H11: Virgin R., below Hurricane basalts S28, 29 T41S R13W 11/02/05
This area can be accessed at the east end of paved 600N, in Hurricane Sky Mountain golf course, walking
north along the fence there. Alternatively, there is
an old ranch road west of the golf course, which
snakes around to the Virgin rim.
The Gould wash is reached initially (just before
emptying into the Virgin), showing that there are two
basalt flows on the north side of the canyon, whereas
3 flows are seen in the south wall. The canyon drops
precipitously at the end of the golf course, at least
50 meters. The additional basalt flow on the south
side originates at the Sullivan's Knoll (10,000 year
cone), whereas the deeper flows originate from the
east, in the city limits of Hurricane. Below the lower
basalt flow, there is a layered rock, appearing to be
a meter thick sandstone- flat and almost level,
definitely not
conformable with the underlying
cross-bedded Jn. This was investigated on 11/02/05,
and found to be a flat and almost level sandstone
covered by a flat and almost level tuff. This Pss
(Pleistocene to Pliocene sandstone) is seen in three
dimensions at the Gould’s Wash, where it remains
constant in thickness (1 foot), elevation (3100 feet),
and layering (flat and pancake-like) over a plane ¼ x
¼ mile in two dimensions. This stratum shows that
there was no canyon at the present Gould’s Wash (now
200+ meters deep), nor at the present Virgin River in
Pss time. However, Pss does not occur on the north
side of the present Virgin, showing that it terminated
at the cross bedded Jn which now occurs there. The
lower and middle basalt flows have been almost
peneplaned before the upper basalt layers were
deposited. This relates that significant time
intervened between the three basalt flows; all three
have cones which occurred in later stages
of
volcanism, compared to the basalt flows.
The middle basalt flow terminates against a hill of Jn
(fossil dunes, from the cross bedding), and
immediately to the side of this Jn hill, there is a
flat and almost level conglomerate, Pc (Pleistocene
conglomerate), at a base of 2830 feet, with top of
2880. Hence there was a canyon present in the Jn to
the east in Pleistocene time, which allowed a swift
current to carry basalt boulders to a large valley
where the large meander now occurs. The current
dropped off in this valley and allowed a wide deposit
(about 300 meters) of Pc to occur. This conglomerate
is mostly basalt boulders initially, but becomes more
sand-colored and contains smaller boulders and sand to
the west (within 400 x 400 meters in area). Looking to
the east, at the Hurricane cliffs, it is obvious that
the basalts lying near the top of the rim of the
Hurricane fault have originated from the same
volcanoes as
occurred in the Hurricane city limits;
these basalts were banked against the preceding
Hurricane cliffs, showing that there was a fault scarp
at the initial deposition of basalts, about 30 meters
in height (just east of the town of Hurricane).
In the meander, which is horseshoe shaped, the origin
seems to be partly from large fractures, and an
obvious fault occurs in the middle of the horseshoe
turn, in the Jn on the west side of the river, running
NW-SE. Fractures on the hogback to the NW, on the west
side of the river, also run NW-SE. However, the fault
points to a large ravine on the south side of the
Virgin, where the fissure makes an abrupt
re-orientation to the N-S.
The large meander now lies deeper than the 2830 Pc,
and it could be that the rise of the C.P. at the Zion
Park has initiated a swifter flow which has cut
through the Pc and deeper into Jn, resulting in a
river canyon below the present Pc. The uplift of
the
Colorado is seen as a normal fault down dropping Jn to
the west, causing renewed canyon-cutting through the
conglomerate. Some evidence of this is noted as a
west-oriented slot in the Pc (and now abandoned).
The reason that a large horseshoe-shaped meander
occurs lower in elevation than the Pleistocene
conglomerates Pc is found by an investigation of the
Virgin surroundings downstream. There is a lava dam
caused by Sullivan volcano deposits to the west, which
apparently created a lake on its east side. So long as
this lake remained static, the river created meanders
upstream of the lake. I will try to answer the
following questions:
a. Is the extensive volcanism the reason for the large
canyons occurring in the Virgin, all the way from the
town of Virgin to this investigated meander (if so,
the course of the river would have been determined by
the location of the basalts)?
b. Remembering that there was a disruption
of the
basalt on the north wall of the Virgin, immediately
west of the town of Virgin, it appears that other
uplift has occurred on the west side of the Hurricane
fault, separately from the uplift on the east side.
