STRUCTURAL GEOLOGY
Lectures 39
Paleozoic Geology of the Cordillera
(Rocky Mountain Geology)

Late Proterozoic and Paleozoic Stratigraphy of the Western U.S. reflect the first three of the four stages of the Wilson Cycle including

1.)  The Rift Stage:

Proterozoic Basins largely reflect the configuration of western North America as a consequence of the breakup of Rodinia.  Some of the more important stratigraphic units include:

--  The Belt Supergroup of Montana and Alberta overlain by the Windermere Series
--  The Uinta Mountain Supergroup of northeastern Utah
--  The Grand Canyon Supergroup of northern Arizona

2.)  The Drift Stage:  

Initial sedimentary blanket accompanying the drift stage is a basal Cambrian quartzite which is found within the Cordillera as well as the Appalachian Mountains.  This basal quartzite is given different names depending on location:

--  Flathead Sandstone (Montana)
--  Prospect Mountain Quartzite (Central Nevada)
--  Tintic Quartzite (West-central Utah)
--  Tapeats Sandstone (Arizona)
--  Lodore Sandstone (Northeastern Utah)

The continental platform was then dominated by carbonates through the Carboniferous.  The area is the Paleozoic Miogeocline (Figure 39-1).  Some of the more famous carbonate units include:

Cambrian:

--  Bonanza King (Southwestern Nevada)
--  Mauv (Grand Canyon)
--  Snowy Range (Montana)

Ordovician

--  Big Horn Dolomite (Montana-Wyoming)
--  Ely Springs Dolostone (Westcentral Utah)

Devonian

--  Jefferson-Three Forks (Montana-Wyoming)
--  Temple Butte Limestone (Grand Canyon)

Mississippian

--  Madison (Montana-Wyoming)
--  Madison (Westcentral Utah)
--  Redwall  (Grand Canyon)

Paleozoic Tectonics of Western US  -  Extensional tectonism with the formation and modification of shelf basins and offshore region, seems to have characterized most of the late Paleozoic.

Figure 39-1 -  Paleogeographic and tectonic map of late Paleozoic time showing Cordilleran elements as well as intracratonic Ancestral Rocky Mountains and Ouachita orogenic belt (adapted from Burchfiel et al., 1992).  The eastern edge of the Havallah Basin in central Nevada is the location of the Golconda Thrust of the Sonoma Orogeny.  The foredeep of the Antler Orogeny is shown as EFAF.

3.)  The Convergence Stage:  During the later Paleozoic, the western edge of the North American Plate became a backarc basin with island arcs forming somewhere off shore.  Convergence was marked by clastic wedges starting in Mississippian time with the advent of the Antler Orogeny.

Compressional tectonic events along the shelf edge were restricted to two short intervals about 10 to 20

. duration in the earliest Mississippian and in the Late Permian to earliest Triassic.

Antler Orogeny - Earliest Mississippian - lower Paleozoic chert-shale sequences and associated mafic volcanic rocks of the Roberts Mountains allochthon were complexly deformed and thrust eastward over the edge of the continental shelf.  

Strata of the Roberts Mountains allochthon represent continental slope, rise, and basinal settings inferred to have been deposited west of the coeval, early Paleozoic continental shelf.  

1.)  Rocks that now form the allochthon most likely represent the deposits of a series of extensional basins developed intermittently along the edge of the continental margin in early Paleozoic time.  Basins are underlain by rifted continental crust.

2.)  Pelagic and hemipelagic sediments and alkalic basalt of the Slaven Chert constitute the youngest and structurally lowest units of the Roberts Mountains allochthon, implying that an extensional or transtensional tectonic setting characterized the region immediately offshore the continental margin in the Late Devonian.  

3.)  Deformation appears to be more penetrative and is associated with lower greenschist-facies metamorphism westward indicating deformation within a tectonic setting characterized by a relatively elevated, rather than depressed geotherm, but no detailed work has been done to address this question.

Rocks of the Roberts Mountains allochthon are overlain by upper Paleozoic shallow-marine sequences.

Havallah Basin:  shortening ended abruptly in early Late Mississippian time with onset of extension.  

Antler foredeep basin accumulated in gradually shoaling waters.  

Havallah basin and other offshore basins formed as a series of rift basins by continued episodic extension into the late Paleozoic.  Island arcs were separated from rift basins.

Ancestral Rockies formed in the Pennsylvanian:   This series of uplifts and adjacent deep basins are most pronounced in Colorado and Utah.  The timing for the Ancestral Rockies corresponds not with west-coast tectonics but rather the formation of the Ouachita Mountains of Oklahoma.  This foreland fold-thrust belt formed as a consequence of South American converging upon the Gulf Coast region of the USA.

Some of the more famous basins of this time include the Paradox basin of southeastern Utah and the Eagle basin of central Colorado.

Oquirrh Basin :  300 m of shallow sedimentary rocks of Atokan time.  (OB in Figure 39-1).

The complex transition to extensional faulting in the Ancestral Rockies reflects tectonic activity along the western margin , which may have acted as a “free-face” tectonic boundary at this time.  

Limestone turbidites flooded offshore basins like the Havallah.

Sonoma Orogeny:  By Late Permian time, the McCloud island arc moved closer to the continental margin by shortening or subducting the depositional basement of the Havallah sequence, resulting in imbricate thrust faulting.

Golconda allochthon - deep-marine sedimentary rocks and associated volcanic rocks of late Paleozoic age, the Havallah sequence, were emplaced onto the outer shelf above the Golconda thrust during the Late Permian to Early Triassic Sonoma orogenic event.

Rocks of the allochthon range from latest Devonian to Late Permian age.

Evidence for age of thrusting:  

Triassic to Jurassic marine sequences were deposited both east of and depositionally above the Golconda allochthon.

Autochthonous Permian strata of the overlap sequence are conformably overlain by fossiliferous lowest Triassic, hemipelagic sedimentary rocks that , in turn, are overlain by Lower Triassic submarine-fan deposits derived from the encroaching allochthon.