Original article

Gregoryceras in the Oxfordian of Kachchh (India):Diverse eventful implications§

Gregoryceras de l’Oxfordien de Kachchh (Inde) : implications diverses

Jai Krishna *, Bindhyachal Pandey, Jai Ram OjhaDepartment of Geology, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India

Received 13 November 2007; accepted 6 March 2008

Available online 27 December 2008

Geobios 42 (2009) 197–208

Abstract

The Kachchh Oxfordian ammonoid stratigraphic record for over 150 years remained restricted largely to Early Oxfordian condensed ooliticfacies in the relatively distal Mainland Kachchh. Recently, it has been enlarged with the discovery in 1994 of over 200 m-thick uncondensedMiddle to Late Oxfordian succession at Kantkote in the proximal part of the basin. Apart from abundant Perisphinctinae and Mayaitinae, a 10 m-thick sediment interval in the lower half of the above succession yielded a few examples of Gregoryceras. The specimens are here identified asG. gr. devauxi Bert and Enay and in view of their association with Larcheria subschilli (Lee) are assigned to the Middle Oxfordian SubschilliHorizon of the Schilli Subzone. Gregoryceras distribution south of the equator in Kachchh, Chile, Mexico and Madagascar, all within 35–408latitude marks the southern limit of its latitudinal expansion during the first order maximum flooding surface (MFS) of the Schilli Subzone of theKachchh Toarcian–Hauterivian Sequence. The maximum ammonoid diversity, density and frequency of the Kachchh first order sequence coupledwith dominance of mayaitins and perisphinctins suggest over 20 m depth in the Gregoryceras interval. Distinctly greater bathymetry in the 200 kmdistally away basinal part causes sediment starved situation, and explains the Middle Oxfordian to early Early Kimmeridgian submarinenondepositional hiatus in the Mainland Kachchh.# 2008 Elsevier Masson SAS. All rights reserved.

Résumé

Depuis plus de 150 ans, le registre fossile associé aux ammonoïdes oxfordiennes de Kachchh s’est limité aux faciès oolithiques condensés del’Oxfordien inférieur, rencontrés dans la partie distale du plateau continental. Il a été récemment augmenté, avec la découverte en 1994, dans lapartie proximale du bassin, à Kantkote, d’une série non condensée d’âge oxfordien moyen à supérieur, de plus de 200 m d’épaisseur. À côtéd’abondants Perisphinctinae et Mayaitinae, un intervalle sédimentaire d’une puissance de 10 m situé dans la moitié inférieure de cette série a livréquelques exemplaires de Gregoryceras. Ces spécimens sont ici attribués à G. gr. devauxi Bert et Enay ; compte tenu de leur association avecLarcheria subschilli (Lee), ils sont rapportés à l’Horizon à Subschilli de la Sous-Zone à Schilli, Oxfordien moyen. La distribution australe desGregoryceras, présents à Kachchh, au Chili, au Mexique et à Madagascar jusqu’à la latitude de 358, souligne leur limite d’expansion vers le suddurant la surface d’ennoiement maximale (MFS) de premier ordre de la Sous-Zone à Schilli dans la séquence toarcienne–albienne de Kachchh. Lemaximum de diversité en ammonoïde, la densité et la fréquence de la séquence de premier ordre de Kachchh, associés à la dominance desmayaitinés et des périsphinctinés, suggèrent une profondeur de dépôt supérieure à 20 m pour l’intervalle à Gregoryceras. Une bathymétrienettement plus profonde dans la partie distale du bassin, située à 200 km de là, est à l’origine d’une situation de « famine sédimentaire » quiexplique le hiatus de dépôt sous-marin observé à Kachchh depuis l’Oxfordien moyen jusqu’à la base du Kimméridgien inférieur.# 2008 Elsevier Masson SAS. Tous droits réservés.

Keywords: Oxfordian; Ammonoid; Gregoryceras; Perisphinctinae; Tethys; India

Mots clés : Oxfordien ; Ammonoïde ; Gregoryceras ; Perisphinctinae ; Téthys ; Inde

§ Corresponding editor: Pierre Hantzpergue.* Corresponding author.

E-mail address: krish_j28@yahoo.co.in (J. Krishna).