Block faulting in the same area, of Mesozoic redbeds,
not in the principal directions of NE-SW or NW-SE,
occurs in the city limits of Laverkin. Can it be that
the latest Hurricane fault first occurred south of
Laverkin (and not in its now further east scarp
location?
2nd foray into the Horseshoe-shaped meander:
Findings (3/27/05):
1. The dip of the Pc (Pleistocene conglomerate, which
is spread over a km distance) is 1-2 degrees up to the
west! This is against the present flow of the river,
so there must have been uplift on the west side of Pc,
since deposition. Since down-cutting is omni-present
now, there had to be a period of deposition- such as
with a lake to the west;
2. There are 2 ravines running E-W
through Pc (inside
the meander), becoming progressively lower in
elevation to the north, before the present stream bed
is reached on the north side (where the meander
terminates against the hogback of the Virgin
anticline); it is impossible for the river to move
further northward, until the hogback is eroded;
3. The small fault running NW-SE from the hogback
center is only a meter in displacement (maximum, as
seen on the north hogback wall) although it points to
a deep ravine on the south side of the river,
indicating that this created a path for the river to
erode downward below Pc.
4. The ravine aligning with the small fault is some
1-2 meters wide, indicating a major influence on the
river path- if it continued under the present river
path. However, although the fractures on the north
side of the river indicate a weakening of Jn for
canyon cutting, there is hardly any displacement. This
same action has been reported for
Zion Park, where
small streams are initiated by fractures, as
contrasted to major rivers crossing these fractures
(which were entrenched. from previous drainage in
large canyons, before the present new fracture
orientation).
New Tentative Conclusions
A. The local area was fairly flat, in Pre-basalt times
(as indicated by the thin bed at base of Pb-
Pleistocene basalt). with a small scarp on the east
side of Hurricane, shown by the top of the basalt
along the Hurricane cliffs. This scarp was only 30-100
meters high, as compared to the 200-300 meter scarp
now in place.
B. Outside of the Laramide Virgin anticline the land
was planed off, not quite flat, but somewhat sloping,
as indicated by the incised Jn to the north of the
river near the Virgin Meander. Zion was rising (more
than the rebound noticed in the conglomerate to the
south), as was the whole C.P., as shown by the small
scarp at the Hurricane cliffs.
C.
In the present episode, the Hurricane fault was
re-activated, causing uplift along the fault; with
increase of fracturing and N-S shear, and vulcanism
commenced. This laid a wall of basalt against the old
30-100 meter scarp (blocking further advance of lava
flow).
D. Vulcanism continued to the west, evidently having
started in Zion Park (measured dates of .2-.5 my).
Basalt extruded first, and later ash fall and cones
developed. The last extrusion was in Snow Canyon, near
St.George.
E. With uplift accelerated, erosion also increased and
the Virgin River enlarged. It was blocked for awhile,
but it worked around the three flows- eroding all of
them partially.
F. After each interval of vulcanism, the top was
peneplaned (or was laid almost flat at termination);
the last flow was almost flat initially..
G. Uplift continued to the west (the slope of Pc is 1
degree upward to the west) with unloading or rebound,
and the river
abandoned Pc and cut canyons in it
(there are two of these, most noticeable in the Virgin
meander).
H. With more uplift, the river downcut further, to its
present elevation of 40 meters below Pc.
Overall Assessment of movements of the Hurricane Fault
1. Prior to the last uplift of the Zion Park, the
Navajo sandstone was fairly well peneplaned, with no
evidence of the Virgin River;
2. With uplift of the Colorado Plateau in the
Pleistocene, new drainages were established- the old
N-S or NE-SW drainage of the Laverkin Creek was
captured by E-W flow of the Virgin;
3. A scarp to the east of the town of Hurricane
created large stream gradients to the west, and caused
the old drainages along the Hurricane scarp to be
abandoned;
4. After deposition of sandstone to the west of this
subtle scarp, an ash flow deposited tuff conformably
on the tan sandstone, and this later was overlain by
basalts (the basalts are associated
with heat
previously contained under the C.P, but now released
by extensional fractures);
5. This lava flow created blockages to westward river
flow, and was finally breached- resulting in a 50 foot
thick layer of conglomerate, with many basalt
boulders; and
6. Later basalt flows dammed the Virgin again, and
were breached -allowing canyon cutting to the present
time. Harold L. Overton