PREFERENTIAL PRESERVATION OF SEDIMENTARY FABRICS AND TEXTURES IN MUDROCK CONCRETIONS FROM THE HORNBROOK FORMATION (UPPER CRETACEOUS) IN SISKIYOU COUNTY, CALIFORNIA

The Hornbrook Formation (Upper Cretaceous) is made-up 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). Concretions provide important depositional and sedimentological information that may not have been preserved and/or was obliterated by compaction in surrounding mudrocks. Calcareous concretions were collected from mudrocks in the Blue Gulch Mudstone Member of the Hornbrook Formation near the town of Hilt in Siskiyou County, California.

Calcareous concretions in the Blue Gulch are 10 to 50 cm in diameter with an elliptical shape elongated parallel to bedding and are typically encased within dark gray, siliceous silty mudrock. The concretions occur as either isolated bodies within silty mudrock, or as bands along bedding within silty mudrock. These bands have been observed to extend up to 30 meters in the field. Ten samples of calcareous concretions were collected for further study. The concretions were cut using a water-lubricated rock saw, and surfaces polished for examination.

At least six of the concretions contain preserved trace fossils, including Chondrites, Helminthopsis, Phycosiphon, Planolites, and Thalassinoides. These burrows are only slightly compacted, suggesting early cementation of these concretions in a shallow diagenetic environment. In three of the concretions, mollusk fragments with only minor fragmentation are present, also suggesting an early diagenetic origin for these calcareous concretions. At least three of the concretions contain disseminated pyrite with faint zoning within the concretion, attesting to localized reduced conditions during their formation. Two concretions contain several fractures formed by dewatering of the mud prior to cementation. These cracks have subsequently been filled with siderite and/or calcite. Although these concretions have varied features, it is apparent from sedimentological observations that they formed early during diagenesis.