169 NEW RECORD OF A CORAL- MOLLUSCA - IISER Kolkata

169

NEW RECORD OF A CORAL- MOLLUSCA REEFAL COMMUNITY FROM THE EARLY TERTIARY OF KUTCH, GUJARAT

DEVAPRIYA CHATTOPADHYAY

Department of Geological Sciences* Jadavpur University, Kolkata 700 032

ABSTRACT

The Matanomadh subbasin in Kutch, Gujrat exposes a well preserved early Tertiary succession.

The present study describes a new and rich assemblage of benthic invertebrates from two calcareous horizons, the older being a reefal limestone. Corals are particularly diverse and abundant. Among eight species described, two are new. Besides, the faunal community is also represented by bivalves, gastropods, bryozoans and serpulid worms. Previously these beds were included within the Naredi Formation, but they lack typical foraminiferal assemblage. The present faunal analysis also does not support the assignment and the stratigraphic position of these beds still remains elusive. The palaeoecological significance of the development of coral reef has also been discussed.

INTRODUCTION

The Kutch sea during Tertiary occupied an east-west pericratonic rift basin at the westernmost periphery of the Indian craton (Biswas, 1987, Biswas et al, 1994). A thick pile of the Tertiary sediments consisting mainly of mud and carbonate developed in a stable shelf environment. Biswas and Raju (1971, 1973) subdivided the rocks into several formations and assigned their ages based mainly on microfossil association (see also Bhatt, 1968; Guha, 1961, 1968, 1974; Khosala and Pant, 1988, 1989) that spanned from the Palaeocene to Pleistocene.

The oldest unit, the Matanomadh Formation according to Biswas and Raju (1971, 1973) represents a heterolithic volcanoclastic association consisting mainly of conglomerate bearing trap pebbles, tuffaceous shales, sandstone, bentonite clay stone etc. These were belived to be deposited on the Deccan trap basement in a restricted but terrestrial setting.

The stratotype of the Matanomadh Formation is exposed in the Madhuali river section, near Matanomadh along Bhuj-Lakhpat road. A reefal limestone buildup has been discovered during the present investigation. The limestone has been previously classified as a part of the Naredi

Formation by Biswas (1992). But the Stratigraphic evidence and faunal content are not compatible with this assignation. Moreover it is very hard compact and contains a rich variety of coral-mollascan association that possibly represents a patch reef. Here, we are reporting the faunal diversity of the reefal association near the Matanomadh village that deserves a special attention from the stratigraphic and palaeoecologic point of view.

STRATIGRAPHY

Tertiary sedimentation, in the basin of Kutch,

took place over a basement of the Deccan Trap during the Late Palaeocene (Tandon et al, 1980; Biswas 1992). Subsurface data however indicate that beyond the extent of Deccan Trap the Tertiary sediments rest directly on the Mesozoic Bhuj Formation (Mukhopadhyay and Shome, 1996 a, b)

The sequenceis generally very low (2°-7°) westernly and north-westernly dipping succession and structurally slightly disturbed. The facies vary laterally both in composition and thickness to a large extenct. Mukhopadhyay and Shome (1996 a) recorded that all the formations are thicker in subsurface than in surface counterpart.

Indian Minerals, Volume 58, No. 3 & 4 (July-December, 2004); pp. 169-182

*Present address: University of Michigan, Ann Arbor, USA

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The first detailed classification of the Tertiary sediments of Kutch was presented by Wynne (1872). The lithology of the Tertiary sediments of kutch has been later extensively worked out by Biswas and Raju (1971, 1973) and Biswas (1992). Matanomadh, Naredi, Harudi, Fulra, Maniyara Fort, khari Nadi, Chasara and Sandhan formations are included in this sequence in ascending order. This stratigraphic framework is followed in the present study. A brief description of the formations that cropout in the present studied area, are given below:

Matanomadh Formation:

It is exposed mainly bordering younger tertiary Formation. The lithology consists of gray coarse-grained sand, greenish gray silty clay, laterite and tuffs. Several horizons contain pebbles and boulders of Deccan trap and seldom lignite bands. The formation generally has a bright-variegated appearance. It does not yield many mega invertebrate.

Naredi Formation:

Matanomadh formation is overlain by a dominantly argillaceous unit, which was defined as Naredi formation by Biswas and Raju (1971). It consists of grayish yellow and gray thinly laminated gypceous shell and dark brown to reddish clay. In the middle part several bands of muddy limestone studded with larger forams alternate with shell. Lignite beds also accur mainly in the middle part. Their thickness laterally varies extensively.

