Early diagenetic origin of mudrock concretions from the Hornbrook Formation (Upper Cretaceous) in Siskiyou County, California: Implications for preservation of primary sedimentary fabrics Michael J. Stephens, William S. Elliott, Jr., Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd., Evansville, Indiana 47712; [email protected] Jad. A. D’Allura, Department of Chemistry, Southern Oregon University, 1250 Siskiyou Blvd., Ashland, Oregon 97520

Abstract Methods Results and Discussion The Hornbrook Formation (Upper Cretaceous) consists of approximately 1,200 m of mudrock, sandstone, and Concretions were collected from several locations within southern Oregon and northern California. Most concretions Figure 15. Geochemical Data Figure 16. Geochemical Data Figure 17. Geochemical Data 22.00 8.50 3.50 conglomerate exposed along the northeastern flank of the Klamath Mountains in southwestern Oregon and northern were collected from the Blue Gulch Mudstone Member of the Hornbrook Formation. Host mudrock samples adjacent to Mudrock Mudrock Mudrock 7.50 3.00 20.00 California. The Hornbrook Formation is subdivided into five members: Klamath River Conglomerate (oldest), the concretions were also collected for geochemical analyses. Field relationships between the concretions and bedding 6.50 2.50 18.00 Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (youngest). planes were recorded. 5.50

3 2.00 O

2 16.00 4.50

For this study, 20 mudrock concretions were examined from the Blue Gulch Mudstone Member collected near the FeO

Mass and volumes of cubed samples of concretions were measured to calculate densities. MgO Al 1.50 3.50 town of Hilt in Siskiyou County, California. 14.00 Concretions Then, dilute hydrochloric acid digestion was used to remove calcite from the samples 1.00 Concretions 2.50 Concretions (Fig. 3). The samples were then centrifuged, decanted, and dried. The mass of the 12.00 In the field, mudrock concretions are spherical- to disk-shaped, 20- to 180-cm in diameter, and are encased in 1.50 0.50 siliciclastic mudrock. The concretions are oriented with the long axes parallel to bedding, and occur as isolated bodies remaining sediments were measured and then subtracted from the original amount. 10.00 0.50 0.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 62.00 64.00 66.00 68.00 70.00 72.00 74.00 76.00 78.00 80.00 62.00 64.00 66.00 68.00 70.00 72.00 74.00 76.00 78.00 80.00 and/or bands within mudrock intervals. In several instances, thin (2 to 4 cm) sandstone beds in close stratigraphic Calcite weight percentages were then calculated. SiO2 SiO2 SiO2 proximity wrap around and drape mudrock concretions. Several concretions contain vertical fractures filled with The samples were analyzed using X-Ray Fluorescence Spectroscopy and the geochemical Figure 3. Samples digested by dilute hydrochloric acid to Figures 15. 16. and 17. Geochemical data (Al O vs. SiO , FeO vs. SiO , MgO vs. SiO ) for concretions and nearby mudrock calcite that produce a brecciated fabric. In the laboratory, concretions contain 65 to 75 wt. % calcite as determined by data were obtained for mudrock and concretion samples. This allowed for comparison of 2 3 2 2 2 dilute hydrochloric acid digestion; mudrocks encasing the concretions are not calcareous. Preliminary geochemical determine the amount of calcite sediments from the Hornbrook Formation. the geochemistry of the concretions and surrounding mudrocks. in concretions. 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 The mudrock concretions in the Hornbrook are interpreted to have formed in an early diagenetic environment. Concretions in the Hornbrook are found oriented along bedding 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. Hand Samples There were no distinguishable differences for TiO2, MnO, CaO, Na2O, K2O, and P2O5. planes, and in several instances, sandstone beds in close vertical stratigraphic proximity Overall, the mudrock concretions in the Hornbrook are interpreted to have formed in an early diagenetic environment, Figure 4. Hand sample of Figure 5. Concretion are contorted (Fig. 18). Moreover, mudrock concretions contain vertical fractures filled concretion with vertical with lamination and as evidenced by contorted bedding and fractures resulting from differential compaction associated with burial after with calcite creating a brecciated fabric (Fig. 4). These fractures have been interpreted to fractures. Note the fractures discrete bioturbation have formed during compaction. their formation. Additionally, early diagenetic formation of the concretions resulted in differing chemical are concentrated near the along bedding planes. transformations of the encasing mudrock. Finally, the early diagenetic origin of these concretions may preferentially center of the concretion. Also Note the weathering The mudrock concretions found in the Hornbrook contain different geochemistry preserve primary sedimentary fabrics and textures that would otherwise be obliterated by compaction. note the presence of burrows. near the top of the than the surrounding mudrock. Preliminary geochemical analyses of the siliciclastic concretion. 2 cm 2 cm sediment from the concretions by X-Ray Fluorescence Spectroscopy yielded 72.7 to Introduction 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, The Hornbrook Formation is a 1,200 m thick deposit of Upper Cretaceous conglomerate, sandstone, and mudrock 18.5 to 19.4 wt. % Al2O3, 2.6 to 3.0 wt. % MgO, and 6.3 to 7.6 wt. % FeO. exposed along the northeastern flank of the Klamath Mountains in southwestern Oregon and northern California (Fig. There were no distinguishable differences for TiO2, MnO, CaO, Na2O, K2O, and P2O5. Chlorite, Figure 18. Mudrock 1). The Hornbrook Formation is interpreted to represent a marine transgression and nonconformably overlies igneous for example, is a mineral associated with diagenesis and is composed of Mg, Fe, Al, and Si. Most concretion with sandstone and metamorphic rocks of the Klamath Mountains. Locally, the Hornbrook is disconformably overlain by the Eocene Figure 6. Mudrock concretion 2 cm of these oxides are enriched within the mudrock further suggesting that the concretions form beds draped around it. Payne Cliffs Formation and the Oligocene Colestin Formation of the Western Cascade Group (Fig. 2). containing fractures and Figure 7. Concretion that is being cut by a early and are not as subjected to geochemical transformations as the surrounding mudrock during 2 cm The Blue Gulch Mudstone Member of the Hornbrook Formation (Upper Cretaceous) is composed of dark gray to burrows. sandstone dike. The formation of the concretion diagenesis. occurs after the intrusion of this sand. greenish gray, silty mudrock. It is interbedded with claystone and sparse thin-to-medium bedded, planar stratified, Concretions provide preferential preservation compared to host rock and contain original fabrics. For example, some of the very fine-grained sandstones with various convoluted bedding and scour features (e.g. flute casts). Within mudrock Thin Sections concretions studied show laminations, intact fossils (e.g. gastropod shells), and bioturbation (see hand sample and thin sections intervals, there are several discrete bands of oblate to spherical concretions that range in size from 20 to 180 cm in images). If not for the preferential preservation of the relatively hard concretions, these features would have been obliterated by diameter. Concretions are hard, spherical to ellipsoidal features that form by the precipitation of mineral cement compaction and diagenesis. within the pore space of existing sediment. The concretions are calcite-cemented and encased by mudrock. These features along with field relationships were analyzed to help determine timing of their formation and any differences Mudrock concretions from the Hornbrook were found to be 65 to 75 wt. % calcite. This high percentage of calcite is probably in geochemical variation from their encasing mudrocks. due to the early cementation of the concretions, preserving the original porosity of the sediment. Thus, initial porosity of these muddy sediments may have been 60 to 70 percent. GENERALIZED GEOLOGIC MAP OF THE STUDY AREA GENERALIZED STRATIGRAPHIC COLUMN X X X X X o o o W123 5 W123 45’ W123 30’ Western Cascades (Paleogene) X X X X X e Colestin Fm. n

