PALEOCURRENT AND PROVENANCE STUDY OF THE UPPER CRETACEOUS WILLAMS FORK FORMATION, WESTERN COLORADO

A paleocurrent and provenance study is being conducted for the Upper Cretaceous Willams Fork (Hunter Canyon) Formation of western Colorado. The Williams Fork Formation (WFF) records the final stage of fill for the Colorado section of the Sevier foreland basin (FLB). The study area is within the Laramide Piceance Basin, and paleocurrent measurements and provenance samples were collected east of Grand Junction along interstate I-70 and Colorado Highway 65. The study interval is within the upper sand-rich portion of the WFF. Ost (2010) subdivided the WFF into 3 informal units, with the upper two units dominated by a repetitive stratal pattern of fluvial deposits. This study focuses on the fluvial sandstones within the lower part of the repetitive cycles: laterally continuous, nested channel-fill complexes which occur in the middle to upper part of the WFF (subunits 2a and 3a of Ost). These intervals contains large-scale trough, wedge-shaped and planar-tabular cross beds which record the downstream migration of large bars within the fluvial systems. These intervals were selected because they provide the best paleocurrent indicators within the WFF. Ost interprets these sandstones as deposits of the lowstand systems tract. Paleocurrents were measured to see if they record different a paleoflow directions, and perhaps axial-parallel flow in the FLB during lowstands events. Paleocurrent measurements were taken using methods outlined by DeCelles and Langford (1983), from foresets with dips of 28⁰ or greater. Eighty four paleocurrent readings were taken. The majority of values ranged between 0 and 180 degrees, with the mean value of S82E. These measurements do not support a change in paleocurrent direction during lowstands.

Sandstones of the WFF are sublithic arenites. Quartz is estimated at 90%, and lithics at <10%. Lithics include amphibole, unidentified rock fragments, and minor amounts of mica. Ongoing thin section analysis will help identify the rock fragments/accessory minerals. Samples are currently being analyzed with Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry for U-Pb geochronology of any rutile and zircon present in the sandstones.