A shallow seismic reflection program at Mud Mountain Dam was successful in defining and delineating stratigraphic and structural features within the Pleistocene sequence that overlies igneous bedrock. An excellent match between the seismic profiles, drill data, and outcrop studies allows confidence in interpreting orientation and localized variability in sedimentation. The technique possesses the potential to resolve beds as thin as 5 m at this site. The goal was to extrapolate/extend sediments and contacts identified in sidehill outcrop into the embayment to help establish a better understanding of horizontal continuity/variability.
The survey was designed to allow 3-D extrapolation of features identified in outcrop and projected from borehole data. The seismic reflection data were acquired on a 96-channel Geometrics StrataView seismograph with an IVI MiniVib and 8-gauge auger gun, and 40 Hz geophones deployed at 2.5-m station intervals.
The 150-m deep igneous bedrock reflector possesses almost 40Emsec or 30 m of variation in elevation within a lateral distance of 500 m. The Mud Mountain Complex (MMC) directly overlies bedrock in this area. A 125-m deep reflector from within the MMC is probably the contact between a layer of fine grain clay particles and gravels/cobbles. A coherent event about 140 m deep and still within the MMC is probably a clayey sand/gravel contact. The MMC, Vashon Lake Bed (VLB) sequences, and Hayden Creek Till contacts are acoustically distinct. A relatively large, well-defined channel within the VLB consistent with boreholes and outcrops could provide a conduit for fluid movement within the embayment.
Kansas Geological Survey
Updated April 17, 1997
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