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g Conclusions Medford Hornbrook Formation Acknowledgements (Late Cretaceous) X X X X X X X Cascades o

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l X X Payne Cliffs Fm.

a o X X Diorite & granodiorite plutons N42 15’ X X (Late Jurassic) P Condrey Mountain Terrane • Mudrock concretions in the Hornbrook are interpreted to have We would like to thank Endeavor and the X Ashland (Late Jurassic) X Rattlesnake Creek Terrane X X Figure 8. Sandstone dike Figure 9. Bioturbation Figure 10. Possible algae Figure 11. Bioturbation cross- Provost Programming Grant at USI for funding X X (Late Triassic to Early Jurassic) formed in an early diagenetic environment, as evidenced by X X X X

X Hayfork & Applegate Terranes s

n through concretion. Mt Ashland (Permian to Early Jurassic) Blue Gulch u patterns in concretion. preserved in concretion. cutting lamination.

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t X Pluton Central Metamorphic Belt Mudstone e contorted bedding and fractures resulting from differential

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H 100 Osburger Gulch Sandstone Oregon. X X X chemical transformations of the encasing mudrock. X X X X X X X X X X Klamath River Conglomerate X X X 20 30 0 X X X X 5 0 10 X X X X X X X X Vesa Kilometers X X X X X X X Mt. Ashland granodiorite pluton (Jurassic) Bluffs X X X X X Yreka • The early diagenetic origin of these concretions may preferentially X Pluton Modified from Mortimer & Coleman (1984). West Hayfork Terrane (Permian to Triassic) preserve primary sedimentary fabrics and textures that would Figure 1. Generalized geologic map of the study area. The Figure 2. The Hornbrook Formation is subdivided into five Hornbrook Formation is exposed along the northeast flank of the members, from oldest to youngest: Klamath River Conglomerate otherwise be obliterated by compaction. o Klamath Mountains, dipping northeast at about 20 . (continental), Osburger Gulch Sandstone, Ditch Creek Siltstone, Figure 12. Bioturbation with Figure 13. Gastropod shell Figure 14. Calcite filled fractures; Rocky Gulch Sandstone, and Blue Gulch Mudstone. parallel lamination. fragment in siltstone. note the angular mudrock fragments. Annual Meeting of the Geological Society of America, Vancouver, 2014