Magnetochronology of the Siwaliks: revision, uncertainties and new data

Abstract:

The Siwaliks sediments are shed from the uplifting Himalaya and stored in its foreland basin. They represent critical archive of climate changes in this region but more specifically may have recorded the flexural, unroofing and shortenning history of the Himalaya. As mainly continental in origine the Siwaliks are often poorly dated via classic biostratigraphy. Hence, numerous magnetostratigraphic analyses have been carried out providing a rather good chronology of the Siwaliks group. However, these studies have several flaws. The proposed correlations to the reference scale of each individual magnetostratigraphic columns remain subjective and therefore ambiguous. Only one unique correlation is often presented while the other likelihood possibilities are not mentioned. Hence, the possible derived uncertainties and ambiguities are never discussed. Moreover, most of the studied sections expose fine grained sediments shed by relatively small rivers which catch only a limited part of the Himalaya. The past sediments of larger Himalayan rivers (Yamuna, Ganga, Karnali, Narayani and Kosi) draining the entire range are more relevant to decipher the tectono-climatic hystory of the Himalayas. However, these rivers mainly deposited in the foreland large and coarse grained fans less suitable for paleomagnetic analyses. At last, very few magnetostratigraphic studies have documented sediments younger than 4-5Myrs while important climate changes occurred during this period.

Therefore our study aims to reconsider the previous magnetostratigraphic correlations using a numerical method based on the dynamic time wraping algorithm. This new method automatically provides a set of reasonably likely correlations and hence enables to discuss possible ambiguities in the magnetostratigraphic correlations. It also provides uncertainties on sediment accumulation rates which will be used to better estimate the flexural and shortening history of the Himalayas.

Our study also aims to better constrain the depositional ages of young Siwalik sediments deposited by a large river. Therefore, we carried out three new magnetostratigraphic sections located in the Valmiki National Park in India which expose coarse and young sediments deposited by the recent Narayani river through a large paleo-fan.