Closure of the Isthmus of : The near- marine record of Costa Rica and western Panama

ANTHONY G. COATES* Department of Geology, The George Washington University, Washington, D.C. 20052 JEREMY B. C. JACKSON Smithsonian Tropical Research Institute, Box 2072, Balboa, Republic of Panama LAUREL S. COLLINS Museum of Paleontology, University of Michigan, Ann Arbor, Michigan 48109-1079 THOMAS M. CRONIN | HARRY J. DOWSETT | U.S. Geological Survey, 970 National Center, Reston, Virginia 22092 LAUREL M. BYBELL j PETER JUNG Naturhistorisches Museum, Augustinergasse 2, CH 4051, Basel, Switzerland JORGE A. OBANDO RECOPE, S.A., Apartado 4351, Zone 1000, San Jose, Costa Rica

ABSTRACT sent on the side. These sections was formed by northwestward movement of the fortuitously include abundant thick intra- South American plate (SO AM) and an easterly The final closure of the Isthmus of Panama formational slumps containing shallow-water vector of both the Nazca (NAZ) and Caribbean at ~3.5 Ma divided the American tropical fauna more appropriate for biological com- (C ARIB) plates that brought the Central Ameri- ocean into two separate and different oceano- parison with the Caribbean biota. Similarly, can in collision with the northwest graphic . Consequences for the marine the -1.9 Ma to 1.5 Ma interval, well con- South American foreland during the biota were profound, but, hitherto, correla- strained by various taxa, includes middle- to (Fig. 1). tion of the Pacific and Caribbean coastal sec- outer-shelf, and inner-shelf to upper-slope The timing and duration of closure of the tions has not been precise enough to track deposits on the Caribbean side, and marginal- Isthmus of Panama is known only in general biologic patterns. We present here a correla- marine to inner-shelf deposits on the Pacific terms. During the middle (12.9- tion of 31 sections from the Pacific and . 11.8 Ma), benthic foraminifera indicate that a sill Caribbean of Costa Rica and western Using our new biostratigraphic framework appears to have formed at -1,000 m (Keller and Panama. Using calcareous nannofossils and to correlate previously poorly constrained Barron, 1983; McDougall, 1985; Duque-Caro, planktonic foraminifera at both the tops and mollusc collections, we show that evolution- 1990b). This was the first tectonic indication of bottoms of each formation, we estimate that ary divergence of the Pacific and Caribbean the collision of the Central American Arc with the Caribbean section ranges from 8.2 Ma to near-shore marine faunas had occurred by northern . Benthic foraminifera 1.7 Ma; and the Pacific sequence, from 3.5 Ma. This strongly suggests that the Isth- also become highly distinct between the Carib- 3.6 Ma to <1.7 Ma. These intervals bracket mus was effectively closed by this time. bean and Pacific at this time, which led Duque- postulated dates for final closure of the Isth- Caro (1990) to postulate a circulation barrier mus and provide the first well-dated record INTRODUCTION caused by the formation of a more powerful, of middle and late faunas from the cool, marginal California . By the latest . The closure of the Isthmus of Panama was an Miocene (7.0-6.3 Ma), similarity of the benthic The Caribbean and Pacific sections include event of fundamental paleogeographic and pa- foraminiferal faunas suggests a return to com- very different environments of deposition, yet leobiologic importance. Two separate and dif- mon circulation between the Pacific and Carib- there is sufficient overlap and diversity of ferent oceanographic regimes were initiated with bean, but the faunas also indicate rapid filling habitats to permit meaningful biological com- profound climatologic implications (Haq, 1984; and shallowing of marginal basins in the parisons. On the Caribbean side, formations Hay, 1988). The Isthmus formed a corridor for Panama Arc and South America to neritic tied together by the overlap of the upper Pli- terrestrial organisms, as well as a major barrier depths (-200 m; McDougall, 1985; Savin and ocene markers Sphenolithus abies and Pseudo- that separated tropical marine organisms of the Douglas, 1985; Duque-Caro, 1990b). A combi- emiliana lacunosa (3.5 Ma to 3.6 Ma) range Pacific and the Atlantic (Woodring, 1954). Clo- nation of marine and terrestrial evidence (Keig- from very shallow to shallow inner shelf sure occurred as a result of a complex interplay win, 1978, 1982a, 1982b; Lundelius, 1987; (<200 m) and upper slope (200-800 m). The of several lithospheric plates, the exact nature of Marshall, 1988) suggests final closure of the Pacific coast sections were mostly deposited which is still debated (Lonsdale and Klitgord, Isthmus at about 3.2 Ma to 2.5 Ma. in a trench slope environment, which is ab- 1978; Pennington, 1981; Pindell and Dewey, Paleobiologically, closure of the Isthmus 1982; Wadge and Burke, 1983; Burke and oth- presents an opportunity to observe patterns of ers, 1984; Dengo, 1985; Duque-Caro, 1990a). *Present address: Smithsonian Tropical Research speciation, extinction, and changes in diversity Institute, Box 2072, Balboa, Republic of Panama. In general, the lower Central American Isthmus resulting from geographic isolation of marine

Geological Society of America Bulletin, v. 104, p. 814-828, 9 figs., 2 tables, July 1992.

814

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biotas. Paleogeographically, profound changes in water depth, sedimentation, and temperature can also be documented and related to the stages of formation of a major tropical biogeographic barrier. Documentation of these patterns re- quires a reasonably complete near-shore marine sedimentary record that is both precisely datable and diversely and continuously fossiliferous. In this paper, we establish the existence of such a record on both the Caribbean and Pacific coasts that brackets all proposed dates for final closure of the Isthmus of Panama. We have measured sections at 31 localities in Costa Rica and west- ern Panama (Fig. 2). Here we present a revised physical stratigraphy and a new detailed biostrat- igraphic correlation for these deposits.

STUDY AREA

A review of the topography, tectonic history, and Neogene regional geology strongly points to the area within Figure 1 as the location of the final closure of the Isthmus of Panama. This region lies between the Chortis continental block in the northwest, whose limit lies approx- imately along the -Costa Rica border, and the South American foreland (SOAM) in Figure 1. Map of the Chorotega and Choco structural blocks, with their defining thrust or the southeast, marked by the Uramita Fault transform boundaries (after Duque-Caro 1990a, 1990b). Principal plate vectors are indicated Zone (UFZ; Duque-Caro, 1990a). Between by the arrows. AF = Atrato fault; CARIB = ; CT = Colombian Trench; GFZ = these two continental crustal units lie two oce- Gatun Fault Zone; MAT = Middle American Trench; NAZCA = Nazca plate; NOAM = North anic blocks. In the west, the Chorotega block American plate; SOAM = South American plate; PFZ = Panama Fracture Zone; UFZ = occupies Costa Rica and western Panama, and Uramita Fault Zone; CHORTIS refers to the continental crustal block underlying Nicaragua to in the east, the Choco block forms eastern southern Mexico. Stippled ornament outlines the approximate distribution of late Miocene to Panama, northwestern , and northern sediments. Ecuador (Dengo, 1985; Duque-Caro, 1990a). The Gatun Fracture Zone (GFZ), lying along the line of the Panama , marks the junc- The lowest topographic pass across the Central sediments are known in the region of San Carlos tion of the two blocks. Both blocks are overlain American Isthmus (34 m; Savin and Douglas, (L. Obando, 1986; Sen Gupta and others, by late Neogene marine sediments (Fig. 1). 1985) lies immediately south of Lake Nicaragua 1986). These deposits contain biotas of Carib- Present-day low-lying transisthmian corridors in the west of the Chorotega block (Fig. 2, bean affinity, yet are only 100 km from the Pa- are likely sites for the last interoceanic . A A'), where marine Neogene to Quaternary cific coast (Figs. 1 and 2). At the site of the GFZ,

Figure 2. Locality map of the 31 sections measured. Locations of the lowest topographic passes across the Isthmus are shown at A-A' and B-B'.