0016-6995/$ – see front matter # 2008 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.geobios.2008.03.005

J. Krishna et al. / Geobios 42 (2009) 197–208198

1. Introduction

The Kachchh basin (Fig. 1) located in the extreme west ofIndia has been known the world over for its rich Jurassicammonoids monographed by Waagen (1873–1875) and Spath(1927–1933). However, recent studies (last few decades) of theammonoid collections made under the rigorest stratigraphiccontrol have revived international interest in the KachchhJurassic. Substantive progress (Fig. 2) has allowed Kachchh toemerge as the prime Gondwanian Tethyan Margin (GTM)region of high potential for the realization of an independentammonoid zonation scheme standard for the Indo-East-Africanfaunal province on GTM. Schemes for the Callovian,Kimmeridgian and Tithonian stages have already beendeveloped and published (Krishna and Ojha, 1996, 2000;Krishna et al., 1996a, 1996b). Among the first Jurassicammonoids recorded and illustrated from Kachchh wasincluded Ammonites maya Sowerby (Sowerby, 1840) ofOxfordian age. It came from Kantkote (Fig. 1) and belongedto Chari Formation (Fig. 3). The first example of Gregoryceras,not only from the Indian subcontinent but from the entire Asianand West-Pacific region, was discovered and reported also fromKantkote by Krishna et al. (1994) as G. gr. transversarium(Quenst.) and assigned to the late Middle Oxfordian SchilliSubzone. The earlier records of the genus besides South Europeand North African countries have been from Madagascar,Mexico and Chile. In a revised systematic evaluation (Krishnaet al., 1995), the said example was found closer to G. fouquei(Kilian) rather than G. gr. transversarium (Quenst.), in spite ofexhibiting slightly greater flexure and retroversity in its middleontogenic stage than in typical G. fouquei (Kilian). The singleincomplete example of Gregoryceras, thus, unfortunately didnot agree with the known species, and we remained hesitant tocreate a new species in view of inadequate material. However,collection of a few additional examples in our subsequent visitsto Kantkote and creation of a closely comparable species,G. devauxi Bert and Enay, in Europe (Bert and Enay, 2004) as

Fig. 1. Schematic geological map of Kachchh with important Jurassic localities and mFault and MH: Median High).

an intermediate species between G. transversarium (Quenst.)and G. fouquei (Kilian) now necessitates formal taxonomicdescription and illustration of the Kachchh examples because oftheir significant intercontinental correlative and biogeographicimportance.

2. Paleontological evaluation

Gregoryceras gr. devauxi Bert and EnayFigs. 4–6Material: four specimens (in situ) numbered 15395 and

15753 from bed I-08, 15447 from bed I-10 from the river bed,east of Kantkote on the Samakhiali–Kantkote road and 15754probably from bed I-08 from the neighbouring Dhandhruvillage.

Description, comparison and remarks: it is a large speciesof Gregoryceras. The Kachchh specimens are septate until theirpreserved ends. The estimated size from the terminal diametersand whorl heights suggest diameter greater than 200 mm. Thecoiling is evolute with umbilicus 37% at 80 mm diameter, 41%at 120 mm diameter and 44% at 137 mm diameter. The whorlsection is somewhat trapezoidal (Fig. 4) with maximum widthat the tubercles and much less at the ventrolateral shoulder. Theumbilical wall is steep to vertical with rounded shoulder. Therib width increases radially, becoming maximum at theventrolateral shoulder in the form of clavicular tubercles.The maximum rib thickness, height and strength of theumbilical as well as ventral nodes/tubercles progressivelyincrease, exhibiting considerable ontogenic variation inindividual examples as also morphological range among thespecimens. The inner whorls in the specimens 15447 and 15753(Fig. 5) up to about 50–60 mm diameter exhibit the ribframework in early ontogenic stage of single and bifurcatedrursiradiate forwardly convex G. devauxi-like ribs (côterétroverse) which, however, are less curved and less convexthan in G. devauxi at this diameter. Ribs at this stage make amuch smaller acute conjugal angle as in the early stage at

ajor structural features modified after Wynne (1872) (KMF: Kachchh Mainland

Fig. 2. Update on Kachchh Jurassic ammonoid zonation vis-à-vis the Submediterranean standard (Cariou and Hantzpergue, 1997) along with their diachronoussecond order sequence stratigraphic framework (sequence boundaries and maximum flooding surfaces [MFS]) (modified after Krishna, 2005).

