Paleomagnetic Stratigraphy and Time in Sediments: Studies in Alluvial Siwalik Rocks of Pakistan

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Abstract
Sediments may acquire magnetic remanence upon deposition and shortly after deposition. Hence, the paleomagnetic record of sedimentary rocks may provide a chronostratigraphic framework for rates and patterns of depositional and post-depositional processes over time scales intermediate between those of modern observation and those of the dated geologic record. Two applications of high-resolution magnetostratigraphy in Miocene, alluvial rocks of Pakistan illustrate this point. (1) Transition stratigraphy-the dense sampling of a magnetic reversal-of correlated sections in the Dhok Pathan Formation revealed high variability in sediment accumulation rates (over several thousand to 10,000 yr), time-transgressive strata representing a paleosol and a floodplain marsh, and a pervasive post-depositional record mainly from pedogenesis. (2) Lateral tracing of paelomagnetic reversal boundaries in the Chinji Formation revealed a secular change in sediment accumulation rate and evidence for increased accumulation rate associated with extensive sandstones and the time-transgressive nature of certain sandstone units. Both studies demonstrate the significant lateral component to accumulation of lithological units, indicating that individual strata may embody considerably greater time spans in their lateral extent than in any vertical transect. Hence, stratigraphic completeness should be evaluated in the lateral as well as the vertical dimension.