Sedimentary structures II

Biogenic Sedimentary Structures

Ichnofossils:

We have discussed trace fossils - records of organismal activity. To a sedimentologist, these are, effectively, sedimentary structures created by critters and are used to infer depositional environment the same as other sed structures. The most common are tracks and burrows. Properly known as ichnofossils, they are studied by ichnologists. These give useful clues to the depositional environment, in particular to water depth and chemistry.

"Taxonomy:" By tradition, identifiable trace fossil forms are given Linnean binomials: "ichnogenus" and "ichnospecies" (and "ichnofamily" and so on), E.G. Trychophycus pedum. Of course, unlike real organisms, they don't represent a nested hiearchy based on patterns of descent!! Indeed:

Really, unless we find the body fossil of a creature that literally "dropped dead in its tracks" or died in its burrow, we have no hope of definitively identifying ancient trackmakers.

So, if traces themselves don't evolve and speciate. What does the hierarchy of ichnotaxomony actually represent? There really are two distinct hierarchies:

Groups by behavior:

Examples: These are of interest to paleontologists for what they say about the behavior of their makers.

Ichnofossils as facies fossils:


Groups by environment:

Recall that a sedimentary facies is characteristic association of sedimentary rock features linked to a distinct sedimentary environment. Applying that concept to trace fossil assemblages, we get ichnofacies - characteristic assemblages of distinct ichnotaxa often found together in particular environments. These are given technical "ichnofacies" names based on some characteristic ichnofossil. The chart above shows the sequence near a rocky shore. Near a sandy shore, we see the following typical:

  • Psilonichnus:
  • Backshore, dune, and supratidal transitional environments. Common ichnofossils include vertebrate trackways and arthropod burrows (such as might be made today by ghost crabs.)

  • Skolithos:
  • Transitional intertidal zone to shallow marine, but above normal wave-base. Typical of environments with rapidly changing conditions (tides, storm-surges, etc.) in which critters may need rapidly to take shelter. Characterized by:

  • Cruziana Ichnofacies:
  • Shallow marine environments, but deep enough that they are below normal wave base, but may be above storm wave base. Typical ichnofossils are shallow horizontal troughs and tunnels: locomotion and feeding traces, including:

  • Zoophycos Ichnofacies:
  • Quiet continental slope and deep marine environments associated with lower oxygen levels, thick sediments, and higher organic matter. Typically characterized by muds and muddy sands. Typical ichnofossils are complex three-dimensional feeding traces of animals burrowing in thick sediment, including:

  • Nereites Ichnofacies:
  • The thin sediment drapes of abyssal plains, often associated with turbidites, deep pelagic muds, probably low oxygen. Typical ichnofossils are meandering feeding traces on bedding planes, superficial horizontal burrows, including:

    Ichnofossils as index fossils:

    Some trace fossils are of great importance as they are recognized as index markers for key boundaries in Earth history. One example is Trychophycus pedum (previously known as Phycodes pedum), the first appearance of which in a homogeneous and continuous succession of rock demarcates the Precambrian-Cambrian boundary.

    Insights from ichnofossils:


    Taucari trace fossil from Live Science
    Even though the trace-makers are enigmatic, the record of marine trace fossils documents the appearance and proliferation of burrowing animals during the Phanerozoic.



    Stromatolite in cross-section

    Stromatolites

    An organo-sedimentary structure produced by sediment trapping, binding, and or precipitation as a result of the growth and metabolic activity of micro-organisms, principally bacterial mats of cyanobacteria (blue-green algae). These have a variety of gross forms, from nearly horizontal to markedly columnar, domed or subspherical. Morphology is more likely to be ecophenotypic (i.e. an indicator of environmental conditions) than strictly genetic. Today, these typically form in carbonate-dominated environments lacking metazoans that might graze on them (because of hypersalinity, rapid current flow, etc.). The classic example: Shark Bay, Western Australia.

    Stromatolites appear ~3.0 Ga and are common until roughly 1 Ga. They nevertheless persist throughout the Phanerozoic.


    How we imagine the fauna of the Ediacaran from the Melbourne Museum