Early Aptian marine incursions in the interior of northeastern Brazil following the Gondwana breakup

This study reports a set of primeval marine incursions identified in two drill cores, 1PS-06-CE, and 1PS-10-CE, which recovered the Barbalha Formation, Araripe Basin, Brazil. Based on a multi-proxy approach involving stratigraphy, microbiofacies, ichnofossils, and microfossils, three short-lived marine incursions were identified, designated Araripe Marine Incursions (AMI) 1–3. AMI-1 and AMI-2, which occur within the shales of the Batateira Beds (lower part of the Barbalha Formation), were identified by the occurrence of benthonic foraminifera, calcareous nannofossils, dinocysts, and a mass mortality event of non-marine ostracods. AMI-3 was recognized in the upper part of the Barbalha Formation, based on the occurrence of ichnofossils and planktonic foraminifera. The observation of the planktonic foraminifera genus Leupoldina for the first time in the basin indicates early Aptian/early late Aptian age for these deposits, and the first opportunity of correlation with global foraminifera biozonation. Our findings have implications for the breakup of the Gondwana Supercontinent, as these incursions represent the earliest marine-derived flooding events in the inland basins of northeastern Brazil.


Geological setting
The Araripe Basin is a large inland basin in northeastern Brazil, associated with the opening of the South Atlantic Ocean 10 . Its development, along several other rift basins in Brazil, was triggered by the breakup of the Gondwana Supercontinent during the Cretaceous [11][12][13][14]21 , and its deposits lay atop Precambrian terrains (Piancó-Alto Brígida and Granjeiro), in the transversal domain of the Borborema Province, to the south of the Patos Shear Zone.
The Santana Group (sensu Neumann and Assine 22 ) is the stratigraphic record of the local Alagoas Stage (Aptian/lower Albian), equivalent to the post-rift I megasequence 11 . The precise age of the deposits of this group is the subject of considerable debate (e.g. 7,15,18,19,[23][24][25]. Though typically assigned to the Aptian, several studies place the upper part of this unit (Romualdo Formation) in the Aptian-Albian interval (e.g. 15,23 ).
The sedimentary succession of the Santana Group is represented by the Barbalha, Crato, Ipubi, and Romualdo formations ( Fig. 1), from base to top, with mixed carbonate-siliciclastic deposits 8,11 . The Barbalha Formation, focus of this study, is divided into two parts 8 . The lower one is essentially siliciclastic, passing from a fluvial to a lacustrine continental environment in a transgressive pattern that culminates in the Batateira Beds, a laterally extensive layer of mudstones and marls that are an important stratigraphic marker for the basin 8 . The type of preservation of the fossils in these beds corroborates an interpretation of continental environment setting, with dysoxic/anoxic intervals 11,12,[26][27][28] . The upper part of the Barbalha Formation is composed of a siliciclastic succession of sandstones and mudstones with a fining upward pattern, that ends with the deposition of the laminated limestones of the Crato Formation reflecting a change from fluvial to hypersaline lacustrine environments (e.g. 29 ).
Widespread marine ingressions have been reported in the Romualdo Formation, during the late Aptian of the basin 8,30 . Its deposits of black shales with fossil-bearing carbonate concretions (e.g. 16,31 ) are famous for their wellpreserved fossils, and include continental (palynomorphs and ostracods 15 ) and marine groups (dinoflagellates 18 , benthonic and planktonic foraminifera 19,20 , marine ostracods 20 , and calcareous nannofossils 20 ). The distribution of these marine organisms, many related to the Tethyan Realm, suggests short marine incursions from the Central Atlantic Ocean (e.g. 7,20,32,33 ), which could have occurred due to sufficient tectonic subsidence rates, accompanied by sea level rise episodes 9,34 at different times during the Early Cretaceous.

Results
Sedimentology and ichnology. We recognized 16 sedimentary facies in the Barbalha Formation (Table 1). The lower part of this unit comprises sandstones and rare muddy deposits, overlain by laminites (Batateira Beds), with low ichnodiversity and low rates on the bioturbation scale (BS). The upper part is characterized by sandstone cosets (locally bioturbated), occasionally intercalated with fine-grained deposits (Fig. 2), overlaid by dominantly muddy deposits with interbedded sandstones (Fig. 3).
