Lake Superior virtual field trip, part 2.

Department of Geography and Geology

University of Nebraska at Omaha

1999 Lake Superior Field Trip - Part 2

Harmon Maher & Bob Shuster

5/15/99 Sunny but windy day along N shore of Lake Superior

Day 6 - Stop 1: Shedding light on magmatic depths.

Location: Outcrops the Split Rock Lighthouse (one of the State Parks) stands on.

Outcrops here are of well layered and coarse grained mafic intrusive rocks.

Figure: Outcrop at base of lighthouse showing layered Duluth complex rocks.

The layers are leuco-gabbros and gabbros, with a preferred orientation of the plagioclase laths. The subvertical position of the layering is so steep it suggests subsequent tilting and deformation. A healed fracture set is also evident. Lee gives his talk on the Duluth complex and indicates this complex has close to a 100 Ma life span of multiple intrusions!

This is a good place to discuss possible mechanisms to explain the oscillatory crystallization history that likely took place. Too bad Werner didn't have these outcrops available to bolster his arguments.

Day 6 - Stop 2: The gabbro's little partner in the bimodal swing.

Location: This is the point just N along the shore from the one harboring the lighthouse. A gravel pull off road just down HW 61 from the entrance to the State Park is connected to a trail that takes one to the intervening bay. This is a diver's paradise since an old ore boat is sunk offshore. A trail then parallels the shore.

Right at the shore a medim grained, red granite can be seen intruding some basalts. Continuing up to the point more outcrops of the red granite occur. While volumetrically minor, this can be considered the plutonic equivalent to the rhyolites associated with the Keweenawan rift.

A question that can be addressed here is - what were the source rocks for the granite?

Day 6 - Stop 3: Green anorthosite clots

Location: Road cut outcrops along HW 61 just S of Silver Bay at the Northshore Mining facilities.

Large xenoliths (10s of square meters surface exposure of anorthosite intruded by gabbro. Some of the anorthosite has a distinctive light green color. Olivine is common in the gabbro, which is actually more of a troctolite. Coarse pockets of plagioclase-magnetite pegmatite exist in the troctolite. The source of the fluids may have been the anorthosite. There is some grain size reduction of the troctolite adjacent to the anorthosite, indicating a chilling effect.

Questions that might be addressed include:

what parts of the magma chamber and at what stages would you expect plagioclase to collect?
what is the 3-D structural position of this anorthosite within the intrusive complex?
why is the plagioclase distinctly green here?

Day 6 - Stop 4: Temperance River State Park

Location: Walk down to river's mouth with lake, from HW 61 bridge. Outcrops are gorgeous.

This locality is well known for the billion year old ropy texture preserved on the surfaces of thin flows. This texture and the thinness of the flows indicates a very low viscosity basaltic lava indeed.

Walking upstream along the trail reveals interesting geomorphology cut into these old flows. Active and inactive potholes abound, and knickpoints are controlled by the lava sequence. One section of the river follows a fault. Given that Wisconsin age glacial ice covered the area some 12,000 or so years ago, the gorge has been formed in relatively quick fashion.

Figure of relatively thin basalt flow layers inclined gently towards Lake Superior exposed in the gorge.

In the upper portion of river, where the falls end, zeolites are well exposed. These include white fibrous stilbite. Janet gives a lecture on these complex hydrous minerals, describing their conditions of formation and their various industrial uses. These are the precipitates from the volcanic pile stewing in its own juices.

Day 6 - Stop 5: Rift red beds and flows

Location: Large road cut on NW side of HW 61.

At the north end of this outcrop are a variety of redbeds. These are thinly intercalated shales, siltstones, and arkoses. Mudcracks are fairly well developed. These sediments give hints as to what the depositional evironment was like in the rift. Overlying theses sediments is a flow that is at least 15 meters thick.

Figure of basalt flow overlying redbeds. Blue arrow points to area of local load casting of the basalt into the underlying sediments.

There are some load cast type structures at the base, but otherwise the underlying sediments were remarkably little disturbed when the flow overran them.

Figure looking NW of fault.

Further S is a well developed fault with the flow in the footwall and agglomerate (scoria clasts) in the hanging wall. One possible interpretation is that this is the top of the thick flow brought down by a normal fault component. Zeolites abound and overprint and are caught up in the faulting, suggesting the two are coeval. As is typical, filled vesicles with zeolites, are more common at the base and top of a flow.There seems to be relatively little in the literature about the faulting associated with the rift, and there seems to be some room here for further research. The agglomerate grades down into a massive basalt flow, and may represent debris flow activity during late stages of an eruption.

Questions that can be considered here include:

What were the fault kinematics, and how were they related to rifting?
Given that basalt has a very low porosity how does enough water infilitrate it and deposit zeolites in the isolated vesicles?

Day 6 - Stop 6: Columnar joints?