J. Krishna et al. / Geobios 42 (2009) 197–208 199

similar diameter of the Italian G. fouquei (Kilian) (D’Arpa andMelendez, 2002: fig. 210) or of the Swiss G. aff. fouquei(Kilian) (Gygi, 1977: Pl. 9, fig. 3). It is noteworthy that Bert andEnay (2004) in their recent revision have synonymised the latter

with their G. devauxi. They have also revised the Italianspecimens as G. aff. fouquei D’Arpa and Melendez (nonKillian). The early part of the middle ontogenic stage of theabove specimens 15447 and 15753 up to 110–120 mm diameter

Fig. 3. Unified Mesozoic lithostratigraphic framework in the Kachchh basin modified after Stoliczka (in: Waagen, 1873–1875), Biswas (1977) and Krishna et al.(1998). The lithological nomenclatures, Patcham Formation (FN.), Chari Formation, Katrol Formation and Umia Formation (UM. FN.) are according to Stoliczka,1867 (in Waagen, 1873–1875) and Jhurio Formation (JH. FN.), Jumara Formation (JUM. FN.), Jhuran Formation and Bhuj Formation as also Khadir Formation andWashtawa Formation (W. FN.) are in accordance with Biswas (1977). MR., CH., W. F., K. F., and U. F. stand for Member, Chari, Washtawa Formation, KatrolFormation and Umia Formation respectively.

J. Krishna et al. / Geobios 42 (2009) 197–208200

Fig. 4. A–D. Whorl sections at different diameters (Diam.) of differentspecimens (A. 15447 at Diam. = 123 mm. B. 15447 at Diam. = 88 mm. C.15753 at Diam. = 63 mm. D. 15395 at Diam. = � 170 mm) of Gregoryceras gr.devauxi Bert and Enay from Kachchh, India.

J. Krishna et al. / Geobios 42 (2009) 197–208 201

exhibits strong and thick single and bifurcated ribs in which thesingle ribs are mostly half when unattached and longer whenattached to the umbilical tubercles. The single ribs in spite ofbeing explicitly rursiradiate are yet relatively straighter than inthe early ontogenic stage. The retroversity of the ribs in earlypart of middle ontogenic stage can be best described as transientor straddling between those of G. devauxi Bert and Enay andG. aff. fouquei D’Arpa and Melendez. Such middle ontogenicstage rib framework, however, is much shortened in G. fouquei(Kilian) only up to 70–80 mm diameter. The preserved septateterminal stage in specimen 15395 (Fig. 6A, B) and somewhatalso in the last three ribs of 15447 (Fig. 5A, B) is nearly similarto the Swiss and Italian specimen. In this context, we wouldhave preferred placement of Gygi’s Swiss ‘‘G. aff. fouquei’’ asintermediate between G. devauxi Bert and Enay andG. aff. fouquei D’Arpa and Melendez instead of synonymisa-tion with G. devauxi Bert and Enay. In somewhat greaterprolongation of the G. devauxi-like middle stage rib frameworkand/or the concomitant late start of the typical G. fouquei ribframework on the phragmocone, in addition of distinctlygreater size, the Kachchh examples straddle betweenG. devauxi Bert and Enay and G. aff. fouquei D’Arpa andMelendez, yet considered closer to G. devauxi Bert and Enay.The evolution in the G. devauxi –gr. devauxi– aff. fouquei isperamorphic as already observed by Bert and Enay (2004). Inthe Kachchh examples, also, there is introduction at theterminus and centripetal extension in the above evolutionarysuccession of species of their typical rib framework.

