geologic atlas of washington county, minnesota

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geologic atlas of washington county, minnesota
MINNESOTA GEOLOGICAL SURVEY
Harvey Thorleifson, Director
Mahtomedi
300
275
27
30
Os
0
1
0
1
WISCONSIN
Thickness (in feet)
Map symbol
<40
30-35
System-Series
Galena
Group
Hydrostratigraphic
properties
140-160
90-120
Prairie du Chien Group
50-80
85-100
27
5
250
225
1
2
3
¤
4
Elevation (feet above sea level)
0
30
50-60
COTTAGE GROVE
FAULT ZONE
D U
m
-200
-400
w e
m
HUDSON–AFTON HORST
A' Cross sections—Every attempt has been
made for the cross sections to match all
geologic interpretations made from the
County Well Index data. Symbology is
the same as on the bedrock map. Only
a small number of drill holes intersected
Mesoproterozoic bedrock, thus the relief is
generalized and inferred from the thickness
of the Mt. Simon Sandstone. Dashed
vertical lines represent Precambrian faults
and the long dashed lines in C–C' represent
volcanic flows inferred from geophysical
imagery.
Ops
Opo
U D
D U
D U
Mpv
Mss
Os
IN
j
ONS
WISC
ST
B
Dellwood
Mss
Lake Elmo Airport
Minnesota Highway 36
1,000
5
LT
FA
U
U
61 ConleyDU
Lake
U
D
s
2
s t
e w
0
1,200
S
6
Unconsolidated
Os
800
Ops
600
j
Opo
400
t
s
Quaternary
200
Os
Ops
Opo
-200
j
s t
w e
m
0
Interstate 94
Opg
sediments
e w
m
Mss
-400
4
A'
1,000
T. 26 N.
¤
10
t
44° 45' N.
s
R. 20 W.
DAKOTA COUNTY
3
5
j
D
River
1
j
t s
200
St. Croix River
Os
-600
T. 27 N.
Ops
Ops
U
SCALE 1:100 000
Manning Avenue
Mss
B'
Lakeland
St. Croix River
t
e
m
HUDSON–AFTON HORST
Mpv
D U
-600
D
D
D
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.
Figure 1. Generalized stratigraphic column depicting
the lithology, thickness, vertical succession, age, and
hydrostratigraphic properties for all units shown on
the map, as well as the schematic depiction of relative
competence in outcrop where exposed. The gamma log is
a compilation of the following borehole geophysical logs
on file at the Minnesota Geological Survey: County Well
Index unique numbers 783609, 777305, and 256005.
Os Opg
Ops
Opo
j
400
Ops
HA
00
44° 45' N.
Universal Transverse Mercator Projection, grid zone 15
1983 North American Datum
GIS compilation by R.S. Lively
Edited by Lori Robinson
U
Os
313
U
Digital base modified from the Minnesota Department of
Transportation BaseMap data; digital base annotation by
the Minnesota Geological Survey.
Elevation contours were derived from the U.S. Geological
Survey 30-meter Digital Elevation Model (DEM) by the
Minnesota Geological Survey.
D
ING
E
OV
GR
E
TT
AG
CO
92° 52' 30" W.
U
U
A' Location of geologic cross section
A
Dolomitic
Jamaica Avenue
Unconsolidated
Quaternary sediments
600
r
1
U
D
2
PIERCE
COUNTY
22
LT
275
FA
U
D
36
D
Mississippi
R. 22 W.
93° W.
300
300
300
M
R. 21 W.
U
t
Military Road
A
800
Rive
6
DENMARK
Opo
U
t
U
Os U D
)
95
ST. CROIX
COUNTY
St. C
roix
s
35
Opo
D
U
D t
0
31
225
D
U
U
D
U
25
Baldwin
Lake t
j
j
5
Ops
DU
44° 52' 30" N.
Ops
275
36
j
D U
10 U
1,000
D
Brook
U D
F
5
j
s
27
Os
Os
¤
¤
s
U
j
300
300
Ops
61
Opo
22
225
s
U
Opo
5
D
U
t
u
Tro
Ops
D U
D
U
U
D
D
Os
27
Mooers Lake
6
30
D
D
U
Opo
t
U
M
F
275
D
275
GREY
CLOUD
ISLAND
225
U
U
U
Opo
0
D
U
w
j
D
F
Ops
D
Opg U
COTTAGE GROVE
Os
T. 27 N.
Bedrock outcrop
D
t
0
275
Cross-bedded (hummocky)
U
Bioturbation
High permeability bedding fracture known to be common
B'
25
r
Rive
250
Cross-bedded (trough)
Geologic contact, approximatly located
Geologic contact, inferred
Fault—Faults are inferred from abrupt changes in the elevation of
stratigraphic units from subsurface and outcrop data. Letters
indicate relative vertical displacement: U—up, D—down. Dashed
lines represent areas where it is inferred.
Fold—Axial trace of anticline, syncline. Fold limbs typically have
shallow dips and are inferred from subsurface data.
