An Introduction to the Geology of the Bocas del Toro Archipelago, Panama

18 Pages·2006·0.61 MB·English

CaribbeanJournalofScience,Vol.41,No.3,374-391,2005
Copyright2005CollegeofArtsandSciences
UniversityofPuertoRico,Mayagu¨ez
An Introduction to the Geology of the Bocas del
ToroArchipelago,Panama
A.G.COATES1, D.F.MCNEILL2, M-P.AUBRY3, W.A.BERGGREN4, L.S.COLLINS5
1SmithsonianTropicalResearchInstitute,Apartado2072,RepublicofPanama
2DivisionofMarineGeologyandGeophysics,RosensteilSchoolofMarineandAtmosphericScience,Universityof
Miami,Florida,33149,USA
3DepartmentofGeologicalSciences,RutgersUniversity,WrightLabs,610TaylorRoad,
Piscataway,NewJersey,08854-8066
4WoodsHoleOceanographicInstitute,DepartmentofGeologyandGeophysics,WoodsHole,Massachusetts,02543,
USAandDepartmentofGeologicalSciences,RutgersUniversity,WrightLabs,610TaylorRoad,Piscataway,New
Jersey,08854-8066,USA
5DepartmentofEarthSciences,FloridaInternationalUniversity,Miami,Florida,33199,USA
Correspondingauthor:[email protected]
ABSTRACT.—WereviewthestratigraphyoftheNeogenerocksoftheBocasdelToroarchipelago,western
CaribbeancoastofPanama,andprovidenewgeologicalmapsandapreliminarydescriptionofnewNeogene
formationsontheislandsofBastimentosandColon.ThePuntaAlegreandValienteformationsrangeinage
from19to12Ma.Afterahiatusfrom12to8Ma,atransgressive/regressivesedimentarycycleisrecordedby
theTobobe,NancyPoint,SharkHole,CayoAgua,andEscudodeVeraguasformations(=theBocasdelToro
Group),thatrangeinagefrom7.2-5.3to1.8Ma.Incontrast,inthenorthernregion,theOldBank,LaGruta,
andGroundCreekunitsandtheSwanCayFormationonlyrangeinagefromabout3.5to0.78Ma.Thehiatus
represented by the unconformity is from 12 to 3.5 Ma. We integrate the geology of the Bocas del Toro
archipelagointoabriefhistoryoftheNeogeneofthelowerCentralAmericanisthmus.
KEYWORDS.—Neogene,BocasdelToro,CentralAmericanIsthmus
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 TheCentralAmericanisthmusformsthe
theBocasdelTororegion.Thestudyofthe westernmarginoftheCaribbeanPlateand
geology of Bocas del Toro was undertaken lies at the center of a complex intersection
aspartofthePanamaPaleontologyProject of the Pacific Cocos and Nazca plates with
(PPP; Collins and Coates 1999), a collabo- the Caribbean Plate and the small Panama
rativeresearchprogramtostudysediments Microplate(Fig.1).Thedominantlyoceanic
deposited during the last 20 million years CaribbeanPlateliesbetweentheNorthand
(involvingtheMiocene,Pliocene,andPleis- South American plates. Its relative east-
tocene Epochs, which are together known wardmotion,withrespecttotheNorthand
as the Neogene) along both sides of the South American plates is accommodated
isthmus of Central America and of north- bystrike-slipfaultstothenorthandinpart
ern South America (Fig. 1). tothesouth(nowconfoundedbycompres-
Wesystematicallymappedthewholere- sionfromthewest-northwestward-moving
gion, named the physical stratigraphic for- South American Plate). In the east, it is
mations (Fig. 2) and located the rich and boundedbytheLesserAntillessubduction
diverse macrofossil sites. After additional zone.ThewesternmarginoftheCaribbean
paleomagneticstudiesandradiometricdat- Plateismorecomplex;inthenorthernpart,
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
GEOLOGYOFBOCASDELTOROARCHIPELAGO 375
FIG.1. MapofsouthernCentralAmerica(darkshading)andthePanamamicroplate(paleshading).Dashed
lineswithteethmarkzonesofconvergence;zipperedlineisPanama-Colombiasuture.Veryheavydashedline
markslocationofNeogenevolcanicarc.FinearrowsarePaleogenefaults;thickarrowsarelateNeogenefaults.
