Saturday, February 07, 2015

Brief Look at some Sabino Canyon Geology

Our Thursday hike at Sabino Canyon took us on a three mile loop through millions of years of history. Our shepherd was Bruce--a volunteer naturalist with a great sense of humor, a zeal and knack for teaching, and an encyclopedic knowledge of the geology of the area. His mantra was "The world is all around you and if you look you will see."

Here are a few of the things he gave us an opportunity to see.

First the basic orientation of the Santa Catalinas front range. 

The slope is about 23 degrees, which was the angle at which the rock gave way or faulted  as it was pulled in the formation of the basin and range topography.   As the area stretched  a whole set of mountains now all the way over by Saguaro West formed the Tucson basin. The basin dropped about 10,000 feet. Five thousand is now full of sediment from the erosion process and helps to form the aquifer that has supplied Tucson with water. See the visual aid .  The Tucson Mountains are to the left, the basin in the middle, and the Santa Catalinas to the right. The general orientation or the pull was south by southwest.

He tugged on a towel on his display table to demonstrate how the pull or stretch develops little parallel humps and valleys,  thus the parallel Bear, Rattlesnake and Sabino Canyons.

The predominant ancient rock we see in the area is the metamorphic Catalina Gneiss. It is banded with alternatelayers of light and dark igneous granite. The dark layers are the 1.4 billion years old  Oracle granite. The lighter bands are the younger Wilderness Granite. (Because of other minerals this material can vary from white and grey to various tans and reddish browns. The granite was subjected to massive pressures during its initial uplift into a huge dome thus creating the metamorphic gneiss. That dome was later stretched and faulted and eroded to produce the Santa Catalina Mountains we see today. Here are some examples of the bands with the grey and white pattern.
Sometimes the pressure swirled the molten lighter injection into Oculas or eyes in the rock 
Eroded rocks and boulders spinning in small depressions in the currents of canyon streams during floods could create holes and depressions in the rocks called Tinajas.  They now retain water for longer periods and offer places for animals to drink.
Later faults also fractured the rock in many places so you may see many areas that seem to be literally chipping away or falling apart.  The reddish cast comes from iron or magnetite.

Pressure can inject molten rock into both horizontal and vertical cracks causing formation of pegmatite. Important mineral deposits can sometimes be found on the edges of these injections according to our guide.

Faults caused some formations to slip by each other producing parallel scratches and shiny surfaces called Slickensides. Here Bruce is showing us how they moved.

A look at the eroded stream channel of Rattlesnake Creek shows how deposits of various sized boulders and gravel can show the strength of floods down the canyon at various times in geologic history.

Globe Mallows near current bottom of Rattlesnake Creek
Oh and if you get lost in this desert and the natural magnetite in the rocks sends your compass awry you can look at the slants of the barrel cacti around you.  Mostly they lean to the south-south west because they grow more slowly in the direction of the strongest sunlight. 

My science is still pretty loose on most of this, but I did buy a book at the visitor center after we returned and do promise to read it and get some items straighter. 

Meanwhile just enjoy Sabino Canyon with or without the Geology


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