COUNTY ATLAS SERIES
ATLAS C-22, PART A
Plate 2—Bedrock Geology
Prepared and Published with the Support of
THE U.S. GEOLOGICAL SURVEY AS PART OF THE 2008 STATE GEOLOGIC MAPPING PROGRAM ELEMENT (STATEMAP)
OF THE NATIONAL GEOLOGIC MAPPING PROGRAM, THE CHISAGO COUNTY BOARD OF COMMISSIONERS, AND
the Minnesota Environment and Natural Resources Trust Fund
as recommended by the Legislative-Citizen Commission on Minnesota Resources
MINNESOTA GEOLOGICAL SURVEY
Harvey Thorleifson, Director
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COUNTY LOCATION
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R. 19 W.
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rise
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320
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Vibo
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Spider
28 Lake
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280
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35
School
Lake
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R. 21 W.
93°00'
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Birch
Lake 31
WASHINGTON COUNTY
SCALE 1:100 000
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2
3
2
0sl
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220
St
R. 19 W.
92°45'
Era
8 KILOMETERS
contour interval 20 METERS
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Vertical exaggeration ~x16
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1 mile
Ocean
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750
850
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Minnesota
700
0
800
Basalt (cf)
0
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80
100 feet
0
80
850
70
m
DOUGL
80
0
Today's Mill Street conglomerate outcrop
Figure 2. Elevation of the stratigraphic tops of the Wonewoc
and Mt. Simon Sandstones in Chisago County, showing geologic
structure of Paleozoic bedrock; scale 1:300,000. Contour
interval is 50 feet (15 meters). In areas where some or all of
the Wonewoc and Mt. Simon Sandstones are missing because
of erosion, the contours are inferred from projection vertically
from other formation contacts, using the estimated full thickness
of the eroded formations. The inferred traces of faults and
folds are shown. Faults in Mesoproterozoic bedrock that do
not displace Paleozoic strata are not shown.
0
The University of Minnesota is an equal opportunity educator and employer
0
75
750
Top of the Mt. Simon Sandstone
(hachures point toward lower
elevation)
70
0
0
70
700
750
©2010 by the Regents of the University of Minnesota
Figure 4. Map of second vertical derivative
magnetic anomaly data in the northern part
of Chisago County showing "stream-like"
anomalies, traced with thin, magenta lines,
that represent buried bedrock valleys. The
buried valleys contain a relatively great
thickness of moderately magnetic glacial
deposits, which is visible as a magnetic
anomaly in areas where the underlying
regional bedrock is comparatively nonmagnetic. Together with drillhole data,
these anomalies were used to produce the
bedrock topography contours shown on
the geologic map in the northwestern part
of the county. See Chandler and others
(2002) for additional information on use
of magnetic anomalies for buried valley
delineation.
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650
700
650
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0
Axial trace, anticline
50
0
The views and conclusions contained in this document are those of the authors and should not be
interpreted as necessarily representing the official policies, either expressed or implied, of the U.S.
Government. This map is submitted for publication with the understanding that the U.S. Government
is authorized to reproduce and distribute reprints for governmental use. Supported by the U.S.
Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No.
08HQAG0095.
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LITHOLOGY KEY
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Sandy
St. Lawrence sl
Formation
Sandstone
Very fine- to fine-grained
G
Fine- to medium-grained
Medium- to coarse-grained
Mazomanie
Formation
Shaly
Lone Rock
Formation
G
G
G
Mazomanie tc
Formation
G
G
Lone Rock
Formation
G
G
Siltstone
Shale
Volcanics
G
G
G
GG
Chert
G
Oolites
G
65
750
Top of the Wonewoc Sandstone
(hachures point toward lower
elevation)
Axial trace, syncline
Oneota
Dolomite
Wonewoc
Sandstone
700
Fault
Every reasonable effort has been made to ensure the accuracy of the factual data on which this map
interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that
there are no errors. Users may wish to verify critical information; sources include both the references
listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In
addition, effort has been made to ensure that the interpretation conforms to sound geologic and
cartographic principles. No claim is made that the interpretation shown is rigorously correct, however,
and it should not be used to guide engineering-scale decisions without site-specific verification.
