Tectonic History of a Jurassic Backarc-Basin Sequence (The Gran Canäon Formation, Cedros Island, Mexico), Based on Compositional Modes of Tuffaceous Deposits

Tectonic history of a Jurassic backarc-basin sequence (the Gran CanÄon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits

Salvatore Critelli* Dipartimento di Scienze della Terra, UniversitaÁ degli Studi della Calabria, 87036 Arcavacata di Rende (CS), Italy Kathleen M. Marsaglia² Department of Geological Sciences, California State University, Northridge, California 91330-8266, USA Cathy J. Busby§ Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA

ABSTRACT ancient volcaniclastic deposits suspected of that re¯ect eruption style, in addition to those having formed in backarc-basin settings. that relate to magma composition. The modal The Jurassic Gran CanÄon Formation analysis of textures makes use of descriptive (Cedros Island, Baja California, Mexico) Keywords: backarc basins, Baja California, classes of shard morphology commonly re- constitutes an unusually well preserved and Jurassic, magmatic arcs, Mexico, prove- ferred to by volcanologists (e.g., Heiken and exposed example of ancient backarc-basin nance, sandstone petrology, volcanology. Wohletz, 1985) in SEM (scanning electron mi- ®ll. Petrofacies analysis conducted on tuff- croscope) studies and only recently quanti®ed INTRODUCTION aceous sandstone and tuff samples from in modal analysis of lithi®ed rocks by De this formation complement and reinforce Rosa (1999). The modal analysis of magmatic Few detailed studies have been conducted prior lithofacies interpretations, but with components to determine their composition on ancient, uplifted, intraoceanic backarc- was developed by sedimentary petrologists some modi®cation. When temporal and basin assemblages, probably because of their spatial trends in petrographic data (detrital (e.g., Dickinson, 1970; Ingersoll and Cavazza, intraoceanic setting and poor preservation dur- 1991; Marsaglia, 1992, 1993; Critelli and In- modes) are analyzed and compared to mod- ing subduction and terrane accretion. The vol- gersoll, 1995; Marsaglia and Devaney, 1995) els based on data collected from Deep Sea caniclastic ®ll of a late Middle Jurassic back- to help ®ngerprint ma®c, intermediate- Drilling Project and Ocean Drilling Pro- arc basin on Cedros Island (Baja California) composition, and felsic components and their gram cores, the trends indicate a second, (Fig. 1), the Gran CanÄon Formation, is re- relative proportions in reworked tuffaceous heretofore unrecognized, phase of backarc markable for its low degree of structural mod- sediments. To quantify these parameters in rifting. Basalt lavas interstrati®ed with da- i®cation, its low to moderate alteration and combination provides a unique approach to in- citic pyroclastic rocks of the primary vol- metamorphism, and its unusually good expo- terpreting the provenance of pyroclastic sedi- canic lithofacies, previously interpreted to sure. Detailed facies analysis of this backarc ments deposited in marine settings, where re- record the eruption of differentiated mag- basin was previously conducted by Busby- mas at the climax of growth of the Gran Spera (1987, 1988). In this paper, we present working and mixing can be common CanÄon island arc, are now as a result of this new petrographic data from the Gran CanÄon processes. study considered to be the product of arc Formation and compare these with petro- Results from the Deep Sea Drilling Project extension and rifting. graphic data from modern backarc basins We (DSDP) and Ocean Drilling Program (ODP) Our method of modal analysis uniquely re®ne the earlier-published model for the tec- have greatly improved our understanding of combines the quanti®cation of textural at- tonic evolution of the Jurassic arc-backarc backarc-basin facies relationships and sedi- tributes of pyroclastic and epiclastic debris system on Cedros Island. In this re®ned mod- ment provenance (Karig, 1983; Taylor and that re¯ect eruption style and magma com- el, we infer that a second phase of arc rifting Karner, 1983; Klein and Lee, 1984; Klein, position, as well as the effects of reworking has been recorded in the backarc-apron se- 1985; Nishimura et al., 1991; Klaus et al., and mixing in marine settings. This study quence (subsequent to the phase that formed 1992; Tappin et al., 1994; Arculus et al., 1995; demonstrates that detailed petrographic the basin). The second rifting event resulted Cambray et al., 1995; Clift, 1995; Clift and analysis is useful in the interpretation of in conversion of the active backarc basin into ODP Leg 135 Scienti®c Party, 1995; Haw- a remnant backarc basin. kins, 1995; Marsaglia, 1995; Marsaglia et al., 1995). However, these widely spaced drill *E-mail: [email protected]. Our petrographic methods include modal ²E-mail: [email protected]. analysis designed to quantify the textural at- holes provide only a limited view of largely §E-mail: [email protected]. tributes of pyroclastic and epiclastic debris submerged, tectonically complex basins, leav-

GSA Bulletin; May 2002; v. 114; no. 5; p. 515±527; 9 ®gures; Data Repository item 2002059.

For permission to copy, contact [email protected] ᭧ 2002 Geological Society of America 515 CRITELLI et al.

which appear to form in 10 m.y. or less (Tay- lor and Karner, 1983).

METHODS

Volcaniclastic Terminology

Volcaniclastic units in the Gran CanÄon For- mation are composed of sand pyroclasts and matrix that are inferred by their textural attri- butes and sedimentary structures to have been transported and deposited in a submarine en- vironment by turbidity currents. If a classi®- cation scheme based on transport and depo- sitional processes (McPhie et al., 1993) is used, these samples would be called sand- stones. According to a classi®cation scheme based on the origin of the particles (Fisher and Schmincke, 1984), however, these samples would be called tuffs because they are mostly composed of compositionally homogeneous pyroclasts that show little to no textural mod- i®cation. Therefore, as described in the pre- vious section and as shown in Figure 2, these deposits are referred to as tuffs, but for pur- poses of comparison with modern backarc- basin deposits described in the literature, they are referred to as tuffaceous sandstones.

