Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

GeneralGeologic origins and diversification of agates

17th Jul 2017 04:43 UTCAshley Wise

I apologize if this is the wrong forum for this type of question, I just joined today.

I've been collecting rocks/agates/minerals my whole life, and am extremely interested in geology.

My inner curiousity has been storming over trying to understand the true origin locations of lake superior agates.

Being from Minnesota, I've been finding them everywhere my whole life, and visit the north shore bedrock yearly and look for signs of lava flows and basalt vesicles.

I've never found obvious signs of the origins of the lake superior agates. There's plenty of basalt and vesicles, but the ones i see in bedrock are always filled with quartz, feldspar, or other minerals, but not true lakers.

I've seen pictures and heard stories of agate veins found underwater. Though those have seemed to be more like the paradise beach variety (opaque pinks and peaches), and not the translucent red and white lakers.

I'm assuming that hundreds of feet or maybe miles of mountain tops have already been eroded away over millions of years from the north shore, and wondering if the true lakers might be primarily in those long ago eroded mountains.

Are the lakers found in the river beaches actuallyeroded from basalt upstream? If so we should be able to find the beds, the rivers tend to lose power and volume several miles inland. Or are they just moving glacial sediments?

Then I wonder if the lakers actually originated farther north, and are only over lake superior from glacier activity?

I have found a couple laker-like agates as far west as central north dakota (which typically only sees north dakota moss agates, which are a snot-colored nodule easily distinguishable but seemingly related to montana moss agates), which could only have arrived via glacier, and those glaciers only came from Canada, not the superior region.

I have found several areas of thompsonite in the basalt bedrock of the north shore. The keweenaw peninsula has several sources of the keweenaw agate in bedrock basalt.

All of these, the thompsonites, paradise beach, and keweenaw agates are all opaque pink-ish material. So this always brings me back to the question, where is the source bedrock for the bright translucent red lakers?

There's some other types of agate and geodes around. The shore of gooseberry has a lot of quarts geodes in the basalt vesicles. And when I was younger I used to find these small bluish-clear vein agates all over the beaches. Though I haven't found any of thos in a while.

At agate days I just picked up a collection of lake superior agates of a variety and coloration I'd never seen before. Similar to paradise beach but with some aspects of lakers.

I'm sure I'm rambling. But I'm just curious if anyone has any geological insight as to where the source bedrock for these different lake superior agates is or was. And also ideas for the diversification, which bedrock layers are originating which of the many types of lake superior agates.

It seems if a particular bedrock layer contains a particular type of agate, you should be able to trace that layer in 3 dimensions as it traverses through the north shore and possibly under lake superior. All of those deep gorges cutting through mountains should be cutting through these different lava flows and exposing the different bedrock sources of the different agates.

For instance, it seems that somewhere in the cascade river gorge, it should have cut through the thompsonite basalt and exposed it.

Does the keweenaw flow reappear up on the north shore of mn?

So many questions, so much curiosity!

17th Jul 2017 04:48 UTCDoug Daniels

Just saw this post, welcome to mindat Ashley! You have a number of good observations, as well as questions. I'm no expert on that area, but there are some here that know quite a bit about up there. You should be getting some interesting conversations soon (ok, maybe tomorrow, or the next day, or...). Give the "knowbies" some time to see your post, and respond. Again, welcome!

17th Jul 2017 12:26 UTCLarry Maltby Expert

Ashley,

The Keweenawan rocks that contain agates outcrop along the Keweenaw Peninsula of Michigan. They dip under Lake Superior and then also outcrop on Isle Royal. The glacial lobe that scoured the bottom of Lake Superior moved toward Duluth and during the melting of the ice a flood of water washed gravel and agates down the Mississippi flood plain. The formation that contains abundant agates has been named the Lake Shore Traps. The traps outcrop along the shore at Keweenaw Point, Copper Harbor and Agate Harbor in Keweenaw County, Michigan. Other than that they run parallel to the Lake Superior shore but are inland and covered by forest. It is likely that the red and white “candy stripers” that you find in the gravel originated in basalts that are now under Lake Superior.

See: https://www.mindat.org/article.php/1593/Keweenaw+%28Part+1%29++Keweenaw+Point+

17th Jul 2017 15:11 UTCJohn Truax

South of the Twin Cities in Dakota County, Minnesota there are areas of glacial till that contain lava from the Keweenaw, thomsonites and agates sometimes large and undamaged. I like to imagine frozen masses of till being pushed along by the glacier and coming to rest at my favorite agate hunting fields.

Glacial till resting on bedrock of sandstone.

21st Jul 2017 19:44 UTCAshley Wise

Yeah, the Keweenawan traps seem to be understood the most, as you can trace that layer of basalt under superior and surfacing again at isle royal. But it's definitely different from the traditional red and white lakers.

I had always assumed that the rivers on the MN north shore were carving the agates out of the rock layers as they dug through their gorges, and thus somewhere upstream there would be the agate sources. Like following gold flakes in a stream until you find the quartz veins up in the mountains.

