LOCATION: 10 miles South of Duluth
AGE: Middle Precambrian
GENERAL COMPOSITION: Slate, Siltstone, Graywacke
The Thompson Dam is only one exposed area of the Thompson Formation. The Thompson formation is composed of Slate, Siltstone, and Graywacke. It is exposed in parts of Carlton, Pine, and St. Louis Counties of East-Central Minnesota. The Formation was folded and metamorphosed about 1.7 billion years ago. Most of the beds are less than one foot thick. Outcrops can rise forty feet above the surroundings. The Thompson Dam area of the Thompson Formation has a low metamorphic grade (greenshist facies) and the textures of the rocks are well preserved.
Most of the Graywacke beds are fine to medium grained, but
some are course grained. Graywacke makes up 1/3-1/2 of the total
thickness of the deposits measured.
Siltstones compose 25-43% of the measured sections. There is more
quartz in these Siltstones than there is in Slate. As a result,
the Siltsones are more massive and have a lighter color.
Slates compose 23-31% of the measured sections. These Slates are
darker in color than the Graywackes and Siltstones.
Carbonate concretions are abundant and characterize the entire
Thompson Formation. These concretions range in length from less
than one inch to three feet long. There are two kinds of Carbonate
concretions:
1) They can be in graywacke beds. The concretions vary the amount
of carbonate and have bedding that is generally parrallel with
the surrounding rock.
2) They can be in Siltstones and Slate. The concretions have two
different parts - and inner core of slaty material, and an outer
rim of recrystallized quartz and carbonate.
Cross bedding is also common in Siltstone beds.