Feldspathic Arenite

Introduction

Feldspathic arenites are sandstones that contain less than 90 percent quartz, more feldspar than unstable rock fragments, and minor amounts of other minerals such as micas and heavy minerals. ^[1]

Their feldspars are the defining feature - not just compositionally but genetically. Feldspars are chemically unstable and are destroyed during prolonged weathering, transport, and diagenesis. A sandstone rich in feldspars therefore signals a sediment system that was disrupted somewhere along that destructive pathway: either weathering was inhibited by climate, transport was short, or both.

Description

Some feldspathic arenites are pink or red owing to the presence of potassium feldspars or iron oxides; others are light gray to white. They are typically medium to coarse grained and may contain high percentages of subangular to angular grains. Matrix content may range from trace amounts to more than 15 percent, and sorting of framework grains ranges from moderately well sorted to poorly sorted. Feldspathic sandstones are therefore commonly texturally immature or submature - that is, they are wackes. ^[1]

Feldspathic arenites are not characterised by any particular sedimentary structures. Bedding may range from essentially structureless to parallel laminated or cross laminated. Fossils may be present, especially in marine beds. ^[1]

Depositional Settings

Feldspathic arenites typically occur in cratonic or stable shelf settings, where they may be associated with conglomerates, shallow-water quartz arenites or lithic arenites, carbonate rocks, or evaporites. Less typically, they occur in successions deposited in unstable basins or deeper-water mobile-belt settings. Feldspathic arenites of the latter type, which are commonly matrix-rich and well indurated owing to deep burial, are often called feldspathic graywackes. ^[1]

The abundance of feldspathic arenites in the geologic record is not well established. Arkoses are estimated to make up about 15 percent of all sandstones; including feldspathic graywackes, the proportion is probably higher. ^[1]

Grus and Clastic Wedges

Some arkoses originate essentially in place when granite and related rocks disintegrate to produce a granular sediment called grus. These residual arkosic materials may be shifted a short distance downslope, deposited as fans or aprons of waste material - commonly referred to as clastic wedges. These fans may extend into basins and become interbedded with better stratified and better sorted sediments. Other feldspathic arenites undergo considerable transport and reworking by rivers or the sea before deposition. These reworked sandstones commonly contain less feldspar than residual arkoses and are better sorted and better rounded. ^[1]

Source Rocks and Climate Controls

Most feldspathic sandstones are derived from granitic-type primary crystalline rocks, such as coarse granite or metasomatic rocks containing abundant potassium feldspar. Feldspathic arenites whose feldspars are dominantly plagioclase, derived from igneous rocks such as quartz diorites or from volcanic rocks, are also known. ^[1]

Preserving large quantities of feldspars during weathering appears to require that feldspathic arenites originate either in very cold or very arid climates, where chemical weathering is inhibited, or in warmer, more humid climates where marked relief allows rapid erosion of feldspars before they can be decomposed. It appears unlikely that sedimentary source rocks alone can supply enough feldspar to produce a feldspathic arenite or arkose. ^[1]

Examples

Feldspathic arenites occur in successions of all ages but are particularly abundant in Mesozoic and Paleozoic strata. Notable examples include the Old Red Sandstone (Carboniferous) in Scotland, the Triassic Newark Group in the New Jersey area, the Pennsylvanian Fountain and Lyons formations of the Colorado Front Range, and the Paleocene Swauk Formation of Washington. The Swauk Formation is notable because it is a plagioclase arkose. ^[1]

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