Salty Matters

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Silica recycling and replaced evaporites - 4. Proterozoic atmospheric transitions and saline microporous chert reservoirs

John Warren - Friday, October 07, 2016

Sulphate and oxygenation levels across the Proterozoic

Proterozoic (2.5 – 0.542 Ga) saline sediments encompass significant transitions in evaporite style and chemistry within an evolving atmospheric and oceanic framework. Lithospheric changes tie to a cooling and biologically-evolving earth as earth-scale plate tectonics move to a system set comparable with that operating today. The Proterozoic eon is divided into three eras: the Paleoproterozoic, Mesoproterozoic and Neoproterozoic. Thick sequences of halite are only found as the actual bedded salts in sediments of the Neoproterozoic (and the Phanerozoic), while calcium sulphate residues and beds occur in all three Proterozoic eras, especially in parts of the Paleoproterozoic and the Neoproterozoic.

Paleoproterozoic era sediments (2.5-1.6 Ga) contain isotopic evidence the first significant oxygenation event in the world's atmosphere, largely driven by the increasing dominance of cyanobacterial photosynthesis. Neoproterozoic sediments (1.0 – 0.542Ga) contain evidence of the second oxygenation event, which is associated with the evolution of widespread multicellular life and CaCO3/siliceous carapaces. By the end of the Neoproterozoic, the world oceans had chemistries, temperatures and salinities similar to those of the Phanerozoic (Blamey et al., 2016). The intervening Mesoproterozoic (1.6- 1.0 Ga) retains evaporitic residues with aspects of both the late Archean and the Phanerozoic.

The oxygenation of Earth’s atmosphere-ocean system occurred in two steps: 1) the Paleoproterozoic “Great Oxygenation Event” (GOE ≈ 2.3 Ga), which refers to the transition from a pervasively reducing Earth-surface system to one with an oxygenated atmosphere and oxygenated shallow seas, and 2) the “Neoproterozoic Oxygenation Event” (NOE), when the Earth’s atmosphere and ocean are understood to have become persistently oxygenated down to the deep ocean bottom (Turner and Bekker, 2016; Scott et al., 2014). The GOE is indicated by a Proterozoic carbon isotope anomaly known as the “Lomagundi event,” a positive carbon isotope excursion between ca. 2.22 and 2.06 Ga, interpreted to be the result of high organic carbon burial and attendant accumulation of atmospheric oxygen (Figure 1; Bekker and Holland, 2012)). A long interval spanning the remainder of the Paleoproterozoic and much of the Mesoproterozoic followed the Lomagundi event, typified by lower levels of atmospheric oxygen and little variation in carbon isotope values. This ended in the late Neoproterozoic with dramatic fluctuations, of escalating magnitude, in the biogeochemical carbon cycle and attendant fluctuations within an overall increasing oxygen content (Figure 2). By the end of the Neoproterozoic, not just shallow shelf waters but much of the deep-ocean water column was consistently oxygenated (see article 1 in this series)


Chert evolution

 We have already seen how low levels of oxygen and high levels of CO2 in the Archean favoured the precipitation of nahcolite, and its hydrothermal silica association, atop subsealevel isolated saline sumps in microcontinents and island arcs in a saline waterworld (article, 28 August 2016). The hydrothermally-dominated silica of the silica-rich Archean oceans is reflected in more negative 30Si isotope values in widespread marine cherts of that time, compared with most Proterozoic cherts (Chakrabarti et al., 2012; see also Figure 3b). However, shorter-term fluctuating levels of atmospheric oxygen in the Proterozoic also influenced drop-out salinities for gypsum in Mesoproterozoic marine brines. In brines derived from the modern, well-oxygenated world oceans, as in evaporite successions deposited throughout the Phanerozoic, the sulphate minerals gypsum and anhydrite precipitated from evaporating seawater after aragonite or calcite, but before halite (see 26 August, 2015, blog for more detail). At lower seawater sulphate levels across the Archean and much of the Proterozoic, gypsum and anhydrite precipitated after halite, even at Na and Cl concentrations similar to those of the modern ocean.

