Rivista Italiana di Paleontologia e Stratigrafia volunre I i 0 no. I pp.181-190 April 2004

ENVIRONMENTS AND FAUNAL PATTERNS IN THE KACHCHH RIFT BASIN. \TESTERN INDIA, DURING THE

FRANZ THEODOR FURSICH', JOHN H. CALLOMON" DHIRENDRA K. PANDEY.E{ ANAND K. TAITLY*

Recit,ed September 19, 2002: accepted July 13,2003

Key-zoords: Jurassic, taphonomy, palaeoecology, cycles, Kach- dominata dai bivalvi, seguiti da brachiopodi, gasteropodi, coralli, ser- chh, India. pulidi e spugne. Lanalisi di 370 campioni statistici ed oltre 27000 esem- plari h;r prodotto più di a0 associazioni e rrggruppamenti bentonici. Abstract. Marine Jurassic sediments (Bajocian-Tithonian) of the Essi mostrano una relazione con molti parametri ambientali, due dei Kachchh Basin were deposited in a ramp setting. Except during the quali, salinità e clima, vengono brevemente discussi. Viene delineata la Middle and Late , when a carbonate reginre became estab- distribuzione areale delle facies e dei biota per due porzioni temporali, lished, the fill of the basin consists predominantly of siliciclastics. The rispettivamente il Bathoniano ed il Calloviano-Oxfordiano. sediments represent environnrents that range from coastal plains (riv- ers and associated flood plains with caliche nodules), deltas, brackish water lagoons, nearshore sand and iron-oolite bars of the inner rlmp, generally situated above fair-weather wave-base, to the middle ramp in- Introduction fluenced by stornr-waves and by storm-generated currents, and finally to the outer ramp which is characterised by lor. energy, fine-grained The rift basin of Kachchh originated at the west- sediments. Changes in relative sea level produced a cyclic sedimenta- ern margin of the Indian Plate in the (e.g. Biswas tion pattern. The rich benthic fauna of macroinvertebrates is dominated 1982). After a phase that was characterised by non-ma- by bivalves, followed by brachiopods, gastropods, corals, serpulids, and rine sediments (alluvial fans, fluvial systems), the sea in- sponges. The analysis of 370 statistical samples and more than 27,000 undated the basin sometime in the early Middle specimens produced more than 40 benthic associations and assemblages. Jurassic, They show a relationship to several environmental parameters, two of when it became an appendix of the so-called Malgassy which, salinity rrnd climate, are briefly discussed. The spatial distribu- Gulf, a southern extension of the Tethyan Ocean. The tion of the facies and biota is outlined for two time slices, the Batho- precise timing of the transgression is still not clear; the nian and the -. resDectivelv. oldest ammonite, a Leptosphinctes, is from the Late Ba- jocian (Singh et al. 1982). However, the bed containing Riass unto. I sedimenti marini giurassici (Bajociano-Tìthoniano) del Bacino di Kachchh furono deposti in un ambiente di rampa. Eccetto this ammonite is underlain by about 250 m of sediments che durante il Bathoniano medio e superiore, quando si stabilì un resin.ìe of partly marine, partly fluvial origin so that the earliest carbonatico, il riempimento del bacino consìste predominantemente di marine influence may date even from the Aalenian (Fùr- materiale silicoclastico. I sedimenti rappresentano ambienti che vanno sich et aI.2001). dalle pianure costiere (fiumi e relative piane d'inondazione con noduli a caliche), delta, lagune d'acqua salmastr:r, barre di sabbia litorale e oo- The Jurassic sedimentary fill consists mainly of sil- liti ferrosi della rampa interna, seneralmente siturti sopra la base d'on- iciclastics (except for a carbonate interlude in the central da di bel tempo, alla rampa media (influenzata dalle onde dì tenrpesta and western areas of the basin during the Middle and Late e dalle correnti generate da tempesta), ed infine alla rampa esterna, che Bathonian) and records a general deepening culminat- è caratterizz-ata da sedimenti a grana fine, di bassa energia. I cambìa- menti nel livello marino relativo hanno prodotto una sedimentazione ing in the Middle Oxfordian Schilli Subzone (Krishna et ad andamento ciclico. Le ricca fauna bentonica dei macroinvertebrati è ai. 2000). This was followed by regression which result-

