Miocene bryozoa from the Kachchh Offshore Basin,Western India

The present paper records for the first time the bryozoan fragments from 4 m core of the Godhra Formation (early Miocene) and from 4m core and 5 m core of the Chhasra Formation (early middle Miocene) of the Kachchh Offshore Basin, Western India. The Godhra Formation has presence of three bryozoans, viz., ?Crassimarginatella sp., Thalamoporella sp. and Vincularia sp. while the Chhasra Formation has presence of four bryozoans, namely, ?Margaretta sp., Steginoporella sp., ?Thalamoporella sp. and Vincularia sp.     

新疆吐哈盆地侏罗纪湖相动物遗迹化石的发现及古环境意义

对首次在吐哈盆地台北凹陷侏罗系发现的分布于不同剖面、不同层位中的动物遗迹化石进行了系统研究。主要分析了研究区内动物遗迹化石的特征 ,将之划分为六大类型 :( 1 )垂直居住迹 ;( 2 )垂直觅食迹 ;( 3 )“U”形管 ;( 4)潜穴系统 ;( 5)逃逸迹 ;( 6)停息迹。较详细地分析了各类型的形态特征和保存特点 ,简要分析了造迹生物的行为习性及环境意义。在此基础上 ,建立了本区动物遗迹化石的环境模式。同时指出 ,遗迹化石在陆相含油气盆地确实大量存在 ,并在储层研究中具广泛的实践意义。     

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

The whole sedimentary succession (ca 600 m thick) of Wagad area ranging in age from Callovian to Early Kimmeridgian has been divided in to three Formations namely Washtawa, Kanthkot and Gamdau in ascending order. Prograding Kanthkot Formation was frequently interrupted by transgressions. Field and petrographic investigations revealed that the Kanthkot Formation represents three fossiliferous marker beds corresponding to Transgressive sequence I; Transgressive sequence II and Transgressive sequence III. These transgressive sequences are composed of two lithounits: medium to coarse grained/gritty, graded to massive, sheetlike, fossiliferous calcareous sandstone (storm lag unit I) and fossiliferous mudrocks (swell lag unit II). The thickness of the unit I varies from 5 to 75 cm and contains mostly 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 fossiliferous mudrocks. This unit is intercalated with moderately bioturbated sandy siltstone. Unit I is dominant over Unit II in the sequences.Study suggests that the transgressive units were deposited close to wave base by high energy storm flows followed by low energy marine swells during transgression. The intense storms played a major role in the distribution of siliciclastics and nonclastic materials. Storms are evidenced by the occurrence of two distinctly different types of units (storm lags and swell lags). High energy levels are characterized by sand dominated sequence, abundance of reworked sediment particles, high proportion broken shells with convex up orientation and erosional and sharp nature of basal contacts of units together with well preserved bioclasts. Sudden short term changes from high to low energy during transgression are explained by the occurrence of medium to coarse grained siliciclastics interbedded with moderately bioturbated mudrocks. Moderately bedded individual strata, high content of coarse clastics along with polished granule size 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 sedimentation in the present study is documented by an abundance of coarse clastics and an over all aggradational nature of transgressive sequences.     

Deciphering condensed sequences: A case study from the Oxfordian (Upper Jurassic) Dhosa Oolite member of the Kachchh Basin,western India

In large parts of the Kachchh Basin, a Mesozoic rift basin situated in western India, the Oxfordian succession is characterized by strong condensation and several depositional gaps. The top layer of the Early to Middle Oxfordian Dhosa Oolite member, for which the term ‘Dhosa Conglomerate Bed’ is proposed, is an excellent marker horizon. Despite being mostly less than 1 m thick, this unit can be followed for more than 100 km throughout the Kachchh Mainland. A detailed sedimentological analysis has led to a complex model for its formation. Signs of subaerial weathering, including palaeokarst features, suggest at least two phases of emersion of the area. Metre‐sized concretionary slabs floating in a fine‐grained matrix, together with signs of synsedimentary tectonics, point to a highly active fault system causing recurrent earthquakes in the basin. The model takes into account information from outcrops outside the Kachchh Mainland and thereby considerably refines the current understanding of the basin history during the Late Jurassic. Large fault systems and possibly the so‐called Median High uplift separated the basin into several sub‐basins. The main reason for condensation in the Oxfordian succession is an inversion that affected large parts of the basin by cutting them off from the sediment supply. The Dhosa Conglomerate Bed is an excellent example, demonstrating the potential of condensed units in reconstructing depositional environments and events that took place during phases of non‐deposition. Although condensed sequences occur frequently throughout the sedimentary record, they are particularly common around the Callovian to Oxfordian transition. A series of models has been proposed to explain these almost worldwide occurrences, ranging from eustatic sea‐level highstands to glacial phases connected with regressions. The succession of the Kachchh Basin shows almost stable conditions across this boundary with only a slight fall in relative sea‐level, reaching its minimum not before the late Early Oxfordian.     