This is why some post-Lomagundi, Paleoproterozoic marine evaporite successions show clear evidence of halite precipitation before gypsum or anhydrite or even an absence of gypsum or anhydrite with halite (e.g., ≈ 1.88 Ga Stark Formation; Pope and Grotzinger, 2003). The post-halite precipitation of calcium sulphate is construed as evidence for a limited marine sulphate reservoir and little atmospheric oxygen (Scott et al., 2014). In contrast, Lomagundi- age sedimentary successions contain evidence for sulphate precipitation before halite (Melezhik et al., 2005; Bekker et al., 2006; Schröder et al., 2008). Sulphur isotope values of marine sulphates (in CaSO4, barite) and sulphides in marine pyrite also record expansion and contraction of anoxic oceanic settings. That is, a higher burial rate of pyrite in anoxic settings is indicated by a positive shift in the sulphur isotope values of sulphates, whereas ocean oxygenation creates a negative shift in values (e.g., Claypool et al., 1980; Strauss, 1997). Furthermore, expansion of the area of anoxic oceanic settings decreases the size of the seawater sulphate reservoir, resulting in more variable sulphur isotope values of sulphate evaporites, barites, and other carbonate-associated sulphates (Figure 2; Kah et al., 2004). This applies in particular in the Mesoproterozoic when only tshallow oceanic waters were consistently oxygenated.

When we look at silica mobility and chert styles across a Proterozoic milieu of evolving oxygen and sulphate levels we see some aspects similar to the Phanerozoic and others more akin to the high-silica oceans of the Archean. Maliva et al. (2005) and Perry and 2014 show that the latter part of the Paleoproterozoic era (post-Lomagundi) is marked by the end of widespread primary and early diagenetic silica precipitation in normal marine subtidal environments. However, silica precipitation continued apace in the deeper marine in waters that were still anoxic. The Paleoproterozoic is defined by the “rusting” of the shallower parts (shelves and upper slopes) of the world’s ocea,n as dissolved oxygen levels increased and the accumulation of widespread Banded Iron Formations (BIFs) occurred, including the huge deposits of NW Australia.

So where and when do we see nodular cauliflower chert after sulphate in the Proterozoic?

Some of the oldest silicified nodular sulphates with cauliflower chert textures and actual relict anhydrite are found in the Huronian Gordon Lake Formation (≈2.4 Ga; Chandler, 1988). The nodules are commonest in mud chip breccia at the base of sandstone, siltstone and mudstone upward-fining storm cycles. Anhydrite nodule relicts are composed of a mosaic and meshworks of blocky crystal laths. Earlier laths are preserved in silicified outer rims of many cauliflower chert nodules, with texture alignments similar to that of Recent displacive sabkha anhydrite nodules. Completely silicified nodules are composed of megaquartz, some calcite-cored, or of jasper, with replacement textures identical those documented the Phanerozoic by Milliken (1979). Similarly-textured cauliflower cherts are found in the Mallapunyah Formation (1650 Ma) in the Paleoproertozoic sediments of the McArthur Basin in the Northern Territory of Australia (Warren, 1999). Thin sections through 5-30cm diameter Mallapunyah nodules (in a redbed host) can still retain small relict highly-birefringent laths of anhydrite, but most of the former felted cores to the nodules are now composed of mimetic silica. There are also older sedimentary chert nodules in the McArthur Basin succession located in units below the level of the Mallapunyah, but they are smooth walled not rugose-surfaced features.

Unfortunately, the term cauliflower chert is loosely defined and is used to describe cemented features in Meosproterozoic and earlier sediments and metasediments, which are not true calcium sulphate evaporite replacements.Although termed cauliflower features, they do not have surface textures resembling the florets of a cauliflower (see article 2 in this series - July 31, 2016). For example, aggregates and clusters of growth-aligned barite crystals in the Archean of South Africa are described as cauliflowers when they should be described as bladed, palmate crystal aggregates (Reimers and Heinrich, 1997). Interstingly, Chowns and Elkins (1974) in a study of cauliflower cherts occurrences across the USA list no examples older than Cambrian. Using a tighter definition of cauliflower chert and recognising that this term should not be interchangeable with crocodile-skin chert it seems that Proterozoic occurrences of cauliflower chert nodules largely mirror times when oxygen levels were sufficiently high in the world's ocean to allow sulphate in solution. In the Paleoproterozoic and Mesoproterozoic only the upper parts of the ocean column, including waters covering the world's continental shelves (and derived evaporite basins) were sufficiently oxygenated to allow the formation of cauliflower chert after nodular anhydrite. However in some Neoproterozoic basins, especially if located in sumps in a highly-restricted brine layered seafloor, the levels of anoxia in the ponded bottom brines facilitated the accumulation of laminar microporous chert in association with evaporites or their early replacements

