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Occurrence of Basalt Flow in Krishna-Godavari Basin-Offshore
Region, Andhra Pradesh, India
Sumangal Prasad Gupta, Email: sumangalprasad@gmail.com
Department of Geology and Geophysics, Gujarat State Petroleum Corporation Ltd, Gandhinagar, Gujarat-382010
Abstract:
There are many published papers reporting the presence of various basaltic flows in the onshore
region of Krishna-Godavari basin India. This paper reports the occurrence of basalt flow of
Cretaceous age in the offshore region of Krishna-Godavari basin in the Bay of Bengal, Andhra
Pradesh India. This basaltic flow occurrence is identified based on seismic data interpretation,
wireline logs, core data analysis, thin section study, XRD analysis and scanning electron
microscopy in two wells, KG-A and KG-B drilled by Gujarat State Petroleum Corporation ltd
(GSPCL). GSPCL is an exploration and production oil company based at Gandhinagar, Gujarat,
India. This basaltic occurrence in offshore region of KG basin may append a new dimension for
establishing the stratigraphy in deep water of KG Basin and may be helpful to study of mass
extinction of species due to change in atmospheric conditions in geological history.
Keywords: Basalt Flow, Offshore, Krishna Godavari Basin, Cretaceous Age, Mass Extinction.
1. Introduction:
The Krishna-Godavari (KG) Basin is a peri-cratonic extensional basin on the rifted east margin
of the Indian continental mass. The basalt flow in this basin has attracted more attention of
several authors worldwide due to eye-opener of KT boundary mass extinction events (Keller et
al. 2008). There is two school of thought regarding origin of basalt lava flow in KG basin. First
postulates that lava has transported along through a paleo valley from western India to eastern
India in Cretaceous time (Baksi et al. 1994; Jay and Widdowson, 2008; Self et al. 2008). Second
thought says that these basalt lavas flowed through fault controlled fissures within KG basin
itself (Reddy et al. 2002; Misra, 2005).
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Fig. 1.
(A) Present extent of Deccan Traps volcanic province
in India
based on outcrop data.
(B) Known extent of subsurface Deccan lava with quarry outcrops near the town of
Rajahmundry. (C) Location map of study area in Krishna Godavari Basin, offshore, Andhra
Pradesh, India (after Keller, 2008).
The wells KG-A and KG-B are drilled by GSPCL from hydrocarbon point of view in Krishna-
Godavari offshore basin. Basalt trap has been encountered while drilling of these two wells.
Depth of sea bed is 200m and 482m at well KG-A and KG-B from mean sea level in KG
offshore region.
2. Stratigraphy:
Generalized stratigraphy of Krishna-Godavari basin from onshore to deep offshore is provided in
Fig-2 (after Shanmugham et al. 2009).
3. Basalt Flow Interpretation: Seismological, petrological and wire line study has been used to
interpret its areal extension, mineralogy and environment in which basalt had flowed.
3.1. Basalt Flow Interpretation from Seismic Data: Three horizons of basalt flow have been
interpreted from seismic data between Lower Cretaceous Unconformity (LCU) and Lower
Cretaceous Unconformity-3 (LCU-3). These horizons are shown in the arbitrary seismic line
along the wells KG-A and KG-B (Fig- 3).
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Fig
-
2
. Generalized stratigraphy of Krishna
-
Godavari basi
n from onshore to deep offshore
(
after
Shanmugham et al. 2009).
Intertrappean rocks of basalt flows are sedimentary rock such as sandstone, siltstone and shale
(Fig-5). The seismic waves passing through the sedimentary to basaltic rocks have very high
amplitude. Seismic RMS amplitude has been extracted for all the three basalt horizons (Fig-4).
RMS Attributes shows that the areal extent of first basalt flow is more than the other two mapped
basalt flow. Seismically the maximum length and width of basalt rock in GSPC area is measured
8 km and 5 km in EW and NS direction respectively. After observing seismic data, it seems that
basalt is extending in South of this block but due to unavailability of data out of block, it could
not be delineated. This huge areal coverage may be only when lava has flown as extrusive
igneous body, not as an intrusive body.
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Fig
-
3
.
Arbitrary seismic line passing through wells KG
-
A and KG
-
B.
Three basalt flows have been
demarcated between LCU and LCU3 Formations.
Fig
-
4. (A) RMS
attribute
of
basalt flow-1.
Fig
-
4. (B) RMS attribute
of
basalt flow-2.
Fig
-
4. (C) RMS attribute
of
basalt flow-3.
