Sedimentary Rocks, Ancient Environments, Layers, and Time
Interpretation of sedimentary rocks:
Depositional environment: Places where sediments are deposited. For various practical reasons, geologists care about what kind of environment sediments that form sedimentary rocks are formed in. The physical characteristics and geographical location determines the type of sediment that will normally be deposited. In broad strokes, we classify depositional environments as:
Note the block diagram format and conventions.
In a more specific sense, we recognize several distinct major associations of sedimentary rock features and depositional environments:
- Continental: Deposited on land or in fresh water. Examples, deposited by:
- Transitional: Deposited in an environment showing influence of both fresh water or air and marine water.
- Deltaic: Deposits at the mouths of large rivers.
- Esturine: Deposits in valleys drowned by rising sea level.
- Lagoonal: Deposits in the waters separating barrier islands from the shore.
- Beach: Deposits in shallowest marine water influenced by waves.
- Marine: Only influenced by sea water.
- Shallow marine clastics: Regions near the mouths of rivers are usually clastic dominated because the critters that secrete CaCO3 tend to have trouble living in muddy water.
- Carbonate shelf: Regions with clear water shallow enough to be penetrated by sunlight are often dominated by the skeletons of marine organisms. Ancient reef.
- Continental slope: Dominated by the deposition of submarine landslides.
- Deep marine: Very thin sediments formed by the slow accumulation of skeletons and clasts dropped into the ocean by wind.
Energy in the depositional environment: The first key to the identification of environments is the relationship between energy and clast size. Generally, the higher the energy (that is, the faster the wind or water is moving) the larger the grains that can be kept moving. Thus:
The existance of strata gives us some powerful tools in understanding the rock record. First, if a single rock is like a snapshop of an ancient environment, layers of strata are like a movie.
- A swift mountain stream might keep even gravel sized objects in suspension, so its deposits would only consist of pebbles and boulders
- A large river might be so slow that it couldn't even keep sand in suspension, so its deposits would contain sand sized grains and larger.
- A body of almost still water, like a lake, migh have so little energy that even mud comes out of suspension and settles to the bottom.
Thus, the smaller the dominant clasts, the lower the energy of the depositional environment.
Sedimentary structures: The second key to the identification environments is small scale physical features they display. Some major ones.
- Cross bedding:- beds (parallel to each other) aligned at an angle to the surface upon which they accumulated
-result of the migation of dunes or ripples, either in stream or desert floor:
- Graded bedding: Mixture of grain sizes carried by a submarine landslide. As current slows down it drops the largest particles first and smallest particles last, resulting in a stratum with the largest clasts on the bottom. Example. Schematic.
- Surface features:
- Ripples: Formed by movement of a current over loose sand to clay sized sediment. Ripples formed by waves have a symmetrical cross section. Those formed by a stream current are asymmetrical. Ancient. Recent
- Rain drop prints Recent
- Mud cracks Ancient. Recent
- Tool marks: Formed when the current carries an object across the sediment. Ancient.
- Sole marks: Formed on the undersurface of a bed that conforms acts as a mold for the surface on which it is deposited. Ancient.
- Stratification - layering / bedding
-individual layers are called strata (sing. stratum) or beds
beds are lain down parallel, with younger beds on top of older ones: Ancient. Recent
The recognition that each stratum was laid down as a layer of sediment enabled Nicholas Steno to develop the following rules regarding their relative ages:
He recognized them to be composed of lithified remains of sediment deposited in layers (or strata) and proposed a set of several principles of stratigraphy by which one coud distinguish younger and older sediments. His results were published in Prodromus, In 1668. The two most significant principles are:
- Original horizontality. Sediments originally deposited in horizontal layers
- Superposition. In undisturbed strata, older layers lie beneath younger ones.
Using these, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association.
This work was furthered by James Hutton who added two additional principles:
And now the task: