1. Introduction

The Marañon Basin is one of the most important basins in Peru in terms of hydrocarbon resources, but despite many decades of active exploration (Calderón et al. 2017a, 2017b; Baby et al. 2018), biostratigraphic research has not been conducted. The Marañon Basin contains Paleozoic and Mesozoic source rocks and Mesozoic reservoirs and subthrust traps. Recent studies by Calderón et al. 2017a, 2017b) have improved understanding and modeling of petroleum systems in this basin, especially regarding the formation and deformation of the subthrust traps. However, uncertainties remain concerning the timing and rate of Cenozoic burial, which forms part of the petroleum kitchen system. This is mainly because the Cenozoic deposits of the Marañon Basin are poorly dated (Roddaz et al. 2010).

The stratigraphic ages of Cenozoic units are generally based on different groups of fossils, including algae (Marocco et al. 1995) and invertebrates, such as mollusks and ostracods, from the Chambira, Pebas, and Solimões formations (Whatley et al. 1998; Muñoz-Torres et al. 2006; Ramos 2006; Wesselingh et al. 2006a; Wesselingh and Ramos 2010), and ostracods, foraminifera, and palynomorphs from the adjacent Solimões Basin (Linhares et al. 2011, 2017, 2019; Leandro et al. 2019), as well as vertebrates (Monsch 1998; Salas-Gismondi et al. 2007). The Pebas/Solimões and Nauta formations were palynologically dated from outcrops and wells in the Solimões and Acre basins (Hoorn 1993, 1994a, 1994b, 2006; Hoorn et al. 1996, 2010a, 2010b, 2017; Rebata et al. 2006a, 2006b; Leite et al. 2016; Leandro et al. 2019; Linhares et al. 2019). These units were also recognized from the Santa Lucia borehole drilled in the southern Marañon Basin (Hermoza 2004) and from three exploration wells (Jibaro 7/1AB-21-181, Huayuri Sur 15/1-AB-15-184D and Capahuari Norte 9/1AB-3-204) drilled in the westernmost part of the Marañon Basin (Hermoza 2004; Hermoza et al. 2005; Wesselingh et al. 2006a). These well sections were dated via mollusks by Wesselingh et al. (2006a) who also correlated them with palynozones (Hoorn 1993, 1994a).

Table 1.

Location of the six studied wells: Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 in the Marañon Basin. N: number of samples analyzed; UTM (18S) coordinates in meters; depth in feet (1 ft = 0.3 meters).

The Neogene palynozones in South America have mainly been established in Colombian, Venezuelan, and Brazilian basins (Van der Hammen 1956; Germeraad et al. 1968; Regali et al. 1974a, 1974b; Lorente 1986; Hoorn 1993; Jaramillo et al. 2011). More specific palynostratigraphic works have been developed in the Solimões and Acre basins (Hoorn 1994a, 1994b; da Silva et al. 2010; Silveira and Souza 2015, 2016; Leite et al. 2016; Leandro et al. 2019; Linhares et al. 2019). Those paleontological studies have helped to establish correlations between Amazonian basins and to reconstruct Neogene biomass evolution (Wesselingh et al. 2006a; Hoorn and Wesselingh 2010; Jaramillo et al. 2010; Boonstra et al. 2015; Antoine et al. 2016; Jaramillo et al. 2017). However, to date, no detailed palynostratigraphic research has been carried out in the Marañon Basin, and the nature, evolution, and age range of the Neogene palynofloras of this basin remain poorly understood.

This study aims to establish a spore/pollen zonation for the Neogene sediments of the retroarc foreland (most subsiding area; Roddaz et al. 2010) of the Marañon Basin based on the identification of qualitative changes in palynomorphs (spores, pollen, and dinoflagellate cysts) through the intervals sampled in six exploration wells (Table 1 and Figure 1).

2. Geological setting

The Marañon Basin covers approximately 320,000km² (Mathalone and Montoya 1995) and is located between 0°N and 7°30′S and 70 and 78°W in northeastern Peru (Figure 1). It is currently considered a foredeep depozone of the northern Amazonian retroarc foreland basin (Roddaz et al. 2005, 2010). The Huallaga and Santiago basins to the southwest and west, respectively, separate the Marañon Basin from the Subandean zone, and to the northeast the basin is bordered by the Iquitos forebulge (Roddaz et al. 2005, 2010). It continues as the Oriente Basin in Ecuador to the northwest and the Putumayo Basin in Colombia to the north. The Guyanese shield and Solimões Basin border the Marañon Basin to the east. To the south, the basin is bordered by the Ucayali and Acre basins, and to the southeast the Contaya arch separates it from the Ucayali Basin (Roddaz et al. 2005).