Primary laminated hypersaline silica chert in an evaporite basin at the Precambrian-Cambrian boundary

An organic-rich laminated porous chert known as the Athel or Al Shamou silicilyte consists of up to 90% microcrystalline quartz along with dolomite, magnesite, anhydrite and halite (Rajaibi et al., 2015). It occurs at the Precambrian-Cambrian boundary in the subsurface of the South Oman Salt Basin, Sultanate of Oman, where it acts as a light-oil reservoir  (Ramseyer et al., 2013; Amthor et al., 2005). Fully encased in variably halokinetic salt masses, it was first discovered during the 1990's hydrocarbon exploration activities of Petroleum Development Oman. This laminated microporous and variably fractured chert, has its source of silica and its mode of precipitation tied to an anoxic, sulphur-rich, stagnant and highly saline basin. Its homogeneous silica distribution and high Si isotope values (avg. d30Si = +0.83 ± 0.28), coupled with a low molar Ge/Si ratio (<0.25 x 10-6) in its microcrystalline quartz matrix imply dissolved silica in concentrated seawater as the Si source, and hydrothermal or biogenic (e.g. sponge-derived) silica are excluded.

Silica precipitation from a seawater-sourced brine was likely the result of a dramatic increase in salinity in response to halokinetic salt dissolution atop and adjacent to the edges of transtensional depressions on a deep basin floor in the South Oman Salt basin, thus markedly reducing the solubility of amorphous silica in these brine-filled seawater depressions. This saturation triggered the formation of silica gel. The gel accumulated at the base of a brine-layer covered basin floor, forming a soft silica-rich layer bound into bacterial mats, giving rise to its fine-scale lamination. The mean number of laminae in this laminated chert is ca. 32 per year suggesting that layering is non-annual and controlled by processes such as fluctuations in nutrient supply, lunar driven re-mixing or diagenetic segregation. The transformation of the silica-gel to microcrystalline quartz occurred below 45°C indicating a less than -4.5‰ d18O composition of the pore-water during microcrystalline quartz formation. The  microporous hydrocarbon filled nature of this ancient chert and the fact the hydrocarbon-filled micropores are still distinct after more than 500 million years after they filled (Figure 4d, e) is  why when artificially fractured the silicilyte can act as a hydrocarbon reservoir (See Rajaibi et al. 2015 and Warren, 2016; Chapter 10 for a summary of relevant literature).


What does this mean for other evaporite-associated laminar cherts in the Phanerozoic?
Beds of laminated chert (not replaced, but precipitated, primary silica accumulations) are unusual across the Phanerozoic, but cauliflower cherts as replacement of calcium sulphate evaporites are not (Chowns and Elkins, 1974). Spanning the Precambrian boundary, the Athel Silicilyte is the first Phanerozoic example of a laminar chert-hypersaline association. Then, there is the somewhat younger but world-famous Devonian to early Carboniferous Caballos Novaculite. This chert location was the site of a world-famous set of arguments as its origin, between two world-renown professors, Dr Earle McBribe and Dr Robert Folk, both on the faculty of the University of Texas at that time. Folk argued for a shallow-water peritidal hypersaline depositional setting, McBride for a deep a marine setting but still with possible hypersaline indicator textures (Folk, 1973; Folk and Mcbride, 1976; McBride and Folk 1977).

