Stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial Siwalik rocks of Pakistan

Abstract

A high-resolution record of a palaeomagnetic reversal is documented in Miocene alluvial rocks of Pakistan. We examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections. Each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal. The position of the reversal illustrates local relief in the study area and the time-transgressive nature of certain stratigraphic units. Variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates. Different characteristics of palaeomagnetic remanence are associated with the depositional and post-depositional history of these sediments. Variability in patterns of remanence behaviour is the basis for inferences about post-depositional processes. We discuss two magnetic parameters that express coherency of palaeomagnetic samples—the maximum angle of deviation and the circular standard deviation. Of particular interest are samples with incoherent palaeomagnetic signals. The incoherency of samples is inversely correlated with the thickness of the transitional interval. A low rate of sediment accumulation, suggested by a thin transitional interval, may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes. Alternatively, transition intervals, denned by coherent magnetization, may be thin as a result of pedogenically induced incoherency.