Biostratigraphic framework and age controls
The absence of globally distributed marine microfossils in the Brazilian interior basins prevented their correlation with global chronostratigraphic charts, and has thus so far only been calibrated using local biozones. Here we report for the first time the occurrence of Leupoldina spp. in the Barbalha Formation, as per observation in thin sections (Fig. 4A-B). Leupoldina is a genus of planktonic foraminifera, with numerous occurrences in the Tethyan region54, that is used in global foraminiferal biozonation schemes55. Among the Leupoldina species (L. cabri, L. pustulans, L. pentacamerata, L. hexacamerata, and L. reicheli), L. reicheli rarely presents bifurcation in the last chamber, which is otherwise very characteristic for the other four species, particularly L. cabri54. Most of the specimens found in borehole 1PS-06-CE (39,20 m) show clear last chamber bifurcation (Fig. 4A), which leads us to exclude L. reicheli as a species candidate for those bifurcate specimens.
This consideration is important, as L. reicheli´s stratigraphic range is the widest among Leupoldina species (from Leupoldina cabri to Globigerinelloides algerianus zones55, while the other four species range from biozones L. cabri to Globigerinelloides ferreolensis55. We assigned the corresponding interval to biozones L. cabri to G. ferreolensis55, which corresponds to the early Aptian/early late Aptian stage1 (Fig. 5). This outcome challenges the local biostratigraphic framework27,56 and implies that the previous understanding of the chronostratigraphic framework for this post-rift sequence in the Araripe Basin must be reviewed.
Concerning the local zonation schemes, the presence of the age-indicative ostracods Pattersoncypris spp. and Theriosynoecum silvai at boreholes 1PS-06-CE and 1PS-10-CE suggests the local Alagoas Stage (Aptian–early Albian) for both sites (Zone 011, sensu Moura58,59; also called the Pattersoncypris and Krommelbeincypris Zone60.
Furthermore, the occurrence of age-indicative pollen grains belonging to the species Inaperturopollenites turbatus (local Zone P-26061) constrains the studied intervals to an early late Aptian age. The uppermost occurrence of this taxon, however, does not represent its last occurrence datum, since a previous study found a continuous record of the same index species through most of the overlying Romualdo Formation30. Moreover, the absence of marker palynomorphs from older biozones (I. curvimuratus and Tucanopollis crisopolensis) also support an age as young as late Aptian61.
The lack of representatives of typical lower Aptian palynomorph-based biozones is probably due to paleoclimate exclusion. Recent reconstructions of atmospheric circulation patterns suggest that a proto-Intertropical Convergence Zone (proto-ITCZ) played a key role in paleoflora distribution in Northeastern Brazil during the Early Cretaceous62. We contend that paleoclimate dynamics might have controlled the local appearance and disappearance of plant species, which directly affected the palynostratigraphic records.
Multi-proxy evidence of marine incursions
Three marine transgressive events, here named Araripe Marine Incursions (AMI), are clearly identifiable in the Barbalha Formation (Fig. 6). The Depositional Sequence 1 (sensu Assine et al.14) starts with very coarse- to coarse-grained sandstone facies (St2), superimposed by layers of fine- to very fine-grained sandstone with ripples (Sr3) (Fig. 7; Table 1). Muddy levels (F6 and F1) are observed in lateral associations, sometimes interbedded with rippled sandstone (Sr3), and as suggested previously, correspond to fluvial braided channels with laterally well-developed overbank63 (Fig. 7; Table 1).
Paleocurrent data reveal preferential flow towards SE14,22,40,63. The presence of agglutinating foraminifera in sandstone deposits (1PS-10-CE) suggests brackish conditions64 and might indicate deltaic influence in this fluvial system.
The fluvial braided channels and overbank deposits are overlaid by fine-grained facies of the Batateira Beds65. The facies observed in the Batateira Beds are siliciclastic (F1 and F2), intercalated with heterolithic facies (Hsr). They have been classified as a lacustrine system (Fig. 7; Table 1), due to the occurrence of non-marine elements such as ostracods, fish, and continental palynomorphs (e.g.,23,66−68). We have also verified the abundance of non-marine ostracod fauna (Pattersoncypris micropapillosa, P. salitrensis, P. angulata, Pattersoncypris sp. 1, Pattersoncypris sp. 2, Candonopsis alagoensis, and Candona? Sp.) in the deposits studied (Fig. 3A-H). Otherwise, ichnological and micropaleontological data support the input of marine waters in two distinct intervals, producing a salinity gradient between brackish to freshwater conditions in a lacustrine paleoenvironment.
The first marine incursion (AMI-1) is observed in both boreholes, just above the transgressive surface TS-1 (Fig. 7). It is marked by the presence of abundant agglutinated foraminifera and the occurrence of calcareous and organic-walled dinocysts. These marine elements occur in association with ichnofabric composed of Planolites, Palaeophycus, and Thalassinoides with low ichnodiversity, localized and restricted to facies F1, which points to the establishment of opportunistic colonization due to the onset of brackish conditions69–71.
The second marine incursion, AMI-2, is observed in the middle part of the Batateira Beds associated with the laminite deposits of the C facies and corresponds to the maximum flooding surface (MFS-1). It is inferred based on the recovery of local marine elements such as calcareous and organic-walled dinocysts72, calcareous nannofossils (Thoracosphaera spp. and ascidian spicules), serpulid tubes, and agglutinated foraminifera (Bathysiphon 1PS-10-CE). The thickness of these lacustrine deposits increases towards SE (borehole 1PS-06-CE: approximately 30 m thick) (Fig. 7). The presence of marine elements just above the TS-1 and in the middle of these lacustrine deposits (MFS-1), resting above the fluvial facies of both wells, reveals that there was a generalized flooding event associated with an increase in the relative sea level.
