Department of Geography and Geology
University of Nebraska at Omaha
1999 Lake Superior Field Trip - Part 1
Harmon Maher & Bob Shuster
Every year we pick a route, pile into various vehicles,
and embark on the very best of learning experiences for a geologist
or earth scientist, a field trip. With the complexity of the landscape
Figure of lighthouse along the shores of Lake Superior and on top of the Duluth Complex - a large layered mafic intrusion associated with a major rift event some 1.1 billion years old.
and rocks in their totality before you, there is no better classroom for learning about the earth. This year we looked at the rocks exposed in the Lake Superior region - rocks that hold insight into some of the earliest history of the earth and rocks exposed by a recent glacial revolution known as the Ice Age or the Pleistocene. This is third time in 15 years we have taken this trip. The lure of this particular area is simple - a geologic richness to mine experience and understanding from. Some of the topics that can be investigated in the region include:
Major geologic features encountered on the field trip include: Archean granulites and migmatites, Archean greenstone belts, Banded Iron formations, the Keweenawan Rift, The Duluth Layered Complex, and the Lake Superior basin.
What follows are notes and images from our department's field trip. By definition they are a bit hodge-podge in character. They may be of interest in of themselves, as a virtual field trip (although the real thing is so much better) or for planning a field trip of your own. Enjoy and send us feedback. Don't hesitate to contact us if you wish to participate in future field trips.
5/11/99 : 11 students and 2 faculty leave Omaha just after 9 AM and drive northwards on I-29 through intermittant rain.
Day 1 - Stop 1: Palisades State Park
Location: Just N of I-90 - its on most maps.
After debating with the State Park attendant, we give up and pay the $5 entrance fee which they will only waive if you have a state vehicle. Educational groups without one are out of luck.
Here a small gorge is cut into excellent outcrops of the Sioux quartzite. This is part of a once much more extensive geologic unit that once spanned an area now covered by three states if not more (Dott, 1983). The Baraboo quartzite in south central Wisconsin is perhaps one of the better known exposure areas of this unit. An interesting speculation is that fact that it is preserved in a spotty fashion across this wide area suggests no major crustal disturbances since its deposition (otherwise wouldn't see equivalent level of exposure and/or wouldn't be preserved.).
Sedimentary features evident in quartzite outcrops by the bridge:
Students can explore what information is available about the depositional environment in these observations. The paleocurrent direction was relatively constant in the small area inspected. The importance of the observation that this is an orthoquartzite can also be addressed?
Other features seen include leisagang staining, very well developed joint sets, and a slight dip. More recent surface features include an abundance of percussion marks on quartzite faces. These were made during time of higher flow regime as large clasts hit the quartzite bedrock. These semi-conical fractures were well developed on upstream and flat faces.
Questions that arise are:
Day 1 - Stop 2: World's oldest rocks, not!
Location: Just S of the river and Granite Falls.
Here a sign commemorates the oldest rocks in the world at 3.8 Ba at small turn off. While geology is getting appropriate attention here, Dr. Bob informs us sign wrong on 2 accounts - these are not the oldest rocks and they are not 3.8 Ba, but more like 3.6 Ba. We are now in the Minnesota River Valley, where Archean bedrock is exposed and has been well studied.
Here we looked at Montevideo gneiss across the road in a cut. A mafic granulite, possibly a metagabbro, the outcrop was cut by numerous north dipping fault zones and veins associated with a fair bit of green staining. The greening coloration is retrogressive metamorphism likely associated with fluid flushing, likely during faulting. The fault textures and mineralogy suggest it was just above brittle-ductile transition at 15 km or so (unless the geothermal gradient was really different at this time in PC) at the time of this deformation change. Some strike-slip striae were present suggesting these are not simple normal faults. Some K-spar bearing granitic veins stand out against the mafic country rock.
A discussion on the conditions necessary to form granulites can lead students to see these as very deep crustal rocks. A natural question that then arises is how do they come to be exposed at the earth's surface?
Day 1 - Stop 3: Granulites and isograds.
