Dual Tectonic-Climatic Controls on Salt Giant Deposition in the Santos Basin, Offshore Brazil GEOSPHERE; V

Research Paper

GEOSPHERE Dual tectonic-climatic controls on salt giant deposition in the Santos Basin, offshore Brazil GEOSPHERE; v. 14, no. 1 C.R. Rodriguez1, C.A-L. Jackson1, A. Rotevatn2, R.E. Bell1, and M. Francis3 1Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, SW7 2BP, United Kingdom doi:10.1130/GES01434.1 2Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway 3WesternGeco Schlumberger, Schlumberger House, Gatwick Airport, Horley, West Sussex, RH6 0NZ, United Kingdom 13 figures; 3 tables

CORRESPONDENCE: ABSTRACT of our study, although based on an analysis of Aptian salts preserved offshore c.rodriguez11@imperial.ac.uk Brazil, offer valuable insights into the sedimentology and stratigraphic archi The stratigraphic evolution of ancient salt giants is controversial, mainly tecture and evolution of other ancient salt giants. CITATION: Rodriguez, C.R., Jackson, C.A-L., Rote- vatn, A., Bell, R.E., and Francis, M., 2018, Dual due to the absence of modern analogues that are of comparable scale and tectonic-climatic controls on salt giant deposition in thickness and that occur in similar tectonic and hydrological settings. Fur the Santos Basin, offshore Brazil: Geosphere, v. 14, thermore, investigating the original stratigraphy of salt giants is often made INTRODUCTION no. 1, p. 215–242, doi:10.1130/GES01434.1. difficult by postdepositional flow and dissolution. Layered evaporites of the Ariri Formation in the Santos Basin (offshore Brazil), deposited during open Ancient saline giants (sensu Hsü, 1972) or salt giants (sensu Hübscher et al., Science Editor: Shanaka de Silva ing of the South Atlantic Ocean, form part of one such salt giant. Despite 2007) are areally extensive (>100,000 km2), hundreds to thousands of meters being well imaged in seismic data and being penetrated by more than 50 thick, evaporite-dominated successions, deposited in hydrologically restricted Received 22 September 2016 Revision received 4 September 2017 boreholes, little work has explored the stratigraphic architecture of this unit basins in a wide range of tectonic settings (Hudec and Jackson, 2007; Warren, Accepted 8 November 2017 and what this may tell us about the syndepositional tectonics, basin physiog 2010). The stratigraphy of salt giants is mainly controlled by the solubility of Published online 12 January 2018 raphy, and variations in climate and sea level. Here we integrate three-dimen different evaporite minerals, which precipitate following an idealized precipi- sional seismic and borehole data from the São Paulo Plateau, deep-water tation sequence; i.e., first carbonates, then gypsum or anhydrite, followed by Santos Basin, to document the intrasalt stratigraphy of the Ariri Formation. halite and ending with bittern salt (K- and Mg-rich salts; e.g., Usiglio, 1849; Our analysis suggests a combination of an arid paleoclimate, low-amplitude Clarke, 1924). During the initial stages of partial restriction and hydrological local sea-level variations, and basin physiography controlled the deposition drawdown in a carbonate-evaporite basin, carbonate platforms are exposed of this thick (2.5 km) salt sequence during a short time span (<530 k.y.). The and gypsum precipitates at the basin margins; if the water level drops below Ariri Formation records at least 12 cycles of basin desiccation and filling, re the basin sill, halite and eventually bittern salts may precipitate in salt pans sulting in the deposition of four key units (A1–A4) that have a distinct com and lakes located in the deepest, most isolated parts of the basin (Fig. 1; e.g., position and therefore seismic expression; i.e., low-frequency, transparent, Hsü, 1972; Tucker, 1991). Breaching of the sill may recharge the basin with chaotic seismic facies represent relatively halite-rich (>85%) units (A1 and saline fluid and halite may fill the basin. A new evaporite cycle starts when the A3), whereas high-frequency, highly reflective seismic facies represent still hypersaline basin is reflooded and gypsum is deposited (Fig. 1; Tucker, 1991). relatively halite-rich (65%–85% halite) units, but contain relatively high pro The applicability of this general model to ancient salt giants is uncertain, partly OLD G portions (15%–35%) of anhydrite and bittern salts (i.e., K- and Mg-rich salts; reflecting the lack of modern analogues that are of comparable scale and thick- A2 and A4 units). Our findings suggest that during salt deposition the Santos ness, and that occur in similar tectonic and hydrological settings. In addition, Basin was characterized by a series of subbasins of varying water depth; as a the original stratigraphy of salt giants is likely to be altered during and/or after result the thickness and composition of these units vary laterally and are spa deposition due to dissolution, changes in mineral phase (i.e., anhydrite to gyp- OPEN ACCESS tially related to structural domains. Overall, thinner salt (~1.8 km) and higher sum and vice versa), and salt flow, which can result in tectonic modification of anhydrite net thickness (~350 m) occur toward the structurally high Sugar the primary depositional stratigraphy due to preferential expulsion of low-vis- Loaf domain, compared to flanking, structurally lower domains where the cosity units (Kupfer, 1968; Warren, 2006; Hudec and Jackson, 2007; Cartwright mean salt thickness is >2.2 km and anhydrite net thickness are less (~180 m). et al., 2012; Jackson et al., 2014a). In addition, stratigraphic variations in the basin suggest that seawater incur Our current knowledge of ancient salt giants is constrained by field data sions came from the south, through the São Paulo and Walvis Ridges; conse (e.g., Jackson et al., 1990; Reuning et al., 2009; Stefano et al., 2010), seismic re- This paper is published under the terms of the quently, more anhydrite was deposited closer to the ridge, whereas more bit flection data that image intrasalt stratigraphy (e.g., Van Gent et al., 2011; Fiduk CC‑BY-NC license. tern salts were deposited in more distal and restricted locations. The results and Rowan, 2012; Schoenherr et al., 2007; Strozyk et al., 2012; Jackson and

GEOSPHERE | Volume 14 | Number 1 Rodriguez et al. | Dual tectonic-climatic controls on salt giant deposition in the Santos Basin, offshore Brazil 215 Research Paper

and volume of fresh-water river discharge, which may modify brine salinity. 4 Basin-fill halite (BFH), early TST gypsum For example, deposition of the Paradox Formation (upper Carboniferous) in North America was primarily controlled by climate and third-order sea-level TST HST variations driven by greenhouse (transgressive phases) and icehouse condi- HST LS-BFH TST tions (regressive phases), thus highlighting the key role of climatic and sea- TST level variations (Eoff et al., 2013). Transgressive phases were characterized by fully marine conditions and the deposition of organic-rich black shales, whereas regressive phases were characterized by basin desiccation and salt Lowstand halite/bittern salts pans and lakes 3 (i.e., anhydrite, halite, bittern salts) deposition. Climate and sea-level varia- seepage tions also influenced the stratigraphy of the Messinian (upper Miocene) salt HST HST in the Mediterranean Sea, where the deposition of gypsum-shale alternations TST TST along the basin margins were controlled by periodic changes in salinity due to low-amplitude, high-frequency, fifth-order, precession-driven climate cycles (Krijgsman et al., 1999; Stefano et al., 2010; Manzi et al., 2012). There is also clear evidence for the impact of syndepositional tectonics on evaporite stratig- 2 Lowstand gypsum wedge during slow sea level fall raphy; for example, the foreland basin setting of the Paradox Basin controlled not only bulk thickness, but also the distribution of lithologies in the Paradox sl HST Formation, with carbonates deposited in the distal forebulge in shallow water sill and/or TST barrier depths, whereas halite and other evaporites were deposited in the basin center during periods of drawdown (Trudgill, 2011; Eoff et al., 2013). Syndepositional tectonics also controlled the thickness and stratigraphic development of the Zechstein Supergroup (upper Permian) in northwest Europe. During deposition 1 Highstand open basin, carbonate rim sl open of the Zechstein Supergroup, basin physiography was controlled by predepo- ocean sitional and syndepositional rift-related normal faulting, with halite-rich units HST sill and/or occurring in the basin center, whereas on intrabasinal highs, carbonate-rich TST barrier and anhydrite-rich units were deposited (e.g., Taylor, 1990; Tucker, 1991; Clark et al., 1998; Stewart and Clark, 1999; Stewart, 2007; Jackson and Lewis, 2013). pinnacle reefs and/or In this study we integrate and analyze three-dimensional (3D) seismic re- mud mounds flection and borehole data from the deep-water Santos Basin, offshore Brazil, to provide new insights into the deposition and evolution of salt giants. Our Carbonates Gypsum/ Halite Bittern salts results show that basin physiography and sea-level variations can play key Anhydrite roles during the deposition of thick (2.5 km) salt sequences.

Figure 1. Conceptual model for the stratigraphic architecture of a carbonate-evaporite basin subjected to complete drawdown (after Tucker, 1991). TST—transgressive systems tract; HST— Early Cretaceous Salt of the South Atlantic: highstand systems tract; LST—lowstand systems tract. 1—Carbonate deposition during high Previous Studies and Controversies stands. 2—Gypsum deposition during slow sea-level (sl) fall. 3—Halite and bittern salts are deposited during lowstands and basin desiccation. 4—Gypsum is deposited during refill of the An Aptian salt giant, covering ~741,000 km2 and as thick as 2.5 km, was basin. deposited during opening of the South Atlantic Ocean (Fig. 2A; e.g., Dias, 2004; Davison, 2007). Although the tectonics and geodynamics of South Atlantic Lewis, 2013; Jackson et al., 2014a, 2014b, 2015b), and boreholes that penetrate opening have been extensively studied, several key questions remain regard- salt-bearing sequences (e.g., Clark et al., 1998; De Freitas, 2006; Massoth and ing the deposition of the Aptian salt giant. (1) How rapidly was the thick (2.5 km) Tripp, 2011; Trudgill, 2011; Eoff et al., 2013; Jackson et al., 2014a, 2014b, 2015b). salt deposited? (2) What mechanism or mechanisms generated accommoda- These data suggest that depositional thickness and intrasalt stratigraphy are tion for thick salt deposition, thus in what tectonic context was the salt depos- controlled by several factors that may act individually or collectively during ited? (3) Was the salt deposited in shallow or deep water? (4) Was the seawater salt giant deposition; i.e., (1) climate; (2) sea-level variations; (3) duration of salt inflow into the Santos Basin from the north or south? In this study we contrib- deposition; (4) syndepositional tectonics and basin physiography; (5) the dis- ute to the debate by analyzing the intrasalt stratigraphy in the Santos Basin, tance to the basin sill and the direction of seawater inflow; and (6) the location but first we review some of the previous studies.

