Understanding palaeoenvironmental constraints on glaucony formation – insights from Late Cretaceous greensand giants
Final Report Abstract
Massive and geologically apparently very rapid shallow-marine glaucony formation was a widespread phenomenon in the Cretaceous greenhouse world that is not matched by the situation today where glauconitic minerals predominantly form rather slowly in cooler and deeper marine settings. This “glaucony paradoxon“ is explained by the different climatic and weathering regimes prevailing during the Cretaceous compared to the Recent: today, concentrations of Si, Fe, Al and K in normal sea-water are commonly too low for direct precipitation of glaucony, even if mean sea-water pH (8.1–8.3) is in a favorable range. Apparently, their supply from the continents to the oceans is today below a critical threshold and only in certain zones of upwelling of element-enriched deep waters, the necessary concentrations are reached, setting the current “deep-, cool- and slow-mode” of glaucony formation on the upper slope and outer shelf. In contrast, the widespread nearshore glaucony formation during Cretaceous times is basically related to the hydrochemical properties of the riverine flux from the low-lying, deeply weathered continents that changed the coastal geochemical environments into glaucony-prone ones. The closest Recent analogue for Cretaceous fluvial systems can be found in today’s low-lying warm-tropical areas with high precipitation rates and deep chemical erosion, such as the Amazon Basin, characterized by tropical streams with low pH (≤ 7) in which Fe, Si and Al contents may be considerably elevated, reflecting a strongly leached tropical environment from which most soluble elements were quickly removed. Glaucony formation in the fluvial systems is prevented by the low pH and the commonly too low concentration of potassium. However, for the latter crucial element, the importance of plant decay as a major factor in river-dissolved potassium must be considered. Even today, potassium released from dead vegetation may be more important than the leaching of K from silicate minerals. Consequently, during the wet Cretaceous with its ubiquitous swamps, bogs and other wetlands, this process should have been a major factor for the availability of potassium in river waters that, finally, also entered the nearshore zone with its suitable pH for glaucony authigenesis. Furthermore, the substantial input of (terrestrial) organic matter favored slightly reducing conditions of pore waters. Our conclusions are in agreement with experimental formation of glauconite that required, beside availability of Fe and Al, sufficiently high concentrations of silica and potassium at sea-water pH under slightly reducing conditions; time, depth and temperature of the waters had no or only minor influence but the generally higher temperatures during the Late Cretaceous led to more rapid and better crystallization. Thus, the results of this research project reveal certain limitations of the uniformitarian approach as the Recent is not always the key to the past.
Publications
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Depositional setting and limiting factors of early Late Cretaceous glaucony formation: implications from Cenomanian glauconitic strata (Elbtal Group, Germany). Facies, 67(3).
Wilmsen, Markus & Bansal, Udita
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Intense chemical weathering of the continents: a key for widespread shallow-marine glaucony formation during the Late Cretaceous. ‒ In: John W.M. Jagt, Elena Jagt-Yazykova, Ireneusz Walaszczyk & Anna Żylińska (eds), 11th International Cretaceous Symposium, Warsaw, Poland, August 22nd – 26th: 373–374; Faculty of Geology, University of Warsaw. [Talk]
Wilmsen, M., Bansal, U. & Böning, P.
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Chemostratigraphy of the lower Danubian Cretaceous Group (Cenomanian–lower Turonian, Bavaria, SE Germany)—A new carbon isotope reference curve and inter-basinal correlation. Cretaceous Research, 149, 105568.
Metzner, Niklas; Niebuhr, Birgit; Pürner, Thomas & Wilmsen, Markus
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Enhanced chemical weathering of the continents promoted fast Late Cretaceous nearshore glaucony formation: implications from the Danubian Cretaceous Group, Germany. Journal of the Geological Society, 180(2).
Bansal, Udita; Böning, Philipp & Wilmsen, Markus
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Integrated stratigraphy of the lower Danubian Cretaceous Group (lower Upper Cretaceous, southern Germany). – GeoBerlin2023, Abstracts
Metzner, N., Niebuhr, B., Pürner, T. & Wilmsen, M.
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Neue Erkenntnisse zur Glaukonitbildung oder: Ist die Gegenwart immer der Schlüssel für die erdgeschichtliche Vergangenheit? – 253. Dresdner Geowissenschaftliches Kolloquium, 18.04.2023; Dresden
Wilmsen, M.
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Geochemical and depositional environment of an Upper Cretaceous greensand giant (Münsterland Cretaceous Basin, Germany). Chemical Geology, 661, 122168.
Wilmsen, Markus; Bansal, Udita; Metzner, Niklas & Böning, Philipp
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Glaucony formation during warm phases of Earth history: new insight from Upper Cretaceous greensand giants. – GeoSaxonia2024, Abstracts. [Talk]
Wilmsen, M., Metzner, N., Bansal, U. & Böning, P.
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Greensand formation, siliceous earth deposition and coastal metal drawdown in the Danubian Cretaceous Basin (Bavaria, Germany). Marine and Petroleum Geology, 170, 107067.
Metzner, Niklas; Wilmsen, Markus & Böning, Philipp
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Greensands and the unique Neuburger Siliceous Earth: Late Cretaceous continent–shelf interactions in the Danubian Cretaceous Basin (Bavaria, SE-Germany) revealed. – 3rd Early Career Sedimentologists Meeting (ECSM), 08.–09.06.2024; Darmstadt. [Talk]
Metzner, N., Wilmsen, M. & Böning, P.
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Greensands and the unique Neuburger Siliceous Earth: Late Cretaceous continent–shelf interactions in the Danubian Cretaceous Basin revealed (Bavaria, SE-Germany). – GeoSaxonia2024, Abstracts. [Talk]
Metzner, N., Wilmsen, M. & Böning, P.
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Integrated stratigraphy and facies development of the lower Danubian Cretaceous Group (Bavaria, SE-Germany): dissecting Cenomanian–middle Turonian onlap patterns onto the Bohemian Massif. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 176(1), 67-94.
Metzner, Niklas; Niebuhr, Birgit; Pürner, Thomas; Švábenická, Lilian & Wilmsen, Markus
