Volcanology and Lava Flow Morpho-types from the Koyna-Warna Region, Western Deccan Traps, India

Abstract

Flow mapping and physical volcanology of 15 basaltic lavas exposed in three critical road pass sections (ghats) in the Koyna-Warna region of the western Deccan Traps is presented in this paper. Transitional lavas like rubbly pahoehoe are most common morpho-type exposed in these ghat sections. Sinking of rubbly breccia into flow interiors and formation of breccia-cored rosette are common in some lava flows. Few rubbly lavas exhibit slabby tendencies. The amount and nature of the associated rubble is variable and result from the mechanical fracturing and auto-brecciation of the upper vesicular crust in response to distinctive stages in the cooling, crystallization and emplacement history of individual lava flows. Occurrence of aa and pahoehoe morpho-types in the lava flow sequence is subordinate. Three prominent pahoehoe flows separated by red bole horizons are seen in the upper parts of the Kumbharli ghat. These are thick, P-type sheet pahoehoe. The pahoehoe lavas represent compound flow fields that grew by budding, endogenous lava transfer and inflation. Presence of pahoehoe lavas in the Koyna-Warna region hints at possible hitherto unrecorded southern extension of Bushe-like flow fields. This study reconfirms the existence of pahoehoe-slabby-rubbly-aa flow fields and transitions even in the upper echelons of the Deccan Trap stratigraphy. The study of morphology and internal structure of lava flows exposed at the ghat sections in the Koyna-Warna region could guide sub-surface core-logging that is critical in deciphering the physical volcanology and emplacement dynamics of basaltic lava flows penetrated by drill holes sunk under the scientific deep drilling programme.

Thickness Characteristics of Pāhoehoe Lavas in the Deccan Province, Western Ghats, India, and in Continental Flood Basalt Provinces Elsewhere

“…Next, we compile lobe thickness measurements from other CFB provinces (for a total of ∼ 3,800 individual measurements) from multiple studies. In concert with other studies (e.g., Bondre et al, 2004;Duraiswami et al, 2017;Jay et al, 2018) our results help illustrate the variety in a specific morphological lava feature (lobe thickness), both within a single CFB formation and for entire CFB provinces. Additionally, we can possibly use lobe thickness distributions to quantitatively compare and group together various CFBs with similar flow emplacement dynamics.…”

“…Deccan data clearly show that lobes in the upper three formations, Mahabaleshwar, Ambenali, and Poladpur (constituting the Wai Subgroup) have thicker median and modal thicknesses than the lower formations. Cores from the Koyna area have the same range as the exposed Wai Sub-group flows which is consistent with the corresponding location of the Koyna cores in the geochemical stratigraphy Duraiswami et al, 2017). However, all formations in the cores contain lobes with smaller thicknesses, like those of the lower formations.…”

“…We use the term (inflated) lava sheet-lobe (as per Self et al, 1997;Self et al, 1998;Thordarson and Self, 1998), shortened to sheet-lobe (SL), to describe widespread lava bodies surrounded by lava crusts. Note that this category would include a number of the transitional flow types such as the rubbly p ahoehoe flows reported in Deccan Traps (e.g., Duraiswami et al, 2017) as well as modern basaltic eruptions (Laki - Guilbaud et al, 2005). These bodies are much more extensive in the horizontal than the vertical axis and are mostly ≥5 m in total thickness.…”

“…An independent Deccan data set comes from lobe thickness measurements on drill core obtained from the Koyna area in the Western Ghats (Sinha et al, 2017;Mishra et al, 2017), near to our Khumbarli Ghat traverse. Although the cored lava formations are not named in these papers, the local geology suggests they are Wai Sub-group (Duraiswami et al, 2017). The distribution of SL thicknesses is like our Wai Sub-group data, seen on Figure 6B, with a mean and mode around 18-20 m. A maximum reported lobe thickness of 165 m for one lobe in the Koyna cores must be treated with suspicion as no other lobes of comparable thickness have been found in our work, but it could be real.…”

Cooling history and emplacement dynamics within rubbly lava flows, southern Deccan Traps: insights from textural variations and crystal size distributions

“…The plateaux of Kas and Thoseghar, west of Satara and around Koynanagar (Patan-Dapharwadi ghat, Arle-Gudhe ghat), and the Kumbharli ghat in the Western Ghat escarpment (Location 8 to 12 in Fig. 1b) expose thick and morphologically diverse pāhoehoe, transitional (slabby and rubbly pāhoehoe) and 'a'ā' flow fields (Duraiswami et al 2017). The surface lava flows and those in the boreholes at Tural-Rajwadi (Location 13 in Fig.…”