DOCSLIB.ORG

Primary Structures in Sedimentary Rocks

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Primary Structures in Sedimentary Rocks Structural Geology (Geo 381) Primary Structures In Sedimentary Rocks Prof. Essam Abd El-Motaal Mohamed Geological structures Nontectonic structures Primary structures Secondary structures During the formation of rocks After the formation of rocks e.g. Folds, Faults, Joints….etc. In Sedimentary In Igneous rocks rocks Primary igneous structures: occur in intrusive and extrusive molten bodies. Primary sedimentary structures: occur in clastic sediments & generated by same processes (currents, etc.) that caused deposition. Importance of Primary Structures in Sedimentary Rocks • Primary sedimentary structure = a syngenetic sedimentary structure determined by the conditions of deposition (primarily current velocity and sedimentation rate), developed before lithification of the rock in which it is found. • Usually form during or shortly after deposition. Significance: 1- Depositional environment. 2- Stratigraphic facing (younging direction i.e. way-up structures) to identify right-side-up or overturned beds. 3- Current direction. Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Bedding • Sedimentary rocks generally have bedding or stratification – Individual layers less than 1 cm thick are laminations • common in mudrocks – Beds are thicker than 1 cm • common in rocks with coarser grains Bedding (Stratification) • Bedding: The primary feature in sedimentary rocks is beds with different composition, texture, color, cement, differential erosion (make sure you recognize beds based on these criteria!). • Different beds represent different sources, sedimentary processes, and environments of deposition. Bedding (Stratification) Bedding (Stratification) Bedding (Stratification) Bedding planes Bedding plane: Nearly flat surface of deposition separating two layers of rock. Every layer has a lower bedding plane (lower surface ) at its bottom and an upper bedding plane (upper surface) at top. Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Fissility in shale Fissility in shale Fissility in shale - Closely-spaced parting in shale • Reorientation of planar flakes (clay minerals) due to post-depositional compaction (squeezing of unlithified sediment due to the weight of the overlying rocks). Clay Shale Clay minerals (Clay particles do not fit tightly) (Compacted clay) Fissility in shale Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Graded Bedding • A layer with a vertical change in particle size (coarse to fine at the top). • Most likely to occur in a turbidity current. Graded bedding • Progressive fining of clast grain size, from the base to the top of a bed; form as a consequence of deposition by turbidity currents (e.g., in turbidite). • Can indicate which way is up. • Flash flood deposits and turbidites are typically graded beds. • Note that debris flows - deposit inverse graded beds. Graded bedding Younger Y Older Graded bedding ? Graded bedding Older Y Younger Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Cross-bedding • Cross beds: Are surfaces within a thicker, master bed that are oblique to the bedding in the master bed. Eolian foresets with truncated tops (arrow),, concave-up morphology. Navajo Sandstone Cross bedding Cross beds formed on the leeward side of a sand dune (top) and on the downstream side of river-deposited sediment (middle). Cross bedding Cross bedding Cross bedding in Dunes Cross bedding from a Water Current Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Convolute structure Convolute structures are intraformational folds occur within a restricted horizon of a sedimentary sequence (one or a few beds only). Due to syndepositional folding or slumping of wet sediments, which may be triggered by gravity or earthquake activity. Convolute structure Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Channel Coarse channel deposit (conglomerate) Channel Channel Channel Channel Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Load-cast structures • Load casts disrupt bedding due to density inversion (sand over mud) Load cast Generation Load cast is similar to flame in terms of generation Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Injection (Flame) structures Less dense material intrudes into material above Caused by rapid loading of turbidite sands Injection (Flame) structures Injection Flame structure Generation Flame is similar to load cast in terms of generation Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Mud cracks Mud cracks in recently dried mud • Polygonal cracks formed in very fine-grained sediment as it dries. • Only form in environments where sediment is exposed to water. • Good facing indicator (individual cracks taper downward). Mud cracks Mud cracks Mud cracks Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Ripple marks Ripples forming on the shore. Ancient ripple marks exposed Sand moved by wind, streams, and coastal waves is often rippled. on the base of a sandstone Useful in determine ancient current directions Good way up indicator Ripple marks Symmetric and asymmetric Ripples