GEOLOGY OF THE TORTOLITA MOUNTAIN AREA

Geographic Setting

The Tortolita Mountains are located just northwest of Tucson in a region known as lush Sonoran Desert. Geography throughout southern Arizona is characterized by numerous mountain ranges separate intervining valleys and basins.

Geologic Overview

Southern Arizona and the Tortolitas are in an area that is geologically young. The Tortolita mountains are typically rugged, erosion is spectacular, (rain water) drainage consists of rivers and streams that are mostly ephemeral. The amount of water these rivers and streams carry varies vastly depending on the time of year. Earthquakes can occur. Soils are often thin or absent. Folds and faults abound in the area. (There are no known major faults in the area; the nearest being in California.) A great variety of different rock is wonderfully exposed. Nearly every kind of rock is present somewhere in southeastern Arizona. And, the Tortolita mountains are no exception.

Geologic History

The geologic history of this region is very complex. Rock forming and other geologic events date back to more than a third of the Earth's history can be found here. Seven major events mark this very, very long time period. These events (from oldest to youngest) are:

1. Creation of a Pre-Cambrian basement of low grade metamorphic rock intruded by several generations of intermediate to granitic plutons.
2. Creation of a Pre-Cambrian sequence of unmetamorphosed sedimentary layers intruded by distinctive diabase sills.
3. A Paleozoic "layer cake" pile of mostly shallow marine sedimentary rock.
4. A Mesozoic morass of sedimentary rocks and igneous materials. These sediments have been laid down on land and sea and/or in shallow seas. They have been intruded several times by plutonic magmas reaching the surface as flows at some locations.
5. A complex and episodic Cenozoic innundation of coarse gravels and rhyolitic volcanic materials. The geologic details of this epoch confound those who study them.
6. A veneer of very young alluvial deposits represent the current phase of geologic history
7. Lastly, a seventh "package" exposed exclusively near Tucson in the Catalina, Tucson and Tortolita Mountains is a special case of its' own. "Catalina gneiss" and associated rocks have driven geologists to distraction for most of this century. Comprising very high grade metamorphic rocks with enigmatic and contradictory contact relationships, these materials are at the core of controversy.