Location: Just E of town campground, road cuts on both sides are some 70 m long. Be very careful of the traffic. We don't know how many points the locals get for a geologist, but they seemed to be eager.
Exposed are mafic granulites, but not the same age as the Montevideo gneiss visited at the previous stop just down the road. These are only 2.65 Ba old. This is suggestive of an intervening fault, which the smaller faults at the previous outcrop may have been related to. While similar in composition these rocks are without significant retrogression. Students can be reminded of the cleavage and color in identifying the metamorphic pyroxene in the rock (some of which is relatively coarse). Biotite is the only hydrous phase. The gneissic texture and a 30 dipping foliation with associated isoclinal and ptygmatic folds of 'sweat out' veins is evident.
This outcrop provides the rare opportunity to easily introduce the concept of mapping metamorphic isograds. At the west end garnet is absent, but as one walk east a point is reached where garnet appears even though the composition has not changed significantly. This then represents the garnet-in isograd, and garnet is likely growing at the expense of biotite.
A mafic dike cuts the west end of the northern outcrop. We discussed with students the ease of assigning it a Keweenawan age.
Camp: We camped at the Upper Sioux Agency Park Campground after some 330 miles of traveling, along the flood plane of a tributory into the Minnesota River that cut through thick glacial deposits? Ticks were found.
5/11/99 Cloudy in the morning with intermittant rain through the night. We continue to investigate the Minnesota River Valley in the morning and take off for the north country in the afternoon.
Day 2 - Stop 1: Minnesota River valley and glaciation.
Location: Along small trail just W of the campgrounds that goes up the slope that defines the edge of the floodplain.
Large clasts of granite and various metamafites littered the ground, a sampling of basement rocks from up ice-stream. The glacier had done the geologic sampling for us. Similar to the Mars Lander landing site sampling breakout flood deposits, here one can be quickly introduced into the geology of the Precambrian shield N of here. The river valley consists of steep walls cut into glacial deposits, and a thin veneer of alluvium on an uneven bedrock surface with occasional basement outcrops. The glacial deposits here seem to be much sandier. Could that be due to reworking of even older glacial outwash material. The present Minnesota River seems incapable of creating the valley before us, it seems underfit. Is that due to a lack of imagination on our part, or does it reflect a much more vigorous river in the past?
Akiko gave the first of the student lectures on the field trip. Her nicely organized lecture stressed differential erosion by ice sheets, and how glacial ice streams can be worked out from the unique debris trail they leave. When we come to Lake Superior the concepts she introduces will be quite useful.
The Ice Age paleogeography can be explored here, and the river Warren draining the paleolake Agassiz mentioned.
Day 2 - Stop 2: Sacred Heart granite margin.
Location: This stop is 5 miles S of Sacred Heart on Route 9/7. There are good road cuts and large riprap blocks on both sides of the road.
The following can be seen in these outcrops:
Figure of Sacred Heart granite intruding and incorporating mafic gneiss. Hammer for scale
Why all of this complexity here? One argument is that this is the complex margin of a deep plutonic body. Towards the river more country rock are seen and away from the river more homogenous granite. The students can be reminded that on a geologic map all of this complexity they see may be reduced to and represented by a contact line.
Questions that can be entertained here are:
Day 3 - Stop 3: Don't take Morton for granite is gneiss (we didn't make this up - its on the sign - see adjacent figure taken from road leading into town).
Location: A knob just behind the school and the Days Inn in the town of Morton - this an overgrown old quarry site, and you need to walk around a bit to find some of the old pits.
Here are extensive exposures of the Morton Gneiss - a migmatitic gneiss of chaotically deformed pink and dark grey layers so striking they have been used as decoration stone. Sampling activity by the students was heavy at this site.
Meg gives the second student lecture on the long history of these rocks involving amhibolites, dioritic gneisses, and a host of younger pegmatitic and aplite granitic phases. Three deformation events span a period from 3 to 2.6 Ba, with evidence of a weak 1.8 Ba Penokean events. No wonder these rocks are so chaotic - they have blended in the crustal mill for at least hundreds of millions of years.
Figures of Morton gneiss quarry and Morton gneiss migmatite.
