CHAPTER 14: Mesozoic Era Geology


Overview of Mesozoic Era

1. Break up of Pangea.

2. Tectonics of eastern & southern North America, northern Europe dominated by extensional tectonics:

(a) Opening of the Atlantic Ocean

(b) Formation of Triassic rift grabens along eastern North America

(c) Opening of the Gulf of Mexico

3. Major tectonic events along western North America dominated by compressional tectonics:

(a) Terrane collision in northern California and Nevada during Permo-Triassic

(b) Formation of Andean-style volcanic arc complex along the western margin

(c) Accretion of exotic terranes along the western margin.

4. Era of warm climate, high sea-level and large continental seaways flooding as much as 1/3 of the earth’s total continental area.

5. Following the Permian extinctions, marine life underwent profound change:

(a) Seas dominated by bivalves, gastropods and echinoids.

(b) Appearance of sea reptiles.

(c) Shell-crushing predators like crabs and lobsters.

(d) Large variety of ammonites

6. Age of dinosaurs.

7. Appearance of flowering plants during the mid-Cretaceous

8. End of Mesozoic marked by a mass extinction possibly caused by a major asteroid impact and/or massive volcanic eruptions.


Breakup of Pangea

1. Figure 14.2: Traces the migration of South Pole positions during the early Paleozoic when Gondwana and Euramerica were separate and each had its own polar track. During Carboniferous time (C), Gondwanaland and North America collided and joined. From there, both had the same polar track. By Permian time (P), the Pangea super-supercontinent was completely assembled. Pangea remained assembled until breakup began during early Mesozoic time (M). Light dashed arrows record seperation of different continents since then.

2. Figure 14.4a: Pangea began to break up near the end of the Triassic period (~215 Ma) when North America and Africa began to separate.

3. Late Triassic to Early Jurassic rifting eventually disrupted the connection between North Africa and Europe. Also during this period, several microcontinents broke away from northern Gondwanaland and eventually collided with southeastern Asia.

4. The Indian Ocean began to open around Middle Jurassic time as evidenced by major transgressions in eastern Africa and Madagascar.

5. India separated from Africa and Australia during the Late Jurassic.

6. Figure 14.4b: The Late Cretaceous (~95 Ma).

7. South America broke away from Africa during the mid-Cretaceous.

8. Australia separated from Antarctica near the end of the Cretaceous.


Extensional Tectonics and Opening of the North Atlantic

The Newark Rifts

1. Figure 14.5: The Newark rifts are a series of trough-like features of Late-Triassic-age which extend along the eastern coast of North America. The Newark rifts are filled mostly with thick sequences of non-marine clastic sedimentary rocks interspersed with basaltic dikes and lava flows. These rift-filled deposits can exceed 6 km in thickness. Formation of the Newark Rifts is attributed to initial opening of the North Atlantic.

2. Figure 14.3: The early break-up of Pangea was initiated by heating and doming of the lithosphere, followed by rifting and sea-floor spreading.

3. Stretching of the Pangean crust initiated Triassic faulting accompanied by basaltic volcanism. The faulting broke up the crust into a series of isolated blocks that formed horst and graben structures. Rivers eroded upraised fault blocks and deposited gravel and sand as broad alluvial fans within the adjacent down-faulted rift basins.

4. The rift basins (including the Newark Rifts), in addition to experiencing volcanism, accumulated thick deposits of red-colored, nonmarine clastic sediments derived from erosion of uplifted fault-blocks. Lakes formed in the central parts of the basins, some accumulating organic-rich muds that eventually became source rock for petroleum.

5. Figure 14.8: As the breakup of Pangea continued, the Newark Rifts eventually became isolated from the growing ocean basin to the east.


Ocean Basins and Aulacogens

1. Figures 14.5: Unlike Newark type basins, marginal rifts continued to expand and were soon flooded by the invading sea. In warm regions such as the early Gulf of Mexico, evaporites were initially deposited by the invading seawater. This was followed by establishment of a broad continental margin as the ocean basin widened.

2. During Jurassic time, the Gulf of Mexico and North Atlantic Ocean Basins continued to grow. By the end of the Jurassic, normal marine conditions were established in the North Atlantic and Tethyan fauna migrated from Eurasia into the Gulf of Mexico.

