Sedimentary structures smallas


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Sedimentary structures smallas

  1. 1. Sedimentary Structures I.G.Kenyon
  2. 2. Give information about thedepositional environment Allow the ‘way-up’ of beds to be ascertained
  3. 3. A Bed A layer of rock separated from the layer above and below by a bedding plane A bed represents a single unbroken episode of sediment accumulationBeds vary in thickness from 1cm to many tens of metres Beds 2 to 5cm thick are called flags or flagstonesBeds may occur in uniform thicknesses over large areas or pinch out laterally
  4. 4. Beds & Bedding Planes, Blue Hills, Near St.Agnes £1 coin for scale Bedding Planes One bed
  5. 5. Bedding Plane Defines the top or bottom of a bed Represents a change in the nature of sedimentation a change in the rate or type of sedimentationa pause where no sediment is deposited a period of erosion where some sediment is removed
  6. 6. Lamination A layer of sediment <1cm thick Common in argillaceous rocks such as siltstone and shale Individual laminations may be just 1mm thick or even lessIf the sedimentary unit is >1cm thick it is a bed
  7. 7. Laminations in Devonian Mylor Beds, Porthleven Laminations here are <1mm thick Difference in colour explained by variation in amount of organic/carbonaceous matter incorporated into the sediment 1cm Mineral content mainly clay minerals such as kaolinite, illite and serecite
  8. 8. Competent Beds A bed of rock, which during folding,flexes and bends without appreciable flow or internal shear to maintain its original thicknessMechanically strong rocks such as limestones and sandstones commonly show this behaviour
  9. 9. Incompetent BedsA bed of rock that deforms internally during folding, resulting in rapid changes in lateral thicknessMechanically weak rocks such as clays, mudstones and shales commonly show this behaviour
  10. 10. Competent and Incompetent Beds at Millook, Near Bude Shale deforms and thins on the fold limbs 1m Tension cracks occur in sandstone around the nose of the foldShale is much thicker inthe nose/core of the fold Sandstone is competent, retaining original thickness in the limbs and nose of the fold
  11. 11. The Law of SuperpositionFirst proposed by Nicolaus Steno in the 17th CenturyIf one bed of sediment lies on top of another, then the one above must be the younger This assumes the beds have not been overturned due to earth movementsSedimentary structures collectively knownas ‘way-up criteria’ can be used to decide if the beds have been overturned or not
  12. 12. Graded Bedding A bed which displays a fining upwards sequence from the base. 3cm 3cmThe fining upward sequence may be produced in several ways
  13. 13. The Formation of Graded Bedding 1 Progressive settling of grade sizes from coarse to fine in comparatively calm bodies of water Example-greywackes on the continental slope,where a poorly sorted sediment is deposited rapidly The larger, denser rock fragments and sand size particles sink first, followed by the smaller and lower density silt and clay particles Greywackes are deposited by turbidity currents which are often initiated by minor seismic events
  14. 14. Formation of Graded Bedding by Turbidity Currents
  15. 15. Graded Bed with an Erosional Base Fining upwards Irregular surface with laminations of shale beneath truncated in places Represents an abrupt change from the much finer grained sediment underneath
  16. 16. The Formation of Graded Bedding 2 Variations in the seasonal supply of sediment, for example deposition from glacial meltwater in a pro-glacial lake In Spring/Summer much meltwater isavailable and coarse sand and gravel may be transported and deposited in the lakeIn Autumn/Winter, the meltwater will be greatly reduced, the lake may even freeze over allowing only the finer silt and clay to settle out from suspension
  17. 17. Millstone Grit showing Graded Bedding Deltaic deposit with seasonal fluctuations in energy conditions Particles 0.5 to 1.0mm at the top Fining upwards sequence 1cm Particles mainly 2-4mm at the base
  18. 18. The Formation of Graded Bedding 3 Seasonal variation in river discharge-in Winter coarse sand and gravel may be deposited during high discharges, in summerfiner sand and silt may be deposited when low flow conditions occur. Example Millstone GritThe stirring up of bottom sediments by storms and their subsequent differentiation on settlingThe stirring up of already deposited sediment by submarine slumping and sliding by turbidity currents followed by gravity settling