Petrographic and Modal Analysis

We selected 32 medium- to coarse-grained Figure 1. Generalized geologic map of Cedros Island (modi®ed from Kilmer, 1977, 1984) tuffaceous sandstone samples from the Gran and location of the measured sections shown in Figure 2. CanÄon Formation for thin-section preparation and modal analysis. These cover the entire formation in both proximal and distal sections ing much to learn about facies and deposi- ation was immediately followed by progra- (Fig. 2; Table DR11 presents ®eld description tional processes in these settings. Our new dation of a deep-water pyroclastic apron into of samples used for petrographic study and petrographic data complement the data set the backarc basin, contemporaneous with the lithofacies subdivision of Gran CanÄon For- from modern basins by documenting the evo- growth of an oceanic arc from deep water to mation). We also examined thin sections of lution of a backarc apron in a more arc- sea level or above. Busby-Spera (1987, 1988) lithic blocks from the tuff breccias, as well as proximal setting than most of the drill sites in divided this pyroclastic apron into tuff, lapilli thin sections of basalt lava ¯ows, to charac- the western Paci®c. Proximal coarse-grained tuff±tuff breccia, and primary volcanic litho- terize better the mineral assemblages and tex- volcaniclastic packages, such as the Gran CanÄ- facies (Fig. 2). Whole-rock analyses and mi- tures of smaller fragments within the tuffa- on Formation, provide more detailed infor- croprobe studies show that the basalts in the ceous sandstones. Some thin sections were mation on the tectonic and volcanic evolution primary volcanic lithofacies are chemically etched and stained for plagioclase and potas- of an intraoceanic arc during backarc-basin and petrographically homogeneous tholeiitic sium feldspar. formation (Marsaglia and Devaney, 1995). basalts that are distinctly more alkalic than The tuffaceous sandstone samples were tholeiitic rocks of the ophiolitic or arc base- point-counted through the use of a petrograph- GEOLOGIC AND STRATIGRAPHIC ic microscope equipped with an automated SETTING OF THE GRAN CANÄ ON ment to the Gran CanÄon Formation (Kim- stage. Five hundred points were counted for FORMATION brough, 1982, 1984). The pyroclastic apron was then blanketed by a relatively thin sheet each sample by using the Gazzi-Dickinson method (Ingersoll et al., 1984; Zuffa, 1985, A rifted-arc and ophiolite assemblage and of siltstone to ®ne-grained sandstone turbi- 1987). Grain parameters (Zuffa, 1985, 1987; overlying volcaniclastic rocks, all of late Mid- dites (``epiclastic lithofacies''; Fig. 2), inter- Critelli and Le Pera, 1994) are de®ned and dle Jurassic age, represent a fragment of the preted to record abrupt extinction and erosion, arc side of a backarc basin, now exposed on but no uplift, of the island arc within 10 m.y. 1 of ophiolite generation. This cycle was in- GSA Data Repository item 2002059, Tables Cedros Island in Baja California (Kilmer, DR1±DR4, is available on the Web at http:// 1977, 1984; Kimbrough, 1984; Busby-Spera, ferred to re¯ect the episodic nature of pro- www.geosociety.org/pubs/ft2002.htm. Requests 1987, 1988; Fig. 1). Backarc ophiolite gener- cesses in backarc basins (Busby-Spera, 1988), may also be sent to [email protected].

516 Geological Society of America Bulletin, May 2002 COMPOSITION OF BACKARC-BASIN VOLCANICLASTIC ROCKS

Figure 2. (A±C) Measured sections of the Gran CanÄon Formation, Cedros Island (modi®ed from Busby-Spera, 1988). Localities of sections A (1, 2, 3), B, and C are plotted in Figure 1. (D) Paleocurrent data indicate a northern source for the Gran CanÄon Formation, and pyroclastic debris coarsens toward the inferred northern source area (from C to B to A). Felsic tuffs T1 to T4 and slumping horizon Sl are marker horizons. GC1 to GC266k refer to samples used for petrographic study (Tables DR1, DR2, and DR3Ðsee footnote 1). See Busby-Spera (1987, 1988) for descriptions and interpretations of the stratigraphy.

Geological Society of America Bulletin, May 2002 517 CRITELLI et al.

Figure 2. (Continued.) raw point-count data are presented in Table resistant grains (e.g., glass vs. crystals or lithic ly labradorite and bytownite-labradorite by DR2 (see footnote 1), whereas recalculated fragments), provide valuable information for optical determination), clinopyroxene as di- modal point-count data are de®ned and pre- inferring eruptive styles and transport histo- opside (optically determined), and iron- sented in Table DR3 (see footnote 1). Grain ries. Note that in the thin sections that were titanium oxides. By using the textural subdi- parameters for the volcanic grains are those of not stained, some zeolitized grains may have vision of volcanic rocks of Williams et al. Dickinson (1970), Marsaglia (1993), and Cri- been inadvertently included in the felsitic (1954), the basalts show intersertal, intergran- telli and Ingersoll (1995), modi®ed to include category. ular, and hyalophitic textures. information on grain shape and fragment mor- phology (e.g., blocky, scoria, bubble wall; PETROGRAPHIC DESCRIPTIONS Dacite Blocks similar to the scheme outlined in De Rosa Basalt Lava Flows [1999]). The types of glassy material (e.g., Phenocrystic phases in dacite blocks in- blocky vs. bubble-wall shards, Table DR2), as Phenocrystic phases in thin sections of ba- clude plagioclase (zoned from labradorite- well as the degree of rounding of fragile and salt lava ¯ows include plagioclase (dominant- andesine to oligoclase by optical determina-