Ostracods. Eight species belonging to three genera of non-marine ostracods were identified in borehole 1PS-06-CE ( Fig. 4a- Comparatively, abundance and diversity are lower in borehole 1PS-10-CE, and fossil preservation is moderate to poor. The samples with the highest abundance are from the laminites of the Batateira Beds (101.54 m). Six species belonging to six genera of non-marine ostracods were recovered (Fig. 4a- We also observed 24 specimens of ascidian spicules at 99.30 m, the most abundant calcareous nannofossil assemblage of the borehole, and which are secreted by small (< 0.1 mm) soft-bodied tunicates that live in shallow water environments. They show diverse morphologies (rounded, spheric, and radial) as well as dissolution/ overgrowth preservation effects. Four specimens of Thoracosphaera spp. found at 99.30 m have morphologies marked by fragments with large inner element sizes and low preservation. Calcareous nannofossil distribution can be observed in Fig. 3.    www.nature.com/scientificreports/ were preserved in these laminites: a packstone with disarticulated (reworked) and compressed ostracod valves ( Fig. 5i-j), and a wackestone with articulated, adult-to juvenile-sized ostracod carapaces, associated with serpulid tubes in an organic-rich matrix (Fig. 5j), interpreted as in situ. The serpulids observed are 50-250 µm long tubes with thin microcrystalline calcite walls (dark color in thin sections) and usually well-developed collars (Fig. 5d). Transverse sections are circular to ovoid, well-preserved, with partial dissolution and locally compressed. They are most abundant on the organic muddy microbial matrix among ostracod valves ( Fig. 5g-j) but are absent in the ostracod packstone. The third horizon is represented by the reddish-brown, cemented mudstone of the F3 facies (39.20 m depth, borehole 1PS-06-CE). Planktonic foraminifera specimens are frequent (Fig. 5a-d) and include the genera Leupoldina and Globigerinelloides. Most specimens have undergone some degree of dissolution, and internal molds, partially dissolved tests, and ghosts are prevalent. Poor preservation hinders species-level identification for leupoldinids (described as Leupoldina sp.) and globigerinelloidids (Globigerinelloides cf. barri and G. cf. ferreolensis) (Fig. 5a-c).
Genus Leupoldina is characterized by pseudo-planispiral coiling, radially elongate chambers, and branched disaxial outer chambers, which makes the last whorl asymmetrical. In thin sections, early chambers are globular, and final chambers may be elongated, ovoid to circular, depending on the orientation of the cut, commonly with paired projections arranged symmetrically on each side of a median plane. Paired projections (branching) of final two chambers are typical for most species of Leupoldina (Fig. 5a). Globigerinelloides specimens were identified by their typical planispiral biumbilicate coiling in equatorial sections. The absence of globally distributed marine microfossils in the Brazilian interior basins has historically prevented their correlation with global chronostratigraphic charts, therefore they have only been calibrated using local biozones so far. Here we report for the first time the occurrence of Leupoldina spp. in the Barbalha Formation, in thin sections (Fig. 5a).
Leupoldina has numerous occurrences in the Tethyan region 35 , and is used in global foraminiferal biozonation schemes 36,37 due to its distinct morphology from other planktonic foraminifera of this interval. Poor preservation and cut orientation limited species-level identification of Leupoldina specimens, but their occurrence, associated with Globigerinelloides, does allow us to assign biozones L. cabri to G. algerianus 36 . corresponding to the early Aptian/early late Aptian stage 1 (Fig. 6). This outcome challenges the local biostratigraphic framework 15,38 showing that the chronostratigraphic framework for the post-rift sequence in the Araripe Basin must be reviewed.
Furthermore, the occurrence of age-indicative pollen grains belonging to the species Sergipea variverrucata (local Zone P-270 40 ) constrains the studied intervals to a late Aptian age.
