GEOL 102 Historical Geology

Spring Semester 2018
The Late Mesozoic: The Cretaceous

Detail of reconstruction of global paleogeography at 90 Ma (Late Cretaceous Epoch) by paleogeographer Ron Blakey

Paleogeography and Geology of the Cretaceous:
Continued breakup of Gondwana (which has been intact since the breakup of Pannotia!). Isolated into Africa and a South America + Indomadagascar + Antarctica/Australia unit connected by now-submerged ridges.

As South Atlantic widens, beginning of Andean Orogeny in western South America (a long ongoing mountain building process, still active today).

The separation of Laurasia and Gondwana becomes wide enough to allow a Circum-Equatorial Current. Some water remains around the equator long enough to continue to warm and evaporate, eventually becoming dense and salty enough to drop to the ocean bottom. The hot bottom water warms the water above. Because warmer water contains less dissolved oxygen and carbon dioxide, the oceans become less oxic and starts become a source rather than a sink of carbon dioxide. So greenhouse warming during the mid-Cretaceous becomes a peak.

The mid-Cretaceous saw some major worldwide events:

During Late Cretaceous, beginnings of regression, but main regression event not until the Maastrichtian (last Age).

In warm Cretaceous seas, important new types of deposits:

In Early Cretaceous, continuation of Nevadan Orogeny.

During the mid-Cretaceous:

During Maastrichtian:

Cretaceous-Paleogene Boundary: impact of a huge (10 km diameter) asteroid at Chicxulub in the Yucatan Peninsula of Mexico.

Marine Life of the Cretaceous:
Radiation of the diatoms, benthic and planktonic forams, and (in Late Cretaceous) coccolithophorids. During Late Cretaceous: massive chalk deposits in epeiric seas.

Also radiations of advanced encrusting bryozoans, burrowing bivalves, predatory gastropods, echinoids, modern-style crabs, and huge diversity of teleosts during the Early Cretaceous. At the same time, huge decline in crinoids, articulate brachiopods, and the like: the Mid-Mesozoic Marine Revolution. Sea floor takes on modern appearance: lots of motile epifauna and infauna in the shallow waters, stalked forms limited to deeper ocean.

New sessile epifauna:

Ichthyosaurs rare in the Early Cretaceous, and die out before the Late Cretaceous. Plesiosaurs remain diverse, with both long-necked and short-necked forms. Rise of three new marine reptile groups:

Terrestrial Life of the Cretaceous:
Flowering plants (angiosperms) may have appeared in the Jurassic, but become more important in the Cretaceous. The basic angiosperm life cycle hinges on co-evolution with animals: Possible angiosperm body fossils are known from the Jurassic, and close relatives of the angiosperms go back to the Permian, but the oldest definite angiosperms are from the Early Cretaceous. Early Cretaceous angiosperm pollen and leaves are known from far off Prince George's County, Maryland, and similar fossils are known from earlier in the Cretaceous in China.

Angiosperms remain small herbaceous weeds for most of the Cretaceous, although during the Late Cretaceous some became arborescent. Some "gymnosperms" (including the Permian "seed-fern" Glossopteris and the bennettitalians) were more closely related to angiosperms than to other gymnosperms.

Insects of the Mesozoic:
Continued insect diversification throughout the Mesozoic, including:

The Cretaceous Terrestrial Revolution
The Cretaceous showed continued diversification of various groups of dinosaurs, pterosaurs, crocodilians, lizards (including snakes), turtles, mammals, amphibians, and freshwater fish. Although overshadowed by the giants of this time, one of the more profound changes was the Cretaceous Terrestrial Revolution (KTR): the diversification of primarily small-bodied forms such as herbaceous flowering plants, their pollinators, herbivorous beetles, frogs, and small predators (snakes, lizards, mammals). This occurred during the middle (late Early and early Late) Cretaceous.

The Cretaceous-Paleogene Mass Extinction Victims include:

Of course, many survivors as well.

Although many untestable hypotheses suggested (hunting by aliens, supernova radiation, etc.), three contributing factors have strong independent physical evidence:

The Maastrichtian Regression:
Draining of epeiric seas would alter terrestrial climate by:

Increased Maastrichtian volcanism, especially the Deccan Traps:

The Chicxulub Impact
1980: Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. Wanted to determine length of time represented by the clay layer. Consulted dad (Nobel winning physicist Luis Alvarez) for possible solution. Suggestion:

The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).

When examined Gubbio clay, found a huge increase in iridium (iridium spike) at base of clay: clearly not an "average" of infall.

Hypothesized: an asteroid impacted Earth at the K/Pg boundary

Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter, the likely consequence to a large-scale nuclear war first proposed shortly after (and suggested by) the Alvarez scenario


Biotic prediction fits most of the predictions; search for geological signature was on.

Shocked Quartz:

Melt Glass (Tektites):

Tsunami ("tidal wave") and ejecta deposits:


So, great evidence for an impact at K/Pg independent of extinction. Also, pattern consistent with proposed effects (although some versions of the superacid rain, global fires, and global super tsunamis do not have good evidence and are probably "overkill" scenarios).

Suggestions that all these systems were in effect:

Extinction of non-avian dinosaurs paves the way for the rise of mammals as the dominant group of terrestrial animals. Marine realm recovery represents survival of many groups, but less change in structure.

Here are some relevant videos:
The Biggest Thing That Ever Flew:

When Did the First Flowers Bloom?

To Lecture Notes.

Last modified: 17 April 2018

Detail from "Dead T. rex" by Douglas Henderson (1999)