Of Foraminifera: Big Things out of Small

Article The Secret ‘After Life’ of Foraminifera: Big Things Out of Small

Jim Buckman 1,* , Carol Mahoney 2, Christian März 2 and Thomas Wagner 3

1 Institute of GeoEnergy Engineering, EGIS, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK 2 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK; [email protected] (C.M.); [email protected] (C.M.) 3 The Lyell Centre, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK; [email protected] * Correspondence: [email protected]

Received: 28 April 2020; Accepted: 15 June 2020; Published: 18 June 2020

Abstract: Calcareous and siliceous microorganisms are common components of mudrocks, and can be important in terms of stratigraphy and environmental interpretation. In addition, such microorganisms can have a significant ‘after life’, through post-mortem alteration, and represent a potential source of additional information about the diagenetic and deformation history of the rock unit. Some examples of the latter are illustrated in this study from foraminifera within a Cretaceous black shale of Colombia. This includes foraminifera tests acting as understudied repositories of authigenic calcite cement, and of elements such as Ba, Zn, Fe and S through the formation of baryte, sphalerite and iron sulphides (pyrite, marcasite). Such repositories, within the body chambers of foraminiferal tests, can provide important windows into the diagenetic processes within mudstones. If calcite cement is not recognised or separated from biogenic calcite, the depositional calcite budget can be easily overestimated, skewing the application of mudrock classification schemes, and affecting environmental interpretation including that of productivity. The elements Ba, Zn and Fe (often in ratio with Al) are commonly utilised as geochemical proxies of environmental parameters (productivity, bottom water redox conditions, etc.). Therefore, the presence of significant amounts of baryte, sphalerite and pyrite-marcasite (within foraminifera) should be noted and their origins (source and timing) investigated based on their spatial relationships before making environmental deductions based on geochemical analysis alone. Additionally, commonly observed marginal shell damage of many of the observed foraminifera is reported. We interpret this damage, for the first time, as an indicator of lateral dissolution, brought about by horizontal foreshortening during orogenesis. This is also supported by the occurrence of microscale anastomosing horizontal to inclined baryte-filled fractures within the mudstone matrix.

Keywords: Cretaceous; diagenesis; foraminifera; carbonate; sulphides; sulphates; foreshortening

1. Introduction Calcareous and siliceous nano- and microorganisms such as coccolithophores, foraminifera, and various forms of spicule are common within many marine mudstones, where they can form a significant component of the rock fabric (Figure1). Such fossilised remains of marine microorganisms can be important in terms of stratigraphy, environmental interpretation (temperature, salinity, anoxia), evidence of transportation and sediment reworking (winnowing), substrate consistency, flow dynamics, rates of sedimentation and as indicators of increased periods of productivity and nutrient upwelling [1–9].

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Figure 1. Backscattered electron (BSE) montage, illustrating the fabric of La Cristalina mudstone, with Figure 1. Backscatteredforaminifera electron (f), foraminifera (BSE) stringers montage, (fs), faecal illustrating pellets (fp), sulphides the fabric and sulphates of La (ss), Cristalina and mudstone, with foraminiferapyriti (f ),sed foraminifera sponge spicule (p). stringers (fs), faecal pellets (fp), sulphides and sulphates (ss),

and pyritised spongeThe current spicule paper(p). illustrates an example of Cretaceous Colombian mudstone that contains foraminifera that display a range of interesting post-mortem features, (i) authigenic mineral infills The currentwithin paper body illustrates chambers and an(ii) unusual example marginal of damage Cretaceous of the tests Colombian. Previous studies mudstonewithin the that contains Eagle Ford Formation (USA), from similar age and environment to the Colombian material, have foraminifera thatindica displayted that a calc rangeite, kaolinite, of interesting pyrite and occasionally post-mortem quartz can occur features, as infill within (i) foraminifera authigenic mineral infills chambers associated with organic material [10–12]. Herein, while kaolinite has not been observed, within body chamberscalcite, andand pyrite (ii) unusualare, with themarginal addition of baryte, damage sphalerite of theand ma tests.rcasite. Previous Marginal damage studies of within the Eagle Ford Formation (USA),foraminifera from tests similarappears not age to have and been environment previously reported to elsewhere the Colombian, including from material, the Eagle have indicated Ford Formation. Both the authigenic minerals and test damage are described herein, and discussed that calcite, kaolinite,in terms pyrite of their andpotential occasionally significance. The quartzpurpose ofcan the present occur paper as is infill not to definitively within foraminiferaanswer chambers associated with organicall questions material about sources, [10 –formation12]. Herein, mechanisms while and the kaolinite ultimate meaning has of not these been features observed,, but calcite, and instead to flag up areas of potential significance that may otherwise be overlooked. The aim is to pyrite are, with thestimulate addition further of research baryte, and sphaleriteinvestigations. and marcasite. Marginal damage of foraminifera tests appears not to have been previously reported elsewhere, including from the Eagle Ford Formation. Both the authigenic minerals and test damage are described herein, and discussed in terms of their potential significance. The purpose of the present paper is not to definitively answer all questions about sources, formation mechanisms and the ultimate meaning of these features, but instead to flag up areas of potential significance that may otherwise be overlooked. The aim is to stimulate further research and investigations.

