Provenance of Plutonic Detritus in Cover Sandstones of Nicoya Complex, Costa Rica: Cretaceous Unroo®Ng History of a Mesozoic Ophiolite Sequence

Provenance of plutonic detritus in cover sandstones of Nicoya Complex, Costa Rica: Cretaceous unroo®ng history of a Mesozoic ophiolite sequence

Claudio Calvo² Anna-Peters-Strasse 51/C, 70597 Stuttgart, Germany

ABSTRACT the Campanian at ca. 75 Ma, ϳ9 m.y. after consisting of basaltic fragments and differen- the intrusive magmatic activity on Nicoya tiated pyroclastic materials (Calvo and Bolz, This study presents new petrologic and Peninsula, and is consistent with the onset 1994). Similarly, Cretaceous cover sandstones sedimentologic data from northwestern Cos- of the Laramide orogeny. of the Nicoya Complex contain a wide spec- ta Rica concerning the provenance of Cre- trum of framework grains from other sources: taceous forearc sandstones that contain plu- Keywords: provenance, modal analysis, radiolarites (including radiolarian cherts), pe- tonic detritus. Plutonic rock fragments are plagiogranites, ophiolite complex, Costa lagic limestones, shallow-marine carbonates, important accessory particles in pyroxene- Rica. andesitic lavas, differentiated ejecta, and ig- bearing sandstones overlying the ophiolite neous intrusive rocks (Calvo, 1998). This named the Nicoya Complex. Through the INTRODUCTION spectrum of grain types suggests that these use of modal analysis of the framework forearc sandstones were derived predominant- grains, I studied three sandstone suites of The ophiolitic basement of northwestern ly from shallow levels of an intraoceanic arc, the El Viejo and Rivas Formations that in- Costa Rica and its sedimentary cover strata but it also re¯ects a signi®cant contribution clude both shallow- and deep-water depos- constitute the most thoroughly studied on-land from uplifted ophiolitic basement areas partly its, ranging from late Campanian to Maas- rock assemblage of the southern Central fringed by shallow-water carbonate deposits trichtian in age. In terms of primary American forearc. This study focuses on the (Calvo, 1998). The plutonic detritus particu- framework components, the sandstones re- origin and provenance of plutonic detritus in larly records an ophiolitic provenance and semble those derived from magmatic arcs. both deep-sea channel and shallow-water bas- deeper unroo®ng of the ophiolitic basement Two modal parameters are introduced to al sandstones from the Cretaceous cover units than previous workers have suggested. evaluate detrital plutonic contributions and overlying the ophiolite named the Nicoya The main objectives of this study are (1) to af®nity of source rocks: the ratio of pluton- Complex. In these forearc sandstones, detrital analyze mineral composition and textural fea- ic to total lithic fragments [(Lp ؉ iQF)/Lt], grains of igneous plutonic origin are important tures of plutonic detritus in order to determine and the ratio of uralitized pyroxene to total accessory constituents, making up as much as likely source rocks and provenance relation- pyroxene grains (uralPx/Px). Modal values 9% of total lithic framework grains. ships; (2) to establish detrital plutonic contri- for (Lp ؉ iQF)/Lt indicate that plutonic Previous studies on petrology and prove- butions and af®nity of source rocks by using fragments comprise up to 9% of total lithic nance of forearc sandstones from northwestern modal analysis of secondary framework pa- fragments. A strong correlation between Costa Rica have suggestedÐon the basis of rameters; and (3) to integrate these new data these two parameters suggests that uraliti- primary framework modesÐthat Cretaceous into a model for the Cretaceous unroo®ng his- zed pyroxene grains were also derived from sandstones were derived from a magmatic arc tory of the ophiolitic sequence of southern intrusive rocks of probably basic and inter- (Lundberg, 1991; Calvo, 1998). Composition- Central America. mediate compositions. In particular, signif- ally, both those sandstones within the Nicoya icant concentrations of lithic fragments ex- ophiolite complex and many of the uncon- TECTONIC SETTING hibiting micrographic textures and formably overlying sandstones are commonly uralitized pyroxene grains are interpreted referred to as basaltic sandstones, derived Southern Central America (Costa Rica and to have been derived predominantly from from erosion of basaltic basement (e.g., Kuij- Panama) represents an island arc that formed eroded plagiogranites. Sandstone suites pers, 1979; Baumgartner et al., 1984; Gursky, over an intraoceanic subduction zone situated containing plutonic detritus signal an un- 1989; Lundberg, 1991). Although basaltic at the western margin of the Caribbean plate roo®ng of deeper levels of the Mesozoic sandstones dominate in some sequences, there (e.g., Kuijpers, 1979; Lundberg, 1982; Wild- ophiolitic sequence as a consequence of are also volcaniclastic sandstones that contain berg, 1984; Calvo and Bolz, 1994). At the strong uplift of the Costa Rican arc in late abundant nonbasaltic framework components. present time, the Cocos plate is being sub- Senonian time. This tectonic event began in For example, volcaniclastic wackes from the ducted beneath Costa Rica at 90 mm/yr (Min- Loma Chumico Formation of the upper Ni- ster and Jordan, 1978). Bathymetric swath ²E-mail: [email protected]. coya Complex exhibit a bimodal composition mapping of the convergent margin offshore

GSA Bulletin; July 2003; v. 115; no. 7; p. 832±844; 10 ®gures; 3 tables; Data Respository item 2003095.

For permission to copy, contact [email protected] 832 ᭧ 2003 Geological Society of America CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

