1. Describe the following fossils and assign to each it’s stratigraphical range and systematic position. (30 Marks)
- Dietyonema flabeliforme:
- Ptilophylum acutifoliam:
- Turrilites indicus:
- Physa prinsepil:
- Productus cora:
This is a well known species of graptolite, ranging between Cambrian to Cretaceous. This is conical or bell shaped, with numerous branches connected by horizontal crossbars. It’s systematic position is given below -
Ptilophylum is a characteristic plant fossil of Upper Gondwana age. This particular species is found in lower Jurassic to Upper Cretaceous beds, in Rajmahal flora.This is in leaf form, compound, unipinnate leaf, the margin is entire or dentante, some times. It’s systematic position is given below -
Turrilites is a late Cretaceous (Cenomanian and Turonian) ammonoid cephalopods, belonging to the Turrilitidae family. This is an example of heteromorphy in ammonoid shell. Unlike ammonoid (most commonly planispiral coiling), they are helically coiled, resembling to auger snails. They almost look like the gastropoda turritella. The shell of Turrilites is tightly wound in a high trochospiral, with an acute angle at the apex. Ribs are weak to strong and may have 3 or 4 rows of equal numbers of tubercles. It’s systematic position is given below -
The genus physa is a small, sinistral gastropoda in the family Physidae. They have thin shell, large and long aperture, pointed spire. Members of the freshwater pulmonate family Physidae possess a complex of muscles that is unique amongst gastropods. This complex was given the name “physid musculature”. The physid musculature has two main components, the physid muscle sensu stricto and the fan muscle. The physid musculature is responsible for a unique ability of physids to rapidly flick their shells from side to side — a reaction that frequently enables them to escape predation. It’s systematic position is as follows -
Nummulite is an extinct species of relatively large, lens shaped formamifera, present in Tertiary (Both Paleogene and Neogene) sediments all over the world. They were particulary abundant in Eocene sediments. They are disk like, spiral, calcareous, very small to several inches. Numerous coils are subdivided by septa into chambers. Sometimes limestones are so abundant in nummulite fossils, that they are called nummulitic limestone. It’s systematic position is as follows –
Fenestella is a genus of extinct marine bryozoans, known from the Middle Ordovician to Upper Triassic, reaching its largest diversity in Carboniferous. They are small colonial marine animals. Small branching network of structures with large elliptical openings and smaller spherical openings that housed individual members of the colony. Some limestones are so rich in these colonies that they are named as Fenestella limestone, as found in Spiti, Kashmir valleys in India. It’s systematic position is as follows –
Productus is a genus of extinct of inarticulate brachiopods, characteristic of Permian and Carboniferous strata. It’s systematic position is as follows –
2. Give a detailed account of the Jurassic of the Kutch region and compare it with formations of the same age in India and elsewhere. With the help of faunal relations, explain in outline the paleogeography of the period. (30 Marks)
The Mesozoic rocks ranging in age from Middle Jurassic to Lower Cretaceous are particularly well developed in Kutch, where they exhibit a complete succession and a total thickness of 600 ft. The sedimentary rocks in the western part of India represent an interplay of marine transgression-regression phases and basin tectonics during the Jurassic Period. The individual rock beds appear to have been deposited under a relatively shallow stretch of marine water and exhibit a gentle slope towards the sea. The enormous thickness of the sedimentary column is probably due to the gradual sinking of the basin along with the deposition of marine sediments. In Kutch region, the Jurassic rocks are exposed in three anticlinal chains of ridges trending in east-west direction.
Characteristically the Mesozoic succession of Kutch region has been intruded by various sills and dykes which are genetically related with the overlying Deccan Traps. The basal conglomerate of thePatcham Formation contains pebbles of crystalline rocks, suggesting a Precambrian basement. There are three typical facies association – a) shale – limestone, b) argillaceous c)areneceous.
The table below shows the litho – stratigraphic succession of kutch basin.
