Geologia Del Bocas Del Toro

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An Introduction to the Geology of the Bocas delToro Archipelago, Panama

A. G. COATES1, D. F. MCNEILL2, M-P. AUBRY3, W. A. BERGGREN4, L. S. COLLINS5

1Smithsonian Tropical Research Institute, Apartado 2072, Republic of Panama2Division of Marine Geology and Geophysics, Rosensteil School of Marine and Atmospheric Science, University of

Miami, Florida, 33149, USA3Department of Geological Sciences, Rutgers University, Wright Labs, 610 Taylor Road,

Piscataway, New Jersey, 08854-80664Woods Hole Oceanographic Institute, Department of Geology and Geophysics, Woods Hole, Massachusetts, 02543,USA and Department of Geological Sciences, Rutgers University, Wright Labs, 610 Taylor Road, Piscataway, New

Jersey, 08854-8066, USA5Department of Earth Sciences, Florida International University, Miami, Florida, 33199, USA

Corresponding author: [email protected]

ABSTRACT.We review the stratigraphy of the Neogene rocks of the Bocas del Toro archipelago, westernCaribbean coast of Panama, and provide new geological maps and a preliminary description of new Neogene

formations on the islands of Bastimentos and Colon. The Punta Alegre and Valiente formations range in agefrom 19 to 12 Ma. After a hiatus from 12 to 8 Ma, a transgressive/regressive sedimentary cycle is recorded by

the Tobobe, Nancy Point, Shark Hole, Cayo Agua, and Escudo de Veraguas formations (=the Bocas del ToroGroup), that range in age from 7.2-5.3 to 1.8 Ma. In contrast, in the northern region, the Old Bank, La Gruta,

and Ground Creek units and the Swan Cay Formation only range in age from about 3.5 to 0.78 Ma. The hiatusrepresented by the unconformity is from 12 to 3.5 Ma. We integrate the geology of the Bocas del Toro

archipelago into a brief history of the Neogene of the lower Central American isthmus.

KEYWORDS.Neogene, Bocas del Toro, Central American Isthmus

INTRODUCTION

Background

This overview paper provides an intro-duction to the geological foundation of themodern biological systems found today inthe Bocas del Toro region. The study of thegeology of Bocas del Toro was undertakenas part of the Panama Paleontology Project(PPP; Collins and Coates 1999), a collabo-rative research program to study sedimentsdeposited during the last 20 million years(involving the Miocene, Pliocene, and Pleis-tocene Epochs, which are together knownas the Neogene) along both sides of theisthmus of Central America and of north-

ern South America (Fig. 1).We systematically mapped the whole re-gion, named the physical stratigraphic for-mations (Fig. 2) and located the rich anddiverse macrofossil sites. After additionalpaleomagnetic studies and radiometric dat-ing, these studies provide a detailed geo-logic history of the region and a temporal

framework for the evolutionary and eco-logical studies of the macrobiota.

Regional Geologic History

The Central American isthmus forms thewestern margin of the Caribbean Plate andlies at the center of a complex intersectionof the Pacific Cocos and Nazca plates withthe Caribbean Plate and the small PanamaMicroplate (Fig. 1). The dominantly oceanicCaribbean Plate lies between the North andSouth American plates. Its relative east-ward motion, with respect to the North andSouth American plates is accommodated

by strike-slip faults to the north and in part

to the south (now confounded by compres-sion from the west-northwestward-movingSouth American Plate). In the east, it is

bounded by the Lesser Antilles subductionzone. The western margin of the CaribbeanPlate is more complex; in the northern part,the Cocos Plate is in contact with the Car-ibbean Plate.

Caribbean Journal of Science, Vol. 41, No. 3, 374-391, 2005Copyright 2005 College of Arts and SciencesUniversity of Puerto Rico, Mayaguez

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In the southern portion of the westernmargin, a triple junction brings the Cocosand Nazca Plates in contact with the smallPanama Microplate (Fig. 1). The PanamaMicroplate appears to have formed bynorthward escape from compression of theSouth American and Cocos/Nazca plates(Burke and Sengor 1986; Coates et al. 2004).Much of Central America lies either on thetrailing western edge of the CaribbeanPlate or on the Panama Microplate but aportion also lies on the southwestern cor-

ner of the North American Plate (Fig. 1).Since their formation in the Miocene, theNazca and Cocos plates have impinged onCentral America with different motions.The Cocos Plate (Fig. 1), with relativenortheasterly motion, is subducting vigor-ously under northern Central America asfar south as the Costa Rica-Panama border

and is associated with active seismicity andvolcanism. Northern Central America alsohas a broad zone of older continental crust,and has a geologic history extending backto the early Paleozoic (Donnelly et al. 1990).In contrast the Nazca Plate has a relativeeasterly motion but is not actively subduct-ing under Panama. The Panama Microplateis formed of oceanic crust that is typical ofthe widespread basalt plateau that under-lies much of the rest of the Caribbean Plate(Case et al. 1990).

