HR: 12:05h
AN: GP42A-08    [Abstracts]
TI: Short Polarity Albian Events in the Cretaceous Normal Superchron - Magnetostratigraphy of the Morelos and Tlayua Formations (Mexico)
AU: Benammi, M
EM: mouloud@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Laboratorio de Paleomagnetismo y Paleoambientes, Instituto de Geofisica, UNAM, Mexico, 04510, Mexico
AU: * Urrutia-Fucugauchi, J
EM: juf@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Laboratorio de Paleomagnetismo y Paleoambientes, Instituto de Geofisica, UNAM, Mexico, 04510, Mexico
AB: Interest in the long period of normal polarity in the Cretaceous has continued to increase since the early study of Helsley and Steiner (1969), who suspected that it corresponded to vast stretches of relatively low amplitude behaviour in the sea-floor magnetic anomaly profiles known informally as the "Cretaceous quiet zone". Since these first studies, paleomagnetists have continued to explore what is now known as the Cretaceous Normal Superchron, an interval of prolonged normal polarity from 118 to 84 Ma. Paleomagnetic data from sediment cores from the DSDP Holes 369, 386, and 364, were interpreted as brief reverse intervals (Keating and Helsley, 1978, 1979). Relatively fewer studies have been reported for land-based sections of pelagic and platform limestones; reports include studies in northern and central Italy (Vandenberg et al., 1978; Tarduno et al., 1992; Tarduno, 1990; Cronin et al., 2001) and Mexico (Urrutia-Fucugauchi, 1988; Benammi et al., 2006). We present here new magnetostratigraphic data for limestone sequences in central and southern Mexico from the Cretaceous Morelos and Tlayua Formations, which record short intervals of reversed polarity during the K normal superchron. The Morelos Formation has been revisited in Guerrero, southern Mexico, where the sequence of platform limestones with almost horizontal thick beds is well-exposed. The magnetostratigraphy shows dominant normal polarity for the sampled sections, with the occurrence of three short reverse polarity intervals corresponding to the Albian, which have been re-sampled and studied in additional detail for magnetic mineralogy and remanent acquisition. The Tlayua Formation is sampled in Tepeji de Rodriguez area, which is known for its rich and well-preserved fossil contents. Age control has mainly been provided from studies on ammonites, belemnites and benthic foraminifera (Cantu, 1987; Seibertz and Buitron, 1987; Kashiyama et al., 2003). Nine polarity intervals are documented in the Tlayua Middle Member, which correlate with middle part of chron C34n (C34n.1n-C34n.2n, 100-105 Ma). Results give further evidence for occurrence of short polarity events within the K normal superchron, and provide additional documentation on the polarity events.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting