Modern Environments and Rock Cycle

• Alluvial Fans (Continental)

  • sediment not transported far so immature + comprises of more easily weathered minerals (feldspars)
  • angular grains and poorly sorted + sedimentary characteristics of braided stream deposits in a fan shape
• Braided Streams (Continental)

  • develop where flows are variable with strong seasonal variation; gravels deposited then plane-bedded sands then cross-bedded sands causing river to change direction creating braided pattern
  • sediment not transported far so immature, poorly sorted, and angular grains + less stable minerals present
• Meandering Streams (Continental)

  • currents not overloaded with sediment + gentler gradient areas - point bars on inside of bends have coarser sediment that grades upwards into finer sands
  • abandoned meanders filled with finer sediments (silts/muds) during floods + become colonised by plants (fossils probable)
• Flood Plains (Continental)

  • layers of silts/muds can build up into thick floodplain deposits; when dry, mud cracks/rain pits/footprints/rootlet beds may be present
  • some deposits are built on top of point bars so whole sequence fines upwards
• Moraines (Glacial/Continental)

  • the till deposited at edge of ice sheets as a hummocky ridge is a terminal moraine + ground moraines are smeared on the ground beneath the ice sheet
  • piles of till deposited on valley sides as lateral moraines + two lateral moraines join near the centre of the valley in a medial moraine

  
• Drumlins (Glacial/Continental)

  • where ground moraines are sometimes moulded by the ice into whale-backed features called drumlins

  
• Kames and Eskers (Glacial/Continetal)

  • meltwater can flow underneath the ice in winding channels to deposit ribbons of sand/gravel called eskers
  • meltwater from on/within the ice flows out to build irregular deposits of sand/gravel called kames

  
• Outwash Deposits (Glacial/Continental)

  • wide sweeps of meltwater flow away from the ice margin in braided outwash deposits and can accumulate in depressions to form glacial lakes
• Varves (Glacial/Continental)

  • lake sediments (from outwash deposits) often contain finely laminated silt layers called varves; they have a thin, dark layer deposited in the winter + thicker, pale layer deposited in summer
• Glacial Environments - when ice melts, it leaves till; a mixutre of angular pebbles and boulders in a matrix of finely ground rock (clay).
• Dunes (Continental)

  • transverse dunes are formed where sediments is abundant + show large-scale cross-bedding + primarily quartz with red surface iron staining
  • transported large distances by wind so sediment is mature, well-rounded and very well sorted
• Playa Lakes (Continental)

  • desert/playa lakes dry out and evaporite minerals form (halite and gypsum) + dinosaur footprints, ripple marks and mud cracks may be present
• Beaches (Coastal)

  • beaches often have coarsening upward fining sequences due to storms + high energy beaches deposit rounded pebbles
  • most beach sediments are mature, rounded and well-sorted + mainly quartz with some shell material
• Deltas (Coastal)

  • pro-delta has mainly mud and silt at the foot + an active sloping delta front formed mainly of sand
• Tropical Shallow Seas (Shallow Marine)

  • limestone forming conditions where seas are clear (no mud brought in by rivers) + limestones form from marine life (abundant where seas are warm/shallow/clear)
  • fine grained, well sorted + limestones can be clastic, biological or chemical
• Reefs (Shallow Marine)

  • sediments rich in calcium carbonate + ooids (spheres of calcium carbonate around 1mm - medium grained)
  • ooids are rolled around by waves/currents to become well-rounded + in low energy areas, fine carbonate mud is deposited
• Barred/Confined Basins (Shallow Marine)

  • barred basin; a sea with a narrow opening + has a shelf at entrance that restricts flow of water -confined basin; lack of circulation and oxygen
  • black stinking muds deposited here (forms source rocks for oil)
• Evaporite Basins (Shallow Marine)

  • ions in seawater become concentrated until gypsum or anhydrite crystallises out and is deposited, then halite and then potassium/magnesium salts (sylvite) form - creates thick salt deposits
• Shallow Marine

  • sediment comes from rivers/coasts (clastic sediments) - redistributed by waves/tides to form ranges of structures and various sizes
  • high energy areas where erosion exposes bedrock/gravels deposited by ice in past + sand/mud deposited in calmer/low energy conditions
• Deep Sea Turbidites (Deep Marine)

  • turbidity currents form due to build up of sand/mud at edge of continental shelves and are triggered by earthquakes
  • turbidite sediment sequence; massive sand grades up through plane-bedded sands to rippled sands then to laminated silt then mud (continued sediment build up)
• Deep Sea Clays (Deep Marine)

  • some ocean floors are covered by deep sea clays; these are red clays that originated as fine silt eroded from deserts or green clays formed from volcanic ash
• Deep Sea Oozes (Deep Marine)

  • where planktonic life is abundant near the surface, dead micro-organisms can accumulate on sea floor in huge numbers forming deep sea oozes
  • some are carbonate oozes (from hard parts of micro-organisms) + others formed from siliceous hard parts of radiolaria/diatoms
• Manganese Nodules (Deep Marine)

  • where ocean floor sedimentation is very slow, manganese nodules can grow (rounded lumps rich in iron, magnesium and other metals)