Multi-scale alluvial fan heterogeneity modeled with transition probabilitygeostatistics in a sequence stratigraphic framework

The complexity of alluvial fan depositional systems makes derailed characte rization of their heterogeneity difficult, yet such detailed characterizati ons are commonly needed for construction of reliable groundwater models. Th e transition probability geostatistical approach provides a means to quanti fy the distribution of hydrofacies in the subsurface. However, a key assump tion used in this and other geostatistical approaches is that of stationari ty. Stratigraphic character often varies within a deposit, making this assu mption tenuous. Sequence stratigraphic concepts help us overcome this probl em by dividing the strata into units that have similar properties, called s equences, based on recognition of unconformities and timelines within the s edimentary record. By using transition probability geostatistics in a seque nce stratigraphic framework, realizations of the alluvial fan facies distri butions are produced that account for multi-scale heterogeneity represented by spatially variable hydrofacies within sequences, laterally extensive aq uitard units at sequence boundaries, and spatial variability attributes tha t are unique to each sequence, incorporation of conceptual geologic informa tion into the Markov chain model of transition probability also allows deve lopment of improved coregionalization models in the typically undersampled, lateral directions. The Kings River Alluvial Fan, located southeast of Fresno, California, prov ides an excellent test case for the approach. Several sequences within the alluvial fan were produced by outwash from Pleistocene glaciations in the S ierra Nevada Mountains. Five sequences, separated by large-scale (>3 km lat erally), mature, red paleosols, were recognized in the alluvial fan strata. Markov chain models were developed to characterize the intermediate-scale (0.3-1.5 km laterally) distribution of hydrofacies in each individual seque nce and to characterize the spatial distribution of paleosols. Separate con ditional simulation of each sequence provides realizations of hydrofacies d istributions. Combining these five sequence realizations into a single real ization, then overprinting the paleosol distributions onto this realization , produced a geologically plausible image of the subsurface facies distribu tion that accounts for non-stationarity between stratigraphic units. Import antly, the resulting realization preserves the lateral continuity of the la rge-scale sequence boundary paleosols, which are potentially important conf ining beds within the fan deposits. Additionally, facies juxtaposition tend encies (e.g. upward fining tendencies of the fluvial deposits) and known di rectional anisotropy and dip of units within the fan are preserved in the r ealization. These physical attributes, accurately reproduced by the geostat istical method, are essential components of the overall hydrogeologic chara cter of the alluvial fan. (C) 1999 Elsevier Science B.V. All rights reserve d.