CHANNEL ARCHITECTURE OF EOCENE AGE IONE FORMATION, AN EXAMPLE OF A SAND-RICH BRAIDED STREAM DEPOSIT

The purpose of this research project was to characterize the architectural style of the extremely sandstone-rich fluvial deposits of the middle Eocene Ione Formation. The exposures of the Ione Formation at Malakoff Diggins, located twenty-six miles east of Grass Valley, northern California, were selected due to the high quality and continuity of the three-dimensional outcrop. Hydraulic mining of auriferous gravels, which occurred at Malakoff Diggins from 1852-1853, has provided one hundred percent exposure of a classic braided stream deposit of the Ione Formation. This study incorporated nine detailed measured sections at close intervals (approximately 15 meters) that were correlated to demonstrate channel architecture. Work included identifying channels elements and associated overbank deposits. Individual channels within the study area are 10 to 15 meters in thickness, show distinct vertical changes and are hundreds of meters in width. Channel bases are typically dominated by clastic conglomerate and grade vertically into coarse-grained, cross-bedded sands. This information was modeled in a three-dimensional computer-aided environment. This model demonstrates the extent of potential fluid communication of the Ione Formation. This information can be used to provide geo-statistical data required to construct geologic (aquifer/reservoir) models in order to better understand and predict fluid flow within similar sedimentary bodies.