It seems more likely that all of them were carved out of the bottom of lake superior by glaciers, shoved up on land, and the streams are just moving them back into lake superior from upland. And also explains why there seems to be 10 or 100 times as many agates found in outwash fields by minneapolis as there actually is around superior.

My parents have a simple 15 acre field south of princeton, and in 20 years we've collected a dozen gallon buckets of lakers just from that small field alone, all from tractors mixing up just the top few inches of soil every year (and I know the difference between chalcedony, jasper, and agate).

Not to mention the landscape river rock from maple grove/rogers.

I just wonder if there's still laker basalt traps hidden in the bottom of lake superior, or exposed in the hills of the north shore somewhere. Or if it's all long eroded.

My big question was just general location. Is it in fact from lake superior region, or were the red/white lakers actually pushed down from canada somewhere?

23rd Jul 2017 03:52 UTCPaul Brandes 🌟 Manager

Hi Ashley, and welcome to Mindat!!!

Forgive me, but I’m having a hard time following what you are actually asking about. Are you asking about the specific source of the “candy stripe” agates, or agates in general? I commend your efforts to learn more about Lakers and their formation, but you’ll soon find out that it is a question that has been debated for years, with no end in sight. I will attempt to shed a little light on your quandary.

The Lakers you find in the prairies of Minnesota were carried to that location by the glaciers from the basalts in current Lake Superior basin. They are likely not from further north in Canada because those rocks are mostly made up of Canadian Shield material which are, for the most part, igneous intrusives; rocks that are not conducive to agate development. Undoubtedly, there are still basalts that hold agates in Lake Superior today. Otherwise, how would we continually get agates washing up on the shores of the Keweenaw and the eastern Upper Peninsula of Michigan? Larry is very correct in that the Lake Shore traps hold a lot of agates, and one can still find plenty if you’re patient. However, you can also find agates far away from Lake Superior in some of the mine tailing piles around the Calumet, Michigan area, some even with copper replacing the bands (those are a whole other animal altogether). These agates are from basalts that are further down section than the traps and even under the Greenstone Flow, the major basalt flow of the Keweenaw (this is actually the flow that can be traced from the Keweenaw under Lake Superior and to Isle Royale).

So to answer what I believe is your original question of where do the candy stripers come from; while it is very well understood where the agates come from (the rift filling basalt flows of the Lake Superior region), I don’t believe anyone knows the exact answer of whether a particular flow produces a particular type of agate. That sounds like it would be a fascinating research study for someone to take on…..

25th Jul 2017 13:30 UTCAshley Wise

Thank you for all the responses.

Yes, my main question is "Exactly where is the source bedrock for the (specifically) red&white lakers". However my interest in where minerals meet geology is broad.

I just find it fascinating that you can find the thompsonites and the keweenaw agates in the source basalt on the shorts, but the red & white lakers are much more elusive.

And I'm fascinated with the idea that these tilted lava flows should be reappearing in the river gorges. I always wonder where is the equivalent lava flow between one river gorge and another. Or will it reappear on the back side of a cliff.

Why is there a thick layer of red sandstone between basalt layers on that road cut by cutface creek?

Etc!

I had the same experience at my in-laws place in Gold Canyon, Arizona. The desert rubble contains numerous examples of jaspers, granites, schists, a multitude of colorful stones. But I've climbed up in the superstition mountains and all the rock is just tuff, dacite, and basalt. Where did all that other rock erode from? It just seems the relative concentrations are out of wack

Same with the lake superior beaches. Some of them are all blue basalt and red rhyolite, and that makes sens. But some beaches are full of a variety of rocks that just doesn't match the surroundings. No doubt the erratics were pushed from glaciers, but it just seems the relative concentrations are off.

1st Sep 2017 10:20 UTCGregg Little 🌟

Hi Ashley;

I just came across your thread and I too welcome you to Mindat.

I'll attempt to address your question about the seeming lack of correlation of beach rock to the surrounding rock in place (outcrop). As you pointed out, glaciers are part of the answer. Ice is different from water and wind as a transportation mechanism in that it disperses rock over a wide areas of lake bottoms and river drainage basins.

In the opposite effect, water (and wind) concentrate sediments within their areas of origin. I would surmise it is this dispersion during the glacial periods then concentration during stretches of non-glacial time that causes the mismatching of beach deposits with the surrounding country rock outcrops.

1st Sep 2017 14:07 UTCGregg Little 🌟

Ashley, you asked, "Why is there a thick layer of red sandstone between basalt layers on that road cut by cutface creek?"

The sandstone simply represents a period of no lava deposition either because volcanic activity stopped altogether during this time of sandstone deposition or no lava flows were deposited in this area at this time of sandstone deposition. Much as is seen in Hawaii, as soon as the lava flow solidifies, it is subject to erosive forces.

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: May 7, 2024 18:42:32

Go to top of page