Stratigraphic and sedimentologic investigation of collapse features in the Mount Baldy Dune in the Indiana Dunes National Park
Introduction
The Mount Baldy area in the eastern part of the Indiana Dunes National Park (INDU) contains the largest active eolian dune by area along the southern shore of Lake Michigan (Fig. 1). Part of the formally named Middle Holocene to Recent Tolleston Beach, the dune is a popular attraction for many visitors to the park. The ongoing landward migration of the dune is related to the loss of vegetation on the lakeward, or stoss, slope caused by heavy human foot traffic, ongoing shoreline erosion, and strong seasonal winds. Vegetation loss became so severe that the landform was declared “impaired” by the National Park Service (NPS) in 2011, requiring active restoration efforts that included restricting visitor foot traffic to designated paths. Then in July 2013, an accident occurred where a 6-year-old boy fell into a narrow hole that opened midway up the dune’s stoss slope. The hole immediately infilled, and the child was buried alive. After more than three hours of excavation, the boy was located and rescued from a depth of more than 3 m below the surface. Park managers restricted access to the dune. While visitors have provided anecdotal reports of previous holes at Mount Baldy through the years, most dismissed them as holes dug by animals, and none were officially reported or studied. A study that immediately followed the accident by Argyilan et al. (2015) showed photographic documentation of holes and buried trees found in the months following the accident and provided a conceptual model for their formation through the ongoing decay of trees buried by the dune’s migration.
Argyilan et al. (2015) proposed that the collapse features, or decomposition chimneys, occur when heavily decayed trees that remain rooted atop the arm of a buried relict parabolic dune are exposed at the surface by the ongoing migration of the overall dune landform. The lack of information concerning the occurrence of collapse features in dunes and the mechanisms for their development highlighted the need for a sedimentological study of the Mount Baldy dune to examine the subsurface architecture of eolian and associated coastal deposits that comprise or underlie the modern dune feature. Of particular importance is the internal architecture of these sediments and the behavior of the dune through time.
This study uses ground penetrating radar (GPR), direct-push sediment cores, and natural exposures to collect sedimentary deposits within the Mount Baldy landform that define the stratigraphy and age of the dune and underlying deposits. We summarize field data collected in the late summer of 2014 and provide cross-sections and block diagrams that illustrate the internal structure of the dune. Conclusions about how and why voids and collapse features form within dunes, their spatial distribution, and the general extent of potential geological hazard areas related to such collapse features are discussed and mapped.
Section snippets
Ground penetrating radar
Approximately 11 km of GPR transect data were collected across the dune before and in conjunction with the collection of sediment cores (Fig. 2). Multispectral GPR profiles were obtained using two Sensors & Software cart-based systems. The highest frequency GPR profiles were collected in common-offset mode every 5 cm using a Noggin 250-MHz SmartCart system triggered by an odometer wheel, with a stack of 16 measurements at each point. The GPR antennas were parallel to each other, separated by
Ground penetrating radar surfaces and packages
Because multiple frequency data were collected along the same transects, cross-sections of the GPR data show similar main reflections, although the detail observed in each cross-section varies with the antenna frequency and position along with the length of the transect on the dune. Using the GPR surface, package, and facies approach of Neal (2004), we recognize three packages separated by two prominent surfaces (Fig. 4). This section will describe the packages and surfaces that will be
Conclusions
The research undertaken in the Mount Baldy area examined the sedimentology and internal architecture of the existing dune landscape and provides the background for assessing the significance of geohazards represented by dune collapse features (i.e., “dune decomposition chimneys;” Argyilan et al., 2015). Using GPR and sediment cores, three depositional facies were defined with distinct sedimentological characteristics and GPR signatures. A paleosol forming atop the lower dune facies defines a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project represented a joint effort by staff at the Indiana Geological and Water Survey, faculty and students from Indiana University Northwest, members from the National Park Service at the Indiana Dunes National Park, and staff of the Midwest Archaeological Center, National Park Service. The authors are grateful for the assistance of students and staff from these institutions. We are also grateful for financial support by the National Park Service through the Cooperative Ecosystem Unit
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