Geologic units in Sibley county, Minnesota

Middle and Upper Cambrian (Middle and Upper Cambrian) at surface, covers 27 % of this area

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

Upper Cambrian (Upper Cambrian) at surface, covers 20 % of this area

Sandstone, siltstone, shale, and dolostone; includes the Jordan Sandstone and St. Lawrence Formation, and the Mazomanie, Lone Rock, and Davis Formations of the Tunnel City Group.

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers 19 % of this area

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

Granitoid gneiss with dioritic to amphibolitic enclaves (Mesoarchean to Paleoarchean) at surface, covers 12 % of this area

Granitoid gneiss with dioritic to amphibolitic enclaves. Produces moderately high and varied gravity and magnetic signatures.

Claystone, siltstone, and sandstone (Albian) at surface, covers 8 % of this area

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

Sandstone, siltstone, and local conglomerate (Mesoproterozoic) at surface, covers 4 % of this area

Sandstone, siltstone, and local conglomerate. Includes the Hinckley Sandstone and Fond du Lac (youngest detrital zircons ~1,000 Ma) and Solar Church Formations; deposition in eolian, fluvial, and lacustrine environments.

Lower Ordovician (Lower Ordovician) at surface, covers 3 % of this area

Dolostone, sandy to silty dolostone, and sandstone; includes the Shakopee Formation and Oneota Dolomite of the Prairie du Chien Group.

Undifferentiated (Cenomanian to Campanian) at surface, covers 3 % 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.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers 0.9 % of this area

Foliated to gneissic tonalite, granodiorite, and diorite. Includes the Lookout Mountain tonalite (~2,718 Ma) of the Giants Range batholith and other intrusions within batholithic complexes defined by a low-to-moderate gravity signature with magnetic foliation apparent from aeromagnetic maps.

Granitic intrusion (Neoarchean) at surface, covers 0.7 % of this area

Granitic intrusion. Includes the Sacred Heart (~2592, 2,603 Ma) and Ortonville granites, the Shannon Lake Granite (~2,674), and other intrusions having low gravity and magnetic signatures.

North Branch mafic volcanic sequence (Mesoproterozoic) at surface, covers 0.6 % of this area

North Branch mafic volcanic sequence.

Granite (Paleoproterozoic) at surface, covers 0.2 % 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.

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).