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a marine, Neogene sedimentary basin that con- were frequent marine connections between the particular localities have proposed stratigraphic nected the Caribbean and the Pacific (Wood- Caribbean and the Pacific (Duque-Caro, 1990b, names. We have used available names where ring, 1978; Dengo, 1985) coincides with a his Fig. 7). The topographic, tectonic, and re- appropriate, but others are duplicates for pre- negative Bouguer gravity anomaly and the sec- gional geologic evidence strongly suggests that viously named lithologic units. ond lowest topographic pass across the Isthmus the stretched from westernmost Sections have been measured on the Nicoya, (85 m; Savin and Douglas, 1985; Fig. 2, B-B'). Costa Rica to the Atrato Valley in Colombia Osa, and Burica on the Pacific coast Extensive Neogene marine sediments cover (Fig. 1) and that this region contains the site of (Fig. 2, localities 22,24-31). In each case, Neo- the Choco block (Bandy, 1970; Bandy and the final marine connection between the Pacific gene sediments are draped over pre-existing Casey, 1973; Dengo, 1985; Duque-Caro, 1990a, Ocean and the . "basement" that hitherto has been lumped into a 1990b) where middle Miocene to lower Plio- single unit, the Nicoya Complex (Berrangé and cene sediments crop out in eastern Panama and LITHOSTRATIGRAPHY others, 1989). Baumgartner and others (1989) northwestern Colombia (Fig. 1). Duque-Caro and Mora and others (1989), however, de- (1990b) outlines the sedimentary and paleocean- In this section, we unify and standardize the scribed the basement at Golfito, Burica, and Caña- ographic history associated with the formation physical stratigraphic nomenclature of the Osa as distinct from the Nicoya Complex and of the Isthmus in this region. It is likely that younger Neogene rocks for the South Central from each other. Thus, in the Osa , the during the late Neogene the Chorotega and American region (Figs. 3A and 3B). Several pre- basement is a subduction complex that Choco blocks formed an archipelago and there liminary studies on the sedimentary history of consists of a mélange, including Paleogene lime-

Figure 3. Lithostrati- graphic correlation charts of the late Neogene units from the Caribbean and Pacific coasts of Panama and Costa Rica. A. Lithostratigraphic correlation chart of the late Neogene Charco Azul Group (Penita, Burica, and Armuelles Formations) and the Montezuma Formation of the Pacific coast of Costa Rica and Panama. The thick- ness datum is zero for the base of the Armuelles For- mation. Calculations of the thickness of the Armuelles Formation are very tentative. The lettered columns corre- spond to the numbered lo- calities in Figure 2: A = 22; B = 24-25; C = 26; D = 27; E = 28; F = 29-30; G = 31. MFZ indicates Medial fault Zone (Corrigan and others, 1990).

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stone turbidites and ophiolitic elements, whereas Formation, but they assigned different ages to they are lithologically distinct and mappable the basement underlying the Burica peninsula is the units named by Terry. throughout the region. They clearly extend into interpreted as a Cenozoic seamount complex. An interpretation of the sedimentary history the Osa Peninsula, where the same sediments Along the Costa Rican Pacific coast, there is of the Burica and Osa Peninsulas is proposed by were later called the Osa Group by Berrange an initial Neogene transgressive unit that com- J. Obando and others (1990), based on exten- (1987), but the name Charco Azul has prece- monly oversteps the basement complex because sive field work in the region. In the thickest and dence. We briefly describe the formations and of the considerable relief developed on the most complete section, along the La Vaca River, members of the Charco Azul Group below, and basement during its long pre-Neogene history. they identified a lower shallow-water facies of their physical relations are shown in Figure 3A. On the Caribbean coast, however, the "base- shoreface, storm-generated, sandy deposits over- Penita Formation. The type locality is along ment" does not directly influence sedimentary lain by a thick sequence of turbidites, which are Penita Creek (Fig. 4), on the western coast of patterns; the Neogene sequence is interbedded capped by coarse, shallow-water, fan-delta the Burica Peninsula in Costa Rica (Corrigan with volcanic units and conformably overlies an complexes and nearshore . These three fa- and others, 1990). The Penita Formation at this extensive Mesozoic and early Cenozoic section. cies correspond to the three physical units, location (Fig. 3A, column F) sits nonconform- Penita, Burica, and Armuelles Members, pro- ably upon a Cenozoic seamount complex whose Charco Azul Group posed by Corrigan and others, and physical basalt surface has lithophagid bivalve borings stratigraphic relationships allow this basic pat- filled with Penita sediment. On the Osa Penin- Terry (1941, 1956) divided the Neogene of tern to be applied to the Osa Peninsula also. sula the Penita Formation is exposed near the the Burica Peninsula into a Miocene Burica Thus, we propose that the sedimentary sequence south coast along Leona Creek (Fig. 4). forms a major volcaniclastic, transgressive/re- Formation, a Pliocene Charco Azul Formation, The Penita Formation is probably early Plio- gressive, sedimentary cycle, which is best ex- and a Pleistocene Armuelles Formation, a cene in age and consists of as much as 1,200 m pressed stratigraphically as the Charco Azul scheme more or less followed by Olsson (1942). of clayey, blue-green siltstone and litharenite, Group, consisting of the Penita, Burica, and Subsequently, Corrigan and others (1990) which is sometimes glauconitic, occasionally rich Armuelles Formations. These units are appro- proposed three units, the Penita, Burica, in molluscs and consistently rich in benthic and priately raised in rank to formations because and Armuelles Members of the Charco Azul planktic foraminifers. The basal unit is coarse,

CARIBBEAN

Bomba Limon Valiente Cayo Escudo de Colon/ Peninsula Agua Veraguas Gatun

BOCAS DEL TORO GROUP Figure 3. (Continued). B. Lithostratigraphic correlation chart of the Limón Group KZX (Rio Banano and Moin For- mations) and the Bocas del LIMON GROUP Toro Group (Nancy Point, Shark Hole Point, Cayo Agua, and Escudo de Vera- H guas Formations) and the 0 m 0 m Gatun Formation. The let- tered columns correspond to liiTTîi the numbered localities in Figure 2: H = 5-6; I = 1-4; i a J = 12-15; K = 7-11; L = 16; Ti H M = 21.

1000 m 1000 m p^A/j'

D

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greenish-blue litharenite with locally channelled tion. At Pavon (Fig. 3A, column D), the La There are additional exposures of the Burica basal volcanic conglomerate; similar conglom- Vaca Member consists of 900 m of coarse, Formation along most of the rivers surrounding erates occur as lenses throughout the lower part grayish-green volcanic conglomerate with large- the central highland of Burica, for example, La of the formation. Turbidite sequences also occur scale crossbeds; channels; armored mud balls; Vaca River (Fig. 3 A, column E; Fig. 4) and San locally. Laterally, the Penita Formation is highly and locally, large thick-shelled oysters, other bi- Bartolo River (Fig. 3A, column F; Fig. 4). variable in thickness and lithology (Fig. 3); this valves, and wood. Eastward, near the La Vaca On the eastern coast of Osa from Sombrero reflects its overstepping relationship to the un- River (Fig. 4), the La Vaca Member consists of Point to Tigre Point (Fig. 4), -800 m of Burica derlying basement, in both the Osa and Burica wedge-bedded, conglomeratic litharenite and Formation turbidites are exposed (Fig. 3A, col- Peninsulas. The La Vaca Member of the Penita fine, evenly bedded litharenite with mud clasts umn C). A 2- to 4-m autobrecciated unit overlies Formation (Fig. 4) is designated for a distinctive (Fig. 3A, column E). a thin (1- to 2-m), basal, coarse volcanic con- suite of coarse conglomerates that stretch from Burica Formation. The Burica Formation glomerate that represents the La Vaca Member east of the La Vaca River westwards to Pavon (Fig. 3A) is late Pliocene in age and conforma- of the Penita Formation, and which rests on the Bay, in the northern and northwestern part of bly overlies the Penita Formation. It consists of subduction complex. Further west near La the Burica Peninsula. The type locality is along about 2,800 m of mostly fine-grained, volcani- Chancha Point (Fig. 4), a distinct lithofacies, the La Vaca River near its headwaters (Fig. 4). clastic turbidite deposits with local megabreccias named by Lew (1983) the "La Chancha Forma- The base rests unconformably upon the Paleo- formed by large-scale intraformational slumps. tion," is considered here to be a member of the gene siliceous limestone that rests on the Ceno- The unit was well described by Corrigan and Burica Formation (Fig. 3A). The La Chancha zoic Seamount Complex, which here forms a others (1990). The type locality is located on the Member of the Burica Formation is exposed for paleocliff. At the top, the La Vaca Member eastern coast of the Burica Peninsula between its ~2.5 km along the coast, east of La Chancha passes conformably up into the Burica Forma- southern tip and Manzanillo Creek (Fig. 4). Point, which is the type section. It consists of

