Geology of “A” Mountain (Sentinel Peak) “A” Mountain Field Trip Outline

Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.. Describe nature of rock, color, rock type, composition, texture, environment of dep

Geology 101 Pima Community College Name _

Dr Jan Rasmussen www.janrasmussen.com jrasmussen@pima.edu Date _

Geology of “A” Mountain (Sentinel Peak)

“A” Mountain Field Trip Outline

This lab exercise involves taking a geologic field trip to “A” Mountain, or Sentinel Peak, on Tucson's west side The objective of the geologic field trip is to discover if “A” Mountain

(Sentinel Peak) is a volcano or and erosional remnant of volcanic rocks For additional

information, go to the Arizona Geological Survey website www.azgs.az.gov and download

DGM-29 (Bedrock geologic map of Sentinel Peak (A-Mountain) and Tumamoc Hill, Pima

County, Arizona) and Open File Report 01-07, Field guide to “A” Mountain and description of

surrounding region, Pima County, Arizona

“A” Mountain Field Trip Stops – Describe these in your report.

Stop 1 – If there is too much traffic, this stop will be omitted

Location: Mission road at base of “A” Mountain on southwest Parking area on east side of road Notes: Safety meeting: Potential traffic dangers You should take care to watch for traffic Skip

this stop when doing the trip on your own Road cut geology Describe nature of rock, color,

rock type, composition, texture, environment of deposition, origin.

Rock type: Porphyritic andesite, rock name Turkey Track andesite, age 28.6 Ma (million years).

Stop 2 – Location: Parking area just after entrance to “A” Mountain Park

Notes: Walk down to west to stream channel to look at channel, caliche in eroded stream bank

Describe origin of caliche Describe nature of rock, color, rock type, composition, texture,

environment of deposition, origin What is rock type of boulders in parking area? Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin Was the rock deposited from area or from outside area?

Stop 3 – Location: Along side road just after “one-way” traffic starts

Notes: On west/south side of road, note the black rock and describe it This is Tb1 - A Mountain basaltic andesite (27.6 Ma)

Then walk across road to outcrop of conglomerate – small pebbles of rounded cinders (vesicular basalt) in matrix of silt, sand, ash This Tc conglomerate is a mudflow/debris flow, matrix

supported, volcanic clasts (particles) Describe nature of rock, color, rock type, composition,

texture, environment of deposition, origin.

Stop 4 – Location: Further up road at contact of tan tuff with agglomerate.

Notes: Notice nature of contact – eroded, baked, type of material at contact, etc Describe tan to

white tuff (Tumamoc tuff (27.4 Ma)) – note pumice fragments in tuff, black mica (biotite) flakes, crystals – feldspar cleavage faces Describe nature of rock, color, rock type,

composition, texture, environment of deposition, origin.

Stop 5 – Location: Slightly further up road at contact of pink to gray tuff over tan tuff.

Notes: This is also called Tumamoc tuff (Tt), with(26.4 Ma) Describe nature of contact

Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin

Stop 6 – Location: At parking area at east side of “A” Mountain under the “A.”

Notes: Look at the monument What is the white stuff? Caliche or whitewash? Find a rock that is in place and not disturbed by man Describe nature of rock, color, rock type,

composition, texture, environment of deposition, origin This is Tumamoc basaltic andesite (23.7 Ma).

Discuss surrounding mountains and rivers Label a map (use matching to assign names to

mountains and rivers on a map Rincon Mountains, Agua Caliente Hills, Redington Pass,

Santa Catalina Mountains to East and North; Santa Rita Mountains and Sierrita

Mountains to the South; Tortilla and Tortolita Mountains to North; Picacho Peak and Picacho Mountains to the Northeast; Tucson Mountains to Northwest and West Discuss basin formation, erosion of mountains, differences between Tucson Mountains and Santa Catalina Mountains and effect on water supply, farming.

Stop 7 – Location: At parking area around top

Notes: Walk up to outcrop and discuss flattened vesicles, rock composition, etc Describe nature

of rock, color, rock type, composition, texture, environment of deposition, origin

From parking area near cars, discuss geology of Tucson Mountains – Black Mountain and

Martinez Hill near San Xavier Mission to the South, Cat Mountain Rhyolite (74 Ma) to the west and at Gates Pass, Safford Peak (Sombrero Peak informally) as possible source of ash for tan tuff and pink tuff Discuss crater – quarry for foundations and rock walls in homes built around 1920 around the University of Arizona Discuss Basin and Range basin

forming event, complexity of rock types and structures in mountains surrounding Tucson Color and label the geologic map

Mountain Ranges surrounding

Tucson-North:

East:

South:

West:

Rivers in the Tucson

valley-Name of river at the base of A Mountain:

Location:

Direction of flow:

Inlet:

Outlet:

Name or river at the base of the Rincon Mountains:

Location:

Direction of flow:

Inlet:

Outlet:

Name of river at the base of the Santa Catalina Mountains:

Location:

Direction of flow:

Inlet:

Outlet:

Tucson Basin

Composition:

Depth:

Caliche

Composition:

Source

Method of transportation

Method of deposition

Extent:

“A” Mountain Rock Sequence

Map

Symbol Rock type Texture Special Features Name of Rock

Formation

Probable Way of Formation

Approximate Age

Tb2

Tt

Tb1

Tc

Ttt

Summarize the Geology of “A” Mountain

Conclusions concerning the origin of “A” Mountain as a volcano and evidences for this

Is “A” Mountain a volcano? Or is it the erosional remnant of lava flows and ash falls?