Stratigraphy, Fossils and Mineral Resources

Fossil taxa morphology

Fossil groups are identified by a small set of diagnostic morphological features, and each taxon is recognized by a structure that is characteristic of it. Bryozoans are colonial animals built from individual zooids, each housed in its own zooecium. Ostracods are small bivalved crustaceans whose two valves together form a carapace that encloses the soft body. Foraminifera are unicellular protists whose test is divided into successive chambers added as the organism grows. Conodonts are extinct chordate-like animals whose feeding elements are phosphatic and built of denticles, so the denticle is the key feature to associate with them.

Brachiopods are bivalved animals with a dorsal (brachial) valve and a ventral (pedicle) valve, and this two-valve plan is the basic diagnostic feature of the phylum. The delthyrium and the pedicle foramen are located in the pedicle valve, not in the brachial valve. The deltidial plate is a pair of plates that partly or completely closes the delthyrium of the pedicle valve, and it is a characteristic brachiopod structure. Brachiopods have bilateral symmetry, so pentameral symmetry is not a brachiopod feature; it belongs to echinoderms. Thoracic segments are likewise not a brachiopod feature, since they are characteristic of arthropods.

Echinoids possess a water vascular system that operates the tube feet, and the periproct is the area of the test surrounding the anus. Trilobites carry a hypostome, a plate lying beneath the mouth on the cephalon.

Bivalve hinge morphology is central to their classification. Cardinal teeth, two adductor muscle scars and a chondrophore that supports the internal ligament are all found in bivalves. A lunule is a shallow, heart-shaped depression lying anterior to the umbo on the bivalve shell.

Bivalve dentition is classified by the arrangement and form of the hinge teeth:

• taxodont — numerous small, similar teeth arranged in sub-parallel or radial rows along the hinge. e.g. A taxodont hinge shows a long row of tiny teeth on each side of the umbo.
• dysodont — small, simple teeth placed near the margin of the valve. e.g. Teeth sit close to the valve edge rather than along the hinge centre.
• desmodont — teeth very reduced or absent, so the valves are held together mainly by the ligament.

The parallel grooves normal to the tooth axis do not define isodont dentition, so a description based on them should be rejected.

Foraminiferal tests may be agglutinated (built of cemented sediment grains), calcareous or siliceous. Miliammina is an agglutinated benthic genus, Cibicides is a calcareous benthic genus, and Guembelitria is a small planktonic genus, so all three are foraminifera. Triceratium, by contrast, is a centric diatom, a siliceous microfossil of the algae, and must not be confused with foraminifera.

Finally, the generic affinity of a fossil must be fixed at family or order level. Gigantopithecus is an extinct genus of the family Hominidae, Equus is a living genus of the family Equidae, and Gomphotherium is an extinct genus of the order Proboscidea.

Geologic time scale periods

The geologic time scale is organized in a hierarchy of eon, era, period and epoch, with the Phanerozoic Eon divided into the Paleozoic, Mesozoic and Cenozoic eras. The Paleozoic Era comprises six periods, from the Cambrian to the Permian, and the Carboniferous and the Silurian both belong to this list. The Paleogene is a period of the Cenozoic Era, and the Cretaceous is the final period of the Mesozoic Era, so neither belongs to the Paleozoic.

The Carboniferous lies roughly between 359 and 299 Ma and the Silurian between about 443 and 419 Ma, so both are firmly placed within the Paleozoic.

Duration of a period is estimated from its boundary ages. The Cambrian lasted about 53 Myr, the Devonian about 60 Myr and the Cretaceous about 79 Myr, whereas the Silurian lasted only about \sim 24 Myr. Among these four Phanerozoic periods, the Silurian is therefore the shortest, and the Cretaceous is the longest.

The Quaternary Period is subdivided into two epochs, the Pleistocene and the Holocene, which together span roughly the last 2.6 million years. The Pliocene and the Miocene are epochs of the Neogene Period, which belongs to the Tertiary and precedes the Quaternary, so they must not be counted as Quaternary epochs.

Example: a rock dated to about 15 Ma falls in the Miocene, which is older than the Quaternary, whereas a deposit formed a few thousand years ago belongs to the Holocene.