Costa Rica shows an oceanic plate covered stones, and minor conglomerates and breccias the modal analysis, because of their possible with numerous seamounts (von Huene et al., (Dengo, 1962; Protti, 1981; Lundberg, 1982; intrabasinal origin and generally minor im- 1995). The most prominent feature is the Co- Rivier, 1983; Baumgartner et al., 1984; Calvo, portance for constraining ophiolitic prove- cos Ridge subducting opposite the Osa Pen- 1998). This unit overlies conformably hemi- nance. Sandstone suites derived from different insula (Fig. 1). pelagic limestones and volcaniclastic rocks of depositional environments were also analyzed The study area is located in northwestern the Sabana Grande Formation of Campanian to evaluate dispersal patterns of detrital sedi- Costa Rica. This region is characterized by age. ment in the marine forearc basin. three major morphotectonic elements that re- In this study, the timing of unroo®ng of the ¯ect an evolved arc edi®ce (Fig. 1): (1) the MATERIALS AND METHODS ophiolitic sequence was established from the emerged outer arc, forming the Nicoya and relative ages of foraminiferal assemblages Santa Elena Peninsulas; (2) the inner forearc Detritus derived from igneous intrusive identi®ed in sandstone samples. Radiometric trough (Tempisque basin), comprising the rocks in Cretaceous cover sandstones was ®rst age determinations of intrusive rocks from the Gulf of Nicoya and lowlands of the Tem- detected during analysis of main framework Nicoya Peninsula by Sinton et al. (1997), cou- pisque River; and (3) the active volcanic cor- grains (Calvo, 1998). On the basis of deposi- pled with the biostratigraphic data presented dilleras of Guanacaste and TilaraÂn, represent- tional environment and age, 20 sandstone here, constrain the age of crystallization and ing the inner magmatic arc. Behind the samples were selected for this study. They earliest erosion of Nicoya Complex intrusive cordilleras are the Guatusos lowlands, repre- were collected from three different locations rocks in northwestern Costa Rica. senting the southern terminus of the exten- in Cretaceous sandstone suites of the inner sional Nicaraguan Depression. forearc: La Tigra, Quebrada Pilas, and Calle RESULTS Codornices (Fig. 1). The sampled units form STRATIGRAPHIC FRAMEWORK part of the sedimentary cover of Nicoya Com- Detailed petrographic examination of plex, recording shallow- and deep-water clas- framework grains reveals that Cretaceous The forearc basement is exposed along the tic sedimentation in the forearc region of forearc sandstones from northwestern Costa Paci®c coast of Costa Rica and comprises a northwestern Costa Rica during the late Sen- Rica contain abundant ophiolitic grains, rang- Mesozoic ophiolitic sequence, the Nicoya onian. Their principal stratigraphic and sedi- ing from basic volcanic fragments and igneous Complex (Dengo, 1962). It is principally com- mentologic characteristics are summarized in intrusive-derived detritus to cherty rock frag- posed of peridotites, basalts (massive and pil- Table 1. Sandstone ages were determined on ments of sedimentary origin (Fig. 2). This lowed ¯ows), basaltic breccias, dolerites, ra- the basis of their contained foraminiferal as- study focuses on plutonic rock fragments and diolarites, limestones, volcaniclastic rocks, semblages. Samples were examined petro- radiolarian chert grains, which are thought to and igneous intrusive rocks (plagiogranites graphically in thin section, cut normal to bed- represent useful distinctive provenance indi- and gabbros). Differentiated pyroclastic de- ding. For analysis of sandstone frameworks, cators re¯ecting unroo®ng of deeper levels of posits of Albian to Campanian age within the an average of 500 grain points were counted the ophiolitic sequence. Petrographically, an- Nicoya Complex record the earliest volcanic in each section. Signi®cant concentrations of alyzed sandstones comprise dominantly lithic activity of the Costa Rican arc orogen (Calvo plutonic detritus in these sandstones make and feldspatholithic wackes, litharenites, and and Bolz, 1994; Calvo, 1998). Overlying the possible the modal analysis of detrital plutonic hybrid arenites as de®ned by Zuffa (1980), in- Nicoya Complex is the sedimentary cover that contributions using the Gazzi-Dickinson cluding arenites bearing Sulcoperculina. includes both shallow- and deep-marine strata point-counting method (Gazzi, 1966; Dickin- ranging from Upper Cretaceous through Pli- son, 1970). Point-count results are listed in Ta- Plutonic Grains ocene age. The forearc rock succession is part- ble DR1.1 Plutonic lithic fragments and ma®c ly covered by Quaternary volcanic rocks mineral grains were counted as separated Detrital grains of plutonic origin comprise erupted from stratovolcanoes of the north- grains. In order to establish plutonic contri- a diverse assortment of grain types, including western cordilleras (Fig. 1). Seismic re¯ection butions, fragments of plutonic rock were in- both lithic fragments and monocrystalline data from the Paci®c margin offshore Costa cluded in tabulations of total lithic grains. Be- grains. Figures 2A through 2D show photo- Rica suggest the seaward continuation of the cause of the scarcity of total counts of ma®c micrographs of representative plutonic frame- Nicoya Complex to the middle slope (Hinz et mineral grains, additional 100 point counts for work grains found in Cretaceous cover sand- al., 1996). total pyroxene grains were necessary to im- stones of the Nicoya Complex. A unique grain The cover sandstone suites studied are strat- prove the statistical reliability of the values for type consists of crystalline lithic grains that igraphically grouped into the El Viejo and Ri- the uralitized pyroxene ratio. Petrologically, exhibit micrographic textures distinctive of ig- vas Formations (Table 1). The El Viejo For- the general correspondence of ratios calculat- neous intrusive rocks (TroÈger, 1967). These mation (late Campanian±Maastrichtian) is a ed here is considered to be signi®cant because micrographic grains are composed of plagio- shallow-marine clastic and related carbonate it re¯ects compositional tendencies observed clase feldspar (albite) and quartz intergrowths reef unit a few meters thick consisting of rud- on the basis of the detailed optical analysis of (Figs. 2A, 2C). Because of the instability of istid framestones, bioclastic grainstones, and framework grains, made prior to the point feldspars in the sedimentary environment rel- sandstones that rest unconformably on rocks counts. Following Dickinson's (1970) conven- ative to quartz, micrographic intergrowths of the Nicoya Complex (Schmidt-Ef®ng, tion, carbonate fragments were not included in generally exhibit differential weathering: In 1975; Ulloa, 1977; Seyfried and Sprechmann, plane-polarized light, albite is seen to have 1985; Calvo, 1987). The Rivas Formation 1GSA Data Repository item 2003095, sedimen- been altered to clay minerals, whereas quartz tary petrography of forearc sandstones from north- (late Campanian±Paleocene) comprises a tur- western Costa Rica, is available on the Web at http: remained essentially unaltered. Other rock biditic slope sequence ϳ1500 m thick com- //www.geosociety.org/pubs/ft2003.htm. Requests fragments include polycrystalline quartz posed of volcaniclastic sandstones, mud- may also be sent to [email protected]. grains and crystalline lithic fragments. Poly-

Geological Society of America Bulletin, July 2003 833 C. CALVO

834 Geological Society of America Bulletin, July 2003 CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

TABLE 1. STRATIGRAPHIC AND SEDIMENTOLOGIC CHARACTERISTICS OF THREE CRETACEOUS FOREARC-SANDSTONE SUITES FROM NORTHWESTERN COSTA RICA

Sandstone suite La Tigra Quebrada Pilas Calle Codornices Formation Rivas Rivas El Viejo Age Late Campanian to early Maastrichtian Maastrichtian² Late Campanian to early Maastrichtian Foraminiferal assemblage Sulcoperculina dickersoni (Palmer); Gansserina gansseri (Bolli); Orbitocyclina Sulcoperculina globosa de Cizancourt; Sulcoperculina globosa de Cizancourt; minima (H. DouvilleÂ); Sulcoperculina sp. Sulcoperculina sp.; Pseudorbitoides cf. Pseudorbitoides sp.; Sulcorbitoides pardoi israelsky Vaughan & Cole BroÈnnimann Thickness ϳ150 m ϳ30 m ϳ5m Type of facies Channel sandstones Channel sandstones Basal sandstones Depositional environment Inner forearc±trough (Tempisque Basin): Island-arc platform: Lowstand submarine slope Arc platform±related submarine slope Transgressive bioclastic shoals ²Age also supported by paleontologic determinations of macrofauna (Fischer and Aguilar, 1994)