During the AMI-2 event, non-marine ostracod carapaces are particularly abundant in borehole 1PS-06-CE (more than 1,000 specimens recovered at the depths of 99.80 and 99.30 m), and the assemblage contains a large amount of well-preserved juvenile instars and adults, all with closed carapaces, indicating that they died at that life stage (Fig. 4G-J). Unusually, carapaces of adult Pattersoncypris, a mixohaline genus, are smaller than usual60. All these features are likely the result of development under a stressful environment and, eventually, a mass mortality event (e.g.,73,74). This could have been caused by salinity changes in the depositional setting of the Batateira Beds, as these ostracods did not tolerate fully marine conditions. This mortality event, associated with the presence of serpulids (Fig. 4I-J), foraminifera, calcareous nannofossils, and dinoflagellate cysts, reinforces the hypothesis of the establishment of full marine conditions during this interval. Towards the top, the Batateiras Bed record reestablishment of lacustrine environments following sea-level drop.
The overlying coarse-grained facies record subaerial exposure and the reactivation of the fluvial system and the beginning of Depositional Sequence 2, as interpreted by Assine et al.14 and Scherer et al.63. The fluvial system strongly eroded the lake system deposits, creating a clear sequence boundary (SB) (Fig. 7). Paleocurrent measurements taken from outcrops (e.g., Rio Batateira) show predominant paleoflow towards a SE direction14,22,40,63,68 Sandy deposits (Gt, Sgt, St1, and St2) related to fluvial channels are predominant in proximal settings (borehole 1PS-10-CE), and facies F2 and F3 represent overbank deposits in an underdeveloped floodplain within the river system40,52,63. The blocky and slickensides structures, associated with rhizobioturbation in F3 facies, also indicate eventual subaerial exposure75.
Down depositional dip and above the SB in the borehole 1PS-06-CE, bioturbation is characterized by ichnofabric composed of Planolites, Palaeophycus, Thalassinoides, and Cylindrichnus, as well as monospecific ichnofabrics Thalassinoides (Hsr, F1, and F3). This set of ichnofabrics with low ichnodiversity and low bioturbation scale (BS = 1–2) reflects stressful conditions69–71 and are related to the colonization of softgrounds, possibly stressed by salinity fluctuations (e.g.,76). They are an expression of the impoverished ichnofacies of Cruziana due to the appearance of brackish conditions in the distal part of the fluvial setting.
Ichnodiversity increases towards the top in distal settings, where bioturbation (Srd, Srw, St4) is represented by ichnofabric composed of Diplocraterion, Ophiomorpha, Palaeophycus, Planolites, Thalassinoides, Chondrites, Helmintopsis, Scolicia, Lockeia, and Skolithos. A moderate to high bioturbation index (BS 4–5) (Fig. 2) indicates the establishment of an impoverished expression of the mixed Skolithos-Cruziana ichnofacies (e.g.,76,77), and marks the beginning of AMI3. 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.,69,78−80), and represents bioturbation by spatangoids echinoderms, which are truly marine organisms (e.g.,81). The record of Scolicia in stressed substrates of distal, brackish, tidally dominated or influenced channels is scarce (e.g.,82). In addition, the higher ichnodiversity (ichnological assemblages) suggests increased marine conditions towards the top (Fig. 5).
The fluvial facies are overlain by muddy deposits (F3, F2, and F1) interbedded with sandstones (Sr, Srd, St2, and Sm). They are related to bayhead delta deposits52 (Fig. 7; Table 1), suggesting the flooding of the fluvial system. The ichnological assemblages and micropaleontological data support this interpretation of flooding in both boreholes.
In borehole 1PS-10-CE, the bioturbation above the TS-2 is characterized by an ichnofabric composed of Diplocraterion, Thalassinoides, Palaeophycus, Planolites, Chondrites, Cylindrichnus, Helminthopsis, Scolicia, Teichichnus, and Skolithos, with BS 3–4. They are restricted to the Srd, and St2 lithofacies (Fig. 7). This ichnological assemblage is indistinguishable from the one observed in the fluvial deposits of the SE portion (1PS-06-CE), and points towards the establishment of an impoverished expression of the mixed Skolithos-Cruziana ichnofacies (e.g.,76,77) and continuous marine influence in the deposits.
The higher ichnodiversity, including Scolicia burrows (1PS-06-CE borehole), suggests marine conditions were more stable in the SE area. In addition, organic-walled-dinocysts are observed associated to this ichnofauna in facies F2 and F3 of both boreholes, reinforcing the input of marine waters in the depositional system. Finally, the occurrence of planktonic foraminifera (Leupoldina and globigerinelloids) in 1PS-06-CE, laterally associated with ichnological assemblages of the impoverished mixed Skolithos-Cruziana ichnofacies of 1PS-10-CE, suggests that marine to brackish conditions followed the onset of the bayhead delta. However, the presence of Leupoldina and globigerinelloids point to the establishment of typical marine conditions in the upper Barbalha Formation corresponding to the second maximum flooding surface (MFS-2) (Fig. 7).