Students can be introduced into coherent vs. incoherent deformation behavior, both of which can be seen here. Deformation episode 9 becomes a bit suspicious.
Questions that can be explored here include:
Day 2 - Stop 3: The Clouds Creek pluton - don't take this for granite either.
Location: Road outcrops on Rt. 23 just NE of the bridge crossing the river in the town of Cold Spring. Again, the traffic moves along here. A small rest stop area occurs just up the road from the outcrops on the NW side.
This is a coarse grained hornblende syenite, with K-feldspar, plagioclase and hornblende. There is a good alignment of many of the large K-spar megacrysts. Discussion can try to decide whether this is a flow foliation in a crystal much in a convecting magma chamber, or some deformation fabric imprinted after the magma had crystallized. A good rapakivi texture can be seen in parts of the outcrop with white plagioclase mantling the brownish K-spar. Subhorizontal loading fractures can also be seen.
Questions that were asked here are:
Long drive up to McCarthy State Beach State Park - in the heart of Archean greenstone country.
5/12/99 Today was focused in on Archean greenstone belts and associates. Initial rain followed by general clearing.
Day 3 - Stop 1: a random green outcrop and typical greenstone.
Location: This was an outcrop on the W side of HW 5 just a mile down from turnoff to the State Park. It was chosen because it was the first good looking crop we came upon. We are making a traverse south to Hibbing and BIF country.
The following features could be seen:
The following questions were addressed here:
The cleavage, lineation, isoclinal and intrafolial folds suggests deformation here is extreme.
Day 3 - Stop 3: What's that outcrop on the right?
Location: Flat pavement outcrop on the W side of Rte 5 several miles south of park turnoff.
Well developed meta-gabbro with green plagioclase laths and hornblende. Little or no deformational fabric is evident in this rock which seems to retain an igneous isotropic character. Again glacial striations were evident, with multiple directions.
Questions asked here include:
Day 3 - Stop 3: SOS?
Location: Another mile or so down Rte. 5, a low pavement outcrop on E side.
Same metgabbro unit, but with a subvertical deformational fabric, and more veining. It is also a bit finer grained we could surmise we are nearer the margin of the gabbro intrusion. Several cm wide thin granitic dikes intrude giving an important age relationship. An outcrop between this and last one suggest there are screens of greenstone in the gabbro, and that the gabbro in part intruded the greenstones.
The point can be made to the students then that the gabbro represents a map size unit.
Day 3 - Stop 4: The other half of granites and greenstones.
Location: Road cuts on the W side, yet another mile or two down the Rt 5. The outcrop is a good 50 m or so long.
This represents a real change from previous stops this
Figure of a mafic dike cutting the leuco gabbro, in turn intruded by a pegmatitic granitoid along the same plane, then cross cut by several other pegmatitic granitoids.
Rock types seen include: a coarse meta-leucograbbro, a finer grained grey granite, a lot of mafic xenoliths (some fairly fine-grained)in the granite, and a series of aplites and pegmatites. The pegmatites represent late stage igneous activity. A subvertical strongly developed foliation exists, with many xenoliths flattened in this plane. Some discussion leads us to postulate that this is a regional foliation, and likely the same as seen in previous outcrops this morning. A distinctive dike set occurs with a coarser less mafic phase cutting the leucogabbro, and a more felsic coarser dike intruding the first one, a beautiful example of dike reactivation. Ubiquitous small brittle faults also occur.
The interpretation is that these were later intrusives into the greenstone rocks, and we are seeing the 'granite' half of granite-greenstone terranes. Another possibility is that this is remobilized basement to the greenstone sequence.
Day 3 - Stop 5: Foliated intrusive.
Location: Still making a traverse down HW 5. Large road cuts exist on either side, and just S of the entrance to the Hibbing taconite mine.
A strongly foliated metadiorite makes up most of the outcrop. Lots of small and flattened mafic xenoliths occcur. Again the foliation is subvertical and oblique to the road suggesting it is a regional fabric. Zoned aplite/pegmatite dikes are oblique to the cleavage and show consistent south-side-down asymmetry. The folds are isoclinal and more similar in style. The asymmetry suggest an antiformal feature to the S using standard models.