The Amazonian retroarc foreland basin system started to form between the late Maastrichtian and early Paleocene, during the first period of Amazonian Andes mountain building (Hurtado et al. 2018; Louterbach et al. 2018). This late Maastrichtian–Paleocene period of Andean tectonic loading was followed by an unloading stage during the Early–Middle Eocene (Roddaz et al. 2010). From the Middle–Late Eocene, the Amazonian retroarc foreland basin was subjected to continuous flexural subsidence driven by the continuous Andean tectonic loading, which promoted high sedimentation rates in the foredeep depozone (Roddaz et al. 2010). The formation and forward propagation of the eastern Amazonian orogenic thrust wedge began in the Oligocene (30–24 Ma) (Eude et al. 2015), causing flexural subsidence and high sedimentation rates in the Marañon foredeep (Roddaz et al. 2010).

The stratigraphy of Amazonian foreland basins has been synthesized by Roddaz et al. (2010). The Neogene sedimentary pile consists of late Oligocene–Miocene Chambira, Early–Late Miocene Pebas, Pliocene Marañon, and Quaternary Corrientes formations. These units are generally poorly dated. The Pebas Formation, with a thickness of approximately 1000 m (Wesselingh et al. 2006a), transitionally overlies the Chambira Formation (Oligocene) and is in underlying concordant contact with the Marañon Formation. The transition is characterized by blue clays, fine-grained lithic sandstones, and lignite layers rich in diverse and well-preserved invertebrate and vertebrate fossils; the base of this formation was dated around 22.5–23.9 Ma (Oligocene–Miocene boundary) (Wesselingh et al. 2006a).

3. Previous studies

Numerous studies have recorded the presence of palynomorph taxa in the Pebas and Solimões formations and other coeval units outcropping in Colombia, Brazil, and Peru (Hoorn 1993, 1994a, 1994b, 2006; Hoorn et al. 1995, 1996, 2010a, 2010b; Räsänen et al. 1995; Gingras et al. 2002a, 2002b; Hoorn and Ramos Feijó 2006; Rebata et al. 2006a, 2006b; Wesselingh 2006; Wesselingh et al. 2006a, 2006b, 2010; Hovikoski et al. 2007b; Latrubesse et al. 2007, 2010; da Silva et al. 2010; Gross et al. 2011; Leite et al. 2017; Leandro et al. 2019; Linhares et al. 2019). These formations generally date from the Early–Late Miocene (Hoorn and Wesselingh 2010; Latrubesse et al. 2010; Roddaz et al. 2010; Boonstra et al. 2015; Hoorn et al. 2017; Jaramillo et al. 2017; Leite et al. 2017). However, it is important to note that no palynological studies exist that deal with the Neogene sedimentary units of the Marañon Basin.

Figure 1.

Location of the six studied deep exploration wells (white-black circles). Structural boundaries of the Marañon Basin (after Roddaz et al. 2005).

Neogene deposits of the Marañon Basin, to date, have been dated mainly based on palynozones defined by Hoorn (1993) for the Solimões and Acre basins, the Malacostraca zones defined by Wesselingh et al. (2006a), and the ostracod zones defined by Muñoz-Torres et al. (2006). Hoorn (1993) defined five palynozones for the Solimões Basin: the Verrutricolporites Acme Zone (Early Miocene), the Retitricolporites Acme Zone (Early Miocene), the Psiladiporites–Crototricolpites Concurrent Range Zone (late Early to early Middle Miocene), the Crassoretitriletes Interval Zone (Middle Miocene), and the Grimsdalea Interval Zone (late Middle to early Late Miocene), which correlated with those by Germeraad et al. (1968) and Lorente (1986). In the same basin, da Silva et al. (2010) defined the Asteraceae–Fenestrites Zone and recognized the Psilatricolporites caribbiensis palynozone of Lorente (1986). More recently, these zones in the Solimões Basin were recognized by (Leite et al. 2017; Linhares et al. 2017, 2019; Leandro et al. 2019), who further identified Lorente's (1986) palynozones: the Crassoretitriletes Interval Zone (Middle Miocene), the Asteraceae Interval Zone (Late Miocene), the Psilatricolporites caribbiensis Interval Subzone (latest Miocene–Pliocene), and the Echitricolporites–Alnipollenites Interval Subzone (Late Pliocene).

