Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the Panama seaway

Palaeogeography, Palaeoclimatology, Palaeoecology, 77 (1990): 203-234 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

203

Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the Panama Seaway HERMANN

DUQUE-CARO

Instituto Nacional de Investigaciones Geologico-Mineras, Bogota (Colombia) (Received May 17, 1988; revised and accepted April 20, 1989)

Abstract Duque-Caro, H., 1990. Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the P a n a m a Seaway. Palaeogeogr., Palaeoclimatol., Palaeoecol., 77:203 234. The Atrato Basin in the Pacific coastal region of NW South America (Colombia) exhibits stratigraphic and biostratigraphic characteristics similar to other Neogene Pacific coastal basins of southern Central America and n o r t h e r n South America. An evaluation of the Neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following. (1) Previous to the middle Miocene, well aerated deep and open oceanic conditions, and free and active water circulation prevailed along the steep continental margins of NW South America. (2) During the early middle Miocene, right after the Neogene Hiatus NH 2 (15.2 Ma), tectonic disturbances triggered the initial uplift of the P a n a m a sill. Bottom water circulation, foraminiferal and sedimentation changes are observed in both the Pacific and Caribbean coastal areas of NW South America. (3) During the middle Miocene, coinciding with the Neogene Hiatus NH 3 (12.9--11.8 Ma), an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the P a n a m a sill to about 1000 m, reflecting the middle Miocene tectonic disturbances in NW South America. (4) During the late middle Miocene, immediately following this uplift a distinctive Pacific benthic foraminiferal fauna abruptly appeared and extended from Ecuador to California. Because these assemblages do not occur in the adjacent Caribbean region, a circulation barrier between the Atlantic and Pacific Oceans as a result of intensification of the cool, marginal California Current is proposed. Anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the Pacific coastal areas of NW South America. (5) During the latest Miocene, immediately following the Neogene Hiatus NH 6 (7.0- 6.3 Ma), surface water circulation between the Caribbean and the Pacific was re-established and the previous influence of the California Current disappeared. A rapid filling and shallowing of the basin to mostly neritic depths is indicated along the Pacific and Caribbean coastal basins of Colombia as well as a progressive increase in similarity between the coastal Pacific and Caribbean benthic biotas. This shallowing in the early late Miocene, also appears to have been associated with the earliest terrestrial i n t e r c h a n g e of ground sloths to North America and raccoons and their allies to South America. (6) Immediately following early Pliocene time, the P a n a m a n i a n isthmus became completely emergent providing a terrestrial e n v i r o n m e n t favourable for intermingling of terrestrial faunas and floras between North and South America. I correlate this Pliocene uplift with the Neogene Hiatus NH 8 (3.7-3.1 Ma). Four regional unconformities associated with tectonic disturbances in the NW corner of South America, have also been recognized and correlated with Neogene hiatuses recorded from the oceans: early middle Miocene, middle Miocene, late Miocene and early Pliocene.

Introduction The region

Atrato of the

0031-0182/90/$03.50

comprises the swampy and forested plains of the Atrato valley area, at the western side of Basin in the Pacific coastal NW corner of South America

the Cordillera Occidental Colombia (Fig.l).

~:~ 1990 Elsevier Science Publishers B.V.

in

northwestern

204

H.DUQUE-CARO 800

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78 °

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LATE MIDDLE MIOCENE TO PLIOCENE MARINE DEPOSITS OF THE ATRATO BASIN EPISODE. GREAT TERRIGENOUS INFLUX AT THE EASTERN SIDE. INCLUDES TERRESTRIAL DEPOSITS, UNDIFFERENTIATED. OLIGOCENE TO MIDDLE MIOCENE DEEP MARINE DEPOSITS OF THE ATRATO PRE-BASIN EPISODE. TERRIGENOUS I N F L U X AT THE EASTERN SIDE. MOSTLY OCEANIC VOLCANIC COMPLEXES OF DIFFERENT AGE (LATE CRETACEOUS TO NEOGENE) OF THE SERRANIA DE BAUDO, NORTHWESTERN FLANKS OF THE CORDILLERA OCCIDENTAL TO SERRANIA DEL DARIEN CHARACTERISTICALLY INCLUDES EXOTIC SEDIMENTARY BLOCKS OF V A R I A B L E SIZE (UP TO KILOMETERS LOP*G) AND OF DIFFERENT AGES (LATE CRETACEOUS TO NEOGEI~IE}. INCLUDES FELSIC INTRUSlVES OF MID-TERTIARY AGE, PARTICULARY AT THE NW FLANKS OF THE CORDILLERA OCCIDENTAL AND SERRANIA DEL DARIEN.

Fig.1. General geology and location of the Atrato Basin. This feature is part of the strait belt of land of s o u th er n Central America (Panama and Costa Rica), presently a geographic bar r i er separating the Pacific Ocean from the Caribbean Sea. Identification of the different phenomena

associated with the emergence and subsidence during the tectonic history of NW South America is critical to r e c o n s t r u c t i o n of the paleogeographic, paleoceanographic and paleobiogeographic interactions between the Pacific and Atlantic Oceans.

NEOGENE ~TRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

Three major and independent approaches have been used to a reconstruction of the emergence history of the Panama land bridge: (1) A physical approach has been used which include mainly observations on the physical and geophysical phenomena both on land and in the oceans. On land (northwestern Colombia and southern Central America), studies combining structural, geophysical, stratigraphical and petrological observations (Nygren, 1950; Lloyd, 1963; Haffer, 1967, 1970; Case and others, 1971; Dengo, 1973, 1983; Barrero, 1979; Weyl, 1980, among others) have indicated that this region has undergone four major periods of tectonic disturbances during the Cenozoic: during the Paleocene, during the middle Eocene, during the middle Miocene, and during the Pliocene-Pleistocene. In the oceans, paleoceanographic and geophysical observations in the equatorial Pacific and in the adjacent Panama and Colombian basins have indicated that: (a) the eastern Panama Basin was formed at 27 Ma (Lonsdale and Klitgord, 1978), (b) the first change in oceanic circulation across Central America, possibly related to a tectonic uplift of the Panama sill, occurred during the early Miocene (Keller and Barron, 1983; McDougall, 1985), (c) the straits of Panama were restricted during the middle Miocene (Holcombe and Moore, 1977; Moore et al., 1978; McDougall, 1985), (d) the shallow water corridor across Panama was restricted during the late Miocene (McDougall, 1985; Savin and Douglas, 1985), and (e) the final closing of the Central American seaway took place during the early Pliocene (Van Andel et al., 1971; Emiliani et al., 1972; Kaneps, 1979; among others). (2) A biostratigraphic and paleobiogeographic approach based on observations of marine and terrestrial fossil faunas and floras has been used to interpret the emergence of the Panama isthmus and its implications in (a) the cessation of the water circulation between the Atlantic and Pacific oceans (Berggren and Hollister, 1974; Saito, 1976; Keigwin, 1978, 1982, among others), and (b) for migration and intermingling of terrestrial faunas and floras

205

between North America and South America (Whitmore and Stewart, 1965; Woodring, 1965, 1978; Keast et al., 1972; Olsson, 1972; Marshall et al., 1982; Petuch, 1982; Marshall, 1985; Jones and Hasson, 1985, among others). There is common agreement among these studies that both the cessation of surface circulation between the Pacific and Atlantic oceans and the great American interchange of terrestrial faunas took place during the Pliocene between 3.1 and 2.4Ma, and that during the early late Miocene, a first interchange occurred between 9.3 and 8.0 Ma (Marshall, 1985; Webb, 1985). (3) A paleobathymetric approach, based on biostratigraphic observations of foraminifera and radiolaria in the Cenozoic record of northwestern Colombia and Panama (Bandy, 1970; Duque-Caro, 1972, 1975; Bandy and Casey, 1973) and comparisons with present distributions of these organisms in the oceans has been used to interpret the resulting paleobathymetric oscillations of southern Central America and northwestern Colombia. These studies have indicated a deep water cycle from Late Cretaceous to middle Miocene time which was followed by a shallowing marine cycle of late Miocene to Pleistocene time. An integration of the different phenomena and corresponding ages given by these interpretations indicates that prior to the early Miocene there is no indication of paleocirculation changes between the Atlantic and Pacific oceans that could be related to the emergence of the Central American isthmus, and that deep water conditions prevailed in most of southern Central America and northwestern Colombia during the Late Cretaceous to the middle Miocene interval. However, the preNeogene record in southern Central America and northwesternmost coastal areas of Colombia is uncoherently exposed and poorly documented. Scattered, discontinuous and chaotic occurrence of Late Cretaceous and Paleogene blocks, including shallow water environments (Duque-Caro, 1990) have prevented coherent paleogeographic reconstructions. Most of the paleoceanographic interpretations dealing with this subject have originated

206 from studies in the equatorial Pacific and in the adjacent Panama and Colombian basins, and none from the geology of the adjacent land areas. Very few stratigraphic and biostratigraphic studies have been done in the land areas of NW South America based on marine fossil faunas, including benthic foraminifera. The known references mostly deal with Neogene molluscan faunas (Olsson, 1932, 1972; Woodring, 1965, 1978; Bandy, 1970; Petuch, 1982; Jones and Hasson, 1985). Therefore, this study aims to document more precisely the late Oligocene to Pliocene stratigraphy and foraminiferal biostratigraphy of the Atrato Basin because of its geographic position and abundant Neogene foraminiferal faunas. This study has benefited from access to basic and unpublished information in the files of Ecopetrol in Bogota, Colombia supplementing field work along the Medellin-Turbo, Medellin-Quibdo and Quibdo-Istmina-Pereira highways. Surface samples from typical stratigraphic localities, and subsurface material from exploratory wells in the Atrato Basin area were made available by Ingeominas, Bogota. Foraminiferal samples from Miocene and Pliocene formations in coastal Ecuador and eastern Panama were kindly supplied by Prof. Alfred G. Fischer, and samples from DSDP Sites 158 and 154A were supplied through the Deep Sea Drilling Project and the international Ocean Drilling Program (ODP). This reference material has allowed more precise correlation o f the benthic foraminiferal biostratigraphy of the Miocene-Pliocene terrestrial sequences in the NW corner of South America.

Stratigraphy Previously, the stratigraphy of the Atrato Basin has not been sufficiently well known. However, an unpublished lithostratigraphic nomenclature originally described by Haffer (1967) has been selected: the Oligocene to Pliocene Uva, Napipi, Sierra and Munguido formations. The Munguido Formation, not described in Haffer's original stratigraphy, is a

H.DUQUE-CARO new name here proposed for the upper part of the Sierra Formation. This stratigraphic nomenclature has been selected for two main reasons: (1) it was originally described from surface sections at the western side of the basin (Fig.l) where benthic and planktic assemblages are very rich, and less terrigenous influx has been observed (Nygren, 1950; Haffer, 1967), and (2) the most continuous and representative subsurface stratigraphic and biostratigraphic section, the Opogado-1 well, is also located at the western side of the basin (Fig.l). Planktic foraminifera and radiolaria predominate in the Oligocene to lower Miocene, whereas benthic foraminifera are dominant within most of the middle Miocene to Pliocene. These features are accompanied by conspicuous occurrences of pyrite, glauconite and volcanic ash layers, and molluscan and fish remains. All of this information has provided most of the data and basis for the stratigraphic and biostratigraphic characterization and interpretation of the Atrato Basin. Ditch cuttings were the only material available from the Opogado-1 well. However, artifical down-hole contamination and mixing of rocks have been minimized, (a) by checking against electric logs and casing levels (b) by using highest abundances of micropaleontologic and lithologic features as major stratigraphic controls, and (c) by checking the highest and lowest occurrences of foraminiferal species, with reference material from surface sections in the surrounding areas, i.e. the Napipi, Munguido and Uva rivers on the western side of the basin, the Medellin-Quibdo highway on the east (Fig.l), and the Carmen-Zambrano highway on the north. These biostratigraphic ranges have been compared with the known ranges on the Caribbean side (Redmond, 1953; Petters and Sarmiento, 1956; Renz, 1948; Blow, 1959) and in the Pacific coastal areas of NW South America and California (Sigal, 1969; Kleinpell, 1938, 1980, among others). The planktic foraminiferal biostratigraphy, in turn, has been checked with some of the most current standards in both the Caribbean and the equatorial Pacific

207

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

(Blow, 1969; Bronnimann and Resig, 1971), and with material from continuous Caribbean and Pacific late Neogene deep-sea sequences (DSDP sites 158 and 154A). The identification of most foraminiferal species was carried out at the U.S. National History Museum, Washington, D.C. The paleobathymetric, paleoceanographic and paleobiogeographic evolution of the Atrato Basin will be here discussed within the frame of the Atrato Pre-Basin and Basin stratigraphic sequences. These sequences represent the pre-middle Miocene episode when the areas to the west of the Cordillera Occidental were open to the ocean, and the post-middle Miocene episode when the Atrato and Chucunaque basins began to develop (Duque-Caro,

1990). Atrato Pre-Basin sequence

This stratigraphic feature corresponds to the upper portion of the Late Cretaceous to middle Miocene deep water cycle recognized in the NW corner of South America (Bandy, 1970). Two stratigraphic units, the Uva and Napipi formations, represent this sequence as described below.

