Rajahmundry, Andra Pradesh
The first direct link between Deccan volcanism and the KTB mass extinction was established in four Rajahmundry quarries (Keller et al., 2008). The longest lava flows of the Phase-2 mega-eruptions are best known from these basalt quarries (Figs. 4, 7). Intertrappean sediments above Phase-2 basalts span up to 8 m of sediments deposited in a shallow near-shore marine environment (Fig. 8). These intertrappean sediments contain earliest Danian planktic foraminiferal assemblages of zone P1a, which mark the evolution in the aftermath of the mass extinction. Deccan Phase-3 volcanic layers overlie the intertrappeans and mark the last Deccan eruptions.
Figure 8. Close-up of intertrappean sediments between Phase-2 and Phase-3 volcanic eruptions at the Gauriputnam Quarry of Rajahmundry.
Figure 9. Intertrappean sediments with bulk mineralogy at the Government Quarry, Rajahmundry. From Keller et al., 2008.
Biostratigraphy
The K-T boundary is easily identified worldwide in planktic foraminifera by the mass extinction of all tropical-subtropical Cretaceous species (2/3 of the assemblages), the immediate increased abundance (up to 90%) of the disaster opportunist survivor Guembelitria cretacea, and the evolutionary first appearances of Danian species (e.g., Parvularugoglobigerina extensa, Woodringina hornerstownensis, Globoconusa daubjergensis, Eoglobigerina eobulloides, Fig. 10). The interval from the first appearance of Danian species to the first appearance of P. eugubina and/or P. longiapertura generally marks the boundary clay zone P0, which frequently is enriched in iridium. The total range of P. eugubina marks biozone P1a (Keller et al., 1995; Keller, 2011). Within this range, the first appearances of Parasubbotina pseudobulloides and Subbotina triloculinoides subdivide biozone P1a into subzones P1a(1) and P1a(2). The first Danian nannofossil biozone NP1 marks this early Danian interval.
Figure 10. KTB identifying characteristics of planktic foraminifera based on the Tunisia stratotype section. From Keller et al., 2008.
Figure 11. Biostratigraphic data from intertrappean sediments in the Government Quarry of Rajahmundry. From Keller et al., 2008.
Planktic foraminifera in sediments of the Government, Balaji, Church and Duddukuru and other sections are rare due to the shallow marine environments (Keller et al., 2008; Malarkodi et al., 2010). Nevertheless, good age control can be obtained from assemblages in marine facies, such as limestone, claystone and clay and mudstone clasts. For this study most of the species were identified from thin sections because the small species are fragile and difficult to preserve during the washing process.
The first Danian planktic foraminifera are found in claystone clasts of unit 2, which overlies the lower Rajahmundry trap (Fig. 11). In the Government quarry, these clasts contain tiny early Danian species, including Parvularugoglobigerina eugubina, Globoconusa daubjergensis and the Cretaceous survivor and disaster opportunist Guembelitria cretacea (Fig. 11). The same early Danian species are also present in claystone clasts of unit 2 in the Balaji quarry. Two additional species, Eoglobigerina eobulloides and Woodringina hornerstownensis, are present in clasts from unit 2 of the Church Quarry (Fig. 9). Unit 2 is thus younger than zone P0 and of early zone P1a age, as also indicated by the presence of the index species P. eugubina in the overlying units. The claystone clasts with early Danian species indicate erosion of a claystone layer that could have been deposited after the arrival of the topmost lava flow of the lower Rajahmundry trap (Keller et al., 2008).
The Rajahmundry quarries thus reveal that the KTB mass extinction occurred at or very close to the end of the main Deccan phase-2. This Deccan Traps/mass extinction juxtaposition within the same outcrops indicates that there is a likely cause-effect relationship. Further work has substantiated this observation.
Depositional Environment
In Rajahmundry quarries, multi-disciplinary studies of intertrappean sediments based on biostratigraphy, sedimentology, mineralogy and geochemistry, reveal that sediments were deposited in a shallow marine environment that fluctuated between supratidal, estuarine, lagoonal and open marine conditions, interrupted by periods of subaerial deposition (paleosoils) (Figs. 12, 13). Changing sea levels are largely related to uplift and subsidence associated with Deccan volcanism. Planktic foraminifera and rare nannofossils mark deposition as early Danian zone P1a, which spans the first 280,000 years after the KTB mass extinction (Keller et al., 2008).
Figure 12. Microfacies and their depositional setting within the shallow estuarine to subaerial environment of the Rajahmundry area. From Keller et al., 2008.
Figure 13. Sea-level changes and paleoenvironmental interpretation of the depositional settings of intertrappean sediments in the Rajahmundry area based on biostratigraphy and faunal assemblages, lithology, bulk rock mineralogy and microfacies analysis. From Keller et al., 2008.
Very shallow, restricted estuarine conditions with high detrital input and a seasonal humid climate mark unit 3a, as indicated by rare ostracods, oysters and benthic foraminifera (Fig. 13). In unit 3b an abundance of basalt, mudstone and claystone clasts with earliest Danian planktic foraminifera reflect intense reworking and current activity. Increasing clay and quartz contents (Fig. 9) indicate high detrital input and a more humid climate, as also suggested by abundant iron nodules eroded from nearby lateritic soils. A rising sea led to lagoonal and coastal marine environments with very low hydrodynamic conditions and a seasonally contrasted climate as reflected by the laminated mudstone with rare marine bioclasts, fish remains) and chalky limestone (units 3c, d, Fig. 13). A pronounced sea-level drop occurred at the top of unit 4, as indicated by the strongly karstified (implying increasing humidity) surface at Bajali and erosion at the other outcrops (Fig. 13) (Keller et al., 2008).
A rising sea returned open marine conditions, depositing first silty claystone (unit 5b) with fish remains, followed by limestone increasingly rich in bioclasts and Danian planktic foraminifera (Figs. 11, 13). A major sea level fall (top of unit 7), possibly related to uplift associated with the eruptions of the upper trap, is marked by increased detrital influx reflecting more humid conditions and erosion. Unit 8 at the top of the intertrappean sediments consists of a thick paleosol enriched in laterite-derived elements (MF0, Fig. 12), which indicates deposition in a terrestrial environment. The paleosol consists of quartz, hematite and smectite, which typically form in warm climates with seasonal contrasts in humidity. The upper lava flows appear to have been deposited in a non-marine environment (Keller et al., 2008).
Summary
Sea level fluctuations interpreted from microfacies indicate a gradual but fluctuating deepening during the early Danian punctuated by significant regressions near the P0/P1a and P1a/P1b transitions. These sea level changes largely reflect local uplift and subsidence related to Deccan volcanism. Despite this local tectonic overprint, the global sea level regressions at the P0/P1a and P1a/P1b transitions are recognizable (Fig. 13).