Active quarry
Shells
T. 28 N.
00
Os31
36
1
Os
Cottage Grove
s
m
Mesoproterozoic and older
rocks, undifferentiated
3
275
M
5
5
27
5
27
j
D
M
F
27
F
300
275
275
U
Opo
D
D
F
6
St Paul
Park
F
Os
MAP SYMBOLS
Relatively low permeability (except for fractures, aquitard)
e
St Mary's
w
Point
F
1
5
61
Ops
D
AFTON
j
95
U
27
pi
Mississip
Opg
Branch
ek
Cre
)
300
w
Lake
St Croix
Beach w
D U
U
Valle
y
Opg
5
¤
¤
10
F
31
36
Os
Os
27
Newport
D
0
44° 52' 30" N.
t
m
Elevation (feet above sea level)
F
30
Opg
t
300
Ops
275
25
Os
5
275
WOODBURY
300
27
Valley
M
Colby
Lake
F
Od
Lake j
Edith
Mt. Simon
Sandstone
Elevation (feet above sea level)
5
27
Opg
5
Os
Phosphate grains
River
5 Powers
Lake
32
§
¦
494
Ph
Relatively high permeability (aquifer)
roix
St. C
5
27
Wilmes
Lake
2
6
300
494
La
Lake
Od
35
t
Lakeland
Shores
275
275
275
§
¦
A
U
§
¦
5
27
300
1
Margrafs
Lake
3
93° W.
R. 22 W.
D
e
Woodbury
Opg
25
RAMSEY
COUNTY
WEST
94
Carver
Lake
95
LAKELAND
31
§
¦
94
Os
)
94
Ops
§
¦
Battle
3060 Creek
Lake
Mpv
Not
Shown
Opo
36
Os
Chert
MESOPROTEROZOIC
HYDROSTRATIGRAPHIC PROPERTIES KEY
T. 29 N.
Os
Lake
G
45° N.
27
Goose
Tanners
Lake
31
e
G
Middle Cambrian
325
Opg
300
275
5
Eau Claire
Formation
Os
Lake
s
Elmo
Point
3Lake
00
32
Os
275
F
Os
Eagle
Mbv
Contact marks a major erosional surface
0
5
Shale
Vugs
25
32
BAYTOWN
0
275
LAKE ELMO
Mss
Glauconite
G
Bayport
275
Os
Siltstone
G
G
225
LAKELAND
225
5
5
T. 28 N.
Os
30
Ops
Sunfish
Lake
27
)
T. 29 N. Os
j 2
5
0
)
unconformity
Pebbles
w
27
McDonald
Lake
5
5
F
G
Opo
Cloverdale
Lake
Os
45° N.
Od
Lone Rock
Formation
G
Middle Cambrian
m
80-100
300
Clear
Lake
694
Od
Oak Park Heights
6
1
§
¦
Oakdale
G
t
Wonewoc
Sandstone
300
Os
e
Stromatolites
~200-280
300
Lake
Jane
unconformity
w
Cross-bedded (planar)
Ops
Ops
36
300
Fine- to medium-grained
Oolites
G
NOT EXPOSED IN WASHINGTON COUNTY
300
)
30
Lake
0
Demontreville
Olson
Lake
Od
G
G
Lily
Lake
31
M
)
Os
Opo
Stillwater
Ops
s
Opo
Long
Lake
36
Os
6
t
Medium- to coarse-grained
j
Upper Cambrian
Very fine- to fine-grained
G
Mazomanie
Formation
250
325
s
Intraclasts
275
Os
Long
Lake
36
Oneota
Dolomite
PALEOZOIC
j
Sandstone
F
Od
k
unconformity
Dolostone
Sandy dolostone
Hager
City Opo
Lower Ordovician
Opo
Shaly
Lake
McKusick275
300
0
31
Ops
St. Lawrence
Formation
Brown's Creek
Ops
30
Pine Springs
Opg
Shakopee
Formation
Jordan
Sandstone
92° 45' W.
T. 30 N.
s
unconformity
COTTAGE GROVE FAULT
Masterman
Lake
Od
Opo
Opo
ee
Twin
Lakes
Ops
LITHOLOGY KEY
4
6
PIERCE
COUNTY
5 MILES
7
8 KILOMETERS
Manning Trail
Forest Lake
C
j
800
Unconsolidated Quaternary sediments
Opo
w
w
e
m
400
j
C'
Opo
Od
s t
t
s
600
St. Croix River
St. Croix Trail
e
m
200
Mbv
0
-200
U
FALLS CREEK GRABEN
300
Lake
White Bear
Pigs
Eye
HASTINGS FAULT
Long
Lake
T. 30 N.
M
Os
Cr
F
)
96
Ops
ve j
r
Opo
1
Middle Ordovician
unconformity
w
95 75
2
STILLWATER
Sil
Os
35-45
Os
ek
Cre
244
Os
Upper Ordovician
Opg
160-180
300
)
w
275
Lake
300
)
Ops
Silver
INTRODUCTION
Od
Tonti
St. Peter
Sandstone
Tunnel City Group
0
Ph
CORRELATION OF MAP UNITS
45° 7' 30" N.