2. Material and Methods Here, we study a mudstone sample previously described from near La Cristalina (Figure2), on the eastern side of the Middle Magdelena Valley (west of the Eastern Cordillera), Cretaceous of Colombia, South America [13]. It was imaged as a polished thin section by scanning electron microscopy (SEM), using a Quanta FEG 650 SEM (Thermo Fischer Scientiﬁc, Waltham, MA, USA), in the low-vacuum mode (0.82 Torr), operating at 20 kV, in association with energy dispersive X-ray (EDX) analysis, utilising an Oxford Instruments X-MaxN 150 mm detector (Oxford Instruments NanoAnalysis & Asylum Research, High Wycombe, UK), with AZtec mapping and particle analysis software (Version 3.3, Oxford Instruments NanoAnalysis & Asylum Research, High Wycombe, UK). Backscattered electron Minerals 2020, 10, x FOR PEER REVIEW 3 of 19

Minerals 2020, 10, 550 2. Material and Methods 3 of 17 Here, we study a mudstone sample previously described from near La Cristalina (Figure 2), on the eastern side of the Middle Magdelena Valley (west of the Eastern Cordillera), Cretaceous of Colombia, South America [13]. It was imaged as a polished thin section by scanning electron (BSE) imaging and EDX mappingmicroscopy (SEM), wereusing a Quanta used FEG 650 in SEM conjunction (Thermo Fischer Scientific, with Waltham, gaseous MA, USA), in secondary electron (GSE) the low-vacuum mode (0.82 Torr), operating at 20 kV, in association with energy dispersive X-ray imaging. The latter uses the(EDX) charge analysis, utilis contrasting an Oxford Instruments imaging X-Max (CCI)N 150 mm technique detector (Oxford Instruments in order to elucidate details of NanoAnalysis & Asylum Research, High Wycombe, UK), with AZtec mapping and particle analysis shell morphology and thesoftware presence (Version 3.3,of Oxford calcite Instruments and NanoAnalysis quartz & Asylum cements Research, High [13 Wycombe,], which UK). gives similar results to Backscattered electron (BSE) imaging and EDX mapping were used in conjunction with gaseous that of cathodoluminescencesecondary (CL) electron microscopy (GSE) imaging. The latter [14 uses]. the charge contrast imaging (CCI) technique in order to elucidate details of shell morphology and the presence of calcite and quartz cements [13], which gives similar results to that of cathodoluminescence (CL) microscopy [14].

Figure 2. Locality map of the La Cristalina sample (yellow star), within its geographic, stratigraphic Figure 2. Locality map ofand the structural La context. Cristalina Red lines = thrust sample faults; blue(yellow lines normal faults. star), After Giuliani within et al. [15]. its geographic, stratigraphic and structural context. RedPolished lines thin =sectionsthrust were prepared faults; using blue the following lines procedure normal. Thefaults. sample wasAfter vacuum Giuliani et al. [15]. resin impregnated to fill any larger pores and stabilise the specimen. It was then trimmed to an appropriate size, using a diamond wafering saw and oil-based lubricating medium, to minimise Polished thin sectionsdamage were to the mudstone prepared microstructure. using After fixation the of following the prepared sample procedure. to a glass thin section,The sample was vacuum resin impregnated to fill any larger pores and stabilise the specimen. It was then trimmed to an appropriate size, using a diamond wafering saw and oil-based lubricating medium, to minimise damage to the mudstone microstructure. After fixation of the prepared sample to a glass thin section, the sample was machine polished to a thickness of 30 µm using a 600 grade aluminium oxide slurry. After cleaning, the sample was further hand polished using a series of diamond pastes, ending with a 1 µm paste. Hand polishing reduces plucking, and the diamond pastes produce a fine mirror-like surface suitable for elemental analysis and charge contrast imaging (CCI). Finally, the polished thin section was cleaned in an ultrasonic bath, using alcohol, to remove any remaining polishing agent. No coverslip was added, as the sample was destined for examination by scanning electron microscopy.

3. Results Foraminifera are common features within the mudstone examined (Figure1), occurring as isolated elements and as associated strings of crowded individuals, parallel to bedding. Many foraminifera are remarkably intact, show little evidence of flattening and have good preservation of shell microstructure. In some cases, the foraminifera chambers have acted as receptacles for authigenic calcite, FeS2, iron oxide, sphalerite and baryte (Figure3A–H). Calcite occurs as a coarse sparry infill (typically 10–20 µm), as a finer microspar (1 µm) coating the inner surfaces of foraminifera tests (Figure3) and as a thin apparently structureless external layer around tests (Figure4). Pyrite and iron oxides occur in the form of framboids (typically 5 µm diameter) and coarser equant crystals (up to ~10 µm). The latter often surround framboids and can totally infill foraminifera chambers. Sphalerite typically occurs as euhedral crystals (5 to 10 µm), surrounding pyrite framboids, and does not completely inﬁll chambers (Figure3). Baryte occurs as euhedral crystals (many up to 25 µm), and typically totally inﬁlls foraminifera chambers (Figure3). Particle distribution analysis for FeS2, iron oxides, sphalerite and baryte illustrates that all phases show some degree of overlap in terms of area of coverage (Figure5). Baryte is the most pervasive and well distributed phase, while FeS2, although more numerous, is restricted in terms of its distribution (Figure5). Sphalerite has a similar distribution to that of FeS 2, but slightly more restricted, and iron Minerals 2020, 10, 550 4 of 17

oxides occur as a halo around the area containing FeS2. Some microquartz is also occasionally observed within foraminifera chambers (Figure6). Although foraminifera in general are well preserved, many are notable for the occurrence of marginal damage to the test (Figures4 and6), which is always restricted to lateral areas within the plane of bedding. The use of charge contrast imaging (CCI) illustrates that both shell and calcite cements are missing from the margins of the tests (Figure4F).

