GEOL 102 Historical Geology
Spring Semester 2017
The Mesozoic Era II: Cretaceous Geology
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:
- Increased rate of sea-floor spreading along mid-ocean ridges
- Decreased rate of magnetic reversals (mid-Cretaceous is marked by the Long Cretaceous Normal, 40 Myr without reversals)
- Highest transgressions of the post-Pangaean world, rivaling the highest Cambrian highstands
- Many continents divided up by epeiric seas: North America divided in three by Western Interior Seaway; European Archipelago isolated from Asian mainland; Africa is divided into three; etc.)
- Black shales and anoxic waters periodically flood continental shelves and epeiric seas: black shales may be generated in part by eutrophication from continental runoff of nutrients, and in part by sluggish oceanic circulation (and thus more anoxia)
- Globally warm and mild climates
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:
- CHALK!!! -- coccolithophorids appear in Jurassic, but take off in producing chalk during mid-to Late Cretaceous. The Chalk of Europe, Niobrara Chalk of the Western Interior Seaway, etc..
- Diatomites: diatoms first appear and become common in Cretaceous
- Foraminiferal ooze: planktonic foraminiferans arise in Cretaceous
- Rudist reefs (particularly in Tethys and along equatorial regions):
rudists are reef-forming bivalves.
In Early Cretaceous, continuation of Nevadan Orogeny.
During the mid-Cretaceous:
- Increased speed of sea-floor spreading means subduction along Pacific margin of
North America at a lower angle
- Various small microplates swept up by western margin of North America
- Subducting Farallon Plate reaches melting point are regions further eastward
- Eastward migration of mountain range from Washington/Oregon to Idaho
- This new style is called Sevier
Orogeny: lasts until near the end of the Late Cretaceous
- Within forearc basin, many regional transgression-regression events
- Slowdown of seafloor spreading rate
- Huge Maastrichtian Regression: Western Interior Seaway drains to near
modern Gulf Coast
- Beginning of Laramide
Orogeny in Cordilleran system: foundering Farallon Plate brings uplift of region,
some volcanism as far east as Colorado, Wyoming, New Mexico: continues well into Tertiary
Deccan Traps: massive flood basalt episode in western India, third largest in
Phanerozoic (begins during last magnetic normal (C30n) interval of Cretaceous, so no closer than
350 kyr from 65.51 Ma (which is during C29r, a reversed chron)
- Cooling, more continental climates
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:
- Inoceramids: huge scallop-relatives
- Rudists: reef-forming bivalves with one large cone-like shell and one flat cap-shaped shell, or in the shape of "longhorns". Displace scleractinians as dominant reef-builders throughout the Late Cretaceous. Almost certainly had symbiotic algae (as do scleractinians, giant clams, etc.)
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:
- Sea turtles: including some up to 4 m long
- Mosasaurs: sea-going true lizards, with internal gestation. Some were ammonoid eaters, some fish eaters, others at marine reptiles
- Marine birds: some were fully aquatic, with wings reduced to tiny spines.
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.
- Bright colors, attractive smells, and interesting patterns on the flowers attract pollinators (typically flying insects). These move pollen (containing the male sex cells) to flowers of other plants (where the female sex cells are)
- Fruit remains bitter, hard, and dull colored until the seeds are ready to grow. At that point, the fruit becomes brightly colored, fleshy or nutty, and sweet and juicy. The fruit is then eaten by a vertebrate, which leaves the area and deposits the seeds in its dung.
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 first flies (
Diptera) in the Triassic
- The first moths & butterflies (
Lepidoptera) questionably in the Jurassic, and definitely in the Cretaceous
- The first wasps (
Hymenoptera) in the Jurassic, evolving into bees & ants in the Cretaceous
- Diversification of beetles (
Coleoptera), roaches, mantids & termites (
Blattaria), and others during the Cretaceous
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
- Many species of coccolithophorid: never recover diversity
- Many species of foram (although these do recover)
- All ammonoids
- All belemnoids
- All inoceramids
- All rudists
- All plesiosaurs
- All mosasaurs
- All pterosaurs
- All non-flying dinosaurs and several clades of birds
- Many mammal groups
- All bennettitalians
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:
- Changing Earth's albedo, and thus its solar budget, and thus its weather
- Producing more continental climates in interiors, changing regional ecosystems
- Change oceanographic conditions, by removing the once-vast epeiric sea systems and by removing a major source of productivity
- Would operate over a 4 million year scale
Increased Maastrichtian volcanism, especially the Deccan Traps:
- Decrease insolation (incoming sunlight) by presence of fine particles in high atmosphere
- Also change Earth's albedo, although not as dramatically
- Would operate on the scale of a few tens of thousands to hundreds of thousands of years
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:
1. Meteors impact the Earth's atmosphere all the time
2. Some chemical elements more common in meteors and such than on Earth's surface: these should be traceable in minute quantities in sediment
3. Find the average infalling rate of these elements today; use this rate and observed amount at the Gubbio clay layer to find out how much time
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
- Calculated probable size need to add this much iridium: suggested a 10-15 km diameter object (Manhattan-Sized).