4. Sample material and procedures

A total of 77 ditch-cutting samples from six exploration wells (Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1) located in the Marañon Basin were studied for their palynological content (Table 2 and Figure 1). The samples varied from silty shale to shale, claystone, sandy clay, and clayey, very fine sandstone. The palynological samples were prepared according to the standard procedure (Wood et al. 1996). The preparation of palynological slides was carried out in the Paleosedes Laboratory in Bogotá, Colombia ( http://www.paleosedes.tk). Samples were processed using hydrochloric acid, hydrofluoric acid, and zinc chloride solutions. The slides were analyzed at the Paleosedes Biostratigraphy Laboratory and the Palynostratigraphy and Paleobotany Laboratory of the institute CICYTTP-CONICET-ER-UADER ( http://www.cicyttp.org.ar) in Argentina. Slides and residues are housed at the Palynostratigraphy and Paleobotany Laboratory of the Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción (CICYTTP) and cataloged under CICYTTP acronyms; Table 2).

Table 2.

Analyzed intervals [depth in feet (1ft = 0.3 meters)] from Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 wells in the Marañon Basin and Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción (CICYTTP-Pl) collection number of each analyzed sample.

Two slides per sample (oxidized and non-oxidized) were scanned for palynomorph identification using transmitted light microscopes with a digital camera (Leitz Labor Lux S and Labomed 10 in Colombia; Nikon E200 and Labomed 5 in Argentina). Well-preserved specimens were selected and illustrated in Plate 1 using England Finder^™ coordinates. Palynomorph counting and logging were done by applying straight transects across each slide. Species names and abundance of taxa were recorded on data sheets. Six palynomorph distribution charts were prepared based on each palynomorph identified in the well sections, and the abundances of spores, pollen grains, fungal spores, algae, dinoflagellate cysts, acritarchs, foraminiferal linings, and copepod eggs were tabulated in the Tilia software (Grimm 2015) (Appendices 1–6). The recovered palynomorphs were gathered in seven morphogroups (Table 3) and percentages for the morphogroups in all wells were calculated (Table 4). Approximately 100–200 specimens were counted per sample. However, some samples with abundances lower than 100 identifications were also included in the qualitative analyses due to their biostratigraphic significance (see Appendices 1–6).

The qualitative vertical arrangement of taxa identified from the wells was used to interpret first downhole occurrence or last appearance datum (LAD) and last downhole occurrence or first appearance datum (FAD) data. Palynostratigraphic studies and online catalogues of species from northern South America (Van der Hammen 1956; Germeraad et al. 1968; Regali et al. 1974a, 1974b; Lorente 1986; Muller et al. 1987; Tryon and Lugardon 1991; Hoorn 1993, 1994a; da Silva et al. 2010; Jaramillo et al. 2011; Raine et al. 2011; Silveira and Souza 2015, 2016; Leite et al. 2017; Williams et al. 2017; Jaramillo and Rueda 2019) were consulted for taxonomic determinations (see the list of species with their authors below) and stratigraphic ranges of the species suitable for biostratigraphy (Table 5). Critical palynomorph taxa were selected according to their stratigraphic value, and the most biostratigraphically significant events are indicated in Figure 2. To validate these findings, the graphic correlation technique of Shaw (1964) was applied to the dataset using GraphCor: Interactive Graphic Correlation software (Hood 1998) to obtain a composite section (Table 6) that was reordered and plotted (Figure 3) according to the stratigraphic succession, thus revealing biostratigraphic units. Details of this method can be consulted in the extensive compilation of Mann and Lane (1995).

Plate 1.

