New observations on the Lameta Formation-Deccan Trap contact in Narmada Basin, Western India--implications for terminal cretaceous events.
The Narmada basin forms one of the important rift basins of western margin of the Indian sub-continent and lies between N 21[degrees],48' ; 23[degrees],08' latitudes and E 72[degrees],58' ; 80[degrees],00[degrees] longitudes, in a linear WSW-ENE direction, covering a distance of about 1100 km and comprise rocks belonging to varying ages from Pre-Cambrian to Recent.
Within this basin the rocks belonging to the Lameta Formation (Maastrichtian, Buffetaut,1987; Joshi,1995) widely referred in the literature as Infratrappeans (Gupta and Mukherjee, 1938) constitutes one of the most important lithological assemblages and occupy a key position in the stratigraphic record from the point of understanding Cretaceous-Tertiary transition. These rocks which have been identified to be the deposits of fresh water inland basins (Joshi and Ganapathi,1990; Joshi,1991) occur along the periphery of Deccan basaltic lava flows in the form of patchy outcrops, having a wide lateral extension throughout the Narmada basin. Although these rocks rest unconformably on different lithological formations of varying ages, however they are capped by the Deccan Traps. The upper most horizons of Lameta succession observed in various locations through out the basin are demarcated by the presence of cherty limestones which are compact, massive and nodular in nature comprising fossilized dinosaurian eggs. Owing to the presence of basaltic cover over these limestones, the contact zone is not clearly discernible in majority of the locations and is found to be concealed below the volcanic flows. However, the dug--well sections exposed near Nimach locality ( N 22[degrees] 41'48.8 ; E 74[degrees] 22' 25.8) in Dahod district of Gujarat state, exhibits an exemplary record of the nature of contact between Lameta limestones and the lava flows. A critical study of these sections has provided interesting informations highlighting the nature of events that occurred during the terminal Cretaceous times, marking the close of Mesozoic era along the western margin of the Indian sub-continent.
The geological succession exposed around the study area comprise sedimentaries belonging to the Lameta Formation (Maastrichtian,Buffetaut,1987;Joshi,1991;1995), resting unconformably over the Pre-Cambrian metamorphics and in turn are overlain by the basaltic flows of Deccan Traps (Upper Cretaceous to Early Paleocene, Sahni and Bajpai, 1988). The rocks of Lameta Formation have been worked out in detail by earlier workers in the context of their depositional environments and the occurrence of dinosaurian remains within them Ample literature is also available on the various aspects related to the Deccan volcanic activity, however the nature of contact between the Lameta Formation and Deccan Traps has not been clearly documented and needs an in depth investigation, which can certainly provide answers to the topical issues of Cretaceous--Tertiary transition and mass extinctions.
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The area around Nimach village (N 22[degrees] 41'48.8 ; E 74[degrees] 22' 25.8) located about 20 km SE of Dahod town comprises exposed dug-well sections excavated along the periphery of Deccan Trap outcrops (Fig. 1). The vertical succession observed in these sections (Figs.2, 3) commences with limestone units representing the uppermost horizon of Lameta Formation. The fresh surfaces of these limestones are dirty white to grayish in appearance, however the weathered portions are yellowish in nature. They are well compacted, massive, siliceous in nature and show evidences of diagenesis. The weathered portion of these limestones distinctly show the presence of fragments of carbon within them. The limestone units are followed upwards by the presence of loosely compacted reddish brown coloured horizon having thickness varying from 2 to 4 m and comprise sediments ranging from fine sand to silt size grade with occasional presence of coarse detrital quartz. This horizon shows effect of weathering processes and is also characterized by the presence of carbon fragments .The contact between this horizon and the Lameta limestones is distinctly sharp and abrupt with no signs of gradation.
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The carbon fragments within the weathered limestones and the reddish brown silty horizon is in significant proportions and occur as broken fragments and as lumps rather than a continuous layer . In order to understand the nature of the carbon, the representative samples of limestones as well as the reddish brown horizons have been subjected to petrographic studies and geochemical analyses (TOC-Total Organic Carbon)
The silty reddish brown horizon is capped over by the basaltic layers which are compact in nature, however, at places on account of weathering they have been disintegrated into angular fragments. Interestingly, the contact between this horizon and the basalts is also sharp and abrupt in nature and do not represent any signatures of hiatus in between. The top of the entire lithological succession is marked by the presence of recent soils and alluvium.
