Dual tectonic-climatic controls on salt giant deposition in the Santos Basin, offshore Brazil

Abstract

The stratigraphic evolution of ancient salt giants is controversial, mainly due to the absence of modern analogues that are of comparable scale and thickness and that occur in similar tectonic and hydrological settings. Fur thermore, investigating the original stratigraphy of salt giants is often made difficult by postdepositional flow and dissolution. Layered evaporites of the Ariri Formation in the Santos Basin (offshore Brazil), deposited during open ing of the South Atlantic Ocean, form part of one such salt giant. Despite being well imaged in seismic data and being penetrated by more than 50 boreholes, little work has explored the stratigraphic architecture of this unit and what this may tell us about the syndepositional tectonics, basin physi og raphy, and variations in climate and sea level. Here we integrate threedimen sional seismic and borehole data from the São Paulo Plateau, deepwater Santos Basin, to document the intrasalt stratigraphy of the Ariri Formation. Our analysis suggests a combination of an arid paleoclimate, lowamplitude local sealevel variations, and basin physiography controlled the deposition of this thick (2.5 km) salt sequence during a short time span (<530 k.y.). The Ariri Formation records at least 12 cycles of basin desiccation and filling, re sulting in the deposition of four key units (A1-A4) that have a distinct com position and therefore seismic expression; i.e., lowfrequency, transparent, chaotic seismic facies represent relatively haliterich (>85%) units (A1 and A3), whereas highfrequency, highly reflective seismic facies represent still relatively haliterich (65%-85% halite) units, but contain relatively high pro portions (15%-35%) of anhydrite and bittern salts (i.e., K and Mgrich salts; A2 and A4 units). Our findings suggest that during salt deposition the Santos Basin was characterized by a series of subbasins of varying water depth; as a result the thickness and composition of these units vary laterally and are spa tially related to structural domains. Overall, thinner salt (~1.8 km) and higher anhydrite net thickness (~350 m) occur toward the structurally high Sugar Loaf domain, compared to flanking, structurally lower domains where the mean salt thickness is >2.2 km and anhydrite net thickness are less (~180 m). In addition, stratigraphic variations in the basin suggest that seawater incur sions came from the south, through the São Paulo and Walvis Ridges; conse quently, more anhydrite was deposited closer to the ridge, whereas more bit tern salts were deposited in more distal and restricted locations. The results of our study, although based on an analysis of Aptian salts preserved offshore Brazil, offer valuable insights into the sedimentology and stratigraphic archi tecture and evolution of other ancient salt giants.

Base‐salt relief controls salt‐tectonic structural style, São Paulo Plateau, Santos Basin, Brazil

“…The survey display follows the Society of Economic Geologists (SEG) normal polarity, where a downward increase in acoustic impedance is represented by a positive reflection event (white on seismic sections) and a decrease in acoustic impedance by a negative event (black on seismic section) (Brown, ). The average dominant frequency in the Aptian salt is c. 36 Hz and the interval velocity is c. 4,400 m/s, yielding a vertical resolution of c. 29 m (Rodriguez et al, ). Overburden strata have a dominant frequency that decreases (from c. 40 Hz to c. 31 Hz) and an average velocity that increases (c. 1900–2015 m/s) with depth, yielding a vertical resolution of c. 12–17 m. Horizontal resolution is twice the seismic line spacing (i.e., 37.5 m in the E–W direction and 50 m in the N–S direction) (Jackson, Jackson, Hudec, & Rodriguez, ).…”

“…We mapped salt structures and minibasins in three‐dimensions using the approach described in in Pichel et al (). Key seismic stratigraphic surfaces (Figure ) were identified using the well control outlined in previous publications (Guerra & Underhill, ; Jackson, Jackson, Hudec, & Rodriguez, ; Rodriguez et al, ). A base‐salt (BoS static‐corrected map, see Pichel et al, ; fig.…”

“…During the late Aptian, fault activity ceased on the SPP and a thick (up to 2.6 km) salt layer was deposited (Figure ) (Davison et al, ). Although salt deposition occurred after active faulting (i.e., it is locally post‐rift), relief inherited from the preceding rift phase controlled salt thickness variations; i.e., salt was thin across rift‐related footwall highs and thick in intervening hanging wall depocentres (Figure ) (Alves et al, ; Davison et al, ; Pichel et al, ; Rodriguez, Jackson, Rotevatn, Bell, & Francis, ).…”

“…The SPP is defined by two large pre‐salt highs: the NNE‐oriented Sugar‐Loaf Sub‐High in the south; and the NE‐oriented Tupi Sub‐High in the north (Mohriak et al, ; Rodriguez et al, ). These highs are connected by an ENE‐oriented structural high (Figure a) and are part of a larger rift‐related structure known as the Outer High (Figure ) (Davison et al, ; Demercian et al, ; Mohriak et al, ).…”

“…The base of salt is typically undeformed, although it is locally offset by a few relatively small (up to 100 ms TWT or c. 450 m of throw) faults. Where closely spaced, the cumulative throw across these faults controls the development of base‐salt ramps and monoclinal geometries at the edge of the largest pre‐salt highs (Figure ) (Alves et al, ; Davison et al, ; Rodriguez et al, ). This indicates that pre‐salt structures and related topography were present before and during salt deposition, with only minor, reduced fault activity during (or after) salt deposition (Davison et al, ).…”