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Cretaceous Period—145.0 to 66.0 MYA

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fossil dinosaur track
Cretaceous age dinosaur track, Denali National Park & Preserve, Alaska.

NPS image

Introduction

In 1882 a Belgian geologist, Omalius d’Halloy, proposed the term “Cretaceous” for strata encircling the Paris Basin in France. The term derives from the Latin word for chalk (“creta”) and describes thick deposits of calcium carbonate (CaCO3) ooze and chert (SiO2). Indeed, the best known and most widespread deposits of chalks are of Cretaceous age, such as those exposed in cliffs on both sides of the English Channel. Though not on a par with the White Cliffs of Dover, England, Cretaceous-age rocks at Big Bend National Park in Texas contain chalk and are noteworthy for documenting the changes in sea level of the Cretaceous Interior Seaway.

Significant Cretaceous events

During the Cretaceous Period the first flowering plants appeared and rapidly diversified. Also, the Rocky Mountains began to rise from the Cretaceous Interior Seaway. However, the event that has caught the public’s imagination is the mass extinction that marks the end of one era with dinosaurs and begins another without them. Current thinking holds that 66 million years ago a large asteroid crashed into Earth near the Yucatan Peninsula of Mexico and caused the most famous, though not the most extensive, mass extinction.

Learn more about events in the Cretaceous Period

Angiosperms (flowering plants) appeared in the fossil record more than 100 million years ago during the Cretaceous Period. Once they appeared, they quickly became the dominant type of plant life on land and remain so today. The earliest angiosperms evolved from a specialized group of seed ferns. Since their first appearance, angiosperms have adapted to nearly every terrestrial habitat from mountains to deserts, and some have adapted to shallow coastal waters. A few varieties are known as “carnivorous” plants.

Not all flowering plants produce the kinds of blossoms you find in florist shops, but along with enclosed seeds, flowers are a key reproductive feature of angiosperms. The evolution of flowers, which attract pollinators, especially insects, and the evolution of a modified seed, with a protective coating and a ready supply of nourishment, enabled flowering plants to populate the world. Angiosperm seeds are dispersed widely in many ways: blown by the wind, transported by rivers and streams, carried in burrs that become attached to animal fur (or your socks), or embedded in fleshy fruit that is eaten and later “pooped out.”

Angiosperms have developed an incredible array of colors, scents, and fruits through their intricate and reciprocal relationship with the animal world. Without the abundance and variety of flowering plants (more than 90% of all land-plant species) known today, the world would be a much duller place.
Near the end of the Cretaceous Period, western North America was once again rising from the sea. During a time of mountain-building known as the Laramide Orogeny, a long series of repeated uplifts, periods of volcanism, and episodes of erosion occurred. The Laramide Orogeny continued into the Cenozoic Era and was a time of active tectonics and block-fault mountain building. The deformation and uplift of the Laramide Orogeny affected an area that had already been the site of large block uplift that formed the Ancestral Rocky Mountains during the Pennsylvanian Period. Preexisting Precambrian faults and shear zones largely controlled Laramide events.

In addition to the mountain uplifts, substantial Laramide deformation occurred in the bordering basins. These basins subsided concurrently with the uplift of the adjoining mountains. Each basin received sediments shed from the uplifted areas. These sediments constitute the principal record of the events of the orogeny. Prior to uplift, thousands of feet of Paleozoic and Mesozoic sedimentary rocks covered Precambrian crystalline rocks. Erosion, during and after the Laramide Orogeny, removed the sedimentary rocks and some of the crystalline rocks from the range. These sediments were deposited in the flanking basins where they are commonly covered by younger Cenozoic rocks.

The Laramide uplift occurred primarily in the middle and southern Rocky Mountains of Wyoming and Colorado, but deformation also took place far to the north and south, in the northern Rocky Mountains of Montana and Alberta, Canada. Some of the national parks in the United States that were affected by the Laramide Orogeny include Glacier National Park (Montana), Bighorn Canyon National Recreation Area (Montana and Wyoming), Rocky Mountain National Park (Colorado), Florissant Fossil Beds National Monument (Colorado), and Great Sand Dunes National Monument (Colorado).
During the Cretaceous, accelerated plate collision caused mountains to build along the western margin of North America. As these mountains were rising, the Gulf of Mexico basin subsided, and seawater began to spread northward into the expanding western interior. Marine water also began to flood from the Arctic region. These bodies of water joined to form the Cretaceous Interior Seaway.

