SAVAGE, Heather,
Geol. Dept., Colgate Univ., Hamilton, NY 13346, hsavage@center.colgate.edu

    The Miocene-Pliocene Usebelli Group and Nenana Gravels record a sequence of non-marine sedimentary environments in southern interior Alaska. Progressive cooling of climate is suggested by floral (Wolfe, 1980) and pollen (Leopold and Liu, 1994) studies of the sequence. Uplift of the Alaska Range is documented in the sequence by changes in sedimentary facies, paleocurrects and clastic sediment provenance (Wahrhaftig, 1987). Tilting of the sequence occurred prior to 2.7 mya (Wahrhaftig, 1987) and is related to uplift and crustal shortening that began in late Miocene - early Pliocene time.

    Calcite-cemented sandstone concretions occur in the upper Suntrana Creek and Lignite Creek Formations of the Usebelli Group, and in the overlying Nenana Gravels. Concretions in the Suntrana and Lignite are elongate, tabular to ellipsoidal in shape; Nanana concretions are generally subspherical. Concretion formation occurred prior to tectonic tilting of the sequence. Calcite is the dominant cementing mineral in the concretions; minor chabazite (Ca-zeolite) is also present. Elevated Ca-concentrations which promoted calcite and chabazite precipitation are linked to alteration of detrital plagioclase. Calcite cements contain trace to minor Fe and Mg. The tabular to ellipsoidal shape of the Lignite concretions indicates formation at or below the water table, with the orientation of long axes related to the direction of groundwater flow (McBride, et al 1994). Subspherical Nenana concretions apparently formed above the water table in the vadose zone.

    Stable carbon and oxygen isotope studies of the calcite cements in the concretions reveal significant stratigraphic variation. Delta ¹⁸O SMOW of calcite increases from +6 to +13 over the stratigraphic interval studied. The progressive change is interpreted to represent the gradual decrease in the temperature of meteoric waters during precipitation of calcite cement. Delta ¹³C PDB of the calcites range from -8 to -18. Carbon isotopes show a general trend of up-section decrease, perhaps linked to changes in the organic input to the soil/water carbonate system. Strongly depleted isotopic values suggest that some calcite may have precipitiated in areas of methane oxidation. The most strongly depleted values are associated with Lignite Creek Formation concretions. Ongoing stable isotope study of the associated organic-rich sediments will provide better understanding of the dynamics of the carbonate cementation system.

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