PALEOZOIC
Os
Brown's
30
GRANT
Os
250
30
0
s
Lake
Pine
Tree
Lake
Os
Upper Ordovician
300
Os Benz
0
30
Dellwood
Loon
Ops
Lake
j
5
Mann
Lake
Opo
6
275
j
Little
Carnelian
Lake
Lake
Od
LOCATION DIAGRAM
225
1
36
s
275
275
ANOKA COUNTY
Os
Louise
6
B
Os
Opo
31
Upper Cambrian
Cre
e
300
31
M
RAMSEY COUNTY
36
Round
Lake
100
0
API-G units
Coon
Valley
Big
Carnelian
Lake
Os
Increasing count
2016
River
Fish
Lake
Lake
Lithology
Platteville and
Opg
Glenwood Formations
5
Ops
Lake
t
27
Bald 31
Eagle
DAKOTA
COUNTY
Ops
School
Section
Lake
Long
j
300
Sunset
Os Lake
w e
ix
St. Cro
5
Cree
k
61
T. 31 N.
j
Square
Lake
0
Composite natural gamma log
Limestone
s
30
Decorah
Shale
95
27
Opo
Os
45° 7' 30" N.
275
Os
Group,
Formation,
Member
)
MAY
300
Rice
Lake
¤
30
Opo0
250
F
Hard
wood
Os
j
Terrapin
275
Lake
Ops
Turtle
Lake
Os
Egg
Os
Lake
275
Mud
Lake
Os
HUGO
Hugo
300
300
300
Ops
Lake
Ops
Clearwater
Opo
Lithostratigraphic
unit
ST. CROIX
COUNTY
6
Marine On
St Croix
0
Os
Oneka
T. 31 N.
Ops
30
Ha
1
w POLK
COUNTY
j
eam
6
31
275
k
s
1
0
d
36
300
s
j
30
oo
rdw
Opo
ill Str
6
Os
Horseshoe
Lake
Creek
Big Marine Lake
t
Old M
j
Sand
Lake
0
Long Lake
31
Os36
Opo
30
Ops
White
Rock
Lake
31
Hay Lake
Era
45° 15' N.
Ops
27
t
Opo
300
Os
Fish
Lake
T. 32 N.
Middle Ordovician
j
Os
Julia R. Steenberg and Andrew J. Retzler
Lower Ordovician
0
300
61
30
SCANDIA
97
Os
250
M
Sylvan
Lake
Ops
)
0
300
s
30
300
¤
w
s
300
s
j
s
j
Opo
225
250
)
j
FOREST LAKE
j
Ops
30
0
275
97
Os
300
Lake
Os
275
s
t
Goose
300
Lake
GermanOps
w Lake
27
5
Clear Lake
45° 15' N.
300
0
30
Opo
By
C'
Opo
300
j
Forest Lake
Lake
T. 32 N.
95
U
D
U
)
BEDROCK Geology
POLK
COUNTY
t
Ops
6
1
j
275
275
t
Forest
Opo
F
ANOKA COUNTY
8
Nielsen
Lake
30
0
t U D
s
t Lake
275
¤
s
Sea
Bone
Lake j
s
6
CHISAGO COUNTY
92° 45' W.
R. 19 W.
R. 20 W.
300
Mud
1
5
35
j
27
§
¦
C
92° 52' 30" W.
R. 21 W.
s
6
THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF ECOLOGICAL AND WATER RESOURCES,
AND the Minnesota Legacy Amendment's Clean Water Fund
M
CHISAGO COUNTY
93° W.
D
Vertical exaggeration = 10x
U
contour interval 25 METERS
G
FC
M
F
Mbv
Elevation of the top of the
Jordan Sandstone in feet
Less than 550
551-600
601-650
50
951-1,000
0
1,001-1,050
HU
DS
ON
-A
CG
FT
ON
F
HO
RS
T
Figure 4. Map of Washington County
depicting the elevation of the stratigraphic
top of the Jordan Sandstone showing the
mapped fold axes (thin black lines) and
faults (thick black lines). Colored intervals
represent 50-foot (15-meter) elevation
intervals; blue colors represent lower
elevations, and orange represents higher
elevations. Dashed lines depict the 25-foot
(8-meter) contour intervals. In areas where
the Jordan Sandstone is absent because of
erosion, the map is not colored, and the
contours are inferred from vertical projection
of the contacts of stratigraphically lower
formations.
