Late Neogene planktonic foraminifera of the Cibao Valley (northern Dominican Republic): Biostratigraphy and paleoceanography
Introduction
The late Neogene history of the Caribbean region presents many questions of geologic and paleobiologic significance. Attempts to address issues concerning patterns of evolutionary stasis and change (Budd, 1990, Budd, 1991, Cheetham and Jackson, 1995, Nehm, 2005, Cheetham et al., 2007), paleoenvironments (Saunders et al., 1982, Saunders et al., 1986, Vokes, 1989, McNeill et al., 2008), changes in biodiversity (Cheetham et al., 1999, Budd, 2000, Klaus and Budd, 2003) and biogeography (Budd, 1989), and their relationship to regional geologic events have produced numerous studies of the sedimentary deposits in the Cibao Valley (northern Dominican Republic) (Fig. 1). The Neogene deposits of the Cibao Valley are comprised of a surprisingly continuous northward-prograding wedge of biogenic carbonates and siliciclastics shed from the nearby Cordillera Central (Mann et al., 1991, McNeill et al., 2008). Several rivers extending from the Rio Yaque Del Norte cross-cut the valley, exposing these sediments. Outcrops in the Rio Gurabo and Rio Cana are the main sequences examined herein. The primary formations that make up these deposits are the Cercado (oldest), Gurabo, and Mao (youngest) Formations (Fig. 2). The Cercado Fm. (158 m thick in the Rio Gurabo and 276 m thick in the Rio Cana) is made up of conglomerates and sands with reef limestones deposited in a nearshore, lagoon-to-reef setting. The Gurabo Fm. (423 m thick in the Rio Gurabo and 299 m thick in the Rio Cana) is a package of reef limestones and massive calcareous siltstones that exhibits a deepening upward sequence from a shallow reef to an outer shelf environment (McNeill et al., 2008). The Mao Fm. (339 m thick in the Rio Gurabo and 615 m thick in the Rio Cana) consists of a basal siltstone overlain by interbedded siltstones and limestones that represents a shallowing upward sequence from an outer shelf to a backreef setting (Saunders et al., 1982, Saunders et al., 1986).
The well-preserved, richly fossiliferous strata of the Cibao Valley provide new information concerning the geologic history of the northern Caribbean, serving as useful tools in understanding how geologic events have shaped depositional environments (Saunders et al., 1982, Saunders et al., 1986, McNeill et al., 2008) and marine community distributions (Klaus and Budd, 2003). In order to properly constrain the timing of such changes, high resolution chronostratigraphy of these deposits is necessary; however, the ages of the Cercado, Gurabo, and Mao Formations have been a matter of debate for more than a century. This study utilizes planktonic foraminifera from a sampling of sediments taken from each formation to provide a higher resolution biochronologic model than has been previously achieved. When possible, samples were taken at ∼ 5 m intervals from outcrops in the Rio Gurabo, Rio Cana, and Arroyo Bellaco. Some large sampling gaps are present, as the lack of accessible outcrops prevents sampling of some portions of the sections. The planktonic foraminiferal data given are part of an ongoing examination of these sample localities, which includes paleomagnetics (McNeill et al., 2008), Strontium isotope dating (Maier et al., 2007), Uranium/Lead dating (Denniston et al., 2008), corals, molluscs, δ13C data, and benthic foraminifers. This multi-faceted investigation promises to enhance understanding of the northern Caribbean region's complex geologic, oceanographic, and paleobiologic history (Nehm and Budd, 2008).
