Title

Stratigraphy of a Deltaic Complex in the Boise Foothills Sandstones: Applications for Environmental and Water Resource Issues

Publication Date

7-1996

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Education, Earth Science

Department

Geosciences

Major Advisor

Walter Snyder

Abstract

Pliocene-Pleistocene sedimentary deposits of the Western Snake River Plain represent lacustrine and fluvial deposits within a fault-controlled basin. This is commonly referred to as ancient "Lake Idaho". These sediments reveal complex stratigraphic patterns that reflect tectonic events and changes in lake levels.

The detailed facies analysis of one of Lake Idaho's deltaic sequences along the northern margin of the Western Snake River Plain in the Boise Foothills, at the Hidden Hollow Sanitary Landfill, reveals a tri-part Gilbert-type bedding complex of topsets, foresets and inferred bottomsets. Steep foresets dipping c. 25° towards the northwest are truncated and overlain by subhorizontal topsets. Channel in-fill, faulting, and mass-gravity flows truncate the foresets, representing synsedimentary tectonism. The sediments of these heterogeneous facies are predominantly well-sorted, uncompacted, and uncemented, ranging from fine-grained, trough cross laminated sands in topsets to granule gravels in the mass-gravity flows. Very coarse sands (1 mm) are the most predominant grain size within the topset, foreset and mass-gravity flow facies.

The northwest prograding deltaic sedimentation and the highly porous and permeable very coarse sands of the deltaic complex serve as primary controls on the flow direction of ground water. These deltaic facies serve as hydrogeologic facies units and can be used to predict movement of contaminated landfill effluent which has the potential to adversely impact the quality of the adjacent ground water resources. Monitor wells, one up-gradient (towards the dip of the sediments) and three down-gradient (against the dip) are periodically tested in the landfill; these tests report little to no contaminants found.

Hidden Hollow Sanitary Landfill management monitors all wastes, screens and stores for shipping those wastes deemed hazardous, and follows strict guidelines for daily burial in "cells" which meet EPA guidelines. Current focus is on methane gas emissions, which surpass EPA standards, requiring its collection.

Because of the excellent exposures in the landfill site, it has become a field trip site used by earth science teachers to show high school students the geology of ancient deltas. The deltaic complex site and many other geologic sites associated with the Western Snake River Plain formation in the Boise foothills are described in a regional field trip guidebook as an appendix to this study.

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