3. Stratigraphic framework

The Kachchh Gregoryceras have been collected under therigorest possible stratigraphic control and are restricted to two

successive levels I-08 and I-10 within a 1 m-thick sedimentinterval (Fig. 7). The stratigraphic restriction of the KachchhGregoryceras in beds (I-08 and I-10) within a much largerstratigraphic interval of Larcheria subschilli (Lee) from bedI-03 to bed I-16 (Fig. 7) clearly suggests that the beds I-08 toI-10 are precisely included in the Subschilli Horizon of theSchilli Subzone. It is significant that neither the underlying bedsI-01 to I-07 (Fig. 7) nor the overlying beds have yielded anyGregoryceras. Thus, the range of Gregoryceras in Kachchh ismuch shortened compared to the range of the genus in Europeas also of the concerned coeval taxa. It also prompts at this pointto consider the suggestions of Gygi (2000) and Głowniak(2006) that the stratigraphic interval of the Schilli Subzone maybe considered younger than the Transversarium Zone instead ofbeing included in its younger part as a subzone and, hence, alsoits elevation to a full zone (Schilli Zone) along withorganization into Schilli Subzone and Rotoides Subzone inthat order. However, we shall await acceptance or rejection bythe European workers of such a Schilli Zone above a truncatedTransversarium Zone in its stratotype in Europe. At present, wehere follow Transversarium Zone sensu Cariou et al. (1997).The Schilli Subzone at Kantkote in Kachchh includessignificant presence of L. subschilli (Lee) in beds I-03 toI-16 (Fig. 7), and we have collected more than 20 specimens ofLarcheria which have been determined mostly as L. subschilli(Lee). Thus, our earlier assignment of the Kachchh Larcheria(Krishna et al., 1994 and later) to Larcheria schilli–iberica ishere revised to L. subschilli (Lee). Beds I-03 to I-16 (Figs. 7and 8) are considered exclusive to Subschilli Horizon ofthe Schilli Subzone. L. subschilli (Lee) is present in at least11 ammonoid levels compared to the records elsewhere onlythrough a maximum of two to four levels. Also, the interval ofL. subschilli (Lee) from beds I-03 to I-16 (Figs. 7 and 8) isfound explicitly differentiable locally at Kantkote in threesubdivisions:

� b

eds I-03 to I-07 without Gregoryceras; � b eds I-08 to I-10 with a few G. gr. devauxi Bert and Enay and/

or G. aff. fouquei (Kilian);

� b eds I-11 to I-16 without Gregoryceras.

4. Sequence stratigraphic interpretation

In the entire Kachchh Mesozoic, the Kantkote beds I-03 to I-16 (Fig. 7) exhibit maximum ammonoid density, diversity andfrequency (Krishna et al., 2000) in spite of this locality being theproximal most exposed part of the basin (Fig. 1). The threeammonoid suborders, Ammonitina, Phylloceratina and Lyto-ceratina are present in significantly greater number (Fig. 9) thannot only in the Wagad Oxfordian but also in the entire ca. 400 m-thick Wagad Jurassic. Among these, there is extremely heavyskewing to the extent of 98% in favour of the suborderAmmonitina in a total of about 800 ammonoid samplescollected. Phylloceratina and Lytoceratina are present only inbeds I-08 to I-10. Within the suborder Ammonitina, theammonoid subfamilies present are Mayaitinae, Perisphinctinae,

Fig. 5. A–D. Gregoryceras gr. devauxi Bert and Enay, Schilli Subzone, Transversarium Zone, late Middle Oxfordian, Kantkote, Kachchh, India. A. 15447, entirelyseptate, lateral view, bed I-10. B. 15447, entirely septate, ventral view, bed I-10. C. 15753, entirely septate, ventral view, bed I-08. D. 15753, entirely septate, lateralview, bed I-08.

J. Krishna et al. / Geobios 42 (2009) 197–208202

Peltoceratinae, Euaspidoceratinae and Taramelliceratinae withheavy dominance of Mayaitinae and Perisphinctinae over theothers. Over all, the Perisphinctinae and Mayaitinae quantita-tively make 90% of the total ammonoid fauna. ThePerisphinctinae genera/subgenera represented in beds I-03 toI-16 (Fig. 8) are Otosphinctes, Dichotomosphinctes, Subdisco-sphinctes, Larcheria, Liosphinctes/Kranaosphinctes, Pachypla-nulites, and Perisphinctes s.s. In comparison, all the abovesubfamilies but Mayaitinae are present on the Eurasian TethyanMargin (ETM), while Cardioceratinae is absent not only inKachchh but on the entire GTM (Fig. 9). All the Perisphinctinaegenera/subgenera present in beds I-03 to I-16 are also commonto ETM with identical or morphologically close species. Thus, itis clear that ca. 55% of the Ammonitina fauna excluding the45% Mayaitinae at the generic/subgeneric level as listed onFigs. 8 and 10 is common with the ETM. At the subfamily orgeneric/subgeneric level, the similarity is of 90%. In comparisonto the underlying Bathonian–Early Oxfordian and the overlyingLate Oxfordian–Tithonian, the ammonoid faunal similarity in

