This report summarizes stratigraphic nomenclature used by the
Minnesota Geological Survey for the Lower and Middle Precambrian
rocks that occur in east-central Minnesota. In addition, it provides
brief descriptions of dike rocks of possible Middle and Upper
A sequence of Lower Precambrian rocks consisting of an older
gneiss terrane and a younger greenstone-granite terrane has been
established on the basis of reconnaissance mapping in the area.
The gneiss terrane is divided into three lithostratigraphic units
of uncertain relative age. These units are the Richmond Gneiss,
the McGrath Gneiss, and the Sauk Rapids Metamorphic Complex. The
Sauk Rapids Metamorphic Complex is further subdivided into three
units of formational status: the Sartell Gneiss, a broadly distributed
quartzofeldspathic unit, and the Watab Amphibolite and St.
Wendel Metagabbro, mappable mafic enclaves within the Sartell
Gneiss. The younger greenstone-granite terrane is poorly exposed,
and these rocks are not described in this report.
The Middle Precambrian consists of both stratified and plutonic
rocks. The stratified rocks lie unconformably on the Lower
Precambrian rocks and on the Hillman Migmatite, a hybrid rock of
uncertain age, and are divided into two groups. From oldest to
youngest these are: (1) Mille Lacs Group, comprising the Denham
Formation, Glen Township Formation, Randall Formation, Little
Falls Formation, and Trout Lake Formation; and (2) the Animikie
Group, comprising the Mahnomen Formation, Trommald Formation,
Rabbit Lake Formation, and the Thomson Formation. An iron-rich
uni t named the Emily Member is also formally recognized in the
Rabbit Lake Formation.
The stratified rocks were deformed and metamorphosed during
the Penokean orogeny, and were subsequently intruded by igneous
rocks of gabbroic to granitic composition. The igneous activity
was characterized by the emplacement of several plutons of widely
varying size. Small dikes or sills of gabbroic to dioritic composition
are inferred to be the oldest Middle Precambrian plutonic
rocks. Probably this period of mafic activity was followed by the
emplacement of several small stocks of granodioritic composition
--the Freedhem and Bradbury Creek Granodiorites of this report-and
later by the emplacement of several large granitic plutons of
generally sodic composition--the Reformatory, Isle, Warman and
Pierz Granites of this report. Plutonic igneous activity culminated
with the emplacement of the Stearns Granitic Complex. This
dominantly granitic unit is subdivided into the St. Cloud Granite
and a border phase named the Rockville Granite. The former is
further subdivided into several unnamed facies. The earlier
gabbroic to dioritic and granodioritic rocks were emplaced during
the waning stages of the Penokean orogeny, whereas the granitic
rocks generally are post-tectonic in age.
The Lower Precambrian rocks and the Middle Precambrian felsic
plutonic rocks are cut by both felsic and mafic rocks of unknown
age, whereas the Middle Precambrian stratified rocks are cut by
several kinds of mafic dikes emplaced during Early and Middle
Keweenawan (Upper Precambrian) time.
RI-21 Lower and Middle Precambrian Stratigraphic Nomenclature for East-Central Minnesota.
Minnesota Geological Survey.
Retrieved from the University of Minnesota Digital Conservancy,