Download MMEW Field Trip-99 - University of Minnesota Duluth

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).