Figure 3. (A,C,E,G,H) Backscattered electron (BSE) images of typical foraminifera and associated secondary minerals. (B,D,F) Energy dispersive X-ray (EDX) phase maps of (A,C,E), showing calcite cement (B,D), pyrite framboids (B), highlighted by dashed black circles, euhedral sphalerite (B), baryte (D), and euhedral pyrite/Fe oxide (hydroxide) (F)—with iron oxide in dark green. Further, small

oxidised pyrite framboids are shown in (D). (G,H) Foraminifera with calcite cement, and FeS2 in the form of pyrite framboids (P) and larger euhedral “marcasite” (M). Note, that diﬀerentiation between calcite test and authigenic calcite is more clearly illustrated1 in the charge contrast images (CCIs) in Figures 4 and6. Minerals 2020, 10, 550 5 of 17 Minerals 2020, 10, x FOR PEER REVIEW 6 of 19

Figure 4. 4. GaseousGaseous secondary secondary electron electron (GSE) (GSE) charge charge contrast contrast images images (CCI (CCIs)s) showing showing the presence the presence and natureand nature of calcite of calcite cement, cement, and and lateral lateral dissolution dissolution of of foraminifera foraminifera tests. tests. Bedding Bedding is is approximately horizontal in all images. ( (AA––EE)) Indivi Individualdual examples examples displaying displaying areas areas of of peripheral damage due due to horizontal foreshortening. All All individuals individuals are are filled ﬁlled with with calcite calcite cement, cement, with with two two phases phases of of cement clearlyclearly observed in (C,)D and). Asterisk (D). Asterisk marks marks locations locations where where calcite calcite cement cement is thickly is thickly developed develo onped the onupper the upper and lower and surfaceslower surfaces of the test.of the (F test.) Enlargement (F) Enlargement of area of marked area marked in (E). in 1—calcite (E). 1—c test;alcite 2—sparry test; 2— scalciteparry calcite cement cement within within test; 3—calcite test; 3—c cementalcite cement on outer on surfaceouter surface of test, of and test, on and broken on broken test and test inner and innersparry sparry cement cement surfaces surfaces (post-dissolution). (post-dissolution). Note Note the the scalloped scalloped dissolution dissolution surface surface between between 3 and3 and 1, 1,as as well well as as 3 and3 and 2. 2.

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FigureFigure 5. Phase 5. Phase distribution distribution maps maps (across (across wholewhole thin section) section),, constructed constructed using using AZtec AZtec Feature Feature and and processed in ImageJ, for (A) pyrite (includes “marcasite”), (B) iron oxide, (C) sphalerite and (D) processed in ImageJ, for (A) pyrite (includes “marcasite”), (B) iron oxide, (C) sphalerite and (D) baryte. baryte. Horizontal field of view is approximately 40 mm. Note that dots only mark the relative Horizontal ﬁeld of view is approximately 40 mm. Note that dots only mark the relative position position of phases, and are not to scale (pixels dilated four times to help with visualisation). Sulphides of phases, and are not to scale (pixels dilated four times to help with visualisation). Sulphides are are both restricted, particularly sphalerite. Iron oxides form a halo around the area dominated by bothpyrite. restricted, Baryte particularly has a more pervasive sphalerite. widespread Iron oxides distribution. form a halo around the area dominated by pyrite. Baryte has a more pervasive widespread distribution.

Within the mudstone matrix, FeS2 framboids and isolated euhedral crystals are also present. However, the dominant authigenic phase is that of microquartz, which is pervasive and occurs as pore-ﬁlling cement and overgrowths to detrital silt grains [13]. Calcite cement is lacking, apart from where it occurs as syntaxial overgrowths of coccolithophore plates within faecal pellets (Figure7). In addition, baryte is also seen to occur as the ﬁll of horizontal to subhorizontal anastomosing microcracks (Figure8).

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Figure 6. GaseousFigure secondary 6. Gaseous electron secondary (GSE), electron charge (GSE), charge contrast contrast images images (CCIs) (CCIs) showing lateral lateral dissolution dissolution of foraminifera tests. Bedding is approximately horizontal in all images. (A) Overview of foraminifera(note tests. some Bedding basal dissolution is approximately due to burial loading) horizontal; (B) and (C)in details all images.of areas marked (A) in Overview (A). Q = (note some basal dissolutionmicro duequartz to burialcement. Arrows loading); indicate (B shell,C) disso detailslution of where areas micro markedquartz contacts in (A foraminifera). Q = microquartz test. cement. Note some limited mechanical breakage (white arrow). Arrows indicate shell dissolution where microquartz contacts foraminifera test. Note some limited Mineralsmechanical 2020, 10, x breakage FORWithin PEER the (white REVIEW mudstone arrow). matrix, FeS2 framboids and isolated euhedral crystals are also present. 9 of 19 However, the dominant authigenic phase is that of microquartz, which is pervasive and occurs as pore-filling cement and overgrowths to detrital silt grains [13]. Calcite cement is lacking, apart from where it occurs as syntaxial overgrowths of coccolithophore plates within faecal pellets (Figure 7). In addition, baryte is also seen to occur as the fill of horizontal to subhorizontal anastomosing microcracks (Figure 8).

FigureFigure 7.7. ((AA)) BackscatteredBackscattered electronelectron (BSE)(BSE) andand ((BB)) gaseousgaseous secondarysecondary electronelectron (GSE)(GSE) chargecharge contrastcontrast imageimage (CCI) (CCI) pair pair of of details details within within faecal faecal pellets, pellet showings, showing disarticulated disarticulated coccolithophore coccolithophore plates. plates. Note Note the blockythe blocky nature nature of calcite of calcite in (A ),in representing (A), representing coccolithophore coccolithophore plates plates plus syntaxial plus syntaxial cement, cement, and pores and (black).pores (black). In (B), platesIn (B and), plates cement and can cement be diﬀ erentiated—coccolithophore can be differentiated—coccolithophore plate = pale grey; plate calcite = pale cement grey; =calcitedarker cement grey. = Therefore, darker grey. original Therefore, porosity original is greater porosity than is apparentgreater than in (Aapparent). Further, in ( theA). pore-ﬁllingFurther, the microquartzpore-filling micro cementquartz is shown cement (bright is shown grain, (bright bottom grain, right bottom (B)). right (B)).

