Geology (Elk Mountains)
Title: Stratigraphy of the Maroon Formation
Entered by: shredthegnar10
Added: 06/12/2010, Last Updated: 06/12/2010
Maroon Peak, North Maroon Peak, Pyramid Peak, Castle Peak, and Conundrum Peak are all composed of interbedded (alternating layers of) sandstones, siltstones, and shales that make up the Permian (299-251 million years ago) Maroon Formation.
The Middle Pennsylvanian (310-304 million years ago) uplift of the Ancestral Rocky Mountains created a sedimentary basin known as the Central Colorado Trough. Cyclothems, or cyclic rises and falls of sea level, resulted in alternating marine and nonmarine deposits in this basin. The narrow linear seaway came in from the north, which is why the Maroon Formation contains finer-grained sediments than its age-equivalent Sangre de Cristo Formation to the south.
Shale is a sedimentary rock composed exclusively of well-sorted clay-sized grains. The processes of deposition and lithification of shale results in the natural preferred orientation of these grains, which creates a strong cleavage plane (direction that the rock likes to break). This is why the rocks of the Maroon Bells and Pyramid are very loose and can be broken off easily.
Geology (Elk Mountains)
Title: Not so Brief Geology of the Elk Mountains
Entered by: WSC_Geologist12
Added: 06/15/2011, Last Updated: 06/15/2011
Sources: Geologic Maps of the Maroon Bells Quadrangle, Snowmass Mountain Quadrangle, Gothic Quadrangle, and Crested Butte Quadrangle. Most of this information comes from the instruction of the very knowledgable geology faculty at Western State College in Gunnison Colorado, whom I have taken many difficult classes with to learn about the origin of the mountains I love to climb.
Most people learn in their Earth Science classes that sedimentary rock is weaker than igneous and metamorphic rock. Where 75% of the time this is true, it leaves people wondering why some of Colorado's highest mountains are composed of sedimentary rocks. After all, shouldn't they be eroded by now?
Obviously, this is not the case, as there are a few ranges that are composed sedimentary deposits AND some of the highest peaks in Colorado.
The Elk range is primarily composed of the Pennsylvanian-Permian Maroon Formation (there are fossils representing both the Pennsylvanian and the Permian in this formation, so it is referred to with both ages). The Maroon Bells, Pyramid, Castle and Conundrum (along with most of the other smaller peaks in the area) are composed almost entirely of the Maroon Formation.
That leaves us with another question, how the heck did all these really old rocks end up so high?
Answer: During the Late Cretaceous Period (cerca 65 million years ago, mmmmm dinosaurs), there was an increase in the rate of subduction (for those who beleive in plate tectonics, which should be everyone) on the west coast of North America. At a normal subduction speed, the ocean crust plunges underneath the continental crust at an angle of 45 degrees or more with the surface. At a rapid subduction speed, this angle is reduced significantly. The reduced angle allowed the ocean crust to travel very far into the continent's interior before melting and becoming volcanic (unlike the Andes Mountains, which form volcanics just off of the coast). The increased friction at the continent's interior caused the ancient basement rock (about 1.7 billion years old) to thrust toward the Earth's surface taking everything on to op if with it. This is the reason we have ancient metamorphic and igneous rocks at the surface(which makes up most of the mountains in Colorado). The Elk Range is the result of one of those thrusted blocks of basement rock with younger Formations above it. The Elk Range Thrust is shown in Image #1 thrusted over Cretaceous Shale.
Had enough? There is more to it. After the thrusting of the basement rock, the subduction in the West stopped. The ocean crust underneath Colorado began to sink and melt, causing the "magma" to rise. In the San Juan's this broke out as violoent volcanism (cerca 34-28 Million Years ago). In the Elk Range, the "magma" didn't quite make it to the surface to produce volcanics, instead it cooled and produced intrusive igneous rocks (mostly granite). This granite makes up Snowmass and Capitol Peak, along with Mt.Crested Butte (in a simple sense). Intrusions have a tendency to intrude (thus the name) into whatever is the easiest pat. So excluding Snowmass and Capitol (which are more complicated), there are many intrusions along old fractures and fault planes throughout the Maroon Formation, thus results in the presence of igneous rocks on Pyramid Peak, and the lower parts of the Maroon Bells. Most people will not notice pieces of igneous rock among the sedimentary rocks of the Maroon Formation.
Now if that wasn't complicated enough, I apologize I will stop here though. If you have any further questions, or I forgot some details please message me and I will be happy to reply.
Geology (Snowmass Mountain)
Title: Young Granites
Entered by: rockdoc53
Added: 10/19/2010, Last Updated: 10/19/2010
Sources: Wallace, A.R., 1995, Isotopic Geochronology of the Leadville 1°x2° Quadrangle, West-Central Colorado- Summary and Discussion: U.S.Geological Survey Bulletin 2104, 51 p.
Snowmass Mountain and Capitol Peak consist of a relatively young granitic intrusive (granodiorite) with an K-Ar age date of 35 million years (Oligocene), and is slightly older than Mount Sopris (34.2 million years) and the mineralization in the Snowmass area.