The closure of the Central American Seaway (CAS) in the early Pliocene had significant oceanographic and biologic implications (Keigwin, 1982a, Keigwin, 1982b, Oberhansli and Hemleben, 1984, Keller et al., 1989, Cronin and Dowsett, 1996, Bornmalm, 1997, Haug and Tiedemann, 1998, Bornmalm et al., 1999, Spezzaferri et al., 2002, Coates et al., 2003, Coates et al., 2004, Lear et al., 2003, Steph, 2005, Schneider and Schmittner, 2006, Jain and Collins, 2007; and others). Prior to closure, the CAS served as a conduit for bidirectional transfer between the Pacific and Caribbean (Keller et al., 1989) (Fig. 3a). With the development of a sill in the CAS during the middle Miocene (Mullins andMullins and Neumann, 1979, Keller and Barron, 1983, McDougall, 1985), Pacific and Caribbean water masses began to differentiate. This separation generated salinity variations (Keigwin, 1982a, Keigwin, 1982b, Oberhansli and Hemleben, 1984, Keller et al., 1989, Bornmalm, 1997, Haug et al., 2001), ocean current reorganization (Cronin and Dowsett, 1996, Haug and Tiedemann, 1998), changes in productivity (Keigwin, 1982a, Keigwin, 1982b, Spezzaferri et al., 2002, Schneider and Schmittner, 2006, Jain and Collins, 2007) and ventilation (Wright et al., 1991, Billups, 2002), evolutionary divergence (Coates et al., 1992, Collins et al., 1996a, Collins et al., 1996b), and significant changes in global climate (Berggren, 1972, Berggren and Hollister, 1974, Rind and Candler, 1991) that are suggested as early as 12 Ma (Duque-Caro, 1990). The final closure of the CAS between 4.2 and 3.5 Ma (Keigwin, 1982a, Keigwin, 1982b, Haug and Tiedemann, 1998, Jain and Collins, 2007) also intensified the Gulf Stream by deflecting westward-moving waters to the northeast, which directed warm, saline waters into higher latitudes (Shackleton and Kennett, 1975, Shackleton and Opdyke, 1977, Keigwin, 1978, Shackleton and Hall, 1984, Keller et al., 1989, Raymo et al., 1989, Cronin and Dowsett, 1996) (Fig. 3b). Changes in marine community distributions within the Cibao Valley likely reflect both of these oceanographic changes, as well as local influences. Faunal changes may also provide insight into the scope and timing of the uplift of Central America and the resulting loss of communication between the Pacific and Caribbean.
Section snippets
Materials and methods
Sedimentary samples were collected by McNeill, Klaus, Budd, and Corbi in 1996, 2005, 2006, and 2007 and by Ishman and Lutz in January of 2007. These samples were taken from outcrops of the Yaque Group in the Rio Gurabo, Rio Cana, and Arroyo Bellaco. The samples were soaked for two weeks in deionized water, washed at ≥ 63μm size fraction, and dried in an oven. Of the 125 samples collected, 89 yielded sufficient planktonic foraminifers for biostratigraphy. A minimum of 100 specimens were obtained
Biostratigraphy and biochronology
Concurrent range data of planktonic foraminifers based on regional first and last appearances were used to generate an age model (Fig. 2, Fig. 4) for the three primary formations of the Yaque Group. A summary of biostratigraphic events can also be found inTable 1. The Cercado Fm. is of late Miocene age (Zone N17 and Zone N18, ∼ 7.0–5.9 Ma). The Gurabo Fm. ranges from Zone N18 to mid-Zone N19 (∼ 5.9–4.5 Ma). The Mao Fm. is early/middle Pliocene in age (upper Zone N19, ∼ 4.5–3.6 Ma). Assemblages
Conclusions
The planktonic foraminiferal assemblage described herein gives insight into the late Neogene geochronology of both the Cibao Valley and the Caribbean region. The three primary formations of the Yaque Group have been biochronologically dated using planktonic foraminifera, providing enhanced age restriction of the Cercado (7.0?–5.9 Ma), Gurabo (5.9–4.5 Ma), and Mao (4.5–3.a) Formations (Fig. 3). A set of plaeoceanographic events has been suggested with timing that correlates to known geologic
Acknowledgements
Funding for this research was provided to McNeill and Budd through NSF Grant EAR-0446768. Thanks are extended to Harry Dowsett for assisting with identification and to Brian Huber for allowing the use of the Smithsonian's Cushman Collection to compare specimens. This manuscript benefited greatly from constructive comments by Ellen Thomas, Harry Dowsett, and an anonymous reviewer.
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