PrincipalNeogenesedimentarybasinslocatedbystripedovals.SpottedpatterndefinestheCocosRidge.Arrows
ontheinsetindicatedirectionsofrelativemotionsoftheplates.PFZ=PanamaFractureZone
In the southern portion of the western andisassociatedwithactiveseismicityand
margin, a triple junction brings the Cocos volcanism. Northern Central America also
and Nazca Plates in contact with the small hasabroadzoneofoldercontinentalcrust,
Panama Microplate (Fig. 1). The Panama and has a geologic history extending back
Microplate appears to have formed by totheearlyPaleozoic(Donnellyetal.1990).
northwardescapefromcompressionofthe In contrast the Nazca Plate has a relative
South American and Cocos/Nazca plates easterlymotionbutisnotactivelysubduct-
(BurkeandSengor1986;Coatesetal.2004). ingunderPanama.ThePanamaMicroplate
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 liesmuchoftherestoftheCaribbeanPlate
portion also lies on the southwestern cor- (Case et al. 1990).
ner of the North American Plate (Fig. 1). Three major movements have affected
SincetheirformationintheMiocene,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- theeasternPacificsubductionzone(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.COATESETAL.
western South America (Coates et al. 2004;
Silveretal.1990;MannandKolarsky1995).
The relative northwestward movement of
South America (Fig. 1) has increasingly
compressed the southern Caribbean Plate
margin in the late Neogene. This conver-
gencehasupliftedeasternPanamaandthe
northern Andes of Colombia and Venezu-
ela.
The resulting shoaling of the CAI (de-
tected paleoceanographically by a marked
divergence between planktonic foraminif-
eraloxygenisotoperecords(aproxyforsa-
linitychanges)wastolessthan100mfrom
4.7-4.2Ma(Keigwin1982;HaugandTiede-
mann 1998; Gussone et al. 2004). This initi-
atedthedevelopmentofthemodernAtlan-
tic-Pacific contrast in sea surface salinities
(SSS), with higher SSS in the Caribbean.
RecentstudiesbyTiedemann,Steph,and
Groenveld and others (Poster sessions,
American Geophysical Union Meeting,
2004andpers.comm.2005)havesuggested
2.8 as the time of final closure of the CAI.
FIG. 2. Physical stratigraphic nomenclature of the UsingCa/Mgmeasurementsofplanktonic
NorthernandSouthernregionsoftheBocasdelToro foraminifera,theyshowthatCaribbeanand
archipelagowithagesinmillionsofyears Pacific Sea Surface Temperature (SST) rec-
ordsaresimilarfrom5-2.8Ma.Thenlarger
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 plificationoftheNorthernHemispheregla-
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. seastandspreventingcoolerlesssalinewa-
1995;Collinsetal.1995),alighterandrela- ter from entering the Caribbean from the
tively thick welt of Pacific oceanic crust, Pacific (the CAI is emergent), and the re-
undertheCentralAmericanvolcanicarcin verseappliesininterglacialswhensealevel
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 triggeredprofoundenvironmentalchanges
CostaRicatoElValleinPanama(deBoeret on land and in the sea. The formation of a
al. 1988) culminating in the center where bridge between the North and South
theTalamancaRangerisestoabout4000m. American continents gave rise to the Great
TheelevationoftheTalamancarangeprob- American Biotic Interchange on land
ably substantially reduced the number of (Webb and Rancy 1996; Webb 1999). Less
marineconnectionsbetweenthePacificand well known are the timing and nature of
Caribbean. thechangesinthesea,consequentuponthe
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
CentralAmericanvolcanicarc(thewestern America.Thiscreatedamarinebarrierthat
margin of the Caribbean plate) with north- increasinglyaffectedoceancirculationdur-
GEOLOGYOFBOCASDELTOROARCHIPELAGO 377
ingNeogenetime.Intheprocess,astriking inthesetwoareas(Fig.2).Wethenattempt
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-
beanandthemoreseasonal,lesssalineand gions in Costa Rica, Darien, Panama, and
morepelagicallyproductiveeasternPacific 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,SanCristobal,Cari-
planktonic organisms like radiolaria and nero, and Bastimentos islands, and the
diatoms from about 15 Ma, and then suc- Zapatillo Cays.
