Coastal ecosystem responses to late stage Deccan Trap volcanism: the post K-T boundary (Danian) palynofacies of Mumbai (Bombay), west India
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Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303 – 332
www.elsevier.com/locate/palaeo
Coastal ecosystem responses to late stage Deccan Trap volcanism:
the post K–T boundary (Danian) palynofacies of Mumbai
(Bombay), west India
J.A. Crippsa,*, M. Widdowsonb, R.A. Spicerb, D.W. Jolleyc
a
School of Earth Sciences and Geography, Kingston University, Kingston-upon-Thames, KT1 2EE, United Kingdom
b
Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
c
Centre for Palynology, University of Sheffield, Sheffield, S3 7HF, United Kingdom
Received 24 March 2004; received in revised form 23 August 2004; accepted 12 November 2004
Abstract
The Deccan Trap continental flood basalt eruptions of India occurred c. 67–63 Ma, thus spanning the Cretaceous–Tertiary
boundary (65 Ma). Deccan eruptions were coeval with an interval of profound global environmental and climatic changes and
widespread extinctions, and this timing has sparked controversy regarding the relative influence of Deccan volcanism upon end-
Cretaceous catastrophic events. If Deccan Trap activity was capable of affecting global ecosystems, evidence should be present
in proximal Indian sedimentary facies and their palaeontological contents. The impact of late stage Deccan volcanism upon
biota inhabiting Mumbai (Bombay) Island’s post K–T boundary lagoonal systems is documented here. Sediments (or
bintertrappeansQ) which accumulated within these lagoons are preserved between Trap lavas that characterise the closing stages
of this flood basalt episode.
Mumbai Island Formation intertrappean faunal and floral communities are conspicuously distinct from those common to
many pre K–T boundary, late Maastrichtian intertrappeans across the Deccan province. The latter sedimentary intercalations
mostly developed in cognate semiarid, palustrine ecosystems; by contrast, those around Mumbai evolved in sheltered,
peripheral marine settings, within subsiding continental margin basins unique to this late Deccan stage, and under an
increasingly humid Danian climate. Geochemical analyses reveal that Mumbai sedimentation and diagenesis were intimately
related to local explosive volcanic and regional intrusive activity at c. 65–63 Ma. Although tectonic and igneous events
imprinted their signatures throughout these sedimentary formations, organisms usually sensitive to environmental perturbations,
including frogs and turtles, thrived. Critically, palynofacies data demonstrate that, whilst plant material deposition was
responsive to environmental shifts, there were no palpable declines in floral productivity following Mumbai pyroclastic
discharges. Therefore, it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally.
D 2004 Elsevier B.V. All rights reserved.
Keywords: K–T boundary; Deccan Traps (India); Flood basalt; Mass extinction; Palaeoecology; Palynofacies
* Corresponding author. Fax: +44 20 8547 7497.
E-mail address: j.cripps@kingston.ac.uk (J.A. Cripps).
0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2004.11.007304 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
1. Introduction emplacement of an entire flood basalt province would
theoretically prove more detrimental than a series of
Continental flood basalt provinces are laterally events separated by protracted dormant intermissions.
extensive lava accumulations of substantial thickness Proof of quiescent phases exists in the form of
and low topographic relief (Rampino and Stothers, sedimentary sequences that accrued between the
1988). India’s dominantly tholeiitic Deccan Trap Traps. Subsequent extrusives often preserved these
flood basalt province presently extends across approx- bintertrappeansQ, and evidence can be sought within
imately one sixth of the subcontinent, encompassing them regarding the influence of volcanism upon
up to 106 km2 of its western portion (Deshmukh, sedimentary systems, microclimates and biota.
1982; Fig. 1). The basalts include Traps downfaulted Because substances released during mafic erup-
into the Arabian Sea west of Mumbai (Bombay) and tions are less likely to reach potentially damaging
forming part of the Seychelles microcontinent (Tan- stratospheric levels than those expelled by felsic
don, 2002; Devey and Stephens, 1991), and possibly volcanism, the effects of late stage, increasingly
originally occupied a volume of up to 106 km3 prior to felsic, explosive Mumbai volcanism are of interest.
their erosion (Courtillot et al., 1986). Controversially, a study of massive, well-constrained
The duration of the whole Deccan volcanic episode pyroclastic events (Erwin and Vogel, 1992) found
remains a polemic issue, and advocates exist for both that these did not reduce the ecological diversities of
a brief (b1 m.yrs., e.g., Duncan and Pyle, 1988; land and marine ecosystems on regional or global
Hofmann et al., 2000) and extended (e.g., Widdowson scales, and hence were unlikely to be responsible for
et al., 2000; Sheth et al., 2001a) period of activity. mass extinctions. A bolide impacting Mexico’s
This theme is particularly pertinent when assessing Chicxulub platform (Hildebrand et al., 1991) is
the effects of flood basalt volcanism upon local, broadly accepted to have exacerbated, if not singu-
regional and even global ecosystems. A rapid larly forced, end Maastrichtian extinctions across the
Fig. 1. Present-day Deccan Trap outcrop extent. Major tectonic structures redrawn from Biswas (1991).J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 305
planet (e.g., Pope et al., 1994; Sweet et al., 1999; first attempts to evaluate ecosystems within a flood
Vajda et al., 2001). basalt succession using an integrated palaeobotanical,
The literature review we offer draws together c. geochemical, geochronological and sedimentological
100 years of disparate observations, with the benefit approach.
of a much improved chronostratigraphic framework, A similar study was conducted for central India’s
and represents the most comprehensive overview yet Jabalpur region, near the Narmada–Tapti rift zone,
produced on Mumbai sequences. Data presented here and the Nagpur area to the south, by Tandon (2002;
are placed within this context, to illustrate the Fig. 1). Tandon’s article described the environmental
ecology of a Deccan volcanic region towards the changes leading up to the onset of local Trap
close of this flood basalt episode. This is one of the emplacement that are recorded in central Indian
Fig. 2. Mumbai District, including localities visited, adapted from Subbarao and Sukheswala (1979).306 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Table 1 Table 1 (continued)
Intertrappean sample lithologies chosen for palynomorph analyses Section Sample Description
and additional techniques
Worli tunnel Wo 2736 2736 m west from shaft,
Section Sample Description organic-rich shale
Bandra tunnel B 2800 2800 m From entrance: Worli tunnel Wo 2850 2850 m west from shaft,
coaly layer organic-rich shale
Bandra tunnel B 3000 3000 m From entrance: Worli tunnel Wo 3128 3128 m west from shaft,
dark, flat-laminated shale organic-rich shale
Bandra tunnel B 3130 3130 m From entrance: Worli tunnel Wo 3408 3408 m west from shaft,
compact, flat-laminated shale organic-rich shale
Bandra tunnel B 3510 3510 m From entrance:
dark shale with pyrite cubes
Jogeshwari Bom 1/98 Fairly coarse, carbonate-rich
Jogeshwari Bom 2/98 Dark, carbon-rich, laminated Lameta Formation sediments. Here, topographic
Jogeshwari Bom 3/98 Coarse, pale and dark laminations adjustments caused fluvial currents to redirect, and
Jogeshwari Bom 4/98 Thick, carbon-rich, periodically submerged terrain to became increasingly
burrows, pyrite subaerial. Although this dynamic landscape was
Jogeshwari Bom 5/98 Rippled silt
influenced by regional volcanic activity, it was
Jogeshwari Bom 6/98 Predominantly coarse
Jogeshwari Bom 7/98 Tuff exploited by sauropod dinosaurs prior to the first
Jogeshwari Bom 8/98 Fissile, laminated local lava incursion (Tandon, 2002).
