(a) Classify the conglomerate rocks on the basis of clast composition and grain-matrix ratio and discuss their genetic…

(a) Classification of conglomerates and their genetic importance

A conglomerate is a sedimentary rock of rounded clasts coarser than 2 mm set in finer material. It is classified in two ways.

By clast composition.

(i) Oligomictic (monomictic) conglomerate has clasts of one stable kind, such as quartz, quartzite or chert. It shows long transport and recycling, in stable settings.

(ii) Polymictic (petromictic) conglomerate has many kinds of rock fragments, such as granite, basalt and schist. It shows short transport from a high-relief source in an active tectonic setting.

(iii) Extraformational clasts come from outside the basin, and intraformational clasts (shale or limestone flakes) are reworked from the same basin.

By grain–matrix ratio and support.

(i) Orthoconglomerate is clast-supported, with less than 15% matrix of sand, mud or cement. The matrix was winnowed out by currents, as in braided rivers, alluvial fans and beaches.

(ii) Paraconglomerate is matrix-supported, with more than 15% matrix and clasts floating in it. Its origin is mass flow, such as debris flows, or glacial deposition as diamictite.

Genetic importance. Conglomerates give the nature of the source rocks, the distance and energy of transport, the palaeocurrent direction, and the tectonic setting. Clasts in vertical order show unroofing of a source: younger rocks appear first and deeper rocks later. Basal conglomerates mark unconformities.

Indian examples. The Talchir Boulder Bed (Permo-Carboniferous) is a glacial paraconglomerate of the Gondwana, the boulder conglomerate of the Upper Siwaliks is a fan deposit of rising Himalaya, and the basal conglomerates of the Dharwar and Vindhyan mark unconformities.

(b) Mechanisms of sediment gravity flows and their features in rocks

A sediment gravity flow is a flow of a sediment and water mixture down a slope, driven by the pull of gravity on the sediment. Four mechanisms are recognised, according to the way grains are supported.

(i) Turbidity current: grains are held by fluid turbulence. The deposit is graded and forms the Bouma sequence, with its divisions: Ta (massive graded), Tb (parallel lamination), Tc (ripple and convolute lamination), Td (upper parallel lamination) and Te (hemipelagic mud). The base carries sole marks such as flute and groove casts. These rocks are the turbidites, and the flysch of the Andamans is an example.

(ii) Debris flow: grains are held by the strength and buoyancy of a muddy matrix. The bed is unsorted, unstratified and matrix-supported, with outsized floating clasts and sharp top and base.

(iii) Grain flow: grains are held by collisions between them, in steep slopes of sand. The deposit is well-sorted and shows inverse grading.

(iv) Fluidised or liquefied flow: grains are held by upward-escaping pore water. It leaves dish and pillar structures and sand volcanoes.

A flow can change, as when a debris flow dilutes into a turbidity current. A neat labelled sketch of a Bouma sequence explains the features.

(c) Mineral-based methods for source and transport history

The sediment mineral content records the source rock and how far and how long it was transported.

(i) Heavy minerals. Grains heavier than 2.9 g/cm³ are separated with bromoform and identified. The assemblage shows the source rock. The zircon–tourmaline–rutile (ZTR) index shows maturity, and grains that survive many cycles of recycling dominate.

(ii) Grain shape. Roundness and surface texture of zircon, tourmaline and quartz increase with distance and with recycling. Euhedral zircon is first-cycle and from a nearby source.

(iii) Quartz types. Undulose and polycrystalline quartz indicate metamorphic sources, and non-undulose monocrystalline quartz indicates plutonic or volcanic ones.

(iv) Single-grain methods. U–Pb dating of detrital zircon gives source ages, and the chemistry of garnet, tourmaline and Cr-spinel gives the source rocks.

(v) Stability. Minerals vary in survival, in the order of Goldich, so the absence of olivine or pyroxene suggests a long route or intense weathering.

Diagnostic minerals.

(i) Igneous: olivine, pyroxene and chromite (mafic and ultramafic); hornblende, biotite, apatite, sphene, magnetite, and euhedral zircon (intermediate to felsic); and volcanic quartz and sanidine.

(ii) Metamorphic: chlorite and epidote (low grade); garnet, staurolite and kyanite (medium grade); sillimanite (high grade); andalusite (contact metamorphism); glaucophane and lawsonite (high-pressure, blueschist); graphite and tremolite.