Uva Formation General characteristics This formation (Haffer, 1967), the oldest outcropping unit in the basin area, was originally described from the Uva river, on the western side of the basin (Fig.l). There, it consists of a carbonate sequence of white and gray foraminiferal and radiolarian limestones and mudstones with inclusions of volcanic glass. The Uva facies appear typically developed at the outer margins of the basin where they rest on mostly oceanic igneous rocks, both at the Serrania de Baudo (Uva river section) and the northwestern flanks of the Cordillera Occidental (Quibdo-Medellin highway). On the western flanks of the Cordillera Occidental, along the rivers flowing into the Atrato River, and along the Medellin-Quibdo highway, inclusions of fine to medium grain

sandy to silty interbeds to the north, and an increase in both grain size and thickness in the south overlying the basal limestones and calcareous mudstone interbeds are characteristic. At the Opogado-1 well (Fig.2), this formation was recognized between 2759.1m (9050 ft) and the 3186 m (10,450 ft) depth interval. Uphole, it mostly consists of a light to yellowish brown (2.5Y) and very dark brown (10YR) carbonate sequence, with cherty interbeds at the base (Fig.2), followed by olive gray (5Y) and brown to dark brown (10YR) calcareous mudstones including volcanic sand layers at the top. Great abundance of planktic foraminifera and radiolaria, and a planktic to benthic ratio greater than 100:1 are the most characteristic features. The benthic assemblages are poor and mostly represented by the Cibicidoides-Uvigerina assemblage (Table I). The upper boundary is marked at 2759.1m (9050 ft) by the occurrence of volcanic sand interbeds and an abrupt change in both the microfaunal abundance and lithology (mostly less calcareous mudstones). The planktic to benthic ratio decreases to less than 80:1. The base is marked at 3186m (10,450ft) by the occurrence of an unknown turbidite sequence (Duque-Caro, 1990).

Age The Uva Formation ranges in age from the Oligocene Zone P.21, Globigerina angulisuturalis/Globorotalia opima opima to the earliest middle Miocene zone N.9 Globorotalia peripheroronda (Blow, 1969). At the Opogado-1 well these zones were recognized as follows (Table II): Zone P.21 (Oligocene): between the highest occurrence of Globigerina angulisuturalis at 3064 m (10,050 ft) and the top of the unknown turbidites at 3186 m (10,450 ft; Fig.2). No evidences for underlying older Oligocene zones were found. Zones P.22 to N.4 (Oligocene-Miocene): these zones were not clearly recognized due to the very poor recovery immediately above the P.21 stratigraphic interval and they are possibly missing. Zones N.5 to N.6 (early Miocene): between

208

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Fig.2. General stratigraphic characteristics of the Uva and Napipi formations(Pre-Basin Sequence) at the Opogado-1well. the highest occurrence of Catapsydrax dissimilis at 3003 m (9850 ft) and the top of zone P.21. Zone N.7: between the highest occurrence of Catapsydrax stainforthi at 2972 m (9750 ft) and the top of Zone N.6. Zone N.8: between the highest occurrence of Praeorbulina glomerosa curva at 2789.6m (9150 ft) and the top of Zone N.7. Zone N.9 (early part, earliest middle Miocene): between the co-occurrence of Orbulina suturalis, Globigerinoides sicanus and Globigerinoides diminutus at 2759.1 m (9050 ft), immediately above the highest occurrence of Praeorbulina glomerosa curva. The top of this zone is recognized within the overlying Napipi Formation (see below).

Environment The predominance of planktic foraminifera and radiolaria and the few associated benthic

foraminiferal faunas, and the occurrence of Melonis pompilioides and Gyroidina soldanii (Table I; Fig.2) recall comparable associations from DSDP sites in the equatorial Pacific (cf. Douglas, 1973) and the Ocean Margin/High Sedimentation assemblages (Woodruff, 1985), at water depths about 2000 m or deeper (cf. Bandy, 1961; Ingle, 1967, 1980). Likewise the characteristic occurrence of carbonate strata accompanied by great abundance of planktic foraminifera indicate well-aerated waters (Berger, 1970).

Correlation The planktic and benthic foraminiferal faunas of the Uva Formation are also characteristic of the Oligocene to early Miocene formations in coastal Ecuador (Galloway and Moorey, 1929; Sigal, 1969; Fig.3), Panama (ICSC, 1968), NW Colombia (Petters and Sarmiento,

NEOGENESTRATIGRAPHYAND EVOLUTIONOF PANAMASEAWAY

209

TABLE I Benthic foraminifera of common occurrence within the Oligocene to middle Miocene Uva and Napipi formations (Pre-Basin Sequence), Opogado-1 Well Level of highest occurrence 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2225.6 2314.0 2314.0 2314.0 2314.0 2314.0 2347.5 2347.5 2347.5 2359.7 2423.8 2515.2 2515.2 2515.2 2713.4 2881.0 2942.0

m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m

(7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7300 ft) (7590 ft) (7590 ft) (7590 ft) (7590 ft) (7590 ft) (7700 ft) (7700 ft) (7700 ft) (7740 ft) (7950 ft) (8250 ft) (8250 ft) (8250 ft) (8900 ft) (9450 ft) (9650 ft)

Martinottiella sp. MartinottieUa communis D'Orbigny Stilostomella nuttalli Cushman and Jarvis Bolivina pisciformis Galloway and Morrey Bulimina cf. aUigata Cushman and Laiming t Bulirnina mexicana Cushman Sphaeroidina bulloides D'Orbigny Globocassidulina subglobosa (Brady) Melonis pompilioides (Fichtell and Moll) Melonis barleeanus (Williamson) PuUenia bulloides D'Orbigny Oridorsalis urnbonatus (Reuss) Oridorsalis ecuadorensis (Galloway and Morrey) t Uvigerina carapitana Hedberg Uvigerina rustica Cushman and Edwards Cibicidoides crebbsi (Hedberg) Cibicidoides floridanus (Cushman) Hanzawaia mantaensis (Galloway and Morrey) tPlanulina renzi Cushman and Stainforth Cibicidoides wuellerstorfi (Schwager) Gyroidina soldanii (D'Orbigny) *Siphogenerina lameUata Cushman t* Siphogenerina collorni Cushman tAnomalinoides trinitatensis (Nuttall) tUvigerina mantaensis Cushman and Edwards Nodosaria stainforthi Cushman and Renz Nodosaria longiscata D'Orbigny Siphogenerina hubbardi Galloway and Heminway tPlanulina karsteni Petters and Sarmiento t Uvigerina gaUowayi Cushman Pseudonodosaria comatus (Batsch) Anomalinoides cicatricosa (Schwager) Bulimina alazanensis Cushman tAnomalinoides pornpilioides (Galloway and Heminway) Guttulina irregularis (D'Orbigny) t Vulvulina spinosa Cushman t Uvigerina schwageri Brady tCibicidoides perlucidus (Nuttall) tCibicidoides mexicanus (Nuttall)

*Species with highest occurrence within the lower portion of the overlying Atrato Basin Sequence. tRobust species. 1956), a n d o t h e r a r e a s i n t h e C a r i b b e a n margins.

coastal

Napipi Formation General characteristics The type locality of this formation crops out along the Napipi river (Fig.l), at the west of the Atrato R i v e r ( H a f f e r , 1967). A m o s t l y

hemipelagic mudstone facies of nodular gray mudstones including scattered rounded calcareous nodules and lenticular nodular limestone interbeds conformably overly the Uva Formation. On the eastern side, along the northwestern flanks of the Cordillera Occidental, more terrigenous sediments are found and the typical Napipi mudstone facies are missing. There, these facies are replaced by a monotonous sand

210

H. DUQUE-CARO TABLE II Planktic foraminifera of common occurrence within the Oligocene to middle Miocene Uva and Napipi formations (Pre-Basin Sequence), Opogado-1 Well Level of highest occurrence

2225.6 2225.6 2271.3 2484.7 2500.0 2637.2 2759.6 2789.1 2820.1 2881.1 2972.5 3003.0 3018.3 3064.0 3064.0

m m m m m m m m m m m m m m m

*Orbulina universa D'Orbigny *Orbulina suturalis B r o n n i m a n n *Globorotalia siakensis Le Roy Globorotalia mayeri Cushman and Ellisor Globigerinoides subquadratus B r o n n i m a n n Globorotalia praemenardii Cushman and Stainforth Globorotalia peripheroacuta Blow and B a n n e r Globorotalia praefohsi Blow and B a n n e r Globorotalia peripheroronda Blow and B a n n e r Globigerinoides sicanus De Stefani Praeorbulina glomerosa curva Blow Globigerina pseudociperoensis Blow Globigerina angustiumbilicata (Bolli) Catapsydrax stainforthi Bolli, Loeblich and Tappan Catapsydrax dissimilis (Cushman and Bermudez) Globigerina tripartita Koch Globorotalia opima opima Bolli Globigerina angulisuturalis (Bolli)

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*Species with level of highest occurrence within the overlying Sierra and Munguido formations.

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211

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAV

and mudstone sequence which increase in both grain size and thickness to the south. At the Opogado-1 well, the Napipi Formation is present between the 2759.1m (9050ft) and the 2225.6 m (7300 ft) depth interval (Fig.2). There, it consists of a brown and dark brown (10YR) mudstone accompanied by conspicuous pyritic foraminiferal and organic remains, particularly in the uppermost 91 m. In contrast to the abundance and predominance of planktic assemblages of the underlying Uva Formation, this formation is distinguished by much reduced abundance of planktic microfaunas and a gradual increase upward in benthic foraminifera. Robust forms of Cibicidoides, Uvigerina and Planulina accompanied by an arenaceous microfauna are common (Table I; Fig.2). The upper boundary is marked at 2225.6 m (7300 ft) by the highest occurrence of 21 benthic foraminiferal species, not typical of the overlying Sierra Formation, and of critical importance to the paleoceanographic, paleobiogeographic and tectonic evolution of the Pacific-Atlantic seaway. The base is defined by the same features that identify the top of the underlying Uva Formation. Other features of note within this stratigraphic interval are the disappearance of radiolarians at 2515.2 m (8250 ft) after a gradual decrease, accompanied by the highest occurrence of

Anomalinoides pompilioides, Vulvulina spinosa and Guttulina irregularis (Table I). In fact, these bioevents are very useful for correlation because these three species are distinctive of, and occur no younger than early middle Miocene in the coastal areas of NW South America, i.e., in Ecuador (cf. Sigal, 1969), in northern Colombia (Guttulina caudriae Subzone, Petters and Sarmiento, 1956), and in Venezuela (cf. Renz, 1948; Blow, 1959).