GEOSPHERE | Volume 14 | Number 1 Rodriguez et al. | Dual tectonic-climatic controls on salt giant deposition in the Santos Basin, offshore Brazil 216 Research Paper

A B Sergipe- N CAMPOS BASIN Alagoas South 100 km Gabon Reconcavo CABO FRIO HIGH Congo BRAZIL

Camamu-Almada

F Lower ig. 2C JEQUITINHONHA Congo

CUMURUXATIBA

3D survey Mucuri ALBIAN GAP 329D Figure 2. (A) Early Cretaceous paleo Kwanza 711 geography illustrating the South At 369A SAO PAULO PLATEAU SANTOS BASIN lantic salt giant (after Lentini et al., Espirito 532A 2010). COB—continent-ocean bound Santo 709 Angola 723C ary. (B) Geographic setting of the

Brazil 1-ESSO-3 Santos Basin. (C) Geoseismic section Campos CAMPOS illustrating the passive margin struc OUTER HIGH tural domains and illustrating the salt thickness variations across the Santos Fig. 2B Basin. Two-dimensional seismic line thin salt courtesy of WesternGeco. variable thickness JEAN CHARCOT SEAMOUNTS Santos thick salt terminal horsts COB IDGE

Fracture zones R L FFF: Florianopolis E

Fracture zone BIMA LT FLORIANOPOLIS A VOLCANIC BE sill /FFF PLATFORM lcanic 500 km Vo FLORIANPOLIS FRACTURE ZONE Ocean

3D survey / Area of study

Proximal/Extensional domain Sao Paulo Plateau Deep salt basin C Minibasin domain NNW SSE

basinward limit of Albian Gap 1500 1-ESSO-3

3500

5500 Depth (m)

Outer High 7500

9500 Key 50 km post-salt salt pre-salt Faults

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The age of the South Atlantic salt giant is poorly constrained; previous stud- isolated basins by saline waters, as the basin became connected to the open ies suggest a deposition between 10 and 0.4 Ma (Jackson et al., 2000; Szatmari, ocean during rifting, eventually becoming entirely evaporitic by the late Aptian 2000; Dias, 2004; De Freitas, 2006; Davison, 2007; Karner and Gambôa, 2007; (e.g., Bate, 1999; Dingle, 1999; Karner and Gambôa, 2007; Reston, 2010). Moreira et al., 2007; Montaron and Tapponnier, 2010). However, most agree on Most studies suggest that the connection to the open ocean and recur- a rapid deposition (<600 k.y.), based on analysis of core data (Dias, 2004), cor- rent seawater inflow was from the south, where a barrier composed of the Rio relation of evaporites stratigraphy with Milankovitch cycles (De Freitas, 2006), Grande Rise and Walvis and São Paulo Ridges confined the evolving South comparison to other salt basins (Davison, 2007), and the predictions of numer- Atlantic, thus providing the necessary conditions for salt deposition and pres- ical models (Montaron and Tapponnier, 2010). ervation. These volcanic ridges formed during Paraná-related volcanism in the In terms of accommodation for thick salt deposition, three main mecha- Barremian, shortly before salt deposition, and are believed to have collectively nisms have been proposed: (1) salt deposition occurred within a preexisting acted as a barrier to ocean-water circulation before open-marine conditions basin during tectonically quiescent conditions (Szatmari, 2000; Burke et al., were established in the Albian (Evans, 1978; Karner and Gambôa, 2007; Gam- 2003; Dias, 2004; Davison, 2007; Davison et al., 2012); (2) salt deposition oc- bôa et al., 2008; Davison et al., 2012). However, a somewhat more controversial curred during rifting (i.e., it is a synrift deposit), prior to the onset of post-rift model suggests that intermittent seawater incursions flooded from the north thermal subsidence (Karner et al., 2003; Karner and Gambôa, 2007; Gambôa via a series of early rifts or seaways formed in the central Atlantic (Dingle, et al., 2008; Torsvik et al., 2009; Scotchman et al., 2010); and (3) loading of 1999; Bate, 1999; Scotchman et al., 2006; Arai, 2014). For example, according the crust by thick salt drove local subsidence and led to the development of to Scotchman et al. (2006), analysis of ostracod data suggests that the volcanic accommodation (e.g., Dias, 2004; Karner and Gambôa, 2007; Davison et al., Walvis Ridge was not breached until Cenomanian–Turonian time, hence pre- 2012). The first mechanism suggests that rift-related faulting had ceased at the venting marine waters entering during the estimated period of salt deposition. onset of salt deposition, with deformation giving rise to a preexisting basin An alternative entry point for seawater is thus postulated, suggesting seawater defined by several discrete lakes and that was as much as 2 km below sea percolation through fracture zones in the volcanic ridges to the south, thus re- level (Szatmari, 2000; Burke et al., 2003). The lakes were bound by preexist- charging the salt basin (Jackson et al., 2000; Nunn and Harris, 2007; Montaron ing, normal fault–related scarps that controlled salt thickness across the basin, and Tapponnier, 2010). with relatively thick (<2 km) salt deposited within the lows and relatively thin (<1 km) salt across flanking highs (Dias, 2004; Davison, 2007; Davison et al., Stratigraphy of the Ariri Formation: Previous Studies 2012). In contrast, studies favoring a synrift salt interpretation postulate that salt deposition occurred during the latest phase of rifting, characterized by mi- Despite being imaged by high-quality, regionally extensive 2D and 3D seis- nor late Aptian normal faulting and thermal subsidence (Karner et al., 2003; mic reflection data and penetrated by 50–100 boreholes, surprisingly few stud- Karner and Gambôa, 2007; Torsvik et al., 2009; Reston, 2010; Scotchman et al., ies have attempted to characterize the composition and stratigraphic architec- 2010). In addition, local synsalt faulting and accommodation development may ture of the Ariri Formation in the Santos Basin, offshore Brazil (Demercian, have occurred due to salt deposition and crustal loading (Dias, 2004; Karner 1996; De Freitas, 2006; Davison, 2007; Davison et al., 2012); as a result, it is not and Gambôa, 2007; Davison et al., 2012). clear how this regionally extensive stratigraphic unit, which spans much of the Despite uncertainties regarding the precise tectonic setting of Aptian salt Brazilian margin, records syndepositional tectonics, or climatic and sea-level deposition, there is a general consensus with regard to the water depth within variations. Seismic and borehole data from the São Paulo Plateau, which is which the salt was deposited. Except for Karner and Gambôa (2007) and Monta the major structural element in the Santos Basin, indicate that the Ariri Forma- ron and Tapponnier (2010), who suggested salt deposition in relatively deep tion is characterized by highly reflective multilayered evaporites. The unusual waters (1.5–3 km), others support a shallow-water depositional environment seismic character of the Ariri Formation was first recognized on 2D seismic based on the following evidence: (1) the stratigraphic context of the salt, above reflection data by Cobbold et al. (1995) and was assumed to be composed of continental and/or lacustrine deposits and below shallow-water limestones interbedded halite, anhydrite, and clastics (e.g., Mohriak et al., 2012). An ex- (e.g., Szatmari et al., 1979; Moreira et al., 2007; Palagi, 2008); (2) subaerial exploration borehole drilled in 2002 revealed the Ariri Formation was composed posure and karstification of the pre-salt sag sequence (Gomes et al., 2009); and of at least 2.5 km of interbedded evaporites, including halite, anhydrite, and (3) paleontological evidence from the base of Aptian carbonates capping the salt bittern salts (e.g., carnallite, bishofite, sylvite, and tachyhydrite; Poiate et al., on the Angolan margin, suggesting water depths of <500 m (Marton et al., 2000). 2006; Scotchman et al., 2010; Mohriak et al., 2012). Therefore, within this depositional context, the proto–South Atlantic was The De Freitas (2006) correlation of two boreholes separated by more than occupied by lakes of varying salinity and, as salinity increased, these lakes 200 km indicates that halite and bittern salt layers thicken basinward, whereas were replaced by hypersaline lagoons (e.g., Szatmari et al., 1979; Burke et al., interbedded anhydrite layers thicken landward (Fig. 3). De Freitas (2006) sug- 2003; Aslanian et al., 2009). The transition from a lacustrine to a lagoonal, gested that this pattern reflects overall lower salinities near the land due to the salt-precipitating depositional environment was the result of partial filling of periodic input of fresh water. Lateral variations in lithology in the Ariri Forma-

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SPS-04A 270 km RJS-598D GR Density GR Density [m] 0

100

200

300

400

500

66% Halite 18% Anhydrite 16% Potash salts

? 81% Halite ? 5% Anhydrite Carbonates and sandstones 14% Potash salts Carbonates and pelites ?

Anhydrite ? Halite

Potash salts Figure 3. Regional intrasalt well correlation between two boreholes 270 km apart Carbonates and/or marls (after De Freitas, 2006). GR—gamma ray. Volcanics

tion were described by Moreira et al. (2007) and Gambôa et al. (2008), with been unable to constrain the regional compositional variations or the overall halite, anhydrite, tachyhydrite, carnallite, and locally sylvite in the basin center, stratigraphic architecture of the Ariri Formation, or the role syndepositional thinning and passing into main dolomite and anhydrite in the basin margins. tectonics or variations in climate and water depth played during salt deposi- In addition, seismic facies analysis of the Ariri Formation has divided the suc- tion; this is the focus of our study. cession into four key seismic-stratigraphic units (Fig. 4; Gambôa et al., 2008) or six mechanostratigraphic intervals (Fig. 4; Fiduk and Rowan, 2012). Even though the detailed stratigraphic architecture of the Ariri Formation TECTONOSTRATIGRAPHIC FRAMEWORK remains very poorly constrained, previous regional studies in the South Atlan- tic, based largely on 2D seismic reflection data and sparse or no borehole data, The Santos Basin (350,000 km2) is the widest (~500 km) of the South Atlantic suggest that a >2-km-thick, halite-dominated succession was deposited in the salt basins (Fig. 2A; e.g., Davison et al., 2012; Guerra and Underhill, 2012), and basin center, whereas thinner (~200 m) carbonate-, anhydrite-, sylvite- and is bound to the west by the Brazilian mainland, and to the north by the Cabo tachydydrite-bearing successions were deposited on highs along the basin Frio high and the Campos Basin (Fig. 2B). To the south, the basin is bound by margins (e.g., Chang et al., 1992; Karner and Gambôa, 2007; Lentini et al., 2010). the Florianopolis high, and the volcanic belt north of the Florianopolis fracture It is clear that, due to a lack of focus on the salt as a depositional entity zone and the São Paulo Ridge, and to the east by the Jean Charcot volcanic and the unavailability of deep-water borehole data, previous studies have seamounts. The São Paulo Plateau is the main structural element in the Santos

