Monday, September 29, 2008

Alaskan Turbidites

This post is mostly an excuse to show some turbidite photos, turbidtites obliquely mentioned much earlier. The best turbidite photos I've seen are Brian's at Clastic Detritus - like these.

The above photo shows the famous Alaska Turbidite Locality, located on Alaska Route 1 between Mileposts 104 and 105, about 23 miles southeast of Anchorage on the way to Girdwood and points beyond. The photo is taken from the Girdwood side of the roadcut, looking in a northwesterly direction, more or less towards Anchorage.

The location of the next two photos is the in the cliff area above the two dark vehicles that are in front of the red truck and past the leftward turn in the road. The location of the last four roadcut photos is in the cliff area above or to the right of the white vehicle that is between the closest turn and the far, righward turn (past the two dark vehicles).

Parking can be seen on the immediate right side of the road. Parking is tight; use caution. I walked on the outside of the guardrail to get to the roadcut, and then carefully ran across the road when no one was coming - you may have to wait awhile, especially on weekends. Also, there may be parking on the Anchorage side of the roadcut; I didn't, however, check that out.

Turbidites! The cliff above is about 50 to 60 feet high. It exposes turbites of the Cretaceous Valdez Formation or Valedez Group (somewhere between about 146 and 66 million years old).
Above, a somewhat closer view of the same cliff. In general, the dark gray layers are finer grained, composed of sandy to silty mudstone, and the light gray layers are coarser grained, composed of dirty, silty sandstone. The alternating patterns create a banded look typical of turbidites. These are not varves, so each light-dark couplet can't be counted as one year - it counts as one turbidite episode.

The rocks in the cliff show all kinds of sedimentary structures, including soft-sediment deformation, ripple cross-lamination, graded bedding, and rip-up clasts. Non-sedimentary structures include some faults and a pervasive high-angle metamorphic foliation.

Unfortunately, I haven't examined this roadcut in any detail for a number of years. Cars and trucks were whizzing past, and the lighting was poor - dark and cloudy. Some of the best examples of sedimentary structures require more time to find than I had time for (if you can believe that! - but we had dates with fish to keep, remember!). The roadcut is a fun place to stop, however, even if only for a moment or two.

A bit of geologic history: The turbidites in the Valdez Formation formed when submarine slumps and slides, set off by earthquakes along a paleo-subduction zone, flowed along the sea floor carrying sand and mud. Currently, the Alaska subduction zone is off to the east and southeast in the Gulf of Alaska.

The Valdez Group rocks were probably not anywhere near Alaska when they were forming. Some notes of mine from a 1997 field trip suggest they formed at about 40 degrees north latitude, and were moved north by strike-slip faulting along the coast of North America. They have since been uplifted along the Border Ranges fault, which runs along the western base of the Chugach Mountains, and which separates the Wrangellia terrane on the west from the Chugach terrane on the east.

The Google Earth - One Geology image above shows the Chugach terrane in green, with Girdwood smack dab in the middle. To the west of that, the Wrangellia terrane is shown in light yellow. Another terrane or group of rocks is shown to the east of the Chugach terrane in bright yellow. The present Alaska subduction zone is approximately located between the dark blue water and the deeper water shown in a funky purple. The terrane boundaries curve eastward, somewhat parallel to the curve of the Alaska subduction zone. Active volcanoes created by the subduction zone also curve crudely to the east (although to the north of the above image). The Border Ranges fault, an active fault with movement and an earthquake as recent as 1997, runs along the west side of the green Chugach Mountains terrane.

The Chugach terrane is an accretionary wedge, prism, or complex. South-central Alaska is made up largely of several accreted terranes with complex geologic history. I really can't pretend to be up on the details - I took a class at UAA in 1997 and went on a few field trips. Things have probably changed some since then, at least interpretationally speaking!

This Google Earth - One Geology image has been rotated to look straight down the Chugach terrane from northeast of Anchorage, toward Girdwood, and to the Homer spit and Kodiak Island beyond. The turbidite roadcut is just a little west, or right of, the yellow pin marking Girdwood.

Back to the roadcut:

The above photo was taken from the other side of the road, outside the guardrail. The cliff in this part of the roadcut is about 20 feet high, with its base just below road level - there's a dip between the road and the cliff that you can't see from this angle. This photo, and the photos below, show some of the features I was looking for, but the cliff is steep and hard to climb around on!
Above: metamorphic foliation cutting across the turbidite beds. The foliation has been interpreted as indicating top-to-the-right motion, which is more or less to the east or southeast, and the top-to-the-right motion has been taken up somewhat by little bedding plane slips, which are hard to see.These last two photos show me pointing to some beds just above the white flowers that were seen in the lower left of the previous photo.
Turbidite flow direction is to the right in these photos, perhaps best indicated by the upper part of the lower light-colored sand layer - the one that makes it all the way across the photo from the lower left corner to about a third of the way up on the right side. The finger, for scale, is pointing to a thin white layer that may show the same thing.

Comments are welcome - maybe you can spot some things in these photos that I haven't! For lots and lots about turbidites, go see these posts at Clastic Detritus.

6 comments:

BrianR said...

yes!

Good stuff ... so, the interpretation is that these are trench or trench-slope deposits?

Silver Fox said...

Brian, these are described as being trench fill deposits, and were "probably deposited on the downgoing plate in a deep-sea trench and accreted shortly thereafter" (Bradley and Karl, 2000).

Also, the paleocurrents are supposed to be at azimuth 215 (S45W), which is at odds with what I said above (that would be about to the left in the photos). Do you see any indication of the current direction? If so, did I get it backwards? My old notes might be wrong!

References: Field Guide to the Mesozoic Accretionary Complex in Kachemak Bay and Seldovia, South-central Alaska by Bradley and Karl, 2000.

1997 Guide to the Geology of the Kenai Peninsula, Alaska by Karl, Vaughn, and Ryherd, 1997.

BrianR said...

hmmm ... it's really tough to see any paleocurrent indicators in those photos ... with beds like that I might want to see some ripple cross-lamination, but difficult to tell

Silver Fox said...

Well, I might have to revise the post slightly, then. I thought maybe that one layer showed something that matched my old notes, but then there is also the cleavage-foliation cutting everything. What we saw on the field trip - I might have put it in my field book incorrectly, or maybe it wasn't the usual direction.

Hmmm... indeed

Anonymous said...

hye... Im Muhammad Azfar form malaysia... your blog very interesting for me... may u explain boat what is turbidite in detail and what the method the we can recognize the turbidite by looking at the outcrop... it there any special features the we can assumed that the outcrop is from turbidite....my u email me at mohamed.azfar@yahoo.com... im hungry to know detail about turbidite... tq.... have a nice day...:)

Silver Fox said...

I'd suggest the blogs Off the shelf edge and Clastic Detritus (linked to at the "BrianR" comments above). Also, Google Scholar for research articles. I probably won't be gettng into details of turbidites in the near future, it's not really my field.