I Institut fùr Palàontologie der Universitàt rù/ùrzburg, Pleichemall 1, D 9/070 \íúrz-burg, Germany: E-mail: franz.fuersich(ri,nT ail.uni-wuerzburg.de z 129 Moffats Lane, Brookmans Park, Hatfield, Herts. AL9 ZRB United Kingdom. E-mail: johncallomon(r,lineone.net.uk 3 Department of Geology, Universitv of Rajasthan, Jaipur 302 004, India. E-mail: dhirendrap(l.,satyam.net.in + Department of Geology, Banaras Hindu University, Varanasi 221 AA5,India. E-mail: akjaitly(ri'banar,rs.ernet.in r82 F. T. Ftirsicb, J. H. Callomon, D. K. Pandey e. A. K. Jaitly

Great Rann Mauwana Dome Chappar Bet Khadir Is Kala Dongar I Cheriya Betl Chorad I

Khavd

Pachchham Is

Washtawa .te ' ,/ .r;.':;tr

Ìrua^ lr\rr\\trrrfa\ {t i r rr ru \ r i a \\r\ I arrì\\\ì\ )f Iììitù\1rÉír- ,r,,,,,f\ . t\\ tlt\f I !it\ttltì\tfiluIIIìl ntuttt a atlì\ì\rrla\ ÌtrtarÌtÌaa^ !rr\ìilì\ìall tatrrraaauar^ \\ttt\a\ttaa\.R I r r t, r t I Ì I t , r r 11 arra\r\i\\raì1lt ,trrraaruaatrrt Mandavi fii':1"q,;- 7f\ t\tÈ\!\\t\\ata\a r 4-,-,',-, BAY OF tttrr\!rrr\arìra * {,}.wi1í;11í1í;1 BENGAL O!. i' *' *' *' *' *' *t *' *' *.b,14 't)t$\atarttttt\ríi:;l;:;ììiìiii arìtrrrtftuÌ lTIl Washtawa Fm (Callovian-Oxfordian) E::::::::::1 Jhurio Fm, Patcham Fm, Chari Fm (Bajocian-Oxfordian) IITTITTTII Khadir Fm, Gadhada fm (Bajocian-oxfordian) rrFTTrnTI HJ#++ttfH Kaladongar Fm, Goradongar Fm, Patcham Fm, Chari Fm (Bajocian-Callovian)

Generalised geological map of Kachchh with localities nrentioned in the text.

ed in the deposition of marginal marine to non-mlrine Environments sediments by the late Early in most parts of Within the of the Kachchh Basin, five dis- the basin. However, this overall trend was not uniform, Jurassic tinct facies associations can be identified, namely alluvial but con.rposed of numerous smaller cycles of different fan congiomerates, coastal plain sediments, deposits of a hierarchies (e.g. Fùrsich tr Oschmann 1993; Krishna et siliciclastic ramp, mixed carbon- al.2000). and of a carbonate and ate-siliciclastic deposits. In the following, the maior facies The marine Jurassic sedin.rents of the basin are types are briefly summed up. A somewhat more extensive well-known for their rich faunas, both benthic rracro- account is given by Fúrsich et al. (in press). A detailed invertebrates, ammonites, and microfaunas. Good expo- description of the facies pattern is in preparation. sures, the facies n.rosaic and the rich benthic faunas irre ideal prerequisites for an integrated basin analysis. First results towards this goal have already been published Alluvial fans and coastal plains (Fùrsich et al. 1991, 1,992,'1,994a, b, 2001, in press; Fùr- The non-marine sedin.rents occur at the base of the sich tr Oschmann 1993; Fùrsich 8r Pandev 2003). The basin fill and at the margins of the basin. At present-day present paper gives a short outline of the main environ- they are exposed on the islands within the Great Rann ments of the basin ,rnd of the palaeoecology of the ben- of Kachchh (Fig. t). They consist of conglomerates rep- thic n.racroinvertebrates, focussing on their relationship resentins alluvial fans, of fining-upward crossbedded with son.re of the nrain palaeoenvironmental parameters. sandstones and arkoses with erosional bases that repre- A locality map is given in Fie. 1, an outline of the lithos- sent fluvial channels, and of reddish silts with thin inter- tratirrirphy in Fig. 2. calations of fine-crained sandstone. The latter are flood Jurassic environments and biota of the Kachcbh Basin 183

