Late Cretaceous calcareous nannofossil assemblages from Colombia: Biostratigraphic contributions to northwestern South American Basins

Journal of South American Earth Sciences 127 (2023) 104315 Available online 18 April 2023 0895-9811/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Late Cretaceous calcareous nannofossil assemblages from Colombia: Biostratigraphic contributions to northwestern South American Basins Estefanía Angulo-Pardo a , * , Felipe Vallejo-Hincapi´ e a , b , Rodrigo Do Monte Guerra c , Andr´ es Pardo-Trujillo a , Carlos A. Giraldo-Villegas a , d , Jenny García Gonz´ alez e , Sebastian Hern´ andez Duran f , Sergio Herrera Quijano f , Angelo Plata Torres a , b , Raúl Trejos-Tamayo a , b a Instituto de Investigaciones en Estratigrafía (IIES), Grupo de Investigaciones en Estratigrafía y Vulcanología (GIEV-Cumanday) y Departamento de Ciencias Geol´ ogicas de La Universidad de Caldas, Calle 65 # 26-10, Manizales, 1700004, Colombia b Departamento de Geología, Facultad de Ciencias, Universidad de Salamanca, Plaza de Los Caídos, S/n, Salamanca, 37008, Spain c Technological Institute for Paleoceanography and Climate Change (itt Oceaneon), UNISINOS University, S˜ ao Leopoldo, 93.022–750, Brazil d Departamento de Estratigrafía y Paleontología, Universidad de Granada, Avenida Fuente Nueva S/n, Granada, 18071, Spain e Fundaci´ on Universitaria Del ´ Area Andina, Transv 22 Bis #4-105, Valledupar, 200001, Colombia f Departamento de Geociencias, Universidad Nacional de Colombia, Carrera 45 # 26-85, Bogot´ a, 111321, Colombia ARTICLE INFO Keywords: Micropaleontology Caribbean nannofossils La luna sea Eastern equatorial Pacific nannofossils ABSTRACT The Upper Cretaceous deposits of northwestern Colombia accumulated in two regions with distinct tectonic settings. The eastern deposits, consisting of Turonian–Maastrichtian rocks from the Upper Magdalena Valley (UMV) and the Cesar-Rancheria basins, were deposited by an epicontinental sea that partially covered the South American Plate. In contrast, the western deposits, which comprise a series of highly faulted and folded Coniacian–Maastrichtian deposits in the Sinú-San Jacinto Folded Belt (SSJFB), Gorgonilla Island, and Western Cordillera, were influenced by a seaway connecting the eastern Pacific Ocean with the proto-Caribbean Sea and were deposited near the collision zone between the Caribbean and South American Plates. We conducted a biostratigraphic analysis of 119 rock samples from these deposits. Although some well-preserved microfossils were found in the Cesar-Ranchería Basin, most samples exhibited poor to moderate preservation of nannofossils. Biostratigraphic markers identified in the Upper Magdalena Valley Basin were Quadrum gartneri, Micula concava, Micula staurophora, Lithastrinus septenarius, Lithastrinus grillii, Arkhangelskiella cymbiformis, Uniplanarius trifidus, Uniplanarius sissinghii, and Reinhardtites anthophorus. In the Cesar-Rancheria Basin, the markers identified were Arkhangelskiella cymbiformis, Lithraphidites cf. L. praequadratus, and an acme of Kamptnerius magnificus. Based on these taxa, the eastern stratigraphic sections accumulated sometime between UC7 (CC11) and UC20 (CC26) biozones, which is equivalent to an age range of early Turonian to upper Maastrichtian. Key biostratigraphic taxa from the western outcrops were more limited, yielding only Uniplanarius trifidus and Uniplanarius sissinghii, which are indicative of biozones UC15d–UC17 (CC22–CC23). This signifies a sedimentation age sometime from late Campanian to early Maastrichtian. Our results correlate well with previous age models and reveal that undistinguished upper Campanian–lower Maastrichtian deposits of the collision zone can be correlated with the last marine deposits of the epicontinental sea in the UMV. Although calcareous nannofossils from the Cesar- Rancheria Basin displayed the best preservation, low-latitude biostratigraphic markers were absent in this locality, making regional correlations challenging. We hypothesize that these deposits formed during the Maastrichtian, but changes in oceanic water conditions of the proto-Caribbean Sea affected productivity and preservation of biostratigraphic markers. * Corresponding author. E-mail address: [email protected] (E. Angulo-Pardo). Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames https://doi.org/10.1016/j.jsames.2023.104315 Received 16 November 2022; Received in revised form 15 March 2023; Accepted 20 March 2023 Journal of South American Earth Sciences 127 (2023) 104315 2 1. Introduction Upper Cretaceous sedimentary deposits from northwestern South America accumulated in contrasting paleoenvironmental and tectonic settings, which can be separated into eastern and western regions (Fig. 1A and B). Marine deposits from the eastern region, including the Upper and Middle Magdalena Valley and Cesar-Rancheria basins, were deposited under shallow marine conditions of an epicontinental sea known as the La Luna Sea (Villamil and Arango, 1998; Villamil, 1998; Erlich et al., 2000; P´ aez-Reyes et al., 2021). In contrast, the western deposits exposed as deformed belts in the Sinú-San Jacinto Folded Belt (SSJFB), Tumaco and Choc´ o Basin, and Western Cordillera accumulated in deeper marine environments associated with a seaway between the proto-Caribbean Sea and the eastern equatorial Pacific Ocean (Fig. 1B) (Duque-Caro, 1972a, 1990; Mann, 1999; Iturralde-Vinent, 2005; Kerr and Tarney, 2005; Villag´ omez et al., 2011; Spikings et al., 2015; Buchs Fig. 1. Geographic location of the western and eastern Upper Cretaceous deposits studied in this research, as well as previous studies related to calcareous nannofossils. A) Geographic location of the western and eastern deposits studied here and previous studies concerning calcareous nannofossils. B) Paleogeographic reconstruction of northwestern South America during the Late Cretaceous, adapted from Pardo-Trujillo et al. (2020), illustrating the tectonic and paleoenvironmental settings of the different deposits. E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 3 et al., 2018; Pardo-Trujillo et al., 2020; Giraldo-Villegas et al., 2023). Our review of the published literature found that Upper Cretaceous deposits from the eastern region yield more detailed chronostratigraphic and lithostratigraphic information than oceanic rocks from the western region (Appendix A) (Bürgl and Dumit, 1954; Gandolfi, 1955; Etayo-- Serna et al., 1982; Jaramillo and Yepes, 1994; Guzm´ an et al., 1994, 2004; Guerrero et al., 2000; Tchegliakova and Mojica, 2001; Yepes, 2001; Martínez, 2003; Patarroyo, 2011; Patarroyo et al., 2017; 2017, 2022; P´ erez et al., 2018; P´ aez-Reyes et al., 2021). Our review also revealed that little is known about the biostratigraphy of calcareous nannofossils, and that nannofossil assemblages were particularly abundant and diverse in deposits formed near the Cenomanian/Turonian boundary (P´ aez-Reyes et al., 2021) and generally poorly preserved in rocks from the Turonian to the Maastrichtian (De Romero et al., 2003; P´ erez et al., 2018; Barrantes et al., 2019; Pati˜ no et al., 2019; Pardo-- Trujillo et al., 2020). Calcareous nannofossils are exclusively marine and planktonic microfossils, which are found in oceanic deposits as old as the Upper Triassic (Bown and Young, 1998). Their wide distribution, high morphological diversity, and high abundance, as well as their calibration in well-dated stratigraphic sections, make them a valuable biostratigraphic proxy for dating and correlating marine deposits on a latitudinal scale (Odin, 2001; Odin and Lamaurelle, 2001; Lamolda et al., 2014; Walaszczyk et al., 2021). Well-documented biogeographic patterns of these microfossils reveal variations between high (>50◦N and >40◦S) and low latitudes (between 45◦N and 10◦S) during the Late Cretaceous (Thierstein, 1981; Concheyro, 1995; Watkins et al., 1996; Burnett, 1998; Lees, 2002; Moore, 2016). As a result, different biozonations have been constructed for Upper Cretaceous deposits around the globe; those of Sissingh (1977, 1978), modified by Perch-Nielsen (1985), and Burnett (1998) are the most commonly used for biostratigraphic studies in low-latitude areas. This study aims to use well-calibrated biostratigraphic markers to date and correlate Upper Cretaceous deposits from the eastern and western regions of Colombia, building a chronostratigraphic framework based on nannofossils (Roth, 1978; Bralower et al., 1995). 2. Geological and paleoceanographic context During the Late Cretaceous, in the eastern region, thick, organic-rich mudrock deposits of the epicontinental La Luna Sea accumulated on the South American Plate between Colombia and Venezuela, forming some of the best hydrocarbon source-rocks from northern South America (Talukdar and Marcano, 1994; Villamil et al., 1999; Mann et al., 2006) (Fig. 1B). The La Luna Sea experienced maximum flooding between the Turonian and Coniacian and subsequently began retreating during the Campanian–Maastrichtian (Villamil and Arango, 1998; Villamil, 1998; Erlich et al., 2000). The sea’s retreat was due to tectonic adjustments as the Caribbean Plate collided with the western margin of the South American Plate and to global sea-level fall from the late Campanian to Maastrichtian (G´ omez et al., 2003; Villag´ omez et al., 2011; Villag´ omez and Spikings, 2013; Haq, 2014; Bayona, 2018; Pardo-Trujillo et al., 2020). According to micropaleontological evidence, oceanic conditions of the La Luna Sea were similar to those described for tropical and subtropical latitudes of the Tethyan Ocean during most of the Late Cretaceous (Petters, 