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Abstract

Inorganic whole rock geochemical and magnetic susceptibility (χ) data have been gathered from seven sections through the Upper Miocene reef complexes of the Llucmajor Platform, Mallorca. The aim of acquiring these data is to determine what chronostratigraphic information a combination of magnetic susceptibility stratigraphy (MSS) and chemostratigraphy can provide in reef complexes of a carbonate platform with no nearby source of terrigenous material.

Chi values display short-term cyclical fluctuations, maximum values commonly being associated with the upper bounding surfaces of sigmoids and sigmoid sets, the building blocks of the Llucmajor reef complexes. Values of χ are low at the bases of the sigmoids; they reach a maximum at the top of the sigmoids, and then fall rapidly to the base of the next sigmoid, indicating a base-level control on χ values. No longer term variation in the χ values have been identified that would enable differentiation of an old reef complex from one that is demonstrably younger in the cliff sections, and there is no apparent facies control on χ values. While the elemental data do not vary in response to base-level fluctuations and therefore do not define stratigraphic surfaces, they do show long-term variations, enabling reef complexes of differing ages to be geochemically characterized: the key to defining chemostratigraphic schemes. The elements used to characterize different age reef complexes are Cr, Zr, Al2O3, TiO2, Ga, and Rb, which are controlled by changes in the amount and composition of wind-blown terrigenous material and tuffaceous material. As such, the changes are chronostratigraphic events on the scale of the study intervals here. Although the two datasets used are both ultimately controlled by the mineral composition of the sediments, it has proved impossible to find a direct link between the elemental data and χ values.

Despite the lack of an understanding of the relationship between whole rock geochemistry and χ values for this dataset, they clearly provide a means to (1) define stratal surfaces that relate to base-level fluctuation and (2) recognize reef complexes deposited at different times. Therefore, the combined approach adopted here for the Llucmajor reef complex has the potential to provide cross-facies chronostratigraphic correlations in detached platform carbonate settings, and since both methods can be applied to core and cuttings samples, the approach could work in subsurface settings where samples would be from well bores.

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