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Seismic Basin, forming an ~500-km-wide area that is characterized by relatively deep Chronostratigraphy Lithostratigraphy Horizons water (2000–3000 m) and the presence of a prominent intrabasinal structural Period Epoch/Stage Formation high, the Outer High (Figs. 2B, 2C) (Gomes et al., 2002, 2009). The Outer High is characterized by half-grabens developed during Barremian rifting, when non- Pleistocene Top marine to shallow-marine sediments of the Guaratiba Group were deposited Pliocene Marambaia (Demercian, 1996; Meisling et al., 2001; Gomes et al., 2002, 2009; Moreira et al., 2007; Carminatti et al., 2009; Scotchman et al., 2010) (Figs. 2C and 4). Episodic marine incursions and basin desiccation events led to the deposition of as Miocene much as 2.5 km of Aptian salt (called the Ariri Formation in the Santos Basin). The salt was deposited on extended continental crust at the base of a 1500-km- Neogene Figure 4. Santos Basin seismic strati long, north-trending rift-related valley located between the eastern Brazilian graphic framework including mapped horizons. Key intrasalt stratigraphic inter and West Africa conjugate margins, north of a volcanic belt represented by Oligocene vals are specified for this and previous the proto–Rio Grande Rise and the Walvis and São Paulo Ridges (Gladczenko studies. B1, B2, and B3 refer to competent et al., 1997; Figs. 2 and 4). However, some have suggested that the southern Marambaia layers or beams defined by high-ampli Intra- tude continuous reflections, and D1, D2, boundaries of the salt basin correspond to exhumed mantle rocks (Zalán et al., Marambaia and D3 refer to ductile detachment zones 2011). These volcanic features acted as oceanographic sills, impeding influx defined by acoustically transparent, poorly of saline marine waters into the developing Santos Basin, facilitating evapo- continuous reflections (Fiduk and Rowan, ration and salt deposition (Figs. 2A, 2B; e.g., Szatmari 2000; Dias, 2004; Davi- Eocene 2012). Abbreviations: Maas.—Maastrich tian; Cam.—Campanian; San.—Santonian; son, 2007; Karner and Gambôa, 2007; Carminatti et al., 2009; Davison et al., Con.—Coniacian; Tur.—Turonian; Cen.— 2012). During the Aptian, the proto–South Atlantic rift-related valley was as Cenomanian; Alb.—Albian; Apt.—Aptian;

Paleogene much as 500 km wide in the Santos Basin, narrowing northward to <50 km in Barr.—Barremian; Haut.—Hauterivian; Val.—Valanginian. the Sergipe-Alagoas Basin (Fig. 2A; e.g., Jackson et al., 2000; Szatmari, 2000; Paleocene Dias, 2004). During the Albian, a shallow-marine, carbonate platform became established (Guaruja Formation) (Figs. 2C and 4) (Moreira et al., 2007). Rapid

Maas. eustatic sea-level rise during the Late Cretaceous and Paleocene–Eocene led to the deposition of shelfal clastics, shales, and marls of the Itanhaem Forma- tion and the development of a major clastic-dominated progradational basin Cam. margin (upper Itajai-Acu Formation; Figs. 2C and 4) (Modica and Brush, 2004;

Itajai-Acu Moreira et al., 2007). e Lat San. Con. DATA AND METHODS

Tur. Rodriguez Gamboa Fiduk and Our study uses a 20,122 km2, high-quality, three-dimensional, poststack Top Itanhaem et. al et. al Rowan Cen. (this study) (2008) (2012) time-migrated seismic reflection data set located in the central deep-water Santos Basin, covering a large area of the São Paulo Plateau and the Outer A4 Interbedded B1 Itanhaem unit High (Fig. 2B). The 3D seismic volume, courtesy of CGG (http://www .cgg.com Alb. /en), contains trace information from sea level down to 5.5 s two-way travel- n Halite-rich time (TWT), with a vertical sample rate of 4 ms, inline spacing (east-west) of A3 D1 Ariri Top Ariri unit 18.75 m and crossline spacing (north-south) of 25 m. Data from 5 boreholes Top Guaratiba within the study area indicate that the intrasalt vertical seismic resolution is Cretaceous Apt. ~30 m, based on an average intrasalt interval velocity of ~4400 m/s and av- A2 Interbedded B2 unit erage dominant frequency of 36 Hz. Interval velocities were estimated using Early Guaratiba Ariri Formatio calibrated checkshot velocity surveys available at the boreholes locations. The Barr. Group D2 Lower intrasalt interval velocities are low when compared to typical halite velocity halite of 4500 m/s; this is due to the presence of acoustically low-velocity (3500 m/s) Haut. A1 B3 unit carnallite and tachyhydrite layers. Seismic profiles are displayed in Society Val. D3 of Exploration Geophysicists standard normal polarity, where a downward

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increase in acoustic impedance is represented by a positive reflection event Group, i.e., base salt), and between the salt and the overlying carbonates and (black) and a downward decrease in acoustic impedance is represented by deep-water marls (Guaruja Formation, i.e., top salt). Lateral changes in base a negative reflection event (red). Three regional 2D Kirchoff prestack depth- salt reflectivity likely reflect changes in the lithology of rocks underlying and migrated lines were also used in this study, principally to help illustrate the overlying this boundary. regional salt thickness distribution and to tie borehole data located outside the The base salt depth map illustrates the present subsalt structure of the ba- area imaged in the 3D survey (Figs. 2B, 2C). sin, indicating the presence of two prominent structural highs in the south Data from seven boreholes have been used to constrain the age of the (the Sugar Loaf subhigh) and east (the Tupi subhigh) of the study area (Fig. mapped reflection events and the lithology of intrasalt stratigraphic units (Fig. 6A) (e.g., Gomes et al., 2009). North and northwest of the highs, the base salt 2B). These data also allow us to construct regional stratigraphic correlations de- dips gently (5°–7°) landward; this dip is estimated using the base salt depth picting the stratigraphy and thickness of the salt. Synthetic seismograms were map (Fig. 6A). Overall, base salt is concordant with underlying subsalt seismic constructed in order to calibrate borehole lithology with the intrasalt seismic reflections (Fig. 7). However, over some parts in the Sugar Loaf and Tupi sub- reflections. Only two of the boreholes contain a comprehensive set of electrical highs, underlying units are truncated against base salt in the footwalls of sub- logs (e.g., gamma ray, GR; neutron, density, and P-wave sonic), and these have salt, rift-related normal faults (Fig. 7A). Moreover, in the Outer High, a series of allowed us to tie seismic and borehole data, and to assign lithology information predominately north-northeast–south-southwest–striking fault systems offset to the mapped intervals (Fig. 5). Identification of intrasalt lithology is based on the base of the salt by as much as 100 ms TWT (~450 m) (Figs. 6A and 7). The the typical borehole log response expected from a combination of wireline logs, top Ariri depth-structure map illustrates the present salt-related structure of including density (RHOB), sonic (DT), neutron porosity (NPHI) and gamma ray the basin and salt thickness variations (Fig. 6B). The salt is as much as 4.9 km (GR) (Table 1; Schlumberger Limited, 1991; Halliburton Energy Services, 1994; thick in the cores of large salt diapirs and is possibly absent in apparent welds Mohriak et al., 2009). Lithology interpretation was also verified with information underlying deep minibasins (Figs. 6C, 7B; see also Jackson et al., 2014a). from cutting descriptions in the borehole composite logs available for the study. Based on the base salt geometry and variations in salt thickness and salt- To establish the regional thickness and stratigraphic framework of the salt, related structural styles, we identify four main structural domains in the São five well-calibrated regional horizons were mapped (Figs. 4 and 5): (1) top of Paulo Plateau (Table 2; Figs. 6C and 7). (1) The thick salt domain is characterized the Guaratiba Group (base salt); (2) top of the Ariri Formation (top salt); (3) top by the greatest mean salt thickness (~2.2 km), with very thick salt (to 4.9 km) of the Itanhaem Formation; (4) intra-Marambaia Formation; (5) the seafloor. present in salt walls. Furthermore, the base salt is at its deepest in this domain Seismic facies analysis and borehole data allowed us to constrain the four (7.5 km) and no welds are developed; the minimum salt thickness is ~2 km. main intrasalt stratigraphic intervals (A1–A4) and to map three intrasalt seis- (2) The Sugar Loaf domain overlies a semiregional structural high at the base mic horizons: top A1, top A2, and top A3 (Figs. 4 and 5; see also Jackson et al., of salt (i.e., the Sugar Loaf subhigh; Fig. 6A). The mean salt thickness in this 2015b). To convert time-structure maps to the depth domain a velocity model domain is ~1.8 km and the salt is characterized by the development of north- was constructed. The interval velocity assigned to each layer was estimated and northeast-trending salt walls, within which salt is as much as ~3 km thick from the time-depth curves from five of the seven boreholes. Depth values (Table 2; Fig. 6C). Some of these salt walls have flat tops beneath which the from stratigraphic markers in all six boreholes, together with the prestack intrasalt stratigraphy is truncated. Complex intrasalt folding and thrusting is depth-migrated seismic sections, allowed us to check the accuracy of the also observed in salt walls in the Sugar Loaf domain (Fig. 7B; see also Fiduk depth conversion of the generated depth maps (Fig. 6). Intrasalt seismic facies and Rowan, 2012; Jackson et al., 2014b, 2015b). (3) The minibasin domain is interpretations and stratigraphic correlations were supported using seismic at- defined by a structural low at base salt level, a mean salt thickness of ~2 km, tribute analysis (i.e., sweetness and variance; Hart, 2008) and seismic inversion and thick minibasins enclosed by large, north-, east-, and northeast-trending (i.e., relative acoustic impedance; Figs. 7 and 8). These seismic attributes and salt walls (Figs. 6B, 6C). The walls are 50 km long, 5–10 km wide, and as much variations in relative acoustic impedance have been used to identify and map as 4.9 km thick, with salt welds developed beneath many of the minibasins lithology away from areas of well control. The relative acoustic impedance (Table 2; Fig. 6B) (see Jackson et al., 2014a). (4) The Tupi domain overlies the was estimated using the zero-phase wavelet extracted at the well location and Tupi subhigh, a semiregional structural high at the base of salt. The mean salt was also used to identify and correlate the lithology between the wells (Fig. 8). thickness in the Tupi domain is ~1.9 km and the salt-tectonic structural style is characterized by a network of minibasins arranged in a polygonal pattern, STRUCTURAL FRAMEWORK enclosed by ~20-km-long, ~2–4-km-wide, north-, east-, and northwest-trending salt walls (Figs. 6B, 6A, and 7C). Salt diapirs in the Tupi domain are of lower Overall, the base and top of the salt are represented by continuous amplitude (to ~2.7 km) than those in the minibasin domain (to 4.8 km). No high-amplitude regionally mappable seismic reflections (Figs. 4 and 5). The welds are observed in the Tupi domain, where the minimum salt thickness is high amplitudes are produced by the high acoustic impedance contrast be- ~1 km. Salt-related structural styles and thickness variations suggest that less tween the salt and the underlying lacustrine shales and carbonates (Guaratiba salt flow occurred here than in the highly deformed minibasin domain (Fig. 7C).

