Sedimentology, structure, and geochemistry of the Galice Formation: sediment fill of a back-arc basin and island arc in the western Klamath Mountains

The western Jurassic belt of the Klamath Mountains is an ideal locale for the study of ophiolite genesis, island arc development, and study of flysch deposition from a nearby eroding island arc and continental margin. Closure and imbrication of an inferred back-arc basin coincided with the Late Jurassic Nevadan orogeny (ca 150 Ma). The Galice Formation is the youngest formation involved in the Nevadan orogeny (Lanphere et al., 1968). In the type section in southwestern Oregon, the Galice structurally overlies the Rogue Formation volcaniclastics. The Rogue Formation has been previously interpreted as an island arc assemblage (Garcia, 1979). The type Galice consists of thin-bedded, dark grey to black slaty shales with a prominent cleavage. Chert-pebble conglomerate is found locally and most probably represents the basal Galice. Coarse-grained pebbly sandstone beds with large shale rip-up clasts are interpreted as representing water-cut channels. Paleoflow data were calculated from measurements of flute casts, parting lineation, grooves, and longitudinal ridges found across from the town of Galice and along Grave Creek. Paleoflow was found to be dominantly towards the west (Grave Creek section) and north (Galice type section) after corrections for local folding and overturned bedding. The Galice is dated as late middle Oxfordian to middle Kimmeridgian on the basis of Buchia concentrica (Sowerby) (Imlay, 1980). Plant fragments consisting of carbonaceous broken stems and possible leafy material are preserved in Galice shales. Apparent trace fossils are found parallel to sub-parallel to bedding. The Galice Formation contains minor interbedded volcaniclastics which appear indistinguishable from the Rogue Formation volcanics. Flattened shale clasts were observed locally in the tuffaceous units within the Galice. The Rogue and Galice Formations generally strike N to NE and dip to the SE. The Rogue-Galice contact is a probable high-angle fault whose exact displacement and times of displacement are uncertain. Nevadan folds are found throughout the study area cut by an axial-planar cleavage. Post-Nevadan folds are defined as those folds that have refolded the axial-planar cleavage. Bedding-cleavage intersection lineations were used to describe the trend and plunge of Nevadan fold axes. An asymmetric Nevadan fold overturned to the NW was identified across from the town of Galice. The Grave Creek section reveals tight to isoclinal Nevadan folds and fold vergence alternates in a systematic way from NE to SW. Proposed F3 or post-Nevadan folds trend NW-SE and account for the dispersal of Nevadan fold axes. The Nevadan axial plane AP1 strikes 022º and dips 060ºSE. The orientation of the post-Nevadan axial plane AP3 is 119º/080ºNE. F1 and F2 fold axes have similar orientations as they both trend SSW and plunge gently SW at 4-10º. The F1 and F2 fold axes are interpreted as Nevadan fold axes with F2 representing a late-stage Nevadan folding episode. The mean F2 fold axis trends 303º and plunges 14ºNW. The fold interference pattern is transitional between types I and II of Ramsay (1967). XRF and INAA analyses show differences exist between the type Galice and Galice shales that overlie the Josephine ophiolite to the south. The type Galice and Galice shales have higher V, Cr, and Ni abundances than the standard SCo-1, which is thought to represent an arc volcaniclastic component or ophiolitic component. Differences were noted in the mean values of the type Galice and Galice in the abundance of Mn, Mg, Na, K, Sr, and Rb, but these differences are not significant at the 2sigma level. Comparison of immobile elements showed differences in geochemical behavior between the type Galice and Galice shales in the plots of Zr versus A12O3, K2O versus TiO2, K2O versus A12O3, Y versus A12O3, and K2O versus V. These differences in geochemical behavior are presented as preliminary, general findings worthy of more detailed study to more accurately characterize the similarities and differences between the type Galice and Galice shales