N. Suresh

Soft-Sediment Deformation in Contemporary Reservoir Sediment: A Repository of Recent Major Earthquake Events in Garhwal Himalaya

Two discrete 10–20-cm-thick zones of soft-sediment deformation (SSD) structures in horizontal beds occur in a ∼2.35-m-thick sandbar in the Asan reservoir in the northwestern Sub-Himalaya. The sediment column shows uninterrupted fluviolacustrine deposition since 1975. The depositional and deformational features, boundary conditions, and sedimentation rate suggest that the discrete SSD features formed in an apparent reverse density gradient and were triggered by the 1991 Uttarkashi and 1999 Chamoli earthquakes. These triggering events occurred within 100–150 km of the reservoir, which lay in intensity zones V–VI. The observations correspond well to the empirical relationship between distances to a liquefaction zone from an epicenter. The highly liquefiable sediments in the Asan reservoir provide a unique depositional and tectonic setting to record SSD. Triggering of SSD by contemporary seismic events demonstrates the validity of interpretations of similar causes of SSD in older sediments. This is probably the first documented example of earthquake-triggered SSD from the seismically active Himalaya range.

Integrated mineral magnetic and lithologic studies to delineate dynamic modes of depositional conditions in the Leh valley basin, Ladakh Himalaya, India

We present here lithofacies and mineral magnetic results from a ∼50 m thick composite record of fluvial, lacustrine and aeolian facies within the Leh valley basin of Indus River in Ladakh Himalaya. Mineral magnetic studies decipher interplay of two contrasting sediment sources viz., the unimodal ferrimagnetic source derived from Ladakh batholithic glacial domain and mixed ferri-to antiferromagnetic source derived from Indus sedimentary sequence. The lithofacies variability expresses dynamic changes in the depositional regimes controlled by base level fluctuations that are governed by the interaction of basin fill conditions and the response to Late Quaternary climatic perturbations. A three stage evolution of the Leh valley basin is proposed after comparison to other characteristic lithofacies changes within the valley as: (I) the basin under-fill conditions marked by fluvial and fluvio-lacustrine phase till ∼24m (∼64 Ka OSL age) above modern base level followed by (II) predominantly varved, glacio-lacustrine, basin overfill phase till 38m (∼28 Ka) gradually passing into an aeolian phase; and (III) basin incision that began at the earliest Holocene warming. Advancement and retreat of glaciers from the transverse valleys, attributed to climatic oscillations, appears to have greatly controlled the basin-fill conditions in the Leh valley. The present approach demonstrates its larger scope in recording the Late Quaternary response of individual valley basins to delineate local and regional attributes of climate change in the Himalayan and Karakoram region.

Mineral magnetic characterization of the Godavari river and western Bay of Bengal sediments: Implications to source to sink relations

Godavari Drainage Basin (GDB) represents the largest Peninsular source of sediment flux into the Bay of Bengal. Mineral magnetic measurements are carried out on the GDB sediments along with the surface sediment cores from the adjoining western Bay of Bengal to explore the relationship between the two. The sediments from Godavari river transact shows varied floodplain to bed load composition with the former dominated by unimodal soft ferrimagnetic, Deccan basalt source. Whereas, the bed load show polymodal composition of mixed nature dominated by silici-clastic sediments derived from the Precambrian granites and Proterozoic sequences. The surface sediment cores (~100-300 cm) from the Bengal fan region off the Godavari delta broadly display an increasing trend in ferrimagnetic mineralogy towards top. Based on mineral magnetism the ferri- and para-magnetic susceptibilities are assigned to the basaltic and siliciclastic sources, respectively which also represents the low and high rain fall zones. The increasing upwards trend of the ferrimagnetic minerals in the western Bay of Bengal sediments, therefore, can be related to the predominance of basaltic source over the siliciclastic/cratonic source from the GDB. These controls of magnetic susceptibility in the Bengal fan sediments are assigned to the fluctuation of the two sources as a result of differential weathering in response to monsoonal variability.

Situating megalithic burials in the Iron Age-Early Historic landscape of southern India

The megalithic burials of southern India—a wonderfully varied set of monuments—have long needed a chronology and a context. Broadly contemporary with the Roman and Sasanian empires, these dolmens, cairns and cists have continually raised contradictions with their material contents. The authors attack the problem using luminescence applied to pottery at the site of Siruthavoor in north-east Tamilnadu. Although sharing material culture, this first pilot project gave dates ranging from 300 BC to AD 600, so exposing the problem and perhaps, in OSL, its long-term solution.