U.S. DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 1651 U.S. GEOLOGICAL SURVEY CORRELATION OF MAP UNITS PLATE 2 Version 1.1 sbspi ib ub af cf uf mncf mnaf tf bi HOLOCENE LATE LATE DESCRIPTION OF MAP UNITS PLEISTOCENE Sedge bog (late Pleistocene to modern)—Water-saturated ground that is colonized by sb acid-tolerant sedges, grasses, mosses, and willows with pH typically ranging from 3.2 to Transitional colluvial manganocrete and ferricrete (late Pleistocene to modern)— 5.5. Sedge bogs generally form at the base of hillslopes, at sites where the water table is tf Varicolored, brown (predominant), reddish-brown to black stained clasts, iron intersected by the ground surface, and downslope from colluvial deposits on relatively oxyhydroxide-cemented deposits. Textures are similar to colluvial ferricrete described flat lying valley fill, flood plains, and terraces. Substrate is spongy organic material that above; however, manganese is in sufficient abundance to cause outcrop matrix and clast is transitional to buried peat and may include fine-grained silt. Sedge bogs may overlie or coatings to have a brown to black color. The term transitional is used to describe those HPDY004HPDY004 interfinger with iron bog and alluvial terrace deposits. Twigs and logs are present in the deposits that are observed to have a mixed manganocrete and ferricrete composition. mnaf 00CGS115000CGS1150 sedge bog and the peat deposits. Thickness in places may exceed 5 m Deposits are preserved in Placer Gulch. Thickness 0.5 to 3 m mnaf mnaf Iron spring (late Pleistocene to modern)—Varicolored, brown to reddish-brown or Bog iron deposits (late Pleistocene to modern)—Very fine grained, iron spi bi mnaf orange deposits of impure hydrous iron oxides that form where water flows naturally oxyhydroxysulfate deposits consisting of goethite, jarosite, and schwertmannite; from bedrock fractures or soil. Deposits consist of finely laminated deposits of goethite, varicolored, brown (predominant), reddish-brown to brownish-yellow with black-stained Figure 2. Alluvial ferricrete along Mineral Creek between Middle and 99VAS0199VAS01 amorphous iron oxyhydroxides, and locally schwertmannite, as well as acid-tolerant partings. Unit is commonly well indurated and thin bedded but may be moderately South Forks of Mineral Creek (view to west). Outcrop is about 30 m mnaf algae or mosses. Precipitates form where acidic springs discharge to the ground surface indurated and porous. Locally, alternating dark-reddish-brown and brownish-yellow thick and rests on granitoid porphyry bedrock. Ferricretes form in or a water body, often where bedrock fractures are intersected by surface topography. lamina occur, and locally beds are crosscut by fractures filled with laminated bog iron. A paleo-alluvial terrace deposits where terrace sands and gravels are mncf mncf May occur on modern flood plain, on old terraces, or on hillslopes. Conifer needles are finely filamentous texture occurs in some outcrops, which appears to be similar to cemented by iron oxyhydroxide minerals. common; twigs and logs rare. Bedrock clasts are locally preserved. Damp surfaces often textures observed in active iron bogs. Iron compound casts of roots and conifer needles uf Figure 3. Alluvial ferricrete above mouth of Cement Creek (view to cf support growth of sphagnum moss. Thickness 0.1 m to more than 4 m occur locally. Unit in many places grades laterally and (or) vertically into clastic alluvial south). Kendall Mountain in distance. or colluvial ferricrete. Bog iron deposits may be water saturated but are predominantly tf Iron bog (late Pleistocene to modern)—Varicolored, predominantly brown to reddish- dry at present. Dry bog iron deposits were once active iron-rich springs, which ib sb cf brown, but may be whitish-gray to yellow or orange, impure hydrous iron oxide deposits precipitated and deposited iron oxyhydroxides adjacent to them. Wet, active iron-rich tf in water-saturated ground. Consist of oxides of iron, aluminum, and manganese springs such as the spring near the confluence of Middle Fork and Mineral Creek have HPDY0013HPDY0013 uf tf 14 99ABFC17099ABFC170 compound precipitates. Precipitates form in acidic, poorly drained conditions by the deposited a terraced mound. Wood fragments encased in bog iron deposits have C oxidizing action of algae, iron (thiobacillus ferrooxidans)- and sulfur (thiobacillus ages ranging from 380 to 4,010 yr B.P. Outcrop thickness ranges from 1 to 5 m; sb thiooxidans)-oxidizing bacteria, or the atmosphere. Acidophyllic algae and mosses may maximum thickness unknown 99ABFC17299ABFC172 be present. Substrate consists of hydrous iron oxyhydroxides (schwertmannite), af HPDY006HPDY006 amorphous iron oxyhydroxides, and goethite, which have porous textures ranging