LOCAL GEOLOGY AND PETROGRAPHY

The beds in the traversed area show a south-southwesternly younging direction. Traverses were taken to the east and the west of the Bhuj-Lakhpat road along the Madhwali river section (Fig. 1). Broad lithological and faunal variations were used to demarcate different formations and two representative local stratigraphic section (taken at the eastern and the western side of the road respectively) are shown in Table 1 and 2. The main features of the observed lithounits are discussed below:

In the area of Matanomadh the Deccan basalt is found roughly bordering the sedimentary basin. The direct contact between the Deccan Trap and

the overlying Matanomadh Formation is not found. Generally the contact is covered by alluvium. In some places we get a conglomerate bed between the two units. The boulders are mainly made up of lateralized pebbles.

The lowest unit, Bentonite clay is mainly exposed in the eastern part. It contains some lemon yellow interlayering bands, which are possibly caused by the leaching of pyrite. This layer is very characteristic of the Matanomadh Formation. The thickness of this unit varies from 0.15 to 0.7 m. It is probably the weathering product of the ash bed that was generated due to the Deccan volcanism. It might be used as a good marker horizon for correlating the strata over a large geographic span.

Tuffaceous red sandstone is exposed in eastern part of the Bhuj-Lakhpat road in the Madhwali river section near Matanomadh. The colour is red. It is cross-laminated. The thickness of this unit varies from 0.3 to1.5 m. This unit clearly indicates its affinity to the underlying Deccan volcanics. The thin-sections of Tuffaceous red sandstone show that it contains glass shards. Some of the glass shards are devitrified, showing different colours in cross-polars. The glass shards are maffic in nature and therefore show red colour.

Lignite is exposed only in the eastern part of the road. It is often contains sedimentary pyrite. In places it is intercalated with a shale layer. The boundary is gradational. This layer is not continuous. The thickness of this unit varies from 0.7 to 4.0 m. According to Biswas and Raju (1973), this unit demarcates the uppermost part of the Matanomadh Formation. Since it is not continuous throughout the basin, it cannot be considered as the marker horizon. The strong UV Fluoroscence emitted by the samples show that this lignite is sapropelic. It is showing low radioactivity. It is due to the presence of uranium (1.95±0.30 ppm) and thorium (1.06±1.03ppm) (personal observation).

In western part of the Bhuj Lakhpat road in the Madhwali river section various litho units are exposed. The oldest unit whose base is not seen is a grayish yellow, compact limestone. Its surface is very knobby and massive. It yields diverse taxa. Different types of corals are found insitu. Besides many encrusters like serpulid worms (having calcareous tube), gastropods, bivalves, bryozoan

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Fig. 1. Map showing major lithounits of the traversed area.

mats are also abundant. This unit also contains some boring traces of Lithophaga bivalve. The exposed thickness of the unit varies from .05 m to .3 m. The bed can be traced up to a distance of 400m. along the both flanks of the river. It sharply underlies grayish shale intercalated with thin gypsious lenses in the northern flank of the river whereas in the southern flank it is overlain by thin mottled sandstone.

In one place a white patchy lime mud studded with bivalves and opiomorph burrows (Plate.2, Fig.3) overlies the above-mentioned grayish yellow limestone. Strikingly, the gypsum bearing shale is devoid of any macro fauna. It intergrades with carbonaceous shale towards west. The thickness of this unit varies from.05 to 0.8 m.

The thin-section study reveals that it is a fossiliferous micrite. The grayish limestone is much more recrystallised compared to the white lime mud. It contains coral fragments, sponge specule and a few forams. Thin sections of the lime mud show foraminifera of milliolid and globorotalid group and fenestrate bryozoa (Plate.2, Fig.6).

Grey shale is exposed both in the eastern and the western part of the road. It is overlying on a yellow pisolitic layer. In some places it is totally bioturbated whereas in other places it preserves some bedding traces. Bioturbated bed is sometimes containing reddish marks. It may be the iron coating of the burrows. So the iron coating may provide the burrow outline. From this data the extent of burrow infestation can be marked. But, in some sections this layer is conspicuously absent and is replaced by the mottled sandstone unit that directly overlies the limestone. The thickness of this unit varies from 0.03 to 0.8 m.