Figure 8. Examples of baryte-bearing anastomosing microfractures within the mudstone matrix. (A) and (B) Horizontal and inclined examples. (C) Example of three en echelon inclined microcracks. (D) and (E) Details of horizontal microcracks, with en echelon and anastomosing veinlets.

4. Discussion Previous geochemical work [13] indicates that the La Cristalina mudstones were deposited in a high-productivity environment, with relative enrichment in Mo, V, U, and Zn, and high Zn/Al, Cu/Al, and Ni/Al ratios. High productivity is further supported by the abundance of foraminifera tests and the occurrence of faecal strings containing coccolithophore plates (Figures 1 and 7). The occurrence of calcite cements, as well as sulphide and sulphate species within the chambers of foraminifera, may have a number of important consequences that should be taken into consideration when considering environmental settings, and applying interpretations of bulk collected geochemical data.

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Figure 7. (A) Backscattered electron (BSE) and (B) gaseous secondary electron (GSE) charge contrast image (CCI) pair of details within faecal pellets, showing disarticulated coccolithophore plates. Note the blocky nature of calcite in (A), representing coccolithophore plates plus syntaxial cement, and pores (black). In (B), plates and cement can be differentiated—coccolithophore plate = pale grey; Mineralscalcite2020 ,cement10, 550 = darker grey. Therefore, original porosity is greater than apparent in (A). Further, the8 of 17 pore-filling microquartz cement is shown (bright grain, bottom right (B)).

Figure 8. ExamplesExamples of of baryte baryte-bearing-bearing anastomosing anastomosing micro microfracturesfractures within within the themudstone mudstone matrix. matrix. (A) (andA,B ()B Horizontal) Horizontal and and inclined inclined examples.examples. (C (C) )Example Example of ofthree three en enechelon echelon inclined inclined micro microcracks.cracks. (D) (andD,E ()E Details) Details of of horizontal horizontal microcracks, microcracks, with with en en echelon echelon and and anastomosing anastomosing veinlets. veinlets.

4. Discussion PreviousPrevious geochemical wor workk [[1133] indicates that the La Cristalina mudstones were deposited in a high-productivityhigh-productivity environment, environment, with with relative relative enrichment enrichment in Mo, in Mo, V, U, V, and U, Zn,and and Zn, high and Zn high/Al, Zn/Al, Cu/Al, andCu/Al, Ni /andAl ratios. Ni/Al Highratios. productivity High productivity is further is supportedfurther supported by the abundance by the abundance of foraminifera of foraminif tests andera thetests occurrence and the occurrence of faecal strings of faecal containing strings coccolithophorecontaining coccolithophore plates (Figures plates1 and (Figure7). Thes occurrence1 and 7). The of calciteoccurrence cements, of calcite as well cements, as sulphide as and well sulphate as sulphide species and within sulphate the chambers species of within foraminifera, the chambers may have of aforaminifera, number of may important have a consequencesnumber of important that should consequences be taken that into should consideration be taken wheninto consideration considering environmentalwhen considering settings, environmental and applying settings, interpretations and applying of bulkinterpretations collected geochemical of bulk collected data. geochemical data. 4.1. Signiﬁcance of Authigenic Mineral Formation

4.1.1. Calcite

The occurrence of large amounts of authigenic calcite within the chambers of foraminifera has the potential to greatly raise the total carbonate budget of such mudrock. Standard geochemical assay techniques, X-ray fluorescence (XRF) and X-ray diffraction (XRD) do not differentiate biological (primary) calcite from authigenic calcite. Therefore, without additional information, for example, from thin-section analysis (optical or SEM), the amount of primary biological calcite may well be overestimated. This is comparable to the situation of microquartz development within mudstones illustrated by Buckman et al. [13]. A typical area of La Cristalina shale (Figure9A) has approximately 13% calcite, which comprises both biological calcite (foraminifera tests and coccolithophore plates) and later authigenic calcite cement. If a contribution to the carbonate budget from coccolithophore plates is excluded, then dependant on foraminifera morphology (Figure9B,C), the di fference in calcite percentage between uncemented and cemented foraminifera morphotypes could vary between 1/6th and 1/8th (Figure9B–E), suggesting that original biocalcite levels could have been in the range of ~2%, considerably lower than the recorded value (13%). In this example, faecal pellets comprising disarticulated coccolithophore plates (Figure7) also contribute to the overestimation of primary calcite Minerals 2020, 10, 550 9 of 17 budget due to the occurrence of syntaxial calcite overgrowths (Figure7B)—observed using the charge contrast imaging technique [14]. Overestimation of the primary biological calcite is of significance, as this will affect estimation of changes in rates of productivity, primary producer community, as well as how the mudrock is classified (i.e., calcareous, siliceous, terrigenous, etc.). For some materials, if the presence of authigenic cement is not recognised and taken into account, carbon and oxygen isotope analysis will be compromised, affecting, for example, estimates of seawater temperature [16,17]. In addition, the elemental composition of cements may differ radically from that of the primary biogenic calcite, depending on the timing of cement emplacement and fluid composition. For example, enrichment in Mn, Fe, Mg or Sr [14,18], which could impact the study and interpretation of shell chemistry (high-/low-Mg calcite, aragonite), while cements that are compositionally similar to shelly material may not be recognised (unless using CCI or CL), leading to the masking of taxonomic criteria such as the presence and character of pore structures. Minerals 2020, 10, x FOR PEER REVIEW 11 of 19

