On the Origins of Eogenetic Chlorite in Verdine Facies Sedimentary Rocks from the Gabon Basin in West Africa T

Marine and Petroleum Geology 112 (2020) 104064

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Research paper On the origins of eogenetic chlorite in verdine facies sedimentary rocks from the Gabon Basin in West Africa T

∗ Branimir Šegvića, , Giovanni Zanonia, Andrea Moscariellob a Department of Geosciences, Texas Tech University, 1200 Memorial Circle, Lubbock, TX 79409, USA b Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland

ARTICLE INFO ABSTRACT

Keywords: The verdine facies clay assemblages are regularly encountered in shallow marine sediments of equatorial lati- Green clays tudes where Fe of terrigenous origin is readily available. There, the 7 Å Fe-rich green clay (odinite) of syn- Odinite sedimentary origin commonly occurs. Previous findings suggested that odinite serves as a precursor of diagenetic Odinite-chlorite chlorite whose importance in porosity conservation has been widely acknowledged. The investigation of Fe-rich Diagenetic chlorite clays is, therefore, helpful to constrain the specific conditions and mechanisms, which give rise to chlorite West Africa growth, thus illuminating the history of early burial. In this contribution, we studied Cretaceous sediments from Solid-state transformation Sandstone reservoir the Gabon coast where multiple sedimentary basins came into existence prior, during and after the opening of the South Atlantic. Analyzed core is siliciclastic and petroleum bearing stemming from the fluvio-lacustrine- deltaic environment recovered from depths of ∼1500 m TVD. Electron micro-beam investigation coupled with X-ray diffraction on the clay fraction and spectra fitting revealed odinite, interstratified odinite-chlorite (O-C), and to a lesser extent, illite-chlorite-smectite (I-C-S) and illite-smectite occur as grain coatings and pore-filling phases with a characteristic honeycomb texture. Embryonic chlorite emerges at the contact of odinite and O-C flakes as a particulate authigenic mineralization. Formed from aluminosilicate and Fe-oxyhydroxide debris at sediment-water interface, odinite becomes intrinsically unstable, possibly due to the activity of Fe-reducing bacteria or decaying organic matter creating a prevalently reducing environment that promotes Fe reduction. Such condition facilitated a two-step chlorite growth, which first included a partial chloritization of odinite through the in situ solid-state transformation of 7 Å to 14 Å layers. Then, with increasing burial, at peak diagenetic conditions (50 °C, ∼1.5 km depth), odinite and O-C both recrystallized to form eogenetic chlorite most likely following the Ostwald ripening mechanism.

1. Introduction et al., 1987; Bailey, 1988; Odin and Sen Gupta, 1988; Odin, 1990). Occurrences of odinite and related 7 Å phases have been widely Sediment that is abundant in greenish Fe-rich clay minerals found in documented in estuarine to marine-deltaic sediments of tropical regions tropical oceanic sediments has been known to define a peculiar mi- (e.g. Hillier, 1994; Baker et al., 2000; Jeans et al., 2000). Being syn- neralogical assemblage known as verdine facies (Odin, 1990; Grigsby, sedimentary in origin, these clay minerals form at shallow, near-surface 2001; Ryan and Hillier, 2002; Aplin and Taylor, 2012; Velde, 2013). conditions (< 50 m, 30 °C), usually at the interface between marine and This facies is represented by a variety of 7 and 14 Å clay minerals non-marine conditions (Odin, 1990; Purnachandra Rao et al., 1993; (odinite and chlorite-like) of loosely defined chemistry and distinct Purnachandra Rao et al., 1995). Several studies on clastic sedimentary geological significance (Harding et al., 2014). The verdine facies has rocks have identified the 7 Å clay minerals as precursors of authigenic, currently been identified in modern shallow marine sediments proximal early diagenetic chlorite (Ryan and Reynolds, 1996; Grigsby, 2001; to Fe influxes of terrigenous origin. It is mineralogically and sedi- Ryan and Hillier, 2002; Worden and Burley, 2003). Chlorite is sug- mentologically different from glaucony facies, with no presence of 10 Å gested to originate via mixed-layer intermediates derived from the 7 Å glauconitic minerals. In his pioneer research on verdine facies clay component that was progressively converted to a 14 Å phase (Beaufort minerals, Odin and his co-workers identified a new 7 Å phase, origin- et al., 2015). Usually, chlorite and 7 Å/14 Å mixed-layer clays occur as ally called phyllite-V, which later became odinite (Odin, 1985; Odin pore fillings and grain coatings, which may preserve high porosity in

∗ Corresponding author. E-mail addresses: [email protected] (B. Šegvić), [email protected] (G. Zanoni), [email protected] (A. Moscariello). https://doi.org/10.1016/j.marpetgeo.2019.104064 Received 1 March 2019; Received in revised form 13 September 2019; Accepted 25 September 2019 Available online 28 September 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved. Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Fig. 1. Position of the Gabon Basin along the South Atlantic margin of Western Africa with the indication of study area (modified after Dupré et al., 2011). Generalized stratigraphic column is shown in the right outlying the position of the Gamba Formation (modified after Brownfield and Charpentier, 2006). sandstone by precluding quartz cementation (Billault et al., 2003; the lack of research on green clays in pre-Miocene sediments Bahlis and De Ros, 2013; Chen et al., 2014; Saïag et al., 2016; Cao et al., (Purnachandra Rao et al., 1993; Wiewióra et al., 1999; Meunier and El 2018; Virolle et al., 2018). Exercising such an important role in porosity Albani, 2007), the present study brings new knowledge on the eoge- conservation, the significance of authigenic chlorite and interlayered netic geochemical evolution of verdine clays through a range of de- chloritic phases has been recognized in assessing the impacts of diag- tailed mineralogical, microtextural and geochemical analyses. This re- enesis on the heterogeneity of sandstone reservoirs (Morad et al., 2010 search further reports on the co-existence of verdine facies minerals and and references therein). Diagenetic control over reservoir properties is trioctahedral smectite, which both served as feedstock for chlorite au- becoming more important due to the increased practice of horizontal thigenesis, albeit the latter being commonly reported to form only in drilling in tight sandstone using large frack stimulations (e.g. Burri, comparatively deeper waters farther from the shoreline (Porrenga, 2014). The growth of eogenetic chlorite in verdine facies is based not 1967; Odin and Sen Gupta, 1988; Odin, 1990). only on the 7 Å substrate but also on smectite as suggested by saponite- The aim of this investigation was to find geochemical and miner- chlorite interstratifications in clastic systems usually rich in volcano- alogical clues as to how chlorite is related to and develops from Fe-rich clastic component. With increasing burial depths, saponite-chlorite clays of verdine facies. To address these goals, we formulated the fol- converts to corrensite and is eventually replaced by Mg-Fe chlorite (e.g. lowing research questions. Does chlorite formation operate via a series Brigatti and Poppi, 1984; Hillier, 1994; Biernacka, 2014). In such en- of mixed-layer intermediates derived from an odinite (or Fe-smectite) vironments, mixed-layer clay minerals may be present in all possible component through the process of in situ solid-state transformation or/ combinations of different 10 and 14–15 Å layers (illite-chlorite, and does it involve a bulk recrystallization of precursor phases to yield chlorite-smectite, illite-chlorite-smectite; Bradley, 1953; Weaver, 1956; an embryonic chlorite mineralization? Is there any connection between Solotchina et al., 2000). odinite composition and mechanisms leading to chlorite formation? Are The Cretaceous sandstone reservoir rocks from South Gabon formed there morphological connections between chlorite and its precursors in a depositional system characterized by a major riverine influx onto and, if so, what are their implications? Does microbial activity play a the shallow-marine shelf during periods of relatively low sea-level stand role in the growth of diagenetic chlorite? (Brownfield and Charpentier, 2006). This study builds on the fact that these rocks, subjected only to weak diagenetic changes, are rich in pore filling 7 Å Fe clays (odinite-like minerals), as well as the chlorite- 2. Geological background smectite and 7 Å/14 Å mixed-layer intermediates (Zanoni et al., 2015, 2016). While most of the literature on chlorite in reservoir rocks focuses One of the largest West African costal basins is the Gabon Basin on the formation and inhibition potential of quartz cementation in whose evolution may be divided in the three distinct stages – the rifting evolved verdine facies (e.g. Hillier, 1994; Ryan and Hillier, 2002; phase, the transitional phase, and the passive continental margin phase Huggett et al., 2015; Ma et al., 2017), this research examines the p-t (Teisserenc and Villemin, 1989). A large intraplate rift zone started to realm of incipient burial, which is a stage that observes the evolution of form during Late Jurassic and Early Cretaceous between the African the syn-depositional, odinite-like phases of variable chemistry to more and South American plates as a consequence of the final break-up of stable chlorite-like compositions. Analyzed rocks thus likely represent western Gondwana (e.g. Heine et al., 2013). The newly opened rift juvenile versions of chlorite-bearing sandstones featured by anom- created accommodation that was progressively filled by riverine, del- alously high porosity in deep sedimentary basins. Taking into account taic, and lacustrine sediments. Following a short peneplanation stage,

