Three-Dimensional Architecture of Modern Large-Scale Fluvial Systems Incorporating Geological, Geophysical, and Geotechnical Investigations: An Example From Varanasi City, Central Gangetic Basin, India

Abstract

The sediment architecture (geometry and arrangement) within fluvial reservoirs/aquifers are becoming increasingly important as a means of reducing risk, especially in a challenging economic climate. However, understanding the architecture of ancient fluvial systems can be problematic since certain parts of the system have increased preservation potential (e.g. channel-fill vs floodplain deposits) and are thus likely to be over-represented in the rock record. However, modern systems can be used to bridge this gap between the active process and resultant preserved sedimentary expression, prior to removal by erosive events (i.e. channel avulsion). This study looks at the three dimensional sedimentary architecture in and around the city of Varanasi, Central Gangetic Basin, as a case study to understand the geometry, proportion and spatial distribution of modern fluvial deposits – believed to be one of the first subsurface geological models for an Indian city.

Varanasi is situated along the banks of the River Ganges with the sub-surface characterised by unconsolidated sediments (muddy floodplain and sandy channel deposits). This study elucidates the fluvial architecture of the subsurface by the creation of a quantitative three-dimensional model for the River Ganges in the study area during Quaternary period.

Data collection included the drilling of 91 boreholes (up to 100 m depth) in and around the city limits, encompassing an area of ~280 km². Samples were collected from each borehole at 1.5-3 m interval. Samples analyse includes sedimentary properties (grain size, texture, sorting), mineral composition and geotechnical properties (soil type, plasticity index, SPT value). Downhole borehole geophysics was also conducted on 38 boreholes (spontaneous potential, gamma & resistivity).

The distribution and architecture of the identified sandbodies is believed to be a function of a combination of allogenic and autogenic forcing. Autogenic controls (river avulsion and channel switching) have generated laterally discrete sandbodies, which dominate the shallower level of the subsurface. By contrast, allogenic forcing (likely tectonics) is interpreted to have generated laterally connected high net to gross amalgamated braided channel sands to vertically stacked more argillaceous meandering river channel sands. The quantification and understanding of the effect of facies geometry and arrangement has direct application to analogous conceptualising fluvial reservoir units.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018