FigureFigure 9. (A 9.) ( EnergyA) Energy dispersive dispersive X-ray x-ray (EDX)(EDX) phase image image of of a arepresentative representative area area of La of Cristalina La Cristalina shale.shale. Orange Orange= shale = shale matrix; matrix red; red= =calcite calcite (foraminifera,(foraminifera, coccolithophore coccolithophore plates plates within within faecal faecal pellets pellets and calciteand calcite cement). cement). (B, C(B)) Schematicand (C) Schematic illustrations illustrations of simple of simple uniserial uniserial foraminifera foraminifera morphology morphology (used (used to simplify analysis). Black = foraminifera test; approximately 11% in each case. (D) and (E) As to simplify analysis). Black = foraminifera test; approximately 11% in each case. (D,E) As in (B,C), in (B) and (C), with the addition of calcite cement fill. Black = test and calcite cement; approximately with the addition of calcite cement ﬁll. Black = test and calcite cement; approximately 68% and 82%. 68 and 82%. In all cases, percentage calcite calculated by simple image analysis binary thresholding. In all cases, percentage calcite calculated by simple image analysis binary thresholding. 4.1.2. Sphalerite Zinc is a particularly important element used in geochemical interpretation of environmental change, with the Zn/Al ratio being used to assess relative enrichment or depletion in respect to average shale [19,20]. The Zn/Al ratio for the sample examined and associated analysed samples of 67.53 and 254.92 [13] is high, even in comparison to other related high-productivity shales such as those from the Zipa section in Colombia [13], which is in line with generally high Zn/Al ratios in Cretaceous shales and mudstones [20]. In the current case, given the co-occurrence of pyrite replacing opaline sponge spicules, which have previously been recorded as sources of Zn and Ba in areas of high productivity and upwelling [13,21], and the high degree of early microquartz cement [13], it is likely that both sphalerite and baryte were produced during the early stages of burial (eogenesis)— preserving the environmental signal of high-productivity upwelling waters—although it cannot be

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4.1.2. Sphalerite Zinc is a particularly important element used in geochemical interpretation of environmental change, with the Zn/Al ratio being used to assess relative enrichment or depletion in respect to average shale [19,20]. The Zn/Al ratio for the sample examined and associated analysed samples of 67.53 and 254.92 [13] is high, even in comparison to other related high-productivity shales such as those from the Zipa section in Colombia [13], which is in line with generally high Zn/Al ratios in Cretaceous shales and mudstones [20]. In the current case, given the co-occurrence of pyrite replacing opaline sponge spicules, which have previously been recorded as sources of Zn and Ba in areas of high productivity and upwelling [13,21], and the high degree of early microquartz cement [13], it is likely that both sphalerite and baryte were produced during the early stages of burial (eogenesis)—preserving the environmental signal of high-productivity upwelling waters—although it cannot be excluded that anomalously high Zn/Al values may be due to the concentration of sphalerite within foraminifera chambers. The preferential formation and the concentration of sphalerite within foraminifera tests may in part be due to the occurrence of organic matter, as zinc and organic carbon have previously been shown to be positively correlated [22], and Zn held within organometallic complexes can be released under reducing conditions and incorporated either in pyrite or form sphalerite [23]. It should also be noted that in Figure3B, sphalerite seems to occur around pyrite framboids and was therefore precipitated later. This could be due to decreasing hydrogen sulphide concentrations in the ambient pore waters/within the foraminiferal chamber, since sphalerite has a tendency to form at such lower hydrogen sulphide levels [24]. The relative signiﬁcance of such localised concentrations will depend on the extent to which the system is closed, the type and extent of available pathways for ﬂuid/ion migration, and the scale/resolution that measurements are taken at.

4.1.3. Baryte Barium can have a hydrothermal [25,26] as well as biogenic or detrital origin [21,27–30]. While the origin of biogenic-derived barium is still not fully resolved, recent research supports the idea of baryte precipitation in supersaturated microenvironments within organic particles sinking down through the water column [31], but it has also been associated with siliceous microorganisms in oxygenated waters of coastal upwelling areas [21]. A number of authors have positively correlated excess non-lithogenic (biogenic) barium with water depth with higher concentration at water depths greater than 1000 m [28,30]. Diagenetic dissolution of baryte is commonly observed in marine sediments where pore waters are depleted in sulphate, followed by reprecipitation of authigenic baryte when sulphate becomes available again [28]. In addition, during early diagenesis, dissolution of opaline biogenic quartz is known to result in the high concentration of barium in pore waters [29]. Given the association of pyrite replaced siliceous sponge spicules from the current sample (Figure 10), it is possible that baryte within foraminifera chambers has a biogenic origin, with baryte precipitated from Ba-enriched pore waters (during eogenesis), associated with a deep-water high-productivity environment of deposition. Nevertheless, a hydrothermal origin for such materials is equally possible. It is remarkable that high-resolution mapping of baryte occurrence (Figure5) shows no preferential enrichments of baryte within the scanned samples, in stark contrast to pyrite and sphalerite, instead being pervasively developed throughout. This may indicate that supersaturated microenvironments leading to primary baryte formation might have occurred within foraminifera tests themselves, or such conditions developed following the burial of the tests into deeper sediments. Baryte formation in large fractures is easily observed [26], and therefore avoided, in geochemical surveys for environmental analysis, whereas the same could not be readily said for baryte within foraminifera chambers (which is also equally valid for sphalerite). At least some of the observed baryte may be late (remobilised) and associated with uplift and unloading (telogenesis), although given the occurrence of basin foreshortening (see below), it may have formed during burial (mesogenesis) associated with the formation of bedding parallel cracks. Therefore, it is feasible that baryte within foraminifera chambers, and microcracks, could have formed at any stage from eogenesis, through mesogenesis to late telogenesis, or indeed all three. Consequently, Minerals 2020, 10, 550 11 of 17 the recording of these or other shales as strongly Ba enriched in such cases may not be representative of the shale as a whole and may be of uncertain affinity and of dubious specific environmental significance. In such cases, further detailed sedimentological observation paired with high-resolution geochemical analysis would be beneficial to elucidate the exact origins of the formation of baryte, and barium-rich Mineralsfluids, in2020 order, 10, x toFOR support PEER REVIEW any detailed geochemical/environmental interpretation. 13 of 19

Figure 10. 10. BackscatteredBackscattered electron electron (BSE) (BSE) images, images, illustrating illustrating the typical the typical form of form siliceous of siliceous sponge spicules sponge spiculesreplaced replaced by pyrite. by ( Apyrite.–C) showing (A) to (C slight) showing variations slight in variations morphology in morphology and pyritization. and pyritization.