2 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064 the unconformable deposition of the Gamba sand ensued during Middle lowest unit of Facies 1 evolves sharply into weakly laminated pale sands Aptian transgression (Fig. 1). The Gamba sand interval is overlain by a (1449 m) without any signs of erosion. Such contact renders a vestige of thick series of evaporites. Final separation of Africa from South America accelerated sedimentation rates accompanied by the increase in energy that took place during the Late Aptian (Wilson, 1992), where the level as suggested by a general coarsening upward trend. Progradation margin evolved into a passive continental margin resulting in thermal of a moderate to low energy delta in an estuarine environment may subsidence of the Gabon Basin (e.g. Anka and Séranne, 2004 and re- explain such developments (Burgess and Hovius, 1998; Porębski and ference therein). According to Dupré et al. (2011), the Gabon Basin may Steel, 2003). be divided into three sub-basins – the North Gabon sub-basin, the South Facies 2A – This sub-facies of Facies 2 consists of pale colored fine- Gabon sub-basin, and the Interior basin, with the N'Komi fault zone grained sand with weakly preserved lamination. Similarities with Facies separating the North from the South sub-basins (Wannesson et al., 1 do exist, however, with less pronounced greenish coloration and lack 1991)(Fig. 1). Sediment analyzed in this study was acquired from the of visible bioturbation. The sand is moderately to well sorted with la- Cretaceous Gamba Formation, specifically the homonymous borehole mination defined by darker material, presumably the grain coating that cored the onshore producing field of Tsiengui in the South Gabon preferentially attracted to finer-grained sand. This facies is believed to sub-basin (Fig. 1). Stratigraphically, the Gamba Formation denotes a reflect higher energy conditions characteristic for the more proximal transgressive depositional sequence of fluvial sandstone overlain by zone of deltaic environment. Sample 24 was recovered from this facies. lagoonal deposits (Teisserenc and Villemin, 1989). Maximal thickness Facies 3 – This facies consisted of wavy and ripple laminated fine- of this formation is about 130 m and consists of unlithified sand with no grained sand. The facies is very thin occurring as a single, 65-cm-thick major discontinuities. Sand of the Gamba Formation is one of the main unit, at 1441.5 m (Fig. 2). Texturally, the facies is defined by fine- oil prospection areas in the onshore part of the South Gabon sub-basin grained sand, moderately to well sorted with well-defined lamination, (Zanoni, 2015). which tends to be somewhat disturbed, possibly due to soft-sediment The material sampled for this study originated from the relatively deformation. Significantly lower energy is suggested for this facies shallow, thermally immature sediments, where phase transformations based on its grain size whilst the wavy and ripple cross-lamination in- have just commenced yielding a range of intermediate mixed-layer dicate weak currents to have been at least intermittently active. Sample compositions. Sediment was acquired from the Gamba core recovered Gamba 16 was recovered from this facies. The thin Facies 3 grades from the South Gabon sub-basin at a depth range from 1426 m to upwards into a pale homogenous sand interval (Facies 4, not described 1453.5 m SSTVD (< 50 °C, Fig. 2). Sediment sampled was subangular to here) whose featureless nature is hard to interpret with respect to de- subrounded, moderately to well sorted sand intervals. Sedimentary logs positional environment. And yet, the fact that both of these sands ap- that include the descriptions of the thicknesses, the sedimentary pear to grade into and then out of Facies 1 at around 1436.5 m suggests structures and facies, the grain sizes, and interpreted depositional en- that the two facies are similar in origin. If, therefore, Facies 4 records a vironments are shown in Fig. 2, which was adapted from Zanoni low-mid deltaic setting, then the unit of Facies 3 is likely also associated (2015). Several major sedimentary facies were identified based on the with a non-erosive minor deepening event. existing rock descriptions (Zanoni, 2015). 3.2. Methods 3. Materials and methods 3.2.1. Scanning and automated electron microscopy 3.1. Description of core material Minimally polished thin-sections were prepared for the investigation using scanning electron microscopy with energy-dispersive X-ray The cored sediments are, for the most part, clean unconsolidated spectroscopy (SEM-EDS). Samples were carbon coated and then ana- sands that were stabilized with polyurethane foam prior to slabbing. In lyzed in a Hitachi S-4300 E/N field emission apparatus with two Silicon brief, the core is made of intervals of homogenous sand found at the Drift Energy Dispersive X-ray Detectors (SDD) from Bruker installed at very top of the sequence. It gradually changes downward into the weak- the College of Arts and Sciences Microscopy (Texas Tech University, to well-laminated sand with localized intervals of cross-bedded sand. USA). Images were obtained in secondary electrons and back-scattered Minor traces of bioturbation are reported solely at the very top of the electron modes. A variety of acceleration voltages and beam size con- section. Thin intervals of undulated and ripple-laminated sand also ditions were employed to assure the best imaging conditions. The an- occur. Towards the top of the core, a thin carbonate layer is present cillary microanalytical system Pegasus 4040 was used for EDS spectra (< 50 cm), consisting of dolomite and calcite with some minor silici- acquisition and quantification in a standardless mode. EDS analyses clastic component (Fig. 2). Seven representative samples chosen for this were performed on spots with a diameter of 10 nm, with acquisition live study were acquired from the facies with the highest clay content that time of 20 s. Chemical data were used as atomic percentages and were commonly exhibited a greenish tint (Fig. 2). normalized to 100%.

3.1.1. Sedimentary facies and depositional environments 3.2.2. X-ray diffraction Detailed description of the cored interval reveals five major sedi- X-ray powder diffraction was performed on the clay-size fraction. mentary facies (Fig. 2), which have been further subdivided into sub- Sediment was gently crushed with an agate mortar prior, then subjected facies. Facies and sub-facies concepts are described in detail in Zanoni to 10 wt% H2O2 for at least 24 h to remove organic matter. The clay-size et al. (2015) and Zanoni et al. (2016). Here we refer to three of those fraction was separated from the bulk rock material by centrifugation. facies that are of importance to this study. Na-metaphosphate was added to disperse the clays, while further dis- Facies 1 – This facies consists of green, massive sand with slight aggregation was performed in an ultrasonic bath. Clay-size fractions bioturbation at the top of the section. Samples belonging to Facies 1 are were treated by 10 ml of approximately 4 M MgCl2 to saturate them Gamba 2, 4, 6, 26 and 28. Texturally, the sand is fine-grained, mod- with Mg thus securing a uniform cation exchange. To minimize the free- erately to well sorted with only a fraction of larger grains. The sediment ion content, all the suspensions were centrifuged with distilled water a is of distinct greenish color and in some samples dark green grains are minimum of three times. Pipettes were used to place solutions with the observable. The green color is attributed to the presence of Fe-rich clay-size fraction onto porous ceramic tiles. Such prepared oriented minerals in the verdine facies (Zanoni, 2015). A moderate level of mounts were left overnight to dry prior to XRD measurements. The bioturbation at the top of this facies suggests a transitional deltaic to thickness of oriented mounts surpasses 50 μm, which is a limit of ‘in- estuarine depositional environment where lamination might have been finite thickness’ (Moore and Reynolds, 1997) required for semi-quan- destroyed by sediment biota. Going upwards through the core, the titative determination of the clay mineral content. The measurements

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Fig. 2. Idealized stratigraphic log of the Gamba core showing depths, sedimentary facies, lithologies, sample locations, grain-size scale, and wireline logs (modiﬁed after Zanoni et al., 2016). The Gamba 2 and 4 are dominated by O-C whilst the bottom of the core is characterized by odinite.

4 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064 were carried out in air-dried (AD) conditions, after ethylene-glycol (EG) first two principal components are commonly utilized to describe the saturation, and after heating for 1 h at 300 °C. A Rigaku® Miniflex II complex structure of original data. Data normalization was applied to Desktop XR diffractometer installed at the Department of Geosciences the compositional data to remove the non-negativity and constant-sum (Texas Tech University, USA) was used for XRD analyses. The size of the constraints that are characteristic for compositional variables divergent slit was 1° whilst the receiving one equaled 1 mm. Measure- (Aitchison, 1982). Statistical analyses were completed using the JMP® ment parameters comprised a step scan in the Bragg-Brentano geometry Pro 12.1.0 statistical software. using CuKα radiation (30 kV and 30 mA) with a curved-graphite monochromator. Sample mounts were scanned at a counting time of 4. Results 10 s per 0.02° from 3 to 30 °2θ. 4.1. XRD mineralogy 3.2.3. Interpretation and fitting of X-ray diffraction patterns The Rigaku PDXL - Integrated X-ray powder diffraction software (v. The clay-size fraction is characterized by an assemblage consisting 2.7.2.0) was utilized for identification of mineral phases. X-ray dif- of discrete phases - chlorite, illite, quartz, and odinite. Odinite-chlorite fraction patterns of clay minerals were examined relying on the re- and illite-chlorite-smectite stand for mixed layered clay minerals commendations introduced by Lagaly et al. (1984), Moore and (Fig. 3). Minor phase is illite-smectite. Presence of amorphous matter is Reynolds (1997) and Środoń (2006).Diffractograms were modelled suggested by the shape of background curves that increased from ∼15 using the Sybilla© software (property of Chevron™), which is based on °2θ. In general, the content of amorphous matter was somewhat lower the formalism of Drits and Sakharov (1976). Spectra fitting relies on a in the samples dominated by 7 Å clay minerals compared to the sample trial-and-error procedure that provided optimal clay mineral structural rich in I-C-S (Gamba 2, Fig. 3). and probability parameters to get the best fit between experimental and The identified mineral phases and their relative abundances are calculated patterns, as well as intensities of 00l reflections for each of summarized in Table 2. The major 10 Å phase present is detrital fine- the clay phases present. The number, nature, and stacking sequence of grained mica featured by the main basal peak at d001 9.96 Å not affected different compositional layers in mixed-layer minerals were taken as by glycolation (Fig. 3a,c). The expandable interstratifications of illite modifiable values (e.g. Uzarowicz et al., 2012; Šegvić et al., 2016). and smectite occur in odinite-rich samples (Table 2) and account for Three discrete clay mineral phases were introduced (i.e. illite, chlorite, disordered R0 I-S and R0 I-SS as suggested by pattern fitting. The R0 I-S and serpentine) to produce experimental spectra. Serpentine was uti- an R0 I-SS were defined by basal peaks with d001 10.12 and 10.42 Å, lized in lieu of odinite-like 7 Å Fe clays having a maximal diffraction respectively (Fig. 3d). The 7 Å odinite of broad composition is virtually intensity at its second basal diffraction peak d002 at 3.55 Å. Yet, such an reported in all analyzed samples, but somewhat less abundant towards intrinsic diffraction feature could have not been fully modelled by the top of the sequence in samples rich in mixed-layer chloritic minerals ff fi serpentine due to the di erence in the intensity ratio of its rst and (Table 2). Odinite is defined by the pronounced d002 at 3.55 Å and, not second basal peak (Whittaker and Zussman, 1956; Sengupta et al., as much prominent, d001 at 7.06 Å (Brindley, 1951, Fig. 3b,d). Dif- 2010). A disordered R0 I-S was used to model the immediate low-angle fraction patterns are characterized by the broad and relatively asym- shoulder of the first illite basal diffraction peak, whereas the 001 peak metrical odd order diffraction peaks at ∼14.57 and ∼4.72 Å (Fig. 3b), asymmetry of illite lower 2θ values was compensated by R0 I-SS. The which argues for the presence of 7 Å and 14 Å layer interstratifications. second smectite in I-SS labelling stands for a multiple type of smectite Recent XRD simulations of 7 Å to 14 Å layer conversion have indeed component differing from the preceding one in terms of charge and, revealed diffraction patterns with a wide odd order compared to sharp therefore, in terms of d spacing. It follows that the expandable layers in even order reflections at ∼7.07 and ∼3.53 Å (Reynolds et al., 1992; I-S are bi-hydrated, while in I-SS they are mono- and bi-hydrated Beaufort et al., 2015). Our modelling supported the presence of at least (Ferrage et al., 2007). Chlorite-odinite mixed layered phases were two 7 Å/14 Å phases (odinite-chlorite) with 10 and 78% of 7 Å com- modelled following the recommendations of Beaufort et al. (2015) ponent, respectively (Table 1). The presence of chlorite is inferred fi using the two disordered C-Sr (R0 O-C_1 and O-C_2 in Fig. 3) with based on d001 at 14.15 Å (Fig. 3d). Low intensity of the rst basal peak different proportions of 7 Å layers (15 and 68%). The latter essentially indicates Fe-enriched chlorite (Moore and Reynolds, 1997), which is served to fit the low-angle asymmetry of the main diffraction peak at consistent with EDS chemistry (Table 3) and authigenic morphology of ∼7.15–7.20 Å. Lastly, the two ordered three-component mixed-layer chlorite mineralization. The presence of two three-component I-C-S clay minerals consisting of illite, chlorite and smectite (R1 I-C-S) were phases is further suggested to fit the peaks at ∼14.22 and ∼9.11 Å, introduced to fitdiffraction peaks at ∼14.22 Å (illite-poor variety) and respectively (Fig. 3b). The first I-C-S is chlorite- and smectite-rich ∼9.11 Å (illite-rich variety). Modelling parameters of mixed-layer (∼45%, Table 1), which is in line with a moderate shift exhibited by minerals consisted of (1) the orientations of particles on the mounted X- d001 upon glycolation (Fig. 3a). The second I-C-S species, rich in 10 Å ray slides (σ*), (2) the coherent scattering domain sizes expressed in component (∼71%, Table 1), is denoted by a broad peak at ∼9.11 Å. number of layers (CSDS), (3) the amounts of smectite and chlorite SEM-EDS investigation also indicated the presence of Fe smectite in components in mixed-layer phases, (4) the amount of octahedral Fe, odinite-rich samples, which was not reported by XRD, most likely be- and (5) the inter-layer cation content (Table 1). Clay mineral nomen- cause of its low content and/or low crystallinity. Analyzed material clature followed the AIPEA (Association internationale pour l'étude des must, therefore, have more smectite than initially suggested by XRD. argiles) recommendations with the terms smectite, illite, kaolinite, serpentine, and chlorite used as general expressions for the respective 4.2. SEM-EDS chemistry and mineral morphology mineral groups (Bailey, 1980). Numerous EDS measurements and multiple SEM images indicate 3.2.4. Statistical analysis diverse micromorphology and phase chemistry (Table 3), which in turn Mineral chemistry acquired through the SEM-EDS investigation allowed categorization of five distinct mineral groups (A to F; Figs. 4 (Table 3) was subjected to statistical multivariate analysis (principal and 5). Particles compatible with odinite chemistry (Odin, 1985; Odin component – PCA) to identify possible evolutionary trends of analyzed et al., 1987, Fig. 4c: spectrum 25, 4d: spectrum 3; Fig. 5a–d: groups C clay mineral assemblages. PCA is a tool commonly used to identify and D) are characterized by a typical honeycomb or boxwork mineral different compositional groups within large multidimensional datasets. arrangement where individual crystals are oriented on edge, with faces It operates by introducing a smaller number of artificial variables called being virtually perpendicular to the surrounding non-clay silicate fra- principal components (PCs, Beier and Mommsen, 1994) that should mework. The boxwork structure is diamond-like to irregular in shape retain as much as possible of total variation. Plots constructed using the and consists of edge-to-edge packets usually surrounded by somewhat

