STRATIGRAPHY, SOILS, AND AGE RELATIONSHIPS OF MIMA-LIKE MOUNDS, LARAMIE BASIN, WYOMING

Abstract

Mima-like mounds in the Laramie Basin, Wyoming, are circular in plan view, lens or funnel-shaped in cross-section, and frequently have relief of 15 to 65 cm. They consist of churned materials derived from adjacent/subjacent soils and sediments. Strong intermound (premound) soils, or parts of them, usually collapse beneath the mounds or are sometimes truncated at mound edges. Both active and inactive mounds occur in areas of shallow, impermeable bedrock, thin alluvium overlying the bedrock, and strong premound soil development on terraces and benches. Radiocarbon dating, along with stratigraphic and soil relationships at various sites, indicates that the mounds are late Holocene (Neoglacial) in age. The premound soils and sediments beneath the mounds, in addition to those in intermound areas, appear to be affected chemically by salty groundwater related to mound formation or salt leached from overlying mound material into these units after the mounds formed. Accordingly, in response to the Hilgard reaction, the salts would lead to decomposition of calcium carbonate in Btk horizons and removal of clay, along with breakdown of calcium carbonate and clay removal from alluvium that formerly contained lower parts of the B horizons. As a result, where they appear to be affected directly by groundwater flow, the alluvial gravels and sands are clean, lack coherency, are devoid of much clay and silt, have cavernous openings, contain some secondary selenite crystals, and readily crumble under slight pressure. The degradation of the alluvium and soils, common to areas of mounded topography, conforms to the groundwater vortex hypothesis proposed for mound formation. Further research, however, is needed, including the gathering of additional data to support the hypothesis or change it. [Key words: groundwater, Holocene, mima-like mounds, soil stratigraphy, topography.]