crystalline quartz grains resemble those de- cherts, are typical framework grains found DR1. In order to establish detrital plutonic rived from granitoid source rocks, because with plutonic detritus in studied Cretaceous contributions and af®nity of source rocks, two they are composed of relatively few crystals forearc sandstones (Fig. 2). In thin section, ra- new modal parameters are introduced. They of equant shape and have few or no intracrys- diolarite grains exhibit a dark rusty red to are de®ned as the ratio of plutonic to total lith- talline sutured contacts. Crystalline lithic frag- brown color and bear silica-®lled blebs that ic fragments [(Lp ϩ iQF)/Lt] and the ratio of ments containing polysynthetically twinned are likely microfossil (radiolaria) remains uralitized pyroxene to total pyroxene grains plagioclase and uralitized clinopyroxene are (Fig. 2E). Radiolarian chert grains are poly- (uralPx/Px). Uralitization, the alteration of cli- also present (Fig. 2B). Some of them display crystalline quartz fragments, commonly ex- nopyroxene to amphibole, is characteristic of subophitic textures. Monocrystalline grains hibiting hematitic clusters and vestiges of ra- plutonic terranes (TroÈger, 1967). Modal values include plagioclase, quartz, and pyroxene. diolaria (Fig. 2F). Such grains are of particular for (Lp ϩ iQF)/Lt indicate that plutonic frag- Both plagioclase and quartz grains are, how- interest for provenance of cover sandstones, ments average up to 9% of total lithic frag- ever, dif®cult to interpret, because in many because they probably result from erosion of ments. Values between 4% and 9% are only cases they do not exhibit diagnostic petro- ribbon cherts within the Nicoya Complex that found in sandstones of the Calle Codornices graphic features of a plutonic or volcanic or- are interpreted to form part of a remnant ac- and La Tigra suites (Table 3). Modal content igin. The most important ma®c mineral grain cretionary unit (Baumgartner, 1990), i.e., the of plutonic materials calculated with this is pyroxene, which often appears partly ural- Punta Conchal Formation (Gursky and method represents only a minimum contribu- itized (Fig. 2B). Schmidt-Ef®ng, 1983). tion, because (Lp ϩ iQF)/Lt provides data The predominance of coarse-grained plu- only on the contribution of lithic fragments. tonic fragments showing little evidence of Modal Analysis of Plutonic Detritus Quartz and feldspar grains were not included chemical weathering, in conjunction with an- in tabulations of plutonic grains, because their gular grain textures exhibiting remains of in- In terms of primary framework grains provenance remains in many cases unclear. tercrystalline boundaries (Fig. 2A), indicates (QFL, Qp-Lvm-Lsm), Cretaceous sandstones Because many monocrystalline grains are in- that intrusive rocks in the source area were from northwestern Costa Rica containing plu- dependent of lithic fragments, but some were broken mechanically along intercrystalline tonic detritus have a composition similar to surely derived from plutonic rocks, it is not boundaries and that detrital grain size is, in those derived from magmatic arcs (Fig. 3; possible to calculate the true total contribu- part, controlled by original textures of coarse- Calvo, 1998). Ophiolitic and especially plu- tions of detritus from plutonic rocks. grained source rocks. Although tropical tonic provenances are commonly obscured in weathering conditions prevailed during denu- these sandstones by abundant volcanic frag- Ternary Diagrams dation, as suggested by alterite grains identi- ments, but both sources are recognizable in The ternary diagrams of Figure 4 display ®ed in samples analyzed, the spectrum of plu- channel sandstones and transgressive basal the detrital compositions of sandstones in tonic detritus apparently re¯ects original sandstones from the slope and neritic sequenc- terms of their populations of lithic fragments compositions of igneous source rocks, an in- es, respectively, selected for this analysis. and ma®c mineral grains. The [(Lp ϩ iQF)/ terpretation that implies signi®cant relief in This study focused on the semiquantitative Lt]±Lvm±(rC ϩ Lsm) diagram shows modal the source area. modal analysis of detrital parameters sensitive values of plutonic, volcanic, and sedimentary to plutonic provenance, de®ned in Table 2. rock fragments. Radiolarian chert grains are Cherty Rock Fragments Recalculated modal values are summarized in included in tabulations of lithic grains of sed- Table 3 and presented graphically in Figures imentary origin. This diagram demonstrates Siliceous rock fragments of sedimentary or- 4 and 5. Original point data for primary and that the lithic grain fraction in all analyzed igin, including radiolarian mudstones and secondary parameters are available in Table samples is strongly dominated by volcanic

Figure 1. Geologic map of northwestern Costa Rica showing locations of studied Cretaceous forearc sandstone suites: La Tigra, Quebrada Pilas, and Calle Codornices. Place names discussed in text are also included. From Calvo and Bolz (1994). Inset map shows the regional tectonic framework of the study area (MPFÐMotagua-Polochic Fault; MATÐMiddle America Trench; NDÐNicaraguan Depression; PFZÐPanama Fracture Zone). N

Geological Society of America Bulletin, July 2003 835 C. CALVO

Figure 2. Photomicrographs of ophiolitic detritus found in Cretaceous forearc sandstones from northwestern Costa Rica, including (A± D) plutonic fragments and (E and F) cherty rock fragments. (A) Lithic fragment composed of monocrystalline quartz (Qm) and micrographic intergrowth of quartz and albite (iQF); albite at extinction. Irregular grain outline (right) is interpreted to be a relict intercrystalline contact. Calle Codornices sandstone, cross-polarized light. (B) Uralitized pyroxene grain (urPx) showing cores of unaltered pyroxene (Px). Calle Codornices sandstone, plane-polarized light. (C) Lithic fragment composed of altered plagioclase (P) and intergrowth of quartz and plagioclase exhibiting micrographic texture (iQF); quartz at extinction. Quebrada Pilas arenite, cross-polarized light. (D) Outsized feldspar grain (F) of probably plutonic origin showing differential weathering and thin coatings of hematite. It is composed of fresh albite and altered anorthite-rich zones, replaced by calcite and sericite minerals. Calle Codornices sandstone, plane- polarized light. (E) Radiolarite fragment (Rf), bearing microfossil remains (radiolaria) ®lled with silica. La Tigra sandstone, cross- polarized light. (F) Radiolarian chert grains (rC) showing vestiges of radiolarian microfossils and hematitic clusters; tests of Sulcoper- culina sp. (S), a late Senonian larger foraminifera species, also occur. Calle Codornices sandstone, plane-polarized light.

