WILLIAM KENNETH HAMBLIN Dept. Geology, the University of Kansas, Lawrence, Kans

WILLIAM KENNETH HAMBLIN Dept. Geology, The University of Kansas, Lawrence, Kans. Paleogeographic Evolution of the Lake Superior Region from Late Keweenawan to Late Cambrian Time Abstract: A combined study of regional stratigra- highland persisted as a positive area, and the shape phy, petrology, and paleocurrents was made of the and extent of the basin remained much the same. A Freda sandstone, Jacobsville sandstone, Bayfield lacustrine environment predominated in the central group, Dresbach formation, and Franconia forma- part of the basin, but much of the Jacobsville sand- tion. Data pertaining to the location and nature stone and Bayfield group undoubtedly represents of the source of the sediments were obtained prima- fan deposits which merged northward into sedi- rily from petrology and directional sedimentary ments of an alluvial plain. During Dresbachian time structures. Environmental reconstructions were the Northern Michigan highland remained as a based on patterns of lithologic variations, kinds of positive area, but shallow seas invaded the Lake sedimentary structures, and heavy minerals. Superior region from the northwest and central This information indicates that the Northern Wisconsin from the south. Most of the Dresbach Michigan highland extended through northern sediments accumulated in a beach environment, Wisconsin and northern Michigan and acted as a but in southern Wisconsin an offshore neritic source of sediment from Late Keweenawan through environment predominated. Prior to Franconian Dresbachian time. The Freda formation accumu- time there was a widespread regression of the seas, lated in the Keweenawan basin, which was north and most of the region was subjected to subaerial of the Northern Michigan highland approximately erosion. By Franconian time the Northern Michi- in the present site of Lake Superior but extended gan highland was reduced to a surface of low relief, considerably farther to the southwest. Deposition and the seas re-advanced across the entire area from took place in a flood-plain and lacustrine environ- the southwest. An appreciable amount of the Fran- ment. Prior to the deposition of the Jacobsville- conia sediments accumulated in an offshore environ- Bayfield sediments, the Keweenawan sequence was ment. deformed and eroded, but the Northern Michigan CONTENTS Introduction 2 4. Diagrammatic section of Upper Cambrian de- Freda formation 2 posits in the Lake Superior region showing Jacobsville sandstone and Bayfield group 6 facies variations and nomenclature of rock Dresbach formation 8 units 9 Franconia formation 11 5. Isopach map and cross-stratification directions Paleogeography 13 of the Dresbach formation 10 Late Keweenawan time 13 6. Variations in sorting, mean grain diameter, and Jacobsville-Bayfield time 14 per cent garnet in the Miner's Castle mem- Dresbachian time 15 ber of the Munising formation 12 Franconian time 15 7. Isopach map and cross-stratification directions References cited 18 of the Franconia formation 13 8. Paleogeographic maps of the Lake Superior Figure region during Late Keweenawan, Jacobs- ville-Bayfield, Dresbachian, and Franconian 1. Geologic map of the Lake Superior region show- time 16, 17 ing area! distribution of formations studied 3 2. Cross-stratification directions in the Freda sand- Table stone 6 1. Heavy minerals in the Freda formation, Jacobs- 3. Cross-stratification directions in the Jacobsville ville-Bayfield sandstones, and Dresbach and sandstone and Bayfield group 8 Franconia formations 4 Geological Society of America Bulletin, v. 72, p. 1-18, 8 figs., January 1961 1 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/1/1/3416832/i0016-7606-72-1-1.pdf by guest on 03 October 2021 \V. K. HAMBLIN-PALEOGEOGRAPHIC EVOLUTION, LAKE SUPERIOR REGION from surface and subsurface data collected by INTRODUCTION the writer and from sections published by Berg One of the most significant changes in the et al. (1956). paleogeography of the Lake Superior region occurred near the beginning of the Paleozoic, FREDA FORMATION when the Keweenawan basin ceased to be the STRATIGRAPHY AND PETROLOGY: The Freda site of accumulation of lava and coarse clastic sandstone constitutes what is generally con- sediments and the entire region became inun- sidered to be the youngest formation of the dated by shallow Paleozoic seas. This change is Keweenawan sequence. It is exposed along the recorded in the sandstones deposited during western coast of the Keweenaw Peninsula and this critical interval of geologic time. They are, can be traced southward into Wisconsin (Fig. in ascending order, the Freda sandstone (Upper 1) almost to the 46th parallel. Sandstones Keweenawan), the Jacobsville sandstone and identical to the Freda in