vertebrate fossils are absent, but remains of camarosau- M. P. IIB/M w. romorph have been discovered in flood plain deposits on Khadir Island (Moser et al. submitted). È I Siliciclastic ramp 6) Siliciclastic ramp deposits represent arange of envi- z ronments. Strongly bioturbated silty to sandy sediments containing a low diversity molluscan fauna (see below) are indicative of brackish water conditions and correspond to protected bays and lagoons. Thick, large-scale cross- bedded sandstones of the western islands are most likely Fr deltaic sand bodies. Towards the west (e.g. on the Habo Dome), these delta front sandstones grade into biotur- bated silt and silty sandstone of prodelta origin. bo Several metres thick. well sorted. fine- to coarse- L grained sandstones (e.g. the Upper Callovian Athleta o) oo al k Sandstone of the Jara Dome; Fùrsich et al. 2001) extend V F over tens of kilometres. They are interpreted as sheet v sands that are of deltaic origin, brrt were subsequently spread across the middle ramp during destructive delta tFix phase or transgressive events. Argillaceous silt with thin intercalations of sand- È stones that exhibit an erosional base and hummocky crossbedding was deposited on the storm-wave influ- -l rì U) enced ramp, whereas bioturbated argillaceous silt with intercalations of graded, parallel-laminated, sharp-based, cq fine-grained sandstones with tool marks and flute casts Q rl are evidence of deposition by storm-generated currents Patcham below storm wave-base. Indistinctly laminated to bioturbated argillaceous silt to silty clay with bands of ferruginous concretions t< represent the deepest environment in the Kachchh ba- f+t sin, beyond the influence of storms. These outer ramp - deposits, widespread in the Callovian Chari Formation -l H of Kachchh Mainland, generally reflect low energy con- bo \l ditions. CJ Protected bays with bioturbated iron-oolitic silt '-l Gl were widespread during late sea level highstand in the Late - V Callovian. They record low energy environments. In the Lower Callovian part of the Chari Formation at the Keera Fig. 2 - Outline of the lithostratieraphy ofJurassic rocks of the Kach- chh Basin. The lithostratigraphic units rre prrtl,v those of Dome and in the Jhurio Formation (Bajocian-Bathoni- Biswas (1980), partly they are traditional terms introducccl an) of the Jhura Dome, several metres-thick crossbed- by Vaagen ( 1 8 75). For more detailed inforrlrtion see Fúrs ich ded srainstones composed of Fe-ooids and ferruginized et al. (2001). M: Kachchh M;rinland; P: Pachchhrnr Island; bioclasts orisinated as high energy shoals. They formed K/B/'V: Khadir Island, Bela Island. and Mourvana Donre. above fair weather wave base when the Fe-ooids, produced durine late sea level highstand, were reworked during the subsequent transgression. In the Bathonian Goradongar Yellow Flagstone Member (Jhurio Formation) of Jhura plain deposits, an interpretation which is supported by Dome, thin lenticular bodies of Fe-oolitic srainstones, the occurrence of caliche nodules in a coastal plain set- intercalated between carbonates, are interpreted as chan- ting. Intercalations of grey, highly carbonaceous marly nels, in which Fe-oolitic sediment was transported from silt are interpreted as lake deposits on the flood plain. shoals into deeper parts of the ramp. The fluvial channel fills occasionally contain large wood During the Oxfordian, large parts of the basin ex- pieces, in some cases even wood logs. Rootlets occur in perienced sediment starvation due to a large rise in rela- ripple-laminated fine-grained sandstones, interpreted as tive sea level (for details see Krishna et al. 2000). As a re- levee deposits, near the top of the channel fills. Macroin- sult, highly condensed sediments with ferruginous crusts, 184 F. T. Fùrsich, J. H. Callottton, D. K. Pandey 8x A. K. Jaitly