836 Geological Society of America Bulletin, July 2003 CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

to a magmatic arc source (Calvo, 1998). On the QFL diagram of Figure 3, in particular, such a compositional trend is expressed by a mean plot within the sub®eld for dissected-arc provenance. In contrast, identi®able plutonic detritus in these sandstones occurs only in trace amounts. The Bt-Hbl-Px diagram of Figure 4 dis- plays modal values of the ma®c mineral grain population. Two main types of sandstones can be distinguished: pyroxene- and pyroxene- hornblende±bearing sandstones. Most of these samples contain clinopyroxene and green hornblende. Arenites from La Tigra are predominantly pyroxene-rich, whereas in sandstones from Quebrada Pilas, hornblende dom- Figure 3. QFL and Qp-Lvm-Lsm plots of mean framework values and their standard inates. The Sulcoperculina-bearing sandstones deviations for analyzed Cretaceous sandstone suites from northwestern Costa Rica con- from Quebrada Pilas are extraordinarily rich taining plutonic detritus. Provenance ®elds from Dickinson and Suczek (1979) and Dick- in hornblende, which accounts for 87% to inson (1985). Mean values and standard deviations are represented by distinctive symbols 93% of the ma®c mineral grain fraction. One and polygons, respectively. Magmatic arc provenance ®eld on diagram QFL is divided volcaniclastic sandstone sample from La Tigra into dissected arc (P Ͼ V) and undissected arc segments (P Ͻ V) (PÐplutonic detritus, contains a similar high abundance of horn- VÐvolcanic detritus); nÐnumber of samples. Because plutonic fragments are accessory blende. Arenites from Calle Codornices con- constituents in cover sandstones, plutonic contributions cannot be re¯ected directly by the tain exclusively clinopyroxene, partly urali- primary framework parameters on both diagrams. Note that the QFL plot for the Que- tized, and are quite similar to most of La Tigra brada Pilas sandstone suite within the P Ͼ V sub®eld, in particular, is caused by strong sandstones. The enrichment of detrital pyrox- contents of radiolarian chert fragments in this suite (Table DR1). This origin is demon- ene grains is strongly related to signi®cant strated by a second plot (depicted by a dashed polygon) where these rock fragments are contents of plutonic fragments within the lith- added to the lithic grain populations L and Lsm. The detailed petrographic examination ic grain fraction (Fig. 4). Biotite grains are reveals that the Quebrada Pilas sandstones have relatively low contents of identi®able rare in most sandstones. Only a few samples plutonic detritus. This example shows that the use of secondary parameters is indispens- from Quebrada Pilas and one sample from able to determine the real provenance relationships in cover sandstones. In general, all Calle Codornices contain trace amounts of bi- three cover sandstone suites record volcanic sources derived from the Cretaceous arc otite fragments. activity, in addition to an ophiolitic provenance derived from exposed plagiogranite bodies and radiolarian chert units of the Nicoya Complex. Plutonic Lithic Fragments vs. Pyroxene Grains The (Lp ϩ iQF)/Lt vs. uralPx/Px diagram TABLE 2. MODAL PARAMETERS AND CLASSIFICATION OF FRAMEWORK GRAINS shows that abundance of uralitized pyroxene Primary parameters (after Dickinson and Suczek, 1979) grains can be correlated with increasing Q ϭ total quartz grains amounts of plutonic rock fragments (Fig. 5). F ϭ total feldspar grains Lt ϭ total lithic fragments (including polycrystalline grains, Qp) In sandstone samples from Calle Codornices Lvm ϭ volcanic and metavolcanic rock fragments and La Tigra that contain Ͼ4% of identi®able Lsm ϭ sedimentary and metasedimentary rock fragments plutonic lithic fragments, these two detrital Secondary parameters sensitive to plutonic provenance (de®ned in this study) rC ϭ radiolarian chert grains parameters increase in parallel. This observa- Lp ϭ plutonic lithic fragments exhibiting granular and subophitic textures, composed of plagioclase, quartz, tion allows the following conclusions. First, it and pyroxene indicates that uralitized pyroxene grains also iQF ϭ grains showing micrographic intergrowth of quartz and plagioclase feldspar Hbl ϭ hornblende grains are derived from eroded intrusive rocks. Sec- Px ϭ total pyroxene grains ond, because both uralitized pyroxene and in- Bt ϭ biotite grains Ratios: tergrowths of albite and quartz are character- uralPx/Px, where uralPx ϭ uralitized pyroxene grains istic mineral phases and textures of (Lp ϩ iQF)/Lt, where (Lp ϩ iQF) ϭ total lithic grains demonstrably of plutonic origin plagiogranites (Wildberg, 1984), the strong correlation between these parameters records a plutonic provenance, probably resulting fragments. Sandstone suites of the Rivas For- characterized by extraordinarily high values of from erosion of plagiogranitic bodies. It also mation display, however, divergent trends. sedimentary lithic fragments and especially of documents the predominantly intermediate Sandstones from the La Tigra suite show sig- radiolarian chert grains. In some samples, composition of igneous source rocks (Wild- ni®cant values of plutonic and volcanic frag- these dominate the lithic grain fraction. The berg, 1987; Sinton et al., 1997). ments, very similar to those for Calle Codorn- predominance of radiolarian chert materials in This relationship can also be recognized in- ices arenites. Arenites from Quebrada Pilas this forearc suite is interpreted to suggest a directly, by comparing modal values of rock are a particular petrofacies, because they are subduction-complex provenance, in addition fragments with those of ma®c mineral grains

Geological Society of America Bulletin, July 2003 837 C. CALVO

(Table 3; Fig. 4). Note that signi®cant values of plutonic lithic fragments only occur in those sandstones that contain a ma®c mineral grain population strongly dominated by pyroxene grains. This statement is true for both La Tigra and Calle Codornices sandstone suites. In samples from Calle Codornices, plutonic contributions are, in part, documented by primary detrital modes (Table DR1). Some modal QmPK values fall in Dickinson's (1985) volcanoplutonic suite of circum-Paci®c sandstones (Calvo, 1998).

DISCUSSION

Source Rocks

The stratigraphic context and framework grain compositions of the analyzed sandstones indicate that the source of plutonic material likely lies within the underlying Nicoya Com- plex. Plutonic rocks occur in the lower part of the ophiolitic sequence, where they generally have intruded massive basaltic ¯ows (Ku- ijpers, 1979; Wildberg, 1987). The transgressive sandstones from Calle Codornices provide unequivocal sedimentologic evidence for this interpretation, because they were deposited directly on exposed basement. Thus, non- carbonate detritus reworked during the transgression was derived from basement rocks and/or from volcanic eruptions. The composition of framework grains that include radiolarian mudstones and cherts, pelagic limestones, and basalts records basement source lithologies that are all found in the ophiolitic sequence. Nonophiolitic grains are biogenic carbonate fragments and volcanic grains, principally andesitic grains and ejecta fragments including glass shards and fresh individual crystals. On the other hand, the composition of plutonic fragments clearly indicates that the Figure 4. Ternary diagrams displaying the lithic and ma®c mineral grain populations of dominant plutonic source was basic to inter- forearc sandstones from the El Viejo and Rivas Formations (upper Campanian± mediate in composition. Conglomerate depos- Maastrichtian). Mean values indicated by distinctive symbols; nÐnumber of samples. its within the La Tigra sequence containing gabbro and dolerite clasts support this interpretation. Plagiogranites in the Nicoya Com- vol% of the Nicoya Complex. Plagiogranites fragments in some sandstone samples requires plex constitute a likely speci®c source rock of are composed of plagioclase, quartz, pyrox- igneous source rocks to contain abundant mi- plutonic coarse-grained lithic fragments and ene, opaque minerals, apatite, and zircon. crographic intergrowth textures, rather than related pyroxene grains. Most of them display subhedral granular tex- only accessory amounts. Such sources are the tures. Graphic intergrowth of plagioclase and plagiogranites. Kuijpers (1979) called atten- Plagiogranitic Source Rocks quartz is a further typical feature (Figs. 6A, tion to the abundance of micrographic inter- According to Wildberg (1987), plagiogran- 6C). Secondary amphibole resulting from growths of albite and quartz in some plagio- ites on the Nicoya Peninsula occur within in- complete or partial uralitization of clinopyrox- granites of the Nicoya Complex (e.g., Playa trusions of isotropic gabbros and dolerites. ene is often present (Fig. 6C). Gabbros, dol- El Ocotal; Fig. 1). Petrographic examination They form dikes up to several meters in thick- erites, and diorites also contain accessory min- of these rocks con®rms a wide occurrence of ness and intrusive bodies of considerable size eral phases of uralitized pyroxene and micrographic textures with intergrowths of up with exposures several hundred meters across. micrographic intergrowths (Kussmaul, 1980) to 10 mm across, displaying differential On the basis of the extent of outcrops, plagio- (Fig. 6D). However, the detrital abundance of weathering recognizable in plane-polarized granites probably constitute no more than 5 micrographic grains of up to 9% of total lithic light (Fig. 6A).

838 Geological Society of America Bulletin, July 2003 CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

entiated intrusive rocks displaying a mature island-arc af®nity, previously detected in this region by Wildberg (1984).