Jogeshwari Bom 9/98 Tuff/calcareous mix The Mumbai peninsula is investigated by the
Jogeshwari Bom 10/98 Dark, carbon-rich present authors. Originally a series of islands (e.g.,
Jogeshwari Bom 11/98 Coarse, plainly bedded
Bombay Island, Salsette Island), the landmass projects
Jogeshwari Bom 12/98 Dark shale and pale,
coarser sediment interlaminated southwards into the Arabian Sea at c. 198 north (Fig.
Jogeshwari Bom 13/98 Dark shale 2). Three intertrappean sections on the western side of
Jogeshwari Bom 15/98 From bdoggerQ layer the peninsula were investigated: an outcrop at Amboli
with calcite veins quarry in Jogeshwari, and two tunnel cuttings exca-
Jogeshwari Bom 16/98 Ash containing small white flecks
vated seawards from the coast, just south of Worli and
Jogeshwari Bom 17/98 Light olive-grey silt
Jogeshwari Bom 18 /98 Trap basalt (top of section) near Bandra (Fig. 2). Both tunnels extend westward
Jogeshwari Bom 19/98 Rippled, dark grey silt into the Arabian Sea, and samples were extracted
Jogeshwari Bom 20/98 Finely laminated very along them between 2001 and 3408 m in the Worli
dark grey silt tunnel, and 1890 and 3740 m in the Bandra tunnel
Jogeshwari Bom 22/98 Float crustacean claw
(Table 1). Since completing fieldwork, the Amboli
Jogeshwari Bom 23/98 Fragments from coarse bed,
possible tuff section has been demolished for housing construction.
Jogeshwari Bom 1/99 Phlogopite-rich, ?rhyolitic tuff This work provides a graphic log and field summary
Jogeshwari Bom 2/99 Slatey layers, flat-bedded, of the lost section. A brief description of Amboli,
v.dark, ?organic-rich Worli and Bandra lithologies is given in Table 1.
Jogeshwari Bom 3/99 Volcanic bombs
Worli tunnel Wo 2001 2001 m west from shaft,
organic-rich shale
Worli tunnel Wo 2100 2100 m west from shaft, 2. Geological setting
organic-rich shale
Worli tunnel Wo 2210a 2210 m west from shaft, 2.1. Stratigraphy and field relationships
organic-rich shale
Worli tunnel Wo 2210b 2210 m west from shaft,
organic-rich shale The Mumbai and Salsette Islands landmass com-
Worli tunnel Wo 2600 2600 m west from shaft, prises a linear depression bounded by easterly and
organic-rich shale westerly ridges (Sukheswala, 1956). Muddy sedi-
Worli tunnel Wo 2610 2610 m west from shaft, ments deposited in the central lowland dip 12–158
organic-rich shale
west, and lavas up to 258 west (Sheth et al., 2001a). A
Worli tunnel Wo 2735 2735 m west from shaft,
organic-rich shale separate classification to the Deccan chemostratigra-
phy, established in the Western Ghats and nowJ.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 307
covering much of the main Deccan province (MDP), Table 3
exists for the distinct geochemistries of Mumbai Stratigraphical position of present samples within the Salsette
Subgroup, after Sethna (1999)
intrusives and extrusives (Sethna, 1999; Table 2).
Subgroup Formation Geology Samples
The Amboli (Bom), Worli (Wo) and Bandra (B)
intertrappean shale sections detailed here occur within Salsette Manori Formation Trachyte and –
Subgroup basalt intrusions
the Mumbai Island Formation, the lowermost of the
Salsette Subgroup (Table 3). Sethna (1999) placed this Mahd–Utan Rhyolite lava flows –
above the highest of the MDP, the Wai Subgroup. Formation
According to him, Worli intertrappeans occur strati-
Mumbai Island Hyaloclastites, Bom, Wo, B
graphically above the Malabar Hill flow (Fig. 3). Formation spilites, basalts
Sethna (1999) estimated this shale’s thickness at c. and shales
150 m, interrupted only by a 10-m tuffaceous breccia
(hyaloclastite) horizon, and a 5-m basaltic layer. The
nearby Bandra tunnel also runs through this sedi- Magnetostratigraphical correlations between Mum-
mentary unit, and the onshore Amboli section bai flows and the MDP volcanic pile have been
possibly represents a lateral equivalent. attempted. Vandamme et al. (1991) and Vandamme
Pandey and Agrawal (2000) detected several and Courtillot (1992) detected a reversed-normal
sedimentary basins offshore of Mumbai and in boundary obscured by a secondary palaeomagnetic
adjacent western Indian offshore areas, retaining component in some localities. These authors estab-
India’s largest hydrocarbon reserves (Gombos et al., lished that the changeover occurred at much lower
1995). Stratified intertrappeans in quarries around altitudes than the typical 600-m elevation observed
Jogeshwari (Fig. 2) have been intruded by a columnar elsewhere in the Deccan (e.g., Western Ghats), and
jointed tholeiitic lopolith (Subbarao and Sukheswala, interpreted the Mumbai boundary to possibly repre-
1979) and are conformably overlain by a basaltic lava sent a later, younger magnetic reversal.
flow. The position of Jogeshwari exposures within the
regional stratigraphy, and possible provincial north– 2.2. Age
south correlations, are given in Fig. 4.
An early Tertiary age was first assigned to
uppermost Mumbai intertrappeans by Blanford
Table 2
(1867), and an inferred close affinity of Mumbai
Deccan chemostratigraphy from Mitchell and Widdowson (1991)
intertrappean biota with modern forms led Sukhes-
Subgroup Formation
wala (1956) to support this. However, Singh and
Salsette (4) Manori (4) Sahni (1996) found that several Mumbai taxa addi-
Madh–Utan
Mumbai Island (4)
tionally occurred in intertrappeans as divergent as
Wai (3) Desur Kutch (Gujarat), Jabalpur (Madhya Pradesh), Nagpur
Panhala (Maharashtra), Gurmatkal and Marepalli (Andhra
Mahabaleshwar (1) Pradesh; Fig. 1), indicating correlations between all
Ambenali (1) these sections. Mumbai ostracod assemblages were
Poladpur (1)
Lonavala (3) Bushe (2)
observed to have affinities with late Cretaceous and
Khandala (3) Palaeocene forms. The authors ultimately ascribed a
Kalsubai (3) Bhimashankar (3) Maastrichtian date, attributing contrasts between
Thakurvadi (3) Mumbai and other Deccan facies to environment
Neral (3) rather than age differences.
Igatpuri (3)
Jawhar (3)
Highly accurate radiometric dates of Mumbai
extrusives recently obtained (e.g., Table 4) are closely
Data compiled from: (1) Cox and Hawkesworth (1984), (2) Cox and
Hawkesworth (1985), (3) Beane et al. (1986) and (4) Sethna (1999).
comparable with those received for late stage MDP
Initial Salsette Subgroup eruptions were coeval with Mahabalesh- feeder dykes (Widdowson et al., 2000). Sheth et al.
war-Desur Formations of the Wai Subgroup. (2001a) argued that Mumbai volcanism continued for308 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 3. Schematic section across Mumbai Island identifying the major lava flows, separated by intertrappeans (marked as bIQ), encountered in
boreholes and detected outcropping at Sewri and Malabar Hill, from Sethna (1999).
z1 m.yrs. Hence, it strongly appears that Salsette extinctions, and represents the final throes of the
Subgroup igneous activity was coeval with terminal Deccan flood basalt episode.