Wave-Caused Ripple Marks (Symmetrical) Ripple marks Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Rain drops Rain Drops on Bedding Plane Rain drops Primary Sedimentary Structures Primary structures in sedimentary rocks On Upper Bedding Inside Bed On Lower Surface Surface Fissility in Shale Mud cracks Channels Ripple marks Gradded Load cast bedding Injection structures Rain drops Cross bedding Biogenic structures Convolute structure Dinosaur Trackways Burrows • Vertical, dark-colored areas in this rock are sediment- filled burrows Biogenic structures (Trace fossils) Leaf print Worm burrows Worm burrows Biogenic structures (Trace fossils) Foot print on ripple marks Biogenic structures (Trace fossils) Cretaceous Jurassic Biogenic structures Tracks – Abu Dhabi Snail Trails Burrows Stromatolites Y • Sharks Bay, Australia • Cyanobacteria grow upward toward the surface Stromatolites 3 Billion Year Old Algal Mounds (Stomatolites) Outcrop of a stromatolite “reef” from 1.6- Australia billion year old • These reefs were formed by colonies of photosynthetic “blue- green” algae, cyanobacteria and represent some of the first life forms on earth. Way-up Structures Determining way up using: • Inside bed features - cross-bedding, graded-bedding, convolute structure. • Bottom surface features - channel, load casts, injection (flame) structures. • Top surface features - ripples, mud cracks, raindrops, footprints, burrows, stromatolites. Which Way was Up? Sedimentary Structures are governed by: • Gravity • Exposure to the Surface They all have a “right way up” Way-up Structures Way-up Structures • Here, the mud crack “V” opens toward younger strata, and shape of current ripple marks • Indicate that the youngest layer is lower right. Way-up Structures.
Load more

Recommended publications
  • Sediment and Sedimentary Rocks
    Sediment and sedimentary rocks • Sediment • From sediments to sedimentary rocks (transportation, deposition, preservation and lithification) • Types of sedimentary rocks (clastic, chemical and organic) • Sedimentary structures (bedding, cross-bedding, graded bedding, mud cracks, ripple marks) • Interpretation of sedimentary rocks Sediment • Sediment - loose, solid particles originating from: – Weathering and erosion of pre- existing rocks – Chemical precipitation from solution, including secretion by organisms in water Relationship to Earth’s Systems • Atmosphere – Most sediments produced by weathering in air – Sand and dust transported by wind • Hydrosphere – Water is a primary agent in sediment production, transportation, deposition, cementation, and formation of sedimentary rocks • Biosphere – Oil , the product of partial decay of organic materials , is found in sedimentary rocks Sediment • Classified by particle size – Boulder - >256 mm – Cobble - 64 to 256 mm – Pebble - 2 to 64 mm – Sand - 1/16 to 2 mm – Silt - 1/256 to 1/16 mm – Clay - <1/256 mm From Sediment to Sedimentary Rock • Transportation – Movement of sediment away from its source, typically by water, wind, or ice – Rounding of particles occurs due to abrasion during transport – Sorting occurs as sediment is separated according to grain size by transport agents, especially running water – Sediment size decreases with increased transport distance From Sediment to Sedimentary Rock • Deposition – Settling and coming to rest of transported material – Accumulation of chemical
    [Show full text]
  • Sedimentary Rocks
    OCN 201 Coastal Sediments Lab Sediment Particle Size Distribution and Turbidity Flows Although this laboratory will pertain to oceanic sediments, similar processes can also be observed on land and in other aquatic systems (i.e., lakes, wetlands). This reading should supplement your understanding of the processes that affect particle size distribution across a marine system (i.e., barrier reef). Next week’s laboratory exercises will focus on demonstrating some of these principles, and give you experience in quantifying particle size distributions across a barrier reef. Sediments Sediment, by definition, is any loose or fragmented material. Hence, loose sand, shells and their fragments, dead leaves, and mud can all be categorized as sediment. All sediments have a source from which they originate. Pelagic sediments are those found in the open ocean, and whose origin cannot be traced to a specific landmass. They include red clay, radiolarian ooze, diatom ooze, and calcareous (nanofossil or foraminefera) ooze (see images of selected biogenic tests – see page 8 of Laboratory#5). Terrigenous sediments are those whose origin is traceable to a specific land (terra) area. They include a series of variously colored muds, volcanic debris, coral muds, and turbidity flow deposits. Lithogenous sediments are derived from weathering of rock (lithos) material, but their source cannot be readily identified. Red clay in the abyssal ocean is lithogenous. Much of the sediment on the sea floor of the open ocean is lithogenous clay that was transported thousands of miles from its origin. Calcareous sediments are found over oceanic rises and platforms, whereas red clays are typically distributed in the deep ocean basins.