HF
F
HF
Z
F
FZ
(
(
(
100
901-950
CG
(
H
HA
( HFZ
(
(
HF (
M
(
(
(
(
(
(
(
(
(
(
(
(
M
FZ
F
F
F
F
F
M
M
F
(
F
F
CG
F
(
Carbonate rock within 50
feet of the land surface
M
F
150
851-900
F
(
200
801-850
Quartzose sandrock within
50 feet of the land surface
©2016 by the Regents of the University of Minnesota
The University of Minnesota is an equal opportunity educator and employer
The geologic map, cross sections, and stratigraphic column on this plate depict the type, distribution,
and structure of the bedrock units in Washington County that are either exposed at the land surface or lie
directly beneath unconsolidated Quaternary glacial sediments of variable thickness (see cross sections
and Plates 3, 4, and 5, Surficial Geology, Quaternary Stratigraphy, Depth to Bedrock, and Bedrock
Topography). The map shows how the bedrock surface would appear if it was viewed from an aerial
perspective and the overlying Quaternary sediments were stripped away. The bedrock units near the land
surface in Washington County consist of sedimentary rocks of Paleozoic age that form distinguishable
and mappable layers designated as formations. These units are commonly exposed along the Mississippi
and St. Croix River bluffs, within rock quarries, and along roadcuts within the county. Several of the
Paleozoic bedrock formations are major reservoirs for water supply in Washington County and also
provide a source of crushed carbonate rock and silica sand.
Characteristics of each formation are given in the stratigraphic column (Fig. 1) and in the description
of map units. The accompanying bedrock geologic cross sections add the dimension of depth and illustrate
the stratigraphic, structural, and topographic relationships of the bedrock units, as well as the variable
thickness of the overlying Quaternary sediments. Surfaces representative of the elevation of the tops of
the mapped formations are also available as Digital Elevation Models (DEMs) for use in GIS programs.
The geologic formations are thin in relation to their aerial extent, and would only be one-tenth as thick
as shown on the cross sections if no vertical exaggeration were used. The exaggeration necessary to
show the thin rock formations gives the appearance of steeper slopes on bedrock unit contacts, the land
surface, and bedrock topography. Most of the Paleozoic units shown on this plate, with the exception
of the Mt. Simon and Wonewoc Sandstones and Eau Claire Formation, can be seen at the land surface
in places in Washington County. The sedimentary rocks differ in their resistance to weathering and
erosion, as indicated by the weathering profile on the lithology column of Figure 1. The units that cover
the largest areas of the map are the most resistant to weathering and generally form plateaus composed
of carbonate rock (limestone and dolostone). The soft sandstone and shale formations are more easily
eroded and commonly occur on bedrock plateau or valley walls.
Production of the map and associated products relied on several data sources, including outcrops,
water-well and scientific drilling records from the County Well Index, rock core, drill cutting samples,
borehole geophysical logs, seismic soundings, geophysical images, and previously published geologic
maps of Washington and adjacent counties (Mossler and Bloomgren, 1990; Mossler and Tipping, 2000;
Mossler, 2005a, b, 2006a, b, c, d, 2013; Anderson, 2009). This map supersedes the previous bedrock
geology map of Washington County (Mossler and Bloomgren, 1990). Significant improvements and
modifications were made to the previous map based on additional water-well records, drill cuttings,
borehole geophysics, and refined geophysical images. The Prairie du Chien Group is separated into
the Oneota Dolomite and Shakopee Formation and the St. Lawrence Formation is mapped separately
from the Tunnel City Group (previously the Franconia Formation). Faults and folds in the southeastern
part of the county were mapped to reflect significant offset in the Paleozoic formations. This provides
detail of the subsurface geologic conditions, which has implications for modeling groundwater flow.
The different data sources and their irregular distribution and density can be seen on Plate 1, Data-Base
Map, and these should be considered when assessing the reliability of the map at any particular location.
Areas with a high density of bedrock control points are more likely to have an accurate interpretation
of the bedrock geology, whereas those areas with widely spaced control points may be less reliable
and inappropriate for site-specific needs. During production of this map, the records of over 14,000
located water wells existed within Washington County in the County Well Index and nearly 10,000 of
those reached Paleozoic bedrock. Geologic interpretations of subsurface material described by drillers
or gathered from other data sources were made by the authors and are represented in the County Well
Index records. Every reasonable effort has been made for the geologic interpretations in the County
Well Index to correspond to the map.
Paleozoic bedrock lies on top of a thick sequence of Mesoproterozoic (approximately 1,100 million
years, or Ma, ago) rocks of the Keweenawan Supergroup, associated with the Midcontinent Rift (see
cross sections and Fig. 2). These rocks include sandstone, siltstone, and shale of the Hinckley Sandstone,
Solor Church and Fond du Lac Formations (unit Mss), and volcanic rocks composed mostly of basalt
including the Powder Mill (unit Mpv) and North Branch (unit Mbv) volcanic sequences. Due to their
deep burial and limited subsurface data, the distribution of these individual units is less certain than the
Paleozoic units. Therefore, the contact between the top of these units and the overlying base of Mt.
Simon Sandstone is dashed in cross sections. No new mapping of these units was done for this project;
instead a recent compilation map of the Precambrian bedrock geology of Minnesota is depicted (Fig.
2; Jirsa and others, 2012).
