early neogene history of the central american arc from...

For permission to copy, contact [email protected] 2003 Geological Society of America 271

GSA Bulletin; March 2003; v. 115; no. 3; p. 271–287; 7 figures; 3 tables.

Early Neogene history of the Central American arc from Bocas delToro, western Panama

Anthony G. Coates†

Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Republic of Panama

Marie-Pierre Aubry‡

Institut des Sciences de l’Evolution, Universite de Montpellier, Montpellier, Cedex 5, France

William A. BerggrenDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

Laurel S. CollinsDepartment of Earth Sciences, Florida International University, Miami, Florida 33199, USA

Michael KunkU.S. Geological Survey, M.S. 963, Denver Federal Center, Denver, Colorado 80225, USA

ABSTRACT

A newly discovered sequence of lower tomiddle Miocene rocks from the eastern Bo-cas del Toro archipelago, western Panama,reveals the timing and environment of theearliest stages in the rise of the Isthmus ofPanama in this region. Two new forma-tions, the Punta Alegre Formation (lowerMiocene, Aquitanian to Burdigalian) andthe Valiente Formation (middle Miocene,Langhian to Serravallian), are here namedand formally described. The Punta AlegreFormation contains a diagnostic microfau-na of benthic and planktic foraminifera andcalcareous nannofossils that indicate depo-sition in a 2000-m-deep pre-isthmian neo-tropical ocean from as old as 21.5–18.3 Ma.Its lithology varies from silty mudstone tomuddy foraminiferal ooze with rare thinmicroturbidite layers near the top. The Val-iente Formation, which ranges in age from16.4 to ca. 12.0 Ma, lies with slight angularunconformity on the Punta Alegre Forma-tion and consists of five lithofacies: (1) co-lumnar basalt and flow breccia, (2) pyro-clastic deposits, (3) coarse-grainedvolcaniclastic deposits, (4) coral-reef lime-stone with diverse large coral colonies, and(5) marine debris-flow deposits and micro-

†E-mail: [email protected].‡Present address: Department of Geological Sci-

ences, Rutgers University, Piscataway, New Jersey08854, USA.

turbidites. These lithofacies are interpretedto indicate that after ca. 16 Ma a volcanicarc developed in the region of Bocas delToro and that by ca. 12 Ma an extensivelyemergent archipelago of volcanic islandshad formed. 39Ar/40Ar dating of basalt flowsassociated with the fossiliferous sedimenta-ry rocks in the upper part of the ValienteFormation strongly confirms the ages de-rived from planktic foraminifera and nan-nofossils. Paleobathymetric analysis of thetwo new formations in the Valiente Penin-sula and Popa Island, in the Bocas del Toroarchipelago, shows a general shallowingfrom lower- through upper-bathyal toupper-neritic and emergent laharic and flu-viatile deposits from ca. 19 to 12 Ma. Theoverlying nonconformable Bocas del ToroGroup contains a lower transgressive se-quence ranging from basal nearshore sand-stone to upper-bathyal mudstone (ca. 8.1–5.3 Ma) and an upper regressive sequence(5.3–3.5 Ma). A similar paleobathymetricpattern is observed from the Gatun to Cha-gres Formations (12–6 Ma) in the PanamaCanal Basin area and in the Uscari, Rio Ba-nana, Quebrada Chocolate, and Moin For-mations (8–1.7 Ma) in the southern LimonBasin of Costa Rica.

Keywords: Neogene, Central America,Panama, Bocas del Toro, paleobathyme-try, stratigraphy.

INTRODUCTION

The closure of the Isthmus of Panama trig-gered profound environmental, evolutionary,and ecological changes both on land and inthe sea. The formation of a bridge between theNorth and South American continents gaverise to the great American biotic interchangeon land (Webb and Rancy, 1996; Webb, 1999;Stehli and Webb, 1985). Less well known arethe timing and nature of the changes in thesea consequent upon the rise of a sill or barrierbetween the eastern Pacific and the Caribbean.This marine barrier gradually affected oceancirculation during Neogene time. In the pro-cess, a striking contrast evolved between therelatively warm, more saline, nutrient-poorCaribbean and the more seasonal, less saline,and more productive (for pelagic organisms)eastern Pacific. These environmental changesapparently altered the course of evolution, firstof the deep-water planktic organisms like ra-diolaria and diatoms from ca. 15 Ma, and thensuccessively shallower taxa until completeemergence at ca. 4–3 Ma. The Caribbean be-came dominated by carbonate-associated ben-thic foraminifera and coral-reef–seagrass–mangrove coastal ecosystems and a profoundtaxonomic turnover occurred in corals andmollusks (Budd and Johnson, 1997; Jacksonet al., 1993). In contrast, the Pacific has richpelagic fisheries but poorly developed coralreefs (Jackson and D’Croz, 1999). The abilityto demonstrate a causal relationship between

272 Geological Society of America Bulletin, March 2003

COATES et al.

Figure 1. Geologic map and cross section (a–a9; F—fault) of the Valiente Peninsula, Bocas del Toro, western Panama, showing thedistribution of the Punta Alegre and Valiente Formations. The five lithofacies of the Valiente Formation are indicated by separate colorsand numbers on the key (upper right) as follows: v1—basalt-lava and flow-breccia facies, v2—coarse-grained volcaniclastic facies, v3—pyroclastic facies, v4—coral-reef facies, and v5—marine debris-flow and turbidite facies. See text for descriptions of each facies. Numberson the map locate the sites of samples used for this study (slashes are used to indicate site-number ranges). Details of sites may beobtained in the PPP Database at http://www.fiu.edu/;collinsl/. Vent centers were located at Llorona Hill and ;2 km east of AvispaPoint. The extensive areas to the east of these centers mapped in red (v1 facies) are dominated by flow breccia with minor columnarbasalt. The coarse-grained volcaniclastic fluviatile, estuarine, and shallow-marine facies (v2) always surrounds the v1 facies except alongthe northern margin of Bluefield Bay, where a pyroclastic facies (v3) is developed. Small coral-reef lenses (v4) are associated with thev2 facies, especially near Toro Point. A debris-flow and turbidite facies (v5) is developed along the northern coast around Cusapin Point.

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the environmental and biological changesrests largely on the availability of land-basedisthmian sections bracketing the time involvedand the establishment of a precise correlationwithin and between them.

In earlier papers, we established precisecorrelations for the late Neogene events in thesouthern Limon Basin of Costa Rica (Collinset al., 1995; McNeill et al., 1999), the PanamaCanal Basin (Collins et al., 1996), and the Bo-cas del Toro Basin, Panama (Coates, 1999);here we describe an extensive, newly discov-ered sequence of lower to upper Miocenerocks from the Valiente Peninsula and PopaIsland in the Bocas del Toro archipelago,western Panama (Figs. 1, 2) that provides thefirst complete record for this region of thenearshore history of the early stages of the riseof the Central American isthmus. The se-quence provides a link between the lowerNeogene section previously described in thesouthern Limon Basin of Costa Rica (Casselland Sen Gupta, 1989a, 1989b; Bottazzi et al.,1994) to the northwest and the Panama CanalBasin to the southeast. These sections revealdifferent local geologic histories and thus thenecessity of closely spaced data points if thepaleogeography of the rise of the southernCentral American isthmus is to be accuratelyreconstructed.