3. In Europe, evaporites were deposited in the North Sea rift during Permian and Triassic time. The North Sea eventually became fully marine during the Jurassic.

4. Figure 14.6: During Cretaceous time, chalk (microscopic calcareous skeletons of pelagic organisms) was deposited widely over northern Europe and locally in southeastern North America during major transgressions. Chalk deposits usually are confined to the deep-ocean basins, but sea-level rise during the Cretaceous caused chalk deposition to spread into continental seaways.



Early Mesozoic History of the North American Craton


Triassic Period

1. Figure 14.10: The ancestral Rocky Mountains in Colorado and New Mexico, which formed during Pennsylvanian time, were largely eroded by Triassic time and were almost completely buried by non-marine sediments. Highlands and portions of lowlands were covered by immense forests as evidenced by petrified logs found today in the rock record (e.g. Petrified Forest National Monument in Arizona).

2. An immense alluvial plain, stretching from the highlands westward to the Cordilleran Sea, was inhabited by amphibians, early dinosaurs and various other reptiles.

3. Along the western margin of the craton, Triassic non-marine red sediments graded westward into marine gray shale and limestone. Marine lagoonal and tidal flat deposits are common and attest to periodic transgressions. The thickest section of these marine strata occurs in southeastern Idaho, where nearly 1000 meters of Lower Triassic sediments accumulated.

4. The upper Triassic formations in western North America consists mostly of continental deposits transported by rivers flowing westward across the immense alluvial plain. There were also upland source areas in present day Nevada, Arizona and Utah here sediments were eroded and re-deposited as the sandy Moenkopi Formation and pebbly Shinarump Formation.

5. The Chinle Formation consists largely of shale which, when eroded away, exposes petrified tree trunks in the Petrified National Monument in Arizona. The Painted Desert of Arizona is mostly developed in Chinle Rocks. The Upper Triassic Wingate and Lower Jurassic Navajo sandstones are remnants of ancient desert sand dunes and are beautifully exposed in the walls of Zion Canyon in southern Utah.

6. During the Mesozoic, a steeply dipping subduction zone existed along the western margin of North America. Entire sections of volcanic arcs, fragments of distant continents and pieces of oceanic plateaus were carried to the western margin as well.

7. Triassic rocks of the far western part of the Cordillera included great thicknesses of volcanic material and graywacke sandstone, presumably derived from an island arc.

8. During the Triassic Period, limestone deposition predominated within the Tethyan seaway and encased fossils of clams, snails, crinoids, reef corals and calcareous algae.


Jurassic Period

1. Figure 14.12: Beginning in Early Jurassic time, the western seaway began encroaching upon western portions of the vast alluvial plain.

2. Early Jurassic deposits consist of clean sandstone, such as the Navajo Sandstone, which contains large-scale cross bedding. Thin beds of fossiliferous limestone and evaporites occur locally. The Navajo and associated sand bodies were probably deposited in a near-shore environment, and it is likely that some of the deposits were part of a coastal dune environment. Others have postulated that the Navajo sandstone was deposited within a vast interior desert.

3. Marine conditions became more widespread during the middle Jurassic when the entire west-central part of the continent was flooded by a wide seaway that extended to central Utah. This great embayment has been dubbed the Sundance Sea.

4..Figure 14.15: by Late Jurassic time, much of western North America was covered by an extensive epeiric seaway. This immense seaway formed local deposits of sandstone and widespread deposits of limestone, shale and evaporite collectively called the Sundance Formation. The limestone contains abundant fossil fragments, oolites and algal material. Also present are cross-stratified glauconitic sandstone and fossiliferous shale.

5. The Late Jurassic marked the last time that significant carbonate and evaporite deposition occurred anywhere on the N. American Craton.

6. The Gulf of Mexico during the Jurassic was like a great evaporating basin, concentrating the waters of the Atlantic and precipitating salt and gypsum to thicknesses exceeding 1000 meters. These evaporite beds are the source of the salt domes of the Gulf Coast.

7. Evaporite conditions in the Gulf abated later during the Jurassic and several hundred meters of normal marine limestone, limy mud, shale and sandstone accumulated in the alternately transgressing and regressing seas.

