mini field trip to the Rodeo Cove Thrust

Calling all Franciscan lovers out there - where are you? Are we going extinct? I am busy recruiting for our cult.

The Franciscan is the accretionary complex along the coast of California which preserves records of Mesozoic subduction: the wedge edge of the more famous Sierra Batholith-generating subduction zone. The Farallon Plate subducted under North America for over 100 MY until its tail edge hit the coast (with the Pacific Plate behind it) and the San Andreas Fault was born. While it was active the Farallon Trench subducted, offscraped, underthrusted and underplated a huge amount of marine sediment and a bit of igneous oceanic crust, mostly in discrete fault-bounded packets = terranes.
In the Marin Headlands, just north of San Francisco's Golden Gate Bridge, you can see the contact between two terranes (white teeth) where an ocean-island seamount (Bonita Terrane) was thrust northeastward over a stack of greenstone-chert-sandstone nappes (Marin Headlands Terrane). Within those nappes of the Headlands Terrane, one major fault outcrops on the beach (black teeth): the Rodeo Cove Thrust. Also, it's frickin beautiful out there on Cronkite Beach and there's a really sweet hostel out there. Field trip planners take note.



The "Cron" is probably the most accessible, adequate surf spot in southern marin. The beach is pebbly and composed almost entirely of chert/ jasper in many colors of red, green, black, gray and orange, aka carnelian (apparently orange chert gains some kind of metaphysical significance?)

The beach divides the sea from the Rodeo Lagoon - happy birds. The bridge out to the beach makes it feel like you're leaving Marin behind.
My dear friend Francesca wrote an amazing paper about the Rodeo Cove Thrust*. Sadly she was not able to guide us on this field trip because she was needed in Italy to make soap out of olive oil. I am not making this up. Anyway, Francesca described the intense veined zone in the thrust:


The kiwi delegation checks out the veined zone, comparing it to kiwi veined zones:
At the top of the veined zone, a small, fault-bounded slab of pillows. VT observes that pillows are all nearly the same size, except for some rhyolitic pillows she has seen. I experience the dual sensations of the elation at realization that there is a such thing as a rhyolitic pillow and disappointment that my Geolutions list for 2009 is already full. Where are these rhyolitic pillows of unusual size?

A close-up look at the perlitic texture on the surface of the pillows. This texture forms when volcanic glass slowly hydrates and develops nano, then micro crystals over time. Glass is an uncomfortable state of being for cations and anions. This is evidence that the outside of the pillows were once glassy.

South of the pillows: greenstone cataclasite. Yummy. It's right about here that this old Mesozoic thrust fault is cross cut by a steep, NW-striking smectity gougey fault zone. The San Andreas is right off the beach. I think this young fault is part of the SAF. But this cataclasite... old or new? A bit of both maybe.

Coming next: Ring Mountain.

*Meneghini, F. and Moore, J. C. (2007) Deformation and hydrofracture at seismogenic depths: The Rodeo Cove thrust zone, Marin Headlands, California. Geological Society of America Bulletin 119 1-2, 174-183, DOI: 10.1130/B25807.1

Coupla Christmassy Pictures for ya

Red chert clast in a greenstone cataclasite

Garnets in blueschist with epidote?

And remember: axial tilt is the reason for the season. Have a good one whatever your reason.

Long night flight - back to the action

I flew back to the US for the AGU spectacle and holidays with the family. On the plane, I read Kinabo et al. (2008)'s paper in Tectonics which includes some interpretations of aeromagnetic surveys over the Okavango Rift in NW Botswana (Figure 4c, below).