3.2. Basalt Interpretation from Wireline Logs: Basic wireline logs i.e. gamma, resistivity,
neutron and density logs have been analized to interpret basalt flow of both wells. In basaltic
lava flow at depth interval of 4300-4370m TVDSS of well-A, Gamma ray is very low ranging
from 30 to 35gAPI (Rider, 2002), resistivity is high varying from 25 to 187ohm-m, neutron is
low varying from 0.083 to 0.086m3/m3 and density is high varying from 2.82 to 2.87g/cm3 (
Fig-5). Similar wireline tool reading is also found for drilled well KG-B. Maximum four distinct
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episodes of lava eruption of basalt at depth 4488, 4290, 4198 and 4172 m TVDSS have been
identified from the basic wireline log study (Fig-3) at drilled well KG-B. Intertrappean rocks are
sandstone, siltstone and shale of different thickness which tells that there was unequal time gap
in lava flow. But in seismic data due to very high depth, it is showing only 3 flows of basaltic
rock. Only one phase of lava flow is present at drilled well KG-A (Fig-3) due to pinching out of
structure. Basalt flow at well KG-A is encountered at depth 4300 m TVDSS measured from
wireline log. Thickness of basalt rock in well KG-A is 68m and in well KG-B is 5m, 170m, 12m
and 11m for 1st, 2nd, 3rd and 4th basalt flow respectively (Fig-5).
Fig
-
5
.
Correlation between Wells of KG
-
A and KG
-
B with interpreted lithology.
GR=Gamma
Log), shallow Res=shallow resistivity log, med res=medium Resistivity log, RHOB=Density Log,
NPHI=Neutron Porosity. LCU=Lower Cretaceous Unconformity, LCU3=Lower Cretaceous
Unconformity 3.
3.3. Environment of Basalt Flow from Core Data Analysis:
Core of well KG-B from depth 4232.05 to 4249 m is described from the bottom to the top to
interpret environment in which basalt had flowed. Core interval (4242.55 - 4249.00 m) is
composed of sandstone. The massive to contorted texture of these sediments and poor sorting are
consistent with mud flow deposition on a continental slope, within or adjacent to a slope channel.
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This interval (4237.70 – 4243.55 m) is composed of basaltic rock. Except this interval all cored
section is of sedimentary rocks. This igneous rock is medium dark gray to dark gray groundmass
with light gray to very dark gray phenocrysts. Small pyrite framboids are scattered throughout.
The interval (4236.15-4237.70 m) is composed of argillaceous sandstone. The massive to
contorted texture of these sediments and poor sorting are consistent with mud flow deposition on
a continental slope, within or adjacent to a slope channel. The interval (4232.05 - 4236.15 m) is
composed of conglomerate. Conglomerate is interpreted to have been deposited as a debris flow
on the continental slope and may have been the result of the collapse of the up-dip shelf. The
interval (4226.00 – 4232.05 m) is composed of sandstone. Environment of deposition of all
sedimentary rocks of this cores are marine. Basalts are lying between these sedimentary
depositions. Therefore it is clear from core description that basalt lava flowed abruptly as an
extrusive igneous body over marine sedimentary depositional system.
3.4. Petrography of Basalt: Thin section of basalt of well KG-B has made to study the
mineralogical composition of basalt which may assist in study of basalt origin. Thin section at
depth 4234.51m is andesitic basalt contains euhedral phenocrysts made up of plagioclase laths
partially replaced by calcite and amygdules composed of spherulitic, needle-like, acicular
chalcedony. The groundmass is made up of spiky plagioclase with swallow-tail microlite
morphology, pyrite and undifferentiated amorphous silica. This sample has porphyritic texture,
holo crystallinity and phaneritic granularity (Fig-6A).Thin section at depth at 4240.00m is
andesitic basalt contains euhedral phenocrysts made up of plagioclase laths, polycrystalline
quartz-filled amygdules, calcite-filled amygdules and clinopyroxene. The groundmass is made
up of spiky plagioclase with swallow-tail microlite morphology, volcanic glass (dark material)
and undifferentiated amorphous silica (Fig-6.B).
3.5. XRD Mineralogy of Basalt: XRD mineralogy has been carried out of basalt sample at
depth 4240.33m for well KG-B (Fig-7). The basalt contains plagioclase phenocrysts and
microlites that are carried in glassy groundmass.
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Fig-6. (A) Thin section at depth 4234.51m of
well KG-B.
(B) Thin section at depth 4240.00 m of well
KG-B.