MV^J) GOLFO DULCE

Caribbean

\ Panama J Costa Rica r\j,

Pacific

QUATERNARY ARMUELLES FORMATION

BURICA FORMATION

LaChancha Member

PENITA FORMATION

La Vaca Member CENOZOIC SUBDUCTION (Osa) AND SEAMOUNT)Burica) COMPLEX

Figure 4. Preliminary geological map of parts of the Osa and Burica Peninsulas, showing the distribution of the formations and members of the Charco Azul Group and their relationship to the Cenozoic subduction complex. See text for location and description of the type localities. In the eastern Osa Peninsula, the sections measured between Sombrero Point and Tigre Point are exposed only in the , which is not wide enough to show on the map.

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coarse, irregularly bedded volcanic conglomer- Cocalito (Fig. 5), where they are ex- Shark Hole Point Formation. Conformably ate and volcaniclastic litharenite, and it is inter- tremely rich in molluscs and wood. The forma- overlying the Nancy Point Formation is the preted to represent canyon fill within the trench tion is also extensively exposed along the coastal Shark Hole Point Formation, which ranges from slope on which the Burica turbidites were depos- terraces and in streams inland from Montezuma latest Miocene through early Pliocene. We des- ited. The La Chancha Member is apparently (Fig. 5). ignate as the type section the coastal sequence conformable with the underlying Peñita Forma- from Butchuqua Oust east of Chong Point) to tion and the overlying Armuelles Formation, Bocas del Toro Group Bruno Bluff (Fig. 6). The formation is -205 m but the contacts are not exposed (Fig. 3A, col- thick and consists of micaceous, clayey siltstone umn B). On the Atlantic side of the Isthmus in the apparently pervasively bioturbated, with scat- Armuelles Formation. We designate the region of the Bocas del Toro Province, Panama tered molluscs and abundant large scaphopods. type section of the Armuelles Formation (Terry, (Fig. 6), a previously unrecorded sequence of Rare thin, coarse sand and cross-bedded lami- 1941) to be along the Rabo de Puerco River, highly fossiliferous Neogene sediments, the nated silt horizons also occur and the upper part starting from the town of Puerto Armuelles and Bocas del Toro Group, is here described, and a is distinguished by numerous regular concre- running west-northwestward for ~3 km (Fig. 4). formal physical stratigraphy is proposed (Fig. tionary horizons. The Armuelles Formation, which is early Pleis- 3B, columns J, K, and L). This sequence, rang- At Bruno Bluff the upper Shark Hole Point tocene or younger, is also well exposed along the ing in age from 6.5 Ma to 1.5 Ma, is correlated Formation consists of medium- to massive- San Bartolo and La Vaca Rivers, Burica Penin- with the time-equivalent but lithologically dif- bedded, dark-gray siltstone with a tight clayey sula (Fig. 3A, columns E and F; Fig. 4), and on ferent Limon Group (here designated) in the matrix, and frequent small vertical burrows. A the Osa Peninsula (for example, along the Claro area of Limon, Costa Rica, where we also pro- distinctive, intraformational slump bed, with pil- River, South Osa; Fig. 3A, column B; Fig. 4). In pose a revised stratigraphic nomenclature (Fig. low folds and rip-up clasts, is present in the mid- the lower part, the Armuelles Formation con- 3B, column H and I; Fig. 7). We divide the dle of the exposure, and there are abundant rich, sists of pebbly conglomerate, often occurring in Bocas del Toro Group into three formations. thin shell beds in the lower part. large channels; unconsolidated greenish-blue Nancy Point Formation. The oldest unit of Escudo de Veraguas Formation. We named siltstone; and litharenite. In the upper part, gray- the Bocas del Toro Group is the Nancy Point the formation for the of Escudo de Vera- blue, clayey siltstone and fine litharenite pre- Formation, which is late Miocene. We propose guas, which lies about 27 km east of Nancy dominate with sporadic bedded coarse sand. A the type locality to be the coastal section along Point, Valiente Peninsula (Fig. 6). It ranges in marker bed near the top consists of large (10- to the north coast of the Valiente Peninsula, from age from latest early to late Pliocene. We pro- 20-cm) boulders, richly encrusted with oysters, Nancy Point to Chong Point (Fig. 6). The forma- pose as the type section of the lowest part of the corals, and cheilostome bryozoans, immediately tion is —175 m thick and consists in the lower formation the exposures along the north coast below which is a biostrome consisting of the part of coarse volcanic litharenite, gray mica- for 2 km east of Long Bay Point and, for the bivalve Pinna. The overlying and underlying ceous and clayey silt, and gray-green fine sand upper part of the formation, along the west coast beds contain extremely abundant and diverse with scattered molluscs and lignitic horizons. for 1 km south of Long Bay Point. The total molluscs. The total thickness of the Armuelles The upper part consists of gray-blue clayey silt- thickness is -58 m. The lower part consists of Formation is -370 m. The Armuelles is con- stone, laminated clay, and fine litharenite with coral- and mollusc-rich, pervasively bioturbated, formable upon the underlying Burica Formation volcanic conglomerate and litharenite in con- blue-gray, shelly, clayey siltstone with large logs, over much of the region. Along El Higo Creek, spicuous superposed channels. abundant arthropod burrows, and coral and ser- however, (Fig. 3A, column D; Fig. 4), an outlier of massive, uniform, unbedded litharenite, rich in molluscs, rests directly on the basement sea- mount complex (Fig. 4).

Late Neogene of the Nicoya Peninsula

Montezuma Formation. The Montezuma Formation (Fig. 3A, column A; Fig. 5) is early Pliocene to early Pleistocene and -30 m thick. It crops out in the southeastern part of the Ni- coya Peninsula (Baumgartner and others, 1984; Aguilar and Fischer, 1986; Chinchilla-Chaves, Figure 5. Map showing the 1989; McKee, 1985). The type locality is along extent of the Montezuma For- the road immediately west of the town of Monte- mation and the localities from zuma (Fig. 5). The formation is highly variable, which datable fossils were - containing three major lithofacies (Chinchilla- c tained. At Cocalito Beach, Chaves, 1989) that are interpreted as represent- the <1.7 Ma dated section is ing three environments: an open marginal just above the high-tide mark. marine facies, embayments and sheltered margin- The early Pliocene localities at al marine environments, and estuarine/fluvia- Las Playitas and Montezuma tile environments. Facies typical of sheltered dated by planktic foraminifera marine environments are well exposed at low (McKee, 1985) are 20 m and 70 m tide, to the east-northeast of Montezuma, along above sea level, respectively.

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Figure 6. Map of the Valiente Peninsula, and the of Escudo de Veraguas and Cayo Agua, showing the distribution of the various formations and members of the Bocas del Toro Group. See text for detailed description of the type localities. A biostratigraphic marker bed dated at 3.5-3.6 Ma (co-occurrence of Sphenolithus abies and Pseudoemiliana lacunosa) occurs in all three areas and is denoted here by vertical lines. Dashed lines indicate the fold axes on Cayo Agua. The type section runs from immediately south of the northernmost fold axis near Norte Point to just south of Tiburón Point. The lines of crosses indicate a marker bed of diverse ahermatypic corals. The 3.5-3.6 Ma marker bed occurs north of the fold axis at Norte Point, that is, outside the type section. Projection of this marker (3.6-3.5 Ma) southward into the type section, over the anticlinal fold, suggests a horizon somewhere near Níspero Point.

pulid marker beds. The upper part is massive Veraguas Formation. We show evidence that ~ 160 m thick, fine to coarse sand beds contain gray-blue, silty, bioturbated clay and clayey silt. the Cayo Agua Formation represents a more abundant black basalt grains. Concretions are This formation is known only on the island of shallow-water lithofacies coeval with the bath- common, and there are horizons of phosphatic Escudo de Veraguas. yal to outer-shelf facies of the Shark Hole Point grains, arthropod burrows, and wood. Several Cayo Agua Formation. The Cayo Agua and Escudo de Veraguas Formations. The des- horizons are richly fossiliferous with distinctive Formation is named for the island of the same ignated type locality stretches along the east marker beds of corals; diverse mollusc horizons name (Fig. 6) where it is extensively exposed coast of Cayo Agua, from just south of Norte are very common. along the eastern and southern coasts. At present Point to Tiburon Point (Fig. 6); south of this the unit is recognized only on Cayo Agua but it point the section is repeated as far as Piedra Limón Group almost certainly extends farther west. It ranges Roja Point. The sequence consists of massive, in age from early to late Pliocene and thus over- mostly blue-gray, pervasively bioturbated, Our stratigraphic framework for the newly laps the upper part of the Shark Hole Point clayey siltstone with abundant mollusc hash and designated Limón Group in Costa Rica builds on Formation and the lower part of the Escudo de shells. In the middle of the unit, which is previous work in this region by Taylor (1975),

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Cassell (1986), and Cassell and Sen Gupta Gupta (1989a) described the Moin Clay as a is principally a sandstone lithofacies. We suggest (1989a and 1989b). We propose to unite the separate formation, consisting of 70 m of blue that the topographically conspicuous facies mostly shallow-marine, volcanogenic, Neogene claystone, cropping out only in the area imme- exposed west of Limón and included by Taylor sediments into a new unit, the Limón Group, diately adjacent to Cangrejos Creek (see Cassell (1975) in his Rio Banano Formation is a much consisting of three parts: the Uscari, Rio Banano, and Sen Gupta, 1989a, their Fig. 1). They con- younger unit, not appropriately included in the and Moin Formations. The Limón Group un- sidered only the sandstone facies in their defini- Limón Group (but not to be confused with the conformably overlies basaltic lavas in the central tion of the Rio Banano Formation. We measured biostromal reef lenses interbedded in the Moin part of the Limón Basin (Cassell, 1986) and is several sections near Limon (Fig. 2, localities Formation). unconformably overlain by distinctive reef de- 1-6), including Taylor's type sections of the Rio Uscari Formation. The Uscari Formation posits, characterized by shallow-water branch- Banano Formation immediately west of Bomba was named by Olsson (1922) after the Uscari ing corals, which lie to the west of Limón. and the type Moin Formation at Cangrejos Creek, a small tributary of the Telire River, We have not studied the Uscari Formation Creek (Fig. 7). We propose that the Moin be about 50 km south of Limón (Cassell and Sen but accept the recent revision of this unit by retained as a formation but that it be expanded Gupta, 1989b, Fig. 2). It ranges from late Oligo- Cassell and Sen Gupta (1989a). The Rio in its definition. At Loma del Mar, in west- cene to middle Miocene. Because of the paucity Banano Formation was first described by Taylor central Limon, extensive new exposures in a of exposures along Uscari Creek, Cassell and (1975), who defined three lithofacies: sandstone, housing construction site clearly show that the Sen Gupta (1989b) designated a new reference conglomerate, and reef. In addition, he included Moin Formation consists of blue claystone, silt- section along Terciopelo Creek, ~62 km due the Moin Clay Member as a fourth element, stone, litharenite, and interbedded biostromal west of Limón, where the formation is >565 m using Gabb's (1895) term but defining a type reef lenses. We agree with Cassell and Sen thick (their Fig. 3). The basal 12 m, unconform- locality. Cassell (1986) and Cassell and Sen Gupta (1989a) that the Rio Banano Formation ably overlying a basalt flow, consists of biocal-

Figure 7. Map of the region around Limón, showing distribution and type localities of the formations and members of the Limón Group. The black lenses within the Moin Formation outcrop show the location of the deeper water (>30m) zooxanthellate coral biostromes interbedded within the Moin Formation. A very tentative indication of the distribution of the late Pleistocene shallow-water Acropora palmata reef deposits is also shown as unnamed reef limestone.

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carenite formed of larger foraminifera and red sists of burrow-mottled, blue-gray siltstone and occasional reefal biostromes, consisting of fine, algae rhodolites, with an interbedded, poorly volcanic litharenite with frequent shell-rich silty and clayey sandstone that contain 37 spe- sorted litharenite rich in pyroxene grains and lenses and scattered mollusc hash. Molluscs, cies of mostly plate-shaped and encrusting corals foraminifers. sand dollars, and callianassid burrows are (T. Stemann, 1990, personal commun.). The Cassell (1986) described the overlying 553 m common. reef lenses, which are clearly interbedded with as predominantly well-bedded, soft, dark shale Moin Formation. The Moin Formation was the other Moin lithologies, for example, at Loma containing lenses rich in planktic foraminifera named as the "Moin Member" by Gabb (1895) del Mar and St. Eduviges Creek (Fig. 7), should and mollusc hash. Near the top of the section and defined by Taylor (1975), who designated not be confused with the widespread, branching there is a 140-m-thick basalt sill. The Uscari the type section as the area along the unnamed coral deposits (unnamed reef limestone, Fig. 7) Formation crops out extensively throughout the creek that runs into the Cangrejos housing sub- overlying the Moin Formation. The total thick- southern Limon Basin (Cassell and Sen Gupta, division on the coast northwest of Limon (Fig. ness of the Moin Formation, here considered to 1989, Fig. 2). It is unconformably overlain by 7). The Moin Formation, which is late Pliocene have a broader definition than that of Cassell the Rio Banano Formation. in age, consists primarily of a series of alternat- and Sen Gupta (1989a), is -200 m thick and Rio Banano Formation. This unit was de- ing blue-gray, silty claystone and blocky, shelly, includes both the patch-reef facies and the Moin scribed by Taylor (1975), and Cassell (1986) volcaniclastic litharenite which contains occa- clay, which Taylor had previously lumped to- established the type section west of the railroad sional grit-sized basalt grains. Scaphopods, scat- gether in the Rio Banano Formation. bridge at Bomba on the Banano River, 14 km tered molluscs, and callianassid burrows are south-southwest of Limon (Fig. 7). The oldest present. In the lower part, laminated, pyritic, Other Neogene Formations of the unit of the Rio Banano that we have correlated organic-rich claystone is evident, as is pebbly Caribbean Coast biostratigraphically is late early Pliocene, al- silty litharenite with large, erect, branching though it may be older in the southern Limon bryozoans. Interbedded with the claystone and Gatun Formation. The late Miocene Gatun Basin. The formation is -750 m thick and con- litharenite in the upper part of the formation are Formation was named by Howe (1907). Mon-

Figure 8. Geologic map of R. H. and J. L. Stewart, with the collaboration of W. Woodring (1980), showing the eastern half of the outcrop of the Gatun Formation, including the locations of the type sections. (See text for detailed description.)

