Surficial geology of the Iron Mountain 7.5 minute quadrangle, Dickinson County, Michigan, Florence Marinette counties, Wisconsin

The Iron Mountain 7.5 minute quadrangle lies within complex glacial deposits of the Green Bay Lobe of the Laurentide Ice Sheet. In 2017 the Michigan Geological Survey mapped the quad as part of a USGS STATEMAP project. Surficial mapping was greatly aided by the availability of LiDAR elevation data. This mapping has provided new, detailed information on the surficial landforms and deposits as well as relationships between the glacial deposits and the underlying bedrock. The complexity of the glacial deposits is in part due to high relief on the bedrock surface and complexity of underlying bedrock formations and structure. A new OSL age date was obtained for an outwash deposit. Three ice-margins were mapped across the quad. A deep bedrock trough mapped as part of an earlier environmental investigation was further defined as well as the bedrock topography and drift thickness mapped across the quad. In addition, the mapping identified ice-walled lake plains, eskers, drumlins and terraces that were not previously mapped. A new OSL age date was obtained for an outwash deposit. The sediments include diamicton (till), sand and gravel, boulders and interbedded silt and clay. The glacial deposits are late Wisconsinan (about 14,500 cal yr BP to 12,500 cal yr BP) in age. Over much of its path the Green Bay Lobe flow was southwesterly as evidenced by the prominent southwesterly oriented drumlins in the Menominee drumlin field in Menominee and eastern Dickinson County to the east. Starting around 13 miles to the east of the map, the Green Bay Lobe ice made a 90-degree change in direction to the WNW as it encountered the high WNW-trending resistant bedrock ridges of the Menominee Iron Range. Within the Iron Mountain Quad, this west-northwesterly ice flow direction is reflected in the numerous drumlins on the uplands and streamlined bedrock hills. Rock drumlins occur within the quad and others are exposed in the bottom of some sand and gravel pits. The elevation across the map ranges from 919 feet above mean sea level (AMSL) along the Menominee River to 1572 feet AMSL at the top of Millie Hill. Three distinct bedrock-controlled uplands occur north of the Menominee River: Pine Mountain, Millie Hill and Trader Hill. These uplands are mostly cored by diamicton and strewn with boulders. These boulders extend to depth into the subsurface based on the local water wells logs and the three borings drilled for this project. The diamicton is mostly reddish brown to brown. Although the larger foundation for these uplands is bedrock controlled, drift up to 140 feet thick occurs in places. The broadly N-S trending Green Bay Lobe ice margins are well defined further south in Wisconsin, where they are easily visible on topographic maps. As the ice margins extend north into Michigan they become less well defined, more discontinuous and harder to correlate. Three ice margins are interpreted within the map. From west to east they are the Winegar-Sagola-Early Athelstane Moraine (about 14,500 cal yr BP), the Middle Athelstane ice margin and the Marenisco-Late Athelstane ice margin (about 13,000 cal yr BP). Under the City of Kingsford lies an extensive pitted outwash plain with elevations ranging from 1140 to 1120 feet AMSL formed west and south of the Middle Athelstane ice margin. An OSL age 12,600 ± 1,000 cal yr BP was obtained from a sample taken from the edge of this outwash plain. This outwash plain overlies a lacustrine sequence of mostly silts, clays, sands and some gravels that is as much as 300 feet thick. This lacustrine sequence overlies a deep bedrock trough under the City of Kingsford. A later, short term fluvial event likely occurred over this outwash surface, as evidenced by the sharp, steep, wave cut or fluvially cut scarp at 1140 feet AMSL along the northern side of this surface. A surficial gravel and cobble lag was noted in places on this surface. As the Green Bay Lobe ice retreated to the east, there must have been successive lowering of the outwash outlets to the south and southeast because of the step-like lowering of the outwash terraces to east along the Menominee River. Meltwater was first diverted south to an outwash channel along the ice front through a narrow gap in the bedrock, to an outwash channel south into Wisconsin at an elevation of 1100 feet AMSL. A lower terrace occurs at 1075 feet AMSL. Later, further eastward ice retreat produced another series of outwash terraces descending in a step like fashion (1045, 1035, 1020, 997, and 960 feet AMSL) farther to the southeast. The lowest of these terraces is 180 feet lower than the large outwash plain to the west. Thin loess deposits have been identified to the north, west