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Fig. 3. XRD patterns of the two representative clay-rich samples from the Gamba core: (a) and (c) the air-dried (continuous line) and ethylene-glycole (dashed line) measurements; (b) and (d) fitting of XRD patterns. Mineral abbreviations: I-S – illite-smectite; I-C-S – illite-chlorite-smectite; O-C – odinite-chlorite; R – Reichweit (measure of order). For details see the text. thicker outer rims (Fig. 5a). The EDS analysis of odinite yields the major evolutionary trend is defined by a gradual loss of Al and Si compensated elements of Mg, Al, Si, and Fe (Table 3). While the content of Si and Fe by an increase in Mg and Fe (Table 3). is relatively uniform, different Mg/Al ratios (group C∼1; group D∼0.5) Aggregated illite platelets are occasionally reported among the seem to reflect the unstable octahedral composition of odinite suscep- pore-filling clay minerals (Nadeau and Bain, 1986; Środoń et al., 1986, tible to differences in chemical availability at the time of sediment Fig. 4e: spectrum 6, 5e). The size (up to 10 μm) and shape of these deposition (Wiewióra et al., 1999; Gabon 2 vs Gabon 24, Fig. 2). At irregular clay forms indicates a clear detrital origin (Fig. 5e: group E). edge-to-edge contacts of sub-micron to micron-sized odinite flakes, The last clay mineral species documented is smectite (Nadeau and Bain, microscopically particulated subhedral or rarer colloform habits occur 1986; Christidis and Eberl, 2003, Fig. 4f: spectrum 8). Smectite forms (Fig. 4b: spectrum 8; Fig. 5b–d: group B). The EDS chemistry of such densely packed patches (Fig. 5f: group F) that are found to be well mineralization is stable and suggests a chlorite composition (Wiewióra aligned along the pore rims. Their low Al/Si ratio (∼0.25) and a re- and Weiss, 1990, Table 3). Relatively similar morphology is demon- lative abundance of measured Fe and Mg (Table 3) best correspond to strated by the massive and homogenous particles of irregular shape up saponite mineral chemistry (Mg-Fe trioctahedral smectite; e.g. Treiman to several microns in size (Fig. 5b–c: group A). These massive particles et al., 2014). are characterized by a chemical composition that approaches chlorite- smectite (Inoue and Utada, 1991, Fig. 4a: spectrum 23). Somewhat 4.3. Statistical treatment of EDS mineral chemistry higher K2O content (4.8–7.4 wt%, Table 3) may be attributed to the minor mica-like interlayering as suggested by XRD pattern fitting fi (Fig. 3a, R1 I-C-S). The illite-smectite-chlorite does not reveal any direct PCA analyses indicates the rst three components account for structural relationship with the network of honeycomb odinite, which 99.16% of the total variance of EDS chemistry dataset (71.96%, excludes a possible genetic link between the two. On the other hand, 18.47%, and 8.73%, respectively; Fig. 6). Two groups (C and D), re- chemical makeup of this three-component clay is unstable and readily presenting the chemistry of odinite, form relatively homogenous pro- converges toward the composition of chlorite without any visible jections areas. They either weight positively PC1 and PC2 (group C) or ff ff change in particle morphological characteristics. Described a ect PC1 positively and negatively while a ecting PC2 negatively only (group D) (Fig. 6a). Samples with a composition of I-C-S (group A) and

Table 1 Sybilla© parameters of mixed-layered phases used for XRD pattern modelling of representative samples.

Mixed-layer phase/Phase parameters σ* CSDS Sme (%) Chl (%) Ill (%) Sr (%) FeVI ICC

R0 I-S 11.4 25.4 10 – 90 – 2.2 2.0 R0 I-SS 17.6 30.0 24 – 76 – 0.4 1.8 R0 C-Sr (1) 14.5 7.1 – 90 – 10 0.2 0 R0 C-Sr (2) 20.0 10.0 – 22 – 78 0 1.0 R1 I-C-S (1) 40.0 30.0 47 46 7 – 0.5 (C) 1.0 (C) R1 I-C-S (2) 14.5 22.8 22 7 71 – 4.0 (I) 1.3 (I)

σ* – orientation of particles on the mounted X-ray slide; CSDS – coherent scattering domain sizes expressed in layers; Sme (%), Chl (%) Sr (%), and Ill (%) – smectite, chlorite, serpentine, and illite content in the respective mixed-layered minerals (in %); I-S, C-Sr, I-C-S, and C-S – mixed-layered illite-smectite, chlorite-serpentine, illite-chlorite-smectite, and chlorite-smectite; R – Reichweit (measure of order); FeVI – octahedral Fe in illite (I) and chlorite (C); ICC – interlayer cation content in illite (Ill) and chlorite (Chl).

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Table 2 Relative clay mineral abundances in analyzed sandstones from the South Gabon sub-basin.

Sample Core Depth [m] Chl Ill 7 Å-m R0 I-S R0 I-SS R0 O-C (1) R0 O-C (2) R1 I-C-S (1) R1 I-C-S (2) Facies

Gamba 2 1427 + ++ + * – ++ + ++ + 1 Gamba 4 1428.4 * ++ + –– ++ + ++ + 1 Gamba 6 1436.6 + ++ + –– +* ––1 Gamba 16 1441.5 + ++ ++ + * + * ––3 Gamba 24 1448.8 + ++ ++ + + + * ––2A Gamba 26 1450.1 + + ++ + * + * ––1 Gamba 28 1453.4 + ++ ++ + * + * ––1

Chl – chlorite; Ill – illite; 7 Å-m – odinite; I-S, O-C, and I-C-S – mixed-layered illite-smectite, odinite-chlorite, and illite-chlorite-smectite; R – Reichweit (measure of order); ++ indicates major phases detected by XRD, + indicates minor phases detected by XRD, * indicated phases present, but not unequivocally confirmed by XRD, – indicates phases not detected by XRD; Mineral abbreviations after Kretz (1983) and Whitney and Evans (2010). illite (group E) also depict well-defined projections areas in the left (0.597), whereas the correlation of Al and Si is rather poor (0.110). segment of a diagram (Fig. 6a). Chlorite (group B) falls between the Based on microtextural evidences discussed above, the PCA biplot three major projection realms (i.e. A, C and D). projection patterns and elemental correlations are best explained by the Clay mineral groups demonstrating transitional chemistry show a three compositional vectors depicted in Fig. 6b. The first one (V1) relatively high dispersion defining a compositional shift from the re- outlines a transition of smectite toward chlorite via several inter- spective end-member mineral chemistry toward the composition of mediates consisted of two to three component mixed-layer clay mi- chlorite (Fig. 6a). Elemental projections in the PCA biplot reveal that nerals (C-S and I-S-C), while the second (V2) and third (V3) vectors variances between the projection groups is largely controlled by the describe a transformation of Fe-Mg dioctahedral-trioctahedral 1:1 clay

ﬂuctuations of MgO and FeO and SiO2,orAl2O3, and K2O. Indeed, the mineral (i.e. odinite) to chlorite. elemental correlation matrix (Table 4) unveils a clear negative relationship of ferromagnesian component of analyzed minerals with regard to their alumina and silica abundances. The same is valid for K which is also preferentially aligned with Al and Si. Within a ferromagnesian component, Fe and Mg are reasonably well correlated

Table 3 Representative EDS analyses (wt%) of clay particles from the Gamba formation of the South Gabon basin.