Three major movements have affectedthe Neogene tectonic evolution of thesouthern Central American isthmus (CAI;Kolarsky et al. 1995; Coates and Obando1996). The first is convergent tectonics ofthe eastern Pacific subduction zone (Fig. 1),the primary driving force that created theCentral American isthmus by forming a

FIG. 1. Map of southern Central America (dark shading) and the Panama microplate (pale shading). Dashedlines with teeth mark zones of convergence; zippered line is Panama-Colombia suture. Very heavy dashed linemarks location of Neogene volcanic arc. Fine arrows are Paleogene faults; thick arrows are late Neogene faults.Principal Neogene sedimentary basins located by striped ovals. Spotted pattern defines the Cocos Ridge. Arrowson the inset indicate directions of relative motions of the plates. PFZ = Panama Fracture Zone

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volcanic arc extending southwards fromNorth America. It forms part of an exten-sive zone of subduction that runs the

length of the western margin of North,Central, and South America (Astorga et al.1991).

The second tectonic effect was subduc-tion of the Cocos Ridge (Kolarsky et al.1995; Collins et al. 1995), a lighter and rela-tively thick welt of Pacific oceanic crust,under the Central American volcanic arc inCosta Rica (Fig. 1). This hard-to-subductridge rapidly elevated the Central Ameri-can Isthmus from the Arenal volcano inCosta Rica to El Valle in Panama (de Boer etal. 1988) culminating in the center wherethe Talamanca Range rises to about 4000 m.

The elevation of the Talamanca range prob-ably substantially reduced the number ofmarine connections between the Pacific andCaribbean.

The third tectonic influence on southernCentral America was the collision of theCentral American volcanic arc (the westernmargin of the Caribbean plate) with north-

western South America (Coates et al. 2004;Silver et al. 1990; Mann and Kolarsky 1995).The relative northwestward movement ofSouth America (Fig. 1) has increasinglycompressed the southern Caribbean Platemargin in the late Neogene. This conver-gence has uplifted eastern Panama and thenorthern Andes of Colombia and Venezu-ela.

The resulting shoaling of the CAI (de-tected paleoceanographically by a markeddivergence between planktonic foraminif-eral oxygen isotope records (a proxy for sa-linity changes) was to less than 100 m from4.7-4.2 Ma (Keigwin 1982; Haug and Tiede-mann 1998; Gussone et al. 2004). This initi-ated the development of the modern Atlan-tic-Pacific contrast in sea surface salinities(SSS), with higher SSS in the Caribbean.

Recent studies by Tiedemann, Steph, andGroenveld and others (Poster sessions,American Geophysical Union Meeting,2004 and pers. comm. 2005) have suggested2.8 as the time of final closure of the CAI.Using Ca/Mg measurements of planktonicforaminifera, they show that Caribbean andPacific Sea Surface Temperature (SST) rec-ords are similar from 5-2.8 Ma. Then largerscale sea level fluctuations with 41 kyr. cy-clicity became dominant in response toam-plification of the Northern Hemisphere gla-ciation. After 2.8 Ma, glacial stages, whensea level is lowest, show minimum PacificSSTs, but maximum SSTs occur in the Car-ibbean. This anomaly is explained by lowsea stands preventing cooler less saline wa-ter from entering the Caribbean from thePacific (the CAI is emergent), and the re-verse applies in interglacials when sea levelis high, breaching the CAI.

The closure of the Isthmus of Panamatriggered profound environmental changeson land and in the sea. The formation of a

bridge between the Nort h and SouthAmerican continents gave rise to the Great

American Biotic Interchange on land(Webb and Rancy 1996; Webb 1999). Lesswell known are the timing and nature ofthe changes in the sea, consequent upon thegrowth of the Central American volcanicarc and its eventual collision with SouthAmerica. This created a marine barrier thatincreasingly affected ocean circulation dur-

FIG. 2. Physical stratigraphic nomenclature of theNorthern and Southern regions of the Bocas del Toroarchipelago with ages in millions of years

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ing Neogene time. In the process, a strikingcontrast evolved between the relativelywarm, more saline, nutrient-poor Carib-

bean and the more seasonal, less saline andmore pelagically productive eastern Pacific(Jackson and DCroz 1999). These environ-mental changes apparently altered thecourse of evolution, first of the deep-waterplanktonic organisms like radiolaria anddiatoms from about 15 Ma, and then suc-cessively shallower taxa until completeemergence at about 2.8 Ma. By this time,the eastern Pacific had evolved rich pelagic

biotas but poorly developed, low diversity,coral reefs (Jackson and DCroz 1999). Incontrast, the Caribbean had become domi-nated by coral reef-seagrass-mangrovecoastal ecosystems (Collins et al. 1996) anda profound taxonomic turnover had oc-curred in corals and mollusks (Budd and

Johnson 1997, 1999; Jackson et al. 1993).Much of this history is documented in

the long geological section preservedaround the Bocas del Toro Archipelagowhose deposits range in age from the earlyMiocene (about 20 Ma). The sequence can

be assigned to four phases in the rise of theisthmus as follows: 1) Deposition of lower

bathyal, pre-isthmian, oceanic sediments inearly Miocene time (21.5-18.5 Ma); 2)Growth of a volcanic arc during middleMiocene time (18 to12 Ma) characterized

by terrestrial and marine deposits includ-ing columnar basalt and flow breccia,coarse volcanic sediments, and scatteredsmall coral reefs; 3) Extinction of the arcabout 12 Ma and subsequent extensiveemergence and erosion; and 4) Subsidenceof the volcanic arc during the latest part ofMiocene time (7.2-5.3 Ma), resulting in amarine transgression, followed by a marineregression, that culminated in the wide-spread development of PlioPleistocenereefs (McNeill et al. 2000; Coates et al.2003).

