AGE AND DEPOSITIONAL ENVIRONMENTS OF THE WHITE CLIFFS OUTCROP (ELLENSBURG FORMATION), HWY 10 IN THORP, WA

The White Cliffs Outcrop on Highway 10 is perhaps the most visited site of geologic interest in Kittitas County. However, few studies of the Ellensburg Formation at this outcrop have determined the exact dates of these deposits and the depositional environments of the sediment in the outcrop. To study the outcrop we analyzed the facies, measured paleocurrents within scours, collected primary volcanic material for zircon analysis to date the deposits, and looked closely at the minerology and grain sizes of clasts within the outcrop. To do this we combined fieldwork, lab work, and computer programs such as MetaShape. The use of MetaShape helped to create a three-dimensional model of the outcrop, which can be used for educational purposes in the future, provided the ability to measure the facies thicknesses and make a stratigraphic column for rocks too high to measure in the field, and assist with calculating and drawing paleocurrent diagrams. We used a CAMSIZER to measure the grainsizes of the matrix from the pyroclastic flow, lahar and hyperconcentrated flow sections of the outcrop. We also conducted a pebble count of the main layers and examined the mineralogy of the matrix and clasts in each layer.

The main layers in the outcrop consist of a pyroclastic flow, lahar, hyperconcentrated flow, which can also be argued as a volcanic surge deposit, and paleosols. Zircon analysis of the eruptive age of the pyroclastic flow unit in the outcrop dates the sediment as 10.0 Ma (+/- 0.2 Ma). The mineralogy of the matrix in the volcanic deposits consist of a broad range of minerals, including plagioclase, pyroxenes (both clinopyroxene and ortho pyroxene), biotite, and quartz. The mainly igneous clasts range from mafic to felsic throughout the outcrop. Using the CAMSIZER we found that the average grain size was 1.11mm. Paleocurrent analysis of the outcrop shows that the direction of water flow during the time of deposition was dominantly 130^o southeast. Understanding the processes that formed the White Cliffs outcrop tells us what the depositional environment looked like 10 Ma, which assists in understanding the transition of Kittitas Valley from the Columbia River Basalts (15 Ma) to the well-known fluvial environment of today.