Characterizing sand and gravel deposits in the Midwestern US has been hampered by difficulties in accessing high, unstable outcrops, describing sediments, and measuring elevations by conventional techniques. Based upon geodetic GPS for ground control and reflectorless total station (RTS) for detailed measurement, 6 profiles in complex sediment sequences along a 3.5 kilometer discontinuous section were described and surveyed without the use of prisms at measurement points. Analysis of the complex outwash sequence depended on 1) detailed descriptions using a lithofacies code based on grain size, sorting, and sedimentary structure related to flow-regime bedforms, 2) measurement of bed thickness, and 3) elevation and location of outcrops described along the traverse. Using RTS, detailed descriptions of exposures up to 29 meters high could be made at a safe distance providing bed and unit contact elevations surveyed to a precision of <1 cm. Description of the sand and gravel of the west-sloping Spring Grove kame terrace in northern McHenry County, IL shows an ice-frontal delta that extended into a temporary, ice-dammed lake bounded to the west by late Wisconsin Haeger till. The sequence differs from classic deltas in that 1-11 m thick, planar and massive pebble gravel and pebbly sand foreset beds taper rapidly over less than 400 m from 11-1 m westward away the Haeger ice front. Foreset thicknesses decrease primarily because the beds were deposited over a progressively thicker bottomset sequence aggrading in the lake basin. Bottomsets, the finest sediments in the sequence, consist of well sorted planar-bedded and ripple-drift cross-laminated sands. Melting of the stagnant ice released confinement of the sediments to form a 0.5 km landslide as the present western valley wall of the Fox River. Economically desirable topset beds unconformably overlie the foreset beds and are significantly coarser than the other deltaic facies. The poorly sorted, planar-bedded cobble gravel topset beds are about 11 m thick near the former ice margin thinning to 5 m at the west end of the transect. Because the RTS georeferences each contact of a profile, the data can be used in 3-D models of the kame terrace as excavations continue. The RTS can also be used wherever descriptions of geologic materials in outcrop are needed for 3D geologic maps and models.
|Original language||English (US)|
|Title of host publication||Abstracts with Programs - Geological Society of America|
|Publisher||Geological Society of America (GSA), Boulder, CO, United States (USA)|
|State||Published - 2008|