Abstract
As the global search for oil continues as a direct consequence of incersed demand, new dimensions are needed to define areas with maximum potential for the accumulation of hydrocarbons.
Formation of oil and gas takes place by deposition of organic material under highly selective conditions. As climate affects all living organisms today, in the same way as it has done throughout the geologic past. The climate at the time of deposition affects both the supply of oil-forming organisms and the associated sediments.
Earth’s shape plays a major role in controlling the environment of deposition and also the resulting oil accumulation. The climate of the earth depends on the heat received from the sun. As the earth is nearly spherical due to which a temperature gradient exists from the equator to the poles.
Thus paleoclimatology , reveals the distribution in space and time of its climates of the past, when correlated with known oil accumulation, provides a new dimension in oil exploration. Here, we are considering KG basin (India) as our discussion point of view and trying to construct its paleoenvironment using certain parameters or proxies to find out rate of sedimentation, temperature, organic content and maturity along with the paleomagnetism of the local area.
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INTRODUCTION
A worldwide attention and attraction came for the Rajahmundry Traps of the K-G basin, as they are increasingly considered as an example for long distance lava transportation on Earth (Baksi et al. 1994; Jay and Widdowson, 2008; Self et al. 2008 and references therein) and also for the clarification of K-T boundary mass extinction events vis-??-vis volcanism (Keller et al. 2008). These traps are the only known outcrops of basalt flows along the east coast of India, thus they occupy a place of preeminence in the geology of the Peninsular India coeval to the Deccan Traps (Fig.1A). Besides these, the sedimentary horizons associated with the flows are characterized by the presence of a wide variety of invertebrate fossils.
STRATIGRAPHY
The Pangidi-Rajahmundry area of K-G Basin exposes coastal Gondwana sediments (Cretaceous), Rajahmundry Traps (K/T boundary), Rajahmundry Formation (Tertiary) and the Quaternary sediments (Table 1).
Fig.1.Field photograph of comprehensive stratigraphic succession of Rajahmundry Traps exposed at Gowripatnam quarry.
DESCRIPTION OF STRATIGRAPHIC UNITS
Infratrappeans
The infratrappeans which form the base of the Rajahmundry Traps constitute of eight to ten metre thick succession of sandstone, clay and limestone, sandstone is medium to fine grained, cross stratified and exhibit paleocurrent direction towards southeast. ~ 1.7 m thick brownish to light green clay bed overlies the sandstone horizon. Limestone is 40 to 70 cm thick, and characterized by the presence of a wide variety of invertebrate fossils viz. pelecypods, gastropods, ammonoids, and other microfossils of late Maastrichtian age (Govindan, 1981). There invertebrate shells are profusely distributed throughout the limestone bed, in the form of clusters, heaps/mounds entombed in limestone imparting a ‘packstone’ texture to it (Fig.2A). The mode of preservation of the invertebrate shells indicate as if there was an unusual proliferation and mass mortality of benthic organism in the basin during late Cretaceous prior to the outpouring of basaltic lava flows of the Rajahmundry Traps.
Lower Flow
Several physical volcanological features and geological details have been revealed during the quarrying in the basaltic rocks of the Rajahmundry Traps. Rapid advancement of the quarries are depleting the outcrop extent of the traps. An imminent concern is that some of the features documented in this communication may be mined out in a few years and might not be available for further study. The lower flow which is about 20-30 m thick is actively exploited by the local miners for road metal. It is overlying the infratrappean bed unconformably along a planar contact. The lower flow exhibits columnar and radial joints, massive vesicular basalt and a variety of physical volcanological features such as rootless cones, tumuli and dyke like forms. Rootless cones are prominent in Gowripatnam quarry and their incidence decreases in other quarries located along the strike extension of the flow.
Intertrappean I
Intertrappean I has a thickness of around ~2-3.5 m thick clay, marl and limestone intercalations and is sandwiched between the lower and middle flows. Basal bed is light grey splintery clay with 3 ‘ 10 cm rounded to subangular ball like weathered basalt occurring as isolated or clustered forms (Fig. 3A). This zone appears to be a ‘C’ horizon in the soil profile. This profile begins with unweathered basalt followed by partly weathered basalt (D) and clay corresponding to C, B and A layers of a standard soil profile (Fig. 3A) formed in humid to semi-arid climates. The clay layer (‘A’ horizon) is overlain by an assemblage of limestone and marl intercalations. This layer is ~15 cm thick at Gowripatnam (Pangidi) and attains maximum thickness of 2.2 m in Duddukuru section. It is yellowish brown to brown and attained porcellanitic nature due to baking effect along its contact with the overlying flow. The limestone is white to light purplish, indurated, micritic and at places displays sparite nature. It does not show any evidence of fossils on fresh surfaces. At Pangidi and Kateru (Rajahmundry) areas, several invertebrate fossils have been collected from this limestone horizon that has received great attention in view of their similarity with the intertrappean beds of western and central India. Fossil record has indicated an estuarine environment for the deposition of these intertrappean beds. Govindan (1981) and Keller et al. (2008) have earlier recorded Danian planktonic foraminifera assemblage of zone P1a whose significance is well documented with regard to K-T boundary mass extinction, basalt volcanism and resurgence of biota in Danian period.
