Magnetic polarity stratigraphy of Siwalik Group sediments in Nepal: Diachronous lithostratigraphy and isochronous carbon isotope shift

The middle Miocene-Pliocene Siwalik Group was deposited inthe Himalayan foreland in response to uplift and erosion in theHimalayan fold-thrust belt (Zeitler1985, Hodges and Silverberg1988, DeCelles et al 1998). Thermal demagnetization experimentsdemonstrate that laminated (probably paludal) siltstones yieldpaleomagnetic data useful for tectonic and magnetostratigraphicstudies, whereas other lithologies yield data of questionablereliability (Ojha et al. 2000). Magnetostratigraphic data wereacquired from 297 sites within a 4200-m thick section of Siwalikdeposits at Surai Khola (27.8 °N; 82.7 °E). The observed sequenceof polarity zones correlates with the geomagnetic polarity timescale (GPTS) from chron C5n to chron C2n. This geochronologiccalibration indicates that the lower-middle Siwaliklithostratigraphic boundary at Surai Khola occurs at 7.5 Ma, usingthe GPTS of Cande and Kent (1995). This boundary ischaracterized by an abrupt increase in the thickness of coarse-grained channel sandstones and other sedimentologicalindicators of increased wetness of the floodplain. The cause ofthe change in depositional character at the lower-middle Siwalikboundary could be either tectonic (an increase in subsidencerate) or paleoclimatic (Cerling et al. 1997, Quade et al. 1995,Harrison et al. 1993). Data from other sites in the Nepal foreland,however, suggest that the change is highly diachronous, probablyruling out the paleoclimatic explanation (Hoorn et al. 2000, Quadeet al. 1995, 1997). At Muksar Khola (26.9°N; 86.4°E), 111paleomagnetic sites from a 2600-m thick section define a polarityzonation that correlates with the GPTS from chron C4An to chronC3n. At this locality, the lower-middle Siwalik lithostratigraphicboundary occurs at 8.8 Ma. Previously published results fromBakiya Khola (27.1°N; 85.2°E) indicate that this section wasdeposited from chron C5n to chron C3n and the lower-middleSiwalik lithostratigraphic boundary occurs at 9.2 Ma. Thelithostratigraphic lower-middle Siwalik boundary is thus timetransgressive by at least 1.7 Ma. along strike in the Himalayanforeland of Nepal. On the other hand, at Surai, Muksar, and Bakiyakholas, a shift in δ