These images show the magnetic susceptibility of the subsurface geology. I think that it's basically a measure of how strongly the material responds to a magnetic field - so it relates to the amount of magnetic minerals in the rocks as well as the exact minerals present and their alignment. Anyway like so many geophysical data sets, it can be used for purposes that are not really related to magnetics, like looking for changes and patterns in rocks in a more general sense: mapping. In this case Kinabo et al. have interpreted the purple-red WNW-trending features as Jurassic dolerite dykes (common in this part of the world) and the NNE-trending lines as faults offsetting them. This is reasonable and consistent with everything I know about the region and I believe they are correct. However I couldn't help noticing that the clouds over Frankfurt, Germany also contain Karoo dykes offset by normal faults:
Excuse the sarcasm, what's wrong with me? It's flippin Christmas.
Just trying to express my trepidation about "mapping" by tracing "lineaments" and making guesses about what they really are. I have extreme discomfort about doing this kind of mapping myself but my local experts assure me that I still have a lot to learn. In theory, educated guesses are just fine (as in Kinabo et al. (2008)). However, if there are real geologic surprises in that data set you'd have to be pretty careful and pretty insightful to find them. Anyway if it leads to a ground-truthing campaign in the northern Kalahari, I'm all over it.


Cold: ice pack in Hudson Bay. Hot: the baby blanket I'm knitting for Mini MacStammer the Vth (not the child's real name).

We flew in twilight across the Canadian shield. My photos didn't come out so well but I could see the shapes of the 3 billion year old rocks poking up in roche moutinees through the patterned ground between snowy frozen lakes. It looked cold and very windy and isoclinally folded with a WSW-ENE trend to everything.

Getting ahead of the sun we finally broke into California in the late afternoon. The clouds opened in front of the sea and I saw what I had been waiting for - the active margin. The Crystal Springs Reservoir in San Mateo:
Funny that the wikipedia article on the reservoir mentions that it "... lies in a rift valley created by the San Andreas Fault". This is technically true, although it might be more accurate to say "IS THE SAN ANDREAS FAULT" (see USGS page for proof).
We did the customary spin around the bay, revealing an intersection of the SAF with the coastline - the mouths of both Tomales and Bodega Bays (west Marin County).

Sunset over the Salinian Plate: Point Reyes


And on to the big party!
(Slugs breakin it down, Thirsty Bear Pub, San Francisco. Looks seismic doesn't it. I frickin love San Francisco.)

New Year's Geolutions

I have been in South Africa for 2+ years now and seen very few of the BIG SOUTH AFRICAN THINGS I meant to see. So I'm suggesting a new meme for you geobloggers out there to follow up on Geotripper's list of the 100 things we all "ought" to see.

What are 10 things you have never seen before, which you hope or plan to see in 2009?

Here are my ten GEOLUTIONS for 2009:
1. Wolfberg Cracks/ Wolfberg Arch Cederberg Mountains, South Africa
2. Etosha Pan, Namibia
3. Any really big wall of San Bushman Paintings
4. Coarse Kyanite schist in the Monapo Complex, Mozambique
5. The Vredefort Crater with the mega-pseudotachylyte breccias, shatter cones, the works.
6. Jeffrey's Bay aka JBay, the world-famous SA surf spot
7. Gems in Sri Lanka
8. Forest elephants in Addo National Park
9. The Bushveld Layered Igneous Intrusion
10. Triassic therapids in the Beaufort Formation, Karoo Basin...

Also for good measure (and a spin), 5 things I want to find or see evidence for (or against, as it may come). These are research goals, or ideas, as well as hobbies, or wishes for my students, etc...:

1. Incipient westward propagation of the Okavango Rift across the Botswana-Namibia border
2. Fluidized granular flow of very thick (meter or more) gouge layers in shallow crustal faults
3. Tectonic environment of the deformation of the "Saldanian belt"
4. Syn-crystallization shearing of the pegmatitic ijolites of the Mazeripane Suite, Mozambique
5. Tectonic fabrics in the Cape Granite Suite - timing, spacing and source.

Finally, how about 10 things that should be on the top 100 List? I totally concur with Geotripper's reticence to rewrite the list himself, as it is politically sticky and never unbiased. But for the sake of it, what would you add?