Indian mineral deposit locations

Economic deposits in India are tied to particular geological terrains, so each district is best memorized together with its commodity and its host setting. The main pairings are as follows:

• Wajrakarur (Andhra Pradesh) — a kimberlite field producing diamonds.
• Sukinda (Odisha) — the principal chromite mining area of India.
• Malanjkhand (Madhya Pradesh) — a copper deposit.
• Mangampeta (Andhra Pradesh) — a barite deposit.

Hutti in the Raichur district of Karnataka is the principal operating gold mine in India, with gold hosted in the greenstone rocks of the Dharwar craton. Jaduguda in Jharkhand is an underground uranium mine, Dariba in Rajasthan is a lead–zinc mine with some silver, and the Baula–Nuasahi complex of Odisha is known for chromite and platinum-group element mineralization.

Copper deposits are controlled by their host rocks. Khetri is hosted in garnetiferous chlorite schist, Mosabani in chlorite–biotite schist associated with soda granite, Malanjkhand in a tonalite–granodiorite–granite complex, and Kalyadi in metachert.

Other commodities are linked to their own districts: magnesite is mined at Salem in Tamil Nadu, uraninite is recovered at Jaduguda, clay minerals are exploited in the Bikaner region of Rajasthan, and platinum-group elements are associated with Nausahi.

When a question asks for a location, the commodity is the usual key, so the study of each mine should begin with the mineral it is famous for.

Stratigraphic unit hierarchy

Stratigraphic units are classified according to the property they describe, and the three main systems are lithostratigraphic, chronostratigraphic and biostratigraphic. A group is a lithostratigraphic unit consisting of two or more associated formations, and the formal hierarchy of lithostratigraphic units runs group, formation, member and bed in decreasing rank.

Chronostratigraphic units are rock bodies defined by the time in which they formed. They form the hierarchy eonothem, erathem, system, series and stage, and each has a geochronologic counterpart that denotes time, such as era, period and epoch. A stage is a chronostratigraphic unit that forms part of a series.

A member is a lithostratigraphic subdivision of a formation, defined by its lithology rather than by time. An acme zone is a biostratigraphic unit, defined by the peak abundance of a fossil taxon. A period, by contrast, is a time interval, and its rock equivalent is the system.

The Furongian is the youngest of the Cambrian series and is also an epoch name within the Cambrian Period, which lies below the Ordovician Period. The Triassic and the Cretaceous are Mesozoic periods far younger than the Cambrian, so the Furongian cannot belong to either.

For chronological ordering of Indian successions, use the principle of superposition together with the established age of each unit. In the Kashmir succession the Muth Quartzite lies beneath the Syringothyris Limestone, and in the Gondwana sequence of central India the Barakar Formation is older than the overlying Bijori Formation. The lowest unit of the Vindhyan Supergroup is the Semri Group, which must be placed below the Kaimur and younger units.

Indian geological map units

The Deccan Volcanic Province is a vast basaltic province covering roughly 5\\times10^{5} km^2 of central and western India, and it is the largest unit on the geological map of India. The Vindhyan Supergroup occupies a substantial area in central India, but it is far smaller than the Deccan basalts, and the Singhbhum Granite and the Mesozoic rocks of Kutch cover restricted areas.

Indian sedimentary basins are linked to their characteristic formations. The Cauvery basin contains the Kallamedu Formation, the Damodar valley contains the Raniganj Formation of Gondwana coal measures, the Chhattisgarh basin contains the Lohardih Formation, and the Indravati basin contains the Tiratgarh Formation.

The volcanic episodes of India follow a clear age sequence. The Bijli volcanics are the oldest of the listed units, being Proterozoic in age. The Malani igneous suite of western India is Neoproterozoic to earliest Cambrian, the Rajmahal traps of eastern India were emplaced in the Early Cretaceous, and the Deccan Traps are the youngest, emplaced at the Cretaceous–Paleogene boundary.

The Closepet Granite is a north–south trending batholith that divides the Dharwar Craton into western and eastern domains. The Shimoga and Bababudan schist belts lie to its west, whereas the Kolar and Hutti schist belts lie to its east, with Hutti located in the Raichur district.

When recalling map units, pair each unit with its position relative to a reference feature (such as the Closepet Granite) and with its age, since the geographic and chronologic data are usually tested together.

Indian petroleum basin reservoirs

Reservoir rocks and trap types control the hydrocarbon potential of each Indian basin. Sandstone is the principal reservoir rock of the Krishna–Godavari basin, where production comes from clastic reservoirs of Cretaceous and Tertiary age. The Cambay basin, by contrast, is dominated by Tertiary sandstone reservoirs rather than limestone.

