(a) What is provenance? How can we use clastic quartz, feldspars and lithic grains in provenance interpretation of sandstones?…

(a) Provenance from clastic quartz, feldspar and lithic grains

Provenance is the nature, tectonic setting, relief and climate of the source area that supplied the detritus. Framework grains record it.

Quartz: monocrystalline quartz with non-undulatory extinction suggests plutonic igneous sources; polycrystalline quartz with sutured subgrains and undulatory extinction points to metamorphic sources; rutile or tourmaline inclusions and embayed outlines help separate granitic from volcanic sources.

Feldspar: K-feldspar (microcline, orthoclase) points to granitic rocks, gneisses and pegmatites; calcic plagioclase to basic igneous or volcanic-arc sources. Fresh feldspar shows rapid erosion under high relief or cold or arid climate; sericitised grains show humid weathering or long transport.

Lithic grains: volcanic lithics indicate magmatic arcs, schist and phyllite fragments indicate orogenic belts, and chert, shale and carbonate fragments indicate recycled sedimentary belts.

Plotting modal Q-F-L and Qm-F-Lt on Dickinson's ternary diagrams assigns the sandstone to continental block (quartz-rich), magmatic arc (plagioclase and volcanic lithics) or recycled orogen (polycrystalline quartz and mixed lithics) provenance. In the Siwalik molasse, lithic content rises upward, recording progressive unroofing of the Himalaya.

(b) Facies model and the meandering river

A sedimentary facies model is a generalised summary of the facies and their vertical and lateral relations in one depositional environment. It acts as a norm for comparison, a guide for interpretation and a predictor, through Walther's law, of what lies above and beside.

A meandering river migrates laterally and aggrades, giving a fining-upward association. Sketch 1: a meander bend in plan showing channel, point bar, levee and floodplain. Sketch 2: the vertical log below.

Channel lag: gravel, mud clasts and heavy minerals on an erosional base. Point bar: inner-bend deposits formed by helical flow, with inclined lateral accretion surfaces (epsilon cross-stratification) and a transition from trough cross-bedded medium sand to ripple-laminated fine sand. Levee and crevasse splay: fine sand and silt with climbing ripples, and thin lobate sheet sands from breaches. Floodplain: mudstone, root traces, palaeosols and, in humid settings, coal, as in the Lower Gondwana coal measures of the Damodar Valley.

(c) Palaeocurrent structures

Trough cross-bedding forms by downstream migration of sinuous-crested, 3D dunes; the trough axes plunge downcurrent. Tabular cross-bedding forms from straight-crested, 2D bedforms; the foreset dip azimuth gives the migration direction, unimodal in rivers and bimodal (herringbone) in tidal settings. Current ripples form under lower-flow-regime traction; the gentle stoss side faces upstream and the steep lee side (about 30°) faces downstream. Flute casts form on turbidite soles when turbulent eddies scour mud that sand then fills; the bulbous end points upcurrent and the cast flattens downcurrent.