Age The Napipi Formation ranges from the earliest middle Miocene Zone N.9, Orbulina suturalis-Globorotalia peripheroronda to the middle Miocene Zone N.11, Globorotalia praefohsi (Blow, 1969). At the Opogado-1 well these zones were recognized as follows (Table II):

Zone N.9 (earliest middle Miocene): between the co-occurrence of Orbulina suturalis, Globigerinoides sicanus and Globigerinoides diminutus within the upper portion of the underlying Uva Formation and the co-occurence above of Globorotalia peripheroronda and Globorotalia peripheroacuta at 2667.7 m (8750 ft). Zone N.10: between the co-occurrence of Globorotalia peripheroronda and Globorotalia peripheroacuta and the co-occurrence above of G. peripheroacuta and G. praefohsi at 2500 m (8200 ft). Zone N.11: between the co-occurrence of G. peripheroacuta and G. praefohsi below and the base of the overlying Sierra Formation which is marked by contrasting foraminiferal and environmental characteristics (see below).

Environment Most of the benthic species recognized within this formation are also found within the previous Uva Formation (Table I). Thus, water depths equal or greater than 2000m are indicated, particularly by the co-occurrence of Melonis pompilioides and Gyroidina soldanii. On the other hand, the predominance of robust benthic foraminifera and of the reduced lesser abundance of planktic organisms, accompanied by the common occurrence of Uvigerina spp. and pyritized planktic foraminifera and framboidal pyrite indicate t h a t the Napipi Formation was affected by waters rich in organic nutrients. The preference of Uvigerina for organic-rich sediments deposited under low oxygen conditions (Lohman, 1978; Miller and Lohman, 1982; Woodruff, 1985; Mullins et al., 1985, among others), as well as the planktic to benthic abundance, and low calcium carbonate concentrations associated with poor preservaton in highly productivity areas (Berger, 1970; Berger and Soutar, 1970) have already been pointed out. Additionally, the occurrence of sedimentary pyrite within the Napipi Formation in the Opogado-1 well, can be linked with conditions of high surface productivity due to reduction of organic matter in an anoxic sulfidic environment under the sedim e n t - w a t e r interface (cf. Berner, 1970, 1981).

212

Correlation As with the U v a Formation, the foraminiferal faunas of the Napipi Formation are also common in the middle Miocene marine formations of coastal Ecuador (Sigal, 1969; Fig.3), Panama (Cushman, 1918; ICSC, 1968), NW Colombia (Petters and Sarmiento, 1956), and in the rest of the Caribbean area. Atrato Basin sequence

In contrast to the open marine conditions of the previous Pre-Basin Sequence, this Basin Sequence developed in an inner borderland basin, comparable to those along the present southern California borderland (cf. Douglas and Heitman, 1979). This sequence which was deposited immediately after the middle Miocene tectonic disturbances, is represented by the Sierra and Munguido formations. These two stratigraphic units, now correspond to the lower and upper portions, respectively, of the Sierra Formation as originally described (Hafter, 1967).

Sierra Formation General characteristics The type section of this formation was originally described at the western side of the basin, from the Munguido river (Fig.l), and the Uva and Naipi rivers, as supplementary reference sections. It consists mostly of a calcareous succession of massive dark gray to gray hard siltstones grading locally into silty mudstones or argillaceous fine-grained sandstones, for the lower portion, and mostly mudstones accompanied by medium grained blue-gray sandstones, a few conglomeratic and carbonaceous levels, for an upper portion. The total thickness estimated for this sequence was approximately 3000 m (Haffer, 1967). Enough lithostratigraphic and geomorphic contrast (for mapping purposes) exists in the type area between the scarps built by the lower portion and the valleys of the upper portion of the original Sierra sequence, to formally subdivide this feature into two formations. Furthermore,

H.DUQUE-CARO the stratigraphic interval comprised by the Haffer's Sierra Formation has two lithostratigraphic counterparts in the adjacent eastern Panama and northwestern Colombia (Fig.3). Thus, the Sierra Formation will only comprise here the lower part of the former Sierra sequence. At the Opogado-1 well, this formation was recognized from 1420.7m (4660ft) down to 2225.6m (7300ft) in a dark grayish brown (2.5Y) calcareous mudstone facies including some limestone, dolomitic and silty interbeds (Fig.4). Abundant benthic and planktic foraminifera distinguish this interval where large and robust specimens of Uvigerina, Valvulineria and Bulimina predominate (Tables III and IV). The base of the Sierra Formation is marked by the contrast between the highest occurrence of 21 benthic species at the top of the underlyin~ Napipi Formation, and completely different and abundant benthic species above (Table III). The top is marked at 1420.7 m (4660 ft) by a distinctive glauconite layer accompanied by the highest occurrence and predominance of large and robust benthic foraminiferal species including Uvigerinella and Bolivina (Table III).

Age The Sierra Formation ranges from the middle Miocene Zone N.13, Sphaeroidinellopsis subdehiscens subdehiscens-Globigerina druryi to the early part of late Miocene, Zone N.17, Globorotalia tumida plesiotumida (Blow, 1969). At the Opogado-1 well these zones were recognized as follows (Table IV; Fig.4): Zones N.13 to N.14 (Middle Miocene): between the co-occurrence at 2195.1 m (7200 ft) of Sphaeroidinellopsis subdehiscens subdehiscens, Globorotalia siakensis and Globorotalia continuosa and the highest occurrence of G. siakensis at 1829.3 m (6950 ft). Here, it is important to note that at the Opogado-1 well, the highest occurrence of Siphogenerina coUomi, a middle Miocene marker (Lamb and Miller, 1984) is common and occurs with Bulimina uvigerinaformis uvigerinaformis (Table III), at 1996.9 m (6550 ft). Thus, this boundary should not be

213

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

STRATIGRAPHY

LITHOLOGY

FORAMINI

F ERA

NONIONELLA-BULIMINELLA 100

i~ z"',.,.,

~2 I

>,

;

ca

_

Low diversity

Anoxic bottom, organic matter Water depths less than 50m

L:

3o0 _,:-~ . . . . ... . . .

I.

I,--"

. . . . . . . . . .

ENVIRONMENT

~ It.l!

SO0 " ~J- ~ L =: ~ . ~

--,

" :~:.

~

,~,

Mostly calcareous mudstones ~=<-:=); less calcareous [:~ ~ ) . B O L I V I N A - U V I G E R I N A

i

__ _ = =~ _ v61auconite o l c a n i c t u f [o f s. . .[.. . .1, . . ]. . . . . . . . Pyrite, molluscan, fish -=:: and carbonaceous 900 ' : - ~ : : : : = remains. 7oo , -

---.--?--?-

llOO

N.ll

,7

1300

lu N.17

•

-'

-

'- - - ~

:h~

~._

High diversity, small size. Disappearance of large and robust benthics at 1265.2m.

~:~-

UVIGERINA-VALVULINERIA

N.16 ~

1500 - ~

_~

=) I(

c~

N.15

•

N.14

"-

,~'I

,~

Large and robust benthics ~

1700

Water depths less than 150m

~ -

m

¢..I

Anoxic bottom, organic mattes

......

~'-

Calcareous mudstones

Anoxic.bot.tom~ orgapic mat|or upRer oat hypt oaliths, coot ann aorazeo surl'aco waters

VA LVUL INERIA - GL O B I G E R I N I D S

. . . . . Limestones ~'~e~), ~-_ ___ ~ delemites ~ , lOOO - ~ ~ ~, siltstenes ( = - - ~ - ~ . . . . . . . . 61auconite [. . . . ~1.

Large and robust plonktics

2100 ~ _ ~ - - ~ a

Large and robust bonthics Less abundance of planktics

Anoxic bottom, organic matter Less abundance of benthics upper middle kothyoldepths cool and aerated surface waters. BULIMINA -UVIGERINELLA

""

"~EN.I3

Fig.4. Stratigraphic characteristics of the Sierra and Munguido formations (Basin Sequence) at the Opogado-1 well. y o u n g er th an middle Miocene Zone N.13 (cf. Lamb and Miller, 1984). Zone N.15 (middle Miocene): between the highest occurrence below of G. siakensis and cooccurrence immediately above, at 1585.4m (5200 ft) of Globorotalia acostaensis tegillata and Globorotalia merotumida. At 1631.1 m (5350 ft), is the highest occurrence of Globorotalia paralenguaensis a distinctive marker for recognizing the late Zone N.15 through early Zone N.16. Zones N.16 to early N.17 (late Miocene): between the co-occurrence of G. acostaensis tegillata and G. merotumida above Zone N.15, and the lowest occurrence of Globortalia conomiozea at 1478.7 m (4850 ft) above a casing level at 1581.7 m (5188 ft).

Environment Three characteristic microfaunal assemblages distinguished primarily on major benthic abundances, have been intimately associated with the environmental evolution of the Atrato Basin (Fig.4): (1) Bulimina Uvigerinella Assemblage. This benthic assemblage, at the Opogado-] well has been recognized from 2225.6 m (7300 ft) at the top of the Napipi Formation to 1996.9m (6550 ft). It is distinguished by a predominance of benthic microfauna and by great abundance of large and robust specimens of Bulimina uvigerinaformis uvigerinaformis and Uvigerinella obesa impolita, and minor abundances of

214

H. DUQUE-CARO

T A B L E III B e n t h i c f o r a m i n i f e r a of c o m m o n o c c u r r e n c e w i t h i n t h e late middle M i o c e n e to Pliocene S i e r r a a n d M u n g u i d o f o r m a t i o n s (Basin Sequence), Opogado-1 Well Level of highest occurrence Surface Surface Surface Surface Surface 18.3 m 18.3 m 18.3 m 27.4 m 45.7 m 73.2 m 73.2 m 82.3 m 201.2 m 201.2 m 201.2 m 219.5 m 265.2 m 265.2 m 265.2 m 265.2 m 265.2 m 265.2 m 411.6 m 411.6 m 472.5 m 503.0 m 807.9 m 807.9 m 868.9 m 929.9 m 1112.8 m 1143.3 m 1192.0 m 1192.0 m 1192.0 m 1265.2 m 1265.2 m 1265.2 m 1265.2 m 1297.7 m 1326.2 m 1326.2 m 1387.2 m 1417.7 m 1417.7 m 1417.7 m 1417.7 m 1417.7 m 1417.7 m 1448.2 m 1448.2 m

(60 ft) (60 ft) (60 ft) (90 ft) (150 ft) (240 ft) (240 ft) (270 ft) (660 ft) (660 ft) (660 ft) (720 ft) (870 ft) (870 ft) (870 ft) (870 ft) (870 ft) (870 ft) (1350 ft) (1350 ft) (1550 ft) (1650 ft) (2650 ft) (2650 ft) (2850 ft) (3050 ft) (3650 ft) (3750 ft) (3910 ft) (3910 ft) (3910 ft) (4150 ft) (4150 ft) (4150 ft) (4150 ft) (4250 ft) (4350 ft) (4350 ft) (4550 ft) (4650 ft) (4650 ft) (4650 ft) (4650 ft) (4650 ft) (4650 ft) (4750 ft) (4750 ft)

