STATISTICAL ASSESSMENT OF TURBIDITES IN THE LATE CRETACEOUS HORNBROOK FORMATION, JACKSON COUNTY, OREGON

A sequence of medium- to thick-bedded mudstones interbedded with thin- to very thick-bedded, very fine- to coarse-grained sandstones and wackes make up the Blue Gulch Mudstone Member of the Late Cretaceous Hornbrook Formation. This interval of interbedded sandstones and mudstones is interpreted as turbidites deposited on a submarine fan in a forearc basin. Stratigraphic data were collected, including bed thickness, rock type, grain size, sedimentary structures, and color from an exposure near Ashland, Oregon. Sandstone beds exhibit normal grading and tool marks at the base with planar stratification and ripple cross-lamination near the top. Planar laminated siltstones overlie each sandstone bed, consistent with waning flow of a turbidity current. The mudstones are interpreted to represent pelagic sedimentation between flow events. The gathered bed thickness data of the sandstones were analyzed using statistical methods and the distribution of bed thicknesses within the interval was determined to be consistent with a random distribution. The infrequent number of thick sandstone beds suggests a low frequency of high energy turbidity currents in the studied interval. Although the overall distribution of sandstone bed thicknesses is random, most of the thick- to very thick-bedded sandstones occur in the lower half of the studied stratigraphic section. The abundance of these thick- to very thick-bedded sandstone is attributed to a proximal location to a distributary channel on the submarine fan. This is the first study to apply statistical techniques to a turbidite sequence and contributes to a better understanding of deep marine depositional processes in a forearc setting.