The lack of representatives of typical lower Aptian palynomorph-based biozones is probably due to paleoclimate exclusion 41 . We contend that paleoclimate dynamics might have controlled the local appearance and disappearance of plant species, which directly affected the palynostratigraphic records. Marine Incursions (AMI), are clearly identifiable in the Barbalha Formation (Fig. 7). The Depositional Sequence 1 (sensu Assine et al. 8 ) starts with very coarse-to coarse-grained sandstone facies (St2), superimposed by layers of fine-to very fine-grained sandstone with ripples (Sr) (Fig. 3; Table 1). Muddy levels (F6 and F1) are observed in lateral associations, sometimes interbedded with rippled sandstone (Sr), and as suggested previously, correspond to fluvial channels with laterally well-developed overbank 42 ( Fig. 3; Table 1). Paleocurrent data reveal preferential flow towards SE 8,10,27,42 . The presence of agglutinating foraminifera in the sandstone deposits (1PS-10-CE) indicates brackish conditions possibly due to deltaic influence in this system.
The first marine incursion (AMI-1) can be observed in both boreholes, in marls (facies M) just above transgressive surface TS-1 (Fig. 3 and 7). It is marked by abundant agglutinated foraminifera (Bathysiphon sp.) and  www.nature.com/scientificreports/ the occurrence of calcareous and organic-walled dinocysts, marine elements that occur in association with an ichnofabric composed of Planolites, Palaeophycus, and Thalassinoides. The low ichnodiversity and occurrence restricted to facies F1 points to the establishment of opportunistic colonization due to the onset of brackish conditions 45,46 . We identified the second marine incursion restricted to borehole 1PS-06-CE, AMI-2, in the middle part of the Batateira Beds, associated with its laminite deposits (C facies). It corresponds to a maximum flooding surface (MFS-1) (Figs. 3 and 7), recognized due to recovery of marine elements such as calcareous dinocysts (Thoracoshpaera spp.), calcareous nannofossils (ascidian spicules), and serpulid tubes. The thickness of these lacustrine deposits increases towards southeast, reaching 30 m in borehole 1PS-06-CE (Fig. 3). The presence of these marine elements, resting above the fluvial facies of both wells, reveals a generalized flooding event associated with an increase in the relative sea level.
Non-marine ostracod carapaces are particularly abundant in borehole 1PS-06-CE at the AMI-2 event (more than 1000 specimens recovered per sample at depths 99.80 and 99.30 m). The assemblage contains a large amount of well-preserved juvenile instars and adults, all with closed carapaces, indicating that they died at those life stages (Fig. 5g-j). Interestingly, carapaces of adult Pattersoncypris, a mixohaline genus, are smaller than usual 47 . All these features are likely result of development in a stressful environment and, eventually, a mass mortality event (e.g. 48,49 ). Salinity changes in the depositional setting of the Batateira Beds could be responsible, as these ostracods did not tolerate marine conditions. This mortality event, associated with the presence of serpulids (Fig. 5i-j), foraminifera, calcareous nannofossils, and calcareous dinoflagellates, reinforces the hypothesis of establishment of fully marine conditions during this interval. Towards the top, the Batateira Beds record the reestablishment of lacustrine environments following a sea level drop.
Distally, in borehole 1PS-06-CE and above the SB, bioturbation is characterized by ichnofabrics composed of Planolites, Palaeophycus, Thalassinoides, and Cylindrichnus, as well as monospecific Thalassinoides  (Table 1), indicating establishment of an impoverished expression of the mixed Skolithos-Cruziana ichnofacies (e.g. 50 ). These ichnological characteristics might reflect stressful conditions caused by salinity changes. Moreover, the presence of Scolicia associated with the Srw lithofacies suggests that salinity was sufficient to sporadically support the establishment of a stenohaline fauna.
