STRATIGRAPHY OF A RIMMED RIDGE PLATEAU, STURGIS MORAINE, NORTHEASTERN INDIANA

A geophysical survey of a small rimmed ridge plateau revealed a detailed stratigraphy of diamicton and boulder gravel. This circular plateau is part of the Sturgis moraine, a recessional moraine of the Saginaw Lobe. The moraine has a hummocky topography with meltwater channels and many features similar to this plateau. Large tunnel channels also extend past its margin, and thick outwash fans slope off its distal side. In October 2015, one Schlumberger sounding and three dipole-dipole profiles were collected in conjunction with auger borings. This survey targeted the northern ridge and central depression of the plateau. A 13-meter deep sounding from the central depression was collected using a AGI SuperSting R1 IP Earth Resistivity Meter. Dipole-dipole profiles were collected with a 28-channel automatic switch box and cable. Each roll-along section used an electrode spacing of 2.00 meters. Auger borings were taken along dipole profiles, and at the location of the sounding. The stratigraphy consisted of two distinct units of sediments. For most of the area surveyed, a lower, highly resistive, discontinuous boulder gravel unit was observed between 4-12 meters depth. Overlaying the gravel is a less resistive silt (10YR/2/2) with minor clay and sand unit. Both these units extend laterally until the central depression where the two units are inverted. Two possible interpretations are proposed, the first assuming a constructional landform, the second an erosional remnant landform. First, these sediments could be the remains of an ice-marginal supraglacial lake or moulin, in which melt water cut down through the ice and deposited the boulder gravel with ice blocks. Next, finer sediments were deposited before ice melted and water drained. This interpretation would explain the roughly circular landform and fining-up stratigraphy. A second interpretation is that both units are part of a pre-existing stratigraphy, and glacier and/or subglacial water flow eroded the deposits into the landform present today. Channels adjacent to the feature, and evidence of water erosion on its northeast margin support this mechanism. Further geophysical surveying is required to determine if the lateral stratigraphy continues south of the depression and if similar stratigraphy can be found elsewhere to test these interpretations.