The occurrence of glaucony in the stratigraphic record: distribution patterns and sequence-stratigraphic significance

none1noGlaucony is traditionally regarded as an indicator of low sedimentation rates and represents one of the most reliable stratigraphic markers within marine sediments. An integrated sedimentological, mineralogical and geochemical characterization of glaucony from 39 sites in Western Europe provides a comprehensive framework for the definition of its sequence-stratigraphic significance. Severe restrictions to the practical use of glaucony in sequence stratigraphy are provided by a number of factors. These include: i) correct mineral characterization, ii) sequence-stratigraphic interpretation of the host deposits, iii) distinction between autochthonous and allochthonous glaucony, iv) identification of a hierarchy of glaucony-bearing depositional units. A remarkable amount of data from about 250 glaucony-bearing horizons documents that spatial distribution of autochthonous glaucony follows predictable trends in abundance and maturity, showing a positive correlation between potassium content and the proportion of authigenic glaucony. This can enable differentiation of three basic types of condensed horizons: i) simple omission surfaces, containing less than 20% of poorly-evolved glaucony, ii) condensed sections, including 20-50% of evolved glaucony, and iii) mega-condensed sections, with > 50% of highly-evolved glaucony. By contrast, glauconitic grains of allochthonous origin do not exhibit any peculiar trend in abundance and maturity. Parautochtonous (intrasequential) glaucony can be relatively abundant, whereas a detrital (extrasequential) origin for the green grains accounts for anomalously low concentrations of highly-evolved glaucony. In terms of sequence stratigraphy, simple omission surfaces may correspond to marine transgressive surfaces or any parasequence boundary within the depositional sequence. These surfaces span intervals of time in the order of 104 years and are expected to have low correlation potential. Condensed horizons represent basin-wide stratigraphic markers that may either bracket the TST/HST boundary (maximum flooding surface and the “condensed section” of the Exxon model) or comprise a significant portion of the TST (105 years). Mega-condensed sections imply huge stratigraphic condensation, spanning from systems tracts to third-order depositional sequences with episodicities of 106 years. Mega-condensed sections are likely to be correlative on a global scale.noneAmorosi A.Amorosi A