Magnetic stratigraphy of Kimmeridgian-Aptian sections from Umbria-Marche (Italy): New details on the M polarity sequence

Abstract

The magnetic stratigraphy of the Maiolica and Calcari ad Aptici pelagic limestones is documented from four Umbro-Marchean land sections (Italy). The overall biostratigraphic age of the sampled strata encompasses the whole Kimmeridgian–Lower Aptian. The data were gathered from two new sections (Arcevia, Contessa) and by resampling more densely than in the past two additional classical sections (Bosso, Gorgo a Cerbara). A detailed record of the polarity chrons M21n-M14 and M9-M0 was derived, and no difference was observed with respect to the sequence of M polarity chrons inferred from oceanic magnetic anomalies. Barely measurable remanent magnetization from chron M14 to M10n at Contessa concurred with previous evidence from Bosso, indicating that the Maiolica from Umbria-Marche is characterized by a "recording gap" during chrons M14n to M11, possibly due to diagenetic magnetite dissolution related to the Late Valanginian carbon isotope event. At Arcevia and Bosso, the boundary between Maiolica and Calcari ad Aptici occurs at the top of chron M19n and in the upper part of chron M20n, respectively, showing that the onset of Maiolica sedimentation may be diachronous by 1.5–2 Myr along the Umbria-Marche domain. Arcevia is the most expanded Mid-Upper Tithonian land section documented so far. Here, the subchrons M20n-1 and M19n-1 are found to represent 3.2% and 10.1% in time of chrons M20n and M19n, respectively. Evidence is also provided for a reversal excursion occurring within the M20n-1 subchron. At Bosso, an excursion and a reversal excursion were found within chrons M16n and M16, respectively.