NORTH DAKOTA STATE UNIVERSITY - FARGO, N D

GEOLOGY UNDER THE
FARGO-MOORHEAD
REGION
NORTH DAKOTA - MINNESOTA

Provided in public service by the:

Department of Geosciences
North Dakota State University
Fargo, North Dakota 58108-6050
U.S.A.

Geologic Cross-Section of
Fargo-Moorhead
(Click on image
for enlargement).

Underlying the flat land surface of the Fargo-Moorhead region is a relatively simple stratigraphy. Drilling to a depth of 200 to 300 feet (65 to 90 m), one would encounter PreCambrian granitic and gneissic basement rock of the Superior Province -- generally exceeding 2.5 billion years in age. We know little of the nature or structure of this bedrock in North Dakota, for there are no exposures of it within the state and only a few boreholes have penetrated to this depth. However, to the northeast in Manitoba and Ontario, rocks of the Superior Province have considerable surface exposure.

Overlying the PreCambrian basement rock is 100 to 200 feet (30 to 60 m) of glacial sediments, mostly deposited during the last (Wisconsinan) ice age. These materials are predominantly till, but localized zones of outwash sands and gravels can provide small aquifers of low yield. These glacial sediments have high bearing strengths and are capable of supporting high-load engineered structures, such as bridges, high-rise buildings, and water towers. Thus, piles and concrete piers (caissons) supporting structures such as the FargoDome and the cities' highrise buildings rest rest on top of these glacial sediments.

Forming the remainder of the materials are sediments deposited into Lake Agassiz. Approximately 85 ft (26 m) of the gray, slickensided, fat clays of the Brenna/Argusville Formations are overlain by 20 ft. (6 m) of the tan-buff, laminated silty-clays of the Sherack Formation. We seldom can view the contact between the two formations, although it is occasionally visible along the channel of the Red River during times of low water flow. Within both formations are occasional cobbles and boulders that appear to represent dropstones: rock debris that fell off icebergs floating in Lake Agassiz.

These Lake Agassiz sediments induce challenging geotechnical conditions. At the site of the Fargo Water Treatment Plant (1989-91 data, Midwest Testing Laboratory), typical soil engineering values for the Sherack Fm (depth: 1 - 6 m) are:

  • PL (plastic limit) = 30
  • LL (liquid limit) = 85
  • N (number of blows for Standard Penetration Test) = 12
  • Qu (unconfined compressive strength in lbs/ft2) = 3000.
For the uppermost Brenna Fm (depth: 8 m), typical values are:
  • PL = 31
  • LL = 113
  • N = 6
  • Qu = 1370
Where the Sherack and Brenna formations are unconfined, their high plasticity leads to slope instability. Because the channel of the Red River and many of its tributaries incise across the Sherack/Brenna contact, the weak characteristics of both formations lead to extensive mass wasting problems. Examples are prevalent along the valley walls and channel margins of the Red River and its tributaries throughout the Red River Valley.

Donald P. Schwert

Related Sites:

  • A City Built on "Stilts"   Deep concrete piers and steel pilings are required to keep much of Fargo from sinking into the weak sediments beneath.

  • Toils Induced by Weak Soils: the Stockwood Fill   In 1906, the Northern Pacific Railroad began construction of a railroad grade east of Fargo-Moorhead. Almost immediately, the grade began to sink under its own weight . . .

[ More Brief Insights on Fargo's Geology ]

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Copyright © Donald P. Schwert, North Dakota State University.