Location: Trail upstream of HW 61 bridge.

More basalts. An interesting possibility is the existance of columnar jointing, which would be expected given modern basalt flows. In places an hexagonal pattern is evident, but not consistently. Also, these joints are not lined with zeolites, which suggests they may be much younger than the crystallization age of the flows.

5/16/99 Foggy and rainy day with a seagull wake-up call.

Day 7 - Stop 1: More magmatic layers

Location: Outcrop in the parking lot for the Thomson Hill Information Center off HW35

Here well layered Duluth gabbroic rocks had varying amounts of plagioclase, clinopyroxene, olivine and sphene. In addition one could see both concordant pegmatitic lenses and pipe shaped bodies, up to1 m across. Layers were subhorizontal, although perhaps gently curved.

As is often the case in relatively massive plutonic bodies well developed joints occur. A subhorizontal joint set could be associated with an unloading history.

Questions that can be addressed here include:

does the layering forming by settling, and/or do other mechanisms exist, such as growth on a crystal mat surface that localizes new crystal growth?
could some built in oscillatory mechanism exist where one phase grows until the necessary ions are depleted, at which time a new phase grows, until the surface grows through the depleted material and the initial phase resumes crystallization?
how can pockets of pegmatite seemingly enclosed in the magma be generated?

Day 7 - Stop 2: More layers and cystal-mush cross-beds

Location: This is DNAG Stop 1 of trip 13 (Green, 1987), just S of Barden peak. Road opens May 1st.

Extensive glacial exposures of well developed layering in the Duluth complex, complete with cross-beds and troughs evident on weathered surfaces. The display of 'sedimentary' structures in a plutonic body gives insight into the processes occurring in a magma chamber during a long history of crystallization. Many of the layers are quite delicate, with a thickness of only several centimeters. The overall mineralogy is similar to last stop. The guidebook indicates locally dunite layers occur in this area.

Day 7 - Stop 3: Flow banding and magmatic convection.

Location: DNAG Stop 3 of trip 13 (Green, 1987) on Skyline drive, just .3 km or so NE of Haines Road. Many exposures along ther road in this area.

Here are flow-banded gabbroic anorthosite with interstitial clinopyroxene and magnetite. In some places the flow banding is subvertical. Variation in grain size defines layering. A diabase dike some 10 m or greater wide in a subvertical position, is interpreted to be Keweenawan dike. If so then there must have been deeper, later magma sources that fed Keweenawan dikes and flows. A discussion ensued regarding use of Occum's razor and explanatory power to assign a likely age to dike.

Questions that can be explored here include:

what controlled the grain size of the plagioclase?
what controls the aspect ration of plagioclase laths?
if settling is a mechanism, is it steady state or episodic and if the later what is the cause?
why is the flow banding subvertical (multiple working hypotheses include vertical flow and later deformation)?

A warm lunch was had at the nearby Country Kitchen.

Day 7 - Stop 4: Deformed Early Proterozoic strata.

Location: Reservoir outlet of Thomson Lake in Jay Cook State Park area. This is stop 4 in DNAG Field Trip 14 (Jirsa and Morey, 1987).

In extensive outcrops along the river below the dam a variety of clastic sediments display well developed folds and bedding-cleavage relationships.

Figure: View of an upright antiform in the fog. The dam is just upstream from the bridge. Note the slightly thickened fold hinge. A steeply dipping cleavage also occurs, but is not as readily evident in this image.

These folds are approximately E-W trending and for the most part subhorizontal folds with an upright axial planar cleavage, and the structure influences the topographic pattern. Faults and quartz veins occur, and a good example occurs on the upstream west side of the bridge abuttment. A thick sandstone horizon is a ridge former. It appears to be separated across the river suggesting an oblique fault, explaining the straight path the small gorge takes here.This is an excellent locality to conduct a mapping exercise, although 2-3 hours is probably not enough time.

The following questions can be explored here:

What are the deformation mechanisms involved in the folding?
What is the age of deformation?
What was the depositional environment of these sediments?

We head back for camp.

5/17/98 - Packed up camp in the fog and on to Michigan we go.

Day 8 - Stop 1: Duluth Gabbro rock bottom

Location: Silver Cliff Tunnel on HW 61. Very large new road cut associated with NE tunnel entry. Rocks and features described are from N to S.

There is a lot to this outcrop, and we don't have time to take it all in. At the North end is a series of amygdaloidal basalt flows, some of which show good flattened and stretched vesicles. Folding of some of the flow units is evident, but it is unclear whether that is primary (flow over uneven topography or tectonic).

A distinctive unit consists of basalt clasts in a red aphanitic matrix with small feldspar phenocrysts. We interpret it as a rhyolite with basaltic xenoliths. However, some of the clasts haver diffuse boundaries, and there may be more to this unit. There appears to be a fair bit of hydrothermal alteration.