Sample Gamba 2 Gamba 2 Gamba 2 Gamba 2 Gamba 24 Gamba 2 Gamba 2 Gamba 2 Gamba 24 Gamba 16 Gamba 24 Gamba 16

Mineral I-C-S I-C-S Chl Chl Chl O-C O-C O-C Od Od Od O-C (?)

Group A A B B B CCCD D D DtoB

Anal. No. 22 23 8 11 11 18 20 25 3 6 7 12

SiO2 57.12 58.06 51.24 52.84 47.13 43.64 45.62 44.57 48.05 49.55 48.52 44.34

Al2O3 19.87 16.74 14.17 13.52 19.80 18.11 14.41 14.85 19.25 18.22 17.46 19.02 FeO 8.57 9.17 14.69 15.67 15.31 19.28 18.83 17.21 17.54 17.72 19.67 21.88 MgO 7.02 10.13 15.62 14.99 14.37 17.19 18.93 21.44 10.48 8.83 9.03 11.35 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

K2O 7.42 5.90 4.27 2.99 3.38 1.78 2.21 1.96 4.68 5.67 5.32 3.41 Fe# 0.55 0.45 0.48 0.51 0.52 0.53 0.50 0.45 0.63 0.67 0.69 0.66 Al/Si 0.35 0.29 0.28 0.26 0.42 0.41 0.32 0.33 0.40 0.37 0.36 0.43 Mg/Al 0.35 0.66 1.10 1.11 0.73 0.95 1.31 1.44 0.54 0.48 0.52 0.60 Total 100 100 100 100 100 100 100 100 100 100 100 100

Sample Gamba 28 Gamba 24 Gamba 16 Gamba 16 Gamba 16 Gamba 4 Gamba 4

Mineral Ill Ill Sme Sme Sme I-C-S (?) I-C-S (?)

Group E E FFFAtoBAtoB

Anal. No. 6 10 247421

SiO2 56.12 54.41 62.81 58.61 62.94 51.75 56.12

Al2O3 26.02 24.00 18.51 18.46 15.22 19.47 17.26 FeO 5.39 8.21 6.06 9.94 9.27 9.69 10.57 MgO 4.35 3.73 3.74 6.09 5.50 12.72 11.41 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00

K2O 8.12 9.65 8.88 6.90 7.06 6.37 4.65 Fe# 0.55 0.69 0.62 0.62 0.63 0.43 0.48 Al/Si 0.46 0.44 0.29 0.31 0.24 0.38 0.31 Mg/Al 0.17 0.16 0.20 0.33 0.36 0.65 0.66 Total 100 100 100 100 100 100 100

I-C-S – illite-chlorite-smectite; Ill – illite; Chl – chlorite; O-C – odinite-chlorite; Od – odinite; Sme – smectite; Fe# = FeO/(FeO + MgO); Al/Si= Al2O3/SiO2; Mg/Al]

MgO/Al2O3. Mineral abbreviations after Kretz (1983) and Whitney and Evans (2010).

7 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Fig. 4. Representative EDS spectra for each of the six clay mineral groups identiﬁed in core samples (refer to the text and Table 3 for more information).

5. Discussion on redox reactions, hydrolysis, and the dissolution, and may be accelerated by mechanical disintegration of rocks and consolidated sedi- 5.1. Bio-mediated eogenesis? ment (e.g. Needham et al., 2006; Crompton et al., 2015; Yamashita et al., 2019). In the last two decades, the importance of understanding The inorganic weathering and eogenetic processes inferred to allow the role of microorganisms has risen. Speciﬁcally, the variety of mi- clay mineral neoformation in sedimentary environments typically focus crobial eukaryotes, such as algae and cyanobacteria, interacting with

8 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Fig. 5. Backscattered electron (BSE) photomicrographs of selected areas of clay-rich samples of the Gamba core showing the morphologies of clay mineral groups identiﬁed in core samples. For details see the text and Table 3. clay minerals during the course of their speciﬁc metabolic functions unambiguously exclude a possible impact of microorganisms on commonly leads to enhanced weathering of phyllosilicates and crys- chlorite authigenesis in shallow sediments. Odinite, a precursor phase tallization of new minerals (Fiore et al., 2011; Cuadros, 2017; Li et al., of studied chlorite, with total iron content overwhelmingly Fe3+ 2019). Thus, the formation of Fe-rich smectite in an environment rich in (> 95%, Huggett, 2013), is likely to get reduced by microorganisms shells and faecal pellets near the sediment-water interface may be at- similarly to the most of other phyllosilicates (montmorillonite, non- tributed to higher concentrations of organic matter and related micro- tronite, vermiculite, chlorite; Dong et al., 2009) either as a result of the bial activity (Baldermann et al., 2013). According to Cuadros (2017), respiration process of sediment dwelling microbiota (Ottow and Von the interaction of microorganisms and phyllosilicates is essentially Klopotek, 1969; Dong et al., 2009) or due to the decay of organic matter twofold – with and without contact. The former creates the edges and (Luef et al., 2013). Although iron reduction has been studied mainly in pits that correspond to the shape of living creature(s) creating mineral smectite, it is safe to hypothesize that reduction of structural Fe in 7 Å instability, whilst the latter may result in the variety of morphologies clay minerals would at least destabilize those minerals because of the and patterns observed. Careful inspection of BSE imagery of analyzed change of electric charge. This may eventually make odinite more samples did not reveal any such artefacts. This however does not prone to convert to mixed-layer odinite-chlorite as advocated in this

9 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Fig. 6. The principal component biplot (PC 1 and PC 2) based on major element phase chemistry showing the areas corresponding to various clay mineral groups. Clay particles of distinct chemistry (i.e. groups A to F) and those of transitional compositions (groups A to B, C to B, and D to B) are displayed in the diagram using several distinguishable symbols (a); Projection patterns in the PCA biplot shown by three compositional vectors (b). For details see the text.

Table 4 minerals and corollary grain coatings in sandstone (e.g. Griffiths et al., EDS chemistry based correlation matrix of analyzed argillaceous minerals. 2018; Worden et al., 2018; Wooldridge et al., 2019), we reported no Marked correlation are significant at p < 0.0001. correlation between the clay mineralogy of analyzed rocks and their respective facies (Table 2). This is presumably due to the lack of major SiO2 Al2O3 FeO MgO K2O differences in deltaic facies, which allowed for an equal distribution of − − SiO2 1.000 0.110 0.826 0.650 0.605 amorphous Fe-rich substrate from proximal to more distal delta de- − − Al2O3 0.110 1.000 0.497 0.636 0.644 positional environments (e.g. Grigsby, 2001). Consequently, we report FeO −0.826 −0.497 1.000 0.597 −0.662 MgO −0.650 −0.634 0.597 1.000 −0.928 on the presence of odinite in all analyzed samples (Table 2). Odinite is

K2O 0.605 0.644 −0.662 −0.928 1.000 thought to form from an Fe-rich substrate within the few decimeters below the sediment–water interface (Hillier, 1994; Baker et al., 2000). Further transformations of ferric clay minerals are controlled by early contribution. Taking into account that the investigation on miner- burial diagenesis and not necessarily related to the sediment deposi- al–microbe interaction is still nascent and that the research on biolo- tional environment. gically mediated diagenesis of 7 Å phases is practically lacking, more research is clearly needed to unveil the role, if any, of microorganisms in eogenesis of Fe-rich clays. 5.3. Growth of diagenetic chlorite from 7 Å phase substrate Recent experimental studies have pointed out the effects of sediment ingestion by macrobiota (e.g. lugworms, Needham et al., 2004, Previous research on sedimentary chlorite suggested a range of clay Needham et al., 2006; Worden et al., 2006; bivalves, Engelhardt and minerals such as smectite, glauconite, odinite or kaolinite as plausible Brockamp, 1995; some fish, Anderson et al., 1958) leading to ac- diagenetic precursors (e.g. Hillier, 1994; Worden and Burley, 2003; celerated silicate dissolution and production of clay-rich assemblages Beaufort et al., 2015). The Mg-rich chlorite variety evolves normally consisting of kaolinite, illite, and possibly berthierine and chlorite. Such from saponite following the dissolution-crystallization mechanism for activity of higher organisms is normally recorded in the surface mineral origin (Chang et al., 1986; Humphreys et al., 1994; (crawling, resting, browsing) and/or internal lebensspuren (dwelling, Barrenechea et al., 2000). Conversely, the Fe-rich chlorite series forms feeding, escaping) of affected sediment (e.g. Lewis and McConchie, through the diagenetic replacement of Fe-rich 7 Å phases like odinite, 1994; de Muro et al., 2017). Except for the very top of analyzed core and to a lesser extent berthierine, by 14 Å chlorite. This process is in- (Fig. 2), no visible traces of bioturbation, bioerosion or caprolites have terpreted through the isochemical solid-state mineral transformation been reported, which minimizes a potential role of macroorganisms in (Ehrenberg, 1990; Hillier, 1994; Aagaard et al., 2000; Ryan and Hillier, eogenetic processes described in this study. 2002). Odinite is a di-trioctahedral ferric phase of broad chemistry, which is stable at very low diagenetic conditions, whilst berthierine 5.2. Facies control on burial eogenesis tends to have a more stochiometric composition (Odin, 1985; Odin et al., 1987; Bailey, 1988; Hornibrook and Longstaffe, 1996; Huggett Based on the sedimentology of the cored interval (c.f. Chapter 3.1.), et al., 2015). The formation of both minerals is attributed to the in- the analyzed Cretaceous sandstone from the South Gabon sub-basin is teraction of unstable, reactive and poorly crystalline terrigenous Fe-rich thought to represent a succession of delta progradation/aggradation alumosilicate substrate with reducing pore fluids (e.g. Velde, 1985; that defines a relatively shallow estuarine depositional environment Aplin, 1993; Aplin and Taylor, 2012). that was part of the Western African fluvial system. The fluvial system Our study has shown that odinite, defined by diffraction peaks at sourced the alkaline to calc-alkaline volcanic basement of the craton ∼3.55 and ∼7.06/7.15 Å (Fig. 3b,d), is characterized by variable delivering the poorly crystalline Fe-rich alumosilicate substrate abundances of Mg and Fe (up to 15–20%, Table 3), which resulted in (Thiéblemont et al., 2014; El Houicha et al., 2018). While sedimentary two odinite phases used to model a 25 °2θ triplet in XRD patterns of 7 Å facies differences usually play a major role in the formation of Fe-clay clay minerals (Table 1; Fig. 3). Such compositional variations are

10 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Fig. 7. Schematic overview showing the formation of diagenetic chlorite in four steps according to the model proposed herein. (a) Accumulation of aluminosilicate and Fe-oxyhydroxide terrigenous debris in shallow marine environment and syn-sedimentary formation of odinite; (b) reduction of FeIII most likely as a result of microbial activity or organic matter decay leading to structural instability of odinite; (c) partial chloritization of odinite through in situ SST chloritization (modiﬁed after Beaufort et al., 2015); (d) Recrystallization of both odinite and O-C to form particulate chlorite mineralization following a dissolution-crystallization mechanism at peak diagenetic conditions.

