Grand Unconformity

• Flat to gently rolling surface probably covering most of Colorado (and most of the western North American tectonic plate)
• Underlying rock ranges from Pikes Peak Granite to PreCambrian rock to unmetamorphosed or slightly unmetamorphosed sedimentary rocks deposited in PreCambrian-aged grabens
• 0.5 Billion years in Colorado Springs to 0.8 Billion years at Red Rock Amptheater

Colorado Sag

Probable “graben” related to accretion of multiple island arcs arriving from the south in PreCambrian time. Oldest rock successively overlap.

Sawatch Sandstone

• Deposited in a rising sea level on Grand Unconformity
• Lower portion composed of tidal sedimentary features grading upward to near-shore, marine sandy deposition
• Angular sand grains suggesting close source for sediment (elevated points on Grand Unconformity)
• Thickest portion is in center of Colorado sag (Glenwood Canyon area) and thins to zero edge to northeast and southwest out of sag. Cliff former

Manitou Limestone

Red limestone and dolostone containing abundant trace fossils, shellfish, sponges and echinoderms.  Deposited in vast tidal mudflat with many well-developed, upward shoaling, meter thick, tidal cycles. Step-like cliff former under Harding.  Deposited in Colorado Sag, overlaps Sawatch, covers greater area than Sawatch and deposited upon Grand Unconformity (visible along Shelf Road but visible at other locations). Indicates slowly rising, relative sea level(transgression).

Harding Sandstone

Relative lowering sea-level(regression), unit deposited in a large array of coastal environments (e.g. beaches, deltas, bays, lagoons, etc) in a shallowing sea. Quartz-rich, relatively fine-grained, sandstone indicates source of sand at a distance.  Common primitive fish fossils, fish scales, and trace fossils. Forms slope.

Fremont Dolomite

Deposited in a rising, relative sea level, deepening (transgression) oceanic environment. Massive, gray, crystalline dolostone containing many coral and shell fossils. Remnant of most widespread marine submersion of North American Continent

Manitou, Harding, and Fremont

frequently occur in a stack of red cliff, tan slope, gray cliff.  Post-Early Paleozoic erosion has removed much of these units, leaving isolated but readily visible stacked units in central portion of Colorado.

Silurian Rocks in Colorado

No Silurian rocks are found in Colorado.   Most probable that a widespread limestone was deposited in a sea that covered western North America but was totally removed by post-Silurian uplift and erosion.  Silurian limestone survives as fragments in volcanic diatremes occurring along the Colorado-Wyoming border.

Chaffee Group

Cliff former in western and west-central Colorado, does not occur in Front Range-Wet Mountains-eastern plains of Colorado.  Consists of a lower sandstone (Parting Sandstone-Elbert Fm-shallow, marginal marine) and upper dolomite (Dyer Dolomite-Ouray Limestone-shallowing upward marine environment, relative sea level falling-regression).