Figure looking North of road cut in the fog. G is gabbro, U is ultramafite, and r is extrusives (likely rhyolite).

Next, to the S and structurally higher, is fine grained to medium grained ultramafite. The southerly dipping contact appears to be intrusive. This very well could be an exposed floor to at least portion of the Duluth gabbro chamber. Clinopyroxene and olivine are two major phases.

Higher above that is a subcordant fault zone with a wedge shape, and adundant zeolite mineralization. We collected what appears to be prehnite.

Above the fault is more fairly mafic gabbroic rocks, with some coarse plagioclase-magnetite pegmatite. One could interpret the fault as a low-angle normal fault juxtaposing a structurally higher and more gabbroic portion of the complex against a basal more ultramafic portion. This extends to the top of the hill. Another fault, subvertical and oblique cuts further S and appears to intersect the tunnel.

Day 8 - Stop 2: Finding fault in Wisconsin

Location: Amnicon falls in Amnicon State Park. Stop 46 in Craddock & others (1977)

This stop is well described in the literature (e.g. Craddock, 1973). A sign indicates this is where the Douglas fault, with 9000 feet of throw emplaces middle Keweenawan phenocrystic basalts against arkosic lithic arenites and some conglomerates of the Upper Keweenawan sequence. The fault appears to dip 30 degrees N and is amazingly narrow with very little wall rock deformation given such a large throw. This is also more typical of a thrust fault.

Later compressional reactivation of rift structures is very common, and this is a good example. The natural question is as to the age of the thrusting. Subvertical Keweenawan strata nearby may be associated with folds associated with this shortening event.

Day 8 - Stop 3: Mellen Complex

Location: Road outcrop on Rte. 13, 1.2 miles after golf course, and just after sign for State Park.

Exposed here is a xenolithic breccia with a granite matrix and various gabbro and anorthositic gabbro clasts of various grain sizes. The variety of clasts suggests the mafic portion of the Mellen complex is differentiated similar to the Duluth Complex. This complex is assumed to be of the same age and provides some symmetry across the rift. Elsewhere the Mellen gabbro is quarried as facing stone as a 'black granite'. There is some coarse biotite in the granite, and good exfoliation jointing. Dr. Bob describes a K-Ar age of 1 Ba for the gabbro, and a Rb-Sr age of 970 Ma.

Lunch at the Main Street Cafe in Mellen - very good food and hospitality.

Day 8 - Stop 4: Beguiling pyroclasts.

Location: Outcrops along the east shore of Giles Lake. Stop # 16 in the 1981 Craddock & others field trip. Rocks are quite slippery in the rain.

This is a difficult outcrop for undergraduates to unravel. Here are andesitic-basaltic pyroclastic rocks with some dacite dikes in it. Dacite dikes are folded with axis parallel to a very strong steeply plunging elongation lineation. The aspect ratio of clasts exceeds 10 in many cases.There is no hint of a foliation in this rock- pure lineation - so on horizontal surface outcrop the rocks almost appears undeformed. This is interpreted to be part of an Archean greenstone belt, and pillows are also reported from this locality.

One question that can be explored here is as to the significance of the suvertical orientation of the elongation axis. What tectonic environment might produce such a pattern.

Across the lake one can see tailings from the Gogebic iron range, an equivalent to the Mesabi. So an unconformity is nearby and the geology here closely mirrors that on the other side of the lake.

We arrive at Porcupine State Park and set up camp in the rain. Forecast is for clearing during the night.

5-18-99 Still raining.

Day 9 - Stop 1: Sweet partings

Location: Shore crops at the Union Bay Campground of Porcupine Mountain State Park

A sedimentary sequence along the shore exhibits a suite of sedimentary structures of museum quality. The sequence is dominated by sandstones, with thin mud and conglomeratic horizons. Sedimentary structures include large trough cross-beds, well developed parting lineations, oscillation ripple marks, sheet laminations, mud cracks, and intraformational breccias. Paleocurrent directions are fairly consistent to the east.

Figures of oscillation ripple marks , a good parting lineation, and an intraformational breccia of mud 'flakes' in fine sandstones of the upper Keweenawan sequence.

The inferred depositional environment must include standing water with waves, swift currents, occasional muddy suspension deposits. A good possibility seems to be some shoreline setting. Mirroring the north shore, the beds dip shallowly towards the lake, forming a large gently syncline. It is likely this syncline, filled with softer sediments in the middle, permitted greater glacial excavation, and a combination of isostatic depression and scour produced the subsequent development of the modern Lake Superior.

Day 9 - Stop 2: Copper fever

Location: Clark Mine just 2 miles SE of the town of Copper Harbor (follow the road signs).