4.1.4. Pyrite/Fe Pyrite/Fe Oxides

Timing of pyrite (FeS2) framboid formation and, by inference, the later more massive surrounding formsTiming is uncertain. of pyrite It is possible (FeS2) that framboid the framboids formation grew and in, association by inference with, organicthe later material more within massive the surroundingforaminifera chamber,forms is possiblyuncertain. within It is thepossible water that column the framboids prior to burial, grew or in soon association after burial with near organic to the material within the foraminifera chamber, possibly within the water column prior to burial, or soon seafloor sediment interface. Notably, FeS2 is known to occur in two forms—pyrite (cubic) and marcasite after(orthorhombic)—and burial near to the euhedral seafloor crystals sediment of marcasite interface are. Notably often found, FeS2 associatedis known to with occur framboids in two offorms pyrite,— pyritewhich they(cubic) may and surround marcasite [32 ]. (orthorhombic) Such changes— inand morphology euhedral are crystals postulated of marcasite as recording are often temporary found associatedchanges in with redox framboids levels, specifically of pyrite, which lower pHthey and may more surround oxidising [32]. conditions Such changes [32]. in Di morphologyfferentiation are of postulatedpyrite and marcasiteas recording is di temporaryfficult, and changes morphological in redox comparison levels, specifically with previously lower pH recorded and more materials oxidising is conditionsnot conclusive. [32]. Ideally,Differentiation electron of backscatter pyrite and di marcasiteffraction (EBSD) is difficult, would and resolve morphological this question, comparison but was withnot available previously for recorded this study. materials The presence is not of conclusive. marcasite isIdeally, of particular electron interest, backscatter as this diffraction is at odds with(EBSD) the wouldconditions resolve required this forquestion, the formation but was of pyrite,not available sphalerite for andthis baryte. study. However,The presence this may of marcasite reflect changes is of particularin oxygenation interest, in the as water this columnis at odds or sea with floor, the fluctuating conditions between required reducing for the (anoxic formation/sulphidic) of pyrite, and sphalerite and baryte. However, this may reflect changes in oxygenation in the water column or sea floor, fluctuating between reducing (anoxic/sulphidic) and oxic conditions, followed by increased anoxia during burial. The occurrence of altered framboids and euhedral forms of FeS2 to Fe oxides (Figure 11) represents subsequent oxidation, either during uplift and subsequent weathering, or possibly influenced by a change to more oxidising conditions during burial. The former seems more likely, given the assumed reduced permeability likely after compaction and the early stages of diagenesis, and given that the iron oxides form in a halo around the main area of pyrite distribution (cf [33]; Figures 5 and 11). The iron sulphide/oxide species noted within the foraminifera and

Minerals 2020, 10, 550 12 of 17 oxic conditions, followed by increased anoxia during burial. The occurrence of altered framboids and euhedral forms of FeS2 to Fe oxides (Figure 11) represents subsequent oxidation, either during uplift and subsequent weathering, or possibly inﬂuenced by a change to more oxidising conditions during burial. The former seems more likely, given the assumed reduced permeability likely after compaction and the early stages of diagenesis, and given that the iron oxides form in a halo around the main Minerals 2020, 10, x FOR PEER REVIEW 14 of 19 area of pyrite distribution (cf [33]; Figures5 and 11). The iron sulphide /oxide species noted within themudstone foraminifera matrix and therefore mudstone record matrix a range therefore of geochemical record a range changes of geochemical throughout changesdifferent throughout periods of ditheﬀerent mudstones periods depositional of the mudstones to burial depositional and subsequent to burial uplift and/weathering subsequent history. uplift/weathering history.

FigureFigure 11.11. ExamplesExamples ofof oxidisedoxidised pyritepyrite framboids.framboids. (A) BackscatteredBackscattered electronelectron (BSE)(BSE) imageimage ofof badlybadly alteredaltered pyritepyrite framboid.framboid. Bright points are unalteredunaltered pyritepyrite framboidframboid microcrystalmicrocrystal cores;cores; darkerdarker greygrey areasareas areare depleted in sulphur and and oxidi oxidised.sed. (B (B––DD) )Energy Energy dispersive dispersive X X-ray-ray (EDX) (EDX) maps maps of ofthe the area area in in(A (),A for), for Fe, Fe, S Sand and O O,, respectively. respectively. (E (E) )A A b better-preservedetter-preserved pyrite pyrite framboid, framboid, but but still heavily altered asas inin ((AA).). ((FF,G) and) Detailed (G) Detailed BSE and BSE EDX and map EDX of anmap altered of an pyrite altered framboid pyrite framboid (similar to (similar (E)). In (toF), (E variations)). In (F), invariations grey scale in reflect grey scale different reflect degrees different of loss degree of S ands of oxidation loss of S (seeand Mahoneyoxidation et(see al. [Mahoney33]). Note et that al. (G[33)] is). aNote composite that (G map) is a for composite O and S. map Oxidised for O areas and appearS. Oxidi greensed areas and moreappear pristine green pyriteand more “cores” pristine are yellow. pyrite “cores” are yellow. 4.1.5. Foraminifera Chamber vs. Shale Matrix Diagenesis 4.1.5.Interestingly, Foraminifera the Chamber authigenic vs. processesShale Matrix that Diagenesis occur within the foraminifera are different in character to thoseInterestingly, within the the mudstone authigenic matrix. processes Authigenic that occur processes within within the foraminifera the mudstone are matrix, different while in character to those within the mudstone matrix. Authigenic processes within the mudstone matrix, while including the formation of pyrite framboids, do not include the formation of marcasite, sphalerite or baryte, but instead are dominated by the formation of microquartz [13], which occurs to a much lesser extent within foraminifera. This suggests that the geochemical microenvironments between mudstone matrix “micro” porosity and foraminifera chamber “macro” porosity, and potentially the mechanisms for fluid migration, are significantly different at the small sediment layer scale to promote the observed variance in mineral species. In the case of pyrite and sphalerite, this may be mitigated by the presence of reactive organic matter. This work agrees with that of Camp