Shakopee
Formation
w
0
5 miles
Eau Claire
Formation
G
G
G
Fossiliferous; fossils (symbols not used in
limestone or dolostone units)
Burrows
G
G
e
G
Pebbles
Flat-pebble conglomerate
G
1
Mt. Simon
Sandstone
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Undifferentiated su
sedimentary
Chengwatana
cv
Group
Clam Falls
Volcanics
cf
Cross-bedded (festoon)
Cross-bedded (planar to tangential)
Hummocky/swaley cross-bedded
Ripple cross-laminations
Dolomitic
Contact marks a major erosional surface
*Log is a compilation of four borehole geophysical
logs collected from water wells in Minnesota;
unique numbers 612116, 475354, 706835, 737047
Numerical ages from Palmer (1999).
Maximum eroded thicknesses: units are eroded everywhere in Chisago County.
2
GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA
Glauconite
Microbial domes; stromatolites/thrombolites
800
1 mile
Approximate scale
EXPLANATION
200
St Peter
Sandstone
Jordan
Sandstone
5 miles
Wisconsin
Figure 1. Schematic illustration of the depositional conditions about 490
million years ago in the area known today as Taylors Falls, Minnesota, and
St. Croix Falls, Wisconsin. The Cambrian sedimentary bedrock present in the
area today was deposited in a shallow sea, with resistant knobs of Proterozoic
basalt (<cf) emergent as islands. Very close to these islands, basalt boulders
and cobbles were incorporated into the Cambrian deposits ( _m, _e, _w), locally
forming conglomerate, such as the Mill Street conglomerate (Berkey, 1897)
in Taylors Falls.
100
API-G units
Dolostone
Middle Cambrian (504-501 m.y.)
Pmsu
500
MESOPROTEROZOIC
(1200-900 M.Y)
300
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600
FAUL
T
400
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700
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PINE
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800
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AS FAU
LT
500
Unconsolidated Quaternary sediment
PINE
FAULT
600
DOUGLAS FAULT
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700
900
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Natural gamma log*
Increasing count
0
Osp
Lower Ordovician (488-478 m.y.)
Sunrise River
1,000
800
Taylors Falls
tc (Mill Street
conglomerate)
1,000
DOUGL
North Branch
Figure 3. Faults mapped in Chisago
County superimposed on first vertical
derivative aeromagnetic data (Chandler and
others, 2004). All faults except those in
extreme southeastern Chisago County are
part of major fault zones along the western
side of the Midcontinent Rift System,
recognizable by strong, abrupt, linearly
extensive contrasts in magnetic intensity.
The faults in the southeastern part of the
county do not appear to correspond to
distinct, linear anomalies. Faults that
displace Paleozoic bedrock, shown as solid
pink lines, were reactivated during and after
Early Paleozoic time. Their position at
the bedrock surface does not everywhere
correspond to the magnetic anomaly that
marks the position of the major faults in
underlying Mesoproterozoic bedrock, but
their similar trends and close proximity
indicate they are related features. Dashed
pink line shows another major Midcontinent
Rift System fault that does not appear to
displace Paleozoic bedrock.
AS FAU
Chisago County Wisconsin
ISANTI CHISAGO
COUNTY COUNTY
900
A'
PALEOZOIC
Area in Figure 1
LT
A
_w
_e
_m
Wonewoc Sandstone (Late Cambrian)—This sandstone unit, formerly referred to as
the Ironton-Galesville Sandstone, is composed mostly of fine- to coarse-grained,
moderately to well sorted, light gray, cross-stratified, quartz sandstone (Mossler,
2008). White, brown, and black linguliform brachiopod shells are locally abundant.
The upper part is the coarsest-grained; the lower part is finer-grained, better sorted,
and progressively finer-grained toward its base. The very fine-grained sandstone in
the lower part is feldspathic. The thickness of the formation is 75 to 110 feet (23 to
34 meters), except where it pinches out in unconformable contact against knobs of
Keweenawan basalt near Taylors Falls. The Wonewoc Sandstone is conformable with
overlying and underlying formations; however, there is a subtle unconformity marked
by a pebbly sandstone layer within the formation (Runkel and others, 1998).
Eau Claire Formation (Middle to Late Cambrian)—The formation is composed of
yellowish-gray to pale olive-gray, fine- to very fine-grained, feldspathic sandstone,
siltstone, and shale. White and brown linguliform brachiopod shells are common.
