Benthic foraminifera and Neogene sea level changes in the Pelotas Basin, offshore south Brazil

Abstract

The Neogene benthic foraminiferal ecostratigraphy of offshore Pelotas Basin has not yet been researched. The benthic foraminifera were examined in twenty-nine ditch cuttings samples from Well 1-SCS-3B, present water depth 200 m. These samples ranged from the Lower Miocene Catapsydrax dissimilis planktonic foraminiferal Zone to the Pliocene Globigerinoides trilobus fistulosus Zone. Only ditch cuttings were available, which allowed us to present just broad paleoenvironmental trends within the studied section. Benthic foraminifera indicate deposition mostly between outer neritic to upper bathyal paleodepths. A peak in uvigerinid relative abundance marks a possible flooding surface or condensed section with a high nutrient flux, low diversity (marked by the Shannon Function H), and sluggish circulation within the Globorotalia mayeri Zone. This peak is bracketed by high abundances of Cibicidoides, which indicate stronger current action than within the condensed section. Biostratigraphic hiatuses within these Cibicidoides-rich intervals are concluded not to indicate subaerial exposure, but to reflect current action sufficiently strong to induce either non-deposition or seafloor scouring. A second possible flooding surface occurs at the top of the section, indicated by a decline in Cibicidoides associated with upticks in Sphaeroidina and Bolivina and a dip in H. The top of this younger maximum flood was not encountered, probably occurring above the studied section. That the timing of these floods differs from those recorded elsewhere in the Pelotas Basin implies that they were tectonically induced, perhaps as distal effects associated with the interaction between the South American and Nazca tectonic plates. Future studies with better quality core samples will add detail to this picture; such studies can incorporate eustatic sea-level changes into the model presented.

Descriptors:
Miocene; Uvigerinidae; Argentine Zoogeographic Province; Pelotas Basin; Florianópolis Platform; maximum flooding surface.

INTRODUCTION

The Pelotas Basin is situated in south-easternmost Brazil and southeast Uruguay. Zoogeographically, its offshore portion currently straddles the boundary between the West Indian Province and the North Patagonian Subprovince of the Argentine Province at ~30°S (see Boltovskoy et al., 1980, Marengo, 2015). During the Cenozoic, Uruguayan deposition was mostly continental. However, Sprechmann (1978), in an examination of the paleontology of a series of water wells drilled on the Uruguayan coastal plane, found marine fossils in the Chuy 364 borehole (Lat. 33°42’S, Long. 53°26’W), drilled in the southern portion of the Pelotas Basin. His study ushered in a series of examinations of the Pelotas Basin marine Neogene. Offshore, however, the basin’s marine foraminifera remain poorly known.

Sprechmann (1978) recorded a diverse assemblage of foraminifera (45 taxa, some in open nomenclature), ostracods, mollusks, bryozoans, and fishes all of broadly Miocene age. The assemblages were dominated by Cibicides aknerianus (d’Orbigny) and Cibicides “pseudoungerianus” (Cushman). Of these, Boltovskoy (1959) found C. aknerianus (described initially from the Vienna Basin), to be at present associated with the Malvinas Current, not with sluggish waters farther offshore below the Brazilian Current, which does not reach the seafloor. This species thus indicates that the marine Miocene in borehole Chuy 364 was subject to the intense current action. Sprechmann (1978) recorded Amphistegina gibbosa, which is characteristic of warm waters and is currently unknown in the Miocene of Argentina to the south. Based on these A. gibbosa, Sprechmann (1978) concluded that the shoreline marine current off northern Brazil reached NE Uruguay during the Miocene. This might also be reflected in his recovery of Discorbis rosea (as Rotorbinella rosea (d’Orbigny)), which is common to dominant in back-reef sites in the Caribbean Sea (Culver and Buzas, 1982, Wilson and Ramsook, 2007). He did not record any planktonic foraminifera, attesting to the nearshore situation of the Chuy 364 borehole location during Miocene times.

In addition to global, eustatic events, SE South America was during Neogene times subject to a series of transgressions and regressions, of which Sprechmann (1978) found just one. Marengo (2015) suggested that these were tectonically induced and apparently due to interactions between the South American and Nazca tectonic plates, reflecting distal subsidence associated with Andean tectonics. In contrast, Anjos-Zerfass et al. (2013) concluded that a regional unconformity at the Oligocene/Miocene boundary is of eustatic origin, being coincidental with almost global sedimentation that resulted from a climatic event, i.e., the ‘Mi-1 glaciation’. They proposed that the other regional Neogene unconformities also correspond to global sea-level falls, and so posited that global eustatic movements controlled the development of regional synthem architecture.

