Geology of the Alston district Houghton and Baraga Counties, Michigan

Abstract

The copper-bearing sediments and flows of Keweena age which underlie the northwest side of Keweenaw Point form a conformable series which to the southeast has been thrust over the Cambrian Jacobsville sandstone. The stratigraphic correlation of the Freda sandstone, which lies at the top of the copper-bearing series, to the Jacobsvilie sandstone is uncertain, as are also the definite dating of the periods of deformation and the age of the copper mineralization. Field work was conducted to the southeast of the main copper-bearing area with the hope of establishing: (1) the date of the native copper mineralization, (2) the relations of the Freda and Jacobsville sandstones, and (3) the dates of deformation. The existence of a pre-Jacobsville period of mineralization was established, but its correlation with the native copper mineralization to the northwest cannot be definitely established. The Jacobsville sandstone rests unconformably upon Keweenawan flows; in an adjacent area, the Freda sandstone rests conformably on flows of the same character. Hence the Jacobsville sandstone is thought to be later than and unconformable to the Freda sandstone. At one locality the Keweenann flows are tilted and sheared, while the overlying Jacobsville sandstone is not affected, thus proving the existence of a pre-Jacobsville but post-Keweenawan deformation. Elsewhere, the marked disturbances of Lower Paleozoic strata prove another period of deformation which is definitely later than the Silurian, and probably later than the Devonian.