1955; Martínez, 1989; Jaramillo and Yepes, 1994; Yepes, 2001; Due˜ nas and G´ omez, 2013). These conditions were characterized by reduced oxygenation that caused anoxic events near the Cenomanian/Turonian boundary and the Coniacian (Martínez, 2003; P´ erez et al., 2018; P´ aez-Reyes et al., 2021). The La Luna Sea also recorded conditions of upwelling from the Santonian to Maastrichtian (Fabre, 1985; F¨ ollmi et al., 1992; Etayo Serna, 1994; Villamil and Arango, 1998; Villamil, 1998; Erlich et al., 2000; Sarmiento, 2018), coinciding with a global cooling episode in low latitudes (Barrera and Savin, 1999; Linnert et al., 2014; O’Brien et al., 2017) and a change in marine productivity in northern oceanic basins of South America (Yepes, 2001; Martínez, 2003; Patarroyo et al., 2022). The Late Cretaceous history of the western region is less documented; however, some previous works have characterized turbiditic and hemipelagic deposits accumulated in deep marine environments during the Campanian–Maastrichtian (Duque-Caro, 1972a, 1972b, 1978, 1979, 1984; Duque-Caro and Due˜ nas, 1987; Clavijo and Barrera, 2001; Guzm´ an, 2007; Pardo-Trujillo et al., 2020; Giraldo-Villegas et al., 2023). These deposits were deposited over the allochthonous rocks of the Caribbean Plate, which began to block the marine connection between the Pacific Ocean and the Caribbean Sea as it was colliding with the South American Plate during the Late Cretaceous–early Paleogene (Duque-Caro, 1972a, 1990; Mann, 1999; Moreno-S´ anchez and Pardo-Trujillo, 2003; Guzm´ an et al., 2004; Iturralde-Vinent, 2005; Guzm´ an, 2007; Mora et al., 2017; Buchs et al., 2018; Pardo-Trujillo et al., 2020; Giraldo-Villegas et al., 2023) (Fig. 1B). In the western region, marine sedimentation was largely influenced by volcanic activity from volcanic arcs built on the Caribbean and South American Plates and by erosion of the South American continental margin (Weber et al., 2015; Buchs et al., 2018; Pardo-Trujillo et al., 2020; Zapata-Villada et al., 2017, 2021; Botero-García et al., 2023) (Fig. 1B). In addition, marine deposits from the Gorgonilla Island in the Pacific Ocean show that the Chicxulub bolide impact perturbated sedimentation during the end of the Cretaceous (Bermúdez et al., 2016; Renne et al., 2018). Although the deformation associated with the collision of the Caribbean Plate against the South American margin is still a matter of study, it is thought that complex tectonic interactions caused obduction of a suite of igneous and marine deposits, which are currently exposed along with the western Colombian margin (Western Cordillera, and Gorgonilla Island) and partially buried by Cenozoic sediments in the SSJFB (Duque-Caro, 1972a; Nivia, 1996; Villag´ omez et al., 2011; Echeverri et al., 2015; Pati˜ no et al., 2019). 3. Lithostratigraphy 3.1. Eastern region In the eastern region, we extracted calcareous nannofossils from deposits outcropping in the Upper Magdalena Valley (UMV) and Cesar- Rancheria basins. The Aico Creek in the UMV section encompasses the Loma Gorda Formation, Oliní Group (which includes Lidita Inferior, Aico Shale, and Lidita Superior formations), and Buscavida Formation (Hern´ andez, 2021) (Figs. 2 and 3). The Loma Gorda Formation consists of ~23 m of laminated limestones (mudstones, wackestones, and packstones) interlayered with sporadic phosphate deposits (Fig. 2A–I) associated with low-energy marine environments below the storm-wave base (Hern´ andez, 2021). These rocks are overlain by the Lidita Inferior Formation (Fig. 2A–II), which is mainly composed of ~49 m of interlayered laminated limestones (wackestones) with cherts. This formation also exhibits laminated mudrocks and phosphatic beds accumulated under low-energy marine environments below the fair-weather wave base. Above this lies a ~23-m-thick deposit informally named the Aico Shale formation (Hern´ andez, 2021), which is stratigraphically equivalent to the El Cobre Formation recognized by Garzon et al. (2012) in neighboring outcrops of the Aico Creek section. This unit consists of laminated claystones deposited under offshore conditions. Overlying these rocks are marine rocks of the Lidita Superior Formation, the last formation of the Oliní Group (Fig. 2A–III). These deposits are characterized by alternating laminated cherts and limestones (mudstones and wackestones) deposited in offshore marine environments. In the upper part of the stratigraphic section, sedimentary rocks of the Buscavida Formation (Fig. 2A–IV) measure up to ~81 m-thick (Hern´ andez, 2021). This formation consists of laminated limestones (wackestones, packstones), mudrocks, sandy siltstones, and fine to very fine-grained sandstones with horizontal and flaser laminations accumulated in offshore and upper shoreface settings. The ANH-LA LOMA-1 core-stratigraphic