F3 - Time and Change

• Uniformitarianism - principle first applied in 1788 by James Hutton - 'the present is the key to the past'; the processes we see acting on earth's surface today must have worked the same way in the geologic past.
• Facies - comprise all the characteristics of a rock that are produced by it's environment of deposition, and that allow it to be distinguished from rocks depositied in an adjacent environment; mineral content, grain size, sorting, structures, fossil content and other observable properties.
• • Facies association - a group of sedimentary facies that occur together and typically represent one depositional environment.
  • Lithofacies - include all the physical and chemical characteristics of a rock (eg. mineral composition/grain size).
  • Biofacies - include the palaeontological characteristics of a rock (fossil content).
  • Both related to depositional environment.
• Glacial processes and past environments - glacial environments exist in Artic climates (polar regions/high mountains) + characteristic sediments/structure:

  • fragments of rock at base of glaciers scratch striations on underlying surfaces
  • melting ice deposits boulder clay/till (poorly sorted)
  • sediments produced by meltwater streams are better sorted
  • in spring thaw, streams transport fine sand/silt/clay that settle at bottom of glacial lakes and remain there as ice freezes in winter
• Fossils - remains of once living organisms or evidence of their acitivity; must be older than 10,000 years (lived before end of last glacial period) or it is a sub-fossil.
• • Body Fossils - the remains of teeth, bones, hair, scales, horn, shells and vegetation (leaves, branches and trunks).
  • Trace Fossils - the preservation of burrows, tracks, footprints, trails, toothmarks, stone implements and gastroliths (stones from a reptile's stomach).
  • Coprolites - fossilised droppings, excreta or faecel pellets.
• • when an organism dies, the soft/non-mineral parts are rapidly destoryed; eaten by scavengers, decomposed by bacteria, or decayed (oxidised) to CO2 - unless prevented by rapid burial, acid conditions (inhibit bacterial decay) or anaerobic conditions (inhibit oxidation)
• • hard skeletal parts leave actual remains unless broken by wave action; majority of organisms destroyed before fossilisation by percolating groundwater, recrystallisation in metamorphism, development of slaty cleavage or erosion
• Mould - a negative impression left after the organism has decayed away.

  • a shell enclosed by/filled with a porous sediment may be dissolved by percolating groundwater leaving a negative impression of the outside pattern/ornamentation of the shell (external mould) + internal markings on shell from muscle scars etc. (internal mould)
  • moulds form when fossils are dissolved out of the rock they are in; leaves void in the rock and breaking rock reveals external mould of fossil + if fossil filled with sediment before burial it reveals internal structures made of soft tissue
• Cast - a model of the organism in the form of a mineral, produced by subsequent infilling of a mould by percolating waters with minerals in solution (eg. calcite, quartz and iron pyrite).

  • casts are formed when the void left by the dissolved shell is infilled by another mineral which crystallise from percolating solutions - casts show internal structures + casts are infilled during deposition
• Preservation potential of Vertebrates:

  • skeletons mainly bone made of collagen; a fibrous schleroprotein hardened by calcium phosphate - teeth coated in enamel + to a lesser extent hair, nails, claws, scales and horns are preserved
• Preservation potential of Invertebrates:

  • invertebrates; shells/skeletons are most common and organisms include insects (chitin, a leathery exoskeleton made of fibrous nitrogenous polysacharide)
  • graptolites (scleroprotein, fibrous insoluble protein) + molluscs include bivalves, echinoids, and gastropods (calcium carbonate + aragonite/calcite) + sponges/radiolaria (silica)
  • plants; cellulose (fibrous polysaccharide) and lignin (non-carbohydrate in woody tissue)
• Petrification/Permineralisation

  • most common method of preservation (means 'turning to stone') - the remains of the organism are dissolved away by percolating acidic waters, then the space created is infilled by minerals precipitated from solution by subsequent percolating waters
  • or spaces within the organic structure are impregnated with minerals precipitated from solution by percolating waters
  • common petrifying minerals; quartz, calcite and iron pyrite - fine ornamentation of shell/tissue may be replicated in new material
• Carbonisation

  • plants, trees and soft-bodied animals normally decay by oxidation and action of micro-organisms + in acid/anaerobic conditions (swamps/deel still waters) the plant material only partly decomposes leaving a thin carbon impression
  • after burial, compaction by overlying rocks (increases pressure and temperature) squeezes out volatiles (hydrogen, oxygen, methane, water and nitrogen) and increases carbon content to leave a residuel cabon film (usually within shale/mudstone)
• Replacement - a mineral in the original shell/skeleton is gradually replaced by another mineral; the original mineral is dissolved out by percolating groundwater, and the new minerals crystallises out from percolating ground water.
• Silicification (Replacement):

  • occurs when percolating groundwater rich in silica dioxide moves through the rock; the minerals dissolved in the groundwater crystallise out of solution anf fill any pores present
  • if the fossil has beeen dissolved away from earlier movement of groundwater, the silica dioxide fills the voids in the rock; if wood, shell or bone is present the pores are filled by a mineral increasing the denity, eg. petrified wood
• Pyritisation (Replacement):