Whole rock XRD analysis indicates that the mineralogy is dominated by plagioclase (64.3%),
followed by quartz (9%), calcite (7.8%), dolomite (1.5%), siderite (0.5%), pyrite (0.6%),
magnesite (0.9%) and halite (1.1%). Halite was probably derived from drilling mud
contamination or drying of saline formation brine. Clay mineral XRD analysis reveals that clay
minerals consist mainly of chlorite (9.2%), with lesser kaolinite (3.8%) and illite (1.3%).
Fig-7. Energy dispersive x-ray analysis spectrum of basalt of depth 4240.33m of well KG-B.
3.6. Scanning Electron Microscopy: Scanning electron microscopy has been carried out of
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basalt sample at depth 4240.33m for well KG-B. General view from Fig-8A illustrates basalt, in
which plagioclase (P) phenocrysts and microlites are carried in glassy groundmass (GM). Higher
magnification view illustrates plagioclase phenocrysts (Pp) and microlites (Pm), along with
shattered grains of plagioclase (Fig-8B). EDX spectrum shows the major elements of Si, Al, Ca,
and O that are consistent with plagioclase feldspar (Pp-EDX). By XRD, this sample is dominated
by plagioclase (Fig-7).
Fig-8. (A) Scanning electron microscopy of
basalt sample at depth 4240.33m.
Fig-8 (B) higher magnification view of
scanning electron microscopy at depth
4240.33m.
4. Age Determination of Basalt Based on Biostratigraphy: Biostratigraphic analysis for well
KG-A was carried out based on Foraminifera and Nannofossils from drill cutting samples for age
determination of basalt flow. The Planktonic Foraminifera occurrence of Hebbergella delrioensis
and Ticinella roberti at 4205m MD which just above the basalt flow in well KG-A, if in situ, may
indicate an age not older than Aptian at that depth. The Calcareous Nannoplankton occurrence of
Farhania varolii at 4105m MD which is just above basalt flow may indicate Aptian sediments.
Nannoconus inorhatus in depth interval 4330-4550m MD which is below to basalt flow indicates
the age of Barremian. All theses evidences suggest that basalt flow of well KG-A was happened
in age of Early Cretaceous in offshore region of KG basin. Biostratigraphic analysis of well KG-
B was carried out based on Nannofossils (foraminifera and palynomorphs are absent) from
conventional core intervals 4227.85m to 4233.73m. The uppermost four samples core (4227.85,
4228.30, 4228.80 and 4232.75) are barren of nannofossils, but are considered to be of the same
biofacies as the two lowermost samples which yielded some nannofossils. The lowermost two
samples (4233.5m and 4233.73m MD) which are just below to 3rd flow of basalt of well KG-B
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contain sparse age-diagnostic nannofloras including the long ranging Cretaceous-Jurassic taxon,
Watznaueria barnesae and long ranging Rhagodiscus splendens. The presence of Watznaueria
barnesae in particular also represents strong evidence for a broad Cretaceous to Jurassic age, but
on regional evidence this particular nannofacies can be more tentatively confined to within the
range Early Cretaceous (?mid Aptian) to Late Jurassic (?Oxfordian). All these evidences of
fossils assemblages from both wells indicate that basalt flow occurred in age of Cretaceous.
5. Environment of Basalt Flow from Biostratigraphy: Environment in which basalt flow
occurred has been predicted based on biostratigraphy from well KG-A. Basalt top and bottom is
4331 and 4395 mMD in well KG-A. Succession of depositional environments in well KG-A is
presented depth wise in Table-1 based on biostratigraphy. Interval depth of basalt flow lies
between the intervals depth of intertidal to shallow marine depositional environment. It indicates
that basalt lava have erupted as an extrusive igneous body in an intertidal or shallow marine i.e.
in hydrous environment. Huge shale thickness just above basalt flow, interpreted in well logs
also indicates that marine depositional environment was dominating above basalt flow at that
time (Fig-3).
In Pangidi-Rajahmundry area of KG onshore basin, lava erupted in a hydrous environment has
been reported. It has cylindrical/tube like paths under a water column that is indicated by radial
jointed basalts. Spatial association between rootless cones, columnar joints, and the radial jointed
basalt at the basal part of the lower flow indicates that the lava erupted in a hydrous environment
(Lakshminarayana, et al. 2010).
Table-1: Succession of depositional environments in well KG-A based on biostratigraphy.