section (Fig. 2B), in the E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 4 Cesar-Rancheria Basin, consists in ~626-m-thick deposits of the Molino and Barco formations (Ucaldas-Minciencias-ANH, 2020). In the lower to middle part of the core (~626–~265 m), marine deposits of the Molino Formation comprise an interlayering of massive and laminated mudrocks, claystones, limestones (mudstones, wackestones, and packstones), and shales with bivalves, brachiopods, gastropods, and echinoderms fossils, which were interpreted as outer shelf deposits (Ucaldas-Minciencias-ANH, 2020). The Molino Formation is also found in the middle to upper part of the core (~266–~117 m), composed of very fine-to medium-grained, locally conglomeratic sandstones, which occasionally show horizontal lamination and heterolytic beds with wavy and lenticular lamination. These deposits are associated with external shelf-offshore and coastal plain deposits (Ucaldas-Minciencias-ANH, 2020). In the upper part of the core, above the Molino Formation, lie ~117 m of Barco Formation deposits (Figs. 2 and 3). They are characterized by interlayering of mudrocks and sandstones forming wavy and flaser lamination. Root traces and carbonized-woody fragments are abundant, and massive coal levels interbedded with carbonaceous massive and laminated mudrocks are locally recorded. This interval is characterized by lack of marine microfossils, abundant pollen and Fig. 2. Photographic record of the studied lithostratigraphic units studied in both the eastern and western regions. In the Eastern region, the Aico Creek section (A) displays four different intervals corresponding to laminated limestone of the Loma Gorda Fm. (I), laminated limestone and cherts of the Lidita Inferior Fm. (II), an alternation between laminated cherts and limestones of the Lidita Superior Fm. (III), and sandy siltstones, and fine to very fine-grained sandstones of the Buscavida Fm. (IV). The ANH-LA LOMA-1 cored-stratigraphic section of the Molino Fm. (B) Displays intervals of mudrocks and shales at the base and sandy layers at the top. In the Western region, the San Carlos Quarry section (C) displays intercalation of mudrocks and limestones of the Cansona Fm. The Western Cordillera section (D) shows folded beds of limestones, siliceous limestones, and mudrocks of the Nutibara Mb. Of the Penderisco Fm. In the Uramita-Dabeiba area. The Gorgonilla Island section (E) exhibits tuffaceous sandstones interbedded with mudrocks. The core in ANH-LA LOMA-1 section has depth numbers indicated. E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 5 spores, and scores of dinoflagellates. This segment has been interpreted as deltaic deposits (Ucaldas-Minciencias-ANH, 2020). 3.2. Western region In the western region, we extracted calcareous nannofossils from moderately deformed deposits outcropping in the SSJFB and Isla Gorgonilla in the Tumaco Offshore Basin, as well as from highly deformed and segmented rocks of the Western Cordillera (Fig. 2C–E and 3). The stratigraphic section of the San Carlos Quarry in the SSJFB is composed of ~20 m of medium to thin bioturbated tabular strata of laminated mudrocks interbedded with massive limestones (mudstones); occasionally these strata are interlayered with siliceous mudrocks (Fig. 2C). These rocks, belonging to the Cansona Formation (Fig. 2C), are some of the oldest sedimentary deposits documented in the SSJFB (Duque-Caro, 1972a,b; Geotec, 2003). The analyzed samples from the highly deformed deposits from the Western Cordillera were collected from the best exposures along the Uramita-Dabeiba, El Sireno, and Santa Cecilia roads (Fig. 2D). These strongly folded and faulted outcrops are composed of an alternation of thin tabular strata of limestones and cherts, which are interlayered with greenish bioturbated mudrocks. These rocks belong to the Nutibara Member of the Penderisco Formation (´ Alvarez and Gonz´ ales, 1978). Owing to sedimentological and ichnological features, these rocks, together with those of the Cansona Formation, are interpreted as pelagic and hemipelagic deposits accumulated in deep marine environments (Pardo-Trujillo et al., 2020; Giraldo-Villegas et al., 2023). The Gorgonilla section consists of an alternation of laminated mudrocks and sandstones (locally conglomeratic) bearing fossils of radiolaria, benthic foraminifera, mollusks, and corals (Fig. 2E). Bermúdez et al. (2016, 2019) described these deposits as an intercalation of tuffaceous sandstones, marls, siltstones, and massive gray-yellow tuffaceous clays with soft-sedimentary deformation structures and tektites, which represent turbidites accumulated in bathyal and pelagic environments. These rocks are informally known as Sedimentos de Gorgonilla (Bermúdez et al., 2019). 4. Previous biostratigraphic studies The age assigned to the studied deposits has been mainly based on macrofossils and microfossils such as ammonites, bivalves, foraminifera, palynomorphs, and calcareous nannofossils (Fig. 3 and Appendix A). According to our review, biostratigraphic information for marine deposits from the eastern region is more extensive than for rocks from the western region, which can be explained by higher oil exploration activity in the Magdalena Valley and Cesar-Rancheria basins (Fig. 3). In the eastern region, chronostratigraphy of marine deposits, initially based on well-preserved ammonites and bivalves, displayed an age range from Turonian to Santonian (Bürgl and Dumit, 1954; Bürgl, 1961; Etayo et al., 1969; Etayo-Serna, 1979; Patarroyo, 2011; Patarroyo and Bengtson, 2017; Patarroyo et al., 2017). This biostratigraphic framework was reinforced by data derived from palynomorphs (Sol´ e de Porta, 1972; Jaramillo and Yepes, 1994), micropaleontology, foraminifera (Bürgl and Dumit, 1954; Petters, 1955; Vergara, 1994, 1997; Guerrero et al., 2000; Martínez, 2003; Navarrete-Parra et al., 2018) and calcareous nannofossils (De Romero et al., 2003; P´ erez et al., 2018; P´ aez-Reyes et al., 2021). These studies, showing well-preserved foraminifera and palynomorphs (Bürgl and Dumit, 1954; Gandolfi, 1955; Petters, 1955; Bürgl, 1961; Martínez, 1989, 2003; Due˜ nas, 1989; Martínez and Hernandez, 1992; Jaramillo and Yepes, 1994; Vergara, 1997; Guerrero et al., 2000; Tchegliakova and Mojica, 2001; Yepes, 2001; Terraza-Melo et al., 2002) and scarce calcareous nannofossils (Tchegliakova and Mojica, 2001), particularly helped to improve age constraints for Campanian–Maastrichtian marine deposits. Among these earlier studies, Martínez (2003), Jaramillo and Yepes (1994), and Garzon et al. (2012) built some of the most complete foraminiferal and palynological biozonations for Turonian–Maastrichtian deposits for the UMV. Similarly, the pioneering study of P´ erez et al. (2018) contributed to Coniacian–Campanian calcareous nannofossil biostratigraphy of deposits of the La Luna Formation in the Middle Magdalena Valley (MMV). Finally, the studies of Martínez (1989) and Yepes (2001), based on foraminifera and palynomorphs, respectively, provided useful biostratigraphic information for Campanian–Maastrichtian deposits in the Cesar-Rancheria Basin (Fig. 3). The available biostratigraphic information for Upper Cretaceous Fig. 3. Summary of available chronostratigraphic and lithostratigraphic information about the studied deposits. See text and Appendix A for further details. E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 6 marine deposits from the western region indicates an age range from Coniacian to Maastrichtian (Fig. 3 and Appendix A). Published data show that age control of the Coniacian–Maastrichtian marine deposits from the SSJFB relied mainly on foraminifera (Chenevart, 1963; Duque-Caro, 1967a, 1967b, 1972a, 1967b; Geotec, 1997, 2003; Clavijo and Barrera, 2001; Guzm´ an, 2007; Herrera et al., 2009; Barrantes et al., 2019), although there are a few studies based on palynological results conducted by Due˜ nas and G´ omez (2013). The age of sedimentation of these deposits has also been addressed by studies on ammonites and bivalves (Etayo-Serna et al., 1982; Duque-Caro, 1972a, 1972b, 1973); however, their biostratigraphic value has been questioned because of the abundant reworked fossils in these deposits (Duque-Caro, 1967b, 1972a, b; Etayo-Serna, 1989). Biostratigraphic studies from the Gorgonilla Island, based mostly on foraminiferal assemblages, suggest an age near the Cretaceous/Paleogene boundary (Bermúdez et al., 2016; Renne et al., 2018; Bermúdez et al., 2019). Age determinations for marine deposits from the Western Cordillera are based on ammonites, bivalves (Etayo-Serna, 1985, 1989; Moreno-S´ anchez et al., 2002; G´ omez-Cruz et al., 2002; Pardo-Trujillo et al., 2002a, b; Rodríguez and Arango, 2013; Díaz-Ca˜ nas and Patarroyo, 2014), and some mentions of foraminiferal and calcareous nannofossil assemblages, which are described as poorly preserved and rarely abundant (Barrero, 1979; Th´ ery, 1980; Etayo-- Serna et al., 1982; 1990; Pardo-Trujillo et al., 2002b; Pati˜ no et al., 2019; Pardo-Trujillo et al., 2020). These previous studies indicate age ranges of Coniacian? To Maastrichtian (Fig. 3). 