Trace fossil assemblages in mixed siliciclastic-carbonate sediments of the Kaladongar Formation (Middle Jurassic), Patcham Island,Kachchh, Western India

The Middle Jurassic rocks of the Kaladongar Formation well exposed in the Kaladongar Hill range of the Patcham Island and Kuar Bet of the Northern Kachchh comprises of ∼450 m thick sequence of mixed siliciclasticcarbonate sediments intercalated with shales. These Mixed siliciclastic-carbonate sediments show wide variation in textural and mineralogical composition and represent genetically related six rock types: micritic sandstone, allochemic sandstone, sandy allochemic limestone, micrtic mudrock, sandy micrite and muddy micrite; which are highly bioturbated and show behaviourally diverse groups of trace fossils. Total 34 ichnogenera are identified, which includes, Arenicolites, Asterosoma, Beaconites, Bergaueria, Chondrites, Cochlichnus, Dactylophycus, Daedalus, Didymaulichnus, Diplocraterion, Gordia, Gyrochorte, Gyrolithes, Ichnocumulus, Laevicyclus, Lockeia, Margaritichnus, Monocraterion, Nereites, Ophiomorpha, Palaeophycus, Phoebichnus, Phycodes, Pilichnus, Planolites, Plug Shaped Form, Protovirgularia, Rhizocorallium, Scolicia, Skolithos, Taenidium, Teichichnus, Thalassinoides and Walcottia. These trace fossils are classified into six morphological groups namely, circular and elliptical structures; simple structures; branched structures; rosette structures; spreiten structures; and winding and meandering structures. These trace fossils are further group into eight assemblages which occurred together into mixed siliciclastic-carbonate sediments, include, Asterosoma assemblage, Gyrochorte assemblage, Rhizocorallium assemblage, Thalassinoides assemblage, Planolites-Palaeophycus assemblage, Phycodes assemblage, Ophiomorpha assemblage and Skolithos assemblage. The recurring pattern of these assemblages through the sequence displays the development of Skolithos and Cruziana ichnofacies and at places the mixed Skolithos-Cruziana ichnofacies which suggest a low wave and current energy conditions with intervening period of high wave and current energy conditions and an intermediate period of stressful environments, respectively. Sedimentological and ichnological data suggest that the deposition of the mixed siliciclastic-carbonate sediments of the Kaladongar Formation took place in the foreshore to offshore environment under fluctuating wave and current energy condition.     

Paleoecological significance of the trace fossils of Dhosa Oolite Member (Jumara Formation), Jhura dome,Mainland Kachchh,Western India

Dhosa Oolite Member of the Jumara Formation comprises alternating bands of oolitic limestones and shales, exposed in Jhura dome of Mainland Kachchh, Western India. This sequence is highly bioturbated and exhibits a moderate diversity and behaviourally complex assemblage of ichnospecies. The rhythmically bedded sequence shows three different levels of preservation of traces. Epichnial tiering consists of moderately bioturbated oolitic limestone exhibiting horizontal or low-angle protrusive/retrusive biogenic laminae, commonly dominated by feeding structures like Rhizocorallium jenense, R. irregulare, Zoophycos brianteus and Zoophycos isp. The endichnial structures within the oolitic limestone can be separated into two different preservational trace fossil suites. The endichnial shallow suites consist chiefly of deposit feeders like Chondrites intricatus, C. targionii, Planolites beverleyensis, Taenidium cameronensis, Thalassinoides isp., Z. brianteus, Z. cf circinnatus and Zoophycos isp. and few suspension feeder forms like Palaeophycus tubularis; while endichnial deep suites consist of Chondrites intricatus, Skolithos linearis and Zoophycos isp. Hypichnial structures consists abundant, cylindrical, branched, horizontal, large-sized three dimensional feeding burrows of Thalassinoides isp. and somewhat irregular, obtuse angle ramification burrows of Phycodes isp., which are attached to the lower surface of the casting medium. The trace fossil association indicates Cruziana ichnofacies and abundance of Zoophycus species below the fair weather wave base level is largely a preservational artifact. The preservational processes of the trace fossils indicate soft substrate and diversity and their abundance reflects the other palaeoecological parameters of the open shallow marine environments.     