Photographs of the main palynomorphs from the studied wells. 1. Laevigatosporites catanejensis CICYTTP-2005 Arabela 1X (610–640ft), EF M12/2; 2. Laevigatosporites granulatus CICYTTP-2014 Arabela 1X (1810–1840ft), EF L32/2-4; 3. Polypodiisporites usmensis CICYTTP-2007 Arabela 1X (910–940ft), EF H25/2; 4. Cyathidites congoensis CICYTTP-2074 Frontera 1 (1860–1880ft), EF N35/2; 5. Foveotriletes ornatus CICYTTP-2068 Nahuapa 24X (5720–5750ft), EF N32/1-2; 6. Osmundacidites ciliatus CICYTTP-2074 Frontera 1 (1860–1880ft), EF R35/1; 7. Retitriletes altimuratus CICYTTP-2061 Nahuapa 24X (3740–3770ft), EF W21/3-4; 8. Cicatricosisporites dorogensis CICYTTP-2017 Arabela 1X (3310–3340ft), EF E3/3-2; 9. Crassoretitriletes vanraadshooveni CICYTTP-2052 Nahuapa 24X (2210–2240ft), EF H43/1; 10. Magnastriatites grandiosus CICYTTP-2057 Nahuapa 24X (3110–3140ft), EF O43/2; 11. Grimsdalea magnaclavata CICYTTP-2053 Nahuapa 24X (2300–2330ft), EF W17/3; 12. Gomphrenipollis minimus CICYTTP-2061 Nahuapa 24X (3740–3770ft), EF T34/2-3; 13. Monoporopollenites annulatus CICYTTP-2058 Maynas 1 (3350–3400ft), EF P44/1; 14. Psilamonocolpites medius CICYTTP- 2046 Tigrillo 30X (8370–8400ft), EF Q11/4; 15. Mauritiidites franciscoi minutus CICYTTP-2014 Arabela 1X (1810–1840ft), EF S38/1; 16. Mauritiidites franciscoi franciscoi CICYTTP-2031 Tucunare 1X (6750–6800ft), EF H42/1; 17. Cyclusphaera scabrata CICYTTP-2020 Maynas 1 (4400–4450ft), EF E10/3; 18. Corsinipollenites psilatus CICYTTP-2014 Arabela 1X (1810–1840ft), EF N12; 19. Echitriporites cricotriporatiformis CICYTTP-2006 Arabela 1X (820–850ft), EF F44/3; 20. Proteacidites triangulatus CICYTTP-2061 Nahuapa 24X (3740–3770ft), EF Z33/1; 21. Retitricolpites simplex CICYTTP-2035 Tucunare 1X (7600–7650ft), EF P43/2; 22. Crassiectoapertites columbianus CICYTTP-2061 Nahuapa 24X (3740–3770ft), EF F38/1; 23. Bombacacidites nacimientoensis CICYTTP-2059 Nahuapa 24X (3560–3590ft), EF J35/1-3; 24. Bombacacidites brevis CICYTTP-2017 Arabela 1X (3310–3340ft), EF R40/3; 25. Malvacipolloides maristellae CICYTTP-2034 Tucunare 1X (7350–7400ft), EF N25/2; 26. Margocolporites vanwijhei CICYTTP-2046 Tigrillo 30X (8370–8400ft), EF M23/ 4; 27. Rhoipites guianensis CICYTTP-2014 Arabela 1X (1810–1840ft), EF H35/1; 28. Siltaria hammenii CICYTTP-2072 Frontera 1 (1180–1200ft), EF X48; 29. Ranunculacidites operculatus CICYTTP-2007 Arabela 1X (910–940ft), EF G23/3; 30. Psilastephanoporites tesseroporus CICYTTP-2077 Frontera 1 (2700–2720 ft), EF W34/3; 31. Perisyncolporites pokornyi CICYTTP-2011 Arabela 1X (1480–1510ft), EF V37/3; 32. Echiperiporites akanthos CICYTTP-2027 Tucunare 1X (5500–5550ft), EF U27/2; left high focus and right lowfocus; 33. Echiperiporites estelae CICYTTP-2016 Arabela 1X (2860-2890ft), EF G23/1. Scale bar is 20 µm for all pictures except for Figures 9 and 10 where it represents 30 µm.

5. General characterization of the palynofloral assemblages

Seventy-seven samples from the Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 wells in the Marañon Basin (Table 2; Figure 1) yielded 6262 identifiable grains, as summarized in Table 3. Quantitative distribution of palynomorphs in the wells (Table 3 and 4; Appendices 1–6) reveals a total of 226 morphotypes, including 80 genera and 150 species of pollen grains, 24 genera and 43 species of spores, nine genera and eight species of dinoflagellate cysts, five genera of algae, four morphotypes, and two genera of fungal spores. Foraminiferal linings and copepod eggs (indeterminate copepods) were also identified. Of the identified palynomorphs, 75.1% belonged to pollen and spore groups, 14.3% were algae, and 7.8% included dinoflagellate cysts, acritarchs, foraminiferal linings, and other palynomorph remains. Fungal palynomorphs (2.8%) and abundant organic matter were recovered in all the samples.

Table 3.

Total grains counted per each analyzed well and percentage of each morphogroup from Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 wells in the Marañon Basin. NA: not applicable.

Table 4.

Total grains counted per morphogroup in each analyzed well and percentage of each morphogroup from Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 wells in the Marañon Basin.

Fairly well-preserved pteridophyte–bryophyte spores, angiosperm and gymnosperm pollen grains, phytoplankton (chlorophyceans, acritarchs, and dinoflagellate cysts) and miscellaneous groups (foraminifera, copepods, and fungal remains), characterize the palynoflora obtained from the six wells. The first group was mainly composed of spores such as Azolla, Cicatricosisporites, Crassoretitriletes, Cyathidites, Deltoidospora, Echinatisporis, Foveotriletes, Laevigatosporites, Magnastriatites, Polypodiisporites, Psilatriletes, Striatriletes, and Verrucosisporites–Verrutriletes.