Minerals 2020, 10, 550 13 of 17 including the formation of pyrite framboids, do not include the formation of marcasite, sphalerite or baryte, but instead are dominated by the formation of microquartz [13], which occurs to a much lesser extent within foraminifera. This suggests that the geochemical microenvironments between mudstone matrix “micro” porosity and foraminifera chamber “macro” porosity, and potentially the mechanisms for fluid migration, are significantly different at the small sediment layer scale to promote the observed variance in mineral species. In the case of pyrite and sphalerite, this may be mitigated by the presence of reactive organic matter. This work agrees with that of Camp [10], who indicated that foraminifera chambers, within the contemporaneous Eagle Ford Formation of Texas, can act as crucibles for the study of diagenetic products (e.g., pyrite, quartz, calcite and kaolinite) resulting from thermo-chemical reactions duringMinerals burial, 2020, 10, andx FOR PEER that REVIEW diagenesis can be different between foraminifera15 of 19 chambers and normal pore spaces.[10], who Therefore, indicated that detailed foraminifera analysis chambers, ofwithin the the type contemporaneous and timing Eagle of Ford such Formation diagenetic products (within foraminiferaof Texas, chambers—Figure can act as crucibles for 12the) study and of comparison diagenetic products to cements (e.g., pyrite, within quartz, othercalcite and mudstone matrix kaolinite) resulting from thermo-chemical reactions during burial, and that diagenesis can be porosities shoulddifferent be formative between foraminifera in determining chambers and details normal pore of changesspaces. Therefor in fluide, detailed movement, analysis of the dissolution and mineral diagenesistype and brought timing ofabout such duringdiagenetic burial products (mesogenesis) (within foraminifera and chambers subsequent—Figure 12 uplift) and (telogenesis). comparison to cements within other mudstone matrix porosities should be formative in determining The understandingdetails and of changes diff erentiation in fluid movement, of the dissolution authigenic and mineral processes diagenesis within brought about shale during matrix compared to that of unusualburial microcrucibles, (mesogenesis) and subsequent such as uplift within (telogenesis) foraminifera,. The understanding should and be differentiation taken into of consideration, the authigenic processes within shale matrix compared to that of unusual microcrucibles, such as especially wherewithin geochemical foraminifera data, should is be extracted taken into to consideration interpret, aespecially range ofwhere environmental geochemical data factors, is including productivity, waterextracted depth to interpret and anoxia a range ofamongst environmental others. factors, including productivity, water depth and anoxia amongst others.

Figure 12. SchematicFigure 12. cartoon Schematic ofcartoon diagenetic of diagenetic sequence sequence observed observed within within foraminifera foraminifera tests within the tests within the studied material. (A) Initial formation of pyrite framboids, probably in association with organic studied material.matter (A (black),) Initial either formation in water column, of or pyrite during framboids,early burial. Anoxic probably or dysaerobic in associationconditions. (B) with organic matter (black),Formation either in of water euhedral column, species surrounding or during some early of the burial. pyrite framboid Anoxic cores: or 1— dysaerobicsphalerite conditions. (B) Formation of(reducing euhedral environment) species; 2—p surroundingyrite (reducing environment) some of; the3—“m pyritearcasite” framboid (temporary change cores: to 1—sphalerite more acidic and oxidising conditions). Zn for sphalerite, derived from dissolution of siliceous (reducing environment);(opaline) microorganisms 2—pyrite (reducing (sponge spicules) environment); within mudstone 3—“marcasite” matrix—spicules (temporary pyritised. (C) change to more acidic and oxidisingFormation conditions). of calcite cements, Zn with for two sphalerite, major phases. 1 derived—Fine internal from; 2—later dissolution coarser sparry of cement, siliceous (opaline) infilling chambers (and reducing porosity); 3—resulting in sulphide phases embedded within calcite. microorganisms(D (sponge) Formation spicules) of later-stage within pervasive mudstone baryte in matrix—spiculesforaminifera tests and horizontal pyritised. micro (Ccracks,) Formation in of calcite cements, with twoassociation major with phases. basin foreshortening, 1—Fine internal; sequence thickening2—later and coarser major thrusting. sparry cement,Barium pos inﬁllingsibly chambers (and reducing porosity);sourced from 3—resultingdissolution of opaline in sulphide sponge spicules, phases and later embedded remobilised, within or associated calcite. with (theD ) Formation of migration of hydrothermal fluids during thrusting. (E) Uplift and weathering, resulting in partial and later-stage pervasivevariable baryte degree of in oxidation foraminifera of FeS2, tests both framb andoids horizontal and equant microcracks, crystals. Note crystal in association shapes with basin foreshortening,illustrated sequence are thickening not morphologically and accurate,major thrusting. but rather indicative Barium of phase possibly distribution. sourced Brown from = dissolution pyrite; blue = sphalerite; green = oxidised FeS2 (pyrite or marcasite); grey = marcasite; orange = baryte; of opaline spongeyellow spicules, = calcite; solid and black later = organics. remobilised, or associated with the migration of hydrothermal ﬂuids during thrusting. (E) Uplift and weathering, resulting in partial and variable degree of oxidation

of FeS2, both framboids and equant crystals. Note crystal shapes illustrated are not morphologically accurate, but rather indicative of phase distribution. Brown = pyrite; blue = sphalerite; green = oxidised