Provenance Areas

The geographic distribution of the sandstone suites records very different provenance areas of plutonic detritus in northwestern Cos- ta Rica. On the other hand, the common occurrence of angular, coarse sand-sized plutonic particles requires limited transport and deposition near the source area. The sandstone Figure 5. Correspondence of proportion of lithic fragments that are plutonic and propor- suite from La Tigra, located in the eastern Ni- tion of clinopyroxene grains that are uralitized in pyroxene-bearing sandstones from the coya Peninsula, illustrates the relationship be- El Viejo and Rivas Formations (upper Campanian±Maastrichtian); nÐnumber of tween the source area and site of deposition, samples. because this region exposes several intrusive bodies. In the Quebrada Cuajiniquil, near the locality of La Tigra where the clastic sequence TABLE 3. RECALCULATED MODAL VALUES OF DETRITAL PARAMETERS FOR 20 FOREARC is exposed, Protti (1981) found a plagiogran- SANDSTONES OF EL VIEJO AND RIVAS FORMATIONS (UPPER CRETACEOUS) FROM NORTHWESTERN COSTA RICA itic sill. This rock obviously represents a potential source of plutonic detritus present in ² ²³ ²§ Lithic fragments Ma®c mineral grains Ratios the Cretaceous turbidite sandstones. In this Sample Lvm Lp ϩ rC ϩ Lsm Px Hbl Bt (Lp ϩ uralPx/Px case, the distance from source rock to site of iQF iQF)/Lt deposition is only a few kilometers (Fig. 1). Calle Codornices sandstones Similarly, sandstones from Calle Codornices BB-14/1 81 6 13 96 2 2 5.5 9 and Quebrada Pilas record source areas situ- BB-14/2 81 5 14 100 0 0 4.4 3 BB-14/4 80 5 15 100 0 0 5.0 5 ated on the northeastern margin of the Gulf of BB-1414a 86 6 8 100 0 0 5.1 12 Nicoya (Fig. 1). The intrusive rocks of Cerro BB-14/7 78 9 13 100 0 0 9.0 25 La Tigra sandstones Barbudal, located 2.5 and 10 km from the out- LT-1/1 86 8 6 100 0 0 8.2 22 crops of Quebrada Pilas and Calle Codornices, LT-1/4 87 5 8 97 0 3 5.6 15 respectively, are considered to be possible LT-1/5 80 6 14 100 0 0 6.0 8 sources. These intrusive rocks contain acces- LT-2/2 94 1 5 21 79 0 0.9 0 LT-2/6 83 3 14 100 0 0 3.4 0 sory mineral phases including uralitized cli- LT-4/1 83 Ͻ0.5 17 67 33 0 0.3 0 nopyroxene as well as myrmekitic and micro- LT-4/1a 89 1 10 100 0 0 0.6 0 Quebrada Pilas sandstones graphic intergrowths of plagioclase and quartz BB-4/1b 43 3 54 33 62 5 2.2 0 (Calvo, 1998). Trace amounts of plutonic lith- BB-4/2b 49 2 49 53 47 0 0.9 0 ic fragments are also present in Maastrichtian BB-4/5b 56 0 44 33 67 0 0 0 bioclastic limestones within the rudistid reef BB-4/1a 49 Ͻ0.5 51 40 57 3 0.3 0 BB-4/2a 42 Ͻ0.5 58 63 37 0 0.5 0 facies of Cerro Barbudal (Fig. 1). The lime- BB-4/5a 47 1 52 31 54 15 1.2 0 stones rest unconformably on eroded basaltic BB-4/6b 70 0 30 5 93 2 0 0 BB-4/6a 56 2 42 13 87 0 2.0 0 ¯ows laterally penetrated by the intrusions. ²Values in percent plotted on diagrams of Figure 4 and Figure 5. ³Includes independent monomineralic grains only. Detrital Dispersal Patterns §Values for uralPx/Px based on additional 100 point counts for total pyroxene. Sandstone suites from northwestern Costa Rica record the dispersion of plutonic rock Only very minor amounts of both potassi- Geochemical studies of potential plagio- fragments into both shallow- and deep-water um feldspar and biotite grains in analyzed granitic sources (Wildberg, 1987; Sinton et al., environments during Late Cretaceous time. sandstones argue against granitic source rocks. 1997) suggest that plutonic rock fragments in However, sandstones deposited in the two dif- This inference is consistent with the fact that cover sandstones and megabreccia deposits on ferent environments exhibit very divergent no older rocks of continental af®nity have Santa Elena Peninsula (Tournon and AzeÂma, dispersal patterns. The detrital dispersion ap- been identi®ed in southern Central America 1980) were derived from intrusive rocks of pears to have been controlled mainly by a (Lundberg, 1991). The predominance of so- both island arc± and oceanic plateau±related combination of location and areal extent of dium-rich plagioclase feldspar within micro- origin. Of particular interest is the occurrence exposed source rocks, sedimentary environ- graphic grains and the strong correlation be- of arc-derived plutonic grains containing po- ment, and relative sea-level changes. tween uralitized pyroxene grains and plutonic tassium feldspar in Pliocene sandstones from In the shallow-water clastic rocks (Calle lithic fragments (Fig. 5) also require a plutonic the Montezuma Formation of the southern Ni- Codornices sandstones) and related carbonate source rock of basic to intermediate compo- coya Peninsula (Lundberg, 1991) (Fig. 1). reef deposits of the El Viejo Formation, plu- sition, preferably a plagiogranitic source. They probably record the erosion of differ- tonic detritus commonly appears in lithofacies

Geological Society of America Bulletin, July 2003 839 C. CALVO

Figure 6. Photomicrographs of representative intrusive rocks from the Nicoya ophiolite complex. (A) Plagiogranite exhibiting characteristic granophyric texture. The radiating intergrowth of quartz and albite (iQF) is arranged about a euhedral plagioclase crystal (P); albite at extinction. Rock sample from Playa El Ocotal, cross-polarized light. (B) Dolerite with subhedral texture consisting mainly of plagioclase laths and clinopyroxene crystals. Rock sample from Cerro Barbudal, plane-polarized light. (C) Plagiogranite exhibiting graphic intergrowth of quartz and albite (at the right of the photograph), clinopyroxene (Px), partly uralitized (urPx), and plagioclase phenocryst (P). Rock sample from Playa El Ocotal, cross-polarized light. (D) Accessory micrographic intergrowth of plagioclase and quartz (iQF), albite at extinction, and zoned plagioclase crystals (P) in dolerite from Cerro Barbudal, cross-polarized light. at the base of the sequence. Signi®cant con- Transgressive systems tracts are deposited Quebrada Pilas of the Rivas Formation (Table centrations are found in clastic deposits that during rapid rises in relative sea level when 1) have an extensive vertical distribution of immediately overlie the basement rocks (Fig. little sediment is delivered to the shelf (Po- plutonic detritus, but in generally lower con- 7). Up section, the relative content of plutonic samentier and Vail, 1988). This scenario co- centrations. Identi®able plutonic fragments detritus rapidly decreases in the reef frame- incides with the accumulation of basement- commonly appear in levels of these sequences work and in grainstone facies overlying basal derived detritus at the base of the Calle that contain medium to coarse-grained channel strata. In terms of sequence stratigraphy, these Codornices sequence, proximal to the source sandstones (Fig. 7). Texturally, they comprise facies represent deposits of a transgressive area, indicating moderate reworking during well-sorted, graded and massive sandstones systems tract. Moreover, it is clear from the the transgression. These observations suggest with grain support and without or little matrix presence of both plutonic detritus and weath- that plutonic detritus was deposited in shallow- contents, showing erosional basal contacts. ered basaltic rocks that the unconformity at water environments during rapid marine in- These sedimentologic features indicate that the base of the Calle Codornices transgressive cursions on exposed and previously emergent plutonic detritus in this case was redistributed sequence represents a subaerial erosional sur- source terranes of the Nicoya Complex. into the deep-water environment by turbidity face, a type 1 unconformity following the In comparison with the shallow-water se- currents and/or sandy debris ¯ows. Typical classi®cation of Posamentier and Vail (1988). quence, the slope sequences of La Tigra and ®ning-upward trends in both sequences sug-

840 Geological Society of America Bulletin, July 2003 CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA gest, moreover, a grain transport via submarine channels, probably during periods of relative sea-level lowstands.