  • the replacement of original material by iron pyrite; takes place when the environment is anaerobic and the only organisms are sulfur bacteria - common in deep sea environments with anoxic sea beds or shallow swamps
  • the bacteria use sulfur to respire (reduces sulfur to bisulfate); reacts with iron to form iron pyrite which replacs fossil material, eg. ammonites
• Calcification (Replacement):

  • the original fine calcereous structure of the shell is replaced by coarse calcite
• Preservation, habitat, temporal and collection bias:

  • fossil record incomplete as conditions required to create fossils rare ever since life began on earth (only small % of organisms that have ever lived became fossils)
  • fossil record is biased; organisms that live in more favourable fossilation condition more likely to be preserved + older fossils less likely to be found (more likely to be destroyed by erosion/weathering)
  • not all organisms fossilise well (soft organisms not likely to be preserved) + some organims studied more than others
• Preservation Potential - environmental conditions of ecosystems that affect the potential for an organism to be preserved.
• Conditions favouring fossilisation:

  • Abundance of the organism - greater the number, the greater the chance of preservation.
  • Rapid burial after death - limits decay, erosion by transport and destruction by scavengers after death.
  • Fossilisation in situ - organism not transported by wind, water, ice, gravity or scavengers; results in limited abrasion/better preservation (less likely to be broken).
  • Covered by argillaceous sediment - efficient seal, excludes oxyegn and limits decay + argillaceous sediment gives finest detail.
• Conditions favouring fossilisation:

  • Organisms that have hard parts - bone, teeth, shells, nails, claws, hair and chitinous exoskeletons.
  • Larger forms/adults most common as fossils - skeletons stronger than juvenile forms but smaller/younger organisms more rapidly covered by sediment.
  • Marine enviroment most suitable - abundance of organisms/rapid sedimentation rates; benthonic organisms greatest potential.
  • Limestone reefs - slightly alkaline conditions limit dissolution of shells by percolating waters.
• Conditions limiting fossilisation:

  • Littoral zone (beach environment) has low preservation potential - high-energy environment organismm rapidly broken up by abrasion/attrition in swash and backwash.
  • Terrestrial environments - organisms likely to decay, be scavenged or eroded unless rapidly buried (exceptions; peat bogs, tar pits, swamps and amber).
  • Metamorphism destroys fossils; at low grade they may be present but distorted/deformed with loss of detail.
• Factors that affect fossilisation:

  • original composition - fossils made of calcite/aragonite easily altered + hard parts made of silica (radiolarian micro-fossils and sponges) preserved unaltered
  • energy levels - high energy produces fragments due to collisions + low energy results in more complete fossils
  • transport distance - fossils fragmented in transport due to abrasion and collisions
  • rapidity of burial - faster burial means more chance of body fossils being preserved + less chance of scavengers eating them
• Factors that affect fossilisation:

  • amount of oxygen - oxygen accelerates breakdown of organisms due to bacterial decay
  • size of sediment - fine sediment preserves organisms better + poor quality fossils found in coarse sandstone/gravel
  • diagenesis - composition/acidity of percolating groundwater may dissolve/replace fossil with minerals + compaction may flatten fossils
• Modes of Life:

  Benthonic - organism lives on or in the sediment substrate on the sea floor.

  • Infaunal - organism lives in the sediment, usually in a burrow and many will filter feed eg. bivalve clam.
  • Epifaunal - organism lives on the sediment substrate, eg. bivalve oyster.
  • Vagrant - organims moves around on the sea floor and is usually a scavenger or a predator, eg. echinoid.
  • Sessile - organism doesn't not around on sediment substrate; some attached to sea floor, other lie on sea floor and most will filter feed, eg. bivalve mussel
• Modes of Life:

  Pelagic - organism lives in water column typically in the surface layers.

  • Planktonic - floats in the water column to wherever the current will take the organism (usually filter feeders), eg. graptolite.
  • Nektonic - actively swims in the water column and most are scavengers or predators, eg. ammonite.
• Derived Fossils

  • fossils may be derived from older rocks by erosion and included within younger sediments + show signs of abrasion/polishing with some loss of detail; unless known as a derived fossil, age of beds would be given as older than actual age
  • derived fossils orginally preserved in an older sedimentary layer, then eroded/transported and deposited in a younger sedimentary layer (different fossils give conflicting dates)
• Life Assemblages - a collection of organisms found within sediments in the same position as they would have been when they lived.

  • organisms preserved as they lived + no transport of bodies after death (preserved in situ) + large number of species preserved
  • range of individuals (juveniles/adults) + sudden event buried and preserved community of organisms + good quality (no erosion)
  • no current alignment (randonly orientated) + geopetal structures (determines way up of rock) + no sorting + articulated (whole or connected as in life)
• Death Assemblages - a collection of organisms found in a different place and position than they occupied in life.

  • fragmental shells sorted by size or aligned by a current + made up of disarticulated shells (fragments)
  • not preserved in situ (transported after death) + erosion, sorting and fragmentation by currents + alignment due to current action
  • abensence of juveniles/smaller organism + limited number of types of organism present + high quality preservation unlikely
• Lagerstatten - exceptional quality of preservation of organisms and quantity of organisms preserved; rare occurences that yield much information on the history and evolution of life (soft parts often preserved in great detail).