Wai Subgroup eruptions along the Western Ghats,
although the flow-types are not geochemically related. 2.3. Tectonic setting
By this closing stage, the most intense and volumi-
nous MDP lava formations had already erupted (Table Sukheswala (1956) determined that a narrow basin
2). Locally restricted Mumbai Island magmatism and common volcanic centres occurred along subsur-
directly proceeded the major K–T boundary global face fracture zones, trending north–south across the
Fig. 4. Possible correlation of Mumbai province stratigraphy.J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 309
Table 4
Published ages of a variety of Deccan igneous rocks occurring around the Mumbai and Salsette Islands, in reverse chronological order (dates
acquired from Amboli samples by M. Widdowson); wr=whole rock, pl=plagioclase
Rock Method Date (Ma) Confidence Comments Reference
40
Basalt (tholeiite) Ar/39Ar (wr) 64.55F0.59 2r Sample Bom18/98 Widdowson et al. (2000)
40
Rhyolitic tuff Ar/39Ar (wr) 64.64F0.39 2r Sample Bom1/99
40
Trachyte Ar/39Ar (wr) 60.4F0.3 2r Unaltered sample Sheth et al. (2001b)
40
Trachyte Ar/39Ar (wr) 61.8F0.3 2r Unaltered sample
40
Basalt (tholeiite) Ar/39Ar (wr) 60.5F1.2 2r Unaltered sample Sheth et al. (2001a)
40
Intermediate rock Ar/39Ar (wr) 62.4F1.0 Unspecified From Salsette Island Kaneoka et al. (1997)
Not specified K–Ar (pl) 60.2F2.5 1r Unaltered sample Vandamme et al. (1991)
Not specified K–Ar (pl) 62.8F3.0 1r Unaltered sample
Trachyte Rb–Sr (wr) ~60 – No clear isochron Lightfoot et al. (1987)
Rhyolite Rb–Sr (wr) 61.5F1.9 Unspecified High initial 87Sr/86Sr
Basalt (tholeiite) K–Ar (pl) 88.8F4.0 1r Argon excess Balasubrahmanyan and Snelling (1981)
40
Olivine nephelinite Ar/39Ar (wr) 72.0F6.9 – No plateau ages Kaneoka (1980)
40
Basalt (tholeiite) Ar/39Ar (wr) 74.1F3.3 – No plateau ages
Mugearite K–Ar (wr) 38.7F0.9a 1r Altered sample Kaneoka and Haramura (1973)
a
Age corrected with new decay constants by Vandamme et al. (1991).
Mumbai and Salsette Islands. A regional, oval- extension arguably promoted the mantle upwarping
shaped, 12 km height by 35 km base diameter that resulted in the Mumbai gravity anomaly (Dessai
positive gravity anomaly, with its focus along the and Bertrand, 1995). Lightfoot et al. (1987) consid-
west coast of Salsette Island, coincides with an area of ered this to have triggered partial melting of lower
high heat flow (Negi et al., 1992, 1993; Fig. 1). crust gabbroic complexes and an associated produc-
Hooper (1999) and Sen (2001) inferred that mildly tion of trachytic magmas, whilst contamination from
alkaline and tholeiitic dykes bearing mantle xenoliths, assimilated crust was debated to have generated the
again trending roughly north–south, created this more acidic suites present.
gravity high, and Sethna (2003) associated the Negi et al. (1992) interpreted the Salsette Island
Mumbai anomaly with intermediate and felsic igneous gravity anomaly as a magma conduit, discrete from
rocks underplated by gabbroic intrusive complexes. the main Deccan plume, which breached the con-
Vertical movements played a key role in shaping tinental margin fracture zone offshore of Mumbai.
Mumbai Trap palaeoenvironments. Blanford (1872) This fracture, and the Seychelles block detachment,
proposed a mechanism which instigated alternating were stated to be related to a bolide collision.
rising and sinking events across Mumbai Island, and Chatterjee and Rudra (1996) submitted the Mumbai
structures across the district have recently been High (Fig. 1) oilfield and Deccan intrusives as
attributed to tectonic deformation (Widdowson, evidence of an impact (the bShiva craterQ), embroiling
1997; Sheth and Ray, 2002). North–south trending a putative offshore Mumbai meteorite strike with K–T
fractures through, and the block tilting of, offshore boundary extinctions. Shale organic maturation was
Mumbai basement rock have been related to the allegedly instigated by impact-induced lithospheric
western margin of India rifting from Madagascar, then heating, and the offshore region, uplifted by earlier
the Seychelles bank, respectively, before or during the Deccan magma accumulation, sank in response
Deccan volcanic episode (e.g., Devey and Lightfoot, (Pandey and Agrawal, 2000).
1986; Singh and Sahni, 1996). Mumbai regional tectonic characteristics are more
Inferring a different sequential order from flow- widely implied to be entirely products of terrestrial
mapping, Hooper (1990) concluded that the litho- processes (e.g., Sethna, 2003; Table 5). Gombos et al.
spheric thinning, shearing and rotation which pro- (1995) suggested that India’s west coast hydrocarbon
duced the present regional westward dips only ensued reserves resulted from a Mesozoic collapse of
after Réunion mantle plume emplacement, litho- Proterozoic mobile belts into passive margin basins,
spheric doming and MDP eruptions. This crustal during and following the rifting that produced the310 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Table 5 Structures including ripple marks prompted Sukhes-
Chronology of tectonic events influencing Mumbai Island For- wala (1956) to advocate shallow lakes as likely
mation pyroclastic and sedimentary facies
depositional environments for the lowermost sedi-
Stage Events
ments. Oblong concretions of V10 cm diameter in a
Stage 1 Lithospheric doming above Réunion plume, prominent ash bed exposed along Mumbai Island’s
flood basalt activity across main Deccan province
western ridge were interpreted by this author to
Stage 2 Rifting begins along previously existing
N–S crustal fractures, crustal blocks tilted westward represent ash bombs which coalesced during pyroclas-
Stage 3 Development of shallow gulf as rifting and tic eruptions, and a recurring subaqueous influence was
subsidence propagate, water invades depressions deduced from the widespread occurrence of laminated
Stage 4 Magma upwells beneath thinned crust beds. Deshmukh (1984) recognised that breccias had
and intrudes into tensive crustal fractures
evolved from explosive volcanic activity, such vola-
Stage 5 Mumbai Island Formation explosive
volcanism; shale and ash deposition tility being enhanced by the invasion of water follow-
into basin systems ing subsidence.
Stage 6 Intertrappeans buried as subsidence Sethna (1999) described most Mumbai district
continues and thermally metamorphosed by intrusions flow facies as at least partially subaqueous. Extrusive
Stage 7 Tertiary erosion onshore and deposition
offshore isostatically enhances westward dips
breccias in the Amboli section, Jogeshwari, are
composed of basaltic and altered vesicular glass clasts
in a fine- to medium-grained clay, carbonate and
Mumbai High fault block. Sedimentation into Mum- quartz-rich matrix. Their petrography indicated a
bai High rifts was dominated by organic-rich shales, spilitic origin to Tolia and Sethna (1990), the
with continued subsidence promoting their thermal hyaloclastites having consolidated during phreato-
heating and maturation (Gombos et al., 1995). magmatic basalt effusions. These authors did not
Widdowson (1997) attributed the current western detect volcanic bombs, finding infrequent subcircular
Indian margin geomorphology to simultaneous objects possessing chilled margins to be pillow
onshore erosion and offshore deposition operating structures. The angular shapes of most volcanic
throughout the Tertiary. fragments suggested to Singh (2000) that these
underwent minimal aqueous transportation; conse-
2.4. Facies quentially, the eruptive centres themselves are likely
to have occupied low-grounds.
