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  • APPROXIMATE MEANDECLINATION, 1998

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    FERRICRETE, MANGANOCRETE, AND BOG IRON OCCURRENCES WITH SELECTED SEDGE BOGS AND ACTIVE IRON BOGS AND SPRINGS IN PART OF THE ANIMAS RIVER WATERSHED, SAN JUAN COUNTY, COLORADO

    ByDouglas B. Yager, Stanley E. Church, Philip L. Verplanck, and Laurie Wirt

    2007

    Base from U.S. Geological Survey 1997, 3.3 ft (1 m) resolution digital orthoquadrangles. Quadrangles used include Ironton, Handies Peak, Ophir, Silverton, and Howardsville.

    U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

    PROFESSIONAL PAPER 1651PLATE 2

    Version 1.1

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    Figure 2. Alluvial ferricrete along Mineral Creek between Middle and South Forks of Mineral Creek (view to west). Outcrop is about 30 m thick and rests on granitoid porphyry bedrock. Ferricretes form in paleo-alluvial terrace deposits where terrace sands and gravels are cemented by iron oxyhydroxide minerals.

    Figure 3. Alluvial ferricrete above mouth of Cement Creek (view to south). Kendall Mountain in distance.

    Figure 4. Log in alluvial ferricrete along Middle Fork Mineral Creek is below and about 400 m downstream from Bonner mine. Note log cemented in place by iron oxyhydroxide cement. The radiocarbon age of this log is 790 yr B.P. Logs encased in ferricrete throughout the upper Animas River watershed range in age from modern to 9,150 yr B.P.

    Figure 5. Alluvial ferricrete at mouth of Middle Fork Mineral Creek.

    Figure 6. An active iron spring located about 240 m above the confluence of Mineral Creek and Middle Fork Mineral Creek forms where organics and iron precipitates accumulate. Note acidophyllic, dark-green sphagnum moss at base of spring.

    Figure 7. Iron spring in lower Prospect Gulch near head of Cement Creek (view to south). Bog iron and colluvial ferricrete deposits are preserved adjacent to spring.

    Figure 8. Alluvial ferricrete at base of excavated foundation (town of Silverton). Builders commonly encounter ferricrete "hardpan" when excavating foundations throughout the town.

    Figure 11. Colluvial ferricrete crosscut by vertically layered bog iron, above South Fork Cement Creek (view to east).

    Figure 12. Colluvial ferricrete above South Fork Cement Creek, deposited on a hillslope topographically below altered lavas (upper right). View to east.

    Figure 13. Bog iron outcrop near headwater region of Topeka Gulch. Deposit is essentially clast free and consists of fine, horizontally laminated iron oxyhydroxide (goethite). Vertically laminated bog iron deposits also crop out in upper Animas River watershed (fig. 11).Bog iron deposits are inactive, and are likely the remnants of once active iron springs and iron bogs.

    Figure 14. Iron bog located east of Dry Gulch along Cement Creek. Iron bogs are shallow mixing zones, having at this site a range of pH (3.2 - 5.7) and conductivity (900 - 1,700 microsiemens per centimeter) values. Oxidation of ferrous iron and mixing of ground water result in precipitation of iron trapped in pools or in organic material. Iron bogsare often transitional to sedge bogs.

    Figure 15. Sedge bog in South Fork Mineral Creek subbasin. Sedges, grasses, and willows are prevalent along bog margins. Dark area of photograph is water saturated, about 15 cm deep, and consists of both living and decaying sedges grasses, mosses, and shrubs (photograph by M.R. Stanton, U.S. Geological Survey).

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    HANDIES PEAK

    3800' 3800' 10730' 10745'

    10752'30" 3752'30" 3752'30"

    3745' 3745'

    10737'30"

    3737'30"

    3730'

    3722'30"

    3715'

    IRONTON

    SILVERTON

    OPHIR

    HOWARDSVILLE

    Map area (shaded)

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    10 0 10 20 30 40 MILES

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    COLORADO

    Animas River watershed boundary

    in Colorado

    Figure 1. Index map of Animas River watershed.

    Ferricrete mapping accomplished along Mineral Creek and tributary subbasins by P.L. Verplanck and D.B. Yager (1996 - 2000). Ferricrete mapping along Cement Creek basin and tributary subbasins accomplished by S.E. Church, Laurie Wirt, and D.B. Yager (1996 - 2001). Other areas were mapped in reconnaissance by S.E. Church and D.B. Yager. All data were digitally compiled by D.B. Yager and Tracy Sole.

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    SCALE 1:24 0001/ 21 0 1 MILE

    Sedge bog (late Pleistocene to modern)Water-saturated ground that is colonized by acid-tolerant sedges, grasses, mosses, and willows with pH typically ranging from 3.2 to 5.5. Sedge bogs generally form at the base of hillslopes, at sites where the water table is intersected by the ground surface, and downslope from colluvial deposits on relatively flat lying valley fill, flood plains, and terraces. Substrate is spongy organic material that is transitional to buried peat and may include fine-grained silt. Sedge bogs may overlie or interfinger with iron bog and alluvial terrace deposits. Twigs and logs are present in the sedge bog and the peat deposits. Thickness in pl