8. Also during the Jurassic, shallow seas advanced from both the Tethys and the Atlantic and spread across Europe. Eventually, marine conditions extended from the Tethys across Russia and into the Arctic Ocean.


Review of Mesozoic Breakup of Pangea(Figure 14.4)

1. Pangea began to break up near the end of the Triassic period when North America separated from Africa.

2. Northern Africa also separated from Europe beginning in the Late Triassic.

3. During the later Mesozoic, Greenland and Europe separated from North America during opening of the North Atlantic.

4. India separated from Africa and Australia in Late Jurassic time.

5. South America split from Africa in mid-Cretaceous time


Geologic History of Western North America (the Cordillera)

1. Figure 10.3: The western margin of North America began as a passive margin in the Proterozoic and Cambrian.

2. Figure 14.21: By the Devonian, the western margin had turned into an active subduction zone involving collision of a volcanic arc with the continent (Antler orogeny of central Nevada).

3. Figure 14.16: Subduction along the western margin continued into the Mesozoic, but the axis of subduction shifted as a result of global plate reorganization related to the breaking up of Pangea. The old northeast-southwest trend of the Paleozoic passive margin and Antler orogenic structures was abruptly truncated by a new mountain belt trending northwest-southeast.

4. Figure 14.10: During the Triassic, erosion of the craton in the east blanketed western North America with thick deposits of sand.

5. Figure 14.12: An immense aluvial plain developed along western North America as rivers flowed westward into the Cordilleran Sea. Forests covered the highlands and much of the region was dominated by amphibians, early dinosaurs and various other reptiles. Freshwater molluscs and fish inhabited the swamp and river environments.

6. Figure 14.15: Beginning in the middle Jurassic, an epeiric sea invaded western North America and deposited sandstone, limestone, shale and evaporite. The rock succession deposited by this seaway is collectively known as the Sundance Formation.

7. The epeiric sea developed into a continuous seaway stretching from the Gulf of Mexico northward to the present Arctic.


Suspect Terranes

1. Figure 14.17: Western North America is largely a collage of ancient arcs and microcontinents derived from other places.

2. These suspect terranes consist of old arcs amd microcontinents accreted along the margins of western North America. Each terrane has a distinct stratigraphy, fauna and/or volcanic rock type and are separated from adjacent terranes by faults.

3. Paleomagnetic evidence from several areas of the western Cordillera indicate that some terranes had come from much further south.

4. The Stikinia terrane may have originated much further south and moved northward along stike-slip faults.

5. Wrangellia may have originated from the equatorial region and was displaced 35 to 65 degrees northward.

6. Alaska is composed almost entirely of exotic terranes.

7. Figure 14.18 shows the possible late Paleozoic positions of exotic terranes that were accreted to the Cordillera during the Mesozoic.

8. Figure 14.20 shows application of superpositional principles for dating collision of two suspect terranes with a continent.

9. Most of the Cordilleran terranes were probably transported to their present positions by no later than Cretaceous or early Cenozoic time.


Sonomia and the Sierran Arc

1. Figure 14.17: The Sonomia terrane was sutured to its present location in Nevada by the mid- Triassic.

2. Figure 14.21 shows the possible Mississippian to Jurassic development of western margin of North America. Successive arc collisions of the Antler and Sonoman orogenies are followed by reversal of subduction. The new eastward subduction along western North America produced an Andean arc along the western edge of North America beginning in Late Triassic time.

3. Figure 14.22: The modern Andes provide an analogue of the Sierran Arc in the Mesozoic which include an accretionary prism, melange, blueschist and a forearc basin.


Granitic Rocks of the Sierran Arc

1. Figure 14.23: Melting of subducting oceanic lithosphere produced an Andean-sized volcanic arc along western North America that probably erupted almost continuously from the Late Triassic until the Late Cretaceous.

2. This immense Mesozoic arc system may reflect rapid spreading in the Pacific during this period, resulting in fast, low-angle subduction along western North America.