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key Hill Formation and Mindi Hill beds were ashy siltstone; and thin conglomerate with very tered molluscs. A distinct basal unit, the Toro other names used to refer to this unit (Hill, abundant molluscs, echinoid fragments, and Point Member (Woodring, 1957), is a calcare- 1898), but the name Gatun has worldwide use wood. The type section of the upper part of the ously cemented, coarse, cross-bedded coquina today (Woodring, 1957). The type area is lo- Gatun Formation is exposed about 2-3 km composed mostly of large barnacle plates, but it cated between Gatun, Mount Hope, and Saba- north of Gatun, in the region around Mount also contains mollusc and coral fragments, echi- nita at the northern end of the Hope, and to the west of the Panama Canal in noid spines, and interbedded coarse calcarenite. (Fig. 8). Most of the original sections are now the valley. It is characterized by The Toro Point Member is exposed immediately covered, but many fresh exposures are available. clayey, biocalcarenitic, tuffaceous siltstone and west of Colon and is named for the on The formation is divided into three parts fine silty litharenite that is burrow mottled and the northwest side of Colon harbor. (Woodring, 1957). The type section of the lower rich in scattered molluscs. The total thickness of part is exposed on the transisthmian highway the Gatun Formation, recorded in a borehole BIOSTRATIGRAPHY about 12 km east of Col6n, from Sabanita to near Colon, is -500 m (Woodring, 1957). Cativa, (Fig. 8). The formation consists of bur- Chagres Sandstone. The Chagres Sandstone Samples for microfossils were collected from rowed, concretionary, gray-green, tuffaceous lith- was named by MacDonald (1915) for a massive near the base and top of the 12 formations and 2 arenite, with shell hash and abundant molluscs. sandstone overlying the Gatun Formation that members that constitute our new stratigraphic There is a 2-m basal unit, consisting of volcanic occurs west of Colon between Toro Point and framework (Fig. 9). Nannofossils and plank- conglomerate and cross-bedded, tuffaceous, the Chagres River (Fig. 8). It is a massive, homo- tonic foraminifers were extracted and were used silty litharenite. The type section of the middle geneous, fine-grained, silty sandstone with scat- to correlate each lithostratigraphic unit to the part of the Gatun Formation is located in the tered molluscs of Pliocene age. The unit Geomagnetic Polarity Time Scale of Berggren region between Cativa and Gatun (Fig. 8) and becomes finer grained westward for 50 km and others (1985). We defined 34 planktonic consists of massive, burrow-mottled, silty, tuf- along the coast where it becomes a gray-green foraminiferal and nannofossil events (first [FO] faceous and occasionally calcareous litharenite; siltstone with lithic and quartz grains and scat- or last [LO] occurrences, 1) using

LU Z LU o o H I Armuelles (n Fm ? LU

Moin Fm 1.5 i... 1.8 1 1.74 — 1.7? — 1.7 Î 2- 1.9 T Montezuma Figure 9: Biostratigraph- Rio Banano Fm Fm ic correlation of the late Escudo de —12.4 Veraguas Fm Neogene formations of the LU Burica Fm Z 2.9.1 r Pacific and Caribbean LU 3- 3.0 O coasts of Costa Rica and O western Panama. Arrows 3.5J. . 3.5-1 3.5 I 3.5| 3.5 i indicate age range of up- 3.6Î ' 3.6Î 3.6 T 3.6 T 3.6 T permost and lowermost 4- Cayo Penita Fm samples for each forma- Agua Fm tion. See text for detailed 4.61 - discussion. Shark Hole Point Fm 5- 5.0Î-

