GEOLOGY OF THE BLACK ROCKDESERT

ByCathy Busby

Professor of GeologyUniversity of California

Santa Barbara

http://www.geol.ucsb.edu/faculty/busby

BURNING MAN EARTH GUARDIANSPAVILION 2012

LEAVE NO TRACEPlease come find me and Iʼll give

you a personal tour of the posters!

You are here! In one of themost amazing geologicwonderlands in the world!

Fantastic rock exposure,spectacular geomorphicfeatures, and a long history,including:

1. PreCambrian loss of ourAustralian neighbors bycontinental rifting,

2. Paleozoic accretion ofisland volcanic chains likeJapan (twice!),

3. Mesozoic compression andemplacement of a batholith,

4. Cenozoic stretching andvolcanism, plus a mantleplume torching the base of thecontinent!

*

http://www.terragalleria.com

Let’s start with what you can see on the playa and from the playa:

the Neogene to Recent geology, which is the past ~23 millionyears (= Ma).

Note: Recent = past 15,000 years

Then we’ll “build”the terrane youare standing on,beginning with aBILLION yearsago, movingthrough thePaleozoic (oldlife, ~540-253Ma),

Mesozoic (ageof dinosaurs,~253-65 Ma))and

Cenozoic (ageof mammals,~65 -0 Ma).

Black RockPlaya extends100 miles, fromGerlach to theJacksonMountains.

The BlackRock Desert isdivided into twoarms by theBlack RockRange, andcovers 1,000square miles.

Empire (south of Gerlach)has the U.S. Gypsum mine and drywall factory (brandname “Sheetrock”), and thereʼs an opal mine at base of Calico Mtns.

Neogene to Recent geology

BRP = The largest playain North America

“Playa” = a flat-bottomeddepression, usually a drylake bed

3,500ʼ asl in SW,4,000ʼ asl in N

Land speed record: 1997- supersonic car, 766MPH

Runoff mainly from theQuinn River, which headsin Oregon ~150 milesnorth.

Neogene to Recent geology

>15,000 years (Recent) agothe Humboldt River flowedinto north Lake Lahontan, butsince that dried up, the riverhas diverted south to theCarson Desert sub-basin(Carson Sink). Earliesthumans here lived on theLahontan lake shores.Lake Lahontan was 500ʼdeep in the Black RockDesert, and 900ʼ deep atpresent-day Pyramid Lake.Walker Lake also survives.Lake Lahontan dried up dueto increased evaporation asthe climate warmed.

Neogene to Recent geology

The highest lake level reached an elevation of about 4370 feetabove sea level, evidenced by a wave-built terrace of unsortedgravel called Lahontan Beach. This photo shows numerousflat-lying shoreline deposits. You can also see these at thesouth end of Death Valley, which held Lake Manly.

View 11 miles north of Gerlach on road 34. Playa is to right. Photo by M. Bilbo.

Neogene to Recent geology

The highest LahontanLake levels are alsorecorded by the “High-Dry” micro-playas justeast of the Black Rock(Black Rock hot springson basin floor in front).

Neogene to Recent geology, continued

Top photo by DeloresCates, bottom photoby Mike Bilbo.

These areperfect, beautifullittle playas!

Neogene to Recent geology, continued Over the past ~25million years(Neogene), the crusthas been stretchedor rifted.

That makes grabensdrop down inbetween horstblocks, along normalfaults.

The playa is in agraben. Nevadahas a wholeseries, calledthe “Basin andRange”.

Hot springs of the Black Rock Desert form where hot watercomes up along the faults that make the grabens.

Neogene to Recent geology, continued

Cartoon view of a typical rift basin: A high mountain rangecomes up, next to a basin that sinks along the normal fault.Alluvial fans build out from the mountain front into the basin, andthe center of the basin fills with lake sediment.

From Tectonics ofSedimentary Basinsedited by Busby andIngersoll (1995)

Cross section of a typical rift basin: thousands of feet oflacustrine silt and clay, with alluvial fans on basin margin. Thisexample is actually from the East African Rift.

Neogene to Recent geology, continued

From Ebinger & Scholz, 2012, in Recent Advances in Tectonics ofSedimentary Basins, Busby and Azor editors.

BIG PICTURETECTONICSETTING:Nevada is outlined inyellow.

Ranges of the Basinand Range look like“worms crawling northto Canada” =

ALL NORMALFAULTS.

Neogene to Recent geology, continued

BIG PICTURE TECTONIC SETTING - Stretching of Basin andRange, also Rio Grande Rift. The Colorado Plateau in betweenis too strong to stretch!

BIG PICTURE TECTONICSETTING

Weʼre on the NorthAmerican Plate in the Basinand Range,but weʼre close to theeastern edge of the SierranMicroplate, which has strikeslip faults (as well asnormal faults) that areparallel to the San AndreasFault = WALKER LANEBELT(from Unruh et al., 2003)

Neogene to Recent geology,continued

Most of the Basin and range is now tectonically dead but theWalker Lane belt is seismically active (dots = earthquakes).

Neogene to Recent geology, continued

The Neogene (~23 Ma-present) stretching ofNevada is very obvious onthe geologic map.But now letʼs see how ourcorner of Nevada has beenconstructed over the pastbillion years,starting with the oldestrocks and working towardthe Recent.