FeS2 (pyrite or marcasite); grey = marcasite; orange = baryte; yellow = calcite; solid black = organics. Minerals 2020, 10, 550 14 of 17

4.2. Signiﬁcance of Test Damage The missing lateral margins of the foraminifera test can be hypothesised to have a number of origins: (i) predatory damage; (ii) natural marginal collapse, brought about during burial; (iii) shell damage caused during transportation; or (iv) localised dissolution.

4.2.1. Predation Active predation of foraminifera could conceivably cause damage to the test. However, given the small size of the foraminifera aﬀected, and the localisation of damage to the margins of the test, it is hard to envisage either the instigator or the purpose of such speciﬁc damage. In addition, unless damage is caused all around the periphery of the test, an additional mechanism is required to explain the alignment of tests, so that the damage is commonly observed in the plane of the thin section.

4.2.2. Shell (Test) Collapse Where foraminifera, or other structures, are variable in architecture, some parts may be structurally more robust than others; with spherical structures possessing higher load-bearing capacity than more tapered slender structures. However, in this case, the foraminifera appear robust enough to withstand burial loading (vertical compaction), and in many cases, are further supported by internal strengthening through the precipitation of authigenic minerals such as calcite, pyrite, sphalerite and baryte. In addition, there is no evidence to suggest that the lateral peripheries were architecturally diﬀerent to the rest of the foraminiferal test, and therefore less robust.

4.2.3. Transportation Damage If it is hypothesised that tests were deposited from an ocean current, it may be possible to explain the damage as due to transportation. Similar stringers of foraminifera, and faecal material containing disarticulated coccolith plates (Figures1 and7) can appear similar to bioclastic material transported and deposited by bottom currents [5], but lack characteristic erosive basal surfaces. Additionally, if damage was in this case explained by entrainment and transportation in bottom currents ﬂowing along the sea ﬂoor, then evidence of abrasion might reasonably be expected throughout the tests periphery, which is not generally the case.

4.2.4. Localised Dissolution The samples examined were deposited within a deep-water basin that currently sits within an active thrust belt zone, at the margin of the Eastern Cordillera, and is marked by crustal thickening and structural foreshortening [34–36], and in particular, the site is located between two thrust faults (Figure2). Given this, the lateral damage potentially can be explained as the product of localised dissolution brought about by lateral compression (foreshortening). This is indicated by the association of early microquartz with the damaged areas (Figures4 and6), with the microquartz (and silt sized detrital particles) being pushed into calcite cement and shell during the dissolution process.

4.2.5. Overview of Test Damage None of the above hypotheses unequivocally explain the damage patterns observed. However, localised dissolution due to basin foreshortening appears to be the most likely explanation. Lithologically, mudstones such as the Eagle Ford Formation come from similar deep-water settings with high numbers of foraminifera and faecal lenses. However, there appears to be no mention of test damage within the literature, and none have been personally observed by the authors from representative examined materials. This is perhaps surprising, as the Eagle Ford has also undergone basin compression during the Larimide Orogeny [37]. Minerals 2020, 10, 550 15 of 17

5. Conclusions Foraminifera host a range of authigenic minerals (calcite, pyrite, sphalerite and baryte) which hold clues to the composition and timing on the migration of ﬂuids through mudrocks. The example illustrated in this study demonstrates the potentially complex geochemical and authigenic history of a foraminiferal-rich mudstone, during deposition, burial, and uplift, that can be hidden away within such foraminifera (Figure 12), and may radically diﬀer from that observed within shale matrix materials. This is in agreement with previous studies [10]. It also illustrates how apparently simple forms of foraminifera test damage can be interpreted in a range of ways, from ecological, environmental, taphonomic scenarios, to evidence of structural foreshortening. Although much can often be gleaned from the examination of such materials, it is not always an easy task to separate out the exact meaning and timing of all events that have taken place. Knowledge of such hidden pockets of information may provide additional details on burial and diagenesis, while ignorance may impact on whole rock-derived geochemical analysis of environmental change indices, particularly in respect to both Ba and Zn. Further work on materials within the Eastern Cordillera, and other environmental-time equivalents (e.g., Eagle Ford Formation), and mudstones in general, may prove informative, with the potential to obtain big things out of small.

Author Contributions: Conceptualisation, J.B.; methodology, J.B. and C.M. (Carol Mahoney); formal analysis, J.B.; investigation, J.B.; resources, J.B.; writing—original draft preparation, J.B.; writing—review and editing, J.B., T.W. and C.M. (Christian März); supervision, J.B., T.W., and C.M. (Christian März). All authors have read and agreed to the published version of the manuscript. Funding: This work carries on work initiated from the Ph.D. project of C.M. (Carol Mahoney), which was financially supported by Ecopetrol S.A. Acknowledgments: The Centre for Environmental Scanning Electron Microscopy (CESEM) is thanked for access and use of scanning electron microscopy and computer facilities. Ecopetrol S.A. is acknowledged for access to sample materials and funding. We specifically thank Vladimir Blanco from Ecopetrol S.A. for his research leadership and continued support of C.M. (Carol Mahoney) Ph.D. research studies. Conflicts of Interest: The authors declare no conflicts of interest.

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