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: coatesj@hardynet.com
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
Background
This overview paper provides an intro-
duction to the geological foundation of the
modern biological systems found today in
the Bocas del Toro region. The study of the
geology of Bocas del Toro was undertaken
as part of the Panama Paleontology Project
(PPP; Collins and Coates 1999), a collabo-
rative research program to study sediments
deposited during the last 20 million years
(involving the Miocene, Pliocene, and Pleis-
tocene Epochs, which are together known
as the Neogene) along both sides of the
isthmus 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 and
diverse macrofossil sites. After additional
paleomagnetic 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 the
western margin of the Caribbean Plate and
lies at the center of a complex intersection
of the Pacific Cocos and Nazca plates with
the Caribbean Plate and the small Panama
Microplate (Fig. 1). The dominantly oceanic
Caribbean Plate lies between the North and
South American plates. Its relative east-
ward motion, with respect to the North and
South 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-moving
South American Plate). In the east, it is
bounded by the Lesser Antilles subduction
zone. The western margin of the Caribbean
Plate 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, 2005
Copyright 2005 College of Arts and Sciences
University of Puerto Rico, Mayagu?ez
374
In the southern portion of the western
margin, a triple junction brings the Cocos
and Nazca Plates in contact with the small
Panama Microplate (Fig. 1). The Panama
Microplate appears to have formed by
northward escape from compression of the
South American and Cocos/Nazca plates
(Burke and Sengor 1986; Coates et al. 2004).
Much of Central America lies either on the
trailing western edge of the Caribbean
Plate or on the Panama Microplate but a
portion also lies on the southwestern cor-
ner of the North American Plate (Fig. 1).
Since their formation in the Miocene, the
Nazca and Cocos plates have impinged on
Central America with different motions.
The Cocos Plate (Fig. 1), with relative
northeasterly motion, is subducting vigor-
ously under northern Central America as
far south as the Costa Rica-Panama border
and is associated with active seismicity and
volcanism. Northern Central America also
has a broad zone of older continental crust,
and has a geologic history extending back
to the early Paleozoic (Donnelly et al. 1990).
In contrast the Nazca Plate has a relative
easterly motion but is not actively subduct-
ing under Panama. The Panama Microplate
is formed of oceanic crust that is typical of
the widespread basalt plateau that under-
lies much of the rest of the Caribbean Plate
(Case et al. 1990).
Three major movements have affected
the Neogene tectonic evolution of the
southern Central American isthmus (CAI;
Kolarsky et al. 1995; Coates and Obando
1996). The first is convergent tectonics of
the eastern Pacific subduction zone (Fig. 1),
the primary driving force that created the
Central American isthmus by forming a
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
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 375
volcanic arc extending southwards from
North 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 in
Costa Rica (Fig. 1). This hard-to-subduct
ridge rapidly elevated the Central Ameri-
can Isthmus from the Arenal volcano in
Costa Rica to El Valle in Panama (de Boer et
al. 1988) culminating in the center where
the Talamanca Range rises to about 4000 m.
The elevation of the Talamanca range prob-
ably substantially reduced the number of
marine connections between the Pacific and
Caribbean.
The third tectonic influence on southern
Central America was the collision of the
Central American volcanic arc (the western
margin 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 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.
Using Ca/Mg measurements of planktonic
foraminifera, they show that Caribbean and
Pacific Sea Surface Temperature (SST) rec-
ords are similar from 5-2.8 Ma. Then larger
scale 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, when
sea level is lowest, show minimum Pacific
SSTs, but maximum SSTs occur in the Car-
ibbean. This anomaly is explained by low
sea stands preventing cooler less saline wa-
ter from entering the Caribbean from the
Pacific (the CAI is emergent), and the re-
verse applies in interglacials when sea level
is high, breaching the CAI.