Black shale unit is exposed only in the western part of the road. It is interlayered with sulphur rich bands. It might be a weathering product of sedimentary pyrite. The cleavages and the fractures of the unit are enriched by sulphur concentration. In some vugs, gypsum layers have been formed. The gypsums are often showing purplish colour inside. That is may be due to radiation since the unit contains good amount of uranium (6.07ppm, personal observation). The bed contains plant fossils. It is not continuous throughout the basin, appearing as lenses, with varying thickness ranging from 20 cm. to 4 m. In some areas limonitic shale instead of the black shale is found. The thickness of this unit varies from 0.6 to 2.0 mts..

The thin-section of Black shale shows some dark lumpy patches that are identified as Renalcis, an important blue green algal type which is often associated with reef and very near the reef facies (Scholle, 1973).

Yellow pisolitic bed is mainly present in the western part of the road showing a pisolitic texture. But it is remarkably absent in the southern flank of the river section. The thickness of this unit varies from 0.2 to 3.0 m. In thin section, it shows glassy materials. The pisolitic texture is formed by the rounded to sub rounded grains of clayish materials that are appearing dark in crossed polars.

Variegated sandstone shows a mottled texture in some restricted part of the western side of the road. This unit contains some coral fossils but those are not insitu. The fossils are basically transported from the limestone. The vugs are filled up by gypsum crystals that are slightly reddish in colour. In some places there occurs

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sedimentary pyrite. The shale layer is highly weathered. The thickness of this unit varies from 0.3 to 0.5 m.

Fig 2. Composite section showing the lithounits exposed in the Madhwali river section (a) in the western part of the Bhuj-Lakhpat road and (b) in the eastern part of the Bhuj- Lakhpat road.

FAUNAL ASSOCIATION Among all the above mentioned lithounits,

only two horizons yield diverse taxa of invertebrate megafossils. The hard, compact grayish limestone is rich in corals. Majority of the corals are compound in nature and globular in shape. Only a single unidentifiable fragment of a solitary coral possibly belonging to Montlivaltia has been found. Majority of the corals are found

insitu and no evidence of reworking has been noticed. Among the eight species of corals, faviids are diverse and most abundant. They are represented by genera like Favia and Goniastrea. The other contributors are Astreopora and Actinastrea of Acroporidae. Alveopora and Columastrea are less commonly found. Packing of corals are at places high. The base of the limestone is not exposed in the studied are, but high incidence of compound and encrusting corals suggest the occurrence of hard substrate (see Fürsich et al., 1994). The limestone also contains many other invertebrate taxa such as fenestrate bryozoans, serpulid worms, bivalves and gastropods. Bivalves are represented by oysters and astartids. Gastropods belong to turreted groups, but they are loosely coiled like vermetids of the present day coral reefs.

The soft mudstone overlying the grayish limestone, is devoid of any large corals. Only some coral fragments can be observed in the thin section. This unit is studded with giant bivalves, often opiomorph burrows. It also yields variety of gastropod genera including ill preserved Conus and Oliva. All specimens are kept in the museum of Department of Geological Sciences, Jadavpur University.

Coral:

Family: FAVIIDAE Gregory, 1900 Subfamily: FAVIINAE Gregory, 1900 Genus: Favia Oken, 1815 Type species: Madrepora fragum Esper,

1795

Favia madhuwalensis sp. nov

Plate.1, Fig.1 (a-f) Holotype. JUM/c/1A; original shell preserved.

Type locality. West of Bhuj-Lakhpat road near Matanomadh village in Madhwali river section. Stratigraphic horizon. A grayish compact limestone unit overlying Matanomadh Formation.

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Derivation of name. It is named after the Madhwali river from where the specimens were collected. Diagnosis: High intraspecific variation. Semicereoid, large (except var.B). The coenenchyme is variable, generally broader and having different patterns. Collumella is not seen. Colonies are hemispherical.

Description: Favia madhuwalensis var.A

Large, individual corallum absent, chambers are nearly elliptical, coenenchyme showing honeycomb pattern. Colonies are hemispherical. Favia madhuwalensis var.B

Small, individual corallum present, chambers are nearly circular, coenenchyme showing a radiating pattern. Colonies are hemispherical. Favia madhuwalensis var.C

Large, indivual corallum absent, chambers are nearly circular, conenchyme showing a tubular mesh pattern. Colonies are hemispherical. Favia madhuwalensis var.D

Large, individual corallum absent, chambers are nearly elliptical, coenenchyme is more or less flat with a faint reticulation. Colonies are hemispherical.