cessively shallower taxa until complete 2) The Southern region comprises the is-
emergence at about 2.8 Ma. By this time, landsofPopa,Deer,CayoAgua,andEs-
theeasternPacifichadevolvedrichpelagic cudodeVeraguas,andtheValientePen-
biotasbutpoorlydeveloped,lowdiversity, insula.
coral reefs (Jackson and D’Croz 1999). In
Ingeneral,thenorthernregionexhibitsa
contrast, the Caribbean had become domi-
latePliocene-Pleistocenesuccessionofshal-
nated by coral reef-seagrass-mangrove
low water sediments, especially coral reef
coastal ecosystems (Collins et al. 1996) and
deposits,thateitherunconformablyoverlie
a profound taxonomic turnover had oc-
middle Miocene volcanic arc basalt (Basti-
curred in corals and mollusks (Budd and
mentosIsland)orrestonathick(>2500m)
Johnson 1997, 1999; Jackson et al. 1993).
siliciclastic shale (Colon Island) that is late
Much of this history is documented in
Pliocene at its top and of unknown age at
the long geological section preserved
its base. This suggests that there is a major
around the Bocas del Toro Archipelago
structural break in the volcanic arc base-
whosedepositsrangeinagefromtheearly
ment rocks between Bastimentos and Co-
Miocene (about 20 Ma). The sequence can
lon islands. The southern region reveals a
beassignedtofourphasesintheriseofthe
more extensive volcanic arc suite of lower
isthmus as follows: 1) Deposition of lower
and middle Miocene rocks, including a se-
bathyal,pre-isthmian,oceanicsedimentsin
quenceofdeep-seaooze,basalt,coarsevol-
early Miocene time (21.5-18.5 Ma); 2)
canicsedimentsandsmallscalepatchreefs,
Growth of a volcanic arc during middle
Miocenetime(∼18to∼12Ma)characterized that tracks the onset and rise of an active
volcanic arc in the Bocas del Toro region.
by terrestrial and marine deposits includ-
Subequently, an unconformably overlying,
ing columnar basalt and flow breccia,
non-reefal, upper Miocene and Pliocene,
coarse volcanic sediments, and scattered
marine, transgressive/regressive shelf se-
small coral reefs; 3) Extinction of the arc
quence was deposited.
about 12 Ma and subsequent extensive
emergence and erosion; and 4) Subsidence
of the volcanic arc during the latest part of Stratigraphic definitions
Miocene time (∼7.2-5.3 Ma), resulting in a
marinetransgression,followedbyamarine Stratigraphy of sedimentary rocks in-
regression, that culminated in the wide- volves the integration of four main kinds
spread development of Plio—Pleistocene of units, each of which provides the basis
reefs (McNeill et al. 2000; Coates et al. of a subdiscipline of stratigraphy. The
2003). first,lithostratigraphy,describesthephysi-
cal stratigraphic units and their three-
dimensional geometry across the region
STRATIGRAPHY
studied.EachmappableunitorFormationis
For the purposes of this paper, we first distinguished by its rock type or lithology
describe the stratigraphic units that occur andisnamed,usuallyfromtheplacewhere
inthesouthandnorthoftheBocasdelToro it most typically can be observed. A series
Archipelago. These are discussed sepa- of formations deposited in vertical succes-
rately because the sequences are different sion in a genetically related depositional
378 A.COATESETAL.
episode may be lumped together as a (i.e., Milankovich periodicity), usually in
Group. conjunction with paleomagnetic stratigra-
If the contact of two lithostratigraphic phy, to determine the ages of chronostrati-
units shows evidence of a break in sedi- graphic and biostratigraphic boundaries.