ferruginous ooids, hardgrounds, erosion surfaces, intra- Faunal patterns formational conglomerates and shell concentrations, the Benthic macroinvertebrates are common in Lower to lower Middle Oxfordian Dhosa Oolite Member Jurassic marine sediments of the basin. In nearshore areas they of the Chari Formation, were deposited on the Kachchh tend to occur concentrated in shell beds and shells lens- Mainland beyond the influence of storm waves but sub- es. Usually they have been reworked and transported' al- ject to currents (Singh 1989; Fúrsich et al. 1992,2001). though apparently not for large distances. In more off- The remaining Middle Oxfordian, the Upper Oxfordian, shore areas nrost shells occur scattered or as winnowed and the lowermost Lower Kimmeridgian are represented pavements. Deep burrowine bivalves commonly occur in by a cm-thick ferruginous crust and by stratigraphic gaps. position of growth and the preservation quality is gener- In contrast, from areas closer to the shore (e.g. Wagad ally high. In some cases, even epif:runal taxa such as bra- Dome) a rich arnmonite fauna has been recorded from chiopods are preserved in life position suggesting low these intervals (e.g. Krishna et al. 1998,2000). enersy environments and lack of disturbance by preda- tors or scavengers. Such autochthonous to parautoch- Carbonate ramp thonous faunas are the relicts of former benthic commu- Within the basin, carbonate sediments are largel,v nities, ternred here associations. In addition, shell concen- restricted to the Middle and Upper Bathonian. A fair- trations formed during early transgression (transgressive weather wave-influenced ramp is documented by the lags) are widespread in the Callovian Chari Formation Raimalrho Member of the Patcham Forma- (Fùrsich & Oschmann 1993), in the Oxfordian of the tion of the Pachchham Island. Thickly bedded, laree- Wagad Dome (e.g. the so-called Bharodia Astarte Bands), scale crossbedded grain- and packstones with scour and and in the upper part of the Katrol Formation and the fill structures and shell lenticles indicate constant re- overlying Un.ria Forrnation (Kinrnrerideian to Neocomi- working bv waves. Further offshore (e.g. at the Habo an) (Fúrsich tt Pandey 2003). They are usually heavily Dome), these carbonates change into graded grainstones time-averaged, mixed, and sorted, and therefore represent with sharp, erosional bases, hummocky crossbedding, distorted con.lnunity relicts, ternred assen.rblages in the and occasional amalgamation of successive beds. These following (for terminology see Fùrsich 1984). features are indicative of proximal storm deposits (e.g. Aigner 1985). Benthic associations Still further offshore (e.g. at the Jhura Dome) thin, A quantitative data base consisting of more than craded and laminated grainstones are intercalated between 27,OOO specimens and 370 sanrples was used to define bioturbated, bioclastic marl. The environment was the more than 4O associations and several assemblages with storm-flow influenced middle ramp. Increasingly thinner the help of a cluster ,rnalysis (Fùrsich et al. in and more fine-grained tempestites in combination with an Q-mode press; Table I Table 1 shows that most associations are autochthonous fauna of sponges correspond to the deep- ). dominated by bivalves and only few by other groups est part of the ramp exposed on Kachchh Mainland. such as corals, sponges, gastropods or anellids. As can be also seen from Table 1, most associations come from Mixed carbonate-siliciclastic environments the Callovian part of the basin fill. This is partly a col- Mixed carbonate-siliciclastic sediments are wide- lection bias, because the MiddleJurassic has been n.ruch spread in the Kachchh Basin. They either represent en- more heavily sampled, partly it is a preservational bi- vironments of the carbonate ramp where the autoch- as, because in the lareely sandy sediments of the Kim- thonous carbonate component was diluted by silt and meridgian-Tithonian of the mainland the preservation sand fron.r :r terrigenous source [e.g. the Bathonian Go- potential of shelly benthic macroinvertebrates is much radonsar Yellow Flagstone Member (Goradongar For- lower than in the argillaceous silts of the Callovian Chari nration) of the Jhura Dome and the Pachchham lslandl, Formation. Nevertheless, the occurrence, within a con- or else transsressive lag deposits. In the latter, lowstand fined time-interval, of numerous distinct associations deposits that usually consist of siliciclastic materitrl were in very sinrilar substrate (argillaceous silt) and in what reworked during subsequent transgression. Due to lorv appears to h,rve been similar environments (outer rantp) input of nerv terrigenous material during early trans- suggests that the distribution pattern of the benthic sression, skeletal-derived carbonate particles are inrpor- fauna was governed by very subtle differences in en- tant constituents of the sediment. The resulting rock vironmentirl parameters. The reason for this may be r:lnges fror.n crlcareous sandstone to gravelly or pcbbly that in offshore mid to outer ranrp environments (i.e. rudstones, the pebbles usually consisting of quartz :rnd the equivalents of the middle shelf), the comparatively granite/uneiss. They probably indicate marine rework- high environn.rental stability and low stress conditions ins of fluvial sediments. A characteristic example is the led to a much narrower subdivision of the niches of the Upper Bajoci,rn Leptosphinctes Pebbly Rudstone rt the ecospace conrpared to shallower, frequently disturbed top of the Kaladongar Formation of Pachchham Island environments (e.g. Sanders 1968). lFùrsich et al. 2001). Tab. I - List of the benthic macro-rn- dominant group(s) stratigraphic occurrence r.ertebrate associations and assemblages recognised in Bositrq buchi bivalr,es Bath.-Callov Bositt'a huc hi Praesuccellu.juricutu bi r,alvcs Callovian the Jur;issic of the Kachchh PruesucceIIu jurianu NitanicIIo pisilònnì.s bivalves Callovian Basin. Cin€:cntoliutn pqrtiturn Corhulomínu nutneilli bir alves Callovian C ingenlol iun p0rl if unl bivalves Callovian N u c'tt I

