Continental sedimentation reflects a complex interplay of tectonics and climate. A 2000-km transect from coastal California to the western Great Plains documents a major increase in sedimentation in earliest middle Miocene (ca. 17-15 Ma). Basin and Range-style regional extension following elongation of the Pacific-North American transform boundary at 17.5 Ma provided fault-bounded basins for accumulation of continental deposits. Sedimentation also occurred in transtensional basins along the transform boundary and on unextended erosional surfaces of the Great Plains and Colorado Plateau. Upwelling along the California Coast deposited the hemipelagic Monterey Formation (ca. l6-6 Ma) coeval with continental sedimentation. Three tectonic/oceanographic events that strengthened thermohaline and Pacific gyral circulation were: 1) Opening of Fram Strait (17.5 Ma), 2) growth of the East Antarctic Ice Sheet (14.2-13.8 Ma), and 3) closing of the Indonesian Seaway (12-10 Ma). Upwelling of cold waters along the California Coast, abetted by domination of La Nina phases of ENSO, progressively aridified the Southwest as reflected in both the sedimentary and biologic records. Opening of the Gulf of California (6.4 Ma) intensified the North American monsoon resulting in integration of drainages, incision of uplifts, and exhumation of basin fills. The Miocene ended with the driest climate of the Tertiary accompanied by conversion of savanna to steppe or scrub desert, spread of C4 grasses, and extinction of 35 genera of large mammals. The answer to the long-running controversy over tectonic uplift versus changing climate in continental sedimentation/erosion is not either/or, but the complex interplay of both.

By J. Harrison Daniel, Ph.D, PE.

Travel through the historic mining districts in Colorado to view our mining heritage from the turn of the century when "Mining Was King of the Mountains." The impressive and treasured structures and ruins were largely responsible for the building of our Nation and in establishing the United States as a world leader. Districts include Central City, Leadville, Red Mountain Pass, Summitville, Bonanza and Victor.

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By Matt Morgan

The weather is finally turning warmer as summer nears. Time to dust off the old boots and backpack and head out to the field to look at some rocks. After being in the office for the last 6 months I realized how lucky I am to be involved in mapping the geology of our great State. This leads me to the subject of this month’s president’s message: what is your favorite place to explore the geology of Colorado?

For me, it depends.

Best place to see a geologic mystery:

Unaweep Canyon (Uncompahgre Plateau near Grand Junction). Did glaciers or rivers carve it? A truely spectacular place as impressive as the Black Canyon of the Gunnison.

Best place to view geology in motion: Great Sand Dunes National Monument and Preserve (San Luis Valley). Massive sand dunes are moving before your eyes! You could also check out the Slumgullion earthflow between Creede and Lake City that continues to move 20 feet per year.

Best place to see geology that makes a difference:

The Denver Basin (Greater Denver area). The rocks here are important sources of ground water, oil, gas, coal, minerals, and aggregate which help drive the Colorado economy and allow us to live in relative comfort.

Best place to see evidence of an extraterrestrial impact:

Trinidad Lake State Park (Trinidad). Here the K/T boundary impact layer is exposed and available for you to touch.

Best place to learn about geology on a rainy day: Denver Museum of Nature and Science (Denver). An incredible display of minerals, fossils, and dioramas await you. Alternatives include the Morrison Natural History Museum and Dinosaur Journey Museum in Fruita.

Best place to collect fossils with your kids: Douglas Pass (near Rangely). While I have not taken my kids here yet, I can’t wait. Beautifully preserved plant and insect fossils are preserved in the Green River Formation. At nearly 8300 feet above sea level, the panoramic view from the summit (which can be easily driven to by car) makes the trip even better.

There is more incredible geology in Colorado than a person can explore in a single lifetime. But don’t let that wonder end with you; pass your knowledge of these hidden gems along to your children, friends, and colleagues so that they too can visit the places that still inspire you.

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