Under the microscope, the limestones exhibit crystalline texture comprising micro-crystalline to coarse-crystalline calcites with scattered sub-angular to sub-rounded grains of detrital quartz. These limestones seem to have undergone extensive diagenesis which is clearly indicated by the recrystallisation of calcites, occurring as overgrowth rims around detrital quartz (Fig.4). The petrographic studies of these limestones have clearly shown the presence of fragments of carbon within them (Fig 5). They are irregular in shape, blackish in appearance, isotropic in nature and occur either as isolated fragments or as clusters possibly representing a soot. Along with the carbon, broken and irregular fragments of glass shards exhibiting uneven to conchoidal fractures have been noticed in the limestone samples (Fig. 6).
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The petrogenesis of reddish brown horizon clearly exhibits the presence of a glassy matrix with discontinous pattern of laminations indicating a eutaxitic texture (Fig.7). At places, sub-angular to sub-rounded grains of detrital quartz having numerous fractures and vacuoles are observed. This horizon reveals the presence of glass shards majority of which are free from vesicles. The glass shards are colourless, irregular to rounded in shape, crystalline in nature and exhibits conchoidal to irregular fractures within them (Fig 8 ). These glassy particles appears to have formed by rapid cooling of the droplets of basaltic melts during the lava eruptions. Along with the glass particles, clusters of carbon soot are observed in this horizon (Fig.9). The petrographic characters clearly indicate that this horizon represents glassy welded tuffaceous material, perhaps the consolidated ash layer directly resting on the Lameta limestones.
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Geochemical studies have been carried out for the representative samples of the lithological units along the contact zone. These samples have been subjected for the rare earth (LREE) and trace elements estimation using ICP-MS (Perkin Elmer, Elan DRC-II) at the National Geophysical Research Institute (NGRI) Hyderabad. The plots of normalized concentration values of rare earth elements is given in Fig.10 whereas Fig.11 represents the plots of other trace elements recorded in the samples. The concentration values obtained from the data as well as the nature of plots clearly indicate higher concentration of Cerium and appreciably higher values of trace elements such as Nickel, Vanadium, Strontium, Zirconium, Gallium and Chromium.
Along with the rare earth and trace elements study, the representative samples of limestones and reddish brown horizons were subjected for the total organic carbon (TOC) contents. The samples were subjected to acid treatment to remove carbonates and the TOC was determined using Leco CR-12 (USA make) carbon determinator. The results obtained have indicated the presence of TOC in the samples less than 0.5 wt%. Particularly the weathered limestone samples (N-2) contain 0.412 wt% of TOC while the fresh limestone samples (N-1) and reddish brown horizon (N-3) contains 0.218 wt% and 0.127 wt% of TOC respectively.
The present study initiated on the contact of Lameta Formation and the overlying Deccan Traps has brought out certain significant results focusing on the events occurred during the terminal Cretaceous times in Narmada basin. The sharp and abrupt nature of the contact between the Lameta sediments and the overlying tuffaceous layers aptly corroborates that the Deccan volcanic activity in western India initiated immediately after the Lameta sedimentation ceased . The tuffaceous layer represents the ash transported from the volcanic activities perhaps in the proximal regions, followed by the rapid flows of basaltic material that concealed the below lying units. The sharp and abrupt nature of contact between the tuffaceous layers and the overlying basalts clearly supports the above observation. This further implies that no appreciable time gap was available for the erosion of Lameta sediments and that the close of sedimentation experienced sudden outpouring of Deccan lavas perhaps around 66.5 m.y. The fragmentary presence of carbon within the weathered limestones and the tuffaceous layers probably indicate the phenomena of the burning of terrestrial biomass resulted on account of Deccan activity. Glasby and Kunzendorf (1996) have suggested that the Deccan eruption contributed large amounts of [H.sub.2]S[O.sub.4], HCl, C[O.sub.2] dust and soot into the atmosphere leading to the significant drop in sea level and marked changes in the ocean temperatures. According to them, during the Deccan volcanic event, there was a lowering of sea level about 100m and drop in ocean temperature on account of the volcanic dust and soot clouds that blocked the solar radiations.
