BACTRIAN MOUNTAIN
Bactrian Mountain is a
double-hump, ENE-WSW trending extension of the southern Mt. Irish Range in
southern Nevada, Great Basin, USA. It is northeast of America's Groom Lake
Facility ("Area 51"), and accessed by gravel roads heading north from
the Extraterrestrial Highway (formerly Rt. 375).
The section at Bactrian
Mountain is a nice Devonian to Mississippian succession of mostly carbonates
& siliciclastics. The section has a well-constrained
Frasnian-Famennian boundary (lower Upper Devonian-upper Upper Devonian boundary
& mass extinction horizon). The only known place on Earth having a
Famennian-aged carbonate succession is right here in the Great Basin. The
stratigraphy here also records the Middle Paleozoic transition from a passive
margin to an active margin along Laurentia's Cordilleran margin. This
passive-to-active margin transition was a result of the Antler Orogeny.
Above & below: Looking ~N. The
double-humped ridge in the foreground is the Bactrian Mountain section.
The rocks here are dipping westward. The peak on the right contains
Devonian sedimentary rocks. The peak on the left contains Mississippian
sedimentary rocks. The Devonian-Mississippian boundary occurs somewhere
near the western edge of the saddle between the two peaks.
Stratigraphy at Bactrian Mountain:
Joana Limestone
_____________________________
Pilot Shale ¬ ~Dev.-Miss. bdy.
_____________________________
West Range Limestone
_____________________________
¬ Frasnian-Famennian bdy.
Guilmette Formation
Above & below: Looking W. This is
the southern face of Bactrian Mountain. The eastern hump is to the
right. The western hump is the cliff top at left-center. The rocks
in the slope at center & right are part of the Guilmette Formation
(Upper Devonian), a heterolithic unit containing limestones, quartzites, &
evaporites. The steep cliff face in the distance is the Joana
Limestone (Kinderhookian, lower Lower Mississippian). The Pilot
Shale (~uppermost Devonian to ~lowermost Mississippian) is the gray slope
below the Joana cliffs. The thin limestone cliffs below the Joana are
part of the West Range Limestone (Famennian, upper Upper Devonian).
Above: Looking ~NW.
Southern slopes of Bactrian Mountains's eastern hump. These beds are part
of the Guilmette Formation.
BACTRIAN MOUNTAIN EAST
Above & below: Wonderful views of the
Pahranagat Valley from the eastern hump of Bactrian Mountain, Mt. Irish Range,
southern Nevada, USA.
Above: The Frasnian portion
(lower Upper Devonian) of the Guilmette Formation's upper member. The
light gray rocks are limestones. The dark brownish rocks are quartzites
(well-cemented quartz sandstones). Ernie Slucher for scale (Ohio
Geological Survey).
Above: zebra evaporites in the
Guilmette Formation's upper member (upper Frasnian Stage, lower Upper
Devonian).
Above: geologists examining the
Frasnian-Famennian boundary (= gray limestone bedding plane marked with stake,
just below camera bag at center-left). The Frasnian-Famennian boundary
divides the Upper Devonian into two parts. The F-F boundary marks one of
the top five most significant mass extinction events in Earth history (others
include the end-Ordovician m.e., the end-Permian m.e., the end-Triassic m.e.,
and the Cretaceous-Tertiary boundary). The F-F boundary in the Bactrian
Mountain East section occurs at the top of unit 4 of Morrow (1997, Courier
Forschungsinstitut Senckenberg 219).
Above: Bedding plane view of the
Frasnian-Famennian boundary, Guilmette Formation's upper member, mid-Upper
Devonian.
Above: Ernie Slucher (Ohio
Geological Survey) resting on the Frasnian-Famennian boundary.
Above & below: plants of the genus Yucca
are moderately common in southwestern America's deserts. The green leaves
are extremely stiff & sharp-pointed. They could easily serve as
daggers. Extreme care is advised when walking along & among Yucca
plants.