The crustal subsidence that accompanied Mumbai Sharma and Pandit (1998) assigned ignimbrite
Island Formation activity represents the waning phase facies to cycles of felsic tuffs overlying intermediate
of Deccan activity (Singh and Sahni, 1996). Con- to mafic pyroclastic flows in the Sasunavghar–
sequentially, Mumbai intertrappeans are generally Juchundra area, c. 5 km north of Salsette Island.
much thicker than MDP sequences. An exceptionally The greater pyroclastic content of such sequences
thick shale overlying the Malabar Hill flow reflects a around Mumbai than other Deccan fringe regions was
prolonged volcanic hiatus (Sethna, 1999; Fig. 3), and regarded by Singh and Sahni (1996) to reflect a closer
The Worli and Bandra tunnels cut into extensive, proximity to their volcanic source, their evolved
carbonaceous shales (Sethna, 1999). Sukheswala chemistries pointing to the termination of Deccan
(1956) described the western ridge at Malabar and events.
Worli as composed of a repetitive series of green and Igneous, tectonic and hydrological activity greatly
black ashes, and similar facies occur further north, influenced Mumbai shale as well as ash facies.
around Jogeshwari (Sukheswala and Awate, 1957; Amboli intertrappeans display a hardened, baked
Fig. 2). Volcanic and pyroclastic units were substan- margin where they contact the tholeiitic lopolith,
tially reworked during repose phases, becoming and elsewhere exhibit plastic deformation (Tolia and
increasingly clay and organic-rich, as reflected in a Sethna, 1990). Singh (2000) attributed shale baking to
transition from greenish ashes and rhyolites to dark, heat conducted from overlying lavas. Mumbai shales
fossiliferous shales in the Malabar and Worli hills of are indicative of sedimentation under waters with low
the western ridge (Sukheswala, 1956). oxygen concentrations (Singh and Sahni, 1996), asJ.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 311
reflected in pyrite precipitation along many carbona- (1867), and additional species of frogs, the most
ceous laminations (Singh, 2000). However, occasional abundant faunal element, by Chiplonkar (1940).
subaerial exposure led to desiccation and swamp Sukheswala (1956) extracted two Carteremys leithii
formation under semiarid climes, as evidenced by freshwater Pelomedusidae turtle specimens, and a
calcite-filled rain prints and mud cracks (Singh, tooth later diagnosed as crocodilian (Singh and Sahni,
2000). 1996).
According to Singh and Sahni (1996), preservation
2.5. Geochemistry within the Mumbai shales is unique to the Deccan,
being superior to that within most MDP intertrap-
Sukheswala (1956) identified pyroxenes and feld- peans. These authors examined the faunal component
spars flanking calcite crystals in a Worli ash, and thus of sections at Worli Hill, Amboli and Malabar,
inferred a mafic chemistry. Partially decomposed unearthing Shweboemys (Carteremys) leithii skull
feldspars, pyroxenes and biotite also occur in Jogesh- and carapace fragments within the latter. This genus
wari tuffs, with calcite and quartz forming the major was further documented in MDP sediments at Nagpur,
minerals here. Amboli hyaloclastites contain higher Marepalli and Kutch (Fig. 1). Similarly, the Mumbai
H2O and Na2O proportions than the local tholeiites, ostracod genera Mongolianella, Altanicypris, Cypri-
these enrichments in hydrous and alkali phases having dea (Pseudocypridina), Timiriasevia and Cyprois
been influenced by magma contacting water during its were associated with those from MDP intertrappeans
crystallisation according to Tolia and Sethna (1990). (e.g., Bhatia et al., 1990; Whatley et al., 2003). A new
These authors recognised Amboli plagioclase to be a pelomedusoid turtle species, Sankuchemys sethnai,
sodium-rich variety, and found that much of the has recently been extracted at Amboli (Gaffney et al.,
calcite and quartz occurred as secondary minerals 2003).
filling veins alongside zeolites. Metasomatism related Genera common to inland and marginal marine
to tectonism and intrusions is likely to have instigated ecosystems signify that either lagoon waters were
zeolite precipitation across the Mumbai district occasionally virtually freshwater, or that central
(Sabale and Vishwakarma, 1996). Indian lakes tended towards brackish. However, Singh
Evidence of pyroclastic activity associated with and Sahni (1996) emphasised that dinosaur and fish
terminal Deccan tensional regimes is preserved in the taxa, important in several widely distributed MDP
clay fractions of Mumbai shales. An X-ray diffraction localities, are entirely absent in the Mumbai shales
(XRD) study of Amboli, Worli and Malabar inter- (Table 6). The lack of fish was attributed to water
trappean mineralogies (Singh, 2000) revealed match- turbidity or contamination, conditions frogs were
ing mineral suites that indicated a mafic ash capable of tolerating (Singh and Sahni, 1996),
provenance for the shales’ clastic components. Pyrox- although turbid waters of modern coastlines are often
enes degraded, glass devitrified and smectitic clays colonised by fish. Even the Mumbai Leptodactylidae
evolved during reworking, the smectites producing frog Indobatrachus was distinguished from MDP
few reflection angle peaks due to their weak crystal Pelobatidae and Discoglossidae forms (see also
structure development (Singh, 2000). Smectites and Khosla and Sahni, 2003, and references therein).
chlorite constitute the most important Mumbai clays, The absence of some important MDP taxa around
and combine to form a mixed-layer superlattice. Mumbai, despite favourable preservation conditions,
led Blanford (1867) to speculate that the cumulative
2.6. Palaeontology effects of previous Deccan volcanism suppressed
rainfall and damaged Mumbai environments to the
Owen (1847) assigned frog remains within shales extent that most MDP organisms lapsed into extinc-
underlying the Malabar Hill Trap at Worli Hill the tion. He interpreted poorly fossiliferous volcaniclas-
species Rana pusilla, although the fossil evidence for tics low in the Malabar and Worli sequences to signify
Maastrichtian Indian ranids has since been queried originally barren ecosystems, and suggested that
(Bossuyt and Milinkovitch, 2001). Turtles and mol- Mumbai communities were a replacement biota to
luscs from this section were detailed by Blanford MDP fauna. Sukheswala (1956) reasoned that con-312 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Table 6 Table 6 (continued)
Important organism groups in the Poladpur, Ambenali and Mumbai Organism Poladpur Ambenali Mumbai
Island Formations (based upon a collation of results presented in Island
Cripps, 2002 and references therein)
Algae Acritarcha Y Y –
Organism Poladpur Ambenali Mumbai Botryococcus Y Y Y
Island Dinoflagellate Y Y –
Dinosaur Y Y – Zygnemataceae Y Y –
Crocodile – Y Y
Fish Apateodus Y Y –
Lepisosteus Y Y – temporaneous local, rather than preceding regional,
Phaerodus Y – – volcanic activity generated a terrain inhospitable for
Pycnodus Y Y –
Mumbai life. A thick basal greenish ash was thought
Ray Y Y –
Stephanodus Y – – to indicate an extended extrusive episode prior to a
Turtle – Y Y period of diminishing volcanism and community
Frog Y Y Y regeneration, represented by upper dark, fossiliferous
Gastropod Lymnaea Y Y – shales.
Paludina Y Y –
Physa Y Y –
According to Mumbai Trap radiometric dates
Planorbis Y – – (Table 4), the diverse shale communities survived
Bivalve Unio – Y – regional and global K–T boundary events. Bossuyt
Ostracod Altanicypris Y Y Y and Milinkovitch (2001) detailed archaeobatrachan
Bisulocypris – Y – frog lineages enduring the Deccan volcanic episode
Candona Y Y –
along the Indian island’s peripheries, and thriving
Cypridea – Y Y
Cyprinotus Y – – during the early Tertiary, notwithstanding their
Cypris – – Y probable confinement along the western fringe by
Cyprois Y Y Y volcanism to the east and an ocean to the west.