Depth
Interval (m)
Environment Criteria
4005, 4105,
4205 and
4305
Marine, upper
bathyal
Abundant nannofossils, rare to few planktonic foraminifera. This indicates that open marine
conditions prevailed throughout the interval. Benthonic fauna characterized by
redominance occurrences of deep-water arenaceous taxa such as Ammobaculites,
Ammodiscus, Bathysiphon, Budhasevaella, Glomospira, Gaudryinopsis, Karreriella and
Verneuilina. Deep-water calcareous benthonics (Gavelinella, Gyroidinoides, Lenticulina,
Marginulina) are also sparse recorded.
4330 ?Intertidal to
shallow marine,
inner neritic
This sample yielded on few indeterminate arenaceous benthonics. If in situ, and not as
cavings, may suggest an intertidal to shallow marine, probably inner neritic condition.
4335-4705 Marine, inner
neritic.
All samples are barren of foraminifera, but continued occurrences of nannofossils
(including age-diagnostic forms) suggest marine, inner neritic settings down to 4705m.
Palynomorphs are sparse and poorly preserved suggesting oxic and/or low energy
depositional settings. Note: Similar assemblages can be transported into much deeper
water.
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6. Conclusions: Occurrence of Basalt flows is established in two offshore wells of Krishna
Godavari basin in Bay of Bengal drilled by GSPCL Company. Maximum three horizons of
basalt flow are interpreted in seismic section. RMS amplitude indicates that basalt flow-1 has a
highest flow compared to other two flows. One flow of basalt in well KG-A and four in KG-B
(due to high resolution than seismic data) has been identified from the basic logs. Plagioclase is
the dominating mineralogy as studied from thin sections and XRD analysis of basalt sample.
Conventional core description of well KG-B indicates that the basaltic flow occurred in
continental slope of marine depositional system. Most of fossil assemblages recognized from
overlying and underlying sedimentary rocks indicate that basalt flow occurred in age of
Cretaceous at sea bed of shallow marine depositional environment.
7. References:
1. Baksi, A.K., Byerly, G.R., Chan, L.H., Farrar, E., 1994. Intracanyon flows in the Deccan
province, India? Case history of the Rajahmundry Traps. Geology, 22, pp.605-608.
2. Baksi, A.K., 2001. The RTB, Andhra Pradesh: Evaluation of their Petrogenesis relative to the
Deccan Traps. Journal of Earth System Science, 110, pp397-407.
3. Baksi, A.K., Byerly, G.R., Chan, L.H. and Ferar, E., 1994. Intracanyan flows in the Deccan
Province India? Case History of the Rajahmundry Traps. Geology, v.22, pp.605-608.
4. Jay, A.E. and Widdowson, M., 2008. Stratigraphy, structure and volcanology of southeast
Deccan continental flood basalt province: implication for eruptive extent and volumes. Jour.
Geol. Soc. London, v.165, pp.177-188.
5. Keller, G., Adatte, T., et al, 2008. Main Deccan volcanism phase ends near the K–T boundary:
Evidence from the Krishna–Godavari Basin, SE India. Earth and Planetary Science Letters 268,
pp.293–311.
6. Lakshminarayana, G., Manikyamba, C., et al. 2010. New Observations on Rajahmundry Traps
of the Krishna-Godavari Basin. Jour.Geol.Soc.India, Vol.75, June 2010, pp.807-819.
7. Misra, K.S., 2005. Distribution pattern, age and duration and mode of eruption of Deccan and
associated volcanics. Gondwana Geol. Spec. Mag., v.8, pp.53-60.
8. Rider, M. 2002. The Geological Interpretation of Well Log. 2nd edition, whittles publishing.
pp.82-83.
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9. Shanmugam, G., Shrivastava, S. K., and Das, B., 2009. Sandy debrites and tidalites of
Pliocene reservoir sands in upperslope canyon environments, Offshore Krishna-Godavari Basin
(India): Implications. Journal of Sedimentary Research, v.79, pp. 736-756.
10. Self, S., Jay, A. E., Widdowson, M. and Kaszthelyi, L. P., 2008. Correlation of the Deccan
and Rajahmundry Trap lavas: Are these the longest and largest lava flows on Earth? Jour.
Vol.Geo. Res., v.172, pp.3-19.
11. Sen, B., Sabale, A. B., 2011. Flow-types and Lava Emplacement History of RTB, West of
River Godavari, Andhra Pradesh. Journal of Geological Society of India, 78, pp.457-467.
12. Reddy, P. R., Venkateswarulu, N., Prasad, A. S. S. S. R. S. and Koteswar Rao, P., 2002.
Basement Structure Below The Coastal Belt of Krishna-Godavari Basin: Correlation Between
Seismic Structure and Well Information, Gondwana Res., v.5, pp.513-518.