Also referred to as the Western Interior Seaway, the seaway was an elongate basin, stretching 3,000 miles (4,800 km) between the Gulf of Mexico and the Arctic Ocean. During periods of maximum transgression the width of the basin was at least 1,000 miles (1,600 km), stretching across Utah into Iowa. The western margin of the seaway coincided with the rising mountain range to the west. Sedimentation into the Cretaceous interior basin from the uplifted area was rapid, and sediment loading led to downwarping of the basin along the western margin.

As the shoreline continued to advance into eastern Utah and western Colorado, the river-dominated system changed into a coastal plain with swamps, lagoons, and beaches. The inland sea advanced, retreated, and re-advanced many times during the Cretaceous Period until the most extensive interior seaway ever recorded drowned much of western North America.
Though not the largest, the most famous of all mass extinctions marks the end of the Cretaceous Period. As you may know, this was the great extinction in which the dinosaurs died out. Other lineages, including marine ichthyosaurs, mosasaurs, and plesiosaurs also went extinct by the end of the Cretaceous, as did the flying pterosaurs. However, some reptile groups such as ichthyosaurs were probably extinct a little before the end of the period. All told, 16% of marine families and 18% of land vertebrate families, including dinosaurs, became extinct.

Climate change and the resulting glaciations and changes in sea level were the causes of other mass extinctions, for example at the end of the Ordovician and Devonian periods. The cause of the Cretaceous-Tertiary mass extinction may at first seem a bit obscure, but as scientists have accumulated more and more evidence, opposition to the idea has dwindled. The main contender for the Cretaceous mass extinction event is a huge asteroid striking Earth about 66 million years ago.

Eleven other impact structures are known from the Cretaceous, but none rival the terminal event of the Cretaceous. The asteroid that hit Earth north of the Yucatan Peninsula in what is now the Gulf of Mexico was 6–12 miles (10–20 km) in diameter, resulting in a crater that is nearly 120 miles (190 km) wide, the second largest structure of its kind known. The asteroid would have been traveling at a speed between 22,000 and 45,000 miles per hour (10 and 20 km per second) when it entered Earth’s atmosphere. At such mass and speed, both the air and any water would have provided negligible resistance, and the resulting impact with Earth’s surface would have kicked up billions of tons of debris. The dust entered the atmosphere, presumably blocking out sunlight for an extended period of time, perhaps six months. When the dust literally settled, a layer that geologists call the “K–Pg boundary” (short for “Cretaceous–Paleogene boundary”) or “KT boundary” (short for "CretaceousTertiary boundary") had formed across the globe.

Source: The Glenn T. Seaborg Center.

K–T boundary

Though not up to date with current geologic-time nomenclature, the term “KT boundary” will probably stick around for quite some time. The letter “K” serves as the symbol for Cretaceous Period following the German terminology “Kreide” (“C” is actually used for the Carboniferous). The “T” represents the Tertiary. The International Stratigraphic Commission now uses Paleogene instead of Tertiary.

Rocks deposited during the Cretaceous and Paleogene (Tertiary) periods are separated by a thin clay layer that is visible at sites around the world. A team of scientists led by Luis Alvarez, a Nobel Prize-winning physicist, and his geologist son Walter discovered that the clay layer contains a strikingly high concentration of iridium, an element that is much more common in meteorites than in Earth’s crustal rocks. Like meteorites, asteroids also have relatively large abundances of iridium. Consequently, these scientists proposed that an impacting asteroid hit the Earth, generating the iridium anomaly and causing the mass extinction event. The discovery of high iridium concentrations in the clay layer around the world suggests the impact was a large one.

Though scientists are still looking, the KT boundary (iridium) layer has yet to be located in a national park. Big Bend National Park in Texas is a candidate because it hosts an uninterrupted suite of Cretaceous and Tertiary rocks. However, scientists have not identified the actual iridium-rich layer there.

Visit—Cretaceous Parks

Every park contains some slice of geologic time. Here we highlight a few parks associated with the Cretaceous Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Mesozoic

Last updated: September 11, 2020