gross lithology, heavy Bayfield group (Cambrian or Precambrian?), minerals, and sedimentary structures are ex- the Dresbach formation (lower St. Croixan), posed intermittently along the east coast of and the Franconia formation (middle St. Lake Superior from Gros Cap to Alona Bay. Croixan). Preliminary work by the writer on These rocks apparently occupy the same strati- the Jacobsville and Munising formations graphic position as the Freda and are therefore (Hamblin, 1958) indicated that although considered to be its equivalent. This correla- many of the stratigraphic problems of the tion is substantiated by paleomagnetic measure- "Lake Superior sandstones" could not easily ments made by P. M. Du Bois (Personal com- be solved, a regional study of stratigraphy, munication), who found that the paleomagnet- petrology, and paleocurrents of these rocks ism of the Freda equivalent of Canada is very would shed much light on the paleogeographic similar to that of the Freda of Michigan and evolution of the region. Thus in 1958 the work Wisconsin but dissimilar to that of the Jacobs- was extended to include the Freda sandstone ville sandstone and Bayfield group. of Michigan and the Franconia sandstone, The Freda formation lies conformably upon Dresbach sandstone, and Bayfield group of older Keweenawan sediments, but the upper Wisconsin and Minnesota. contact is not exposed, so its stratigraphic re- This study was supported by a research grant lationships with the overlying Bayfield group from the University of Kansas general research are still a matter of conjecture. Outcrops of the fund. The writer gratefully acknowledges the Freda are too scattered to permit accurate assistance extended by members of the Michi- measurements of its maximum thickness; how- gan Geological Survey in supplying a boat and ever, semicontinuous exposures along the access to unpublished data and expresses his Montreal River on the Michigan-Wisconsin appreciation to Mr. Harry Sorensen and Mr. boundary and along the coast south of the Por- Charles Bondurant for assistance in the field. cupine Mountains indicate that the Freda Dr. Louis I. Briggs and Dr. Ralph H. King could be more than 14,000 feet thick. In both kindly read the manuscript and suggested vari- areas detailed examination of the section re- ous improvements. veals a gradual change in grain size, type of METHODS: The writer examined essentially bedding, and sedimentary structures from bot- every outcrop of sandstone on the southern tom to top, indicating that there is probably coast of Lake Superior and selected exposures no important repetition due to faulting. inland in Michigan, Wisconsin, and Minnesota The Freda consists principally of alternating during this study. Cross-stratification directions layers of fine arkosic sandstone and red mica- were measured at each locality, and samples ceous silty shale. The texture in the lowest were collected for petrographic and heavy- 1500 feet of the section is generally coarser, and mineral analysis. Except in the Dresbach for- a conglomerate 15 to 150 feet thick is found in mation, all cross-stratification measurements many places several hundred feet above the were made on a plan view of the structure, and base of the formation. Pebbles from the con- at least three measurements were made on two glomerate average 3 inches in diameter, but stratigraphic horizons at each outcrop. Petro- several boulders exceed 1 foot in their longest graphic and heavy-mineral studies were made dimension. Most are composed of basalt and on selected samples in order to supplement quartzite, although an appreciable number published data. Isopach maps were constructed were derived from the Huronian Iron forma- for the Dresbach and Franconia formations tion. The upper part of the Freda is character- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/1/1/3416832/i0016-7606-72-1-1.pdf by guest on 03 October 2021 FREDA FORMATION EZJ St. Croixon Series Jacobsville - Bayneld Sandstones MINNESOTA IOWA Pre - Freda Rocks Source of data: Geologic map of the United States, (932; Michigan Geological Survey Publication 51, 1958 , Field work by the writer, 1958 - 1959. FIGURE \.—GEOLOGIC MAP OF THE LAKE SUPERIOR REGION SHOWING AREAL DISTRIBUTION OF FORMATIONS STUDIED ized by a monotonous sequence of alternating lens of shale-pebble conglomerate can be traced beds of siltstone and shale 3 to 4 feet thick. laterally to the truncated edge of the shale bed Shale-pebble conglomerates are common at from which it was derived. This is especially many horizons throughout the Freda forma- true of the larger blocks, which were obviously tion but are especially abundant in the lower transported only a few feet. part of the section. The individual pebbles Although quantitatively

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