The Eau Claire Formation gradually coarsens to the north across Chisago County,
and as a result is composed dominantly of very fine- to fine-grained sandstone in
the northern one-half of the county. The formation ranges from 60 to 90 feet (18
to 27 meters) in thickness, except where it pinches out in unconformable contact
against knobs of Keweenawan basalt near Taylors Falls. The contact with the Mt.
Simon Sandstone is conformable.
Mt. Simon Sandstone (Middle Cambrian)—The Mt. Simon Sandstone is pale yellowishbrown to grayish-orange-pink to light gray, medium- to coarse-grained, quartz
sandstone. Interbeds of shale, siltstone, and very fine-grained, feldspathic sandstone
are common, particularly in its upper half (Mossler, 1992), and in the lower several
tens of feet of the Mt. Simon Sandstone in the North Branch area. Inarticulate
brachiopod shells are locally common in the upper one-third of the formation. Thin
beds of quartz-pebble conglomerate occur at several stratigraphic positions, and
are especially abundant near the base of the formation. The Mt. Simon Sandstone
unconformably overlies Mesoproterozoic rocks, burying a surface of erosion that had
hundreds of feet of relief within the county. As a result, the thickness of the Mt. Simon
Sandstone is markedly variable even within small areas such as near Taylors Falls.
Based on a limited number of full penetrations of the formation, it appears to have
a maximum thickness of about 250 feet (76 meters) across most of the county.
MESOPROTEROZOIC ROCKS
KEWEENAWAN SUPERGROUP
<su
<cv
<cf
Sandstone, siltstone, and minor shale—Sedimentary rocks consisting largely of
reddish-brown mudstone and siltstone, and lithic and feldspathic sandstone (Morey,
1977). They are poorly known in Chisago County, represented only by a handful
of borehole cuttings samples, gamma logs, and two video logs. Therefore they
cannot be confidently assigned to individual formations, but likely correlate to parts
of the Solar Church and/or Fond du Lac Formations. Mooney and others (1970)
suggested that some parts of the sedimentary package may be interbedded with mafic
volcanic rocks. Geophysical modeling implies that this unit is more than 0.6 mile
(1 kilometer) thick in the area between the Pine and Douglas faults, and more than
2 miles (3 kilometers) thick to the west of the Douglas fault, beneath the Paleozoic
rocks (Allen and others, 1997).
Chengwatana Volcanic Group—A thin panel of dominantly basaltic volcanic rocks
located between the Pine and Douglas faults (Cannon and others, 2001). Rocks of
the Chengwatana Group are not exposed in Chisago County, but likely are similar
to exposures in nearby Pine County, which are composed of steeply east-dipping
flows of basalt, porphyritic basalt, and interlayered conglomeratic sedimentary rocks
(Boerboom, 2001).
Clam Falls Volcanics—A thick succession of largely mafic volcanic rocks between
the Pine fault on the west and the Cottage Grove–Lake Owens fault on the east in
Wisconsin (Cannon and others, 2001). Outcrops near Taylors Falls consist of thick,
coarsely-grained, ophitic basalt flows with thick, fragmental flow tops, and thinner
flows of fine-grained, intergranular basalt and porphyritic basalt. All the exposed
flows contain abundant epidote and actinolite, which indicates that the flows were
deeply buried (approximately 4.7 miles [7.5 kilometers]; Wirth and others, 1998)
prior to uplift of the St. Croix Horst (see Plate 2, Fig. 2). Based on deep seismic
profiles the thickness of the remaining volcanic rocks in the St. Croix Horst beneath
the Clam Falls Volcanics is estimated to be approximately 5 miles (8 kilometers).
The Clam Falls volcanics underlie Paleozoic bedrock across a large expanse of
southeastern Chisago County. They subcrop beneath unconsolidated Quaternary
material in deep bedrock valleys, and are exposed as the uppermost bedrock in the
Taylors Falls area.
ACKNOWLEDGMENTS
Funding was provided by the Minnesota Environment and Natural Resources Trust Fund as
recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR), and the
U.S. Geological Survey as part of the 2008 State Geologic Survey Mapping Component (STATEMAP)
of the National Geologic Mapping Program. Ted Starns assisted with field mapping. V.W. Chandler
(Minnesota Geological Survey) compiled and in part revised seismic and airborne geophysical data.
B.J. Bonin, WSB and Associates, provided data and insight into the interpretation of the geologic
setting of the North Branch area.
