Reversed polarity characteristic magnetizations in the Albian Contessa Section, Umbrian Apennines, Italy: Implications for the existence of a Mid‐Cretaceous mixed polarity interval

Abstract

Paleomagnetic and paleontologic data from the Valle della Contessa (Umbrian Apennines, Italy) span a 14 m.y. gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization (ChRM) in limestones and marls of the Albian Marne a Fucoidi. Rock magnetic data indicate that the reversed ChRM is carried by hematite having high unblocking temperatures (∼ 625–690° C). Hematite with these unblocking temperatures also carries a normal polarity ChRM in other intervals. A potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component, carried by magnetite and hematite, is isolated at lower unblocking temperatures (∼300–600°C). Bulk magnetic properties vary throughout the measured section. In the portion containing most of the reversed ChRM intervals, hematite is the dominant carrier of natural remanent magnetization, while magnetite is the dominant carrier stratigraphically above and below. These changes correspond approximately with lithology. The zone of dominant hematite magnetization is marked by reddish bands (pigmentary hematite) which can be correlated throughout Umbria and are thought to record changes in the oxidation state of the seafloor during deposition. Two models may account for the observed magnetization directions. The reversed magnetizations may record a remagnetization which occurred at least 18 m.y. after deposition, while the beds were still flat‐lying, in reversed polarity chron 33R (83–79 Ma) or later. If the reversed ChRM represents a remagnetization, the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals. Alternatively, the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid‐Albian (107–104 Ma) and may record unrecognized intervals of reversed geomagnetic field polarity. Milankovitch‐like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals. Using these estimates, two of the potential reversed polarity intervals are of sufficient duration (> 100 kyr) to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys. A primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the Biticinella breggiensis and Ticinella primula foraminiferal zones and within the Prediscosphaera cretacea nannofossil zone on a worldwide basis.