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overtheedge
03-14-2013, 07:15 PM
It isn't quite yet spring and I admit I start getting antsy this time of year, but I AIN"T depressed.

I'm working on my summer itinerary and getting maps ready and I found a strange depression.
1863

The area was heavily glaciated, but the hole is 500+ feet deep and about 1/4 mile across.

The depression is in meta-sedimentary rock.

And now I feel compelled to go a few miles out of my way to check this anomaly out. I am baffled by its existence.

Ideas?
eric

overtheedge
03-15-2013, 12:00 AM
Hole is just under 500' deep. This map segment shows some finer detail. Satellite photo was horrible.

1864

Normally I would just accept it as a cirque tucked up against an aręte, but the depression is the puzzler.

eric

Steve Herschbach
03-15-2013, 08:04 AM
That really is interesting! I honestly have no idea.

Reno Chris
03-15-2013, 08:17 AM
Hard to say just from the maps but two possibilities include:

1. Glaciers - depending on softness of rock, etc. you can get a glacier that rotates around a horizontal axis that carves out a divot like that.

2. Volcanic explosion: There is some tertiary dike rock on the north end of the depression. It is possible that water contacting hot rock caused a surface explosion.

chickenminer
03-15-2013, 08:38 AM
If it was in the valley I might think old remnant Pingo. Too weird of an area for that though I'd say.

geowizard
03-15-2013, 09:37 AM
The depression is inconsistent with the mountain building. It is post-mountain building. If you eliminate cirque - then there remains only one possibility = an external force. Since there is only one outside force; a meteorite is the most plausible.

Volcanic activity would be noted in the geologic mapping. Igneous - lava related rock would be evident to the most inexperienced - it wasn't and probably isn't volcanic.

It's a meteorite crater.

Has DGGS done geophysical mapping over this area?

- Geowizard

overtheedge
03-15-2013, 10:00 AM
As it stands, this area is under snow until mid-July, early August.

I put the geologic map segment first because of the dike cluster and bedrock being meta-sedimetary, typically greywacke in this area. The Chugach Terrane is not that old. About 20 miles away, a dacite dike in the same terrane was dated at about 50MA (Tertiary). But no dating any closer. Though it is thought the terrane was accreted during the late Cretaceous.

The dike clusters have me thinking there is a possibility that this was a diatreme under the ice. But the closest dikes seem oriented about 90° from what I would expect. But what if the diatreme happened at the same general time as the dike clusters were created?

But the heavy glaciation presents another possibility. Most of us have seen potholes in the bedrock in rivers. Alpine glaciers have a thing called ice mills or moulins that carve a hole down through the ice. It would take a big moulin to do it, but there were at least a couple thousand feet of ice over this part of the Chugach Mountains. The remnants of a channel on the north edge of the rim provide evidence of a water course flowing north and east out of the depression. Remember, point of V in contour lines is upstream.

Lakes and ponds in cirque remnants are fairly common. The problem here is the short distance from the arręte makes a terminal moraine this tall and close doubtful. Then there is the talus cone/rock glacier aspect.

Were it on the flats, I agree with Dick that it would make a kettle pond or pingo suspect.

Anyway, it'll have to wait until summer. Figured I'd toss it out for your entertainment. Stinking glaciers have a way of muddying up reading the past.
eric

And yes, I'm keeping the co-ordinates to myself for the time being. It is that "thrill of discovery" thingy. Doubtful that there is an economic value connected with this. But ...

Reno Chris
03-15-2013, 10:55 AM
Geowiz - tertiary dike rock = intrusive rock, meaning near surface melted material that was intruded into the older rock, that is what an intrusive dike is by definition. To intrude material into much older rock, the dike material pretty much has to melted to flow into the faults and cracks that the dikes occupy. Intruded melted rock is a source of which can cause steam related explosions. Just because no surface volcanic rocks are noted in the little section of the geo map shown, does not mean there was no volcanic activity in the area. What is likely is that volcanic surface rocks were removed by repeated glaciation since tertiary times, and only the root source dikes remain. Yes, in theory it could be a meteorite impact site as well, but that is a very unlikely possibility. A low spot carved out by a glacier is the most likely option.

geowizard
03-15-2013, 11:38 AM
The tertiary dike rock is a mile away southeast of section 10 and across the wide creek bed. It's a separate event and unrelated geologic structure. IMO. The creek bed probably rests on a fault zone and represents structural demarcation.

The Quaternary Talus could have been caused by a meteor impact. We don't know the composition of the talus.

- Geowizard

overtheedge
03-15-2013, 12:06 PM
geowizard, I pretty much ruled out a meteorite crater.

In the army I got some training in artillery crater analysis. Artillery craters have a couple of slightly different features, but all in all pretty much the same. The differences are determined by air-burst versus surface/subsurface.

The depression has the appearance of high angle impact. It had to have been almost vertical and subsurface detonation (30-50 miles/second velocity) to get the conic shape. But here is where the problem comes in. Being tucked up against the aręte would have forced the ejecta on the west side to fall back into the crater. It couldn't be almost 500' deep without another mechanism to remove the clastics a second time.

Most of the continental glaciation in this area flowed toward the north. This is based upon striations and the drumlin fields at the south end of the Copper River Basin. Any time there is an abrupt drop in elevation, an alpine glacier presents an ice fall. This would have presented a weak spot for a surface stream on the glacier to form a moulin. My experience and training with alpine glaciers is a poor substitute for understanding underneath continental glaciers.

