Wednesday, March 30, 2011

Turbid Conglomerates And The Point Reyes Lighthouse

My early–March geology adventure at Point Reyes National Seashore was slowly winding down, ebbing like high tide turning to low. Earlier in the week, I had walked the Earthquake Trail at the Bear Valley Visitor Center, discovered the granites at Point Reyes headlands, unmasked the Salinian Block, and found the Franciscan Mélange at Shell Beach. Next on my travel agenda was a visit to Point Reyes Lighthouse.
 
By the time I got there the fog was so thick it seemed I could barely see thirty feet in front of me. The 20-mile drive along the peninsula had been fairly clear but as I neared the parking lot a coastal cloud crept in to shroud the headlands. No wonder they put a lighthouse here! 
 
IMG_5846PathToPointReyesLighthouse
Foggy Path to Point Reyes lighthouse

There were not many other visitors as I ambled up the path. This paucity of people is one reason I enjoy traveling in the off-season. I like chatting with the odd person here or there, but I am not a fan of crowds.
 
IMG_5849 PointReyes LighthouseVisitor Center
Visitor center at lighthouse


IMG_5851NationalRegisterPlaque


IMG_5856 PointReyes LighthouseVC
Point Reyes conglomerate and whale remains near visitor center

The lovely rocks peeking out from behind the whale remains are the Point Reyes conglomerate of Early Tertiary age, around 65–55 million years old. It is the oldest marine sediment found on top of the granite bedrock of Point Reyes. Quartzite and feldspar cobbles are found in a matrix of sandstone and granitic rocks. Marine fossils indicate an origin deep in the ocean. 

The similarity of these rocks with some found 100 miles to the south on the Monterey Peninsula suggests that the Point Reyes conglomerate was formed while it was adjacent to Monterey in Early Tertiary time. As the Salinian Block traveled northwest along the western edge of North America, so did Point Reyes and its conglomerate.

IMG_5855Point Reyes Conglomerate
Graded bedding in Point Reyes conglomerate

IMG_5857 PointReyes Conglomerate
Point Reyes conglomerate

The cobbles and sandstones most likely were deposited in a submarine canyon during the slurry of water and sediment of a submarine landslide or turbidity current. As the current flowed down the canyon slope towards the ocean floor, larger coarse cobbles and boulders settled out first followed by finer sediments of sand and then mud or silt.  This formed what it known as graded bedding and is a good indication of submarine landslide activity. If the turbidity deposits are composed mostly of sandstones, the submarine canyon was closer to shore. If the deposits are mostly shale (mud or silt deposits), the canyon was farther out in the ocean.

The time span between landslides flowing down these submarine canyons could be tens, hundreds, or thousands of years; most likely earthquakes were responsible for them.  These outcrops appear to represent several episodes of landslide deposits closer to shore, with coarse cobbles of a later landslide sitting on top of finer-grained sandstones of an earlier deposit.

The fog was not going anywhere as I started down the first of 300 steps. I could barely see the lighthouse so I did not expect any great views of gray whales today.
 
IMG_5860300StepsToLighthouse
300 steps to lighthouse

 I watched through my binoculars as a peregrine falcon searched for lunch on the cliffs…
 
IMG_5861PeregrineFalcon
Peregrine falcon

…and later wondered if any surf scoters were likely to be its next meal.
 
IMG_5890 SurfScoters
Surf scoters enjoying the Pacific Ocean

It took me probably an hour to walk down the steps. Although it was chilly and a bit windy, I was in no hurry.  The fog seemed to be lifting, and there were quite a few little “balconies” where I could pause, out of the path of what little visitor traffic there was. I thought these balconies would surely come in handy on the trek back up those 300 steps!
 
IMG_5872Windy HairDay
Yowza!  I hoped my toupee would not fly off in the wind


IMG_5869Point ReyesLighthouse
Steps descend the Point Reyes conglomerate


IMG_5879Point ReyesLighthouse
Point Reyes lighthouse
For some lighthouse workers, it apparently was quite a desolate, unforgiving place at which to live and work.  Early logbook entries tell of men going insane while staying out here for weeks at a time.
Remnants of the historic lighthouse operation are still on display.

IMG_5885Point ReyesLighthouse

 
IMG_5892Point ReyesLighthouse


 




The upper part of the lighthouse with the lamps and lenses is closed to visitors but anyone can walk around outside. 