This is a tailings pile from an old mining operation and is a good collecting locality. Good native copper can be found in veins and small faults that cut through the gabbroic host rocks. Small nuggets of copper were also found. Gabbro specimens show a wide range in grain size suggesting it might be layered or the mine is near a chilled contact. Most veins show a 2 to 3 stage history of development with copper in the center. Azurite is a nicely visible clue to the likely presence of nearby copper. Again we are reminded of the juices circulating through this thick pile of volcanics, basal intrusives and overlying sediments. Clearly the mineralization was basically syn rift development.

Jeremy gives a talk on history of copper mining in the area. He also discusses the three or so different types of copper deposits in the area. He discusses the various mineral phases that occur with increasing O2 content of the waters (cuprite - azurite - malachite. The White Pine deposits in the Nonesuch shale seem to have ore values controlled by a syndepositional syncline, and may represent exhalative deposits.

Day 9 - Stop 3: Life on the shores Lake Keweenawan?

Location: Somewhere along the shore.

Here coarse conglomerates dip shallowly towards the lake. Clasts are well rounded and predominantly volcanic material. Thin discontinuous sands are intercalated. The depositional enviornment of such deposits can be difficult to unravel. However, close inspection of a thin white layer provides a clue. A horizon of stromatalites mantles a conglomeratic horizon. This suggests a shoreline of a standing body of water with a period of nondeposition existed for some time.

Figure: Image of stromatalites (blue arrows mantling clasts.

The red-beds attest to an oxygen rich atmosphere by this point in earth history, and the stromatalites remind us that life was present.

A question that can pondered here is as to the chemistry of the waters. Stromatalites are commonly found in a marine setting - is that the case here?

Day 9 - Stop 4: Raiding the spoils.

Location: Spoil piles several blocks N of in the town of Allouez.

Figure: Image of conglomerate with 'spotted' clasts that are rhyolite with feldspar and quartz phenocrysts, and red basalt clasts.

A last collecting frenzy took place here. A well cemented and coarse grained conglomerate has copper mineralization in the interstices of the clasts. Cuprite, azurite, and some native copper all occur here. Some of the clasts were of rhyolite with very large K-spar phenocrysts. Are the rhyolite clasts overrepresented here because of greater surface durability or was this conglomerate near to a rhyolitic source?

Day 9 - Stop 5: A final view.

Location: Lake of the Clouds scenic overlook.

Nestled between two ridges composed of basalt flows, the lake lies in a trough scooped in the softer intervening Keweenawan conglomerates. Again, the landscape is shaped by the long preceding geologic history. One could say this is the land of fire and ice, fire some billion years ago and ice some ten thousand years ago. Lindsey and Jessica give their lectures, encapsulating the history of the Keweenawan rift, covering many interesting background facts. Subsidence started before the first appearance of the volcanics, and after it was all over the fill was in excess of 12 km thickness. That is a bit less than one third the normal thickness of continental crust! This is substantially thicker than many other rifts! In the last decade geologists have begun to recognize the significance of Large Igneous Provinces (LIPs), and the Keweenawan can be recognized as one of these.

Many questions remain about the Keweenawan. Was this rift fed by some super plume, and if so was it stationary for the almost 100 Ma years of the rift development? What controlled the original direction of rifting? Could such effusive terrestrial volcanism have perturbed atmospheric development? How does the Keweenawan compare to the East African Rift, its closest modern counterpart?

Day 10 - Long drive back to Omaha, Nebraska!

Links:

Minnesota Geological Survey

Wisconsin Geological Survey

References:

Craddock, C., 1973, Structural evolution of the Keweenawan province: Geology, vol. 1, # 4, p. 190.

Craddock, C., Montgomery, W., & Mudrey, M. G. Jr., 1977, Lake Superior Field Trip; Third Symposium on Antarctic geology and Geophysics: Madison, WI, 69 p.

Craddock, C., Mudrey, M. G., Jr. & Ali, H. M., 1981, Lake Superior Field Trip; International Proterozoic Symposium, Department of Geology & Geophysics, University of Wisconsin - Madison, 99 p.

Dott, R. H., Jr., The Proterozoic red quartzite enigma in the north central United States: Resolved by plate collision?; GSA Memoir 160, p. 129-141.

Green, J. C., 1987, The Middle Proterozoic Duluth Complex at Duluth, Minnesota: in Biggs, D. L., GSA Centennial Field Guide - North Central Section Volume 3, p. 63-66

Jirsa, M. A. & Morey, G. B., 1987, Jay Cooke State Part and Grandview areas: Evidence for a major Early Proterozoic-Middle Proterozoic unconformity in Minnesota: in Biggs, D. L., GSA Centennial Field Guide - North Central Section Volume 3, p. 67-70

Southwick, D. L., 1987, Geologic higlights of an Archean greenstone belt, western Vermilion district, northeastern Minnesota; in Biggs, D. L., GSA Centennial Field Guide - North Central Section Volume 3, p. 53-58