RI-21 Lower and Middle Precambrian Stratigraphic Nomenclature for East-Central Minnesota

Abstract

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 Precambrian age. 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.