Weathering Products of a Dismantled Variscan Basement. Minero-Chemical Proxies to Insight on Cretaceous Palaeogeography and Late Neogene Palaeoclimate of Sardinia (Italy)

Paola Mameli,Giovanni Mongelli,Rosa Sinisi,Giacomo Oggiano

doi:10.3389/feart.2020.00290

Paola Mameli, Giovanni Mongelli + Show 2 more

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https://doi.org/10.3389/feart.2020.00290

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Abstract

This study compares, for the first time, the mineralogy and geochemistry of two residual-clay deposits in NW Sardinia (Nurra district) that formed at different times in tropical and sub-tropical climates. Both deposits represent palaeosols with deep-weathered residual profiles and overlie Mesozoic carbonate rocks that were deposited on the south European palaeomargin. The older alterite is Cenomanian–Turonian in age and grades upward into a horizon of karstic bauxite, whereas the younger unit occurs within alluvial deposits of Late Neogene age. The Cretaceous palaeosol represents the precursor of the overlying bauxite and formed from unknown sedimentary parent rocks. In contrast, the Messinian weathering products formed by alluvium that was sourced from the Variscan metamorphic basement. Chemical Index of Alteration values, REE fractionation index values, and the results of R-mode factorial analysis suggest a common initial weathering path and a common precursor for the deposits. However, the latter stages of weathering of the Cretaceous palaeosols resulted in lateritic alteration and bauxite production, whereas weathering of the Late Neogene, palaeosols produced 2:1 clay minerals under less extreme conditions. Comparison of these residual products constrains the parental material and weathering trends and allows insight into the relationship between palaeoclimate and regional/local palaeogeography of southern Europe during Upper Cretaceous and during Messinian.

Highlights

Breakdown of rock at the Earth’s surface produces a thin porous covering that represents a section of the recently defined “Critical Zone” (National Research Council, 2001; Brantley et al, 2006), and according to Eggleton (2001), referred to as “regolith.” Typical regolith displays considerable vertical and lateral heterogeneity (Taylor and Eggleton, 2001; Anderson et al, 2007)
In detail we aim to provide further insights into: (1) the provenance of the Cretaceous alterite, which is the parental material of bauxite; (2) the palaeogeography of the south European margin during the Late Cretaceous; (3) the palaeoclimatic conditions in the western Mediterranean during the Messinian
Cretaceous palaeosols (CP) samples were collected in two vertical profiles (3 per each profile from bottom up to the boundary with bauxite) in a mined open pit, other 6 come from individual surficial outcrops scattered over a wide area, in order to get samples representative of different Mesozoic bedrocks

Summary

Introduction

Breakdown of rock at the Earth’s surface produces a thin porous covering that represents a section of the recently defined “Critical Zone” (National Research Council, 2001; Brantley et al, 2006), and according to Eggleton (2001), referred to as “regolith.” Typical regolith displays considerable vertical (distinguishable layers of weathered rocks) and lateral (variations in landscape and associated soils) heterogeneity (Taylor and Eggleton, 2001; Anderson et al, 2007). The low field strength elements (LFSE; e.g., Na, Ca, and Sr) have a high affinity for aqueous phases and are removed from exposed rocks in wet regions (Nesbitt et al, 1980; Nesbitt and Markovics, 1997). Other elements, such as K, Rb, Mg, and Ba, are removed from primary minerals but are often rapidly incorporated into secondary clay minerals and insoluble hydroxides within weathering profiles and residual deposits. Mineralogical and geochemical investigations of alterites (including palaeosols) can constrain both the palaeoclimate and the nature of the parent rocks

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