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Rocker-Ramsey area WGI has completed a geologic map of the Rocker-Ramsey area, southwestern Montana. This map is at the 1:100,000 scale (topographic metric). The map area covers the Rocker Operable Unit, part of the Silver Bow Creek Streamside Tailings Unit, and continues south of these areas for approximately 10 miles. Cenozoic units are mapped in the sequence stratigraphic format, and are age-constrained by unpublished vertebrate land mammal ages and radiometric age dates. Consequently, it was possible to map detailed structure within the Cenozoic deposits, most of which have not been recognized and mapped prior to our work. Project Abstract(The Rocker-Ramsey map was done to supplement a project done at the Rocker Operable Unit by students from the Keck Consortium. The following abstract is from this project with the addition of the geologic map data by WGI. This abstract was presented at the Geol. Society of America National Meeting, 1997, by Hanneman, D. L., Conaway, J., Feiveson, T. D., Hammar-Klose, E., and Sneeringer, J.) CENOZOIC GEOLOGY OF A PART OF THE SILVER BOW CREEK/BUTTE AREA SUPERFUND SITE, SOUTHWESTERN MONTANA Detailed surface mapping and near-surface seismic reflection surveys are integrated to delineate Cenozoic geology on the Rocker Operable Unit and an adjacent part of the Streamside Tailings Operable Unit of the Silver Bow Creek/Butte area Superfund Site in southwestern Montana.The Rocker Operable Unit is underlain by late Eocene (Chadronian) mudstone, sandstone, and cobble conglomerate that are interpreted as flood-plain and channel-fill deposits. Unconsolidated Quaternary sediments veneer the Tertiary strata. The late Eocene strata onlap early Eocene volcanic and late Cretaceous granitic rocks on the east end of the Rocker area. The basin-fill/bedrock interface is steep near the east edge of the Rocker site, and the late Eocene strata thicken to approximately 600 feet within about 0.3 miles west of the east edge of an erosional paleovalley. Approximately 3 miles west of the Rocker site, within the Streamside Tailings Operable Unit, late Tertiary sandstone and mudstone (late Hemingfordian-early Barstovian) are downfaulted against late Eocene strata. It is in this area that the Cenozoic basin becomes structurally controlled by a major north to northwest-trending normal fault zone.Note: This interpretation of the Cenozoic geology of the Rocker and part of the Streamside Tailings operable units negates the existence of the Rocker Fault zone, a major normal fault zone that has been inferred to lie within the Rocker area by numerous past workers, and an associated thick deposit of unconsolidated Quaternary sediment located adjacent to the inferred Rocker Fault. The newly collected data also indicate a thick Tertiary section to be present beneath the Rocker area, in excess of several hundred feet over previous subsurface models for this area. Implications for site remediation as related to groundwater flow should now be centered around Tertiary lithologic change, thickness of Tertiary strata, and the erosional interface of basin-fill and bedrock rather than on flow controlled by geologic structure and Quaternary sediment distribution.
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Volcanic ash in Sequence 3, located a few miles north of Rocker.
Fine- coarse-grained couplets in Sequence 4 unit located a few miles west of
Rocker.