Value of Water Well Drilling Data in
Understanding the Precambrian Geology
of Northeastern Minnesota
Jim Miller
University of Minnesota Duluth
Precambrian Bedrock Geology of NE MN
The
Geologic
Record in
Minnesota
LAURENTIA
KENORALAND
Terrane 2:
The Superior
Province
(GraniteGreenstone
Terrane)
2.7-2.5 Ga
GREENSTONE
Metamorphosed Basalt Flows
Pillowed Structures indicative
of submarine eruption
SEDIMENTARY ROCKS
BANDED IRON FORMATION
GRAYWACKE
GRANITE
Crystallized from Magmas intruded into the crust
The Superior
Province
Multiple
Accreted
Terranes
TECTONIC MODEL FOR THE SUPERIOR PROVINCE
Accretion of Island Arcs, Oceanic Plateaus, and Protocontinents
Terrane 3:
The Penokean
(and Yavapi
and Mazatzal)
Orogen
1.9-1.6 Ga
Making
Laurentia
MESABI-TYPE IRON FORMATION
Geologic Marker of a Major
Evolutionary Event
Penokean
Orogeny
1.85 Ga
Thomson Dam
Eveleth
St. Cloud
Penokean Mountains (1.85 -1.75 Ga)
Terrane 4:
The
Midcontinent
Rift
1.1 Ga
An Attempt
at Unmaking
Laurentia
MIDCONTINENT RIFT
A 1.1 Ga attempt at continental
separation
Gabbro
Sandstone
Basalt
Precambrian
Geology of
Minnesota
Direct Evidence:
OUTCROP
Glacial “Overburden”
Indirect Evidence: GEOPHYSICAL DATA
More Direct Evidence: DRILL CORE
Image from Dale Setterholm, MGS
Location of core hole
Drill core
1st vertical derivative of aeromagnetic data
Drill hole locations provided by Minnesota Department
of Natural Resources, Division of Lands and
Minerals, Hibbing 2002
Even in Areas of “Good” Outcrop
Bedrock Information is Sparse
Usefulness of (well located)
Water Well Drilling
Information
to Precambrian Geology
- Depth to bedrock info
- General rock descriptions
- Well chip samples
Helpful Crystalline Rock Descriptions
in Water Well Logs
Color – can be a good indicator of minerals in the rock
Coarseness – can individual mineral be seen with the
naked eye?
Internal Fabrics – layers, bands, laminations
Fragment Shapes - irregular chunks, thick flakes,
thin flakes
Usefulness of Water Well Rock Chips
The
Precambrian
Research
Center
at UMD