3D FRAMEWORK MODELS AND SEDIMENTATION CONTROLS IN BASINS FROM THE NORTHERN AND CENTRAL CALIFORNIA COAST

Tectonism and eustatic sea level changes are two processes that control sediment distribution and basin aggradation in the groundwater basins of coastal California. Three-dimensional hydrogeologic framework models (3D HFMs) constructed in support of water-resource assessments of the Salinas Valley and Eel River Valley coastal basins provide contrasting examples of the relative importance of these fundamental drivers on basin sedimentation. For both basins, digital input data were compiled from multiple sources and 3D HFMs were constructed by combining three framework elements: 1) elevation and thickness of each hydrostratigraphic unit; 2) bounding polygons depicting the subsurface extent of each unit; and 3) location of faults. Grids representing hydrostratigraphic tops were stacked to construct 3D volume models of each basin that portray the stratigraphic setting and geologic structure of each basin.

In the Salinas Valley, poorly-consolidated Pleistocene and Pliocene alluvial, fluvial, eolian, and estuarine deposits are up to 750 m thick. Borehole lithologic logs and the 3D HFM show that these deposits are organized into sequences of relatively coarse-grained and fine-grained materials forming four to five aquifers that are vertically separated by regionally extensive confining units. The continuity and ordered stacking of these hydrostratigraphic units suggest basin-wide controls on sedimentation. Sediment age control from previous work supports the conclusion that the basin-wide alternation of estuarine and fluvial deposition relates to marine transgressions and regressions associated with glacio-eustatic sea level fluctuations and not to tectonism.

The Eel River Basin in northern California is a structurally controlled basin directly north of the Mendocino Triple Junction. Neogene deep-water to shallow marine siliciclastic sediments are up to 3,300 m thick. These sediments display an upward-coarsening, regressive sequence that previous work suggests represents emergence of the basin as the Mendocino Triple Junction approached from the south and impacted local geodynamics. Laterally continuous aquitards are not apparent from subsurface lithologic data, indicating that tectonically-driven sedimentation exceeds and masks any signal from eustatic sea level changes.