Study Guide: Paleozoic & Mesozoic Geology of Northern NM

Major Physiographic Provinces

• Colorado Plateau

• Southern Rockies

• Rio Grande Rift

• Great Plains

• Jemez Lineament (Late Cenozoic Volcanics ~15 Ma)

Key Geographic Features

• Jemez Lineament: Linear volcanic alignment

• Rio Grande Valley: Major drainage basin

• Late Cenozoic Volcanism (40-15 Ma): Includes Jemez Mountains

Precambrian Geology of New Mexico

• Timeframe: ~1.75 Ga to 1.35 Ga (first 4 billion years of Earth's history)

• Formation of North America: Laurentian Shield formed by continental block suturing

• Exposure: Precambrian rocks mostly buried but visible in Canada & Rocky Mountains

Orogenic Events

• Yavapai Orogeny (1.75-1.68 Ga): Island arcs attached to North America, expanding the crust

• Mazatzal Orogeny (1.65-1.60 Ga): Further crustal expansion

• Grenville Orogeny (1.3-1.0 Ga): Laurentia collided with African/South American cratons

The Great Unconformity

• Major gap in geologic time with older Precambrian rocks overlain by much younger Cambrian sandstones (~540 Ma).

• Exposed in the Sandia Mountains, with Pennsylvanian-age rocks directly above Precambrian granite.

Picuris Mountains & Precambrian Formations

• Vadito Group (1.7-1.72 Ga): Metavolcanics, schists, quartzites

• Hondo Group: Includes Ortega (quartzite), Rinconada (schists), Pilar & Piedra Lumbre Formations

• Marqueñas Formation (~1.47 Ga): Cross-bedded quartzite; revised age via zircon dating

• Structural Geology: Overturned synclines, detrital zircon dating refined ages

Gilman Tunnels

1. Precambrian rock type?
   Precambrian granite, later metamorphosed into granitic gneiss.

2. Overlying formation and environment?
   Osha Canyon (limestone), Sandia (shale), Madera (sandstone). Represents Mississippian-Pennsylvanian shallow sea.

3. Persistence of this environment?
   Until Late Pennsylvanian; early Permian saw sea regression & arid climate.

4. Environmental change recorded in Abo & Yeso sediments?
   Transition from marine to river system (Abo: fluvial; Yeso: wind-blown dunes).

5. Shinarump Formation sediment source & fossils?
   Northwest (from Texas); contains Coelophysis bauri & Limnoscelis fossils.

Ghost Ranch & Vicinity

6  . Key Chinle Formation fossils?
Coelophysis bauri (dinosaur), Limnoscelis (primitive reptile).
7. Time gap & unconformity between Chinle & Entrada?
44 million years; disconformity.
8. Entrada Sandstone structures & interpretation?
Cross-beds indicating eolian dunes; wind direction south/southwest.
9. Gypsum formation in Todilto Fm?
Evaporation of saline, oxygen-poor water body.
10. Environmental change from Todilto to Morrison?
Transition from arid (Summerville Fm) to humid floodplain (Morrison Fm).
11. Sea level changes recorded in Dakota Sandstone?
Shale & sandstone reflect Western Interior Seaway fluctuations.

Uranium Mining in the Grants District 12. Mining start date & prior minerals mined?
Uranium mining began in 1948; previous mining focused on other minerals.
13. Role of USAEC?
U.S. Atomic Energy Commission (1947) regulated uranium mining.
14. Peak uranium production year?
1987 (9,371 tons).
15. Reasons for mine closures by 1989?
Overproduction, Three Mile Island incident, rising costs, better deposits elsewhere.
16. Primary uranium-bearing formation?
Morrison Formation (Late Jurassic), Westwater Canyon Member.
17. Three uranium deposit types?

• 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. Uranium deposit type in Saltwash Member?
    Primary deposits.

19. Unique uranium deposits in Todilto Formation?
    Limestone-hosted uranium (rare), rich in organics, over Entrada Sandstone.

20. Source of Jurassic uranium?
    Altered volcanic ash & shale from Jurassic volcanic arcs west of Morrison Basin.

Mesozoic Stratigraphy of Northern New Mexico

• Triassic: Moenkopi Fm (Middle Triassic), Chinle Group (Upper Triassic)

• Jurassic: Entrada Sandstone (eolian), Todilto Fm (gypsum-rich), Morrison Fm (dinosaur fossils)

• Cretaceous: Dakota Sandstone (shallow sea), Mancos Shale (marine), Mesaverde Group

Tectonic & Paleogeographic History

• Sonoma Orogeny (~150 Ma): Subduction off western North America.

• Sevier Orogeny (~65 Ma): Subduction shifts inland.

• Cretaceous Interior Seaway: Divided North America, deposited marine rocks.

• Transcontinental Arch: High-elevation landmass prevented deposition until Pennsylvanian.

• End-Permian Mass Extinction (~260-300 Ma): ~90% marine species loss due to Siberian Traps volcanism.

This study guide covers key geological events, formations, and concepts in New Mexico’s Paleozoic and Mesozoic history.