The Caballos Novaculite outcrops in the Marathon Uplift of Texas, while its lithologic and time equivalent, the Arkansas Novaculite, outcrops in the Ouachita Mountains of Arkansas and Oklahoma (Figure 5). Novaculite chert) in outcrop is very resistant to erosion so that layers of novaculite stand out as characteristic ridges and dip slopes in the Ouachita and Marathon mountains (Figure 5). This outcrop forms and its hard abrasive nature gives it the name novaculite, which in its Latin root novacula, means razor-stone. When some novaculite is fractured in the subsurface, there is sufficient connected porosity to form a fractured reservoir play, as in Arkansas and Texas. There, some 30 years ago, oil and gas fields such as Isom Springs in Oklahoma and McKay Creek, Pinion and Thistle fields in West Texas were discovered in the Caballos and Arkansas novaculite-chert. The chert reservoir is most productive when it is highly fractured, occurs within complex thrust faults and has had any enclosed carbonate material leached from its chert matrix, so creating microporosity (Figure 6; Godo et al., 2011).

Chert beds in the Caballos Novaculite are composed of equant grains of microcrystalline quartz, minor amounts of illite and radiolaria, and trace amounts of pyrite, carbonate and other minerals and organic matter. The chert beds are generally interpreted as having formed by the silicification and alteration of precursor sediment, sometimes massive, other times finely laminated. Some beds retain occasional evidence biogenic silica derived from radiolaria, while underlying levels retain can contain abundant siliceous sponge spicules. Fractures and crackle breccias developed in a grey chert following lithification; green siliceous sediment, whose lithification was impeded by clay, filled these pre-orogenic fractures.
Beds of red shale, chert pebble and cobble conglomerate, sandstone, limestone, dolomite, and lumpy manganiferous and jasperitic chert make up no more than 3% of the chert and shale members of the Caballos, but still are of controversial origin and environmental significance. The chert conglomerate beds, for example, are interpreted as tidal-channel deposits by Folk and as mass-flow deposits by McBride. Jasper beds texture are considered bizarre by many geologists who have worked on them: they are lumpy, uneven beds 0.2 to 2 m thick composed of cherry-red chert with local geopetal cavities, contorted laminae. manganiferous zones, cauliflowerlike quartz-filled nodular cavities sometimes with hollow centres and variably filled or partially filled with zebraic chalcedony, lutecite, quartzine, pseudocubic quartz crystals, and filamentous structures resembling algae. These beds are interpreted by Folk as the product of diagenetic alteration of sabkha evaporite nodules and siliceous ooze, during and following subaerial exposure with soil development, and by McBride as the product of diagenetic alteration of evaporite beds deposited in deep water and sandwiched between radiolarian ooze. Synthesis of evidence on the origin of both the novaculite and chert and shale members leads to contrasting interpretations of water depth during deposition. However, if the evaporite solution breccias, recognised as such by both authors and the cauliflower cherts as replaced diagenetic anhydrite clusters then the depositional setting is akin to that of the Athel Silicilyte (namely a deepwater holokinetic hypersaline evaporite setting not unlike a siliceous DHAL association (see Warren, 2016; Chapter 9, for discussion of the DHAL literature).
Economic implications of understanding what defines a silicilyte versus novaculite versus tripolite versus diatomaceous ooze
We have already seen that microporous cherts when fractured are possible reservoir rocks as illustrated by the saline-associated Athel Silicilyte and the Caballos Novaculite. The former retains its microporosity because of an early hydrocarbon charge into existing microporosity (Rajaibi et al. 2013; Amthor et al., 2005), while the reservoir quality of the latter was enhanced by diagenetic leaching of finely dispersed carbonate material, likely when it was caught up in the Ouachita Orogeny (Figure 6; Godo et al., 2011). Like the Athel, the Arkansas (Caballos) Novaculite is thought to be self-sourcing in terms of reservoir hydrocarbons (Zemmels et al., 1985).
The term silicilyte is defined by Rajaibi et al. (2013) as a "locally-used" term to describe porous organic-rich laminated chert, it is a succession of microcrystalline quartz that is preserved within salt-encased slabs, 300 to 400 m thick, at a depth of 4 to 5 km in the South Oman Salt Basin. As mentioned earlier, the term novaculite comes from its outcrop expression and its "razorstone" properties and does not necessarily have a direct connection to microporosity in the reservoir portions of the unit (Figure 5). Outcrops of weathered microporous chert zones in the upper part of the Arkansas (Caballos) Novaculite are called tripoli or rottenstone (Figure 6). When present in this finely powdered microporous form, it is quarried and crushed for use as a polishing abrasive in metalsmithing and woodworking. When present as a very hard dense rock, it can be cut and shaped for use as a whetstone or razorstone. Before European settlement, novaculite was a source for numerous arrow-tips, spear tips and knives. 
