Journal of Asian Earth Sciences 34 (2009) 310–316

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Journal of Asian Earth Sciences

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High energy transgressive deposits from the Late Jurassic of Wagad,Eastern Kachchh, India

Diwakar Mishra *

Department of Geology, Banaras Hindu University, Varanasi 221 005, Uttar Pradesh, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 October 2006Received in revised form 20 February 2008Accepted 27 May 2008

Keywords:Shell rich beds (formation/memberboundaries)Storm dominated transgressive sequencesLate JurassicWagadEastern Kachchh

1367-9120/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jseaes.2008.05.010

* Tel.: +91 542 2307311; fax: +91 542 2368174.E-mail address: d_mishra5@yahoo.co.in

The whole sedimentary succession (ca 600 m thick) of Wagad area ranging in age from Callovian to EarlyKimmeridgian has been divided in to three Formations namely Washtawa, Kanthkot and Gamdau inascending order. Prograding Kanthkot Formation was frequently interrupted by transgressions. Fieldand petrographic investigations revealed that the Kanthkot Formation represents three fossiliferous mar-ker beds corresponding to Transgressive sequence I; Transgressive sequence II and Transgressivesequence III. These transgressive sequences are composed of two lithounits: medium to coarsegrained/gritty, graded to massive, sheetlike, fossiliferous calcareous sandstone (storm lag unit I) and fos-siliferous mudrocks (swell lag unit II). The thickness of the unit I varies from 5 to 75 cm and containsmostly convexly oriented shell fragments and whole shell of Pelecypods, Cephalopods and Brachiopods.Unit II (5–15 cm) is distinguished by sheetlike, massive or laminated, yellowish colour, soft fossiliferousmudrocks. This unit is intercalated with moderately bioturbated sandy siltstone. Unit I is dominant overUnit II in the sequences.Study suggests that the transgressive units were deposited close to wave base by high energy storm flowsfollowed by low energy marine swells during transgression. The intense storms played a major role in thedistribution of siliciclastics and nonclastic materials. Storms are evidenced by the occurrence of two dis-tinctly different types of units (storm lags and swell lags). High energy levels are characterized by sanddominated sequence, abundance of reworked sediment particles, high proportion broken shells with con-vex up orientation and erosional and sharp nature of basal contacts of units together with well preservedbioclasts. Sudden short term changes from high to low energy during transgression are explained by theoccurrence of medium to coarse grained siliciclastics interbedded with moderately bioturbated mud-rocks. Moderately bedded individual strata, high content of coarse clastics along with polished granulesize quartz and abundance of comminuted shells indicate a significant change in depositional setting,possibly closure approach of the source of terrigenous fraction or source uplift. Synrift sedimentationin the present study is documented by an abundance of coarse clastics and an over all aggradational nat-ure of transgressive sequences.

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1. Introduction

Transgressive deposits can be of various types (fully marine,estuarine/lagoonal or fluvial) and represent broad range of litholo-gies depending upon the variability driven by changes in rate of sealevel rise, sediment supply, textural character of sediments, shelfgradient and basin morphology (Cattaneo and Steel, 2003). Littlechanges in one or combination of the controlling factors can pro-duce sand dominated (high energy) transgressive deposits ormud dominated (low energy) transgressive deposits. Ancienttransgressive successions have been well studied in sand domi-nated outcrop of limited lateral extant that provide good local doc-

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umentation of key surfaces; however, modern transgressivesystems are better studied in mud dominated environment (Berg-man and Snedden, 1999a,b). Based on recent studies on modernsediment transporting events on shelves, Keen et al. (2006) con-cluded that storm moves largest mass of sediment and are poten-tial cause of offshore transport. The several papers have providedthe important effect of storms in generating sedimentary struc-tures and textures in recent and ancient marine environment(Brenner and Davis, 1973; Brenner, 1980; Kreisa, 1981; Keen andSlingerland, 1993; Tamura and Masuda, 2005). Hayes (1967) dem-onstrated that in 1961 Hurricane Carla scoured the barrier islandoff laguna Madre, and that sediment was re-deposited as an inten-sive graded bed.