Black Rock Desert,from a billion years tonow.

From NevadaBureau ofMines andGeology

Quaternary sedimentsand volcanic rocks(<2.6 Ma)

Miocene-Pliocenevolcanic andsedimentary rocks

Oligocene volcanic rocks

Mesozoic strata andintrusions

Upper Paleozoicaccreted oceanic rocks

Lower Paleozoic:accretedcontinental margin

NO PRECAMBRIANHERE

Black Rock Desert, from a billionyears to now.

Rocks from other parts of Nevada show that, a billion years ago,Gondwana (Australia and Antarctica) rifted westward off Nevadaand an ocean opened in between.

Black Rock Desert, from a billion years to now.

By Early Paleozoic time (~650 - 450 Ma), an “Atlantic-type”passive margin developed, with a broad continental shelf, andwarm water (see outline of Nevada).

Black Rock Desert, from a billion years to now.

Black Rock Desert, from a billion years to now.

Paleozoic sedimentaryrocks host the “Carlin-type gold deposits”,produced byhydrothermalcirculation of hot waterthrough them.

Nevada has thelargest gold deposit ofthis type in the world!

The origin of the heatsource for the “CarlinTrend” remainscontroversial.

Paleozoic sedimentary rocks deposited on the broad continentalshelf are mainly bedded limestones and mudstone. They gotfolded during the mountain building events (orogenies) in theLate Paleozoic to Mesozoic.

Black Rock Desert, from a billion years to now.

Examplefrom DeathValley, fromMiller, 2005.

Somewhere offshore of the broad early Paleozoic shelf, anoceanic arc a formed above a subduction zone (see next).

Black Rock Desert, from a billion years to now.

Black Rock Desert, from a billion years to now.

An “arc” is a chain ofvolcanoes that fromabove a subductionzone. Arcs may formislands that lieoffshore of a continentlike Japan (shown attop )or they may formabove sea level oncontinental crust(shown at bottom, likethe Cascadevolcanoes today).

In Middle Paleozoic time, the oceanic/island arc approached andthen got pushed up onto Nevada along THRUST FAULTS,causing the Antler Orogeny (~370 to 340 Ma).

Then in LatePaleozoic time, thesame thinghappened all overagain with adifferent arc,causing theSonoma Orogeny(~ 250 Ma)

Black Rock Desert, from a billion years to now.

THRUSTFAULTS

SONOMIA

The Black Rock is part of the “Sonomia” oceanic rockassemblage. It is Permian volcanic rock - probably a shallowintrusion, not a lava flow.

Black Rock Desert, from a billion years to now.

All through the Mesozoic Era(~251 to 65 Ma), subductionoccurred under the edge ofthe continent, forming acontinental arc. This producedthe great batholiths of westernNorth America, which gotexposed by uplift and erosionof the volcanic cover

Black Rock Desert, from a billionyears to now.

During the Mesozoic,the subducting slab gotshallower andshallower,until it was grindingalong the base of thecontinent.

This causedcompression of thecontinent,

causing uplift(orogeny), and erosionof the volcanic rocksarc to expose thebatholith below.

The Mesozoic batholithsare thus the deeply-eroded roots of theMesozoic subductionvolcanoes.

Big pink feldspars in agranite, typical of thebatholithic rocks.

Black Rock Desert, from a billionyears to now, continued.

Oligocene volcanic rocks

In Oligocene time (~34-23 Ma), giantcontinental calderas producedSUPERVOLCANOES.

SUPERVOLCANOESproduce gigantic,highly explosiveeruptions.

Black Rock Desert, from a billionyears to now, continued.

Miocene-Pliocenevolcanic andsedimentary rocks

In Miocene and Pliocenetime (~23 to 2.5 Ma),Basin and RangeExtension was in fillswing! Grabens filled withvolcanic and sedimentaryrocks.

Black Rock Desert, from a billion years to now,continued.

Stretching of the continental crustproduced the Lahonton Basin,ultimately giving us our playa!

Black Rock Desert, from a billion years to now,continued.

(from Catchings, 1992).

Cross section through Black Rock Range (BRR), Pinto Mountain(PM), Black Rock Desert (BRD), and Jackson Mountains (westto east). Shown to a depth of 12 km.

Black Rock Desert, from a billion years to now, continued.

From Lerch et al. 2008. NOTE: the second author of thispaper is a Burner. Can anyone guess who she is?????

Vapid Transit by Elizabeth Miller, Dwight Harbough & ClayHamilton

Miocene-Pliocenevolcanic andsedimentary rocks

NOTE ALSO:

Some of the volcanicrocks (orange) arerelated to theYellowstone hot spottrack………

Black Rock Desert, from a billion years tonow, continued.

Yellowstone hot spot track: an eastward-migrating series of giantcontinental calderas (16 Ma in west, Yellowstone in east).

Also

MOP =ModocPlateau,a mile-highlavaplateau.

Pleistocenewoolymammothskeletonexcavatedfrom theBlack RockDesert.

Thank you for your interest and have a good burn!

THANKS to Dr. Graham Andrews for assistance with posterprinting! And to the National Science Foundation for supportingresearch and outreach.