11 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064 related to the early diagenetic origin of odinite that is dependent on the transition by proposing a syn-sedimentary formation of odinite in a terrestrially derived aluminosilicate and Fe-oxyhydroxide debris irre- warm shallow sea (Fig. 7a). Being of broad chemical composition odi- gularly supplied to the sediment close to shorelines (Bailey, 1988; Aplin nite is readily destabilized most likely as a result of the reduction of Fe and Taylor, 2012; Velde, 2013). The feedstock for the formation of 7 Å by microbial activity or decaying organic matter, which rendered the clays was likely related to the Proterozoic fold and thrust Ogooué mineral structure unstable and facilitated chlorite interlayering orogenic belt of the eastern Gabon highlands, which stands for the (Fig. 7b). The latter took place by the in situ SST process, which gave closest large mass of crystalline rocks, drained by the westward-going rise to the chlorite-rich mixed-layer odinite-chlorite in the immediate African rivers such as Ogooué (Moussavou and Edou-Minko, 2006; subsurface (Table 1; Figs. 3 and 7c). Our model further assumes the Seranne et al., 2008; Thiéblemont et al., 2014). The compositional crystallization of the monocomponent eogenetic chlorite through the heterogeneity of odinite can also be related to chlorite interlayering, dissolution-crystallization process (Fig. 7d) at burial temperatures that which controls ferromagnesian content in the newly formed odinite- did not exceed ∼50 °C, given an average geothermal gradient of 35 °C/ chlorite intermediates (Hillier, 1994; Aagaard et al., 2000; Ma et al., km taken for the passive continental margins of West Africa (Addax 2017). Modelling of XRD spectra is also consistent with the existence of petroleum, pers. com.). We hypothesize that this final step of chlorite a mixed-layer odinite-chlorite with only 22% of the chlorite component eogenesis might have also involved the nucleation of the “transitional”, (Table 1; Fig. 3). Flakes of odinite fill the porous space defining a thermodynamically unfavored phases or compositions (Morse and characteristic honeycomb or boxwork arrangement with micro- Casey, 1988) as suggested by several EDS analyses (e.g. anal. Gamba scopically particulated mineralization of chlorite at edge-to-edge con- 24_12, Table 3). More work is required to fully address this question, tacts (Fig. 5a and b). An analogue microtexture is documented in including microprobe and transmission-electron microscopy investiga- samples where in lieu of odinite the honeycomb mineral arrangement is tion of the newly formed chlorite mineralization. consisted of odinite-chlorite (Fig. 5c and d) with 90% of chlorite layers (Table 1; Fig. 3). Discrete chlorite mineralization developed at odinite 5.4. Growth of diagenetic chlorite from smectite and O-C flake contacts permits the following inferences: (i) the chloritization of odinite was not complete and is related to the particular set The less significant precursor for diagenetic chlorite in analyzed of geochemical conditions (availability of Fe and Mg, Si deficiency, sediment is smectite. Densely packed mineralization reported by the microbial activity; e.g. Beaufort et al., 2015) that favored a 7 Å to 14 Å SEM-EDS investigation (Fig. 5f: group F) is characterized by the low Al/ layer conversion in sediments that were buried just beneath the sedi- Si ratio (∼0.25) and an elevated ferromagnesian content (Table 3), ment-water interface; and (ii) the progressive chloritization by solid- which best corresponds to the trioctahedral smectite of saponite com- state transformation (SST; e.g. Xu and Veblen, 1996; Ryan and Hillier, position (Treiman et al., 2014). The tricomponent mixed-layer clay 2002) cannot be considered as a sole mechanism to govern the for- phase – R1 I-C-S – with about 45% of chlorite layers (Table 1) was also mation of eogenetic (i.e. monocomponent) chlorite. As discussed ear- documented and represents a transition toward a chlorite composition. lier, the concentrated organic matter or microbial activity in the sea Multicomponent clay interstratifications are intrinsically unstable with bottom sediment could have generated a reducing environment that sediment burial and tend to converge toward simpler compositions would lead to the reduction of ferric Fe in odinite, thus enhancing the without a significant change in particle morphologies (e.g. Inoue and odinite compositional instability. The geochemical and microtextural Utada, 1991; Cuadros, 2010); hence, chlorite is also expected to have data are also illustrative in this regard; namely, the compositional followed this path. Gradual conversion from smectite to smectite- vectors V2 and V3 (Fig. 6b), which adumbrate the transition from chlorite, and then eventually to diagenetic chlorite, is clearly portrayed odinite-chlorite and odinite to chlorite, are defined by a decrease in Al/ by the net loss of silica and an increase in Fe and Mg (Fig. 6; Table 3)as Si ratio and a net loss of Mg and Fe (Table 3; V2: odinite-chlorite to the chloritization proceeds. We, however, do not have firm evidence chlorite transition) and a Si-Mg increase coupled with an Al-Fe decrease that supports the origin of the monocomponent 14 Å phase from R1 I-C- (Table 3; V3: odinite to chlorite transition). After chlorite became a S because the latter contained barely half of the chlorite layers dominant component in O-C, the further growth of discrete chlorite (Table 1). Having reached the composition of R1 I-C-S, the chloritiza- must have included a dissolution of precursor phases and precipitation tion of smectite apparently ceased. of chlorite in an open, or at least partially open diagenetic system. The The newly formed diagenetic chlorite demonstrated in this study process was presumably boosted by a high gradient zone (fluid-flow (Fig. 5a–d) is thus related to the 7 Å precursor. This conforms well with event) in which sediment attained burial depths of ∼1.5 km (Fig. 2). the literature data, which advocates the conversion of odinite to Hence, formed crystallites of eogenetic chlorite are larger in size in chlorite at lower temperatures when compared to the conversion from comparison to the flakes of odinite and odinite-chlorite that comprise saponite to chlorite. The latter process is principally temperature driven the mineral framework of the pore space (Fig. 5a–d). The mechanism of and is thought to take place at the eo-to mesogenesis transition from 40 Ostwald ripening might have therefore played an active role in the to 120 °C (Worden and Morad, 2003; Aagaard et al., 2000; Beaufort coarsening of chlorite crystallites (Jahren, 1991; Haile et al., 2015). et al., 2015), while burial temperatures in analyzed sediment did not The herein proposed diagenetic scenario on the origin of shallow exceed ∼50 °C. Moreover, the trioctahedral smectites like saponite do eogenetic chlorite from verdine facies (< 1500 m; Fig. 2) contrasts with not have an optimal Al/Si ratio needed to attract interlayer cations the model used to explain the formation of diagenetic chlorite at depths during diagenesis, rather chemical changes in their octahedral sheet are greater than 2000 m from the same type of sedimentary environment. sluggish as a result of divalent cations occupancy, which adds to their In the deeper burial model, the Ibb layers of 7 Å phase are gradually thermal stability during sediment burial (e.g. Eberl et al., 1978; Ferrage converted to Iaa layers of 14 Å phase by SST until the single component et al., 2011). chlorite composition is achieved (Ehrenberg, 1990; Hillier, 1994; Smectite in clastic sediment is usually related to the alteration of a Aagaard et al., 2000; Ryan and Hillier, 2002). This transition is con- volcaniclastic component but saponite occurrences are also known from sidered isochemical and, thus, a polymorphic reaction (e.g. Jiang et al., various evaporite facies and their origin is often associated to eva- 1992; Xu and Veblen, 1996). There are, however, disagreements that poration controlled Mg-rich brines that invade the sediment at near- such a structural model bears on thermodynamic instability of chlorite surface conditions (Hover et al., 1999; Ryan and Hillier, 2002). The characterized by minor 7 Å interlayering because no electrostatic dif- upper portion of the Gamba Formation, containing the I-S-C samples ference occurs between the two types of chlorite octahedral layers when (Gamba 2 and 4; Fig. 2), is separated from the massive Ezanga eva- it theoretically should (i.e. the t-o-t octahedral layer and brucite layer; porites by a thin-layer of Vembo Shale (Teisserenc and Villemin, 1989; e.g. Banfield and Bailey, 1996; Beaufort et al., 2015). Our model ac- Esestime et al., 2018). The latter is not laterally continuous, enabling commodates this criticism as well as the non-isochemical phase direct contact with evaporite deposits and sediments of the Gamba

