H15: Virgin Oilfield S13 T41S
R12W 1/2/06
In this country, the highest basalt is the oldest one- quite the
opposite of Sedimentary beds which have the oldest at the bottom of the
stratigraphic column. Since this land is rapidly eroding down, the earliest
basalt flow would have over-ridden and protected the soft beds beneath, when
the land was much higher in elevation (pre-recent erosion). The oldest basalt
column will lie at the highest elevation, and will protect the conglomerates
which were in place previously; this oldest basalt will yield the oldest Pc
(uneroded), and will help establish the landscape, pre-vulcanism.
In S13
T41S R12W, hikers climbed to the base of the Pb (Plio-Pleistocene basalt) via
a small arroyo, which leads into North Creek. There is an erosional break
(saddle) in the basalt at the NE corner of this section, indicating an NW-SE
fault, but this and the oilfield were not investigated. The creek drainage
direction on both sides of the valley indicates the orientation of the supposed fault. The
stratigraphic section can be clearly seen across the N. Creek valley, since it
is not covered by volcanic rubble- as is the one climbed. The entire climb was
underlain with Triassic, mostly Moenkopi, but increasing petrified fragments
(becoming finally very large- up to foot length)- indicated that under the
rubble there was Chinle. Before the final loose conglomerate just below the
Pb, there were many thin beds of shaly sand, on the order of a centimeter
thickness (soft to the point of breaking in my pocket). These overlay 3-5
meter thick layers of sandstone, which contained occasional layers of
inch-thick pebble conglomerate.
The loose
conglomerate Pc, lies at an elevation of 4800 feet, by Topo map and altimeter,
and contains round boulders of Pine
Valley intrusive? rock, meter-thick
rounded pieces of red sandstone, and some limestone clasts with fossil
remains. If these clasts are confirmed, this will indicate that there was no
scarp in the Colorado Plateau in this location in Pc time- the boulders would
have rolled down from the Pine
Valley
Mountains unimpeded, in large
storms, with no scarp to re-direct them.
The time
of the gravels may be calculated
using the approximation that there are 400 feet erosion per million years-
comparing the valley elevation of 3700 feet (Topo) to the 4800 feet basalt
base elevation. This yields at least an age of 2.5 m.y. for Pb. This would be
among the first eruptions of basalt for the area, and would establish that the
last uplift of the Hurricane fault had not commenced; the previous uplifts
would have been peneplaned for the path going NW to the Pine Valley Mountains
(This is not exactly a peneplane, since the lava would have flowed down a
pre-existing water course- it is more likely an ancient wide valley).
Peneplanes may be seen on the next mesa to the west and on Hurricane Mesa
(5000 feet elevation), hinting that the valley may have been several hundred
feet deep.
Should
this analysis of the time sequence be confirmed, this would place an age of
the Pc at Pliocene. The basalt would have been emplaced later, only to be
found by radioactively measuring the flow. This would fit with the other
calculations made by the author, for an age of the last uplift of the CP near
2 million years. Previous observations indicate that the Colorado
River cut through the Kaibab uplift, and the N-S fractures
supplanted NW-SE orientations at this same time. Down cutting of the Plateau,
as indicated by mesa tops giving way to steep slopes occurred at about this
time also, which all indicates that a major uplift of more than a thousand
feet was initiated then.
CONCLUSIONS;
1.
The last cycle of the uplift of the Hurricane fault
(and CP uplift) started after the estimated Pc gravels' age of 2.5 m.y;
2.
Basaltic eruptions started after the last uplift,
erupting near the Hurricane fault;
3.
Previous cycles of uplift were peneplaned in some
areas (but not at the Hurricane town location) before final
uplift;
4.
Eruption centers moved westward through time in the
last cycle; and
5.
Rivers re-oriented after the rejuvenation of the CP
uplift.
Review of Findings:
In this country, the highest basalt is the oldest one- quite the
opposite of Sedimentary beds which have the oldest at the bottom of the
stratigraphic column. Since this land is rapidly eroding down, the earliest
basalt flow would have over-ridden and protected the soft beds beneath, when
the land was much higher in elevation (pre-recent erosion). The oldest basalt
column will lie at the highest elevation, and will protect the conglomerates
which were in place previously; this oldest basalt will yield the oldest Pc
(uneroded), and will help establish the landscape, pre-vulcanism.