Sanguinetti (1980), who studied ostracods, suggested that during the Miocene (Sphenolithus heteromorphus-Discoaster quinqueramus calcareous nannofossil Zones), within a Henryhowella evax ostracod Zone of local extent, the Pelotas Basin was uniformly covered by a sea with maximum depths of ~200 m (outermost neritic to uppermost bathyal). This is within the planktonic foraminiferal Globorotalia acostaensis and Globorotalia humerosa Zones, N16/N17 (see figure 2 in Bolli et al., 1985).

Carreño et al. (1999) examined the ostracods in onshore boreholes of the Pelotas Basin. Correlation of the ostracod biozones placed maximum transgressive events in the latest Miocene and Pleistocene. Ostracod paleoecology indicates the depositional system to be a barrier-lagoon. Of the transgressions recorded, the oldest (1^st T of Carreño et al. (1999)) peaked during calcareous nannofossil Zone NN11, equivalent to the Globorotalia acostaensis and Globorotalia humerosa Zones, N16/N17.

Malumián and Nánez (2011) reported Atlantic transgressions on the Patagonian Platform, southernmost South America, which they characterized as comprising a passive margin on one of the most extended continental shelves in the world. The Colorado Basin, centered around 40°S, and the Salado Basin, at ca. 37°S, were the most north-easterly basins reported by these authors (see figures 1, 2 in Malumián and Nánez, 2011). They noted three transgressions to occur in the Late Oligocene, Early Miocene, and Middle Miocene. The Late Oligocene transgression, represented in the Colorado Basin by the Elvira Formation, was of limited extent, inducing shallow waters with abundant Buccella and conspicuous Discorotalia. The Early Miocene transgression, the rocks for which have not been named in the Colorado Basin but which might have been coincident with the Laguna Paiva Transgression (TLP) of Marengo (2015), transported the extinct Antarctic genus Ammoelphidiella northwards and saw the development of the modern, low diversity Patagonian coastal assemblage with dominant Buccella. The Middle Miocene transgression reported by Malumián and Nánez (2011), represented in the Colorado Basin by the Barranca Final and Belén Formations, covered mainly northern Argentina and maybe the TEP of Marengo (2015). Malumián and Nánez (2011) noted that this Middle Miocene transgression was coeval with a relative climatic optimum. Windhausen (1931) recorded tropical mollusks in the Golfo San Jorge Basin at this time. Amphistegina, though found along the Uruguayan coast during the Middle Miocene, is not recorded in the Argentine Salado Basin, though equatorial microfossil bryozoa (e.g., Cupuladria canariensis) are there common in subsurface samples but not in the more austral Colorado Basin. This distribution provides evidence of a clear Middle Miocene thermal gradient (Malumián, 1999).

In contrast to Malumián and Nánez (2011) finding three transgressions on the Patagonian Platform, Marengo (2015) suggested that two major transgressive-regressive cycles affected the Chacoparanense Basin of onshore northern Argentina during the Neogene and that both of these impacted the Pelotas Basin. The deposits of both transgressions are micropaleontologically rich, allowing reliable estimates of their ages and the main environmental changes. The first of these transgressions, known as the Patagoniense on the Patagonian coast, is in northern Argentina called the Laguna Paiva Transgression (TLP). It started during the latest Oligocene (NP25, = Globorotalia kugleri Zone, P22) and persisted through the Early Miocene (NN1, = Globigerinoides primordius Zone, N4) (Marengo, 2015, figure 2.13). The second transgression, which occurred between the Middle Miocene and the early Late Miocene (NN5 - NN8, = latest Globigerinatella insueta to Globorotalia mayeri Zones, latest N7 - N14) was referred to by Marengo (2015) as the Entrerriense-Paranense Transgression (TEP).

Within the Pelotas Basin, Anjos and Carreño (2004) examined the planktonic foraminifera in Well 1-SCS-3B (Figure 1). They concentrated on the Neogene section. Although they found that the well penetrated Oligocene sediment, there was too much caving of Miocene age material into that Oligocene section, and recrystallization of the in situ Oligocene foraminifera, to allow reliable zonation of it. The Neogene section, between 360 - 1230 m below the seafloor, was assigned by Anjos and Carreño (2004) to planktonic foraminiferal zones using the zonal scheme for tropical and temperate areas of Bolli and Saunders (1985) (see Table 1). Anjos and Carreño (2004) recorded eight zones within this Neogene interval. The ages of the zonal boundaries recognized by these authors were based on the extinction of taxa established by Berggren et al. (1995). Based on the zonal scheme, four hiatuses were observed in the studied sedimentary sequence, situated at the Oligocene-Miocene boundary, in the Early Miocene, at the Early Miocene-Middle Miocene boundary and the Miocene-Pliocene boundary.

Figure 1
Location of Well 1-SCS-3B, within the Pelotas Basin.

Table 1
Planktonic foraminiferal biostratigraphy of the Neogene in Well 1-SCS-3B, Pelotas Basin.