mentation this line is called an unconfor- i.e. the stratigraphic level common to two
mity. Unconformities usually reflect major successive stages or biozones. Numerical
geologicalevents(liketectonicupliftorsea ages vary depending on the criteria se-
levelchange)intheareawheretheyoccur. lected in the building of a gechronologic
The significance of unconformities will frameworkortimescale.Theagesgivenbe-
vary depending on how different the units lowrefertothetimescaleofBerggrenetal.
are above and below, and how much time (1995). The boundaries between lithostrati-
thebreakrepresents.Fromthenatureofthe graphic units may be of the same age re-
different sediments and their associated gionally, but over larger regions they may
sedimentary structures and fossils, the become younger or older in age as the
physical environment and water depth in physical environments they represent mi-
which the formation was deposited can be grated across the sedimentary basin
reconstructed.Fig.2outlinesthenomencla- through time. They are then said to be di-
tureofthelithostratigraphyoftheNeogene achronous.
formations of the Bocas del Toro archi-
pelago.
The southern region
The second subdiscipline is biostratigra-
phy, which takes advantage of the fossil
content of rocks, and is based on the range Eachlithostratigraphicunit,startingwith
ofspecies,i.e.,thestratigraphicintervalbe- the lowest, will first be named and de-
tween two specific stratigraphic horizons scribed, and then evidence for its age and
corresponding to the lowest (LO) and the environment of deposition will be added.
highest(HO)occurrenceofataxon.Theba- For a more detailed description of the
sic unit of biostratigraphy is the biozone, a stratigraphy of these units see Coates et al.
stratigraphic interval defined between LO (1992, 2003) and Coates (1999).
and HO of taxa selected for their distinct Punta Alegre Formation.—This formation
morphology and vast geographic distribu- is named for Punta Alegre, the nearest
tion.Biozonesallowstratigraphiccorrelation, smallvillagetoitssolitaryexposure.Punta
the fundamental procedure of establishing Alegre is located on the western tip of the
that two horizons have the same relative northcoastofBahiaAzul(BluefieldBay)in
age. Chronostratigraphy, the third element the northern part of the Valiente Peninsula
ofstratigraphy,isasystemofreferencefor (Fig.3).Theformationcanbeobservedina
the relative dating of sediments. Its basic prominent small cliff about 1.3 km north-
unit is the stage, defined by its base (= a westofthevillage,onthewest-facingcoast
stratigraphic horizon) and a stratotype (a 1 km south of Valiente Point. It consists of
stratigraphic interval). Correlation of rocks clay and silty ooze containing abundant
inagivenbasintoastageisestablishedvia calcareous nannofossils and benthic and
biostratigraphy. Stages are grouped in a planktonic foraminifera (see Coates et al.
nested hierarchy of Series, Systems, and Er- 2003,Tables1,2),manyofthelatterareeas-
athems. Their temporal equivalents are ily seen by a hand lens and by the naked
called Ages, Epochs, Periods, and Eras, respec- eye. The formation can be seen to lie un-
tively. derneath weathered coarse volcanic flow
The fourth subdivision, geochronology, breccia(thebrokenupboulderyrockthatis
is the science of numerical time. Whereas crated when lava flows and cools at the
chronostratigraphydealswithrelativetime same time). From its abundant planktonic
(younger/older), geochronology trans- foraminifera and calcareous nannofossils,
forms specific stratigraphic horizons into the age of the Punta Alegre Formation is
numerical markers. Geochronology uses 19-18.3 Ma. (Coates et al. 2003), and by its
radioisotopic ages and/or astrocyclicity benthic foraminifera it is interpreted to
GEOLOGYOFBOCASDELTOROARCHIPELAGO 379
have been deposited in water depths of flows and coral reef lenses. The latter con-