The closure of the Isthmus of Panama
triggered profound environmental changes
on land and in the sea. The formation of a
bridge between the North and South
American continents gave rise to the Great
American Biotic Interchange on land
(Webb and Rancy 1996; Webb 1999). Less
well known are the timing and nature of
the changes in the sea, consequent upon the
growth of the Central American volcanic
arc and its eventual collision with South
America. This created a marine barrier that
increasingly affected ocean circulation dur-
FIG. 2. Physical stratigraphic nomenclature of the
Northern and Southern regions of the Bocas del Toro
archipelago with ages in millions of years
A. COATES ET AL.376
ing Neogene time. In the process, a striking
contrast evolved between the relatively
warm, more saline, nutrient-poor Carib-
bean and the more seasonal, less saline and
more pelagically productive eastern Pacific
(Jackson and D?Croz 1999). These environ-
mental changes apparently altered the
course of evolution, first of the deep-water
planktonic organisms like radiolaria and
diatoms from about 15 Ma, and then suc-
cessively shallower taxa until complete
emergence 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 D?Croz 1999). In
contrast, the Caribbean had become domi-
nated by coral reef-seagrass-mangrove
coastal ecosystems (Collins et al. 1996) and
a 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 preserved
around the Bocas del Toro Archipelago
whose deposits range in age from the early
Miocene (about 20 Ma). The sequence can
be assigned to four phases in the rise of the
isthmus as follows: 1) Deposition of lower
bathyal, pre-isthmian, oceanic sediments in
early Miocene time (21.5-18.5 Ma); 2)
Growth of a volcanic arc during middle
Miocene time (?18 to ?12 Ma) characterized
by terrestrial and marine deposits includ-
ing columnar basalt and flow breccia,
coarse volcanic sediments, and scattered
small coral reefs; 3) Extinction of the arc
about 12 Ma and subsequent extensive
emergence and erosion; and 4) Subsidence
of the volcanic arc during the latest part of
Miocene time (?7.2-5.3 Ma), resulting in a
marine transgression, followed by a marine
regression, that culminated in the wide-
spread development of Plio?Pleistocene
reefs (McNeill et al. 2000; Coates et al.
2003).
STRATIGRAPHY
For the purposes of this paper, we first
describe the stratigraphic units that occur
in the south and north of the Bocas del Toro
Archipelago. These are discussed sepa-
rately because the sequences are different
in these two areas (Fig. 2). We then attempt
to synthesize the geologic history of Bocas
del Toro and compare it with adjacent re-
gions in Costa Rica, Darien, Panama, and
the Atrato Valley, Colombia.
1) The northern region comprises Swan
Cay, Colon, Pastora, San Cristobal, Cari-
nero, and Bastimentos islands, and the
Zapatillo 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 a
late Pliocene-Pleistocene succession of shal-
low water sediments, especially coral reef
deposits, that either unconformably overlie
middle Miocene volcanic arc basalt (Basti-
mentos Island) or rest on a thick (>2500 m)
siliciclastic shale (Colon Island) that is late
Pliocene at its top and of unknown age at
its base. This suggests that there is a major
structural break in the volcanic arc base-
ment rocks between Bastimentos and Co-
lon islands. The southern region reveals a
more extensive volcanic arc suite of lower
and 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 kinds
of units, each of which provides the basis
of a subdiscipline of stratigraphy. The
first, lithostratigraphy, describes the physi-
cal stratigraphic units and their three-
dimensional geometry across the region
studied. Each mappable unit or Formation is
distinguished by its rock type or lithology
and is named, usually from the place where
it most typically can be observed. A series
of formations deposited in vertical succes-
sion in a genetically related depositional
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 377
episode may be lumped together as a
Group.
If the contact of two lithostratigraphic
units shows evidence of a break in sedi-
mentation this line is called an unconfor-
mity. Unconformities usually reflect major
geological events (like tectonic uplift or sea
level change) in the area where they occur.
The significance of unconformities will
vary depending on how different the units
are above and below, and how much time
the break represents. From the nature of the
different sediments and their associated
sedimentary structures and fossils, the
physical environment and water depth in
which the formation was deposited can be
reconstructed. Fig. 2 outlines the nomencla-
ture of the lithostratigraphy of the Neogene
formations of the Bocas del Toro archi-
pelago.
The second subdiscipline is biostratigra-
phy, which takes advantage of the fossil
content of rocks, and is based on the range
of species, i.e., the stratigraphic interval be-
tween two specific stratigraphic horizons
corresponding to the lowest (LO) and the
highest (HO) occurrence of a taxon. The ba-
sic unit of biostratigraphy is the biozone, a
stratigraphic interval defined between LO
and HO of taxa selected for their distinct
morphology and vast geographic distribu-
tion. Biozones allow stratigraphic correlation,
the fundamental procedure of establishing
that two horizons have the same relative
age. Chronostratigraphy, the third element
of stratigraphy, is a system of reference for
the relative dating of sediments. Its basic
unit is the stage, defined by its base (= a
stratigraphic horizon) and a stratotype (a
stratigraphic interval). Correlation of rocks
in a given basin to a stage is established via
biostratigraphy. Stages are grouped in a
nested hierarchy of Series, Systems, and Er-
athems. Their temporal equivalents are
called Ages, Epochs, Periods, and Eras, respec-
tively.
