Lithic Arenite
Introduction
Lithic arenites are compositionally immature sandstones - rocks that were deposited before the weathering and transport system had time to destroy the unstable rock fragments and minerals they contain. Their diversity reflects the diversity of source rocks and depositional environments. A lithic arenite can form at the foot of a collapsing mountain range or at the toe of a turbidite fan in the deep ocean. What all lithic arenites share is an abundance of mechanically or chemically fragile material that will not survive a second sedimentary cycle.
Description and Composition
Lithic arenites are an extremely diverse group of rocks characterised by generally high content of unstable rock fragments, such as volcanic and metamorphic clasts, though they may also contain some stable clasts such as chert. They contain less than 90 percent quartzose grains and more unstable rock fragments than feldspars. [1]
Colours may range from light gray or salt-and-pepper to uniform medium to dark gray. Many lithic arenites are poorly sorted, though sorting ranges from well sorted to very poorly sorted. Quartz and many other framework grains are generally poorly rounded. Lithic arenites tend to contain substantial amounts of matrix, much of which may be of secondary origin. Most lithic sandstones are therefore texturally immature to submature - they are lithic wackes. [1]
Lithic arenites and graywackes together make up nearly one-half of all sandstones. [1]
Sedimentary Structures and Associated Rocks
Lithic arenites range from irregularly bedded, laterally restricted, cross-stratified fluvial units to evenly bedded, laterally extensive, graded marine turbidite units. They may occur in association with fluvial conglomerates and other fluvial deposits, or in association with deeper-water marine conglomerates, pelagic shales, cherts, and submarine basalts. [1]
Graywacke Overlap
Lithic arenites include sandstones that many geologists continue to refer to as graywackes. These differ from “normal” lithic arenites in being dark gray to dark green, well indurated or lithified, and commonly having a matrix of secondary chlorite. The term graywacke is used so loosely, however, that simply dropping it in favour of wacke is probably advisable. [1]
Origin and Source Areas
Lithic arenites are typically compositionally immature sandstones that originate under conditions favouring the production and deposition of large volumes of relatively unstable materials. The mechanically weak character of many of their lithic fragments suggests derivation from rugged, high-relief source areas. [1]
Lithic arenites may be deposited in nonmarine settings in proximal alluvial fans or other fluvial environments. Alternatively, they may be deposited in marine foreland basins adjacent to fold-thrust belts, or transported by rivers into deltaic or shallow shelf environments. Lithic sediments deposited in coastal areas may be retransported into deeper water by turbidity currents or other sediment gravity-flow mechanisms. These deeper-water sediments are particularly likely to undergo deep burial and incipient metamorphism, leading to development of the characteristics generally attributed to graywackes. [1]
Examples
Common examples of lithic sandstones include the Paleozoic sandstone successions of the central Appalachians in the eastern United States (the Ordovician Juniata Formation, the Mississippian Pocono Formation, and the Pennsylvanian Pottsville Formation); sandstones associated with the Coal Measures throughout the world; Jurassic and Cretaceous sandstones of the U.S. and Canadian Rocky Mountains and the U.S. West Coast (including the Cretaceous Belly River Sandstone of Canada and the Jurassic Franciscan Formation of California); and Tertiary sandstones of the Gulf Coast, the West Coast, and the Alps. [1]
Related Topics
Sedimentary Structures
Sedimentary structures are large-scale features of sedimentary rocks - including parallel bedding, cross-bedding, ripples, and mudcracks - that form as a direct result of depositional or...
Weathering
Weathering is the physical disintegration and chemical decomposition of older rock that produces solid particulate residues - resistant minerals and rock fragments - and dissolved chemical...
Sandstone
Sandstones make up 20-25 percent of all sedimentary rocks. They are common in geologic systems of all ages and are distributed throughout the continents. They occur in beds ranging in thickness...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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