The Point Reyes conglomerate outcrops nicely underneath the lighthouse structures…
 
IMG_5899Point ReyesLighthouse OnConglomerate

IMG_5898Point ReyesHeadlands



 …and southeastward along the headlands for a bit.















It was soon time to scamper back up those 300 steps.
 
IMG_5905Point ReyesLighthouse Steps

I paused at each balcony and watched scoters surf the waves, buzzards glide on thermals, and gulls gossip on the sea rocks, but I never again saw the peregrine falcon I had watched earlier.
 
IMG_5908GullsOn RocksAtPointReyes Lighthouse
Hundreds of gulls can be seen on the rocks below

Although the sun did not come out all day, it was nice that the fog lifted and the chill dissipated. I walked slowly back to my car as a few more visitors appeared, embraced in the beauty that is Point Reyes.
 
IMG_5876 CaliforniaPoppy
California poppy


IMG_5917DeerOn PointReyes Headlands
Deer grazing on the headlands


IMG_5901Point Reyes Conglomerate
Life as Art - Point Reyes conglomerate


References:

Evens, J.G., 2008, Natural History of the Point Reyes Peninsula, second edition, California Natural History Guides: University of California Press.
 
Sloan, D., 2006, Geology of the San Francisco Bay Region, California Natural History Guides: University of California Press.

Monday, March 28, 2011

This Week’s Southwest Geology Symposium

If you happen to be in or near southwest Utah this week, make plans to attend the First Annual SUU Geology Symposium being held on the campus of Southern Utah University in Cedar City.  There will be a variety of speakers on Thursday March 31 (lunch is included!) followed by a field trip on Friday and Saturday (April 1-2).  
 
Compelling speakers and fascinating geology of the Great Basin/Colorado Plateau transition zone  are two of the fascinating features of this timely symposium.
 
Program - SUU Geology Symposium, Spring 2011
Program - SUU Geology Symposium, Spring 2011_Page2
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Program - SUU Geology Symposium, Spring 2011_Page7

Friday, March 25, 2011

Finding The Franciscan Complex

Mélange: A body of rock mappable at a scale of 1:24,000 or smaller, characterized by a lack of internal continuity of contacts or strata and by the inclusion of fragments and blocks of all sizes, both exotic and native, embedded in a fragmental matrix of finer-grained material.
 
Tectonic mélange: A mélange produced by tectonic processes.
 
IMG_5787 Franciscan MelangeAtShell Beach
The Franciscan Melange at Shell Beach, Sonoma County CA

It was not strictly serendipity that found me at Shell Beach on the Sonoma Coast of California, a 45-mile drive north of Point Reyes National Seashore on Route 1, but the internet. I had originally planned to visit Mount Diablo State Park east of San Francisco, where rocks of the Late Jurassic Franciscan Complex mélange present themselves for examination by anyone who is interested. I had heard of these rocks for years but had not been able to wrap my mind around them. Mount Diablo, it appears, is a park dedicated to the Franciscan rocks – just what I was looking for! 
 
IMG_5761 Shell_Beach

However, an excursion from Point Reyes to Mount Diablo seemed less than appealing after an 800-mile marathon drive from southern Utah. For that reason, before leaving home I googled “Franciscan Complex” and hoped for something closer. What I discovered was a field trip to Shell Beach, complete with photos of what to look for. Geology on a beach! It does not get much better than that.
 
Serendipitously, it was not even raining on that particular day of my mission to investigate these remarkable coastal rocks. The geology gods and goddesses were certainly watching out for me.
 
After parking my car, I tell everyone who happens to walk by that I have driven nearly 900 miles from southern Utah to come to this particular beach.
 
Why? They ask with smiles, not a little perplexed. 
 
I explain that the rocks exposed here are from a subduction zone, and that they are around 150 million years old. I smile gleefully as if I know what I am talking about. In my positive uncertainty, I hope that the age is at least nearly correct and that this is a good enough answer to satisfy these friendly folk. Frankly, that was pretty much all I (barely!) understand of the Franciscan Complex at the moment.
 
They have no idea what I am talking about and are duly astounded, taking my word for it all. We smile our goodbyes and then, with camera around my neck and field guide printout in hand, I stumble down the trail steps to peer at the jumbled eroding edge of a continent.
 