5.64, 5.6¿

6- Nancy Point Fm = Nancy Point • O tongue ? in O 6.5 T Limon Basin

Gatun Fm

8.2 Î

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TABLE 1. LIST OF PLANKTTC FORAMINIFERA AND CALCAREOUS NANNOFOSSIL RANGES USED constrained at 1.7-1.5 Ma (earliest Pleistocene) Limón Group TO DATE THE FORMATIONS LISTED IN TABLE 2 by the presence of both large Gephyrocapsa sp. and Calcidiscus macintyrei. The overlying part The Uscari Formation is the oldest unit in the Taxon Type FO LO of the Armuelles Formation cannot be biostrati- Limón Group. The basal part of the formation graphically correlated as yet; we assign an age of 1. Pseudoemiliana lacunosa N 3.6 0.5 contains Globorotalia kugleri (earliest Miocene, 2. Helicosphaera setlii N 1.4 <1.7-1.5 Ma (early Pleistocene) based on <23.7 Ma). The upper part contains several spe- 3. Calcidiscus macintyrei N 1.5 4. Gephyrocapsa sp. (large) N 1.7 superposition. cies of Globigerinatella, Praeorbulina, and Glo- 5. Globigerinoides extremus F 1.8 6. Gbbowtalia truncatulinoides F 1.9 borotalia, which Cassell and Sen Gupta (1989b) 7. Globorotalia exilis F 6.5 2.1 Bocas del Toro Group refer to the earliest middle Miocene (Zone N8 of 8. Discoaster brouweri N 1.9 9. Discoaster surculus N 2.4 Blow, 1969) at -16.5-15.2 Ma. In the reference 10. Gephyrocapsa sp. (small) N -2.5-3.0 11. Discoaster pentaradiatus N 2.4 The oldest unit in the group is the Nancy section of the Terciopelo Creek, Cassell (1986) 12. Discoaster tamalis N 3.8 2.6 Point Formation. We suggest an age not older refers to 20 m of siltstone overlying the shales of 13. Dentoglobigerina altispira F 2.9 14. Sphaeroidinella dehiscens F 3.0 than 6.5 Ma (middle late Miocene) for its base the Uscari Formation, apparently disconform- 15. Sphaeroidinellopsis F 3.0 ably, which he tentatively assigns to the Rio 16. Globorotalia margaritae F 5.6 3.4 because of the presence of Globorotalia exilis 17. Sphenolithus abies N 3.5 (FO = 6.5 Ma). The age of the boundary with Banano Formation. He identifies Sphaeroidi- 18. Reticulofenestra pseudoumbilica N 3.5 19. Globigerina nepenthes F 4.0 the overlying Shark Hole Point Formation is not nella dehiscens in this unit, which has an FO of 20. Discoaster asymmetricus N 4.1 2.2 3 Ma (late Pliocene). If correct, this suggests a 21. Globigerinoides conglobaos F 5.3 well constrained (Fig. 9; Table 2). The nanno- 22. Discoaster quinqueramus N 8.2 5.6 fossil Discoaster berggrenii is found at the top of significant hiatus in this area between the Uscari 23. Discoaster berggrenii N 5.6 24. Ceratolithus rugosus N 4.5 the Nancy Point Formation and near the base of and Rio Banano Formations as indicated by 25. Ceratolithus acutus N 5.0 4.6 Cassell and Sen Gupta (1989b, Fig. 1). In the 26. Neogloboquadrina humerosa F 7.5 the Shark Hole Point Formation, indicating only 27. Globoquadrina dehiscens F 5.3 that the contact is >5.6 Ma (late Miocene). The southernmost part of the Limón Basin, however, top of the Shark Hole Point Formation is (for example, along Sandbox Creek and Bribri Note: the ranges are after Berggren and others (1985) and Dowsett (1989). N = nannofossil; F = planktic foraminifer. FO = first occurrence; LO = last precisely dated at 3.6-3.5 Ma (latest early Plio- road), sediments were sampled that strongly re- occurrence. cene), based on the co-occurrence of Sphenoli- semble the lithology of the Nancy Point Forma- thus abies (LO = 3.5 Ma) and Pseudoemiliana tion. Abundant Neogloboquadrina acostaensis lacunosa (FO = 3.6 Ma). and very few Neogloboquadrina humerosa sug- gest an age range of 10-5 Ma for these southern The age range of the base of the Escudo de Limón Basin deposits. We have also tentatively Berggren and others (1985) and Dowsett Veraguas Formation is also 3.6-3.5 Ma, which identified Globorotalia juani in these sediments, (1989). We place the first occurrence of Pseudo- correlates precisely with the uppermost Shark which, if correct, would constrain the age to not emiliana lacunosa at 3.6 Ma (Table 1), rather Hole Point Formation (again based on the co- older than 6-5 Ma, which is coeval with the than 3.4 Ma as in Berggren and others (1985), occurrence of S. abies and P. lacunosa). The Nancy Point Formation of the Bocas del Toro because of the consistent occurrence of this spe- top of the Escudo de Veraguas Formation is also Group. Our preliminary conclusion is that the cies with Sphenolithus abies (1.0-3.5 Ma) and tightly constrained at 1.9-1.8 Ma (latest late Pli- hiatus between the Uscari Formation and the Reticulofenestra pseudoumbilica (LO = 3.5 Ma) ocene) by the presence of Globigerinoides ex- Rio Banano Formation, noted by Cassell and in Panama and in samples from the Atlantic tremus (LO = 1.8 Ma) and Globorotalia Sen Gupta (1989b) for the northern part of the . Table 2 gives an estimate of the truncatulinoides (FO =1.9 Ma). Limón Basin, may in part be filled in the south- age and the taxa upon which the estimate is The age of the Cayo Agua Formation remains ern portion of the basin by a tongue of the based for each formation and member (except problematic. This unit represents a different Nancy Point Formation. We have no biostrati- the Chagres Formation, where no evidence was lithofacies from the other units of the Bocas del graphic data for the lowest portion of the Rio available). Our stratigraphic framework (Fig. 9) Toro Group and is not physically correlated Banano formation; however, the middle part of combines the dates or ranges provided by bio- with them. The base of the Cayo Agua Forma- the Rio Banano Formation at Quitaría (Fig. 7), stratigraphy with lithostratigraphic information. tion is defined at 5.0-4.6 Ma on the presence of along the Rio Banano River, dates from the nannofossil Ceratolithus acutus. This places 3.6-3.5 Ma (latest early Pliocene). This corre- Charco Azul Group the base of the unit equivalent in age to some lates precisely with the uppermost Shark Hole portion of the upper part of the Shark Hole Point Formation and the base of the Escudo de The Penita Formation is the oldest in the Point Formation (Fig. 9). The highest unit of the Veraguas Formation, based on the presence of group, but the age of its base is poorly con- Cayo Agua Formation is poorly constrained at S. abies and P. lacunosa. The highest samples in strained. All samples at present indicate an age >2.9 Ma by the presence of Dentoglobigerina the Rio Banano Formation, from the type sec- of 3=3.5 Ma (early Pliocene or older). Sphenoli- altispira. A sample in the middle part of the tion at Bomba (Fig. 7), contain Discoaster pen- thus abies occurs from the top to the base of the formation, however, contains Sphenolithus abies taradiatus (LO = 2.4 Ma) and have small unit, and in the lower samples, Reticulofenestra and Pseudomiliana lacunosa, identifying the Gephyrocapsa sp. (FO = 2.5 Ma), giving an age pseudoumbilica and Sphaeroidinellopsis are also biostratigraphic marker horizon at 3.6-3.5 Ma, of 2.5-2.4 Ma (late Pliocene). present. The lowest sample from the overlying which also occurs at the base of the Escudo de Burica Formation is more precisely dated at Veraguas Formation. Thus, the upper Cayo Discoaster pentaradiatus also occurs in 3.6-3.5 Ma by the overlap of S. abies and Agua Formation is coeval with the lowest part the lower part of the Moin Formation with Pseudoemiliana lacunosa, confirming that the of the Escudo de Veraguas Formation (Fig. 9). Sphaeroidinellopsis (LO = 3.0 Ma) and Sphae- age of the top of the Penita Formation is There are no lithostratigraphic controls because roidinella dehiscens (FO = 3.0 Ma). Sphaeroidi- >3.5 Ma. The top of the Burica Formation and the Cayo Agua Formation is only recognized to nellopsis, however, is rare in these samples. If the base of the Armuelles Formation are well date on Cayo Agua. this date is correct, then the lower portion of the

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TABLE 2. LIST OF THE CALCAREOUS NANNOFOSSIL AND PLANKTONIC FORAMINIFERA ably overlies parts of the Moin and Rio Ba- USED TO DATE THE LOWEST AND HIGHEST PORTIONS OF EACH PHYSICAL STRATIGRAPHIC UNIT nano Formations and conceals their contact. As yet unmapped, the reef deposits will almost cer- Group Formation Age Samples Nannofossits Planktic forams tainly merit separation as a distinct lithostrati- graphic unit. Thomas Stemann (1990, personal Annuelles top ? commun.) has indicated that these reef deposits base 1.5-1.7 CJ 89-10-4 large Gephyrocapsa Calcidiscus macintyrei are distinguished by 38 species of zooxanthellate corals, including Acropora palmata, which is Burica Charco Azul top 1.5-1.7 O 89-9-16 large Gephyrocapsa only common in late Pleistocene to Holocene Calcidiscus macinerei deposits throughout the Caribbean (Frost, >1.8 CJ 89-10-1 Globigerinoides obliquus base 3.5-3.6 CJ 88-11-5 Sphenolithus abies 1977); the oldest record of A. palmata appears Pseudoemiliana lacunosa Peflita to be about 560,000 yr B.P. (Bender and others, top >3.5 O 89-12-6 Sphenolithus abies 1973). base 2*3.5 CJ 89-3-2 SphenolUhus abies Sphaeroidinellopsis Reticulofenestra pseudoumbilica >3.0 CJ 89-3-3 Sphaeroidinellopsis SEDIMENTARY HISTORY

Escudo de Veraguas top 1.8-1.9 CJ 88-30-3 small Gephyrocapsa Globigerinoides extremas Pseudoemiliana lacunosa Globorotalia truncatulinoides Although as little as 100 km apart, the Pacific Calcidiscus macintyrei and Caribbean sections are strikingly different, base 3.5-3.6 CJ 87-9-3 Sphenolithus abies Pseudoemiliana lacunosa reflecting their contrasting active and passive Cayo Agua margin tectonic settings. Bocas del Toro top >2.9 CJ 88-26-5 Dentogbbigerina alüspira base 4.6-5.0 CJ 86-37-1 Ceraiolithus acutus Shark Hole Point top 3.5-3.6 CJ 88-32-1 Sphenolithus abies Globorotalia margaritae Pacific Convergent Margin Sequence Pseudoemiliana lacunosa Dentogbbigerina altispira base >5.6 CJ 88-36-1 Discoasier berggrenii Nancy Point Exposed on the Burica and Osa Peninsulas top >5,6 O 88-49-1 Discoaster berggrenii and Discoaster quinqueramus are inliers of Cenozoic basaltic oceanic crust or base 6.5 CJ 88-51-1 Globorotalia exilis complex Paleogene ophiolite suites, part of

Moin which apparently originated from aseismic top <1.7 CJ 89-18-1 large Gephyrocapsa Globorotalia truncatulinoides ridges or seamounts associated with limestone base >2.4 CJ 89-16-3 Discoaster pentaradiatus -3.0 CJ 89-18-9 Sphaeroidinellopsis turbidites (J. A. Obando, 1986; Baumgartner Puerto Limón Sphaeroidinelb dehiscens Rio Banano and others, 1989). The Neogene sedimentary top 2.4 CJ 89-20-11 Discoasier pentaradiatus small Gephyrocapsa sequence was deposited in part as these base- base 3.5-3.6 CJ 89-21-1 Sphenolithus abies ment elements were involved in subduction or Pseudoemiliana lacunosa obduction (Corrigan and others, 1990; J. A.