REFERENCES
STRATIGRAPHIC COLUMN
220
GIS compilation by R.S. Lively
Edited by Lori Robinson
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APPROXIMATE MEAN
DECLINATION, 2009
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Duck
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CHISAGO LAKE SOUTH
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WYOMING
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TRUE NORTH
MAGNETIC NORTH
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IN
45°22'30''
TA
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Franconia
21
280
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280
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Blooms
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280
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Chisago
Kroon
Lake
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26
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Little
Green
Lake
South
Center
Lake
280
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Chisago
City
31
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ANOKA COUNTY
28
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Lindstrom
Lake
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79
6
0
28
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Middle Ordovician
(460 m.y.)
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No. 1
Center
City
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Pool
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36
School
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320
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280
80
280
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Center
28
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0
28
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28
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North
North
Lindstrom
Lake
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Little
Lake
T. 34 N.
28
Colby
Lake
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80
2
0
SHAFER
9
20
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45°22'30''
Lake
16
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20
Sunrise
280
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CHISAGO LAKE NORTH
No. 3
35
12
300
61
0
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280
280
LENT
Sunrise
Pool
18
24
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ISANTI COUNTY
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60-90
45°30'
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Thickness (feet)
95
Sun
greatly variable; maximum of about 250
A
T. 35 N.
1000s
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AMADOR
702
Bra
SUNRISE
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302
NORTH BRANCH
300
N.
12
River
T. 35 N.
cf
St. Peter Sandstone and Prairie du Chien Group (Early to Middle Ordovician)—Limited
information (one set of borehole cuttings and one natural gamma log) indicates that
the St. Peter Sandstone and Prairie du Chien group locally unconformably overlie
the Eau Claire Formation and Wonewoc Sandstone west of the Douglas fault at Rush
City. The St. Peter Sandstone consists largely of fine- to medium-grained, quartzose
sandstone, with interbeds of very fine-grained, feldspathic sandstone that may be
the Pigs Eye Member (Mossler, 2008). Strata beneath the St. Peter Sandstone in
this area are largely fine- to coarse-grained quartzose sandstone, with interbeds of
sandy, oolitic dolostone. They most likely correlate to the Shakopee Formation, but
could instead be equivalent to the lowest part of the Oneota Dolomite (Coon Valley
Member) of the Prairie du Chien Group. The maximum thickness of this map unit
is about 70 feet (21 meters).
Prairie du Chien Group–Oneota Dolomite (Early Ordovician)—Dominantly tan to gray,
medium to thickly bedded, finely crystalline dolostone, present only in the extreme
southeast corner of the county. Microbial build-ups (thrombolites) are common in
some beds. The lowermost few feet include interbeds of fine- to coarse-grained,
intraclastic, quartzose sandstone, very fine-grained, feldspathic sandstone and
siltstone, shale, and sandy dolostone of the Coon Valley Member. The maximum
preserved thickness of the Oneota Dolomite is about 30 feet (9 meters).
Jordan Sandstone (Late Cambrian)—Dominantly tan to light gray sandstone characterized
by coarsening-upward sequences consisting of two interlayered facies (Runkel,
1994), which are not separated on the map. They are medium- to coarse-grained,
cross-stratified, generally friable, quartz sandstone; and very fine-grained, commonly
bioturbated, feldspathic sandstone and lenses of siltstone and shale. The major
part of the very fine-grained facies forms a regionally continuous interval that
gradationally overlies the St. Lawrence Formation (unit _sl ), although there are
lithologically similar intervals intercalated with the medium- to coarse-grained
facies at higher stratigraphic intervals. The Jordan Sandstone is generally 60 to
80 feet (18 to 24 meters) thick where uneroded. An unconformity, locally marked
by thin beds of quartz pebble conglomerate and silcrete-cemented sandstone clasts
(Runkel and others, 2007), separates the Jordan Sandstone from the overlying Oneota
Dolomite.
St. Lawrence Formation (Late Cambrian)—The St. Lawrence Formation is principally
light gray to yellowish-gray and pale yellowish-green, dolomitic, feldspathic siltstone
with interbedded, very fine-grained sandstone and shale. Lenses and layers of light
gray, finely crystalline, sandy dolostone occur locally, especially in the lowermost
few feet of the formation (Runkel and others, 2006). The formation is 25 to 40 feet
(8 to 12 meters) thick. The upper contact with the Jordan Sandstone is conformable
and gradational. The gradational nature of the contact in well-cuttings and on
natural gamma logs can make selecting a precise contact between these formations
problematic.
