Chapter 5: Sandstone and Conglomerates

Terrigenous “From the Earth” Terrigenous clastic sedimentary rocks are composed of clasts of pre-existing rocks and minerals. Also referred as: detrital (detached from pre-existing rocks by erosion or weathering), or; Epiclastic (derived from the surface).

Silisiclastic Because most terrigenous clastics are especially rich in quartz and other silicates minerals, the term silisiclastic is also used. Clast formed by physical weathering are eroded and transported by mass wasting, wind, water, and ice and are deposited as discrete, unconsolidated fragments that are eventually lithified.

Three distinct groups are recognized: Terrigeneous sediments and sedimentary rocks are defined on the basis of clast diameter. Three distinct groups are recognized: Conglomerates and breccias Sandstones mudrocks

Conglomerates and Breccias Conglomerates is lithified gravel made up of rounded to subrounded clasts whose diameters exceed 2 mm. They are also called roundstone or puddingstone. Breccias is lithified rubble made up of angular clasts coarser than 2 mm. They are also called sharpstone.

Conglomerates and Breccias The roundness (angularity) of the grains is measured using standard grain silhouettes. Very coarse clastic rocks are collectively referred to as rudites or rudaceous sedimentary rocks.

Composition Most clasts on conglomerates and breccias are fragments of rocks and minerals produced by the disintegration of bedrock. These occur both as coarser-grained fragments and finer-grained matrix filling the space between fragments. Clasts are typically glued together by a small amount of siliceous, calcareous, or ferruginous cement.

Conglomerates and Breccias Three principal categories of clasts are distinguished: Mineral fragments that occur as major components, Mineral fragments that occur as accessory constituents, and Fragments of rock.

Mineral fragments occurring as major constituents (5% or more) Clasts of a single mineral such as quartz or feldspar tend to be less abundant in conglomerates and breccias than in sandstone because few igneous, metamorphic, or sedimentary rocks have original grains coarse enough to disintegrate into pebbles and coarser detritus. Source rocks with mineral grain diameters coarser than 8 mm (fine pebble) include quartz veins, pegmatites, deep-seated plutons, high-grade metamorphic rocks, breccias and conglomerates.

Quartz is the most abundant major mineral in conglomerates and breccias. It is harder than other rock-forming mineral Has no cleavage Practically insoluble Large clasts of K-feldspar, plagioclase feldspar, and mica can also be abundant but seldom last as long as quartz because they corrode, disaggregate, and abrade with transport. The sand matrix is similar in composition to sandstone interbedded with the conglomerate or breccia.

Mineral class occurring as accessory constituents (less than 5%) Other fragments occur as accessory, their presence is incidental. They occur as accessory minerals either because their original abundance in source rocks is low or because they are easily destroyed by weathering. Micas and heavy minerals such as: olivine, pyroxene, amphibole, zircon, magnetite and hematite.

Rock fragments Rock fragments are typically the most abundant component in very coarse-grained terrigenous rocks and are invariably the most interesting. Careful analysis of their composition provides us with direct information on provenance.

Texture Conglomerate and breccia textures are studied at the outcrop using methods of quantitative grain size analysis that differ from those used for sandstone. Grain diameters of particles coarser than sand (>2mm). The interstitial space between framework grains can be empty; filled with finer-grained detrital matrix; or occupied by cement, fluid (oil, water) or natural gas.

Two distinctive varieties of conglomerates (and breccias) are defined on the basis of texture: Orthoconglomerates (literally, “true” conglomerates) consist mainly of gravel-sized framework grains. Matrix (sand or finer) is less than 15%. So, grain-supported framework. Paraconglomerate have a matrix of sand and finer clasts. Matrix is at least 15%; most have 50% matrix and are actually sandstone and mudrocks in which pebbles, cobbles and boulders are scattered.

Extraformational or intraformational They are separated by comparing the composition of framework and matrix grains. Intraformational- conglomerates and breccias have an interior (intrabasinal) source: that is; they are eroded from the same sedimentary rock unit they are a part, rather than being derived from rocks located outside the depositional basin. Consequently intraformational conglomerates and breccias have framework grains identical in composition to those in the matrix.

Extraformational or intraformational Extraformational- conglomerates and breccias are derived from sources areas outside the depositional basin. Detritus weathered from external sources is carried away and deposited elsewhere. As a result framework clasts differ markedly in composition from matrix.Framework matrix is exotic; that is, not derived by the erosion and redeposition of matrix material.

Orthoconglomerate and Paraconglomerates (Orthobreccias and Parabreccias) They are separated by examining the proportion of matrix. Orthoconglomerate are matrix poor, Paraconglomerates are matrix rich.

Orthoconglomerates They are matrix-poor (80% or more framework grains) and have an intact, stable, grain-supported fabric. They are transported and deposited on a grain-by grain basis by fluids, specifically water or air. Oligomict or petromict-cngl are further divided into these on the basis of framework grain composition.

Oligomict or petromict In oligomict (orthoquartzose) conglomerates (or breccias), more than 90% of the framework clasts consist of fragments of only a few varieties of resistant rocks and minerals as metaquartzite, vein quartz, and cherts. In petromict (polymict) clasts of many different composition of metastable and unstable rocks are abundant; for example, basalt, slate, and limestone.

A more precise classification can be given by specifying predominant clast size and lithology: quartz pebble oiligomictic orthoconglomerate slate coble petromictic parabreccia

Oligomict orthoconglomerates imply wholesale decomposition and disintegration of immense volumes of rocks, reflecting climate and topography that promote chemical decomposition and physical disintegration of all but the most resistant components. Typically stream channels deposits and bars deposits, or near shore marine settings.

Petromic are much more abundant than oligomict orthoconglomerates and are mainly alluvium eroded from high-relief areas.

Paraconglomerates

Paraconglomerates Paraconglomerates and parabreccias are further divided on the basis of their inferred origin as well as the size and internal organization of their matrix. Is the matrix sand or mud? Is the matrix internally laminated or chaotic? Is the framework imbricated, sorted, and vertically graded? Is the deposit sheetlike or lenticular? With what other types of sediment is the deposit associated?

Paraconglomerates Paraconglomerates containing a matrix of delicately laminated mudrocks in which coarser framework grains float are called laminated pebbly (or cobbly, or bouldery) mudrock. Dropstone- ice rafting

Paraconglomerates Paraconglomerates in which the matric is disorganized and non-laminated are either tillite (only if glacial origin can be inferred) or tilloid (deposited by mass movement).