Favia madhuwalensis var.E

Large, individual corallum present, sprouting out from each chamber, coenenchyme showing an acicular pattern. Colonies are lowly convex. Discussion: The described species is different from all other previously described species in having broader coenenchyme. F.fragum (see Wells, 1956, Fig. 296.4a) differs from F.madhuwalensis in having relatively raised stereozone.

Genus: Goniastrea M. Edw-H., 1848

Type species: Astrea retiformis Lamarck, 1816

Goniastrea matanomadhensis sp. nov

Plate.1, Fig. 2 (b, c) Holotype. JUM/c/2A; original shell preserved.

Type locality and Stratigraphic horizon. As for Favia madhuwalensis. Derivation of name. It is named after the Matanomadh village since the specimens were collected from the near by area.

Diagnosis: Corallites permanently monocentric, colony formation by mono- to tristomodaeal budding; septa with scarely dentate paliform lobes, Columella feeble. Description: Cereoid, raised stereozone present; chambers of varying geometry, from nearly elliptical to pentagonal; very feeble septal insertion. Discussion: The larger size, the presence of feeble columella and weak insertion of septa distinguishes matanomadhensis from all previously described species of Goniastrea.

Goniastrea sp. Plate.1, Fig.2a Material. JUM/c/3A; original shell preserved.

Type locality and Stratigraphic horizon. As for Favia madhuwalensis.

Description: Strongly developed numerous septa, which are fused together to form a pseudocolumella. Discussion: Septal insertion much prominent in this species compared to the previously discussed Goniastrea matanomadhensis. The geometry of the chambers are nearly rounded whereas it ranges from subelliptical to pentagonal in Goniastrea matanomadhensis.

Family: ACROPORIDAE Verrill, 1902 Genus: Astreopora Blainv., 1830


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Plate 1.

1a. Favia madhuwalensis var. A x 1b. Favia madhuwalensis var. B x 1.1 1c. Favia madhuwalensis var. B x 2 1d. Favia madhuwalensis var. C x 2.4 1e. Favia madhuwalensis var. D x 2.4 1f. Favia madhuwalensis var. E x 2.4 2a. Goniastrea sp. x 2.4 2b. Goniastrea matanomadhensis x 1.4

2c. Goniastrea matanomadhensis x 2.4 3a. Alveopora sp. aff. verrilliana 3b. Alveopora sp. aff. verrilliana x 2.4 4. Astreopora sp. 5. Actinastrea sp. x 2.4 6a. Columastrea sp. x 1.1 6b. Columastrea sp. x 2.1 6c. Columastrea sp. x 2.3

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Type species: Astrea myriophthalma

Lamarck, 1850 Astreopora sp. Plate.1, Fig. 4 Material. JUM/c/4; original shell preserved.


Description: Massive, no axial corallite, corallite walls solid. Discussion: Coenenchyme is coarse with a stereozone forming ridge. Stereozones thickens during ontogeny.

Family: ASTROCOENIIDAE Koby, 1890 Subfamily: ASTROCOENIINAE Koby, 1890 Genus: Actinastrea Orb., 1849 Type species: Actinastrea goldfussi Goldfuss,

1826 Actinastrea sp. Plate.1, Fig. 5 Material. JUM/c/5; original shell preserved.


Description: Cerioid, incrusting; extratentacular budding, columella well developed, styliform. Discussion: It is larger, coenenchyme is coarse forming ridge compared to that of the A.guadalupae (see Wells, 1956, Fig. 262, 6d).

Family: PORITIDAE Gray, 1842 Genus: Alveopora Blainv., 1830 Type species: Madrepora daedalea Forskål,

1775 Alveopora sp. aff. verrilliana

Plate.1, Fig. 3 (a, b) Material. JUM/c/5; original shell preserved.


Description: Massive, septa represented by nearly horizontal spines projecting inward from mural trabeculae. Discussion: It is smaller, having very short blunt septal spines and thus differs from A. verrillina of Recent (see Wells, 1956, Fig. 288, 1b).

Family: ASTROCOENIIDAE Koby, 1890 Subfamily: ASTROCOENIINAE Koby, 1890 Genus: Columastrea Orb., 1849 Type species: Astrea striata Goldf., 1827 Columastrea sp. Plate.1, Fig. 6 (a, b, c) Material. JUM/c/6 ; original shell preserved.


