Study Guide: Paleozoic and Mesozoic Geology of Northern New Mexico

Study Guide: Paleozoic & Mesozoic Geology of Northern NM

Gilman Tunnels

1. What type of Precambrian rocks are the Gilman Tunnels carved into?
   Precambrian granite, later metamorphosed into granitic gneiss.

2. What formation overlies these rocks, and what does it represent?
   Osha Canyon Fm (limestone), Sandia Fm (shale), Madera Group (sandstone). Represents a shallow sea from the Mississippian to Pennsylvanian.

3. Until when did this environment persist?
   Late Pennsylvanian; early Permian saw sea-level regression and arid climate.

4. What change is recorded by Abo and Yeso sediments?
   Transition from marine to river system: fluvial Abo Fm and wind-blown Yeso dunes.

5. What direction was the source of the Shinarump Formation? What fossils does it contain?
   Northwest, from central Texas. Contains Coelophysis bauri and Limnoscelis fossils.

Ghost Ranch & Vicinity 6. Significant fossils from the Chinle Formation?
Coelophysis bauri (early dinosaur), Limnoscelis (primitive reptile).
7. Time gap and unconformity between Chinle Group and Entrada Sandstone?
44 million years; disconformity.
8. Sedimentary structures of the Entrada Sandstone?
Cross-beds indicating eolian dunes, wind direction toward south/southwest.
9. Cause of gypsum in the Todilto Formation?
Evaporation of a saline, oxygen-poor water body.
10. Environmental change from Todilto to Morrison Fm?
Transition from arid (Summerville Fm) to humid floodplain (Morrison Fm).
11. What records sea level changes in the Dakota Sandstone?
Shale alongside sandstone, reflecting Western Interior Seaway fluctuations.

Uranium Mining in the Grants District 12. When did uranium mining begin, and what was mined before?
1948; prior mining focused on other minerals.
13. What was the USAEC?
U.S. Atomic Energy Commission, established 1947, regulated uranium mining.
14. Peak uranium production in NM?
1987 (9,371 tons).
15. Why did uranium mines shut down by 1989?
Overproduction, Three Mile Island incident, rising costs, better deposits elsewhere.
16. Main uranium-bearing formation?
Morrison Formation (Late Jurassic), particularly Westwater Canyon Member.
17. Three types of uranium deposits?

• Primary (tabular): Flat-lying, 0.20% ore, parallel to beds.

• Redistributed: Irregular, >8 ft thick, 0.16% ore, altered by oxidation.

• Remnant: Preserved primary deposits, 0.20% ore.

18. What type occurs in the Saltwash Member?
    Primary deposits.

19. Unique feature of uranium in the Todilto Formation?
    Unusual limestone-hosted uranium, rich in organics, overlies permeable Entrada Sandstone.

20. Source of Jurassic uranium deposits?
    Alteration of volcanic ash and shales from Jurassic volcanic arcs west of Morrison Basin.