Buliminella curta C u s h m a n (2) Nonionella basispinata ( C u s h m a n a n d Moyer) (1) Bolivina acutula B a n d y (2) Textularia panamensis C u s h m a n Buccella frigida (Cushman) (1) Cancris auricula (Fichtell a n d Moll) (1) Lenticulina rotulata ( C u s h m a n ) aUvigerina curticosta ( C u s h m a n ) Fursenkoina pontoni ( C u s h m a n ) Lenticulina vaughani ( C u s h m a n ) Uvigerina adiposa (White) Hanzawaia nitidula (Bandy) (2) Buliminella elegantissirna (D'Orbigny) (1) Bolivina decurtata C u s h m a n (3) Bolivina charapotoensis C u s h m a n a n d S t e v e n s o n Cassidulina californica C u s h m a n a n d H u g h e s (3) Bolivina acuminata N a t l a n d (2) Bolivina rnarginata C u s h m a n (3) Epistominella bradyana C u s h m a n (2) aUvigerina juncea C u s h m a n a n d Todd (2 a n d 3) Nonion obducum C u s h m a n a n d S t e v e n s o n Bolivina bicostata C u s h m a n (2) aUvigerina incilis Todd (2) Bolivina floridana ( C u s h m a n ) (4) Sphaeroidina bulloides D ' O r b i g n y (4) Valvulineria depressa C u s h m a n (2) Hanzawaia illingi (Nuttall) (2) aUvigerinella obesa C u s h m a n var. (3) Cassidulina carinata Silvestri (3) Gypsina vesicularis ( P a r k e r a n d Jones) Epistominella srnithi (Stewart a n d Stewart) (4) Buliminella subfusiformis C u s h m a n (3) Bolivina cf. turnida C u s h m a n (3) Valvulineria araucana malagensis Kleinpell (4) Epistorninella exigua (Brady) (3) Chilostomella ovoidea Reuss (4) aUvigerina peregrina C u s h m a n (4) Bulimina pagoda C u s h m a n (4) Valvulineria ecuadorana C u s h m a n a n d S t e v e n s o n (3) bBolivina sinuata Galloway a n d Wissler (4) Nonion goudkoffi Kleinpell Uvigerina kernensis B a r b a t a n d v o n Storff (3) bRotalia ecuadorana C u s h m a n a n d S t e v e n s o n bSphaeroidina variabilis Reuss (4) bpullenia salisburyi S t e w a r t a n d S t e w a r t bGyroidina multilocula Coryell a n d M o s s m a n (4) a'bUvigerineUa obesa obesa C u s h m a n (3) Bolivina girardensis R a n k i n (4) bUvigerinella californica cf. ornata C u s h m a n (3) Planulina ornata (D'Orbigny) ( ) Quadrimorphina sp. bBolivina dispar C u s h m a n a n d S t e v e n s o n

215

NEOGENE STRATIGRAPHYAND EVOLUTIONOF PANAMASEAWAY TABLE III (continued)

Level of highest occurrence 1448.2 1448.2 1448.2 1448.2 1448.2 1478.6 1509.1 1509.1 1509.1 1554.9 1554.9 1554.9 1554.9 1692.0 1996.9 1996.9 1996.9 1996.9 1996.9 1996.9

m m m m m m m m m m m m m m m m m m m m

(4750 (4750 (4750 (4750 (4750 (4850 (4950 (4950 (4950 (5100 (5100 (5100 (5100 (5500 (6550 (6550 (6550 (6550 (6550 (6550

ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft) ft)

bBolivina bramlettei Kleinpell (3) bBolivina interjuncta C u s h m a n (3 and 4) Concavella gyroidinaformis ( C u s h m a n and Goudkoff) (4) bValvulineria araucana (D'Orbigny) var. (4) bEpistominella s u b p e r u v i a n a (Cushman) (3) bUvigerinella californica californica C u s h m a n (3) Valvulineria californica californica C u s h m a n (3) bUvigerina joaquinensis Kleinpell (4) bValvulineria alicia Pierce Bulimina uvigerinaformis charapotensis C u s h m a n and S t e v e n s o n Bolivina benedictensis Pierce (3) Buliminella ecuadorana C u s h m a n and S t e v e n s o n (2) bGlobobulirnina ovula (D'Orbigny) (4) bValvulineria araucana araucana (D'Orbigny) (4) bSiphogenerina collomi C u s h m a n (4) bBulimina uvigerinaformis uvigerinaformis C u s h m a n and Kleinpell a'bUvigerinella obesa impolita C u s h m a n and Laiming (3) bLenticulina smiIeyi (Kleinpell) bCancris baggi C u s h m a n and Kleinpell (3) bBulimina ecuadorana C u s h m a n and S t e v e n s o n

aCostate species. ~Large and robust species. ( ) P a l e o b a t h y m e t r i c i n d i c a t o r s after Ingle (1980), and S m i t h (1964). (1) I n n e r Shelf Biofacies: 0 50 m. (2) Outer Shelf Biofacies: 50 150 m. (3) U p p e r B a t h y a l Biofacies: 150 500 m. (4) U p p e r Middle B a t h y a l Biofacies: 500 1500 m.

Siphogenerina collomi, Cancris baggi, Bulimina ecuadorana, among others (Table III), accompanied by occurrences of framboidal pyrite and pyritized planktic foraminifera. The inferred paleodepth for most of these species (Table III; Ingle, 1980), the abundance of costate Uvigerina spp. (cf. Douglas, 1979; Table III), and the large and robust size of these species (cf. Bandy, 1963; Table III) are predominantly indicative of upper middle bathyal depths (500-1000m, cf. Ingle, 1980; Douglas and Heitman, 1979). Likewise, the abundance of Uvigerina and the presence of Globobulimina ovula, associated with the occurrence of pyrite and pyritized planktic foraminifera also indicates association with organic rich sediments under low oxygen conditions (Lohman, 1976; Ingle, 1980; Miller and Lohman, 1982; Woodruff, 1985; Mullins et al., 1985).

(2) Valvulineria-globigerinids Assemblage. This assemblage at the Opogado-1 well has been recognized from 1996.9m (6550ft) to 1585.4 m (5200 ft). In contrast with the previous benthic assemblage, this one is distinguished by a great abundance of floods of large and robust planktic foraminifera accompanied by lesser numbers of benthic foraminifera (planktic to benthic ratio more than 70:1). Valvulineria araucana araucana predominate associated with lesser numbers of mostly Bolivina and Uvigerinella (Table III). This benthic association, like the previous Bulimina-Uvigerinella Assemblage also indicates upper middle bathyal depths associated with organic-rich sediments within the oxygen minimum zone. The occurrence of costate Uvigerinella obesa obesa and Globobulimina ovula accompanied by increasing abundance of Bolivina sp. (Doug-

216

H. DUQUE-CARO TABLE IV Planktic foraminifera of common occurrence within the late middle Miocene to Pliocene Sierra and Munguido formations (Basin Sequence), Opogado-1 Well Level of highest occurrence Surface Surface Surface 14.6 m 14.6 m 14.6 m 14.6 m 18.3 m 27.4 m 45.7 m 45.7 m 73.2 m 128.0 m 210.4 m 350.6 m 411.6 m 746.9 m 777.4 m 868.9 m 868.9 m 868.9 m 899.4 m 990.8 m 1082.3 m 1143.3 m 1143.3 m 1265.2 m 1265.2 m 1265.2 m 1448.2 m 1448.2 m 1448.2 m 1448.2 m 1631.1 m 1692.1 m 1814.0 m 2149.4 m

(48 ft) (48 ft) (48 ft) (48 ft) (60 ft) (90 ft) (150 ft) (150 ft) (240 ft) (420 ft) (690 ft) (1150 ft) (1350 ft) (2450 ft) (2550 ft) (2850 ft) (2850 ft) (2850 ft) (2950 ft) (3250 ft) (3550 ft) (3750 ft) (3750 ft) (4150 ft) (4150 ft) (4150 ft) (4750 ft) (4750 ft) (4750 ft) (4750 ft) (5350 ft) (5550 ft) (5950 ft) (7050 ft)

Globigerina rubescens Hofker Globigerina decoraperta Takayanagi and Saito Globigerinoides quadrilobatus trilobus (Reuss) Globigerina bulloides bulloides D'Orbigny Globigerina calida praecalida Blow Globoquadrina altispira altispira (Cushman and Jarvis) Globorotalia conomiozea subconomiozea Bandy Globigerinoides ruber (D'Orbigny) Turborotalita quinqueloba (Natland) Globorotalia acostaensis tegiUata Bronnimann and Resig Globigerinoides obliquus obliquus Bolli Hastigerina siphonifera involuta (Cushman) Globigerinoides quadrilobatus quadrilobatus (D'Orbigny) Globorotalia cultrata limbata (Fornasini) Globigerinoides quadrilobatus sacculiferus (Brady) Globigerina falconensis Blow Globigerina juvenilis Bolli Globorotalia acostaensis acostaensis Blow Sphaeroidinellopsis seminulina seminulina (Schwager) Orbulina suturalis Bronnimann Orbulina universa D'Orbigny Globorotalia merotumida Blow and Banner Globorotalia cultrata menardii (Parker, Jones and Brady) Globigerinita incrusta Akers Sphaeroidinellopsis subdehiscens subdehiscens (Blow) Globigerina nepenthes Todd Globigerina foliata Bolli Globorotalia scitula Brady Globigerinoides obliquus extremus Bolli and Bermudez Globigerina venezuelana Hedberg Globigerinoides elongatus (D'Orbigny) Globorotaloides hexagona hexagona (Natland) Pulleniatina praepulleniatina Bronnimann and Resig Globorotalia paralenguaensis Blow Globorotalia lenguaensis Bolli Globorotalia siakensis Le Roy Globorotalia fohsi lobata (Bermudez)

l a s , 1979; R e s i g , 1981; I n g l e , 1980; W o o d r u f f , 198,5) s u p p o r t t h i s i n t e r p r e t a t i o n . O n t h e o t h e r hand, the very abundant large and robust planktic microfauna, characterized by a low d i v e r s i t y a s s o c i a t i o n o f Globigerina, Globoquadrina, Turborotalia a n d Globigerinoides is accompanied by very few and scarce occurr e n c e s o f Globorotalia. T h i s c o n t r a s t i n g a s s o c i aton indicates that this interval was also under the influence of well aerated cool surface

w a t e r s (cf. B a n d y , 1960, 1969; K e n n e t t a n d o t h e r s , 1985). I n t e n s i f i c a t i o n o f m a r g i n a l c u r r e n t s , i.e. t h e C a l i f o r n i a C u r r e n t , c o u l d e x p l a i n t h i s p h e n o m e n o n , a s is d i s c u s s e d b e l o w . (3) U v i g e r i n a - V a l v u l i n e r i a A s s e m b l a g e . A t t h e Opogado-1 well, this assemblage was recogn i z e d b e t w e e n 1585.4 m (5200 ft), i m m e d i a t e l y o v e r l y i n g t h e p r e v i o u s Valvulineria-globigerin i d s A s s e m b l a g e a n d 1420.7 m (4660 ft) a t t h e t o p

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

of the Sierra Formation. This benthic association is also very distinctive and is characterized by great abundance of large and robust Uvigerinella, Valvulineria and Bolivina (Table III) accompanied by less abundant large and robust planktic foraminifera (Table IV). Most of the species listed within this stratigraphic interval belong to both upper middle bathyal and upper bathyal depths (cf. Ingle, 1980). However, the great abundance of Uvigerinella obesa obesa and U. californica ornata, indicates a shallowing from upper middle to upper bathyal depths. This is supported by the abrupt change both in the planktic abundance and in the size of the benthic microfauna at the top of the Sierra Formation as is discussed below. Likewise, the increasing abundance of large and robust Bolivina interjuncta and Bolivina dispar (Table IID and the abundance of costate Uvigerina indicate association with organic-rich sediments within the oxygen minimum zone (cf. Douglas, 1979; Resig, 1981, among others). The environmental conditions indicated by the Uvigerina Valvulineria Assemblage agree with those indicated by the associated occurrences of glauconite. The known setting for glauconite formation involves a marine environment, water depths between outer neritic and upper bathyal, temperatures in the range of 7 15°C, with decaying organic matter and weakly reducing conditions (Cloud, 1955; Porrenga, 1967; Odin and Letolle, 1980; Odin and Matter, 1981; Berner, 1981). In summary, the Sierra Formation indicates a shallowing episode from upper middle to upper bathyal depths in an inner borderland basin (Douglas and Heitman, 1979), all within the oxygen minimum zone. This in turn was accompanied by well-aerated cool surface water currents, particularly during the late middle to late Miocene Valvulineria-globigerinids and Uvigerina Valvulineria assemblages.