Traps are the geometric or stratigraphic structures that hold hydrocarbons. In the Upper Assam basin the traps are mainly structural, such as anticlines and faults, and salt-dome traps are not a characteristic trap type there. In the Mumbai offshore basin the traps are largely drape and fault-block structures over carbonate build-ups, so fold-and-thrust traps are not the common type.

For a basin to be a commercial producer, it must have a proven, economically viable petroleum system. The Krishna–Godavari and Cauvery basins are producing basins with commercial gas and oil output. The Ganga basin shows hydrocarbon indications and minor gas occurrences, but it is not a commercial producer, and the Kerala–Konkan basin is a frontier area without established commercial production.

The western offshore basin of India contains the Tapti and Panna fields, which are developed together with the Mukta field in the Mumbai offshore area. Lakwa is an onshore field of the Upper Assam basin, and Ravva is an offshore field in the Krishna–Godavari basin on the eastern margin, so neither belongs to the western offshore area.

When linking a field to its basin, recall the basin's location first, since the field name alone does not reveal whether it is onshore, western offshore or eastern offshore.

Mass extinction events

A mass extinction is a geologically brief interval in which a large fraction of the Earth's species disappears, usually recognised as a sharp break in marine fossil diversity. Five major Phanerozoic events are conventionally recognised, and each has its own proposed trigger. The end-Ordovician event, centred on the Hirnantian stage at the Ordovician–Silurian boundary, is linked to continental glaciation over Gondwana. The resulting fall in global sea level removed shallow-marine habitats and eliminated more than half of marine genera. The end-Permian event, which removed the largest proportion of species, is associated with volcanism-driven warming, ocean anoxia and acidification. The end-Cretaceous event at the Cretaceous–Paleogene boundary is linked to a bolide impact with secondary environmental effects, and it removed non-avian dinosaurs and many marine groups. Boundaries such as the Paleogene–Neogene and the Precambrian–Cambrian are not standard mass extinction boundaries. The Precambrian–Cambrian boundary is instead remembered for the rapid diversification of skeletal animals. The Holocene extinction is an ongoing event attributed to human activity, not to a natural cooling episode. Among Paleozoic groups, trilobites such as Phacops and Aulacopleura, together with the tabulate coral Favosites, were extinct before or by the end of the Permian, whereas the lingulid brachiopod Lingula ranges from the Cambrian to the Recent.

VMS deposit types and host rocks

Volcanogenic massive sulphide (VMS) deposits form on or just beneath the sea floor, where hydrothermal fluids exhale metal-rich solutions during submarine volcanism. They are classified by the composition of their host volcanic and sedimentary rocks, and this classification is the key to matching deposit types with their hosts. Kuroko-type deposits are hosted mainly by felsic volcanic rocks. Cyprus-type deposits are hosted by mafic volcanic rocks of oceanic, ophiolite-related affinity. Besshi-type deposits occur in mafic volcanics with siliciclastic sediments, whereas Bathurst-type deposits occur in felsic volcanics with siliciclastic sediments. The characteristic composition of the host rock must therefore be memorised for each type.

Massive sulphide bodies are dense and electronically conductive, which gives them a distinctive geophysical signature. Their high density produces a positive gravity anomaly against the surrounding rocks, and their metallic conduction gives low electrical resistivity, which is the basis of electromagnetic and induced polarisation surveys. Pyrite, chalcopyrite and sphalerite are the typical sulphides, and this is why a high-gravity, low-resistivity combination is diagnostic.

Magmatic ore deposit types

Magmatic ore deposits concentrate metals directly from crystallising magma, and the main processes are fractional crystallization, immiscible sulphide segregation and magmatic-hydrothermal exsolution. Fractional crystallization removes early-formed minerals from the residual melt, so dense phases accumulate as layers. Chromite layers in peridotite of layered intrusions and Fe–Ti oxide layers in anorthosite complexes are classic products of this process. By contrast, komatiite-hosted Ni–Cu sulphide deposits form by segregation of an immiscible sulphide liquid, not by fractional crystallization of a magma body.

