Geology of the Precambrian Sangre De Cristo Range and Picuris Mountains of New Mexico

 Angelique Northcutt, Garrett Owen, Chris Perdue, Bo Price, Tory Rogers

Where are we going?

Background

-The Sangre de Cristo fault is a west dipping fault in New Mexico -It forms the border between the Sangre de Cristo Mountains and the San Luis Basin.

-The Sangre de Cristo fault extends from poncho Pass, Colorado to near where we’ll be in Taos, NM.

Background

-The section we will be focused on extends from the San Pedro Mesa creek south to the intersection with the Embudo Fault at Rio del Rancho which is about 8 km south of Taos.

-The Sangre de Cristo is part of the more recent Rio Grande Rift. -It is a normal fault that moves less than 0.2 mm/year.

History

-The Basement rocks in this location are Precambrian in age. -The rocks of Colorado and Northern New Mexico are juvenile Volcanic- Plutonic, Ocean Arc rocks that are approximately 1.78 to 1.65 billion years old. -In New Mexico these rocks have been assigned to the Yavapi and Mazatzal provinces.

- These Rocks were deformed during 2 major Proterozoic orogenies.

History

-First the Colorado Orogeny has a U-Pb date that goes through 1.78-1.75 Ga.

-It was a prolonged thermotectonic episode resulting from collision, subduction, and continued convergence.

-This occurred along the paleosuture known as the Cheyenne Belt along the Archean Wyoming Province.

History

-Second the Berthoud Orogeny has a U-Pb date that started 1.45 Ga and went through 1.40 Ga. -This was a thermo tectonic episode that produced NE trending ductile shear zones and related folds. -The mechanism powering this Orogeny was granitic plutonism. -This Orogeny also formed many high grade metamorphic rocks through generally pervasive metamorphism.

Pre-Cambrian rocks

-Isoclinal folding -East-west strike -Southward dips (60-70 degrees)

Folds

-2 major anticlines and 2 major synclines -Wave lengths 1 to 2 miles -Doubly plunging (~20-30 degrees)

Folds

Pilar Anticline -Widely displaced by the Pilar-Vadito tear fault -Slightly overturned to the north -Axial plane dipping south (60 degrees) Hondo Syncline -Overturned -Axial plane dips south (60-70 degrees) -Eastern segment plunges 30-40W -Western segment plunges 10-20E

Folds

Copper Hill Anticline -Axial plane plunges 30-35W -N85W, 50W -Can be traced 9 miles east of Copper Hill Harding Syncline -Structural detail is obscure -Strikes E-W -Plunge ranges 25-17NE

Jointing

-Abundant/wide variability in pre-Cambrian rocks -3 predominant sets -N10W to N10E (almost vertical) -N50W to N70W (dipping steeply NE) -N20E to N40E (dipping steeply SE)

Jointing

-N-S joints followed almost exclusively by copper ore-bearing quartz veins -Mineralization after granitic intrusion -Harding pegmatite estimated 800myo -Indicates time of jointing

Stretched Pebbles

-Conglomerate of the Vadito -Pebbles have average axial ratio of 1:2:3 -Shortest axis perpendicular to foliation -Some wedge-like shaped with apex oriented down the lineation -Pebbles in close contact have greater impacted shape

Three Major Fault Systems

•

Picuris-Pecos fault system

•

Embudo transfer fault

•

Sangre de Cristo fault zone

Picuris-Pecos Fault System

84 km long fault system consisting of five parallel fault zones: • Picuris-Pecos Major crustal boundary juxtaposing two Proterozoic rock sequences: the Hondo Group and the Miranda Granite • La Serna -East-down fault separating Miranda Granite and Picuris Formation • Miranda -North-striking strike slip fault • McGaffey -West-down branching fault splay • Rio Grande del Rancho -Kilometer-wide, west-down fault zone

Embudo Fault Zone

• Sinistral, antithetic transfer zone which forms border between the Española Basin and the San Luis Basin • 64 km long fault thought to be part of Jemez lineament • Fault consists of two sections based on reversal of throw • Strike of N60E • Sinistral net slip rate of .15 mm/year

