Pyroclastic rocks. In Füchtbauer, H.(Ed.), Sediments and sedimentary rocks. I: Stuttgart(Schweizerbart), p. 160-189.

1976. The geology of the Canary Islands. InKunkel, G. (Ed.), Biogeography and ecology in the CanaryIslands: The Hauge (W. Junk), p. 67-184.

Schmincke, H. U. and Staudigel, 1976. Pillow lavas on cen-tral and eastern Atlantic islands (La Palma, Gran Ca-naria, Porto Santo, Santo Maria). Preliminary report, Soc.Géol. France Bull, v. 18, p. 871-883.

716

397-79-2 397-79-3


PLATE 1397-79-3 397-79-4 397-80-1

1-100 s = ^ = = . — 5 5

- 6 0

TOPOFV-1

-100

- 5 0 -150

•0

79-4

80-1 -100

-150

-100

' - 5 5

V-1

Shipboard core photos of volcaniclastic debris flow V-1 (Cores397-79 and 397-80). Note graded character in upper part (Sections 397-79-2 and397-79-3); very coarse conglomeratic intercalations near Samples 397-79-4,40-50 cm and397-80-1, 0-15 cm; and sharp, graded corse-grained base of flow. For detils see Plate 2.

717


PLATE 2

Shipboard close-up photos of parts of volcaniclastic debris flowV-l (Cores 397-79 and 397-80). Only top of debris flow (Figure 1) isa medium-grained sandstone; the remainder is a very poorly sorted,coarse sandstone to conglomerate, full of basaltic and sedimentaryrock fragments (e.g., large marlstone clast at Sample 397-79-4, 40cm), glass, quartz, and volcanic minerals (clinopyroxenes, alteredolivine, feldspar, etc.), all set in a dark brown clayey matrix (seeTable 1). Note that the coarsest grain size occurs high above thebase of the flow (Figure 3). The basal 4 cm of the unit (Figure 4)show a light colored marly matrix which was probably churned upby the turbulent flow from the underlying pelagic marlstone.

718


PLATE 2

397-79-4 397-80-1

—90

719


PLATE 3Photomicrographs of characteristic types of volcaniclastic

sandstones (direct projection of thin-section image on largenegative).

Figure 1 Sample 397-79-3, 91-93 cm: Volcaniclasticsandstone to conglomerate made up mostly ofepiclastic basalt fragments ranging in texturefrom palagonitized sideromelane (a) to tachy-lite (b) to crystalline basalt (c) to "microgab-bro" (d). Matrix is dominantly montmorillon-ite.

Figure 2 Sample 397-79-3, 141-143 cm: Volcaniclasticsandstone rich in highly vesicular (b :) and non-vesicular tachylite (b2), crystalline basalt (c),and clinopyroxene (cpx) crystals.

Figure 3 Sample 397-79-4, 92-94 cm: Volcaniclasticsandstone with palagonitized, vesicular "sidero-melane " shard (a; see also Plate 5, Figure 3),vesicular (bj) and non-vesicular tachylite (b2),crystalline basalt (c), trachyte fragment (e; seealso Plate 4, Figure 4), clinopyroxene (cpx),calcite-replaced olivine (ol), and roundedquartz (q). Large tachylite fragment (top) isshown in more detail in Plate 4 (Figure 2).Note size contrast between tachylite (1 to 4mm), crystalline basalt (1 to 2 mm), and crys-tals (


PLATE 3

lmm

•• • • . ,

• •

E* ~m JFJÈJ*i

< .< . *'. -

Si; , v- -Λ , •• >' -

‰«*

• - ; " f *' •

• >*L

I

p

t r •

•• . i ^

* *'; £

"« '

lmm

721


PLATE 4Typical fabrics and components of volcaniclastic sandstones

Unit V-l, Figures 1 through 4) and hyaloclastites (Unit V-3,Figures 5 and 6).

Figure 1 Sample 397-79-4, 92-94 cm: Crystalline alkalibasalt fragment with fresh clinopyroxene pheno-cryst and microlites, and partly altered plagio-clase and titanomagnetite microlites. Mesostasisreplaced by clay.

Figure 2 Sample 397-79-4, 92-94 cm; crossed nicols:tachylite rock fragment showing fresh clinopy-roxene phenocryst (lower right) with oscillatoryzoning and altered plagioclase phenocryst (up-per left). Dark ore-rich matrix contains ran-domly oriented plagioclase microlites.

Figure 3 Sample 397-79-4, 92-94 cm: Moderately freshmugearite(?) with clinopyroxene phenocrysts;irregular vesicles (filled with clay and carbon-ate); and flow-banded groundmass of alteredplagioclase laths, clinopyroxene, and titanomag-netite.

Figure 4 Sample 397-79-4, 92-94 cm: Trachytic rock frag-ment consisting dominantly of flow-banded al-kali feldspar laths.

Figure 5 Sample 397-84-4, 2-5 cm: Poorly vesiculated,angular, palagonitized "sideromelane" shardsfrom hyaloclastite (Unit V-3). Note planktonicforaminifer (center right) and discoaster (upperright) in brown smectite matrix.

Figure 6 Sample 397-84-4, 2-5 cm: Highly vesicular, pal-agonitized "sideromelane" shard with flat-ellip-soidal vesicle completely filled with smectiteand zeolite (same sample as Figure 5).

722


PLATE 4

100 µm

723


PLATE 5Glass, secondary minerals, and typical non-volcanic

components of volcaniclastic sandstones.

Figure 1 Sample 397A-23-3, 30-33 cm: Montmorillonit-ized trachytic(?) glass shard, tricuspate and pu-miceous glass shards in oldest (19.3 m.y.B.P.)ash layer of Site 397. Most of the other glassshards are completely altered to clay minerals.

Figure 2 Sample 297-79-3, 91-93 cm: Palagonitized side-romelane shard showing olivine(?) phenocrystreplaced by smectite (light area) and concentricbands (Ti-phases?) in palagonite. Volcaniclasticdebris flow V-1.

Figure 3 Sample 397-79-4, 92-94 cm: Palagonitized,slightly vesicular sideromelane shard showingpartially replaced clinpyroxene (lower left andupper right) and completely replaced olivine(?) phenocrysts (upper left) and several genera-tions of clay minerals lining vesicles. Debrisflow V-l.

Figure 4 Sample 397-79-3, 141-143 cm: Vesicle in side-romelane shard partially filled with a rim ofprismatic zeolites (?phillipsite) and spheruliticor dense phyllosilicate minerals (?chlorite). Re-maining pore space is either open (here) orfilled with calcite (nearby). Debris flow V-l.

Figure 5 Sample 397-79-3, 21-23 cm: Spherulitic bun-dles of zeolites (?phillipsite) filling interstices ofgreenish brown ?smectite matrix.

Figure 6 Sample 397-78-4, 39-43 cm; crossed nicols:Large metaquartzite fragment (elongated quartzgrains with crenulated borders) in a pebbly mud-stone which also contains some volcanic rockfragments (V-la).

Figure 7 Sample 397-78-4, 39-43 cm: Oncoid overgrownby coralline algae (?"Lithothamnium") and?bryozoan (same debris flow as in Figure 6).Dark interior of oncoid shows concentric growthrings.

Figure 8 Sample 397-79-4, 37-40 cm: Coralline algalfragment (?Lithothamnium) in volcaniclasticdebris flow V-l indicating derivation from thephotic littoral zone. Planktonic foraminifer atlower right.

724


PLATE 5

100 µm

725

35. neogene evolution of canary island ...deepseadrilling.org/47_1/volume/dsdp47pt1_35.pdfin the...

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