This satellite photo shows the entire area, with towns, roads and some landmarks identified
consolidated to form a tuff called the
Huelster Formation in the Davis
Mountains and the Pruett Formation
further south. Similar tuffs occur
throughout the Big Bend and northern
Mexico. A cobble found near the base of
the Huelster Formation was dated at
38.4 Ma and gives a starting date for vol-
canic activity although no one knows
how long it took for the Huelster and
Pruett Formations to accumulate.
Two million years later, the main vol-
canic sequence in the Davis Mountains
began. It lasted 1.5 million years and con-
sisted of six main lava eruptions inter-
spersed with explosive ash eruptions.
A certain amount of magma solidified
underground to form igneous intrusions,
several of which have been uncovered by
erosion. Geologists classify intrusions by
shape, and to some extent, size.
A sill is a tabular or sheet-like body
injected parallel to the bedding in its host
rock. The shape of an uncovered intru-
sion depends partly on the shape of the
original intrusion, so sills, being parallel
to the beds in which they were injected,
are usually horizontal or near-horizontal,
and often create mesas when uncovered.
A laccolith is a mushroom-shaped sill
that domed the beds above it. Lacoliths
are common in this area and tend to pro-
duce mushroom-shaped mountains,
Sawtooth Mountain, for example.
A dike is a tabular-shaped intrusion
that crosscuts the bedding of its host
rock. Dikes are usually nearly vertical.
They tend to be harder than lavas and
stand up above the surface, an example
of differential erosion, creating ridges
with coxcomb or iguana lizard profiles.
A plug is a vertical, pipe-like body,
sometimes a volcanic neck or solidified
intrusion feeder. Plugs narrow toward
their peaks when eroded. A good exam-
ple is Mitre Peak north of Alpine.
The greatest concentration of intru-
sions is in the Sawtooth Mountain-
Mount Livermore area where a sill
extends across Highway 166 and a series
of intrusions caps Mount Livermore.
These intrusions have been eroded into a
craggy ridge culminating in the high bare
Baldy Peak, the highest peak in the
mountains. The volcanic rocks around
Mount Livermore have been uplifted by
as much as 2,000 feet, most likely
because of a large intrusion under-
ground. None of these intrusions have
been dated.
Four intrusions between Fort Davis
and Alpine create distinctive domes. Two
of these, the Western Intrusion and the
Barillos Dome, are 32.8 million years
old. A series of sizeable intrusions occurs
south of Alpine and several large sills out-
crop west of Balmorhea. The group of
small intrusions occurring in the Paradise
Mountain Caldera near Highway 166,
are thought to be part of a dome that
rose up when all the lava in the caldera
had erupted, a resurgent dome as such
bodies are called.
In parts of the area, forces deep in the
crust have raised the Earth’s surface up,
creating uplifts. One such uplift that
began in the Permian period has exposed
ancient Precambrian and Ordovician
rocks north and west of Van Horn and
another in the Marathon area east of
Alpine has exposed Permian strata.
The central North American conti-
nent began being uplifted along a ridge
roughly coincident with the Frontal
Range of the Rocky Mountains about 25
million years ago, soon after the main
volcanic phase ended. This uplift affect-
ed the entire United States, most of
Canada and northern Mexico. As a
result, Cretaceous limestones in the Kent
area, for example, are now some 4,500
feet above sea level although they origi-
nated below sea level.
Just before the uplift began, a rift zone
began to develop along the crest of the
uplift about 27 million years ago, a zone
known as the Rio Grande Rift that is
now followed by the Rio Grande from
Colorado to the Big Bend. In this zone,
the Earth’s crust was stretched or extend-
ed and broke into fault blocks. Some
blocks stayed high while others sank
down into grabens (the German word
for ditch). Basalt lavas erupted through
the faults and spread over the landscape
in thin flows from about 25 to 18 mil-
lion years ago.
South of Socorro, New Mexico, the
zone branches out into multiple seg-
ments. One of these segments, called the
Salt Basin Rift after the salt basin north
of Van Horn, runs southwest of the
Davis Mountains. Another segment runs
along the Rio Grande Valley and joins
the Salt Basin Rift at the Big Bend to
continue into Mexico as the Sunken
Block.
The Davis Mountains volcanic field
continued on page 27
Cenizo
Third Quarter 2018
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