Drawinula Y – – Although many frog groups are environmentally
Eucandona Y – –
sensitive, some Leptodactylidae species have broad
Metacyprois Y – –
Mongolianella Y Y Y physiological tolerances, and today populate habitats
Mongolocypris Y – – undergoing ecological or climatic disturbances (Kai-
Paracypretta Y – – ser, 1997).
Paraconadona Y – –
Talicypridea – Y –
2.7. Palaeobotany
Timiriasevia – Y Y
Charophyte Harrisichara – Y –
Microchara Y – – Mumbai intertrappean plant megafossils are
Peckichara Y Y – uncommon and distinct from those of the MDP
Platychara Y Y – (Blanford, 1867), but have similarly originated from
Stephanochara – Y –
land plants (Sukheswala, 1956). Bande et al. (1988)
Angiosperm Aquilapollenites Y Y –
Arecaceae Y Y Y and Bande (1992) found limited Bambusaceae and
?Betulaceae Y Y – Podocarpaceae wood, leaflets of possible Acacia
?Caprifoliaceae Y Y – (Leguminosae) affinity, and seeds similar to Artabo-
?Mimosaceae Y Y – trys (Annonaceae). Megafloral remains are allochth-
Gymnosperm ?Araucariaceae Y Y –
onous within Mumbai basin facies, and the buoyancy
Bennettitaceae Y Y –
Ginkgoaceae Y Y – of such organs as bamboo cane probably assisted their
Pinaceae Y Y – transportation. Leptodactylidae frog taxa that cur-
Podocarpaceae – – Y rently inhabit marine supra- to intertidal zones and
Pteridophyte Gleicheniaceae Y Y – consume saline marine food must regulate their
Osmundaceae – Y –
osmotic balance (Abe and Bicudo, 1991). It thus
Polypodiaceae – Y –
Salviniaceae Y Y Y seems plausible that Indobatrachus consumed terri-
genous plant detritus washed down from vegetatedJ.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 313
areas, a diet that ostensibly safeguarded the frogs from frequently exhibit fine, laterally continuous
any effects of temporary productivity declines driven organic drapes.
by volcanic disturbances (cf. Sheehan and Fastovsky, A 1.22-m ash, Bom 1/99, forms a salient,
1992). continuous bed through the section’s centre.
Palynofacies analyses are useful in combination This resistant unit yields virtually unaltered
with sedimentological investigations, potentially crystals the potassic mica phlogopite and quartz.
distinguishing environmental transitions before mac- Beneath, the uppermost fraction of Bom 8/98
roscopic change is visible (Tyson, 1985). An amal- consists of a series of fining-upwards beds.
gamation of the ecology of organic matter (OM) Fining-upwards cycles throughout this section
producers, palynodebris transportation, decomposition tend to be continuous but thin, containing
prior to burial and alterations during diagenesis ge- neither body nor trace fossils. However, small
nerates a sediment’s palynofacies characteristics. (1–2 cm) internal moulds of bivalves and
According to Cross and Taggart (1982), the principal gastropods occur sporadically elsewhere. An
factors determining plant fossilisation are tissue upper bedding plane exposed upon the quarry
durability, transportation distance, the existence and floor is pitted by common burrows (cf. Thalas-
persistence of viable preservation sites, and sedimen- sinoides), these being virtually absent in higher
tation rates and consistency. No palynofacies analyses beds. These are subhorizontal, smooth-walled,
have previously been performed upon Deccan inter- pellet back-filled, c. 1.5 cm diameter and 6 cm
trappean floral material. length, connecting at triple-junctions. Slightly
oblate features of 1–1.5 cm diameter in Bom 6/
98, viewed in cross section in the quarry face,
3. Data collection initially appear to be higher, slightly com-
pressed, vertical expressions of these horizontal
3.1. Field observations traces. However, laminations cup underneath
them and, when excavated, their true subspher-
(1) Amboli quarry, 19808V03WN; 072850 V30WE, 10 m ical rather than cylindrical shape becomes
a.s.l. exposes an intertrappean of z10 m thick- apparent.
ness, dipping westward c. 88, terminating in a Several ash beds are indistinctly stratified, either
junction with basalt above (Fig. 5). Its base is coarsening or fining-upwards. Some layers are
obscured by the quarry floor (the underlying dominated by grains, commonly feldspars, of up
flow, occurring c. 3–4 m beneath ground level to 1 cm, horizontally aligned in thin, parallel
here, outcrops to the northeast). Sediments range bands. A coarse carbonate cement envelops the
from dark grey, flat-laminated shales, through Bom 1/98 and Bom 2/98 matrices. Crystalline
silts, to pale grey, cross-rippled sands (the latter cement is particularly evident towards the
occurring exclusively around Jogeshwari). uppermost basalt. Slickensides both follow and
Coarse grains, rarely present along certain cross bedding planes. Sediments contacting the
laminations, include well-rounded c. 0.4 mm columnar lopolith exposed in the quarry face
diameter carbonate clasts and rounded quartz exhibit polygonal cracks. In sharp contrast with
sands (e.g., Fig. 6b). Dark, laminated horizons many MDP sections, no reddened ashes are
(e.g., Bom 4/98 and Bom 12/98; Table 1) present.
contain pyrite framboids. The majority of units (2) The Worli and Bandra Tunnels are inacces-
are planar-bedded, although one chaotically sible, hence their overall sedimentological con-
deposited, coarser layer contains btabletsQ of text is impossible to gauge. However, cuttings
flat-laminated sediment. Ripples of 0.1 cm reveal that the tunnels pass through similar
amplitude by 2 cm wavelength traverse another lithologies to those present in the Amboli
upper bedding plane, and some ripple crests section, except that the sediments generally lack
have been transformed into flame structures cemented layers, being dominated by shale and
(e.g., Fig. 7c). Undulose upper bedding planes OM (Table 1).314 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 5. Amboli sedimentary summary log (for detailed log, refer to Cripps, 2002).J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 315
Two Amboli and two Worli tunnel samples were
chosen for stable carbon isotope composition deter-
mination. Kerogen palynological residues (outlined
next) were prepared for stable carbon isotope analyses
by repeatedly centrifuging dry samples in 9:1 dichlor-
omethane:methanol solvent. Stable isotope ratios were
measured on an elemental analyser-isotope ratio-mass
spectrometer.
3.3. Palynofacies
A sediment’s palynofacies is its content remaining
after maceration in hydrochloric and hydrofluoric
acids (Combaz, 1964). The desired end products of
palynofacies maceration processes are slides clearly
displaying an optimum number of phytoclasts (clasts
of plant origin), with as little accompanying extra-
neous material as possible. Ideally, techniques
employed should not alter the proportions of phyto-
clasts as they occur in their host sediment by biasing
particular grain sizes or types. Standard palynological
processing techniques to produce kerogen slides
(Moore et al., 1991) were followed for the present
study.
Fig. 6. Thin-section micrographs (plane-polarised light). (a) Fine- 4. Results
grained clay and OM laminae undulating and bifurcating around
coarser ash clasts and cement in silt sample Bom 3/98; (b)
laminations compressed and distorted about a coarse, weathered 4.1. Geochemistry
pyroclast in ash sample Bom 16.
The Amboli spherical clast Bom 3/99 is domi-
nated by calcite, quartz, smectite and feldspar (Fig.