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B SW 723C 709 NE

Figure 5. Seismic and geoseismic sections across two boreholes in the study area illustrating the mapped horizons and de fined key stratigraphic intervals. TWT— two-way travel time. (Seismic data cour Top A3 tesy of CGG; http://www.cgg.com/en.) See Figure 2B for location. Top A1 Top A2

Top A1 (TWT)

5 km ms 400

C SW 723C 709 NE

A2 A3 A2 A1

TWT) (

5 km ms 400

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TABLE 1. TYPICAL EVAPORITE BOREHOLE LOG RESPONSE EXPECTED FROM WIRELINE LOGS Density Δt Neutron porosity Gamma ray MineralFormula (g/cm3) (μs/ft) (p.u.) (API) HaliteNaCl 2.03 67 00–very low

AnhydriteCaSO4 2.98 50 –1–0 0

Gypsum CaSO4 (H2O)2 2.35 52.5 49 0 Sylvite KCl 1.86 74 0500+

PolyhaliteK2SO4·MgSO4·2CaSO4·2H2O 2.79 57.5 15 180–200

Carnalite KMgCl3·6H2O 1.57 78 65 200–220

Tachhydrite CaCl2(MgCl2)2(H2O)12 1.66 92 No datalow Note: Shaded area is bittern salts (i.e., K- and Mg-rich salts; see text); t is time. Information was gathered from a literature review (after Mohriak et al., 2009; Schlumberger Limited, 1991; Halliburton Energy Services, 1994).

In the transition between the structurally high Tupi and Sugar Loaf domains in the minibasin domain penetrates a relatively thin (~22 m), halite-absent salt and the structurally low minibasin domain and thick salt domain, the base salt sequence, with the Ariri Formation composed of anhydrite (40%; ~8.7 m) and dips landward and shortening-related salt-cored buckle folds are observed nonevaporitic lithologies (i.e., sandstone, ~3.5 m or 16%; carbonate, ~8.9 m (Figs. 7A, 7B) (Cobbold and Szatmari, 1991; Jackson et al., 2015b). or 40%; and marl, ~0.8 m or 4%; borehole 329D; Fig. 9B; see also Jackson Our observations suggest that intrasalt deformation is relatively mild in the et al., 2014a). locations of borehole 532A and 723C, where little postdepositional salt flow has occurred; these two boreholes are the key to describing intrasalt evapo- Seismic Expression of Intrasalt Units rite cycles and the associated impact that syndepositional basin physiography had on salt deposition (Table 2). For example, near borehole 532A, intrasalt The good seismic resolution within the salt (~29–33 m) permits evaporite deformation is modest and expressed by several low-relief, locally developed, layers of specific composition to be tied to individual reflections or reflection salt-cored buckle folds, whereas borehole 723C is located near the flank and at packages (Fig. 8). Overall, high-density anhydrite layers tie with high-ampli- the tip of a salt wall, where internal deformation is overall less than observed tude positive reflections (black peaks), whereas low-density carnallite layers tie elsewhere. In addition, mean salt thickness at borehole 532A (2.3 km) is also with high-amplitude negative reflections (red trough) (Fig. 8). Thinner (<24 m) consistent with previous estimations made of the depositional salt thickness bittern salts and anhydrite layers interpreted from the borehole logs are not in the central deep-water Santos Basin (Davison et al., 2012). Previous studies resolvable in seismic data, whereas thicker (~34–100 m), halite-rich layers have also documented shortening of the salt layer in the study area in the broadly correlate with packages of low-amplitude, moderately chaotic reflec- form of low-amplitude buckle folds, intrasalt shear zones, diapir squeezing, tions (Fig. 8). Evaporite beds of varying composition can also be identified in and local thickening of the salt layer, and support that no major translation of a relative acoustic impedance (RAI) volume (Savic, 2006; Suarez et al., 2008), the salt layer occurred within the study area after salt deposition (e.g., Cobbold with carnallite and anhydrite defined by low and high acoustic impedance val- et al., 1995; Gambôa et al., 2008; Jackson et al., 2015a). ues, respectively.

ARIRI FORMATION: COMPOSITION AND SEISMIC EXPRESSION ARIRI FORMATION: KEY STRATIGRAPHIC INTERVALS Before we can investigate the controls on salt deposition, we need to understand the intrasalt composition and architecture. In this section we de- Based on seismic reflection and borehole data, we define four intra–Ariri scribe the overall composition and seismic expression of the Ariri Formation Formation stratigraphic intervals (A1–A4; Figs. 5 and 8). Considerable post- before describing its constituent subunits. depositional and possibly syndepositional salt flow has occurred in the San- tos Basin (e.g., boreholes 329D, 709, 711). For example, salt flow may have Composition modified the original depositional thickness near the salt diapirs (e.g., borehole 709). However, the stratigraphic order or near-depositional evaporite Borehole data indicate that the Ariri Formation is mainly composed of stratigraphy seems to be reasonably preserved away from the main diapirs halite (82%), anhydrite (11%), and bittern salts (7%), the latter being composed and, even within these structures, intrasalt stratigraphy can be mapped with of mainly carnallite (6%) and tachyhydrite (1%) (Fig. 9). The borehole located reasonable confidence (Figs. 7 and 9; see also Jackson et al., 2014b, 2015b).

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A 3D survey outline C 3D survey outline Depth (m) Thickness (m) 4500 Figu 4500 r 5000 e

329D 7 329D C 5500 3500

7 6000 2500 711 711 Figure 6500 minibasin 1500 7000 domain 500 Tupi 7500 0 Tupi sub-high domain F igur

e 7A 369A 369A

532A 532A

thick salt domain 709 709 723C 723C N Sugar Loaf N domain Sugar Loaf sub-high 50 Km 50 Km

B 3D survey outline Depth (m) 2500

3500 329D

4500 711

5500

6500 Figure 6. Present-day structural framework of the study area. (A) Base of salt depth-structure map illustrating the subsalt basin physiography and the presence of the Sugar Loaf and Tupi

369A subhighs. (B) Top of salt depth-structure map illustrating the salt-related structural domains. (C) Salt thickness depth map; top salt to base salt isochore map.

532A

709 723C N

50 Km

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thick salt domain Sugar Loaf domain NW SE 500 ms (TWT) 532A 723C

A A4 A2 A4 A3 A3 A1 A2 A1 0 10 km

thick salt domain Sugar Loaf domain NW SE 500 ms (TWT) 532A 723C

Salt-cored folds

Thrust faults

Figure 7 (on this and following two pages). (A) Northwest-southeast seismic and geoseismic cross sections and mapped horizons illustrating the salt thickness variations and the seismic ex pression of the intrasalt key stratigraphic intervals across the Thick Salt and Sugar Loaf domains. TWT—two-way travel time. (B) North-northeast–south-southwest seismic and geoseismic cross sections with the mapped horizons illustrating the salt thickness variations and intrasalt seismic expression from the minibasin domain toward the Sugar Loaf domain.

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minibasin domain Sugar Loaf domain

NNE SSW

329D 709

B A4 A4 A1 A3 A2 A3 A1 A2 05km

minibasin domain Sugar Loaf domain NNE SSW 329D 709

Minibasin

Short-wavelength salt-cored folds

Figure 7 (continued). (B) North-northeast–south-southwest seismic and geoseismic cross sections with the mapped horizons illustrating the salt thickness variations and intrasalt seismic expres sion from the minibasin domain toward the Sugar Loaf domain.

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Tupi domain

NNW SSE

369A 711

C A4 A4 A3 A3 A2 A1 A2 A1 ) 500 ms 10 km (TWT

Tupi domain NNW SSE

369A 711 )

10 km 500 ms (TWT

Figure 7 (continued). (C) North-northwest–south-southeast seismic and geoseismic cross section illustrating the salt thickness variations and intrasalt seismic expression across the Tupi domain. See Figure 5C for locations. (Three-dimensional seismic data courtesy of CGG, http://www.cgg.com/en.)

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Well 709 MD P-Sonic Density GR Cycles Lithology Synthec Seismic data Horizons and Units Relave acousc m 50 us/ 85 1.5 g/cm3 3.0 0 API 200 (orbital extracon) impedance Top Ariri or Top A4 unit (orbital extracon) 9 8 A4

A1 Allochthonous sheet

Top A3 unit

6 A3

5 Top A2 unit

A2 3 2

Top A1 unit

1 A1

Top Guaraba

Key Lithology Relave acousc impedance 0.5 Cycles = tachyhydrite = carnallite Figure 8. Seismic-well tie for the intrasalt key stratigraphic Transgressive, deepening upward = halite 0 intervals. MD—measured depth; GR—gamma ray.