Tunnel City Group (Late Cambrian)—The Tunnel City Group, formerly named the
Franconia Formation (Berg, 1954), varies from about 150 to 180 feet (46 to 55
meters) in thickness in the map area, except where it pinches out in unconformable
contact against knobs of Keweenawan basalt near Taylors Falls. It is divided into
two formations: the Mazomanie Formation and the Lone Rock Formation (Mossler,
2008). The Mazomanie Formation is dominantly white to yellowish-gray, fine- to
medium-grained, cross-stratified, generally friable, quartz sandstone. Glauconitic
grains typically are absent and never exceed 5 percent (Berg, 1954). Some beds
contain brown, intergranular dolomite as cement. Skolithos burrows and sandstone
intraclasts are common along discrete horizons. Individual tongues of Mazomanie
Formation are up to 50 feet (15 meters) thick. The Mazomanie Formation and the
Jordan Sandstone are sometimes mistaken for one another in the southern Chisago
County area because of their lithic similarities and similar thickness. The Lone
Rock Formation underlies the Mazomanie Formation and intertongues with it. It
consists of pale yellowish-green, very fine- to fine-grained glauconitic, feldspathic
sandstone and siltstone, with thin, greenish-gray shale partings. Thin beds with
dolomitic intraclasts are common. In the Taylors Falls area, the lower part of the
Lone Rock Formation locally includes a basalt cobble to boulder conglomerate,
informally named the "Mill Street conglomerate" (Berkey, 1897; Yochelson and
Webers, 2006). Intervals of Lone Rock Formation are as much as 60 feet (18 meters)
thick in Chisago County.
The upper contact of the Tunnel City Group with the St. Lawrence Formation
is conformable. The contact is fairly sharp and the contrast between the siltstone
and shale of the St. Lawrence Formation and underlying fine- to medium-grained,
quartzose sandstone in the Mazomanie Formation of the Tunnel City Group is
distinct.
60-80
0
28
U D
North
Branch
Mud
Lake
Osp
25-40
6
150-180
280
m
e
Universal Transverse Mercator Projection, grid zone 15
1983 North American Datum
36'
1
m
Sunrise
e
92°45'
Nomenclature has been revised for some Paleozoic formations in Minnesota, and some formation
names formerly in use at the Minnesota Geological Survey have been replaced by names widely
accepted elsewhere in the region (Mossler, 2008). Rocks formerly included in the Franconia
Formation are now assigned to the Tunnel City Group, which is subdivided into the Mazomanie
Formation and the Lone Rock Formation. The interval formerly referred to as the Ironton-Galesville
Sandstone is now named the Wonewoc Sandstone. More detailed discussion of these revisions is
given in Mossler (2008).
75-110
260
6
31
0
1
36
240
0
Digital base modified from 1990 Census TIGER/Line Files
of U.S. Bureau of the Census (source scale 1:100,000);
county border files modified from Minnesota Department of
Transportation files; digital base annotation by Minnesota
Geological Survey.
m
30
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28
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6
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260
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Chain
Lake
R. 22 W.
31
280
28
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36
30
31
8
D U
R. 20 W.
DU
cv
PINE
FAULT
36
260
280
tc
92°52'30''
260
26
2
9
PALEOZOIC ROCKS
Lithology
Harris
e
80
0
30
0
280
Fish
Lake
This geologic map, cross section, and stratigraphic column depict the bedrock formations exposed
at the land surface or lying directly beneath unconsolidated Quaternary deposits of variable thickness
across Chisago County. The map shows how the bedrock would appear if it were viewed from an
aerial perspective, as if all the overlying unconsolidated material was stripped away. Quaternary
glacial till, lake sediments, outwash, and river alluvium cover the bedrock across most areas (Meyer
and Lusardi, 2001). The unconsolidated deposits reach a maximum thickness of about 500 feet
(152 meters). Bedrock exposures are limited to the eastern part of the county, mostly along the St.
Croix River and its tributaries near Taylors Falls.