STRATIGRAPHY

For the purposes of this paper, we firstdescribe the stratigraphic units that occurin the south and north of the Bocas del ToroArchipelago. These are discussed sepa-rately because the sequences are different

in these two areas (Fig. 2). We then attemptto synthesize the geologic history of Bocasdel Toro and compare it with adjacent re-gions in Costa Rica, Darien, Panama, andthe Atrato Valley, Colombia.

1) The northern region comprises SwanCay, Colon, Pastora, San Cristobal, Cari-nero, and Bastimentos islands, and theZapatillo Cays.

2) The Southern region comprises the is-lands of Popa, Deer, Cayo Agua, and Es-cudo de Veraguas, and the Valiente Pen-insula.

In general, the northern region exhibits alate Pliocene-Pleistocene succession of shal-low water sediments, especially coral reef

deposits, that either unconformably overliemiddle Miocene volcanic arc basalt (Basti-mentos Island) or rest on a thick (>2500 m)siliciclastic shale (Colon Island) that is latePliocene at its top and of unknown age atits base. This suggests that there is a majorstructural break in the volcanic arc base-ment rocks between Bastimentos and Co-lon islands. The southern region reveals amore extensive volcanic arc suite of lowerand middle Miocene rocks, including a se-quence of deep-sea ooze, basalt, coarse vol-canic sediments and small scale patch reefs,that tracks the onset and rise of an active

volcanic arc in the Bocas del Toro region.Subequently, an unconformably overlying,non-reefal, upper Miocene and Pliocene,marine, transgressive/regressive shelf se-quence was deposited.

Stratigraphic definitions

Stratigraphy of sedimentary rocks in-volves the integration of four main kindsof units, each of which provides the basisof a subdiscipline of stratigraphy. Thefirst, lithostratigraphy, describes the physi-

cal stratigraphic units and their three-dimensional geometry across the regionstudied. Each mappable unit orFormationisdistinguished by its rock type or lithologyand is named, usually from the place whereit most typically can be observed. A seriesof formations deposited in vertical succes-sion in a genetically related depositional



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episode may be lumped together as aGroup.

If the contact of two lithostratigraphicunits shows evidence of a break in sedi-mentation this line is called an unconfor-mity. Unconformities usually reflect majorgeological events (like tectonic uplift or sealevel change) in the area where they occur.The significance of unconformities willvary depending on how different the unitsare above and below, and how much timethe break represents. From the nature of thedifferent sediments and their associatedsedimentary structures and fossils, thephysical environment and water depth inwhich the formation was deposited can bereconstructed. Fig. 2 outlines the nomencla-ture of the lithostratigraphy of the Neogeneformations of the Bocas del Toro archi-pelago.

The second subdiscipline is biostratigra-phy, which takes advantage of the fossilcontent of rocks, and is based on the rangeof species, i.e., the stratigraphic interval be-tween two specific stratigraphic horizonscorresponding to the lowest (LO) and thehighest (HO) occurrence of a taxon. The ba-sic unit of biostratigraphy is the biozone, astratigraphic interval defined between LOand HO of taxa selected for their distinctmorphology and vast geographic distribu-tion. Biozones allowstratigraphic correlation,the fundamental procedure of establishingthat two horizons have the same relativeage. Chronostratigraphy, the third elementof stratigraphy, is a system of reference forthe relative dating of sediments. Its basicunit is the stage, defined by its base (= astratigraphic horizon) and a stratotype (astratigraphic interval). Correlation of rocksin a given basin to a stage is established via

biostratigraphy. Stages are grouped in anested hierarchy ofSeries, Systems, and Er-athems. Their temporal equivalents arecalledAges, Epochs, Periods, and Eras, respec-

tively.The fourth subdivision, geochronology,is the science of numerical time. Whereaschronostratigraphy deals with relative time(younger/older), geochronology trans-forms specific stratigraphic horizons intonumerical markers. Geochronology usesradioisotopic ages and/or astrocyclicity

(i.e., Milankovich periodicity), usually inconjunction with paleomagnetic stratigra-phy, to determine the ages of chronostrati-graphic and biostratigraphic boundaries.i.e. the stratigraphic level common to twosuccessive stages or biozones. Numericalages vary depending on the criteria se-lected in the building of a gechronologicframework ortime scale.The ages given be-low refer to the time scale of Berggren et al.(1995). The boundaries between lithostrati-graphic units may be of the same age re-gionally, but over larger regions they may

become younger or older in age as thephysical environments they represent mi-grated across the sedimentary basinthrough time. They are then said to be di-achronous.

The southern region

Each lithostratigraphic unit, starting withthe lowest, will first be named and de-scribed, and then evidence for its age andenvironment of deposition will be added.For a more detailed description of thestratigraphy of these units see Coates et al.(1992, 2003) and Coates (1999).

