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Middle Miocene closure of the Central American Seaway

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Science  10 Apr 2015:
Vol. 348, Issue 6231, pp. 226-229
DOI: 10.1126/science.aaa2815

Early closing between oceans

The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10 million years earlier than is believed. Montes et al. report that certain minerals of Panamanian provenance began to appear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marine basins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange between the Americas need to be reconsidered.

Science, this issue p. 226; see also p. 186

Abstract

Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.

Closure of the Central American Seaway, defined here as the deep oceanic seaway along the tectonic boundary of the South American plate and the Panama arc, is thought to have modified the salinity of the Caribbean Sea, ultimately affecting ocean circulation patterns and global climate (1), as well as to have triggered the Great American Biotic Interchange (2). However, the role of the formation of the Panamanian Isthmus in such global changes remains controversial, in part because of the difficulty of establishing a precise chronology of seaway closure (3). Data on the chronology of Isthmus emergence suggests that the closure not only occurred earlier than previously thought (4) but also may have resulted from factors other than the emergence of currently high terrain in Panama (5, 6).

The Uramita suture (7) separates the young Panama arc to the west from the old Andean terranes to the east (Fig. 1). These are mutually exclusive geochronological domains that are ideally suited for documenting the time of detrital exchange. The young Panama magmatic arc was built on an oceanic plateau substrate (8) during latest Cretaceous to Eocene times [67 to 39 million years ago (Ma), with a peak around 50 Ma] (6, 9), with renewed magmatic activity as young as 19 Ma (10) east of the Canal Basin and 10 Ma and younger west of it (11). To further characterize the Panama magmatic arc fingerprint, we dated a string of incompletely mapped granitic plutons along the northeastern coast of Panama and western Colombia (Fig. 1C), obtaining eight U/Pb magmatic zircon ages ranging between 59 and 42 Ma (217 U/Pb analyses; table S1). The northern Andes, in contrast, include magmatic rocks accreted during latest Cretaceous times (8) to a core of plutonometamorphic rock of late Precambrian (1214) and Permo-Triassic age (14, 15), and plutonic rocks of Jurassic to Cretaceous age (16). Middle Eocene magmatism is absent in the northern Andes (1719). Therefore, detrital zircons of Eocene age can be used to track the detrital contribution of the Panama arc to sedimentary basins of northwestern South America.

Fig. 1 Tectonic setting of the study area and location of samples.

See Table 1. Thick lines represent major boundaries (28). Zero-milligal (0 mgal) contour (29) highlights the geodynamic continuity of the Panama arc; there are no structural breaks between the Uramita suture and the Canal Basin. (A and B) Detrital zircon ages recovered from (A) lower Miocene strata in the Canal Basin (6) and (B) Oligocene-Miocene strata in the Nuevo Mundo Syncline (18) and rivers draining the Eastern and Central cordilleras (19). (C) New U/Pb zircon ages for granitoids of the Panama arc; data point error ellipses are 68.3% confidence (see table S1).

To track detrital contributions from the Panama arc, we sampled fluvial strata at the western flank of the Central Cordillera of Colombia (site SA in Fig. 1), following a roughly northeastern trend toward shallow marine strata (sites SMP and BH1 to BH8) in the Lower Magdalena Basin. Seven stratigraphic levels of middle Miocene age and 11 stratigraphic levels of Oligocene to early Miocene age were sampled in eight boreholes and two surface stratigraphic sections (Table 1). We obtained 18 U/Pb detrital zircon ages, as well as petrographic and heavy mineral analyses of the sedimentary rock (1654 U/Pb analyses; tables S2 to S5). All detrital zircon samples recovered from Oligocene and Miocene strata contained typical north Andean detrital signatures that included late Precambrian, Permo-Triassic, and Late Cretaceous populations. Middle Miocene strata, however, contained an additional Eocene magmatic zircon population that was absent in older strata (Fig. 2).

Table 1 Sample summary.

See fig. S1 for stratigraphic location of samples.

View this table:
Fig. 2 Detrital U/Pb zircon populations for all the samples recovered.

(A) Middle Miocene strata; (B) Lower Miocene and Oligocene strata of the northern Andes. Data are binned insections of 10 million years (see tables S2 and S3). The youngest detrital zircon is labeled on top of the leftmost bin. See Table 1 and Fig. 1 for sample location.

To establish the age of strata sampled in the Lower Magdalena Basin, we used published foraminiferal and palynological studies performed on the same boreholes where we sampled, further bracketed by our detrital zircon minimal ages (Fig. 2). In the western flank of the Central Cordillera, we relied on mapped cross-cutting relationships (20) of volcanic and subvolcanic rocks interbedded with and intruding fluvial, coal-bearing strata. Available geochronology (21) and palynological (22) studies along with our detrital zircon minimal ages were used to establish ages of the strata sampled (fig. S3).