There are two fundamental kinds of bedrock mapped across Chisago County. Mesoproterozoic
bedrock of the Keweenawan Supergroup is about 1.1 billion years old, and consists of reddish-brown
mudstone and siltstone, lithic and feldspathic sandstone, and volcanic rocks such as basalt. The
Keweenawan Supergroup accumulated as part of the development of the Midcontinent Rift System
(for example Cannon and others, 2001), a large tear in the crust of North America. A second type
of bedrock consists of relatively thin layers of sandstone, siltstone, dolostone, and shale deposited
in shallow seas during the Paleozoic Era, about 500 million years ago (for example Mossler,
2008). These bedrock layers eventually buried the Keweenawan Supergroup across nearly all the
county. An exception was at Taylors Falls, where resistant knobs of Keweenawan basalt stood as
topographic highs, emergent as islands in the Paleozoic seas (Fig. 1). This setting also led to local
deposition of unusual Paleozoic bedrock that includes boulders and cobbles of basalt that were
shed from these islands into the shallow sea. These deposits have been informally called the "Mill
Street conglomerate" (Berkey, 1897).
The most prominent feature of the bedrock topography (Plate 6) is an anomalously deep and
linearly extensive buried valley in the southern half of the county that approximates the position
of the modern Sunrise River system and chain of lakes around the cities of Lindstrom and Chisago
(see cross section). The buried valley deepens to the north, and crosses the St. Croix River into
Wisconsin near the town of Sunrise, where the base of the valley reaches an elevation of less than
400 feet (122 meters). Other highlights of the bedrock topography are the knobs of relatively hard,
resistant Mesoproterozoic basalt (Clam Falls Volcanics, unit <cf) that stand at high elevation near
Taylors Falls. The tops of some of the highest knobs exceed 1,050 feet (320 meters), about 300
feet (91 meters) above the bedrock surface across most of the rest of the county.
Buried stream channels incised into bedrock, in combination with faults with up to about 200
feet (61 meters) of displacement, and a gentle, overall southward dip of Paleozoic bedrock (Fig.
2), are a primary influence on the map distribution of bedrock geologic units. Across much of the
county the uppermost bedrock consists principally of moderately consolidated layers of sandstone,
siltstone, and shale of Cambrian age. The southward dip of the Paleozoic strata is reflected on the
map by a pattern of progressively older formations northward across the county. Mesoproterozoic
rocks of the Midcontinent Rift System are the uppermost bedrock largely where deep buried
valleys were incised entirely through Cambrian strata, and where resistant knobs of basalt stood
as topographic highs when Paleozoic strata were deposited. This general distribution of bedrock
units is interrupted by faults such as those along the Douglas fault zone on the western boundary
of the Mesoproterozoic Midcontinent Rift (Sims and Zeitz, 1967), which were reactivated during
and after early Paleozoic time (Figs. 2, 3; cross section). As a result, Mesoproterozoic sedimentary
and volcanic rocks are the uppermost bedrock in some areas where they have been displaced
upward between faults. Conversely, down-dropped blocks of bedrock have preserved relatively
young formations such as the Early Ordovician Oneota Dolomite in a small area in the extreme
southeastern part of the County.
The production of this map and cross section relied on a number of sources of information,
including water-well and scientific drilling records (including holes drilled for this project), outcrop
mapping, geophysical surveys, and published geologic maps of parts of Chisago and adjacent
counties (Berkey, 1897; Cordua, 1989; Mossler and Bloomgren, 1990; Johnson, 2000; Boerboom,
2001). The location of faults is inferred from borehole data, with additional constraints provided by
patterns in aeromagnetic anomalies (Fig. 3). Seismic-refraction techniques were used to determine
the elevation of the bedrock surface in some areas, and linear, sinuous, aeromagnetic anomalies
were used to constrain the position of channels on the bedrock surface in the northwestern part
of the county (Fig. 4). The geophysical data were collected by the Minnesota Geological Survey
(Chandler and others, 2004), the Minnesota Department of Natural Resources, and the Geology
and Geophysics Department of the University of Minnesota. The distribution of all these data is
shown on the map. Because the bedrock is concealed by Quaternary sediments across most of the
county, the distribution of bedrock units and the pattern of faults are strongly dependent on borehole
records. The depiction of uppermost bedrock units in the northern approximately one-half of the
county is more tenuous than in the southern half. Borehole data are relatively sparse in the north,
and the lithic differences between the Cambrian formations are so subtle that they are unresolvable
on many drilling records. For example, parts of the Eau Claire Formation and Tunnel City Group
are dominated by beds of tan, white, or brown sandstone that on the basis of drilling records alone
commonly cannot be distinguished from the Mt. Simon and Wonewoc Sandstones. Many driller's
logs of bedrock wells describe only tan or white sandstone, which is a substantial component of
all the Cambrian formations below the St. Lawrence Formation.
