Paleosecular variations recorded by Holocene sediments from a maar lake (La Hoya San Nicolas) in Guanajuato, Mexico

MARCOS A.E. CHAPARRO; HARALD N. BÖHNEL; ROBERTO S. MOLINA GARZA; ROGER BYRNE

Puerto Vallarta, Jalisco, Mexico

Congreso; Reunión Anual 2007 – Unión Geofísica Mexicana (UGM); 2007

Unión Geofísica Mexicana (UGM)

In this work we report new Paleomagnetic data from a maar lake in central Mexico, La Hoya de San Nicolas, which was previously sampled and studied for rock-magnetic, palynological and geochemical analyses. The Paleomagnetic investigation was carried out on three sediment cores (SN-4, SN-5 and SN-6) of around 5 m in length. Although other available data have been obtained from different archives in Mexico, e.g. volcanic rocks, arqueomagnetic material and cave deposits, there is a need of new data from continuous records like as lake sediments in order to describe in detail the Holocene geomagnetic Paleosecular variation (PSV) in this region. Rock-magnetic results –from studies of magnetic susceptibility, hysteresis cycles, anhysteric and isothermal remanent magnetization– indicate that the remanent magnetization of these sediments is controlled by pseudo-single domain (titano)magnetite, which are suitable recorders of the geomagnetic field. The age-depth model implies average deposition rates of about 0.5 mm/yr and a basal age of about 10,600 cal. years BP. Magnetic susceptibility and natural remanent magnetization logs were used to correlate the Paleomagnetic (i.e. declination and inclination) data of all sediment cores. As result of applying a stretching and stacking process, as well as the age-depth conversion, a composite PSV record was obtained. In this PSV curve, declination and inclination results show an oscillatory behavior varying in a narrow range, and an interesting swing is observed around the basal ages. Furthermore, a comparison between these results and independent data from lava flow is done.