3. Most of the volcanic rocks have since eroded away, exposing the plutonic roots of this once vast arc system.

4. The modern Sierras are mostly comprised of batholiths that once comprised the core of this vast arc system.


Foreland and Forearc Basins

1. Figure 14.26: East of the arc are latest Jurassic non-marine sedimentary rocks collectively called the Morrision Formation. The Morrison Formation is a small clastic wedge composed of shale, sandstone, rare conglomerates and volcanic ash The Morrison Formation is famous for its abundant dinosaur skeletons and many other kinds of fossils.

2. The Morrison Formation (part of the Great Foreland Basin) is a clastic wedge (vast alluvial plain) extending from Canada to southern Arizona and was deposited in a vast foreland basin by prograding rivers and swamps.

3. Progradation of the Morrison was in an eastward direction. Sediments are coarsest and thickest next to the Sevier orogenic belt but thin and become towards the east. Volcanic ash deposits indicate volcanic eruptions primarily to the west.

4. Morrison sediments provide evidence of the first major mountain building episode within the Cordilleran region.

5. Figures 14.28: On the Pacific side of the Sierra Nevada arc complex was the huge accretionary prism of the Franciscan Melange which is today found in the Coast Ranges of central California. This melange is at least 7,000 meters thick and consists of a chaotic assemblage of highly sheared and deformed graywacke, siltstone, black shale, chert and basalt. These rocks were apparently scraped off from the down-going oceanic lithosphere near the trench axis.

6. Behind the Franciscan accretionary prism was a great forearc basin composed of deep marine shale and sandstone of Jurassic and Cretaceous age. Today the Great Valley Group underlie the Central Valley (San Joquin) of California.


The Nevadan, Sevier and Laramide Orogenies

1. Figure 14.26: The first major phase of Cordilleran mountain building (Nevadan orogeny) occurred during the Late Jurassic and Early Cretaceous, during which time much of the Great Foreland Basin was deposited. The vigorous erosion of highlands during the Cretaceous added alluvial fan deposits to the foreland basin to produce a total accumulation over 3,000 meters thick adjacent to the mountainous highland.

2. The peak of mountain building occurred during the Late Cretaceous and is known as the Sevier Orogeny.

3. The Siever orogeny was marked by a slight eastward shift of arc volcanism into Nevada and Idaho due to a slight shallowing of the angle of subduction of the downgoing slab. This eastward migration of the arc was accompanied by increasing K in the igneous rocks due to the greater influence of continental rocks and the depth of melting becoming shallower.

4. Figure 14.30: The Sevier orogeny was characterized by unusually high compressional forces accompanied by massive backarc thrusting (thin-skinned thrusting). Slabs of continental rock was sheared off and displaced tens of kilometers eastward, resulting in an extensive belt of stacked thrust sheets extending from Nevada to the Canadian Rockies. This thrusting caused over 100 km of crustal shortening.

5. The final phase of Cordilleran mountain building, which began at the end of the Cretaceous and continued until the Eocene, is called the Laramide orogeny. Unlike the Sevier orogeny, the Laramide orogeny was characterized by large folds accompanied by steep thrusts along the flank. These broad anticlinal uplifts occurred further east of the Sevier belt, through central Colorado and Wyoming. Eventually these basement uplifts enclosed enormous sedimentary basins.

6. Figure 14.32: The eastward shift of Laramide deformation was accompanied by a shutoff of volcanic activity in the Sierran arc, although normal arc volcanism continued north (Pacific Northwest) and south (Mexico) of the Laramide tract.

7. This peculiar phenomenon may indicate that the downgoing Farallon Plate became so shallow that the plate scraped along horizontally beneath the continent and would no longer reach melting depth beneath the Sierran arc. In addition, this shallow subduction would transfer the compressive stresses much farther eastward into Colorado and Wyoming.

8. The shallow, near-horizontal subduction along western North America may have been due to rapid seafloor spreading in the Atlantic during this period, causing North America to ride over the Pacific Plate much faster than before.


Cretaceous Transgression and Sedimentation


The Great Cretaceous Interior Seaway

1. Figure 14.27: A worldwide transgression during the Cretaceous Period formed a great Cretaceous interior seaway spanning the length of the western interior of North America. Similar cratonic flooding occurred on practically every continent, including Europe.

2. In western North America, the center of this vast seaway contained areas where calcareous algae rained down from the surface to form calcareous ooze (which lithified into chalk). The shoreline of this vast inland sea typically contained interfingering sequences of marine shale and deltaic sandstone while coal swamps developed behind the shoreline sandstone.