Angiosperm pollen grains were frequent and more diverse, including Bombacacidites, Corsinipollenites, Crassiectoapertites, Cyclusphaera, Echiperiporites, Echipollenites, Echitriporites, Inaperturopollenites, Ladakhipollenites, Malvacipolloides, Mauritiidites, Paleosantalaceaepites, Perfotricolpites, Perisyncolporites, Proxapertites, Proteacidites, Psilamonocolpites, Psilastephanocolporites, Psilatricolporites, Retimonocolpites, Retipollenites, Retistephanoporites, Retitrescolpites, Retitricolpites, Retitricolporites, Scabrapollenites, Siltaria, Spinizonocolpites, and Striatopollis.

Gymnosperm pollen grains were rare and less diverse: Araucariacites, Cyclusphaera, Inaperturopollenites, Podocarpidites, and Striapollenites. Chlorophycean algae such as Botryococcus, Oedogonium, Pediastrum, and Pterospermella were obtained. Other phytoplankton, including acritarchs and dinoflagellate cysts, such as Apteodinium, Batiacasphaera, Bosedinia, aff. Ceratium, Cribroperidinium, Leiosphaeridia, Quadrina, Selenopemphix spp., and Operculodinium and Polysphaeridium groups, were common and always accompanied by foraminiferal linings, copepod eggs, and fungal palynomorphs, such as Fusiformisporites, Microthyraceae, and Tetraploa. The distribution of the palynomorphs in the abovementioned wells, their relative abundances (range charts), and extended Tilia diagrams are shown in Appendices 1–6, and key markers are illustrated in Plate 1.

6. Definition of palynozones

Fifty-two biostratigraphic marker species were recognized in the six boreholes (Table 5). Some occurred in all the wells, whereas others did not (Appendices 1–6), but all have a wide geographic distribution and well-known stratigraphic ranges given as FADs and LADs in millions of years ago for each taxon and/or based on stratigraphic ranges that have been used in biostratigraphic definitions in northern South America (Lorente 1986; Muller et al. 1987; Hoorn 1993; da Silva et al. 2010; Jaramillo et al. 2011) and elsewhere (Bujak and Williams 1985; Macphail 1999; Williams et al. 2017). Therefore, they are useful for dating the studied successions in this basin and for proposing regional correlations. Six palynozones named from the oldest to the youngest, namely Mar-A to Mar-F (‘Mar' is the abbreviation for ‘Marañon') are proposed considering the vertical distribution of the taxa (Figures 2–4; Appendices 1–6) based on the analysis of qualitative biostratigraphic events. A quantitative deterministic method of graphic correlation (Shaw 1964) in order to test this scheme was run in GraphCor: Interactive Correlation Software Hood (1998). Maximum stratigraphic ranges of species from previously known appearances and disappearances in each well (Table 6) allowed the construction of a biostratigraphic ordination in a composite section (Figure 3) according to last appearances of species. This method corroborates the proposed zonation based on a qualitative analysis, also evidenced by stratigraphically constrained cluster analysis (CONISS) in the Tilia software (Grimm, 2015) for each well (Appendices 1–6). The upper two zones are preliminarily characterized until further studies with more materials are carried out. The presence of well-known diagnostic species (FADs and LADs; Figures 2–4; Tables 5 and 6) supported the ages of the palynozones. In addition, seven timelines and a west–east and north–south correlation among the six wells were drawn (Figure 5), and a correlation of our palynozonation with other palynozones from northern South America (Figure 6) is also addressed below.

Table 5.

Marker age of palynomorphs recovered in the Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 boreholes in the in the Marañon Basin, in alphabetical order (occurrence levels are included; depth in meters). (1) da Silva-Caminha et al. (2010), (2) González-Guzmán (1968), (3) Hoorn (1993), (4) Jaramillo et al. (2011), (5) Eisawi and Schrank (2008), (6) Cole (1992); (7) De Verteuil et al. (1992), (8) Raine et al. (2011) and (9) Williams et al. (2017).

Figure 2.

Range chart of taxa as used in this work, indicating the boundaries of the palynozones Mar-A to Mar-F.

6.1. Mar-A Corsinipollenites oculusnoctis Interval Zone

Reference section. Arabela-1X interval 2890–1660ft (881–506m).

Distribution. This zone was recognized in all the wells: Arabela-1X interval 2890–1660ft (881–506m), Tigrillo-30X interval 10,680–7620ft (3255–2323m), Nahuapa-24X interval 4610–3230ft (1405–985m), Maynas-1 interval 8030–5250ft (2438–1600m), Tucunare-1X interval 7650–7400ft (2332–2255.5m) and La Frontera-1 interval 4420–4100ft (1347–1250m) (see Appendices 1–6 for details and Figures 2–6 for summary and correlation).

Description. We define the Mar-A Corsinipollenites oculusnoctis palynozone as occurring from the LAD of Cicatricosisporites dorogensis to the FAD of Malvacipolloides maristellae or, alternatively, it is from the FAD of Corsinipollenites oculusnoctis, the FAD of Retitricolporites wijmstrae, and/or the FAD of Acaciapollenites myriosporites to the FAD of Malvacipolloides maristellae as occurring respectively in the Maynas-1, Tigrillo-30X, and La Frontera-1 wells, to the FAD of Malvacipolloides maristellae.