1. A famous "big wave" e.g. Maverics or Dungeons, breaking.
2. A glacier calving into the sea
3. Pink sand beaches (e.g. Bahamas)
4. Singing beaches or dunes
5. Walk across and observe a metamorphic aureole
6. Experience an earthquake
7. See the snowball earth stratal assemblage (e.g. diamictites+carbonates)
8. An earthquake damaged area, e.g. Earthquake Park in Anchorage
9. A Bore tide
10.Hear the sound of waves in a fjord.

I wonder why we geologists emphasize the fast/rare things on this list, instead of the slow/common things... which are more truly special to observe...

Apologies for missing the geobloggers dinner on Weds, I was too jetlagged/overwhelmed to stay so I went and worked on my talk in bed.

11. Hoping i get another chance to meet Kim Hannula

New Lows Achieved.

Two things I said I would not do: join facebook; do a blog meme. Today I did both. Anybody want to come over and give me a tattoo or a makeover or something? Just so I can become totally unrecognizable to myself.

This one is pretty fun. Geotripper is responsible for this. Following Chris Rowan's lead, I'll post a photo where possible. Bold ones are CHECKED OFF. Italicized are checked off sensu lato.

1. See an erupting volcano
(erupting what? if steam and gas, YES (Yellowstone). if magma, NO.)
2. See a glacier (unID'd glacier near Whittier, Alaska)

3. See an active geyser such as those in Yellowstone, New Zealand or the type locality of Iceland
4. Visit the Cretaceous/Tertiary (KT) Boundary. Possible locations include Gubbio, Italy, Stevns Klint, Denmark, the Red Deer River Valley near Drumheller, Alberta.
(according to my field books, I saw it in the Elk River Basin, WY. However, I don't recall this.)
5. Observe (from a safe distance) a river whose discharge is above bankful stage
6. Explore a limestone cave. Try Carlsbad Caverns in New Mexico, Lehman Caves in Great Basin National Park, or the caves of Kentucky or TAG (Tennessee, Alabama, and Georgia)
7. Tour an open pit mine, such as those in Butte, Montana, Bingham Canyon, Utah, Summitville, Colorado, Globe or Morenci, Arizona, or Chuquicamata, Chile.
8. Explore a subsurface mine.
9. See an ophiolite, such as the ophiolite complex in Oman or the Troodos complex on the Island Cyprus (if on a budget, try the Coast Ranges or Klamath Mountains of California).
10. An anorthosite complex, such as those in Labrador, the Adirondacks, and Niger (there's some anorthosite in southern California too).
11. A slot canyon. Many of these amazing canyons are less than 3 feet wide and over 100 feet deep. They reside on the Colorado Plateau. Among the best are Antelope Canyon, Brimstone Canyon, Spooky Gulch and the Round Valley Draw.
12. Varves, whether you see the type section in Sweden or examples elsewhere.
(Smith College - Paradise Pond!)
13. An exfoliation dome, such as those in the Sierra Nevada.
14. A layered igneous intrusion, such as the Stillwater complex in Montana or the Skaergaard Complex in Eastern Greenland.
15. Coastlines along the leading and trailing edge of a tectonic plate
16. A gingko tree. (Smith College again!)
17. Living and fossilized stromatolites (Glacier National Park is a great place to see fossil stromatolites, while Shark Bay in Australia is the place to see living ones)
18. A field of glacial erratics Smith college rugby field! also freshies in Alaska and fossil in South Africa
19. A caldera Long Valley! Yellowstone! Haleakela!
20. A sand dune more than 200 feet high Namib Desert, Sossusvlei
21. A fjord (unknown fjords in Kenai Mts, Alaska; photo by my colleague francesca)

22. A recently formed fault scarp Lone Pine, CA
23. A megabreccia Death Valley
24. An actively accreting river delta
25. A natural bridge Natural Bridges Beach, santa cruz!
26. A large sinkhole
27. A glacial outwash plain Alaska
28. A sea stack
29. A house-sized glacial erratic
30. An underground lake or river
31. The continental divide
32. Fluorescent and phosphorescent minerals
33. Petrified trees fossil ridge Y-stone, also in Anza Borrego but just pieces
34. Lava tubes Bend, OR and hilo
35. The Grand Canyon. All the way down. And back.
36. Meteor Crater, Arizona, also known as the Barringer Crater, to see an impact crater on a scale that is comprehensible
37. The Great Barrier Reef, northeastern Australia, to see the largest coral reef in the world.
38. The Bay of Fundy, New Brunswick and Nova Scotia, Canada, to see the highest tides in the world (up to 16m)
39. The Waterpocket Fold, Utah, to see well exposed folds on a massive scale. I think the Cape Fold Belt qualifies for this one:

40. The Banded Iron Formation, Michigan, to better appreciate the air you breathe.
41. The Snows of Kilimanjaro, Tanzania,
42. Lake Baikal, Siberia, to see the deepest lake in the world (1,620 m) with 20 percent of the Earth's fresh water.
43. Ayers Rock (known now by the Aboriginal name of Uluru), Australia. This inselberg of nearly vertical Precambrian strata is about 2.5 kilometers long and more than 350 meters high
44. Devil's Tower, northeastern Wyoming, to see a classic example of columnar jointing
45. The Alps.
46. Telescope Peak, in Death Valley National Park. From this spectacular summit you can look down onto the floor of Death Valley - 11,330 feet below.
47. The Li River, China, to see the fantastic tower karst that appears in much Chinese art
48. The Dalmation Coast of Croatia, to see the original Karst.
49. The Gorge of Bhagirathi, one of the sacred headwaters of the Ganges, in the Indian Himalayas, where the river flows from an ice tunnel beneath the Gangatori Glacier into a deep gorge.
50. The Goosenecks of the San Juan River, Utah, an impressive series of entrenched meanders.
51. Shiprock, New Mexico, to see a large volcanic neck
52. Land's End, Cornwall, Great Britain, for fractured granites that have feldspar crystals bigger than your fist.
53. Tierra del Fuego, Chile and Argentina, to see the Straights of Magellan and the southernmost tip of South America.
54. Mount St. Helens, Washington, to see the results of recent explosive volcanism.
55. The Giant's Causeway and the Antrim Plateau, Northern Ireland, to see polygonally fractured basaltic flows.
56. The Great Rift Valley in Africa.
57. The Matterhorn, along the Swiss/Italian border, to see the classic "horn".
58. The Carolina Bays, along the Carolinian and Georgian coastal plain
59. The Mima Mounds near Olympia, Washington
60. Siccar Point, Berwickshire, Scotland, where James Hutton (the "father" of modern geology) observed the classic unconformity
61. The moving rocks of Racetrack Playa in Death Valley
62. Yosemite Valley
63. Landscape Arch (or Delicate Arch) in Utah
64. The Burgess Shale in British Columbia
65. The Channeled Scablands of central Washington
66. Bryce Canyon
67. Grand Prismatic Spring at Yellowstone
68. Monument Valley
69. The San Andreas fault
70. The dinosaur footprints in La Rioja, Spain
71. The volcanic landscapes of the Canary Islands
72. The Pyrennees Mountains
73. The Lime Caves at Karamea on the West Coast of New Zealand
74. Denali (an orogeny in progress)
75. A catastrophic mass wasting event
76. The giant crossbeds visible at Zion National Park
77. The black sand beaches in Hawaii (or the green sand-olivine beaches)
78. Barton Springs in Texas
79. Hells Canyon in Idaho
80. The Black Canyon of the Gunnison in Colorado
81. The Tunguska Impact site in Siberia
82. Feel an earthquake with a magnitude greater than 5.0. Loma Prieta!
83. Find dinosaur footprints in situ Dinosaur Tracks, MA
84. Find a trilobite (or a dinosaur bone or any other fossil)
85. Find gold, however small the flake
86. Find a meteorite fragment
87. Experience a volcanic ashfall
88. Experience a sandstorm
89. See a tsunami
90. Witness a total solar eclipse
91. Witness a tornado firsthand.
92. Witness a meteor storm, a term used to describe a particularly intense (1000+ per minute) meteor shower
93. View Saturn and its moons through a respectable telescope.
94. See the Aurora borealis, otherwise known as the northern lights.
95. View a great naked-eye comet, an opportunity which occurs only a few times per century
96. See a lunar eclipse
97. View a distant galaxy through a large telescope
98. Experience a hurricane
99. See noctilucent clouds
100. See the green flash

Score: 42 sensu stricto
ps. I did post a bunch of rock pictures on facebook though.

goin to the BIG SHOW!

