view from southeast Silver Reef
Silver Reef, Utah, Geology Highlights
Geological Formations as viewed from the Museum at Silver Reef, Utah
By Robert F. Biek
Utah Geological Survey
Still farther southeast, through a gap in the Virgin anticline, we can see the Hurricane Cliffs with the town of Hurricane at their base; the cliffs, remember, are an eroded fault scarp of the Hurricane fault, southwest Utah's largest, most active earthquake fault. On the skyline, the same brightly colored layers of the Moenkopi Formation form Gooseberry Mesa, again capped by the resistant Shinarmp Conglomerate, the basal member of the Chinle Formation.
Near Hurricane, the bulk of the Hurricane Cliffs are eroded from gray limestone that was deposited in Lower Permian time about 270 million years ago. The top of the cliffs are capped by a yellowish-brown limestone deposited almost 250 million years ago at the beginning of the Triassic Period. The boundary between these two rock formations is called the TR-1 unconformity, the first major unconformity of the Triassic Period in southwest Utah representing a gap in the rock record of some 10 million years. Worldwide, the end of the Permian Period corresponds to the great Permian mass extinction, a time 252 million years ago when as much as 95% of all marine species and 70% of all land vertebrates on earth vanished. That great mass extinction left countless ecological niches open and ushered in the age of the dinosaurs at the beginning of the Triassic Period.
Like at Black Ledge to the north, the Hurricane Cliffs here are in essence an eroded fault scarp of the Hurricane fault, the largest and most active earthquake fault in southwest Utah and one that is active and capable of producing damaging earthquakes. At Hurricane, the fault juxtaposes Upper Cretaceous strata west of the fault against Permian strata east of the fault, revealing about 3600 feet of displacement over the past several million years. The fault also offsets comparatively young basaltic lava flows, parts of which are preserved as remnants in the cliffs east of town.
The Virgin River cuts through the cliffs between Hurricane and LaVerkin revealing an impressive cross section through the fault zone at the entrance to Timpoweap Canyon. Just upstream of the fault are Pah Tempe hot springs. The springs used to flow about 5000 gpm with a very high 7000 to 10,000 TDS. Unfortunately, the flow of springs disrupted by Washington County Water Conservancy District pipeline work in channel and the hot springs are no longer open to the public. Still, with a water temperature of 100 to 108º F (42º C), Pah Tempe has the highest recorded spring-water temperatures in Washington County. The high TDS means that Water Conservancy District extracts Virgin River water from upstream of the springs and pipes it to Quail Creek and Sand Hollow Reservoirs for storage. The springs are present here because this is a natural discharge point for regional groundwater — the groundwater rises through permeable and cavernous Permian strata east of the fault, but the Triassic red beds and the fault zone itself act as barriers to groundwater flow.
The Permian-Triassic unconformity is marked by river-channel deposits eroded down into the Lower Permian Kaibab Formation, and by old soil deposits developed on former paleotopographic high areas. Near Hurricane, these channel and soil deposits are mostly inconspicuous, but southwest of St. George, channels several hundred feet deep are cut down into Kaibab strata. The unconformity represents an episode of dramatic, worldwide sea-level drop and the largest global extinction event in Earth’s history.
The Moenkopi Formation of southwestern Utah, so well exposed at Gooseberry Mesa with its alternating reddish-brown, white, and gray layers, documents renewed shallow-marine sedimentation along the western margin of Pangea. The Moenkopi consists of three transgressive members (the Timpoweap, Virgin Limestone, and Shnabkaib Members that each record an interval of sea-level rise), each of which is overlain by an informally named regressive red-bed member (the lower, middle, and upper red members, respectively, which record sea-level fall); the Rock Canyon Conglomerate Member locally forms the base of the Moenkopi Formation. These members record a series of incursions and retreats of a shallow ocean across a gently sloping continental shelf, where sea-level changes of several feet translated into shoreline changes of many miles.