Depth in Well	Zone	Age
360 - 510 m	Globigerinoides trilobus fistulosus Zone	Lower Pliocene
	Hiatus
510 - 630 m	Globorotalia margaritae evoluta Zone	Lower Pliocene
630 - 780 m	Globorotalia menardii / Globorotalia acostaensis Zone	Middle to Upper Miocene
780 - 1080 m	Globorotalia mayeri Zone	Middle Miocene
1080 - 1200 m	Globigerinoides ruber Zone	Middle Miocene
	Hiatus
1200 - 1230 m	Globorotalia fohsi Zone	Middle Miocene
	Hiatus
1230 - 1260 m	Catapsydrax stainforthi Zone	Lower Miocene
1260 - 1320 m	Catapsydrax dissimilis Zone	Lower Miocene

Some caving of material was noted throughout the open drill hole of Well 1-SCS-3B. For example, specimens of Orbulina universa and O. suturalis were recovered from the Catapsydrax dissimilis Zone. However, Anjos and Carreño (2004) did not tabulate the number of specimens per species in each sample, so the severity of the caving is unknown. Nor did these authors provide details of casing points. The total recovery of specimens per sample was generally low, ranging between ~10 - 125 (see figure 3 in Anjos and Carreño, 2004), suggesting mostly shallow water conditions. Several intervals in the Neogene section contained what Anjos and Carreño (2004) called ‘oxidized’ tests. These intervals were mainly concentrated above 855 m depth. There was also an interval with fragmentary planktonic foraminiferal tests at ~1245 m depth. Anjos-Zerfass et al. (2014) suggested that concentrations of oxidized tests may be related to regressive events that promoted the reworking of the Miocene deposits in the Pelotas Basis by exposing areas of the Florianópolis Platform. Oxidized tests in Well 1-SCS-3B are found mainly in the late Globorotalia mayeri Zone and younger. The TLP was not recorded by Anjos and Carreño (2004), but rocks representing this transgression may be found within the unstudied Oligocene interval. Some other transgressions, perhaps reflecting the greater depth at 1-SCS-3B, were apparently recorded by Anjos and Carreño (2004). The maximum flood recorded in 1-SCS-3B coincides with the TEP. The transgression at the very top of the studied interval in 1-SCS-3B is apparently associated with 1st T of Carreño et al. (1999). The authors were unable to infer the causes of the hiatuses but hypothesized that they were associated with transgressive events. They suggested that an analysis of benthic foraminifera might test this hypothesis. Such an analysis is presented in this paper.

The onshore wells studied by Carreño et al. (1999), with their maximum transgressive events in the latest Miocene and Pleistocene, lay somewhat west of Well 1-SCS-3B. Anjos and Carreño (2004) did not study the Pleistocene in Well 1-SCS-3B. Nor is it examined here. However, the 1st T of Carreño et al. (1999), which reached its maximum in the Globorotalia acostaensis and Globorotalia humerosa Zones, occurs within the single most extensive interval of deposition found by Anjos and Carreño (2004), these Zones specifically being within the 630 - 780 m interval in Well 1-SCS-3B.

Coimbra et al. (2009) expanded on Anjos and Carreño ‘s (2004) study of the biostratigraphy and paleoceanographical significance of the Neogene planktonic foraminifera from the Pelotas Basin, southernmost Brazil. Coimbra et al. (2009) examined the assemblages in 1-SCS-2 drill-hole of Pelotas Basin, ~70 km NW of 1-SCS-3B. In 1-SCS-2, they recognized the Catapsydrax dissimilis, Catapsydrax stainforthi, Globorotalia fohsi robusta, Globorotalia mayeri, Globorotalia margaritae evoluta and Globigerinoides trilobus fistulosus Miocene and Pliocene zones. There are thus four important hiatuses, though they do not precisely correlate with those in Well 1-SCS-3B. The Miocene planktonic foraminifera constitute a tropical/sub-tropical assemblage. In contrast, the Pliocene yielded scarce species associated with subantarctic water masses, suggesting that the Malvinas Current had reached the area by then, but it was not a controlling factor concerning paleoenvironmental conditions.

Anjos-Zerfass et al. (2014) used strontium isotope ratios to identify hiatuses in the Oligocene and Miocene of the Pelotas Basin. Most of these were related to unconformities and coincided with those hiatuses identified by Anjos and Carreño (2004). However, Anjos-Zerfass et al. (2014) concluded that a hiatus in the early-late Oligocene hiatus reflected a maximum flooding surface. No such condensed sections were noted in the Miocene sections examined by Anjos-Zerfass et al. (2014).

Here we examine the timing of maximum transgressions (maximum floods) in the offshore Pelotas Basin. To do so, we use benthic foraminifera in Well 1-SCS-3B, drilled on the Florianópolis Platform, Pelotas Basin, in the State of Santa Catarina, Brazil (28º29’34.18” S, 47º29’10.8” W; present water depth ~200 m).

