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Tenth Annual Minnesota Minerals Education Workshop Duluth, Minnesota Geologic Field Excursion of the Duluth Area and Southern Part of the North Shore of Lake Superior Wednesday, August 2, 2006 Compiled by Jim Miller Minnesota Geological Survey University of Minnesota It can be argued that nowhere else in the state is geology such an integral part of the natural landscape as along the rocky North Shore of Lake Superior. The field trip stops associated with the 2006 Minnesota Minerals Education Workshop are intended to give an introduction to that geology and to the mineral resources that the rocks and surficial sediments provide. For a more complete accounting of the geology of the North Shore, see Prof. John Green's book Geology on Display: Geology and Scenery of Minnesota's North Shore State Parks (1996, MNDNR). Given here is a brief overview of the geologic story of the North Shore— an area truly born of fire and ice. Overview of North Shore Geology The rocks of the North Shore of Lake Superior tell the story of an aborted attempt by the North American continent (Laurentia) to break apart about 1,100 million years ago. Continental rifting is a common and recurring process in the history of the earth that leads to break-up of continents and the formation of ocean basins. A geologically recent example is the ongoing expansion of the Atlantic Ocean that began with the breakup of the supercontinent Pangea about 200 million years ago. The Midcontinent Rift began to form 1109 million years ago along a 1500-km-long arcuate break that extended in two arms from the Lake Superior region - one to the southwest to Kansas; the other to the southeast to Lower Michigan (Figure 1). As the crust broke and thinned, basaltic magma generated 50 to 100 kilometers deep in the earth rose to the surface. Most of this magma erupted layer upon layer of lava flow into an ever widening and deepening rift valley (Fig. 1A). We now see some of these lavas as the North Shore Volcanic Group. At times, the magma ponded and slowly solidfied in chambers within the lava pile to form coarse-grained gabbro intrusions, some of which we now see as the Duluth Complex and the Beaver Bay Complex. After almost complete separation of the originally 40-km-thick continental (granitic) crust and its replacement by oceanic (basaltic) crust, magmatic activity and rifting stopped about 1086 million years ago. The cooling of dense lava rocks caused continued downwarping of the rift valley and led to its infilling with sediments (Fig. 1B). We now see these sediments as the sandstones of east-central Minnesota and northwestern Wisconsin (Bayfield Peninsula). Sometime later, compression, probably generated by the plate collision creating the Grenville Mountains to the east, caused the central block of lavas to be thrust back up toward the surface (Fig. 1C). The Midcontinent Rift and its failed attempt at continental break-up was the last great tectonic event in this part of the North American continent as subsequent events proceeded to add new terranes to the continental margins. Although the rocks of the Midcontinent Rift would come to be weathered and locally buried under younger sediments, this tectonic scar across the North American continent would be revealed again by the recent erosive action of mammoth continental glaciers and come to play a controlling role in the sculpting of Lake Superior. Duluth Complex--- ?? Figure 1. Structure and generalized geology of the Midcontinent Rift. Geology shown south of dashed lines is buried beneath younger rocks. Formation of Lake Superior Part I – FIRE Figure 2. Simple idealized cross sections showing the main stages of the Midcontinent Rift. The creation of the Lake Superior basin and the ruggedness of its shoreline reflects the awesome erosive power of recent continental glaciers. Mile-thick sheets of ice advanced and retreated over Minnesota several times in the past two million years. However, the exact shape of Lake Superior can be directly correlated with the geology of the Midcontinent Rift and the relative erodability of the lavas, intrusions and sandstones contained within it. Enormous glaciers flowing over what would become the Lake Superior basin (Fig. 3), found it easy to erode loosely cemented sandstone that filled the axis of the rift valley, but encountered more resistance from the crystalline igneous rocks forming the flanks of the rift - now the margins of the lake basin. In the final retreat of the ice from the Lake Superior basin about 11,000 years ago, meltwaters filled the scoured-out sandy core of the rift. As the last glacier retreated west to east across the basin, meltwaters at times filled the lake to as high as 500 feet above the current level; at other times, water drained from the lake to levels about 250 feet below the present 600' elevation (Fig. 4). Nowhere else in the state did the bedrock geology exert such strong control on the glacial landscape. Formation of Lake Superior Part II - ICE Figure 3. Stages of deep weathering and glacial erosion that resulted in the scouring of the Lake Superior basin. Figure 4. Stages of Lake Superior following the last glacial retreat. Field Trip of the Duluth area and southern part of the North Shore Because of the large number of participants for this field trip, we will run it in two groups. One group will progress through the eight stops in the order described below and the other group will run through the stops in the opposite order. Figure 5. Generalized geology of the Duluth area showing locations of field trip stops 1-6. Stop 1 (8:30-9:25) - EARLY PROTEROZOIC THOMSON FORMATION/ MIDCONTINENT RIFT