12 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Formation (Esestime et al., 2018). The share of smectite and mixed- margeo.2004.06.007. layer smectite-chlorite largely diminishes farther from the contact with Aplin, A.C., 1993. The composition of authigenic clay minerals in recent sediments: links to the supply of unstable reactants. In: In: Manning, D.A.C., Hall, P.L., Hughes, C.R. the Ezanga evaporites and is replaced with odinite and odinite-chlorite (Eds.), Geochemistry of Clay-Pore Fluid Interactions: London, vol. 4. Mineralogical (Table 2), suggesting a supporting role of evaporite processes in the Society Series, pp. 81–106. formation of diagenetic smectite-chlorite. Aplin, A.C., Taylor, K.G., 2012. Mineralogy of marine sediment systems: a geochemical framework. In: Oberti, R., Vaughan, D.J., Wogelius, R.A. (Eds.), Environmental Mineralogy II: London. Mineralogical Society of Great Britain and Ireland, pp. 6. Conclusions 123–175. https://doi.org/10.1180/EMU-notes.13.4. Bahlis, A.B., De Ros, L.F., 2013. Origin and impact of authigenic chlorite in the Upper (i) Thermally immature Cretaceous sediment studied in this con- Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil. Pet. Geosci. 19 (2), 185–199. https://doi.org/10.1144/petgeo2011-007. tribution has been recovered from relatively shallow depths of the Summary of recommendations of AIPEA nomenclature committee on clay minerals. South Gabon sub-basin in western Africa. Bailey, S.W. (Ed.), Am. Mineral. 65, 1–7. (ii) Analyzed material is found to be rich in green clays (odinite, Bailey, S.W., 1988. Odinite, a new dioctahedral-trioctahedral Fe3+-rich 1:1 clay mineral. Clay Miner. 23 (3), 237–247. https://doi.org/10.1180/claymin.1988.023.3.01. odinite-chlorite, chlorite, illite-chlorite-smectite) as well as smec- Baker, J.C., Havord, P.J., Martin, K.R., Ghori, K.A.R., 2000. Diagenesis and petrophysics tite and illite-smectite. of the early permian moogooloo sandstone, Southern Carnarvon basin. Western – (iii) Odinite, O-C, I-C-S and I-S occur as grain coatings and pore-filling Australia: AAPG Bull. 84 (2), 250 265. https://doi.org/10.1306/C9EBCDBF-1735- 11D7-8645000102C1865D. phases with a characteristic honeycomb texture, while embryonic Baldermann, A., Warr, L.N., Grathoff, G.H., Dietzel, M., 2013. The rate and mechanism of chlorite emerges at the contact of odinite and O-C flakes as a deep-sea glauconite formation at the Ivory Coast-Ghana Marginal Ridge. Clay Clay particulate authigenic mineralization. Miner. 61 (3), 258–276. https://doi.org/10.1346/CCMN.2013.0610307. Banfield, J.F., Bailey, S.W., 1996. Formation of regularly interstratified serpentine- (iv) Odinite is suggested to have formed from aluminosilicate and Fe- chlorite minerals by tetrahedral inversion in long-period serpentine polytypes. Am. oxyhydroxide debris at sediment-water interface; a seasonality in Mineral. 81 (1–2), 79–91. https://doi.org/10.2138/am-1996-1-211. Fe supply may explain its broad phase chemistry. Barrenechea, J.F., Rodas, M., Frey, M., Alonso-Azcárate, J., Mas, J.R., 2000. Chlorite, corrensite, and chlorite-mica in Late Jurassic fluvio-lacustrine sediments of the (v) Odinite is readily destabilized presumably as a result of the re- Cameros basin of Northeastern Spain. Clay Clay Miner. 48 (2), 256–265. https://doi. duction of octahedral Fe and the subsequent change of electric org/10.1346/CCMN.2000.0480212. charge due to the microbial activity or decaying organic matter Beaufort, D., Rigault, C., Billon, S., Billault, V., Inoue, A., Inoue, S., Patrier, P., 2015. that consumes O more quickly than it can diffuse from the water- Chlorite and chloritization processes through mixed-layer mineral series in low- temperature geological systems – a review. Clay Miner. 50 (4), 497–523. https://doi. sediment interface. org/10.1180/claymin.2015.050.4.06. (vi) Once destabilized, odinite will undergo a two-step diagenetic Beier, Th., Mommsen, H., 1994. A method for classifying multidimensional data with evolution to yield particulate chlorite mineralization; first step respect to uncertainties of measurement and its application to archaeometry. Naturwissenschaften 81, 546–548. https://doi.org/10.1007/BF01140001. includes partial chloritization of odinite through in situ solid-state Biernacka, J., 2014. Pore-lining sudoite in rotliegend sandstones from the eastern part of transformation of 7 Å to 14 Å layers, while at peak diagenetic the Southern permian basin. Clay Miner. 49 (5), 635–655. https://doi.org/10.1180/ conditions (50 °C, ∼1.5 km depth), the remaining odinite and O-C claymin.2014.049.5.02. Billault, V., Beaufort, D., Baronnet, A., Lacharpagne, J.-C., 2003. A nanopetrographic and both recrystallize to form diagenetic chlorite following a dissolu- textural study of grain-coating chlorites in sandstone reservoirs. Clay Miner. 38 (3), tion-crystallization mechanism. 315–328. https://doi.org/10.1180/0009855033830098. (vii) The chlorite formation model proposed in this study sheds light on Bradley, W.F., 1953. Analysis of mixed-layer clay mineral structures. Anal. Chem. 25 (5), 727–730. https://doi.org/10.1021/ac60077a012. the origin of sedimentary chlorite in its juvenile stage; deeper in Brigatti, M.F., Poppi, L., 1984. Crystal chemistry of corrensite: a review. Clay Clay Miner. sedimentary basins such formed chlorite mineralization may lead 32 (5), 391–399. to anomalously high porosity of clastic reservoir rocks. Brindley, G.W., 1951. X-ray Identification and Crystal Structures of Clay Minerals. pp. 345 London. Brownfield, M.E., Charpentier, R.R., 2006. Geology and Total Petroleum Systems of the Acknowledgments West-Central Coastal Province (7203). U.S. Dept. of the Interior, U.S. Geological Survey, West Africa: Reston, Va. Financial support for this research was provided by Texas Tech Burgess, P.M., Hovius, N., 1998. Rates of delta progradation during highstands: consequences for timing of deposition in deep-marine systems. J. Geol. Soc. 155 (2), University and Addax Petroleum Corp. (Geneva office) (168/141715) 217–222. https://doi.org/10.1144/gsjgs.155.2.0217. which is gratefully acknowledged. We extend our appreciation to the Burri, P., 2014. Hydraulic fracturing - postscriptum. A geologist's attempt to summarize – company for giving us the access to their core material and to some of what we know and where we go. Bulletin fuer Angewandte Geologie 19 (2), 143 150. https://doi.org/10.5169/seals-583934. the unpublished research on the Gabon area, as well as for granting us a Cao, Z., Liu, G., Meng, W., Wang, P., Yang, C., 2018. Origin of different chlorite occur- permit to publish this study. Javier Cuadros provided a helpful dis- rences and their effects on tight clastic reservoir porosity. J. Pet. Sci. Eng. 160, cussion on the microorganisms-clay minerals interaction. A great ap- 384–392. https://doi.org/10.1016/j.petrol.2017.10.080. ć Chang, H.K., Mackenzie, F.T., Schoonmaker, J., 1986. Comparisons between the diag- preciation is further addressed to Dustin E. Sweet and Aleksandar Risti enesis of dioctahedral and trioctahedral smectite, Brazilian offshore basins. Clay Clay for English improvements. We also wish to acknowledge the support of Miner. 34 (4), 407–423. https://doi.org/10.1346/CCMN.1986.0340408. NSF grant MRI 04–511 to College of Arts & Sciences Microscopy, TTU, Chen, B.-Y., Li, R.-X., Liang, J.-W., Huang, K.-Z., 2014. Authigenic chlorite and its im- paction on reservoir physical properties: an example from the Yanchang formation of for enabling the access to SEM-EDS equipment. Gratitude is extended to the Southwest Ordos Basin. Bull. Miner. Petrol. Geochem. 33 (3), 389–394. https:// Bo Zhao for providing the excellent microbeam conditions. Finally, the doi.org/10.3969/j.issn.1007-2802.2014.03.014. constructive review by Richard H. Worden, as well as the editorial Christidis, G.E., Eberl, D.D., 2003. Determination of layer charge characteristics of fi smectites. Clay Clay Miner. 51 (6), 644–655. https://doi.org/10.1346/CCMN.2003. handling and suggestions by Ihsan Al-Aasm, contributed signi cantly to 0510607. the manuscript quality. Crompton, J.W., Flowers, G.E., Kirste, D., Hagedorn, B., Sharp, M.J., 2015. Clay mineral precipitation and low silica in glacier meltwaters explored through reaction-path – References modelling. J. Glaciol. 61 (230), 1061 1078. https://doi.org/10.3189/ 2015JoG15J051. Cuadros, J., 2010. Crystal-chemistry of mixed-layer clays. In: In: Fiore, S., Cuadros, J., Aagaard, P., Jahren, J.S., Harstad, A.O., Nilsen, O., Ramm, M., 2000. Formation of grain- Huertas, F.J. (Eds.), Interstratified Clay Minerals: Origin, Characterization and coating chlorite in sandstones. La. Synth. vs. Nat. Occur.: Clay Miner. 35 (1), Geochemical Significance: Bari, AIPEA, Educational Seried, vol. 1. pp. 11–33. 261–269. Cuadros, J., 2017. Clay minerals interaction with microorganisms: a review. Clay Miner. Aitchison, J., 1982. The statistical analysis of compositional data. J. R. Stat. 611 Soc. B 52 (2), 235–261. https://doi.org/10.1180/claymin.2017.052.2.05. 44, 139–177. de Muro, S., Pusceddu, N., Buosi, C., Ibba, A., 2017. Morphodynamics of a Mediterranean Anderson, A.E., Jonas, E.C., Odum, H.T., 1958. Alteration of clay minerals by digestive microtidal wave-dominated beach: forms, processes and insights for coastal man- processes of marine organisms. Science 127 (3291), 190–191. https://doi.org/10. agement. J. Maps 13 (2), 26–36. https://doi.org/10.1080/17445647.2016.1250681. 1126/science.127.3291.190. Dong, H., Jaisi, D.P., Kim, J., Zhang, G., 2009. Microbe-clay mineral interactions. Am. Anka, Z., Séranne, M., 2004. Reconnaissance study of the ancient Zaire (Congo) deep-sea Mineral. 94 (11–12), 1505–1519. https://doi.org/10.2138/am.2009.3246. fan. (ZaiAngo Project). Mar. Geol. 209 (1–4), 223–244. https://doi.org/10.1016/j. Drits, V.A., Sakharov, B.A., 1976. X-ray Structural Analysis of Mixed-Layer Minerals.