N eo tt'tts s i n u .s tt hde p r cs s tt bivalves Callovian High lv divcrse soligrouncl bivalves Callovian

Eom i odon hq ro n i - P ro to(qrd id l.t (' e t I i bivalves Bajocian Eontiodon huroni Placunopsis soc'iqlís bivalves Bajocian lnclocorbulct ltt'ula bivalves Bajoc.-Bath. lndoqt'hula lyrotu - Protoutrdiu gruruliclieri bivalvcs ?Aal.-Bajoc. Tltrqùd fiLeIiurcn.\i.\ bivalvcs Callovian Ps eu.lo I í n e.t .lu p I i utÍ 0 bivalves Callovian Nunogtt'tt turno PolueonuttrIu

Lo p h ro t h.tt' i s e u r.ryt h' c h u brachiopods Callov.-Oxf . Ku I c' h i t h.tt'i s hypsogo n i u brachiopods Cal lovian R hrn <'lrm e I I o i cle I I u b rcv i cos I u l.l brachiopods Cal Iovian

K u t t h i t h_tt' i.y b rc v i p I i c u I u brachiopods Callovian C.r'c'lo,serpulu annelids,/brachiopods Cal Iovian

P I qn'c h on ia .s ú I ot h c i n i sponges Bathonian

assemblages Sonru I i rh.vt t h i u ctf i i ut rut brachiopods/bivalvcs Callovian

KaI I irh)trc h ia vcrsqh i I is brachiopods 'lBajocian Crinoid crinoidsr'annelids Callovian Telroserpulet annelids/brachiopods Callovian Bucqrdknt.tu Iirutu bivalves Callovian Sphaaro ntgcri bivalvcs Callovian ,4mphieslrqeQ corals Tithonian Gnphoeu bivalves Tithonian Inclolrigoniu Pi.rotrigoniu bivalves Tithonian Trigonioid Hcr:oginu bivah,es Neocomian Sechu< hiu Dlvalves Oxf.-Kimm. Ìr4e guc ucu I I oea - Sae bcrt' h i u bivalves Tithonian

On the whole, the environmental specrrum within in the Jurassic. They include the Eomiodon baroni - Pro- the basin was fairly wide. Consequenrly, many environ- tocardia lycetti, the Eomiodon baroni - Placunopsis socialis, mental parameters differed strongiy, say, between coastal the Indocorbula lyrata, and the Indocorbula lyrata - Proto- and outer ramp settings and exerred a major influence on cardia grandidieri associations. The latter consists only of the distribution of the fauna. To illustrate this point, the the two name-e;vine taxa occurring in grey marly silt in influence of two of these parameters, salinity and clin.rate, the ?Bajocian/Bathonian Khadir Formation of Mouwana in particular water temperature, on the benthic fauna is Dome. More than 98"/" of the individuals are articulated briefly discussed in the following. (Fig. :B). Size sorting or selective dissolution of shells can be excluded, as a wide range of sizes are present, al- though aduits clearly dominate, and the originally arago- Environmental control of the benthic macrofauna nitic shells have been replaced by calcite, but are otherwise well preserved. The fact that the strata below and above

Salinity. Several of the associations listed in Table 1 the Indocorbula-Protocardia Bed are unfossiliferous and are characterised by a low species diversity and are domi- that three metres above the bed there occurs a well devel- nated by taxa, whose close relatives are knos/n to have been oped root horizon in a silty fine-grained sandstone (Fig. able to tolerate brackish water conditions somewhere else 3A), suggest iowered salinity values as the main stress fac- 186 F. T. Fi.irsich, J. H. Callomon, D, K. Pandey 8*. A. K. Jaitly

Khadir lsland o/o '40

20 D

Khadir lsland Yo 60

40

20 c

;,,r,*';',;'f's'in I

The Indocorbula lyrata, Protocardia grandidieri association from the Bathonian of Mouwana Dome (B) and related assemblages from the ?Bajocian of Khadir Island (C, D) are interpreted as having been strongly controlled by freshwater input. Given is the relative abun- dance of the species (n refers to the number of individuals). B-C-D is thought to reflect increasing salinity values, from oligohaline/ lower mesohaline conditions (B) to brach;'haline conditions (C, D).