In recent decades, the biotic field observable features carry thesignature of formational processes and are useful environmental

D. Mishra / Journal of Asian Earth Sciences 34 (2009) 310–316 311

indicator (Fursich and Oschmann, 1986, 1993; Banerjee and Kid-well, 1991; Kidwell, 1986).

Wynne (1872) was first to described a detailed account of thegeology of Kutch. Other studies focused on various aspects suchas palaeontological, stratigraphic, structural and depositional set-ting of the area includes (Waagen, 1873–1875; Krishna et al.,1998; Deshpande and Merh, 1980). Late Jurassic shallow marinesedimentary succession of Wagad has been divided in to variousdepositional units by prominent fossiliferous marker bedsnamely fossiliferous conglomerate beds, Lower Astarte bedsand Upper Astarte beds corresponding to transgressive se-quences. These beds are considered as formation/memberboundaries or synchronous surfaces by earlier workers. Shell richbeds can be valuable tools for stratigraphic, sequence strati-graphic and sedimentological analysis (Kidwell, 1989). Despitetheir importance, little sedimentological analyses have been at-tempted. Considering the significance of these shell rich bedsas formation boundaries, the purpose of this paper is to show

Fig. 1. Geological map and

the nature and distribution of various lithounits within the mar-ker beds, to describe the sedimentary processes for the genesisof these prominent shell rich beds and to reconstruct their depo-sitional environment.

2. Geological setting and stratigraphy

Kachchh sedimentary basin is a pericratonic rift basin in the ex-treme West of Indian Peninsula. The Mesozoic rocks ranging in agefrom the Middle Jurassic to Upper Cretaceous occur conspicuouslyin the various major uplifts and are exposed in the Kachchh Main-land, Wagad, the ‘Islands’ of Pachcham, Khadir, Bela and the Chorarhills. The geological map along with location of study area (Wagad)is given in Fig. 1.

The stratigraphy of Wagad has been worked out in some detailby Biswas (1977). Instead of extending Stolickzka’s stratigraphicunits to Wagad, he created two formations in Wagad: 210 m thickshale dominated Washtawa Formation overlain by 480 m thick

location of study area.

Table 1Lithostratigraphic framework of Jurassic rocks of Wagad

After Krishna (2002) integrated to Stoliczka(in Waagen, 1873–1875)

After Biswas (1971) After Deshpande and Merh (1980)

Jurassic Callovian to EarlyKimmeridgian

Umia Formation(Gamdau Member)

Callovian toNeocomian

Wagad SandstoneFormation

GamdauMember

Gamdau Formation

Katrol Formation(Kanthkot Member)

KanthkotMember

KanthkotFormation

Upper Uppere Astarte bandAdhoi Member

Lower Lower Astarte bandFort SandstoneMemberPatasar Shale Member

Chari Formation Washtawa Formations Washtawa Formation FossiliferousConglomerate bandNara Shale MemberKharol SandstoneMember

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sand dominated Wagad Formation. The Wagad Formation has beenfurther subdivided into two members: Kanthkot and Gamdau. Heintroduced three fossiliferous marker beds: Kanthkot Ammonitebed, Kanthkot Fossiliferous bed and Bharodia Astarte bed occurat the base, middle and top of the Kanthkot Formation respectively.The stratigraphy of Wagad area is controlled by these fossiliferousmarker beds. All along since 1971 Biswas has considered Wagadrocks as ranging from Callovian to Neocomian. Deshpande andMerh (1980) have included three formations in Mesozoic sedi-ments of Wagad i.e. Washtawa, Kanthkot and Gamdau in ascend-ing order. Thus, apparently they have raised the status of

Fig. 2. Geological map of Wagad Hill

Kanthkot Member and Gamdau Member of Biswas as distinct for-mations. They have considered two significant Astarte beds one inthe middle and other at the top of Kanthkot Member/Formation asmarking disconformities. According to recent stratigraphic studies(Krishna et al., 1998; Krishna, 2002) the Mesozoic succession ofWagad ranges from Callovian to Early Kimmeridgian. Stratigraphicnomenclature used for Late Jurassic rocks and bounding rock unitsin Wagad is given in Table 1. Present study is confined to shell richbeds (formation/member boundaries namely Fossililiferous con-glomerate bed, Lower Astarte bed and Upper Astarte bed) exposedin south western part of Wagad uplift.

and locations of studied sections.