MATERIALS AND METHODS

In this study, we use the samples of well cuttings from Well 1-SCS-3B studied for planktonic foraminifera by Anjos and Carreño (2004). As noted by Loboziak et al. (2005) and Jones (2014), of all types of samples recovered from wells, cuttings are the least desirable, caving of material from higher in the well into lower sections causing problems of stratigraphic mixing. Olson and Thompson (2005) recorded that not only do drilling fluids obscure and dilute the true sample volumes of well-cutting samples, but those drilling-fluid additives may also damage microfossils fossils by chemical reactions. Unfortunately, for Well 1-SCS-3B, cuttings are the only available source of micropaleontological data. Nevertheless, they can provide valuable data. Orndorff and Culver (1998) studied the Early Miocene age upper part of the Anahuac Formation of Louisiana, USA, using 109 rotary cuttings samples, each representing a ca. 10 m interval. Despite potential problems from caving, they were able to identify a general up-well shallowing trend punctuated by some possible flooding surfaces. However, as is the case with Well 1-SCS-3B, the lack of magnetostratigraphic and seismic control precluded the identification of sequence boundaries and correlation of maximum flooding surfaces with sea-level cycle charts. Here we concentrate on identifying possible maximum flooding surfaces in the earlier Neogene of Well 1-SCS-3B and placing them within the biostratigraphic scheme of Anjos and Carreño (2004).

Given the damage from drilling (breakage) and drilling fluids (dissolution), we were able to identify only to genus level, despite the availability of local taxonomic papers (Marengo, 2015, Finger, 2013, Sprechmann, 1978). Dissolution had affected, for example, the surfaces of Lenticulina spp., rendering them opaque (cf. Murray, 1967) and making it challenging to identify species with confidence. This was especially so in the oxidized parts of the section. Pekar et al. (2001) found that, in Oligocene of the North American Atlantic passive margin, condensed sections and flooding surfaces are characterized by the peak abundance of uvigerinids, which led to a decrease in biodiversity. We use peaks of Uvigerina + Trifarina to identify such possible flooding surfaces, and measure diversity using the Shannon function H ( = -Σp_i×ln(p_i), in which p_i is the proportional abundance of the ith genus). For details of H, see Hayek and Buzas (2013).

We use the benthic foraminiferal high productivity index (BFNI) of Martins et al. (2007) as a proxy for the organic carbon flux, where the BFNI is the sum of the proportions of Bolivina, Bulimina, and Uvigerina. The percentage of the benthic assemblage as serial tests (defined as P_{se: sp} = N_se/(N_se + N_sp) × 100, in which N_se and N_sp are the numbers of serial and spiral [= planispiral + trochospiral] tests respectively), has been used as a potential indicator of diminished ventilation (Murray, 2006) and as an indicator of the redox state of the sediments (Tetard et al., 2017). Here we examine the pattern of P_se:sp through our Neogene section in Well 1-SCS-3B. All calculations were made using the PAST paleontological freeware of Hammer et al. (2001), version 4.01.

RESULTS

A total of 7942 benthic foraminiferal specimens were recovered from the samples. These were placed in 89 taxa (primarily genera), of which 39 were rare, being represented by < 4 specimens, and 21 were singletons. Indeterminate agglutinated taxa comprised 52 specimens only (0.6% of total recovery), while indeterminate uniserial chambers formed only 69 specimens (0.9%). Total recovery was dominated by Cibicidoides (23.9%; principally, C. cf. coryelli [~12% of total recovery] and C. pachyderma [~9%]) with lesser Uvigerina (15.6%; especially U. parvula) (Figure 2). Genera forming >5% of the total recovery each were: Bulimina (7.6%; primarily B. striata mexicana), Siphonina (7.5%; S. tenuicarinata only), Bolivina (7.4%) and Lenticulina (6.5%). Of the total recovery, 4759 specimens (59.9%) had spiral (trochospiral and planispiral) morphologies, including such globose genera as Globocassidulina and Sphaeroidina. A further 3120 specimens (39.3%) had serial (elongate) morphologies. The small remainder was either unilocular genera or indeterminate agglutinants.

Figure 2
The distributions of selected genera or genera groups in the Neogene of Well 1-SCS-3B, Pelotas Basin. A. Uvigerinids (Uvigerina + Trifarina). B. Cibicidoides. C. Bulimina. D. Bolivina. E. Sphaeroidina. F. Siphonina. The grey bar outlines the peak in uvigerinids.

Uvigerinids ranged from 0-69% of each sample (x̅ = 15.8%), with a pronounced peak in relative abundance between 900-945 m depth (Figure 2A). The Shannon function ranged from H = 1.19-2.71 (x̅ = 2.23, s.d. = 0.33), with particularly low values between 900-975 m depth (which encompasses the section with peak uvigerinids), and at the top of the studied section at 360-375 m depth (Figure 3A). The correlation between the transformed proportions of total uvigerinids [ln(x + 1)] and H was both significant and negative (r = -0.664, p < .0001, n = 29).