    [Show full text]
  • Origin and Evolution Processes of Hybrid Event Beds in the Lower Cretaceous of the Lingshan Island, Eastern China
    Australian Journal of Earth Sciences An International Geoscience Journal of the Geological Society of Australia ISSN: 0812-0099 (Print) 1440-0952 (Online) Journal homepage: http://www.tandfonline.com/loi/taje20 Origin and evolution processes of hybrid event beds in the Lower Cretaceous of the Lingshan Island, Eastern China T. Yang, Y. Cao, H. Friis, K. Liu & Y. Wang To cite this article: T. Yang, Y. Cao, H. Friis, K. Liu & Y. Wang (2018) Origin and evolution processes of hybrid event beds in the Lower Cretaceous of the Lingshan Island, Eastern China, Australian Journal of Earth Sciences, 65:4, 517-534, DOI: 10.1080/08120099.2018.1433236 To link to this article: https://doi.org/10.1080/08120099.2018.1433236 Published online: 16 May 2018. Submit your article to this journal Article views: 9 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=taje20 AUSTRALIAN JOURNAL OF EARTH SCIENCES, 2018 VOL. 65, NO. 4, 517–534 https://doi.org/10.1080/08120099.2018.1433236 Origin and evolution processes of hybrid event beds in the Lower Cretaceous of the Lingshan Island, Eastern China T. Yang a,b,c, Y. Cao a, H. Friisc, K. Liu a and Y. Wanga aSchool of Geosciences, China University of Petroleum, Qingdao, 266580, PR China; bShandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266580, Shandong, PR China; cDepartment of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark ABSTRACT ARTICLE HISTORY On the basis of detailed sedimentological investigation, three types of hybrid event beds (HEBs) Received 13 September 2017 together with debrites and turbidites were distinguished in the Lower Cretaceous sedimentary Accepted 15 January 2018 – sequence on the Lingshan Island in the Yellow Sea, China.
    [Show full text]
  • Page -  Paleo Lab #4 - Sedimentary Environments
    page - Paleo Lab #4 - Sedimentary Environments 1. CHARACTERISTICS OF SEDIMENT Grain size and grain shape: The sizes and shapes of sedimentary particles (grains) are modified considerably during their transportation to the site of deposition. For example, abrasion of particles tends to round off jagged edges and corners, producing smaller grains with smooth, rounded surfaces and spherical shapes. Thus, a sand grain eroded from a hill in Wisconsin and carried down the Mississippi River will be smaller, better rounded, and more nearly spherical when it gets to New Orleans than it was when it passed Memphis. In addition, the higher the velocity (or energy) of the transporting agent (wind, water currents), the larger the grains that can be moved. An example of this is commonly seen in marine environments where, as the distance from shore increases, particle size of the sediments decreases due to the decreasing energy of water currents and increasing distance from the sediment source (i.e., land). Finally, continuous transportation tends to produce a well-sorted sediment in which all of the particles are more or less of the same size. A good example is a beach, where the constant swash and backwash of waves tend to produce well-sorted sand deposits. Mineral composition: Detrital grains (composed of particles derived from pre-existing rocks), which occur in a variety of sizes, (e.g., boulders, pebbles, sand, silt, and mud) may be found in almost any environment. The percentage of quartz present is commonly taken as a measure of the maturity (degree of weathering) of the detrital sediment prior to deposition.