518 Geological Society of America Bulletin, May 2002 COMPOSITION OF BACKARC-BASIN VOLCANICLASTIC ROCKS tion), green hornblende, minor quartz, and rare (0%±32%), and calcite is rare (0%±16%), ex- sample has either blocky shards (of hydro- potassium feldspar. Groundmass minerals in- cept for one sample with abundant late poi- clastic origin) or scoria fragments (of explo- clude plagioclase, quartz, hornblende, and kilotopic cement (GC242, 16%); authigenic sive magmatic origin) (Table DR2Ðsee foot- magnetite. The dominant groundmass texture clay minerals (0±6.4%), zeolites (laumontite note 1), and some of the scoria fragments are is felsitic granular, consisting of an anhedral and heulandite; 0±4.8%), and albite (0±0.8%) blocky in shape (indicating phreatomagmatic microcrystalline mosaic composed mainly of are subordinate (Table DR2). eruptions, Heiken and Wohletz, 1985). The quartz and feldspar with uniform, very ®ne glass in Gran CanÄon ma®c tuffaceous sand- grain size. Hyalopilitic texture in which col- Lithic Fragment Types in Tuffaceous stone samples is sideromelane (i.e., light orless glass occupies the minute interspaces Sandstone brown colored), but most of these grains are between randomly oriented microlites of pla- at least partially altered to clay or zeolite min- gioclase is also common. In some cases, this Volcanic lithic grains in Gran CanÄon sam- erals (Fig. 3, C±E). Palagonite alteration is glass is largely recrystallized. Vitrophyric tex- ples of intermediate to ma®c composition are dominant, accompanied in some samples by ture in which phenocrysts lie in a matrix of variably devitri®ed and altered to clay min- lesser montmorillonite and minor laumontite. glass (with or without minor alteration and/or erals and zeolites; they show lathwork, mi- Orange, brown, and black vitric lithic frag- devitri®cation of the glass) also occurs. crolitic, and vitric textures (Fig. 3). Volcanic ments in Gran CanÄon tuffaceous sandstone lithic fragments in Gran CanÄon felsic samples samples are similar to glassy material in the Volcaniclastic Sandstone show predominantly felsitic granular and vit- groundmass of intergranular-, hyalophitic-, ric textures with rare microlitic volcanic lithic and intersertal-textured basalt lavas that are Most samples come from graded beds, or grains. interbedded with the tuffaceous sandstone. upward-®ning and upward-thinning sequences Lathwork volcanic lithic fragments (Lvl; Vitric volcanic lithic fragments (Lvv) in the of beds interpreted as turbidites (Table DR1Ð see Figs. 3 and 4), as ®rst de®ned by Dick- felsic samples are colorless and consist of bub- see footnote 1). The samples are poorly sorted, inson (1970), have sand-sized phenocrysts in ble-wall shards and pumice (Fig. 3H). and the clasts are largely angular, although a groundmass of glass or devitri®ed glass (Fig. Felsitic volcanic lithic fragments (Lvf) may rare subangular to subrounded grains occur in 3, A and B). This texture is characteristic of include two types, granular and seriate (Dick- most samples. They exhibit a range of com- basaltic and basaltic andesite lavas and pyro- inson, 1970; Ingersoll and Cavazza, 1991). positions, from ma®c to intermediate to felsic. clasts. In the more ma®c samples from the Felsitic granular texture consists of anhedral Volcanic lithic fragments are the dominant Gran CanÄon Formation, the phenocrysts are microcrystalline mosaics, with uniform, very component. The rare nonvolcanic grains con- laths of labradorite plagioclase and clinopy- ®ne grain size, composed mainly of feldspar sist solely of intrabasinal carbonate bioclasts roxene, and the glassy groundmass is black, and quartz and/or ma®c minerals. Vitric grains and intraclasts. Crystals, counted as mono- brown or orange, or is microlitic (see subse- may grade into granular grains through devit- crystalline grains, are all euhedral. In the fel- quent discussion). These lathwork volcanic ri®cation. Felsitic granular texture is typical of sic samples, they are dominantly zoned pla- lithic grains are similar in texture to the rhyolites and dacites (Ingersoll and Cavazza, gioclase (andesine-labradorite to oligoclase) groundmass of both intergranular and hyalo- 1991). Felsitic seriate texture is an anisometric with lesser quartz and hornblende, and rare phitic textures in the basalt lava ¯ows. mosaic, with a wide range of crystal sizes and potassium feldspar. The intermediate- Microlitic volcanic lithic fragments (Lvmi; shapes, composed mainly of feldspar, quartz, composition to ma®c samples have plagio- see Figs. 3 and 4) are de®ned as fragments and/or ma®c minerals. Felsitic seriate texture clase (bytownite to labradorite) and clinopy- that contain variable amounts of microlites of is typical of dacites and andesites (Ingersoll roxene crystals. Accessory detrital minerals plagioclase or ferromagnesian minerals that and Cavazza, 1991). Seriate felsitic fragments include (in decreasing order of abundance) are visible at high magni®cation and are are rare in Gran CanÄon samples, and granular magnetite and epidote in the more ma®c sam- Ͻ0.0625 mm long (Dickinson, 1970). Mi- felsitic grains are abundant, suggesting that ples, and ilmenite, epidote, magnetite, and rare crolitic texture is typical of andesites, but it they are silicic rather than intermediate in apatite in the more felsic samples. also commonly occurs in basalts and basaltic composition. In addition, the dominance of Interstitial materials within the Gran CanÄon andesites. In Gran CanÄon samples, plagioclase plagioclase and hornblende (Fig. 3, F and G), Formation samples include detrital matrix and microlites occur in a dominantly black, brown rather than potassium feldspar and quartz phe- authigenic cement. Detrital matrix is repre- (Fig. 3C), or orange vitric groundmass. The nocrysts, suggests a dacitic, rather than rhyo- sented by very ®ne volcanic ash, but it is dif- microlitic texture in the tuffaceous sandstone litic, composition for these fragments. ®cult to estimate original abundances because samples is similar to the interstices of inter- the degree of recrystallization or replacement granular and hyalophitic textures in the basalt COMPOSITIONAL MODES AND varies from sample to sample; therefore, only lava ¯ows. Felsic microlitic fragments are rare PETROFACIES OF THE GRAN CANÄ ON minimum estimates of original ash matrix can and consist of pumice fragments with micro- FORMATION be made by using present abundances. These lites of plagioclase. are 0±1.6% for the ma®c samples and 0± Vitric volcanic lithic fragments (Lvv; see The Gran CanÄon samples are feldspatho- 13.6% for the felsic samples (Table DR2Ðsee Figs. 3 and 4) are de®ned as pumice or scoria lithic, plotting along the base of a standard footnote 1). Calcite cement is present in all of and glass shards, but also include partially to Qm-F-Lt ternary diagram (Fig. 4A) used for the ma®c samples (6%±51%) and includes wholly altered glass (Dickinson, 1970; Inger- tectonic-provenance analysis of sandstones pore-®lling (0%±23%) and poikilotopic (0± soll and Cavazza, 1991; Marsaglia, 1992). (e.g., Dickinson and Suczek, 1979; Dickin- 47.6%) types. Authigenic silica, laumontite, Volcanologists refer to these as vitric frag- son, 1982, 1985, 1988). Table DR4 (see foot- heulandite, and clay minerals serve as minor ments, rather than vitric lithic fragments, in note 1) shows recalculated average vitric, (0%±7%) cements (Table DR2). Authigenic order to distinguish them from nonglassy frag- microlitic, lathwork, and felsitic volcanic silica cement is present in the felsic samples ments, which they call lithic fragments. Every lithic grains of published volcaniclastic

Geological Society of America Bulletin, May 2002 519 CRITELLI et al.