Description: Subcerioid, coatae short, septa with minutely dentate margins. Columella styliform. It is previously reported from the horizons ranging from Upper Jurassic to Upper Cretaceous. Discussion: It is quite comparable to C. striata (Polšak, 1981, Fig. 4C) showing a botroidal nature with one crown of 9 pali before second cycle; broad, prominent columella present. However it is reported from older horizons ranging from Upper Jurassic to Upper Cretaceous.

Gastropoda: Family: TURRITELLIDAE Woodword,

1851 Genus: Turritella Lamarck, 1799

Type species: Turbo terebra Linné, 1758 Turritella spA.


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Plate 2.

1a. Bourguetia sp. (lateral view) x 2.1 1b. Bourguetia sp. (apertural view) x 2.1 2a. Unidentified nerenid gastropod sp.

(apical view) x 1.4 2b. Unidentified nerenid gastropod sp.

(apertural view) x 1.4 3. Opiomorph sp.

4. Turritella cf. mortoni (apical view) x 3.1 5. Turritella spA. (lateral view) x 2.7 6. Fenestrate bryozoa x 1.1 7. Pyrazus? sp. (lateral view) x .5 8a. Lucina sp. (lateral external view) x .4 8b. Lucina sp. (dorsal viw) x .4

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Plate.2, Fig.5 Material. JUM/g/1; composite state of

preservation, internal mould and part of the original shell is preserved. Type locality and Stratigraphic horizon. As for Favia madhuwalensis.

Description: The internal mould of the broken spire with remnants of original shell near the suture is preserved. Small, slender; whorls are convex. The original shell is showing axial ribbing pattern near the suture. The suture is impressed. Discussion: The overall shape and the nature of suture are quite comparable with internal mould of the Turritella mortoni, reported from Aquia Formation (Ypressian, Belvedre Beach, Recent) Virginia (Case, p.70, pl. 5.22). Though the same species, which is reported from Turritella beds of Eocene age from King George County, Virginia is much more uncoiled than the present form.

Genus: Turritella Lamarck, 1799

Type species: Turbo terebra Linné, 1758

Turritella cf. mortoni Conrad Plate.2, Fig.4 Material. JUM/g/2; internal mould.


Description: Internal mould; apical part is missing; small, whorl convex, uncoiled; translation rate is very high. Discussion: It is quite comparable with internal cast of the Turritella mortoni, reported from Turritella beds of Eocene age from King George County, Virginia in having the same translation rate. It is larger and the translation rate is higher compared to the above-described Turritella spA.

Family: BATILLARIIDAE

Genus: Pyrazus Montfort, 1810 Pyrazus? sp. Plate.2, Fig.7 Material. JUM/g/3; internal mould.

Type locality and Stratigraphic horizon. Soft mudstone, overlying gray limestone.

Description: Poorly preserved internal mould; moderate in size, slender, turriculate, multi-whorled and elongate cerithiform in outline. Apical angle 25º. Protoconch missing. Whorls about 7 in number, rounded, isometrically expanded and separated by impressed suture. Body whorl with roundly convex sides and weakly convex base, without any angulation between them; nodes appears in late ontogeny. The aperture is more or less rounded with thickened inner margin.

Family: PSEUDOMELANIIDAE Fischer,

1885

Subgenus:RHABDOCONCHA Gemmellaro, 1878

Genus: Bourguetia Terquem & Jourdy, 1870 Bourguetia sp. Plate.2, Fig.1(a, b) Material. JUM/g/4; internal mould.


Description: It is an internal mold; phanaramphalous, degree of overlapping of whorls moderate. The overlapped part when exposed show prominent spiral growth lines; canaliculated suture; aperture elliptical, outer lip straight. Only the body whorl and the penultimate spire are preserved.

Unidentified nerenid gastropod sp. Plate.2, Fig. 2(a, b)


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Material. JUM/g/5; internal mould.

Type locality and Stratigraphic horizon. As for Lucina sp..

Description: Shell large, only last one and half whorl preserved, internal mould, widely phaneromphalous; whorls are separated by canaliculate suture. Aperture subquadrangular in outline, outer lip abapically inclined; cross section shows three prominent folds of which one parietal and two columellar; parietal fold is thicker than columellar folds which are of more or less equal strength. Fold spacing increases during ontogeny. Degree of overlapping of whorls moderate%; two sets of prominent inner ribs.

Bivalves: Family: LUCINADAE Fleming, 1828

Subfamily: LUCINANAE Fleming, 1828

Genus: Lucina Bruguière, 1797 Type species: Lucina (Lucina) gratis Olsson, 1965

Lucina sp.

Plate.2, Fig. 8(a, b) Material. JUM/b/5; internal mould.

Type locality and Stratigraphic horizon. Soft mudstone overlying grayish limestone.

Description: Large, thick, obese, subtrapizoidal; more or less flattened with well-marked dorsal areas. Prosogyral beak, anticarinal ridge prominent, broad; escutcheon broad, raised; lunule small. The shell develops a radial ribbing pattern that is prominent near the ventral commissure.

Family: OSTREIDAE Rafineque 1815

Subfamily: OSTREINAE Rafineque 1815

Genus: Crassostrea Sacco, 1897

Type species: Ostrea (C.) virginica Gmelin, 1791

Crassostrea virginica

Plate.3, Fig.3(a, b)

1886 Gryphaea Fischer, p.927 1971 C. virginica Stenzel p.N1129, Fig.

J101, 1 Material. JUM/b/1; original shell preserved.

Type locality and Stratigraphic horizon. As for Favia madhuwalensis. Description: Left valve, small, high and slender spatulate forms with subparallel anterior and posterior margins seeming to preponderate. Surface rough, with irregularly spaced growth sequence, frilled along free ends; Imbricated ornamentation. Umbonal cavity is well-developed. Chomata present. Hinge straight with v-shaped resilifer. The curveture of the shell decreases in the later ontogenetic state giving the shell an overall spatulate shape. Discussion: The described specimen is comparable to C. virginica (see Stenzel, 1971, Fig. J101, 1e-h) in having the same size, v-shaped resilifer.

Crassostrea anguasta

Plate.3, Fig.2(a, b)

1936 Angustostrea Vyalov 1971 C. anguasta Stenzel p. N1129, Fig.J101, 2 Material. JUM/b/1 ; original shell preserved.


Description: The collected specimen is a left valve. Medium sized, anterior-posterior margins are straight where as the dorsal margin is rouded giving the shell an overall globular shape. Prominent resilifer ligament groove, ornamentation imbricated.

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Plate 3.

1a. Flemingostrea sp. , right valve (lateral external view) x 3.1 1b. Flemingostrea sp., right valve (lateral internal view) x 3.1 2a. Crassostrea anguasta (lateral external view) x 1.6 2b. Crassostrea anguasta (lateral internal view) x 1.6

3a. Crassostrea virginica (lateral internal view) x 1.5 3b. Crassostrea virginica (lateral external view) x 1.5 4. Flemingostrea sp., left valve (lateral internal view) 5a. Astarte sp., internal mould (dorsal view) x 2.1 5b. Astarte sp., external mould x 2.4


180

Genus: Flemingostrea Vredenburg, 1916

Type species: Ostrea (Flemingostrea) flemingi D’Archiac & Haime, 1853 p. 275

Flemingostrea sp.

Plate.3, Fig.1a, 4

1948 Dsolidostrea Vyalov p.24 Material. JUM/b/3; original shell is preserved.


Description: The two valves of two different individuals were collected. The left valve is of an adult individual where as the right valve is of a younger one. The left valve is large, overall shape flattish. Umbo is not prominent; terebratuloid fold broad and gentle, arising gradually at later growth stage; no radial sulcus delimiting fold on its posterior flank which has gradual slope. Chomata is present, but restricted near the umbonal part. Resilifer prominent flattish. The right valve shows small but prominent insertion of Quenstedt muscle.

Family: ASTARTIDAE d’Orbigney, 1844

Subfamily: ASTARTINAE d’Orbigney, 1844

Genus: Astarte J.Sowerby, 1816

Type species: Venus scotica Maton & Rackett, 1807

Astarte sp.

Plate.3, Fig. 5(a, b) 1778 Pectunculus sulcatus Da Costa 1971 Astarte astarte Stenzel p. N562, Fig.

E64.1 Material. JUM/b/4; Two specimens represent

internal and external moulds respectively.

Type locality and Stratigraphic horizon. The internal mould is collected from soft mudstone overlying grayish limestone. The external mould is collected from the hard grayish limestone overlying Matanomadh Formation.

Description: The internal mould show triagono-elliptical shape, small prominent beaks. Impressed lunule and escutcheon can be marked. External mould shows prominent growth lines.