  19. 19. Cross BeddingAlso known as Current Bedding and False Bedding If very large scale it is termed Dune Bedding If very small scale it is termed Cross Lamination In each case the sediment is beingmoved and accumulated at an angle to the principal bedding direction Produced by a uni-directional current ofwind or water moving sediment as a series of asymmetrical ripples or dunes
  20. 20. The Formation of Cross Bedding Topset beds are truncated Erosion surface Foreset beds 2m Bottom set beds are preserved Layers curve in towards the horizontal (asymptotically) at the base of a cross bedded unitErosion surface-truncated topset beds Foreset beds 10cm Bottom set beds
  21. 21. Dune Bedding – Large Scale Cross Bedding 2m
  22. 22. Large Scale Cross Bedding – Dune Bedding Topset beds are truncated Palaeo-wind direction indicated by yellow arrows Foreset and bottom set beds preserved People for scale
  23. 23. Herring Bone Cross Bedding Upper Unit Middle Unit Lower Unit Penknife for scaleRepresents a current reversal through 180°. Blue arrows indicate the direction of sediment movement in each of the 3 units above
  24. 24. Cross Lamination (Very small scale cross bedding) Truncation/erosion surface of topset beds Current direction Approximate base of Pen top cross laminated unit for scale Individual laminations 2 to 4mm thick Fine sandstone unit, Compass Point near Bude
  25. 25. Convolute Bedding/Slump Bedding 1 Common on deltas where sediment is saturated with water and easily mobilised Occurs frequently in interbedded sandstone and shale sequences Shales deform internally and flow showing incompetence Sandstone layers break into rigid blocks which become displaced and Incompetent shale Competent sandstone show competence
  26. 26. Convolute Bedding/Slump Bedding 2 Incompetent shaleOften initiated by which has flowed or deformed internally a minor tectonic disturbance or slope failureCan also be formedby the rapid expulsion of pore water The example here isfrom the Carboniferous beds at Compass Point near Bude Rigid, competent sandstone blocks
  27. 27. Included/Derived FragmentsYounger upper series Older beds may be Derived fragments from eroded before the older lower series deposition of the next bed in the sequence The eroded fragments unconformity are then included as clasts in the bed above 1m Younger upper series Lower older series Derived Lower older series fragments
  28. 28. Imbricate Structure Common in rudaceous rocks Deposited under the influence of a powerful current Long axes of clasts lie sub-parallel with one another ‘leading’ in direction of current flow Arrows indicate direction of flow
  29. 29. Mud CracksFormed when sediment isexposed to the atmosphere Common in tidal flats, Note how the edges curl mudflats and playa lakes up to accentuate the V shaped gap between them Mud cracks form as desiccation polygons The sediment dries outand shrinks as water is evaporated from itContraction centres developand a polygonal pattern of cracks develop Analogous to columnar jointing in cooling lavas 30cm
  30. 30. Mudcracks The mud cracks are widest at the surface tapering to a point at a depth of 0.5 to 2.0 cm Often later infilled with finer, wind blown sediment of a different colour or calcareous material if in a playa lake
  31. 31. Mud Cracks and Rain Pits Rain pits formed by impact of raindrops on an exposedsediment surface. They appear as small rounded depressions up to 1cm in diameter, sometimes with a small raised rim. Rain pits mark the top of the sediment
  32. 32. Wash-Out in Fine Grained Sediment Scremerston, Northumberland Coin for Small scale channel – base scale is convex downwards Older laminations truncatedFormed as a result of Scour and Fill
  33. 33. Load Casts and Flame Structures Common in sandstone and shale sequencesLocally, the denser sandstone sinks down into the less dense shale below as bulbous protrusionsThe shale is incompetent and deforms/flows upwards into the spaces between the bulbous sandstone protrusions The rounded protrusions mark the base of the sandstone bed, whilst the flames mark the top of the shale bed Sometimes a globule of sandstone becomes completelydetached from the bed above and sinks into the shale below distorting the laminations to form a teardrop structure