IMG_5830 ShellBeachParkingWithStepsToBeach
Overlooking the Franciscan Melange at Shell Beach

 During the Late Jurassic Period, nearly 145 million years ago, the eastward moving Farallon plate commenced its collision with the North American plate. For the next 100 million years, the two plates would continue colliding as the Farallon was subducted beneath the North American plate.
 
Over time, yet another island arc terrane was “rafted in” as a geologic passenger on the Farallon plate. This island arc came in from an unknown somewhere, docking itself against earlier accreted terranes already attached to this western edge of a future California. As the subducting Farallon plate melted many miles beneath the surface, large magma chambers formed and rose to become a chain of volcanoes in what is today eastern California. Subduction processes changed around 90 million years ago and volcanism ceased; the magma chambers slowly cooled and crystallized to form the granitic batholith of the Sierra Nevada Mountains. 
 
Meanwhile, where seafloor of the Farallon plate was subducted beneath the North American plate, a trench formed and top layers of the seafloor were scraped or sheared off as an accretionary wedge or prism. These slivers of seafloor and arc ended up attached to the North American plate, emplaced among the formations of the continent.
 
accretionary_wedge
Image courtesy of US Geological Survey

 A large sort of “berm” formed from these slivers and became the California Coast Range. This berm includes large slices of the seafloor and a jumble of oceanic and continental materials now known to geologists as the Franciscan Mélange.
 
Extensive field studies reveal that there is no stratigraphy in the mélange, no consecutive story of deposition – as John McPhee so succinctly described it, the Franciscan mélange is “just mountains of bulldozed hash.”  Sandwiched as it was between the incessant grinding of the Farallon and North American plates, the Franciscan mélange appears today as a pervasively sheared, multihued 145-million year old fruitcake smorgasbord of pillow basalt, sedimentary rocks such as shale, chert and greywacke sandstone, and assorted metamorphic rocks.
 
So as I slowly move down the sea cliff at Shell Beach, I gaze thoughtfully at this smorgasbord of jumbled hash tumbling out of the eroding gully.
 
IMG_5784Eroding Franciscan MelangeAtShell Beach
Eroding cliffs of the Franciscan Melange

 The first boulder I investigate (and sadly take bad blurry close-up photos of) is a hard block containing eclogite, an oceanic crustal rock derived from lava and basaltic tuff (ash) or from gabbro;  it had been metamorphosed in a continental plate subduction zone at relatively low temperature. There are some other conditions under which an eclogite will form, but this simple definition will suffice well enough for now.
 
IMG_5764Eclogite
Eclogite boulder

Red garnets pockmark the boulder, and flaky white mica crystals glimmer in the winter sunshine. I cannot distinguish any green omphacite pyroxene crystals, perhaps because the blue stripes of spray-painted graffiti are so distracting.  I sigh.  If people only knew what they were defacing…would they still do it?
 
IMG_5767Eclogite
Eclogite with garnets and mica




The next boulder I stop at is a blueschist, again formed in the regional metamorphic environment of our low temperature, high pressure subduction zone. 
 
IMG_5773 Blueschistof Franciscan Melange
Blueschist of the Franciscan Melange













 I learn that the blue mineral is glaucophane, found only in these low temperature, high pressure metamorphic rocks. Here, the alternating streaks are blue (glaucophane) and green (eclogite, which I can actually see in this rock). The sheared streaks are evidence of flow in the rocks as they are intensely squeezed through the subduction zone.
 
IMG_5772 Blueschistof Franciscan Melange
Sheared blueschist



 Next, I climb about in the eroding gully with its gray-green clay matrix containing sheared serpentine boulders.

IMG_5768 FranciscanRocksAtShellBeach
Serpentine boulders and clay matrix eroding in gully

Serpentine occurs from the transformation of olivine and pyroxene found in peridotite, the dominant rock found in the upper part of Earth’s mantle. The iron-rich rocks combine with hot water to change the olivine to serpentine. 
 
IMG_5778BlocksOfSerpentine
Blocks of serpentine in fine-grained clay matrix

The slick, sheared appearance of these rocks is evidence of their being caught up in a fault zone.
 