The Bocas del Toro sequence reveals fourphases in the rise of the isthmus: (1) deposi-tion of lower-bathyal, pre-isthmian, oceanicsediments in early Miocene time (21.5–18.5Ma), represented by the Punta Alegre For-mation; (2) growth of a volcanic arc duringmiddle Miocene time (17–12 Ma) with sub-sequent development of an extensive volcanicarchipelago characterized by columnar basaltand flow breccia, coarse-grained volcaniclasticdeposits, and diverse coral reefs during lateMiocene time (12–8 Ma); (3) extinction andsubsidence of the volcanic arc during the latestpart of Miocene time (8–5 Ma), resulting in amarine transgression, represented by the To-babe and Nancy Point Formations of the lowerBocas del Toro Group; and (4) early Miocene

regression in the upper Bocas del Toro Group,starting at ca. 5 Ma and culminating in thelower Pleistocene forereef limestone that com-poses the Swan Cay Formation (Coates et al.,1992; Coates and Obando 1996; Collins andCoates, 1999).

We present a formal lithostratigraphic de-scription of the new units, followed by a bio-stratigraphic and radioisotopic analysis. Wethen reconstruct the geologic history of therise of the Isthmus of Panama in the Bocasdel Toro region by using paleobathymetricdata from benthic foraminifera. The recordfrom Bocas del Toro is compared with that ofthe Limon Basin in Costa Rica and the Pan-ama Canal Basin and Darien region in easternPanama.

LITHOSTRATIGRAPHY

Lower and middle Miocene rocks of theBocas del Toro Basin are everywhere overlainby the Bocas del Toro Group, a richly fossil-iferous series of marginal-marine to upper-bathyal formations of latest Miocene to earlyPleistocene age. The lithostratigraphy of theBocas del Toro Group and the major micro-and macrofossil groups collected from the var-ious formations within it have already beendescribed by several authors (Collins andCoates, 1999, and references therein). Theolder Miocene rocks (Figs. 1, 2) are assignedto two new formations. (1) The lower Mio-cene Punta Alegre Formation consists oflower-bathyal clayey and silty ooze containingabundant calcareous nannofossils and benthicand planktic foraminifera (Tables 1, 2). Itcrops out only on the western tip of the Val-iente Peninsula. (2) The Valiente Formationunconformably overlies the Punta Alegre For-mation and crops out extensively on the Val-iente Peninsula and nearby Popa Island.

The Valiente Formation is a lithologicallycomplex and highly variable formation be-cause it represents the suite of facies—marineand terrestrial, igneous and sedimentary—that

is associated with active volcanic island arcsin tropical waters. It includes columnar basaltflows, basaltic flow breccia, pyroclastic tuffand ignimbrite, laharic breccia, fluviatile/estuarine conglomerate, coral-reef lenses, andmarine debris flows. These facies intercalateand replace each other over very short dis-tances, both laterally and vertically. The geo-graphic and stratigraphic distribution of theselithofacies is shown in Figures 1–3. Thebasalt-lava and flow-breccia facies forms acore around which the other facies are distrib-uted peripherally as terrestrial, coastal, or ma-rine slope deposits.

Punta Alegre Formation

The formation is named for the nearest vil-lage, Punta Alegre, and the prominent lowbluffs that lie along the coast of the westernlimit of the Valiente Peninsula, north of Blue-field Bay form the stratotype (Llorona HillSection, Figs. 3, 4A). They lie 1 km south ofBluefield Point (known locally as Punta Val-iente) and 1.3 km northwest of the village ofPunta Alegre (Fig. 1). This is the only expo-sure of the formation.

The formation is 19 m thick at the type lo-cality, where a disconformable upper contactis marked by the scoured basal surface of theValiente Formation and an overlying coarse-sand unit with up to 30-cm-diameter slumpedblocks. Laterally, the Valiente Formation lieswith up to 208 of angular unconformity on thePunta Alegre Formation (Fig. 4B). The lowercontact of the Punta Alegre Formation is notexposed.

The Punta Alegre Formation consists ofthin- to medium-bedded, gray-weathering,blue-green, siliceous, clayey siltstone andblocky mudstone (Fig. 4A). Harder,conchoidal-fracturing, 3–5-cm-thick horizons,spaced 5–20 cm apart, are common and areprobably bentonitic. Many horizons containvery abundant, diverse, benthic and plankticforaminifera, but macrofossils and burrowing

Geological Society of America Bulletin, March 2003 273

EARLY NEOGENE HISTORY OF THE CENTRAL AMERICAN ARC, WESTERN PANAMA

structures are absent. The Punta Alegre is in-terpreted to be a pre-isthmian lower-bathyaloceanic deposit; several horizons approach fo-raminiferal ooze in lithology.

Valiente Formation

The Valiente Formation is named for theValiente Peninsula (Fig. 1) on which the for-

mation crops out extensively. The stratotypestretches 270 m southwestward along thecoast (Toro Point section, Fig. 3) from thenorthwest corner of Toro Point, Valiente Pen-


COATES et al.

Figure 2. Geologic map (located in Fig. 1) and section (b–b9; F—fault) showing the Val-iente Formation on Popa Island, where the Valiente is unconformably overlain by thePliocene Cayo Agua Formation (ca). Numbers and symbols as for Figure 1. On PopaIsland, only the basalt-lava and flow-breccia facies (v1) and the coarse-grained volcani-clastic facies (v2; not associated with reef lenses) are present, apparently with thin layersof low-rank coal, an example of which is exposed along the coast immediately north ofPunta Laurel. A prominent basalt dike is exposed at the tip of the Punta Laurel (hereshown in red as v1 facies) where it cuts the Valiente Formation.

insula. The formation is 135 m at the typesection, but varies greatly across its outcrop.Additional reference sections include LloronaHill (upper part), Avispa and Sirain Points,Cusapin Village and Cusapin Creek (Figs. 1,3), Deer Island, and southeast Popa Island(Figs. 2, 3). The Valiente Formation overliesthe Punta Alegre Formation with angular un-conformity and is unconformably overlain bythe Tobabe Formation, the basal unit of theBocas del Toro Group. It is characterized byfive lithofacies, described next.

Basalt-Lava and Flow-Breccia LithofaciesBasalt lava is commonly interbedded as thin

flows in other lithofacies of the Valiente For-mation, but it is the dominant lithology in thenorthern part of the Valiente Peninsula (Fig.1), north of Bluefield Bay, and in the regionof the Grape and Wanchi Points, Plantain

Cays, and Tobabe Point. It also forms inter-calated units in the type section of the ValienteFormation (Toro Point section, Fig. 3) alongthe western coast of the Toro Point peninsula.Lava flows vary from 2 to 3 m to several tensof meters in thickness and are commonly co-lumnar, with columns averaging 20–30 cm indiameter. These columns are well displayed inthe Plantain Cays, on the coast west of Cu-sapin and north of Punta Alegre (Fig. 1). Thebasalt is fine-grained, aphyric, or aphanitic,with up to 1-cm-long feldspar crystals, or hol-ocrystalline and medium grained with pyrox-ene and plagioclase.

More commonly, basalt occurs as flow brec-cia up to 30 m thick (Fig. 4C). These brecciaunits are monomict, with a very wide rangeof clast sizes, ranging up to 30 cm on average,but with occasional blocks up to 50 cm, in afine-grained basalt matrix. Flow breccia dom-

inates the region around Sierra Llorona in theValiente Peninsula, where flows are spectac-ularly exposed along the coast for 1 km to thesouth and 1 km to the east of Bluefield Point(locally known as Punta Valiente) and alongthe coast east of Cusapin (Fig. 1). They appearto be proximal flows radiating down the flanksof a vent center located near Llorona Hill (Fig.1). The total thickness of the flows in this re-gion is ;130 m.