The Effects of Diagenesis on the Reservoir Characters in Ridge sandstone of Jurassic Jumara dome,Kachchh, Western India

Ridge sandstone of Jurassic Jumara dome of Kachchh was studied in an attempt to quantify the effects of diagenetic process such as compaction, cementation and dissolution on reservoir properties. The average framework composition of Ridge sandstone is Q₈₀F₁₇L₃, medium-to coarse grained and subarkose to arkose. Syndepositional silty to clayey matrix (3% average) is also observed that occurs as pore filling. The diagenetic processes include compaction, cementation and precipitation of authigenic cements, dissolution of unstable grains and grain replacement and development of secondary porosity. The major cause of intense reduction in primary porosity of Ridge sandstone is early cementation which include silica, carbonate, iron, kaolinite, illite, smectite, mixed layer illite-smectite and chlorite, which prevents mechanical compaction. The plots of COPL versus CEPL and IGV versus total cement suggest the loss of primary porosity in Ridge sandstone is due to cementation. Cements mainly iron and carbonate occurs in intergranular pores of detrital grains and destroys porosity. The clay mineral occurs as pore filling and pore lining and deteriorates the porosity and permeability of the Ridge sandstone. The reservoir quality of the studied sandstone is reduced by clay minerals (kaolinite, illite, smectite, mixed layer illitesmectite, chlorite), carbonate, iron and silica cementation but on the other hand, it is increased by alteration and dissolution of the unstable grain, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to serve as a reservoir is strongly related to sandstone diagenesis.     

Facies analysis and depositional environment of the Jurassic, Jumara Dome sediments, Kachchh, western India

The study deals with the depositional environment of Jumara Dome sediments. The Jumara Dome is an important outcrop of Bathonian to Oxfordian sediments amongst the Kachchh Mainland exposures. On the basis of facies analysis three associations have been documented, namely, G-1 consisting of low energy facies comprising of cross-bedded sandstone, massive sandstone, grey shale and thin bedded sandstone, bioclastic — lithoclastic grainstone, bioclastic — lithoclastic packstone, microbioclastic packstone/wackestone, bioturbated laminated wackestone to mudstone and pelagic lime mudstone; G-II consisting of moderate energy facies comprising of laminated sandstone and grapestone or agglutinated grainstone; G-III consisting of high energy facies comprising of interbedded gypsiferous shale and sandstone/siltstone, oolitic grainstone to conglomerate and bioclastic grainstone. The facies associations reflect an ideal shallowing upward sequence representing slope, bioclast bar, lagoon and inner shelf. Presence of wide range of facies indicates that the rocks of the studied area were deposited during the fluctuating sea level, interrupted by the storms, in the shallow marine environment.     

Arm hooks of coleoid cephalopods from the Jurassic succession of the Wessex Basin,Southern England

The Jurassic succession of the Wessex Basin – especially that cropping out within the Dorset and East Devon Coast World Heritage Site – contains important lagerstätten for coleoid cephalopods. The Blue Lias and Charmouth Mudstone formations of West Dorset, the Oxford Clay Formation of North Wiltshire and the Kimmeridge Clay Formation of Purbeck have provided large numbers of important body fossils that inform our knowledge of coleoid palaeobiology, including the hooks present in the arms. Isolated hooks are also found in the processed residues studied by micropalaeontologists and these occurrences can be used – in some cases – to record the presence of key taxa in the absence of well-preserved body fossils. While some hook morphotypes can be attributed to known species, there are many forms of hook described where the parent animal remains unknown. The present state of our knowledge of the Jurassic assemblages in the Wessex Basin is presented and remaining issues identified.     