The fourth subdivision, geochronology,
is the science of numerical time. Whereas
chronostratigraphy deals with relative time
(younger/older), geochronology trans-
forms specific stratigraphic horizons into
numerical markers. Geochronology uses
radioisotopic ages and/or astrocyclicity
(i.e., Milankovich periodicity), usually in
conjunction with paleomagnetic stratigra-
phy, to determine the ages of chronostrati-
graphic and biostratigraphic boundaries.
i.e. the stratigraphic level common to two
successive stages or biozones. Numerical
ages vary depending on the criteria se-
lected in the building of a gechronologic
framework or time 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 the
physical environments they represent mi-
grated across the sedimentary basin
through time. They are then said to be di-
achronous.
The southern region
Each lithostratigraphic unit, starting with
the lowest, will first be named and de-
scribed, and then evidence for its age and
environment of deposition will be added.
For a more detailed description of the
stratigraphy of these units see Coates et al.
(1992, 2003) and Coates (1999).
Punta Alegre Formation.?This formation
is named for Punta Alegre, the nearest
small village to its solitary exposure. Punta
Alegre is located on the western tip of the
north coast of Bahia Azul (Bluefield Bay) in
the northern part of the Valiente Peninsula
(Fig. 3). The formation can be observed in a
prominent small cliff about 1.3 km north-
west of the village, on the west-facing coast
1 km south of Valiente Point. It consists of
clay and silty ooze containing abundant
calcareous nannofossils and benthic and
planktonic foraminifera (see Coates et al.
2003, Tables 1, 2), many of the latter are eas-
ily seen by a hand lens and by the naked
eye. The formation can be seen to lie un-
derneath weathered coarse volcanic flow
breccia (the broken up bouldery rock that is
crated when lava flows and cools at the
same time). From its abundant planktonic
foraminifera and calcareous nannofossils,
the age of the Punta Alegre Formation is
19-18.3 Ma. (Coates et al. 2003), and by its
benthic foraminifera it is interpreted to
A. COATES ET AL.378
have been deposited in water depths of
100?2000 m (Coates et al. 2003).
The Valiente Formation.?This unit over-
lies the Punta Alegre Formation and crops
out 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 because
it represents the suite of facies, marine and
terrestrial, igneous and sedimentary, that is
associated 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-
cluding M. imperatoris and M. canalis), mas-
sive Porites waylandi and thin branching
Stylophora (possibly S. granulate; Ann Budd
pers. comm. 2005). These facies intercalate
and replace each other over very short dis-
tances, both laterally and vertically (for de-
tailed descriptions of the facies, see Coates
et al. 2003). Fig. 3 shows that the basalt lava
and flow breccia facies form a core around
which the other facies are distributed pe-
ripherally as terrestrial, coastal or marine
slope deposits. Planktonic microbiotas and
radiometrically 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 five
lithofacies of the Valiente Formation are indicated by separate colors and numbers on the key (upper right) as
follows; v1) basalt lava and flow breccia facies; v2) coarse volcaniclastic facies; v3) pyroclastic facies; v4) coral
reef facies; v5) marine debris flow and turbidite facies.
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 379
Fi 3 li 4/C
quence indicate that the Valiente Formation
ranges in age from 16.5-11.5 million years
old and includes marine deposits from
near-shore to as deep as 1000 m, as well as
several types of terrestrial deposits (Coates
et al. 2003).
Bocas del Toro Group.?Columnar basalt
and pyroclastic and fluviatile rocks of the
Valiente Formation required extensive
emergence of parts of the growing volcanic
arc (columnar basalt is formed by the cool-
ing of lava ponded lakes). About 12 Ma, the
arc had become inactive and recent studies
in Darien, Panama (Coates et al. 2004), sug-
gest that the collision of the southern tip of
the Central American volcanic arc with
South America was initiated at the same
time. These events may explain the major
unconformity in the Bocas del Toro Archi-
pelago above the Valiente Formation, sig-
nifying extensive emergence and erosion of
the volcanic arc from ?11.5-?7.2 Ma (Fig. 2,
3). This unconformity can be clearly ob-
served in the inner islands of the Plantain
Cays and along the coast immediately to
the west. The underlying columnar basalt
of the Valiente Formation can be seen on
the north side of the Cays and the fossilif-
erous conglomerate and sandstone of the
Tobobe Sandstone on the south side.