Punta Alegre Formation.This formationis named for Punta Alegre, the nearestsmall village to its solitary exposure. PuntaAlegre is located on the western tip of thenorth coast of Bahia Azul (Bluefield Bay) inthe northern part of the Valiente Peninsula(Fig. 3). The formation can be observed in aprominent small cliff about 1.3 km north-west of the village, on the west-facing coast1 km south of Valiente Point. It consists ofclay and silty ooze containing abundantcalcareous nannofossils and benthic andplanktonic foraminifera (see Coates et al.2003, Tables 1, 2), many of the latter are eas-ily seen by a hand lens and by the nakedeye. The formation can be seen to lie un-

derneath weathered coarse volcanic flowbreccia (the broken up bouldery rock that iscrated when lava flows and cools at thesame time). From its abundant planktonicforaminifera and calcareous nannofossils,the age of the Punta Alegre Formation is19-18.3 Ma. (Coates et al. 2003), and by its

benthic foraminifera it is interpreted to

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have been deposited in water depths of1002000 m (Coates et al. 2003).

The Valiente Formation.This unit over-lies the Punta Alegre Formation and cropsout extensively on the Valiente Peninsula(Fig. 3) and nearby Deer and Popa islands(Fig. 4). The Valiente Formation is of com-plex and highly variable lithology becauseit represents the suite of facies, marine andterrestrial, igneous and sedimentary, that isassociated with an active emergent volca-nic island arc. It includes columnar basalt,

basalt flow breccia, pyroclastic tuff (air-borne volcanic ash deposits), fluviatile/estuarine conglomerate, marine debris

flows and coral reef lenses. The latter con-sist of several species of Montastraea (in-cludingM. imperatorisandM. canalis), mas-sive Porites waylandi and thin branchingStylophora(possiblyS. granulate; Ann Buddpers. comm. 2005). These facies intercalateand replace each other over very short dis-tances, both laterally and vertically (for de-tailed descriptions of the facies, see Coateset al. 2003). Fig. 3 shows that the basalt lavaand flow breccia facies form a core aroundwhich the other facies are distributed pe-ripherally as terrestrial, coastal or marineslope deposits. Planktonic microbiotas andradiometrically dated basalt in the se-

FIG. 3. Geological map and cross section (a ) of the Valiente Peninsula, Bocas del Toro, western Panama,showing the distribution of the Punta Alegre and Valiente formations and the Bocas del Toro Group. The fivelithofacies of the Valiente Formation are indicated by separate colors and numbers on the key (upper right) asfollows; v1) basalt lava and flow breccia facies; v2) coarse volcaniclastic facies; v3) pyroclastic facies; v4) coralreef facies; v5) marine debris flow and turbidite facies.



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quence indicate that the Valiente Formationranges in age from 16.5-11.5 million yearsold and includes marine deposits fromnear-shore to as deep as 1000 m, as well asseveral types of terrestrial deposits (Coateset al. 2003).

Bocas del Toro Group.Columnar basaltand pyroclastic and fluviatile rocks of theValiente Formation required extensiveemergence of parts of the growing volcanicarc (columnar basalt is formed by the cool-

ing of lava ponded lakes). About 12 Ma, thearc had become inactive and recent studiesin Darien, Panama (Coates et al. 2004), sug-gest that the collision of the southern tip ofthe Central American volcanic arc withSouth America was initiated at the sametime. These events may explain the majorunconformity in the Bocas del Toro Archi-

pelago above the Valiente Formation, sig-nifying extensive emergence and erosion ofthe volcanic arc from11.5-7.2 Ma (Fig. 2,3). This unconformity can be clearly ob-served in the inner islands of the PlantainCays and along the coast immediately tothe west. The underlying columnar basaltof the Valiente Formation can be seen onthe north side of the Cays and the fossilif-erous conglomerate and sandstone of theTobobe Sandstone on the south side.

The Bocas del Toro Group signifies a newdepositional cycle as a marine transgres-sion submerged the older eroded volcanicarc rocks. The oldest sediments thus repre-sent the first shallow-water, beach, andnear-shore sand deposits. As the transgres-sion developed, subsequent overlying ma-rine units reflect deeper water depositionuntil the reverse occurred and the sea re-gressed to shallow water again. Thus, theunits of the Bocas del Toro Group are ge-netically related and document a single ma-rine transgressive/regressive event. Themost continuous section lies along the eastand west coasts of the Valiente Peninsula(Fig. 3) where it ranges in age from lateMiocene (Messinian, 7.2-5.3 Ma) to latePliocene (3.5 Ma).

Tobobe Sandstone.This lowest unit is apebble conglomerate that passes up intoclean, hard quartz sandstone containingsand dollars, spatangoid echinoids, mol-lusks, including the large, thin-shelled bi-valve Amusium, vermetids and serpulids,as well as an array of infaunal burrowstructures. It is best observed on the smallPlantain Cay and an adjacent smaller un-named island to the west (Fig. 3). The de-posit represents a beach and near-shoresand body and documents the earliest stageof the marine transgression. Planktonicforaminifera from laterally equivalent de-posits on Toro Cay (Fig. 3) indicate that theage of the unit is Messinian (7.2-5.3 Ma).

Here the deposit contains a variety of mol-lusks including turritellids and pectens,erect bryozoa, and echinoids. Classic ex-amples of complex thalassinoid burrowsystems constructed by callianasid crusta-ceans are also present.

Nancy Point Formation.It was namedfor the promontory called Nancy Point

FIG. 4. Geological map of Popa Island. Section (b-b)shows the Valiente Formation on Popa Island, whereit is unconformably overlain by the Pliocene CayoAgua Formation (ca). Numbers and symbols as forFigure 3. On Popa Island, only the v1 basalt flow faciesand the v2 coarse volcaniclastic facies (not associatedwith reef lenses) are present, with thin layers of lowrank coal, an example of which is exposed along thecoast immediately north of Punta Laurel. A prominent

basalt dike is exposed at the tip of the Punta Laurel(here shown in red as v1 facies) where it cuts the Va-liente Formation.

