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The Fountain Formation (PPf) is a Pennsylvanian-Permian aged conglomerate sandstone located along the east flank of the Front Range, CO.

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Photo of the Fountain Formation outcropping near Red Rocks Park, west Denver, CO. Height is about 15 feet. [1]

DescriptionEdit

The Fountain Formation is an arkosic, conglomeratic sandstone. Colors range from maroon to rust-brown, to a light grey. It is thickly bedded and often banded with white layers. It has extremely poor sorting, and no uniform grain size. Grains can range from medium-fine to very coarse. There is large clast variation, with sizes ranging from 1 cm to cobble sized. Notable features include lenses within the bedding, often with reverse grading. Sedimentary structures include 1-2m dune scale cross beds. In many places, the Fountain formation lies directly on top of pre-cambrian basement. The unit is rich in quartz and k-spar. In some places there are thin layers of maroon and green shales. The Glen Eyrie Shale member is a grey sandstone layer approximately 30m thick, with thin coal beds. The Fountain formation is not well indurated, and as such does not form the ridges that the nearby Lyons formation does. The Fountain formation is approximately 1350m thick. Notably, it outcrops in Boulder, CO as the Flatirons, and in Colorado Springs, CO as one of the units of the Garden of the Gods.

InterpretationEdit

The Fountain Formation is generally associated with the uplift and erosion of the Ancestral Rocky Mountains, which started in the Pennsylvanian. The heavy erosion that occurred during this time stripped the marine sediment from the area down to the 1.8Ga (pre-cambrian) basement (Pike's Peak Granite in the Colorado Springs region). This is represented as the unconformity between the granites (such as Pike's Peak Granite), and the Fountain formation. At the time of the Ancestral Rockies Orogeny, the uplift would have caused steep slopes, and heavy sediment load in stream runoff. It is this runoff that is the supposed source for the fountain formation, specifically a large fan type area, possibly based in the still receding waters in the large paleooceans. Of great significance are the large channels noted above, which are commonly interpreted to be debris flow channels, and stream channels that have been filled with sediment. There are also some channel patterns that appear to be braided streams, which are characteristic of a fan or fan delta type environment. The dark red and maroon layers are likely secondary features caused by the precipitation of hematite into existing rock from groundwater.


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Detail of layering within the Fountian formation. Photo provided by paleocurrents.com


Fountain Formation Characteristics
ColorPrimarily reddish-brown to maroon with some lighter grey layers
Thickness~1350m
SortingExtremely Poor
RoundingSub-Angular to Sub-Rounded
SizeFine Grained Sand to Cobbles
Mineral CompositionLots of Feldspar, Quartz Rich. Pre-cambrian granitic clasts
CementSilica cement, with Hematite cement in places
BeddingThickly bedded, with some coal beds in the Glen Eyrie Shale member ~30m thick
OtherPoorly indurated, laterally extensive





Scholarly Works about the Fountain FormationEdit

External LinksEdit

USGS Website Geology of the Boulder Flatirons: Fountain Formation

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