Diversity and origin of quartz cements in continental carbonates: Example from the Lower Cretaceous rift deposits of the South Atlantic margin

Abstract

Silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis. The Lower Cretaceous rift deposits of the South Atlantic equatorial margin, which are intensively explored by petroleum companies, provide good examples of such silicifications in carbonates, exhibiting a wide diversity of petrographic habit of early to late quartz cements. In order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity, we integrated detailed petrography of diagenetic sequences and quartz habit with δ 18 O quartz measurements (by SIMS) of individual cements observed in samples from the offshore and onshore basins of the West African margin. The petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length-fast or length-slow habit, in addition to laminated microquartz and micro-or mega-quartz forms. Amongst the isotopic analysis, the δ 18 O quartz data show that length-slow cements are generally strongly enriched in 18 O (δ 18 O quartz ranging from 31 to 37‰ SMOW), whereas length-fast forms show less elevated values ACCEPTED MANUSCRIPT 2 (<32‰ SMOW). The highest δ 18 O quartz values for fibrous microquartz are interpreted to reflect precipitation from evaporated meteoric fluids at temperatures >25°C and <100°C. The alkalinity required to favor the precipitation of length-slow fibrous microquartz cements is probably related to fluid/rock interactions with underlying mantel-related or basic volcanic rocks. Such interactions would be in agreement with the recent geodynamic models of the South Atlantic passive margin. The length-fast fibrous microquartz associated with δ 18 O quartz values ranging from 27 to 32‰ SMOW, probably reflect precipitation from moderately to non evaporated, fairly neutral to acid, fluids. The partial dissolution of carbonate cements prior to quartz cementations represents the signature of those acidic conditions. We therefore suggest that acidic pH was obtained through fluid/rock interactions with the intermediate to acid volcanic rocks encountered along the palaeohydrological pathway. Other quartz phases, such as the megaquartz cement, exhibit highly variable δ 18 O quartz values ranging from 20 to 40 ‰. This variation may reflect significant variation in temperature conditions (between 100 and 200°C) or changes in fluid δ 18 O at the small scale. For these populations of non-fibrous quartz cement, the very high δ 18 O quartz values may reflect a contribution of fluids that have either suffered strong evaporation or strong water/rock interaction.

Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon‐oxygen isotopes

“…On the contrary, the “pristine” (non‐corroded) chert cluster shows positive δ 30 Si (>0.1‰, mean of 1‰) and δ 18 O values ranging from 22 to 25‰; literature data (Abraham et al, 2011; Chakrabarti et al, 2012; Teboul et al, 2019; van den Boorn et al, 2010) consider these values as a possible indicator of direct precipitation from marine‐derived pore water during diagenesis. High values of δ 18 O in silica (>20‰–25‰) are usually interpreted in the context of diagenetic fluids (Marin‐Carbonne et al, 2014; Teboul et al, 2019), whereas depleted δ 18 O is considered to reflect either a mantle or a meteoric source (Hemond et al, 1993; Kleine et al, 2018). Hydrothermal‐derived quartz and silica usually show more dispersed δ 18 O values, in the range of 13%–36% (Clayton et al, 1972; Liu et al, 2022).…”

“…To assess the origin of silica dissolution‐precipitation processes in the Calixto Cave, we studied samples consisting of megaquartz (euhedral quartz crystals larger than 100 μm, generally up to 1–2 mm; Marin‐Carbonne et al, 2014; Teboul et al, 2019), chalcedony and chert (lithotype composed of microcrystalline quartz). Quartzite samples from the Chapada Diamantina Group (Figure 1b) were analyzed for comparison.…”

“…Undulose extinction and deformation lamellae in megaquartz crystals also suggest that these crystals were subject to deformation after their deposition (Figure 4e,f). Megaquartz and chalcedony in marine carbonate sequences are usually products of silica recrystallization and cementation in a deep-burial setting or due to later hydrothermal/metamorphic processes (Marin-Carbonne et al, 2013;Teboul et al, 2019). In Calixto Cave, the colloform-plumose textures and the coexisting solid (S), two-phase (V + L) and single-phase (V) primary inclusions are diagnostic features of hydrothermal processes, possibly associated with boiling solutions (Albinson et al, 2001;Moncada et al, 2017;Roedder & Bodnar, 1997).…”

“…Cavernous, sometimes mineralized solutional voids attributed to hydrothermal speleogenesis have been described in quartzites and skarns in Kyrgyzstan (Kornilov, 1978; Leven, 1961), USA (Colorado; Lovering et al, 1978) and Ukraine (Tsykin, 1989). Silicified reservoirs are typical of the lower Cretaceous pre‐salt sequences of the Campos and Santos basins in Brazil (Lima et al, 2020; Lima & De Ros, 2019), the Kwanza‐Congo Basins of Africa (Poros et al, 2017; Teboul et al, 2019) and the Parkland gas field in Canada (Packard et al, 2001). On the basis of petrographic, isotopic and fluid inclusion studies, pervasive silicification and porosity enhancement processes in these basins were ascribed to the circulation of high‐temperature fluids (De Luca et al, 2017; Girard & San Miguel, 2017; Lima et al, 2020; Poros et al, 2017; Teboul et al, 2019).…”

“…Silicified reservoirs are typical of the lower Cretaceous pre‐salt sequences of the Campos and Santos basins in Brazil (Lima et al, 2020; Lima & De Ros, 2019), the Kwanza‐Congo Basins of Africa (Poros et al, 2017; Teboul et al, 2019) and the Parkland gas field in Canada (Packard et al, 2001). On the basis of petrographic, isotopic and fluid inclusion studies, pervasive silicification and porosity enhancement processes in these basins were ascribed to the circulation of high‐temperature fluids (De Luca et al, 2017; Girard & San Miguel, 2017; Lima et al, 2020; Poros et al, 2017; Teboul et al, 2019). However, the link between silicification and hypogene karst dissolution, and their evolution in space and time, are still poorly understood.…”