13 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Russian Federation, Nauka, Moscow, pp. 256. Luef, B., Fakra, S.C., Csencsits, R., Wrighton, K.C., Williams, K.H., Wilkins, M.J., Dupré, S., Cloetingh, S., Bertotti, G., 2011. Structure of the Gabon Margin from integrated Downing, K.H., Long, P.E., Comolli, L.R., Banfield, J.F., 2013. Iron-reducing bacteria seismic reflection and gravity data. Tectonophysics 506 (1–4), 31–45. https://doi. accumulate ferric oxyhydroxide nanoparticle aggregates that may support planktonic org/10.1016/j.tecto.2011.04.009. growth. ISME J. 7 (2), 338–350. https://doi.org/10.1038/ismej.2012.103. Eberl, D.D., Whitney, G., Khoury, H., 1978. Hydrothermal reactivity of smectite. Am. Ma, P.J., Lin, C.Y., Zhang, S.Q., Dong, C.M., Xu, Y.F., 2017. Formation of chlorite rims Mineral. 63, 401–409. and the impact of pore-lining chlorite on reservoir quality: a case study from Ehrenberg, S.N., 1990. Relationship between diagenesis and reservoir quality in sand- Shiqianfeng sandstones in upper Permian of Dongpu Depression, Bohai Bay Basin, stones of the Garn formation, Haltenbanken, mid-Norwegian continental shelf(1). eastern China. Aust. J. Earth Sci. 64 (6), 825–839. https://doi.org/10.1080/ AAPG Bull. 74 (10), 1538–1558. 08120099.2017.1351394. El Houicha, M., Pereira, M.F., Jouhari, A., Gama, C., Ennih, N., Fekkak, A., Ezzouhairi, H., Meunier, A., El Albani, A., 2007. The glauconite-Fe-illite-Fe-smectite problem: a critical El Attari, A., Silva, J.B., 2018. Recycling of the Proterozoic crystalline basement in review. Terra. Nova 19 (2), 95–104. https://doi.org/10.1111/j.1365-3121.2006. the Coastal Block (Moroccan Meseta): new insights for understanding the geody- 00719.x. namic evolution of the northern peri-Gondwanan realm. Precambrian Res. 306, Moore, D.M., Reynolds, R.C., 1997. X-Ray Diffraction and the Identification and Analysis 129–154. https://doi.org/10.1016/j.precamres.2017.12.039. of Clay Minerals. Oxford University Press, pp. 400. Engelhardt, H.-J., Brockamp, O., 1995. Biodegradation of clay minerals: laboratory ex- Morad, S., Al-Ramadan, K., Ketzer, J.M., De, R., 2010. The impact of diagenesis on the periments and results from Wadden Sea tidal flat sediments. Sedimentology 42 (6), heterogeneity of sandstone reservoirs: a review of the role of depositional fades and 947–955. https://doi.org/10.1111/j.1365-3091.1995.tb00419.x. sequence stratigraphy. AAPG Bull. 94 (8), 1267–1309. https://doi.org/10.1306/ Esestime, P., Eastwell, D., Rodriguez, K., Hodgson, N., 2018. Shallow water 3D in South 04211009178. Gabon: a new generation of hydrocarbon prospects. First Break 36 (9), 59–64. Morse, J.W., Casey, W.H., 1988. Ostwald processes and mineral paragenesis in sediments. Ferrage, E., Lanson, B., Sakharov, B.A., Geoffroy, N., Jacquot, E., Drits, V.A., 2007. Am. J. Sci. 288 (6), 537–560. https://doi.org/10.2475/ajs.288.6.537. Investigation of dioctahedral smectite hydration properties by modeling of X-ray Moussavou, M., Edou-Minko, A., 2006. Contribution à l’histoire thermo-tectonique diffraction profiles: influence of layer charge and charge location. Am. Mineral. 92 précambrienne du complexe annulaire de N’goutou par la géochimie et la (10), 1731–1743. https://doi.org/10.2138/am.2007.2273. géochronologie U/Pb sur minéraux accessoires (Bassin Francevillien d'Okondja, Ferrage, E., Vidal, O., Mosser-Ruck, R., Cathelineau, M., Cuadros, J., 2011. A re- Gabon). Afr. Geosci. Rev. 13, 53–61. investigation of smectite illitization in experimental hydrothermal conditions: results Nadeau, P.H., Bain, D.C., 1986. Composition of some smectites and diagenetic illitic clays from X-ray diffraction and transmission electron microscopy. Am. Mineral. 96 (1), and implications for their origin. Clay Clay Miner. 34 (4), 455–464. https://doi.org/ 207–223. https://doi.org/10.2138/am.2011.3587. 10.1346/CCMN.1986.0340412. Fiore, S., Dumontet, S., Huertas, F.J., Pasquale, V., 2011. Bacteria-induced crystallization Needham, S.J., Worden, R.H., McIlroy, D., 2004. Animal-sediment interactions: the effect of kaolinite. Appl. Clay Sci. 53 (4), 566–571. https://doi.org/10.1016/j.clay.2011. of ingestion and excretion by worms on mineralogy. Biogeosciences 1 (2), 113–121. 05.005. https://doi.org/10.5194/bg-1-113-2004. Griffiths, J., Worden, R.H., Wooldridge, L.J., Utley, J.E.P., Duller, R.A., 2018. Detrital clay Needham, S.J., Worden, R.H., Cuadros, J., 2006. Sediment ingestion by worms and the coats, clay minerals, and pyrite: a modern shallow-core analogue for ancient and production of bio-clays: a study of macrobiologically enhanced weathering and early deeply buried estuarine sandstones. J. Sediment. Res. 88 (10), 1205–1237. https:// diagenetic processes. Sedimentology 53 (3), 567–579. https://doi.org/10.1111/j. doi.org/10.2110/jsr.2018.56. 1365-3091.2006.00781.x. Grigsby, J.D., 2001. Origin and growth mechanism of authigenic chlorite in sandstones of Odin, G.S., 1985. La verdine, faciès granulaire vert, marin et côtier, distinct de la glau- the lower Vicksburg formation, South Texas. J. Sediment. Res. 71 (1), 27–36. conie: distribution actuelle et composition. C. R. Acad. Sci. 301, 105–108. Haile, B.G., Hellevang, H., Aagaard, P., Jahren, J., 2015. Experimental nucleation and Odin, G.S., 1990. Clay mineral formation at the continent-ocean boundary; the verdine growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces. facies. Clay Miner. 25 (4), 477–483. Mar. Pet. Geol. 68, 664–674. https://doi.org/10.1016/j.marpetgeo.2015.02.006. Odin, G.S., Sen Gupta, B.K., 1988. Chapter B6 geological significance of the verdine fa- Harding, S.C., Nash, B.P., Petersen, E.U., Ekdale, A.A., Bradbury, C.D., Dyar, M.D., 2014. cies. Dev. Sedimentol. 45 (C), 205–219. https://doi.org/10.1016/S0070-4571(08) Mineralogy and geochemistry of the main glauconite bed in the Middle Eocene of 70064-5. Texas: paleoenvironmental implications for the verdine facies. PLoS One 9 (2), Odin, G.S., Debenay, J.-P., Froget, C., Rigolot, P., 1987. Le faciès verdine: néoformation e87656. https://doi.org/10.1371/journal.pone.0087656. d’une phyllite en milieu subrécifal. Geodynamique 2 (1), 27–41. Heine, C., Zoethout, J., Müller, R.D., 2013. Kinematics of the South Atlantic rift. Solid Ottow, J.C.G., Von Klopotek, A., 1969. Enzymatic reduction of iron oxide by fungi. Appl. Earth 4 (2), 215–253. https://doi.org/10.5194/se-4-215-2013. Microbiol. 18 (1), 41–43. Hillier, S., 1994. Pore-lining chlorites in siliciclastic reservoir sandstones; electron mi- Porębski, S.J., Steel, R.J., 2003. Shelf-margin deltas: their stratigraphic significance and croprobe, SEM and XRD data, and implications for their origin. Clay Miner. 29 (4), relation to deepwater sands. Earth Sci. Rev. 62 (3), 283–326. https://doi.org/10. 665–679. 1016/S0012-8252(02)00161-7. Hornibrook, E.R.C., Longstaffe, F.J., 1996. Berthierine from the lower Cretaceous Porrenga, D.H., 1967. Glauconite and chamosite as depth indicators in the marine en- Clearwater formation, Alberta, Canada. Clay Clay Miner. 44 (1), 1–21. vironment. Mar. Geol. 5 (5), 495–501. https://doi.org/10.1016/0025-3227(67) Hover, V.C., Walter, L.M., Peacor, D.R., Martini, A.M., 1999. Mg-smectite authigenesis in 90056-4. a marine evaporative environment. Salina Ometepec, Baja California: Clay Clay Purnachandra Rao, V., Lamboy, M., Dupeuble, P.A., 1993. Verdine and other associated Miner. 47 (3), 252–268. https://doi.org/10.1346/CCMN.1999.0470302. authigenic (glaucony, phosphate) facies from the surficial sediments of the south- Huggett, J.M., 2013. Minerals: Glauconites and green Clays☆. In: Reference Module in western continental margin of India. Mar. Geol. 111 (1), 133–158. https://doi.org/ Earth Systems and Environmental Sciences. Elsevier. https://doi.org/10.1016/B978- 10.1016/0025-3227(93)90193-Y. 0-12-409548-9.02893-1. Purnachandra Rao, V., Thamban, M., Lamboy, M., 1995. Verdine and glaucony facies Huggett, J.M., Burley, S.D., Longstaffe, F.J., Saha, S., Oates, M.J., 2015. The nature and from surficial sediments of the eastern continental margin of India. Mar. Geol. 127 origin of authigenic chlorite and related xements in Oligo-Miocene reservoir sand- (1–4), 105–113. https://doi.org/10.1016/0025-3227(95)00056-5. stones, Tapti gas fields, Surat depression, offshore Western India. J. Pet. Geol. 38 (4), Reynolds, R.C., Di Stefano, M.P., Lahann, R.W., 1992. Randomly interstratified serpen- 383–409. https://doi.org/10.1111/jpg.12618. tine/chlorite; its detection and quantification by powder X-ray diffraction methods. Humphreys, B., Kemp, S.J., Lott, G.K., Bermanto, D. A. Dharmayanti, Samsori, I., 1994. Clay Clay Miner. 40 (3), 262–267. Origin of grain-coating chlorite by smectite transformation: an example from Ryan, P.C., Hillier, S., 2002. Berthierine/chamosite, corrensite, and discrete chlorite from Miocene sandstones, North Sumatra back-arc basin, Indonesia. Clay Miner. 29 (4), evolved verdine and evaporite-associated facies in the Jurassic Sundance Formation. 681–692. https://doi.org/10.1180/claymin.1994.029.4.21. Wyoming: Am. Mineral. 87 (11–12), 1607–1615. Inoue, A., Utada, M., 1991. Smectite-to-chlorite transformation in thermally metamor- Ryan, P.C., Reynolds Jr., R.C., 1996. The origin and diagenesis of grain-coating serpen- phosed volcanoclastic rocks in the Kamikita area, northern Honshu, Japan. Am. tine-chlorite in Tuscaloosa Formation sandstone. U.S. Gulf Coast: Am. Mineral. 81 Mineral. 76, 628–640. (1–2), 213–225. Jahren, J.S., 1991. Evidence of Ostwald ripening related recrystallization of diagenetic Saïag, J., Brigaud, B., Portier, É., Desaubliaux, G., Bucherie, A., Miska, S., Pagel, M., 2016. chlorites from reservoir rocks offshore Norway. Clay Miner. 26 (2), 169–178. https:// Sedimentological control on the diagenesis and reservoir quality of tidal sandstones doi.org/10.1180/claymin.1991.026.2.02. of the upper Cape Hay formation (permian, Bonaparte basin, Australia). Mar. Pet. Jeans, C.V., Wray, D.S., Merriman, R.J., Fisher, M.J., 2000. Volcanogenic clays in Jurassic Geol. 77, 597–624. https://doi.org/10.1016/j.marpetgeo.2016.07.002. and Cretaceous strata of England and the North sea basin. Clay Miner. 35 (1), 25–55. Šegvić, B., Benvenuti, A., Moscariello, A., 2016. Illite-smectite rich clay parageneses from Jiang, W.-T., Peacor, D.R., Slack, J.F., 1992. Microstructures, mixed layering, and poly- Quaternary tunnel valley sediments of the Dutch Southern North Sea - mineral origin morphism of chlorite and retrograde berthierine in the kidd creek massive sulfide and paleoenvironment implications. Clay Clay Miner. https://doi.org/10.1346/ deposit, Ontario. Clay Clay Miner. 40 (5), 501–514. https://doi.org/10.1346/CCMN. CCMN.2016.064026. 1992.0400503. Sengupta, A., Kadam, R.M., Rajeswari, B., Dhobale, A.R., Babu, Y., Godbole, S.V., 2010. Kretz, R., 1983. Symbols for rock-forming minerals. Am. Mineral. 68, 277–279. Characterization of Indian serpentine by X-ray diffraction, photoacoustic spectro- Lagaly, G., Barrer, R.M., Goulding, K., 1984. Clay-organic interactions [and discussion]. scopy and electron paramagnetic resonance spectroscopy. Appl. Clay Sci. 50 (3), Phil. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 311 (1517), 315–332. https://doi. 305–310. https://doi.org/10.1016/j.clay.2010.08.013. org/10.1098/rsta.1984.0031. Seranne, M., Bruguier, O., Moussavou, M., 2008. U-Pb single zircon grain dating of Lewis, D.W., McConchie, D., 1994. Sedimentary structures. In: Lewis, D.W., McConchie, present fluvial and cenozoic aeolian sediments from Gabon: consequences on sedi- D. (Eds.), Practical Sedimentology. Springer US, Boston, MA, pp. 89–113. https://doi. ment provenance, reworking, and erosion processes on the equatorial West African org/10.1007/978-1-4615-2634-6_4. margin. Bull. Soc. Geol. Fr. 179 (1), 29–40. https://doi.org/10.2113/gssgfbull.179. Li, G.L., Zhou, C.H., Fiore, S., Yu, W.H., 2019. Interactions between microorganisms and 1.29. clay minerals: new insights and broader applications. Appl. Clay Sci. 177, 91–113. Solotchina, E.P., Kameneva, M. Yu., Vasilevsky, A.N., Solotchin, P.A., 2000. X-ray iden- https://doi.org/10.1016/j.clay.2019.04.025. tification of mixed-layer illite/smectites from terrigenous deposits of the West