520 Geological Society of America Bulletin, May 2002 COMPOSITION OF BACKARC-BASIN VOLCANICLASTIC ROCKS

Figure 3. Photomicrographs of grain tex- tures in the tuffaceous sandstone petrofa- cies of the Gran CanÄon Formation. (A±E) Ma®c petrofacies. (F±H) Felsic petrofacies. (A and B) Lathwork texture (volcanic lithic grainsÐLvl in Fig. 4) of basalt grains with labradorite laths in (A) orange and (B) black vitric groundmass. (C) Microlitic tex- ture (volcanic lithic grains with microlitic textureÐLvmi in Fig. 4) of basalt and ba- saltic andesite grains, with plagioclase mi- crolites and brown groundmass. (D and E) Vitric volcanic lithic grains (volcanic lithic grains with vitric textureÐLvv in Fig. 4) occur as (C) brown and (D, E) orange glass. (D) Ma®c vitric tuff (Lvv; arrows indicate examples of shards). (E) Detail of a basaltic vitric grain (Lvv) with light brown sidero- melane largely replaced by orange palagon- ite and laumontite (L) vesicle ®lling. (F) Crystal-lithic felsic tuff, with abundant pla- gioclase and hornblende (hbl). (G) Felsitic granular texture (volcanic lithic grains with Figure 4. Triangular plots of Gran CanÄon Formation tuffaceous sandstone samples: Poly- felsitic granular and seriate texturesÐLvf gons show mean and standard deviation for felsic and ma®c petrofacies. Polygons for the in Fig. 4) with plagioclase and partially cor- syneruptive mixed petrofacies and the posteruptive mixed petrofacies are not shown be- roded hornblende phenocrysts in a silicic cause there are so few examples. (A) QmÐmonocrystalline quartz, FÐfeldspars, LtЮne- granular groundmass. (H) Felsic vitric tuff grained lithic grains. (B) LvvÐvolcanic lithic grains with vitric texture, LvmiÐvolcanic with bubble-wall shards (arrows indicate lithic grains with microlitic texture, and LvlÐvolcanic lithic grains with lathwork texture. examples of shards). A, B, C, E, F, and G (C) LvfÐvolcanic lithic grains with felsitic granular and seriate textures, LvmiÐvol- are in cross-polarized light; D and H are in canic lithic grains with microlitic texture, and LvlÐvolcanic lithic grains with lathwork plane-polarized light. White bar length rep- texture. resents 0.2 mm. N ments, largely exhibiting lathwork and mi- fragments, with lesser vitric lithic fragments sand(stone) suites for which raw data were crolitic textures and subordinate vitric lithic (Fig. 4, B and C). On both Figure 4B and published or could be obtained from the au- fragments (Fig. 4, B and C). Single crystals Figure 4C, the felsic samples lie in ®elds en- thors responsible (including our own unpub- of plagioclase (4.6%±31%) and of clinopyrox- tirely separate from the ma®c petrofacies. Sin- lished raw data). Comparison of the sand- ene (1%±7%) are less abundant in the ma®c gle crystals are more abundant in the felsic stones' compositional modesÐincluding samples than they are in the felsic samples samples than in the ma®c samples (Fig. 4A) textural proportionsÐto volcaniclastic sed- (Table DR2Ðsee footnote 1). Basalt clasts and consist of plagioclase (12.2%±40%), iments of known ma®c, intermediate- range from dense nonvesicular types to highly quartz (2.6%±13%), hornblende (0.8±7.2%), composition, and felsic provenance (Table vesicular scoria fragments and are generally and K-feldspar (0%±4%) (Table DR2Ðsee DR4; Fig. 5)Ðsuggests that there are two angular to subangular. Every ma®c sample has footnote 1). The ratio of unaltered green horn- distinct clusters, one more ma®c and the either blocky shards (0.8%±20.8%) or frag- blende to total hornblende is high (0.9±1.0), other more felsic. The seven samples that plot ments of scoria (0±11.2%) (Table DR2), some putting Gran CanÄon samples in the range of between the distinct ma®c and felsic ®elds are of which are blocky. The former are generally syneruptive facies as de®ned by Erskine and referred to as mixed felsic ϩ ma®c. These considered to be the result of nonexplosive to Smith (1993) and Smith and Lotosky (1995). compositional groups roughly correlate with mildly explosive thermal contraction and shat- Furthermore, a pyroclastic source is indicated Gran CanÄon lithofacies; for example, the maf- tering of glass in hydrovolcanic to phreato- by this ratio because unaltered green horn- ic group is mainly composed of samples from magmatic eruptions, typical of deep-water blende is not found in lava ¯ows (Smith and the tuff lithofacies and lapilli-tuff lithofacies, volcanism (see references in Heiken and Woh- Lotosky, 1995). Like the ma®c samples, whereas the felsic group correlates with the letz, 1985, p. 13). The latter are attributed to grains in the felsic samples show little to no primary volcanic lithofacies. Because of this magmatic vesiculation during explosive mag- evidence of textural modi®cation. Some sam- stratigraphic connection, we refer to these matic or phreatomagmatic eruptions (Heiken ples are composed largely of glass. groups as petrofacies. and Wohletz, 1985). Every felsic sample has bubble-wall shards or pumice, except for samples we did not Ma®c Tuffaceous Sandstone Petrofacies Felsic Tuffaceous Sandstone Petrofacies point-count from the two ®ne-grained felsic marker horizons (T1 and T2) near the base of The ma®c tuffaceous sandstone samples The felsic tuffaceous sandstone samples the section, which have dominantly platy consist dominantly of volcanic lithic frag- consist dominantly of felsitic granular lithic shards and bubble-wall shards. The platy

Geological Society of America Bulletin, May 2002 521 CRITELLI et al.

are from the epiclastic lithofacies at the top of the Gran CanÄon Formation and are typi®ed by mixed compositions as well as a notable lack of shards, pumice, or scoria (Busby-Spera, 1988).

