BASIC GEOLOGY OVERVIEW
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Geological Location
The monument is located in the transition zone of central Arizona, between the Colorado Plateau Province to the Northeast and the Basin and Range Province to the Southwest. The AFNM is situated between the New River mountians (Moore gulch shear zone) to the East and the Bradshaw mountains (Shylock shear zone) to the West. Just North of the monument is the Estler basalt volcanic center (Estler peak area) and South is the Black Canyon Dispositional Basin (Chalk Canyon & Hickey Formations).

Basic Geology
The AFNM is located on basement rock, which is called the Cherry Springs batholith. This composite (~10 plutons?) batholith is pre-tectonic(?) and was implaced at ~1.74 to 1.72 Ga*. It consists mainly of granodiorites and quartz diorites. Most of the monument sits on the Badger Spring and Bumblebee granodiorite plutons. The batholith intruded older altered volcanic rocks of which some still remain after uplift has unroofed most of the batholith. It is theorized that their was one or two main pulses of tectonic activity that built this part of the continental crust in a SE to NW direction in the monument area.
* The oldest pulse occured at ~=1.74Ga
* The last pulse was at ~=1.7Ga
The oldest pulse may have been contemporaneous with collision of island-arc terranes.

Most of the surface rock is Tertiary in age (Hickey formation) with small amounts of Quaternary sediments at the north end of the monument. A small part of the north end of the Chalk Canyon formation is exposed at the south end of the monument where it contacts the Hickey formation. The last exposure seen going north on I-17, is just past exit 244 and the first exposure seen going south, on the opposite side of I-17, starts about the middle of mile marker 247.
There are a number of locations along the West side of the monument, near I-17, where protozoic rocks are exposed. Going north on I-17, the proterozoic rocks start just north of the Bumble Bee exit-248, along the right side of the road cut. The Joes Hill, hawaii type, basalt shield volcano (10.8 My) can be seen as a low rounded hill directly to the east at I-17, Sunset Rest area, at mile marker 252. At the Northwest end of the monument, granodorite basement is exposed along with minor amounts of proterozoic mafic lava flows which may represent a small volcanic center or is an extended part of the Cordes volcanic center to the West. Going south on I-17, proterozoic schist is exposed along I-17 around Black Mesa and at the South end of the monument, greenstone, altered andesite, migmatite and green schist is the proterozoic basement.

A SIMPLIFIED PLATE TECTONICS MODEL FOR PROTEROZOIC CRUST CREATION IN THE AFNM AREA
((( Note: there are other versions of this model )))

1. A southeast dipping subduction zone created volcanic piles and volcanic centers in a deep ocean environment via island arc’s between ~1800 to 1740Ga.

2. The subduction zone flipped direction ~1.745Ga.

3. Arc magmatism changed from volcanism to pluton and batholith formation.

4. The new northwest dipping subduction zone coleased, uplifted, deformed and metamorphosed the Basalt, Andesite and Rhyolite lava’s to greenschist grade. These products were rotated the to near vertical and a northeast strike (~9 to 25deg) was imprinted on the whole sequence. Calcic Granodiorite Plutons (Bumblebee & Badger Spring) then invaded the resultant modified volcanic piles around 1.74 to 1.735Ga.

5. These older granite family rocks may have been invaded by the 1.43Ga anorgenic(?) granite plutons that caused more uplift and stabilization of the new crust. None of the anorgenic(?) granites have been found in the AFNM yet.

6. NOTE: For more information, refer to “Geology 1982, V10, p37-42, K.C. Condie."Plate-tectonics model for Proterozoic continental accretion in the southwestern US".

Below is a list of general area formations names by increasing age;

Tertiary
  • Hickey formation (HF)...There is some time overlap between the HF & CCF.
  • Chalk Canyon formation (CCF)
    Proterozoic
  • Yavapai schist or Yavapai group ...general name for the oldest rocks in central Arizona. This name was given by E. Wilson in 1939.
    There are several discontinuous sequences of volcanic-sedimentary rocks, both metamorphosed and unmetamorphosed.
  • Big Bug Group...between the Shylock and Chaparral fault, west of the AFNM.
  • Ash Creek Group...east of the Shylock Shear Zone. The AFNM is in this group.
    NOTE1: There are younger Proterozoic rocks (1.0 to 1.4Ga) in Arizona but none in AFNM area.
    NOTE2: * Ga equals giga or 1X10(9), a billion years.
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