Correlation Most of the foraminiferal assemblages listed for the Sierra Formation are typical of the northeastern Pacific basin, and were originally

217

described from the Californian borderland basins, i.e. Mohnian Stage (cf. Monterey Formation, Kleinpell, 1938, 1980; Ingle, 1967, 1980, among others). Comparable benthic associations have also been found in coastal Ecuador within the latest middle to late Miocene Charapoto, Angostura and Onzole formations (cf. Cushman and Stevenson, 1948; Sigal, 1969; Figs.3 and 5), and in the late Miocene of DSDP Site 570, offshore Guatemala (McDougall, 1985). The corresponding late middle to late Miocene formations along the Caribbean coastal margins of NW Colombia, i.e. the Cuesta Formation, in the Carmen-Zambrano area, bearing the Bulimina carmenensis Zone (Petters and Sarmiento, 1956) and the upper portion of the Perdices Shale (Duque-Caro, 1990) bear different benthic foraminiferal assemblages. The same differences are also observed in the whole Caribbean coastal areas of NW South America (cf. Renz, 1948; Blow, 1959).

Munguido Formation General characteristics As stated earlier, this new name is proposed to formally discriminate the upper portion of Haffer's Sierra Formation in the Munguido river area (Fig.l; Haffer, 1967). This feature comprised by a disected valley (Haffer, 1967), conformably overlies the Sierra Formation. The top is unconformable (see below). A predominantly gray mudstone interval features some medium-grained sandstone interbeds accompanied by distinctive occurrences of molluscan beds, few conglomerates and carbonaceous zones, which are the lithostratigaphic features that comprise the original description. At the Opogado-1 well, the Munguido Formation was recognized from the surface down to 1420.7m (4660ft). There, a comparable lithology was recognized consisting mostly of olive gray (5Y5/2) calcareous mudstones including a pyroclastic layer (tufts in carbonate and zeolitic matrix) at 304.9 m (1000 ft), a thin

218

H.DUQUE-CARO PACIFIC IATES~r MIDDLE 20 IATE MIOCENE FORMATIONS NW COLOMBIA Sierra Formation Opogado-i Well

ECUADOR Charapoto Angostura

Uvigerlna peregrina Bulimina pagoda Valvulineria ecuadorana Bolivina sinuata Nonion goudkoffi Uvigerina kernensis Rotalia ecuadorana Sphaeroidina variabilis Pullenia salisburyi Cyroidina multilocula Uvigerinella obesa obesa Bollvina glrardensis Uvigerinella californica cf. ornata Planulina ornata Quadrimorphina sp. Bolivina dispar Bolivina bramlettei Bolivina interjuncta Concavella gyroidinaformie Valvulineria sraucana var. Epistominella subperuviana Uvigerinella califarnica californica Uvigerina joaquinensis Uvigerina hootsi Valvulineria alicia Bulimlna uvigerinaformis charapotoensis Bolivina benedictensis Buliminella ecuadorana Globobulimin~ ovula Valvulinerla araucana araucana Siphogenerlna collomi Bulimina uvigerinaformis uvigerinaformis Uvlgerinella obesa impolita Lenticulina smileyi Cancris baggi Bulimina ecuadorana

X X X X

SUAT~|ALA

CALIFORNI~

Site 570

Monterrey

Onzole X X X X

X X X X X X

X X X X X

X X X X

X X X X X X X X X

X X

X

Fig.5. Selected benthic foraminifera of the Sierra Formation arranged by highest occurrence (Table III), and comparable occurrences in some Pacific latest middle to late Miocene formations. glauconitic layer at 183 m (600 ft), and thin limestone interbeds within the uppermost 300m (Fig.4). Mostly benthic foraminifera (Table III) accompanied by conspicuous occurrences of pyritic, carbonaceous, molluscan and fish remains are also characteristic features of the Munguido F o r m a t i on at the Opogado-1 well. A marked co n tr as t in size, an average of 3 to 1, approximately is observed between the benthic assemblages of the Sierra and Munguido formations, respectively. Whereas the Si-

erra F o r m a t i o n is distinguished by large and robust benthic assemblages, part i cul arl y including Bolivina and Uvigerina, the same taxa within the Munguido Form at i on are smaller. An abrupt decrease both in abundance and diversity of planktic foraminifera also takes place at the base of the Munguido Formation. This cont rast is marked by the glauconite layer separating these two formations. The Munguido Form at i on at the Opogado-1 well exhibits very abundant and diverse benthic assemblages (more t han 500 specimens

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

per sample) below 244m (800ft) and less diverse and abundant (less t h a n 300 specimens per sample) above, accompanied by conspicuous occurrences of pyritic, carbonaceous, molluscan and fish remains.

Age The age of the Munguido Formation both in the type section and at the Opogado-1 well, is difficult to assess due to the rarity of diagnostic planktic foraminifera. This is particularly noticeable above 747 m (2450 ft) where very rare occurrences of globigerinids are noted (Table IV). However, the co-occurrence of

Globigerina rubescens, Globoi~uadrina altispira, Globigerina decoraperta, Globorotalia conomiozea subconomiozea indicates a chronostratigraphic interval not younger t h a n the late Pliocene Zone N.21 (Berggren et al., 1985), based on the highest occurrence of Globoquadrina altispira (cf. extinction datum at 2.8 Ma, Berggren, 1973). However, I consider the top of the Munguido Formation not being younger than the extinction datum of Globorotalia margaritae margaritae at 3.4 Ma (Berggren and others, 1985). The planktic assemblages noted within the upper Munguido Formation occur associated with Globorotalia margaritae in NW Colombia, i.e. the Tubara beds, Barranquilla area. The Globorotalia margaritae Zone in the interior basins of NW Colombia marks the closing of the marine episodes prior to the middle Pliocene tectonic disturbances (cf. Sincelejian Stage, Duque-Caro, 1984). The base of the Munguido Formation is dated within the late Miocene Zone N.17 based on the continued occurrence of Globorotalia conomiozea subconomiozea immediately above the Sierra Formation. Similarly, the top of the early Pliocene Zone N.18 has been placed at 899.4 m (2950 ft; Fig.4) at the highest occurrence of Globorotalia merotumida.

Environment As with the Sierra Formation, the Munguido Formation is distinguished by two benthic assemblages associated with the final stage of the environmental development of the Atrato Basin.

219

(1) Bolivina-Uvigerina Assemblage. This assemblage is characterized by very abundant benthic foraminifera where Bolivina bicostata and costate species of Uvigerina (U. curticosta, U. incilis and U. juncea, Table III) are common. At the Opogado-1 well this zone was recognized from the top of the Uvigerina-Valvulineria Assemblage, at the top of the Sierra Formation (Fig.4) to 201.2 m (660 ft). Within the lowermost portion, below 1265.2 m (4150 ft), the large and robust benthic species distinctive of the upper portion of the Sierra Formation, i.e. Sphaeroid-

ina variabilis, Rotalia ecuadorana, Uvigerina kernensis and Bolivina sinuata (Table III) gradually disappear, and are replaced by the characteristic smaller benthic species of this biostratigraphic interval. Most of the species listed (Table III) have their upper depth boundaries within upper bathyal depths in the California borderlands (cf. Bandy, 1961; Bandy and Arnal, 1969; Ingle, 1980, among others). However, the abrupt decrease in abundance and diversity of planktic foraminifera and the variation in size in the benthic microfauna immediately above the Sierra Formation, indicate a drastic paleobathymetric change to 150 m water depth, immediately above the shelf slope juncture (cf. Bandy, 1963; Smith, 1963; Ingle, 1980). It follows that during the latest Miocene Pliocene time, along the Pacific coastal areas of the NW corner of South America, the former upper bathyal biofacies species apparently extended their upper bathymetric boundaries into the outer shelf domain. Investigations performed on living benthic foraminifera off the Pacific coast of Central America (Bandy and Arnal, 1957; Smith 1963, 1964; Thompson, 1982) have recognized the common benthic taxa of California, and have also indicated greater depth boundaries for these biofacies, i.e., in the Middle America Trench sediments. However, this situation is complicated by the fact t h a t during the latest Miocene-Pliocene time, the Atrato Basin was an '~inner borderland basin" (sensu Douglas and Heitman, 1979). These kinds of basins are relatively shallow (900 m and less) and are anoxic at

220

water depths corresponding to the lower slope and basin floor because their sills are within the oxygen minimum zone. Also, the depth distribution of slope and basin benthic foraminifera, and especially the considerable variation in species depth limits, complicate the application of empirical paleoecologic models (cf. Douglas and Heitman, 1979). As described earlier conspicuous occurrences of pyritic and carbonaceous remains is a characteristic feature of the Munguido Formation and also of the Uvigerina-Valvulineria Assemblage Zone. Therefore, this indicates the association of these faunas with organic rich sediments within the oxygen minimum zone during the final development of the Atrato basin. (2) Nonionella-Buliminella Assemblage. This assemblage distinguishes the uppermost part of the Munguido Formation and marks the closing of the marine episode in the Atrato Basin. A new and contrasting benthic foraminiferal association, characterized by major abundances of Nonion.ella basispinata (70-98%) and Buliminella curta (Table III), overlies the previous Bolivina-Uvigerina asemblage, above 201.3 m (660 ft) at the Opogado-1 well. Low microfaunal diversity, occurrence of framboidal pyrite, either as free grains or as infillings of foraminiferal tests (i.e. Nonionella basispinata, in particular), occurrence of phosphatic and fish remains, and molluscan shell fragments are also distinctive features within this uppermost biostratigraphic interval. All are suggesting shallowing depths in an environment rich in organic matter under very low oxygen conditions. In fact, the great abundance of Nonionella basispinata accompanied by the occurrence of Buccella frigida, Cancris auricula, Buliminella elegantissima and Buliminella curta (Table III) suggest shallower depths than the underlying Bolivina- Uvigerina assemblage, all within the outer shelf domain of the near-shore basins (Douglas and Heitman, 1979).

Correlation Only a few references are known for the latest Miocene to Pliocene stratigraphy and

H. DUQUE-CARO

foraminiferal biostratigraphy of the NW corner of South America. However, the known data indicate comparable benthic foraminiferal associations (Fig.6) in coastal Ecuador within the Borbon, Progreso and Daule formations (Sigal, 1969), in the Panama Canal Zone, within the Gatun Formation (Cushman, 1918; Vaughan, 1919), on the Caribbean side of NW Colombia within the Tubara beds (Redmond, 1953), and in the Uvigerina subperegrina Zone and Ammonia beccarii Zonule (Petters and Sarmiento, 1956) within the Cuesta Formation in the Carmen-Zambrano area. Here, it is important to note that the basal parts of the former stratigraphic units, particularly those of coastal Ecuador and NW Colombia which also bear Woodring's well known Miocene molluscan faunas, are apparently synchronous (cf. Sigal, 1969; Duque-Caro, 1971), and bounded by a regional unconformity.

