Abstract

In the summer of 1992, during construction of Little Dell Dam, faulted Quaternary-age fluvial sediments were recognized in the borrow area. Investigation of these faults revealed that they were consistently associated with highly sheared, altered beds in the Cretaceous-age Frontier Formation, and exhibited reverse-fault geometry. The reservoir borrow area for the earthfill dam had four discrete fluvial units, termed fluvial cycles 1 through 4, from youngest to oldest. The upper two cycles had been removed before recognition of the faulting. Evidence that the two oldest fluvial cycles, 3 and 4, had been offset was documented, but no displacement was recognized above the basal unit of fluvial cycle 3.

Part of the Corps of Engineers criteria for active (capable) faulting is displacement within the past 35,000 years. A total of twenty-five trenches were excavated in the attempt to confirm continuity of faulting and to establish a minimum age for the most recent displacement. Radiocarbon dating, paleontological evidence, and stratigraphic relationships were used to establish age constraints. Radiocarbon dating of two samples indicated an age of deposition for the upper portion of the youngest offset unit to be 37,000 years B.P. Paleontological evidence could only classify several fossils as Quaternary age. A study correlating the fluvial stratigraphy with late Quaternary glacial events, suggests an age of greater than 100,000 years B.P. for the youngest offset unit

We have no specific explanation for the genesis of these compressional features in a known extensional area, but other studies have indicated that compressional features can occur in extensional regimes.