Rajahmundry Traps has been under geo-scientific investigations for over half a century; yet sketchy information is available on morphology and internal architecture of lavas. This study bridges this gap by describing the morphology of lava pile of Duddukuru area following Hawaiian scheme that relates a lava-type with a particular emplacement style.The lowest flow (Flow 1) at Duddukuru, overlying an infra-trappean bed is a thick pahoehoe sheet lobe with intermittent basal hummocky, P-type pahoehoe lobes representing slow, sub-aerial emplacement of lava parcels. Some of them also have cherty intercalations and morphology suggestive of possible interaction of lava with water or water-laden sediments. Hummocky lobes were over-ridden by subaerial emplacement of voluminous sheet lava, which developed multi-tier columnar joints probably due to inundation of lava-surface by water. Inter-trappean sediments were deposited above Flow 1 in the ensuing period of volcanic quiescence. The eruptive history of Duddukuru ended with sub-aerial outpouring of two consecutive a'a flows (Flows 2 and 3) separated by a thin red bole horizon.As pahoehoe lavas are known to travel long distances due to emplacement through thermally efficient mechanism of endogenous inflation and insulated melt transport whereas a'a lavas, in contrast, are cooling-limited flows which rarely attain lengths covered by pahoehoe lavas; it is probable that the pahoehoe and a'a flows of Duddukuru have not traveled comparable distances. A'a flows of Rajahmundry Traps traveling from far-off vents are, therefore, more unlikely despite temporal and chemical similarity of this lava-pile with upper parts of Western Deccan Basalt Group.
Morphological evolution of classical pāhoehoe to transitional flow-type in a compound basalt flow occurring in the lower pile of Deccan traps is described here from the southern slope of Mesan Dongar in Manmad-Chandwad area, western part of Maharashtra plateau, India. The transitional flow lobes, largely identified as rubbly pāhoehoe-type, are characterized by a crust of breccias, with thickness ranging from 10 to 20 m. The breccia is generally a welded one and consists of mm to m-size, angular to sub-angular basaltic clasts that vary in vesiculation (vesicle size, shape and population). The crust, at times, is supported by an inner vesicular zone coherently developed above a persistent core that varies in thickness from 10 to 15 m. Base of these lobes is smooth, sparsely vesicular, without any basal breccia.
Many of the transitional lobes of Mesan Dongar, however, shared a few common morphological traits with slabby pāhoehoe and this inherent uncertainty in classifying them as rubbly pāhoehoe has been greatly reduced using the semi-quantitative tool of Keszthelyi (2002). Moreover, it is also proposed that this morphological evolution of flow lobes in Mesan Dongar represents a NW-SE trending, southerly propagating and at least four kilometre long flow-front of pāhoehoe lava. Identification of flow fronts is important, even in this ∼65 Ma old, dissected Deccan flood basalt province, as these can reveal vital information on flow dynamics, slope and cooling histories of individual flows/lobes.
Elephanta Island near Mumbai is an important area for understanding the stratigraphic and structural framework of the Deccan flood basalt province in the tectonically disturbed Panvel flexure zone on the western Indian rifted margin. Elephanta exposes a west-dipping, 66–65 Ma sequence of tholeiitic lava flows and dykes. Geochemical correlations with the thick, horizontal, 66–65 Ma Western Ghats sequence to the east show that lava flows of the Khandala and Ambenali formations are present at Elephanta, with two lava flows probably being locally derived. The Elephanta tholeiites have experienced crystal fractionation and accumulation, particularly of olivine. They have εNd(t) ranging from +5.4 to −7.9 and (87Sr/86Sr)t from 0.70391 to 0.70784, with most tholeiites little contaminated by continental lithosphere, probably lower crust. Field and geochemical data indicate a normal fault along the central part of Elephanta with a 220 m downthrow, consistent with a domino-type block-faulted structure of Elephanta, and the surrounding area as previously known. Seventeen of the 20 analyzed Elephanta intrusions, striking ~N–S, belong to the Coastal dyke swarm of the western Deccan province. Several of these are probable feeders to the Ambenali Formation in the Western Ghats sequence, requiring reconsideration of the current view that the voluminous Wai Subgroup lavas of the Western Ghats were erupted without organized crustal extension. East–west-directed extensional strain was already active at 66–65 Ma along this future (62.5 Ma) rifted continental margin. A young (~62 Ma) ankaramite dyke on Elephanta Island is a probable feeder to the Powai ankaramite flow in the 62.5 Ma Mumbai sequence 20 km to the northwest.
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