The Dillon Reservoir
Dillon Reservoir, referred to as Lake Dillon, is a large fresh water reservoir located in Summit County, Colorado, bordered by the towns of Frisco, Silverthorne, and Dillon
Activities include boating, fishing, bicycling, camping, canoeing, cross country skiing, and hiking.
Unlike some Colorado reservoirs, Dillon looks like it has always been there, but this isn’t the case. Deep beneath the surface of the water sits what was once the largest town in Summit County—a community called Dillon—that was home to more than 800 residents as recently as 1960.
It’s fascinating to think that a decent-sized town was once located here.
Now, on to our lesson.
Earth Science Lesson
In this lesson you will learn about The Morrison Formation and The Dakota Formation.
The Jurassic Period and The Morrison Formation
Morrison Formation
The Morrison Formation is named for Morrison, Colorado, a small town just west of Denver in the foothills of the Front Range where some of the first fossils in the formation were discovered by Arthur Lakes in 1877.
Nearly-complete dinosaur skeletons have been found in quarries in the Morrison in Dinosaur National Monument and in Jurassic National Monument. Bone fragments can be found in sandstones that deposited in stream channels. Other types of fossils found in the Morrison including those of freshwater bivalve mollusks, petrified logs and other plant fossils, pollen, and insects.
The Morrison Formation is somewhat variable throughout its geographic area, but it largely consists of sediments deposited in a continental basin containing rivers, floodplains, lakes, and wetlands. The Morrison basin was bounded on the west by a volcanic mountain range in what is now Nevada. Rivers draining these highlands flowed west to east into the Morrison basin, and volcanic ash was carried in on wind currents from massive volcanic eruptions that also took place in the western uplands. The northern part of the Morrison basin bordered the Western Interior Seaway, and some sediments deposited under marine conditions are present there.
(What is the Western Interior Seaway? It is the rising sea level in the Cretaceous coupled with the foreland basin formation resulting in a seaway splitting the North American continent. The deposition of the near shore marine Dakota sandstone is the first manifestation of the seaway in Colorado.)
The Morrison Formation was deposited between 150-157 million years ago during the latter part of the Jurassic Period. The climate in the Morrison basin was mostly semi-arid. Water flowed into the basin from the western highlands via rivers and as groundwater. Despite the relatively dry climate, the area had a high water table as evidenced by the numerous lakes that dotted the landscape. At this time North America was farther south, and the Rocky Mountains did not yet exist.
It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho.
It covers an area of 1.5 million square km (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east and much of the rest was destroyed by erosion as the Rocky Mountains rose to the west
In the Colorado Plateau region, the Morrison Formation is further broken into four sub-divisions, or members. From the oldest to the most recent, they are:
1. Windy Hill Member: The oldest member. At the time, the Morrison basin was characterized by shallow marine and tidal flat deposition along the southern shore of the Sundance Sea. The Sundance Sea was an arm of what is now the Arctic Ocean, and extended through what is now western Canada into the central western United States.
2. Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.
3. Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats. It is about 82 m thick, and an interval of crossbedded sandstone and conglomeratic sandstone with a few interbeds of reddish-brown mudstone.
4. Brushy Basin Member: Much finer-grained than the Salt Wash Member, the Brushy Basin Member is about 105 m thick and is dominated by mudstone rich in volcanic ash. It’s strata are mostly variegated pale greenish gray, grayish yellow green, pale olive, yellowish brown, and pale reddish brown zeolitic mudstone with a few beds of trough-crossbedded pebbly sandstone.
The Cretaceous Dakota Formation
Sandstone
Red Colorado Sandstone Cliff
Sandstone
Sandstone is a sedimentary rock, composed of fragments of pre-existing minerals and rock. These minerals and rock fragments are usually sand-sized, thus the name. It consists of sandy, shallow-marine deposits with intermittent mud flat sediments, and occasional stream deposits.
It may be any color due to impurities within the minerals, but the most common colors are tan, brown, and yellow. However, It can also be red, grey, pink, white, and black depending on the minerals present. It is created in layers that are often horizontal or perhaps with a slight wavy pattern.
Dakota Sandstone
The Dakota Formation, called Dakota Sandstone, and also referred to as the Dakota Group, is a geologic formation composed of sedimentary rocks deposited on the western side of the Late Cretaceous Western Interior Seaway.
Deposition of the sediments that would become the Dakota Formation began during the early Late Cretaceous. This deposition marked a reversal from millions of years of erosion. This reversal was due to rising of the mouth of the rivers, called a rise in base level, as the Cretaceous Seaway formed. This rise lowers the gradient of the rivers causing them to deposit sediment because their velocity can no longer sustain high volumes of sediment.
This shift means that the rivers had completely eroded away the Paleozoic rocks in the river source area by the time the Seaway rose high enough for the rivers to deposit sediments in Iowa. The very top of the Dakota Formation was deposited along the coast as indicated by some fossil marine invertebrates. Fossil plants, coal deposits and kaolinite clays show that the climate was warm and wet during deposition of the Dakota Formation. Some of the ancient preserved soils show that an extensive flood plain forest was present.
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Please answer the following questions—DO NOT POST THE ANSWERS IN YOUR LOG!
***Send your answers only to me.***
At the Dillon Dam Overlook (this is at GZ—the western end of the dam), look across the road at the outcrop of Morrison Formation.
Questions:
1. Describe the layers (Formations) you see. Which Formation is on top and which on the bottom? Do any of them display banding? Is it horizontal or vertical?
2. The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock. What sedimentary rocks is it composted of? What color are these rocks?
3. Which of the four sub-divisions of the Morrison Formation is this an example of?
4. OPTIONAL PHOTO. Please take lots of photographs here as it is so beautiful. Post one on the page so others may appreciate it.