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which lies 2.5 km southwest of the villageof Tobobe (Fig. 3). The formation is almostcontinuously exposed southward along thecoast as far as Chong Point (= NisperoPoint). The Nancy Point Formation is con-formable with the Tobobe Formation belowand the Shark Hole Point Formation above(Fig. 2 and 3). It consists of shelly, muddyand silty sandstone, and muddy siltstonewith occasional coarse volcaniclastic and

bioclastic sandstone beds. It contains scat-tered mollusks and occasional leaves andplant fragments with diverse moderatelyabundant molluscan assemblages through-out the section and several, rich, low-diversity, thick-shelled mollusk beds at the

base.The transition from Tobobe Sandstone to

Nancy Point Formation is best seen on ToroCay (Fig. 3) where dark blue-gray, siltysandstone, typical of the Nancy Point For-mation, contains occasional, clearly definedinfaunal burrow systems and extremelyabundant and diverse mollusks. It overliescoarse, channeled Tobobe Sandstone withonly a 10-m exposure gap. Nancy PointFormation benthic foraminifera indicatethat deposition of the Nancy Point Forma-tion was in 200-500 m. water depth (Collins1993) demonstrating that relatively rapiddeepening took place from near-shore (To-

bobe Sandstone) to upper slope (NancyPoint Formation). This represents the maxi-mum depth of the transgressive/regressivecycle. Planktonic foraminifera and calcare-ous nannofossils date the Nancy Point For-mation as Messinian (7.2-5.3 Ma), the sametime range as for the Tobobe Sandstone.Within that time frame the Nancy PointFormation is younger because it overliesthe Tobobe Sandstone.

Shark Hole Point Formation.It is namedfor the promontory of the same name thatlies 3 km east of Chong Point (Fig. 3) andthe stratotype lies along the coast between

Chong Point and Bruno Bluff, south of OldBess Point. The Shark Hole Point Forma-tion conformably overlies the Nancy PointFormation (Fig. 2 and 3) and is overlain byan unnamed conglomeratic, cross-bedded,coarser grained sequence of volcaniclasticscontaining large pieces of wood and plantfragments. This unnamed unit is exposed

only along the southern coast of the Va-liente Peninsula, east of Secretario. TheShark Hole Formation consists of mica-ceous, clayey siltstone that is pervasively

bioturbated and rich in large scaphopods.The uppermost part of the formation alsocontains abundant, thin, shelly beds and in-traformational slumps with pillow foldsand rip-up clasts. Benthic foraminifera in-dicate that the paleobathymetry of this unitranges from 100-200 m (Collins 1993). Thisrepresents the first stage of the regressivephase of the Bocas del Toro Group trans-gressive/regressive cycle. Calcareous nan-nofossils and planktonic foraminifera areabundant in several horizons and suggestthe age of the formation is early Pliocene(5.3-3.6 Ma).

Escudo de Veraguas Formation.Th estratigraphic order of the formations de-scribed above has been determined byphysical superposition. The three remain-ing formations of the Bocas del Toro groupare known only on islands and their posi-tion relative to the other units has been de-termined through stratigraphic correlation.

The Escudo de Veraguas Formation isknown only from the island of the samename that lies 27 km east of Nancy Point(Fig. 5) . Its stratotype, for the lower part ofthe formation, is along the coast on the eastside of the V-shaped embayment situatedin the central part of the north coast, about1 km east of Long Bay Point (Fig. 5). Thestratotype for the upper part of the forma-tion lies along the west coast for about onekm south of Long Bay Point.

Lithologically, the Escudo Formationconsists of, in the upper part (1.8 Ma), per-vasively bioturbated clayey siltstone andsilty claystone, with frequent concretions,and scattered shelly hash, often with scat-tered whole and diverse mollusks, small,cornute, ahermatypic corals. The lower partof the formation (3.5 Ma) is more indurated

with very common and densely packed ce-mented burrow concretions and thalassi-noid galleries at the base, scattered aher-matypic corals through the middle part,as well as variably abundant mollusks, andat the top a coral biostrome dominated byStylophora and sand dollars. The top and

botton units suggest deposition in inner



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neritic water depths, but the middle part ofthis lower succession has abundant benthicforaminifera indicating deposition in outerneritic to upper bathyal water depths (Col-lins 1993).

Cayo Agua Formation.This formationwas named for the island that lies about 6km to the west of Toro Point, Valiente Pen-insula. The formation is well exposed alongthe east coast (Fig. 6) and consists lithologi-cally of pervasively bioturbated, muddy,silty sandstone with common horizons of

abundant thick-shelled mollusks and aher-matypic corals. Occasional horizons ofpebble conglomerate and very coarse-grained volcaniclastic sandstone are com-

mon in the middle of the formation. Com-pared to the Shark Hole and Escudo deVeraguas formations, the Cayo Agua For-mation is consistently coarser-grained,with common basalt grains and granules,phosphatic pebbles, and wood fragments.A distinctive marker bed of corals occursnear the top of the formation and is wellexposed at Tiburon Point and the unnamedpoint to the south. The corals appear to bemostly free living, hermatypic, grass flatcorals, such asPlacocyathusandManicina(=Teleiophyllia), as well as ahermatypic spe-cies, and they suggest deposition in waterdepths of 30-50 m (Ann Budd, pers. comm.2005). Evidence from benthic foraminifera(Collins 1993) indicates paleobathymetry of20-80 m, overlapping the inference from li-thology and coral fauna that the Cayo AguaFormation represents a shallow-water fa-cies.