DISCUSSION

Tuff Lithofacies and Lapilli-Tuff Lithofacies

Busby-Spera (1988) interpreted the lapilli- tuff lithofacies to be a more proximal equiv- alent of the tuff lithofacies of the Arroyo Choyal area and Arroyo Gran CanÄon sections, and this understanding is supported by the compositional similarities of these subunits (Figs. 6 and 7). The compositional modes for the Gran CanÄon tuff lithofacies and lapilli-tuff lithofacies plot in the undissected-arc ®eld of Dickinson et al. (1983) and the intraoceanic- arc to continental-arc sub®elds of Marsaglia and Ingersoll (1992) on Qm-F-Lt and Qm-K- P plots (Fig. 6). As shown in Figure 5, the Figure 5. Triangular plot of aphanitic volcanic lithic grains in Gran CanÄon Formation mean volcanic-lithic textural proportions of tuffaceous sandstone samples, compared with detrital (compositional) modes of ma®c, the lithofacies are most similar to (1) upper felsic, and mixed felsic ؉ ma®c and/or intermediate-composition volcanic provinces from Miocene sand from DSDP Site 451 on the various published sources (Table DR4; see footnote 1). Key shows symbols for mean val- remnant arc ¯ank of the active Mariana Basin, ues. In order to simplify the diagram, only selected ®elds of variation (polygons de®ned (2) Pleistocene(?) sand at ODP Site 787 lo- (by one standard deviation on either side of mean) are shown. (A) Lvf±Lvv±(Lvmi ؉ Lvl)Ð cated in the Izu-Bonin forearc region, and (3 felsitic±vitric±(microlitic ؉ lathwork). (B) Lvf±Lvmi±LvlÐfelsitic±microlitic±lathwork. the ma®c examples, particularly Hawaiian (C) Lvv±Lvmi±LvlÐvitric±microlitic±lathwork. Ma®c examples are (1) eruption-fed, hy- sand. All three of these sand groups were like- drovolcanic (blocky nonscoriaceous shards) in the Topanga Group (Critelli and Ingersoll, ly derived from emergent volcanic islands: 1995) and (2) modern beach sands from the island of Hawaii (Marsaglia, 1993). Felsic The Hawaiian samples are modern beach examples are (1) ¯uvial volcaniclastic sandstones (Cordito petrofacies) from the Rio sands; the Site 451 samples are from volcanic- Grande rift (Ingersoll, 1984; Ingersoll and Cavazza, 1991) and (2) marine volcaniclastic apron facies associated with an at least partly sands deposited within the Gulf of California (Marsaglia, 1991). Mixed (felsic ؉ ma®c) emergent volcanic arc (Klein, 1985; Kroenke and/or intermediate-composition examples are from DSDP and ODP sites located in the et al., 1980); and sand at Site 787 was likely Parece Vela Basin and Mariana Trough (Packer and Ingersoll, 1986; Marsaglia and De- derived via submarine canyon from subaerial vaney, 1995) and an incipient backarc basin (Sumisu rift) in the Izu-Bonin arc (ODP Leg ma®c to intermediate-composition volcanic 126 Shipboard Scienti®c Party, 1989; Nishimura et al., 1992). The synrift sequences drilled ¯ows exposed on the island of Hachijo in the in the Sumisu rift are compositionally bimodal (Marsaglia, 1992). Similar compositions Izu-Bonin arc (Marsaglia, 1992). Thus, the characterize synrift sequences in the Mariana backarc basin (Marsaglia and Devaney, high proportion of microlitic and lathwork 1995). volcanic lithic fragments that characterize these lithofacies may be due to some combi- nation of their composition, basalt to basaltic shards near the base of the section indicate posteruptive, on the basis of the presence or andesite, and proximity to and/or derivation granulation by contact of magma with water absence of glass (shards, pumice, or scoria), from an emergent eruption center. (Heiken and Wohletz, 1985). The abundance respectively. These syneruptive and posterup- Despite the fact that their textural attri- of pumice and bubble-wall shards in the upper tive mixed petrofacies are distinguished best butes are most similar to sand with ma®c part of the Gran CanÄon Formation and the ab- on ternary plots emphasizing volcanic lithic and/or intermediate-composition provenance, sence of platy or blocky shards indicate ex- textures (Fig. 4), where it is apparent that the the presence of quartz and felsitic volcanic plosive magmatic eruptions in the absence of syneruptive samples, generally, contain more lithic fragments in many of those samples pre- external water. vitric lithic grains. The syneruptive mixed cludes a purely ma®c source for either the tuff petrofacies occurs within the upper third of Mixed Felsic ؉ Ma®c Tuffaceous lithofacies or the lapilli-tuff lithofacies. This Sandstone Petrofacies: Syneruptive and the pyroclastic apron sequence (in the tuff felsic contribution may have been admixed Posteruptive and lapilli-tuff lithofacies) and contains during eruption or transport, given the pres- shards, scoria and pumice shreds (Table ence of discrete interbeds of felsic tuff in this The mixed felsic ϩ ma®c petrofacies can be DR2Ðsee footnote 1), whereas samples in- interval (un-point-counted intervals as well as subdivided into two types, syneruptive and cluded in the posteruptive mixed petrofacies samples GC219 and GC17).

522 Geological Society of America Bulletin, May 2002 COMPOSITION OF BACKARC-BASIN VOLCANICLASTIC ROCKS

Busby-Spera (1988) qualitatively noted an up-section decrease in blocky shards and an up-section increase in scoria fragments in the Gran CanÄon Formation, which we can now quantitatively demonstrate in the tuff lithofa- cies and lapilli-tuff lithofacies (ma®c petrofa- cies) by using modal analysis. To illustrate this trend, we use a discrimination plot pro- posed by De Rosa (1999) where the axes are JVI (100 ϫ the ratio of juvenile vesiculated glass to total juvenile glass fraction) versus FCrl (100 ϫ the ratio of single [free] juvenile crystals to total juvenile crystals). The Gran CanÄon data plot in two distinct groups at ex- treme values of JVI (high and low) and a range of FCrl values (Fig. 8). There are several possible causes for the range of FCrl values. De Rosa (1999) related the FCrl to the variability of the initial crystal- size distribution of the fragmenting magma, to the mechanical energy of the eruption, and to transport and depositional effects. De Rosa (1999) argued that the effect of the initial crystal-size distribution is negligible when dealing with sand (2.0±0.0625 mm). The Gran CanÄon samples are products of submarine eruptions that were subsequently transported and redeposited into deeper water by turbidity currents. Thus, the variation in their FCrl val- Figure 6. Evolutionary trends from the base to the top of the Gran CanÄon Formation: ues is probably a product of sorting and/or Qm-F-Lt and Qm-K-P diagrams (with superposed provenance ®eld of Dickinson, 1985, mixing during transport. and Marsaglia and Ingersoll, 1992). With respect to JVI values, the Gran CanÄon samples exhibit very high and very low ve- sicularities (Fig. 8). The three highly vesicu- lated samples were collected in the upper part of the lapilli tuff±tuff breccia lithofacies, in some cases, just below the pillow lavas and breccias of the primary volcanic lithofacies (Fig. 2). Busby-Spera (1988) interpreted this trend as recording a decrease in hydrostatic pressure as the summit of the volcanic source grew nearer to sea level. Those samples with low vesicularity (low JVI) are equivocal in terms of the depth of water in which they formed. Vesiculated glass is commonly pro-

duced by CO2 outgassing of basaltic magma during subaerial eruptions or in water depths of Ͻ800 m (Moore and Schilling, 1973; Moore, 1979). Highly vesiculated basaltic breccia has been recovered in the basement rocks of the Sumisu rift (Taylor et al., 1990), which Gill et al. (1990) interpreted as the

products of the explosive eruption of H2O-rich magma in a relatively deep-water (2000 m depth; as suggested by paleontological data) basin formed by arc extension and nascent rifting. As no scoriaceous breccias were noted Figure 7. Evolutionary trends from the base to the top of the Gran CanÄon Formation: in the Gran CanÄon section, it is more likely A) Lvf±Lvmi±Lvl diagram. (B) Lvf±Lvv±(Lvmi ؉ Lvl) diagram. (C) Lvv±Lvmi±Lvl di- that the vesiculated basaltic tuffs in the Gran) agram. For explanation of abbreviations, see Figure 5 caption and the text. CanÄon accumulated adjacent to a shallowly

