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

Sedimentary structures, facies models and conglomerates are key tools for reconstructing ancient flow, basin geometry and source areas.

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 by suspended load settling on a bed surface, may also give a vector 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.

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. The deltaic facies model distinguishes river-dominated, wave-dominated and tide-dominated systems, and also separates proximal distributary-channel, mouth-bar, shoreface, offshore and basin-floor facies. River-dominated Gilbert-type deltas are coarse-grained, progradational clinoforms with steep foresets; low-gradient bird-foot deltas, such as the Mississippi, are different, having elongate distributary-mouth bars and finer sediment. Wave-dominated deltas are reworked, shallow, and show swash bars, fining-upward sand and onlapping facies. Tide-dominated deltas contain herringbone cross-bedding, tidal flats, muds and intertidal structures. Vertical profiles are commonly progradational and fining-upward; Walther’s Law links vertical facies succession to lateral facies changes, helping predict reservoir sandbodies. Indian examples include Kutch deltaic sequences used to interpret progradation and shoreline migration.

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. 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 Siwalik conglomerates, which record fluvial palaeocurrents and Himalayan provenance, and Karewa conglomerates, used for palaeocurrent and source-area analysis. Together with Kutch deltaic sequences, they help reconstruct Himalayan foreland basin evolution.

Together, palaeocurrent structures, facies models and conglomerate fabrics allow reconstruction of basin evolution, sediment routing, source-area tectonics and hydrocarbon exploration targets.