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

Provenance Interpretation from Sandstone Petrography

Provenance encompasses the composition, tectonic setting, climate, and relief of the source terrain supplying detritus to a sedimentary basin. Detrital framework grains—quartz, feldspars, and rock fragments (lithics)—provide primary genetic signatures:

Clastic Quartz: Monocrystalline quartz (Qm) with straight/non-undulatory extinction typically indicates stable, plutonic igneous (granitic) sources. Polycrystalline quartz (Qp) with multiple sutured subgrains and undulatory extinction points to high-to-medium-grade metamorphic parentage. Micro-inclusions such as rutile needles, tourmaline, or embayed volcanic margins further differentiate source lithologies.

Feldspars: Potassium feldspar (microcline, orthoclase) reflects acidic plutonic rocks and pegmatites, whereas plagioclase (calcic to intermediate) denotes intermediate-to-basic igneous or volcanic arc sources. Pristine, unaltered feldspars indicate rapid erosion under arid or cold climates with high tectonic relief, while heavily sericitised or saussuritised grains signify intense chemical weathering in humid conditions or prolonged sediment transport.

Lithic Fragments (L): Supracrustal lithics directly mirror the unroofed source. Volcanic lithics (Lv) characterise magmatic arc systems; metamorphic lithics (Lm) like schist and phyllite fragments identify dynamic orogenic belts; sedimentary lithics (Ls) such as chert, shale, and carbonate clasts signify recycled sedimentary fold-and-thrust belts.

Using the QFL and Qm-F-Lt ternary discrimination diagrams of Dickinson et al., sandstones are systematically categorized into distinct tectonic provenances: Continental Block (craton interior and transitional continental crust; quartz-rich), Magmatic Arc (undissected to dissected volcanic-plutonic arcs; rich in plagioclase and Lv), and Recycled Orogen (subduction complexes and collision belts; dominated by Qp and diverse lithics). A prime Indian example is the Siwalik molasse of the Himalayan foreland basin, where the upward increase in metamorphic and sedimentary lithics records the progressive unroofing of the rising Himalayan orogen.

`` Q (Total Quartz) /\ / Continental Block / \ / Recycled\ / Orogen \ /_____________\ F L (Feldspars) (Lithics) ``

Sedimentary Facies Model: Meandering Fluvial System

A sedimentary facies model is an idealized, predictive summary that synthesises the physical, biological, and chemical characteristics of a specific depositional environment. It acts as a norm for comparison, a framework for hydrodynamic interpretation, and a predictive tool via Walther’s Law of Facies.

`` [ Vertical Succession ] Floodplain : Mudstone, caliche paleosols, root traces (Suspension settling) Crevasse Splay: Sheet-like thin sandstones with climbing ripples Levee : Interbedded silt-fine sand with ripple laminations Point Bar : Fine-medium sand; climbing ripples -> tabular/trough cross-beds : Lateral Accretion Surfaces (Epsilon Cross-Stratification) Channel Lag : Coarse gravels, rip-up clasts on basal erosional scour ``

In a meandering fluvial environment, continuous lateral migration and vertical aggradation generate an asymmetrical, fining-upward facies association:

Channel Floor and Lag Facies: Formed at the base of the thalweg by intense scouring; composed of poorly sorted gravels, intraformational mud clasts, and heavy mineral concentrates.

Point Bar Facies: Deposited by helicoidal flow on the inner meander bend, exhibiting characteristic inclined lateral accretion surfaces (epsilon cross-stratification). Hydrodynamic energy wanes upward, generating a vertical transition from basal trough cross-bedded medium sand (3D dunes) to upper planar-laminated and ripple-cross-laminated fine sand.

Levee and Crevasse Splay Facies: Overbank flooding builds wedge-shaped levee ridges (fine sand with climbing ripples) flanking the channel. Breaching yields crevasse splay deposits characterized by thin, graded, lobate sheet sandstones interbedded with mud.

Floodplain / Overbank Facies: Settling of suspended silt and clay in low-energy backswamps. Dominant features include massive mudstones, pedogenic calcretes, root bioturbation, and coal seams, as seen in the Lower Gondwana fluvial coal measures of the Damodar Valley basin.

Sedimentary Structures for Palaeocurrent Analysis

Palaeocurrent analysis reconstructs ancient sediment dispersal pathways and regional palaeoslopes using directional structures:

Trough Cross-Bedding: Generated in the lower flow regime by the downstream migration of three-dimensional, sinuous-crested dunes. The curved foresets produce spoon-shaped scours whose symmetry axes plunge strictly downcurrent, providing a reliable unidirectional vector in fluvial channels.

Tabular Cross-Bedding: Produced by the downstream migration of two-dimensional, straight-crested sand waves and dunes. The maximum dip azimuth of the planar foresets records the local bedform migration direction, exhibiting unimodal trends in rivers and bimodal-herringbone patterns in tidal regimes.

Current Ripple Marks: Formed by low-velocity traction currents under lower flow regime conditions. They display an asymmetric cross-sectional profile with a gentle stoss slope (8^°–10^°) facing upstream and a steep lee slope (angle of repose,∼ 30^°–34^°) facing downstream, reliably defining the current vector.

Flute Casts: Formed on the soles of turbidite beds when high-energy turbulent eddies scour cohesive mud substrates, followed by immediate sand infill. Flutes exhibit a bulbous, deeply excavated upstream nose that tapers, widens, and flattens downcurrent, serving as precise indicators of turbidity flow pathways.

Palaeocurrent measurements across the Bhander Sandstone of the Proterozoic Vindhyan Basin systematically demonstrate sediment transport directed towards the west and north-west into the intracratonic depocentre. Integrating sandstone petrofacies, vertical facies models, and directional sole structures provides an indispensable basis for reconstructing basin evolution, paleogeography, and regional tectonic histories.