Wood's Canyon (T16 R6E S30),
near Jack's Canyon
The nearest butte close
to hiway 179, on the east side, after crossing Jack's canyon creek, was
ascended up to the level of Ts (tertiary sediments, just below Tvolc
extrusives). These sediments are on top of Ps (Permian redbeds-
This white ls. was
viewed close-up on this butte. It was composed of lake carbonates with
considerable amounts of Pk- Kaibab- fossils and angular pebbles and of Ps
cobbles and shards, which had been layered and distorted in the soft bottom
carbonate (Ps stones were not generally touching, but isolated as if ooze had
supported them above the bottom of the lake). They lay in rounded arrangements
and other non-level configurations, while the containing ls. was generally
flat-lying. The overall analysis was that the ls. was deposited in stagnant
(except for storms) lake waters, and it is still
presently level- as is the top of the formation. The limestone is very
coherent and strong against breakage, but generally clean and low in silt and
clay- it gives the crushing appraisal of being early Tertiary in age. It is not
travertine, but is fossiliferous limestone, with occasional slats of tan
sandstone- probably Ps, since it overlies Ps, but possibly Pc or even Trm. If
it were Moenkopi, this would give it an age much earlier than middle Tertiary
(due to the estimated erosion rate of the edge of the Colorado Plateau).The
fact that Pk is overwhelmingly dominant suggests that the slats are likely Ps
or Pc.
Across the
canyon, similar deposits were still level, although faulted and
lowered relative to those seen close up at 4800 feet. These have no similarity
to Laramide gravels, and they have no extrusive clasts in them, but appear to
be intermediate between Laramide gravels and Tbb. Further, since they are
almost 300 meters above the normal elevation of 3900 feet, they have been
influenced by the raising of the Mogollon rim. This would not preclude them
from having Laramide gravels at the base, but this was obscured by rubble
(unlike the same outcrop seen across the canyon). Since it would have required
4 more hours work to climb across to view them, this was deferred. Why is it
likely that the formation is early Tertiary? Because the unconformity
between the ls. containing no extrusives and the overlying Th is
clean, ungravelled, level and flat, and the part at the base is larger grained
and sitting on top of Ps; this would make it older than Tbb and younger
than my Kg.
The following scenario
is proposed for these Ts limestones, which would have been deposited some
distance from the rim (if it existed at the time) at the time of their
deposition (not as now, when we were sitting on the rim):
This location, which is
about 3 km from the estimated position of the rim which is expected in
Miocene times, was in a small valley, which was first filled with sand from
faster moving streams. Gradually, the valley became a lake as it clogged
itself, and eventually became a closed stagnant pond. Storms brought in
occasional clasts from the nearby Ps, which were floated in algae and organic
debris-influenced calcareous mud. This was pre-vulcanism time, hence somewhat
later than Laramide gravels but earlier than Hickey time- 20+ m.y.? The
elevation of 4800 feet requires that uplift (thermal expansion?) has elevated
the formation sometime later than its deposition.
No indications of fractures
deriving from the Montezuma linear were found, and the expected location of
these is 2-3 miles further to the NE, in Wood's canyon. Ps manifests many
orientations of fractures in this faulted canyon, but the few TKs did not.
A tentative observation
is that my Hackberry- Montezuma Well linear does not occur where the
Mogollon Rim is present (roughly starting just west of the I-17 freeway). If
this is found to be generally true, this means that the NW fractures dominate
the surface, whenever there is a rim, but where there is no rim, the N-S
fractures can manifest themselves.
Harold L. Overton