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.
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).
Conglomerate: rounded clasts Gravel-sized rocks Gravel Sized Detrital Rocks are Subdivided Based on Grain Roundness into: general term for lithified gravel that has been eroded and transported Breccia: angular clasts "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
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 size quartz, feldspars, rock fragments and heavy minerals. The matrix may be cemented with quartz, calcite, hematite, clay, or other cements.
Rounding of Gravel Particles Talus slope Angular = 5 km of transport River gravels Sub-rounded Beach gravels Effective rounding by waves
Rounding of Gravel Particles Little change in sphericity Rapid initial change in roundness
Surface Textures of Gravel Ventifacts wind sculpturing Striated clasts glacial grinding
Gravel Fabric a, b = clast axes p = parallel i = imbricated Gravel carried in suspension: streamlined transport with a-axis parallel to flow Imbricated gravel Gravel carried as bedload: rolling or sliding with a- axis transverse to flow 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
Gravel Fabric: Unordered
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
Major types Epiclastic conglomerate and breccia Volcanic breccia Cataclastic breccia Solution breccia Meteorite impact breccia Fundamental Genetic Types of Conglomerates and Breccias Subtypes Extraformational conglomerate and breccia Intraformational conglomerate and breccia Pyroclastic breccia Autobreccia Hyaloclastic breccia Landslide and slump breccia Tectonic breccia: fault, fold, crush breccia Collapse breccia Origin of clasts Breakdown of older rocks of any kind through the processes of weathering and erosion; deposition by fluid flows (water, ice) and a sediment gravity flows Penecontemporaneous fragmentation of weakly consolidated sedimentary beds; deposition by fluid flows and sediment gravity flows Explosive volcanic eruptions, either magmatic or phreatic (steam) eruptions; deposited by air-falls or pyroclastic flows Breakup of viscous, partially congealed lava owing to continued movement of the lava Shattering of hot, coherent magma into glassy fragments owing to contact with water, snow, or water-saturated sediment (quench fragmentation) Breakup of rock owing to tensile stresses and impact during sliding and slumping of rock masses. Breakage of brittle rock as a result of crustal movements Breakage of brittle rock owing to collapse into an opening created ed by solution or other processes Insoluble fragments that remain after solution of more soluble material; e.g., chert clasts concentrated by solution of lime-stone Shattering of rock owing to meteorite impact
Classification of epiclastic conglomerate???? Extraformational origin I. Intraformational II. Orthoconglomerate Intact, grain-supported fabric composition texture III. Paraconglomerate Unstable, matrix-supported fabric A. Oligomict Only a few kinds of resistant rocks and mineral clasts B. Petromict A wide variety of Unstable rock and mineral clasts A. Laminated Matrix B. Unlaminated Matrix A. Oligomict Orthoconglomerate B. Petromict Orthoconglomerate Laminated Conlomeratic mudrock 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 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.
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
Epiclastic conglomerate Tectonic breccia Tectonic breccia Collapse breccia
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.
Oligomictic Conglomerates They are composed of only the most stable and durable kinds of clasts (quartzite, chert, vein- quartz). Stable conglomerates composed mainly of a single clast type are referred to by Pettijohn (1975) as oligomictic conglomerates.
Oligomictic Conglomerates Most oligomictic conglomerates were probably derived from mixed parent-rock 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.
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.
Oligomictic 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. Polymictic conglomerate 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.
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").
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. 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
Conglomerates Based on Fabric If clasts touch, 60-65% 65% clasts by volume (depends on sorting and shape) Clast-supported supported ("orthoconglomerate orthoconglomerate") pores filled with: cement (openwork) matrix (closed-work) = bedload transport Matrix-supported ("paraconglomerate paraconglomerate" " or "diamictite") = suspension in turbulent flow, or glacial ice
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 clast- supported 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( diamictite).
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.
Classification of Conglomerates and Diamictites on the Basis of Clast Stability and Fabric Support Percentage of ultrastable clasts Clast-supported supported Type of fabric support Matrix-supported >90 <90 Quartzose conglomerate Petromict conglomerate Quartzose diamictite Petromict diamictite
Classification of conglomerate on the basis of clast lithology and fabric support. (From Boggs, 1992)