Gatun Obando and others, 1990). top >5.6 CJ 86-19-3 Discoaster quinqueramus Other Neogene base <8.2 CJ 86-19-3 Discoaster quinqueramus The Penita, Burica, and Armuelles Forma- formations Montezuma youngest <1.7 CJ 88-15-1 Gephyrocapsa oceanica tions form a complex transgressive/regressive oldest ? Pliocene cycle -4,000 m thick, that was rapidly and con- tinuously deposited. The basal Penita Formation Note: for each date, the sample numbers and the age range in millions of years are also given. was apparently deposited around an island or seamount and represents marginal marine, shore- face sand and inner shelf deposition with the Moin Formation overlaps the upper part of the <1.7 Ma (latest Pliocene or younger). McKee presence of local fan deltas (La Vaca Member) Rio Banano Formation in age. The youngest (1985), however, identified a suite of planktic on the top of a basement topography that has sample from the Moin Formation is constrained foraminifera from higher terraces near Monte- some relief (Fig. 4). The overlying Burica For- as « 1.7 Ma (latest Pliocene or younger) by zuma and Las Playitas (Fig. 5), which suggests mation formed as a thick turbiditic apron during the presence of large Gephyrocapsa sp. that the oldest part of the formation is early deposition within the Middle American Trench (FO =1.7 Ma) and Globorotalia truncatuli- Pliocene. (Fig. 1). This almost 3,000-m-thick unit was ex- noides (FO = 1.9 Ma). On the Caribbean coast we obtained Discoas- tensively deposited along the length of the Mid- ter quinqueramus from the middle part of the dle America Trench as evidenced by numerous Other Neogene Formations and Gatun Formation near Gatun. This indicates a seismic lines (Buffler, 1982; Crowe and Buffler, Unnamed Units range of 8.2-5.6 Ma (late Miocene) correlating 1983, 1986). Also characteristic of the Burica it with the Nancy Point Formation of the Bocas Formation are 30- to 50-m-thick intraforma- On the Nicoya Peninsula of the Pacific coast del Toro Group. On the basis of planktonic fo- tional slumps and marginal marine debris flows of Costa Rica, the Montezuma Formation is dis- raminifera, Whittaker and Hodgkinson (in containing blocks several meters wide. These tributed on a series of terraces near sea level Woodring, 1982) had correlated the lower part deposits are rich in shallow-water, thick-shelled (Cocalito Beach) and inland (Fig. 5). At the of the Gatun Formation as midzone N16 (or mollusc, coral, bryozoan, and coralline algal as- lowest (youngest) exposures on Cocalito Beach younger) and the upper part as N17-18. semblages; diverse microfaunas; large volcanic (Fig. 5) the Montezuma Formation contains Immediately to the west of Limon, a series of boulders; and abundant wood fragments and Gephyrocapsa oceanica, indicating an age of topographically distinct reef deposits unconform- logs.

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f Steady shoaling is suggested by benthic fora- macrofossil-rich shelf facies that are comparable Isthmus is provided by the dating of the first minifera (Corrigan and others, 1990) for the to the Caribbean sequence. vertebrates to cross the isthmian corridor from transition from the Burica Formation to the North to South America or vice versa. The first Armuelles Formation. Deep-water (>2,000-m) Constraints on Closure of diverse assemblage of large South American ver- turbidites in the middle part of the Burica For- the Isthmus tebrates likely to have walked across the Isthmus mation gradually pass up into shallower turbid- appears in during late Blancan ites in the upper part of the Burica, followed by To date, attempts to constrain the timing of time at about 2.7-2.5 Ma (Lundelius and others, assemblages that indicate a dramatic shallowing closure of the Isthmus of Panama have been 1987). These ages are verified by magnetic stra- during deposition of the Armuelles Formation derived from four principal lines of evidence. tigraphy and radiometric dates. The arrival of (Corrigan and others [1990] suggested that this With the exception of information from terres- North American immigrants to South America uplift occurred at 1 Ma), and transition to trial vertebrates, the evidence suffers from at- is dated at 2.8-2.5 Ma (Marshall, 1988), but marginal-marine, fluviatile and mangrove facies. tempts to use patterns involving deep-water sections in South America with vertebrate data Corrigan and others (1990) have suggested that phenomena to interpret an event that ultimately are not as reliably correlated with magnetic stra- the extremely rapid uplift at the end of the cycle occurred in shallow water. tigraphy or radiometric dates as the North was due to vertical isostatic readjustment of the 1. Differences in stratigraphic ranges and American sections. If ages estimated from plank- arc in response to underlying subduction of the coiling patterns in planktonic foraminifera as tonic microfossils in Deep Sea Drilling Cores Cocos Ridge. This, however, does not explain well as the distributions of radiolarian and ben- reflect the rising Isthmus before emergence, and the apparent deepening during deposition of the thic foraminifera have been attributed to isola- ages based on large vertebrate migration, which Peñita and Burica Formations. This led Obando tion due to formation of the Isthmus of Panama effectively rules out waif dispersal, represent and others (1990) to propose a more complex (Casey and others, 1979; Gartner and others, "latest" estimates (since they document the first subduction history for the region that involves 1987; Keigwin, 1978, 1982a; Keller and others, preserved vertebrate deposits formed after emer- gence of the Isthmus), then combining the earlier subduction of a proto-Cocos ridge. 1989; McDougall, 1985; Saito, 1976). These organisms are open oceanic or deep-water forms "early" marine with the "late" terrestrial esti- mates produces from these data an estimated age Caribbean Passive Margin Sequence whose distributions would probably have been affected by the rising Isthmus, but which are not of about 3.2-2.5 Ma for closure of the Isthmus of Panama. The Bocas del Toro and Limón Groups (ex- easily correlated to the date of final closure. cluding the Uscari Formation) form a much 2. Abrupt changes in patterns of carbon and Data from our newly measured sections, thinner sequence than the contemporaneous oxygen isotopes from Deep Sea Drilling Pro- however, strongly suggest that closure had al- Charco Azul Group, which is consistent with gram cores have also been attributed to the ready occurred by at least 3.5 Ma. This date is their position on the tectonically passive Carib- emergence of the Isthmus (Keigwin, 1982b; derived using our new biostratigraphic frame- bean margin of the arc. The Bocas del Toro Hodell and others, 1985). These changes were work to refine the correlation of Olsson's (1922, Group has a total thickness of -1,000 m, as interpreted to reflect differences in oceanic circu- 1942) Pacific coast localities. These localities compared to -4,000 m for the coeval Charco lation brought on by the development of the yielded 50 mollusc species from the Penita Azul Group. The Caribbean sequence consists of Isthmus, but, as in the case for planktonic ma- Formation, 87 species from the Burica Forma- massive, shelly, clayey siltstone; original bedding rine organisms, these are difficult to relate to the tion, and 130 species from the Armuelles and other sedimentary structures were mostly date of final closure. Based on planktonic Formation, none of which is recorded from ap- destroyed by pervasive bioturbation. Although marine organisms and oxygen/carbon, an age proximately coeval units on the Caribbean side the Bocas del Toro and Limón Groups were of final closure of -3.2-2.8 Ma has been of the Isthmus. The Pacific collections of Olsson more slowly deposited with respect to the estimated. come from sections we have measured in detail. Pacific sequence, we have not found evidence of 3. Hiatuses and disconformities in the sedi- The faunas are diverse; those from the Penita breaks in sedimentation such as hardgrounds, mentary record are perhaps the weakest line of and Armuelles Formations are shallow shelf in concentrations of phosphatic , or lag evidence, because such features are the most origin and of comparable facies to their Carib- deposits. susceptible to explanations unrelated to closure bean equivalents. Those from the Burica Forma- The Bocas del Toro and Limón Groups have of the Isthmus. Most of these arguments imply tion come from resedimented slump deposits certain features in common with Atlantic Coast- that hiatuses represented by disconformities which brought shallow-water species into a al Plain Cenozoic sediments (Kidwell, 1988). were generated by increased winnowing due to deep-water facies. We have observed that these Both regions have pervasively bioturbated, rela- the reinforcement of the Stream after clo- slumps are the only sites of diverse mollusc fau- tively thin sequences which were reworked on a sure (Brunner, 1984; Emiliani and others, 1972; nas in the Burica Formation. For example, the stable shelf. They both are rich in shell beds and Holcombe and Moore, 1977; Mullins and oth- Charco Azul locality of Olsson (1922, p. 15) is show minor transgressive-regressive pulses re- ers, 1987). In several cases, there are multiple described as "approximately 10 km south of flected by near-shore coarse clastic input. This disconformities that are not easily explained by Puerto Armuelles," where we have observed contrasts strongly with the Burica Formation, such a mechanism, and both the timing and and collected a major, fossiliferous slump hori- which consists of thick, well-bedded turbidite cause of such features are difficult to isolate. zon. Similar slumps occur 1 km to the south at sequences that are poor in macrofauna and Most of the examples are also relatively distant Olsson's "Quebrada Mellisa" locality (1942, range to >2,000 m in depth of deposition. Both from the Isthmus, further diminishing the confi- p. 16). Thus, we are confident that Olsson's Pa- the Peñita and Armuellas Formations within the dence with which they may be linked to closure. cific coast faunas are from broadly comparable facies to the coeval Caribbean sequences. The Charco Azul Group, however, contain massive, 4. Positive evidence for emergence of the