Folks, I am about to get on the plane to go to AGU in San francisco and I can't remember the last time I was so excited. I got all the Christmas shopping done, cleaned the house for the housesitter (although he is my student and I did leave him a six-pack so who's complaining). I downloaded all the session info.

I used to think GSA was so much better than AGU. People wear field-fashion there and bring rocks to their posters for goodnessakes. Now I think they're just different. I like going to GSA when I want to reconnect with my geology roots and especially for teaching/education recharge. I like to go to AGU when I want to stretch myself quantitatively and party with everybody I ever knew in geology, ever, while spending a month's income on food and drink!

I like to give a talk when I want a lot of people to hear what I have to say. I like to give a poster when I want to hear what a lot of people have to say to me. Also talks are nice when you're flying from overseas, and say, not ready to go to print yet.

This year my needs/wants are perfectly aligned with my AGU/talk. There is a lot I love about being in South Africa but one of the drawbacks is a small community of very busy scientists working on very different things. In order to have the real exciting science discussions and challenging collaborations, I have picked up some very new and strange research directions. Now I have some great stuff going on and some very interesting people to work with on it. But I miss the energy and intensity of a being at a pumped-up research institution where everybody is jazzed about science all the time. Also, there are so many people available to talk to that questions get opened and answered all the time. Heavy trading on the ideas floor.

Bring on the BIG SHOW! I want to see all you friends of kodiak at thirsty bear on Tuesday night. I want to talk about SUBDUCTION and ACTIVE FAULTS with a beer in front of me that is made of BEER INGREDIENTS and not CORN! Ahhh the brew pub options within 3 blocks of Moscone center trounce the entire nation of South Africa subsaharan Africa.

If you're at AGU and you make it to Friday Afternoon I hope you'll check out my talk. Yahooo!

Eric was here.

My brother was here.

Whoops, wrong photo, sorry: Eric enjoys the view at Blouberg Beach. Eric, Sila and I checked out Shipwreck Trail in Cape Point National Park.


Unfortunately Eric had to ruin it by stealing a sandwich from a nice lady.

Visitors are Fun.

For those of you considering a visit to Cape Town, get on with it already! Visitors are Fun! When Visitors arrive I go out to Eat! I see the Sights! and I do ridiculously short field trips to Far-Away Turbidites!

I have had three "reunions" recently as a result of the AAPG meeting - two with blogosphere "colleagues" (Jeannette and Chris), and one with an an actual real-world acquaintance, the notorious Daniel Minisini. These visitors are very intrepid - Chris circumambulated Cape Town to see the Sea Point Contact on his own, and gave an excellent talk in our department. Jeannette found her way (taxi-wise) to campus on her own for Chris's talk, and brought me an amazing and unexpected present without sticking around long enough to be compensated by a trip to above-referenced turbidites. Come back Jeannette! Anyway, watch your mailbox.

Daniel materialized a few days after the meeting was over. Daniel is one of the "crazy Italians" who held court in my grad school lab at UC Santa Cruz - there were always one or more around during the 5.5 years I was there - We were part of some Appeninic exchange program! Daniel probably inspired the most mayhem during his tenure (excepting our RESIDENT Italian, Stefano, the undisputed Maestro of Mayhem).

I won't tell you what happened on Friday because... um... it's blurry.

On Saturday we made lots and lots of food and went swimming with penguins. Can you see how this photo is weirdly funny?
On Sunday, we drove to Laingsburg to see turbidites. Yes, the Laingsburg, which is 3.5 hrs away from Cape Town. We spent an hour or so pondering weird sedimentary structures in the Laingsburg Formation.

