Geology and Fossilization
Fossilization
Fossils are most often formed from organisms that have hard parts, such as bones and teeth. Because they are resistant to decomposition, they are more likely to survive the ravages of time than their softer counterparts, which tend to decompose rapidly. When an organism with such hard parts dies, it can be quickly buried, setting the stage for the long, slow process of fossilization. Once buried, the hard parts of the organism transform.
Over time, minerals from the surrounding sediment seep into the pores of the bones or shells. This mineral infusion can turn organic material into stone, preserving the original organism’s shape and structure. In some cases, the original material is completely replaced by minerals, creating a detailed stone replica. Some soft tissues can fossilize, though this is rare. Insects trapped in amber or mammoths preserved in ice are examples of how soft tissues can be fossilized under exceptional conditions.
Stratigraphy
Stratigraphy is a scientific subject that focuses on the description of rock layers and their interpretation in terms of the geologic time scale.
Stratigraphic studies are primarily done with sedimentary rocks but may also involve layered igneous rocks in certain situations, such as when they form layers due to successive lava flows, or metamorphic rocks formed either from such surface-cooled igneous material or from sedimentary rocks.
A common goal of stratigraphic studies is to divide a sequence of rock layers into mappable units, determine the time relationships involved, and correlate units of the sequence—or the entire sequence—with rock strata elsewhere.
Laws of stratigraphy
Law of superposition: younger layers of rock sit on top of older layers of rock.
Law of original horizontality: layers of sedimentary rock are originally deposited as flat layers
Law of cross-cutting relationships: Rock layers that are intruded by another rock must be older than the intrusion.
Law of later continuity: Lawyers of rock are continuous until they hit other solid boundaries that block their deposition or until acted upon by forces after deposition.
Why is stratigraphy important?
Stratigraphy is important for fossils because it can help us understand the geological processes that occurred in the area after a fossil was deposited. They can also help us to date fossils.
What is Marl?
Marl is common in post-glacial lake-bed sediments. It is a very fine-grained sediment–this means that the grains that make it up are small and pack together well. When a super fine-grained mineral like marl or clay mixes with water, it forms a mud or paste that is exceptional at preserving fossils because it can completely encase them and reach into all the cracks without damaging them.
Marl is usually a mixture of clay and calcium carbonate, making it a chemically stable compound that doesn’t change a lot over time. Calcium carbonate creates an alkaline environment, which helps buffer the material by keeping things in and out. When acids are produced during decomposition, the calcium carbonate reacts with them (instead of letting the acids sit and affect the organic material), stabilizing the environment’s pH. This alkaline environment also protects against bacteria and other microorganisms that might speed up decomposition.
Marl environments are often low in oxygen, which slows decay, leaving fossils intact for long periods. This is also why, upon exposure to air, uncovered fossils like this mastodon tend to decay (even crumble) quite quickly.