5. Methodology We analyzed calcareous nannofossils from 119 rock samples distributed over nine localities (Fig. 1A; Table 1). Analyzed samples from the eastern area were collected from two stratigraphic sections, one cropping out along the Aico Creek section in the UMV and one coredsection named the ANH-LA LOMA-1 and drilled by the Agencia Nacional de Hidrocarburos (ANH) in the Cesar-Rancheria Basin (Fig. 1A). The samples from the western region belong to two stratigraphic sections, one outcropping in the San Carlos Quarry in the SSJFB of the Caribbean region and one from the Gorgonilla Island in the Tumaco Offshore in the Pacific region (Fig. 1A). In addition to this, we sampled five localities of folded-bed outcrops from the Western Cordillera (Table 1). Sampling resolution varies depending on the thickness and state of preservation of the outcrops. Samples were taken every meter in the San Carlos Quarry (~20 m thick) and every one to 2 m in the Gorgonilla Island (~11 m thick), whereas samples from the Western Cordillera lack consecutive sampling because of discontinuity and extensive deformation of the outcrops (Table 1). In the Aico Creek section (~284 m thick), sampling was performed every ~15 m, and in the ANH-LA LOMA-1 core (~626 m in thickness), every ~10 m. We used the classification of for siliciclastic rocks with grain size <63 μ m and for limestones. Slides of calcareous nannofossils were prepared using the standard technique of the smear slide (Bown and Young, 1998). Quantitative analyses of nannofossils were done using a Nikon polarized light microscope at 1000×magnification and counting up to ~600 fields of view (Appendix B). We used the qualitative scale of Roth and Thierstein (1972) to evaluate preservation of calcareous nannofossils: (G) good preservation: scarce or no evidence of dissolution and/or recrystallization, (M) moderate preservation: slightly dissolved and/or recrystallized microfossils, and (P) poor preservation: species strongly dissolved and/or recrystallized microfossils. Determination of calcareous nannofossils relied on taxonomic schemes of Bown and Young (1997) and Perch-Nielsen (1985), as well as information available on the online database Nannotax Our biostratigraphic analysis was based on the Upper Cretaceous standard biozonations for low-latitude by Sissingh (1977, 1978) and Burnett (1998). We used the chronostratigraphic framework and nomenclature of the International Subcommission on Cretaceous Stratigraphy https://stratigraphy.org/. 6. Calcareous nannofossil assemblages and biostratigraphic assignments Micropaleontological analyses show 35 (morpho) genera and 50 (morpho) species of calcareous nannofossils (Fig. 4, Appendix B). Nannofossils from the eastern deposits were rarely to commonly abundant and assemblages were dominated by Watznaueria and Micula. This assemblage was accompanied by Prediscosphaera, Cribrosphaerella, and Retecapsa, in the Aico Creek section and peaks of abundance of Kamptnerius magnificus in the ANH-LA LOMA-1 core (Fig. 4, Appendix B). Nannofossil abundance from the western stratigraphic sections ranged from common to very abundant, and calcareous nannofossils were dominated by the genera of Watznaueria, Micula, and Uniplanarius (Fig. 4, Appendix B). Preservation of microfossils varied from poorly to well preserved in eastern deposits, but preservation was uniformly poor in western deposits (Appendix B). We observed that eastern deposits yield a higher variety of taxa (73%) in comparison to nannofossils recovered from western rocks (27%). In contrast, our counts show that western samples have higher abundance of calcareous nannofossils (54%) than eastern samples (46%) (Appendix B). Among the studied sections from the eastern region, micropaleontological assemblages of the ANH-LA LOMA-1 core were more diverse but less abundant than those from the Aico Creek. The analyzed sections of the San Carlos Quarry and El Purgatorio Quarry, together with the Gorgonilla Island, recorded the most abundant and diverse nannofossils among the western sections (Appendix B). 6.1. Aico Creek section Micropaleontological results from the Aico Creek section showed that calcareous nannofossils are poorly to moderately preserved and abundance can vary from rare to very abundant. A total of 20 genera were quantified in this section. The most abundant genera were Watznaueria (63%) and Micula (19%); taxa of lower abundance were Table 1 Studied localities with their respective lithostratigraphic information, number of analyzed samples, thickness and/or depth. Three types of deposits were sampled: slightly deformed outcrops (O: outcrops), cored-stratigraphic section (C: core), and highly folded outcrops (CP: check point). Region Locality Location Lithostratigraphic unit Coordinates Number of samples O/C/CP Eastern region 1 ANH-LA LOMA-1 coredstratigraphic section Molino Fm. 9◦37′56.26′′ N 73◦28′36,01′′ W 58 C (~626 m) 2 Aico Creek Loma Gorda, Buscavida fms. and Oliní Group 3◦42′49.62′′ N 75◦31′25.13′′ O 20 O (~284 m) Western region 3 San Carlos Quarry Cansona Fm. 9◦14′38.28′′ N 75◦47′18.07′′ W 21 O (~20 m) 4 Gorgonilla Island Sedimentos de Gorgonilla 2◦56′00′′ N 78◦12′00′′ W 8 O (~11 m) 5 El Purgatorio Quarry Cansona Fm. 8◦41′11.26′′ N 75◦50′0.85′′ W 1 CP 6 Road Uramita-Dabeiba Nutibara Mb. (Penderisco Fm.) 6◦54′47.38′′ N 