Flathead Sandstone - Cambrian Beach Deposit

Outcrop on Hillside

        Located in trees above the Miami Beach chairlift on Red Lode Mountain are outcrops of thick layers of sandstone. Upon close examination one can see that there are cross-bedded units of quartz sandstone. These beds are part of the geologic formation known as the Flathead Sandstone, named for similar rocks found near Flathead Pass in the northern part of the Bridger Range, Montana. The Flathead is a quartzite in places and frequently appears red or reddish-brown in color. The color is primarily due to iron staining, with iron oxide cementing many of the grains. At its contact with the underlying Precambrian granites and granite gneisses, the Flathead commonly has a large proportion of pink potassium feldspar and appears pebbly and arkosic.

Cambrian Seaway

        Approximately 530 million years ago (Middle Cambrian time) this region was a seaway with a beach area on the western flank of what was then the "Transcontinental Arch" of what became the North American continent. (On the map the sandstone is shown in a stippled pattern. Carbonate Platform deposits, shown as brick-like pattern, occur farther from the continent and Open Shelf depostis, shown as fine dashed lines, occur even farther from shore. All of these represent deposits in a shallow marine sea. Deeper oceanic waters probably occurred outside this shallow seaway.) The map shows there were similar beach deposits all around the Arch. In this region of what is now Montana/Wyoming, the seaway slowly migrated east as sea level rose relative to the continent. The sandy beach migrated slowly eastward with this rise in sea level. By late Cambrian time (nearly 500 million years ago) the beach area reached what is now the Black Hills region. There the Upper Cambrian Deadwood Sandstone is the lateral equivalent of this Middle Cambrian Flathead Sandstone. In other words, it took about 30 million years or so for sea level to rise enough for the shoreline to migrate from this part of Montana/Wyoming the approximately 300 miles (450-500 km) to the region of the Black Hills.

Flathead Formation = Quartz Sandstone

        As described above, this formation consists of nearly pure quartz pebbles and sand (silicon dioxide = SiO2), with Some impurities, such as iron minerals like pyrite, weather to give the Flathead Sandstone its color. Elsewhere such formations are quarried to make window glass. This formation is similar to many east coast beaches (New Jersey to South Carolina), with nearly pure quartz sand and minor iron impurities. The composition of this formation suggests that many igneous and metamorphic rocks were exposed at the surface and were being subjected to weathering and erosion to produce the quartz sands. Many minerals that make up the igneous and metamorphic rocks are chemically dissolved, leaving mainly quartz grains, with some feldspars. These quartz grains are carried to the oceans where currents distribute them along beaches. These currents carry the sand grains over other layers, allowing the grains to accumulate along sloping surfaces, producing the cross-bedded character of the Flathead Sandstone.

Cambrian Portion of Stratigraphic Section

        In this area the Flathead Sandstone is the oldest Cambrian unit, sitting on top of eroded Precambrian "basement" rocks
Other Cambrian rocks occur higher in the stratigraphic section (to the north in this area). They include limestones and shales in the saddle between this outcrop and the more resistant limestones and dolomites forming the high Palisades on the northern skyline

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