3. Waters from both the Arctic and Gulf region merged over the present Plains area in mid-Cretaceous time, depositing marine strata from Minnesota to western Wyoming and lengthwise from the Gulf Coast to the Arctic.

4. The western part of the seaway contained a great clastic wedge which graded westward (that is, towards the Siever Highlands) from black shale with thin limestone layers and volcanic ash through massive conglomerates and into thick, massive, cross stratified sandstone containing coal seams.

5. The inter-tonguing of marine and shoreline deposits indicate oscillation of the shoreline where deltas and associated swamps shifted back and forth with each transgression and regression.

6. Abundant vegetation grew in the swamps and gave rise to widespread coal seams as well as providing a habitat for dinosaurs.

7. Dinosaurs also roamed the vast alluvial plain that extended back to the Sierra Nevada mountains.

8. Along the Gulf of Mexico, sandstone, shale and carbonate were deposited along the southern United States. In Mexico, limestone deposits occurred widely throughout most of the period since the region was nearly in tropical latitude.

9. In general, the Gulf Coast Region was subsiding rapidly and accumulating vast thicknesses of passive continental margin sediments.

10. The Cretaceous worldwide transgression is attributed to acceleration of seafloor spreading during this period, which caused the enlargement of ocean ridges. The larger ridges displaced large volumes of water, causing the continents to flood. Circum-Pacific mountain building and the uplifting of shorelines deposited large volumes of shoreline sediments. Large-scale continental breakup caused an increase in area of continental shelves and led to progradation of deltas and alluvial plains.

11. Regression of the North American seaway commenced during the Late Cretaceous and by early Cenozoic time, the waters had drained completely from the craton both northward and southward.


Late Mesozoic Paleoclimatology


1. Figure 14.35: During the late Mesozoic, water covered a larger proportion of the earth's surface than today. Solar radiation effectively warmed the water and the heat was distributed pole-ward by the ocean currents, producing an overall warm climate with ice-free poles.

2. Conifers and ginko forests grew as far north as 80o N while Magnolia and Sequoia trees thrived in western Greenland at 70o N.

3. The Cretaceous fossil record indicates that North America was largely subtropical.

4. Widespread uniformity of Cretaceous plants indicates a lack of sharp climatic zonation of the continent.

5. Although evaporites occurred in Jurassic times, their general absence from Cretaceous sediments suggests relatively humid conditions and open-marine circulation.

6. Figure 14.34: Oxygen isotopic analysis of marine fossils confirm mild ocean temperatures on the order of 20-25o C at present middle latitudes of 30o to 70o N., comparable with today's surface ocean temperatures along the coasts of Florida and Mexico.


The Black Shale Problem

1. Large quantities of black, organic rich shale was deposited on the central part of the N.A. craton during the Cretaceous and black muds occurred in deeper parts of the sea. Widespread black shales were also deposited in Europe during the Jurassic.

2. Organic-rich mud requires (1) a source of organic carbon and (2) isolation from oxygen subsequent to deposition in order to maintain preservation.

3. During the Cretaceous, production and deposition of organic carbon may have been rapid enough such that the material was quickly buried and isolated from oxidation. This may have occurred in shoreline areas adjacent to highlands where sedimentation was rapid.

4. Alternatively, organic-rich material was deposited under stagnant anoxic conditions.

5. Ancient black shale deposition corresponded with times of unusually deep transgressions where the interactions among climate, paleogeography and sedimentation enhance preservation.

6. Figure 14.36: Major transgressions result in warm global temperatures (increase in organic productivity). Oxygen is less soluble in warmer water, resulting in less oxygen uptake by seawater and greater preservation of carbon in seafloor sediments. In addition, a global warm climate would reduce the temperature contrast between the poles and equator, thus inhibiting vigorous circulation of deep seawater.

7. The earth during middle Cretaceous time may have been a greenhouse world where abundant atmospheric CO2 (from volcanic activity) held significant amounts of heat, raising the global temperature to as much as 8o C higher than present. Such a greenhouse world would reduce oceanic circulation and promote anoxic conditions on the seafloor (see Figure 14.36).