Paleocurrent analysis for the Late Pleistocene–Holocene incised-valley fill of the Yangtze delta, China by using anisotropy of magnetic susceptibility data

Abstract

Anisotropy of magnetic susceptibility (AMS) analysis has been conducted on samples from borehole core CM-97 from the Yangtze River (Changjiang) incised-valley fill, China, to determine the paleocurrent directions to help in reconstructing sedimentary paleoenvironments. Borehole CM-97 consists of fluvial (Unit 1), estuarine (Units 2–6), and deltaic (Units 7–9) sediments after the Last Glacial Maximum in ascending order. The AMS results show that the paleocurrent directions for the tide-dominated estuarine and deltaic sediments were westerly or northwesterly directed (flood-tide dominated), but give no definite trend for the fluvial sediments. Comparison between the paleocurrent directions inferred from primary sedimentary structures and in situ AMS data shows that they are in good agreement, confirming the applicability of AMS as a good paleocurrent indicator for sediments deposited in coastal tide-dominated environments. Considering that these sediments were strongly tide-influenced and the tidal pattern since 12 kyr bp has not changed significantly, we think that the westerly or northwesterly current direction most probably resulted from flood-tidal currents, and the sedimentary paleoenvironment was a flood-tide dominated estuary or delta. Additionally, it has been found that downhole changes of some AMS parameters, including the mean magnetic susceptibility ( K), the corrected anisotropy degree ( P j) and the magnetic foliation ( F), clearly mark the defined stratigraphic boundaries in the borehole. This further extends the validity of AMS as a good stratigraphic marker in addition to its long known credibility as a sensible paleocurrent recorder.