The Bocas del Toro Group signifies a new
depositional cycle as a marine transgres-
sion submerged the older eroded volcanic
arc rocks. The oldest sediments thus repre-
sent the first shallow-water, beach, and
near-shore sand deposits. As the transgres-
sion developed, subsequent overlying ma-
rine units reflect deeper water deposition
until the reverse occurred and the sea re-
gressed to shallow water again. Thus, the
units of the Bocas del Toro Group are ge-
netically related and document a single ma-
rine transgressive/regressive event. The
most continuous section lies along the east
and west coasts of the Valiente Peninsula
(Fig. 3) where it ranges in age from late
Miocene (Messinian, 7.2-5.3 Ma) to late
Pliocene (?3.5 Ma).
Tobobe Sandstone.?This lowest unit is a
pebble conglomerate that passes up into
clean, hard quartz sandstone containing
sand dollars, spatangoid echinoids, mol-
lusks, including the large, thin-shelled bi-
valve Amusium, vermetids and serpulids,
as well as an array of infaunal burrow
structures. It is best observed on the small
Plantain Cay and an adjacent smaller un-
named island to the west (Fig. 3). The de-
posit represents a beach and near-shore
sand body and documents the earliest stage
of the marine transgression. Planktonic
foraminifera from laterally equivalent de-
posits on Toro Cay (Fig. 3) indicate that the
age 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 burrow
systems constructed by callianasid crusta-
ceans are also present.
Nancy Point Formation.?It was named
for the promontory called Nancy Point
FIG. 4. Geological map of Popa Island. Section (b-b?)
shows the Valiente Formation on Popa Island, where
it is unconformably overlain by the Pliocene Cayo
Agua Formation (ca). Numbers and symbols as for
Figure 3. On Popa Island, only the v1 basalt flow facies
and the v2 coarse volcaniclastic facies (not associated
with reef lenses) are present, with thin layers of low
rank coal, an example of which is exposed along the
coast 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.
A. COATES ET AL.380
Fi 4 li 4/C
which lies 2.5 km southwest of the village
of Tobobe (Fig. 3). The formation is almost
continuously exposed southward along the
coast as far as Chong Point (= Nispero
Point). The Nancy Point Formation is con-
formable with the Tobobe Formation below
and the Shark Hole Point Formation above
(Fig. 2 and 3). It consists of shelly, muddy
and silty sandstone, and muddy siltstone
with occasional coarse volcaniclastic and
bioclastic sandstone beds. It contains scat-
tered mollusks and occasional leaves and
plant fragments with diverse moderately
abundant 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 Toro
Cay (Fig. 3) where dark blue-gray, silty
sandstone, typical of the Nancy Point For-
mation, contains occasional, clearly defined
infaunal burrow systems and extremely
abundant and diverse mollusks. It overlies
coarse, channeled Tobobe Sandstone with
only a 10-m exposure gap. Nancy Point
Formation benthic foraminifera indicate
that deposition of the Nancy Point Forma-
tion was in 200-500 m. water depth (Collins
1993) demonstrating that relatively rapid
deepening took place from near-shore (To-
bobe Sandstone) to upper slope (Nancy
Point Formation). This represents the maxi-
mum depth of the transgressive/regressive
cycle. Planktonic foraminifera and calcare-
ous nannofossils date the Nancy Point For-
mation as Messinian (7.2-5.3 Ma), the same
time range as for the Tobobe Sandstone.
Within that time frame the Nancy Point
Formation is younger because it overlies
the Tobobe Sandstone.
Shark Hole Point Formation.?It is named
for the promontory of the same name that
lies 3 km east of Chong Point (Fig. 3) and
the stratotype lies along the coast between
Chong Point and Bruno Bluff, south of Old
Bess Point. The Shark Hole Point Forma-
tion conformably overlies the Nancy Point
Formation (Fig. 2 and 3) and is overlain by
an unnamed conglomeratic, cross-bedded,
coarser grained sequence of volcaniclastics
containing large pieces of wood and plant
fragments. This unnamed unit is exposed
only along the southern coast of the Va-
liente Peninsula, east of Secretario. The
Shark Hole Formation consists of mica-
ceous, clayey siltstone that is pervasively
bioturbated and rich in large scaphopods.
The uppermost part of the formation also
contains abundant, thin, shelly beds and in-
traformational slumps with pillow folds
and rip-up clasts. Benthic foraminifera in-
dicate that the paleobathymetry of this unit
ranges from 100-200 m (Collins 1993). This
represents the first stage of the regressive
phase of the Bocas del Toro Group trans-
gressive/regressive cycle. Calcareous nan-
nofossils and planktonic foraminifera are
abundant in several horizons and suggest
the age of the formation is early Pliocene
(5.3-3.6 Ma).
Escudo de Veraguas Formation.?The
stratigraphic order of the formations de-
scribed above has been determined by
physical superposition. The three remain-
ing formations of the Bocas del Toro group
are 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 is
known only from the island of the same
name that lies 27 km east of Nancy Point
(Fig. 5) . Its stratotype, for the lower part of
the formation, is along the coast on the east
side of the V-shaped embayment situated
in the central part of the north coast, about
1 km east of Long Bay Point (Fig. 5). The
stratotype for the upper part of the forma-
tion lies along the west coast for about one
km south of Long Bay Point.
Lithologically, the Escudo Formation
consists of, in the upper part (1.8 Ma), per-
vasively bioturbated clayey siltstone and
silty claystone, with frequent concretions,
and scattered shelly hash, often with scat-
tered whole and diverse mollusks, small,
cornute, ahermatypic corals. The lower part
of 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, and
at the top a coral biostrome dominated by
Stylophora and sand dollars. The top and
botton units suggest deposition in inner
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 381
neritic water depths, but the middle part of
this lower succession has abundant benthic
foraminifera indicating deposition in outer
neritic to upper bathyal water depths (Col-
lins 1993).
Cayo Agua Formation.?This formation
was named for the island that lies about 6
km to the west of Toro Point, Valiente Pen-
insula. The formation is well exposed along
the 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 of
pebble conglomerate and very coarse-
grained volcaniclastic sandstone are com-
mon in the middle of the formation. Com-
pared to the Shark Hole and Escudo de
Veraguas 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 occurs
near the top of the formation and is well
exposed at Tiburon Point and the unnamed
point to the south. The corals appear to be
mostly free living, hermatypic, grass flat
corals, such as Placocyathus and Manicina (=
Teleiophyllia), as well as ahermatypic spe-
cies, and they suggest deposition in water
depths of 30-50 m (Ann Budd, pers. comm.
2005). Evidence from benthic foraminifera
(Collins 1993) indicates paleobathymetry of
20-80 m, overlapping the inference from li-
thology and coral fauna that the Cayo Agua
Formation represents a shallow-water fa-
cies.
The age of the Cayo Agua Formation is
dated at the base ?5.0-3.5 Ma and at the top
3.7-3.4 Ma, which suggests it is a contem-
porary, shallow water equivalent of the
Shark 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 the
Bocas del Toro archipelago have a different
geologic history than the southern region.
Volcanic arc columnar basalts of the Va-
liente Formation form the basement that
underlies the marine late Neogene succes-
sion, as in the southern region. While no
radiometric dates for these basalts are
available, we can presume they are part of
the same volcanic arc that went extinct and
cooled after the middle Miocene (?12 Ma).
In all the Northern Region, the sediments
unconformably overlying the Valiente For-
mation basalt are Plio-Pleistocene in age.
The upper Miocene succession of Tobobe
Sandstone, Nancy Point, and Shark Hole
formations are absent. Furthermore, the
northern units are characterized by a major
component of coral reef limestone.
The geology of Colon and Bastimentos
islands are currently being studied and the
formal designation of the lithostratigraphic
units 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 lower
and upper parts of the Escudo de Veraguas Forma-
tion.
FIG. 6. Geological map of Cayo Agua. Ages in mil-
lions of years.
A. COATES ET AL.382
Fi 5 li 4/C Fi 6 li 4/C
general, the sequence is younger than the
section to the south with the exception of
the Escudo de Veraguas Formation. The
oldest units on Colon and Bastimentos is-
lands may overlap in time with the top of
the Cayo Agua and Shark Hole Point se-
quences at ?3.5 Ma.
Valiente Formation.?The spectacular ex-
posures along the northwestern coast of
Bastimentos Island (Fig. 7) of columnar ba-
salt and flow breccia are assigned to the
Valiente Formation solely on their great
similarity in lithology. The exposures range
from Toro Point to the eastern side of
FIG. 7. Geologic map and section (a-a?) of Bastimentos
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 383
Fi 7 li 4/C
Dreffe Beach and occur also at the western
end of Long Beach. These basalt units prob-
ably form the basement of the entire Bocas
del 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).?This
is 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 del
Drago road along which there are several
good exposures. It also occurs in the north-
east of the island where it forms a distinct
escarpment parallel to the coast. Many
streams have waterfalls where they cross
this scarp with good exposures. The same
unit crops out on Bastimentos Island (Fig.
7). There are exposures from Juan Brown
Point eastwards along, and inland of the
south 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 with
occasional thin sandstone stringers, fine
volcanic conglomerate and volcanic boul-
der beds. On Bastimentos, it may be mica-
ceous and sandier, with wood fragments
and sparse mollusks including turritellids
and Anadara. Preliminary field observa-
tions suggest it is an inner shelf deposit
flanking volcanic islands and that it is
about 3.5-2.0 Ma.