76◦13′19.97′′ W 6 CP 7 El Sireno Pathway Nutibara Mb. (Penderisco Fm.) 6◦23′20.42′′ N 76◦14′51.99′′ W 1 CP 8 Road to Santa Cecilia Nutibara Mb. (Penderisco Fm.) 5◦20′51.50′′ N 76◦6′28.33′′ W 1 CP 9 El Naranjo Quarry Cisneros Fm. 3◦46′56.14′′ N 76◦43′12.88′′ W 3 CP E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 7 Fig. 4. Microphotographs of key calcareous nannofossil taxa identified in this work. (AC): Aico Creek section. (LL): ANH-LA LOMA-1 cored-stratigraphic section, (WC)*: Western Cordillera, (SC): San Carlos Quarry section, (GI): Gorgonilla Island section. Scale bar =10 μ m *Calcareous nannofossils from folded beds. E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 8 Prediscosphaera (4%), Retecapsa (3%), Cribrosphaerella (2%), Zeugrhabdotus (2%), Eiffellithus (1%), Uniplanarius (1%), and Reinhardtites (1%) (Appendix B). In the base of the section (from ~7 m to ~13 m), calcareous nannofossils were rare and their preservation was poor; we recovered only sporadic forms of Quadrum gartneri from ~9 m. This biozonal marker, whose first appearance occurs at the beginning of zone UC7 (CC11), indicates an age not older than early Turonian (Fig. 5). The abundance and preservation of nannofossils improved after ~19 m, when the biozonal marker Micula concava appeared for the first time. This taxon has its first appearance in zone UC11c (CC16) of the upper Coniacian (Burnett, 1998; Sissingh, 1977), thus constraining the interval between ~9 m and ~19 m (~10 m thick) to a zonal range from UC7 to UC11b (CC11 to CC15), equivalent to an age range from lower Turonian to Coniacian (Fig. 5). The UC11c (CC16) biozone is delimited by the first occurrence of M. concava and the last occurrence of Lithastrinus septenarius (Burnett, 1998). Although these bioevents were not identified in the section, both taxa were recovered simultaneously at ~34 m, corroborating the presence of this biozone and assigning an age range from upper Coniacian to lower Santonian to this part of the section. The biozonal marker Lithastrinus grillii, which appears for the first time in the base of zone UC11, was similarly found at ~34 m, supporting our zonal assignment of UC11c (Fig. 5). Above this interval, nannofossil counts revealed the first occurrence of A. cymbiformis at ~59 m and the absence of L. septenarius and L. grillii, which became extinct for the rest of the section. The first occurrence of A. cymbiformis defines the base of the zone UC13 of Burnett (1998), equivalent to CC17 biozone of Sissingh (1977), close to the Santonian/Campanian boundary. Therefore, the stratigraphic interval between ~34 m and ~59 m (~24.7 m) is restricted to zone UC12 (Burnett, 1998) and biozonal range CC16-17 (Sissingh, 1977), thus indicating an age range from Santonian to the lower Campanian (Fig. 5). Micropaleontological recovery from ~59 m up to ~156 m (~97 m) was characterized by two samples that contain poorly preserved taxa of Eiffellithus spp., Micula spp., Prediscosphaera spp., Retecapsa spp., and Watznaueria spp., making biostratigraphic constraints difficult in this part of the section. However, the first occurrence of Uniplanarius trifidus at ~156 m indicates that this level cannot be older than zone UC15d of the upper Campanian and helps to restrict the interval between ~59 and ~156 m to biozones UC13–UC15c (CC17–CC21), equivalent to the lower part of Campanian (Fig. 5). An increase in preservation and abundance after ~221 m enabled us to observe the co-occurrence of Reinhardtites anthophorus and U. trifidus, confirming that the interval between ~156 m and ~221 m (~65 m) belongs to the UC15d–UC15e (CC22) subzones of the upper Campanian (Fig. 5). Subsequently, calcareous nannofossils showed a decline in preservation and abundance from ~237 m until the extinction of several taxa at ~259 m (Fig. 5). Among them, U. trifidus and Reinhardtites levis were observed for the last time at this level, denoting the top of biozones UC17 (CC23b) and UC18 (CC24), respectively, of the early Maastrichtian. These biostratigraphic markers help to constrain the upper part of the section to biozones UC16–UC17 (Burnett, 1998) and CC23 (Sissingh, 1977), indicating an age range from the uppermost Campanian to lowermost Maastrichtian (Fig. 5). This age is also supported by the common occurrence at ~259.5 m of Uniplanarius sissinghii, which has its last occurrence in the early Maastrichtian (Burnett, 1998). In the last analyzed sample at, ~282 m, calcareous nannofossils were poorly preserved and scarce, having an assemblage composed of Micula spp., Retecapsa spp., and Watznaueria spp. (Appendix B). These genera are characterized by their wide biostratigraphic range during the Late Cretaceous, making it difficult to establish the age more precisely. Although the number of identified reworked taxa in the section was low, some specimens of P. columnata occur sporadically between ~19 m and ~221 m. This taxon, which has a biostratigraphic distribution from the Albian to Turonian (Burnett, 1998), was found together with younger assemblages from the Coniacian to Maastrichtian (Fig. 5). In summary, the Aico Creek section between ~9 m and ~259 m covers a biozonal range from undifferentiated zones UC7–UC11b (CC11) to UC16–?UC17 (?CC23), which is equivalent to an age range from Turonian–Coniacian to the latest Campanian–early Maastrichtian (Fig. 5). 