Table 2Name and location of studied sections

S.No.

Name of thesections

Location Thickness (in m)

1. Washtawa Near Washtawa village(Southern flank of Washtawa Dome)

5

2. Kanthkot Near Kanthkot village(Kanthkot Dome)

100 (regressive part:not studied)

3. Adhoi 3 km West of Adhoi village(Adhoi – Kanthkot road)

5

4. Bharodia 1.5 km SW of Bharodia village 20 (studied part: 5 m)

D. Mishra / Journal of Asian Earth Sciences 34 (2009) 310–316 313

3. Methodology

Four sections namely Washtawa, Kanthkot, Adhoi and Bharodiaof Wagad uplift were measured and described (Fig. 2 and Table 2).A vertical column has been prepared by integrating all the sections(Fig. 3). Special attention was paid on sedimentary structures, tex-ture, bedding characters and distribution of fossils as well as con-tact relation of individual beds. Megascopic fabric was describedprior to thin section preparation. For petrographic investigation,microscope (Nicon Japan) was used to examine the various physi-cal characters i.e. size, shape, sorting, mineral composition and nat-ure of cementing material.

4. Descriptions of lithounits

Field and petrographic investigations revealed that the trans-gressive sequences (shell rich beds) are composed of two lithoun-its: medium to coarse grained fossiliferous calcareous sandstone(storm lag unit I) and calcareous mudrocks (swell lag unit II).Descriptions of lithounits are given below and also tabulated in Ta-ble 3.

4.1. Medium to coarse grained fossiliferous calcareous sandstone(storm lag unit I)

Unit I is characterized by medium to coarse grained/gritty(quartz and altered feldspars), sheetlike, graded to massive, thinlyto moderately bedded, grey to brownish red, hard and compact,moderately to poorly sorted, calcareous sandstone containing subangular to sub rounded and polished quartz granules. The thick-ness of this unit varies from 5 to 75 cm and contains whole shellof Pelecypods (Astarte) and Cephalopods (Ammonoids) (Plate 1Aand B). Polished quartz granules are scattered on the surface ofthe sandstone. Sorting of biotic and abiotic elements increases up-ward. The biofabric consists of fining upward, mostly convex-uporiented, parallel to bedding plane, strongly abraded, disarticulatedbivalve shells fragments (Plate 1C). Fragmented shells dominateover whole shells. Thin sections of few samples reveal quartz, feld-spars and carbonate bioclasts embedded in fine recrystallised san-dy calcareous cement. Most of the quartz grains are sub angular tosub rounded though angular grains are also observed.

4.2. Mudrocks (swell lag unit 2)

Unit II is distinguished by sheetlike, massive or laminated, bio-turbated, yellowish colour, soft mudrocks containing completelypreserved epifaunal bivalve shell with convex side up. However,this unit is intercalated with well laminated, moderately bioturbat-ed calcareous siltstone with individual units generally less than15 cm in thickness. The thickness of intercalated unit (calcareoussiltstone) varies from 2 to 5 cm. The complete shell concentratesare difficult to demonstrate because of the poor exposure of the

units in which they occur. The presence of beds made up of thesewhole shells can be observed in Upper Astarte bed.

5. Transgressive sequences

A stratigraphic column having thickness of 150 m has been pre-pared by integrating all the sections measured in the field (Fig. 3).It is composed of mainly two depositional units i.e. regressive(progradational) and transgressive (aggradational). Regressiveunits (that are not dealt with in detail in this paper) are character-ized by relatively thick, coarsening upward succession. The trans-gressive units (formation/member boundaries namelyFossiliferous conglomerate bed, Lower Astarte beds and Upper As-tarte beds) are relatively thin and represent aggradational stackingpattern. The description of each marker bed is given below:

5.1. Transgressive sequence-I (Fossiliferous conglomerate bed)

It represents fining and thinning upward units comprising alter-nations of medium to coarse grained/gritty, graded to massivesandstone (unit I) and mudrocks (unit II) (Plate 1D). The sandstoneunit contains mostly bivalve shell fragments. The total thickness ofthis unit is 5 m. It is sand dominated sequence and occurs at thebase of the Kanthkot Formation. It can be observed near Washtawavillage (Southern flank of Washtawa Dome).