Trace fossils and their palaeoenvironmental significance in the Lameta Formation of Salbardi and Belkher area, district Amravati, Maharashtra, India

Trace fossils namely, Planolites, Planolites montanus, Thalassinoides and stuff burrows belonging to the Lameta Formation of Central India are reported for the first time from two new localities, i.e., Salbardi and Belkher area. Both the exposures are well represented by the typical marker lithounits as of Jabalpur, Nand-Dongargaon area of central India, i.e., greenish yellow sandstone, clays of various shades, chertified and brecciated limestones, etc. These successions are devoid of megaforal remains, therefore, these biogenic structures play a significant role in the interpretation of palaeoenvironment and palaeoecology during the deposition of the Lameta sediments. The trace fossils assemblage shows that the benthonic community was mainly polychaetes and crustaceans.     

Overview on the middle to upper Jurassic sedimentary succession of Gangta Bet in the Kachchh basin,western India,with special emphasis on its lithostratigraphy,biostratigraphy, and palaeoenvironment

The Jurassic succession of Gangta Bet in the Kachchh basin of western India comprises around 130 m of mostly siliciclastic rocks. The strata belong to the Gangta Member of the Gadhada Formation and are herein sub-divided into four units: the Gangta sandstone beds, the lower silty sandstone beds, the upper silty sandstone beds, and the Gangta ammonite beds. These units can be separated by three marker horizons: the Brachiopod bed, the Gangta Conglomerate Bed, and the Gervillella Bed. Ammonites indicate an Oxfordian age for the upper half of the succession, but the scarcity of identifiable fossils in its basal part so far prevented precise biostratigraphic assignments. The shallow-water sediments can be interpreted as parasequences as a result of minor sea-level changes. Deposition took place close to the palaeo-coastline at water depths around the fair-weather wave-base.     

Trace fossils from Bhuban Formation,Surma Group (Lower to Middle Miocene) of Mizoram India and their palaeoenvironmental significance

A detailed ichnological study performed on the Bhuban Formation, Surma Group (Lower to Middle Miocene) of Mizoram, India reveals the occurrence of rich and diverse trace fossils. These have been collected from the two localities in Aizawl, i.e., Bawngkawn and Ropaiabawk, where sandstone—shale sequence is well exposed. Total 20 ichnospecies of 14 ichnogenera have been identified which include Arenicolites isp., Cochlichnus anguineus, Helminthopsis abeli, Laevicyclus mongraensis, Ophiomorpha borneensis, Palaeophycus tubularis, Palaeophycus heberti, Palaeophycus sulcatus, Palaeophycus alternatus, Pholeus abomasoformis, Pholeus bifurcatus, Planolites beverleyensis, Planolites annularis, Polykladichnus irregularis, Rhizocorallium isp., Skolithos linearis, Taenidium satanassi, Teichichnus rectus, Thalassinoides horizontalis and Thalassinoides paradoxicus. Ethologically these ichnogenera display dwelling and feeding activities of the infaunal organisms. Arenicolites, Ophiomorpha, Polykladichnus and Skolithos are the members of the Skolithos ichnofacies while Palaeophycus, Planolites, Rhizocorallium and Thalassinoides are the members of the Cruziana ichnofacies. The presence of Skolithos ichnofacies indicates sandy shifting substrate and high energy conditions in foreshore zone while the Cruziana ichnofacies indicate unconsolidated, poorly sorted soft substrate and low energy condition in the shoreface/offshore zone. These ichnogenera indicate foreshore to shoreface-offshore zone of shallow marine environment for the deposition of the rocks of the Bhuban Formation of Mizoram.     