The age of the Cayo Agua Formation isdated at the base5.0-3.5 Ma and at the top3.7-3.4 Ma, which suggests it is a contem-porary, shallow water equivalent of theShark Hole Point Formation and the lower-most part of the Escudo de Veraguas For-mation.

The northern region

The islands in the northern region of theBocas del Toro archipelago have a differentgeologic history than the southern region.Volcanic arc columnar basalts of the Va-liente Formation form the basement thatunderlies the marine late Neogene succes-sion, as in the southern region. While noradiometric dates for these basalts areavailable, we can presume they are part ofthe same volcanic arc that went extinct andcooled after the middle Miocene (12 Ma).In all the Northern Region, the sedimentsunconformably overlying the Valiente For-mation basalt are Plio-Pleistocene in age.The upper Miocene succession of Tobobe

Sandstone, Nancy Point, and Shark Holeformations are absent. Furthermore, thenorthern units are characterized by a majorcomponent of coral reef limestone.

The geology of Colon and Bastimentosislands are currently being studied and theformal designation of the lithostratigraphicunits and their ages is not yet finalized. In

FIG. 5. Geological map of Escudo de Veraguas is-land. Ages in millions of years in brackets for lowerand upper parts of the Escudo de Veraguas Forma-tion.

FIG. 6. Geological map of Cayo Agua. Ages in mil-lions of years.

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general, the sequence is younger than thesection to the south with the exception ofthe Escudo de Veraguas Formation. Theoldest units on Colon and Bastimentos is-lands may overlap in time with the top ofthe Cayo Agua and Shark Hole Point se-quences at 3.5 Ma.

Valiente Formation.The spectacular ex-posures along the northwestern coast ofBastimentos Island (Fig. 7) of columnar ba-salt and flow breccia are assigned to theValiente Formation solely on their greatsimilarity in lithology. The exposures rangefrom Toro Point to the eastern side of

FIG. 7. Geologic map and section (a-a) of Bastimentos



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Dreffe Beach and occur also at the westernend of Long Beach. These basalt units prob-ably form the basement of the entire Bocasdel Toro Archipelago.

Unnamed Formations of Colon and Basti-mentos islands.There are three major litho-stratigraphic units (Fig. 2).

Old Bank Unit (informal field name).Thisis the oldest unit, and it crops out exten-sively in the southern half of Colon Island(Fig. 8) where it is crossed by the Bocas delDrago road along which there are severalgood exposures. It also occurs in the north-east of the island where it forms a distinctescarpment parallel to the coast. Manystreams have waterfalls where they crossthis scarp with good exposures. The sameunit crops out on Bastimentos Island (Fig.7). There are exposures from Juan BrownPoint eastwards along, and inland of thesouth coast, in steep stream courses flow-ing down from the ridge formed by its out-crop, which parallels the coast.

On Colon Island the Old Bank Unit con-sists mostly of a blue-grey mudstone withoccasional thin sandstone stringers, finevolcanic conglomerate and volcanic boul-der beds. On Bastimentos, it may be mica-ceous and sandier, with wood fragmentsand sparse mollusks including turritellidsand Anadara. Preliminary field observa-tions suggest it is an inner shelf depositflanking volcanic islands and that it isabout 3.5-2.0 Ma.

La Gruta Unit (informal field name).OnColon Island (Fig. 8), an extensive recrys-tallized reef and reef rubble deposit is ex-posed in the north central part of the island.Eastward from Hill Point it forms distinc-tive ridge running parallel to the northcoast towards Mimitimimbi Creek. Thelimestone extends southward as far as LaGruta, a locally famous bat cave. AroundLa Gruta, the limestone is a reef depositwith numerous large coral colonies but the

limestone grades into fore-reef rubblenortheastwards toward the mouth of Mim-itimimbi Creek where it is well exposed onthe coast. Both the reef and fore-reef depos-its are heavily fractured to produce a rub-

bly rock unit when exposed. Earthquakesand uplift are likely responsible for thispervasive fracturing.

Twenty species of corals have been iden-tified from the limestone near Hill Point,the most abundant being Caulastraea porto-ricensis,Stylophora granulata,Agaricia (= Un-daria agaricites), Colpophyllia natans, Mon-tastraea faveolata,and Mycetophyllia danaana(Ann Budd, pers. comm. 2005). Exposure ofsimilar limestone also occurs immediatelyto the North of Paunch, on the east coast ofColon Island, and on Carinero Island (Fig.8). At Paunch, an entirely extant fauna ofcorals includes Colpophyllia natans, Diploriastrigosa, Meandrina meandrites, Montastraea

faveolata, and Agaricia (Undaria) agaricites(Ann Budd, pers. comm. 2005).

