Study Guide: Paleozoic and Mesozoic Geology of Northern New Mexico

What type of Precambrian rocks are the Gilman Tunnels carved into?

Precambrian granite later metamorphosed into granitic gneiss.

What formation overlies the Precambrian rocks in the Gilman Tunnels?

Osha Canyon Formation, contains limestone.

What sedimentary environment does the Osha Canyon Formation represent?

Shallow sea from the Mississippian to Pennsylvanian period.

Until which period did the shallow sea environment persist in New Mexico?

Until the late Pennsylvanian.

What major environmental change during the Permian is recorded by the Abo and Yeso groups?

Sea level dropped, and the climate became warmer and drier.

What type of deposits does the Abo Formation consist of?

Fluvial (river) deposits.

What type of deposits does the Yeso Formation consist of?

Cross-bedded wind-blown dunes (desert environment).

What was the direction of sediment transport that formed the Shinarump Formation?

Northwest, from central Texas.

What is the significant fossil found in the Shinarump Formation?

Coelophysis bauri (early dinosaur).

What time gap exists between the Chinle Group and the overlying Entrada Sandstone?

44 million years.

What type of unconformity exists between the Chinle Group and Entrada Sandstone?

Disconformity.

What sedimentary structure indicates the depositional environment of the Entrada Sandstone?

Several foot-high cross-beds.

What environment is indicated by the sedimentary structures in the Entrada Sandstone?

Deposited in a desert environment (eolian dunes).

What caused the abundant gypsum in the Todilto Formation?

Evaporation of an isolated, oxygen-poor, saline water body.

What does the environmental change from the Todilto to Morrison formations indicate?

Transition from a flooded region to a muddy floodplain.

What additional rock type does the Dakota Sandstone contain?

Shale.

What changes does the Dakota Sandstone record?

Oscillating sea level changes in the Western Interior Seaway.

When did uranium mining first begin in New Mexico?

Before uranium mining, other minerals were mined.

What does USAEC stand for and what was its role?

United States Atomic Energy Commission; it regulated uranium mining.

When did annual uranium production peak in New Mexico?

1980, with 9,371 tons mined.

What caused the shutdown of conventional uranium mines in New Mexico?

Public fear from the Three Mile Island disaster and economic factors.

Which geologic formation contains the majority of uranium deposits in New Mexico?

Morrison Formation.

What are the three types of uranium deposits in the Morrison Formation?

Primary, Redistributed, and Remnant Deposits.

What type of uranium deposit occurs in the Saltwash Member?

Primary uranium deposits.

What is unusual about the uranium deposits in the Todilto Formation?

Uranium is found in limestone, which is rare for such deposits.

What is the likely source of uranium in the deposits in the Morrison Formation?

Volcanic ash and shale alteration.

How did uranium get concentrated in the Morrison Formation?

Groundwater transported uranium from volcanic rocks into permeable sandstone beds.