Addendum to Report High Resolution Seismic Reflection Survey at Mud Mountain Dam near Enumclaw, Washington
Richard D. Miller
Joe M. Anderson
David R. Laflen
Brett C. Bennett
Choon B. Park

The shallow seismic reflection program at Mud Mountain Dam focuses on defining and delineating stratigraphic and structural features within the Pleistocene sequence that overlies igneous bedrock. The portion of the project contained in the first report consisted of a series of acquisition/feasibility tests followed by two CDP profiles along gravel logging roads. The goal of the first CDP profiles was to extrapolate/extend sediments identified in sidehill outcrop into the embayment to establish some understanding of horizontal continuity. The second part of the seismic program (the portion reported here) included a series of source tests and a 300+ shotpoint CDP profile along the main access road to the Mud Mountain Dam overlook. This second survey line was designed to allow extrapolation and enhancement of interpretations from the first set of survey lines immediately north of the main dam road. Improvements in acquisition and processing techniques and equipment on the second trip allowed a much improved image of the subsurface and confidence in interpretation of the entire sedimentary sequence beneath the main access road. Interpretations of the CDP profile collected during March of 1996 are consistent with drill holes 38, 23, 24, and 34. The bedrock reflection possesses almost 40 msec or about 100 ft of variation across this profile. An apparent bedrock high is interpreted beneath and slightly east of the logging road bridge. The configuration and topography of the bedrock surface suggests borehole 23 must have a TD very near the bedrock surface. The bedrock is deepest at the west end of the line beyond borehole 34. The Mud Mountain Complex (MMC) is stratigraphically the next higher unit. Several reflection events can be interpreted within this interval that are consistent with logs from on-line boreholes which show discrete changes in material. Reflections from the MMC and bedrock surface can be interpreted with a high degree of confidence and are both very consistent with logs. The division between the MMC and Vashon Lake Bed (VLB) sequences and Hayden Creek Till (HCT) is acoustically distinct and can be confidently interpreted 2 everywhere except beneath the bridge where the fold drops and the lithology appears to change dramatically. It is difficult to determine with confidence whether the MMC possesses a relative elevation high near the center of the line due to missing data, hence two possible interpretations are provided. The interpretation suggesting a local thickening of the MMC near the center of the line is consistent with off-line borehole data. Above the MMC there appears to be a very complex set of erosional and depositional sequences that are made up of VLB, HCT, and Vashon Outwash (VO). The most likely interpretation of this sequence puts a complete relatively undisturbed section of bedrock, MMC, VLB, and VO at borehole 34 with dramatic cut-and-fill features possessing little or no distinct horizons above the deltaic layer. A very well-defined erosional channel cut-and-fill within the VLB is interpreted at boreholes 38 and 23. The high amplitude nature of the reflection with the VLB is likely related to saturation and possibly the pooling of water at that interface. Between boreholes 38 and 23 a mound of HCT can be interpreted which is consistent with borehole 24, previous seismic lines, and hillside outcrop maps. The distinct character, reflection orientation, and shadowing effects of this area on deeper reflections is strong evidence to support a distinct horizontal change in material within this area. The borehole data was used to help identify the possible source of this acoustically unique area within the context of the entire seismic profile line. Preliminary processed sections possess distinct high angle apparent reflection arrivals with apparent dip to the west that are likely indicators of the western boundary of the HCT mound. The geologic interpretation of this HCT mound was only possible with the inclusion of borehole logs and interpretations. Intermediate processed seismic sections enhanced the identification of secondary channels and approximate amounts of fill material redeposited into these apparent deep cuts into the VLB sequences. Without digital filtering and spectral balancing the channel feature between stations 1070 and 1110 is quite pronounced, possessing well-defined sides and top. The unique acoustic characteristics of this feature as displayed on the intermediate processed section is not consistent with the final processing flow that was designed to enhance reflections from deeper more consistent and competent layers. A much less well-defined channel is discernable between stations 1220 and 1280 on this same intermediate processed section. It is reasonable to suggest the strong reflection altered by these channels is the top of the VLB sequence. The high amplitude reflection truncated by the western edge of the western channel is consistent with the missing lower outwash layer as interpreted in borehole 34. It is not unreasonable to suggest the section as sampled by well 34 might be unaltered while the section encountered in boreholes 38 and 23 have the upper portion of the VLB altered by erosional fill.

Full Paper KGS-97-65.PDF 4.68MB
Orignal Mud Mountain Dam Abstract KGS-95-8.PDF 4.90MB

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