On Bastimentos Island (Fig. 7), similarreef deposits appear to sit directly on theMiocene basalt of the Valiente Formationwhere they are well exposed on Wild CaneKey and along the coast for 2 km to the east.The reef limestone here is extensively re-crystallized but many coral colonies can beobserved. A particularly well preserved ex-ample of the La Gruta reef, packed withlarge and diverse coral colonies is exposedat the base of the cliffs in three small bays atFish Hole, at the southern end of LongBeach. The most abundant species are Di-chocoenia stokesi, Manicina (Teleiophyllia) gei-steri, Montastraea faveolata, M. canalis, Placo-cyathus variabilis, Porites waylandi, P.macdonaldi, Antillia gregorii, Goniopora im-

peratoris, Agaricia (Undaria) crassa, and Sty-lophora granulata. Because >50% of thisfauna is extinct, the La Gruta Formation onBastimentos appears to be older than thaton Colon Island (Ann Budd, pers. comm.2005).

Ground Creek Unit (informal field name).Interbedded with and overlapping onto theLa Gruta Unit are back reef/reef flank de-posits, dominantly shelly coral bearing bio-clastic carbonate and volcaniclastic sand-stone and siltstone. They are typicallyexposed in several stream courses in the

northwest of Colon Island in a regionknown as Ground Creek (Fig. 8). NearGround Creek, the unit has yielded 90 gen-era of bivalves and gastropods in siliciclas-tic mudstone and fine sandstone, wherethey are intercalated with thin carbonatesand containing poritid thickets and otherreef patches with platy Caulastraea portori-

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censis, Porites baracoaensis, Agaricia, ?Clado-cora,small domed ?Favia, Manicina (Teleio-

phyllia) and serpulids. Also occasionallyinterbedded are horizons of reworked largecoral heads.

The Ground Creek unit also crops out onBastimentos Island (Fig. 7), where it is in-terbedded with and overlying the La Grutaunit on Wild Cane Key, and it may alsocrop out around the Valiente Formation

FIG. 8. Geologic map and section (a-a) of Colon Island.



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outlier at the northern end of Long Beachand overlie the reefs at Fish Hole.

Swan Cay Formation.The formation is

named for the small island of the samename that lies 1.7 km off the northern coastof Colon Island (Fig. 8). The stratotype runsfrom north (youngest) to south (oldest)across Swan Cay. The formation has threecomponents. The lower 15 m is exposed onthe southerly low hill of the island and con-sists of silty sandstone and shelly calcaren-itic siltstone, with coral rubble and red al-gal fragments and balls. The middle 4 mconsists of calcarenitic clayey siltstone,with dense, fine shell-hash horizons, andabundant large coral colonies in the lowerpart. The upper 60 m of the formation con-

sists of massively thick-bedded, pale tan-white limestone. The upper 30 m includesa 4-m-thick coral bed with large Mon-tastraea colonies, other corals and mol-lusks. The lower 30 m contains silty cal-carenite with common red algae and largeforaminifera, shell hash, and micromol-lusks. This limestone also shows evi-dence of frequent microfracturing, many ofwhich are healed with secondary calcite ce-ment. Cave deposits, about 5 m above the

base of the calcarenite, consist of silty,shelly, volcaniclastic sandstone, mixedwith abundant volcanic cobbles and boul-

ders, and calcareous reef rubble contain-ing an abundant and diverse molluscan as-semblage. The most abundant corals are

Acropora palmata, A. cervicornis, Diplorialabyrinthiformis, Montastraea faveolata,Porites furcata, Agaricia (Undaria) agaracites,

Mean dr in a mean dr it es , an d Dich oc oe ni astokesi. Organisms from a range of depthsindicate that the deposit is reworked fore-reef debris formed at about 100 m (Collinset al. 1999). Microfossils are similarly re-worked. Abundant large and freshly pre-served globigerinid planktonic foraminif-era combined with paleomagnetic data

strongly suggest that the deposit is earlyPleistocene (0.78-1.77 Ma).

GEOLOGICHISTORY

The oldest sediments recorded in the Bo-cas del Toro Archipelago (Fig. 9) belong to

the Punta Alegre Formation and are earlyMiocene in age (ca. 19-18.3 Ma). They re-cord oceanic muddy and silty ooze depos-ited in lower bathyal depths (1000-2000 m)and the unit is interpreted to represent thepre-isthmian Neotropical ocean. Coeval de-posits, like the Uscari Formation in thesouthern Limn Basin (Cassell and SenGupta 1989; Bottazzi et al. 1994), the ClaritaFormation of the Darien (Coates et al. 2004)and the Uva and Naipipi formations of theAtrato Basin (Duque Caro 1990) in north-west Colombia were also deposited at

bathyal depths. Thus, prior to the collisionof the southern Central American arc withSouth America, upper Cretaceous to lowermiddle Miocene rocks were deposited inabyssal to lower bathyal depths in an open-o cean, lo w e ne rgy, e ssentially no n-siliciclastic sedimentary environment thatwas distant from South America (Coates etal. 2004).

Evidence from the overlying ValienteFormation indicates the rapid growth of apre-isthmian volcanic arc (whose southernend was still some distance from SouthAmerica) from about 16.5-12.3 Ma. Duringthe deposition of the Valiente Formation(Coates et al. 2003), paleodepths (Fig. 9) inthe region shallowed in general to upper

bathyal depths (200-600 m). However, sub-marine topographic relief was high as vol-canoes in the chain grew and coalesced,giving rise to steep sided slopes on whichthe sediments were deposited. Thus, nearPunta Alegre in the Valiente Peninsula, andDeer Island (Coates et al. 2003) sedimentswere deposited in middle bathyal depths(1000-600 m) and at Avispa Point, ToroPoint and Cusapin Village deposition wasat upper bathyal paleodepths (600-200 m).Widespread development of columnar ba-salt lava, flow breccia, fluviatile to estua-rine, coarse volcaniclastic deposits, lahars(terrestrial mudflow deposits), and interca-

lated reef lenses, packed with large coralheads, testify to the extensive emergence ofthe isthmian volcanic arc in the Bocas delToro area by 12 Ma (Fig. 3).