Geological Society of America Bulletin, May 2002 523 CRITELLI et al.

recovered in deep-ocean cores (see discussion in Marsaglia, 1995). Alternatively, the high quartz content could be attributed to a ``quasi-continental'' more highly evolved magmatic-arc source. Also, given the age of the Gran CanÄon Formation and associated degree of burial diagenesis, the felsitic textures could be a product of post- burial devitri®cation or alteration of originally colorless felsic glass, a major component in volcanic sediment produced during arc rifting (Marsaglia, 1992; Marsaglia and Devaney, 1995).

Epiclastic Lithofacies

The compositional modes of sandstone from the epiclastic lithofacies are somewhat unique from underlying units. They plot in the undissected-arc ®eld of Dickinson et al. Figure 8. Discrimination plot (proposed by De Rosa, 1999) where the axes are JVI (100 (1983), and the continental-arc sub®elds of ؋ the ratio of juvenile vesiculated glass to total juvenile glass fraction) vs. FCrl (100 ؋ Marsaglia and Ingersoll (1992) on Qm-F-Lt the ratio of single [free] juvenile crystals to total juvenile crystals). The Gran CanÄon data and Qm-K-P plots (Fig. 6). In addition, they for the ma®c petrofacies plot in two distinct groups at extreme values of JVI (high and are characterized by relatively high plagio- low) and a range of FCrl values. Our subdivision of the surge deposits into ma®c, inter- clase (Fig. 6), intermediate felsitic, and low mediate-composition, and felsic members is a function of the relative proportion of glass vitric volcanic lithic proportions (Fig. 7) in types in each: Ma®c (dominantly composed of brown and black glass), intermediate comparison to other intraoceanic-arc exam- (brown glass dominant, less colorless glass), and felsic (colorless dominant over brown ples. Because of the relatively low quartz con- glass). MF represents the explosive magmatic fall ®eld samples, and the PF represents the tent of the epiclastic sandstones, it is unlikely phreatomagmatic fall ®eld samples. that they were derived from the erosion of un- derlying more quartzose units. submerged volcanic center (Ͻ800 m depth), to that seen in sand derived from subaerial fel- Summary of Vertical Trends as originally interpreted by Busby-Spera sic volcanic provinces (e.g., Rio Grande rift (1988). Again, the sedimentary structures in and Sierra Madre Occidental of North Amer- There are distinct up-section shifts in detri- these deposits indicate that the pyroclasts were ica). This result might relate to the more prox- tal compositional modes as de®ned by mean reworked from where they were produced imal setting interpreted for the primary vol- compositions for Gran CanÄon lithofacies (Ta- such that they were transported downslope canic lithofacies, as slowly cooling volcanic ble DR3Ðsee footnote 1). Arrows de®ning into deeper water. ¯ows produced more microcrystalline frag- compositional trends are shown in Figures 6 ments and liberated more quartz phenocrysts. and 7. These trends show that the sand frac- Primary Volcanic Lithofacies Higher quartz content in more proximal fa- tion of the Gran CanÄon Formation is generally cies can be seen on the scale of the Gran CanÄ- of ma®c or intermediate composition in both Compositional modes for the primary vol- on outcrop. Compositional modes for tuffa- the tuff lithofacies and lapilli-tuff lithofacies canic lithofacies are characterized by (1) high- ceous sandstone from location A (more and generally felsic in the primary volcanic er feldspar (plagioclase and potassium feld- proximal facies) are more quartz rich than lithofacies (Table DR3). The epiclastic litho- spar) and quartz content than the underlying those of location B (more distal facies) in the facies has a more intermediate composition. lithofacies (Fig. 6) and (2) volcanic propor- Gran CanÄon section (Fig. 6), indicating a pro- This overall compositional trend has impor- tions that are dominantly felsitic, with lesser gressive decrease in quartz away from the tant implications for the interpretation of the vitric, and microlitic components (Fig. 7). source volcanoes (location A to B), a trend tectonic evolution of the Gran CanÄon arc sys- Samples of the primary volcanic lithofacies that probably continued into more distal ba- tem, as discussed next. plot in the undissected-arc ®eld of Dickinson sinal settings. Equivalents of these distal fa- et al. (1983) and the continental-arc sub®elds cies drilled by DSDP or ODP contain only REFINED TECTONIC MODEL FOR of Marsaglia and Ingersoll (1992) on Qm-F- trace amounts of quartz. Additional support GRAN CANÄ ON FORMATION Lt and Qm-K-P plots (Fig. 6); these Gran from the rock record includes a study by Gi- CanÄon samples are unique among deep- meno (1994), who reported quartz phenocryst The backarc setting of the Gran CanÄon For- marine intraoceanic arc-related sand recovered concentrations in proximal epiclastic deposits mation has been inferred, in part, from the pe- by DSDP and ODP in their high proportion of associated with subaqueous felsic domes. In trology and geochemistry of underlying Juras- felsitic volcanic lithic fragments and quartz. sum, abundant quartz and felsitic volcanic sic basement complexes, particularly the The proportion of felsitic volcanic fragments lithic fragments could be characteristic of Cedros Island ophiolite. The basement lithol- in the primary volcanic lithofacies is similar proximal felsic volcanic facies not previously ogy changes northwestward, across the Pinos

