Geochemistry and depositional environments of two black shale formations: the Baltoscandian Cambrian-Ordovician Alum Shale and the Cretaceous Lokpanta Oil Shale
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2024-05-30
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Abstract
Mustad kildad on kogu maailmas laialt levinud settekivimid, pakkudes väärtuslikke teadmisi Maa geoloogilisest ajaloost. Nende teket mõjutavad erinevad keskkonnategurid, settimise kiirus ja merevee koostis. Tuntud mustade kildade näited on Paleosoikumi Alum-kilt Põhja-Euroopas ja Kriidiajastu Lokpanta põlevkivi Nigeerias. Mõlemad kildad sisaldavad suures koguses orgaanilist ainet ja kasulikke elemente, mistõttu on need olnud nii metallide kui süsivesinike uurimiseks atraktiivsed. Selles töös uuriti võrdlevalt Alum-kilta ja Lokpanta põlevkivi, et mõista paremini nende kivimite geokeemiat ja settekeskkonda ning metallidega rikastumise mehhanisme.
Mineraalse koostise uuring näitas, et Alum-kilt on rikas kvartsi, K-päevakivi ja K-vilgu/illiidi mineraalide poolest, mis moodustavad kuni 95% selle koostisest, samas kui Lokpanta põlevkivi K-päevakivi sisaldused on madalamad, kuid karbonaadid (kaltsiit ja dolomiit) moodustavad ligi poole selle koostisest. Mõlemad kildad on rikastatud uraani (U), molübdeeni (Mo) ja vanaadiumi (V) poolest. Eesti Alum-kilt (graptoliit-argilliit) on nende metallidega rikastatud rohkem kui Lokpanta põlevkivi, eriti kihi alumises osas. Seda erinevust võis mõjutada orgaanikarikka muda aeglasem settimise kiirus vähese või ilma hapnikuta merekeskkonnas. Lokpanta põlevkivi tekkis samuti hapnikuvaestes meretingimustes, kuid settimiskiirus oli oluliselt suurem kui Alum-kildas. See näitab et settimiskiirus mõjutab erinevusi U, Mo ja V rikastumisel mustades kiltades.
Suurem osa mõlemat kilta moodustavast purdmaterjalist erodeeriti ja kanti sisse keskmise kuni happelise koostisega tard- ja moondekivimitest ja/või vanematest setetest ning settekuhjumine toimus mandrite passiivsel äärealal. Keemilise murenemise astme näitajad viitavad sellele, et settinud purdmaterjal Alum-kildas ja Lokpanta põlevkivis on olnud mõjutatud soojale ja niiskele kiimale iseloomulikust intensiivsest murenemisest.
Black shales are widespread globally, offering valuable insights into Earth's geological history. Various environmental factors, sedimentation rate and seawater composition, influence their formation. Notable examples of black shales include the Paleozoic Alum Shale in Northern Europe and the Cretaceous Lokpanta oil shale in Nigeria. Both shales contain high levels of organic matter and useful elements, making them attractive for metal and hydrocarbon exploration. In this work, the Alum Shale and Lokpanta oil shale were studied to understand better their geochemistry and depositional environment and to model the element distribution and accumulation mechanisms. A study of mineral composition shows that the Alum Shale is rich in quartz, K-feldspar, and K-mica/illite minerals, making up to 95% of its composition, whereas the Lokpanta oil shale has lower K-feldspar but abundant in carbonates (calcite and dolomite) which comprise half of its composition. Both shales are enriched in uranium (U), molybdenum (Mo), and vanadium (V). The Alum Shale (graptolite argillite) in Estonia shows higher enrichment in these metals than the Lokpanta shale, particularly in the basal part of the bed. This variation may be influenced by the slow sedimentation rate of the organic-rich mud during its deposition in the marine environment with little or no oxygen. While the Lokpanta oil shale was also deposited under oxygen-depleted conditions, the sedimentation rate was much higher than in the Alum shale. This difference may have contributed to the enrichment of U, Mo and V in both shales. The bulk of the materials that make up both shales were eroded from rocks of intermediate to felsic composition and/or older sediments and deposited on a passive margin setting. Indicators of the degree of chemical weathering suggest that these materials in Alum shale and the Lokpanta oil shale have experienced intense and moderate–intense weathering, characteristic of warm and humid paleoclimate.
Black shales are widespread globally, offering valuable insights into Earth's geological history. Various environmental factors, sedimentation rate and seawater composition, influence their formation. Notable examples of black shales include the Paleozoic Alum Shale in Northern Europe and the Cretaceous Lokpanta oil shale in Nigeria. Both shales contain high levels of organic matter and useful elements, making them attractive for metal and hydrocarbon exploration. In this work, the Alum Shale and Lokpanta oil shale were studied to understand better their geochemistry and depositional environment and to model the element distribution and accumulation mechanisms. A study of mineral composition shows that the Alum Shale is rich in quartz, K-feldspar, and K-mica/illite minerals, making up to 95% of its composition, whereas the Lokpanta oil shale has lower K-feldspar but abundant in carbonates (calcite and dolomite) which comprise half of its composition. Both shales are enriched in uranium (U), molybdenum (Mo), and vanadium (V). The Alum Shale (graptolite argillite) in Estonia shows higher enrichment in these metals than the Lokpanta shale, particularly in the basal part of the bed. This variation may be influenced by the slow sedimentation rate of the organic-rich mud during its deposition in the marine environment with little or no oxygen. While the Lokpanta oil shale was also deposited under oxygen-depleted conditions, the sedimentation rate was much higher than in the Alum shale. This difference may have contributed to the enrichment of U, Mo and V in both shales. The bulk of the materials that make up both shales were eroded from rocks of intermediate to felsic composition and/or older sediments and deposited on a passive margin setting. Indicators of the degree of chemical weathering suggest that these materials in Alum shale and the Lokpanta oil shale have experienced intense and moderate–intense weathering, characteristic of warm and humid paleoclimate.
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