1. LECTURE 12. LATE MESOZOIC GEOLOGY.
HISTORICAL GEOLOGY
LECTURE 12. LATE MESOZOIC GEOLOGY.
CRETACEOUS ( MYBP)
Paleogeography: The Cretaceous begun with the continuation of the ZUNI TRANSGRESSION, which resulted in the largest marine invasion since the end of the Pennsylvanian, 200 million years before.
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In north Texas, Cretaceous rocks cover the surface and dip gently to the southeast (the Gulf) and grade into Cenozoic rocks farther south.
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If you travel west (which we do on the field trip) you come to the edge of the Cretaceous bedrock (Cretaceous rocks farther west have been eroded) and move onto the underlying Pennsylvanian and then Permian rocks, which dip to the northwest.
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Here’s a smaller scale map showing the bigger picture. Arrows show dip, the red lines show the locations of the following upper and lower cross sections. Also notice the Llano Uplift (LU) and the outliers of Cretaceous rocks (OL). OL
upper
lower LU
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Permian Pennsylvanian Crtetaceous
Uplifted craton
Ouachita Orogenic Belt
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Permian Basin
Llano Uplift
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The result, in North America and elsewhere, was the establishment of large EPEIRIC SEAS, characterized by carbonate deposits especially CHALK, from the remains of golden-brown algae (earlier blue-green + green algae did not secret shells)("Creta" = Latin for chalk).
Coccoliths
(golden-brown algae shells).
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There are many cretaceous chalk deposits around the world. One example is the famous WHITE CLIFFS OF DOVER.
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Tectonics and Sedimentation
The east coast continued to be a passive divergent margin in the Cretaceous; the Appalachians continued to erode. The greatest tectonic activity was on the west coast. Orogenic activity, resulting from plate convergence, subduction and terrane accretion, had been going on since the Early Paleozoic - the peak periods being the Antler, Sonoma and Nevadan Orogenies.
By the Late Cretaceous, the mountainous Cordillera of the west coast of North America was well established. Two new orogenies (periods of intense mountain building) occurred in the Cretaceous:
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THE SEVIER OROGENY (mid-Cretaceous)
The "crumpling up" of the west coast, which intensified with the break up of Pangaea, was transmitted from west to east through the Mesozoic. The Sevier Orogeny is notable for low angle thrust faults which formed further inland (e.g. Nevada, Utah, Montana, B.C. and Alberta), as compressional stress from the west coast was transmitted eastwards. In many cases Proterozoic rocks were thrust up over Mesozoic rocks - in the case of the famous Lewis Thrust (Montana, B.C., Alberta) over a distance of 65 km (40 miles). The structure produced is known as a decollement (old rocks pushed up on top of younger rocks).
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The Sevier orogeny thrust belts
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THE LARAMIDE OROGENY (Late Cretaceous)
Orogenic activity continued into the Late Cretaceous, again further eastward than previously. This resulted mostly in folding of strata in the site of the present-day Rocky Mountains of New Mexico, Colorado and Wyoming, producing uplifted domes and anticlines, and basins. MOST OF THE UNDERLYING STRUCTURES OF THE PRESENT ROCKY MOUNTAINS WERE THE RESULT OF THE LARAMIDE OROGENY; HOWEVER, THE ACTUAL LANDSCAPE WE SEE TODAY RESULTED FROM EPISODES OF UPLIFT AND EROSION IN THE LATER CENOZOIC ERA.
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SEDIMENTATION
Apart from the chalk deposition already mentioned, much of the sedimentation in North America occurred in the backarc basin of the growing Cordillera region. This basin stretched from Utah to Kansas. It contains a gradation of clastics eroded from the Cordillera, from proximal conglomerates and coarse sandstones to distal marine shales A famous example is the Dakota sandstone, a beach deposit formed along the eastern front of the Rockies
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This is similar to the situation in the east during the Pennsylvanian – a clastic wedge created lowlands that were periodically submerged by fluctuating sea levels. The result was cyclotherms and coal in states such as Colorado.
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COAL
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Coal seam in Raton Pass, NM.
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EURASIA AND THE TETHYS SEAWAY
The Tethys seaway was a deep marine depositional trough between Gondwanaland and Eurasia during the Mesozoic. The trough was a site mainly of limestone deposition. Soon after the break up of Pangaea, Africa begun to converge on Eurasia. The result, mainly in the Cretaceous, was the closing and compression of the Tethys Sea, along what is now southern Europe. This caused orogenic activity and volcanism along the southern margin of Europe, which continued and intensified in the later Cenozoic. At the same time, India had broken free from Gondwanaland and was heading towards the site of the present-day Himalayas.
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Tethys 2 4
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