3.2. Geochemistry 8a). A minor peak at 3.6 2 in the clay separate
diffraction profile (Fig. 8b) denotes the presence of
A thorough account of Mumbai clay mineralogy is an ordered super-lattice, produced by two different
given in Singh (2000). To provide comparison, the minerals alternating regularly, constituting the mixed-
mineralogy of Bom 3/99, a spherical clast from layer clay corrensite. Peak positions confirm the
Amboli quarry, was assayed by X-ray diffraction super-lattice to be chlorite interleaved with a
(XRD) for this work, after preparation using standard saponitic smectite (approximately 80:20 chlorite:s-
whole-rock and clay-separate methods (Hardy and mectite; Clayton, personal communication). A trace
Tucker, 1988). The clay separate was subjected to of kaolinite is evident in the whole-rock profile,
glycolation and heating, to distinguish between smec- although, interestingly, this clay is unusual in MDP
tites, chlorites and kaolinites. Element concentrations intertrappeans (Cripps, 2002).
were established using X-ray fluorescence spectro- XRF results reveal that, although Amboli ash and
scopy (XRF). Analyses of major elements were tuff chemistries vary considerably, all the samples
performed on glass discs, and powder pellets were possess elevated Na2O levels (Table 7). The two
used for trace element analyses. Losses on ignition Amboli ashes analysed for stable carbon isotopic
(LOI) were recorded to account for volatile contents. composition exhibit marginally lighter d 13C values316 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 7. Amboli section photographs. (a) Entire section, (b) typical organic-rich shale to marly sandstone bedding cycles, (c) flattened ripples on
upper bedding plane of siltstone.
than the Worli shales (Table 8). The significance of characteristics. Mumbai shales and silty sands are
these findings is discussed in Section 5. suited to palynofacies investigations due to their high
concentrations of well-preserved, structured organic
4.2. Palynofacies clasts. Seventeen Amboli (Bom), 11 Worli (Wo) and 4
Bandra (B) specimens were examined; samples were
Although significant volumes of organic residue selected to typify the range of sediment types present
remained after macerating Mumbai intertrappeans, (Table 1).
palynomorphs supplied a negligible contribution. Two hundred phytoclasts were logged for each
Spinizonocolpites palm pollen, Azolla water-fern sample, and grains allocated 1 of 16 designated
massulae, Botryococcus algal colonies and various microfloral categories (Table 9; Fig. 9). Palynodebris
fungal spores were exceptionally logged in some percentages are displayed at their stratigraphical
shales. While this paucity means that a comprehensive positions through the Amboli sequence in Fig. 10.
palynological interpretation is unfeasible, similar Six Amboli ashes proved unproductive (Table 1), and
lithologies through the Amboli, Worli and Bandra only one ash horizon macerated trapped significant
sections permit comparisons of their palynodebris quantities of organic clasts (Bom 16/98). By contrast,J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 317
Fig. 8. Amboli XRD profiles. (a) Bom 3/99 whole-rock profile, (b) Bom 3/99 clay separate profile. sme=smectite, cal=calcite, qtz=quartz,
feld=feldspar, latt and csme=chlorite:smectite superlattice (corrensite), kao=kaolinite.
all 11 Worli and 4 Bandra samples contained abundant by marked increases in fragments displaying tracheids
palynodebris. (Fig. 9). Following a different trend, low amounts of
Changes in absolute palynodebris abundances angular black clasts in Amboli samples generally
occur with lithology transitions through these beds, accompany augmented amorphous organic matter
the changes being accompanied by variations in the (AOM) and branching leaf-like fragment percentages
relative percentages of some phytoclast categories to (Fig. 10).
others. For example, taking into account that drops in Small angular black clasts are consistently present
angular black clast numbers will force rises in other in high percentages; the largest concentration occurs
category percentages, decreases in small and large in Bom 4/98, a laminated, pyrite-rich bed (Table 9).
angular black clasts in Worli samples are accompanied Large angular black clasts are less concentrated, but318 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Table 7
Infratrappean and intertrappean major (wt.%) and trace (ppm) element compositions received from XRF analyses (for lithologies, refer to
Table 1)
Sample Bom Bom Bom Bom Bom Bom Other
1/98 9/98 16/98 23/98 1/99 3/99 Deccan
SiO2 60.62 52.13 37.15 71.46 64.59 36.76 42.46
TiO2 0.729 0.544 0.557 0.635 0.751 0.893 1.655
Al2O3 14.25 11.9 3.78 12.09 15.93 9.06 11.01
Fe2O3 5.25 4.5 5.52 3.57 4.24 9.74 11.66
MnO 0.085 0.103 0.161 0.048 0.077 0.19 0.18
MgO 2.43 4.4 11.53 1.5 1.74 6.83 3.78
CaO 3.82 8.43 16.16 1.04 2.3 15.49 13.07
Na2O 5.52 5.66 0.28 3.21 6.89 1.76 0.17
K2O 1.72 1.01 0.05 3.21 1.92 0.41 1
P2O5 0.154 0.077 0.147 0.19 0.107 0.103 0.09
LOI 4.33 11.18 24.34 2.51 1.88 15.38 15.16
Rb 56.8 26.4 2 113 55.5 12.8 34.87
Sr 159 148 155 166 143.6 87.1 106.5
Y 35.1 24.8 18.4 33.3 35.3 26 22.71
Zr 472 420 74 360 582.8 69.1 108.7
Nb 110.6 97.9 10.8 79.1 142.6 9.8 10.58
Ba 500 220 30 919 537 80.6 131.4
Pb 12 10 1 9 14.7 2.1 5.35
Th 23 19 2 17 29.6 0 3.93
U 3 5 0 4 4.9 2.2 1.372
Sc 13 10 17 12 9.8 39.8 30.53
V 217 76 156 103 67 273.4 243.9
Cr 228 222 41 272 212.8 115 111.5
Co 21 7 16 32 9.8 35.1 25.92
Ni 81 24 24 309 3737 58.6 47.15
Cu 51 42 39 56 22.7 58.7 136.7
Zn 57 32 38 74 49.8 51.9 46.12
Ga 14 11 6 11 15.2 15.1 14.9
Mo 0 0 7 1 0.4 0 0.564
As 4 4 4 8 6.7 6.9 3.32
S 232 415 2571 125 424 705 258.8
Other Deccan=mean result obtained from a variety of ash intertrappeans from the Western Ghats, the Krishna–Godavari basin and the Mandla
Lobe (Fig. 1).