524 Geological Society of America Bulletin, May 2002 COMPOSITION OF BACKARC-BASIN VOLCANICLASTIC ROCKS syncline (Fig. 1), from the Cedros Island ophiolite to the intraoceanic-arc rocks of the Choyal Formation. Nowhere on Cedros Island is the contact between arc and ophiolite base- ment exposed, and it probably lies under the Pinos syncline. The compositional similarity of volcaniclastic sequences on the northwest- ern and southeastern ¯anks of the Pinos syn- cline, as determined in this study, suggests that the Gran CanÄon Formation is draped across the transition from arc basement to backarc basement, as depicted by Busby- Spera (1988). The fact that the Gran CanÄon Formation straddles this crustal boundary helps set limits on the likely depositional set- ting during the initial stages of sedimentation in the basin, as shown in Figure 9A. The compositional modes of the tuff litho- facies and lapilli-tuff lithofacies provide in- sight into the early history of the Gran CanÄon magmatic arc (Fig. 9A). The oldest samples examined at location A and location B (Fig. 1) are felsic (GC17) and ma®c (GC205a), re- spectively. We suggest that there was bimodal magmatism during basin inception. Such bi- modal magmatism has been documented and suggested for the early-rift and sea¯oor- spreading phases of modern backarc basins in the western Paci®c. For example, Marsaglia and Devaney (1995) found greater quantities of felsic components (colorless glass, quartz, and volcanic lithic fragments with felsitic tex- tures) in synrift sediment in the Mariana re- gion. This pattern has also been observed in Figure 9. Schematic diagram of the tectonic setting during accumulation of the various the Lau Basin (Clift, 1995; Clift and ODP Leg lithofacies of the Gran CanÄon Formation. Note that volcanoes shown as submerged may 135 Scienti®c Party, 1995) and Sumisu rift have been emergent islands. GC refers to the approximate setting of the Gran CanÄon (Taylor et al., 1990). The relatively high mean Formation (Cedros Island) in each time frame. Approximate ages for the early tuff and proportion of felsic detritus in the tuff litho- primary volcanic lithofacies are indicated. (A) The oldest samples analyzed above base- facies and lapilli-tuff lithofacies indicates that ment are bimodal, suggesting deposition in a nascent backarc (BABÐbackarc basin). (B) the arc volcanoes were somewhat evolved. With continued subduction and sea¯oor spreading in the backarc basin, the arc matures. The shift from arc basement overlain by (C) Eventually, the arc starts a new extensional phase. The Gran CanÄon Formation is proximal facies to backarc ophiolite overlain rifted away from the frontal arc to form a remnant arc, and magmatism wanes. (D) The by more distal facies across the Pinos syn- extension leads to another phase of backarc-basin formation. (E) Eventually, the remnant cline, as well as the apparent continuous rec- arc undergoes thermal subsidence and becomes partly draped by extrabasinal turbidites. ord of volcanism, suggests that rifting oc- Stage E depicts a possible scenario just prior to accretion of the Gran CanÄon terrane to curred behind the arc axis (i.e., backarc the North American continent (NA). The switch in polarity of subduction pictured in D rifting). As discussed by Marsaglia and De- and E requires the frontal arc (shown in A±D) to be removed by either translation or vaney (1995), forearc rifting results in pro- subduction erosion. gressive temporal and spatial shifts in volca- nism and associated depocenters in the backarc basin. In contrast, the locus of mag- cludes basalt ¯ows and coarse felsic tuffs and that extended down the apron within the back- matism is more static in the case of backarc breccias, indicating more proximal facies of a arc basin as a form of intraplate volcanism on rifting, resulting in the superposition of vol- volcanic center or centers. the ``hot,'' arc side of the basin, unaccompa- canic centers through time, producing a semi- In Busby-Spera's (1988) original interpre- nied by faulting. Drilling in the Izu-Bonin re- continuous record on the arc side of the back- tation of the Gran CanÄon Formation, dacite gion showed that rifting of the arc and the arc basin (i.e., toward the trench) (Marsaglia pyroclastic rocks of the primary volcanic lith- resultant change in stress regime can be re- and Devaney, 1995). The model depicted in ofacies were inferred to record the eruption of sponsible for bimodal volcanism, including Figure 9 for the Gran CanÄon magmatic arc differentiated magmas, which climaxed the the development of silicic calderas (Gill et al., shows initiation of rifting behind the arc axis growth of the adjacent island arc. Basalt lavas 1992); in the Izu-Bonin case, these calderas (backarc). The primary volcanic lithofacies in- were inferred to have been fed from ®ssures provide an abundant supply of dacite pyro-