IMG_5775Sheared Serpentine
Sheared serpentine with digits for scale




 My mountain of references say that serpentine is generally green, but it looks blue to me.

IMG_5783Sheared Serpentine
Sheared serpentine - hand lens for scale

















The entire north slope of the gully is composed of shimmering, sheared serpentine blocks in a finer-grained clay matrix. These clays of the south slope are eroding into a badlands topography.
 
IMG_5785Eroding ShaleMatrixOf Franciscan Melange
Eroding badlands of fine-grained clay matrix

 Descending to the beach, I putter around in the wreckage of a subduction zone.
 
IMG_5794 FranciscanBlocks AtShell Beach
Blocks of Franciscan Melange at Shell Beach

IMG_5796Huge BlocksofFranciscanMelange
Huge block of Franciscan Melange
























I am thinking this possibly may be a chunk of mantle peridotite, with its black surface on a light-green rock containing greenish-white veins…
 
IMG_5791Possible Peridotite
Possible mantle peridotite?


…and this looks like pillow basalt next to greywacke sandstone. Pillow basalts form when molten lava erupts underwater. The greywacke sandstone is a “dirty” sandstone, originating in an underwater landslide off an ancient continental shelf.
 
IMG_5790         (L)Graywacke SandstoneWith(R)PillowBasalt
(L) Graywacke sandstone with (R) possible pillow basalt



IMG_5803 Conglomerate
Conglomerate of Franciscan Melange




Conglomerates would have formed from stream gravels or the turbidity currents of underwater landslides.






As I wander about, I think that I could surely spend a lifetime in this microcosm of the larger Franciscan Complex. However, I must soon leave this amazing geology even though I have barely begun to see what there is to see. Low tide is starting to turn as the sun sinks incrementally lower towards the Pacific horizon. 
 
I take a few minutes to leisurely walk along the top of the sea cliffs, fervently hoping that the sandy prominence on which I pause does not slump off into the ocean because the San Andreas Fault suddenly takes a notion to shift its britches.
 
IMG_5833TopOf SeaCliffsAt ShellBeach
Trail along sea cliffs at Shell Beach



References:
 
Evens, J.G., 2008, Natural History of the Point Reyes Peninsula, second edition, California Natural History Guides: University of California Press.

McPhee, J., 2000, Assembling California, in McPhee, J., Annals of the Former World: Farrar, Straus, and Giroux, p. 429-621.

Minerals of the World, Johnson, O., Princeton Field Guides, Princeton University Press

Simon and Schuster’s Guide to Rocks and Minerals (my well-thumbed copy dates from the early 1980s).

I am particularly indebted to Terry Wright at http://terrywrightgeology.com/shellbeach.html  (accessed 2/18/2011) for his field guide to Sonoma County Geology at Shell Beach.  It was this website which ultimately led me to drive those 900 miles to a better understanding of the Franciscan Mélange.  For this I am grateful.

Sunday, March 20, 2011

The Salinian Block Unmasked At Last

This really must stop. I have been procrastinating entirely too long and now is the perfect first-day-of-spring Sunday morning to get busy – gray overcast skies, blustery winds, temps in the 40s, a nice pile of reference books, and a carafe full of coffee. I just need to sit down, put pen to paper (or fingertips to keyboard, as it were) and figure out this whole “Salinian Block-Point Reyes” conundrum.
 
There are, as usual, so many questions. Where did the block allegedly come from? How long has it purportedly been on the move? Who are its suspected geologic relatives? Where does it think it’s going?
 
A brief review may be helpful here. To appreciate any sort of past (and, ultimately, future) movement of our Salinian Block, what do you think we should consider first?
 
Plate Tectonics, of course — nothing happens without it! 
 
After the recent earthquakes and accompanying tsunami in Japan, the “Ring of Fire” has been in the news a great deal lately.
 
map_plate_tectonics_worldUSGS

Interestingly, the Pacific plate rotates around a point which is located south of Australia. However, unlike the mainly subduction action of the tectonic plates at Japan, transform or right-lateral strike-slip movement occurs along the 800-mile long San Andreas Fault  (aka SAF). The average movement of the Pacific plate relative to the North American plate at the SAF is about two inches per year. Geologists believe that, over the past 20 or so million years since the SAF came into existence, the total horizontal displacement of the Pacific plate along the fault is around 350 miles. Geologically similar terranes on opposite sides of the SAF have been found hundreds of miles apart. 
 