Paleoceanography and paleobiogeography Uva Formation The abundance of planktic foraminifera and radiolaria in most carbonate and hemipelagic strata within the late Oligocene to earliest middle Miocene is a common feature along the Pacific and Caribbean coastal areas of NW South America (cf. Cushman and Stainforth, 1951; Riedel, 1959; Sigal, 1969; Bandy 1970; Bandy and Casey, 1973; Duque-Caro, 1975, 1984). It indicates free and active communication between the Atlantic and Pacific oceans during this interval (cf. Keller and Barron, 1983; Romine and Lombari, 1985; Figs.7,8), at water depths equal to or greater than 2000 m without indication of uplift or any sill development along the present coastal areas of NW South America, including southern Central America. Similarly, the great abundance of planktic foraminifera and radiolaria, a planktic to benthic ratio greater than 100:1 for the Uva Formation, and the gradually decreasing abundance of microfaunal assemblages within the Napipi Formation mark a contrasting

NEOGENE STRATIGRAPHYAND EVOLUTIONOF PANAMASEAWAY

221

PACIFIC NW COLOMBIA Munguido Formation Opogado-1 Well Buliminella curta Nonionella basispinata Bolivina acutula Textularia panamensis Buccella frigida Cancris auricula lenticulina rotulata Uvigerina curticosta Fursenkoina pontoni lenticulina vaughani Uvigerina a~iposa Hanzawaia nitidula Buliminella elegantissima Bolivina decurtata Bolivina charapotoensis Cassidulina californica Bolivina acuminata Bolivina marginata Epistominella ~ a d y a n a Uvigerina juncea Nonion obducum Bolivina bicostata Uvigerina ineilis Ammonia heecarii Bolivina floridana Sphaeroidina bulloides Valvulineria depressa Hanzawaia illingi Uvigerinella obesa Cassidulina carinata Gypsina vesieularis Epistominella smithi Buliminella subfusiformis Bolivinacf. tumida Valvulineria araucana malagensis Epistominella exigua Chilostomella ovoidea Uvigerina peregrina Bulimina pagoda Valvulineria ecuadorana Iblivina s i n ~ t a Nonion goudkoffi Uvigerina kernensis Rotalia ecuadorana Sphaeroidina variabilis

CARIBBEAN

ECUADOR Bor bon Progreso Daule X X

PANAMA Gatun

NW COLOMBIA Tubara

CarmenZamhrano

X X

X X

X X

X X X X X X X

X X X X X X X

X X X X X X X

X

X X X X X X X X X X X

X X

X X

X X X X X

X X

X

X

X

X

X X X

X X X

X X X

X X

X X X

X X X

X X

X

X

X X X X

X X

X X X

X X

X

Fig.6. Selected b e n t h i c f o r a m i n i f e r a of the M u n g u i d o F o r m a t i o n a r r a n g e d by h i g h e s t o c c u r r e n c e (Table III), and c o m p a r a b l e o c c u r r e n c e s in the Pacific and Caribbean latest Miocene to Pliocene f o r m a t i o n s in the NW c o r n e r of S o u t h America.

boundary within the earliest middle Miocene Zone N.9 (Fig.2). This boundary, at about 15.2Ma (cf. Berggren et al., 1985) which coincides with the disappearance of radiolarian faunas in the Caribbean (Riedel, 1959) and with the changeover in silica deposition from

the Atlantic to the Pacific ocean (Keller and Barron, 1983), also coincides with Miocene hiatus NH 2 (Keller and Barron, 1983). According to these workers intensification of bottom water flow due to a change in circulation during the late early to middle Miocene was

-

MIOCENE TO 6.3 Ma

TO 11.1 Mu

MIOCENE

PLIOCENE

Atlantic to Pacific shallow water connection restricted Anoxic bottom accompanied by shallowin~ and sea level rise. Caribbean affinities. Woodring's M~oceneCaribbean Province.

LATE MIOCENE TO EARLY 6.3 TO 3.1 Me

Partial emo.~enee of the Panama Isthmus and disruption of the Atlaoflc to Pacific flow. Closing of the intermediate water connection and onset of the cool California Current flow. Sea level drop

]2.D

MIDDLE

7.D Mu

MIOCENE

PL I O C E N TO 3.1 Ma

E

Uplift and complete emergence of the Panama Isthmus and ¢losi~ of the Atlantic to Pacific shellow water connection. Onset of the Great American Terrestrial Interchange.

EARLY 3.7

Shallow water connection open but disrupted by the ¢ool and well aerated surface waters of the California Current. Anoxic bottom accompanied by sea level rise and drops. CaUforn0an benthic foraminiferal affinities. First recorded intermingling of terrestrial faunas -9.3 to S.O Ma.-

11.11 TO

LATE MIDDLE TO LATE

Fig.7. Neogene paleoceanographic and paleogeographic evolution of the NW corner of South America. Hypothetical surface circulation resulting from th~ disruption of the warm Caribbean flow and intensification of the cool California current. Supplemented with data by Keller and B a r r o n (1983, 1986), Vail and Hardenbol (1979), McDougall (1985) and Webb (1985).

Uplift to water depths less than 150m and restriction of the shallow water connection. The cool California Current influence ends, and the shallow Atlantic to Pacific flow is re-establisbod

LATE , 7.0

Atlantic to Pacific ntermed ate and shallow water connection open. Common benthic foraminiferal faunas. Isolation of North American and South American vertebrate fauna.

?,

MIDDLE MIOCENE 15.1 TO ]2.! Ma

©

c~

L~

223

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

r'

'

A 6 ES

T ECT 0 N I CS

EARLY PLIOCENE UPLIFT AND OF COMPLETE EMERGENCE THE 3.7.3.1 Ma PANAMAiSTHMUS.

PALEOCEANOGRAPHY SHALLOW CLOSED. WATERCONNECTION SEA LEVEL DROP.

SHALLOWWATERCONNECTION LATE MIOCENETO SHALLOWINGTO WATER RESTRICTED. EARLY PLIOCENE ANOXICBOTTOM. 6.3-3.7 Ma OEPTHSLESS THAN SOre SEA LEVELRISE.

PA LE 0 B 1 0 6 M AR I NE DIMINISHING OF CARIBBEAN AFFINITIES

E 0 URA PHY TERRESTRIAL

HIATUSES CLIMATE

ONSETOF THE GREAT NH 8 AMERICANINTERCHANGE COLD

CALIFORNIAN AND WARM

CARIBBEAN AFFINITIES

SHALLOWWATERCONNECTION LATE MIOCENE UPLIFTTO WATERDEPTHS RESTRICTED. END OF THE CALIFORNIA CURRENTACTION. 7.0- 6.3 Ma LESS THAN 150 m SEA LEVEL DROP.

NH 6 COLD

CALIFORNIAN LATE MIOCENE SHALLOWINGTO UPPER SHALLOWWATER CONNECTION OPEN. COOL AND WELL 8.6-7.0 Ma BATHYALDEPTHS. AERATED SURFACEWATERSOF THE CALIFORNIACURRENT. ANOXIC BOTTOM. LATE MIDDLE TO SEA LEVEL RISE. LATE MIOCENE APPARENTSTABILITY 11.$- B.5 Ma UPLIFT TO UPPERMIDDLE DEPTHS. MIDDLE MIOCENE BATHYAL INNER BORDERLAND BASINS FORMED. 12.9-11.8 Ma PARTIAL EMERGENCEOF THE PANAMAISTHMUS.

CLOSINGOF INTERMEDIATE WATER CONNECTIONAND ONSETOF THE CALIFORNIA CURRENT ACTION. SEA LEVEL DROP.

MIDDLE MIOCENE APPARENTSTABILITY 15.1 -12.9 Ma

INTERMEDIATE AND SHALLOW WATERCONNECTIONOPEN. SEA LEVEL RISE.

EARLY-MIDDLE MIOCENE 1B.1-15.1 Ma EARLY MIOCENE ~, 23.7-16.2 Ma

UPLIFT TO LOWER BATHYAL DEPTHS APPARENTSTABILITY

AFFINITIES

EARLIESTRECORDOF INTERMINGLING 9.3 TO S.OMa PROCYONIDS (RACCOONS) COOL AND MEGALONYCHIDS (GROUND SLOTHS)

ABRUPT END OF

NH ]

CARIBBEAN AFFINITIES

COLD

CARIBBEANAFFINITIES

NO

WARM

INTERCHANGE

CLOSINGOF THE DEEP WATER CARIBBEANAFFINITIES. CONNECTION. RADIOLARIANSDISAPPEAR SEA LEVEL DROP. IN THE CARIBBEAN.

NH 2 COLD

DEEP WATER CONNECTION OPEN. SEA LEVEL RISE.

WARM

CARIBBEANAFFINITIES

Fig.8. Neogene tectonic, paleoceanographic and paleobiogeographicevolution of the Pacific NW corner of South America. Supplemented with data by Keller and Barron (1983, 1986),Vail and Hardenbol (1979), McDougall (1985), and Webb (1985). the major cause of this hiatus (see early-middle Miocene Un co nf or m i t y below).

Napipi Formation The contrasting much reduced abundance of planktic microfaunas and the disappearance of radiolarians in the Napipi formation, and the corresponding disappearance of radiolarians in the nearby Caribbean during the earliest middle Miocene time (Riedel, 1959) indicate a change in surface w a t e r circulation in the Atrato Basin and adjacent borderland basins in the NW co r n er of South America. an initial uplift of the Darien-San Blas and Baudo sills, as part of the P a n a m a sill (Keller and Barron, 1983) could be sufficient to disrupt bottom water circulation into the Atrato, C hucunaque

and Sambu basin areas during the earliest middle Miocene. On the other hand, all of the benthic species listed for both the Uva and Napipi formations (Table I) are common in, and have been reported from, the Oligocene t h r o u g h middle Miocene formations in both the Pacific and Caribbean coastal areas of the NW corner of South America (Fig.3). This indicates t hat water connection along the Pacific and Caribbean coastal areas of NW South America included depths in the range of 2000m t h r o u g h o u t all of the late Oligocene to middle Miocene. Similarly, microfaunal similarities, particularly Uvigerina, also indicate t hat organic-rich waters prevailed along these coastal areas. This suggests t hat the middle Miocene high surface productivity after 16 Ma (Woo-

224 druff, 1985) also extended into the present coastal areas of NW South America. This period of high surface productivity, on the other hand, corresponds with a general warming of the equatorial sea surface (cf. Savin et al., 1975; Savin et al., 1981; Woodruff et al., 1981; Savin et al., 1985) and with a maximum sea level rise (Vail and Hardenbol, 1979; Figs.7 and 8). The Colombian middle Miocene transgression reached interior basins as distant as the eastern Llanos region in the vicinity of the Guyana Shield and the borders with Peru and Ecuador. There, I have observed an ingression of middle Miocene neritic benthic foraminifera marking a single Cenozoic marine event, interbedded within predominantly terrestrial sequences.

Sierra Formation As it has already been noted, there is a marked contrast in benthic composition between the Napipi and Sierra Formations of the Pre-Basin and Basin Sequences respectively (Tables I and III). Except for Siphogenerina collomi and Siphogenerina lamellata, the Napipi and Sierra formations have different benthic faunas. On the other hand, the benthic foraminiferal faunas of the Uva and Napipi formations, as mentioned earlier, are common and widely recognized elements within the Oligocene to middle Miocene biostratigraphy of the Caribbean and Pacific costal areas of NW South America. In contrast, the late middle to late Miocene benthic faunas of the Sierra Formation, typical of the higher latitude borderland basins off California, with no Caribbean affinities, appear restricted to the coastal Pacific areas of NW South America and Central America. The corresponding late middle to late Miocene benthic foraminiferal assemblages in the immediately adjacent Caribbean coastal areas of NW Colombia are markedly different (cf. Bulimina carmenensis Zone, Petters and Sarmiento, 1956). Therefore, a change from a tropical (Napipi Formation) to a temperate cool water (Sierra Formation)

H.DUQUE-CARO regime, and the emergence of a circulation barrier disrupting the flow of the warm Caribbean current through the Panamanian straits, is indicated. The appearance of this barrier seems to be associated with the paleobathymetric variants already noted between the Napipi and Sierra formations, and with the intensification of ocean circulation during a rapid fall of sea level and declining temperatures in associaton with the growth of the Antartic ice sheet (Shackleton and Kennett, 1975; Vail and Hardenbol, 1979; Keller and Barron, 1983; McDougall, 1985; Savin and Douglas, 1985). The timing of this phenomenon coincides with the Neogene hiatus NH 3 (12.9-11.8 Ma, Keller and Barron, 1983, 1986) which was associated with a cool period along the Pacific coastal areas of Ecuador, Colombia and Central America (cf. McDougall, 1985). This cool regime prevailed throughout the latest middle to late Miocene, represented by the Sierra Formation. The abrupt faunistic contrast between the top of the Bulimina-Uvigerinella Assemblage (Fig.4) where the highest Bulimina uvigerinaformis uvigerinaformis and Siphogenerina collomi, are replaced above by the low diversity planktic association of the Valvulineria-globigerinids Assemblage apparently coincides with the Miocene hiatus NH 4. This contrasting faunal turnover correlates fairly well with the timing of other foraminiferal changes occurring during this hiatus such as the disappearance of the middle Miocene planktic assemblages and replacement by a relatively cool, low diversity fauna that persists through the late Miocene (cf. Keller and Barron, 1983). This hiatus, dated with the planktic zones N.14 to N.15 (10.2-9.5 Ma, Keller and Barron, 1986) corresponds to another cold pulse associated with increased polar glaciation (Keller and Barron, 1983; Fig.8). No comparable evidences for recognizing the Miocene hiatus NH 5 which is placed within the planktic Zone N.16 (Keller and Barron, 1983), has been noted in the Atrato Basin and vicinity. However, the top of the Valvulineria-globigerinids Assemblage (Fig.4) is also

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

marked by a foraminiferal change which indicate another major paleoceanographic event, and which could be an expression of this hiatus. Similarly, Miocene hiatus NH 6, placed within the planktic Zone N.17, could also correspond with a major faunal paleobathymetric and paleoceanographic change noted between the Sierra and Munguido formations (Fig.4; see below).