Scolicia has been mainly recorded in marine environments, from shoreface to deep-sea (e.g. 45 ), and represents bioturbation by spatangoids echinoderms, which are truly marine organisms (e.g. 51 ). The record of Scolicia in stressed substrates of distal, brackish, tidally-dominated, or -influenced channels is scarce (e.g. 52 ). In addition, the higher ichnodiversity (marine elements) suggests increased marine conditions towards the top (Fig. 3).
These fluvial facies are overlain by muddy deposits (F3, F2, and F1) interbedded with sandstones (Sr, Srd, St2, and Sm), interpreted as bayhead delta deposits 34 and suggesting flooding of the fluvial system ( Fig. 3; Table 1). The ichnological assemblages and micropaleontological data support this interpretation of flooding in both boreholes.
The higher ichnodiversity, including Scolicia burrows in 1PS-06-CE, suggests that marine conditions were more stable in the southeastern area. In addition, the occurrence of planktonic foraminifera (Leupoldina sp. and Globigerinelloides spp.) in 1PS-06-CE, laterally associated with ichnological assemblages of the impoverished mixed Skolithos-Cruziana ichnofacies of 1PS-10-CE, shows that marine to brackish conditions followed the onset of the bayhead delta. The presence of Leupoldina sp. and Globigerinelloides spp. also points to the establishment of typical marine conditions in the upper Barbalha Formation, configuring the second maximum flooding surface (MFS-2) (Fig. 3).

Conclusions
We dated the deposits of the Barbalha Formation for the first time based on the recovery of the planktonic foraminifera genera Leupoldina and Globigerinelloides. These genera mark the L. cabri to G. algerianus zones, which corresponds to the early Aptian/early late Aptian interval. Three marine incursions (AMI-1 to AMI-3) were identified in the Barbalha Formation based on a multi-proxy analysis (micropaleontological, ichnological, and sedimentary analyses). The lower two occur in the Batateira Beds (lower Barbalha Formation), recording the primeval marine incursions into the Araripe Basin related to the breakup of the Gondwana, and the opening of the South Atlantic Ocean.
AMI-1, near the base of the Batateira Beds, is characterized by an abundance of the agglutinated foraminifera, and minor occurrence of organic-walled dinocysts and calcareous dinocysts.
AMI-2, in the laminite deposits of the Batateira Beds of borehole 1PS-06-CE, is characterized by calcareous dinocysts, ascidian spicules, microbial peloids, and abundant serpulid tubes. The mass mortality event of nonmarine ostracods reinforces this marine incursion event.
AMI-3, in the upper part of the Barbalha Formation, is characterized by the occurrence of bioturbation mainly represented by Scolicia, Diplocraterion, Thalassinoides, Ophiomorpha, Skolithos, and Chondrites, as well as the presence of planktonic foraminifera Leupoldina spp. and Globigerinelloides spp.
These incursions are the oldest recorded so far related to the breakup of the Gondwana Supercontinent and the opening of the South Atlantic Ocean. Future studies with fossil records, geochemistry, and paleomagnetism will contribute to the characterization of these marine deposits, as well as the routes taken by these marine incursions.

Materials and methods
Sedimentary sections. We described and analyzed core samples taken from boreholes 1PS-06-CE and 1PS-10-CE, drilled by the Santana II Project 53 in the eastern portion of the Araripe Basin, northeastern Brazil. Lithological, ichnological, microbiofaciological, and micropaleontological analyses were completed, preferentially done in the fine siliciclastic and limestone lithologies (shales, mudstones, siltstones, marls, and laminites), but also in some sandstone levels. For further details, we provide the full dataset of microfossils/microbiofacies in the supplementary materials (SM).
Sedimentological, ichnological, and stratigraphic analysis. The characterization of the sedimentary facies of boreholes 1PS-06-CE and 1PS-10-CE followed the usual methods, with description of physical sedimentary structures and basic lithology, focusing on lithofacies and ichnology. Intervals with no recovery were interpreted based on the accompanying well drilling data, namely gamma-ray values (indirect data); a small portion was obtained from cut samples. For the purposes of stratigraphic correlation, the laminite deposits of the Batateira Beds, in the lower part of the Barbalha Formation, were chosen as our datum; specifically, the levels containing serpulids and the ostracods mass mortality event, which occur on both drill cores.