5.2. Transgressive sequence-II (Lower Astarte bed)

The thickness of this unit is 5–6 m and composed of mudrocks(unit II) interbedded with medium to coarse grained/gritty sand-stone (unit I). Sandstone unit shows fining and thinning upwardand contains mostly bivalve shell fragments (Astarte). It occurs inmiddle of Kanthkot Formation and exposed 3 km west of Adhoi vil-lage (Southern flank of Adhoi anticline).

5.3. Transgressive sequence-III (Upper Astarte bed)

The thickness of this unit is 5–6 m and comprises alternationsof coarse grained/gritty, hard sandstone (unit I) and soft calcareousmudrocks (unit II). Both the units contain well preserved epifaunalbivalve (Astarte) shell with convex side up. Ammonites are rarelyobserved. Wave ripples are locally observed on top of the sand-stone unit. There is no remarkable change in bed thickness in thesequence. It occurs at the top of Kanthkot Formation and exposed1.5 km SW of Bharodia village.

From the distribution pattern of lithotypes in the various trans-gressive sequences (formation/member boundaries) it is com-monly observed that the unit I is inter bedded with fine grainedlithology (unit II) and showing fining and thinning upward witherosional basal contact. The unit I rest very sharply on bioturbatedmudrocks.

6. Discussion

6.1. Depositional processes

The stratigraphic units (Washtawa, Kanthkot and Gamdau For-mations) distinguishable in the south – western part of Wagad up-lift were deposited in a variety of environment ranging fromshallow marine to fluvio-deltaic and were strongly influenced byfluctuation of relative sea level (Deshpande and Merh, 1980; Bis-was, 1981). These fluctuations resulted in various types of keystratigraphic surfaces, mainly transgressive and regressive. Thetransgressive key surfaces (shell rich beds) have been mapped asformation/member boundaries by earlier workers (Biswas, 1977;

Fig. 3. Lithologies and depositional processes of transgressive sequences with stratigraphic nomenclature used for Late Jurassic rocks.

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Deshpande and Merh, 1980) but there has been no discussion ofhow these key surfaces were deposited during transgression.Lithology and depositional processes of transgressive sequencesof the present study are shown in Fig. 3. The study reveals thatthe transgressive units accumulated under the dominance of stormand wave processes in a generally shallow marine environment.The storms played an important role in generating the sedimentarystructures and textures during deposition of these shell rich beds.These fossiliferous beds with mudrocks are interpreted to havebeen deposited below wave base and are believed to be part of

transgression as evidenced by wide occurrence in the area, relativethickness (<5 m), fining upward trend of individual beds and interbedding of fine grained lithology together with calcareous compo-sition (Tamura and Masuda, 2005). In the present case, high energylevels are characterized by sand dominated sequence, abundanceof reworked granular to sand sized sediment particles and ero-sional and sharp nature of basal contacts of units together withwell preserved bioclasts. Low proportion articulated, high propor-tion broken shells and various state of wear of fossils support highenergy and transportation effect. The intense storm has played a

Table 3Description of Lithounits

Sedimentaryvariables

Medium to coarse grained calcareoussandstone (storm lag unit I)

Mudrocks (swell lagunit II)

Sorting Moderately to poorly sorted, sortingincreases upward

Moderately to poorlysorted

Structure Distinct gradation in grain size (normallygraded bedding)

Massive or laminatedand moderatelybioturbated

Grain size Medium to coarse grained, polishedquartz granules scattered on the surface

Fine grained

Geometry Sheetlike SheetlikeThickness 5–75 cm 5–15 cmContacts Planar erosive Non-erosiveFossils Mostly bivalves (Astarte and pectenlike),

ammonoidsMostly bivalves (Astarteand pectenlike)

Biofabric Usually convex-up oriented invariablyfragmented and disarticulated shellsparallel to bedding plane