Age. Aquitanian to Early Burdigalian (23.03–17.71Ma).

Characteristics. The presence of Paleogene taxa whose extinction occurred in the Neogene, such as Bombacacidites nacimientoensis, Corsinipollenites psilatus, Magnastriatites grandiosus, Mauritiidites franciscoi var. franciscoi, Mauritiidites franciscoi var. minutus, Retitricolpites simplex, and Spirosyncolpites spiralis, among others, is notable. This zone is characterized by the inception of other species, such as Apteodinium australiense, Leiosphaeridia, and Polysphaeridium groups (dinoflagellate cysts); the end of the acme of dinoflagellate cysts (Late Oligocene/Burdigalian); underlying Malvacipolloides maristellae (FAD); the first appearance of Acaciapollenites myriosporites, Corsinipollenites collaris, Corsinipollenites oculusnoctis, Corsinipollenites psilatus, Ctenolophonidites suigeneris, Multiporopollenites aff. pauciporatus/M. pauciporatus, Cyathidites congoensis, Retitricolporites wijmstrae, and Selenopemphix nephroides (dinoflagellate cysts); and the LAD of Bombacacidites gonzalezii. They frequently occur along with Clavainaperturites aff. clavatus, Laevigatosporites spp., Polypodiisporites specious, Polypodiisporites spp., and Psilamonocolpites spp., and seldom with Magnaperiporites spinosus (see Appendices 1–6 for details and Figures 2–6 for summary and correlations).

Comparison. The co-occurrence of several taxa (see Tables 5–6 and Figures 2–4) supports the correlation (summarized in Figure 6) of this zone with the palynostratigraphy of Venezuela (Lorente 1986), northern South America (Muller et al. 1987), and the Solimões Basin in Western Amazonia (Hoorn 1993). It is coeval with the T-12 Horniella lunarensis Zone from Llanos in Colombia (Jaramillo et al. 2011), the zonal boundaries of which are coeval with the Mar-A Zone. However, the T-12 taxon Horniella lunarensis is not present in the Mar-A Corsinipollenites oculusnoctis Zone (Figures 2–4)

Table 6.

Maximum stratigraphic ranges of species among the studied wells, constructed in a composite section from previously known appearances and disappearances of species in each well obtained by means of graphic correlation.

6.2. Mar-B Malvacipolloides maristellae Interval Zone

Reference section. Arabela-1X 1660–1210 ft (506–369 m).

Distribution. This zone was recognized in the intermediate section of the six wells analyzed: Arabela-1X interval 1660–1180ft (506–360m), Maynas-1 interval 4450–3350ft (1356–1021m), Tucunare-1X interval 7400–5350ft (2255.5–1631m), Tigrillo 30X interval 7350–6450ft (2240–1966m), Nahuapa-24X interval 4430–2600ft (1350–792m), and La Frontera-1 interval 3220–3120ft (981–951m) (see Appendices 1–6 for details and Figures 2–6 for summary and correlation).

Description. We propose the Mar-B Malvacipolloides maristellae Zone as corresponding to the interval from the FAD of Malvacipolloides maristellae to the LAD of Retitricolporites wijmstrae and/or from the LADs of Retistephanoporites angelicus or Cyclusphaera scabrata to the FAD of Grimsdalea magnaclavata in the upper zone.

Age. Burdigalian (17.71–16.1Ma).

Characteristics. This zone is characterized by the first appearance and continuous presence upward of Malvacipolloides maristellae, often accompanied by Retistephanoporites angelicus, Corsinipollenites psilatus, and Retitricolporites wijmstrae; the rare occurrence of Bombacacidites nacimientoensis, Crototricolpites cf. annemariae, Ctenolophonidites suigeneris, Gomphrenipollis minima, and Retitriletes altimuratus; and the continued presence of Cyathidites congoensis. Furthermore, Clavainaperturites aff. clavatus, Laevigatosporites spp., Magnastriatites grandiosus, Mauritiidites franciscoi var. franciscoi, Mauritiidites franciscoi var. minutus, Perisyncolporites pokornyii, Polypodiisporites specious, Polypodiisporites spp., Polypodiisporites usmensis, Psilamonocolpites spp., Retipollenites spp., and Retitricolporites sp. are commonly present in moderate abundances. Acritarchs and dinoflagellate cysts, such as Batiacasphaera sp., Leiosphaeridia, Selenopemphix spp., and Operculodinium group, also occur. A peak abundance of Pterospermella spp. (prasinophycean algae) is noticeable (see Appendices 1–6 for details and Figures 2–6 for summary and correlations).