Middle Flow
The middle flow unconformably rests on an uneven erosional surface as revealed by the presence of clay and limestone at the base of this flow exposed at different places. It is 6 ‘ 10 m thick greenish grey vesicular basalt. Millimeter size vesicles are noticed throughout the flow. Due to weathering, basalt attained conspicuous horizontal fissility which gives the appearance of bedding planes in a sedimentary rock. However, any relevant features of flow layering could not be deciphered. At some places, caughtup patches of underlying sediment are also observed. The middle flow is devoid of the physical volcanological features (rootless cones, and tumulii) that are commonly observed in the lower flow. Presence of caught-up patches of limestone is observed in the lower part of the middle flow. The infillings shown in Fig. 5B could be the caught up patches of intertrappean I preserved in the middle flow.
Development of wollastonite layer (~6 cm), observed at the contact between the intertrappean I and middle flow could be due to the conversion of calcite into wollastonite. This phenomena is localized along NW-SE fault near Pangidi which could probably be the path way for the middle flow.Such types of features have also been documented earlier (Knight et al. 2005; Humane et al. 2007).
Intertrappean II
It is well exposed in Gowripatnam (Fig. 4A) and Duddukuru quarries (Fig. 4B), which is ~ 2 ‘ 4 m thick, consisting of red clay/bole sandwitched between the middle and upper flow. In the Gowripatnam section, the basal part
of the intertrappean II is around two meter thick red clay in which rounded to sub rounded basalt fragments are
embedded (5 to 15 cm size). The contact between the basalt and the red clay is sharp (Fig. 4A). In Duddukuru section, this unit has weathered basalt at the base overlain by reddish brown clay (Fig. 4B). Difference of opinion exists about the origin of red bole as volcanic ash or sedimentary origin.However, further petrological and geochemical studies are under progress to evaluate the nature and composition of the red clay to decipher their origin.
Upper Flow
The upper flow is about 5 to 17 m thick, exposed in Gowripatnam and Duddukuru quarries, and overlies the red clay / bole horizon of the intertrappean II along a sharp contact. This flow consists of mainly fine-grained basalt which on weathering gave rise to spheroidal boulders of various sizes. The unweathered central part of the rock is massive, glassy, light colour, unusually hard and breaks with conchoidal fractures. Maximum thickness of the flow (~10 m) is recorded in Rajahmundry quarries. The upper flow is overlain by sandstone/conglomerate of the Cenozoic Rajahmundry Formation. A 60 ‘ 80 m thick clay present on the top of the upper flow represents a paleosol formed prior to the onset of sedimentation of the Rajahmundry Formation.
Fig.2. Field photographs of (A) Fossiliferous limestone infratrappean bed showing profuse gastropod shells; (B) conical mounds/rootless cones overlying the infratrappean beds; (C) calcite coating observed in the basal fragments of rootless cones; (D) loose (powdery material with caughtup limestone blocks in the rootless cones; (E) lava type flow marked by upper rubbly zone and lower massive basalt.
Fig.3. Field photographs exhibiting the (A) soil profile in the intertrappean I; (B) the squeezed-up intertrappean sediments within the middle flow.
Fig. 4. Field photographs of (A) Rounded to subrounded weathered basalt fragments in the intertrappean II at Gowripatnam and (B) Intertrappean II overlain by upper flow at Duddukuru.
CONCLUSIONS
‘ Rajahmundry Traps comprises of three basalt flows (lower, middle and upper) interbedded with two intertrappean sedimentary horizons identified as intertrappean I and II.
‘ These flows underlain by fossiliferous infratrappean bed that represents a marker zone of K-T boundary mass extinction.
‘ The succession of flows and intertrappeans are overlain by Cenozoic Rajahmundry sandstone.
‘ Physical volcanological features identified from the lower flow such as rootless cones, tumuli and dyke like forms indicate hydro volcanic origin for the formation of the lower flow.
‘ Soil profiles that developed in the intertrappean I and II indicate a change in palaeoclimatic conditions from humid to arid.
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References
‘ Baksi et al. 1994; Jay and Widdowson, 2008; Self et al. 2008 and references therein
‘ Keller et al. 2008.
‘ Govindan, 1981.
‘ Knight et al. 2005; Humane et al. 2007.