Map symbol
k
Cree
su
DESCRIPTION OF MAP UNITS
Geologic contact—Approximately located.
Fault—Faults in Paleozoic rocks are interpreted to be dip-slip. Letters indicate what is inferred
to be the most recent relative vertical displacement: U, up; D, down. Faults are concealed
by Quaternary sediments and recent alluvium and are inferred from subsurface geologic
data supplemented by aeromagnetic and gravity data. Contrasts in magnetic intensity on
the aeromagnetic map are inferred to be largely the expression of faults in the underlying
Mesoproterozoic volcanic and sedimentary rocks. Offsets of Paleozoic strata indicate
that some of these faults were rejuvenated during and after Early Paleozoic time.
Bedrock outcrop
Not present?
e
w
31
MINNE
Goos
300
9
T. 36 N.
Introduction
Group,
Formation
300
Robour
Horseshoe
Lake
Lake
SUNRISE
e
SOTA
WISCO
NSIN
Mud
Lake
10
MESOPROTEROZOIC
45°37'30''
e
cv
93°7'30''
Lithostratigraphic
units
tc
FISH LAKE
4
Middle Cambrian
cf
w
T. 36 N.
MAP SYMBOLS
e
31
1
35
Neander
Lake
280
45°37'30''
1
280
Lake
m
Late Cambrian
2010
PALEOZOIC
su
280
6
Terrence J. Boerboom
w
Goose
7
Anthony C. Runkel
unconformity
ix
30
0
280
30
Osp
280
31
8
m
St. C
ro
280
e
tc
Creek
280
ISANTI COUNTY
D U
D
260
5
7
Early Ordovician
sl
River
U
e
0
Opo
By
j
T. 37 N.
cv
e
Early to Late
Ordovician
unconformity
RUSHSEBA
28
30
Osp
unconformity
Rush
City
1
CORRELATION OF MAP UNITS
3
Prairie du Chien Group
NESSEL
T. 37 N.
e
m
e
361
BEDROCK Geology
6
1
m
DOUGLA
280
4
Rush
0
28
6
1
e
2
92°52'30''
R. 21 W.
Tunnel City Group
6
93°00'
PINE COUNTY
System-Series1
300
R. 22 W.
Upper Cambrian (501-488 m.y.)
93°7'30''
S FAULT
KANABEC
COUNTY
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of the North American Midcontinent Rift System: New interpretations for western Lake Superior,
northwestern Wisconsin, and eastern Minnesota: Geological Society of America Special Paper
312, p. 47-72.
Berg, R.R., 1954, Franconia Formation of Minnesota and Wisconsin: Geological Society of America
Bulletin, v. 66, p. 857-882.
Berkey, C.P., 1897, Geology of the St. Croix Dalles: American Geologist, v. 20, p. 345-383.
Boerboom, T.J., 2001, Bedrock geologic map and sections, pl. 2 of Boerboom, T.J., project manager,
Geologic atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13,
pt. A, scale 1:100,000, 7 pls..
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geology of North American Mid-continent Rift: Eos, v. 82, p. 97-101.
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anomalies in Pine County, Minnesota, in Boerboom, T.J., project manager, Contributions to the
geology of Pine County, Minnesota: Minnesota Geological Survey Report of Investigations
60, p. 43-53.
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On file at the Minnesota Geological Survey.
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1 pl.
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1:100,000.
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75, no. 26, p. 5056-5086.
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Geological Survey Report of Investigations 16, 67 p.
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southeastern Minnesota: Minnesota Geological Survey Report of Investigations 40, 71 p.
———2008, Paleozoic stratigraphic nomenclature for Minnesota: Minnesota Geological Survey
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eds., Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County
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scale 1:250,000.
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1 p.
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Sandstone, southeastern Minnesota, in Southwick, D.L., ed., Short contributions to the geology
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Stratigraphy across the Sauk II–Sauk III boundary in the upper Mississippi valley: Geological
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