Because the geochronological makeup of the northern Andes is incompletely known, we used published U/Pb detrital zircon data as proxies for the magmatic age distribution of the eastern Panama arc and the northern Andes. We used the Oligocene-Miocene strata of the Canal Basin as a proxy for the Panama arc (Fig. 1A) (6). For the northern Andes (Fig. 1B), we used two data sets as proxies of its geochronological makeup: Oligocene-Miocene strata of the Nuevo Mundo Syncline (18) and active-sediment river samples draining the Eastern and Central Cordilleras (19). We found that detrital zircons from basins and rivers in the northern Andes are decidedly older than those from Panama; the mean for Nuevo Mundo Syncline and active-sediment river samples is 304.1 Ma, whereas the mean for Panama is 49.7 Ma (Kolmogorov-Smirnov test, P < 0.001, D = 0.98; fig. S2, A and B). Nuevo Mundo Syncline and active-sediment river samples have an age range of 51.2 to 2675.4 Ma, with only 16 ages between 51.2 and 63.1 Ma (mean 56 Ma). Panama ages range from 17.6 to 65.1 Ma (fig. S2B).

Oligocene to middle Miocene strata sampled in the northern Andes can be separated into two age groups according to their detrital zircon populations: one containing an Eocene population and another missing it (Fig. 2). The Oligocene–early Miocene strata show an age range of 54 Ma to 3103.6 Ma. Only two of 1045 zircons have ages younger than 65.1 Ma (54 and 64 Ma). In contrast, middle Miocene sandy strata in the same sampling sites (Fig. 2A) show an age range from 13.1 Ma to 3189.9 Ma. A large number of them (103 of 609) have ages younger than 65.1 Ma, with a mean age of 36.8 Ma, slightly younger than the mean age of Panamanian detrital zircons (mean Panama = 49.7 Ma, Kolmogorov-Smirnov test, P < 0.001, D = 0.53; fig. S2).

The Eocene detrital zircon population documented in middle Miocene strata of northwestern South America (Fig. 2A) could have only been derived from the emerged Panama arc, as there are no igneous bodies of that age in the northern Andes (18, 19). The magmatic roots of the Panama arc had been cooling (5, 6), emerging, and eroding (23) since at least late Eocene times (6); therefore, they were available as source areas by middle Miocene times. Both fluvial coal-bearing strata (20, 24) and shallow marine strata of middle Miocene age contain the Panama arc signature (Fig. 2). This signature—in fluvial strata to the south and in shallow marine strata to the north—suggests that the Panama arc had docked and emerged and was shedding detrital material to north-bound currents parallel to the Uramita Suture, similar to today’s Cauca River (Fig. 1), and to northeast-bound coastal currents (Fig. 3).

Fig. 3 Paleogeographic reconstruction (28) of the Panama arc and northwestern South America during middle Miocene times (13 to 15 Ma).

The first detrital loads from Panama arrived in one of two, or both, paths to the basins of northwestern South America: (i) along coastal currents transporting detritus product of the erosion of exposed plutonic rocks along the northern coast of the Panama arc and/or (ii) along fluvial channels draining emerging ranges parallel to the length of the Isthmus. The El Valle volcano, an edifice rising from sea level starting before 10 Ma (11), would only allow shallow and transient seaways between 15 and 10 Ma, as the Canal basin was connected to North America by a land bridge from Oligocene until middle Miocene times (30).

Our results imply that by middle Miocene times (13 to 15 Ma), rivers originating in the Panama arc were transporting sediment to the shallow marine basins of northern South America. This implies that (i) at least a segment of the Panama arc, including an emerged (6) Mande batholith and San Blas Range (Figs. 1 and 3), had already docked and (ii) the Central American Seaway was closed. Continued Caribbean-Pacific water exchange may have taken place along narrow, shallow, and transient channels that fragmented (5) the Isthmus west of the Canal Basin (4) (Fig. 3). These results support recent paleoceanographic studies (25, 26) that show a decrease in the transport of deep and intermediate Pacific waters into the Caribbean by 10 to 11 Ma, probably related to a closing Central American Seaway. Recorded changes in Caribbean water salinity at ~4.2 Ma (1), and a delay of nearly 10 Ma in the Great American Biotic Interchange (2) after the first detrital loads crossed the Isthmus, could be unrelated to seaway closure and instead may be linked to Plio-Pleistocene global climatic transitions (3, 27).

Supplementary Materials

www.sciencemag.org/content/348/6231/226/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S3

Tables S1 to S5

References (3142)

References and Notes

  1. Acknowledgments: Supported by Ecopetrol-ICP “Cronología de la Deformación en las Cuencas Subandinas,” Smithsonian Institution, Uniandes P12. 160422.002/001, Autoridad del Canal de Panama (ACP), the Mark Tupper Fellowship, Ricardo Perez S.A.; NSF grant EAR 0824299 and OISE, EAR, DRL 0966884, Colciencias, and the National Geographic Society. We thank N. Hoyos, D. Villagomez, A. O’Dea, C. Bustamante, O. Montenegro, and C. Ojeda. All the data reported in this manuscript are presented in the main paper and in the supplementary materials.
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