the referred interval is not only maximum between Kachchh andEurope on the two margins of the Tethys during the entireMesozoic, but also is far greater than any time earlier or later inthe Jurassic. This spectacular ammonoid faunal similarity isaccompanied with many other important features of the bedsI-03 to I-16 (Fig. 7) in contrast to the immediately underlyingand overlying ca. 500 m-thick succession (Fig. 8). Thesefeatures are:

� th

e highest ammonoid faunal density, diversity and frequencyof the Kachchh Mesozoic in terms of individuals; � e xclusive presence of Phylloceratina and Lytoceratina; � e xclusive presence of Tethyan Gregoryceras in beds I-08 to

I-10 and Larcheria at Kantkote in beds I-03 to I-16;

� th e presence of ammonoids in beds I-03 to I-16 of a

condensed to starved succession of hard grounds stacked overone another here interpreted as transgressive with minimalintervention of softer shale/silt/marl as the correspondingregressive components in between;

Fig. 6. A–C. Gregoryceras gr. devauxi Bert and Enay, Schilli Subzone, Transversarium Zone, late Middle Oxfordian, Kantkote, Kachchh, India. A. 15395, entirelyseptate, lateral view, bed I-08. B. 15395, entirely septate, ventral view, bed I-08. C. 15754, entirely septate, lateral view, bed I-08.

J. Krishna et al. / Geobios 42 (2009) 197–208 203

� m

igration of slowly sedimented, low energy sedimentaryfacies until Kantkote in the proximal most exposed part of thebasin; � th e spread of ammonoids to proximal most exposed part of

the basin at Kantkote in beds I-01 to VII-20 (Fig. 8);

� e xtreme slowly sedimented and fine textured character

of beds I-03 to I-16 included in the Schilli Subzone (Figs. 7and 8).

On the basis of the above lithological, facies and ammonoidcompositional features, our earlier interpretation (Krishnaet al., 1998, 2000) of the maximum flooding/maximum

transgression (Fig. 2) of the entire Kachchh Mesozoic in bedsI-03 to I-16 is here further strengthened.

Another significant observation is the drastic qualitative andquantitative reduction in terms of individuals (Figs. 9 and 10) inthe ammonoid fauna immediately above the bed I-16 whichcontinues until the complete withdrawal of the sea fromKantkote region in early Early Kimmeridgian. This is incontrast (Figs. 2, 9 and 10) to the global sea level and Europeansequence frameworks (Haq et al., 1987; Graciansky et al., 1993,1998; Gradstein et al., 2004) in which these workers on theETM indicate the first order maximum flooding near the LateKimmeridgian Eudoxus Zone/Beckeri Zone boundary above,

Fig. 7. Lithologic section of the Chari Formation (uppermost part) at Kantkotewith well labeled beds (beds I-01 to I-16) showing ranges of Larcheria andGregoryceras.

J. Krishna et al. / Geobios 42 (2009) 197–208204

and a second order maximum flooding surface (MFS) belownear the Callovian/Oxfordian boundary. Moreover, in the ETMframework the major part of Oxfordian until Bifurcatus Zone isincluded in a second order regressive component much contraryto the inclusion of Early and Middle Oxfordian until the SchilliSubzone in Kachchh in a second order as also first ordertransgressive component. We relate the above first and secondorder chronological discordance of the sequence frameworkacross the Tethys between ETM and GTM to their respective,yet, widely differing regional extensional tectonic frameworksin Europe and Kachchh (Krishna, 2006a). In Kachchh the firstorder MFS may be genetically linked to the reactivation ofmedian high and climax of rifting in the pericratonic rift basinof Kachchh at the Schilli Subzone/Rotoides Subzone boundary.Thus, shallowing and gradual withdrawal of the Tethys fromKachchh at the start of Rotoides Subzone is tectonicallyinduced as part of an overall reorganization of the plateframework in this region. The postulated regional tectonicgovernance of the first and second order sequence framework inKachchh is further supported by the termination of theMayaitinae at the end of the Schilli Subzone on the entire GTM(Krishna, 2006b). This first and second order discordancebetween GTM and ETM contrasts with the earlier demonstra-tion of the excellent accord of the third order frameworkbetween Kachchh and Europe (Krishna et al., 2000). Our aboveobservations allow us to postulate that the low magnitude highfrequency eustatically guided third order globally remainsundisturbed by interference or distortion linked to the lowfrequency high magnitude extensional tectonics.

