Geology & Minerals: Coconino NF, Red Rock District, Bell Rock

In geology, sedimentary rocks are divided by stratigraphers into groups, which may then be divided into formations, which may then be divided into members.  A formation generally is a section of rock that was deposited under a certain set of environmental conditions.  Members within a formation usually record minor environmental fluctuations during the deposition of the formation.  The place where each group, formation, or member is first described in detail is known as the type section for that unit.  Bell Rock is the type section for the Bell Rock Member of the Schnebly Hill Formation. 

Bell Rock

The Schnebly Hill Formation plays a leading role in the Sedona area, making up most of the spectacular red rock cliffs, including Bell Rock, Courthouse Rock, Cathedral Rock, and Coffee Pot Rock.  All of the bright orange sandstone in Oak Creek Canyon is also part of the Schnebly Hill Formation.

The Schnebly Hill Formation has been divided into four members in the Sedona area, from youngest to oldest:

  • Sycamore Pass Member - up to 700 feet of red to orange sandstone containing high-angle cross-beds

  • Ft. Apache Member -  a limestone, dolomite, and siltstone unit, 8 to 10 feet thick at the top of Bell Rock, over 100 feet thick to the southeast near Ft. Apache

  • Bell Rock Member - over 500 feet of flat-bedded, ripple-marked sandstone and siltstone, slightly darker in color than the Rancho Rojo

  • Rancho Rojo Member - 20 to 40 feet of orange sandstones containing low-angle cross-beds

The Schnebly Hill Formation is separated from the underlying Hermit Formation by an unconformity representing less than one million years of erosion or non-deposition.  While the Hermit Formation was being deposited, the Sedona area was an arid floodplain surrounding river channels draining the Ancestral Rocky Mountains. 

Less the one million years later, the Pedregosa Sea had encroached upon Sedona and sand was blown into the sea from the nearby desert to form submarine sand dunes of the Rancho Rojo Member

As the sea retreated, Sedona emerged from the sea as a tidal flat.  Still, sand was blown out of the desert to the north into the Sedona area.  The sand was reworked by the tides and deposited as the flat-bedded Bell Rock Member.  Near Bell Rock, over 500 feet of sand accumulated before the sea advanced over the tidal flat for one last brief period, during which 8 to 10 feet of limestone, dolomite, and siltstone were deposited in the Sedona area.  About 272 million years later, the limestone, dolomite, and siltstone would be called the Ft. Apache Member

Near Ft. Apache, southeast of Sedona, the Ft. Apache Member is over 100 feet thick, indicating that the Pedregosa Sea had advanced from the southeast.

Soon, the sea receded for good, and the dunes advanced into the area.  By 270 million years ago, near the middle of the Permian period, Sedona was entirely covered by sand dunes of the Sycamore Pass Member.