HOW THE CRB TAUGHT US TO INTERPRET LAVAS IN THE DECCAN PROVINCE

In 1981, Thor Thordarson and I started work on the Columbia River Basalt Group (CRBG), concentrating on physical lava characteristics. We chose to log sections and apply observations Thor had made on Icelandic lavas. Slowly a commonly repeated pattern emerged in many flows, we were often logging sheet-like lobes 10s of m thick and 100s (minimum) of m long. We saw a three-fold division in these lobes – upper crust (UC), core (CO), and lower (or basal) crust (LC), and logged characteristics of these zones. This included banded vesicular UC and a zone or two of mega-vesicles, a massive CO with few vesicles, and a tiny LC, rarely > 50 cm thick irrespective of lobe thickness. We noted that COs sometimes had vesicle cylinders and sub-horizontal vesicular sheets near the top; the COs also carried coarser joints (sometimes well-enough developed to be called colonnade) while the UCs had finer, untidy, hackly joints. Finally, we noted that UC was often very thick and we amassed enough data to show that mean UCs varied from 40-70% of the lobe thickness. We didn’t know how to interpret this data but we’d also been working on Kilauea (active all these years) and knew Ken Hon et al’s inciteful 1984 GSA paper on inflation in Kilauean pāhoehoe (phh) flows; CRB lavas were all phh. We put out a paper in 1996 based on CRBG observations and the role played by sheet-lobe inflation.

In late 1996, I was offered a trip to a conference in Bombay, India, on the Deccan Province (DVP) – with several DVP “Gods” in attendance – Subburao, Cox, Hooper. I presented a paper on our CRBG observations; there had been nothing like this coming out of India. I didn’t know what to expect on the field trip; I’d hardly seen lavas as old as 66 Ma before! Sure, they were quite altered but some characteristics were recognizable, especially those familiar phh sheet-lobes! Gradually, we were able to make out UCs (usually by the near-pervasive amygdaloidal character), COs, and sometimes LCs. Was this similarity to the CRB lucky or was this a common pattern in CFB provinces? There was no literature to guide us! One thing was different, DVP lavas have fewer vesicular features – and we wondered why? I was requested to submit a paper on CRBG lavas to the post-Conference Volume, and soon papers interpreting what we recognized in the CRBG in DVP lavas by younger Indian researchers began to appear in Indian journals.