Several other deposit classes are often confused with magmatic types. SEDEX deposits can be both exhalative, formed by hydrothermal discharge on the sea floor, and replacive, formed within sediments beneath it. Orogenic gold deposits are epigenetic because they form from metamorphic fluids after the host rocks were emplaced. In porphyry Cu–(Mo) systems, boiling of exsolved magmatic-hydrothermal fluid causes pressure drop, phase separation and precipitation of sulphides. Rampura–Agucha is a sediment-hosted SEDEX-type Pb–Zn deposit, not an MVT deposit.

Magmatic-hydrothermal deposits are linked to aqueous fluid exsolved from a cooling granitic pluton, and greisen-related Sn–W mineralisation is a classic example. Mississippi Valley-type deposits, in contrast, form from basinal brines moving through carbonate sequences, and ophiolite chromitites form from mafic-ultramafic melts.

Bivalve ecology and burrowing

Bivalve ecology is classified by life habit, and the main categories are infaunal burrowers that live within soft sediment, borers that excavate hard substrates, epifaunal forms that attach to or rest on hard surfaces, and free-swimming forms that move through the water column. Venus is a shallow infaunal burrower that lives in the upper layers of soft sediment. Pholas is a rock and clay borer, and Spondylus is an epifaunal form that cements to hard substrate, so neither is a burrower in soft sediment.

Swimming bivalves escape predators by rapid valve closure or jet propulsion. Scallops such as Pecten and file clams such as Lima are the typical swimmers. Tellina is an infaunal bivalve that remains buried in sediment, and Aspergillum is a semi-infaunal form that lives partly embedded and does not swim.

Taphonomy processes

Taphonomy is the study of all processes affecting an organism from death to its eventual preservation as a fossil or its destruction. It is subdivided into biostratinomy, which covers processes acting between death and burial under sediment, such as decay, scavenging, transport and abrasion, and diagenesis, which covers the changes after burial. Biostratinomy is therefore the branch that answers questions about the interval between death and burial. Biostratigraphy is different, because it uses fossil assemblages to establish the relative ages and correlation of strata, and taxonomy is the classification of organisms.

Time averaging occurs when shells or skeletons from different times accumulate together in a single stratigraphic layer, so the fossil assemblage represents a span of time rather than an instant. Bioturbation mixes sediment vertically and combines shells of different ages. Erosion and redeposition rework older shells into younger sediment, which also mixes ages within one bed. Compaction reduces porosity and thickness but does not mix shells of different ages, and the formation of faecal pellets packages sediment without mixing ages.

Indian sedimentary basin stratigraphy

Indian sedimentary basins are identified by their named stratigraphic units, and each unit must be linked to its basin and to its geological period. The Ramgundam Sandstone belongs to the Godavari basin, and the Raipur Formation belongs to the Chhattisgarh basin. The Bagalkot Group is the stratigraphic unit of the Kaladgi basin, and the Sonia Sandstone belongs to the Marwar basin. Formation ages must be memorised separately. The Kopili Shale of the Assam–Meghalaya region is Eocene. The Raniganj Formation is a Permian Gondwana unit. The Quilon Formation of the Kerala coast is Miocene, and the Dalmiapuram Formation of the Cauvery basin is Cretaceous. A common error is to assign the Permian Raniganj or the Eocene Kopili to the wrong period, so each formation should be learned with its age and basin together.

Gondwana Supergroup formations

The Gondwana Supergroup of peninsular India is a sequence of Permian to Triassic continental strata, and its formations are named in ascending order such as Talchir, Barakar, Karharbari, Panchet and others. The Talchir Formation is the basal glacial unit made largely of diamictite and carries only thin carbonaceous shale. The Barakar Formation is the principal coal-bearing unit of the Gondwana Supergroup, with the thickest and most extensive seams of economic coal. The Karharbari Formation contains only minor coal seams, and the Panchet Formation consists mainly of sandstone and red beds with negligible coal. Students should therefore remember that the most coal-rich formation is the Barakar, not the glacial base or the younger red-bed units.

Ore grade calculation

The grade of an ore is the percentage by weight of the metal, which often must be calculated from volume percentages of minerals and their densities. The procedure has four steps: convert each volume fraction into mass by multiplying by the mineral density, calculate the mass of metal in each mineral from its formula, sum the total mass of ore, and divide the metal mass by the ore mass. The metal fraction in a compound is the atomic weight of the metal divided by the molar mass of the compound. For galena (PbS), the Pb fraction is 206/(206+32) \approx 0.8655.