Regressive, brining upward = anhydrite Salinity increase = carbonate -0.5

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TABLE 2. STRUCTURAL DOMAIN DESCRIPTION INCLUDING STRUCTURAL CONTEXT OF THE BOREHOLES USED FOR THE ANALYSIS Depth to base salt Mean salt thickness Salt-related deformation Salt thickness Structural domains (km) (km)Borehole at the borehole location at the borehole location Area characterized by locally developed Thick salt domain 6–7.5 2.2 532A 2.3 km salt-cored buckle folds 4.4–5.5 1.8 723C Located near the flank and at the southern 1.8 km tip of a salt wall Sugar Loaf domain (overlies 1-ESSO-3 Area characterized by locally developed 1.3 km the Sugar Loaf subhigh) salt-cored buckle folds (two-dimensional seismic line) 709C Near a salt wall flank 1.4 km Penetrates a salt weld within a deep Minibasin domain 5.5–7.5 2 329D 22 m minibasin Tupi domain (overlies the 4.4–5.5 1.9711 Penetrates a salt-cored diapir 2.2 km Sugar Loaf subhigh) 369A Near the flank of a salt-cored high 1.9 km

As described here, the seismic expression of the key stratigraphic intervals can have moved less, seismic and borehole data suggest that A1 thickness ranges be correlated to the composition of the salt from the boreholes, where highly between 500 and 1200 m (Figs. 9A and 10). Overall, A1 is moderately thin- reflective continuous intervals like A2 and A4 correspond to high acoustic im- ner (by ~200 m) in the distal structurally higher domains than in the proximal pedance anhydrites interbedded with low acoustic impedance bittern salts and structurally deeper domains (boreholes 532 and 723C; Fig. 10A). halite; these units are also characterized by several complete evaporite cycles Borehole data available indicate that A1 is dominated by halite (87%), and (boreholes 709 and 723C; Figs. 8 and 9). In contrast, A1 and A3 are more rela- relatively small amounts of anhydrite (10%), bittern salts (2.7%), and carbonate tively halite rich and contain lesser proportions of anhydrite and bittern salts, (0.3%) (Fig. 10). Although A1 is generally halite rich, lateral stratigraphic varia- and therefore contain less complete evaporite cycles (boreholes 709 and 723C; tions are observed between boreholes located on different structural domains. Figs. 8 and 9). Detailed variations in the thickness and composition, and the Anhydrite net thickness increases by more than 50 m from structurally deeper seismic expression of the intrasalt units are described in the following sections. locations (borehole 532A) toward structurally higher locations (borehole 723C; Figs. 7A and 10A). An increase in anhydrite proportion toward the Sugar Loaf A1: Seismic Expression, Thickness, and Composition domain is complemented by a decrease in the amount of halite and bittern salts (Table 3; Fig. 10A). Bittern salts are rare in Al, especially in distal bore- The top of A1 is defined by a positive reflection that correlates to an an holes located in the structurally high Sugar Loaf domain. However, bittern salts hydrite layer (Fig. 8). The base of A1 is defined by a positive seismic reflection, are volumetrically important (>~23 m) in the present-day structurally high Tupi which correlates to an anhydrite layer overlying the carbonates and shales of domain (Table 3; Fig. 10B). We identify three complete evaporite cycles where the Barra Velha Formation (Figs. 8 and 10). Seismic data indicate that A1 is borehole data penetrates A1 (borehole 723C; Fig. 10A). generally characterized by low-frequency, discontinuous to locally transparent, chaotic seismic facies, although stronger, more continuous reflections A1: Interpretation are locally observed (Figs. 5 and 7). The chaotic seismic facies are particularly evident in the proximal, structurally lower domains (near boreholes 532A Our analysis reveals an overall link between the seismic expression, thick- and 711 in the thick salt domain and Tupi domain; Fig. 7). Conversely, con- ness, and composition of A1 (Table 3). Where A1 is more continuous, highly tinuous high-amplitude reflections are more common on structurally higher reflective and thinner (~500 m), anhydrite is more common (>15%) and bittern locations (near borehole 723C and 369A in the Sugar Loaf domain and Tupi salt is rare (<1%). In contrast, where A1 is discontinuous, transparent, and thicker domain; Fig. 7). (>600 m), bittern salt is more common (>3%) and anhydrite is relatively rare Seismic data indicate pronounced thickness variations (0–2500 m) in A1, (<8%). Based on this relationship we interpret that the syndepositional basin principally related to salt tectonics (Fig. 9A). Where A1 can be identified in the physiography directly controlled deposition of A1, with gypsum and/or an study area, major thickness variations occur in the thick salt domain and lo- hydrite deposition occurring preferentially in the Sugar Loaf domain, which we cally in the Sugar Loaf domain, where this unit has been expelled from below suggest was structurally higher than the surrounding domains at the onset and minibasins into flanking walls (borehole 709; Fig. 9A). In contrast, thickness during A1 deposition, thus promoting vertical salinity variations in the São Paulo variations and, we infer, salt flow is more subtle in the structurally higher Tupi Plateau. In contrast, the Tupi domain, minibasin domain and the thick salt do- domain, the Sugar Loaf domain near borehole 723C, and in the thick salt domain were structurally lower and more hydrologically isolated than the Sugar main near borehole 532A (Fig. 9A). In these areas, where the salt seems to Loaf domain, thus promoting more halite and bittern salt deposition (Fig. 10A).

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Thickness (m) Thickness (m) Key 750 A Key 2500 C boreholes 2000 329D boreholes 600 329D

area where the intrasalt key 1500 area where the intrasalt key 450 intervals can be idenﬁed intervals can be idenﬁed 711 711 1000 300 top A1 150 Tupi 500 Tupi domain 0 domain 0 A1 isochore 369A A3 isochore 369A

532A 532A

thick salt domain 709 709 723C N 723C N

Sugar Loaf 50 km Sugar Loaf 50 km domain domain

Thickness (m) Thickness (m) D Key 500 B Key 2000 boreholes 400 boreholes 1600 329D 329D area where the intrasalt key area where the intrasalt key 300 intervals can be idenﬁed 1200 intervals can be idenﬁed 711 200 800 711 top A1 top A1 100 400 Tupi Tupi 0 0 domain domain A4 isochore 369A A2 isochore 369A

532A 532A

709 709 723C N 723C N

Sugar Loaf Sugar Loaf 50 km 50 km domain domain

Figure 9. (A) A1 thickness map in depth (isochore). (B) A2 thickness map in depth (isochore). (C) A3 thickness map in depth (isochore). (D) A4 thickness map in depth (isochore). The isochores illustrate intrasalt diapirism by A1 (in pink).

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thick salt domain Sugar Loaf domain 532A 723C

0.00 EHGR gAPI A GRtest DTC lithology 32 km Density lithology Lithology 200.00 1.1011 g/cm3 4.9495 Seismic facies Proporons Units SeismicSeismic facies GRgAPI 200.00 90.004P-Sonius/ c 5.00 GR Density Lit Proporons

12 18% 12 13% A4 A4 20% 11 34% 11 69 % 46% A3 A3 14% 10 10 2% 3% 3%

9 9 84% 8 8 94% Repeon and 24% 7 8% A2 salt-related deformaon 6

7% 13% 5 7 A2 4 68%

6 5 80 % 4 18% 1.8% 3 A1 0.2% 2 3 5% 3% 80% A1 1

92% 1

200 m

Figure 10 (on this and following page). (A) Well correlation across the Thick Salt and Sugar Loaf domains illustrating variations in intrasalt thickness and lithology proportions. GR—gamma ray; Lit—lithology.

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Tupi domain 711 369A B Lithology 32.7 km Proporons Seismic facies GREHGR P-SonicPSonic GREHGR DensitRHOZ_ed [Neural net 10] y Lilithologyt Seismic facies Proporons 0.00 gAPI 200.00 90.005us/ 0.00 Lithology 0.00 gAPI 200.00 1.5653 g/cm3 2.9679 A4 11% A4 78 45% 19% A3 A3 A3 25% 30%

70% 2% 5%

2% A2 93% 12% A2 A2 9% 86% 9%

82%

8% A1 A1 8% 1% 2% A1 84%

97%

250 m

KEY Lithology Proporons Tachyhydrite Bierns salts Figure 10 (continued). (B) Well correlation across the Minibasin and Sugar Loaf domains illus Carnallite trating variations in intrasalt thickness and lithology proportions. Halite Halite Anhydrite Anhydrite Salt-related Carbonate deformaon

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TABLE 3. STRATIGRAPHIC INTERVALS Anhydrite net thickness Halite net thickness Bittern salt net thickness (m) (m) (m) Thickness Evaporite cyles Interval (m)* (723C) TSDTupiSugar Loaf TSD Tupi Sugar Loaf TSD Tupi Sugar Loaf A1 500–1200 34040100 720950 400205010 A2 400–750 5.5 50 40 100560 650400 90 85 30 A3 150–300 2101535350 140240 10 95 5 A4 150–250 1.5 65 15 20 90 15 120402520 Ariri Formation 1200–2500 12 165145 2551720 1755 1160 160255 65 Note: A1–A4 are key intervals. TSD—thick salt domain. *Thickness ranges are estimated from isochores and borehole data in areas where less postdepositional salt flow has occurred.Evaporite proportions across domains are also included.

We suggest that higher gypsum (now anhydrite) deposition in the Sugar Loaf Borehole data indicate that A2 is also relatively halite rich (79%), although domain occurred during highstands due to renewed seawater inflow and it contains slightly less halite than A1 (i.e., 87%) and, as a result, more an lower salinities. In contrast, the basin was more isolated and deeper toward hydrite (12%) and bittern salt (9%). We observe lateral compositional variations the minibasin domain, Tupi domain, and the thick salt domain, thus promoting between boreholes located in different structural domains For example, anhy- higher salinities during lowstands and hence more halite and bittern salt depo- drite thickness increases (by >50 m) and halite and bittern salt decreases (by sition when the salt basin became partially or almost completely desiccated. ~250 m and ~55 m, respectively) from structurally deeper (i.e., boreholes 532A, The number of evaporite cycles we identify in A1 suggests that the basin was 369A, and 711) to structurally higher locations (borehole 723C; Table 3; Figs. almost or completely desiccated (lowstands) and refilled (highstands) at least 7A, 10A, and 11). We identify at least five and a half evaporite cycles within A2; three times during the deposition of this unit. this is almost twice the number of cycles identified within A1 (boreholes 532A and 723C; Fig. 7A).

