EARLY DIAGENETIC ORIGIN OF MUDROCK CONCRETIONS FROM THE HORNBROOK FORMATION (UPPER CRETACEOUS) IN SISKIYOU COUNTY, CALIFORNIA: IMPLICATIONS FOR PRESERVATION OF PRIMARY SEDIMENTARY FABRICS

The Hornbrook Formation (Upper Cretaceous) consists of approximately 1,200 m of mudrock, sandstone, and conglomerate exposed along the northeastern flank of the Klamath Mountains in southwestern Oregon and northern California. The Hornbrook Formation is subdivided into five members: Klamath River Conglomerate (oldest), Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (youngest). For this study, 20 mudrock concretions were examined from the Blue Gulch Mudstone Member collected near the town of Hilt in Siskiyou County, California.

In the field, mudrock concretions are spherical- to disk-shaped, 20- to 180-cm in diameter, and are encased in siliciclastic mudrock. The concretions are oriented with the long axes parallel to bedding, and occur as isolated bodies and/or bands within mudrock intervals. In several instances, thin (2 to 4 cm) sandstone beds in close stratigraphic proximity wrap around and drape mudrock concretions. Several concretions contain vertical fractures filled with calcite that produce a brecciated fabric. In the laboratory, concretions contain 65 to 75 wt. % calcite as determined by dilute hydrochloric acid digestion; mudrocks encasing the concretions are not calcareous. Preliminary geochemical analyses of the siliciclastic sediment from the concretions by X-ray fluorescence spectroscopy yielded 72.7 to 78.0 wt. % SiO₂, 12.8 to 15.8 wt. % Al₂O₃, 0.6 to 1.0 wt. % MgO, and 1.7 to 3.8 wt. % FeO compared to encasing mudrocks with values of 63.5 to 64.9 wt. % SiO₂, 18.5 to 19.4 wt. % Al₂O₃, 2.6 to 3.0 wt. % MgO, and 6.3 to 7.6 wt. % FeO. There were no distinguishable differences for TiO₂, MnO, CaO, Na₂O, K₂O, and P₂O₅.

Overall, the mudrock concretions in the Hornbrook are interpreted to have formed in an early diagenetic environment, as evidenced by contorted bedding and fractures resulting from differential compaction associated with burial after their formation. Additionally, early diagenetic formation of the concretions resulted in differing chemical transformations of the encasing mudrock. Finally, the early diagenetic origin of these concretions may preferentially preserve primary sedimentary fabrics and textures that would otherwise be obliterated by compaction.