Jurassic in other regions in India:
Apart from Kutch, there is well exposure of Jurassic in the following areas –
- Spiti basin – In Spiti basin there is good exposure of Jurassic beds along with significant faunal content like Kutch.
- Spiti Shales (Black shale with pyrite nodules)
- Sulcactus beds (Black ferrogeneous oolites)
- Kioto limestone
- Nucula, Ostrea, Pecten, Phylloceras, Belemnites are some important fosssils found.
- Kashmir basin – Same succession as Spiti basin can also be found in Kashmir basin.
- Salt Range – In the western part of Salt Range, series of fossil bearing limetone, shale, sandstones of Jurassic age can be found. Fossil assemblage is similar to Kutch basin, most important being ammonites and plant fossils.
- Western Rajasthan – There is very good development of Jurassic rocks in Jaisalmer, which links Kutch with Salt Range. Triassic strata is not exposed here , Jurassic stratas are transgressive.
Jurassic succession is given below-
- Bedasir Beds (Fe sandstone, grits with ammonites)
- Baisakhi Beds (Sandstone, limestone with little fossil assemblages)
- Jaisalmer Limestone (Oolitic limetone)
- Lathi Beds (Sandstone, limestone with molluscan fossils and gymnospermous wood)
Badhaura Formation (Permo-Carboniferous)
The lowest beds of Patcham Formation yield a rich pelecypod fauna (Corbula lyrata, and other species, Eomiodon, Protocardia, Trigonia etc) and corals. The only ammonite found in these beds was a Stephanoceratid. Formations overlying the Shell and Coral limestone are characterized by the presence of Macrocephalites triangularis, Sivajiceras congener.
The Chari Formation contains five groups. The lowest is the Macrocephalus beds, which have yielded a rich collection of Callovian ammonites and gastropods. They chiefly yield ammonite – Indocephalites diadematus. The species of Nucula and Astarte are also very common in the Macrocephalus group. The lower portion of this bed is the Rehmanni zone in which Macrocephalites persists and Phylloceras and Lytoceras begin to appear; Idiocycloceras and Subkossmatia are characteristics, as also some Terabratulids and Trigonias. Anceps beds succeeding the Rehmanni zone contain ammonites, brachiopods and lamellibranches. The Athleta beds above them composed of white limestone with a similar fauna. The topmost bed of the Chari Formations are the Dhosa Oolites, very rich in Oxfordian fossil assemblage like Mayaites maya, Epimayaites polyphemus, Perisphinctes indogermanus, Peltoceretoides semirugosus and some teribratulites. The Dhosa Oolites contains the late Macrocephalitids such as Mayaites, Epimayaites and Dhosaites.
Below the Katrol beds chief fossil being Epimayaites transience, Prososphinctes virguloides and Torquatisphinctes torquatus of Upper Oxfordian Age. The lower and middle Katrol Beds Proper contains Oppelides and Perisphinctids (Taramelliceras, Glochiceras, Streblites, Aspidoceras; Torquatisphinctes, Pachysphinctes, Katroliceras, Subplanites, etc.) but the upper part tapped by gritty sandstone so it is generally devoid of fossils. The Gajansar beds contain a fauna in which appear species of Glochiceras, Phylloceras, Belemnopsis etc. The Katrol succession has also Mioflora and other plant fossils. The microfaunal record however suggests an uninterrupted phase of sedimentation between the Katrol and the Chari Formations.
The bottommost formation contain Haploceras elimatum, Hildoglochiceras spp., Phylloceras, Belemnopsis. The Umia Group consists mainly of barren sandstones with fossiliferous intercalations. The Ammonite beds are green oolites with abundant ammonites with some brachiopods and corals. Large Virgatosphinctes are characteristics but Aulacosphinctes, Ptychophylloceras, Umiaites and Micracanthoceras also occurs; other fossils are Acanthorhynchia, Lobothyris and Trigonia retrorsa. The Trigonia beds are characterized by Trigonia ventricosa and T. crassa. The Umia group is therefore Upper Tithonian and Neocomian. The Ukra beds contain Australiceras, Cheloniceras, Tropaeum.The overlying Bhuj beds yields abundant plant fossils including Philicales, Cycadophyta, Conifers etc. The Saurian remains of Pleisiasaurus indica have also been reported and also contain Ptilophyllum. The plant fossils are closely related to those from the Jabalpur Formation of Post Aptian Age.