- Calculated probable effects of impact of an asteroid this size:
- Short term:
- Release lots of energy near impact, form huge crater: 1.8 x 108 megatons!!
- Burst of light would vaporize material for kilometers around, just like thermonuclear weapons
- Blast wave would devastate nearby region; it would be felt around the world, but decrease with distance
- Shockwaves from impact would generate huge tsunamis ("tidal" waves)
- Newly recognized minutes-to-hours event: the "Easy Bake Oven Effect":
Material that was thrown up above atmosphere and reentered generates substantial
infrared radiation. This heat raises air temperature by only about 10C° (18F°), but would be fully absorbed by rock, leaf, flesh, and any other opaque material. It is predicted that the increase in infrared radiation would be 8-10x that of high noon at the hottest spot of the Earth, and persist for many minutes to hours. Living tissue would bake, unless underground 10 or more cm (heat wouldn't have time to make it that deep) or underwater (upper few microns of water might boil off, but that would be it).
- Longer term:
- Material vaporized by impact kicked high up in atmosphere: reduced amount of incoming sunlight
- Observations on Mars showed big temperature drops due to high-level particles
- In human history, eruption of Tambora in Indonesia in 1815 produced chilling effects worldwide for more than a year later
- Dust and ash would block out sunlight, reducing photosynthesis and killing off plants on land and surface algae in water; herbivores feeding on these would die; carnivores feeding on these would starve (after a brief feast): Impact Winter
- Estimates of duration of the Impact Winter have varied from a year or so to a few months to just a few weeks, but a model published in January 2017 puts a duration for a 26°C temperature drop in global surface temperature for 3-16 years and a greater than 30 year duration until recovery!
- Collapse of foodwebs would require long term to recover, as many parts of each foodchain might be lost
- Paleoceanographic evidence suggests that seas were cooled considerably, and took 10s of kyr to reheat at depth
- "Greenhouse Summer": release of substantial carbon dioxide from Deccan Traps and vaporization of carbonates at Chicxulub raised carbon dioxide levels from ~500 to ~2300 ppm, with long-term (100s of kyr) greenhouse warming
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
- Animals with larger total food requirements die more those with less
- In marine communities, foodwebs tied into photosynthesis (that is, direct from the phytoplankton) would be hit harder than bottom feeders (which feed on the accumulated decayed remains of organisms)
- Additionally, taxa dependent on symbiotic algae would be devastated
- Some geologic record other than just iridium might remain
- Effects would be global and essentially instantaneous: hours to days to months to a few years
Biotic prediction fits most of the predictions; search for geological signature was on.
- Quartz is one of the most common of all minerals
- When subjected to intense heat & pressure, forms shock planes
- Shocked quartz has been found in over 100 K/Pg boundary sites worldwide
Melt Glass (Tektites):
- Material thrown up by impact would melt during reentry, form glassy spheres
- These have been found at some K/Pg sites
Tsunami ("tidal wave") and ejecta deposits:
- Thick units probably formed by tsunami found at K/Pg in Carribbean, Gulf Coast of Texas, Mexico, Central America, and South America
- Thinner but widespread deposits of ejecta (material flung through the air) at K/Pg in Caribbean, Gulf Coast of Texas, Mexico, Central America, and South America
- Chances were that the impact was in ocean basins, but most Cretaceous ocean basins have been recycled by plate tectonics
- Some early leads were in Siberia (too early); Manson, Iowa (too small and too early (within Late K))
- Nearly all geological lines of evidence (tektites, tsunami deposits, ejecta deposits, shocked quartz, etc.) were more abundant in Western Hemisphere, and especially in the Gulf of Mexico, than the rest of the world: pointed to impact in that region!
- In Yucatan, Mexico: disrupted layers at K/Pg boundary in buried rock
- Seismic and gravity scan suggested a crater 180 km across: the right size!
- Although not visible as a crater because buried under 300-1000 m of Cenozoic rock, it
can be seen using sensitive satellite and other data
- Crater was named Chicxulub, after nearby town
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:
- Some suggestion of million-year scale decline in some groups, but not as strong as once thought
- Change in flora of western North American Interior, consistent with climate/ecosystem changes due to Regression
- Many extinctions, however, seemingly instantaneous
- In marine realm, planktonic forms and creatures that eat them (and those that ate them, and so on) suffered greater than benthic detritivores, consistent with shut down of photosynthesis. Impact Winter seems to have been the primary killing agent. (Ammonoids were doubly-hit: their larval forms were plankton, and they ate plankton!)
- In terrestrial realm, basic pattern is that animals dependent on large food supply and/or metabolism: larger and/or fully terrestrial creatures survive better than smaller and/or subaquatic forms. A combination of the "Easy Bake Oven" and "Impact Winter" seem to have been the main killing agents, with the longer "Greenhouse Summer" picking up many of the survivors.
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.
To Lecture Notes.
Last modified: 19 January 2017