La Gruta Unit (informal field name).?On
Colon 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 north
coast towards Mimitimimbi Creek. The
limestone extends southward as far as La
Gruta, a locally famous bat cave. Around
La Gruta, the limestone is a reef deposit
with numerous large coral colonies but the
limestone grades into fore-reef rubble
northeastwards toward the mouth of Mim-
itimimbi Creek where it is well exposed on
the coast. Both the reef and fore-reef depos-
its are heavily fractured to produce a rub-
bly rock unit when exposed. Earthquakes
and uplift are likely responsible for this
pervasive 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 of
similar limestone also occurs immediately
to the North of Paunch, on the east coast of
Colon Island, and on Carinero Island (Fig.
8). At Paunch, an entirely extant fauna of
corals includes Colpophyllia natans, Diploria
strigosa, Meandrina meandrites, Montastraea
faveolata, and Agaricia (Undaria) agaricites
(Ann Budd, pers. comm. 2005).
On Bastimentos Island (Fig. 7), similar
reef deposits appear to sit directly on the
Miocene basalt of the Valiente Formation
where they are well exposed on Wild Cane
Key and along the coast for 2 km to the east.
The reef limestone here is extensively re-
crystallized but many coral colonies can be
observed. A particularly well preserved ex-
ample of the La Gruta reef, packed with
large and diverse coral colonies is exposed
at the base of the cliffs in three small bays at
Fish Hole, at the southern end of Long
Beach. 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 this
fauna is extinct, the La Gruta Formation on
Bastimentos appears to be older than that
on Colon Island (Ann Budd, pers. comm.
2005).
Ground Creek Unit (informal field name).?
Interbedded with and overlapping onto the
La Gruta Unit are back reef/reef flank de-
posits, dominantly shelly coral bearing bio-
clastic carbonate and volcaniclastic sand-
stone and siltstone. They are typically
exposed in several stream courses in the
northwest of Colon Island in a region
known as Ground Creek (Fig. 8). Near
Ground Creek, the unit has yielded 90 gen-
era of bivalves and gastropods in siliciclas-
tic mudstone and fine sandstone, where
they are intercalated with thin carbonate
sand containing poritid thickets and other
reef patches with platy Caulastraea portori-
A. COATES ET AL.384
censis, Porites baracoaensis, Agaricia, ?Clado-
cora, small domed ?Favia, Manicina (Teleio-
phyllia) and serpulids. Also occasionally
interbedded are horizons of reworked large
coral heads.
The Ground Creek unit also crops out on
Bastimentos Island (Fig. 7), where it is in-
terbedded with and overlying the La Gruta
unit on Wild Cane Key, and it may also
crop out around the Valiente Formation
FIG. 8. Geologic map and section (a-a?) of Colon Island.
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 385
Fi 8 li 4/C
outlier at the northern end of Long Beach
and overlie the reefs at Fish Hole.
Swan Cay Formation.?The formation is
named for the small island of the same
name that lies 1.7 km off the northern coast
of Colon Island (Fig. 8). The stratotype runs
from north (youngest) to south (oldest)
across Swan Cay. The formation has three
components. The lower 15 m is exposed on
the 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 m
consists of calcarenitic clayey siltstone,
with dense, fine shell-hash horizons, and
abundant large coral colonies in the lower
part. The upper 60 m of the formation con-
sists of massively thick-bedded, pale tan-
white limestone. The upper 30 m includes
a 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 large
foraminifera, shell hash, and micromol-
lusks. This limestone also shows evi-
dence of frequent microfracturing, many of
which are healed with secondary calcite ce-
ment. Cave deposits, about 5 m above the
base of the calcarenite, consist of silty,
shelly, volcaniclastic sandstone, mixed
with 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, Diploria
labyrinthiformis, Montastraea faveolata,
Porites furcata, Agaricia (Undaria) agaracites,
Meandrina meandrites, and Dichocoenia
stokesi. Organisms from a range of depths
indicate that the deposit is reworked fore-
reef debris formed at about 100 m (Collins
et 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 early
Pleistocene (0.78-1.77 Ma).
GEOLOGIC HISTORY
The oldest sediments recorded in the Bo-
cas del Toro Archipelago (Fig. 9) belong to
the Punta Alegre Formation and are early
Miocene 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 the
pre-isthmian Neotropical ocean. Coeval de-
posits, like the Uscari Formation in the
southern Lim?n Basin (Cassell and Sen
Gupta 1989; Bottazzi et al. 1994), the Clarita
Formation of the Darien (Coates et al. 2004)
and the Uva and Naipipi formations of the
Atrato Basin (Duque Caro 1990) in north-
west Colombia were also deposited at
bathyal depths. Thus, prior to the collision
of the southern Central American arc with
South America, upper Cretaceous to lower
middle Miocene rocks were deposited in
abyssal to lower bathyal depths in an open-
ocean, low energy, essentially non-
siliciclastic sedimentary environment that
was distant from South America (Coates et
al. 2004).