The Paleozoic rocks of Washington County are characterized by relatively thin, widespread layers of
sandstone, shale, and carbonate deposited in shallow seas during the Cambrian and Ordovician Periods
of the Paleozoic Era, from about 500 to 450 million years ago. The older Cambrian-age formations are
dominated by siliciclastic sedimentary rock including sandstone and siltstone with minor shale, such
as the Jordan Sandstone and Mazomanie Formation (Tunnel City Group). Carbonate rock occurs only
as relatively thin layers within these units. Ordovician-age formations, in contrast, are dominated by
thicker units of carbonate rock with less sandstone and shale, such as the Prairie du Chien Group and
the Platteville Formation. Where carbonate rock and silica-rich (quartzose) sandstone exist near the land
surface (or within 50 feet [15 meters]), they are considered a valuable geologic resource. Carbonate
rock products are crushed, sorted, and used as construction material for roads and buildings, and in
concrete operations. The silica-rich sandstone is texturally mature and well-rounded, which makes it a
sought after resource in the oil industry. It also has a wide variety of other uses including glassmaking,
foundry operations, ceramics, filtration, and agriculture. However, viability of extraction is dependent
on many other factors, including detailed geologic conditions at individual sites, proximity to bulk
transportation, current land ownership and use, market prices, regulatory requirements, and many others.
Figure 3 depicts where carbonate or quartzose sandrock is within 50 feet (15 meters) of the land surface
and highlights the active quarry operations at the time this map was created.
Across most of Washington County, bedrock units are slightly tilted (less than 1°) southwest towards
the central Twin Cities metropolitan area, as part of the eastern margin of a shallow structural depression
known as the Twin Cities basin. As a result, progressively younger bedrock formations subcrop from
eastern to western Washington County. This general trend is locally interrupted by deep valleys that
incise older formations, and by faults and folds. A fault is recognized where changes in the elevation of
a bedrock contact occur within a very short distance, generally elevation changes of 50 feet (15 meters)
or more within a distance of 1,000 feet (305 meters). A fold is inferred where these elevation changes
indicate more gradual slopes. The stratigraphic top of the Jordan Sandstone was contoured at 25-foot
(15-meter) intervals to show the inferred location of faults and folds in Washington County (Fig. 4).
The top of the Jordan Sandstone was selected to portray these structures because it is a well-recognized
and distinct contact, has numerous control points including outcrops that expose the contact, and has
water wells that penetrate it. Displacement along faults in Washington County is on the order of 25 to
300 feet (8 to 91 meters), which is sufficient in places to juxtapose several different formations along
the fault contact (see cross sections and Fig. 4). Bedrock that has dropped alongside a fault preserves
relatively younger formations (shown with a D on the map) at the surface and bedrock that has been
uplifted brings older formations closer to the surface (shown with a U on the map). Most faults trend
in the northeast direction but several trend northwest.
Several of the faults observed in Paleozoic strata are subparallel to faults in the underlying
Mesoproterozoic Midcontinent Rift rocks, as inferred from geophysical imagery, and are therefore
interpreted to represent reactivation of these older structures (Fig. 2). The aeromagnetic and gravity
data show where a great thickness of dense and magnetic basalts have been brought near the surface in
an inverted graben known as the Hudson–Afton horst (Sims and Zeitz, 1967; Cannon and others, 2001).
Paleozoic rocks overlying the Hudson–Afton horst in Washington County are also uplifted relative to
the rocks on either side, although the displacement is of a lesser magnitude than that in the underlying
Mesoproterozoic bedrock (Fig. 4). Lower Ordovician Prairie du Chien Group rocks thin (less than 50
feet [15 meters]) within the uplifted horst and thicken along either side (less than 300 feet [91 meters]),
suggesting displacement was occurring during Early to Middle Ordovician time (see cross section A–A'
and Fig. 5).
Figure 5. Color shaded map indicating
the thickness of the Prairie du Chien
Group where it exists below the St. Peter
Sandstone. Blue colors represent areas where
the Prairie du Chien Group is thinner and
orange represents areas where it is thicker.
There is progressive thinning toward the
north, where it thins to 50 feet (15 meters)
thick. Thicknesses reach nearly 300 feet (91
meters) in the southern part of the county
except within the Hudson–Afton horst
(HAH), where it thins to less than 50 feet
(15 meters) in places. Small areas where
the Prairie du Chien Group appears to thin
abruptly are interpreted to represent areas of
significant erosion during a hiatus marked
by an unconformity prior to the deposition
of the St. Peter Sandstone. CGF—Cottage
Grove fault, CGFZ—Cottage Grove fault
zone, HF—Hastings fault, HFZ—Hastings
fault zone.