Pyroclastic LithofaciesAlong the northern shore of Bluefield Bay,

for ;3 km eastward from Punta Alegre, im-mediately south of Llorona Hill, thin colum-nar basalt-lava and flow-breccia units are in-terbedded with white to cream, laminated tomassive, tuffaceous sandstone, ashy tuff andfine breccia whose welded crystalline tuffa-ceous matrix suggests a pyroclastic origin.

Coarse-Grained Volcaniclastic LithofaciesThis lithofacies consists of three subtypes.

(1) Volcanic boulder conglomerate, in units upto 8 m thick, is strongly mud-matrix supportedand has no trace of internal stratification; theconglomerate contains both rounded and an-gular clasts with an extreme size range fromsmall pebbles to blocks 50 cm in diameter aswell as some sandstone and basalt blocks 3–4 m in diameter. (2) Sandy, pebble breccia, upto 2 m thick, is distinctive. (3) Very coarsegrained volcanic sandstone, ranging from 2 to10 m thick, is distinctly bedded (although thebedding is streaky) and commonly containsscattered boulders up to 20 cm in diameter.The sandstone also contains lenses and chan-nels of pebble breccia and cobble conglom-erate (Fig. 4D); these depositional bodies bearmostly basalt clasts but also laminated sand-stone, siltstone, and rip-up mudstone clasts.The sandstone units commonly have scouredbases and high-angled cross-beds.

The chaotic, matrix-supported boulder bedsare interpreted as lahars because of the ex-treme size range of the clasts, the lack of in-ternal bedding, and the absence of marine fos-sils. They commonly grade laterally or passup into the streaky-bedded pebble breccia andmore consistently stratified coarse-grainedvolcanic sandstone whose scoured horizons,high-angled cross-beds, and channels suggesta fluviatile origin. Local clayey siltstone lens-es, with comminuted shell hash and a few tur-ritellid gastropods and other mollusks, indicateperiodic marginal-marine deposition. This se-quence is interpreted as fluviatile reworking ofthe distal toes of lahars, perhaps in small deltasat the margin of active volcanic islands, given



TABLE 1.

the common intercalation, within this lithofacies,of thin (1–2 m thick), basaltic flow breccias.

South of Toro Point, along the west coastof the peninsula (Toro Point section, Fig. 3),

the coarse-grained volcaniclastic facies isfound intercalated with basaltic flow brec-cia, 3–5 m thick, that is presumably distalbecause thin reef lenses are also locally pre-

sent. Good exposures are also seen, withoutreef lenses, along the northern margin of theValiente Peninsula between Cusapin Creekand the village of Cusapin as well as along


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TABLE 2.



TABLE 2. (Continued.)

the northern margin of the Plantain Cays(Fig. 1).

Coral-Reef LithofaciesAt several points on the Toro Point penin-

sula (Fig. 1), small reef limestone lenses (Fig.4E) are intercalated with the coarse-grainedvolcaniclastic and basaltic flow-breccia litho-facies. The most extensive reef limestone unitcovers the northern tip of the Toro Point pen-insula (Toro Point section, Fig. 3) and consistsof thick-bedded, rubbly, bioclastic, cream-weathering, pale blue limestone that containsvolcanic sand and silt grains in varyingamounts. The limestone is ;37 m thick andhas interbeds up to 6 m thick of volcanic-cobble conglomerate with well-rounded clastsand a graywacke matrix. Large coral coloniesup to 50 cm in diameter, including Montas-traea, Porites, Stylophora, and Diploria, arecommon at several levels. Other similar, coral-rich, reef-rubble units can be observed ;100m to the south along the western coast of theToro Point peninsula. These deposits appearto represent local reef buildups of limited lat-eral and vertical extent, fringing active vol-canic islands.

Marine Debris-Flow and TurbiditeLithofacies

This distinctive lithofacies is well devel-oped immediately to the west and east of Cu-sapin on the north coast of the Valiente Pen-

insula (Fig. 1). To the east of Cusapin, startingat Cusapin Point (Cusapin Village section,Fig. 3) and continuing east for ;1.3 km, theunits consist of steeply dipping, thick-bedded(up to 2 m thick), coarse-grained polymictbreccia and boulder conglomerate, all matrixsupported. The matrix is coarse, blue-greengraywacke and siliceous mudstone in which‘‘floats’’ a chaotic array of very coarse grainedboulder- to pebble-sized blocks. The clasts aredominantly basalt, including up to 1-m-longsegments of well-formed hexagonal basaltcolumns and up to 60–80 cm diameter blocksof laminated and convolute-bedded coarse-grained sandstone, pebble breccia, and lami-nated to thin-bedded siltstone and lignite. Atseveral places, the matrix contains ahermatyp-ic corals, echinoids (spines), algal (Archeolith-othamnion?) balls and scattered mollusks, in-cluding conid and olivid gastropods. Thesevery coarse grained boulder beds are interbed-ded with well-laminated, cross-bedded,coarse-grained volcanic sandstone and thick-bedded graywacke, which also containmollusks.

The association of very large, unabraded,polymict clasts and perfectly preserved seg-ments of hexagonal basalt pillars in a sup-porting matrix of upper-bathyal graywacke,with associated shallow- and deep-water or-ganisms, strongly suggests rapid resedimen-tation. The coarse-grained fluviatile, laharic,and marginal-marine volcaniclastic deposits

appear to have been redeposited as debrisflows into nearby deep water on the steep, un-stable, submarine slopes of the volcanic islandarc.

To the west of Cusapin, these boulder bedsare less common and are associated with moretypical turbidite sequences (Fig. 4F) that arewell exposed at the point that forms the west-ern limit of Cusapin harbor and along thecoast extending ;5 km to the west. Turbiditeunits range from 1 to 15 cm thick and weatherin places a sulfurous yellow. Blue-gray, sili-ceous and carbonaceous blocky mudstone al-ternates with finely and sometimes sublenti-cularly laminated sandstone units that havescoured bases. At Toro Point, pebble brecciapacked with wood fragments is common asare intraformational slumps (Fig. 4F).

On Popa Island the Valiente Formation isintruded by basalt dikes. At Punta Laurel (Fig.2) the dike yielded 40Ar/39Ar ages of 8.4 6 0.5and 8.5 6 0.6 Ma.

BIOSTRATIGRAPHY

Punta Alegre Formation

Both planktic foraminifera and calcareousnannofossils point to an early Miocene (Bur-digalian) age (ca. 18.3–19 Ma) for the PuntaAlegre Formation. Six sites (PPP2794–PPP2799) in the lower part of the Llorona Hillsection (Fig. 3) yielded a rich and well-


COATES et al.

Figure 3. Geochronological relationships of the principal sections measured in the Punta Alegre Formation and the Valiente Formation.Numbers represent PPP sites analyzed, and columns give the ranges of biostratigraphically useful nannofossils (white) and plankticforaminifera (black). Sections are located on Figure 1; numbers 1–5 (in boxes adjacent to sections) refer to lithofacies of the ValienteFormation described in the text. (There is no pattern in the symbol key for v3 as it does not crop out along the line of the section.)Crosses in the Llorona Hill and Toro Point sections represent 40Ar/39Ar dates of basalt lavas (for exact ages, see Fig. 6).

