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