5. Interdisciplinary biogeographic implications

The distribution of Gregoryceras in Madagascar and India(Fig. 11) appears linked to an important widespread geologicalevent in southern hemisphere. It represents the southernmostexpansion limit of Gregoryceras around 35–408S on GTM. Inaddition to its distribution on the GTM, Gregoryceras is alsoknown from the SE Pacific margin in Chile in a similar 35–408Slatitudinal framework. Chile is longitudinally several thousandkilometers away from Madagascar and India. Thus, thedistribution of Gregoryceras in the South Hemisphere insimilar latitudinal framework has almost a hemisphericaldimension. Absence in the Himalayas, Indonesia and NewGuinea is probably on account of unfavourable cooler climateand lower temperatures in relatively higher latitudes comparedto Kachchh and Madagascar. Stratigraphically, the Kachchhrecord is in the first order Schilli Subzone MFS marking themaximum bathymetric event of the Jurassic on the GTM(Krishna, 2006a). Climatically and latitudinally, Gregorycerassuggests stronger constraints than its predecessors (Peltocer-atoides and Peltoceras) by way of far less density, diversity andfrequency in Kachchh (five specimens, one species and twosuccessive levels). Apart from the influence of eustatic rise inthe expansion of Gregoryceras in South Hemisphere, it alsomarks a widespread tectonic event at least on the GTM. TheSchilli Subzone/Rotoides Subzone interval marks a tectonicallyforced first order regression not only in the Kachchh Mesozoic

Fig. 8. Lithostratigraphic differentiation and ranges of the stratigraphically significant genera in the late Middle to early Late Oxfordian stratigraphic section atKantkote, Kachchh: stratigraphic ranges of the genera corresponding to the sediment intervals I to VII; ranges of Larcheria and Gregoryceras are further precisedwithin the sediment intervals I and II, on the right side. Middle part of the figure.

J. Krishna et al. / Geobios 42 (2009) 197–208 205

Fig. 9. Comparison of ammonoid compositional and quantitative variation and relative rate of sedimentation between the Gregoryceras bearing maximum floodinginterval of Schilli Subzone and underlying and overlying subzonal transgressive intervals (o: common elements; �: indigenous elements).

J. Krishna et al. / Geobios 42 (2009) 197–208206

but also in several other basins of the GTM (Krishna, 2006a).Paleontologically, the Kachchh Gregoryceras record coincideswith sudden drastic reduction of overall ammonoid density anddiversity across the Schilli Subzone/Rotoides Subzone bound-

Fig. 10. Comparison and correlation of ammonoid chronology and second and thiKrishna et al., 2000) and European (Jacquin et al., 1994) margins of the Tethys.

ary from seven subfamilies to a single subfamily. There is alsodisappearance of Gregoryceras, Euaspidoceras, Larcheria,Taramelliceras, Subdiscosphinctes and Mayaites along withorigin of just one genus, Dichotomoceras, in the Rotoides

rd order sequence stratigraphy framework between the Indian (modified after

Fig. 11. Late Middle Oxfordian, distribution of Gregoryceras in Chile, Mada-gascar and India on the Tethyan and Southeast Pacific margins of the formerGondwanaland in a simplified schematic paleogeographic reconstruction afterKrishna (1983).