Other workers (Wolbach et al, 1985, 1988, 1990; Anders et al, 1986, 1991; Gilmour et al 1990; Heymann et al, 1998) have considered the presence of large quantities of soot, attributed to the global wild fires that could remain suspended in the atmosphere for months after the terrestrial impacts. Wolbach et al (2003) are of the view that the element carbon and soot cannot survive long periods of oxic diagenesis and that chemically reducing conditions are necessary for its preservation. They have also suggested that the thermal energy radiated from re-entering ejecta from K/T impact is likely to have caused global wild fires, leading to the formation of soot. The extensive burning of the biomass and injection of soot into the atmosphere is considered by them to have played an important role in the severity of K/T mass extinctions. However, Hallam (1987) has pointed out that soot is commonly observed in the stratigraphic record and that its occurrence cannot be used to support the asteroid impact hypothesis.
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In context of the study area and its environs, since no concrete evidences have so far been recorded indicating the role of extra-terrestrial impacts, it is more suitable to consider the presence of carbon resulted due to the burning of biomass on account of Deccan volcanic activity. The active role played by this volcanic event is also substantiated by the presence of glass shards, resulted on account of the consolidation of melt during the eruption.
The elevated concentrations of elements such as Cerium and other trace elements such as the Nickel, Vanadium, Strontium, Zirconium and Chromium in the samples of the study area are suggestive of the volcanic source. The petrogenesis of the basaltic samples collected from several locations of the Deccan Trap region including the Narmada basin (Mahoney 1988) shows higher concentrations of trace elements such as Zirconium, Strontium, Nickel and Vanadium. The REE studies initiated by Alexander and Gibson (1977) for the Deccan basalts of Dhandhuka area, Western India have shown the significant concentrations of Cerium. According to Macleod and Irving (1996), the rise in the concentration of Cerium is indicative of the palaeo-environmental conditions. They have further suggested that the variation of Cerium anomalies is influenced by a number of factors, including terrigenous input, depositional environment and diagenetic conditions. According to Wilde (1987), the changes in the value of Cerium anomaly in the whole rock analyses could be attributed to the redox conditions. Negative values are found during warmer climates and transgressive conditions whereas positive values indicate cooler to glacial climates and regressive conditions.
The overall observations on the nature of contact of Lameta sediments and Deccan Traps, indicates that the end of Maastrichtian within the Narmada basin in particular and western India in general was marked by Deccan volcanic activity concomitant with the close of Lameta sedimentation. This activity perhaps elevated the atmospheric temperatures initially, however later contributed to the drop in ocean temperatures, sea level regressions and atmospheric dysoxia which is clearly indicated by the presence of carbon and elevated concentrations of Cerium. The observations made during the present investigation have brought out interesting details on the events recorded in the Indian sub-continent during the close of Mesozoic era. Further in-depth study on the nature of the carbon recorded during the present work and its relation to the K-T events is in progress.
The present study forms a part of the work carried out under the major research project (No. F.5-7/2001, SR-I) funded by the University Grants Commission, New Delhi, which is gratefully acknowledged. The author also thanks Prof. K.C.Tiwari, Department of Geology, The Maharaja Sayajirao University of Baroda, Vadodara and Prof. M. Sisodia, Department of Geology, Jai Narayan Vyas University, Jodhpur for their useful discussions and suggestions. The help extended by NGRI, Hyderabad for REE and Trace elements analyses and ONGC Ltd. Vadodara in carrying out the TOC studies is duly acknowledged. The author also acknowledges the help extended by Department of Archeology, M.S.University of Baroda for photomicrographic work. The opportunity to present this paper and financial assistance extended by the organizers of the 7th International Seminar on Cretaceous,5-9 Sep.2005,Neuchatel,Switzerland is gratefully acknowledged.
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A. V. Joshi
Department of Geology, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara--390 002, India. E email@example.com.
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|Publication:||Bulletin of Pure & Applied Sciences-Geology|
|Date:||Jan 1, 2007|
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