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preserved planktic foraminiferal fauna domi-nated by globoquadrinids, Globigerinoides tri-lobus, Catapsydrax dissimilis, and C.stainforthi. This fauna is suggestive of ZonesN5–N6 (ca. 17–21 Ma). Calcareous nannofos-sils are common and moderately well pre-served. Common species include Reticulofe-nestra floridana, R. pseudoumbilicus,Coccolithus pelagicus, and C. miopelagicus,Helicosphaera carteri, Sphenolithus spp. andDiscoaster gr. deflandrei. Assignment of thesection to Biozone(s) NN2 or NN3 is basedon the occurrences of Discoaster druggi,Sphenolithus disbelemnos (in all samples), andSphenolithus belemnos (common and charac-teristic only at site PPP2796). Although theLO of D. druggi defines the base of ZoneNN2, the lowest occurrence of the two Sphen-olithus species are located in upper ZoneNN2. In addition, two fragments (pristine) ofTriquetrorhabdulus carinatus (whose highestoccurrence defines the top of Zone NN2) werefound at site PPP2795. Because of the extremescarcity of T. carinatus (at site PPP2795), itis uncertain whether this interval belongs toZone NN2 or NN3. However, the co-occurrenceof S. disbelemnos and S. belemnos restricts theage of the Punta Alegre Formation as repre-sented in the Llorona Hill section to not olderthan the upper part of Zone NN2 (ca. 19.5 Ma)and not younger than Zone NN3 (ca. 18.3Ma).

Valiente Formation

The oldest deposits of the Valiente Forma-tion are found in the 40-m-thick section of re-sedimented shelly bioclastic limestone, mud-stone, and conglomerate at Avispa Point (Figs.1, 3). They belong to Zone NN4 (entire sec-tion) and Zone N8 (5 M5a; lower part of thesection). The four sites (PPP2801–PPP2804)examined for calcareous nannofossils, taken atregularly spaced levels throughout the out-crop, yielded sparse, low-diversity assemblag-es with Helicosphaera ampliaperta (common)and Sphenolithus heteromorphus, whose con-current ranges define Zone NN4 (ca. 18.2–15.6 Ma). Together with R. pseudoumbilicus,D. deflandrei, C. miopelagicus, Discoaster cf.D. exilis and the planktic foraminifera Glo-borotalia peripheroronda, G. birnageae, Glo-

bigerinoides sicanus, G. bisphericus, and G.altiaperturus (sites PPP2801, PPP2803), thesetaxa indicate Zone N8 (5 M5a) and yield anestimated age of 16.4–16.1 Ma for the lower;15 m of the Avispa Point section.

A slightly younger age (ca. 15.6–16.1 Ma)is estimated for the turbiditic sequence at Cu-sapin Village (Figs. 1, 3). The section is re-ferable to Zones NN4 (entire section) and N8(5 M5b; lower part of the section). Calcare-ous nannofossils are common in site PPP3148and rare in site PPP3200. Zonal assignment toZone NN4 is firm, based on the co-occurrenceof H. ampliaperta and S. heteromorphus. Theoccurrence of Praeorbulina circularis and theabsence of true orbulinids in site 3148 indicatean early middle Miocene (early Langhian)age.

Slightly younger rocks overlie the PuntaAlegre Formation in the Llorona Hill section(Fig. 3). The uppermost part of the section be-longs to Zone NN5 (sites PPP3650–PPP3654;15.6–13.6 Ma) as indicated by the occurrenceof nannofossil S. heteromorphus and the ab-sence of H. ampliaperta. Sites PPP3650–PPP3655 also yielded the planktic foraminif-era Praeorbulina circularis, P. glomerosa (aswell as true orbulinids), Globigerinoides bi-sphericus, G. sicanus, G. peripheroronda,Globorotalia archeomenardii, and G. praes-citula. These taxa are assignable to Zone N9(ca. 15.1–14.8 Ma). Site PPP2800, located 2–3 m below PPP3654, also contains a ZoneNN5 nannofossil assemblage, and its similarplanktic foraminiferal assemblage resemblesthe overlying sites, but it does not yield trueorbulinids and is therefore assigned to upper-most Zone N8 (M5b; ca. 15.1–15.5 Ma). SitePPP3655 is located 14 m stratigraphicallylower than PPP2800, with an angular uncon-formity between, and yields abundant calcar-eous nannofossils although the zonal markersare rare. The co-occurrence of S. heteromor-phus, S. belemnos, and H. ampliaperta inPPP3655 may suggest a stratigraphic positionnear the NN3/NN4 (ca. 18.2 Ma) zonal bound-ary. It is unclear, however, whether the rangesof S. belemnos and S. heteromorphus over-lapped, and it may be that the co-occurrenceis due to reworking, particularly since the latePaleogene species Reticulofenestra bisecta iscommon. Indeed, provisional planktic fora-

miniferal assignment to Zone N8 would sup-port reworking. Site PPP3655 contains Par-agloborotalia mayeri, Globoquadrinaaltispira, G. cf. birnageae, and G. bisphericus.In the absence of praeorbulinids (Zone N8)and catapsydracids (highest occurrence at thetop of Zone N7), this site is probably referableto a level stratigraphically correlative withZone N8. Thus, the upper part of the LloronaHill section spans the N8/N9 zonal boundary.

The section at Cusapin Creek (Figs. 1, 3) ismiddle Miocene (Serravallian, ca. 13.6–12.7Ma) and notably younger than any of the sec-tions described so far in this paper. SitePPP3146, located toward the base of the sec-tion, yields common calcareous nannofossilswith frequent R. pseudoumbilica and C. mio-pelagicus and rare, poorly preserved discoas-ters. The co-occurrence of Triquetrorhabdulusrugosus, Discoaster musicus, C. miopelagi-cus, and Catinaster mexicanus suggests ZoneNN6 (ca. 13.6–12.7 Ma). The planktic fora-minifera from the same site, with Globorotal-ia peripheroacuta, G. partimlabiata, Orbulinasuturalis, Globoquadrina dehiscens, and G.venezuelana, characterize Zone N10 (ca.14.8–12.7 Ma).

The age of the Sirain Point section is poorlydefined. The planktic foraminifera are silici-fied and the calcareous nannofossil assem-blages are poorly preserved and of low diver-sity. The occurrence of C. miopelagicus, D.musicus, and D. exilis (without S. heteromor-phus) indicates an interval correlative withZones NN6–NN8 (ca. 13.6–11.2 Ma).

The youngest rocks of the Valiente For-mation occur at the type locality at Toro Point(Figs. 1, 3) in the Valiente Peninsula. Mud-stone alternating with fine-scale turbidites(PPP3134, PPP3136, PPP2730/PPP2731;slashes indicate site number ranges) span;3.1 m.y. of the middle Miocene from ZoneN8/M5b (15.5–15.2 Ma) to Zone N11/M8(12.7–12.5 Ma). However, on biostratigraphicevidence, we suspect the presence of an un-conformity near the base of the section withinthe 3 m interval between sites PPP3136 (ZoneN8 5 M5b) and PPP2730 (Zone N11). Cal-careous nannofossils occur only at sitesPPP2730 and PPP2731. The co-occurrence ofT. rugosus (lowest occurrence in Zone NN6)and T. milowii (highest occurrence in Zone


COATES et al.

Figure 4. Photographs of exposures of the Punta Alegre Formation and various lithofacies of the Valiente Formation. (A) Stratotype ofthe Punta Alegre Formation on the Valiente Peninsula, 1 km south of Bluefield Point, Valiente Peninsula, Bocas del Toro. Possiblebentonite appears as a dark horizon above the hammer. (B) Slightly angular unconformable contact of the upper surface of the PuntaAlegre Formation and a flow breccia of the Valiente Formation, 1 km south of Bluefield Point, Valiente Peninsula, Bocas del Toro. (C)Typical basaltic flow breccia in the stratotype of the Valiente Formation, along the coast, 900 m south of Toro Point, Valiente Peninsula,Bocas del Toro. (D) Coarse-grained volcaniclastic facies in the stratotype of the Valiente Formation, along the coast ;850 m south ofToro Point, Valiente Peninsula, Bocas del Toro. (E) Bedded reef-rubble limestone in the stratotype of the Valiente Formation at ToroPoint, Valiente Peninsula, Bocas del Toro. (F) Overturned block of the turbidite lithofacies of the Valiente Formation, 1 km south ofToro Point, Valiente Peninsula, Bocas del Toro. In the photographs with a hammer for scale, the hammer head is 14 cm long and thehandle is 34 cm long.