GEOLOGIC ATLAS OF WASHINGTON COUNTY, MINNESOTA
HYDROSTRATIGRAPHY
The Paleozoic bedrock formations contain significant sources of groundwater, which provide the
majority of the water supply for Washington County. This map and associated products, such as the
bedrock unit DEMs, provide a three-dimensional depiction of the rock properties that control flow
in these water-bearing layers. Such rock properties are called hydrostratigraphic properties. The
hydrostratigraphic classification, shown by brown and blue colors on the hydrostratigraphic column of
Figure 1, distinguishes layers that are dominated by relatively high permeability (easily transmitting
water) material, versus layers dominated by lower permeability (relatively more difficult to transmit
water) material. This generalized characterization for Washington County is based on hydrogeologic
reports by Runkel (1996), Paillet and others (2000), Runkel and others (2003, 2006a, b, 2014a, b),
Tipping and others (2006), Anderson and others (2011), Luhmann and others (2011), Green and others
(2012), and unpublished borehole and core data collected by the Minnesota Geological Survey. The
high permeability layers are potential aquifers, able to yield economic quantities of water in most places.
The low permeability layers are potential aquitards that retard vertical flow, hydraulically separating the
aquifer layers from one another in many places, and protecting water resources in the underlying layers
from surface contamination. Fractures parallel to bedding that have been demonstrated to have high
permeability are likely to be present, at least locally, in all formations in Washington County, but are
placed on the column where the hydrogeologic reports cited above indicate they are most common. The
locations of springs in Washington County most commonly occur within the lower Jordan Sandstone,
St. Lawrence Formation, and upper Tunnel City Group, where high permeability bedding fractures
are known to be common. The hydrogeologic properties of the Mesoproterozoic bedrock are poorly
understood. It does not supply water to Washington County at least in part because sufficient water
resources are available at shallower depths in the Paleozoic rocks.
In Washington County, most aquifers are layers dominated by relatively coarse-grained sandstone,
such as within the upper part of the Jordan Sandstone, in which water can be fairly easily transmitted in
both horizontal and vertical directions through the pore spaces between sand grains, as well as through
fractures. Other aquifers, such as the upper part of the Prairie du Chien Group, are composed mostly
of carbonate rock in which water is transmitted through a relatively dense network of fractures and
solution cavities. However, layers designated as aquifers can locally contain low permeability strata
that serve as small, internal aquitards including parts of the upper Mt. Simon Sandstone and the lower
Jordan Sandstone.
Most layers designated as aquitards in the county have a much lower permeability in the vertical
direction than do aquifers. Examples in Washington County include the lower Jordan Sandstone, St.
Lawrence, and Eau Claire Formations, whose rocks are composed mostly of very fine-grained sandstone
and shale with small, poorly connected pore spaces. Carbonate rock with relatively sparse fractures,
such as the lower part of the Prairie du Chien Group (Oneota Dolomite), are also aquitards. However,
layers designated as aquitards with very low permeability in the vertical direction may locally contain
horizontal fractures that are conductive enough to yield large quantities of water, including the St.
Lawrence Formation.
Horizontal and vertical fractures are more common where bedrock layers are at or near the bedrock
surface. As a result, aquitards in such conditions are likely to have higher permeability compared with
more deeply buried portions of the same formation, and may have a diminished ability to retard water
flow to underlying aquifers. There is no precise boundary between shallow and deep conditions of
burial, but in most areas of southeastern Minnesota about 50 feet (15 meters) of depth below the bedrock
surface is considered a best approximation (Runkel and others, 2006a).
In addition to this hydrostratigraphic classification, the Minnesota Department of Natural Resources,
as Part B of the Washington County atlas, will conduct a thorough hydrogeologic study of the groundwater
flow system, aquifer capacity, and aquifer sensitivity, which may result in modifications to this
classification. Furthermore, designations of aquifers versus aquitards made here may not correspond
precisely with those made for regulatory purposes by the Minnesota Department of Health.
250
751-800
M
(
M
M
CG
300
701-750
F
(
Mss
Active quarry
Figure 3. Map showing where carbonate
and quartzose sandrock are present within
50 feet (15 meters) of the land surface in
Washington County. Brown represents
quartzose sandrock (including the Jordan,
Wonewoc, and St. Peter Sandstones) and
beige represents carbonate rock (including
the Platteville and Shakopee Formations
and Oneota Dolomite). The active quarry
operations in Washington County are shown
in black. This figure was generated using
the bedrock topographic surface and bedrock
geology polygons, which are standard
Geographic Information System (GIS)
products of a county geologic atlas.
Thickness of Prairie du Chien
Group rocks in feet
651-700
F
Mpv
Figure 2. Faults that displace Paleozoic
bedrock in Washington County superimposed
on a map of the first vertical derivative
aeromagnetic data (Chandler, 1991) and
the underlying Mesoproterozoic rock units
(Jirsa and others, 2012). Paleozoic faults
and fault zones where they cluster in the
southern part of the map (CGF—Cottage
Grove fault, CGFZ—Cottage Grove fault
zone, HF—Hastings fault, HFZ—Hastings
fault zone) are subparallel to strong, abrupt,
linearly extensive contrasts in magnetic
intensity in the underlying Mesoproterozoic
rocks associated with the Midcontinent Rift.
The aeromagnetic data display where a great
thickness of magnetic Mesoproterozoic rift
basalts (unit Mpv) have been brought near
the surface in an inverted graben known as
the Hudson–Afton horst (HAH), bounded by
thrust faults, shown by black lines with teeth.
