An unconformity is created when these depositional environments change to a regime of no-net accumulation so that the deposition of sediments, which records time, ceases. In some cases, sediment accumulation simply stops, and more often erosion begins stripping rock layers away.

geology. : a part of a stratigraphic unit in which the fossil fauna or flora differs significantly from that found elsewhere in the same unit.

A lithofacies is a lateral mappable subdivision of a designated stratigraphic unit, distinguished from adjacent subdivisions on the basis of lithology, including all mineralogic and petrographic characteristics and those paleontologic characteristics that influence the appearance, composition, or texture of the rock ( …

facies. / (ˈfeɪʃɪˌiːz) / noun plural -cies. the general form and appearance of an individual or a group of plants or animals. the characteristics of a rock or series of rocks reflecting their appearance, composition, and conditions of formation.

Stratigraphy, scientific discipline concerned with the description of rock successions and their interpretation in terms of a general time scale. It provides a basis for historical geology, and its principles and methods have found application in such fields as petroleum geology and archaeology.

The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top. Based on this, layer C is oldest, followed by B and A.

But carbon-14 dating won’t work on dinosaur bones. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old. To determine the ages of these specimens, scientists need an isotope with a very long half-life.

As its name suggests, uranium-series dating uses the radioactive decay of uranium to calculate an age. When uranium decays, it goes through a series of decays until it eventually reaches a stable isotope. So, for example, uranium 238 will decay to uranium 234, which will decay to thorium 230.

The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth….

All lead does not come from decayed uranium. Lead is also be formed with the other heavy metals in a supernova, through various different processes. There are different isotopes of lead that are naturally occurring, forming from both supernovae and uranium decay.

To determine the age of a dinosaur fossil, carbon dating can never be used. By the time a dinosaur fossil was found, any carbon-14 the organism would have taken up and incorporated into its tissues during its life, would have decayed too much to be useful as a tool to determine its age.

Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. Histories of archaeology often refer to its impact as the “radiocarbon revolution”.

RADIOMETRIC TIME SCALE

You might not be surprised to learn that eating large doses of a radioactive substance leads to an increased chance of developing a cancer. Fortunately, while there’s plenty of reason to believe uranium is lethal in high doses, there are no known human deaths from “oral exposure” to uranium [source: Keith et al.].