REMARKS

The studied area represents a very small

geographic span with an enormous variety of faunas. It is quite remarkable to find this faunal bloom just in the vicinity of Matanomadh Formation made up mainly of volcanoclastic rocks. This carbonate facies was previously included within the Naredi Formation, but due to the lack of microfossils, it would be really a very ambitious claim to include it in that formation. Majority of the forms show a wide time span, ranging from Cretaceous to Recent. Some of them even are restricted to Eocene only (Turritella cf. mortoni). The corals are not very good biostratigraphic markers, still it is to be mentioned that the reefal limestone has yielded some forms which have previously been reported from the Upper Jurassic to Upper Cretaceous horizons (Columastrea sp.). The discovery of a Montlivaltia fragment has added more controversy since this genus got extinct in Cretaceous. So it is really very difficult to comment on the precise age of the limestone unit; the controversy continues whether to place it with other Eocene limestone buildups or it is only a spatial variation of the Matanomadh Formation.

The tertiary succession of Kutch has been preceeded by long and wide spread sedimentation during Mesozoic (Middle Jurassic to Early Cretaceous; see Biswas, 1977). Despite the presence of stunning diversity and absence of many invertebrate faunas, corals significantly show limited temporal distribution. They occur only in two stratigraphic levels in the Middle Jurassic and could not build true reefs. Fürsich et al. (1994) attributed this failure to form reefs mainly due to siliciclastic input in to the Kutch basin. Moreover, the large parts of the basin

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represented offshore midshelf region and hence coral reefs which generallythrive in shallow waters could not grow (see also Fürsich and Oschmann, 1993).

Reef building corals in modern seas prefer not

only shallow water, they flourish best in warm water. All major recent coral reefs have therefore, latitudinal limitsand are confined to tropical seas.

The Kutch basin during the Jurassic was situated near the subtropical (see Shome et al., 2004). It was only during the earliestr Tetiary Kutch reached the palaeoequator (see Gombos et al., 1995, and Bardhan et al., 2002) as the Indian plate drifted northward. This resulted in the great proliferation of organisms, especially foraminifera and widespread development of biogenically derived carbonates. This carbonate build up, we speculate, provided the necessary substrate for the development of the present reef.

The present faunal community is also significant from evolutionary palaeobiological viewpoint. If the carbonates which harbour them, are coeval with the Matanomadh Formation (Palaeocene). The organisms are stragglers of the great K-T boundary mass extinction event, which took the toll of most of the then living marine invertebrate species and corals especially were worst hit. The present faunal diversity, thus speaks for quick recovery and speciation of the surviving taxa. The coral reef may be of small aerial extent, perhaps rest on the Deccan Trap which is believed to be the chief destroyer of the Cretaceous life of the world (Hallam, 1987). The detailed study of the evolutionary consequence of this early Tertiary life, which appeared like the phoenix from the pyres, and ashes of basaltic lava, will be undertaken in a later endeavor.

ACKNOWLEDGEMENT

I am grateful to Prof. S.Bardhan, Department

of Geolgical Sciences, Jadavpur University for overall supervision and constant guidance. The present work is an extended part of my M.Sc. dissertation, carried out under the supervision of S. Banerjee, Department of Earth Sciences, IIT Bombay. Prof. P.K.Saraswati helped me by giving critical comments and suggestion. Thanks are due to P.Roy, Jadavpur University for taking photographs and S.Das, Indian Statistical Institute

for providing valuable suggestions regarding gastropoda taxonomy. Infrastructural help including laboratory facilities and financial support rendered by IIT Bombay authority is also acknowledged.

REFERENCES

Bardhan, S., Gangopadhyay, T.K. and Mandal, U.

2002: How far did India drift during the Late Cretaceous? – Placenticeras kaffrarium Etheridge, 1904 (Ammonoidea) used as a measuring tape. Sediment. Geol. 147, 193-217

Biswas S.K. 1977:Mesozoic Rock stratigraphi of Kutch, Gajarat. Quarternary Journal of Geological, Minning and Metallurgical Society of India, Vol.49, 1-62

__________ 1987: Regional tectonic framework, structure and evolution of the western marginal basins of India. Tectonophysics, V-135, 307-327.

___________1992: Tertiary Stratigraphy of Kutch, Journal of The Palaentological Society of India; Volume 37, 1-29

Bisws S.K., Bhasin A.L. and Ram, J. 1994: Classification of sedimentary basins of India in the framework of plate tectonics. Proc.Second Symp. Petroliferous basins of India, KDMIPE, Dehradun, v. 1, pp.1-42.