100—2000 m (Coates et al. 2003). sist of several species of Montastraea (in-
The Valiente Formation.—This unit over- cludingM.imperatorisandM.canalis),mas-
lies the Punta Alegre Formation and crops sive Porites waylandi and thin branching
out extensively on the Valiente Peninsula Stylophora(possiblyS.granulate;AnnBudd
(Fig. 3) and nearby Deer and Popa islands pers. comm. 2005). These facies intercalate
(Fig. 4). The Valiente Formation is of com- andreplaceeachotheroververyshortdis-
plex and highly variable lithology because tances,bothlaterallyandvertically(forde-
itrepresentsthesuiteoffacies,marineand tailed descriptions of the facies, see Coates
terrestrial,igneousandsedimentary,thatis etal.2003).Fig.3showsthatthebasaltlava
associated with an active emergent volca- and flow breccia facies form a core around
nic island arc. It includes columnar basalt, which the other facies are distributed pe-
basalt flow breccia, pyroclastic tuff (air- ripherally as terrestrial, coastal or marine
borne volcanic ash deposits), fluviatile/ slope deposits. Planktonic microbiotas and
estuarine conglomerate, marine debris radiometrically dated basalt in the se-
FIG.3. Geologicalmapandcrosssection(a–á)oftheValientePeninsula,BocasdelToro,westernPanama,
showingthedistributionofthePuntaAlegreandValienteformationsandtheBocasdelToroGroup.Thefive
lithofaciesoftheValienteFormationareindicatedbyseparatecolorsandnumbersonthekey(upperright)as
follows;v1)basaltlavaandflowbrecciafacies;v2)coarsevolcaniclasticfacies;v3)pyroclasticfacies;v4)coral
reeffacies;v5)marinedebrisflowandturbiditefacies.
Fi 3 li 4/C
380 A.COATESETAL.
pelago above the Valiente Formation, sig-
nifyingextensiveemergenceanderosionof
thevolcanicarcfrom∼11.5-∼7.2Ma(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.
TheBocasdelToroGroupsignifiesanew
depositional cycle as a marine transgres-
sion submerged the older eroded volcanic
arcrocks.Theoldestsedimentsthusrepre-
sent the first shallow-water, beach, and
near-shoresanddeposits.Asthetransgres-
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-
FIG.4. GeologicalmapofPopaIsland.Section(b-b’)
neticallyrelatedanddocumentasinglema-
showstheValienteFormationonPopaIsland,where
it is unconformably overlain by the Pliocene Cayo rine transgressive/regressive event. The
Agua Formation (ca). Numbers and symbols as for most continuous section lies along the east
Figure3.OnPopaIsland,onlythev1basaltflowfacies and west coasts of the Valiente Peninsula
andthev2coarsevolcaniclasticfacies(notassociated (Fig. 3) where it ranges in age from late
withreeflenses)arepresent,withthinlayersoflow
Miocene (Messinian, 7.2-5.3 Ma) to late
rankcoal,anexampleofwhichisexposedalongthe Pliocene (∼3.5 Ma).
coastimmediatelynorthofPuntaLaurel.Aprominent
basalt dike is exposed at the tip of the Punta Laurel Tobobe Sandstone.—This lowest unit is a
(hereshowninredasv1facies)whereitcutstheVa- pebble conglomerate that passes up into
lienteFormation. clean, hard quartz sandstone containing
sand dollars, spatangoid echinoids, mol-
lusks, including the large, thin-shelled bi-
quenceindicatethattheValienteFormation
valve Amusium, vermetids and serpulids,
ranges in age from 16.5-11.5 million years
as well as an array of infaunal burrow
old and includes marine deposits from
structures. It is best observed on the small
near-shore to as deep as 1000 m, as well as
Plantain Cay and an adjacent smaller un-
severaltypesofterrestrialdeposits(Coates
named island to the west (Fig. 3). The de-
et al. 2003).
posit represents a beach and near-shore
Bocas del Toro Group.—Columnar basalt sandbodyanddocumentstheearlieststage
and pyroclastic and fluviatile rocks of the of the marine transgression. Planktonic
Valiente Formation required extensive foraminifera from laterally equivalent de-
emergenceofpartsofthegrowingvolcanic positsonToroCay(Fig.3)indicatethatthe
arc(columnarbasaltisformedbythecool- age of the unit is Messinian (7.2-5.3 Ma).