  34. 34. Load Casts and Flame Structures Competent sandstone Incompetent shale
  35. 35. Load Casts and Flame Structures Bulbous protrusions from base of overlying sandstone bedCompetent sandstone Incompetent shale Shale squeezed up between sandstone protrusions as a flame structure
  36. 36. The Life Position of Fossils Organisms preserved in life position such as trees can indicate if the beds are the ‘right way up’
  37. 37. The Life Position of FossilsAlgal mounds are convex upwards Top Convex upwards growing towards the light 4cm Bottom Modern day stromatolites Stromatolites 2.5 billion years old from Cordoba Provence, ArgentinaOrganisms preserved in life position such as stromatolites (algal mounds) can indicate if the beds are the ‘correct way up’
  38. 38. The Life Position of Fossils 3cmThe Great Barrier Reef, Australia Corals preserved in limestone Organisms preserved in life position such as corals can indicate if the beds are the ‘correct way up’ Corals indicate clear water less than 50m deep, well oxygenated envi. with normal salinity (3.5%), Living corals form the upper part of the reef temperatures 22-28°C and located within 30° latitude of the equator
  39. 39. Sole Structures Formed in interbedded sandstone and shale sequencesThese are preserved on the base of the overlying sandstone bed Main processes are scouring and erosion of the soft shale accumulation surface by currents and tools Classified according to shape They include Flute, Groove, Bounce and Prod/Tool casts
  40. 40. Flute Casts on the underside of a Greywacke BedCowpeel Bridge, Peebleshire, Scotland 10cm Palaeo-current Direction
  41. 41. Flute CastsPlan View Cross section showing scouring of fluted hollows in soft mud by current vortices
  42. 42. Groove cast on the under surface of aGreywacke bed, Hartland Quay, North Devon Formed by a pebble rolling across a soft sediment surface and cutting a groove into it. Preserved as a cast on the under surface of the overlying bed Pen top for scale Possible palaeo-current directions
  43. 43. Prod/Tool Casts on the underside of aGreywacke Bed, Hartland Quay, North Devon Possible palaeo- current directions 5cm
  44. 44. Bounce, Groove and Prod Casts1cm Prod Cast Bounce cast formed by a saltating fish vertebra Groove cast
  45. 45. Geopetal Structures Partially infilled shells of marine organisms Also known as ‘fossil spirit levels’ They indicate how much tilting has taken place since deposition When mud originally entered the cavities it would have settled horizontally due to the influence of gravitySubsequent tilting results in the level of mud being moved to a new inclination and different from today’s horizontal If the cavities are empty or completely filled with sediment, then they cannot be used as geopetal structures
  46. 46. Geopetal Structures – Brachiopods in Reef Limestone Only partially infilled brachiopod shells can be used as geopetal structuresEmanual Range Western Australia
  47. 47. Ripple Marks-Symmetrical Mark the top of the bed and imply the sediment was under the influence of wave action Minibus key for scaleCarboniferous sandstones, Compass Point near Bude
  48. 48. Concretions 30cm Concretion within a fine sandstone bed, Compass Point, Near BudeA roughly spherical or ellipsoidal body produced as aresult of early localised cementation within a sediment.Often found with a fossil as the nucleus of the concretion
  49. 49. Trace Fossils - Burrows ‘Right way up’ Organisms such as bivalves and marine Open at the surfaceworms burrow from the surface downwards Tapers to a point into unconsolidated soft sediment The burrows are openat the ancient sediment surface and taper downwards to a point ‘Right way up’
  50. 50. Trace Fossils – Horizontal Burrows Limestone showing tube-like traces of burrowing animals, Port Issol, France
  51. 51. Trace Fossils-Trails and Footprints Trilobite trail Winding trails-Repichnia Cruziana and Pasichnia Represents the upper sediment surface over which organisms walked or crawledSauropod footprints
  52. 52. Relationships between types of TraceFossils and Sedimentary Environments
  53. 53. Halite Pseudomorphs 1cm Halite pseudomorph, the original crystal has been dissolved away and the mould has been infilled by mudPreserved on the base of the overlying bed as a cast
  54. 54. The End