We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are more complex, and that a polygenetic origin best explains them. We suggest that combined bioturbation, seasonal frost action, and erosion processes, with occasional eolian inputs, best account for the mounds, their well sorted stone borders, and the poorly sorted rubbly soil intermound pavements. We propose a transitional, eight-stage conceptual model to explain this complex landscape. The model may generally explain the origin of other similar strongly bioturbated, cold winter-impacted, erosion-prone mounded tracts in the Pacific Northwest.

Newly recognized cyclomedusoid fossils in the Antelope Mountain Quartzite confirm that it is latest Neoproterozoic (Ediacaran) in age. Biogeographic affinities of the cyclomedusoid fossils suggest that the Yreka subterrane and its close associate, the Trinity subterrane, formed after the breakup of Rodinia in an ocean basin bordering Australia, northern Canada, Siberia, and Baltica. Reevaluating biogeographic, geological, and paleomagnetic evidence in the context of this starting point, the Yreka subterrane and Trinity subterrane may have been located at either 7°N or 7°S latitude ca. 580–570 Ma, but were not necessarily close to Laurentia. Continental detrital zircons (3.2–1.3 Ga) in the Antelope Mountain Quartzite most likely came from Australia or Siberia rather than Laurentia. The Yreka subterrane and Trinity subterrane record ∼180 m.y. of active margin events somewhere in Panthalassa (Proto-Pacific Ocean). Paleozoic biogeographic data, paleomagnetism, and regional relationships indicate that Yreka subterrane and Trinity subterrane were located throughout the early Paleozoic in the part of Panthalassa surrounded by Australia, NW Laurentia, Siberia, China, Baltica, and the Uralian terranes. By the mid-Devonian they were located at 31°N or 31°S in a somewhat isolated location, probably in a Northern Hemisphere oceanic plateau or island chain well outboard of other tectonic elements, and by the Permian they were almost completely isolated from other tectonic elements. The Yreka subterrane, as part of the Klamath superterrane, was not native to North America and did not accrete to it until the Early Cretaceous.

The Bear Peak intrusive complex is a Late Jurassic (ca. 144 Ma) composite plutonic suite that ranges in composition from ultramafic to silicic. Clinopyroxene- and hornblende-rich ultramafic cumulate rocks form an intrusion breccia that is complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. The bulk of the intrusive complex consists of mappable gabbroic to quartz dioritic to tonalitic/granodioritic units. The Bear Peak intrusive complex was emplaced into rocks of the Rattlesnake Creek terrane, producing a dynamothermal contact aureole. Contact metamorphism was chiefly at hornblende-hornfels-facies conditions and grades into regional greenschist-facies metamorphism. Andalusite, cordierite, and chloritoid form small porphyroblasts in some of the more aluminous metasedimentary rocks, indicating low-pressure contact metamorphism (<4 kb). Al-in-hornblende geobarometry in quartz dioritic to tonalitic rocks also suggests pressure conditions of ∼4 kb. Pseudomorphs of original chiastolite porphyroblasts developed during contact metamorphism of pelitic horizons in the Upper Jurassic Galice Formation, which lies in the footwall of the regional Orleans thrust fault, indicate that the Bear Peak intrusive complex was emplaced after regional contraction related to the Nevadan orogeny. The Bear Peak intrusive complex is an example of the extended compositional range characteristic of some oceanic-arc plutonic suites and demonstrates how multiple, chiefly magmatic processes, can yield a broad range of rock compositions within a single intrusive complex. Mafic magmatic enclaves are common in most of the plutonic units of the Bear Peak intrusive complex, and distinctive migmatitic amphibolite enclaves indicate that magma temperatures were sufficient to facilitate dehydration-melting of metabasic rocks. The distribution of host-rock enclaves and screens suggest that much of the gabbroic to quartz dioritic parts of the Bear Peak intrusive complex were emplaced as magmatic sheets that coalesced into mappable, relatively homogeneous units that grew by piecemeal intrusion. Ultramafic-mafic cumulates and hornblende gabbro crystallized from a high-Mg, low-Al basaltic parent, whereas high-Al, low-Mg contents in quartz dioritic rocks suggest an evolved basaltic or basaltic andesite parent. The biotite tonalite/granodiorite rocks have high Sr values (>700 ppm), large Sr/Y and Ba/Y ratios, and reverse J-shaped rare-earth-element (REE) patterns. These features are characteristic of partial melting of metabasic rocks in which amphibole ± garnet are residual phases. Thus, major, trace, and REE compositions indicate at least two batches of magma were involved in the petrogenesis of the Bear Peak intrusive complex. Complex field relationships and geochemical data suggest that multiple magmas passed through the cumulates and presumably fed structurally higher mafic units in the complex.