follow a similar pattern up the samples. Branching
leaf-like clasts are important in Amboli and Worli Table 8
sediments, and Bandra cuttings are dominated by Results of stable carbon isotope analyses of kerogen samples
AOM. Amorphous matter and parenchymatous tissues (PDB=Peedee belemnite standard)
are more abundant in Amboli than Worli samples, Sample d 13Cx PDB Mean d 13Cx PDB Standard
while fragments displaying tracheids are only impor- deviation
tant in Worli sediments. As with the small and large Bom 5/98 26.39 26.68 0.409
26.97
black clasts, branching leaf-like fragments and black
Bom 16/98 25.4 25.58 0.261
laths typically exhibit angular edges. 25.77
Phytoclast colours are recorded in Table 10, Wo 2001 24.78 24.89 0.148
following the thermal maturity scheme of Batten 24.99
(1996). Derived plant material is dominantly black- Wo 2850 24.86 24.94 0.12
25.03
ened, creating high thermal maturity estimationsTable 9
Relative percentages of palynofacies categories for productive B, Bom and Wo samples
Sample AOM Black Black, Branching Brown, Brown, Fungal Large, Palynomorph Parenchyma Small, Small, Subspherical Tracheid Cuticle? Noncellular
J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
lath (?wood) porous (?leaf) angular porous black, (non fungal) black, translucent black membrane
angular angular
B 3510 51.5 3.5 0 0 0 0 0 8 0 0 33.5 0 3.5 0 0 0
B 3130 68.5 0 0 0 0 0 0 2 0 0 28.5 0 1 0 0 0
B 3000 44.5 3 0 0 0 0 0 11 0.5 0 38 0 0.5 0 0 2.5
B 2800 35 6 0 0 0 0 0 17.5 0 0 41.5 0 0 0 0 0
Bom 20/98 2 3 1 0 0.5 0 0 23 0 70.5 0 0 0 0 0 0
Bom 19/98 60.5 2 0 0 0 0 0 6.5 0 0 31 0 0 0 0 0
Bom 16/98 28 0.5 0 31 0.5 1.5 0 8 0 1.5 20.5 0 0.5 0.5 0 7.5
Bom 15/98 17.5 3.5 0 10.5 0 0 0 14.5 0 3.5 36 8.5 4 0 0 2
Bom 13/98 0.5 6.5 0.5 2.5 0 0 1 26.5 0 0 60 1.5 0.5 0 0 0.5
Bom 12/98 72 2.5 0 0 0 0 0 3.5 0 0 21.5 0.5 0 0 0 0
Bom 10/98 2 0 1 49 0 0 0 10.5 0.5 13 9.5 0 0 0 0 14.5
Bom 8/98 42 1 0 0 0 0 0 11 0 0 45.5 0 0 0 0 0.5
Bom 5/98 0 4.5 19.5 1 0.5 1 0 28 0 0 44.5 0 0 1 0 0
Bom 4/98 8.5 1.5 0 2 0 0 0 12.5 0.5 0 74 0.5 0.5 0 0 0
Bom 3/98 0.5 0 0 0 0 0 0 30 0 0 66.5 1 2 0 0 0
Bom 2/99 47.5 2 8.5 6.5 0 0 0 13 0 0 21 0 1.5 0 0 0
Bom 2/98 0.5 1.5 0 0 1 0 0 19 0.5 0 72 2.5 0.5 2.5 0 0
Bom 1/98 2.5 1.5 0 1.5 0 2 0 12 0.5 0 64.5 11.5 2 2 0 0
Wo 3408 5 1.5 7.5 8.5 4.5 9 1.5 20 0 12 17 3.5 0.5 3 0 6.5
Wo 3128 1.5 4 0 0 24.5 0 0.5 14 0.5 0 25.5 0 1.5 22 3 3
Wo 2850 2 2.5 2.5 19.5 6.5 5 7 9 0 5 24.5 1.5 3 4 0 8
Wo 2736 0 5 0 0 2.5 9 0 11 0 0 30.5 11.5 3 27 0 0.5
Wo 2735 0 5 5 1 1 0 0 24.5 0 0 60.5 0.5 2.5 0 0 0
Wo 2610 1.5 4.5 2.5 15 2.5 6.5 0 8 0 0.5 37 6 5.5 1.5 0 9
Wo 2600 0 6 0 0 4 11.5 0 24 0 0 29.5 0 1 22 1.5 0.5
Wo 2210b 2 7 0 0 3.5 0 0 5 1.5 0 53 16 3 7.5 0.5 1
Wo 2210a 0.5 6 2.5 4.5 6.5 2 0 18.5 0 0 56.5 1.5 1 0.5 0 0
Wo 2100 2.0 30.2 0 1.2 0 0 0.8 12.7 0 0 23.7 0 15.9 6.1 6.9 0.4
Wo 2001 3.5 1.5 0 10.5 1.5 2.5 0 16 0 11.5 34.5 0.5 5.5 1.5 0 11
319320 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 9. Relative percentages of palynofacies categories for Bandra (B) and Worli (Wo) samples.J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 321 Fig. 10. Distribution of palynofacies types with height through the Amboli section (details given in Table 9). Grey bands mark the positions of unproductive ashes.
322 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 10 (continued).J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 323
~30%
~30%
~40%
3408
(Amboli mean 6.3; Worli mean 5.6). E:L ratios
5–6
Wo
37
13
(equant to lath-shaped clasts; Table 10) were received
~40%
~45%
~15%
3128
from counts of 50 black wood grains. Mean E:L ratios
Wo
33
17
5
~30% (38.4:11.6 for Amboli, and 35.8:14.2 for Worli) are
~40%
~30%
similar, equant-shaped grains dominating over lath-
2850
Wo
32
18
shaped in both sequences. Fig. 11 compares thermal
5
~30%
~35%
~35%
maturity with black wood shape and size ratios
2736
5–6
Wo
29
21
through the Amboli section. Overall, b40 Am grains
~35%
~40%
~25%
marginally form the greatest black wood size compo-
2735
6–7
Wo
41
nent, although there is a relatively even distribution of
9
b40 Am, 40–80 Am and N80 Am clasts.
~45%
~30%
~25%
2610
Wo
42
5
8
Black wood phytoclast size, colour and shape statistics for productive Bom and Wo samples (thermal maturation after Batten, 1996)
~30%
~40%
~30%
2210a 2210b 2600
5–6
Wo
5. Interpretation
28
22
~70%
~25%
~5%
5–6
5.1. Facies
Wo
34
16
~40%
~25%
~35%
The conspicuous absence of archetypal MDP boles
Wo
44
6
6
and calcretes in Mumbai Island intertrappeans high-
~20%
~50%
~30%
2100
lights a general lack of sediment subaerial exposure.
Wo
40
10
6
Tectonic adjustments controlled the subaqueous
~40%
~25%
~35%
10/98 13/98 15/98 16/98 2001
nature of Mumbai sediments and Traps, allowing
5–6
Bom Bom Bom Bom Wo
34
16
water to flood into the developing shallow basins as
~45%
~30%
~25%
rifting and foundering of the margin progressed.
45
Slickensides that both follow and cross bedding
6
5
~45%
~25%
~30%
planes probably developed during this period of
36
14
tectonism. Substantial intertrappean thicknesses are
6
partly due to the extent of contemporaneous regional
~35%
~35%
~30%
42
subsidence.
7
8
Shale laminations indicate a lack of bioturbation,
~40%
~35%
~25%
suggesting that infauna were unable to exploit these
28
22
5
sediments, possibly due to inadequate interstitial
~40%
~25%
~35%
Bom
5/98
oxygen levels. The combination of swamp facies
45
7
5
and anoxic laminated sediments implies that water
~40%
~35%
~25%
Bom
4/98
6–7
levels were generally very shallow, yet liable to
43
7
stagnation. This was perhaps a consequence of
~40%
~30%
~30%
Bom
3/98
restricted water mixing through a low-energy column,
6–7
23
27
the aqueous body being isolated from a fully open
~40%
~30%
~30%
Bom
2/98
marine influence.
6–7
43
7
A stratified water column with a high potential
~40%
~40%
~20%
Bom
towards basal anoxia may have resulted from a subtly
1/98
41
6
9
more dense, brackish layer separating surficial,
Medium (40–80 ı̀m)
aerated freshwater from the sediments, such circum-
Equant (out of 50)
Thermal maturation
Lath (out of 50)
stances being liable to occur in partly enclosed,
Small (b40 ı̀m)
Large (N80 ı̀m)
Pytoclast shape:
Phytoclast size:
sheltered lagoons fed by rivers. Shale carbon concen-
trations appear to have been optimised by low clastic
Table 10
Sample
sediment input combined with high terrigenous
organic productivity, and OM decomposition would324 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Fig. 11. Log of Amboli section palynofacies characteristics.J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332 325
in turn have depleted oxygen resources. Clastic When present, bivalve and gastropod internal
sediments are dominated by volcanic material, signi- moulds are of small (1–2 cm) sizes. This might be
fying that sedimentation rates diminished during consequential to oxygen deficiency having stunted
nonvolcanic periods. Although OM was largely growth and/or caused large proportions of the mollusc
introduced, the dearth of eroded clastic material points populations to die prior to reaching maturity. The
to hinterland gradients having been negligible. sizes of feeding traces upon a quarry floor bedding
Water energy infrequently increased, and undulat- plane point to excavation by small crustaceans, and
ing or rippled horizons became deposited above flat- float crustacean claw sample Bom 22/98 (Table 1)
laminated sediments. Paler, ash-rich units typify may have originated from this horizon. Subhorizontal
these faintly higher energy facies, the water move- burrowing activity suggests sedimentation rates were
ment perhaps initiated by ash introductions that low when organisms exploited the sediments. Their
triggered minor density currents. Tablets of flat- near absence in higher beds might be consequential to
laminated shale in one sandy ash appear to have subsequent ash injections.