tor responsible for the low species diversity. Both taxa are Sample 3C is regarded as representing a somewhat lower well known members of salinity-controlled environments salinity than sample D, despite its higher species richness, in the Middle Jurassic of Morocco (Mongin 1967) andTu- because it seems to be more time-averaged. The evenness nisia (Holzapfel tllS; and related species are dominant el- is, however, still lower than that of sample 3D. ements of many Middle and UpperJurassic brackish water Climate (Table 2). Precise measurements of pa- biota (e.g. Fúrsich 1994).In the Kachchh Basin the two laeotemperatures are only possible with the help of oxy- species vrere restricted to the ?Bajocian-Bathonian part of gen isotopes. This requires well preserved, diagenetically the sequence. They were no true brackish water endem- unaltered skeletal hardparts, a requirement not fulfilled ics, but managed to thrive in a range of salinity regimes. by most benthic faunal elements within the basin. How- Judging from its low diversity, the sample in Fig. 38 most ever, large-scale shifts in climatic conditions and hence likely corresponds to the lower mesohaline or even oligo- also in temperatures can be deduced from the analysis haline regime, whereas in the other two samples (Fig. 3C- of the benthic macrofauna in connection with changes D), the salinity probably was in the brachyhaline range. in the facies pattern.

parameters Late Bathonian Callovian observations; terrigenous input low high sedimentary regime carbonate siliciclastic faunal diversity high medium characteristic faunal elements diverse coral meadows nuculid bivalves E I i gmus, Opinae ft ivalves)

inferences.' Tab.2 - Environmental change be- water temperature high medium tween the Late Bathonian nutrient supply low high rnd Errlv Callovian as in- overall palaeoclimate hot, semi-arid warm, humid ferred from the f.rcies regimes and the benthic macrofauna. I arid climate r carbonate regime during humìd interludes coastal red beds r@f brackish lagoo I n d oco rb u I a - P roto c a rd i a fair weather-wave influenced siliciclastic carbonate ramD shoreline siliciclastic ramp: dune fields

Scm | storm-flow influenced carbonate ramp

ffiE/lgmus ass. ffil',' ffi meadows transgressive carbonate coral meadows sand sheet

FiO J Bathonian pf,Lrcoenvironnrents ;rncl biota in the Kachchh Basin. Shos'n are charlcteristic facies and benrhic,rssociations, sonrc of rvhich are p,irt of ir rîr[]onatc r,rnrp, others part of l silicichstic r;ìnrp. Note thît not all facies are contemporaneous. lÌrr example, the coral me,rclou,s and sponge merdosrs rre seperated in tinre bv the trrnsgressive c,rrbonate sand shcet.

In terms of shifts in facies pi'rttern, one of the nopis and Pacbopis, which do not occur elsewhere in the most distinct changes within the basin is the change Jurassic of Kachchh. Similarlv, the diversity of gastro- from a carbonate-dominated sedimentation reginre in pods (n-rore than l0 species) is quite unique, considering the Middle and Upper Bathonian (Jhurio Formation, that in most associations from the Jurassic of Kachchh Patcham Formation) to a siliciclastic resin.re in the Cal- no or only 1-2 species of gastropods occur. In beds be- lovian (Chari Formation). This facies change is acconr- low the coral meadows the epibyssate bivalve Eligmus panied by a distinct change in the benthic macrofauna: rollandi is a ch,rracteristic element. The bivalves men- at Jumara Dome, fine-grained carbonates with rare in- tioned ,rbove are characteristic elements of low-latitude tercrlations of distal tempestites of the middle to outer carbonate environments in the Jurassic. Their occur- ramp contain high diversity coral rneadows, rhe À4icro- rence, tosether with the high diversity coral meadows, solena amorpba - Montlicaltia frustriforzis association in the Upper Bathonian of Kachchh points to tropical, (Fùrsich et al. tll+b). Apart from more than 60 taxa of high water temperatures in the basin at that time. This corals there occurs a rich f:runa of bivalves, brachiopods, vien'is corroborated bv the carbonate sediment. Appar- and uastropods. The overall species diversity is three to entl,v, the climate at that time was hot and at least semi- four times higher thirn that of average benthic associn- arid, so that input of terrisenous siliciclastics from the tions occurrine in older and younger beds. The good hinterland was greatly reduced. Low run-off probably preservation of the fauna and limited biogenic alteration :rlso resulted in ;r low nutrient supply and hence oligo- of shells (borine, encrustation, biogenic bre:rkage) pre- trophic conditions. This view is supported b;, the high cludes time-:rveraging as the decisive factor for the high faunal diversity and by the presence of sponge mead- diversity. A characteristic faunal eler.nent of the coral ows, consisting of "lithistid", hexactinellid and calcarean meadows are bivalves of the subfamilv Opinae. They are sponses (Mehl & Fùrsich 1992). These sponge meadows, represented by five species of the genera Coelopis,Trigo- occurrins in fine-grained carbonate sediments, are the 188 F. T. Ftirsich, J. H. Callomon, D. K. Pandey t< A. K. Jaitly

