The Hilt Bed, an Upper Cretaceous compound basin–plain seismoturbidite in the Hornbrook Forearc Basin of southern Oregon and northern California, USA

Abstract

Abstract The Hilt Bed is a thick, laterally continuous marker bed of fine- to medium-grained sandstone that crops out for a NW–SE along-strike distance of about 50 km on the southwestern flank of the Cretaceous Hornbrook forearc basin of northern California and southern Oregon. The Hilt Bed forms part of the Blue Gulch Mudstone Member of the Hornbrook Formation and is everywhere underlain and overlain by basin–plain mudstone that contains thin interbeds of turbidite sandstone and siltstone. The bed is as thick as 4.71 m in the central part of the outcrop belt and thins progressively to the NW and SE. It was derived from erosion of igneous and metamorphic basement rocks of the Klamath Mountains, which flank the basin to the SW. Paleocurrents measured from flute casts at the base of the bed indicate sediment transport to the NE. The Hilt Bed can be divided into a compound succession of nine Bouma divisions, all of which are generally present in outcrop. The succession consists in ascending stratigraphic order of Bouma abababcde divisions. The basal a division is typically the thickest and coarsest layer, with overlying divisions generally thinner and more variable in thickness. The bed appears to have been deposited as a compound turbidite by at least three separate turbidity currents that appear to have been generated either by the same triggering event or by a rapid succession of related events. Only the uppermost part of the bed thus contains a complete Bouma sequence. The unidirectional paleocurrent indicators, lateral continuity and extent of the three flows that make up the bed, incompleteness of the two lower Bouma sequences, and absence of interbedded shale or bioturbated levels suggest that the three turbidity currents reached the basin floor at almost the same time to yield the bed. The compound nature of the Hilt Bed suggests that a major seismic event triggered the three large flows, which probably originated in either a single submarine canyon or three adjacent canyons.