2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): REBOULET, Edward C.,ecr@cgiss.boisestate.edu; and BARRASH, Warren, Center for the Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise, Idaho, 83725
Keywords: Grain-size, Alluvial-aquifer, Braided-stream, Porosity, Heterogeneity
The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield consisting of 18 wells emplaced in a shallow alluvial aquifer (~20 m thick) composed of coarse braided-stream deposits. Grain-size distribution (GSD) data of core samples from 11 of these wells are analyzed in conjunction with porosity data from neutron logs. The objectives of this research are to: (1) determine textural variation within the deposits as a whole; (2) determine vertical spatial dimensions and distribution of the lithofacies within five subsurface stratigraphic units interpreted from porosity logs and GPR data; and (3) use this information to interpret the subsurface stratigraphic units as facies. Five GSD types (i.e., lithofacies) have been recognized and characterized at the BHRS: one sand and four cobble-bearing or cobble-dominated GSD types. These GSD types do not correspond directly with subsurface units, but form the basic building blocks of the coarse braided stream deposits at the BHRS. Three GSD indices (large cobbles, all cobbles, and the medium sand fraction) are combined with porosity values using principal component analysis; 90% of this four-way system variation is explained by two principal components (PC1 and PC2). PC1 corresponds to the five GSD types and PC2 corresponds to the packing variation within the types. Spatial characteristics of the GSD types in the stratigraphic units show a trend in all four of the cobble-dominated units toward increased average thickness and thickness proportions as GSD types become more cobble dominated. Analysis of vertical succession tendencies of GSD types with first-order embedded Markov chain models and transition probability geostatistics indicates that: (1) variable porosity units (Units 2 and 4) are statistically different from random independent and are interpreted to be cobble-bar and associated bar-margin deposits; and (2) low-porosity cobble-dominated units (Units 1 and 3) show random independent transition structure and are interpreted to be hyper-concentrated traction-carpet deposits.
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