r humid climate r siliciclastic regime

delta

nearshore sandy reworked fauna dominated by regime See b ach ia and I n dotrigon i a Fe-ooid bar storm-influenced ramp IA- à | ff":î":T:",:5:,,*. Pholadomya lirata ass.

f\sfY epiraunalassociations Kutchithyris breviplicata ass.

infaun€-dominated W Praesaccella juriana- Main factors governing the benthíc fauna: N ica n i el I a o is ifo rm is ass. I substrate (grain size, consistency) r energy level (storrn/wave/tidal influence) l@,rry r rate ol sedimentation II condensed shell beds

Fio q Callovian to Oxfordian palaeoenvironments and biota in the Kachchh Basin. Shown are characteristic facies and benthic associarions, which are part of a siliciclastic ramp, Note that not all facies are contemporaneous. For example, the condensed shells beds are a result of a major rise in relative sea level during the Oxfordian.

characteristic biofacies of the Upper Bathonian Patcham mate is envisaged for the Callovian-Oxfordian. This is Formation of the Jumara and Jhura domes. In conrrasr also supported by the presence of ferruginous ooids at to the classical UpperJurassic microbial-sponge reefs on several levels in the sedimentary succession. the northern margin of the Tethyan Ocean (e.g. Keupp These general statements about the palaeoclimate et al. 1990), no microbial crusts are associared with the disregard the fact that the climate was not uniform with- sponges from the Kachchh Basin, probably as a result in either the Bathonian or Callovian, but that short-term of oligotrophic conditions. temperature and humidity fluctuations occurred repeat- The hot, arid to semi-arid climate apparenrly ex- edly. For example, ferruginous ooids occur intermittently isted for much of the early Middle Jurassic, which is in- also in the Upper Bathonian of the islands and may indi- dicated by the widespread occurrence of red beds, part- cate brief spells of more humid conditions. Similarly, the ly with caliche nodules, in Baiocian-Bathonian rocks of presence of stacked fluvial channels and brackish water the islands. faunas in the Bajocian/Bathonian of the islands points to In the Callovian, in conrrasr, the predominanrly periods of greater humidity. silty sediment of offshore areas and the f resence of large deltaic sand bodies near rhe easrern margin of the basin in- dicate increased sediment input from a terresrrial source, Onshore-offshore distribution pattern of facies and ben- probably caused by a wetter climate. The abundant depos- thic fauna during the Middle Jurassic it-feeding nuculid bivalves Palaeonwcuk, Nuculoma, Meso- saccelk, Praesaccella, and DacryomVa potnt to an abundant Integrating sedimentological and palaeoecologi- cal data, some characteristic facies food source within the sediment in connection with a high the distribution of nutrient supply. Eutrophic conditions are therefore envis- and associations on the ramp morphology is depicted in aged to have prevailed during the Callovian. As a result, Figs. 4 and 5 for the Bathonian and Callovian-Oxford- species diversity of benthic associations was lower than ian, respectively. in the Late Bathonian. Corals occur either as scattered Bathonian individuals or as monospecific patch reefs (A.mphiastraea In the Bathonian (Fig. a), the arid climate resulted in a carbon- pirifurmis association) indicating that conditions for reef ate sedimentary regime, which was characterised by fair-weather wave- corals were no longer ideal. Overall, a warm, humid cli- influenced crossbedded grainstones and packstones in the coastal zone Jurassic environments and biota of the Kachchh Basin 189

(e.g. the Raimalro Limestone Member at the top of the Patchanr l;or- Kutchith),ris breriplicata association) or by infaunal bivalves, among mation in the Pachchham Island). In an offshore direction, this facies n'hich deposit-feeding nuculids and small suspension-feedin g corbulids gave way to grainstones and packstones subject to stornr \!aves (e.g. the and ;rstartids v/ere pxrticularly abundant (e.g. the Praesaccella juriana