    [Show full text]
  • Sedimentary Structures
    E-PG PATHSHALA IN EARTH SCIENCE Content Writers Template 1. Details of Module and its Structure Module details Subject Name Earth Science Paper Name Stratigraphy and sedimentology Module Name/Title Sedimentary Structures Module Id ES 251 Pre-requisites Before learning this module, the users should be aware of The types of sedimentary rocks Mode of formation of sedimentary rocks Sediment generation, Transportation and depositional processes Objectives Objectives: The aim of this lesson is to introduce the sedimentary structures. The objectives of learning this module are to highlight the following: a) Identification of various structures on sedimentary rocks b) Mechanism of structures formed in sedimentary rocks c) Pre and post depositional process of sediments. After attending this module, the learner will be able to explain about the various types of sedimentary structures and its importance in stratigraphic construction and geologic interpretation. Sedimentary structures are key to defining depositional environments, geological history, and surface processes. Keywords bedding, deformation, deposition, erosion, interbed, intrabed, sediment 2. Structure of the Module-as Outline : Table of Contents only ( topics covered with their sub-topics) 1. INTRODUCTION 2. PRIMARY STRUCTURES 2.1. PREDEPOSITIONAL SEDIMENTARY STRUCTURES 2.1.1.Channels 2.1.2.Sole marks 2.1.2.a. Scour-and-fill 2.1.2.b. Tool Marks 2.1.3.Flute Marks 1 2.1.4.Groove Marks 2.2. SYNDEPOSITIONAL (INTRABED) STRUCTURES 2.2.1. Bedding and lamination 2.2.1.a. Massive Bedding 2.2.1.b. Flat-Bedding 2.2.2. Graded Bedding 2.2.3. Cross-Bedding 2.2.4. Ripples and Cross-Lamination 2.2.5.
    [Show full text]
  • Sedimentary Structures Mud Cracks
    1 Summary of common chemical sedimentary rocks nLimestone (organic and inorganic), coquina, and chalk nChert nEvaporites nCoal - today 2 8) Coal (organic) LIGNITE Soft, brown coal; moderate energy 3 Table 6.2 of textbook. Rocks to know 4 SEDIMENTARY ENVIRONMENTS MARINE AND CONTINENTAL 5 Transitional (shoreline) environments Tidal flats, Deltas, Estuaries 6 Grand Canyon 7 Sedimentary facies 8 Sedimentary structures nProvide information useful in the interpretation of Earth history 9 Beds of ripple marks 10 “fossil” ripple marks 11 Symmetrical and assymetrical ripples 12 Formation of cross beds 2) Cross-bedding Strata (beds) are inclined at an angle to the horizontal bedding plane 13 Environments in which ripple marks and cross bedding occurs nSand dunes in deserts and beaches nSandy ocean beaches nPoint bars in river channels 14 Cross bedding in ancient sand dunes 15 Graded Bedding 16 Graded beds Particle size varies from coarse to fine 17 Mud cracks 1 18 What sedimentary environment is represented by these fossil mud cracks? 19 Fossils: Evidence of past life nBy definition, fossils are the traces or remains of prehistoric life now preserved in rock 20 Some types of fossils nOriginal mineral matter nAltered e.g. original mineral replaced by another mineral nCasts and molds nFoot prints nTrails nBurrows 21 Spider preserved in amber 22 Natural casts of shelled invertebrates 23 Dinosaur footprint in limestone 24 Animal trail 25 Geologically fossils are important for several reasons nAid in interpretation of the geologic past nServe as important time indicators nAllow for correlation of rocks from different places 26 Example: Using interpretations of sedimentary environments and fossil assemblages to understand the ancient climate of Antarctica 27 Map of Antarctica 28 When did the major Ice sheets come into existence? 29 The effect of melting about 75% of the West Antarctic 2 Ice Sheet 30 Lat.
    [Show full text]
  • Grain-Size Characteristics of Turbidites Geological Society, London
    Geological Society, London, Special Publications Grain-size characteristics of turbidites Kate Kranck Geological Society, London, Special Publications 1984; v. 15; p. 83-92 doi:10.1144/GSL.SP.1984.015.01.05 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by Oregon State University on 17 November 2010 © 1984 Geological Society of London Grain-size characteristics of turbidites Kate Kranck S U M M A R Y: Detailed sampling using a very small sample size and grain-size analysis with a Coulter Counter of three fine-grained turbidites enabled a distinction to be made between the well-sorted single-grain Stokes'-deposited and unsorted "whole suspension' floc-deposited grain-size populations. The results indicate that each turbidite is a continuous sequence deposited from the same source suspension. Particles settle to the bottom as flocculated masses. Initially flocs are broken up by near-bottom shear forces and only the coarsest silt and sand remains on the bed. The remaining mud forms a temporary mud suspension near the bottom which reflocculates and intermittently deposits at some critical concentration producing mud interlayers between silt laminae. Decrease in current velocity eventually allows simultaneous deposition of single grains and flocs with the latter becoming progressively more abundant resulting in formation of graded beds. Eventually, all deposition occurs in the form of mud flocs and a massive sediment results.