Unroo®ng History of the Ophiolite Sequence and Tectonic Implications

Basaltic sandstones directly overlying the Nicoya Complex were derived from eroded basaltic rocks (massive and pillowed ¯ows). Cover sandstones bearing plutonic detritus record the erosion of deeper levels of the ophiolitic sequence. Biostratigraphic age determinations of sandstones based on their foraminiferal assemblages indicate that the earliest unroo®ng of these levels took place during late Campanian and Maastrichtian time (Fig. 8). This ®nding implies that intrusive Figure 7. Model suggested here for the dispersion of plutonic detritus in the marine en- bodies of the Nicoya Complex must have in- vironment during relative sea-level changes, based on detrital dispersal patterns observed truded before the late Campanian (ca. 75±71 in (A) the neritic sequence of Calle Codornices, and (B) the slope sequences of La Tigra Ma, Gradstein et al., 1994). In fact, radiomet- and Quebrada Pilas. Dispersion of plutonic detritus succeeded periods of signi®cant uplift ric ages of two intrusive rocks (a gabbro and and subaerial erosion that likely cut deep into the basement sequence to expose the plu- a plagiogranite) from the Nicoya Peninsula as tonic bodies (e.g., plagiogranites). UÐCampanian unconformity representing an erosion determined by the 40Ar-39Ar method indicate surface on exposed basement areas. See text for discussion. that intrusive magmatic activity occurred at ca. 84 Ma (Sinton et al., 1997). The unroo®ng of plutonic rocks probably required long periods of subaerial exposure and concomitant denudation of the forearc ophiolite complex during evolution of the Costa Rican orogen, as suggested by a marked Campanian unconformity. Relationships between crystallization and earliest erosion ages of Nicoya Complex intrusions suggest that unroo®ng had been ef- fectively accomplished ϳ9 m.y. after the ca. 84 Ma intrusive activity on northern Nicoya Peninsula (Fig. 8). On the other hand, geochemical studies of these plutonic rocks (e.g., Wildberg, 1987) suggest that plutonic detritus originated, in part, from intermediate arc- related intrusions. Campanian sandstones and tuff interbeds of the Sabana Grande Formation in southern Nicoya Peninsula (Lundberg, Figure 8. Comparing crystallization and earliest erosion ages of intrusive rocks from the 1982), as well as Albian to Campanian pyro- Nicoya ophiolite complex of northwestern Costa Rica. Radiometric age determinations of clastic rocks of the Loma Chumico Formation Nicoya Peninsula intrusive rocks from Sinton et al. (1997); the two rocks come from occurring within the Nicoya Complex (Calvo outcrops located in the northwestern part of the peninsula. Biostratigraphic ages of sand- and Bolz, 1994), indicate that the arc was ac- stones bearing plutonic detritus established on the basis of their foraminiferal assemblages. tive at the time of intrusive activity. Time scale and planktic foraminiferal zones from Gradstein et al. (1994) and Erba et al. Like late Senonian carbonate reefs and plat- (1995), respectively. See text for discussion. forms, which grew on both ma®c and ultra- ma®c substrates in northwestern Costa Rica (Ulloa, 1977; Seyfried and Sprechmann, 1985; most important Cretaceous unconformity in (Ͼ100 km/m.y.) of the Farallon plate with re- Calvo, 1987), plutonic and radiolarian chert southern Central America and one that marks spect to North America (Engebretson et al., detritus in cover sandstones points to strong the boundary between the ophiolitic basement 1985). uplift of the Costa Rican orogen, which in- rocks and their sedimentary cover strata. This Two volcanic rock units of note are: (1) the cluded the formation of emergent terranes of tectonic uplift is coeval with the onset of the vesicular basaltic lavas intercalated within the ophiolitic basement (Fig. 9). Therefore both Laramide orogeny in the Late Cretaceous. In upper Campanian carbonate slope sequence of grain types in these forearc sandstones can be terms of relative plate motions, the onset of BahõÂa Santa Elena, located in northern Santa considered as detrital paleotectonic indicators the Laramide orogeny at ca. 75 Ma coincides Elena Peninsula (Baumgartner et al., 1984), of the regional Campanian unconformityÐthe with the beginning of rapid convergence and (2) the basaltic lavas and breccias inti-

Geological Society of America Bulletin, July 2003 841 C. CALVO

plagiogranites implies signi®cant strong uplift and subsequent deep erosion of the ophiolite sequence. The Cretaceous megabreccia deposits containing plagiogranite boulders (Tournon and AzeÂma, 1980) suggest a similar pro®le of erosion on Santa Elena Peninsula. In contrast, framework grains derived from shallow levels of the Nicoya ophiolitic sequence principally include basaltic and tachylite grains (partly showing vesicular textures) as well as radiolarian mudstone, pelagic limestone, and tuff- aceous lithic fragments. In general, the spectrum of ophiolitic grains in cover sandstones points to an ophiolitic source-rock assemblage that does not differ from that of the exposed modern forearc basement (Fig. 10).

