Flow-types and lava emplacement history of Rajahmundry Traps, west of River Godavari, Andhra Pradesh

Abstract

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.

No Cretaceous‐Paleogene Boundary in Exposed Rajahmundry Traps: A Refined Chronology of the Longest Deccan Lava Flows From40Ar/39Ar Dates, Magnetostratigraphy, and Biostratigraphy

“…Our observations are in agreement with the previous conclusion that the first lava flow was emplaced into shallow water, corroborated by the shallow marine infratrappean and carbonate intertrappean units, whereas the upper two flows and the red bole were emplaced subaerially (e.g., Keller et al, 2008;Self et al, 2008;Sen & Sabale, 2011). We interpret the Knight et al (2003), Self et al (2008), and Keller et al (2008) "upper flow" to be equivalent to our Flow 2 on the western side of the river, as there is limited exposure of the Flow 3 in those quarries, and Flow 1 is easily distinguished from Flow 2 because of the highly visible…”

“…However, in Gowripatnam Quarry, Flow 2 has a second thin (1–2 m) lobe at the base with a finely jointed and highly vesicular texture, indicating rapid cooling (Figures S5a and S5b). Flows 2 and 3 are separated by a 1–2 m thick sediment layer which is formed of red clay and is extremely similar in appearance to a very thick “red bole” of the main Deccan province (e.g., Ghosh et al, 2006; Sen & Sabale, 2011; Widdowson et al, 1997) (Figure S6). There are no fossils of any kind, suggesting negligible marine influence.…”

“…Previous 40 Ar/ 39 Ar dates indicated the RT are late Cretaceous to early Paleocene in age (∼63–67 Ma) (Knight et al, 2003), and paleomagnetic analyses found that flows with normal magnetic polarity overlie reversely magnetized flows (Baksi, 2001; Vandamme & Courtillot, 1992). Additionally, Danian fossils found in a carbonate sequence between the lower two flows (e.g., Baksi et al, 1994; Keller et al, 2008; Sen & Sabale, 2011), and Maastrichtian fossils in the sediments below the lowest flow (e.g., Keller et al, 2008; Mukherjee et al, 2013) suggested that the lowest flow was emplaced in the latest Cretaceous. These chronologic constraints led to the hypothesis that the lowest flow is part of an extremely voluminous Deccan eruptive episode just prior to the mass extinction and Cretaceous‐Paleogene boundary (e.g., Keller et al, 2008, 2012; Lakshminarayana et al, 2010).…”

“…Consequently, it is challenging to assemble literature data, collected over decades, into a consistent chronostratigraphic framework. For instance, earlier studies describe the RT as having either two or three exposed flows depending on the sampling locality (e.g., Baksi et al, 1994; Keller et al, 2008; Sen & Sabale, 2011). Thus, for example, the precise stratigraphic location of the reverse to normal polarity transition is unclear due to these sampling uncertainties (Baksi, 2001; Knight et al, 2003; Subbarao & Pathak, 1993).…”

Emplacement history and evolution of the Deccan Volcanic Province, India

“…They indicate a much larger aerial expanse of the DVP that may have been eroded away during the Tertiary and Quaternary times. Whether the basaltic flows exposed around Rajahmundry and the adjoining Krishna-Godavari basin represent long lava flows originating from a single edifice in the western DVP (Baksi, et al, 1994;Self et al, 2008b) or represent an independent volcanic sequence altogether (Bastia et al, 2010;Sen and Sabale, 2011;Duraiswami et al, 2014;Manikyamba et al, 2015) is open to debate.…”