Geological Society of America Bulletin, May 2002 525 CRITELLI et al. clastic debris, which dominates the strati- right). The fate of the frontal-arc part of the Boles, J.R., and Landis, C.A., 1984, Jurassic sedimentary melange and associate facies Baja California, Mexico: graphic record of the backarc basin (Nishi- Gran CanÄon arc system (subducted, translat- Geological Society of America Bulletin, v. 95, mura et al., 1991, 1992). Thus, the abundance ed?) remains equivocal (Fig. 9E). p. 13±21. of dacitic volcanic and pyroclastic debris and Faulting obscures the contact of the Gran Busby-Spera, C.J., 1987, Lithofacies of deep marine basalts emplaced on a Jurassic backarc apron, Baja California the alkalic tholeiitic composition of the basalts CanÄon with overlying units. We have ob- (Mexico): Journal of Geology, v. 95, p. 671±686. in the Gran CanÄon Formation are consistent served continentally derived coarse-grained Busby-Spera, C.J., 1988, Evolution of a Middle Jurassic back-arc basin, Cedros Island, Baja California: Evi- with the interpretation of arc extension. detritus in the overlying Upper Jurassic Co- dence from a marine volcaniclastic apron: Geological Based on comparisons with models con- loradito and Eugenia Formations, which sug- Society of America Bulletin, v. 100, p. 218±233. structed from DSDP and ODP drilling of Ce- gests a much different tectonic setting more Cambray, H., Pubellier, M., Jolivet, L., and Pouclet, A., 1995, Volcanic activity recorded in deep-sea sedi- nozoic examples, we suggest that the up- proximal to a continental margin (e.g., conti- ments and the geodynamic evolution of western Pa- section shift from tuff of ma®c and/or nental margin to right in Fig. 9D). These for- ci®c island arcs, in Taylor, B., and Natland, J., eds., intermediate composition to a bimodal com- mations have been interpreted as recording the Active margins and marginal basins of the western Paci®c: American Geophysical Union Monograph 88, bination of basaltic ¯ows and dacitic pyro- docking of the Gran CanÄon basement with the p. 97±124. clastic rocks indicates that the Gran CanÄon arc North America continent (Boles and Landis, Clift, P.D., 1995, Volcaniclastic sedimentation and volcanism during the rifting of western Paci®c backarc basins, in underwent another rifting episode. Given the 1984). Taylor, B., and Natland, J., eds., Active margins and constraints of our model for the setting of the marginal basins of the western Paci®c: American Geo- lower Gran CanÄon Formation (tuff and lapilli SUMMARY AND CONCLUSIONS physical Union Monograph 88, p. 67±96. Clift, P.D., andODP Leg 135 Scienti®c Party, 1995, Vol- tuff) and associated basement rocks, the likely canism and sedimentation in a rifting island-arc ter- setting during accumulation of the bimodal Studies of modern backarc basins demon- rain: An example from Tonga, southwest Paci®c, in primary volcanic lithofacies was an arc edi®ce strate that the history of basin formation and Smellie, J.L., ed., Volcanism associated with extension at consuming plate margins: Geological Society [Lon- that was undergoing extension and rifting evolution is re¯ected in the sedimentary ®ll of don] Special Publication 81, p. 29±51. (Fig. 9C). Evidence of success of this last pe- the basins, particularly the composition and Critelli, S., and Ingersoll, R.V., 1995, Interpretation of neo- volcanic versus paleovolcanic sand grains: An exam- riod of extension, in other words, whether ex- texture of volcanic and volcaniclastic com- ple from Miocene deep-marine sandstone of the To- tension progressed to the sea¯oor-spreading ponents. In this study, we show that this his- panga Group (southern California): Sedimentology, phase (as pictured in Fig. 9D) or ceased, might tory can be deciphered in ancient backarc- v. 42, p. 783±804. Critelli, S., and Le Pera, E., 1994, Detrital modes and prov- be found in the overlying sedimentary se- basin sequences, even if they are only partly enance of Miocene sandstones and modern sands of quence. Facies recovered across the Mariana preserved in accreted terranes. Speci®cally, the southern Apennines thrust-top basins (Italy): Jour- Basin (see Marsaglia and Devaney, 1995) we re®ne a tectonic model for the Gran CanÄon nal of Sedimentary Research, v. 64, p. 824±835. De Rosa, R., 1999, Compositional modes in the ash fraction show that if extension and rifting progress to Formation (Busby-Spera, 1988) by proposing of some modern pyroclastic deposits: Their determi- sea¯oor spreading, then the remnant arc be- that the primary volcanic lithofacies formed nation and signi®cance: Bulletin of Volcanology, v. 61, p. 162±173. comes isolated from volcaniclastic input, and during a second arc rifting event (the ®rst be- Dickinson, W.R., 1970, Interpreting detrital modes of gray- the cover is largely pelagic. If rifting was not ing the creation of the backarc basin). We il- wacke and arkose: Journal of Sedimentary Petrology, successful, then subduction may have contin- lustrate this evolution in proximal volcanic fa- v. 40, p. 695±707. Dickinson, W.R., 1982, Compositions of sandstones in cir- ued, with another phase of magmatic-arc de- cies, thus expanding current models based on cum-Paci®c subduction zones and forearc basins: velopment superimposed on the extended arc. distal facies that were more easily cored and American Association of Petroleum Geologists Bul- The composition of sand within the epiclas- recovered during deep-sea drilling. letin, v. 63, p. 2±31. Dickinson, W.R., 1985, Interpreting provenance relations tic lithofacies (high plagioclase content and from detrital modes of sandstones, in Zuffa, G.G., ed., volcanic fragments exhibiting microlitic and ACKNOWLEDGMENTS Provenance of arenites: Dordrecht, Netherlands, D. lathwork textures) can be explained in several Reidel, NATO Advanced Study Institute Series 148, Discussions with and comments by J. Boles, R.V. p. 333±361. ways. First, an intermediate-composition Ingersoll, and E. Le Pera improved the text. We are Dickinson, W.R., 1988, Provenance and sediment dispersal source may have been made up of more grateful to GSA Bulletin reviewers Rebecca Dorsey, in relation to paleotectonics and paleogeography of Allen Glazner, Raymond Ingersoll, Douglas Smith, sedimentary basins, in Kleinspehn, K.L., and Paola, evolved arc volcanoes; however, the lack of C., eds., New perspectives in basin analysis: New glass-rich pyroclastic-apron facies does not and Michael Underwood for reviews, helpful dis- York, Springer-Verlag, p. 3±25. cussions, and comments on an earlier version of the support arc rejuvenation. Likewise, it is un- Dickinson, W.R., and Suczek, C.A., 1979, Plate tectonics manuscript. Salvatore Critelli was supported by a and sandstone compositions: American association of likely that the epiclastic lithofacies was de- 1995±1996 Consiglio Nazionale delle Ricerche Petroleum Geologists Bulletin, v. 63, p. 2164±2182. rived from underlying units or lateral equiva- (C.N.R.)-North Atlantic Treaty Organization Dickinson, W.R., Beard, L.S., Brakenridge, G.R., Erjavec, lents because it has little compositional (N.A.T.O.) advanced fellowship at the University of J.L., Ferguson, R.C., Inman, K.F., Knepp, R.A., Lind- berg, F.A., and Ryberg, P.T., 1983, Provenance of af®nity with them, particularly with respect to California, Santa Barbara. Work supported by grants of the National Science Foundation (EAR-9304130 North American Phanerozoic sandstones in relation to quartz content. If the epiclastic facies was de- to C. Busby), Italian National Council of Research, tectonic setting: Geological Society of America Bul- rived from the erosion of underlying units, letin, v. 94, p. 222±235. Ministero dell'UniversitaÁ e della Ricerca Scienti®ca Erskine, D.W., and Smith, G.A., 1993, Compositional char- then one might expect it to be more enriched (M.U.R.S.T.), and a N.A.T.O.-C.N.R. grant (to S. acterization of volcanic products from a primarily sed- in quartz than the underlying units as a func- Critelli). imentary record: The Oligocene Espinaso Formation, tion of weathering and transport; however, the north-central New Mexico: Geological Society of America Bulletin, v. 105, p. 1214±1222. opposite is true. 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MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 4, 2000 REVISED MANUSCRIPT RECEIVED SEPTEMBER 28, 2001 Klaus, A., Taylor, B., Moore, G.F., Mackay, M.E., Brown, Moore, J.G., 1979, Vesicularity and CO in mid-ocean ridge 2 MANUSCRIPT ACCEPTED OCTOBER 31, 2001 G.R., Okamura, Y., and Murakami, F., 1992, Structural basalt: Nature, v. 282, p. 250±253. and stratigraphic evolution of Sumisu Rift, Izu-Bonin Moore, J.G., and Schilling, J.-G., 1973, Vesicles, water, and Printed in the USA

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