SUMMARY AND CONCLUSIONS

Detrital plutonic grains, previously overlooked in earlier studies, comprise accessory framework components of Cretaceous forearc sandstones in northwestern Costa Rica. Inte- grated petrographic, sedimentologic, stratigraphic, and ®eld evidence clearly indicate that plutonic detritus was derived from eroded intrusive rocks of the Nicoya ophiolite com- Figure 9. Cartoon depicting model of evolution for the Mesozoic ophiolite sequence of plex. Moreover, the plutonic detritus corre- Costa Rican arc orogen during the late Senonian, including (A) intrusive magmatic activ- sponds, compositionally and texturally, to ity, probably arc-related, in the Santonian±early Campanian (at ca. 84 Ma, Sinton et al., source rocks of basic and intermediate com- 1997), and (B) uplift and subsequent erosion in the late Campanian, resulting in unroo®ng positions. An important new petrologic result of ophiolitic intrusive bodies: dolerites, gabbros, and plagiogranites. Both episodes were is the parallel increase of plutonic lithic frag- accompanied by andesitic arc and basic forearc volcanism. In late Campanian and Maas- ments [(Lp ϩ iQF)/Lt] and uralitized pyrox- trichtian times, transgressive shallow-marine carbonate reef and platform deposits of the ene grains (uralPx/Px) identi®ed in the frame- El Viejo Formation prograded over exposed basement. (MATÐMiddle America Trench). work population of pyroxene-bearing arenites. This observation indicates that uralitized py- mately commingled with hemipelagic lime- reconstruction of a Cretaceous erosion pro®le roxene grains probably also resulted from stones of Maastrichtian age, exposed on the through the ophiolitic sequence (Fig. 10). On eroded intrusive rocks; the observation also western coast of Nicoya Peninsula, between the basis of the earliest assemblage of pseu- re¯ects more clearly a predominantly inter- Garza and Puerto Carrillo (Schmidt-Ef®ng, dorbitoidal foraminifera present in analyzed mediate composition of plutonic source rocks. 1979) (Fig. 1). These two volcanic units in- samples, the late Campanian (ca. 75 Ma) is In particular, signi®cant concentrations of lith- dicate that both uplift and unroo®ng of the assumed for this reconstruction. As can be de- ic fragments exhibiting micrographic textures ophiolitic sequence in the late Senonian were duced from framework grain compositions, and uralitized pyroxene grains are interpreted accompanied by ma®c forearc volcanic activ- erosion at that time affected both the upper to be predominantly derived from eroded pla- ity (Fig. 9). These processes together attest to and lower parts of the Nicoya ophiolite com- giogranitic intrusions. This study demon- coeval Cretaceous subduction and erosion, plex. Because intrusive rocks appear in the strates, moreover, that both newly introduced probably related to rapid convergence of the lower part of the Nicoya Complex, plutonic parametersÐi.e., (Lp ϩ iQF)/Lt and uralPx/ Farallon plate in southern Central America rock fragments in cover sandstones record the PxÐcan be used as semiquantitative modal also. In addition, the presence of alterite unroo®ng of the Lower Nicoya Complex in parameters to determine af®nity of source grains, indicative of tropical weathering en- northwestern Costa Rica since at least ca. 75 rocks and detrital plutonic contributions in vironments (Johnsson, 1990), in cover sand- Ma. Such unroo®ng is also well constrained sandstones derived from ophiolitic sequences. stones provides strong evidence for the resi- by the Maastrichtian cherty sandstones from Sedimentologically, dispersal patterns of dence of the Costa Rican arc in low latitudes the Quebrada Pilas suite, whose detrital modes detrital sediment suggest that plutonic frag- near the equator during Cretaceous time, as (Q23F50L27 and Qp41Lvm46Lsm13; Table DR2) ments, and all ophiolitic grains in general, constrained by paleomagnetic data (de Boer, plot close to Dickinson's dissected-arc and were deposited in shallow-water environments 1979; Sick, 1989; Frisch et al., 1992). subduction-complex provenance ®elds, re- during rapid marine transgressions on exposed spectively (Calvo, 1998) (Fig. 3). Considering basement areas. The detritus was apparently Cretaceous Erosion Pro®le the minimum depth of formation of 4 km ob- distributed into the slope and deep-water en- Provenance relationships recorded by detri- served for MORB-related intrusions (S. Foley, vironments by turbidity currents and sandy de- tal compositions of cover sandstones allow the 2002, personal commun.), the unroo®ng of bris ¯ows, principally during relative sea-level

842 Geological Society of America Bulletin, July 2003 CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

acaste, Costa Rica: San JoseÂ, Costa Rica, Instituto GeograÂ®co Nacional de Costa Rica, 112 p. Dickinson, W.R., 1970, Interpreting detrital modes of gray- wacke and arkose: Journal of Sedimentary Petrology, v. 40, p. 695±707. Dickinson, W.R., 1985, Interpreting provenance relations from detrital modes of sandstones, in Zuffa, G.G., ed., Provenance of arenites: Dordrecht, Netherlands, D. Reidel Publishing Company, p. 333±361. Dickinson, W.R., and Suczek, C.A., 1979, Plate tectonics and sandstones compositions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2164±2182. Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Rel- ative motions between oceanic and continental plates in the Paci®c Basin: Geological Society of America Special Paper 206, p. 1±59. Erba, E., Premoli Silva, I., and Watkins, D.K., 1995, Cre- taceous calcareous plankton biostratigraphy of Sites 872 through 879, in Haggerly, J.A., et al., Proceedings of the Ocean Drilling Program, Scienti®c results: Col- lege Station, Texas, Ocean Drilling Program, v. 144, p. 157±169. Fischer, R., and Aguilar, T., 1994, Paleontology of an evolv- ing island arc: Pro®l, v. 7, p. 391±400. Frisch, W., Meschede, M., and Sick, M., 1992, Origin of the Central American ophiolites: Evidence from pa- Figure 10. Cartoon showing a hypothetical pro®le of erosion through the ophiolitic se- leomagnetic results: Geological Society of America quence of Nicoya Complex in the Late Cretaceous (at ca. 75 Ma), deduced from framework Bulletin, v. 104, p. 1301±1314. Gazzi, P., 1966, Le arenarie del ¯ysch sopracretaceo compositions and biostratigraphic ages of forearc sandstones of the Rivas and El Viejo dell'Apennino modenseÐCorrelazioni con il ¯ysch Formations (upper Campanian±Maastrichtian). This ophiolitic source-rock assemblage is Monghidoro: Minerallogica et Petrographica Acta, v. 16, p. 69±97. very similar to that currently exposed in the forearc region of northwestern Costa Rica. Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., The erosion unroofed deeper levels of the sequence, where intrusions occur. The erosional van Veen, P., Thierry, J., and Huang, Z., 1994, A Me- surface corresponds with the Campanian unconformity. No scale implied. sozoic time scale: Journal of Geophysical Research, v. 99, p. 24.051±24.074. Gursky, H.-J., 1989, Presencia y origen de rocas sedimen- tarias en el basamento o®olõÂtico de Costa Rica: Re- vista GeoloÂgica de AmeÂrica Central, v. 10, p. 19±66. lowstands. Provenance areas were apparently ACKNOWLEDGMENTS Gursky, H.-J., and Schmidt-Ef®ng, R., 1983, Sedimentol- located on Nicoya Peninsula as well as in the ogy of radiolarites within the Nicoya ophiolite com- I thank A. Bolz for determination of larger fo- plex, Costa Rica, Central America: Amsterdam, El- area of Cerro Barbudal, north of the Gulf of raminifera in thin section and S. Kussmaul for pro- sevier, v. 36, p. 127±142. Nicoya. viding representative samples of the plagiogranites Hinz, K., von Huene, R., and Ranero, C.R., 1996, Tectonic structure of the convergent Paci®c margin offshore from Playa El Ocotal. M. Meschede and S. Foley Tectonically, Cretaceous sandstone suites Costa Rica from multichannel seismic re¯ection data: containing plutonic detritus provide evidence provided additional data on plutonic rocks. Reviews Tectonics, v. 15, p. 54±66. by J. Mezger, A. Bolz, and H.-J. Gursky improved for the unroo®ng of deeper levels of the Mes- Johnsson, M.J., 1990, Overlooked sedimentary particles the early manuscript version. I am also indebted to from tropical weathering environments: Geology, v. 8, ozoic ophiolitic sequence of Costa Rica in late R.J. Dorsey, N. Lundberg, and G.H. Girty for help- p. 107±110. Senonian time. Sandstone ages show that the ful reviews of the submitted manuscript. Kuijpers, E.P., 1979, La geologõÂa del complejo o®olõÂtico de earliest erosion of Nicoya Complex intrusive Nicoya, Costa Rica: San JoseÂ, Costa Rica, Instituto GeograÂ®co Nacional de Costa Rica, Informe Semes- rocks began at least by late Campanian time REFERENCES CITED tral Julio±Diciembre 1979, p. 15±75. (ca. 75 Ma), ϳ9 m.y. after cessation of the Kussmaul, S., 1980, PetrografõÂa de Rocas Igneas: San JoseÂ, intrusive magmatic activity on northern Ni- Baumgartner, P.O., 1990, Mesozoic and Tertiary arcs, sea- Costa Rica, Escuela Centroamericana de GeologõÂa, mounts and accretionary terranes of Costa Rica (Cen- Universidad de Costa Rica, 122 p. coya Peninsula. Geochemical signatures of tral America): Geological Society of America Ab- Lundberg, N., 1982, Evolution of the slope landward of the potential plutonic sources (Wildberg, 1987; stracts with Programs, v. 22, no. 7, p. A338±A339. Middle America Trench, Nicoya Peninsula, Costa Rica, in Leggett, J.K., ed., Trench-forearc geology: Sinton et al., 1997) suggest coeval erosion of Baumgartner, P.O., Mora, C.R., Butterlin, J., Sigal, J., Gla- cËon, G., AzeÂma, J., and Bourgois, J., 1984, Sedimen- Geological Society [London] Special Publication 10, both island arc± and oceanic plateau±related tologõÂa y paleogeografõÂa del CretaÂcico y Cenozoico p. 131±147. intrusive rocks. The unroo®ng of the ophiolit- del litoral PacõÂ®co de Costa Rica: Revista GeoloÂgica Lundberg, N., 1991, Detrital record of the early Central American magmatic arc: Petrography of intraoceanic ic sequence concomitant with the beginning of de AmeÂrica Central, v. 1, p. 57±136. Calvo, C., 1987, Las calizas nerõÂticas de la Vertiente PacõÂ®- forearc sandstones, Nicoya Peninsula, Costa Rica: neritic carbonate sedimentation records strong ca del norte de Costa Rica y sur de Nicaragua: EÂ pocas Geological Society of America Bulletin, v. 103, uplift of the Costa Rican orogen, including the y sistemas asociados con la apertura y evolucioÂn del p. 905±915. margen convergente de AmeÂrica Central meridional Minster, J.B., and Jordan, T.H., 1978, Present-day plate mo- formation of emergent ophiolitic basement ar- [Tesis de Licenciatura]: San JoseÂ, Costa Rica, Escuela tions: Journal of Geophysical Research, v. 38, eas in the forearc. This tectonic event marks Centroamericana de GeologõÂa, Universidad de Costa p. 5331±5354. the onset of the Laramide orogeny in Late Rica, 165 p. Posamentier, H.W., and Vail, P.R., 1988, Eustatic controls Calvo, C., 1998, Kretazische Subduktionsprozesse in SuÈd- on clastic deposition IIÐSequence and systems tract Cretaceous time. In this context, plutonic and zentralamerika: Pro®l, v. 15, p. 1±161. models, in Wilgus, C.R., et al., eds., Sea-level chang- associated radiolarian chert grains in Creta- Calvo, C., and Bolz, A., 1994, Der aÈlteste Inselbogen-Vul- es: An integrated approach: Society of Economic Pa- ceous cover sandstones are considered as de- kanismus in Costa Rica, Marine Pyroklastika der For- leontologists and Mineralogists Special Publication mation Loma Chumico (Alb bis Campan): Pro®l, v. 7, 42, p. 125±154. trital paleotectonic indicators of the prominent p. 235±264. Protti, R., 1981, GeologõÂa de la planicie costera desde Ji- Campanian unconformity separating the de Boer, J., 1979, The outer arc of the Costa Rican orogen caral, Puntarenas hasta Santa Rita, Nandayure, Pen- (oceanic basement complexes of the Nicoya and Santa Ânsulaõ de Nicoya, Costa Rica: San JoseÂ, Costa Rica, ophiolitic basement from its sedimentary cov- Elena Peninsulas): Tectonophysics, v. 56, p. 221±259. Informe TeÂcnico del Servicio Nacional de Aguas Sub- er strata. Dengo, G., 1962, Estudio geoloÂgico de la regioÂn de Guan- terraÂneas (SENAS), 12 p.