been ripped up and reworked after their compaction The prevalence of shales through the extensive
but before lithification. Horizons bearing asymmetric Worli and Bandra sequences points to continually
ripples indicate directed flow, potentially having low sedimentation rates here, and therefore substan-
resulted from such ash-bearing currents progressing tial sedimentation durations. Discrepancies in ash
across lagoon floors. Ripple tops were sometimes and Trap frequencies between Amboli and the Worli
preserved flattened or altered into flame structures and Bandra tunnels indicate that either volcanic
during their rapid deposition, dehydration and centres were closer to Amboli, or activity was more
collapse (e.g., Fig. 7c). intense at the time of Amboli deposition. Worli and
The Mumbai lagoons were stable environments Bandra shales are not as well-cemented as the
that were disrupted by ash eruptions. Rare fine, Amboli sediments, suggesting cement migrated from
laterally continuous organic drapes settled above ash horizons. Diagenetic events have altered the
rippled layers, as the water reverted to its calm state. Amboli section, and recrystallisation during lithifi-
Repetitive pyroclastic influxes established the series cation is particularly evident towards the uppermost
of fining-upwards, ash-rich rhythms through the basalt. Polygonal cracks in sediments contacting the
Amboli section. The transition from Bom 8/98 to columnar lopolith are likely to have evolved simul-
Bom 1/99 appears to equate to a gradual increase in taneously with the intrusion’s contraction upon
pyroclastic activity, culminating in a major local cooling.
event. Many ash beds are indurated, their matrix,
having been welded. 5.2. Geochemistry
Spherical to ovoid objects, constituting Bom 3/99,
lack internal structure, more closely resembling the The XRD profile of a volcanic bomb (Bom 3/99)
coalesced ash bombs described by Sukheswala (1956) exhibited numerous, clearly defined reflection peaks
than the spilitic fragments or pillows detailed by Tolia at positions signifying well-developed corrensite
and Sethna (1990), occurring in an ash rather than a crystals (Fig. 8). Relatively fresh feldspars produce
flow breccia. Laminations cup underneath these peaks; thus, it seems unlikely that sedimentary
bombs, as though the pyroclasts dropped upon and processes occurred over an extended enough period
depressed unconsolidated sediments. These accre- to permit the development of regularly alternating
tionary lapilli strongly suggest that ejecta cones were chlorite:smectite lattices. Rather, increasing diagene-
in close proximity to the Amboli lagoon. The sis temperatures and durations transformed smectites
lamination deficit through most ashes probably into this mixed-layer, chloritic clay, by means of
resulted from their accelerated, chaotic deposition repeated dissolution and precipitation events. The
styles. Air-fallen and fluvially deposited loose pyro- ratio of chlorite to smectite (c. 80:20) indicates a
clastics were possibly aerated enough to support heating event of z100 8C during lithification,
burrowing organisms that obscured original bedding possibly accompanied by a degree of saline fluid
features. flow (Beaufort et al., 1997; Murakami et al., 1999).326 J.A. Cripps et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 216 (2005) 303–332
Kaolinite forms a minor contribution to the 5.3. Palaeontology
volcanic bomb. As weathering continues, smectite
can alter to kaolinite through a succession of Although molluscs are sporadically distributed
smectite–kaolinite mixed-layer transitions. Its near through Amboli shales, no typical MDP genera
absence in weathered Deccan volcanics suggests these (e.g., Physa gastropods, Unio bivalves) were identi-
fossilised at early stages of modification. Kaolinite fied during the present study. Since shale faunal
crystals can, however, grow within substrates sub- material possessed high preservation potentials, the
jected to prolonged waterlogging, and while MDP absence of ubiquitous MDP forms almost certainly
boles were largely too well-drained to promote its reflects their intolerance to marginal marine environ-
precipitation, the Mumbai lagoonal basins provided ments. Investigations are required to ascertain whether
more favourable precipitation sites. Kaolinite is a these genera continued to occupy contemporaneous
common alteration product of felsic igneous rocks, MDP Danian, ?Desur Formation palaeoenvironments
and phlogopite micas present through tuff sample (e.g., Singh and Kar, 2002), and thus survived the full
Bom 1/99 may be indicative of a transformation to effect of the Deccan episode proximal to the principal
more felsic late stage volcanism as the region rifted focus of flood basalt activity. Invertebrates which did
and subsided. inhabit Mumbai lagoons were periodically capable of
The varied chemistries of Amboli ashes are a exploiting oxygenated surface sediments, as demon-
reflection of their occasional explosive genesis in strated by the pellet back-filled feeding traces.
aqueous facies, clastic contamination, element mobi- No macroflora was recovered from the Amboli
lisation prior to lithification and hydrothermal alter- section by the present authors, although this sequence
ation resulting from nearby intrusions. High sodium is extremely rich in disseminated plant matter. Parent
levels through these relative to MDP ashes (Table 7) plants possibly colonised firm terrain tens of metres
may be consequential to their deposition in saline from the low-angled, muddy lagoon shores and,
lagoons, although sodium from albites present would consequentially, intact plant organs were seldom
have augmented these concentrations. fluvially transported into the lagoons.
Amboli kerogen possesses marginally lower
carbon isotopic signatures than those of Worli 5.4. Palynofacies analyses
(Table 8). Thermal maturation, induced by local
intrusion emplacement, is one means by which Of the 16 palynofacies categories selected to
original Amboli OM d 13C could have been low- represent the Mumbai phytoclasts (Table 9), 14
ered. Dykes cross-cut intertrappeans offshore Mum- symbolise land-derived plant fragments which
bai (Sethna, personal communication); if these received their shapes, colours and sizes from their
imparted a greater influence on Amboli than Worli parent plant and organ varieties and taphonomic
sediments, they might additionally have been (including sedimentological) effects. (AOM is of
responsible for the darker Amboli phytoclast col- unknown derivation, and fungal remains are virtually
ours (Table 10). ubiquitous.) To classify the OM according to kerogen
Relative depletions in Mumbai shale 13C through type (Tyson, 1985), these palynofacies are rich in
heating was possibly influenced by a selective humic kerogens (higher plant wood and parenchym-
preservation of organic fractions with augmented atous tissues), much of this having altered to inertinite
12
C comparative to the total OM. Lipids, the most (carbonised black wood). The sapropelic kerogen
stable of plant constituents, are enriched in 12C by component (structureless matter, largely plankton-
up to 8x compared with other biogenic com- derived) is negligible and fusinite (fossil charcoal) is
pounds (Faure, 1986), and their hydrocarbon rare. Any volcanogenic charcoal potentially entered
composition closely resembles that of petroleum. the open sea due to its slow waterlogging rate (cf.
Smectitic clays catalyse lipid transformations to Nichols et al., 2000).
hydrocarbons virtually identical to petroleum All phytogenic clasts of known origin are terri-
(Faure, 1986), and the offshore Mumbai region is genous, reflecting deposition proximal to land, shel-
rich in source rocks. tered from a strong marine influence. The lack ofYou can also read