DIABASE DIKE. St. Louis River at Thomson Dam. 1 mile east of Carlton on MN Hwy 210. Fine-grained sedimentary rocks that display moderate degrees of metamorphism and deformation are exposed in the streambed of the St. Louis River downstream of the Thomson Dam. These graywackes and slates were deposited about 2 billion years ago along what was at the time the edge of the North American continent. Sedimentary structures such as ripple marks, graded bedding, and cross bedding are evident in several locations. Deformation and metamorphism resulting from the 1.85 billion-year-old Penokean Orogeny (mountain building event) has recrystallized the rocks (e.g., made slate from shale), tilted and folded the sedimentary bedding, and produced a steeply inclined deformational fabric (cleavage). The Penokean was a Rocky Mountain-type mountain belt formed across central Minnesota and northern Wisconsin. The metasedimentary rocks are cut by several northeast-trending diabase dikes related to the magmatism of the younger (1.1 Ga) Midcontinent Rift. These dikes were likely feeders to basaltic lava flow that have been eroded away in this area. Notice the columnar jointing developed at right angles to the margins of the dikes. Stop 2 - NORTH SHORE VOLCANIC GROUP - ELY’S PEAK BASALTS. Ulland Bros. Aggregate Quarry. Midway/Becks Rd. approximately 1.5 miles south of I-35 interchange. (CAUTION: DO NOT CLIMB ON THE BLASTED ROCK PILES AS THEY ARE VERY UNSTABLE). Exposed in this aggregate quarry are up to four basalt flows from the lower part of the North Shore Volcanic Group called the Ely’s Peak basalts. A variety of volcanic features can be observed in the many blasted blocks in the lower part of the working area including vesicular and amygdaloidal basalt, large cavities filled with secondary minerals (mostly epidote, chlorite, and feldspar), and ropy (pahoehoe) flow tops. Hints of copper mineralization are also evident in many blocks. In the upper ledge of the quarry, a prominent fault zone cuts the basalt and is marked by sheared and altered basalt and calcite mineralization. Toward the west end of the cliff face, a lava flow contact between amygdaloidal and massive basalt can be observed. The crushed rock derived from this quarry is used for a variety of purposes including asphalt filler, landscaping stone and road ballast. Stop 3 – LAYERED SERIES OF THE DULUTH COMPLEX/ST. LOUIS RIVER ESTUARY. Overlook on Skyline Parkway (gravel road) about 2 miles to the east of Midway/Beck’s Road. From Midway Road, Skyline Parkway climbs up the back side of Ely’s Peak and over the lava flows observed at the Ulland Bros. quarry. Past a subtle saddle, the parkway crosses the contact with the gabbroic rocks of the Duluth Complex and over Bardon Peak to a spectacular overlook of Duluth and the St Louis River Estuary (barring any fog!). From this vantage point, the entire breadth of the 5 km-thick intrusion of the Duluth Complex can be seen; the top of the east dipping intrusion being located at the radio tower hill above downtown Duluth. In the outcrops at the overlook, the gabbros of the Duluth Complex Elevation (feet above present day sea level) display a layering that is defined by changes in the proportions of dark and light minerals, (olivine and plagioclase, respectively). This layering is created by different rates of accumulation of minerals of different densities along the floor of the magma chamber. The settling of these crystals causes the magma to change composition as it crystallizes, causing it to become a more iron- and alkali-rich composition up section and causing the rock to change from troctolite (olivine + plagioclase) to gabbro (olivine + plagioclase + pyroxene + Fe-Ti oxide; see figure 5). This mineral layering and compositional layering gives rise to the name of this intrusion as “the Layered Series at Duluth”. Looking out over the St. Louis River estuary, we will discuss changes in the lake level over the past 12,000 years. As shown in the figure below, the lake was at times 500’ higher and 250’lower than the present lake level. These changes were caused by the damming of meltwaters from the retreating of the Superior Lobe glacier and varied rates of crustal rebound across the lake basin. 1200 Basin filled with ice GLACIAL LAKE DULUTH 1000 800 Lake level rising due to uplift of outlets Present Lake Level 600 GLACIAL LAKE NIPISSING 400 200 12 GLACIAL LAKE HOUGHTON Ice occupying northeast part of basin, melting back 11 10 9 8 7 6 5 Thousands of years ago 4 3 2 1 0 From Green (1978) After Farrand (1969) Figure 6. Variations in levels of Lake Superior during past 12,000 years (modified from Green, 1996) Stop 4 - ANORTHOSITIC SERIES OF THE DULUTH COMPLEX - 57th Ave. Quarry, West Duluth. This quarry was worked in the early part of the century for pier construction in Duluth Harbor. The rock is a variation of gabbro that contains more plagioclase than a normal gabbro (which should contain 65-70% plagioclase). A rock composed entirely of plagioclase is called an anorthosite. This rock contains approximately 80% plagioclase, so it is called gabbroic anorthosite. This rock tends to form in the roof zone of most Duluth Complex intrusions, composing what is called the anorthositic series. It was formed before the layered series as indicated by the fact that gabbroic anorthositic rocks occur as inclusions in layered series rock. Also, in the high wall of the quarry, you can make out a sheet-like body of dark rock. This is a fine-grained gabbro related to the layered series which is intruding into this large mass of anorthositic series rock. The gabbroic anorthosite commonly display a foliated texture, which is defined by the parallel alignment