We pull out the state map and review the various geologic units we have seen in the traverse this morning. The general N70E cleavage trend is reflected in the map pattern. What tectonic event created it?
A very large tailings settling pond is evident in the distance, and the rain has ceased.
Day 3 - Stop 6: Hull Rust Mine Overlook
Location: Follow the signs in Hibbing to the Hull Rust Mine Overlook.
This 5 mile long trench and scar and associated
tailings piles is impressive to some and depressing to others.
Figure looking at a small part of the Hull Rust Mine. The terraced piles in the distance are tailings (mine waste). Whitish layers at the base may be a sandier basal unit.
It is a now a taconite mine in the Mesabi Iron Range. The roles of geologists in mineral exploration, extraction, and also the associated environmental remediation and clean up comes to mind when confronted with this overlook. A guide mentions that there are asbestoid concerns associated with dust from the mine since there are some amphiboles in the banded iron formation being formed.
Students are asked about how these rocks differ from the ones we had seen in the morning. In this context, noting a slight southerly dip of these relatively undeformed rocks, and looking at the geologic map brings to light a major unconformity between the Mesabi Iron range and the underlying Archean greenstone-granite terrane we visited to the north in the morning - an Archean basement embedded in the Precambrian basement of north-central Minnesota.
On the way out we visited MN DNR Mineral Division facility, and were given a very nice summary discussion by a staff geologist on the mineral exploration in the various Precambrian entities. We thank them for their hospitality and information. A air photo of the area had the Mesabi Fe range super imposed is on the foyer wall. A well developed drumlin field was evident to the SE.
Day 3 - Stop 7: Stop on a whim - greenstone.
Location: Roadcut on S side of HW 169, just N of Virginia.
These rocks are very much like first stop of the day - complexly deformed, massive greenstone. A post metamorphic fold of the subvertical cleavage was associated with slip surfaces, suggesting a flexural slip component, and a polyphase deformation history. That is the way it is with these terranes - polyphase this and polyphase that. So how did these greenstones form?
Day 3 - Stop 8: Granite-greenstone boundary.
Location: Wayside rest area for the Laurentian divide on HW 169, and only a mile or less from last outcrop. Good road cuts.
Here is well exposed a complex array of layered, deformed
tonalite and quartz diorite intrusives into mafic rocks with local local
Figure: Image of complex intrusive relationshop between the mafic rocks (greenstone protolith) and the granitic light colored material.
The preponderance of hornblende, the grain size, and the lack of green minerals suggests these rocks were deformed and formed at amphiblite facies conditions. Thin mafic layers are cut and extended by thin and somewhat regularly spaced granitoid dikes. Under these conditions amphibolite is acting more brittlely and more competently than granitoid rocks, and this is a fundamental relationship. One can make a leap from the outcrop to Wegener, and the trouble his continental drift theory had with the relative strength of continental versus oceanic crust.
The interpretation is that this is the intrusive margin of a greenstone belt and the higher grade is due to the intrusives.
Sheryl gave a nice presentation on greenstone belts identifying many of the components we had seen and were to see today. These certainly are intriguing bodies - primitive arcs, collapsed rear arc basins, ophiolites, intracratonic rifts, are astroblemes? Why are they unique to the Precambrian or are they?
Day 3 - Stop 9: 'Upper' greenstone belt folded turbidites.
Location: Stop 6 of the DNAG guidebook article11 (Southwick, 1987).
Figure of light colored and coarse grained, graded, volcanic wackes intercalated with darker mudstones. These are interpreted to be turbidites.
The DNAG volume does a very nice job of describing the next suite of outcrops of graded volcanic wackes and mudstones. Forewarned by the guidebook one could see abundant evidence of the soft-sediment deformation component that complicates the folding style. Sand dikes/intrusions were particularly common. The foliation has a bit more of a dip here, and again a shallowly plunging lineation (some 30 or so) was evident.
It might be interesting if someone took more of a basin evolution approach to greenstones.
