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Introduction
The Chalk Canyon
formation is a series of Early Miocene (15 - 22My) aged basaltic
volcanic and tuffaceous limestone lacustrine sedimentary deposits
in Central Arizona. It is subdivided into an upper and lower
member by an unconformity, the lower member, 22 to 17 million
years old consisting of alkaline basalts interbedded with crystal
and lithic tuffs, and the upper member, which is 17 to 15 million
years in age is primarily fluvial and lacustrine (lake deposit)
sediments with a subordinate amount of basaltic rocks. Although
the Chalk Canyon formation extends from southwest of Lake Pleasant
to north of Black Canyon City, it is the upper lacustrine and
fluvial member that is of primary interest to sedimentologists
and paleontologists.
The upper member
can most easily be recognized by the extremely white chalky sediments
extending along I17, the main north - south freeway in the state,
from about North of Cave Creek up to about a few miles north
of Black Canyon City. This area represented a lower depression
in Early Miocene times, and quickly filled in with the lake and
river sediments we see today as limestones, coarse conglomerates
and tuffaceous "shales". The white limy beds are interbedded
with thin brown coarse conglomerate deposits, with clasts containing
a variety of rocks with the exception of basalts.
Fossils
and sedimentary structures
While most of the
beds in the chalk canyon formation are a white amorphous and
thickly bedded sequence of tuffaceous rocks, there are also many
areas with finely bedded laminar beds of very finely grained
sediments with sedimentary structures. Most common are mudcracks.
We have found many thousands of pure white slabs with fossil
mudcracks ranging from 1/16 inch cells up to many feet across.
Most are in the range of 1 inch cell size. Along with mudcracks,
crossbedding, channel fill deposits, and raindrop impressions
were seen.
Fossils are rare.
Most abundant are highly bioturbated worm burrow slabs, with
small tubular burrows about 2 to 5mm across saturating the surfaces
of localized areas. This is the only fossils mentioned in the
literature. But we also found other types of trace fossils including
Aulichinites - attributed to small gastropods grazing traces,
and a few vertebrate toe and foot imprints similar in size to
a small carnivore type animal might produce. Its key to note
that any areas with mudcracks always indicates a period of subareal
exposure, and the possibility of animals leaving traces is always
present. One body fossil of a small annelid worm was also found,
similar if not identical to Deros sp., a small tube building
worm, that lived on rotting vegetation in lakes and streams as
today.
Finally, we found
the Chalk canyon formation difficult to photograph because of
its extreme whiteness. It is even whiter than the tertiary marls
in the Verde formation and challenging to get satisfactory images.
We also found some interesting heavy manganese minerals in flat
sheet-like beds in the formation, and a few areas with abundant
cherts and psudomorphs.
GENERAL APPEARANCE
Erosion of the
soft limy sediments create low rounded hills and cliffs such
as this butte in Black Canyon city, just east of the freeway
I17. The slopes of the hills in this area contained a large amount
of chert and calcedony nodules, along with psudomorphs of calcite
after actinolite like mineral crystals.
This image best
reveals the color variations in the upper Chalk Canyon formation
beds, seen here in Black Canyon city. The topmost bed here is
extremely white and consists of a limy facies composed of limestone,
chert and subordinate conglomerates.
Massively bedded
tuffaceous materials are also very common in the upper member.
They are unfossiliferous, and have few sedimentary structures.
However, this outcrop also had beds of re-vitrified tuff, seen
as a very white porcelain like material, that was glassy in appearance
and very hard. Black Canyon City.
Blinding white
shaley ledge in a wash, north of New River along Highway I17.
At this locality, which is on the southernmost end of the depositional
basin of the lacustrine deposits we find sheet like beds of tuffaceous
shales, and large numbers of fossil mudcracks.
Interbedded conglomerate
bed in the upper member of the formation. This ledge is in a
wash west of Black Canyon City, and was overlain by the sheetlike
white tuffaceous shaley beds. It appears to be a fluvial deposit
of river bed origin, that proceeded the deeper water lacustrine
environment.
Close up of the
very typical interbedded sandy conglomerate beds in the upper
member of the formation. The keys are shown for scale. Interestingly,
the clasts do not contain any basalt, and appear to predate the
Hickey formation volcanics.
Dick and Mardy
Zimmerman examine the hordes of mudcracked shales north of New
River. Several of the slabs found here contained what looked
like fossil salt crystal casts, in addition to extensive bioturbation
from large worms. The rolling hills and ridges seen here are
typical of the deposit border localities.
The Author searches
for more annelid worm fossils in the dry wash. The thinly bedded
shales contained beds of both smooth shales and those with extensive
mudcracks. Certainly, some of the slabs made for very nice display
pieces.
FOSSILS AND
SEDIMENTARY STRUCTURES
Close up of some
of the extremely white "fossil" mudcracks found in
Black Canyon City. Close examination of some of them revealed
the toe prints of small carnivores, perhaps cat-like in nature.
The material weathers to a very chalky consistency, and you soon
realize you are covered with a white powder!
Spectacular fossil
mudcracks found north of New River. This was at the bottom of
a dry wash, and was over 50 feet long. This represents and ancient
shore environment with at least temporary subareal exposure.
(above water) No trace fossils were found here, indicating that
the moist period was very short.
Primarily square
shaped mudcracks from north of New River. Although these were
about 1/2 inch in size, the smallest were only 1/16 inch across!
"Extreme bioturbation"
best describes the worm burrows found in the Chalk Canyon Formation.
This three inch slab was found on the top of one the rolling
hills found so commonly in the formation near New River. This
is the most common fossil in the formation.
Gastropod trackway
Aulichnites found north of New River. Close examination will
show the double ridged tracks of the snail approaching what appears
to be the imprints of some plant material, perhaps grasses either
in the water or just on the shoreline.
The two vertebrate
toe impressions seen here (two dimples on right edge) are a sure
sign that some 15 million years ago, a small carnivore type mammal
crossed this mudflat. Although quite rare in the Chalk Canyon
formation, these trace fossils provide a great deal of paleo
environmental data for researchers.
Fossil foot print
of a small carnivore in a huge slab bed in a dry wash in Black
Canyon City. Although toe prints were more common, we did find
several complete footprints in other areas. They are difficult
as you see here, to photograph unless the sun angle is low and
just the right angle.
A small tube building
annelid worm, similar to Deros sp., found in the deposits north
of New River. The "worm" itself is about 1/2 inch long,
and shows remarkable detail. It is a carbonized film from the
upper member of the Chalk Canyon formation - a simple imprint
of a small creature that lived 15 million years ago.
This latest find,
from a new outcrop south of Black Canyon City are fossil plant
rhizomes, the root molds of land plants that were later infilled
with siliclastic ash. These are the only plant fossils we have
ever found in the Chalk Canyon formation at this point, so I
decieded to add them in as an update.
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