Geological Society of America Bulletin, July 2003 843 C. CALVO

Rivier, F., 1983, SõÂntesis geoloÂgica y mapa geoloÂgico del Rica and Panama): TuÈbinger Geowissenscha¯iche Ar- Florez, H.A., Escobedo, Z.D., Leon, R., and Barrios, aÂrea del Bajo Tempisque, Guanacaste, Costa Rica: San beiten, v. 4, p. 1±108. L.O., 1995, Morphotectonics of the Paci®c convergent JoseÂ, Costa Rica, Instituto GeograÂ®co Nacional de Sinton, C.W., Duncan, R.A., and Denyer, P., 1997, Nicoya margin of Costa Rica, in Mann, P., ed., Geologic and Costa Rica, Informe Semestral Enero±Junio 1983, Peninsula, Costa Rica: A single suite of Caribbean tectonic development of the Caribbean plate boundary p. 7±30. oceanic plateau magmas: Journal of Geophysical Re- in southern Central America: Geological Society of America Special Paper 295, 291±308. Schmidt-Ef®ng, R., 1975, El primer hallazgo de amonites search, v. 102, p. 15,507±15,520. Wildberg, H.G.H., 1984, Der Nicoya-Komplex, Costa Rica, en AmeÂrica Central meridional y notas sobre las facies Tournon, J., and AzeÂma, J., 1980, Sobre la estructura y la petrologõÂa del macizo ultrabaÂsico de Santa Elena Zentralamerika: Magmatismus und Genese eines po- cretaÂcicas en dicha regioÂn: San JoseÂ, Costa Rica, In- lygenetischen Ophiolith-Komplexes: MuÈnsterische stituto GeograÂ®co Nacional de Costa Rica, Informe (Provincia de Guanacaste, Costa Rica): San JoseÂ, Cos- ta Rica, Instituto GeograÂ®co Nacional de Costa Rica, Forschungen fuÈr Geologie und PalaÈontologie, v. 62, Semestral Enero±Junio 1974, p. 53±61. p. 1±123. Informe Semestral Enero±Junio 1980, p. 17±54. Schmidt-Ef®ng, R., 1979, Alter und Genese des Nicoya- Wildberg, H.G.H., 1987, High level and low level plagio- TroÈger, W.E., 1967, Optische Bestimmung der gesteinsbil- Komplexes, einer ozeanischen PalaÈokruste (Oberjura granites from the Nicoya ophiolite complex, Costa denden Minerale, Teil 2: Textband: Stuttgart, Germa- Rica, Central America: Geologische Rundschau, bis EozaÈn) im suÈdlichen Zentralamerika: Geologische ny, Schweizerbart, 822 p. Rundschau, v. 68, p. 457±494. v. 76, p. 285±301. Ulloa, F., 1977, Aspectos de la GeologõÂa de la parte este de Zuffa, G.G., 1980, Hybrid arenites: Their composition and Seyfried, H., and Sprechmann, P., 1985, Acerca de la for- la PenõÂnsula Santa Elena (desde CuajiniquõÂl a Santa classi®cation: Journal of Sedimentary Petrology, macioÂn del puente-istmo centroamericano meridional, Rosa de la Cruz de Guanacaste) [Tesis de Licencia- v. 50, p. 21±29. con eÂnfasis en el desarrollo acaecido desde el Cam- tura]: San JoseÂ, Costa Rica, Escuela Centroamericana paniano al Eoceno: Revista GeoloÂgica de AmeÂrica de GeologõÂa, Universidad de Costa Rica, 76 p. MANUSCRIPT RECEIVED BY THE SOCIETY 30 APRIL 2001 Central, v. 2, p. 63±87. von Huene, R., Bialas, J., Flue, E., Cropp, B., Csernok, T., REVISED MANUSCRIPT RECEIVED 24 SEPTEMBER 2002 MANUSCRIPT ACCEPTED 12 NOVEMBER 2002 Sick, M., 1989, Paleomagnetism of the ophiolite complexes Fabel, E., Hoffman, J., Emeis, K., Holler, P., Jeschke, from southern Middle America land bridge (Costa G., Leandro, M.C., Perez-Fernandez, I., Chavarria, S., Printed in the USA

844 Geological Society of America Bulletin, July 2003