NN6), together with C. miopelagicus, D. exi-lis, and C. mexicanus, allow confident assign-ment of this level to Zone NN6 (ca. 13.6–11.8Ma). The presence of the planktic foraminiferaGloborotalia praefohsi and G. fohsi fohsi atsites PPP2730–PPP2731 further restricts theage of this level to Zone N11 (5 M8; ca.12.7–12.5 Ma). The lowest site (PPP3136) inthe section, located 3 m below site PPP2730,contains a planktic foraminiferal fauna that in-cludes P. circularis, P. glomerosa, G. vene-zuelana, and Globigerinella praesiphoniferaand clearly indicates Zone N8/M5b (ca. 15.5–15.2 Ma).

Two sections were measured along thecoast of Deer Island (Figs. 2, 3). The sectionalong the eastern coast of Deer Island is older.Sites PPP3150 and PPP3153 contain S. het-eromorphus and Discoaster formosus, whichindicate that the upper part of the eastern sec-tion belongs to the lower part of Zone NN5(note that the range of D. formosus has notbeen calibrated to the geomagnetic polaritytime scale [GPTS] of Cande and Kent, 1992,1995). The planktic foraminifera at sitePPP3150 include Orbulina suturalis and P.circularis, which define the age to (basal)Zone N9 (5 basal Zone M6; ca. 15 Ma). Thesection on the northwestern coast of Deer Is-land (Fig. 3) is younger. The calcareous nan-nofossil assemblages are scarce and poorlypreserved, but S. heteromorphus was not en-countered, indicating an age younger thanZone NN5. The planktic foraminifera includeParagloborotalia mayeri, G. praefohsi, and G.cf. peripheroacuta, suggesting Zone N11.

A section in the Valiente Formation wasalso measured along the southeastern coast ofPopa Island, immediately north of LaurelPoint (Figs. 2, 3). Stratigraphic relationshipsare complicated in this section by the intrusionof a basalt dike (40Ar/39Ar dates of 8.46 6 0.04and 8.52 6 0.06 Ma), which locally bringssome of the coarse-grained volcanic conglom-erate units to vertical dips. Preservation of mi-crofossils is poor, but they are abundant atmany levels. Sites PPP2845–PPP2846 yieldedR. floridana, R. pseudoumbilicus, C. miope-lagicus, Sphenolithus neoabies, Helicos-phaera carteri, D. deflandrei, D. variabilis,and D. musicus, which indicate an age youn-ger than Zone NN5 (ca. 13.6 Ma). SitePPP2845 also yielded planktic foraminifera G.trilobus, G. altispira, G. venezuelana, G. bis-phaerica, Paragloborotalia mayeri, P. bella,and G. peripheroronda (but no true orbulin-ids), seemingly indicative of Zone N8/M5 (ca.16.4–15.1 Ma). Sphenolithus heteromorphus(indicative of Zones NN4–NN5) would havebeen expected if the older Zone N8 assign-

ment were correct, but no specimens werefound at sites PPP2824–PPP2827 orPPP2844–PPP2846, and we prefer the youn-ger nannofossil age determination.

GEOCHRONOLOGY

We dated samples of unaltered plagioclasephenocrysts (Fig. 5, Table 3) from a basaltflow from the Llorona Hill section (sitePPP2148), a basalt flow from the Toro Pointsection (Fig. 3, PPP2157 and PPP2158), anda large dike that intrudes the Valiente For-mation and extends along and just inland ofthe south coast of Popa Island (Fig. 3). Sam-ples of the dike (Fig. 2) were taken at PuntaLaurel (PPP2160) and ;2 km to the west(PPP2161). Analysis of the groundmass ofthese samples produced very disturbed 40Ar/39Ar age spectra that were not interpretable,and thus data from the basalt age spectra arenot included in the following discussion. Stan-dard methods of mineral separation, includingcrushing, pulverizing, magnetic separation,heavy liquids, and handpicking were used toseparate .99.9% splits of plagioclase foranalysis.

The age spectrum for site PPP2148 in theLlorona Hill section (Fig. 5) yielded a plateauage of 11.98 6 0.07 Ma that includes 63.2%of the 39Ar released in the 1100 8C throughthe 1300 8C steps. The apparent K/Ca ratio ofthe first step in the analysis (0.34) is signifi-cantly higher than the ratios of the rest of theanalyses, which range from 0.08 to 0.09. In-verse isotope-correlation analysis of the age-spectrum data, including all of the data, resultsin an age of 12.0 6 0.6 Ma with an initial40Ar/36Ar ratio that is indistinguishable fromthat of the modern atmosphere. The goodagreement of the correlation age with the pla-teau gives a high degree of confidence in theresults.

The 40Ar/39Ar age spectrum for sitePPP2157 in the Toro Point section (Fig. 5)yielded a plateau age of 11.88 6 0.07 Ma thatincluded 99.6% of the 39Ar released in all butthe first step of the analysis. Apparent K/Caratios of all of the steps in the age spectrumrange from 0.06 to 0.07, indicating a high-purity plagioclase separate. Inverse isotope-correlation analysis of the age-spectrum data,excluding the first step, results in an age of11.9 6 0.3 Ma with an initial 40Ar/36Ar ratiothat is indistinguishable from that of the mod-ern atmosphere. The good agreement of thecorrelation age with the plateau gives us ahigh degree of confidence in the results.

The age spectrum for sample PPP2158 (Fig.5), from the same flow as PPP2157, yielded a

plateau age of 12.08 6 0.07 Ma that includes99.3% of the 39Ar released in all but the firststep of the analysis. Apparent K/Ca ratiosrange from 0.09 to 0.07 on the plateau agesteps. The K/Ca ratio of the first step in theage spectrum is much lower (0.01) than theratios on the plateau and suggests the presenceof a small amount of some other phase in thesample. Inverse isotope-correlation analysis ofthe age-spectrum data, excluding the first step,results in an age of 12.1 6 0.5 Ma with aninitial 40Ar/36Ar ratio that is indistinguishablefrom that of the modern atmosphere. The goodagreement of the correlation age with the pla-teau age gives us a high degree of confidencein the results. There is also a remarkableagreement between the estimated age of thisflow (ca. 12 Ma) and Zone N11 (5 M8; 12.7–12.5 Ma; Berggren et al., 1995) assigned tosites PPP2730 and PPP2731, which lie ;15m stratigraphically lower.

The average age for the Toro Point basaltbased on plagioclase samples from PPP2157and PPP2158 is 11.98 6 0.14 Ma (1s). Thisage is indistinguishable from our age of 11.986 0.06 Ma for the Llorona Hill plagioclase.The 40Ar/39Ar dates and similar K/Ca valuesfrom all three samples suggest that the samebasalt flow is most likely represented in bothoutcrops.