Paleobathymetric constraints The closing of the Atrato Pre-Basin Sequence as described earlier, was characterized by the abrupt disappearance of the Caribbean benthic: biotas in the pacific coastal areas of NW South America, including southern Central America, and by the abrupt replacement of these faunas by completely different assemblages with no apparent evolutionary relation or Caribbean affinities. How can this phenomenon be interpreted and what happened in the Caribbean coastal areas of NW Colombia?

Pacific Coast The abrupt disappearance of the Caribbean benthic biotas in the Pacific coastal areas of NW South America is associated with the abrupt paleobathymetric contrast between the Atrato Pre-Basin and Basin Sequences from lower to middle bathyal depths, a shallowing in the range of 1000 m. Thus, the Darien-San Bias and Baudo sills underwent another major uplift, and a circulation barrier emerged, i.e., the cool California Current, resulting from middle Miocene intensification of water circulation, sea level drop and declining temperatures following Miocene hiatus NH 3 (Keller and Barron, 1983). Therefore, none of the deep • benthic dwellers could adapt to the new middle bathyal environment in the resulting inner borderland basins, except Siphogenerina collomi and S. lameUata (Tables I and III). It is interesting to note the apparent coincidence of this phenomenon with the extinction of Paleogene benthic foraminiferal faunas during this middle Miocene interval (Woodruff and Douglas, 1981; Keller and Barron, 1983).

225

As to the abrupt replacement by completely different benthic microfaunas within the Sierra Formation, with no apparent evolutionary relation to those of the underlying Napipi Formation, it can be said that most of the species occurring within the Sierra Formation are typical of, and have originally been described from, the Miocene and Pliocene formations of the Californian borderland areas. There, species such as Uvigerinella obesa, Valvulineria californica, Valvulineria araucana, among others (Table III) occur associated with more diverse benthic assemblages, and display longer stratigraphic ranges and phyletic origins extending into the Oligocene (cf. Kleinpell, 1938, 1980; Ingle, 1980, among others). Therefore, a northern origin for the late middle to late Miocene benthic assemblages characteristic of the Pacific coastal areas of NW South America, including southern Central America, seems likely. Intensification of a cool marginal current coming from the northern latitudes, i.e. the California Current, bringing benthic foraminifera as far south as the Ecuadorian latitudes (Golfo de Guayaquil) could explain this phenomenon. Presently, the California Current flow slowly equatorward delivering cool waters south along the Pacific margin, only to about 25°N latitude where it turns west. However, at the end of the middle Miocene (Fig.7), during the major cooling episode accompanied by the growth of the Antarctic ice cap, an intensification of the California Curent should occur. A bifurcation of this current at about 2 f i n in the fashion described by Abbott (1966) to explain the factors influencing the zoogeographic affinities of the equatorial Galapagos Island inshore faunas, would also explain these observations. An eastern segment of this current continued on farther southward along the coasts of Mexico and Central America, distributing California borderland benthic foraminiferal faunas along the Pacific coastal areas of NW South America. The California Current, according to this interpretation of the biostratigraphy recorded in the Atrato Basin, influenced the entire interval represented by the

226 Sierra Formation, above which other major faunal contrast within the Munguido Formation occurs.

Caribbean side With the exception of two species of Siphogenerina (Table III) none of the benthic assemblages reported from the late middle to late Miocene of the Pacific coastal areas of NW South America have been reported from correlative Caribbean formations. In NW Colombia the late middle to late Miocene Bulimina carmenensis Zone (Petters and Sarmiento, 1956), bears typical and distinctive benthic foraminiferal species for the lower Magdalena Basin and offshore areas of the Guajira peninsula: Bulimina carmenensis Petters and Sarmiento, and Bulimina dentoni Petters and Sarmiento, which are accompanied by Bolivina marginata multicostata Cushman, Bolivina rioridana, Cibicidoides crebbsi, Gyroidinoides venezuelana and Uvigerina peregrina, among others. To date, no occurrence nor mention of the first two species is known from surrounding areas in the Caribbean. This indicates that late middle to late Miocene provincialism spread over other coastal areas of NW South America (cf. Petters and Sarmiento, 1956). Vertebrate terrestrial faunas Within the interval covered by this study, being mostly Neogene, the first recorded intermingling of terrestrial faunas occurs during the late Miocene (9.3-8.0 Ma, Marshall et al., 1979; Marshall, 1985; Webb, 1985). Procyonids (raccoons and their allies), a group of North American origin, migrated to South America and megalonychids and mylodontids (ground sloths) from South America migrated to North America. Based on the present paleobathymetric interpretation, the Atrato and Chucunaque inner borderland basins began to develop at upper middle bathyal depths (1000 m or slightly less) immediately after the middle Miocene tectonic disturbances. Therefore, the intermediate and deep w a t e r connection across the isthmus of Panama was closed (cf. McDougall, 1985). In

H.DUQUE-CARO the same fashion, the paleogeographic panorama of the late Miocene depicts a partially emerged land mass, the Serrania de San BiasDarien, as a major topographic feature between the Caribbean Sea and the Pacific ocean (Fig.7), and the Baudo sill at water depths not deeper than 1000 m. All of these features were accompanied by decreasing temperatures and sea level falls possibly below the shelf edge (cf. Vail and Hardenbol, 1979). By this argument, the '~new island hoppers" (Simpson, 1950), i.e., the raccoons and ground sloths, being good swimmers (Webb, 1985), could easily migrate to either South America or North America.

Munguido formation The abrupt contrast in benthic faunal size, and the abundance of pyrite and organic matter between the Sierra and Munguido formations, and the similarities of benthic biotas between the latest Miocene to Pliocene Pacific and Caribbean formations indicates: (1) the cessation of the cool California Current, involving the entire interval represented by the Sierra Formation in the Atrato and Chucunaque basins, (2) re-establishment of warm water circulation between the Caribbean Sea and Pacific Ocean, (3) restriction of the water circulation in the Atrato and Chucunaque basins due to the abrupt paleobathymetric change which left these basins at shallower depths (less than 150 m), and (4) a new episode, characterized by very abundant organic matter and sediment supply, replacing the former Sierra Formation episode. The timing of these phenomena coincides with Miocene hiatus NH 6 (Keller and Barron, 1983). In fact, the upper boundary of this hiatus, dated at 6.3 Ma (Keller and Barron, 1986) also coincides with the late Miocene Carbon Shift and the isolation of the Mediterranean Basin (cf. Vincent et al., 1980), and with the increase in biologic productivity, and the rate of supply of sediments and organic carbon from coastal lowlands and exposed shelves (cf. Vincent et al., 1980; Barron, 1980; Keller, 1980). These phenomena, which have been recorded throughout the world oceans,

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

are recognized in the latest Miocene to Pliocene stratigraphic record in the NW corner of South America. In NW Colombia (the Tubara Group and the Cerrito and Cuesta formations), I have observed a great accumulation of terrigenous sediments in excess of 2O0Om, associated with deltaic environments, both very organic-rich and containing costate Uvi-

gerina (Uvigerina peregrina). The coincidence of most Miocene hiatuses recorded in the oceans with biostratigraphic and sedimentation phenomena in the Pacific and Caribbean coastal stratigraphy of NW Colombia might well indicate that the marked change between the Bolivina-Uvigerina and the NonioneUa-Buliminella assemblages (Fig.4) and the occurrence of glauconite are effects of the cooling pulse associated with the Neogene Hiatus NH 7 (Keller and Barron, 1983). In the inner borderland basins of NW South America, this phenomenon also marks the onset of a gradual impoverishment of the benthic microfauna on a regional scale. There, Nonionella Buliminella and Ammonia are the most common and characteristic faunas (cf. Renz, 1948; Blow, 1959, in Venezuela; Redmond, 1953; Petters and Sarmiento, 1956, in NW Colombia; Cushman, 1918; Bandy, 1970, in eastern Panama; and this study). Because the biostratigraphic interval encompassing the Munguido Formation episode, the Globorotalia margaritae Zone, corresponds to a general sea level rise (Vail and Hardenbol, 1979; Haq et al., 1987), this shallowing cycle (regression) apparently resulted from a very rapid rate of emergence due to the tectonic interaction between southern Central America and the NW corner of South America (cf. Lloyd, 1963; Dengo, 1973). On the other hand, the end of this shallowing cycle also coincides with the Neogene Hiatus NH 8 (3.7 3.1 Ma, Keller and Barron, 1983, 1986). This date coincides with that given for the final emergence of the P a n a m a n i a n isthmus from 3.5 to 3.1 Ma (Saito, 1976; Keigwin, 1978, 1982) which finally closed communication between the Atlantic and Pacific oceans. This event also marks the closing of the Neogene marine

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episode and the emergence of the interior coastal areas of Colombia (Duque-Caro, 1984).

Paleobiogeographic constraints The latest Miocene to Pliocene (6.0 3.1 Ma), in the NW corner of South America (Fig.6) includes similar benthic foraminiferal faunas for both the Pacific and Caribbean coastal areas. It also includes Woodring's Neogene molluscan beds of the Pacific and Caribbean coastal areas of NW South America, (Fig.3). These beds served as the basis for the well known "Miocene Caribbean Province" (Woodring, 1965, 1978), originally dated too old (Petuch, 1982). Thus, the latest Miocene to Pliocene species, including benthic foraminifera, from Peru, Ecuador, Colombia and eastern Panama are closely related and often identical to the common latest Miocene to Pliocene forms of the Caribbean. When the Panamanian isthmus finally emerged, it isolated the eastern Pacific region from the Caribbean and disrupted the seaway which had previously existed. The terrestrial environment which replaced the previous latest Miocene to Pliocene shallowing marine episode in the Atrato and Chucunaque basins (after 3.5 and 3.1 Ma) also marks the onset of the great American interchange of vertebrate faunas. Unconformities Previously, three Neogene unconformities on the Caribbean side of NW Colombia have been discussed, based mainly on the structural and tectonic development of the adjacent Sinu and San Jacinto belts (Duque-Caro, 1984). However, the timing of these phenomena has not been precisely defined. Based on the new information gathered from the paleoceanographic, paleobathymetric and paleobiogeographic events recorded in the Atrato Basin, the timing of the former pre-middle and middle Miocene, and early Pliocene unconformities is more precisely defined and a new late Miocene unconformity is identified.