Ejecta from a diatreme woulda/coulda been long since transported by glaciation/glacial outwash to elsewhere as Chris pointed out. This depression sort of reminds me of some of the stuff you see in the Channeled Scablands of SE Washington.

But all this is speculation. Like so much in our field of endeavor, it needs boots on the ground. I stated at the beginning of this post, "pretty much ruled out", and that still leaves a little wiggle room for me being wrong about a meteorite crater.

eric

Reno Chris
03-15-2013, 02:33 PM
The tertiary dike rock is a mile away southeast of section 10

Huh?? The tertiary dike rock is marked as purple lines on the map and these appear at both the northern and southern margins of the crater. I think you may be mis-reading the geologic map.

geowizard
03-15-2013, 04:29 PM
The only point I'm trying to make is that the intrusive tertiary dikes aren't volcanic. The intrusives are hydrothermal events of a very small size.

Here's a paper on the geology. It shows most of the structure is from folding and faulting of sedimentary formations. There is a short reference to volcanism.

http://www.dggs.alaska.gov/webpubs/dggs/pr/text/pr094.pdf

Ote won't divulge the location but it' may suffice to say, it's in the Northern Chugach Mtns. Just a WAG? :)

- Geowizard

overtheedge
03-15-2013, 05:41 PM
Yes, Northern Chugach Mtns in the Chugach Terrane but not in the McHugh Complex according to the map. The dikes are sorta ubiquitous and are associated with what little mineralization there is. There were a couple three pocket mines, but none really produced much on this side of Thompson Pass (a few tens to a few hundred ounces). The same general geologic event did produce in the Valdez area (Cliff Mine being one in particular).

Lat/Long approx. 61°21'30" x 145°23'30"

Keeping it secret isn't that important. The area is not completely barren, but it is awful lean diggings.

This might help:
http://www.dggs.dnr.state.ak.us/pubs/id/12848

eric

geowizard
03-15-2013, 06:05 PM
Ote,

Geo Jim and others will be visiting Ophir this summer. You are welcome to visit.

I can get you on to some elephant tracks and we can see where they lead. ??

- Geowizard

Bob(AK)
03-15-2013, 11:03 PM
I'm no expert, but sure looks glacial to me. Many other holes like that around, just not so deep

geowizard
03-17-2013, 10:03 AM
Ote,

Thanks for the added information.

Not likely a meteorite after all. (depressing) :)

I viewed over 3000 square miles of Chugach Mountain glacial terraine and found NO OTHER features similar to your depression. There are many references to Talus cone deposits in the geologic map which provides further enlightenment. Yes, now, looking at the bigger picture with discussion of the rock types, the Td intrusives are described as Andesite - definitly igneous. The larger view shows small dike structures covering four square miles or more locally.

There is some discussion of metamorphic marble rock types and carbonates. I have seen cases where interpretation of rock types from a helicopter i.e. quartz intrusives may be incorrect. Laurel Burns is the chief geophysicist at DGGS and a well respected authority.

- Geowizard

overtheedge
03-17-2013, 07:50 PM
Yep weird all right. Tarns are common enough and there is usually a shallow depression in the cirque, but this is a bit extreme. And so close to the aręte. I could easily see it as a terminal moraine if it was further away.

Well it will have to wait until mid-July or later for the snow to melt enough. Ain't much summer in the high country.

eric

Tarn: a pond or lake in the depression of a cirque formed from seasonal snow melt and rain.

Late July or early August 1966, I made the mistake of going swimming in a tarn on Broken Top (or was it South Sister?) in Central Oregon. I won't make that mistake again. It was bloody cold. Hmm, a depression filled with snow melt didn't ring any bells? One trick to getting old is surviving your youth.

fineswme
03-19-2013, 12:41 PM
OTE , Would you need a permit of any sort to dredge the pond in that depression ??

overtheedge
03-19-2013, 10:26 PM
I really don't know. The premise behind the permitting is your mess leaking out into the watershed and causing problems. Here is a case of a deep hole that leaks back into the hole. Not a clue. I was a bureaucrat for awhile and got into trouble. I contend I was right, they signed the time card. I'm no longer working for the bureaucracy.

I would bet that there won't be many folks watching to see if you are operating outside your permit conditions.

Now look at it from a different perspective. The permit is free. Why not get a permit?

Have I applied for a permit? No. Why would I without sampling?

For other folks, you have the lat/long. To the best of my knowledge it is not claimed. All I want is credit for noticing it. The best science is NOT the eureka moment. It is that, "Now that is really weird" moment.

eric

fineswme
03-20-2013, 02:56 AM
Ote,
The reason I asked is in the area I'll be at this season I was looking at the feeder creeks dropping into the main river when I noticed on the topo, say 1500 feet above river elevation one of the feeder creeks has 3 ponds at the very top in like a saddle area with the surrounding hills rising again about 1500' first draining into the ponds then flowing out a fairly steep creek .. seems like it could be a legitimate gold trap , lord only knows how deep the gravel may be in there and if it is indeed gold bearing ... I agree with you on the fact it is fun to search around with the computer and notice different things that gets a guys curiosity up ... Looking at this small creek I really hope it has a bit of gold worthy of going after ; I would love to make use of gravity with the ponds providing ample water supply .... 15,000' distance and 1,500' elevation change ; never used gravity before but this seems like a prime candidate to me !!!!