(a) Discuss the genesis of any four sedimentary structures which are helpful in palaeocurrent analysis. How are the palaeocurrent…

(a) Palaeocurrent structures

Four useful structures are ripple marks, cross-bedding, flute casts and clast imbrication. Unidirectional current ripple marks form on migrating bedforms; the stoss side faces upstream and the slip face dips downstream, so the vector is taken from stoss to slip face. Cross-bedding records inclined foresets deposited on migrating dunes or bars; the foresets dip downstream, and the palaeocurrent is down the foreset, perpendicular to the strike of the cross-bedding surface. Flute casts are streamlined grooves formed by turbulent flow eroding a mud surface and later filled by sand; the head points upstream and the tail downstream, giving a clear flow vector. In clast imbrication, pebbles lie with their long axes transverse to flow and their intermediate axes dipping upstream; the ab plane dips upstream, so the current flowed opposite to the clast dip. Parting lineation, formed under upper-flow-regime plane-bed conditions, may also give a trend parallel to flow. These vectors are measured in the field, plotted as rose diagrams, and analysed as mean resultant vectors. A unimodal pattern suggests a single dominant current, as in fluvial channels or deep-marine turbidites; a bimodal pattern, especially with herringbone cross-bedding, indicates tidal flow; eolian systems may be unimodal to bimodal according to wind regime; shallow-marine and estuarine patterns can be unimodal, bimodal or polymodal depending on wave, storm and tidal processes. The pattern, with grain size and facies, establishes the depositional environment; for example, Siwalik sandstones show dominantly southward to south-westward palaeocurrents recording transverse Himalayan rivers, and palaeocurrent reversals across a basin help locate its axis and source.

(b) Facies and deltaic models

A facies model is a generalised summary of lithofacies, grain size, sedimentary structures, facies associations and vertical successions that characterise a depositional system. Such models are used to correlate outcrops and wells, identify reservoirs and traps, and assess basin evolution. It links lateral facies changes to vertical successions and helps predict reservoir sandbodies. A delta is divided into the delta plain (distributary channels, levees, crevasse splays, marshes), the delta front (mouth bars, distal bars) and the prodelta (offshore muds). Galloway's ternary scheme classifies deltas as river-, wave- or tide-dominated. River-dominated deltas, such as the bird-foot Mississippi, have elongate distributary-mouth bars (bar-finger sands) and fine sediment. Coarse-grained Gilbert-type deltas, with topset–foreset–bottomset geometry and steep foresets, form where rivers enter lakes or deep water, as in fan deltas. Wave-dominated deltas are reworked, shallow, and show swash bars, coarsening-upward shoreface sand and beach-ridge facies. Tide-dominated deltas contain herringbone cross-bedding, tidal flats, muds and intertidal structures. Vertical profiles are commonly progradational and coarsening-upward; Walther’s Law links vertical facies succession to lateral facies changes, helping predict reservoir sandbodies. Indian examples include the modern Krishna–Godavari and Mahanadi deltas (wave-influenced), the tide-influenced Ganga–Brahmaputra delta with the Sundarbans, and ancient deltaic sequences such as the Jurassic Bhuj Formation of Kutch and the Krishna–Godavari basin reservoirs.

(c) Conglomerates

Conglomerates are coarse-grained sedimentary rocks with clasts larger than 2 mm, usually rounded to subrounded, and may be clast-supported or matrix-supported. Angular clasts define breccia, whereas rounded clasts indicate longer transport or reworking. Clast-supported types show clast-to-clast contacts, while matrix-supported types have clasts suspended in finer sediment. Their fabrics include clast orientation, imbrication, packing, sorting, clast shape and matrix composition, such as sand, silt, clay or cement. They are classified by genesis as residual, lag, till, fluvial, alluvial-fan or turbiditic, and by composition as petromict or oligomict, the latter indicating a more restricted source. Pettijohn separated orthoconglomerates (clast-supported, deposited by traction currents) from paraconglomerates (matrix-supported, deposited by mass flows or ice, e.g., tillites), and intraformational conglomerates (clasts from the same basin, such as mud-flake conglomerates) from extraformational ones. Geologically, conglomerates indicate high-energy transport, proximity to source, tectonic setting and palaeogeographic connections; they are important for provenance studies, placer deposits and aquifers. Indian examples include the Upper Siwalik Boulder Conglomerate, which records fluvial palaeocurrents and Himalayan unroofing; the Talchir boulder bed, a glacial paraconglomerate at the base of the Gondwana; basal quartz-pebble conglomerates of the Dharwar Supergroup, studied as uranium–gold placers; and Karewa conglomerates of Kashmir. Together they help reconstruct foreland, glacial and cratonic basin evolution.