We also visited my favorite folds at the "LilyPad Pond". Can you see how this photo is also weirdly funny?
We had the chance to follow up on rumors from past students that there was a cave on the "Whale Back" doubly-plunging anticline that had Bushman paintings. We found them. Small and perfect (but potentially NSFW).
You be the judge.

We stopped at Matjiesfontein for a beer on the way home and arrived in CT at 11pm.
On Monday Daniel and Sila went surfing and hiked all over Cape Point and we cooked another HUGE dinner.
On Tuesday Daniel and Sila are climbing Table Mountain (via Platteklip Gorge) and then ALSO climbing Lion's Head.
On Wednesday I need a nap.

Landmarks

Next week my first generation of advisees (BSc Honours students) will turn in their theses. In keeping with the tradition in which I was raised, I'm having a dinner party for them. Tacos.

In another, perhaps more poignant benchmark moment, when they say:

"Hey, what's the time?"

and I say:

"It's time to get ILL!"
"Hey what's the time! It's time to get ILL!"

They don't get it.

They are all getting CDs for graduation presents. You know, to make me feel younger.

Anybody hear me on this one?

Lazy post - funnier than me anyway

FSP I love you. That's Female Science Professor to those of you who don't currently substitute her in your mind into the vacuum of lady-scientist role models at your institution.

FSP has written of a recent search to fill a tenure track position as her cat in the fall of 2007.

FSP has just announced that happily Kitten X has been awarded tenure and promoted to Associate Cat X.

For those of you involved in/subject to either of these processes I think you will enjoy this.

Rock (photo) of the Week

Special treat today folks. We have a reader submission for rock (photo) of the week. This means somebody besides Mom reads my blog. It also means I'm not 100% sure what the hey is going on in this picture, only that there are lots of things going on. The first thing is that somebody is gonna need a bigger hammer to get this done right.

So what's up folks? What do you see in this Rorschach Ink Blot of an outcrop?

I want to recognise Elisabeth for authorising the use of this photo for you to analyse. See what I did there? Just finished this book, which I enjoyed very much. So much that I have been making a bunch of word nerd puns since I read it.

I would like Elisabeth to recognise that her institution stole the world's greatest university president from my institution. No hard feelings though.

Some people we met in Mozambique

I've had this post in the "draft" folder for a while, trying to decide what to write about these photos. But I think they are expressive enough on their own. Nampula Province, Mozambique, July 2008.

Carrots and brains

I made these! I fought the snails.


My lithops are turning themselves inside out for spring. Absorbing their old leaves and making new ones. Resurfacing like Venus. Looks like brains.


You know what else is cute, but I don't have a picture? My students had a slumber party on the roof of the geology building last night.

Sockeye Salmon Pattern for Knitters

Sure, I publish. I publish knitting patterns! 

This pattern "Sockeye Salmon" is available on Ravelry.com

See pattern here
or

buy now

Link will remain in the sidebar - in case you're not sure but you want to buy it for somebody!
If you're not a Ravelry user but would like to purchase the pattern,  leave a comment or email me and I will contact you.

100% of proceeds go to the Cordova District Fishermen United Scholarship Fund which supports continuing education for fishermen and their families.

I present you the Sockeye Salmon, prototype #1

running up Turnagain Arm, Alaska :

He's got character.

He now has two more prototypes to spawn with (here posed in mid-flop

among my chili plants)

The final pattern incorporates the head from the front right salmon with the body

and fins from the left front salmon below. 

Dwykacious Injectites

The famous Dwyka "Tillites". As previously ranted on this blog, these are not lithified tills! They are glaciomarine. As proof, I offer the drop pebble. Isn't he cute. (top of photo is stratigraphic top)

The Dwyka Group contains one formation, the Elandsvlei Fm., making it the city and county of San Francisco of geologic Groups. Sorry if that's too corny for you. The whole package is matrix-supported, laminated and massive diamictites. The massive ones are ridge-formers in the field area and the laminated ones are valley-formers. They are informal called "coarse" for the ridge formers and "fine" for the valley formers but I actually think the difference is in cementation rather than grain size, possibly having to do with more abundant clays in the "fine" laminated units prohibiting silica circulation. The matrix is glacial rock flour, a very fine sediment (quartz/felspar ground down to clay-size particles) which is unique to glacial erosion.

