Title	Petrophysical properties of the entire Deccan basalt stratigraphy: Sample collection campaign

Work Description:

The Deccan volcanic province is an important large igneous province in the world, as it marks the Cretaceous-Palaeogene boundary that witnessed a global change of climate and biogeosphere. The Deccan province currently exposes about 500,000 km2 of basaltic lava flows with a stratigraphic thickness of more than three kilometers. It is underlain by Precambrian rocks and Gondwana sedimentary rocks. The Deccan Group of lava flows consists of three Subgroups: Kalsubai, Lonavala and Wai. The Kalsubai Subgroup form the lower level of the succession, containing Jawhar, Igatpuri, Neral, Thakurvadi, Bhimashankar Formations. It is overlain by Lonavala Subgroup that consists of Khandala and Bushe Formations. The Wai Subgroup containing Poladpur, Ambenali, Mahabaleshwar, Panhala and Desur Formations comprise the upper level of the sequence. A number of individual lava flows constitute each formation. The Wai Subgroup constitutes about 50% of the Deccan volcanic province, while the Kalsubai and Lonavala Subgroups make up the rest of the sequence. The startigraphy of the Deccan basalt was established based on field, geochemical and palaomagnetic data. The complete stratigraphic sequence of the Deccan basalts is well exposed on the Western Ghats escarpment, which is a spectacular geomorphic feature that divides the Konkan coastal plains in the west and the Deccan plateau in the east. The Western Ghats escarpment provides a great opportunity to petrologists and geochemists to sample the basaltic lava flows of all formations for documenting the petrological and geochemical stratigraphy. Although geochemical stratigraphy of the Deccan basalts is well established, there has been no study on how petrophysical properties (e.g., density, compressional and shear wave velocity, and porosity) vary between different formations of the Deccan basalts. Therefore, we started a research project to characterize the entire Deccan basalt sequence, exposed on the Western Ghats escarpment. For this, we have selected four regional traverses: (1) Nasik – Jawhar, (2) Pune – Lonawala, (3) Mahad-Mahabaleshwar and (4) Chiplun – Koyna. These traverses covered 11 Formations of the Deccan basalts. We collected 158 block samples along these traverses (Figure 35.1). We collected 20 cm × 20 cm × 20 cm size block rock samples from fresh rock exposures on the quarry walls and road cuttings without any alteration. These samples preserve original mineralogical, petrographic and micro-structures. From the block samples, we are obtaining three to five cylindrical cores (30 mm diameter and 60 mm length) at different orientations, which account for textural and mineralogical anisotropy in the rocks. These samples will be characterized in the near future for obtaining petrophysical properties.

2. A study of the Western Ghats escarpment around Mahabaleshwar and Koyna

We present petrophysical properties (density, P and S wave velocity, porosity and Poisson's ratio) of the Deccan basalts from the Western Ghats escarpment around Mahabaleshwar and Koyna and characterize the Dhawar basement rocks around Goa. This study presents the petrophysical stratigraphy of the southwestern Deccan Traps for the first time. These properties would grossly reflect the compositional variation, textural properties, and vesicles/amygdales. The Western Ghats sections near Mahabaleshwar expose more than a kilometer thick basaltic sequence belonging to Bushe, Poladpur, Ambenali and Mahabaleshwar Formations, which represent the upper levels of the sequence, widespread in the southwestern Deccan Traps region. We collected fresh quarry samples of basalts from the Mahabaleshwar and Koyna sections and the Dharwar basement rocks (granites, gneisses, and greywackes) from Goa area. This study is aimed at understanding the petrophysical properties that would reflect the original petrological characteristics, but not the effect of weathering on these properties. In the 1300 m thick Mahabaleshwar section, the basalts show a considerable variation between the Formations (Figure 35.2). The Bushe Formation is characterized by a wide range in these properties, low density (2.61 g/cm³), P-wave (4.53 km/s) and S-wave velocity (2.74 km/s), and high porosity (4.6%). The Poladpur and Ambenali Formations show a narrow range, but higher density and velocities, but lower porosity. The Poladpur Formation is characterized by the mean density of 2.9 g/cm³, P-wave velocity (5.78 km/s), S-wave velocity (3.32 km/s), and porosity (1.3%). The Ambenali Formation is similar to the Poladpur Formation, has a mean density of 2.9 g/cm³, P-wave velocity (5.78 km/s), S-wave velocity (3.30 km/s), and porosity (1.2%). In contrast, the Mahabaleshwar Formation shows a wide variation in these properties, but has higher density (2.86 g/cm3), P-wave velocity (5.61 km/s) and S-wave velocity (3.20 km/s), and porosity (2.3%), compared to the Bushe Formation. In the 750 m thick Koyna section, the basalt samples are broadly similar to the Poladpur and Ambenali Formations of the Mahabaleshwar section (mean density: 2.94 g/cm³, P-wave velocity: 5.70 km/s, S-wave velocity: 3.29 km/s) (Figure 35.3). The Dhawar rocks (granites/gneisses and greywackes) exposed in and around Goa represent the basement of the basaltic sequence around Koyna-Mahabaleshwar region. They are characterized by lower density (2.63 and 2.76 g/cm3), P-wave velocity (5.13 km/s and 5.52 km/s) and S-wave velocity (3.15 and 3.40 km/s), respectively (Figure 35.3). This study clearly documents the lateral and vertical variation of density, velocity, and porosity in the Deccan basalts, which are very sensitive to the vesicle/amygdale content of the flows that depend on the volatile release from these lavas. The petrophysical stratigraphy is also broadly similar to the geochemical stratigraphy. Gravity and seismic studies in these regions would greatly benefit from the present study, because precise models of the subsurface density and velocity structures from the field geophysical data requires inputs from the measurements of petrophysical properties of the rocks in these regions. The Poisson's ratio of the Deccan basalts is largely affected by the vesicular porosity, aspect ratio of the amygdales, and zeolite content. High porosity of the Bushe and Mahabaleshwar Formations constitute a multi-layered aquifer system in the Deccan volcanic province. In the Koyna earthquake zone, these formations may provide an effective groundwater connectivity between the reservoir and earthquake focal areas. The new petrophysical data of the Deccan basalts and Dharwar basement rocks will help to refine the geophysical models of the southwestern Deccan basalt province.

Fig.35.1:
ASTER DEM showing the topography and sample locations.

Fig.35.2:
Petrophysical stratigraphy of the Deccan basalts in and around Mahabaleshwar.

Fig.35.3:
Petrophysical data of Koyna basalt and Dharwar basement rocks.