Figure 3
Selected measures within the Neogene of Well 1-SCS-3B, Pelotas Basin. A. The Shannon Function, H. B. The nutrient flux indicator BFNI. C. The serial:spiral coiling ratio, Pse:Psp. The grey bar outlines the peak in uvigerinids.

Among the remaining genera, Cibicidoides showed peaks in relative abundance both above and below the peak in uvigerinids but also showed a marked decline in percentage abundance towards the top of the section (Figure 2B). Cibicidoides pachyderma (1080-1235 m depth) was most abundant in the lower part of the studied section, and C. cf. coryelli in the upper part (360-855 m depth). There was a significant correlation between the transformed proportions of the assemblages as Cibicidoides and uvigerinids (r = -0.493, p = .007, n = 29). A peak in Bulimina (48.4-58.6% per sample) occurred immediately below the peak in uvigerinids, at 960-1005 m depth (Figure 2C), while Bolivina peaked both above and below the peak in uvigerinids, and towards the top of the section (Figure 2D). Sphaeroidina (represented by S. bulloides), showed a remarkable distribution, though forming at most only ~5% of any sample. It occurred principally between 780-1110 m depth, with a minimal increase also in the uppermost sample (Figure 2E). Siphonina (represented by S. tenuicarinata) increased in abundance from ~0.5-30% per sample between 900-780 m depth, immediately above the peak in uvigerinids (Figure 2F).

The BFNI ranged from 0.024-0.779 (x̅ = 0.331, s.d. = 0.216) (Figure 3B). The graphical analysis showed that the values were approximately normally distributed with no outliers. The values reached a maximum between 900-975 m depth, coincident with the peak in uvigerinids, and at the top of the studied section between 360-420 m. The percentage serial specimens (P_se:sp) ranged between 8.9-83.8% (x̅ = 43.7%, s.d. = 20.4). The wide range in the percentage of serial specimens reflected the abundance of serial genera not included in the calculation of the BFNI. As with the BFNI, values showed maxima between 900-975 m and 360-420 m, but with a peak also at 1050-1065 m (Figure 3C). The correlation between suitably transformed values of the BFNI and P_se:sp was thus strongly significant (r = 0.936, p < .00001, n = 29).

DISCUSSION

Anjos and Carreño (2004) noted the occurrence of some caving of planktonic foraminifera within Well 1-SCS-3B. They did not quantify its extent; they aimed to correlate the Neogene succession with the zonal scheme of Bolli and Saunders (1985) using stratigraphic tops of planktonic foraminiferal species. No extraction and deletion of caved benthic foraminiferal specimens are possible within our dataset, many of the species found in Well 1-SCS-3B ranging biostratigraphically throughout the entire Neogene (Bolli et al., 1994, van Morkhoven et al., 1986, Holbourn et al., 2013). Thus, we record only the broadest of Neogene benthic foraminiferal trends and events in the studied sedimentary record. A peak in the abundance of uvigerinids was noted between 900-945 m depth within the well. The more-or-less distinct occurrences of Cibicidoides cf. coryelli above this peak and C. pachyderma below allow us to be confident that caving has not distorted benthic foraminiferal signals in the studied samples. The second interval of low H occurred at the top of the section examined.

Some remarks can be made regarding paleodepth using depth ranges provided by van Morkhoven et al. (1986) and Holbourn et al. (2013). Cibicidoides pachyderma, which has its maximum abundance near the base of the studied section, is considered an upper bathyal species (Holbourn et al., 2013) primarily though it has been recorded over a more extensive depth range (van Morkhoven et al., 1986). Uvigerina parvula was initially described from upper bathyal depths (309 m) and was noted by Cushman (1923) to be most abundant at such depths. Sphaeroidina bulloides, the high abundances of which occurred above and below the peak in uvigerinids, is a neritic to abyssal species. However, Siphonina tenuicarinata, which had its maximum abundance above the uvigerinids peak, is an outer neritic to the middle bathyal taxon. We conclude that the studied Neogene section in Well 1-SCS-3B was deposited between the outer neritic and upper bathyal paleodepths. Inner neritic Amphistegina and Discorbis rosea, recorded in the onshore Pelotas Basin by Sprechmann (1978), were not found in the analyzed samples.