Day 3 - Stop 10: Dacitic volcanics of the Vermillion greenstone belt.
Location: Stop 6 of the DNAG guidebook article11 (Southwick, 1987)
This was a new stop for us this year. At this outcrop one could see a variety of dacitic pyroclastic units with large feldspar phenocrysts. Some interflow sediments, muddy interbeds of some sort, are now good phyllites. This represents yet another component of the Vermillion greenstone belt. What was driving the evolution of volcanics in a greenstone belt? Could it be crustal thickening and differentiation and evolution with time, or evolution of tectonic setting (rift to arc)? So many questions so little time.
Day 3 - Stop 11: More turbidites and great fault geometries.
Location: Stop 7 of the DNAG guidebook article 11 (Southwick, 1987).
The following eatures can be seen here:
Just N were very good outcrops of the dacitic pyroclastic unit in a large highway cut we will visit the next day.
Well past 5 PM and time to head back to camp. Drying sleeping bags in Virginia. Giant Loon on the lake. Thanks to kind ranger showers upon return. Sprinkles at night. Where is that Red Lobster?
Day 4: 5/14/99 Light sprinkle in the morning otherwise clear day with only patchy clouds. This day was spent in the Soudan-Eli area with an emphasis on Banded Iron Formations.
Day 4 - Stop 1: A different BIF.
Location: Stop 4 of the DNAG guidebook article 11 (Southwick, 1987)
Figure of the folded Vermillian range, Soudan BIF. Hematite, siderite, and jasper layers are all present here.
This consists of a beautiful pavement crop at the top of a ridge, with an old mining pit right behind it. The banded iron formations often are somewhat more resistant to weathering and form topographic ridges in this country.
BIF with the hematite, jasper and a few siderite layers. The following could be seen:
Rich gave his talk on the mineralogy of BIFs, describing the various mineral-sediment facies as a function of water depth and organic content.
Deb gave her a very informative talk on the overall formation of BIFs, discussing the role they played as chemical sinks during chemical and biologic evolution of the hydro- atmos- and litho- spheres.
Day 4 - Stop 2: Deep, deep underground.
This stop consisten of a visit to the Soudan Iron Mine State Park with a tour of underground workings. We highly suggest this tour because it is fun to plunge a half mile into the ground in a rattling metal cage. We learned that ore bodies were pipe like in form following a plane at a steep angle (some 70 degrees) into the ground. What caused the pipe-like form? Considering the degree of deformation and the style of deformation one possibility is large steeply plunging fold hinges where the BIF was thickened to minable amounts. This phenomena is known from elsewhere. Much of the surrounding rocks was metavolcanics.
Day 4 - Stop 3: Associates of BIF
Location: Stop 3 of the DNAG guidebook article 11 (Southwick, 1987).
A good lunch and collecting stop here. Large blocks of the BIF can be found, along with a variety of dacite porphyry with large plagioclase and quartz phenocrysts. Outcrops show mafic, dacitic and andesitic volcanics to be intercalated with the BIF. These gives credence to hydrothermal venting possibly playing some role in BIF development. Pyroclastics are common. Some in-place outcrops of the BIF show the folds to have steeply plunging axes.
The van is beginning to ride significantly lower due to rock loading.
Day 4- Stop 4: Hypabyssal metadacite.
Location: Just N on Rte. 77 from juncture with HW 1, and N of Stop 7 of the DNAG guidebook article 11 (Southwick, 1987). Large new road cuts.
We know from the map that this should be the meta-dacite unit here, and the mineralogy is consistent with that. However, the geometry of mafic xenoliths, and the presence of finger intrusions, suggest this is intrusive. Students can be led to the conclusion these are very shallow intrusive (hypabyssal) equivalents to the dacitic pyroclastic rocks seen earlier.
Some coarse and very green chlorite was found in some vein material.
Day 4 - Stop 5: Iron Man Chisolm Stop
Location: Large, several-story high, statue of iron miner on the N side of the road.
We collect here as an appropriate spot for Gabrielle to give a lecture on the history of BIF mining. We learn alot, including about the different iron ranges, types of ore, and the significance of the taconite process.