Point Reyes National Seashore, its headlands protruding ten miles out into the Pacific Ocean on the Pacific Plate, is today separated from the rest of Marin County, California and the North American plate by the San Andreas Fault. Apparently, sometime in the geologic past Point Reyes was not located here. It was somewhere else.
 
PointReyesGoogle
 

pic_sanandreasfault_285x285_3
 

So where did it come from? Where is it going? We can decipher some answers from the rocks of Point Reyes and others along the San Andreas Fault.
 
That’s the plan, anyway.  As always, I must leave much information out - this is a blog, after all, not a dissertation.  That said, I devote a lot of energy to making sure what information I do include is accurate.  Enjoy the tectonic ride! 
 
Around twenty-five to thirty million years ago, as the western edge of the Farallon plate was being subducted and consumed beneath the North American plate, a jagged eastern tip of the Pacific plate first touched North America at present-day Los Angeles and Santa Barbara. It was at this time that the transform boundary which would eventually become known as the San Andreas Fault came into existence
 
Six million years ago, a terrane was moving northward along the west side of the San Andreas Fault on the Pacific plate. The actual origins of the sediments comprising this early terrane are obscure, an “unknown elsewhere” from probably hundreds of millions of years ago, lost in the pages of time. It appears, though, that as early as 110 million years ago molten rock from deep within the mantle had enveloped these sedimentary rocks.
 
In the fullness of geologic time this terrane would come to be known as the Salinian Block. It carried on its back the future sites of Salinas, Monterey, Big Sur, San Luis Obispo, Santa Barbara, Los Angeles, and San Diego. Today, a part of northern Salinia is the Point Reyes peninsula – the granites west of Tomales Bay and the San Andreas Fault.  The San Francisco Bay Area is the northernmost place where our Salinian rocks occur.
 
IMG_5752ViewTo SalinianBlock_ PointReyes Peninsula_Across TomalesBay_SAF

Plutonic rocks (the granites of the molten intrusion) of the Salinian Complex or Salinia Terrane occur only west of the SAF. They were formed in the “unknown elsewhere” of a subduction zone setting during Mesozoic plate collisions starting about 145 million years ago and lasting nearly 100 million years. These plutonic rocks formed deep in magma chambers as the subducting Farallon plate, predecessor of the Pacific plate on the west coast of North America, partially melted and slowly cooled underground. 
 
The rocks of the Salinian Complex and thus Point Reyes are similar in silica composition to the granitic rocks found in the Sierra Nevada Mountains. Both are similarly 80-110 million years old.
 
IMG_5698Point Reyes

Evidence suggests that the granites of Point Reyes formed as a southern extension of the granitic Sierra Nevada Mountains, later to be separated from them by motion on the San Andreas Fault. The granites on the west side of Tomales Bay broke away from the southern Sierra Nevada and have traveled north along the fault at least 300 miles, one earthquake at a time.
 
IMG_5697Granite AtPointReyes

A comparison of the Point Reyes rocks with similar ones in the Santa Cruz Mountains and on the Monterey Peninsula to the south of San Francisco indicates that they are the same rocks and were once contiguous. The Point Reyes rocks have been offset about 90 miles by movement along the San Andreas and San Gregorio Faults; according to Evens, they sat docked next to the Monterey Peninsula from approximately 60 million to 15 million years ago.
 
The Salinian Block will one day move out to sea towards the northwest. The rocks of Point Reyes are only transient visitors, fleeting geologic travelers attached to the ephemeral Pacific plate as it moves past California on its journey towards some future Alaska.
 
IMG_6004Looking SouthTowardPoint ReyesHeadlands
 

References:
Evens, J.G., 2008, Natural History of the Point Reyes Peninsula, second edition, California Natural History Guides: University of California Press.
McPhee, J., 2000, Assembling California, in McPhee, J., Annals of the Former World: Farrar, Straus, and Giroux, p. 429-621.
Sloan, D., 2006, Geology of the San Francisco Bay Region, California Natural History Guides: University of California Press.
http://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html (pacific plate rotation and movement) - page accessed 3/20/11
http://pubs.usgs.gov/gip/earthq3/safaultgip.html (rate of plate movement) - page accessed 3/20/11