Sangre de Cristo Fault Zone

• West-dipping normal fault that forms border between the Sangre de Cristo Mountains and the San Luis Basin • Beginning of Sangre de Cristo Fault forms the terminus of the Embudo fault • The southern area of the fault is divided into five sections. From north to south, the sections are: - San Pedro Mesa - Urraca - Questa  The northern three strike north-south - Hondo  Strikes N30W - Cañon  Strikes N20E

Aerial View of Faults in Taos Region

Pre-Cambrian Formations:

•

The Hondo Group

-The Pilar Formation -The Riconada Formation -The Ortega Formation •

The Vadito Formation

The Ortega Formation

• Estimated 2,500 ft. thick • Gray to very light gray in color • Mostly Quartzite • Thin beds of sillmanite - kyanite gneiss • Bands of schistose with muscovite • Tourmaline

Riconada Formation -Richly micaceous foliated rocks -Consists of four distinct beds:

• Andalusite-biotite Hornfels bed Muscovite and quartz matrix that contains biotite, nodular masses of quartz and andalusite • Staurolite Schist and Gneiss Bed Soft and light gray in color micaceous matrix speckled with biotite, contains staurolite crystals Staurolite Andalusite

The Riconada Formation

• Quartzite bed – Grayish white in color, contains glassy- white and translucent quartzite • Muscovite-quartz-biotite-garnet Phyllite bed Muscovite rich phyllite containing garnet crystals and biotite, sheen that ranges from pearly gray to greenish

Pilar Phyllite Formation

• Estimated 2,300 ft. thick • Black to gray-black in color with a gray sheen • Contains muscovite flakes • Quartz veins • Limonite masses

Vadito Formation

• Occupies the southern 1/3 of the Picuris Range -Named after the village of Vadito • Best outcrops come from within a one-mile radius of the Harding Mine -Total thickness: 4,500 feet • Comprised of a Lower Conglomerate Member and an Upper Schist Member

Marquenas Formation

• Total thickness: ~ 2,000 feet • Composed of Quartz Conglomerate and Quartzite, Felsites and Meta-andesites • Quartz conglomerate outcrops in a quarter-mile wide east-west belt near Picuris Canyon but grow sparser farther east • Composed of gray coarse to pebbly Quartzite with fine-grained micaceous quartzite matrix • Thickness of the Quartz Conglomerate: 500 - 1,000 feet

Marquenas Formation

• Felsites occur to the east and west of the canyon but appear to pinch out to the west along the Ortega-Vadito contact • Composed of meta-rhyolite that grades into coarser granite representing partial replacement by granite • Gray-white to pink-white and has a dense felsitic texture with micas and feldspars present • Thickness of the Felsites: 50-100 feet

Marquenas Formation

• Meta-andesites occur abundantly to the south and east of the Harding mine • Greenish-gray to gray-black in color • Hornblende occurs in all of these rocks as prisms and give the variable darkness in color to them • Rocks show strong pleochroism in thin-section.

• Represent thin-layered volcanic material of dacitic and andesitic composition

Schist Member

• Consists of a schist and phyllite composed of quartz-muscovite and a quartz-biotite granulite • The schist is similar to the underlying conglomerate quartzite with more densely disseminated flakes of muscovite • Phyllite is a lustrous, silvery-gray rock with stubby biotite porphyroblasts • Granulite is fine-grained, sandy, and crudely foliated with micaceous surfaces with flakes of biotite • Thickness: At least 1,250 feet and no more than 2,500 feet

Amphibolites

• The amphibolites found in the Vadito Formation are split into two zones based on lithology: -One of the zones occurs in the Lower Conglomerate and one occurs in the Upper Schist • These amphibolites show effects of intrusion by granitic and pegmatitic magma • Thickness: 750 feet in Lower Conglomerate, and 1,250 feet in Upper Schist

Questions?