Caribbean Journal of Science, Vol. 41, No. 3, 374-391, 2005 Copyright 2005 College of Arts and Sciences University of Puerto Rico, Mayagu¨ez An Introduction to the Geology of the Bocas del Toro 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 Panama 2Division of Marine Geology and Geophysics, Rosensteil School of Marine and Atmospheric Science, University of Miami, Florida, 33149, USA 3Department of Geological Sciences, Rutgers University, Wright Labs, 610 Taylor Road, Piscataway, New Jersey, 08854-8066 4Woods 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, USA 5Department 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, western Caribbean 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 age from 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 Toro Group), 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 hiatus represented 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 framework for the evolutionary and eco- logical studies of the macrobiota. Background This overview paper provides an intro- Regional Geologic History duction to the geological foundation of the modern biological systems found today in The Central American isthmus forms the the Bocas del Toro region. The study of the western margin of the Caribbean Plate and geology of Bocas del Toro was undertaken lies at the center of a complex intersection as part of the Panama Paleontology Project of the Pacific Cocos and Nazca plates with (PPP; Collins and Coates 1999), a collabo- the Caribbean Plate and the small Panama rative research program to study sediments Microplate (Fig. 1). The dominantly oceanic deposited during the last 20 million years Caribbean Plate lies between the North and (involving the Miocene, Pliocene, and Pleis- South American plates. Its relative east- tocene Epochs, which are together known ward motion, with respect to the North and as the Neogene) along both sides of the South American plates is accommodated isthmus of Central America and of north- by strike-slip faults to the north and in part ern South America (Fig. 1). to the south (now confounded by compres- We systematically mapped the whole re- sion from the west-northwestward-moving gion, named the physical stratigraphic for- South American Plate). In the east, it is mations (Fig. 2) and located the rich and bounded by the Lesser Antilles subduction diverse macrofossil sites. After additional zone. The western margin of the Caribbean paleomagnetic studies and radiometric dat- Plate is more complex; in the northern part, ing, these studies provide a detailed geo- the Cocos Plate is in contact with the Car- logic history of the region and a temporal ibbean Plate. 374 GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 375 FIG. 1. Map of southern Central America (dark shading) and the Panama microplate (pale shading). Dashed lines with teeth mark zones of convergence; zippered line is Panama-Colombia suture. Very heavy dashed line marks 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. Arrows on the inset indicate directions of relative motions of the plates. PFZ = Panama Fracture Zone In the southern portion of the western and is associated with active seismicity and margin, a triple junction brings the Cocos volcanism. Northern Central America also and Nazca Plates in contact with the small has a broad zone of older continental crust, Panama Microplate (Fig. 1). The Panama and has a geologic history extending back Microplate appears to have formed by to the early Paleozoic (Donnelly et al. 1990). northward escape from compression of the In contrast the Nazca Plate has a relative South American and Cocos/Nazca plates easterly motion but is not actively subduct- (Burke and Sengor 1986; Coates et al. 2004). ing under Panama. The Panama Microplate Much of Central America lies either on the is formed of oceanic crust that is typical of trailing western edge of the Caribbean the widespread basalt plateau that under- Plate or on the Panama Microplate but a lies much of the rest of the Caribbean Plate portion also lies on the southwestern cor- (Case et al. 1990). ner of the North American Plate (Fig. 1). Three major movements have affected Since their formation in the Miocene, the the Neogene tectonic evolution of the Nazca and Cocos plates have impinged on southern Central American isthmus (CAI; Central America with different motions. Kolarsky et al. 1995; Coates and Obando The Cocos Plate (Fig. 1), with relative 1996). The first is convergent tectonics of northeasterly motion, is subducting vigor- the eastern Pacific subduction zone (Fig. 1), ously under northern Central America as the primary driving force that created the far south as the Costa Rica-Panama border Central American isthmus by forming a 376 A. COATES ET AL. western South America (Coates et al. 2004; Silver et al. 1990; Mann and Kolarsky 1995). The relative northwestward movement of South America (Fig. 1) has increasingly compressed the southern Caribbean Plate margin in the late Neogene. This conver- gence has uplifted eastern Panama and the northern Andes of Colombia and Venezu- ela. The resulting shoaling of the CAI (de- tected paleoceanographically by a marked divergence between planktonic foraminif- eral oxygen isotope records (a proxy for sa- linity changes) was to less than 100 m from 4.7-4.2 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, and Groenveld and others (Poster sessions, American Geophysical Union Meeting, 2004 and pers. comm. 2005) have suggested 2.8 as the time of final closure of the CAI. FIG. 2. Physical stratigraphic nomenclature of the Using Ca/Mg measurements of planktonic Northern and Southern regions of the Bocas del Toro foraminifera, they show that Caribbean and archipelago with ages in millions of years Pacific Sea Surface Temperature (SST) rec- ords are similar from 5-2.8 Ma. Then larger volcanic arc extending southwards from scale sea level fluctuations with 41 kyr. cy- North America. It forms part of an exten- clicity became dominant in response toam- sive zone of subduction that runs the plification of the Northern Hemisphere gla- length of the western margin of North, ciation. After 2.8 Ma, glacial stages, when Central, and South America (Astorga et al. sea level is lowest, show minimum Pacific 1991). SSTs, but maximum SSTs occur in the Car- The second tectonic effect was subduc- ibbean. This anomaly is explained by low tion of the Cocos Ridge (Kolarsky et al. sea stands preventing cooler less saline wa- 1995; Collins et al. 1995), a lighter and rela- ter from entering the Caribbean from the tively thick welt of Pacific oceanic crust, Pacific (the CAI is emergent), and the re- under the Central American volcanic arc in verse applies in interglacials when sea level Costa Rica (Fig. 1). This hard-to-subduct is high, breaching the CAI. ridge rapidly elevated the Central Ameri- The closure of the Isthmus of Panama can Isthmus from the Arenal volcano in triggered profound environmental changes Costa Rica to El Valle in Panama (de Boer et on land and in the sea. The formation of a al. 1988) culminating in the center where bridge between the North and South the Talamanca Range rises to about 4000 m. American continents gave rise to the Great The elevation of the Talamanca range prob- American Biotic Interchange on land ably substantially reduced the number of (Webb and Rancy 1996; Webb 1999). Less marine connections between the Pacific and well known are the timing and nature of Caribbean. the changes in the sea, consequent upon the The third tectonic influence on southern growth of the Central American volcanic Central America was the collision of the arc and its eventual collision with South Central American volcanic arc (the western America. This created a marine barrier that margin of the Caribbean plate) with north- increasingly affected ocean circulation dur- GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 377 ing Neogene time. In the process, a striking in these two areas (Fig. 2). We then attempt contrast evolved between the relatively to synthesize the geologic history of Bocas warm, more saline, nutrient-poor Carib- del Toro and compare it with adjacent re- bean and the more seasonal, less saline and gions in Costa Rica, Darien, Panama, and more pelagically productive eastern Pacific the Atrato Valley, Colombia. (Jackson and D’Croz 1999). These environ- mental changes apparently altered the 1) The northern region comprises Swan course of evolution, first of the deep-water Cay, Colon, Pastora, San Cristobal, Cari- planktonic organisms like radiolaria and nero, and Bastimentos islands, and the diatoms from about 15 Ma, and then suc- Zapatillo Cays.