sedimentary environments

• Geopetal structure - bedding planes are assumed horizontal when deposited. When a hollow shell fills with water, water lies horizontally. The sediment settles in the bottom of the shell - can be a calcite crystal. 
• Sedimentation:

  • Varves - each laminae has two parts, fine sediment from still water - dark (winter) and coarser sediment in active water- light (summer) 
  • Formed in Periglacial lakes 
  • Used for dating - each pair is equal to one year 
• Sedimentation:

  • At any one point in time the three rocks were being deposited in different places 
  • Ammonite zones show that different rock types are the same age 
  • Beds are diachronous 
• Diachronous - a bed deposited across a range of time periods. 
• Deltaic environments:

  • Layers of sediment become diachronous   
• Walther's law - a vertical succession of facies to represent environments that once existed side by side but have migrated throughout time. Assumes no unconformities 
• Cyclotherm - delta sequence from carboniferous, provide coals. 

  • Changes in sea level control delta development 
  • A change may result in a change of deposition 
• Rises in sea level:

  • Deposition of marine sediments on top of the delta topsets 
  • Deposition of a new delta front sands on the topsets 
  • A rise is more likely to be preserved in geological succession 
  • NB subsidence of the delta has the same effect as rise 
• Falls in sea level:

  • Results in erosion followed by deposition of new topsets on top of earlier front sands 
  • No as obvious as sequence not altered