6.2. ANH-LA LOMA-1 cored-stratigraphic section Micropaleontological analyses from this cored-stratigraphic section show that 26 samples were barren and 32 yielded calcareous nannofossils with moderate to good preservation and few to common in abundance (Fig. 6; Appendix B). The recovered assemblages were diverse (29 genera) but dominated by high abundance of Micula (29%), Watznaueria (20%), and K. magnificus (19%), followed by Prediscosphaera (4%), Chiastozygus (4%), Cribrosphaerella (3%), Retecapsa (3%), Zeugrhabdotus (3%), Eiffellithus (37%), Microrhabdulus (2%), Calculites (2%) and Staurolithites (1%) (Appendix B). Examined samples from the basal part of the core (first ~21 m) are barren, but several peaks of poorly and well-preserved nannofossils occurred between ~599 m and ~144 m (Fig. 6). Abundance and preservation of microfossils improved from ~204 m to ~313 m before they dramatically disappeared after ~138 m in the upper part of the core (Fig. 6). From ~599 m to ~144 m we identified the sporadic occurrence of Arkhangelskiella cymbiformis, Calculites obscurus, Micula staurophora, and Prediscosphaera cretacea, indicating a zonal range from UC13 (UC13a in boreal provinces) to UC20 (CC17–CC26) of the early Campanian to Maastrichtian interval (Fig. 6). This biozonal assignment is supported by the recovery of Lithraphidites cf. praequadratus at ~144 m, which is a biostratigraphic marker commonly used for recognition of zones UC15d–UC20 (CC22–CC26) of the Campanian–Maastrichtian (Roth, 1978; Burnett, 1998). Quantitative results also show that Micula adumbrata, which has a biostratigraphic range from Turonian to Coniacian (Sissingh, 1977; Burnett, 1998), was found irregularly between ~144 m and ~295 m, indicating reworking of older Upper Cretaceous deposits. Likewise, the recovery at ~313 m of Prediscosphaera columnata, whose last occurrence has been registered in the Turonian (Burnett, 1998), also indicates the presence of reworking in this core (Appendix B). 6.3. San Carlos Quarry section Our analyses showed abundance patterns that vary from common to very abundant in the first ~13 m of the section and from few to barren from ~14 m up to the top (Fig. 7). Nannofossils were generally poorly preserved, showing overgrowth and recrystallization; however, moderate preservation of some taxa allowed taxonomic determination (Figs. 7 and 4). Our counts indicate the recovery of an assemblage dominated by high abundance of Watznaueria (61%), Micula (23%), and Uniplanarius (11%) and containing low occurrences of Quadrum (3%) and Retecapsa (2%) (Appendix B). Our counts revealed a uniform assemblage characterized by the co-occurrences of U. sissinghii and U. trifidus, whose appearance denotes the base of UC15d biozone, suggesting that the basal segment of the section cannot be older than zone UC15d (CC22) of the upper Campanian (Burnett, 1998; Sissingh, 1977) (Fig. 7). Likewise, the continuous record of U. trifidus and U. gothicus up to ~13 m indicates that this stratigraphic level is not younger than UC17 (CC23), since these species have their last occurrence in the early Maastrichtian (Sissingh, 1977; Burnett, 1998). Therefore, the interval between 0 and ~13 m is restricted to the biozonal range UC15d–UC17 (CC22– CC23) of the upper Campanian–lower Maastrichtian. At ~14 m abundance of calcareous nannofossils changed markedly, with distribution patterns becoming discontinuous due to the absence of nannofossils in some samples up to the top of the section (Fig. 7). The observed assemblage in this interval (last ~6 m of the section) consisted of sporadic forms of Micula spp., Watznaueria spp., Q. gartneri, and Retecapsa spp. Although this association is characteristic of the Late Cretaceous, the scarce recovery prevented us from performing a more detailed biostratigraphic analysis. E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 9 Fig. 5. Distribution patterns of the calcareous nannofossil markers (in bold), biozonal assignments, and age constraints of the Aico Creek section. “X" represents the presence of the specimen in the sample. The stratigraphic log was taken from Hern´ andez (2021). E. Angulo-Pardo et al. Journal of South American Earth Sciences 127 (2023) 104315 16 Concheyro, G.A., 1995. 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