    [Show full text]
  • Chapter 6 Sediments and Sedimentary Rocks
    Chapter 6 Sediments and Sedimentary Rocks Learning Objectives After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to: • Describe the differences between cobbles, pebbles, sand, silt, and clay and explain the relationship between clast size and the extent to which clasts can be transported by moving water or by wind. • Describe the characteristics of the various types of clastic sedimentary rock, including the significance of differences in the composition of sandstones. • Explain the differences in the characteristics and depositional environments of various types of chemical sedimentary rocks. • Differentiate between various sedimentary depositional environments in both terrestrial and marine environments, and explain how the formation of sedimentary basins can be related to plate tectonic processes. • Apply your understanding of the features of sedimentary rocks, including grain characteristics, sedimentary structures, and fossils, to the interpretation of past depositional environments and climates. • Explain the importance of and differences between groups, formations, and members. 180 181 Steven Earle Figure 6.0.1 The Cretaceous Dinosaur Park Formation at Dinosaur Provincial Park, Alberta, one the world’s most important sites for dinosaur fossils. The rocks in the foreground show cross-bedding, indicative of deposition in a fluvial (river) environment In Chapter 5, we talked about weathering and erosion, which are the first two steps in the transformation of existing rocks into sedimentary rocks. The remaining steps in the formation of sedimentary rocks are transportation, deposition, burial, and lithification (Figure 6.0.2). Transportation is the movement of sediments or dissolved ions from the site of erosion to a site of deposition; this can be by wind, flowing water, glacial ice, or mass movement down a slope.
    [Show full text]
  • Sedimentary Structures
    Sedimentary Structures Sedimentary structures: Features preserved in sedimentary strata that indicate physical, biological, and/or chemical processes during and after deposition. These features may be indicative of sedimentary depositional environment. I. Primary Bedding – Tabular or lenticular layers of sedimentary rock having characteristics that distinguish them from strata above and below Beds are a function of and are distinguished by the composition, size, shape, orientation, and packing of sediment Beds are separated by bedding planes (well defined planar surfaces that separate one layer from another in sedimentary rocks, each plane marks a break in deposition) Massive Bedding – Beds of sedimentary rock contain few or no visible forms or structures. Graded Bedding – Strata characterized by gradual but distinct vertical changes in grain size. The figure above shows normal grading, where grain size gets smaller moving up section. Imbrication – Sedimentary structure consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, like toppled dominoes. Planar Bedding – Layers of sediment with flat, parallel bedding planes that were originally deposited nearly horizontal. Lamination is bedding that is <1 cm thick. Cross-bedding – One of the most common structures in sedimentary rocks, which is usually occurs in sandstone (but not always). This is strata in which internal layers dip at a distinct angle to the surface that bound the sets of cross-beds. Tabular cross bedding has bounding surfaces that are planar. Trough cross beds have bounding surfaces that are curved. To distinguish tabular and trough cross beds you must have an exposure that is cut perpendicular to flow direction.