Biswas, S.K. and Raju, D.S.N. 1971: Note On the rock Stratigraphic classification of Tertiary Sediments of Kutch, Bull. Geol. Min.Met. Soc.India;

__________ and ___________1973: The rock-stratigraphic classification on the Tertiary sediments of Kutch. Bull. ONGC, 10, (1&2): 37-46

Bhatt, D.K. 1968. Planktonic foraminifera from Lower Eocene sediments of Kutch, India. Bull., ONGC, 4(2): 13-17

Case, G.R. 1982 : A pictorial guide to fossils, Van nostrand reinhold company, Newyork, 515p.

Fürsich, F. T. and Oschmann, W. 1993: Shell beds as tools in basin analysis: the Jurassic of Kuchch, western India. J.Geol.Soc.London, 150,169-185

Fürsich, F.T., Pandey, D.K., Oschmann, W., Jaitly, A.K. and Singh, I.B. 1994: Ecology and adaptive strategies of corals in


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unfavourable environments: example from Middle Jurassic of Kuchchh Basin, Western India. N.jb.Geol. Palaont. Abh., 194, 269-303

Gombos, A. M., Powell, W.G. and Norton, I.O. 1995: The tectonic evolution of Western India and its impact on hydrocarbon occurrences: an overview. Sediment. Geol. 96, 119-129

Guha, D.K. 1961: Anote on the ostracods from Lower Miocene of Chasra, Kutch. Bull. Geol. Min. Met. Soc. India, 24:1-6

_________ 1968: Ostracods from Middle Eocene of Kutch, Gujrat state, Western India. Bull. ONGC, 5(1): 83-92

_________ 1974: Observation on the Cenozoic and some Mesozoic ostracoda of India. Publ. Centre Advance study Geology. Punjab University, 7: 205-212

Hallam, A. 1987: End-Cretaceous mass extinction event: argument for terrestrial causation. Science, Vol. 238, 1237-1241

Khosla. S.C. and Pant, S.C. 1988: Ostracods from Eocene and Oligocene beds of Kutch, Gujrat, Part 1: Families Cyllerellidae, Bainidiidae and Trachylebiridiidae. Indian Jour. Earth Sci., 15(4): 325-346

_________ and _________ 1989: Ostracods from Eocene and Oligocene beds of Kutch, Gujrat, Part 2: Families Citheridae, Hemicytheridae, Laxoconchoidae, Paracytheridae, Xestoleberididae and Candonidae. Indian Jour. Earth Sci., 16(1): 1-10

Mukhopadhyay, S.K. and Shome, S.K. 1996 a: Depositional environment and basin development during Early Palaeogene Lignite deposition, Western Kutch, Gujrat. Jour. Geol. Soc. India. Vol. 47, 579-592

________________and ______________ 1996 b:

Lithostratigraphy of Pre-Fulra supratrappean (Early Tertiary) sequence of

NW Kutch – A Reappraisal. Geol. Surv. Ind. Spl. Pub. No. 20, 39-48

Polšak, A. 1981: Upper Cretaceous biolithitic complexes in a subduction zone: examples from the inner Dinarides, Yugoslavia. In Toomey, D.F. (eds.) Eoropean fossil reef models, Soc. of Economic palaeontologists and mineralogists, Spl. Pub. 30, 546p.

Scholle, P.A. 1973: A colour illustrated guide to carbonate rock constituents, textures, cements and porosities.

Shome, S., De, S., Roy, P. and Das, S.S. 2004: Ammonites as biological stopwatch and biogeographical black box – a case study from the Jurassic-Cretaceous boundary (150Ma) of Kutch, Gujarat. Current Science, Vol.86, No.1, 197-202

Stenzel, H.B. 1971: Oysters. In Moore, R.C. (ed.) Treatise on invertebrate paleontology, Part N, Vol. 3, 954-1224

Tandon, K. K. and Srivastava, D. K. 1980a: A new genus and species of Clypeasteroid echinoid from the Middle Eocene Rocks of Kutch. Jour. Pal. Soc. India. Vol23-24: 1-3

____________ and _______________ 1980b: Hercoglossa kutchensis – a new species from the Middle Eocene Rocks of Kutch, India. Jour. Pal. Soc. India. Vol. 23-24: 55-57

Wells, J.W. 1956: Scleractinia. In Moore, R.C. (ed.) Treatise on invertebrate paleontology, Part F, 1-497

Wynne,A.B. 1872: Memoir On The Geology of Kutch, Mem.Geol.Surv.lndia, Vol IX Pt.1;

Received 23.08.2004; Revised manuscript accepted 01.06.2005.

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