(a) Discuss the various factors that control the composition of sandstone. (15 marks) (b) What do you understand by facies model…

Factors Controlling Sandstone Composition

Sandstone composition is governed by the interplay of source rock lithology, tectonic setting, climate, relief, transport distance, depositional environment, and diagenesis. The primary mineralogy—represented on Pettijohn’s and Dott’s (1964) QFL ternary classification (quartz, feldspar, lithic fragments)—is initially determined by parent rock lithology.

Climate and relief directly control the rate of chemical versus mechanical weathering. In humid, tropical conditions with low relief, intense chemical breakdown destroys unstable feldspars and lithic fragments, producing compositionally mature quartz arenites, such as the Proterozoic Vindhyan Supergroup sandstones. Conversely, in arid or cold climates with high tectonic relief, rapid physical erosion and unroofing preserve labile feldspars and rock fragments, resulting in immature arkoses and lithic arenites, such as the Permian Barakar Formation of the Gondwana Supergroup.

Transport distance and hydrodynamic energy further alter composition; extended transport causes selective abrasion and elimination of softer minerals, progressively increasing Folk’s compositional maturity. Post-depositional diagenesis modifies composition through the dissolution of unstable grains, authigenic cementation, and the albitization of feldspars.

Facies Models and Fluvial Environments

A facies model is an idealized, generalized summary of a specific sedimentary depositional environment, synthesized from modern analogues and ancient rock records. It acts as a norm for comparison, a framework for observations, and a predictive tool for spatial and vertical distribution of strata. In contrast to a single facies—which represents a distinct rock unit with specific lithological, structural, and paleontological characteristics—a facies association is a group of genetically related facies reflecting a broader environmental setting.

In a meandering fluvial environment, the facies association characteristically displays a fining-upward cycle (Allen model), comprising three distinct facies:

Channel facies: Situated at the base above an erosional scour surface, consisting of basal intraformational conglomerate or lag gravels that grade upward into medium-to-coarse, trough and planar cross-bedded sandstones deposited by migrating dunes and point bars.

Levee and crevasse splay facies: Intermediate units composed of fine-grained sandstones and siltstones exhibiting ripple cross-laminations, climbing ripples, and small-scale scours formed during overbank flood stages.

Floodplain facies: Uppermost vertical accretion deposits consisting of thinly laminated mudstones, shales, and silts, often containing pedogenic features like paleosols, root traces, and calcrete nodules.

Heavy Minerals: Separation and Provenance Interpretation

Heavy minerals are accessory detrital mineral grains having a specific gravity greater than 2.85 to 2.89 g/cm³, noticeably higher than framework minerals like quartz and feldspar.

Separation is carried out systematically on disaggregated, sieved sand fractions (typically 63–250 µm). Preliminary concentration is achieved by panning. Dense liquid separation is subsequently conducted using bromoform (specific gravity 2.85) or tetrabromoethane (specific gravity 2.96) in a separating funnel or centrifuge tube, allowing the heavies to settle while quartz and feldspar float. The separated heavy fractions are further divided using a Frantz isodynamic magnetic separator based on differential magnetic susceptibility (e.g., separating magnetite, ilmenite, garnet, and tourmaline from non-magnetic zircon and rutile), followed by optical mounting for petrographic study.

In provenance interpretation, heavy mineral suites provide diagnostic fingerprints of the parent source terrane:

Source lithology: Acidic plutonic sources yield assemblages of zircon, tourmaline, monazite, and apatite; high-grade metamorphic sources contribute kyanite, sillimanite, garnet, and staurolite; and mafic to ultramafic rocks supply chromite, olivine, and pyroxenes.

Transport history and maturity: The ZTR index (percentage of ultra-stable zircon, tourmaline, and rutile) assesses the degree of chemical and mechanical weathering, sediment recycling, and transport distance.

Stratigraphic correlation: Characteristic heavy mineral suites allow basin-wide correlation of strata and delineate unroofing histories.

This utility is exemplified by the heavy mineral assemblages of the Siwalik foreland basin, which record the sequential tectonic uplift and erosion of the Himalayas, and the economically vital coastal beach placers of Chavara, Kerala, where ilmenite, monazite, and sillimanite suites trace sediment derivation from the high-grade metamorphic Southern Granulite Terrane.