Geologic units in Meeker county, Minnesota

Little Falls Formation (Paleoproterozoic) at surface, covers 35 % of this area

Schist and slate of graywacke-mudstone protolith.

Middle and Upper Cambrian (Middle and Upper Cambrian) at surface, covers 22 % of this area

Sandstone, siltstone, and shale; includes the Wonewoc Sandstone, Eau Claire Formation, and Mt. Simon Sandstone.

Rockville Granite and rocks inferred to be related (Paleoproterozoic) at surface, covers 15 % of this area

Rockville Granite and rocks inferred to be related. Coarse-grained and pink to white (~1,780 Ma).

Undifferentiated (Cenomanian to Campanian) at surface, covers 9 % of this area

Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.

Gray granodiorite to granite intrusions (Paleoproterozoic) at surface, covers 4 % of this area

Gray granodiorite to granite intrusions. Includes the Reformatory (~1,783 Ma), Freedhem (~1,775, 1,776 Ma), Isle (~1,779 Ma), and Warman (~1,787 Ma) intrusions.

Claystone, siltstone, and sandstone (Albian) at surface, covers 4 % of this area

Claystone, siltstone, and sandstone. Albian age is tentatively inferred from a single analysis of pollen.

Granite (Paleoproterozoic) at surface, covers 4 % of this area

Granite, reddish, variably porphyritic, massive; includes the St. Cloud Red (~1,779 Ma), Foley (~1,774, 1,779 Ma), Pierz (~1,779 Ma), and Pease granites, and Richmond charnockitic granite (~1,772 Ma). An unnamed granite in south-central Minnesota is slightly older at ~1,792 Ma.

Granitic rocks, undifferentiated (Paleoproterozoic) at surface, covers 3 % of this area

Granitic rocks, undifferentiated. Largely inferred from geophysical maps.

Mafic intrusions (gabbro, diabase) of unknown age (Paleoproterozioc or Mesoproterozoic) at surface, covers 2 % of this area

Mafic intrusions (gabbro, diabase) of unknown age. Includes the LL, BKV, and Lake Washington intrusions in Meeker County.

Granitic to granodioritic orthogneiss (Neoarchean) at surface, covers 1 % of this area

Granitic to granodioritic orthogneiss. Includes the McGrath Gneiss (~2,752 ± 15, 2,557 Ma) and unnamed units.

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers 0.3 % of this area

Granitic orthogneiss and migmatite. Geophysical map patterns imply this unit intruded other gneisses.

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre (Paleoproterozoic) at surface, covers 0.1 % of this area

Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre. Defined largely by magnetic signature. One intrusion in Morrison County is ~1,791 Ma, and lithologically similar intrusions cut the Foley Granite (~1,774 Ma).

Interlayered volcanic, volcaniclastic, sedimentary, and hypabyssal intrusive rocks (Paleoproterozoic) at surface, covers < 0.1 % of this area

Interlayered volcanic, volcaniclastic, sedimentary, and hypabyssal intrusive rocks. Includes parts of the Mille Lacs Group, North and South Range Groups, and Glen Township Formation.

Gabbro, pyroxenite, diorite, and lamprophyre (Paleoproterozoic) at surface, covers < 0.1 % of this area

Gabbro, pyroxenite, diorite, and lamprophyre. Includes the Watab quartz diorite (~1,780 Ma), St. Wendell quartz gabbro, and an unnamed diorite (~1,786 Ma).