Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 241, part 3
Prev Page--Plattsburg Limestone, cont. || Next Page--Discussion
Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 241, part 3
Prev Page--Plattsburg Limestone, cont. ||
Next Page--Discussion
Merriam Limestone Member (ML3)--The argillaceous limestone at the top of unit ML3 was well imaged between stations 30 and 315 (fig. 10), probably because of the large dielectric-constant contrast between it and the overlying lowermost limestone bed in unit ML2. The recessive weathering pattern of this unit suggests that it is predominantly a thin-bedded, shaly limestone that becomes more argillaceous downward (fig. 9). The high clay content of this unit may have caused the relatively low internal reflectivity and greater signal attenuation.
Merriam Limestone Member (ML4)--Unit ML4 is a shaly limestone that is more argillaceous and thinner bedded than unit ML3. Reflections from this unit were imaged only near the surface between stations 320 and 360. As in unit ML3, reflections from within unit ML4 were relatively weak, possibly due to high clay content at bounding surfaces and within the limestone unit, lack of significant internal dielectric-constant contrasts, and thin bedding. Diffractions from within unit ML4 occurred near the surface near station 340 (fig. 10). They may have been due to open fractures seen on the outcrop near the surface, the edges of collapse features, or modern weathering of this relatively easily erodable unit. The diffractions were not likely to have been caused by cobbles or out-of-plane reflections because of the GPR-profile distance from the outcrop face.
Merriam Limestone Member (ML5)--Unit ML5 is primarily a thin-to-medium-bedded limestone similar to unit ML2. It is relatively resistant and is visible in outcrop between stations 270 and 400 (fig. 9). This unit was imaged by GPR between stations 355 and 400 (fig. 10). The top of unit ML5 was a relatively strong reflection that was reduced in strength downdip due to signal attenuation caused by unit ML4. Internal reflections within unit ML5 were also attenuated by unit ML4. As with unit ML2, the high-amplitude reflectivity at bed boundaries was probably due to changes in clay content or compaction. Diffractions occurred along many of the bed boundaries in unit ML5 and were most apparent close to the surface at the top of the outcrop. As with the diffractions in unit ML2, they were probably caused by open or soil-filled joints and other fractures seen on the outcrop; they could possibly have been caused by the edges of modern collapse features seen in exposures at some locations. The base of this unit is an erosional contact between the Merriam Limestone Member and underlying Bonner Springs Shale. The interpretation of a hemi-channel form for the contact was supported by the westward thickening of the Merriam member visible on the outcrop (fig. 9), the GPR data (fig. 10), and measured stratigraphic sections (Enos et al., 1989) (fig. 8).
Bonner Springs Shale (BS1)--GPR successfully imaged the erosional contact (hemi-channel form) between the Bonner Springs silty shale lithology (BS1) and the overlying Merriam Limestone Member (ML5). The contact was visible on both the outcrop and GPR profile between stations 360 and 400, where measured section IX is located (fig. 8). The contact is expressed on the outcrop by low-angle truncation of dipping shale, silty shale, silty limestone, and siltstone layers in the Bonner Springs Shale (BS1) by the overlying Merriam Limestone Member (ML5). The Bonner Springs correlates with the distinct siltstone and shale-filled hemi-channel form below the Merriam member and above channel-filling sandstones and siltstones (measured sections VIII, IX, and X in fig. 8). This hemi-channel form was also supported by a very gradual thickening of unit BS1 downdip (fig. 9). The contact between the Bonner Springs Shale and Merriam Limestone Member is a relatively high amplitude reflection, but no more reflective than those between limestone beds in the overlying ML5 unit (fig. 10). Although the erosional truncation of beds is apparent on the outcrop, study of interference patterns did not show the termination of unit BS1 against the Merriam Limestone Member, probably because it was at too low an angle to image with the 500-MHz GPR antenna.
Bonner Springs Shale (BS2)--The top of unit BS2 is an erosional contact with a hemi-channel form that cuts across flatter-lying beds below (fig. 9). This unit correlates with the lower channel-filling sandstone and siltstone in measured sections VIII, IX, and X (fig. 8), which occur below the siltstone- and shale-filled hemi-channel form. The upper contact is obvious on the outcrop, but it was much less so on the GPR data. The relatively horizontal beds of siltstone, shaly siltstone, sandy siltstone, and sandstone are visible on the outcrop, but they were not imaged by GPR. The poor reflectivity along the top and within unit BS2 was probably due to signal attenuation caused by the siltstones in unit BS2, the silty to shaly layers in unit BS1, or a lack of significant dielectric-constant contrasts within the siltstones of BS2. The events seen at 80-100 ns between stations 390 and 400 were not related to beds within unit BS2, but instead were noise that appeared parallel to the ground surface.
Prev Page--Plattsburg Limestone, cont. || Next Page--Discussion
Kansas Geological Survey
Web version September 15 1998
http://www.kgs.ku.edu/Current/1998/martinez/martinez8.html
Comments to webadmin@kgs.ku.edu