Direct macroscopic observation was used to describe trace fossils, following the main ichnotaxobases applicable to ichnofabrics: burrow limit, burrow infill, branching pattern, and presence/absence of spreiten. The guidelines used for ichnotaxonomy classification followed Bertling et al. 54 . In some cases, identification of ichnotaxa was hampered by the loss of ichnotaxonomical features, dark lithology, and/or core deformation, which erased this data, as well as the restrictions inherent to observations made using two-dimensional core surfaces.
The amount of bioturbation was quantified considering the average diameter of 4 cm for the cores and using the bioturbation scale (BS) proposed by Reineck 55 , which ranges from 0 (non-bioturbated) to 6 (completely bioturbated). Data from the Rio da Batateira outcrop 8,33 was taken into consideration for the creation of the stratigraphic section, due to its geographical proximity and relevant correlation to the stratigraphic sequence.
Calcareous microfossils. Samples were prepared for the recovery of both ostracods and foraminifera using the methodology developed for the rocks of the Romualdo Formation used by Bom et al. 49 , and consisted of the immersion of 20 g of sediment in 200 mL of deionized water with 3 mL of Extran for 24 h. The sediments were then washed through 250, 180, 63, and 45 µm sieves, and dried for 48 h in a lab oven at 40 ºC. After hand picking, we imaged the most representative specimens in an EVO/MA15 Zeiss scanning electron microscope (SEM).
No planktonic foraminifera species were recovered in washed samples, probably due to the strong cementation and poor preservation of the material in the sampled interval. Thus, their analysis was only possible through the thin sections made for microbiofacies studies.
All the studied material is currently stored in the micropaleontologic collection of the Museu de História Geológica do Rio Grande do Sul (MHGRS), Unisinos University, Brazil, under the curatorial numbers ULVG 13482 to ULVG 13491.
Organic-walled microfossils. We processed approximately 40 g of each sediment sample for palynology analysis, following standard techniques 56 . Minimums of 200 palynomorphs were counted in each sample for the palynological method. Species scanning, identification and counting were carried out with a Zeiss Imager. A2 microscope, using bright field illumination and incident blue light (fluorescence mode) at 200× , 400× , and 1000× (oil) magnifications. Photomicrographs were taken using a Zeiss AxioCam MRc (Micropaleontology Reference Center) digital camera. Our palynological analysis recorded pollen grains, spores, dinocysts, and freshwater green algae. The slides are stored in the collection of the MHGRS, Unisinos University, Brazil, under the curatorial numbers ULVG 13595 to ULVG 13682. www.nature.com/scientificreports/ Calcareous nannofossils. Sample preparation for calcareous nannofossil analysis followed the decantation methodology described in Bown and Young 57 . Each sediment sample was fragmented in an agate mortar and placed in a Falcon tube with 40 mL of deionized water. The solution was stirred for 30 s and then set to decant for five minutes. The supernatant (approximately 0.2 mL) was then collected, poured onto a coverslip, and placed in a hotplate (60 °C) to dry. After dried, the coverslip was placed on a slide with Norland optical adhesive 61 and cured under UV light. The slides were examined using a Zeiss Axio Imager.A2 microscope, at 1000× magnification. Data was processed using the software Zen 3.0 (blue edition) for micrometric measurements. The slides are stored in the collection of the MHGRS, Unisinos University, Brazil, under the curatorial numbers ULVG 13492 to ULVG 13594.
Microbiofacies. Lithology and preservation degree throughout the cored sections controlled the sampling interval. Select intervals of alternated mudstone-packstone and shale-siltstone required higher sampling density, while coarser-grained intervals were strategically undersampled.
We used a ZEISS Axioscope 5 petrographic microscope for microfossil identification and lithologic analysis, with transmitted and polarized light, objectives of 2.5× , 5× , 10× , and 20× magnification, and an attached digital camera. Data was processed using the software Zen 3.0 (blue edition) for micrometric measurements.