Subaerial exposure surfaces in the Middle and Upper Mississippian Slade Formation of northeastern Kentucky are largely composed of cutanic concentrations of micritic calcite within the former Ccam horizons of caliche soils. The association of this material with soil horizons and structures, as well as with abundant root traces, strongly indicates a pedogenic origin. In fact, the contribution of plants and small soil organisms was far greater than has been previously recognized. The caliches occur as “interformational” profiles on disconformities separating lower Slade members and as “intraformational” profiles within three lower Slade units. Paleoexposure was related to position on a structurally active margin of the Appalachian Basin and to episodes of regional and local regression. The caliches resulted from soil and ground-water conditions in a semi-arid climate characterized by seasonal rain and drought and an overall net moisture deficit. Growth of roots, desiccation, and displacive crystallization broke up parent limestones, allowing access of vadose waters and creating framework (skeleton) grains that were easily transformed into a mobile plasma fraction by solution. Solution of carbonate grains and eluviation of carbonate-bearing solutions primarily occurred during the moist rainy season, whereas illuviation rapidly followed the onset of drought. The calcium carbonate was deposited largely as internal, laminar plasma concentrations called cutans, which have been incorrectly referred to as “crusts” in previous work on the Slade. Accumulation of these cutanic laminae formed indurated laminar calcrete deposits near the bases of the caliche profiles. These calcretes may be of physicochemical or rhizocretionary origin, depending on conditions of exposure. More diffuse, irregular calcretes apparently developed along avenues of porosity and were formed by plasma separation, the in situ micritization of other limestone textures. Although climate in the Meramecian and earliest Chesterian epochs was the major factor responsible for caliche formation, the length of exposure and the type of carbonate lithology controlled the nature and thickness of caliche profiles. “Intraformational” profiles are always thin and immature, representing short-lived exposure on porous lithologies like calcarenite. Conversely, “interformational” profiles are always mature or composite and represent longer periods of exposure on more impermeable lithologies such as calcilutite. Impermeable lithologies were important, because they prevented migration of soil waters and plasma below the soil profile. By late Early Chesterian time, the climate had become more humid, and the latest formed caliches were partially destroyed by solution, creating a leached, clayey residual soil on top of earlier caliche soils. On structurally elevated areas, where exposure was long and drainage was good, this period of humid pedogenesis resulted in composite terra rossa paleosols produced from the humid weathering of older caliche profiles.

Kimberlites in the eastern United States contain two suites of megacrysts/inclusions that are mineralogically similar but compositionally distinct. One suite (olivine, garnet, diopside, Cr-spinel) has higher Cr and Mg than the other (olivine, garnet, diopside, picroilmenite). Based on detailed petrologic studies of megacrysts from the Fayette County, Pennsylvania, kimberlite, Hunter and Taylor (1984) suggested that these two suites represent the crystallization products of separate magmas that mixed in the low-velocity zone (LVZ) to form kimberlite magma. Major and trace element abundances of individual garnet megacrysts from eastern U.S. kimberlites (i.e., from Kentucky, New York, Pennsylvania, and Tennessee) support the magma-mixing hypothesis but also indicate additional complications. Eclogite garnets have Cr 2 O 3 <0.3 wt.%, CaO >7 wt. %, and chondrite-normalized Lu/Hf <<1. Peridotite garnets have Cr 2 O 3 >2 wt.%, MG# >83, and chondrite-normalized Lu/Hf <1. Garnet megacrysts from Kentucky and Pennsylvania form two groups, one with TiO 2 <0.5 wt.%, and one with TiO 2 >0.5 wt.%. Both groups span a similar range in Cr 2 O 3 (≅ 1.0 to 9.0 wt.% Cr 2 O 3 ), but the high-Ti garnets may have Cr 2 O 3 as low as 0.1 wt.%. The low-Ti garnets have chondrite-normalized Lu/Hf <1 and are probably derived by the disaggregation of peridotite xenoliths and wall rock. The high-Ti garnet megacrysts have chondrite-normalized Lu/Hf ≥1 and are interpreted here as cognate “phenocrysts” that crystallized in a kimberlite or proto-kimberlite magma. Two suites of high-Ti garnet megacrysts are recognized: a low-Cr to very low-Cr suite (Cr 2 O 3 <4 wt.%) with flat to slightly positive heavy rare-earth element (HREE) slopes, and a high-Cr suite with steeply negative HREE slopes. These suites correspond to the “Cr-poor” and “Cr-rich” suites, respectively, defined by Hunter and Taylor (1984) for the Pennsylvania kimberlite. These data are consistent with the mixing of two magma batches to form kimberlite, as proposed by Hunter and Taylor (1984). Mixing prpobably occurred in the LVZ prior to eruption of the hybrid kimberlite magma.