Anyways, along some of these facies-boundaries we have channel sands. These are often called "eskers" but they are not true eskers as this is not a ground moraine.
Super TA Nic perched on the stratigraphic top of one of these channel sands - the dip is to the left on the photo at about 40*S and Nic is sitting on the top of the deepest part of the channel. Original vertical thickness is about 2m and the sand body tapers to the foreground and background (these represent the edges of the channel). This particular channel deposit has a nice coarse, well-rounded conglomerate around the edges - like a gravel bar? The matrix of the conglomerate is greenish-gray rock flour, resembling the rock into which the channel cut.

Here's where things get even better (and by "better" I mean "more structurally interesting"). See that long spindly (~15cm thick) sandy arm reaching gently UPSECTION from the sandstone channel? IT'S A SANDSTONE INJECTITE! If you look carefully at the photo (click to enlarge if you need to) you will see that the lamination in the gray-green matrix is going roughly across the photo while the dyke cuts upsection (and up-photo) to the left.

Injectites usually form when a porous, saturated sediment is overlain by a less porous sediment. The overlying rock acts as a seal and doesn't allow the water to escape from the porous sediment. Pressure increases as the sediments are buried and eventually the porous rock can become very "overpressured", with the trapped water in the pores carrying the weight of the rocks above. This is an unstable state and can only persist as long as the overlying seal rock can withstand the pressure! Eventually, the sealing rock fails - usually along a planar or curviplanar fracture - this occurs when the overpressure reaches a greater magnitude than the weight of the rock, or some kind of disturbance (earthquake, passing landslide or debris flow) triggers the failure. The high pressure fluid/sediment mixture escapes its former captivity by injecting outward and upward along the fractures. When the pressure is released, the water is free to move off but the sediment is left behind in the fractures, forming "injectites" or "sandstone dykes".

The injectites are found in a particular stratigraphic horizon in the Dwyka Group (2c/3f contact for those of you in the know) where small sandy bodies are common. The sand is coarse, well-sorted and nearly pure quartz (C. Herbert pers. comm. last week in the computer lab). Injectites are curviplanar with roughly parallel surfaces (although they sometimes undulate out of phase). Thickness varies from about 25cm to 3cm in the several examples Nic and I stumbled upon while looking for faults fortuitously discovered. The outer surfaces of the injectites are very smooth and polished. They are now quartz cemented and weather out relative to the finer-grained, less well-cemented rock flour matrix.


The outer wall surfaces of the injectites have a very distinctive texture - I don't know if it has a name, but it's something like flute marks but sort of braided looking.... Can anyone help me out here? Has this been described before? Perhaps my dear friend the "former" geologist can help.

Have I ever told you that you can't un-geologist yourself? It's like finding out about santa claus carter. Your world has rocks in it and that makes you different... forever.

Eh hem, excuse me. Anyway, the anastomosing flutes are about a centimeter to 3cm in wavelength, with high amplitude (~0.5x wavelength) and vary quite a bit in length. Sources say that the famous Panoche Hills injectite complex in central California may show similar clastic-dike-margin-textures... but on a larger scale...



The geometries fo the injectites can be rather complex - they are even sorted with coarsening towards the center - reflecting increased flow velocities with distance from the conduit walls. Here's Nic again sitting on what is either a) a folded injectite or b) the complex branching/intersection of multiple injectites - Somebody should find out!
What can these surface textures tell us about viscosity and velocity of injectites, strength of sediments, and fluid pressure in the ancient sub-glacial-icey seas? Somebody should find out! Who should that somebody be? I'm hoping one of my future honours students....


In case you doubt that these sandstone channels are submarine, I present you... the drop-boulder. I'm sorry, so sorry for this but each time I look at this picture I think of Cornwallis.