Fig. 10 outlines paleogeographic recon-structions for these events. The volcanic arcunderlying the Neogene sediments of Bo-cas del Toro was an active and emergent

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FIG. 10. A. Schematic paleogeographic reconstruction of the Panamanian Isthmus at 15-16 Ma. Circles rep-resent reliably dated sections that have yielded rich benthic foraminiferal assemblages useful for paleobathy-metric analysis. The Central Cordilleran Volcanic Arc is shown as a line of islands; the triangles representvolcanoes. Evidence of middle Miocene volcanism in western Panama is from de Boer et al. (1988, 1991) andreferences therein. Land areas to the south of the arc are interpreted as exotic terrains. The Panamanian Isthmusat this stage was a volcanic island arc with a narrow neritic zone. The Southern Limn and Bocas del Toro Basinsediments indicate bathyal paleodepths in contrast to the Panama Canal Basin where neritic to emergentcondition persist through most of the Cenozoic. B: Schematic paleogeographic reconstruction of the PanamanianIsthmus at 11-12 Ma. Symbols as for part A. The neritic zone has by this time expanded significantly and anemergent active volcanic back-arc has developed in the Bocas del Toro Basin. We assume that the CentralCordilleran volcanic arc had become emergent at this time.

FIG. 9. Chronological chart showing bathymetric ranges of various formations from the Valiente Peninsula,Popa Island, Cayo Agua, and the southern Limn and Panama Canal basins. Dashed lines enclose the samegeologic section. Dotted lines show the biochronologic ranges of sections that are placed according to physicalstratigraphy.



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A. COATES ET AL.388


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back-arc, parallel to the main Central Cor-dilleran arc. The environment of the Bocasdel Toro back arc archipelago, by 12 Ma,had patchy shallow-water fringing reefsaround active volcanic islands whose emer-gent flanks were characterized by laharic

breccia and basalt flows. The lahars andflows graded laterally into fluviatile anddeltaic sequences, and then into resedi-mented near-shore deposits. Major volcanicactivity in the area ceased about 12 Ma (theyoungest radiometric date on columnar ba-salt flows,) although there are younger ba-salt dykes intruding the Valiente Formationsediments, e.g., at Laurel point, Isla Popa(Fig. 4), dated about 8.5 Ma.

To the northwest, in the southern LimnBasin, a similar general shallowing patternis observed (Coates et al. 2004, Fig. 9). From18-13 Ma, the Uscari Formation shallowsfrom middle bathyal (1000-600 m) to outerneritic (100-200 m) depths. The record isabsent after 13 Ma. The Gatun Formation(11.8-11.4 Ma) an inner neritic (


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rapid growth of an active volcanic arcwhich by 12 Ma was an extensiveemergent archipelago of volcanic is-lands flanked by coral reefs.

3) After a prolonged period of emergenceand erosion, marine sediments began tocover the extinct volcanic arc startingabout 7.2-5.3 Ma.

4) The deposits of this transgressive/regressive marine cycle are named theBocas del Toro Group. The oldest units(Tobobe Sandstone and Nancy PointFormation), record deepening to 200 m.The younger units (Shark Hole Point,Cayo Agua, and Escudo de Veraguasformations) represent the regressive se-quence.

5) In the Plio-Pleistocene, an extensivecoral reef system developed either on a

basement of siliciclastic sediments (Co-lon Island, Swan Cay) or on the basalt ofthe Miocene Valiente Formation (Basti-mentos Island), reflecting the oceano-graphic changes taking place in thewestern Caribbean as a result of shal-lowing and closure of the Isthmus ofPanama.

Acknowledgments .This research wassupported by the National Science Founda-tion grants BSR-9006523, DEB-9696123 andDEB-9705289 to AGC, LSC, and Jeremy

Jackson; several grants from the NationalGeographic Society to AGC and LSC, andScholarly Studies and Walcott Fundawards from the Smithsonian Institution toAGC. We are grateful to the staff of theSTRI field station in Bocas del Toro and tonumerous field assistants of the PanamaPaleontology Project, who have helpedwith the complex logistics of fieldwork inthis region since 1986 (see Collins andCoates, 1999 for details), but particularly toBeatrice, Lucien Ferrenbach and DoroteoMachado, and whose constant efforts to

maintain motors and vessels, and to orga-nize food and gasoline, allowed us to workwith maximum efficiency. We are also verygrateful to Xenia Saavedra for laboratorytechnical and logistical support, includingorganizing (and sometimes preparing)samples, tracking them in the data base,producing field maps, and numerous other

tasks. Janet Coates, Gloria Jovane, LidiaMann, Isis Estribi, and the crews of the RVBenjamin and RV Urracca provided invalu-able logistical support over many years. Wealso thank Rachel Collin, Ann Budd, andAaron Odea for very helpful reviews.

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