A2: Seismic Expression, Thickness, and Composition A2: Interpretation The top of A2 is defined by a negative reflection and correlates with a laterally extensive carnallite layer (Fig. 8). Seismic data indicate that A2 is charac- Like A1, our analysis reveals a strong link between the seismic expres- terized by high-frequency, highly reflective, subparallel to parallel reflections sion and composition of A2, with the high-frequency and strongly reflective (Figs. 7 and 8). The highly reflective packages within A2 are more evident in nature of this unit reflecting vertical changes in acoustic impedance related the distal and structurally higher Sugar Loaf domain (i.e., near boreholes 723C to the interbedded nature of the contained lithologies (i.e., anhydrite, halite, and 709; Fig. 7). In the thick salt domain and Tupi domain, A2 is characterized and bittern salts). We suggest that higher proportions of anhydrite and bit- by more transparent or lower amplitude seismic facies (i.e., near boreholes tern salt in A2 compared to A1 record an increase in the frequency and mag- 532A, 369A, and 711; Fig. 7). nitude of fluctuations in basin salinity at this time, possibly driven by varia- Seismic data indicate that A2 varies from 0 to 2000 m (Fig. 9B). However, tions in water depth. More specifically, based on the occurrence of at least six local thickening of A2 is associated with shortening, folding, and thrusting of intra-A2 evaporite cycles, we infer that, during A2 deposition, the basin was the entire salt sequence, whereas localized thinning of A2 is associated with partially or completely desiccated and refilled at least six times (borehole expulsion of A1 into diapirs and, in and some cases, allochthonous salt sheets 723C; Fig. 7A). (Figs. 5 and 7; see also Jackson et al., 2014b). In areas where we infer the salt We interpret that the syndepositional basin physiography controlled depo- has moved less, seismic and borehole data indicate that A2 ranges between sition of A2, in a manner similar to that inferred for A1. A thinner salt sequence 400 m and 700 m (Figs. 7A and 9B). Overall, A2 is thinner (by ~65 m) in the and more gypsum (now anhydrite) were deposited onto the structurally higher distal, structurally higher domains than in the proximal, structurally deeper and shallower Sugar Loaf domain, in comparison with the surrounding do- domains (see boreholes 532 and 723C; Figs. 7A and 10A). It is important to mains where the salt sequence was thicker, gypsum (now anhydrite) poor, and note that this estimation takes into consideration structurally induced thick- halite rich (Figs. 11 and 12). However, more subtle thickness and stratigraphic ness variations, such as the intrasalt shear zone–related stratigraphic repeti- variations in A2, in comparison to A1, suggest subtler basin relief and broadly tion observed in borehole 532A (Fig. 7A). similar water depths between structural domains at the time (Figs. 11 and 12).

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Sugar Loaf domain Tupi domain 0 10 km 500 ms 709 369 A 723C 711

0 10 km 500 ms 709 369 A 723C 711

Ariri Fm.

Sugar Loaf domain Tupi domain Figure 11. Southwest-northeast detailed intrasalt well correlation across the Outer Fence alignment: Top Fence alignment: Top Fence alignment: Top Distance: 28282 m. Distance: 72367 m. Distance: 32602 m. High. The detailed correlation was done 4-BRSA-723C-SPS723C [MD] 4-BRSA-709-SPS709 [MD] 1-BRSA-369A-RJS369A [MD] 4-BRSA-711-RJS711 [MD] Density GR lithology 28 km Density GR Lithology - Key Well: 709 72 km Density GR lithology 33 km Density [Neural net 1] GR lithology using interpreted seismic events and the 3 3 3 1.3 g/cm 3 0 gAPI 150 1.3 g/cm 3 0 gAPI 150 datum = top Ariri Formaon 1.3 g/cm 3 3.0 0 gAPI 150 1.3 g/cm 3 0 gAPI 150 relative acoustic impedance to identify

evaporite layers between the boreholes. A4 A3 GR—gamma ray.

Allochthonous

A3 sheet A2 n A2 Ariri Formao A1 A1 Lithology key tachyhydrite carnallite halite anhydrite carbonate 200 m

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hole 369A to borehole 711 in the Tupi domain is due to erosion and/or dissolu- ?------erosion and/or dissoluon------? tion at the crest of a salt wall (Fig. 7C). Apparent thickening of A3 in borehole 709 in the Sugar Loaf domain is due to the emplacement of an A1-sourced allochthonous salt sheet (Figs. 5 and 8; see also Jackson et al., 2014b). Seis- Zone 2 Zone 1 mic data and boreholes that penetrate A3 away from areas of major intrasalt deformation indicate that A3 thickness ranges between 150 m and 300 m and A3 that only minor lateral thickness variations are observed between structural domains (Fig. 9C). For example, A3 thins by only ~21 m (5%) from the proximal and structurally low thick salt domain toward the distal and structurally high Sugar Loaf Sugar Loaf domain (boreholes 532A and 723C; Figs. 7A and 10A). sub high A3 is composed of halite (87%), anhydrite (8%), and bittern salts (5%), although lateral variations in the proportions occur between structural domains Anhydrite % (Table 3). Higher proportions of anhydrite occur in the distal and structurally Zone 3 Zone 2 Zone 1 high Sugar Loaf domain (average ~35 m; boreholes 723C, 1-ESSO-3) when compared with the structurally low thick salt domain (~10 m; borehole 532A) A2 and more distal Tupi domain (~15 m; boreholes 711 and 369A; Table 3; Fig. 10). We identify two evaporite cycles in locations where A3 is less deformed Sugar Loaf (borehole 723C; Fig. 10A). sub high A3: Interpretation Anhydrite % Zone 4 Zone 3 Zone 2 Zone 1 We interpret that the weakly reflective, chaotic seismic expression of A3 reflects the fact this unit is more halite dominated, and thus compositionally and A1 Sugar Loaf acoustically homogeneous than A1 or A2. In turn, this interpretation suggests sub high that, during deposition of A3, the Santos Basin saw an increase in halite precipitation and overall consistently higher salinities than during deposition of A2 (Fig. 12). During A3 deposition, the basin was desiccated only once, as sug- Halite Anhydrite Biern salts gested by the intra-A3 evaporite cycles (borehole 723C; Fig. 10A). Our analysis shows that syndepositional basin physiography likely influenced A3 thickness Figure 12. Proposed model for A1, A2, and A3 intrasalt key stratigraphic intervals illustrating the and stratigraphy, but to a more limited extent than for A1 or A2; this interpre- influences of the syndepositional basin physiography in the resulting stratigraphic architecture. tation is based in the more subtle thickness and compositional changes that occur between structural domains in A3. From this, one may infer that A1 and A3 Interval: Seismic Expression, Thickness, and Composition A2 had largely filled the predepositional or syndepositional relief, thus resulting in an overall flatter basin (Fig. 12). The top of A3 is defined by a positive reflection event and, in some locations, such as in the Tupi domain, intra-A3 reflections are truncated beneath A4: Seismic Expression, Thickness, and Composition top salt (borehole 711; Fig. 7C). Overall, A3 is characterized by transparent, discontinuous, subparallel reflections (Figs. 5 and 8). Although the seismic ex- The top of A4 (top salt) is interpreted as a positive reflection event. Lo- pression varies across the study area, in the distal and structurally high Sugar cally, the top of A4 is defined by an angular and erosional unconformity, with Loaf domain, A3 is highly reflective and dominated by continuous reflections, intra-A4 and A3 reflections being truncated beneath it (Fig. 7). Borehole data whereas in the more proximal Tupi domain and structurally lower thick salt indicate that top A4 correlates to a thick anhydrite layer (Figs. 8 and 10). Seis- domain, it is defined by more transparent, chaotic reflections (Fig. 7). mic data indicate that A4 is generally characterized by high-frequency, highly Seismic and borehole data indicate that A3 varies between 0 and 750 m reflective, subparallel reflections. No significant lateral changes in intra-A4 and it is characterized by structurally induced thickness changes related to seismic facies are observed between structural domains (Fig. 7). shortening of the salt sequence, and associated folding and thrusting (Fig. 9B; Seismic and borehole data indicate that A4 thins toward and is absent at see also Jackson et al., 2014b). A3 is thinner and truncated near and on top of the crest of salt walls, and thickens toward the flanking minibasins ranging in some of the large salt structures. For example, 12% thinning of A3 from bore- thickness from 0 and 500 m (Fig. 9D). Postdepositional, structurally induced

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variations in the thickness of A4 are particularly clear in borehole 709C, where whereas high-frequency, highly reflective seismic facies represent still rela- they are related to the emplacement of an A1 salt sheet (see also Jackson tively halite rich (65%–85% halite) units, but contain relatively high proportions et al., 2014a; Figs. 7B and 8). Seismic data and boreholes that penetrate A4 (15%–35%) of anhydrite and bittern salts (Fig. 7). In addition, our findings sug- indicate that in less-deformed areas, the A4 thickness ranges between 150 m gest that variations in the thickness and composition, the latter inferred from and 250 m. In areas where A4 is not truncated at the top of the salt, borehole seismic expression or directly constrained by boreholes, of the Ariri Forma- data indicate that only very minor thinning (~1 m; 0.8%) of the unit occurs tion are spatially related to structural domains. Overall, thinner salt (mean salt between the structurally low thick salt domain (borehole 532A) and the struc- thickness <1.9 km) and higher anhydrite net thickness (~350 m) occur toward turally higher Sugar Loaf domain (borehole 723C; Figs. 7A and 10A). the structurally high Sugar Loaf domain, compared to flanking, structurally Borehole data indicate that A4 is composed of halite (64%), anhydrite (17%), lower domains where the mean salt thickness is >2.4 km and anhydrite net and bittern salts (19%); A4 is thus the most halite-poor, bittern salt–rich unit de- thicknesses are less (~180 m). We also note that anhydrite proportions sub- veloped within the Ariri Formation. However, in contrast to A1–A3, A4 contains stantially differ between the Tupi (5% average) and Sugar Loaf (19% average) significantly lower proportions of anhydrite and bittern salt proportions in the domains, even though both occur in the Outer High and have depths compara structurally higher Sugar Loaf domain (~20 m, ~20 m) when compared to the ble to the present-day base of salt. structurally lower thick salt domain (~65 m, ~40 m) (Table 3; Fig. 10A). We iden- We observe four key controls on compositional variations and the over- tify one and a half evaporite cycles in A4 (borehole 723C; Fig. 10A). all stratigraphic architecture of the Ariri Formation: i.e., climate, sea level, syndepositional basin physiography, and hydrology (i.e., variations in basin A4: Interpretation salinity driven by varying contributions of continental fresh water and saline seawater). These results raise some key questions. Can these elements pro- The highly reflective character of A4 reflects its compositional and acous- vide any clues about duration of deposition? How rapidly was the thick salt tically heterogeneous characters, with halite being interbedded with high deposited? Can the basin physiography be constrained by using the observed acoustic impedance anhydrites and low acoustic impedance carnallite (Fig. 8). stratigraphic and thickness variability, and what does this reveal regarding the We interpret that the heterogeneous character of A4 reflects higher salinity basin origin and accommodation development? Are there any indications of fluctuations in the basin during its deposition, with these fluctuations perhaps fresh-water inflow from the continent during salt deposition, or any evidence being of a similar magnitude to those occurring during A2 deposition. During about the direction of seawater inflow? A4 deposition, the basin was partially or completely desiccated at least twice, based on our identification of evaporite cycles in a location where the salt is relatively undeformed (borehole 723C; Fig. 10A). We suggest that thinning of Climate, Water Depth, and Duration of Deposition A4 toward the largest salt walls is due to postdepositional salt flow, erosion, and dissolution (Figs. 7 and 10). The top of A4 defines the end of evaporite We suggest that compositional variations defining the four intrasalt units deposition, and a transition to more fully marine conditions characterized by (A1–A4) arose due to changes in salinity, possibly related to changes in the fre- deposition of deep-water carbonates and marls. The Albian thus defines a rela quency of marine incursions and near-desiccation episodes during late Aptian tively desalinization of the basin related to basin deepening and an influx of development of the Santos Basin. More specifically, based on the identifica- seawater (Modica and Brush, 2004; Moreira et al., 2007). tion of 12 more-or-less complete evaporite cycles, we propose that the Santos Assessing the role of basin physiography on A4 deposition is difficult due Basin underwent 12 cycles of low and then high salinities, which may, but need to the significant postdepositional variations in thickness caused by salt flow not, be related to fluctuations in sea level (e.g., carbonate and gypsum, now and related dissolution. However, only minor thinning of A4 toward the struc- anhydrite, deposition during periods of high sea level, and halite and bittern turally higher Sugar Loaf domain may indicate that earlier deposited units (A1– salt deposition during periods of low sea level). During deposition of the as A3) had filled and smoothed out any predepositional or syndepositional relief much as ~1000-m-thick halite-rich A1 unit, we infer that the salt basin was partly by the onset of A4 deposition (Fig. 12). or completely desiccated and refilled 3 times; in contrast, during deposition of the thinner (to ~600 m), relatively halite poor A2 unit, the basin was partly or ARIRI FORMATION: KEY CONTROLS ON THE completely desiccated and refilled at least 6 times. Similar intervals are inter- COMPOSITION AND STRATIGRAPHIC ARCHITECTURE preted in the northern Espirito Santo Basin, where analysis of 345 m of cores AND THEIR SIGNIFICANCE from 5 boreholes indicates that the lower part of the Aptian Ibura salt sequence is dominated by halite, suggesting it was deposited during a relative sea-level We found that the composition and stratigraphic architecture of the Ariri lowstand, whereas the upper part is dominated by anhydrite deposited during Formation directly controls its seismic expression. Low-frequency, acoustically periods of rising relative sea level (Dias, 2004). Our analysis of the salt in the transparent, chaotic seismic facies represent relatively halite rich (>80%) units, Santos Basin indicates that each filling episode was immediately followed by