of plagioclase crystals. Dark minerals of pyroxene, olivine and iron oxide are entirely interstitial to the rectangular plagioclase crystals and commonly occur as clots up to 10 cm across. The plagioclase-rich nature of these rocks is thought to result from the physical enrichment of low density plagioclase crystals in otherwise normal basaltic (mafic) magmas, thus creating a crystal mush. These crystal mushes are thought to have been derived from deep crustal magma chambers where plagioclase would be very buoyant in their host magmas due to high pressures. Although plagioclase and mafic magma have similar densities in the upper crust, the high- pressure environment of the lower crust would increase the density of the magma to a greater degree than that of the solid plagioclase crystals. Stop 5 – CROSS-BEDDED INTERFLOW SANDSTONE AND BASALT. Shoreline adjacent to Leif Erickson Park off London Road, Duluth. Immediately down from the park amphitheatre, exposures of basalt similar to that observed at stop 2 are observed. Progressing to the northeast, one comes abruptly to outcrops of very obviously cross-bedded, purplish to green sandstone. This unit is approximately 100’ thick and indicates a prolonged hiatus in volcanic activity. The dark color of the sandstone indicates that the sand particles are dominantly fragments of basalt (like you would find on a black sand beach in Hawaii). This indicates that the sediment source for this sandstone was local - the volcanic rocks within the rift basin. The green comes from abundant epidote alteration of the sandstone (also found in the basalt). Epidote is a moderately high temperature (200-350ºC) hydrothermal mineral that indicates heating due to deep burial within the volcanic pile and probably thermal metamorphism by the nearby Duluth Complex. Stop 6 - RHYOLITE AND BASALT LAVA FLOWS. Shoreline NE of Brighton Beach on Scenic 61. Park in 3rd pull off northeast of the park entrance and walk back (SW) 100 yards to a trail through a roadside clearing that takes you down to the shore. Beginning at the top of a rhyolite flow, we will walk through a sequence of basalt flows. Features to take note of along this stretch of shore include: • Convoluted flow banding in the flow top of the rhyolite flow • Occurrence of a thin siltstone unit of variable thickness at the basalt-rhyolite contact • Transition of massive to amygdaloidal basalt within individual flows • Ropy (or pahoehoe) flow tops capping some basalt flows • Amygdule cylinders in the interiors of some basalt flows. • Polygonal columnar jointing in some basalt flows. • Smooth flow contacts Try to determine the number of basalt flows we cross from the top of rhyolite to the creek where we hike back to the parking area? ________ Stop 7 - SILVER CREEK DIABASE AND ANDESITE FLOWS. Pull-out on northeast side of tunnel. Walk SW up trail to old road bed. The Silver Creek diabase forms an irregular subhorizontal intrusion that is at least 200 feet thick. The diabase forms a prominent highland that projects inland several miles from Silver Cliff at Lake Superior. The Highway 61 tunnel has created excellent exposures of the contact between both the top and bottom of the diabase with adjacent volcanic rocks, and has exposed a north striking, 55-degree east dipping brittle fault that cuts the base of the diabase. The andesitic flows beneath the sill show a very irregular, rather chaotic flow contact, a rubbly flow top, vuggy quartz-lined stretched vesicles and amygdules of gray agates that have been recrystallized by contact metamorphism from the adjacent diabase. The margins of the diabase are marked by a mix of commingled fine-grained, strongly magnetic, dark gray diabasic rock and pink granite. The lower diabase contact strike is approximately N-S, 70 degrees west, and the upper contact (exposed at the south end of the tunnel) strikes N40E, 70 degrees northwest. Out on the old roadbed the diabase displays prominent columnar joints that plunge approximately 60-65 degrees east. From the north edge of the old roadbed, one can look back across the highway down the length of the brittle fault that cuts the base of the diabase. The fault is about 3 meters thick and filled with a mixture of pink zeolites and calcite-filling voids around altered diabase gouge. The old roadbed affords an excellent view of the Lake Superior coast. Stop 8 - NORTH SHORE VOLCANIC GROUP — BASALT. Gooseberry Falls State Park. Park in rest area, proceed past interpretive center to Middle Falls. At the Middle Falls of the Gooseberry River, one can view the basic features of basaltic lava flows, the dominant rock type along the North Shore. A lava flow contact is exposed in the cliff face of the Main Falls and is marked by the break between a vesicular (gas bubble-rich) lava flow top and a massive, vesicle-poor base and interior of an overlying flow. Note that most of the vesicles are filled with whitish and greenish minerals, which formed from hot fluids passing some time after the lava solidified. Filled vesicles are called amygdules and the rock is called an amygdaloidal basalt. Flow contact surfaces are smooth and billowing, whereas in other types of flows they can be brecciated (broken blocks). The polygonal fracture pattern of columnar joints, which commonly form during cooling of the massive flow interiors, can be recognized in the pavements above and below the Middle Falls. The falls at Gooseberry reflect the differential erosion of the flow tops and interiors (Fig. 7). Figure 7. Schematic profile of the relationship of the falls of the Gooseberry River and the gently-dipping sequence of basaltic lava flow in Gooseberry Falls State Park (after Green, 1996).