Etymological variations of the terms tripoli, novaculite and silicilyte as forms of chert, as currently used in the geological literature are interesting and geologically confused. This is particularly true if the writer did not 1) understand there are various origins to laminar sometimes microporous cherts, and  2) that there various possible silica sources and precipitation/replacement mechanisms can be halotolerant bacteria, or other silica sources that can be tied to marine sponges and yet others to radiolaria and diatomaceous oozes. Hence, there is the time-related aspect of biogenic chert evolution tying back to the source of the silica in some cherts and the presence or lack of salinity indicators in a laminar chert and chert nodules (e.g., cauliflower versus crocodile-skin cherts).
A diatomaceous ooze is a form of opaline silica made up of accumulations of siliceous frustules of diatoms in normal marine pelagic sediments and when it retains microporosity is sometimes called tripolite or tripolitic earth. Diatomaceous oozes, the precursor to this type of tripolitic earth is often laminated, but is harsh to the feel and scratches glass. To add to the confusion there are microporous mesohaline diatomaceous oozes called the Tripoli unit in many Messinian subbasins.
Tripoli or tripolite powder is also the term used to describe the form of microporous laminar chert used as an abrasive and collected from weathered zones of the Caballos and Arkansas Novaculites. The Palaeozoic age of the Caballos and Arkansas Novaculite means it cannot contain diatoms. Diatoms evolved in the Cretaceous and have been the dominant source of remobilised silica in marine chert nodules ever since. Palaeozoic silica remobilized into chert is often related to nearby sponge spicule horizons, or less often to radiolarian beds. The further back in time, and perhaps the more saline the marine bottom water, the greater the likelihood of a microbial association with chert precipitation and remobilisation.
Even in Tertiary strata some microporous diatomaceous earths can have a normal-marine, organic-enriched depositional association and can constitute a fractured microporous hydrocarbon reservoir. This is the case with the Miocene-age fractured microporous chert reservoirs that produce today in the Santa Barbara Basin of offshore California (Reid and McIntyre, 2001). Cores and coastal outcrops of the Monterey Formation show this type of marine-deepwater diatomaceous ooze is interlayered with microbial (bacterial-archeal) methanogenic organic-rich dolomites. Then, there are the deepwater diatomaceous saline oozes in the Miocene units that immediately underly Messinian evaporites in the Lorca and other sub-basins across the Mediterranean (Rouchy et al., 1998). These diatomaceous organic-rich oozes were deposited as pelagic sediments in saline waters on stratified bottoms that herald the creation of saline bottom water layers related to the onset of hypersaline conditions. Their depositional setting is in restricted basins with increasingly saline bottoms, driven by tectonic isolation and drawdown that soon after precipitated the salts of the Messinian Salinity Crisis. Finally, there are the lacustrine diatomaceous oozes accumulating at the base of density-stratified water columns in lakes of African Rift Valley. This type of laminar ooze occurred in the deeper parts of the lake floor and in Lake Magadi and Lake Natron define units that immediately predate significant lake drawdown episodes that are defined by the type 1 (sodium bicarbonate) evaporite layers. These laminar diatomaceous chert beds contain nodules with the characteristic surface shrinkage textures of crocodile-skin chert (see article 1 in this series of four).
So, in terms of silica with an evaporite association, "one size does not fit all.' There are multiple saline settings, both depositional and diagenetic, with silica sources evolving with life across the Proterozoic into the Phanerozoic. Interactions between biology and brine chemistry control the accumulation of silica in various forms in a range of evaporite settings, ranging across the marine to deep halokinetic seafloors to lacustrine basins. Once we understand how to recognise cauliflower chert and its possible association with laminar saline cherts of the Proterozoic and the Phanerozoic, then particular chert styles can help to define the evolution of atmospheric oxygen and the saline versus non-saline origins of some organic-rich laminar biogenic microporous cherts.



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