The Hornbrook Formation (Upper Cretaceous) is exposed along the northeastern margin of the Klamath Mountains in southwestern Oregon and northern California. The sediments of the Hornbrook Formation are interpreted to have been deposited in a forearc basin similar to the Cretaceous sediments of the Great Valley forearc in central California. Typically, volcanic rock fragments and feldspars are common in forearc basin settings and are derived from denudation of the adjacent magmatic arc. Previous petrographic studies of Hornbrook sandstones, however, reported an absence of volcanic rock fragments and 15 to 30 percent feldspars. The absence of volcanic rock fragments and low concentration of feldspars in Hornbrook sandstones may be the result of: (1) compaction and/or obliteration of volcanic rock fragments, e.g. formation of pseudomatrix; (2) alteration of volcanic detritus and/or feldspars to clay minerals by circulating diagenetic fluids; and/or (3) absence of volcanic detritus and/or feldspars during deposition. Within the Hornbrook Formation, there are numerous sandstone concretions ranging in size from 2 to 3 cm to over 2 meters in diameter cemented by poikilotopic calcite interpreted to have formed by early cementation in a shallow diagenetic environment. These poikilotopic calcite cements may preserve ductile rock fragments and/or feldspars that otherwise would be obliterated by compaction and/or altered by diagenetic fluids.

Twelve samples of sandstone concretions were collected from each member of the Hornbrook Formation and thin-sections prepared. Detailed petrographic examinations and point count analyses following the Gazzi-Dickinson method reveals a higher abundance of feldspars in the concretions compared with previous petrographic studies. In addition, Q_tFL and Q_mFL_t modal plots of petrographic data from the concretions are consistent with compositions of Late Cretaceous sandstones of the Great Valley Sequence. These compositions are interpreted to represent the final stages of unroofing associated with denudation of the Sierra Nevadan magmatic arc. In summary, this study highlights the usefulness of sandstone concretions formed by early cementation in petrographic studies and provides valuable insights into the Late Cretaceous tectonic setting of the Hornbrook Formation.