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Cypress, CA, United States

Raymond L.A.,Appalachian State University | Raymond L.A.,Coast Range Geologic Mapping Institute
International Geology Review | Year: 2015

Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the Yolla Bolly Terrane and the Central Terrane. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen. © 2014 © 2014 Taylor & Francis. Source

Raymond L.A.,Appalachian State University | Raymond L.A.,Coast Range Geologic Mapping Institute | Webb Jr. F.,Appalachian State University | Love A.B.,Appalachian State University
Southeastern Geology | Year: 2014

Silurian sandstone-dominated units, including the Tuscarora Sandstone, Rose Hill Formation, and Keefer Sandstone, are the preeminent ridge-capping units within the Clinch Mountain Wildlife Management Area of the Saltville and Broadford quadrangles of Southwestern Virginia. These three units constitute all but the <1m-25m of Tonoloway Limestone that caps the Silurian section. Traditional features used in mapping the Silurian siliciclastic units in southwestern Virginia include: 1) sandstone character, including grain size and cement type; 2) abundance of shale; and 3) stratigraphic position with predominantly light gray to white sandstones plus minor graybrown shale and white granule conglomerate of the Tuscarora Sandstone at the base of the section succeeded upward by light gray to hematitic dusky red to black Rose Hill sandstone, conglomerate, siltstone, and shale; and overlying, light gray to white Keefer sandstone and granule to pebbly sandstone. Tuscarora quartz arenites are cemented by locally abundant silica cement, but some thin arenite beds are cemented by hematite, as are joints and ichnofossil traces. Large-scale trough cross-beds and cross stratifiction are common. In contrast, the Rose Hill Formation is characterized by abundant, multicolored mudrocks and similarly colored sandstone and conglomerate. Rose Hill sandstones composed of quartz grains plus localized lithic clasts, fossils, and ooids are typified and generally defined by hematite cement that may have been enhanced twice within a complex diagenetic history involving quartz, calcite, and hematite precipitation. In a few sandstones, phyllosilicate matrix and cements bind the grains. Coarsegrained, cross-bedded, hematite-rich sandstones and pebble- to granule-bearing sandstone and conglomerate are noteworthy, but very light gray quartz arenite beds are present and locally form packages that define members. Keefer quartz arenites are similar to Thscarora quartz arenites in their framework compositions, but tend to be coarser grained than Tuscarora sandstones. Local Keefer cements include silica and a MnOmineral. Gray-brown shales and sandstone breccias are minor constituents of the formation. Mapping the contacts between units through moderate to poor exposures is problematic. Difficulties are a consequence of poor exposure, conformable to interfingering contacts, the local presence of hematite-cemented sandstone typical of the Rose Hill Formation within the Tuscarora and Keefer sandstones, very light gray quartz arenites within the Rose Hill Formation, and the variable thickness of the diagenetically distinctive Rose Hill Formation. Consequently, mapping and recognizing the limits of the Rose Hill Formation locally, by necessity, is subjective and is based on hematite-bearing sandstone character rather than abundance of mudrocks, because the latter are rarely exposed in off road terrains. Source

Raymond L.A.,Coast Range Geologic Mapping Institute | Raymond L.A.,Appalachian State University | Bero D.A.,Coast Range Geologic Mapping Institute | Bero D.A.,Sonoma State University
Geosphere | Year: 2015

Understanding details of accretionary complex architecture is essential to understanding construction of oceanic "outer" sides of orogens. The architecture of the Franciscan Complex (California), considered by many to be the "type" accretionary complex, is widely viewed in the context of terranes or belts delimited by reconnaissance mapping that reveals neither regional variations within terranes nor critical details of stratigraphy and structure. The architectural importance of Franciscan mélanges is recognized, but the importance of sandstone-matrix mélanges and olistostromal sandstones is not. Large-scale mapping in Sonoma and Marin counties, California, shows that Franciscan rocks are deformed, submarine-fan units of Facies A-E, plus Facies F olistolith-bearing submarine channel sandstones and olistostromal sandstone- and shale-matrix mélanges. Some mélanges are polygenetic with a sedimentary origin and a tectonic overprint. Glaucophane schists were recycled into conglomerates and olistostromes. Mappable units constitute members, broken and dismembered formations, and mélanges. Considering the stratigraphy and structure evident at the 1:24,000 scale, accretion via a subduction channel mechanism is impossible. The Sonoma-Marin Central belt or Central terrane (mélange) is not a monolithic shale-matrix mélange and lacks this characteristic of rocks assigned the same name to the north. Franciscan rocks here structurally underlie thrust-faulted fragments of a regional ultramafic sheet and, locally, an underlying exotic block-bearing serpentinite-matrix mélange. The detailed mapping shows that regional relations among and within Franciscan terranes and belts are poorly understood and suggests that such mapping is needed to clarify accretionary complex architecture and history. The implication for accretionary complex studies, in general, is that, while terrane or belt designations provide a general picture of the collage nature of accretionary complexes and clarification of regional relationships, only large-scale structural and stratigraphic studies can elucidate the architectural details of these orogenic complexes. © 2015 Geological Society of America. Source

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