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lack of taxa in common at 3.6-3.5 Ma suggests younger portion of the Montezuma Formation) 3.6-3.5 Ma, strongly suggesting that the Isthmus the Isthmus was by then an effective barrier for on the Pacific Coast. had effectively closed by this time. shallow-water molluscs, to the point of evolu- Our study also provides a clearer understand- tionary divergence of congeneric species. ing of the age and setting of the classic ACKNOWLEDGMENTS "Gatunian" facies, which was described origi- nally by Woodring (1957) as the Gatun Forma- Laurie Anderson, John-Mark Coates, Tim DISCUSSION tion, at the northern end of the Panama Canal, Collins, Mathew Cotton, Dana Geary, Carl but subsequently extended to other units Hansen, René Panchaud, Stephen Rhodes, Tom The data presented here provide the first de- throughout the lower Central American Isth- Stemann, Omar Sugasti, and "Chena" and Rey- tailed biostratigraphic correlation between late mus. The Gatun Formation is a sandy, near- naldo Tapia were valuable members of various Neogene deposits from the Caribbean and Pa- shore marine facies famous for its remarkable field expeditions. cific sides of the Isthmus. The 4,000-m-thick Pa- molluscan diversity (Woodring, 1957). Nanno- In Costa Rica, our field work was greatly cific coast sequence ranges in age from >3.6 to fossils from the middle part of the 500-m-thick facilitated by Professors Teresita Aguilar, < 1.7 Ma. Most of this interval is represented by formation indicate an age range of 8.2-5.6 Ma, Peter Baumgartner, and Gisela Mora. Gener- the biostratigraphically well-constrained, 2,800- correlating it with the late Miocene Nancy Point ous logistical support was provided by Lie. m-thick late Pliocene Burica Formation; ages of Formation of the Bocas del Toro Group. This José F. Castro M., Director de Geología y older and younger deposits are as yet poorly age estimate agrees with those of Whittaker and Minas; Lic. Maria Elena Rodríguez A., Sec- defined. In contrast, the Caribbean sequence is Hodgkinson (in Woodring, 1982), who dated ción Ambiental; Sr. José Luis Sibaja H., Ofic- only 1,000 m thick but ranges from ~8.2 to the lower Gatun as midzone N16, -8.5 Ma. ina Regional, Puerto Jimenez; and all of the <1.7 Ma. Both the Pacific and Caribbean sec- "Gatunian" facies have also been recorded Dirección de Geología y Minas, Ministerio de tions bracket comfortably the most likely inter- from the Limon Basin where the Rio Banano Recursos Naturales, Energía y Minas, San val for the final closure of the Isthmus between Formation was previously referred to as "Gatun José, Costa Rica. Sra. Lorena San Roman, Di- 3.5 Ma and 2.5 Ma. The Caribbean sequence, Formation" (Olsson, 1922; Bold, 1967). The rectora del Museo Nacional de Costa Rica, represented by the Bocas del Toro and Limon type "Gatun" facies in Panama, however, is and Sr. Franz Ulloa were also extremely help- Groups, overlaps and extends upwards by ~1.5 correlated with the bathyal facies of the Nancy ful. In Panama, Mercedes Arosemena, Argelis Ma, the time represented by the important, con- Point Formation in western Panama and (?) Ruiz de Guevara, Hector Guzman, Elena tinuously fossiliferous sequences of the Domini- eastern Costa Rica (8.2-5.6 Ma, late Miocene), Lombardo, and Hanibal Velende provided in- can Republic (Saunders and others, 1986), one whereas the Rio Banano Formation ranges from valuable logistical planning and support. To of the few Caribbean Neogene sequences that 3.6-2.4 Ma. Thus, the "Gatunian" facies persists all, we are extremely grateful. has been used for detailed evolutionary studies in Costa Rica during the Pliocene but is absent The Government of Panama kindly assisted (Cheetham, 1986; Foster, 1986). in Panama after the earliest Pliocene at the latest us with geological reports. Field work for this The Caribbean and Pacific sections include (Whittaker and Hodgkinson, in Woodring, project was generously supported by three very different environments of deposition, yet 1982). grants from the National Geographic Society, there is sufficient overlap and diversity of In contrast, Pleistocene deposits are more and by extensive logistical support from the habitats to permit meaningful biological com- poorly constrained and less well developed on Smithsonian Tropical Research Institute parisons. On the Caribbean side, formations tied both sides of the Isthmus. On the Caribbean side (STRI). Coates was supported by awards from together by the overlap of the late Pliocene the reef facies unconformably overlying the The George Washington University, and Jung markers Sphenolithus abies and Pseudoemiliana Moin Formation in the Limon Region contains and Panchaud, by the Hans G. Kugler Fund, lacunosa (3.6-3.5 Ma) range from very shallow abundant Acropora palmata. Because this spe- Basel Naturhistorischesmuseum, Switzerland. to shallow inner shelf (Rio Banano Formation, cies is known throughout the Caribbean only in Coates and Jung received short-term visitor Cayo Agua Formation) to outer shelf (Escudo deposits younger than 500,000 yr (T. Stemann, fellowships from STRI, and Collins received a de Veraguas Formation), to outer shelf and 1990, personal commun.), these reef deposits are post-doctoral fellowship from STRI and grants upper slope (upper Shark Hole Point Forma- almost certainly late Pleistocene in age. No other from Sigma Xi and the Roger Tory Peterson tion). On the Pacific coast, stratigraphically unequivocal Pleistocene deposits, however, have Institute. equivalent Pacific Burica Formation represents a been described. On the Pacific coast, the upper trench slope environment absent on the Carib- part of the Armuelles Formation and the bean side, but also includes abundant thick younger part of the Montezuma Formation are REFERENCES CITED intraformational slumps containing shallow- <1.7 Ma in age. Aguilar, T., and Fischer. R., 1986, Moluscos de la Formation Montezuma (Plioceno-Pleistoceno; Costa Rica): Geologica et Palaeontologica, v. 20, water fauna more appropriate for comparison In conclusion, our results demonstrate a well- p. 209-241. Bandy, O. L., 1970, Upper Cretaceous Cenozoic paleobathymetric cycles, east- with the Caribbean biota. 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