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The Sands of Time: Determining the Age of Sedimentary Rocksby Allyson Mathis
Ammonites are important index fossils in Upper Cretaceous rocks. This species (Collignoniceras woollgari) is found in rocks that are 91-92 million years old. Photo courtesy of Jim Kirkland.

The phrase “the sands of time” is understood by almost everyone to mean the passage of time as in an hourglass. This antiquated device measured time based on the rate that sand passed through a narrow opening between two glass chambers. The sand itself wasn’t the time-keeper, but instead the size of the aperture and the quantity of sand in the hourglass allowed the measurement of an interval of time.

In geology, sand also isn’t a time-keeper that geologists can use to determine how old a rock is. But that doesn’t mean that geologists haven’t figured out ways to determine when sandstones and other sedimentary rocks formed. The methods that geologists use to determine how old sedimentary rocks are range from those that predominantly rely on common sense to highly technical techniques that depend on very precise analyses on individual grains of particular minerals and calculations using radioactive decay rates of uranium and other radioisotopes.

Common sense comes into play when geologists determine the relative age of rocks. Put simply, older rocks are at the bottom of a sequence of sedimentary rocks, and younger rocks are at the top since younger sediments must progressively bury older ones. Fossils also help geologists tell time, as certain animals and plants only lived during distinct time intervals, such as the Mesozoic (“middle life” or the age of the dinosaurs) between 252 and 66 million years ago. The most important of these are index fossils. Index fossils are to paleontology what bell bottoms were to fashion; i.e., distinctive, nearly ubiquitous, but only around for a short time period.

A volcanic ash near the top of the Morrison Formation. Geologists dated zircon crystals from this ash at 150.6 million years old. Photo courtesy of Jim Kirkland.

The youngest detrital zircon grains in a sedimentary rock constrain its maximum age as the sedimentary rock must be younger than the detrital zircons it contains. Photo courtesy of Karl Karlstrom.

Naturally-occurring radioactive elements like uranium and thorium (along with certain isotopes of potassium) provide the clocks that geologists use to determine when rocks and minerals formed. Radioactive decay occurs at known rates, so it serves as the pendulum (of sorts) for geologic dating techniques. The specifics of radiometric dating techniques are complicated, but they generally involve measuring the amount of the radioactive parent isotope and the daughter element (e.g., the element formed by radioactive decay) in rock or mineral samples to determine when igneous rocks or minerals first crystallized, such as during a volcanic eruption or within the Earth’s crust in plutonic rocks such as those that form the La Sals Mountains. These analyses show that the La Sals formed between 29.1 and 27.7 million years ago.

But most rocks found near Moab are sedimentary, meaning that they are made up of sediment which is mostly recycled rock fragments such as sand grains and other particles that were eroded from older rocks. Most sand grains are made up of quartz (silicon dioxide; SiO2). But neither silicon or oxygen have radioactive isotopes that can be used to determine the age of sand. As a result, geologists have no way of determining the age of the majority of grains in most sedimentary rocks.

The good news is that sometimes there are other sedimentary grains that geologists can date using radiometric methods. The most important are those made of the mineral zircon (ZrSiO4) which crystallizes in some igneous rocks and contains small amounts of uranium which means that geologists can use it to tell time. Detrital zircons are zircon crystals that were eroded from igneous rocks and later deposited in sedimentary rocks. These grains can provide important age information for sedimentary rocks, especially those that do not contain fossils. A limit of this technique is that detrital zircons can only be used to infer the maximum age of a sedimentary rock, and don’t date sedimentary deposition.

Former Moab Museum Director John Foster on a volcanic ash layer in the upper part of the Morrison Formation near Blanding. Photo courtesy of Jim Kirkland.

The most direct way for geologists to determine the numeric age of a sedimentary rock is when they contain volcanic ash deposits. Volcanic ash itself is igneous in origin. It consists of glass fragments and crystals ejected from volcanoes in explosive eruptions. Volcanic ash can be incorporated in sedimentary rocks in one of two ways. Ash from an erupting volcano can fall directly into a sedimentary basin. Secondly, ash deposits may be reworked by rivers and streams and then incorporated in sedimentary rocks. Ashfall layers in sedimentary rocks are the same age as the enclosing sediments as they were deposited concurrently. Reworked ash layers provide less precise ages because an unknown time interval may have passed between eruption and the ash being incorporated into a sedimentary rock (although it is usually a short period of time).

Geologists can date feldspar or zircon crystals present in ash deposits to determine the age of sedimentary rocks that contain them. Ash layers have been dated in some rock layers found near Moab. For example, the Morrison Formation was deposited during a time when volcanoes were erupting in the region. A number of ash beds have been found among the Morrison’s sandstones and shales, including the 150.6-million-year-old ash bed found near the top of unit. (See the volcanic ash photos above.) While the Morrison Formation is best known for the dinosaur fossils that it contains, it is the inconspicuous ash beds that have allowed geologists to determine its precise age.

Hourglasses in the ancient world were often ornate and beautiful works of art. But the most important timepieces in sedimentary rocks are usually not the most spectacular fossils nor most beautiful outcrops. Instead, it is the lowly ash beds, detrital zircons, and index fossils that tell time.

Special thanks to Jim Kirkland, Utah State Paleontologist.

You can read more geology articles HERE

A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab. A native of suburban South Florida, Allyson now lives by the motto: give me sandstone and open spaces.

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