Faults in the northeastern corner of the county
that bound what is known as the Falls Creek
Graben (FCG) are subparallel to foliation
trends of underlying Mesoproterozoic lava
flows (unit Mbv ). Due to their similar
trends and close proximity, all Paleozoic
faults are interpreted to have originated
from reactivation of deep Mesoproterozoic
structures. Abrupt thickness changes in
the Prairie du Chien Group across the
Hudson–Afton horst indicate that faults
were reactivated during Early Paleozoic
(Early to Middle Ordovician) time.
F
Mss
COUNTY ATLAS SERIES
ATLAS C-39, PART A
Washington County
Plate 2—Bedrock Geology
Prepared and Published with the Support of
THE WASHINGTON COUNTY BOARD OF COMMISSIONERS,
DESCRIPTION OF MAP UNITS
Decorah Shale (Upper Ordovician)—Dominantly grayish-green shale interbedded with thin
beds of fossiliferous limestone. Fossiliferous, yellowish-brown limestone beds are most
common at the base of the Decorah Shale, and are recognized as the basal Carimona
Member (Mossler, 2008). The Decorah Shale is present as erosional remnants capping
Platteville mesas in the southwestern part of the county. Though no exposures exist
in the county, water well logs indicate the Decorah Shale has a maximum preserved
thickness of 40 feet (12 meters).
Opg
Platteville and Glenwood Formations (Upper Ordovician)—The Platteville Formation is
generally tan to gray, fossiliferous limestone and dolostone. The underlying Glenwood
Formation is dominantly a green-gray, sandy shale. The Platteville Formation is the
dominant uppermost bedrock unit across a large expanse of the southwestern part of the
county. The combined thickness of the formations is 30 to 35 feet (9 to 11 meters).
Platteville Formation—The Platteville Formation is 25 to 30 feet (8 to 9 meters) thick. It is
composed of tan to gray limestone and dolostone. It is commonly burrowed, mottled,
and fossiliferous. It contains fine- to coarse-grained quartz sand and phosphate grains
in the lowermost 2 feet (0.6 meter).
Glenwood Formation—The principal rock type of the Glenwood Formation is a grayishgreen to brownish-gray, calcareous, sandy, and phosphatic shale. The Glenwood Formation
is 3 to 7 feet (1 to 2 meters) thick.
Os
St. Peter Sandstone (Middle to Lower Ordovician)—The upper 100 to 140 feet (30 to 43
meters) of the St. Peter Sandstone is mostly a white to tan, fine- to medium-grained,
friable quartzose sandstone. Bedding and structures are generally absent. It is exposed
in patchy outcrops in the southern half of the county where glacial sediments are thin.
The lowermost 10 to 40 feet (3 to 12 meters), referred to as the Pigs Eye Member,
includes white to gray feldspathic shale and siltstone interbedded with coarser-grained
sandstone similar to that of the Tonti Member. The Pigs Eye Member is not exposed
in Washington County. The thickness of the St. Peter Sandstone varies from about 130
to 160 feet (40 to 49 meters). The basal contact of the formation with the underlying
Shakopee Formation (unit Ops) is a major erosional unconformity (Smith and others,
1993).
Prairie du Chien Group (Lower Ordovician)—Dominated by dolostone interlayered with
lesser amounts of quartz sandstone. Outcrops are exposed along the tops of bluffs of
the Mississippi and St. Croix River valleys; small patchy outcrops are also present in
southeastern Washington County where the Quaternary sediments are thin. The Prairie
du Chien Group is a significant source of rock aggregate in this part of the county. The
Prairie du Chien Group is formally divided into two formations: the Shakopee Formation
and underlying Oneota Dolomite. Geophysical logs and drill cuttings in the northern part
of the county and within parts of the Hudson–Afton horst that span the St. Peter through
the Jordan Sandstones are lacking and water well records are typically inadequate to
Od
distinguish the Oneota Dolomite from the Shakopee Formation and St. Peter Sandstone
from the sandy upper Shakopee Formation. Therefore, the mapped distribution of these
units is more speculative in these areas. The thickness of the Prairie du Chien Group
beneath the St. Peter Sandstone varies greatly across Washington County, from less than
50 to nearly 300 feet (15 to 91 meters; Fig. 5). This is interpreted to be due to several
factors, including syndepositional faulting along the Hudson–Afton horst, and erosion of
the Shakopee Formation prior to deposition of the St. Peter Sandstone. This erosional
surface is the unconformity that marks the contact between the Shakopee Formation and
overlying St. Peter Sandstone.
Ops Shakopee Formation (Lower Ordovician)—A heterolithic unit composed mainly of light
brown, thin- to medium-bedded dolostone, sandy dolostone, sandstone, and shale. It
contains oolites, intraclasts, fossilized microbial mounds, chert nodules, quartz sandstone,
and green-gray shale partings. Thickness of the Shakopee Formation beneath the St.