ingoflavapondedlakes).About12Ma,the Here the deposit contains a variety of mol-
archadbecomeinactiveandrecentstudies lusks including turritellids and pectens,
inDarien,Panama(Coatesetal.2004),sug- erect bryozoa, and echinoids. Classic ex-
gestthatthecollisionofthesoutherntipof amples of complex thalassinoid burrow
the Central American volcanic arc with systems constructed by callianasid crusta-
South America was initiated at the same ceans are also present.
time. These events may explain the major Nancy Point Formation.—It was named
unconformity in the Bocas del Toro Archi- for the promontory called Nancy Point
Fi 4 li 4/C
GEOLOGYOFBOCASDELTOROARCHIPELAGO 381
which lies 2.5 km southwest of the village only along the southern coast of the Va-
of Tobobe (Fig. 3). The formation is almost liente Peninsula, east of Secretario. The
continuouslyexposedsouthwardalongthe Shark Hole Formation consists of mica-
coast as far as Chong Point (= Nispero ceous, clayey siltstone that is pervasively
Point). The Nancy Point Formation is con- bioturbated and rich in large scaphopods.
formablewiththeTobobeFormationbelow The uppermost part of the formation also
andtheSharkHolePointFormationabove containsabundant,thin,shellybedsandin-
(Fig. 2 and 3). It consists of shelly, muddy traformational slumps with pillow folds
and silty sandstone, and muddy siltstone and rip-up clasts. Benthic foraminifera in-
with occasional coarse volcaniclastic and dicatethatthepaleobathymetryofthisunit
bioclastic sandstone beds. It contains scat- ranges from 100-200 m (Collins 1993). This
tered mollusks and occasional leaves and represents the first stage of the regressive
plant fragments with diverse moderately phase of the Bocas del Toro Group trans-
abundant molluscan assemblages through- gressive/regressive cycle. Calcareous nan-
out the section and several, rich, low- nofossils and planktonic foraminifera are
diversity,thick-shelledmolluskbedsatthe abundant in several horizons and suggest
base. the age of the formation is early Pliocene
ThetransitionfromTobobeSandstoneto (5.3-3.6 Ma).
NancyPointFormationisbestseenonToro Escudo de Veraguas Formation.—The
Cay (Fig. 3) where dark blue-gray, silty stratigraphic order of the formations de-
sandstone, typical of the Nancy Point For- scribed above has been determined by
mation,containsoccasional,clearlydefined physical superposition. The three remain-
infaunal burrow systems and extremely ingformationsoftheBocasdelTorogroup
abundant and diverse mollusks. It overlies are known only on islands and their posi-
coarse, channeled Tobobe Sandstone with tionrelativetotheotherunitshasbeende-
only a 10-m exposure gap. Nancy Point terminedthroughstratigraphiccorrelation.
Formation benthic foraminifera indicate The Escudo de Veraguas Formation is
that deposition of the Nancy Point Forma- known only from the island of the same
tionwasin200-500m.waterdepth(Collins name that lies 27 km east of Nancy Point
1993) demonstrating that relatively rapid (Fig.5).Itsstratotype,forthelowerpartof
deepening took place from near-shore (To- theformation,isalongthecoastontheeast
bobe Sandstone) to upper slope (Nancy side of the V-shaped embayment situated
PointFormation).Thisrepresentsthemaxi- inthecentralpartofthenorthcoast,about
mumdepthofthetransgressive/regressive 1 km east of Long Bay Point (Fig. 5). The
cycle. Planktonic foraminifera and calcare- stratotype for the upper part of the forma-
ousnannofossilsdatetheNancyPointFor- tionliesalongthewestcoastforaboutone
mationasMessinian(7.2-5.3Ma),thesame km south of Long Bay Point.
