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Department of Petroleum Geology & Sedimentology, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia EPS 421 CLASTIC SEDIMENTARY ROCKS Lecture 08: Composition, textures and Classification of conglomerates and breccias; part I Prof. Dr. Mahmoud A. M. Aref The Classification of Clastic Sedimentary Rocks A very basic classification of all sedimentary rocks is based on the type of material that is deposited and the modes of deposition. EPS 421 Lecture 08 Conglomerates The term conglomerates is used as a general class name for sedimentary rocks that contain a substantial fraction (at least 30 percent) of gravel-size (> 2 mm) particles. Conglomerates are common in stratigraphic successions of all ages but probably make up less than 1 percent by weight of the total sedimentary rock mass. They are closely related to sandstones in terms of origin and depositional mechanisms, and they contain some of the same kinds of sedimentary structures (e.g., tabular and trough cross bedding, graded bedding). EPS 421 Lecture 08 Gravel-sized rocks Gravel Sized Detrital Rocks are Subdivided Based on Grain Roundness into: Conglomerate: Breccia: rounded clasts angular clasts general term for lithified gravel that has been eroded and transported EPS 421 Lecture 08 "non-transported" fragments (faults, solution collapse, volcanoes, meteorite impacts) Particle Composition of conglomerate-1 Conglomerates are composed of gravel-size pieces of: 1. Framework grains 2. Matrix 3. Cement. F C M 1) Framework grains composed of: C A) Most of the gravel-size framework grains are rock fragments (clasts). - Any kind of igneous, metamorphic, or sedimentary rock may be present in a conglomerate, depending upon source rocks and depositional conditions. B) Individual minerals such as quartz EPS 421 Lecture 08 Particle Composition of conglomerate-2 The matrix of conglomerates commonly consists of various kinds of clay minerals and fine micas and/or silt-or sand-size quartz, feldspars, rock fragments and heavy minerals. The matrix may be cemented with quartz, calcite, hematite, clay, or other cements. EPS 421 Lecture 08 Rounding of Gravel Particles Talus slope – Angular = 5 km of transport River gravels – Sub-rounded Beach gravels Effective rounding by waves EPS 421 Lecture 08 Rounding of Gravel Particles Little change in sphericity Rapid initial change in roundness EPS 421 Lecture 08 Surface Textures of Gravel Ventifacts – wind sculpturing Striated clasts – glacial grinding EPS 421 Lecture 08 a, b = clast axes p = parallel i = imbricated Gravel Fabric Imbricated gravel Gravel carried in suspension: streamlined transport with a-axis parallel to flow Gravel carried as bedload: rolling or sliding with aaxis transverse to flow EPS 421 Lecture 08 Gravel particles not able to adjust individually to flowing medium, e.g. encased in ice, or in debris flow Gravel Fabric: Imbrication A-axis transverse to flow B-axis dips upflow scour EPS 421 Lecture 08 Gravel Fabric: Unordered EPS 421 Lecture 08 Classification of Conglomerates Conglomerates can originate by several processes (See next table). The present course is interested mostly in epiclastic conglomerates, which form by breakdown of older rocks through the processes of weathering and erosion. Major types Epiclastic conglomerate and breccia Volcanic breccia Cataclastic breccia Solution breccia Meteorite impact breccia EPS 421 Lecture 08 Fundamental Genetic Types of Conglomerates and Breccias Major types Epiclastic conglomerate and breccia Volcanic breccia Cataclastic breccia Subtypes Origin of clasts Extraformational conglomerate and breccia Breakdown of older rocks of any kind through the processes of weathering and erosion; deposition by fluid flows (water, ice) and sediment gravity flows Intraformational conglomerate and breccia Penecontemporaneous fragmentation of weakly consolidated sedimentary beds; deposition by fluid flows and sediment gravity flows Pyroclastic breccia Explosive volcanic eruptions, either magmatic or phreatic (steam) eruptions; deposited by air-falls or pyroclastic flows Autobreccia Breakup of viscous, partially congealed lava owing to continued movement of the lava Hyaloclastic breccia Shattering of hot, coherent magma into glassy fragments owing to contact with water, snow, or water-saturated sediment (quench fragmentation) Landslide and slump breccia Breakup of rock owing to tensile stresses and impact during sliding and slumping of rock masses. Tectonic breccia: fault, fold, crush breccia Breakage of brittle rock as a result of crustal movements Collapse breccia Breakage of brittle rock owing to collapse into an opening created by solution or other processes Solution breccia Insoluble fragments that remain after solution of more soluble material; e.g., chert clasts concentrated by solution of lime-stone Meteorite impact breccia Shattering of rock owing to meteorite impact Classification of epiclastic conglomerate ???? origin Extraformational I. Intraformational texture II. Orthoconglomerate III. Paraconglomerate Intact, grain-supported fabric Unstable, matrix-supported fabric composition A. Oligomict Only a few kinds of resistant rocks and mineral clasts A. Oligomict Orthoconglomerate B. Petromict A wide variety of Unstable rock and mineral clasts A. Laminated Matrix B. Petromict Orthoconglomerate Laminated Conlomeratic mudrock EPS 421 Lecture 08 B. Unlaminated Matrix 1. Tillite Glacial deposition 2. Tilloid (a) Debris flow (b) Grain flow CLASSIFICATION OF CONGLOMERATES AND BRECCIAS On the basis of sediment fabric, there are two types: clast-supported and matrix-supported conglomerates. The latter have been termed diamictites. Two particular types of breccia are slump breccias, consisting of broken and brecciated beds derived from downslope slumping, and solution breccias, resulting from the dissolution of evaporites and the collapse of overlying strata. EPS 421 Lecture 08 1. Epiclastic conglomerate and breccia 1. 