The age spectra of plagioclase samplesfrom sites PPP2060 and PPP2061 (Fig. 5),which are from the dike that intrudes the Val-iente Formation, are very similar. Both haveslightly U-shaped spectra suggesting the pres-ence of excess argon. Nonetheless, the sam-ples develop age plateaus of 8.42 6 0.05 and8.50 6 0.05 Ma, respectively. The minimumages in both age spectra agree, within the lim-its of analytical precision, with their respectiveplateau ages. Inverse isotope-correlation anal-ysis of the gas from these samples producesapparent ages of 8.4 6 0.4 Ma and 8.5 6 0.5Ma, respectively, with initial 40Ar/36Ar ratiosthat are atmospheric within the limits of ana-lytical precision. Our best estimate for the ageof this dike is the average of the two plateauages, 8.46 6 0.05 Ma (1s).

PALEOBATHYMETRY

Figure 6 shows the paleobathymetry of Val-iente Peninsula and Popa Island determinedfrom 27 samples examined by this study, com-bined with data from previous studies of theBocas del Toro, Limon, and Panama CanalBasins (summarized by Collins et al., 1999).Paleobathymetry was inferred for each samplefrom the combined ecologic ranges of the ben-thic foraminifera, based on their known paleo-


COATES et al.

Figure 5. 40Ar/39Ar age spectra and inverse isotope-correlation diagrams for the sites an-alyzed in this study. Boxes for individual analyses in age spectra are plotted at 2s. TheMSWD (mean square of weighted deviates) in the inverse isotope-correlation diagramsare goodness-of-fit indicators, and boxed points in these diagrams indicate analyses notused in the correlation.

bathymetric distribution (e.g., Phleger andParker, 1951; Pflum and Frerichs, 1976; vanMorkhoven et al., 1986; Boersma, 1984).

The benthic foraminiferal assemblages (list-ed in Table 2) are generally diverse and wellpreserved. The taxa are especially character-istic of the Miocene southern Caribbean de-posits of Panama, Costa Rica, and Venezuela(Renz, 1948; Blow, 1959). Typical deepest-dwelling taxa (and their preferred waterdepths) of the Punta Alegre Formation(PPP2796) include Melonis sphaeroides (low-er bathyal–abyssal), Uvigerina rugosa (lowerupper bathyal–middle bathyal), and Rectuvi-gerina multicostata (bathyal).

The deepest-dwelling taxa from the Valien-te Formation occur in the upper Llorona Hill(PPP2800) and Deer Island (PPP3151) sec-tions, which are middle bathyal. Taxa includeCibicides wuellerstorfi (middle bathyal–lowerbathyal), Melonis pompilioides (upperbathyal–middle bathyal), Siphonina tenuicar-inata (outer neritic–middle bathyal), and La-ticarinina pauperata (bathyal-abyssal). Thereare also many upper-bathyal sections in theValiente Formation, including Avispa Point(PPP2803), Cusapin Village (PPP3148,PPP2836), Cusapin Creek (PPP3146), ToroPoint (PPP2730, 2731), and southern Popa Is-land (PPP2825, PPP2827, PPP2844,PPP2845, PPP2846). Taxa include the follow-ing deepest-dwelling species that are mostlyupper bathyal: Cibicidoides crebbsi, C. com-pressus, Bulimina mexicana, Uvigerina mexi-cana, Siphogenerina transversa, and Planuli-na subtenuissima.

Taxa characteristic of the Bocas del ToroGroup (upper Miocene–lower Pliocene for-mations, PPP2839, PPP2828, PPP2850,PPP2852, PPP2853), which are mostlymiddle- to outer-neritic deposits, are neriticElphidium spp., Hanzawaia concentrica, Tri-farina eximia, and Reussella atlantica; middle-neritic Neoeponides antillarum; outer-neriticto bathyal Cassidulina laevigata, Hanzawaiaisidroensa, Gyroidina regularis, and Sipho-generina lamellata; and outer-neritic to bathy-al Hoeglundina elegans.

The paleobathymetric history of the newlystudied sections of the Punta Alegre Forma-tion and the Valiente Formation in the Valien-te Peninsula and Popa Island agree with andsupplement those of the Limon and PanamaCanal basins (Fig. 6). From Burdigalian toLanghian time, lower-bathyal depths (PuntaAlegre Formation) in the Bocas del Toro Ba-sin shallowed to middle-bathyal depths (Val-iente Formation), and middle-bathyal waterdepths of the Limon Basin shallowed toupper-bathyal depths (Uscari Formation).



TABLE 3. ANALYTICAL DATA FOR SAMPLES ANALYZED IN THIS STUDY

Temp(8C)

%39ArK

of total%

radiogenic%39ArK

† 40Ar*/39ArK ApparentK/Ca

Apparent age andprecision (Ma)‡

#2148 J 5 0.001205 6 0.50% Plagioclase Sample wt 0.2140 g600 0.2 9.6 0.009 4.523 0.34 9.81 6 9.60700 1.1 51.4 0.056 5.345 0.10 11.59 6 0.64800 3.6 18.5 0.186 5.672 0.09 12.29 6 0.09900 9.3 78.0 0.484 5.562 0.08 12.05 6 0.161000 17.3 90.9 0.896 5.492 0.08 11.90 6 0.031100 23.6 88.3 1.227 5.520 0.08 11.96 6 0.021200 27.4 93.6 1.424 5.539 0.09 12.00 6 0.031300 12.2 81.6 0.631 5.557 0.09 12.04 6 0.131400 5.4 72.6 0.279 5.422 0.08 11.75 6 0.05Total gas 100.0 84.5 5.193 5.525 0.09 11.9763.2% of gas on plateau in 100 8C through 13000 8C steps Plateau age 5 11.98 6 0.07

#2157 J 5 0.001207 6 0.50% Plagioclase Sample wt. 0.2494 g600 0.4 9.7 0.018 3.621 0.06 7.88 6 1.00700 2.1 65.1 0.101 5.438 0.07 11.80 6 0.69800 6.1 27.4 0.296 5.553 0.07 12.05 6 0.39900 13.9 77.4 0.680 5.424 0.07 11.77 6 0.141000 18.2 82.0 0.890 5.455 0.07 11.84 6 0.071100 22.4 89.3 1.094 5.485 0.07 11.90 6 0.031200 23.7 93.3 1.159 5.447 0.07 11.82 6 0.071300 9.8 84.9 0.480 5.563 0.06 12.07 6 0.141400 3.4 65.9 0.167 5.686 0.07 12.34 6 0.63Total gas 100.0 81.5 0.000 5.473 0.07 11.8899.6% of gas on plateau in 700 8C through 14000 8C steps Plateau age 5 11.88 6 0.07

#2158 J 5 0. 001195 6 0.50% Plagioclase Sample wt. 0.2502 g600 0.7 51.9 0.029 16.321 0.01 34.85 6 3.08700 2.9 72.5 0.126 5.688 0.09 12.22 6 0.79800 10.1 34.7 0.443 5.564 0.08 11.96 6 0.14900 18.5 87.0 0.810 5.594 0.08 12.02 6 0.151000 22.4 89.5 0.985 5.626 0.08 12.09 6 0.101100 19.1 82.3 0.838 5.629 0.07 12.10 6 0.041150 13.2 89.2 0.581 5.608 0.07 12.05 6 0.101200 13.2 90.5 0.578 5.591 0.07 12.02 6 0.11Total gas 100.0 81.5 0.000 5.681 0.08 12.2199.3% of gas on plateau in 700 8C through 1200 8C steps Plateau age 5 12.08 6 0.07

