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Quaternary alluvial-fan deposits in Birch Creek Valley are poorly sorted carbonate gravels that have undergone diagenesis in the meteoric realm through the dissolution and precipitation of calcium carbonate. Three diagenetic zones are documented on the basis of cement morphologies and paragenesis: (1) near-surface vadose, (2) vadose, and (3) "vadose-phreatic."
Cements formed in the near-surface vadose zone result from both pedogenic and nonpedogenic processes. Pedogenic processes predominate within the upper meter of fan surfaces, whereas nonpedogenic processes cause case-hardening on steep, unvegetated outcrops. Pedogenic cementation proceeds in a series of four morphologic stages and is characterized by clotted micrite and fibrous sparry calcite, commonly with gravitational morphologies and intricate banding. Nonpedogenic cements are primarily micritic to finely crystalline with homogeneous or clotted textures; microdigitate cements are common on the undersides of clasts.
Dissolution and incipient cementation are typical in the vadose zone; cements are best developed beneath large clasts. Thin, banded, gravitational cement, grain-contact cement, rare syntaxial overgrowths, and the lack of clotted micrite are indicative of vadose cementation.
Well-cemented fanglomerate reflects progressive cementation in the "vadose-phreatic" zone, or in a zone of water-table fluctuation. Two generations of cement are generally apparent. Early micrite cement forms discontinuous to continuous rims and is followed by an isopachous sparry cement. Syntaxial overgrowths are relatively common on monocrystalline grains. The degree of cementation is variable and appears to be related to grain size, sorting, and packing geometries.
The distribution and nature of the cements suggest that cementation is initiated soon after deposition and proceeds simultaneously in each diagenetic zone.
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