新疆柯坪地区志留系依木干他乌组的遗迹化石

通过对塔里木盆地柯坪露头志留系依木干他乌组中遗迹化石的研究,鉴别出6 属7 种。化石分子有Chondrites isp.,Gordia.isp., Helminthopsis aff. abeli, Ophiomorpha nodosa, Planolites beverleyensis, Planolites montanus, Thalassinoides isp.。这些遗迹化石具有较高的分异度和较低的丰度。按古生态或造迹生物的行为习性分类,可分为3 种生态类型：①居住迹（Domichnia）,Ophiomorpha nodosa, Thalassinoides isp.;② 进食迹（Fodinichnia）,Planolites beverleyensis, Planolites montanus, Chondritesisp.;③觅食迹（Repichnia）,Gordia isp.,Helminthopsis aff. abeli。根据遗迹化石的古生态特征、沉积序列上的分布特点及围岩和沉积构造特征,依木干他乌组的遗迹化石可归为Planolites -Chondrites 遗迹组合,属于Cruziana 遗迹相,形成于泥坪沉积环境。     

Trace fossils from the cambro-ordovician rocks of North Wales and their stratigraphic significance

In North Wales, Cruziana semplicata (Salter) is recorded from three otherwise unfossiliferous localities and Teichichnus sp. (Seilacher) and Phycodes circinatum (Richter) are shown to be widely distributed within the lower Arenig (Didymograptus extensus zone) sediments. P. circinatum is also recorded from the otherwise unfossiliferous Upper Pennant Quartzite. From a discussion of the stratigraphic ranges of these trace fossils it is suggested that the rocks of the C. semplicata bearing localites are Upper Cambrian (Ffestiniog) in age while the Upper Pennant Quartzite is Ordovician, probably Arenig.     

Palaeoenvironmental significance of trace fossils from the palaeocene sediments of the Pondicherry area,South India

Sediments of Manaveli Formation (Palaeocene) are studied from the Thondaimanattam quarry section and Manaveli section of the Pondicherry area, which comprises of bioturbated arenaceous limestone-shale and shaly-sandstone sequences respectively. These sediments consist of high density and low diversity trace fossil genera which includes Lanicodichnus, Ophiomorpha, Planolites, Palaeophycus, Skolithos, Thalassinoides, and Polychaetes tubes. The abundance of horizontal structures of the Thondaimanattam quarry section indicate low to moderate wave and current energy in fully marine subtidal environments while the presences of vertical structures of the Manaveli section indicate moderate to high wave and current energy in the marginal marine environment of deposition for the Palaeocene sediments of the Pondicherry area.     

Nongeniculate coralline algae from early middle Miocene offshore sequence of Kachchh basin,Western India: Paleoenvironmental significance

The present paper reports five nongeniculate coralline algal species, viz., Lithothamnion valens Foslie, Mesophyllum roveretoi Conti, Phymatolithon calcareum (Pallas) Adey and Mckibbin, Melobesioideae gen. et spec. indet. and Lithoporella melobesioides Foslie form the early middle Miocene Chhasra Formation of Offshore Sequence of Kachchh basin, western India. The present day depth distribution of Lithothamnion and Mesophyllum and growth forms of five nongeniculate coralline algal species points that the two cores belonging to the Chhasra Formation of offshore sequence of Kachchh basin were deposited in inner shelf environment at 60-100m depth in moderate-energy conditions.     

Petrography and geochemistry of Jurassic sandstones from the Jhuran Formation of Jara dome,Kachchh basin,India: Implications for provenance and tectonic setting

Sandstones of Jhuran Formation from Jara dome, western Kachchh, Gujarat, India were studied for major, trace and rare earth element (REE) geochemistry to deduce their paleo-weathering, tectonic setting, source rock characteristics and provenance. Petrographic analysis shows that sandstones are having quartz grains with minor amount of K-feldspar and lithic fragments in the modal ratio of Q ₈₉:F ₇:L ₄. On the basis of geochemical results, sandstones are classified into arkose, sub-litharenite, wacke and quartz arenite. The corrected CIA values indicate that the weathering at source region was moderate to intense. The distribution of major and REE elements in the samples normalized to upper continental crust (UCC) and chondrite values indicate similar pattern of UCC. The tectonic discrimination diagram based on the elemental concentrations and elemental ratios of Fe₂O₃ + MgOvs. TiO₂, SiO₂ vs. log(K₂O/Na₂O), Sc/Cr vs. La/Y, Th–Sc–Zr/10, La–Th–Sc plots Jhuran Formation samples in continental rift and collision settings. The plots of Ni against TiO₂, La/Sc vs. Th/Co and V–Ni–Th ?10 reveals that the sediments of Jhuran Formation were derived from felsic rock sources. Additionally, the diagram of (Gd/Yb) _N against Eu/Eu ^? suggest the post-Archean provenance as source possibly Nagar Parkar complex for the studied samples.     