#2160 J 5 0.001250 6 0.50% Plagioclase Sample wt. 5 0.2501 g600 0.9 31.2 0.053 4.935 0.10 110.70 6 1.53700 5.1 75.8 0.289 3.786 0.08 8.22 6 0.25800 13.0 43.0 0.736 3.879 0.08 8.42 6 0.02900 22.2 69.4 1.257 3.908 0.08 8.48 6 0.051000 23.9 90.5 1.356 3.891 0.08 8.44 6 0.071100 18.5 81.8 1.047 3.877 0.08 8.41 6 0.031200 11.7 80.7 0.662 3.980 0.08 8.64 6 0.051300 4.8 65.2 0.270 4.403 0.08 9.55 6 0.16Total gas 100.0 74.4 0.000 3.930 0.08 8.5383.6% of gas on plateau in 600 8C through 1100 8C steps Plateau age 5 8.42 6 0.05

#2161 J 5 0.001204 6 0.50% Plagioclase Sample wt. 5 0.2495700 0.7 41.7 0.041 5.151 0.08 11.16 6 1.38800 5.7 18.4 0.330 4.203 0.08 9.11 6 0.28900 13.6 75.5 0.783 3.980 0.08 8.63 6 0.121000 23.2 90.6 1.333 3.946 0.09 8.55 6 0.051100 21.3 86.0 1.225 3.893 0.08 8.44 6 0.041200 18.0 90.1 1.034 3.935 0.09 8.53 6 0.041300 11.7 86.5 0.670 3.911 0.09 8.48 6 0.081400 5.8 69.9 0.332 4.153 0.09 9.00 6 0.14Total gas 100.0 81.3 0.000 3.969 0.09 8.6087.8% of gas on plateau in 9000 8C through 13000 8C steps Plateau age 5 8.50 6 0.05

Notes: 40Ar/39Ar analyses were conducted using a MAP 215 rare gas mass spectrometer. About 0.25 mg ofplagioclase from each of our samples was irradiated in the core of the U.S. Geological Survey TRIGA reactor(Dalrymple et al., 1981) to convert a portion of the 39K in the sample to39Ar. FCT-3 sanidine was irradiated alongwith the samples to monitor this conversion (McDougall and Harrison, 1988). An age of 27.791 0.07 Ma wasused for FCT-3 sanidine (Kunk et al., 1985). The irradiated plagioclase samples were heated in a low blankfurnace similar in design to that of Staudacher et al. (1978) and were degassed in a stepwise manner to producean age spectrum. Individual increments of gas were cleaned using SAES ST 707 and ST101 getters, and a Tigetter prior to analysis in the MAP 215 mass spectrometer. The resultant argon isotopic data were reduced andanalytical errors calculated with an updated version of the computer program ArAr*(Haugerud and Kunk, 1988),using the decay constant values recommended by Steiger and Jaeger (1977). Corrections used for the productionof interfering isotopes during irradiation are those of Dalrymple et al. (1981) and Roddick (1983). All uncertaintiesare stated at 1o. The occurrence of age plateau was determined using the definition of Fleck et al. (1977) asmodified by Haugerud and Kunk (1988). A rigorous discussion of the error propagation calculations, as well asage calculations, can be found in Haugerud and Kunk (1988). The40Ar/39Ar age spectrum data for all of thesamples can be found in Table 3 and Figure 4.

†39ArK gas quantities are in moles 3 10–1.‡All precision estimates are at the one sigma level. Ages of individual steps do not include error in the irradiation

parameter J. No error is calculated for the total gas age. The plateau age includes the error in the irradiationparameter J. Ages calculated assuming an initial40Ar/36Ar 5 295.5 6 0.

From the Langhian to middle Serravalliantime there was no discernible bathymetricchange in either the Bocas del Toro or Limonbasins (Fig. 6). In late Serravallian time (ca.12 Ma), upper bathyal deposition (ValienteFormation) in the Bocas del Toro Basinchanged to alternating deposition of terrestrialand nearshore sedimentary rocks, as indicatedby coarse-grained basaltic flow breccia andcoral-reef limestone with large colonies ofhermatypic corals that crop out in the ToroPoint section. Upper Serravallian to middleTortonian deposits of the Panama Canal Basin(Gatun Formation) also reflect inner- to middle-neritic depths (Collins et al., 1996). Shallowdepths persisted in the Bocas del Toro Basinuntil late Tortonian to Messinian time, as in-dicated by the Tobabe Sandstone, coarse-grained nearshore marine conglomeratic sand-stone packed with large sand dollars and largebivalves. Later in the Messinian, the depthsdecreased to upper bathyal, represented by theNancy Point Formation. By Zanclean time,water depths had shallowed from upper bathy-al to neritic in both the Bocas del Toro Basin(Shark Hole Point and Cayo Agua Forma-tions) and the Limon Basin (Rio Banano, Que-brada Chocolate, and Moin Formations).

DISCUSSION

The discovery of the Punta Alegre Forma-tion and Valiente Formation extends the geo-logic history of the Bocas del Toro Basin fromca. 8 Ma (the base of the Bocas del ToroGroup; Coates et al., 1992; Coates, 1999) backto early Miocene time, ca. 18–21 Ma. Thelower-bathyal (1000–2000 m), muddy andsilty ooze of the Punta Alegre Formation re-cords the pre-isthmian tropical Americanocean. Coeval deposits, like the Uscari For-mation in the southern Limon Basin (Casselland Sen Gupta, 1989b; Bottazzi et al., 1994)and the Clarita and Tapaliza Formations of theDarien region (Collins et al., 1998), were alsodeposited at bathyal depths. This finding sug-gests that an extensive oceanic gap existed be-tween the Central American volcanic arc andSouth America.

Evidence from the overlying Valiente For-mation indicates the rapid rise of the pre-isthmian sill between 18 and 16 Ma (Figs. 6,7A). Middle Miocene (Langhian; 16.4–14.8Ma) deposits at Avispa Point, Toro Point, andCusapin Village indicate upper-bathyal (200–600 m) paleodepths, and those at Llorona Hilland Deer Island record middle-bathyal (600–1000 m) paleodepths. These turbidite and re-sedimented shelly units presumably formed onthe unstable slope of the rising isthmian arc.


COATES et al.

Figure 6. Chronological chart showing bathymetric ranges of PPP sites (numbered) determined by benthic foraminifera for various agesfrom the Valiente Peninsula, Popa Island, and the southern Limon and Panama Canal basins. Dashed lines enclose the same geologicsection. Dotted lines show the biochronologic ranges of sections that are placed according to physical stratigraphy.

A similar bathymetric sequence is recordedfor the Uscari Formation in the southern Li-mon Basin.

Middle- and upper-bathyal paleodepthswere sustained through the lower part of theSerravallian in the upper Toro Point and Cu-sapin Creek sections (Fig. 6). Further rapidshallowing in late Serravallian time is, how-

ever, signaled by the widespread developmentof columnar basalt lava; flow breccia; fluvia-tile to estuarine, coarse-grained volcaniclasticdeposits; lahars; and intercalated reef lensespacked with large coral heads. These rockstestify to the extensive emergence of the isth-mian volcanic arc in the Bocas del Toro areaby 12 Ma (Fig. 7B).