    [Show full text]
  • Sedimentary Structures & Palaeocurrent Analysis
    LLAABB 55:: SSEEDDIIMMEENNTTAARRYY SSTTRRUUCCTTUURREESS && PPAALLAAEEOOCCUURRRREENNTT AANNAALLYYSSIISS Sedimentary Structures Sedimentary structures are an important attribute of sedimentary rocks because they can be used to deduce the processes and conditions of deposition, the direction(s) of the currents that deposited the sediments and even the original way-up of the strata. Structures developed through physical and/or chemical (inorganic) processes before, during and after deposition can be categorized as either depositional, erosional or post-depositional. Those formed by organisms are termed biogenic. Depositional structures occur on the upper surface of and within sedimentary beds. The simplest depositional structure is plane bedding, which is ubiquitous to sedimentary rocks. Finer-scale plane bedding (<1 cm in thickness) is known as lamination. Both are produced by changes in the pattern of sedimentation and may be defined by changes in colour, mineralogy or grain size. Bed/laminae boundaries may be sharp, irregular or gradational and can be modified by soft-sediment compaction, tectonic movements and the subsequent development of cleavage. Bed thickness is also an important parameter and is gauged as follows: Very thickly bedded 1 metre------------ Thickly bedded 0.3 metres--------- 10 centimetres---- Medium bedded 3 centimetres----- Thinly bedded 10 millimetres---- Very thinly bedded 3 millimetres----- Thickly laminated Thinly laminated Ripples, sand waves and dunes are bedforms which develop with increasing flow velocity, mainly in sand-sized sediments, limestones or sandstones, and even some chemical sedimentary rocks. Ripples are formed by either unidirectional currents or wave/wind action and are generally asymmetrical, with steep lee (downstream) and gentle stoss (upstream) sides. Sand waves and dunes form by the same processes as ripples, only on a much larger scale.
    [Show full text]
  • The Bouma Sequence and the Turbidite Mind Set
    EARTWSCIENCE ELSEVIER Earth-Science Reviews 42 (1997) 201-229 The Bouma Sequence and the turbidite mind set G. Shanmugam * Mobil Technology Company, P. 0. Box 650232, Dallas, 7X 75265-0232, USA Received 12 September 1996; accepted 28 April 1997 Abstract Conventionally, the Bouma Sequence [Bouma, A.H., 1962. Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168 pp.], composed of T,, T,, T,, Td, and T, divisions, is interpreted to be the product of a turbidity current. However, recent core and outcrop studies show that the complete and partial Bouma sequences can also be interpreted to be deposits formed by processes other than turbidity currents, such as sandy debris flows and bottom-current reworking. Many published examples of turbidites, most of them hydrocarbon-bearing sands, in the North Sea, the Norwegian Sea, offshore Nigeria, offshore Gabon, Gulf of Mexico, and the Ouachita Mountains, are being reinterpreted by the present author as dominantly deposits of sandy debris flows and bottom-current reworking with only a minor percentage of true turbidites (i.e., deposits of turbidity currents with fluidal or Newtonian rheology in which sediment is suspended by fluid turbulence). This reinterpretation is based on detailed description of 21,000 ft (6402 m) of conventional cores and 1200 ft (365 m> of outcrop sections. The predominance of interpreted turbidites in these areas by other workers can be attributed to the following: (1) loose applications of turbidity-current concepts without
    [Show full text]
  • Chapter 6: Sedimentary and Metamorphic Rocks NOTES
    Chapter 6: Sedimentary and Metamorphic Rocks NOTES (Can be accessed on notes section of website) 6.1 Formation of Sedimentary Rocks Main Idea: Sediments produced by weathering and erosion form sedimentary rocks through the process of lithification. Weathering (1) Sediments: small pieces or fragments of rock that created by weathering. There are two types of weathering: physical and chemical During physical weathering, minerals remain chemically unchanged. Rock fragments break off of the solid rock along fractures or grain boundaries. Plant growth, frost wedging, and animal activity can expedite this process. Chemical weathering is when minerals in a rock are dissolved or otherwise chemically changed (moving water or acid rain). When exposed to both chemical and physical weathering, granite eventually breaks apart and might look like this decomposed granite. Erosion (2) The removal and transport of sediment is called erosion. The four main agents of erosion are wind, moving water, gravity, and glaciers. After rock fragments and sediments have been weathered out of the rock, they often are transported to new locations through erosion. Eroded material is almost always carried downhill. Deposition (3) Deposition occurs when transported sediments are deposited on land or sink to the bottom of a body of water, forming layers with the largest, most dense grains at the bottom. As moving water slows down, some sediment deposits are sorted into layers of different-sized particles. The largest particles settle out first, then the next largest, and so on. Wind only moves small grains, so deposits made by wind are usually fine and well-sorted. Some sediment deposits contain particles of all sizes because they are dumped in unsorted piles when, for example, a glacier melts or there is a landslide.
    [Show full text]