time range as for the Tobobe Sandstone. Lithologically, the Escudo Formation
Within that time frame the Nancy Point consists of, in the upper part (1.8 Ma), per-
Formation is younger because it overlies vasively bioturbated clayey siltstone and
the Tobobe Sandstone. silty claystone, with frequent concretions,
Shark Hole Point Formation.—It is named and scattered shelly hash, often with scat-
for the promontory of the same name that tered whole and diverse mollusks, small,
lies 3 km east of Chong Point (Fig. 3) and cornute,ahermatypiccorals.Thelowerpart
the stratotype lies along the coast between oftheformation(3.5Ma)ismoreindurated
ChongPointandBrunoBluff,southofOld withverycommonanddenselypackedce-
Bess Point. The Shark Hole Point Forma- mented burrow concretions and thalassi-
tion conformably overlies the Nancy Point noid galleries at the base, scattered aher-
Formation(Fig.2and3)andisoverlainby matypic corals through the middle part,
an unnamed conglomeratic, cross-bedded, aswellasvariablyabundantmollusks,and
coarser grained sequence of volcaniclastics at the top a coral biostrome dominated by
containing large pieces of wood and plant Stylophora and sand dollars. The top and
fragments. This unnamed unit is exposed botton units suggest deposition in inner
382 A.COATESETAL.
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
exposedatTiburonPointandtheunnamed
point to the south. The corals appear to be
mostly free living, hermatypic, grass flat
corals,suchasPlacocyathusandManicina(=
Teleiophyllia), as well as ahermatypic spe-
cies, and they suggest deposition in water
depthsof30-50m(AnnBudd,pers.comm.
FIG. 5. Geological map of Escudo de Veraguas is-
2005). Evidence from benthic foraminifera
land. Ages in millions of years in brackets for lower
(Collins1993)indicatespaleobathymetryof
and upper parts of the Escudo de Veraguas Forma-
tion. 20-80 m, overlapping the inference from li-
thologyandcoralfaunathattheCayoAgua
Formation represents a shallow-water fa-
neriticwaterdepths,butthemiddlepartof
cies.
thislowersuccessionhasabundantbenthic
The age of the Cayo Agua Formation is
foraminifera indicating deposition in outer datedatthebase∼5.0-3.5Maandatthetop
neritictoupperbathyalwaterdepths(Col-
3.7-3.4 Ma, which suggests it is a contem-
lins 1993).
porary, shallow water equivalent of the
Cayo Agua Formation.—This formation
SharkHolePointFormationandthelower-
was named for the island that lies about 6
most part of the Escudo de Veraguas For-
kmtothewestofToroPoint,ValientePen-
mation.
insula.Theformationiswellexposedalong
theeastcoast(Fig.6)andconsistslithologi- The northern region
cally of pervasively bioturbated, muddy,
The islands in the northern region of the
silty sandstone with common horizons of
BocasdelToroarchipelagohaveadifferent
abundant thick-shelled mollusks and aher-
geologic history than the southern region.
matypic corals. Occasional horizons of
Volcanic arc columnar basalts of the Va-
pebble conglomerate and very coarse-
liente Formation form the basement that
grained volcaniclastic sandstone are com-
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
thesamevolcanicarcthatwentextinctand
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
northernunitsarecharacterizedbyamajor
component of coral reef limestone.
The geology of Colon and Bastimentos
islandsarecurrentlybeingstudiedandthe
FIG.6. GeologicalmapofCayoAgua.Agesinmil- formaldesignationofthelithostratigraphic
lionsofyears. units and their ages is not yet finalized. In
Fi 5 li 4/C Fi 6 li 4/C
GEOLOGYOFBOCASDELTOROARCHIPELAGO 383
general, the sequence is younger than the Valiente Formation.—The spectacular ex-
section to the south with the exception of posures along the northwestern coast of
the Escudo de Veraguas Formation. The BastimentosIsland(Fig.7)ofcolumnarba-
oldest units on Colon and Bastimentos is- salt and flow breccia are assigned to the
lands may overlap in time with the top of Valiente Formation solely on their great
the Cayo Agua and Shark Hole Point se- similarityinlithology.Theexposuresrange
quences at ∼3.5 Ma. from Toro Point to the eastern side of
FIG.7. Geologicmapandsection(a-a’)ofBastimentos
Fi 7 li 4/C