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Early-Middle Miocene Unconformity As described earlier, the stratigraphic boundary between the Uva and Napipi formations correlates with the Miocene hiatus NH 2 (Keller and Barron, 1983). One of the alternatives proposed to explain this hiatus was an initial uplift of the Panama sill. Lithostratigraphic and sedimentological evidence in NW Colombia, both in the Pacific and Caribbean coastal areas, may be associated with some kind of tectonic activity during this interval. In the Atrato Basin, volcanic activity related to that reported in southern Central America (Weyl, 1980) is indicated by volcanic sand interbeds at the top of the Uva Formation (Fig.2). In the adjacent San Jacinto and Sinu belts (Duque-Caro, 1984), and in the lower Magdalena Basin (Cf. Plato Geofracture, Duque-Caro, 1979), contrasting stratigraphic phenomena can be correlated with the timing of this paleoceanographic event: (a) in the Sinu Belt, the stratigraphic boundary between the upper Oligocene to lower Miocene pelagic and hemipelagic Aguas Vivas Formation (Fig.3), and the overlying middle Miocene turbidite strata of the Sinuian Series (Duque-Caro et al., 1987), and (b) the stratigraphic boundary between the upper Oligocene to uppermost lower Miocene hemipelagic strata of the Carmen Shale Formation, and the overlying middle Miocene clastic Rancho Formation (Fig,3; Duque-Caro et al., 1983, 1987) both in the San Jacinto Belt and lower Magdalena Basin areas. This boundary in the Sinu Belt marks the closing of mostly pelagic and hemipelagic low energy carbonate and silicic sedimentation and the beginning of high-energy clastic sedimentation during the initial phase of diapiric deformation and uplift of the Sinu Belt (early Sinuian Diapirism, Duque-Caro, 1984). In the San Jacinto Belt, this boundary also represents the closing of low-energy, hemipelagic sedimentation of the Carmen Formation in a deep environment, and the onset of a marine, high-energy, clastic episode of fluvial origin represented by the Rancho Formation also in a

I4. DUQUE-CARO

deep environment. This phenomenon correlates with the time when the Proto-Magdalena River formerly flowing into the Maracaibo Basin (Forero-Esguerra, 1974), due to some kind of tectonic instability diverged its course from the northeast to the northwest and began to shed its sediments into the Lower Magdalena Basin (Duque-Caro, 1979; Duque-Caro et al., 1983; Kolla et al., 1984). In summary, these data indicate that the early middle Miocene Unconformity, now correlated with the Neogene Hiatus NH 2 (16.1-15.1 Ma, Keller and Barron, 1983, 1986) was associated with major tectonic disturbances along the present Pacific and Caribbean coastal areas of NW South America.

Middle Miocene Unconformity The middle Miocene Unconformity coincides with the Miocene hiatus NH 3 (12.9-11.8 Ma, Keller and Barron, 1983, 1986). The disappearance of 21 benthic species (Table I) at the top of the Napipi Formation, accompanied by an abrupt paleobathymetric displacement in the range of 1000 m, and the absence of planktic Zone N.12, mark the middle Miocene Unconformity in the Atrato Basin. This feature, dated within the Zone N.11, was previously recognized in the Pacific and Caribbean coastal areas of NW South America (Duque-Caro, 1972, 1975), primarily based on the absence of the wholly carinated forms of the Globorotalia fohsi lineage group, mainly Zone N.12, i.e., in northern Peru (Weiss, 1955), Ecuador (Sigal, 1969), and northwestern Colombia (DuqueCaro, 1972). In other areas of the Caribbean, this unconformity has also been recognized, i.e., in Trinidad (Bolli, 1957; Stainforth, 1968; Blow, 1969). In fact, widespread middle Miocene tectonic disturbances in NW Colombia (Haffer, 1970; Van Houten, 1976; Duque-Caro, 1979, 1984, 1990) have been reported and coincide with the timing of this phenomenon. In the Atrato Basin area, this unconformity is marked by the contact between the low relief mudstone lithology of the Napipi Formation and the higher relief of the overlying Sierra

NEOGENE STRATIGRAPHY AND EVOLUTION OF PANAMA SEAWAY

Formation (Duque-Caro, 1990). This feature in the adjacent Sinu Belt marks the climax of diapirism and major deformation of this belt (Duque-Caro, 1984). This unconformity throughout the Circum-Pacific region appears to have major tectonic implications (cf. Ujiie, 1984; Duque-Caro, 1990).

Late Miocene Unconformity Tectonic disturbances at the end of Sierra deposition are also indicated by an abrupt paleobathymetric contrast between the upper bathyal depths of the Sierra Formation, and the shallower depths (less than 150 m), of the overlying Munguido Formation, as earlier discussed. This unconformity, not previously recorded in the NW corner of South America, and coinciding with the Neogene Hiatus NH 6 (7.0-6.3 Ma, Keller and Barron, 1983, 1986), marks contrasting formational boundaries in NW Colombia, i.e., between the late middle to late Miocene shaly Perdices and Cuesta formations, and the overlying latest Miocene to Pliocene Tubara Group, a thick terrigenous unit rich in organic remains. This major unconformity has also been recorded at the top of the Californian Monterey Formation, within the Miocene Hiatus NH 6, and between a cool water event below and warm water event above (Barron, 1986).

Early Pliocene Unconformity In the Atrato Basin area, the end of the latest Miocene to early Pliocene shallowing marine episode of the Munguido Formation, and replacement by a terrestrial environment is marked by an unconformity. This event, also coincident with the Neogene Hiatus NH 8 (3.7-3.1Ma, Keller and Barron, 1983, 1986) marks the base of terrestrial formations associated with the initiation of the major intermingling of terrestrial faunas between North and South America. In the San Jacinto Belt area, NW Colombia (Duque-Caro, 1984), this unconformity has also been recognized marking the contact of the marine Carmenian Series

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(Duque-Caro et al., 1987) and the overlying late Pliocene to Pleistocene terrestrial Sincelejian Stage.

Summary and conclusions The Atrato Basin is an inner borderland basin which exhibits stratigraphic and biostratigraphic characteristics similar to other Neogene Pacific coastal basins in southern Central America and northern South America. Three major stratigraphic sequences have been identified and include, (1) a late Oligocene to middle Miocene sequence composed of mainly pelagic and hemipelagic formations, deposited in open and deep oceanic conditions, and preceding effective development of the Atrato and Chucunaque basins, (2), a late middle Miocene to early Pliocene sequence, composed of hemipelagic and terrigenous sediments, deposited from middle bathyal to neritic depths, and developed in inner borderland basins. Contrasting benthic assemblages typical of the Californian Neogene biostratigraphy with no evolutionary relation with those of the early to middle Miocene of NW South America are distinctive, and lastly (3) a post-early Pliocene sequence, composed of mostly fluvial and lacustrine deposits not specifically discussed in this study. An evaluation of the data with emphasis on paleoceanography, paleobathymetry and paleobiogeography has indicated (Figs.7 and 8): (1) Prior to the middle Miocene, at about 16 Ma, well-aerated, deep, open oceanic conditions and free active water circulation prevailed along the steep continental margins of NW South America. (2) During the early middle Miocene Hiatus NH 2 (16.1-15.1 Ma), a change in both bottom water circulation and sedimentation occurred due to tectonic disturbances that triggered the initial uplift of the Panama sill. These changes, which are observed in both the Pacific and Caribbean coastal areas of NW South America, continued through most of the middle Miocene. Waters rich in organic nutrients and depleted in oxygen, a general warming of the

230

equatorial sea surface, a maximum sea level rise, and bathyal depths of about 2000 m. were distinctive. (3) During the middle Miocene, coinciding with the Miocene Hiatus NH 3 (12.9-11.8 Ma), an abrupt foraminiferal paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the Panama sill to about 1000 m, reflecting the middle Miocene tectonic disturbances in NW South America. (4) Immediately following this uplift in the late middle Miocene a distinctive benthic foraminiferal fauna with homogeneous coastal Pacific assemblages appeared and extended from Ecuador to California, following the Pacific coastal areas of NW South America and Central America. These Miocene to Pliocene assemblages are characteristic of the Californian borderland biostratigraphy and appear abruptly with no evolutionary relation with the early to middle Miocene faunas of NW South America. Because these assemblages do not occur in the Caribbean region I interpret: (a) that the prevous uplift which left the Panama isthmus partially emergent apparently developed a circulation barrier between the Pacific and Atlantic Oceans; and (b) that this was the result of intensification of the cool, marginal California Current, bringing benthic foraminifera to the south as far as the Golfo de Guayaquil. This phenomenon coincided with documented drops in sea level and cooling episodes associated with the growth of a major Antarctic ice cap. Middle to upper bathyal environments were anoxic under wellaerated cool surface waters in the Pacific coastal areas of NW South America. The resulting paleogeographic picture included a partially emergent Serrania de San Bias-Darien as a major island mass between nuclear Central America and the corner of NW South America. This situation gave rise, for the first time during the Cenozoic to a paleobiogeographic panorama favourable for the earliest intermingling of terrestrial faunas between North and South America, i.e., procyonids (raccoons and their allies), megalonychids and mylodontids (ground sloths).

H. DUQUE-CARO

(5) During the latest Miocene, coinciding with the Neogene Hiatus NH 6 (6.3-7.0 Ma) water surface circulation between the Atlantic and Pacific oceans was re-established and the influence of the cool California Current disappeared. This major paleoceanographic event coincided with the late Miocene Carbon Shift, the isolation of the Mediterranean Basin and the global increase of biologic productivity and the rate of supply of sediments. A rapid filling and shallowing to mostly neritic depths of the Pacific and Caribbean inner borderland basins of NW South America is indicated by the great thickness of terrigenous sediments, gradual impoverishment of microfaunal assemblages, and a progressive increase in similarity between benthic biotas. (6) Immediately following early Pliocene time, the P a n a m a n i a n isthmus became completely emergent providing a terrestrial environment favourable, to intermingling of terrestrial faunas and floras between North and South America. I correlate this Pliocene uplift with the Neogene Hiatus NH 8 (3.7-3.1 Ma). (7) Four regional unconformities, always associated with tectonic disturbances in the NW corner of South America, have also been recognized and correlated with some of the Neogene hiatuses recorded from the oceans: early-middle Miocene, middle Miocene, late Miocene, and early Pliocene. (8) The paleobiogeographic evolution interpreted in this study agrees with the existing data both on land and in the oceans. However, a new result has emerged: the appearance of the cool California Current in the Pacific coastal areas of NW South America, and the resulting disruption of the surface water circulation between the Atlantic and the Pacific oceans during the late middle to late Miocene. (9) All of these data from the Pacific and Caribbean coastal areas of the NW corner of South America supplement the corresponding paleoceanographic data recorded in the adjacent oceans. However, the existing correlation between the timing of tectonic disturbances recorded in the continental margins of NW South America and some of the ocean-wide

NEOGENE STRATIGRAPHYAND EVOLUTIONOF PANAMASEAWAY

Neogene hiatuses is significant. This has major implications for interpreting aspects of continental margin evolution, at least in the NW corner of South America: climatic changes, and ocean circulation as factors associated with sedimentation and tectonics. In the NW corner of South America, i.e., in the adjacent Sinu and San Jacinto belts tectonic disturbances (diapirism) appears associated with abrupt sedimentological changes from low energy (pelagites and hemipelagites) to high energy (turbidites; Duque-Caro, 1984). There, the corresponding hiatuses (cold pulses, Keller and Barron, 1983; Fig.8) are represented by stratigraphic planes separating these contrasting lithologies. These hiatuses, in this continental margin, appear associated with low sedimentation rates due to inactivity of terrigenous sources, widespread erosion, sea level drops, and intensification of the cool marginal currents. In contrast, the high-energy sedimentation periods, coinciding with both warm periods (Keller and Barron, 1983; Fig.8) and sea level rises (Vail and Hardenbol, 1979; Fig.8), result from great terrigenous influx and high sedimentation rates due to greater sediment transport and activity of terrigenous sources. Acknowledgements This paper is part of my Ph.D. thesis at the Department of Geological and Geophysical Sciences, Princeton University. In this matter, I want to express my sincere thanks to A. G. Fischer, F. B. Van Houten, G. Keller and N. Lundberg for their invaluable critics and suggestions during various stages of its completion. Similarly, my sincerest thanks go to J. P. Kennett for reading and helping to improve this particular paper. References Abbot, D. P., 1966. Factors influencing zoogeographic affinities of Galapagos inshore marine faunas. In: R. Bowman, (Editor), The Galapagos. Univ. California Press, Berkeley, Calif., pp. 108-122. Bandy, O. L., 1960. Planktonic foraminifera criteria for paleoclimatic zonation. Sci. Rep. Tohoku Univ., 2nd Ser. (Geol), Spec. V o l , 4:1 8.

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