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an increase in the salinity of the brine due to lower water depths with partial Syndepositional Basin Physiography or complete desiccation of the basin. We suggest that gypsum (now anhydrite) deposition occurred during periods of relatively high water depth that were Our results show that thickness and stratigraphic variations within the salt associated with lower salinities due to episodic marine incursions; in contrast, were directly controlled by the depth of the basin at the onset and during salt we infer that halite was precipitated during periods of relatively low sea level deposition. We propose that A1 was deposited in an irregular subbasin, within when brine salinity was higher. During periods of extreme or complete desic- which the Sugar Loaf subhigh represented an intrabasin structural high cov- cation, bittern salts were deposited in the more isolated parts of the salt basin. ered by relatively shallow water (Fig. 10). Given the possibly rapid depositional Our analysis suggests that at least part of the Ariri Formation (i.e., A2) is time frame estimated for the salt (~500 k.y.), we propose that A1 largely filled characterized by regular; high-frequency, short-term cycles (≤30 k.y.; assum- preexisting rift-related relief and that A2 was also deposited in a basin defined ing a depositional rates ≥0.4 cm/yr; boreholes 532A, 723C; Fig. 13). Based on by accordingly more subdued relief (Fig. 10). The heterogeneous, cyclic, bittern the evaporite cycles interpreted and assuming depositional rates measured in salt-rich character of A2 was probably the result of a combination of reduced ac- analogue ancient salt giants (i.e., Paradox Formation, 0.4 cm/yr, Trudgill, 2011; commodation in a subbasin with subtle relief, low-magnitude episodic marine Mediterranean salt, 0.66 cm/yr, Clauzon et al., 1996) and a modern rift-related incursions, and an overall increase in accommodation mainly driven by salt salt basin, Lake Assal (1 cm/yr; Imbert and Yann, 2005), we suggest that the salt loading. A combination of these factors may have promoted higher frequency sequence in the central deep-water Santos Basin was deposited in 190 to 529 desiccation of the salt basin when compared with A1 deposition (Figs. 4 and k.y. (Fig. 13). Considering the duration of each cycle (Fig. 13), and based on the 10). A3 and A4 display more subdued thickness and compositional variability, fact salt is almost entirely composed of evaporites (e.g., Fig. 10A), we suggest suggesting that A1 and A2 had filled most of the preexisting relief. We therefore that this deposition time scale may have been driven by low-amplitude, fourth- propose that a series of subbasins, defined by preexisting rift-related relief, ex- to fifth-order sea-level changes, such as those characterizing the greenhouse isted at the onset of salt deposition. The associated relief of such subbasins pro- conditions and an arid paleoclimate; such conditions have previously been moted vertical salinity variations that controlled the thickness and stratigraphic proposed for the late Aptian of the South Atlantic (Evans, 1978; Taylor, 1990; variations within the salt. During periods of relatively high sea level, thinner salt Dias, 2004; Gambôa et al., 2008). and higher proportions of gypsum and/or anhydrite were deposited along the A depositional time scale of <500 k.y. is also broadly consistent with the basin margins and on shallow intrabasinal structural highs such as the Sugar high rates of deposition scenario proposed by Dias (2004; 600 k.y.), De Freitas Loaf domain. In contrast, during periods of relatively high salinities, thicker salt (2006; 500–800 k.y.), and Davison (2007), Davison et al. (2012), and Montaron and higher proportions of halite and bittern salts were deposited in the basin and Tapponnier (2010) (500 k.y.). A lithology-time-climate correlation is sug- center and deeper isolated subbasins, such as those now represented by the gested for the Messinian salt in the eastern Mediterranean by correlating ob- thick salt, minibasin, and Tupi domains. These lateral intra-A1 stratigraphic vari- served lithological variations to precessional climate cycles of ~21 k.y. duration ations have been previously defined as separate, individual units for the Ariri (Stefano et al., 2010). Astronomical tuning of the Primary Lower Gypsum of the Formation (i.e., B3 of Fiduk and Rowan, 2012). Our interpretation, that changes Messinian salt allows banded selenites to be correlated to the acme of the arid- in basin physiography controlled regional changes in salt thickness and intra ity peak in the precessional climate cycle (insolation minima), whereas shale salt stratigraphic variations are consistent with those previously proposed from layers (comparable to anhydrite layers in our model) are correlated to humid regional analysis of the South Atlantic salt basins, is discussed in the introduc- phases (Stefano et al., 2010). tion (see also Davison and Bate, 2004; Dias, 2004; Davison, 2007; Davison et al., We suggest that a combination of an arid paleoclimate, low-amplitude 2012). In addition, our observations support previous studies suggesting that sea-level changes, and relatively rapid deposition (~190–529 k.y.) of a thick the Santos Basin Outer High has been a positive structural element since at (1.2–2.5 km) salt sequence also prevented the deposition of nonevaporite lith- least the onset of salt deposition (i.e., Aptian), and that it was periodically emer- ologies in the central deep-water Santos Basin. Although broadly similar in gent before and during salt deposition (Gomes et al., 2002; Carminatti et al., terms of overall thickness, the depositional time scale and at least the sea- 2009; Gomes et al., 2009; Scotchman et al., 2010). Our findings thus challenge level controls we infer for salt deposition in the Santos Basin are unlike those previous studies arguing that the Outer High developed as a Late Cretaceous suggested for the Zechstein Supergroup, northwestern Europe (~1.5 km thick flexural bulge by vertical loading and perhaps by horizontal compression (e.g., and deposited in in 5 m.y.) and the Paradox Formation, North America (~2 km Cobbold et al., 2010). Although our results are not conclusive regarding the in- thick and deposited in 7 m.y.). The presence of nonevaporite lithologies (e.g., fluence of tectonic events on salt deposition, we suggest that, due to the rapid sandstone, mudstone) within the Zechstein Supergroup and the Paradox (<530 k.y.) salt deposition discussed herein (climate, water depth, and duration Formation suggest that fully marine conditions periodically occurred in the of deposition), synsalt subsidence and accommodation were only important basins and that the salt sequences were deposited during less arid condi- locally and were likely driven by salt loading (see also Davison et al., 2012). In tions, higher amplitude sea-level changes, and relatively slower deposition addition, postrift thermal subsidence and/or fault-related subsidence was prob- (>5 m.y.). ably insignificant (cf. Karner et al., 2003; Dias, 2004; Karner and Gambôa, 2007).

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thick salt domain Sugar Loaf domain borehole 532A borehole 723C Duraon of deposion (k.y.) Duraon of deposion (k.y.) cle cle

Unit Cy 10 20 30 40 50 60 70 80 90 100 Cy Unit 10 20 30 40 50 60 70 80 90 100

12 deposional rates 12 deposional rates 0.4 cm/yr 0.4 cm/yr Paradox Basin, Utah Paradox Basin, Utah A4 A4 (Trudgill, 2011) (Trudgill, 2011) 11 11 0.66 cm/yr 0.66 cm/yr Mediterranean salt Mediterranean salt (Clauzon et al., 1996) (Clauzon et al., 1996) 1 cm/yr

10 1 cm/yr A3 A3 Lake Assal, Djibou 10 Lake Assal, Djibou (Imbert and Yann, 2005) (Imbert and Yann, 2005)

9 9 8

A2 short-term regular cycles 8 7 ≥ 10 k.y. , ≤ 30 k.y. A2 short-term regular cycles Figure 13. Duration of deposition esti 7 ≥ 7 k.y. , ≤ 31 k.y. mated based on the thickness of each A2 A2 6 evaporite cycle and depositional rates 6 from ancient and modern salt basins. 55 5 5 4 4 4 4

3 Ariri Formaon total 2 duraon of deposion for

A1 Ariri Formaon total A1 each deposional rate 2 duraon of deposion for 212 k.y. 320 k.y. 529 k.y. each deposional rate 190 k.y. 287 k.y. 474 k.y. 1 inial deposion of A1 at least 1 twice the deposion of A2 inial deposion of A1 at least twice the deposion of A2