Comparison. The co-occurrence of taxa (see Tables 5–6 and Figures 2–4 for details) supports the correlation of this zone with others in South America (summarized in Figure 5; Lorente 1986; Muller et al. 1987; Hoorn 1993; Jaramillo et al. 2011). The record of the Early Miocene Malvacipolloides maristellae (Muller et al. 1987) in the six wells (Figures 3 and 5) reinforces the age assigned to this zone. The zonal limits of the T-13 Malvacipolloides maristellae Zone from Llanos in Colombia (Jaramillo et al. 2011) were identified in one of the six wells, the Nahuapa-24X (Figure 3), supporting their correlation (Figures 4 and 5).

Figure 3.

Maximum stratigraphic ranges of species ordered by last appearance datum (LAD) among the studied wells, constructed in a composite section from previously known appearances and disappearances of species in each well obtained by means of graphic correlation.

Figure 4.

First palynological zonation for the Marañon Basin (this work) and correlation (time-lines). See Appendices 1–6 and Table 5 for details, and see Figures 2–3 and 5–6 for timeline correlation and correlation with other South American palynozones.

Figure 5.

First palynological zonation for the Marañon Basin (this work), and south–west and north–east correlation and timelines between boreholes of the six exploration wells Arabela-1X, Maynas-1, Tucunare-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 in the Marañon Basin.

Figure 6.

First palynological zonation for the Marañon Basin (this work) and its suggested correlation with other South American palynozones and the geological column.

7. Correlation with other Western Amazonian basins and discussion

The vertical succession of species in the six wells investigated from the Marañon Basin (see Appendices 1–6 for details) allowed us to propose the first palynozonation in a chronostratigraphic context (Figure 5). The presence of selected species in some or all of the boreholes and their stratigraphic ranges (FAD and LAD ages; Figures 2–3 and Tables 5–6) were used to date our zones and establish correlations with relatively coeval palynozones in South America (Figure 6). Some diachronisms exist between the ages of published palynozones from northern South America and ours. For instance, the Early Miocene to earliest Middle Miocene in Venezuela was characterized by the successive Verrutricolporites and Psiladiporites–Echitricolporites pollen zones (Lorente 1986). These zones are equivalent to the Verrutricolporites rotundiporus–Echidiporites barbeitoensis and Echitricolporites maristellae–Psiladiporites minimus zones defined by Muller et al. (1987). The Verrutricolporites Interval Zone and the Verrutricolporites rotundiporus–Echidiporites barbeitoensis Zone are defined as extending from the first records of Verrutricolporites rotundiporus to the first occurrence of Psiladiporites minimus or Echitricolporites maristellae.

Hence, the Verrutricolporites (Lorente 1986) and the Verrutricolporites rotundiporus–Echidiporites barbeitoensis (Muller et al. 1987) zones can be correlated with our Mar-A Corsinipollenites oculusnoctis Interval Zone. In addition, the Psiladiporites–Echitricolporites Interval Zone (Lorente 1986) and the Echitricolporites maristellae–Psiladiporites minimus Zone (Muller et al. 1987) are defined by the first occurrence of Psiladiporites minimus or Echitricolporites maristellae at the base, and the first occurrence of Crassoretitriletes vanraadshooveni at the top. These markers also correlate with our Mar-B Malvacipolloides (Echitricolporites) maristellae and Mar-C Mauritiidites crassibaculatus zones. Hence, our Mar-A, Mar-B, and Mar-C zones are within the Early Miocene to earliest Middle Miocene pollen zones of Lorente (1986) and Muller et al. (1987). Our Mar-A, Mar-B, and Mar-C zones are also coeval with the T-12 Horniella lunarensis, T-13 Echitricolporites maristellae, and T-14 Grimsdalea magnaclavata zones, respectively, of Jaramillo et al. (2011).

The late Early Miocene in Venezuela and in Western Amazonia was characterized by Lorente (1986) and Hoorn (1993), respectively, by the Crassoretitriletes Interval Zone. This zone is defined from the first records of Crassoretitriletes vanraadshooveni and Trichotomocolpites sp. to the first occurrence of Grimsdalea magnaclavata in the latest Middle Miocene (Lorente 1986; Hoorn 1993). The top of this zone was defined by Muller et al. (1987) based on the first record of Echitricolporites spinosus in the earliest Tortonian. The FAD of Grimsdalea magnaclavata sensu Jaramillo et al. (2011) is the oldest at ca. 16.1 Ma, meaning that this FAD would occur before that of Crassoretitriletes vanraadshooveni.