Peter Sandstone is quite variable within the area of the Hudson–Afton horst, ranging
from almost absent to nearly 200 feet (61 meters) thick. It appears to be thickest in the
most southeast part of the county, east of the Hastings fault, where it is nearly 200 feet
(61 meters) thick. On the opposite side of the horst, on the west side of the Cottage
Grove fault, it reaches thicknesses of 115 feet (35 meters) and appears to progressively
thin towards the northwest. Based on a limited amount of drill cuttings and geophysical
data within the Hudson–Afton horst, it appears that the Shakopee Formation thins to
less than 50 feet (15 meters) and may even be absent beneath the St. Peter Sandstone at
several locations (see cross section A–A').
Opo Oneota Dolomite (Lower Ordovician)—Predominantly a yellowish-gray to light brown,
medium- to thick-bedded dolostone that generally lacks sedimentary features such as
oolites and quartz sand characteristic of the Shakopee Formation, except in its lowermost
part. The formation contains two members, the Hager City and the Coon Valley, but
they are not mapped separately. The basal Coon Valley Member is a heterolithic unit
composed of thinly bedded dolostone, sandy dolostone, and beds of fine- to coarsegrained, poorly sorted quartz sandstone. Thickness of the Coon Valley Member is quite
variable, it is locally absent to 30 feet (9 meters) thick. There appears to be a slight
trend in its thickness along the Hudson–Afton horst whereby it is thickest on the down
dropped sides of the faults (20 to 30 feet [6 to 9 meters]) and thinner between the faults
(10 to 15 feet [3 to 5 meters]). It also appears to be thin to absent towards the western
edge of the county and within the central and northern parts of the county. The Hager
City Member is primarily very finely crystalline dolostone, with microbial textures. Its
thickness also shows a similar trend to the Coon Valley Member in being thicker along
the down-dropped sides of the Hastings and Cottage Grove faults (50 to 70 feet [15 to
21 meters]) and thinner between them (40 to 50 feet [12 to 15 meters]).
_j
Jordan Sandstone (Upper Cambrian)—Dominantly white to yellow, very fine- to coarsegrained, friable quartz sandstone characterized by coarsening-upward sequences consisting
of two interlayered facies (Runkel, 1994). They are medium- to coarse-grained,
cross-stratified, generally friable, quartz sandstone; and very fine-grained, commonly
bioturbated, feldspathic sandstone with 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 _s), although there are lithologically similar
intervals intercalated with the medium- to coarse-grained facies at higher stratigraphic
intervals. An unconformity, locally marked by thin beds of quartz pebble conglomerate
and silcrete-cemented sandstone clasts (Runkel and others, 1999, 2007), separates the
Jordan Sandstone from the overlying Oneota Dolomite. The Jordan Sandstone is exposed
along the Mississippi and St. Croix River bluffs and ranges in thickness from 80 to 100
feet (24 to 29 meters). In the northernmost part of the county near Scandia, it appears
to thin to 65 to 70 feet (20 to 21 meters) thick.
_s
St. Lawrence Formation (Upper 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, 2006a). The formation is 35 to 45 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 difficult.
_t
Tunnel City Group (Upper Cambrian)—The Tunnel City Group, formerly named the Franconia
Formation (Berg, 1954), varies from about 160 to 180 feet (49 to 55 meters) in thickness
across Washington County. It is formally divided into two formations: the Mazomanie
and the Lone Rock Formations (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 rare to absent and never exceed 5 percent
(Berg, 1954). Some beds contain brown, intergranular dolomite cement. Skolithos
burrows and sandstone intraclasts are common along discrete horizons. 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 northeastern Washington County, individual tongues of Mazomanie
Formation are as thick as 50 feet (15 meters), and the Mazomanie Formation as a
whole can reach thicknesses of 100 feet (30 meters). The Mazomanie Formation thins
to the south, where it is progressively replaced laterally by the Lone Rock Formation.
As a result, in southern Washington County the Mazomanie Formation is less than 25
feet (8 meters) thick to absent. 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 and typically marked by an intraclastic conglomerate.
_w
Wonewoc Sandstone (Upper 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 45 to 75 feet (14 to 23 meters). 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).
_e
Eau Claire Formation (Middle to Upper 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
formation ranges from 80 to 100 feet (24 to 30 meters) in thickness. The contact with
the Mt. Simon Sandstone is conformable.
_m
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). 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.
Based on a limited number of full penetrations of the formation, it appears to have a
maximum thickness of about 280 feet (85 meters).
MESOPROTEROZOIC
Keweenawan Supergroup
Mss
Mbv
Mpv
Sandstone, siltstone, and local conglomerate (shown on cross sections and Fig. 2)—Includes
the Hinckley Sandstone and Fond du Lac (youngest detrital zircons ~1,000 Ma) and
Solor Church Formations; deposited in eolian, fluvial, and lacustrine environments.
North Branch volcanic sequence (shown on cross section C–C' and Fig. 2)—Part of the
St. Croix horst.
Powder Mill volcanic sequence (~1,099 Ma; shown on cross sections and Fig. 2)—Part of
the St. Croix horst.
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