Facies controlled synaeresis cracks in mixed siliciclastic-carbonate sediments of Middle Jurassic of Patcham Island, Kachchh, Western India

Synaeresis cracks are observed at different stratigraphic levels in shallow marine mixed siliciclastic-carbonate sediments of the Middle Jurassic rocks of the Patcham Island, Kachchh, Western India. Cracks are preserved as cast or grooves in micritic sandstone of the Kuar bet member of Kaladongar Formation and sandy allochem limestone of the Raimalro Limestone member of Goradongar Formation. It bear distinct morphology of simple, straight to gently curved, spindle-shaped, irregular, unbranched to branched at acute angle; interconnected curlicue forms of non-orthogonal pattern. The X-radiography shows sharp margin and tapering twigs which support to nullify the possible biogenic origin. These cracks are developed at sediment-water interface and sediment-sediment interface in aqueous conditions, where partial dewatering of sediments causes reduction of sediment volume and loss of plasticity. Formation of cracks are also post-depositional phenomenon operated during initial phase of diagenesis where induced stress is generated due to compaction of sediments and neomorphism/recrystallisation of the susceptible carbonate grains.     

Trace fossils of a cyclic chalk-marl succession; the upper Maastrichtian Rørdal Member, Denmark

Trace fossils from an upper Maastrichtian cyclic chalk-marl succession, the Rørdal Member, exposed in the Rørdal quarry, Denmark, are analysed in order to test whether the changes in substrate lithology exerted any control over the ichnodiversity, tiering complexity, and density of the infauna. The cyclicity is interpreted as caused by orbital changes within the Milankovitch frequency band. The carbonate content varies between 71 and 82 weight% in the marl and 82-92 weight% in the chalk beds. The material is based on 19 samples collected from six chalk and marl beds. The investigated bedding-normal sample surfaces vary in area between 29 and 155 cm². Eight ichnogenera and two undetermined ichnogenera are recognised. The member is characterised by three ichnofabrics (A, B and C). The ichnofabric analysis is based on texture and internal structure of the sediments resulting from bioturbation. Ichnofabric A is found only in chalk samples and shows a poor preservation of trace fossils, whereas ichnofabric C is found in a few chalk and all marl samples and comprises a very dense, diverse and well preserved ichnofauna representing a high tiering complexity. Ichnofabric B represents an intermediate situation between ichnofabrics A and C and occurs in chalk samples immediately adjacent to marl beds. The observed changes in ichnofabrics between chalk and marl are related to the amount of clay in the samples and the differences in the occurrence of trace fossils are interpreted as due to differences in the visibility of traces between chalk and marl and not due to differences in ecological stress upon the endobenthic community of the two lithologies. The study thus provides an excellent example of how the effect of taphonomic factors may give a misleading and biased impression of apparent differences in the endobenthic community between chalk and marl.     

Stratigraphy and Sedimentology of Middle Jurassic Sequence of Chorar Island,Patan District,Kachchh Basin,Gujarat

Chorar Island exposes ～109 m thick middle Jurassic (Bathonian-Callovian) succession in the eastern most part of the Kachchh Basin, Patan District, Gujarat and is divided into two, Khadir and Gadhada formations. It mainly comprises of mixed siliciclastic-carbonate sediments and limestones; the shales dominate the lower part of the succession while the top of the succession is marked by ferruginous sandstone which also forms the vast, prominent peripheral zone of the Chorar dome. The field and laboratory analysis of the succession reveals nine lithofacies which includes ferruginous sandstone, cross bedded white sandstone, micritic sandstone, allochemic sandstone, sandy micrite, mudstone, sandy allochemic limestone, coralline limestone and shale. The mix siliciclastic-carbonate sediments and ferruginous facies are fossiliferous in nature and display sedimentary structures, like ripple marks, cross- and planar- lamination with biogenic sedimentary structures. Coralline limestone facies comprise of large size (>1m diameter) corals, which are diagenetically modified severely and have lost its original internal structures. The sediment characteristics and associated bioclasts indicates low to moderate wave and current energy in shoreface-offshore subsequently changing to wave dominated shoreface during the deposition of the middle Jurassic sediments of the Chorar Island.