Day 4 - Stop 6: Basal Mesabi Fe-range.
Location: road cut formed by the E bound exit ramp from HW 169 for Iron World.
Here another sedimentary facies associated with the BIF can be seen. Arkosic sands and sideritic muds are intercalated in shallowly dipping, undeformed and relatively unmetamorphosed strata. We must be near the base of the sequence.
Questions that can be asked here are:
We return to camp in the sunshine.
5/14/99 Packing up camp in the sun, we head for Ely, and then the north shore of Lake Superior.
Day 5 - Stop 1: Where are the pillows?
Location: Some 9 miles W of Eli on 169; road cut on the N side of the road.
We were searching for an outcrop with abundant good pillows exposed on a glacially polished and striated pavement that we remember from the past. We suspect it was destroyed by the highway widening crew. Another case of blatant geologic vandalism!
Here are pillow basalts at good greenschist facies condition. They are best seen on the top of the outcrop. Selvages and interpillow material are apparent. Abundant sulfides are present. Are these sulfides related to smoker vents or secondary? The pillows are deformed, and a cleavage exists. At the E end of outcrop a granodiorite or hypabyssal dacite intrudes. See good xenoliths and a wedge-crack intrusion geometry exists. It doesn't seem that you've seen all the components of a good greenstone until you've seen the pillow basalts.
Day 5 - Stop 2: A rock of a different color.
Location: On 1 just S of Eli, pink outcrops on both sides of the road, with dirt road turnoff just to SE
We stopped here to see what the pink rock was. A good medium to coarse grained pink granite with the following:
A granite portion of the Arcehan terrane?
Day 5 - Stop 3: First introduction to the Duluth gabbro.
Location: On HW1, 11 miles S of Eli at the turnoff to Outward Bound (gravel road).
Here we found a leucogabbro displaying an ophitic texture of plagioclase in orthopyroxene. We are clearly in the Duluth gabbro, the major intrusive body associated with the Keweenawan rift. Some of the enclosing orthopyroxene 3-4 inches in width. Some clinopyroxene , biotite and opagues (oxides and sulfides) were observed. Locally the rocks contain a good flow foliation. As we drive along many more outcrops of this same coarse grey rock are evident, giving the students some idea of the size of this body!
Day 5 - Stop 4: A rhyolitic mess.
Location: Outcrop on S side of HW 1 just up from junction with 69.
The rock type is rhyolite with large K-spar and small quartz phenocrysts. The K-spar phenocrysts are highly zoned and often rounded. This suggests a complex history of growth, including a late stage of partial resorbtion. This can be explained by changes in P or T or refreshment of the magma chamber from which these crystal were later spewed. Dr. Bob sees flattened pumice and argues for welded tuff and eutaxitic structure. A very well developed layering and irregular folds remind me very much of a flow banded rhyolite. There is also a lot of vuggy mineralization with zeolite minerals. Some of the zeolite phases had become powdery to the touch reminding one of their hydrous character.
Questions that can be pondered here include:
Day 5 - Stop 5 - the red basalts of the Keweenawan
Location: Large road cuts on HW 69 just NE of the junction with HW 1. We concentrated on the lake side outcrop.
We have troubled upwards in the crust well into the volcanic fill of the Keweenawan rift. Here we see the base of a slightly tilted massive basalt flow with underlying interflow breccia (debris flow deposit), a thin mafic sill, and some interflow sediment. These are all truncated by a subvertical fault that is at a high angle to the road, with a massive basalt flow on the south side. In this vesicular flow large xenoliths of anorthosite are common, providing some important relative timing. In addition, it has some interesting implication for the depth of the Duluth gabbro intrusion, suggesting it may have been quite shallow. Such a large shallow intrusion could be expected to drive quite a vigorous hydrothermal cell and could contribute to the zeolitization of the rift sediments. Are there any places where zeolites are found in the gabbro??
This is a good zeolite collecting locality, and the vans ride even lower.
Just why are so many of the Keweenawan basalts dark red in color instead of the normal black?
We finish the day by setting up camp at Gooseberry falls just down the road, and setting people free to hike.
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