Geology of Arvada, Colorado

Arvada is a city located in Jefferson and Adams counties in the U.S. state of Colorado, part of the Denver-Aurora metropolitan area. Arvada has a diverse and interesting geology shaped by ancient seas, volcanic activity, and erosion over millions of years.

This article will provide an overview of the key geological features and history that have shaped the landscape of Arvada.

Geologic History

Precambrian Basement Rocks

The oldest rocks beneath Arvada formed during the Precambrian era, over 1 billion years ago. These ancient basement rocks consist of metamorphic gneiss, schist, amphibolite, and granite formed deep underground. Tectonic activity brought these rocks to the surface in some areas. Precambrian rocks underlie the entire region but are not usually visible on the surface around Arvada.

Paleozoic Sedimentary Layers

During the Paleozoic era, around 300 to 550 million years ago, what is now Colorado was covered by warm, shallow seas. Thick layers of sedimentary rock formed from the accumulation of sand, gravel, limey mud, and skeletal debris deposited on the sea floor.

These major Paleozoic formations include the Sawatch Quartzite, Peerless Formation, Morgan Formation, Leadville Limestone, Belden Formation, Minturn Formation, Maroon Formation, and Chaffee Group. These layers can be seen in some exposures around Arvada.

Mesozoic Sandstone and Shale

During the Mesozoic era, the area was again repeatedly covered by advancing and retreating seas. Different sedimentary layers formed, including sandstone, shale, and limestone.

Notable formations in the area from this era include the Lykins Formation, Morrison Formation, and Dakota Group. These rocks record the transition from marine to coastal to continental environments. Fossils are found in many of these units.

Cenozoic Volcanic Activity

The modern Rocky Mountains began rising during the Laramide orogeny around 80-55 million years ago, associated with subduction along the western edge of North America.

This mountain building event was accompanied by widespread volcanic activity, including in areas near what is now Arvada. Major igneous intrusions formed, including the Silver Plume granite. Volcanic ash layers are found within sedimentary units.

Later volcanic activity between 35-25 million years ago deposited layers of welded tuff and volcaniclastic sedimentary rocks. Notable formations include the Denver Formation and the Dawson Arkose, which can be seen around Arvada. The minerals biotite and sanidine are common.

Local Geology

Arvada’s landscape consists of weathered sedimentary rocks dipping gently to the west, along with some igneous intrusions. Here are some of the specific geologic features visible in the area:

Ralston Dike

A prominent igneous dike cuts through older sedimentary layers in western Arvada. This vertical sheet-like intrusion called the Ralston dike is made of alkali-feldspar granite and associated aplite dikes. It extends over 12 miles and is part of a larger dike swarm.

Table Mountains

Arvada is located along the edge of the hogback ridges of the Table Mountains southwest of Denver. These flat-topped ridges are the eroded remnants of tilted sedimentary strata, consisting of sandstone, shale, and conglomerate. The rock layers have been uplifted and tilted by tectonic activity.

Clear Creek

Clear Creek flows down from the mountains through Arvada, carrying sediments eroded from various geologic formations. The alluvial deposits along the creek include sand, gravel, cobbles, and boulders. The creek sediments record the local erosion history.

Fossils

Numerous fossils can be found in the sedimentary layers around Arvada. Common fossils include brachiopods, pelecypods, gastropods, horn corals, stromatoporoids, and crinoids from Paleozoic layers. Dinosaur bones and trace fossils occur in the Mesozoic units like the Morrison Formation. Plant fossils are found in the Cenozoic Denver Formation.

Mineral Resources

Clays, sand, and gravel deposits around Arvada are mined as construction aggregate. Some limestone is also quarried. There is also some local deposits of gold, silver, lead, zinc and other metals associated with veins cutting the sedimentary layers.

Geologic Structures

Faults

The Dakota Hogback, Ralston Shear Zone, and other faults cut through Arvada and the surrounding area. These fractures in the earth’s crust have shifted and tilted the geologic strata and influenced erosion patterns. For example, the Ralston Shear Zone contains the Ralston Dike intrusion along its length.

Folds, Dip, and Strike

The sedimentary layers around Arvada show broader folding and tilting to the southwest, part of the regional dipping structure. More localized folding and deformation of the rocks is visible around faults and igneous intrusions. The strike and dip of rock units provides clues to the tectonic history.

Unconformities

Surfaces where rock layers are missing or distinctly shifted record major periods of erosion or non-deposition. These unconformities show gaps in the geologic record and shifts in the landscape over time. Several major unconformities bound the major geologic eras in the Arvada area strata.

Unique Local Formations

Berkley Blue Limestone Lentil

This 10 foot thick limestone layer within the Peerless Formation contains large chert nodules weathered into a pitted surface. Its blue color comes from ferroan calcite.

Arvada Mudstone

A dark grey to greenish-gray mudstone found within the Denver Formation south of Arvada. It represents a low-energy swamp environment.

Van Bibber Creek beds

A sequence of sandstone and conglomerate layers interbedded with the lowest part of the Denver Formation. Contains fossil leaves and petrified wood.

Significance for Arvada

Arvada’s geologic history and resources have played a key role in shaping the city’s growth and economic development. Here are some of the ways Arvada’s geology has been important:

• Clays used to make bricks and tiles by early settlers
• Sand, gravel, and limestone provided construction aggregate
• Weak and erosion-prone mudstones and shales determined development patterns
• Mineral deposits spurred early mining and industry
• Flat iron-rich sandstones formed mesas for housing and agriculture
• Creek drainages shaped transportation corridors
• Fossils and geology inspired early scientists and naturalists
• Rock formations added aesthetic value and local identity

Conclusion

From ancient oceans to dinosaurs to volcanic eruptions, Arvada preserves a diverse geological history spanning over one billion years. The local rocks, minerals, and landforms continue to shape the city today. Understanding Arvada’s unique geology provides insights into the natural landscape and how the area has evolved over deep time to create the environment we see today.