The uvigerinid peak at 900-945 m marks a possible flooding surface or condensed section within the Globorotalia mayeri Zone. This peak is associated with low values of the Shannon Function H and peaks in the BFNI and P_{se: sp}. This is coincident with the latest EPT of Marengo (2015). However, it is challenging to discern the entire EPT in Well 1-SCS-3B, as its commencement during the Globigerinatella insueta Zone coincides with a hiatus in the well. The BFNI maximum reflects the association of uvigerinids with a high nutrient flux (Altenbach et al., 1999; Wilson and Hayek, 2017), Uvigerina spp. preferring a continuous flux of organic matter (Nomaki et al., 2006). The decline in Cibicidoides towards the top of the section, coupled with (1) increases in the relative abundances of Sphaeroidina and Bolivina in the uppermost two samples, (2) an associated decrease in H, and (3) peaks in the BFNI and P_{se: sp}, mark some degree of transgression during the Lower Pliocene Globigerinoides trilobus fistulosus Zone, perhaps leading towards a younger possible flooding surface.

This latter maximum flood is somewhat younger than the 1st T of Carreño et al. (1999), which attained maximum transgression during the Globorotalia acostaensis and Globorotalia humerosa Zones. This indicates that times of maximum flooding varied throughout the Pelotas Basin. This may, in turn, indicate that the maximum floods detected at different sites within the basin reflect not global eustatic events, but tectonic interactions between the Nazca and South American plates affecting different parts of the basin at different times. The shallower, Cibicidoides-rich interval above the uvigerinids peak, might reflect the filling of accommodation with sediment.

Of the three hiatuses found by Anjos and Carreño (2004), the one at 510 m depth occurred above the uvigerinid peak, while the two at 1200 m and 1230 m are situated below that peak. All the hiatuses are associated with abundant Cibicidoides. Anjos-Zerfass et al. (2014) suggested that areas of the Pelotas Basin may, by the lifting of the Florianópolis Platform, have been exposed during regressive events. However, this appears unlikely given the ecology of Cibicidoides. Schoenfeld (2002) noted that Cibicidoides spp. in the Gulf of Cadiz live on elevated substrates subject to the current enhanced action. The distribution of Cibicidoides throughout the sedimentary section in Well 1-SCS-3B indicates that bottom current strength varied over time, it being depressed around the peak in uvigerinids and at the top of the section. Such stagnation is expected at times of maximum transgression (Catuneanu et al., 2011). Elsewhere, the distribution of Cibicidoides suggests current action was enhanced and that conditions were somewhat oligotrophic. It is possible that current action became strong enough to cause non-deposition or to erode bottom sediment, leading to the development of the hiatuses. Similar causes have been invoked for hiatuses in the Miocene deep-sea that coincided with cooling (Barron and Keller, 1982). The reason for the intervals with oxidized foraminiferal tests in Pelotas Basin is unclear. However, we note that such tests are currently widespread on continental shelves around the world (Emery, 1968) and are relict, having been deposited during an early Holocene transgression (Wilson, 2010). Further work, using better quality samples, seismic sections, and well logs, is needed to assess the sequence stratigraphic significance of the oxidized intervals in 1-SCS-3B and elsewhere in the Pelotas Basin.

CONCLUSIONS

Benthic foraminifera of the Pelotas Basin Neogene have previously been studied using onshore wells, and Amphistegina sp. and Discorbis rosea reflect how these sections were deposited nearshore. Those in ditch cuttings from the offshore Well 1-SCS-3B, however, reflect deposition at outer neritic to upper bathyal paleodepths with moderate to intense current action, as indicated by the abundance of the genus Cibicidoides. Hiatuses in the Well, which coincide with abundant Cibicidoides, are concluded to have resulted from either non-deposition or erosion due to current scouring. A significant peak in uvigerinids within the Globorotalia mayeri Zone marks a possible flooding surface or condensed section with possible stagnant bottom water and high nutrient flux. Within the Lower Pliocene Globigerinoides trilobus fistulosus Zone, there occurs a decline in Cibicidoides coupled with higher relative abundances in Sphaeroidina and Bolivina, an associated decrease in the Shannon Function H, and an increase in nutrient flux. This is concluded to mark some degree of transgression during this time, perhaps leading towards a younger possible flooding surface.

The timing of maximum flooding identified in Well 1-SCS-3B differs from that found elsewhere in Pelotas Basin. We conclude that the maximum floods were therefore not eustatically induced, though examining core samples from future wells will undoubtedly recover such a signal. The broader trends recorded here instead indicate that the Pelotas Basin had a complex tectonic evolution, with the timing of subsidence differing from one place to another. We suggest that this subsidence was a distal response to plate tectonic interactions between the South American and Nazca Plates.

ACKNOWLEDGEMENTS

The authors are grateful to Petróleo Brasileiro S.A (Petrobras) for providing the samples. JCC thanks the National Council for Scientific and Technological Development (CNPq) for the grant 305128/2017-5. BW would like to thank the University of the West Indies, St. Augustine Campus, for support during the early stages of this work. We thank the two anonymous reviewers for their constructive comments on our work.