We have identified in Arabela-1X, Tigrillo-30X, Nahuapa-24X, and La Frontera-1 wells an interval similar to the Crassoretitriletes Interval Zone, by the presence of three biostratigraphic markers, Crassoretitriletes vanraadshooveni, Psilastephanoporites tesseroporus, and Retipollenites crotonicolumellatus (Tables 5–6) co-occurring at the base, which belong to the bases of both the T-15 Crassoretitriletes vanraadshooveni palynozone (Jaramillo et al. 2011) and the Crassoretitriletes palynozone (Lorente 1986). GraphCor indicated the FAD of Retipollenites crotonicolumellatus was slightly below the FADs of Crassoretitriletes vanraadshooveni and Psilastephanoporites tesseroporus. However, this might be explained by the quality of sampling. The top is identified by Corsinipollenites oculusnoctis, Cyathidites congoensis and Mauritiidites crassibaculatus at ca. 11.62 Ma (Serravallian/ Tortonian limit), as shown in the Crassoretitriletes vanraadshooveni Interval Zone of Muller et al. (1987). Therefore, the base of our Mar-D Crassoretitriletes vanraadshooveni Zone is coeval with the bases of the Crassoretitriletes vanraadshooveni zones of Hoorn (1993), Lorente (1986), and Jaramillo et al. (2011), whereas the top is coeval with the top of the Crassoretitriletes vanraadshooveni Interval Zone sensu Muller et al. (1987). Our Mar-D Crassoretitriletes vanraadshooveni Zone is also coeval with the Crassoretitriletes Interval Zone of Lorente (1986), recognized by Leite et al. (2017) in the Solimões Basin (Brazil).

The late Miocene from northern South America is characterized by the Echitricolporites spinosus Zone (Muller et al. 1987), so our Mar-E zone is coeval and can be also compared with the upper part of the T-16 Fenestrites spinosus Zone of Jaramillo et al. (2011), where the LAD of Ladakhipollenites? caribbiensis occurs. It is also correlated to the lower part of the T-17 Cyatheacidites annulatus Zone of Jaramillo et al. (2011), in which the LAD of Psilastephanoporites tesseroporus occurs (upper boundary of our Mar-E zone: 5.48Ma). Although sediments referable to Mar-E and Mar-F zones were studied only in the Fontera-1 well, GraphCor displays the events limiting these zones. Furthermore, due to the abovementioned correlations this interval has a preliminary zonal character which can be improved with additional biostratigraphic studies above the upper section of the studied wells.

Some authors have suggested that there is no record of Pliocene deposits in the Solimões Basin (Latrubesse et al. 2007, 2010). However, recent palynostratigraphic studies (da Silva et al. 2010; Silveira and Souza 2015, 2016; Leite et al. 2017) have documented Pliocene sedimentation in some areas of the Solimões Basin. These palynological studies are based on the presence of the Psilatricolporites caribbiensis Interval Subzone (Late Miocene to Early Pliocene: 5.6–3.7 Ma; Lorente 1986) and the Alnipollenites verus Interval Subzone (Late Pliocene to Holocene: 3.7 to present; Lorente 1986) found in the studied Solimões wells. Similarly, our study showed that Neogene sedimentation in the Marañon Basin continued during the Pliocene, where at least 360 ft (103 m) of the Marañon formation sediments were deposited (see La Frontera-1 well, Appendix 6 and Figures 4–6). We propose that the Mar-F Ctenolophonidites suigeneris Zone (Late Miocene to Early Pliocene, 5.48–3.6 Ma) comprises the youngest deposits dated here. However, since we did not analyze sediments above 1180 ft (356 m), we cannot exclude the possibility of the younger sediments belonging to the Alnipollenites verus Interval Subzone sensu Lorente (1986) or any coeval zone at depths over 1180 ft (356 m), and/or any coeval zone at depths over those of the last intervals from the other wells studied herein for the Marañon Basin.

8. Conclusions

This study of the Neogene biostratigraphic record obtained from six industrial wells in the Marañon Basin allowed us to define six palynozones (Mar-A to Mar-F); five of them (Mar-A to Mar-E) were validated by GraphCor. They range from the Aquitanian (Early Miocene) to the Messinian–Zanclean (latest Miocene to earliest Pliocene):

The biostratigraphic scheme proposed for the Marañon Basin is based on the presence of species in common with Miocene palynozones of northern South America (Colombia, Venezuela, and western Brazil), and a correlation between them is also established. Our study corroborates recent studies regarding the palynostratigraphy of the Miocene Solimões and Acre basins, and documents that Pliocene sedimentation occurred in Peru (western Amazonia). Future study of the interval above the analyzed wells may provide information to characterize the Pliocene palynofloras of the Marañon Basin. A comparison with Amazonia and elsewhere in northern South American basins will achieve new insights for the less well-known evolution of floras during the Pliocene.