from 99ABFC17099ABFC170 Sample site (see this volume, Chapters E14–E17) 99ABFC17399ABFC173 mncf thinly layered to irregular aggregates. Algal mats trap freshly formed oxyhydroxide- 99ABFC17499ABFC174 bi HPDY0016HPDY0016 oxyhydroxysulfate precipitates. Clasts, logs, and twigs are locally preserved. Observed Photograph location, figures 2–15 af F7 bi thicknesses range from 0.1 to 0.5 m sb mnaf 99ABFC17599ABFC175 sb Undifferentiated bog (late Pleistocene to modern)—Sedge bog or iron bog, as bi ub sb described above. Unit is undifferentiated because deposits were either identified from af aerial photograph interpretation or were not accessible due to land ownership issues af 95ABS11995ABS119 Alluvial ferricrete (late Pleistocene to modern)—Brown to yellowish-brown, iron cf bi af oxyhydroxysulfate-cemented sandstone or conglomerate; cement consists principally of af 99ABFC17699ABFC176 af goethite. Deposits are bedded to weakly stratified and consist mostly of heterogeneous cf 99ABFC16599ABFC165 af cf bi cf uf subrounded to subangular pebbles and cobbles with occasional boulder-size clasts in an cf af IDY0014IDY0014 iron oxyhydroxide-cemented, clast-supported matrix, of coarse sand- to pebble-size cf cf 99ABFC17799ABFC177 af mncf 99ABFC16999ABFC169 sediment. Clasts are imbricated and dip upstream. Pebbles and cobbles in some places af 99ABFC10599ABFC105 99ABFC10499ABFC104 99ABFC10399ABFC103 are coated with a fine filamentous iron oxyhydroxide cement similar in appearance to Figure 4. Log in alluvial ferricrete along Middle Fork Mineral Creek is uf af 99ABFC18199ABFC181 99ABFC10799ABFC107 99ABFC10699ABFC106 algae. Exceptional (as much as 30 m thick) alluvial ferricrete exposures preserved along below and about 400 m downstream from Bonner mine. Note log 99ABFC10599ABFC105 sb 99ABFC11099ABFC110 99ABFC15599ABFC155 the west side of Mineral Creek between South Fork and Middle Fork of Mineral Creeks, cemented in place by iron oxyhydroxide cement. The radiocarbon age of 99ABFC10199ABFC101 uf 99ABFC10299ABFC102 cf and near the mouth of Cement Creek near Silverton, consist of sandstone layers among bi 99ABFC16099ABFC160 99ABFC15699ABFC156 this log is 790 yr B.P. Logs encased in ferricrete throughout the upper sb sb af coarse-grained graded beds of gravel, which are indicative of high-energy stream 00ABFC21100ABFC211 99ABFC14999ABFC149 99ABFC11599ABFC115 99ABFC10399ABFC103 Animas River watershed range in age from modern to 9,150 yr B.P. af uf transport. Alluvial ferricrete deposits are either wet or dry at present. Along active 99ABFC10399ABFC103 cf sb af 99ABFC15999ABFC159 cf flood-plain channels, such as Cement and Mineral Creeks, seeps and springs flow from 99ABFC11199ABFC111 00ABFC21200ABFC212 cf IDY0029A&BIDY0029A&B premining ferricrete terraces. Sphagnum moss is frequently observed at seep zones, from Figure 5. Alluvial ferricrete at mouth of Middle Fork 99ABFC11699ABFC116 99ABFC11299ABFC112 0 to 2 m from stream beds. Conifer logs are locally found within these deposits and the Figure 9. Alluvial manganocrete near the former Lake Emma in Eureka Gulch. Photo on left shows 99ABFC11399ABFC113 af Mineral Creek. 99ABFC14399ABFC143 99ABFC15899ABFC158 bi bi af alluvium locally interfingers with peat. Alluvial ferricrete preserved several meters above black, manganese-stained, crudely bedded paleo-alluvial fan deposit. Photo on right is closeup view of sb 99ABFC11799ABFC117 99ABFC11499ABFC114 99ABFC16699ABFC166 99ABFC14499ABFC144 cf the active channel is often dry and represents cemented alluvial fan remnants and stream stratigraphy. 99ABFC11899ABFC118 99ABFC16199ABFC161 mnaf 99ABFC14699ABFC146 14 99ABFC11999ABFC119 terrace deposits. C ages from logs and twigs recovered from these deposits range in sb 99ABFC16799ABFC167 99ABFC12099ABFC120 cf age from modern to 4,960 yr B.P. Thickness, 0.5 to 30 m 99ABFC10899ABFC108 99ABFC16899ABFC168 00ABFC20400ABFC204 cf 99ABFC15499ABFC154 99ABFC12199ABFC121 bi sb ib af cf Colluvial ferricrete (late Pleistocene to modern)—Iron oxyhydroxysulfate-cemented 99ABFC12299ABFC122 cf cf Afaf bi deposits; varicolored, brown (predominant), reddish-brown to brownish-yellow with 99ABFC12399ABFC123 dark-brown stained clasts. Cement consists principally of goethite. Deposits are massive cf af to weakly stratified subparallel to the current slope or drape existing topography; consist 99ABFC12499ABFC124 cf bi of mostly homogeneous angular, subangular, to subrounded pebbles, cobbles, and 99ABFC15399ABFC153 af uf boulders in an iron oxyhydroxide-cemented, finer grained clastic to relatively clast free cf 99ABFC16399ABFC163 matrix. Cobbles are weakly imbricated and dip downslope. Clasts consist of subangular af 99VMS1499VMS14 bi to subrounded pebbles in contact with silt- and sand-size sediment. Pebbles and cobbles sb bi