14 Š ć B. egvi , et al. Marine and Petroleum Geology 112 (2020) 104064

Siberian Plate: surface Investigation X-Ray. Synchrotron Neutron Tech. 15 (11), Wiewióra, A., Giresse, P., Jaunet, A.M., Wilamowski, A., Elsass, F., 1999. Crystal chem- 1621–1628. istry of layer silicates of the Miocene green grain (Congo Basin) from transmission Środoń, J., 2006. Identification and quantitative analysis of clay minerals. In: Handbook electron microscopy (TEM) and analytical electron microscopy (AEM) observations. of Clay Science. Elsevier Ltd., Amsterdam, pp. 765–787. Clay Clay Miner. 47 (5), 582–590. Środoń, J., Morgan, D.J., Eslinger, E.V., Eberl, D.D., Karlinger, M.R., 1986. Chemistry of Wilson, M., 1992. Magmatism and continental rifting during the opening of the South illite/smectite and end-member illite. Clay Clay Miner. 34 (4), 368–378. Atlantic Ocean: a consequence of Lower Cretaceous super-plume activity? Geol. Soc. Teisserenc, P., Villemin, J., 1989. sedimentary basin of Gabon–Geology and oil systems. Lond. Special Publ. 68 (1), 241–255. https://doi.org/10.1144/GSL.SP.1992.068. AAPG Special Vol. 132, 117–199. 01.15. Thiéblemont, D., Bouton, P., Préat, A., Goujou, J.-C., Tegyey, M., Weber, F., Ebang Wooldridge, L.J., Worden, R.H., Griffiths, J., Utley, J.E.P., 2019. How To Quantify Clay- Obiang, M., Joron, J.L., Treuil, M., 2014. Transition from alkaline to calc-alkaline Coat Grain Coverage in Modern and Ancient Sediments. J. Sediment. Res. 89 (2), volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern 135–146. https://doi.org/10.2110/jsr.2019.6. Gabon (Western Central Africa). J. Afr. Earth Sci. 99 (Part 2), 215–227. https://doi. Worden, R.H., Burley, S.D., 2003. Sandstone Diagenesis: The Evolution of Sand to Stone. org/10.1016/j.jafrearsci.2013.12.007. In: Burley, Stuart D., Worden, Richard H. (Eds.), Sandstone Diagenesis. Blackwell Treiman, A.H., et al., 2014. Ferrian saponite from the Santa Monica Mountains Publishing Ltd., pp. 1–44. (California, U.S.A., Earth): Characterization as an analog for clay minerals on Mars Worden, R.H., Morad, S., 2003. Clay minerals in sandstones: controls on formation, dis- with application to Yellowknife Bay in Gale Crater. Am. Mineral. 99 (11–12), tribution and evolution. In: Worden, R.H., Morad, S. (Eds.), Clay Mineral Cements in 2234–2250. https://doi.org/10.2138/am-2014-4763. Sandstones. Special Publication of the International Association of Sedimentologists, Uzarowicz, Ł., Šegvić, B., Michalik, M., Bylina, P., 2012. The effect of hydrochemical Blackwells, Oxford,, pp. 3–41. conditions and pH of the environment on phyllosilicate transformations in the Worden, R.H., Needham, S.J., Cuadros, J., 2006. The worm gut; a natural clay mineral weathering zone of pyrite-bearing schists in Wieściszowice (SW Poland). Clay Miner. factory and a possible cause of diagenetic grain coats in sandstones. J. Geochem. 47 (4), 401–417. https://doi.org/10.1180/claymin.2012.047.4.01. Explor. 89 (1), 428–431. https://doi.org/10.1016/j.gexplo.2005.12.011. Velde, B., 1985. Clay Minerals: a Physico-Chemical Explanation of Their Occurrence, vol. Worden, R.H., Utley, J.E.P., Butcher, A.R., Griffiths, J., Wooldridge, L.J., Lawan, A.Y., 40. Elsevier, Developments in Sedimentology, Amsterdam, New York, pp. 427. 2018. Improved Imaging and Analysis of Chlorite in Reservoirs and Modern Day Velde, B., 2013. Green Clay Minerals. In: Treatise on Geochemistry, second ed. pp. Analogues: New Insights for Reservoir Quality and Provenance, vol. 484 Special 351–364. https://doi.org/10.1016/B978-0-08-095975-7.00712-9. Publications, Geological Society, London. https://doi.org/10.1144/SP484.10. Virolle, M., Brigaud, B., Bourillot, R., Féniès, H., Portier, E., Duteil, T., Nouet, J., Patrier, SP484.10. P., Beaufort, D., 2018. Detrital clay grain coats in estuarine clastic deposits: origin Xu, H., Veblen, D.R., 1996. Interstratification and other reaction microstructures in the and spatial distribution within a modern sedimentary system, the Gironde Estuary chlorite-berthierine series. Contrib. Mineral. Petrol. 124 (3–4), 291–301. https://doi. (south-west France). Sedimentology. https://doi.org/10.1111/sed.12520. org/10.1007/s004100050192. Wannesson, J., Icart, J.-C., Ravat, J., 1991. Structure and Evolution of Adjoining Yamashita, S., Mukai, H., Tomioka, N., Kagi, H., Suzuki, Y., 2019. Iron-rich Smectite Segments of the West African Margin Determined from Deep Seismic Profiling. In: Formation in Subseafloor Basaltic Lava in Aged Oceanic Crust. Sci. Rep. 9, 1. https:// Meissner, R., Brown, L., Dürbaum, H.-J., Franke, W., Fuchs, K., Seifert, F. (Eds.), doi.org/10.1038/s41598-019-47887-x. Continental Lithosphere: Deep Seismic Reflections. American Geophysical Union, pp. Zanoni, G., 2015. Reservoir Characterization, Geochemistry and Clay Mineralogy of the 275–289. Dentale and Gamba Reservoirs (South Gabon Sub-basin, Equatorial West Africa). Weaver, C.E., 1956. The Distribution and Identification of Mixed-Layer Clays in MSC Thesis. Université de Genève, Geneva 130 p. Sedimentary Rocks. Am. Mineral., vol. 41, 202–221. Zanoni, G., Segvic, B., Moscariello, A., 2015. Reservoir geology, mineralogy and geo- Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. Am. chemistry of the Dentale and Gamba Formations (Early Cretaceous, Gabon): new Mineral. 95, 185–187. https://doi.org/10.2138/am.2010.3371. insights both at regional and pore scale. Modelling the Earth. University of Basel, Whittaker, E.J.W., Zussman, J., 1956. The Characterization of Serpentine Minerals by X- Basel, pp. 228–229. ray Diffraction. Mineral. Mag. 31 (233), 107–126. https://doi.org/10.1180/minmag. Zanoni, G., Šegvić, B., Moscariello, A., 2016. Clay mineral diagenesis in Cretaceous clastic 1956.031.233.01. reservoirs from West African passive margins (the South Gabon Basin) and its impact Wiewióra, A., Weiss, Z., 1990. Crystallochemical classifications of phyllosilicates based on regional geology and basin evolution history. Appl. Clay Sci. 134 (3), 186–209. on the unified system of projection of chemical composition: II. Chlorite Group: Clay https://doi.org/10.1016/j.clay.2016.09.032. Miner. 25 (1), 83–92. https://doi.org/10.1180/claymin.1990.025.1.09.