Preliminary magnetic study of the quaternary red-soil bed on Linkou Terrace, northern Taiwan

This study presents the preliminary magnetic results from analyses of the Q uaternary red-soils and the fine grain sediments within the underlying cong lomerate bed of the Linkou Terrace in northern Taiwan. Magnetic susceptibility measurements indicate that the fine grain sediments taken from the conglomerate bed have extremely low susceptibilities relati ve to those of the red-soils. This phenomenon suggests that the source of t he red-soil probably did not originate from the weathering of the conglomer ate bed. From both paleomagnetic and rock magnetic results, it is thought t hat the red-soil bed was deposited during the period between the Jaramillo normal event and the Brunhes normal epoch, or later. Stratigraphic variations of magnetic susceptibility of the red-soil samples before and after CBD (citrate-bicarbonate-dithionite) treatment show the s ame trend: the lower part of the red-soil bed has values about two times hi gher than those of the upper part. In addition, susceptibility of the upper part after CBD treatment has been reduced by more than 60%, but that of th e lower part only has been reduced by less than 40%. It is known that CBD t reatment can resolve hematite, goethite and ultra fine-grained magnetite. S o, the results of this study might suggest that the upper part has much mor e soil development and lateritization than the lower part. Furthermore, the major (and possibly the original) magnetic mineral of the red-soil is magn etite. In the area neighboring the Linkou Terrace to the north, there was a lot of volcanic activity during the early Quaternary. Magnetite has been identifi ed as the major magnetic mineral of the volcanic rocks. Thus, it is propose d that the source of the red-soil bed in the Linkou area developed, at leas t partly, from the volcanic rocks. In addition, the boundary between the tw o groups, with distinct very different magnetic susceptibilities, might be used as an indicator for stratigraphic correlation in the study area.