Quartz Arenite

Introduction

Quartz arenites represent the end point of sedimentary maturation - the rock that forms when everything less stable than quartz has been removed by weathering, abrasion, and multiple recycling. They are compositionally the simplest of sandstones and yet they record some of the most complex depositional and recycling histories. Understanding them requires considering not just where they were deposited but how many sedimentary cycles their grains passed through before reaching their final resting place.

Description and Composition

Quartz arenites are composed of more than 90 percent siliceous grains, which may include quartz, chert, and quartzose rock fragments. They are commonly white or light gray but may be stained red, pink, yellow, or brown by iron oxides. They are generally well lithified and well cemented with silica or carbonate cement, though some are porous and friable. ^[1]

Depositional Setting and Sedimentary Structures

Quartz arenites typically occur in association with rocks deposited in stable cratonic environments, such as eolian, beach, and shelf settings. They tend to be interbedded with shallow-water carbonates and, in some cases, with feldspathic sandstones. ^[1]

Most quartz arenites are texturally mature to supermature; quartz wackes are uncommon. Cross-bedding is particularly characteristic of these sandstones, and ripple marks are moderately common. Fossils are rarely abundant, possibly owing to poor preservation or to the eolian origin of some quartz arenites, but fossils may be present. Trace fossils such as burrows of the Skolithos ichnofacies may be locally abundant in some shallow-marine quartz arenites. ^[1]

Quartz arenites make up about one-third of all sandstones. ^[1]

Origin: Polycyclic vs. First-Cycle

Quartz arenites can originate as first-cycle deposits derived from primary crystalline or metamorphic rocks, but they are more likely to be the product of multiple recycling of quartz grains from sedimentary source rocks. ^[1]

If they are first-cycle deposits, they must have formed under weathering, transport, and depositional conditions so vigorous that most grains less stable than quartz were eliminated. Extensive chemical leaching under hot, humid, low-relief conditions; prolonged eolian transport; intensive surf-zone reworking; or a combination of these processes might be adequate to generate a first-cycle quartz arenite. Most quartz arenites are probably polycyclic, and their history may have included at least one episode of eolian transport, though not necessarily during the final depositional cycle. ^[1]

Examples

Quartz arenites are common in the geologic record, particularly in Mesozoic and Paleozoic stratigraphic successions. Well-known North American examples include the Ordovician St. Peter Sandstone in the midcontinent United States, the Jurassic Navajo Sandstone of the Colorado Plateau, the Ordovician Eureka Quartzite in Nevada and California, parts of the Cretaceous Dakota Sandstone in the Colorado Plateau and Great Plains, and many Cambro-Ordovician sandstones in the Upper Mississippi Valley. ^[1]

Master UPSC Geology Optional

Ex-ONGC Geologist & Rank Holder

Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.

1-on-1 Personalized Mentorship

Elite Batch (Strictly 10 Seats)

Targeted Strategy for AIR 1-100

Bilingual Conceptual Lectures

Join Us

Offline in Delhi