10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100

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Other ancient salt giants evolved under the influence of controls similar et al., 2000). Here seawater incursions occur through a highly fractured, 10-km- to those inferred for the Ariri Formation. For example, the 1.5-km-thick Zech- wide volcanic ridge that bounds the southeastern margin of the lake. Close to stein Supergroup salt sequence in the North Sea was deposited in a basin the seeps that bring seawater to the surface, gypsum precipitation dominates, that largely formed through postrift thermal subsidence following a preced- whereas stacked halite crusts characterize the opposite side of the lake where ing phase of early Permian extension (Tucker, 1991). However, pre-Zechstein salinities are higher; in this location, a 60-km-wide, as much as 80-m-thick, faults, inherited from the earlier Permian rift phase or even formed syndepo halite body developed (Warren, 2006). sitionally during late Permian rifting, also directly controlled thickness and lithology variations in the salt (Clark et al., 1998; Jackson and Lewis, 2013, 2016). Periodically submerged intrabasinal highs formed shallow areas where CONCLUSIONS carbonate and anhydrites were deposited (Taylor, 1990). In deeper waters, a thick succession (as much as 1.5 km) of basin center facies, composed mainly The integration of 3D seismic reflection data and borehole data from the of halite, was deposited (Clark et al., 1998). São Paulo Plateau, Santos Basin, offshore Brazil, has allowed us to investigate the lithology and stratigraphy of a salt giant (i.e., the Ariri Formation), as well as the key controls on the depositional thickness distribution and intrasalt Basin Hydrology stratigraphic variability. The following are the key conclusions of our study. 1. The Ariri Formation in the São Paulo Plateau is mainly composed of De Freitas (2006) proposed that anhydrite is thicker and more voluminous halite interbedded with significant proportions of anhydrite and bittern salts in the west of the Santos Basin due to fresh-water influx from the continent (carnallite and tachyhydrite) as well as minor proportions of carbonates, marls, during the Aptian. Conversely, eastward on the São Paulo Plateau, away from and sandstones locally. this fresh-water source, salinities were higher and halite and bittern salts were 2. The Ariri Formation can be subdivided into four key stratigraphic inter- thicker. However, given the insignificant proportions of interbedded clastics vals based on seismic facies: A1, a variably thick and highly deformed lower found within the salt (see also Jackson et al., 2014a), we argue that continen- unit mostly characterized by chaotic and transparent seismic facies, that varies tal river discharge had only a minor impact on basin salinity, an observation laterally and locally to subparallel and more continuous seismic facies; A2, a consistent with the fact our study area is, and was during the Aptian, located a thick highly reflective unit characterized mainly by continuous parallel to sub- considerable distance (>100 km) from the coeval coastline (Modica and Brush, parallel seismic facies; A3, a relatively thin unit characterized by transparent 2004). Nevertheless, we observe salt thickness and stratigraphic variations seismic facies and subparallel continuous reflections; and A4, a thin, highly between the two structurally higher domains (i.e., the Sugar Loaf and Tupi reflective unit that thins and is truncated beneath the top of salt and toward subhighs) that, at present, have similar depths to the base of the salt (Fig. 9). If the main salt structures. both the Sugar Loaf and Tupi subhighs were shallower than the surrounding 3. Seismic facies within the salt are directly controlled by salt composition. domains at the onset of salt deposition, then the main factor controlling salin- Acoustically transparent chaotic seismic facies are associated with higher halite ity variations and thus lithologies found on the Outer High may be the position proportions (>80%), whereas high-frequency, highly reflective continuous seis- of this area with respect to the putative sill controlling seawater influx into the mic facies represent lower halite proportions (<80%) cyclically interbedded with basin (Figs. 9 and 10). More specifically, to the south, closer to the seawater high-density anhydrite and low-density carnallite. entry point (i.e., in the Sugar Loaf domain), gypsum (now anhydrite) was pre- 4. The Santos salt basin was completely desiccated and refilled at least 12 cipitated in larger quantities, whereas larger amounts of halite and bittern salts times in <530 k.y., as suggested by the 12 complete evaporite cycles that have were deposited farther away from the sill, toward the north, in the deeper, been identified within the salt. The Ariri Formation stratigraphy and cyclicity more isolated domains (i.e., Tupi domain; Figs. 9 and 10). Our observations are suggest that the sea-level variations controlling the salinity during salt deposi- consistent with those of Demercian (1996), who suggested that gypsum and/or tion were low-amplitude, rapid, fourth- to fifth-order sea-level changes typical anhydrite deposition was controlled by the position and direction of the sea- of greenhouse conditions, i.e., few meters to decimeters in ~10 to ~100 k.y. water inflow from the southern Florianopolis high (São Paulo Ridge), with con- 5. Lateral depositional thickness and stratigraphic variations occur within centrations increasing northward. In addition, our interpretation agrees with the salt and between structural domains, which we defined based on mean previous studies suggesting that seawater recharge from the south probably salt thickness and depth to base of salt structure; i.e., (1) the minibasin domain, percolated through fractures in the volcanic barrier (see Jackson et al., 2000; (2) the Tupi domain, (3) the Sugar Loaf domain, and (4) the thick salt domain. Nunn and Harris, 2007). 6. We suggest that an irregular deep basin existed at the onset of salt depo- Salinity variations similar to these interpreted for the Outer High have sition with sufficient accommodation for A1 and possibly A2 deposition. This been observed in Lake Assal, Djibouti, which represents a modern example of basin was characterized by a series of shallower and deeper subbasins, where a marine-fed rift basin actively accumulating interbedded evaporites (Jackson the Sugar Loaf subhigh was shallower with respect to surrounding areas. This

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syndepositional basin physiography controlled depositional thickness and tonics, Sediments and Prospectivity: Geological Society of London Special Publication 363, stratigraphy; overall, thinner salt (mean salt thickness <~1.8 km), highly reflec- p. 449–470, https://doi.org/10.1144/SP363.21. Chang, H.K., Kowsmann, R.O., Figueiredo, A.M.F., and Bender, A., 1992, Tectonics and stratigra- tive and continuous seismic facies, and higher anhydrite proportions (>~10%) phy of the East Brazil Rift system: An overview: Tectonophysics, v. 213, p. 97–138, https://doi characterize the salt in the Sugar Loaf domain, whereas thicker salt (mean salt .org/10.1016/0040-1951(92)90253-3. thickness of >~2.2 km), discontinuous, acoustically transparent seismic facies, Clark, J., Stewart, S., and Cartwright, J., 1998, Evolution of the NW margin of the North Permian Basin, UK North Sea: Journal of the Geological Society [London], v. 155, p. 663–676, https:// and higher bittern salt and halite characterize the structurally lower domains doi.org/10.1144/gsjgs.155.4.0663. (minibasin and thick salt domains). Clarke, F., 1924, The Data of Geochemistry: U.S. Geological Survey Bulletin 770, 841 p. 7. Stratigraphic variations exist between the Sugar Loaf domain and the Clauzon, G., Suc, J.-P., Gautier, F., Berger, A., and Loutre, M.-F., 1996, Alternate interpretation of the Messinian salinity crisis: Controversy resolved?: Geology, v. 24, p. 363–366, https://doi Tupi domain with higher gypsum (now anhydrite) proportions precipitated .org/10.1130/0091-7613(1996)024<0363:AIOTMS>2.3.CO;2. closer to the entry on seawater, whereas higher bittern salts were deposited Cobbold, P.R., and Szatmari, P., 1991, Radial gravitational gliding on passive margins: Tectono- in a more restricted and distal Tupi domain. This also suggests that seawater physics, v. 188, p. 249–289, https://doi.org/10.1016/0040-1951(91)90459-6. incursions came from the south through the São Paulo and Walvis Ridges. Cobbold, P.R., Szatmari, P., Demercian, L.S., Coelho, D., and Rossello, E.A., 1995, Seismic and experimental evidence for thin-skinned horizontal shortening by convergent radial gliding 8. In this study we provide new insights into the deposition and evolution of on evaporites, deep-water Santos Basin, Brazil, in Jackson, M.P.A., et al., eds., Salt Tectonics: an ancient salt giant. Our results suggest that an arid paleoclimate, low-ampli- A Global Perspective: American Association of Petroleum Geologists Memoir 65, p. 305–321. tude sea-level variations, and syndepositional basin physiography controlled Cobbold, P.R., Chiossi, D., Green, P.F., Jaspen, P., and Bonow, J., 2010, Compressional reactiva- tion of the Atlantic Margin of Brazil: Structural styles and consequences for hydrocarbon the deposition of this thick (1.2–2.5 km) salt giant, even though the depositional exploration: American Association of Petroleum Geologists Search Discovery Article 30114. time span was relatively short (<530 k.y.) and sediment accumulation rates Davison, I., 2007, Geology and tectonics of the South Atlantic Brazilian salt basins, in Ries, A.C., were correspondingly high. There is no clear evidence of influence from river et al., eds., Deformation of the Continental Crust: The Legacy of Mike Coward: Geological Society of London Special Publication 272, p. 345–359, https://doi.org/10.1144/GSL.SP.2007 discharge of fresh-water influx, and further studies are required to understand .272.01.18. how salt loading and syndepositional tectonics may have also controlled salt Davison, I., and Bate, R., 2004, Early opening of the South Atlantic: Berriasian rifting to Aptian deposition. salt deposition, in Africa: The Continent of Challenge and Opportunity: Petroleum Exploration Society of Great Britain–Houston Geological Society 3rd International Joint Meeting, p. 7–8. Davison, I., Anderson, L., and Nuttall, P., 2012, Salt deposition, loading and gravity drainage in the Campos and Santos salt basins, in Alsop, G.I., et al., eds., Salt Tectonics, Sediments and ACKNOWLEDGMENTS Prospectivity: Geological Society of London Special Publication 363, p. 159–174, https://doi We thank CGG (Paris, France; http://www.cgg.com/en) for providing access to the three-dimen- .org/10.1144/SP363.8. sional seismic data and for granting permission to publish the results of this study. We also thank De Freitas, R.T.J., 2006, Ciclos Deposicionais Evaporiticos da Bacia de Santos: Una Analise Ciclo Statoil and ANP Brazil for providing access to the borehole data. Schlumberger WesternGeco is estratigrafica a Partir de Dados de 2 Pocos e de Tracos de Sismica [M.S. thesis]: Porto Alegre, gratefully acknowledged for providing two-dimensional seismic regional data, partial funding, and Brasil, Universidade Federal do Rio Grande do Sul, 160 p. support of this work. We thank Ian Davison and Webster Mohriak for insightful reviews that im- Demercian, L.S., 1996, A Halocinese na Evolução do Sul da Bacia de Santos do Aptiano ao Cretá- proved the manuscript. ceo Superior [M.S. thesis]: Porto Alegre, Brasil, Universidade Federal do Rio Grande do Sul, 201 p. Dias, J.L., 2004, Tectônica, estratigrafia e sedimentação no Andar Aptiano da margem leste bra- sileira: Boletim de Geociências da Petrobras, v. 13, p. 7–25. 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