Usually convex-uporiented whole shells

Stratigraphicposition

Lower middle and upper part of section Lower middle and upperpart of section

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major role in the distribution of siliciclastics and non-clastic mate-rials. According to Brenner and Davis (1973) storms are evidencedby the occurrence of two distinctly different types of fossiliferousunits (storm lags and swell lags). Storm lags are sheet like depositsof fragmented shells parallel to bedding plane and coarse siliciclas-tics (fossiliferous sandstone) interbedded with mudrocks. They re-sult from pushing of surge channels during storm and deposition of

Plate 1. Field photographs surface view showing medium to coarse grained, shell rich, cafossils (A); Ammonoids (B). (C) Section view of graded fossiliferous sandstone showing shplane. The size of the shells decreases upward. Granule size polished quartz grains are salternations of medium to coarse grained, calcareous, fossiliferous, graded to massive, h

storm transported skeletal debris and coarse siliciclastics on mud-dy platform areas. In the present sequence, the above charactersare distinctly observed, however each bed becomes finer grainedupward, indicating decrease in the intensity of storm generateddepositional currents (Hayes, 1967). Graded nature of biotic andabiotic components, abraded and fragmented shells, parallel andconvex-up oriented, disarticulated shells and high preservationquality together with erosive base strongly supports that unit Iwas formed by storm generated currents during transgression atdepth bellow wave base (Fursich and Oschmann, 1993; Aigner,1979).

6.2. Transgressive deposits and syn-rift sedimentation

The Wagad sediments described above represent the proximalpart of the Banni Half Graben (BHG) sedimentation domain of theKachchh rift basin. Two separate depositional domains one in themarginal part (BHG) and another in the basinal part Gulf of KachchhHalf Graben (GOKHG) were differentiated by activation of KachchhMainland Fault (KMF) during Oxfordian (Biswas, 2005). This mor-phological subdivision of Kachchh rift basin reflects the changingsediment yield of main source area for different sub-basin types.BHG which is marginal part of rift basin received sediment fromlarge hinterland area and commonly showing sediment over filledor sediment balanced infill type sub-basin as reflected in the presenttransgressive sequences comprising three fold (sandstone–mud-rocks–sandstone) synrift lithology motif (Ravnas and Steel, 1998).Synrift stage in the present study is documented by an abundance

lcareous sandstone contains well-preserved whole shell, convex-up oriented Astarteell fragments are disarticulated, broken convexly up oriented and parallel to beddingcattered on surface of sandstone. (D) Fining and thinning upward units comprisingard sandstone and mudrocks.

316 D. Mishra / Journal of Asian Earth Sciences 34 (2009) 310–316

of coarse clastics and an over all aggradational nature of transgres-sive sequences (Cherry, 1993). These transgressive sequences corre-spond to period of highest sea level with maximum subsidenceduring Callovian–Oxfordian syn-rift phase of basin evolution.

6.3. Source area

Moderately bedded individual strata, high content of coarseclastics along with polished granule size quartz and abundanceof comminuted shells indicate a significant change in depositionalsetting, possibly closure approach of the source of terrigenous frac-tion or source uplift. The medium to coarse grained immature clas-tics and quartz granules indicate moderate to high relief andunstable nature of the basin in which sediments were deposited.Because relief is dependent on the balance between uplift and ero-sion, the character of coarser material is therefore also an index totectonism. Immature sands indicate that their provenance was af-fected by the change in tectonic style and they probably were de-rived from variable continental sources. Presence of gravelydeposits is the result of marked variation in rate of sediment sup-ply controlled by tectonically generated hinterland topography(Frostick and Steel, 1993).

Acknowledgements

The author acknowledge the financial support in the form ofFast Track Project by DST, New Delhi. Thanks to Dr. S.K. Biswas,Ex. Director, KDMIPE.ONGC. Dehradun for supervision in the field.The author wish to thanks Prof. Jai Krishna, Head of GeologyDepartment, Banaras Hindu University for providing laboratoryfacilities. The manuscript benefited from constructive reviews byProf. R.N.Tiwari, Former Head, Department of Geology, BHU.

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