REFERENCES

• ALTENBACH, A. V., PFLAUMANN, U., SCHIEBEL, R., THIES, A., TIMM, S. & TRAUTH, M. 1999. Scaling percentages and distributional patterns of benthic foraminifera with flux rates of organic carbon. Journal of Foraminiferal Research, 29, 173-185.
• ANJOS-ZERFASS, G. S., CHEMALE, F., MOURA, C. A. V., COSTA, K. B. & KAWASHITA, K. 2014. Strontium isotope stratigraphy of the Pelotas Basin. Brazilian Journal of Geology, 44, 23-38.
• ANJOS-ZERFASS, G. S., RUBAN, D. A., CHEMALE, F. & ZERFASS, H. 2013. Cenozoic synthem stratigraphic architecture of the SE Brazilian shelf and its global eustatic context: evidence from the Pelotas Basin (offshore Brazil). Geologos, 19, 273-290.
• ANJOS, G. S. & CARREÑO , A. L. 2004. Bioestratigraphia (Foraminiferida) de Sondagem 1-SCS-3B, Plataforma de Florianopolis, Bacia de Pelotas, Brasil. Revista Brasileira de Paleontologia, 7, 127-138.
• BARRON, J. A. & KELLER, G. 1982. Widespread Miocene deep-sea hiatuses: Coincidence with periods of global cooling. Geology, 10, 577-581.
• BERGGREN, W. A., HILGEN, F. J., LANGEREIS, C. G., KENT, D. V., OBRADOVICH, J. D., RAFFI, I., RAYMO, M. E. & SHACKLETON, N. J. 1995. Late Neogene chronology: New perspectives in high-resolution stratigraphy. Geological Society of America Bulletin, 107, 1272-1287.
• BOLLI, H. M., BECKMANN, J.-P. & SAUNDERS, J. B. 1994. Benthic foraminiferal biostratigraphy of the south Caribbean region, Cambridge, Cambridge University Press.
• BOLLI, H. M. & SAUNDERS, J. B. 1985. Oligocene to Holocene low latitude planktic foraminifera. In: BOLLI, H. M., SAUNDERS, J. B. & PERCH-NIELSEN, K. (eds.) Plankton Stratigraphy Cambridge, England: Cambridge University Press.
• BOLLI, H. M., SAUNDERS, J. B. & K., P.-N. 1985. Comparison of zonal schemes for different fossil groups. In: BOLLI, H. M., SAUNDERS, J. B. (eds.) Plankton Stratigraphy UK: Cambridge University Press.
• BOLTOVSKOY, E. 1959. Foraminifera as biological indicators in the study of ocean currents. Micropaleontology, 5, 473-481.
• BOLTOVSKOY, E., GIUSSANI, G., WATANABE, S. & WRIGHT, R. C. 1980. Atlas of Benthic Shelf Foraminifera of the Southwest Atlantic, Springer Science & Business Media.
• CARREÑO , A. L., COIMBRA, J. C. & APARECIDO DO CARMO, D. 1999. Late Cenozoic sea level changes evidenced by ostracodes in the Pelotas Basin, southernmost Brazil. Marine Micropaleontology, 37, 117-129.
• CATUNEANU, O., GALLOWAY, W. E., KENDALL, C. G. S. C., MIALL, A. D., POSAMENTIER, H. W., STRASSER, A. & TUCKER, M. E. 2011. Sequence stratigraphy: methodology and nomenclature. Newsletters on Stratigaphy, 44/3, 173-245.
• COIMBRA, J. C., CARREÑO, A. L. & DE SANTANA DOS ANJOS-ZERFASS, G. 2009. Biostratigraphy and paleoceanographical significance of the Neogene planktonic foraminifera from the Pelotas Basin, southernmost Brazil. Revue de micropaléontologie 52, 1-14.
• CULVER, S. J. & BUZAS, M. A. 1982. Distribution of Recent benthic foraminifera in the Caribbean region. Smithsonian Contributions to Marine Science, 14, 1-382.
• CUSHMAN, J. A. 1923. The Foraminifera of the Atlantic Ocean, Part 4: Lagenidae. United States National Museum Bulletin, 104, 1-228.
• EMERY, K. O. 1968. Relict sediments on continental shelves of the world. AAPG Bulletin, 52, 445-464.
• FINGER, K. L. 2013. Miocene foraminifera from the south-central coast of Chile. Micropaleontology, 59, 341-492.
• HAMMER, Ø., HARPER, D. A. T. & RYAN, P. D. 2001. PAST: Paleontological Statistics Software Package for Education and Data Analysis. Palaeontologia Electronica [Online], 4.
• HAYEK, L. C. & BUZAS, M. A. 2013. On the proper and efficient use of diversity measures for individual field samples. Journal of Foraminiferal Research, 43, 305-313.
• HOLBOURN, A., HENDERSON, A. S. & MACLEOD, N. 2013. Atlas of Benthic Foraminifera, John Wiley and Sons, Chichester, UK.
