Rock magnetic and dielectric characterizations of a formational contact inCretaceous strata (Eastern Venezuela)

Rock magnetic and dielectric studies have been conducted in sedimentary roc ks close to a lithological contact (Chimana and Querecual formations) at th e Pertigalete sedimentary sequence (Cretaceous northeastern Venezuela). Roc k magnetic results suggest that the presence of secondary magnetic minerals has been conditioned by the transitional zone between Chimana and Querecua l. In fact, this contact might have acted as a conduit for fluid circulatio n and a focus of chemical alteration on most of its adjacent strata. Some o f the secondary magnetic minerals (e.g. magnetite) must be the outcome of t he anoxic conditions that prevailed during times when the sedimentary basin reached its maximum temperatures when oil and gas were generated (Lower Mi ocene). Some others (e.g. hematite) are the likely byproduct of alteration dating from times when oxidizing conditions were dominant. The Thermally St imulated Depolarization Current (TSDC) technique is used here to dielectric ally characterize the same contact. The complex TSDC spectra obtained in th e temperature range from 77 to 230 K for the samples studied, indicate a no table difference between the Chimana (CH) and Querecual (QC) samples. Dryin g and rehydration treatments performed on these samples show that the whole depolarization spectrum is mainly due to moisture trapped in the microporo sity of the material. The broad spectra, separated by using Direct Signal A nalysis, reveal the presence of four overlapping peaks. TSDC results indica te a progressive change in the mean re-orientation energy with the litholog y for the four relaxations. This variation, due to the change in the enviro nment seen by the water dipoles hydrogen-bonded to different sites in the m aterial, is observed for the same lithologies where magnetic anomalies have been reported. Thus, rock magnetic analyses, combined with the TSDC techni que, seem to serve as a sensitive and alternative means of characterizing l ithologies and stratigraphic discontinuities in a sedimentary sequence. (C) 1999 Elsevier Science Ltd. All rights reserved.