Patcham Formation of the Habo Dome). Still further offshore, grad- - N i can i ella pi s iforrni s association). ed, laminated calcarenites produced b1' storm-induced flows alternate As a result of the high rise of relative sea level during the Ox- with a bioturbated bioclastic nrarl, which lepresents the b.rcksround fordian, offshore areas of the basin suffered sediment starvation. High- sedimentation. Autochthonous connrunity relicts dominated either bv lv condensed shell concentrations accumulated in these arers, together bivalves (e.g. Eligmus), sponges or corals characterise the outer ramp. with sedirrentarv features indicative of on.rission, such as hardgrounds The well bedded lorv energy nrud- to wackestones are only occasion- and ferruginous crusts. ally interrupted by thin, graded bioclastic calcarenites, interpreted ;rs distal tempestites. During more humid interludes, higher amounrs of freshr.'ater Conclusions entered the sea. In brackish water lasoons low diversity .rssoci:rtions thrived, dominated by the bivalveJurassicorbula. Above the fair-weather During the environments and biota of the wave base, coastal sands gàve way offshore to subnlarine dune fields lnd Jurassic, to hummocky crossbedded s.rncl hyers indicative of storm-q'ave influ- Kachchh Basin were largely controlled by the interplay of ence. Mixed carbonate-siliciclastics prevailed in a mid-ramp setting (e.!1. general subsidence, local synsedimentary tectonics, eu- the Goradongar Yellow Flagstone Menrber, Goradongar Formation, of static sea level changes, and changes in the regional cli- Pachchharn Island;. Thin intercalations of graded, l;rminated sandy cal- mate. Depending on the influence of the individual fac- carenites are evidence of storm-generated currents. This generally low a re- energv environment was colonised by the high diversity variant of the tors, either a carbonate or siliciclastic sediment^ry Indocorbula lyr.lt.t associarion. During transgression, several m thick gime became established. The spatial distribution of the skeletal sand sheets developed, which consist of crinoid ossicles with benthic macrofauna was mainly governed by substrate, minor amounts of qulrtz grrins. energy level, rate of sedimentation, and nutrient supply, in nearshore areas also by changes in salinity. These en- Callovian-Oxfordian The ramp siruation persisted during the ensuing Callovian-Ox- vironmental parameters, in turn, were controlled by the fordian time interval, but the profound changes in climate, discussed four main exogenic factors mentioned above. They not above, led to a different facies pattern and set of benthic communities only controlled the spatial distribution pattern, but also (Fig. 5). Increased run-off, possibly in combination with increased tec- the temporal changes of the macrofauna. As a result, the tonic uplift in the hinterland supplied much terrisenous material to the cyclic sedimentation pattern within the basin is mirrored basin. In the eastern part of the basin (e.g. Vagad Donre), large deltaic sand bodies developed. \ù7aves and currents distributed the sand either by corresponding changes in the benthic communities during destructive delta phases or rransgressive events rs sand sheets (e.g. Fùrsich et al. 1991). across the basin. In nearshore areas, dune and megaripple fields fornred. Further offshore, storm influence became dominrnt. Only rarely, rel- icts of the benthic communities are preserved. Shells were either dis- Achnouledgemenrs. Ve would like to thank Mr. PH. Bhatti, Bhuj, solved during diagenesis or destrol'ed b,v biostratinomic processes in who throughout our field work in Kachchh has helped us in soh'ing these high energy, shallow water environments. However, when these loeistic problems. The National Geographic Societv (grant #J597-87) faunas were reworked during transgressive events they occasionalli' and the Deutsche Forschungsgemeinschaft (Fu 131/18-l) financially became preserved as shell concentrations (transeressive lags). Typical supported the field work durìng the years 1989-1997. \We also acknowl- faunal elements of such lags are large, thick-shelled infaunal bivalves edge \XI Oschmann, Frankfurt, and I.B. Singh, Lucknov,, companions (l ndotrigon ia, S ee bach ia). during the first field trips, for discussions of palaeoecological and sedi- Towards the outer ramp argillaceous silt was the dominant facies nrentological aspects. \ùúl Vilmsen, Vùrzburg, critically read a draft of containing, in contrast to the usually unfossiliferous sandstones, a rich the manuscript. Finallv. u'e thank S. Fernandez-Lopez and Jai Krishna benthic fauna, characterised either by epibenthic brachiopods (e.g. the for their reviews of the manuscrrpr.

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