Paleogeography of Jurassic:
The supercontinent Pangaea continued to exist in Triassic, and only in the mid – Jurassic time, Pangaea separated into northern Laurentia and southern Gondwana. But by the middle Triassic time, India started to move away from its host Gondwana, this breaking and movement towards north generated tectonism, large scale flooding, sealevel changes.
The Early Jurassic was the time when the Indian Ocean opened along its western margin, resulting into extensive sedimentation in the western coast – the present day Bombay High, Kutch, Kathiawar, Jaisalmer in India and Indus Valley, Baluchistan in Pakistan. In the Late Jurassic time India completely got detached from Africa, Antarctica and Australia, but was still attached to Madagaskar. This reconstruction can be substantiated by the help of paleobiological evidences.
After the Permo – Triassic mass extinction, there was a burst of organic revolution within the Mesozoic marine environments, as well as in land. With close observation of fossils found in Jurassic sediments, a lot of information about the period can be obtained. Jurassic strata preserved first appearances of many important modern biologic groups. Marine invertebrates flourished and the land was dominated by dinosaurs like large reptiles. How the different groups of invertebrates like the bivalves, cephalopods, gastropods have distributed themselves in different present day continents had been the most useful tool in reconstructing the Paleogeography.
3. Write short notes on following: (6 Marks each)
a) Drag Fold
Drag folds are minor asymmetrical fold, produced in incompetent beds lying in between competent beds, because of dragging and shearing along the opposite movement of the competenent beds.
Drag folds are also produced from fault motion, both in normal and reverse faulting. They generally occur in a series of folds.
b) Mohorovicic Discontinuity
In 1910, the boundary between the earth’s crust and the upper mantle was demonstrated by a Yugoslavian Scientist Mohorovicic , so this boundary is known as the Mohorovicic discontinuity.
Mohorovicic discontinuity or M-discontinuity or Moho is a sharp seismic velocity boundary at an average depth of 35 to 40 km below the continental crust and at a depth of 5 to 7 km below the oceanic crust. M-discontinuity is marked by a rapid increase in the P wave velocity from 6.5-7.6 km/second to 8.4 km/sec .This sharp change in velocity at Moho depth is due either to gabbro-eclogite phase transition or basic to ultrabasic transition of silicate. The lower crust is composed of gabbro/basalt or amphibolite/granulite facies rocks. The upper most mantle is composed of Plagioclase/spinel/garnet lherzolite or peridotite. Therefore Moho is considered both as seismic moho as well as petrological moho(petrological boundary of granulite facies rocks and olivine dominated pyrolite(Ringwood) or Peridotite or plagioclase lherzolite).
Often the seismic moho and petrological moho do not coincide exactly.
c) Submarine canyons
Submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls. They serve as channels for the flow of turbidity currents across the seafloor.
The formation of submarine canyons is believed to occur as the result of at least two main process: 1) erosion by turbidity current and 2) slumping and mass wasting of continental slpoes. Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins.
Grand Bahama Canyon is thought to be the highest in the world, rising almost 5 km from the seafloor. Ganges Canyon, extending from the Ganges is another important example.
d) Palaghat Gap
Palaghat or Palakkad Gap is a low mountain pass in Western Ghats between Coimbatore in Tamil Nadu and Palakkad in Kerala, located between Nilgiri hills in north and Annamalai hills to the south.