The reef lenses in the Valiente Formationappear to represent patchy shallow-waterfringing reefs around volcanic islands whoseflanks were otherwise characterized by laharicbreccia and basalt flows. The lahars and flowsgrade laterally into fluviatile and deltaic se-quences and nearshore marine deposits. Vol-canic activity in the area appears to have



Figure 7. (A) Schematic paleogeographicreconstruction of the Isthmus of Panama at15–16 Ma. Circles represent reliably datedsections that have yielded rich benthic fo-raminiferal assemblages useful for paleo-bathymetric analysis. The Central Cordil-leran volcanic arc is shown as a line ofislands; the triangles represent volcanoes.Evidence of middle Miocene volcanism inwestern Panama is from de Boer et al.(1988, 1991) and references therein. Landareas to the south of the arc are interpretedas exotic terranes. The Panamanian Isth-mus at this stage was a volcanic island arcwith a narrow neritic zone. The sedimen-tary rocks of the Southern Limon and Bo-cas del Toro Basins indicate bathyal paleo-depths in contrast to the Panama CanalBasin, where neritic to emergent conditionspersisted through most of the Cenozoic. (B)Schematic paleogeographic reconstructionof the Isthmus of Panama at 11–12 Ma.Symbols as for A. The neritic zone has bythis time expanded significantly, and anemergent active volcanic backarc has de-veloped in the Bocas del Toro Basin. Weassume that the Central Cordilleran volca-nic arc has become more emergent.

ceased by ca. 8 Ma. Two 40Ar/39Ar samplesfrom a basalt dike in the Valiente Formationexposed at Laurel Point on Popa Island yieldages of 8.46 6 0.04 Ma and 8.52 6 0.06 Ma,respectively (Fig. 6). The youngest igneousvolcanic rocks recorded in the Bocas del Toroarea are from the basalt dike exposed at LaurelPoint in Popa Island dated at ca. 8.5 Ma.

Coeval (ca. 11.8–11.4 Ma) inner neritic de-

posits in the Panama Canal Basin (Collins etal., 1996) suggest that the emergence of theisthmus had simultaneously developed to asimilar degree in the Panama Canal Basin(Fig. 7B). To the northwest, in the southernLimon Basin, shallowing was slower becausethere the uppermost Uscari Formation recordsupper-bathyal (200–600 m) paleodepths (Fig.7B) as late as the latest Miocene (Messinian,

7.26–5.32 Ma). Middle Miocene deposits(Langhian and Serravallian; ca. 16.4–11.2Ma) in the Darien region (Clarita and TapalizaFormations; Collins et al., 1998), and Burdi-galian through Messinian sedimentary rocks(ca. 20–6 Ma) of the Atrato Basin in the Pa-cific coastal region of northwest Colombia(Uva, Napipi, and Sierra Formations; Duque-Caro, 1990), also record continuing bathyalpaleodepths until the latest Miocene (Fig. 6).This fact suggests that a deep-water gap stillexisted between the Central American volca-nic arc and South America at this time, pre-sumably responsible for the virtual absence ofexchange of terrestrial vertebrate faunas be-tween North and South America until middlePliocene time (2.8 Ma).

In the Bocas del Toro Basin, the ValienteFormation is unconformably overlain by theTobabe Sandstone Formation (Coates et al.,1992; Coates, 1999), indicating continuedvery shallow, marginal marine conditions.Sedimentary rocks of the conformably over-lying Nancy Point Formation (Messinian,7.26–5.32 Ma; Aubry and Berggren, 1999)show that deepening proceeded to upper-bathyal depths (200–600 m; Collins, 1993) inthe Messinian. This deepening is also record-ed in the Panama Canal Basin (Fig. 6) by the


COATES et al.

rapid transition from the inner neritic (20–40m), middle to upper Gatun Formation to theupper bathyal Chagres Formation (Collins etal., 1996). It is not clear whether this deep-ening represents a regional eustatic event orlocal cooling and sinking of the Bocas delToro volcanic arc. By the early Pliocene (Zan-clean), rapid shallowing again occurred in theBocas del Toro Basin (Shark Hole Point andCayo Agua Formations, Fig. 6) as in thesouthern Limon Basin of Costa Rica (Fig. 4,Limon Group; McNeill et al., 1999).

CONCLUSIONS

The Punta Alegre Formation and the Val-iente Formation document the earliest stagesof the rise of the Isthmus of Panama in theBocas del Toro region. Evidence of the pre-isthmian tropical American ocean is providedby the Punta Alegre Formation, which rangesin age from ca. 18.5 to 21.5 Ma, and accu-mulated in lower bathyal water depths. Coevaldeposits with similar paleobathymetry, likethe Uscari Formation in the southern LimonBasin of Costa Rica and the Uva and NapipiFormations of the Atrato Basin in northwestColombia, confirm the presence of this wide-spread ocean separating North and SouthAmerica.

The distribution of the Valiente Formationfacies suggests that, around the Bocas delToro Basin, the pre-isthmian sill rose to mid-dle bathyal (Llorona Hill and Deer Island sec-tions) and upper bathyal (Avispa and ToroPoints and Cusapin Village sections) paleo-depths between ca. 18 and 15 Ma and lay tothe north of a sparsely emergent Central Cor-dilleran arc (Fig. 7A). These paleodepths per-sisted until early Serravallian time (ca. 15–13Ma). In the late Serravallian, the pre-isthmiansill in the Bocas del Toro region shallowedrapidly, so that by ca. 12 Ma there was anactive, emergent volcanic backarc (Fig. 7B)parallel to the main Central Cordilleran arc.

Coeval sedimentary rocks deposited inshallow water also occur in the Panama CanalBasin (Gatun Formation), but shallowing wasslower in the southern Limon Basin where theupper Uscari Formation indicates upper bathy-al (200–600 m) paleodepths (Fig. 7B). Bathy-al depths also prevailed in the Darien regionof eastern Panama and in the Atrato Basin ofnorthwestern Colombia until the latest Mio-cene. This fact suggests a continued albeitmore narrow oceanic gap between SouthAmerica and the Central American isthmus,also confirmed by the absence of any connec-tion of terrestrial faunas of North and South

America until late Pliocene time (Webb andRancy, 1996).

In the Bocas del Toro region, inner neriticpaleodepths, represented by the Tobabe Sand-stone, prevailed until the Messinian (7.26–5.32 Ma), when widespread deepening to up-per bathyal depths is recorded by theoverlying Nancy Point Formation. This eventwas paralleled in the Panama Canal Basin bythe transition from the Gatun Formation (ca.8 Ma, 20–40 m paleodepth) to the overlyingChagres Formation (ca. 6 Ma, 200–600 m pa-leodepth). In the Bocas del Toro region,depths subsequently shallowed to inner neriticdepths (Shark Hole Point and Cayo Agua For-mations; 5.2–3.5 Ma). Coeval deposits in thesouthern Limon Basin (Rio Banano, QuebradaChocolate, Moin, and Suretka Formations)also record rapid shallowing and emergence inthis region. The cause of the Messinian (7.26–5.32 Ma) deepening event in the Bocas delToro and Panama Canal basins is not clear.Cooling and sinking of the Bocas del Torovolcanic arc, after ca. 8 Ma, is not easily ex-trapolated to the Panama Canal Basin. How-ever, more detailed work in the southern Li-mon Basin and the Darien region is necessarybefore a eustatic cause can be unequivocallyestablished.

ACKNOWLEDGMENTS

We are deeply grateful to Xenia Saavedra deGuerra for editing the manuscript, producing Fig-ures 1, 2, and 3, and compiling the occurrencecharts of nannofossils and planktic foraminifera (Ta-ble 1). We are also grateful to Jijun Zhang and Hui-chan Lin for preparation of foraminiferal and nan-nofossil samples, respectively, and to Deanna Hurtfor preparation of benthic foraminiferal samples. Allmicrofossil samples are deposited in the Smithson-ian Institution, National Museum of Natural Histo-ry. Collins and Coates gratefully acknowledgegrants DEB-9300905 and DEB-9696123 from theNational Science Foundation. Field work and sam-ple preparations were made possible by awardsfrom the National Geographic Society. Coates wasalso supported by grants from the Scholarly StudiesProgram and Walcott Funds of the Smithsonian In-stitution. The manuscript has greatly benefited frominsightful reviews by Susan Kidwell, Jelle Zeilingade Boer, and Nancy Riggs.

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