1. Extraformational conglomerate and breccia Breakdown of older rocks of any kind through the processes of weathering and erosion; deposition by fluid flows (water, ice) and sediment gravity flows 1.2. Intraformational conglomerate and breccia Penecontemporaneous fragmentation of weakly consolidated sedimentary beds; deposition by fluid flows and sediment gravity flows EPS 421 Lecture 08 Epiclastic conglomerate Tectonic breccia Tectonic breccia Collapse breccia EPS 421 Lecture 08 Conglomerates Based on Composition Oligomictic Conglomerates: A conglomerate in which the clasts are made up of only one rock type. Polymictic Conglomerates A conglomerate in which clasts include several different rock types. EPS 421 Lecture 08 Oligomictic Conglomerates They are composed of only the most stable and durable kinds of clasts (quartzite, chert, veinquartz). Stable conglomerates composed mainly of a single clast type are referred to by Pettijohn (1975) as oligomictic conglomerates. EPS 421 Lecture 08 Oligomictic Conglomerates Most oligomictic conglomerates were probably derived from mixed parent-rock sources that included less stable rock types. Continued recycling of mixed ultra-stable and unstable clasts through several generations of conglomerates ultimately led to selective destruction of the less stable clasts and concentration of stable clasts. EPS 421 Lecture 08 Polymictic Conglomerates Conglomerates that contain an assortment of many kinds of clasts are polymictic conglomerates. Polymict conglomerates that are made up of a mixture of largely unstable or metastable clasts such as basalt, limestone, shale, and metamorphic phyllite are commonly called petromict conglomerates (Pettijohn, 1975) Almost any combination of these clast types is possible in a petromict conglomerate. EPS 421 Lecture 08 Oligomictic conglomerate Polymictic conglomerate A conglomerate in which the clasts are made up of only one rock type. Suggests that the source area was nearby or source rock extended over wide geographic area. A conglomerate in which clasts include several different rock types. Conglomerates that include clasts from a widevariety of source rocks, possibly derived over a wide geographical area or a smaller but geologically complex area. EPS 421 Lecture 08 Conglomerates Based on Origin Extraformational Conglomerates A conglomerate in which clasts are exotic (i.e., derived from outside the depositional basin). Clasts are normally very well rounded and well sorted. Intraformational Conglomerates A conglomerate in which clasts are derived locally from within the depositional basin (e.g., clasts composed of local muds torn up by currents; such clasts are commonly termed "rip-up clasts" or "mud clasts"). EPS 421 Lecture 08 Extraformational Conglomerate A conglomerate in which clasts are exotic (i.e., derived from outside the depositional basin). Clasts are normally very well rounded and well sorted. EPS 421 Lecture 08 Clasts derived from a distant source. Intraformational A conglomerate in which conglomerate clasts are derived locally from within the depositional basin (e.g., clasts composed of local muds torn up by currents; such clasts are commonly termed "ripup clasts" or "mud clasts"). Deposition in an environment where muds accumulated. Muds were in very close proximity to the site of deposition as the clasts would not withstand considerable transport. Mudstone fragments in river sands – derived from collapse of muddy river bank EPS 421 Lecture 08 Conglomerates Based on Fabric If clasts touch, 60-65% clasts by volume (depends on sorting and shape) Clast-supported ("orthoconglomerate") pores filled with: cement (openwork) matrix (closed-work) = bedload transport Matrix-supported ("paraconglomerate" or "diamictite") = suspension in turbulent flow, or glacial ice EPS 421 Lecture 08 Conglomerates Based on Texture-1 Orthoconglomerates Epiclastic conglomerates that are so rich in gravel-size framework grains that the gravel-size grains touch and form a supporting framework are called clastsupported conglomerates Paracnglomerates It is a Clast-poor conglomerates that consist of sparse gravels supported in a mud/sand matrix. they are called matrix-supported conglomerates (diamictite). EPS 421 Lecture 08 Conglomerates Based on Texture-2 Conglomerates and diamictites can be further divided on the basis of clast stability into quartzose conglomerate/diamictite and petromict conglomerate/diamictite on the basis of relative abundance of these clast types. Further classification on the basis of clast type (igneous, metamorphic, sedimentary) can be made if desired; however, such classification may not be necessary in many cases. EPS 421 Lecture 08 Classification of Conglomerates and Diamictites on the Basis of Clast Stability and Fabric Support Percentage of ultrastable clasts Type of fabric support Clast-supported Matrix-supported >90 Quartzose conglomerate Quartzose diamictite <90 Petromict conglomerate Petromict diamictite EPS 421 Lecture 08 Classification of conglomerate on the basis of clast lithology and fabric support. (From Boggs, 1992) EPS 421 Lecture 08