J. Krishna et al. / Geobios 42 (2009) 197–208 207

Subzone. It is noteworthy that the ammonoid faunal contrast/change at the first order Schilli Subzone/Rotoides Subzoneboundary MFS in Kachchh on the GTM is stronger compared tothat at the first order Eudoxus Zone/Beckeri Zone boundary onthe ETM. It is also significant that the distribution ofGregoryceras in India, Madagascar and Chile (Fig. 11) is ofmuch shorter duration in the late Middle Oxfordian SchilliSubzone compared to its much longer geological range close toequator in yet lower latitudes. The reduced range in India,Madagascar and Chile is around the first order Kachchh MFSwhich also suggests the possible applicability of the first orderMFS on the GTM and all over South Hemisphere. It isinteresting to note that the relatively high latitude and coldwater Mayaitinae is not distributed north of Mombassa whileGregoryceras is not distributed south of Chile (Gygi andHillebrandt, 1991), Madagascar and Kachchh. Thus, theextreme latitudinal limits of southward Mediterranean expan-sion and northward Indo–East-African expansion and extinc-tions are realized in the late Middle Oxfordian Schilli Subzoneat the first order MFS. Obviously, the southward expansionlimit of Gregoryceras in Chile, Madagascar and Kachchh withreduced range is a cumulative function of tectonoeustacy, andlatitudinally influenced climate.

The discovery of Gregoryceras in Mexico further strength-ens the expansion to Chile via the Central Atlantic as suggestedearlier (Enay, 1972; Enay and Mangold, 1982). Nevertheless,the absence of Gregoryceras in the South Atlantic supports theknown opening of the South Atlantic seaway later than MiddleOxfordian. Similarly, the absence of Gregoryceras farther southof Kachchh and North Madagascar strengthens the earlierassertion (from Krishna, 1983 onwards) that the Gondicshallow marine corridor (Krishna, 1996) across the Gondwana-land from its Tethyan to Pacific margin had yet not originated.In this context, it may be noted that Enay (1972) and Enay andMangold (1982) suspected, even advocated episodic existenceand communication through such a corridor since Toarcian

although favoured undisputed availability of the said corridorseaway not earlier than Late Tithonian.

6. Estimation of relative and absolute bathymetry

The estimate of absolute and relative bathymetry at the firstorder MFS in late Middle Oxfordian Schilli Subzone atKantkote is based on Ziegler (1967), Gygi (1986) and others,using both paleontological and sedimentological considera-tions. Sedimentologically, the first order MFS is included in aslowly sedimented, condensed and hard-grounded interval. It isoverlain by thick shale at the start of the first order regressiveconditions, yet formed under subtidal condition below fairweather wave base of ca. 10 to 15 m. Paleontologically, it is anexceptionally high density, high diversity and high frequencyammonoid-dominated megainvertebrate assemblage with onlyrare benthic macroinvertebrates. The ammonoid morphologicalcomposition is dominated by perisphinctins (45%) andmayaitins (45%) with minor presence of lytoceratins andphylloceratins inclusive of one of the biggest Jurassicammonoids of the world —a giant Pterolytoceras of over2 m in diameter. Compositely, based on the above multipleevidence, greater than 20 m depth at the MFS at Kantkote issuggested here. In this context, it is interesting to take note ofthe submarine stratigraphic gap on the relatively distalMainland Kachchh under yet deeper conditions, e.g. atLakhapur, ca. 100 km west of Kantkote. The gap is alsoknown to increase distally, becoming maximum at the distalmost exposed part of the basin at Lakhapur.

7. Conclusion

The documentation on Kachchh Gregoryceras gr. devauxiBert and Enay and accompanied discussion on ammonoidstratigraphy, paleobiogeography and sequence stratigraphynear the close of Middle Oxfordian holds high significance in:improving correlations between the two margins of theTethys around this time; strengthening the earlier proposedconcordance of the third order maximum flooding during theSchilli Subzone across the Tethys (Krishna et al., 2000) in spiteof the strongly diachronic first and second order sequencestratigraphic framework between the ETM and GTM and,eventually, comprehending biogeographic dynamics in aninterdisciplinary tectonoeustatic–climatic perspective.

Acknowledgement

The authors express their thankfulness to the Department ofGeology, Banaras Hindu University for its all round support. JaiKrishna is grateful to the Department of Science andTechnology, Government of India. B. Pandey (ex-scientist)and J.R. Ojha (ex-pool officer) acknowledge the financialassistance received respectively from the Department ofScience and Technology and the Council of Scientific andIndustrial Research. Assistance received towards logisticslocally in Kachchh from Mr. P.H. Bhatti of the KachchhCollectorate is also thankfully acknowledged.

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