Over 160 million years ago, the supercontinent of Gondwana began breaking up and the continents of Africa, Antarctica and Australlia and South America broke apart initially, Madagaskar and India were still attached for longer period. About 88 million years ago, a major underwater volcanic eruption, the Marian hotspot,which went on for 2 biilon years ultimately separated the two continents. Palakkad gap marks the place where the Indian plate broke.
The gap affects the weather patterns in Southern India as it allows the moisture-laden Southwest monsoon winds into western Tamil Nadu, moderating summer temperatures and generating greater rainfall in the region relative to the rest of lowland Tamil Nadu. It also allows the hot winds coming from Tamil Nadu which warm the eastern part of Kerala compared to the rest of the state and the tropical cyclone winds from Bay of Bengal bearing rain during the summer.
e) Eustatic movements
Eustatic movement is related to a worldwide movement of the ocean floor, due to
- an alteration in the volume of water in the oceans,
- a change in the shape of an ocean basin and hence a change in the amount of water the sea can hold.
During and after an ice age, eustatic change takes place. At the beginning of an ice age, the temperature falls and water is frozen and stored in glaciers inland, suspending the hydrological cycle. This results in water being taken out of the sea but not being put back, leading to an overall fall in sea level. Conversely, as an ice age ends, the temperature begins to rise and so the water stored in the glaciers will re-enter the hydrological cycle and the sea will be replenished, increasing the sea levels.
The shape of the ocean basins can change due to tectonic movement. If the ocean basins become larger, the volume of the oceans becomes larger but the overall sea level will fall since there’s the same amount of water in the ocean. Conversely, if the ocean basins get smaller, the volume of the oceans decreases and the sea level rises accordingly.
Eustatic movement has always a global effect, while tectonic upliftment, subsidence, isostatic movements are localized. All these movements together account for the sea level fluctuations through geologic time and also today.
f) The great boundary fault
In Rajasthan the western boundary of the Vindhyan Basin is characterized by a major fault, extending over 400 km, roughly ENE-WSW direction, known as the great boundary fault ( by Hacket ). Although this fault is regarded as the limit of Vindhyan sedimentation, this is not a boundary fault in strict sense, but has modified the basin margin subsequent to the Vindhyan sedimentation.
The Great Boundary Fault is not a single surface, or a tectonic line, but it is a composite surface modified by secondary faulting. In some places it occurs as a thrust over the decollement, also as wrench in few places.
The Great Boundary Fault zone is a mixture of different type of fault zone rocks, mylonites, successive stages of folding, multiple sets of fractures and faults. Here both mylonites and cataclasite can be found, fault breccias are relatively rare. The breccias or cataclasites and the mylonites have developed in alternate cycles, and the brittle deformation has acted as precursor to the ductile deformation, and the Great Boundary Fault Zone has, in general developed in a brittle-ductile transitional zone.
g) Island arc
An island arc is a chain of volcanoes, commonly arc shaped, situated parallel and close to a boundary between two converging tectonic plates. Island arc when produced at ocean ocean boundary, a volcanic island is formed on the oceanic plate like the Hawaiian island, Japanese volcanic island. On the other hand when volcanism arise at ocean continent convergent boundary, a chain of volcano is formed at the margin of the continental plate, like the large parts of the Andes-Central America-Canadian mountain chain.
In the subduction zone, the addition of volatiles such as water induces partial melting of the oceanic crust because the volatiles (such as water) boil faster than the surrounding rock, forcing the rock apart. This process, called flux melting, generates low-density calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. The resulting volcano chain has the shape of an arc parallel to the convergent plate boundary and convex toward the subducting plate.
On the subducting side of the island arc is a deep and narrow trench, created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple earthquakes occur along this subduction boundary with the seismic hypocenters located at increasing depth under the island arc, these quakes define the Wadati–Benioff zones. In the forearc region accreationary prism, forearc ridge, forearc basin may form. Behind the volcanic chain, is the backarc region, where either extension or compression tectonics goes on.