and southwest of the map but none were encountered in the field. Two eskers were mapped in the southwest part of the map based on LiDAR elevation data. Several ice-walled lake plains were mapped in the uplands just east of the Marenisco-Late Athelstane ice margin, on the east sides of Millie Hill and Trader Hill. A passive seismic instrument using the Horizontal-Vertical Spectral Ratio (HVSR) method was used to gather additional bedrock control for data on bedrock topography and drift thickness. This technique uses the horizontal-to-vertical spectral ratio method to record ambient seismic noise with 3-component geophones. HVSR calibration readings were gathered at 15 wells and borings of known bedrock depth. These data were used to develop a local HVSR bedrock depth calibration curve. Exploration readings were taken at 44 locations within the map. Very good bedrock depth estimates were made in the outwash areas of the map. In the upland morainal area, however, the method yielded depth estimates that were much too shallow relative to the local bedrock elevation of the area. This disconnect is likely due to buried, over-consolidated dense glacial till which was encountered at depth in the three borings drilled for this project. The HVSR bedrock depth estimates at these three borings match well with the depths to the top of the dense till. A significant gamma-ray log kick was also seen in the borings at or near the top of this dense till. Although the glacial deposits in the Iron Mountain Quadrangle average 40 feet thick, numerous bedrock outcrops exist. The drift is maximally 363 feet over the deep bedrock trough in Kingsford. In many places, the land surface topography is controlled not by the glacial deposits, but by the underlying bedrock and bedrock structure. One important exception is a pronounced buried deep bedrock trough that underlies the large pitted outwash plain in Kingsford. Another buried bedrock trough underlies the lowland along the Menominee River in the southeastern part of the map. A poorly defined bedrock low connects the two troughs north of the Menominee River. There is high relief on the bedrock surface ranging from 730 feet to 1530 feet AMSL across the map. Bedrock outcrops and mounds appear throughout the area, even where nearby borings show over 100 feet to bedrock. The bedrock geology exposed at the surface and underlying the glacial deposits in the Iron Mountain Quadrangle is very complex and has had a significant and controlling effect on the overlying glacial deposits. The Iron Mountain Map lies over a major Precambrian terrain boundary named the Niagara fault (suture zone), which runs WNW-ESE across the southern part of the map. This suture zone is part of the significant continental collision boundary of a mountain building event called the Penokean Orogeny about 1.8 billion years ago. North of the fault are mostly complexly faulted and folded Precambrian metasedimentary rocks. The Menominee Iron Range is part of this metasedimentary package and has resulted in the significant historical iron mining district centered around the city of Iron Mountain, specifically along the uplands of Pine Mountain, Millie Hill and Trader Hill; these hills are the result of the complex faulting and folding. South of the Niagara fault are mostly younger metavolcanics and granitic intrusions. Much later, Cambrian sands were deposited, leading to Cambrian Sandstone units. Today, these sandstone outliers appear to be mostly preserved on or near the uplands, and are much more extensive than previous geological maps have indicated. Past local quarrying of the sandstone has produced the distinctive building stone that is common in Iron Mountain. Numerous bedrock outcrops occur across the map. Most of these are due to resistant Precambrian and Cambrian units, especially the Badwater Greenstone and the Hoskins Lake Granite, which tend to form bedrock highlands and outcrops belts. The Michigamme Slate subcrop and outcrop tends to result in bedrock lows. The Menominee River (which forms the border between Michigan and Wisconsin) in places flows over broad lowlands, as in the southeastern part of the map. In other places, it has carved through thick outwash sequences or more resistant bedrock, forming rapids and waterfalls. At Horserace Rapids, the river is cut through a narrow bedrock gorge. Prior to industrial development, there were numerous rapids and several waterfalls along the Menominee River. Dams were later placed at some of these waterfalls. Significant sand and gravel mining operations occur within the map. Most of the gravel pits are associated with pitted outwash plains and outwash terraces. Significant additional sand and gravel deposits may potentially occur within in these deposits and along the newly mapped eskers