• JONES, R. W. 2014. Foraminifera and their Applications, Cambridge, UK, Cambridge University Press.
• LOBOZIAK, S., MELO, J. H. G. & STREEL, M. 2005. Devonian Palynostratigraphy in Western Gondwana. In: KOUTSOUKOS, E. A. M. (ed.) Applied Stratigraphy Netherlands: Springer.
• MALUMIÁN, N. 1999. La sedimentación en la Patagonia extraandina. In: CAMINOS, R. (ed.) La sedimentación y el volcanismo terciarios en la Patagonia extraandina Instituto de Geología y Recursos Minerales.
• MALUMIÁN, N. & NÁNEZ, C. 2011. The Late Cretaceous-Cenozoic transgressions in Patagonia and the Fuegian Andes: foraminifera, palaeoecology, and palaeogeography. Biological Journal of the Linnean Society, 103, 269-288.
• MARENGO, H. 2015. Neogene Micropaleontology and Stratigraphy of Argentina: The Chaco-Paranense Basin and the Península de Valdés, Heidelberg, Germany, Springer.
• MARTINS, V., DUBERT, J., JOUANNEAU, J.-M., WEBER, O., DA SILVA, E. F., PATINHA, C., ALVEIRINHO DIAS, J. M. & ROCHA, F. 2007. A multiproxy approach of the Holocene evolution of shelf-slope circulation on the NW Iberian Continental Shelf. Marine Geology, 239, 1-18.
• MURRAY, J. W. 1967. Transparent and opaque foraminiferid tests. Journal of Paleontology, 41, 791-795.
• MURRAY, J. W. 2006. Ecology and Applications of Benthic Foraminifera, Cambridge, UK, Cambridge University Press.
• NOMAKI, H., HEINZ, P. & NAKATSUKA, T. 2006. Different ingestion patterns of 13C-labeled bacteria and algae by deep-sea benthic foraminifera. Marine Ecology Progress Series, 310, 95-108.
• OLSON, H. C. & THOMPSON, P. R. 2005. Sequence biostratigraphy with examples from the Plio-Pleistocene and Quaternary. In: KOUTSOUKOS, E. A. M. (ed.) Applied stratigraphy The Nertherlands: Springer.
• ORNDORFF, A. L. & CULVER, S. J. 1998. Foraminifera of the early Miocene upper part of the Anahuac Formation from a well in Vermilion Parish, Louisiana, U.S.A. . Journal of Foraminiferal Research, 28, 286-305.
• PEKAR, S. F., CHRISTIE-BLICK, N., KOMINZ, M. A. & MILLER, K. G. 2001. Evaluating the stratigraphic response to eustasy from Oligocene strata in New Jersey. Geology, 29, 55-58.
• SANGUINETTI, Y. T., 1980. 1980. Biostratigrafia (Ostracodes) do Miocene da bacia de Pelotas, Rio Grande do Sul. Pesquisas em Geociências, 13, 7-34.
• SCHOENFELD, J. 2002. Recent benthic foraminiferal assemblages in deep high-energy environments from the Gulf of Cadiz (Spain). Marine Micropaleontology, 44, 141-162.
• SPRECHMANN, P. 1978. The paleoecology and paleogeography of the Uruguayan coastal area during the Neogene and Quaternary. Zitteliana, 4, 3-72.
• TETARD, M., LICARI, L. & BEAUFORT, L. 2017. Oxygen history off Baja California over the last 80 kyr: A new foraminiferal-based record. Paleoceanography, 32, 1-19.
• VAN MORKHOVEN, F. P. C. M., BERGGREN, W. A. & EDWARDS, A. S. 1986. Cenozoic Cosmopolitan Deep-Water Benthic Foraminifera. Bulletin Centres Recherches Exploration-Production Elf-Aquitaine, 11, 1-421.
• WILSON, B. 2010. The significance of iron-stained foraminifera off SE Trinidad, West Indies, Western Central Atlantic Ocean. Geological Magazine, 147, 728-736.
• WILSON, B. & HAYEK, L. C. 2017. Islands, currents, eddies, fronts . . . and benthic foraminifera: Controls on neritic distributions off Trinidad. Micropaleontology, 63, 15-26.
• WILSON, B. & RAMSOOK, A. 2007. Population densities and diversities of epiphytal foraminifera on nearshore substrates, Nevis, West Indies Journal of Foraminiferal Research, 37, 213-222.
• WINDHAUSEN, A. 1931. Geología Argentina, Bueno Aries, Argentina Casa Jacabo Peuser.

Edited by

Associate Editor: Felipe Toledo

Editor: Rubens M. Lopes

Publication Dates

• Publication in this collection
  23 Oct 2020
• Date of issue
  2020

History

• Received
  25 Apr 2020
• Accepted
  27 July 2020