Evidence from the overlying Valiente
Formation indicates the rapid growth of a
pre-isthmian volcanic arc (whose southern
end was still some distance from South
America) from about 16.5-12.3 Ma. During
the deposition of the Valiente Formation
(Coates et al. 2003), paleodepths (Fig. 9) in
the 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 which
the sediments were deposited. Thus, near
Punta Alegre in the Valiente Peninsula, and
Deer Island (Coates et al. 2003) sediments
were deposited in middle bathyal depths
(1000-600 m) and at Avispa Point, Toro
Point and Cusapin Village deposition was
at 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 coral
heads, testify to the extensive emergence of
the isthmian volcanic arc in the Bocas del
Toro area by 12 Ma (Fig. 3).
Fig. 10 outlines paleogeographic recon-
structions for these events. The volcanic arc
underlying the Neogene sediments of Bo-
cas del Toro was an active and emergent
A. COATES ET AL.386
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 represent
volcanoes. Evidence of middle Miocene volcanism in western Panama is from de Boer et al. (1988, 1991) and
references therein. Land areas to the south of the arc are interpreted as exotic terrains. The Panamanian Isthmus
at this stage was a volcanic island arc with a narrow neritic zone. The Southern Lim?n and Bocas del Toro Basin
sediments indicate bathyal paleodepths in contrast to the Panama Canal Basin where neritic to emergent
condition persist through most of the Cenozoic. B: Schematic paleogeographic reconstruction of the Panamanian
Isthmus at 11-12 Ma. Symbols as for part A. The neritic zone has by this time expanded significantly and an
emergent active volcanic back-arc has developed in the Bocas del Toro Basin. We assume that the Central
Cordilleran 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 Lim?n and Panama Canal basins. Dashed lines enclose the same
geologic section. Dotted lines show the biochronologic ranges of sections that are placed according to physical
stratigraphy.
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 387
Fi 9 li 4/C
A. COATES ET AL.388
Fi 10 li 4/C
back-arc, parallel to the main Central Cor-
dilleran arc. The environment of the Bocas
del Toro back arc archipelago, by 12 Ma,
had patchy shallow-water fringing reefs
around active volcanic islands whose emer-
gent flanks were characterized by laharic
breccia and basalt flows. The lahars and
flows graded laterally into fluviatile and
deltaic sequences, and then into resedi-
mented near-shore deposits. Major volcanic
activity in the area ceased about 12 Ma (the
youngest radiometric date on columnar ba-
salt flows,) although there are younger ba-
salt dykes intruding the Valiente Formation
sediments, e.g., at Laurel point, Isla Popa
(Fig. 4), dated about 8.5 Ma.
To the northwest, in the southern Lim?n
Basin, a similar general shallowing pattern
is observed (Coates et al. 2004, Fig. 9). From
?18-13 Ma, the Uscari Formation shallows
from middle bathyal (1000-600 m) to outer
neritic (100-200 m) depths. The record is
absent after 13 Ma. The Gatun Formation
(11.8-11.4 Ma) an inner neritic (<50 m) de-
posit in the Panama Canal Basin (Collins et
al., 1996a) is evidence that the emergence of
the isthmus had developed to a similar de-
gree in that region. In the central Darien
region (Coates et al. 2004), middle Miocene
deposits shallowed from middle bathyal
depths (1000-600 m) in the Clarita and
Tapaliza formations (?16.4-11.2 Ma) to
neritic depths (<200 m) in the Tuira,
Yaviza, and Chucunaque formations (11.2-
5.6 Ma).
In the Bocas del Toro region, the Valiente
Formation is unconformably overlain by
the Tobobe Sandstone Formation (Coates et
al. 1992; Coates 1999), indicating flooding
of the eroded extinct volcanic arc rocks by
very shallow water. Deposition of the mar-
ginal marine conglomerate and sandstone
signals the start of a marine transgressive/
regressive cycle (Fig. 9). Sediments of the
conformably overlying Nancy Point For-
mation (Messinian 7.26-5.32 Ma; Aubry and
Berggren 1999) show that deepening pro-
ceeded to upper bathyal depths (200-600 m;
Collins 1993). This deepening is also re-
corded in the Panama Canal Basin (Coates
et al. 2004, Fig. 9) where a rapid transition
occurs from the inner neritic (20-40 m),
middle-upper Gatun Formation (?11-8 Ma)
to the overlying upper bathyal (200-600 m)
Chagres Formation (?6 Ma; Collins et al.
1996a). It is not clear whether deepening
represents a eustatic (sea level) or a re-
gional tectonic event caused by cooling and
sinking of the Bocas del Toro volcanic arc.
By the early Pliocene (Zanclean, 5.2-3.5
Ma), marine regression had caused rapid
shallowing throughout the Bocas del Toro
Basin (Shark Hole Point and Cayo Agua
formations). Coeval deposits (4-1 Ma) in
the southern Lim?n Basin (Rio Banano,
Quebrada Chocolate, Moin, and Suretka
formations) also record rapid shallowing
and emergence (McNeill et al. 2000). Simi-
larly, shallowing to neritic (<100 m) or
emergent conditions was occurring in the
Darien region (Yaviza Formation, Tuquesa
and Chucunaque formations; Coates et al.
2004). These patterns are evidence of the
shoaling of Central American isthmus vol-
canic arc (4.7-2.8 Ma). In the Northern Re-
gion of the Bocas del Toro archipelago
there is an extensive Plio-Pleistocene se-
quence, ranging in age from about 3.5 to
0.78 Ma, that displays a widespread devel-
opment of coral reef formation. This is in
marked contrast to the very limited coral
reef lenses intercalated into the siliciclastic
Miocene sediments the Southern Region.
Collins et al. (1996b) have demonstrated the
increase, from the late Miocene, of carbon-
ate dwelling benthic foraminiferal species
and of corals associated with increased
warming and salinity (and decreased pro-
ductivity) in the western Caribbean as a re-
sult of closure of the Isthmus of Panama.
The Northern Region of the Bocas del Toro
archipelago demonstrates the same trend.
CONCLUSIONS
1) Prior to the collision of the southern
Central American arc with south
America, upper Cretaceous to lower
Miocene rocks of the Bocas del Toro Ba-
sin (lower Miocene, Punta Alegre For-
mation) were deposited in abyssal to
lower bathyal depths in an open-ocean,
low-energy, essentially non-siliciclastic
sedimentary environment distant from
South America.
2) The Valiente Formation records the
GEOLOGY OF BOCAS DEL TORO ARCHIPELAGO 389
rapid growth of an active volcanic arc
which by ?12 Ma was an extensive
emergent archipelago of volcanic is-
lands flanked by coral reefs.
3) After a prolonged period of emergence
and erosion, marine sediments began to
cover the extinct volcanic arc starting
about 7.2-5.3 Ma.
4) The deposits of this transgressive/
regressive marine cycle are named the
Bocas del Toro Group. The oldest units
(Tobobe Sandstone and Nancy Point
Formation), record deepening to 200 m.
The younger units (Shark Hole Point,
Cayo Agua, and Escudo de Veraguas
formations) represent the regressive se-
quence.
5) In the Plio-Pleistocene, an extensive
coral reef system developed either on a
basement of siliciclastic sediments (Co-
lon Island, Swan Cay) or on the basalt of
the Miocene Valiente Formation (Basti-
mentos Island), reflecting the oceano-
graphic changes taking place in the
western Caribbean as a result of shal-
lowing and closure of the Isthmus of
Panama.
Acknowledgments.?This research was
supported by the National Science Founda-
tion grants BSR-9006523, DEB-9696123 and
DEB-9705289 to AGC, LSC, and Jeremy
Jackson; several grants from the National
Geographic Society to AGC and LSC, and
Scholarly Studies and Walcott Fund
awards from the Smithsonian Institution to
AGC. We are grateful to the staff of the
STRI field station in Bocas del Toro and to
numerous field assistants of the Panama
Paleontology Project, who have helped
with the complex logistics of fieldwork in
this region since 1986 (see Collins and
Coates, 1999 for details), but particularly to
Beatrice, Lucien Ferrenbach and Doroteo
Machado, and whose constant efforts to
maintain motors and vessels, and to orga-
nize food and gasoline, allowed us to work
with maximum efficiency. We are also very
grateful to Xenia Saavedra for laboratory
technical and logistical support, including
organizing (and sometimes preparing)
samples, tracking them in the data base,
producing field maps, and numerous other
tasks. Janet Coates, Gloria Jovane, Lidia
Mann, Isis Estribi, and the crews of the RV
Benjamin and RV Urracca provided invalu-
able logistical support over many years. We
also thank Rachel Collin, Ann Budd, and
Aaron Odea for very helpful reviews.
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