Benjamin A. Morgan

Role of debris flows in long-term landscape denudation in the central Appalachians of Virginia

Four major storms that triggered debris flows in the Virginia‐West Virginia Appalachians provide new insights into the role of high-magnitude, low-frequency floods in longterm denudation and landscape evolution in mountainous terrain. Storm denudation in the Blue Ridge Mountain drainage basins is approximately an order of magnitude greater compared to basins located in the mountains of the Valley and Ridge province. This difference is probably the result of higher storm rainfall from the Blue Ridge storms. Radiocarbon dating of debris-flow deposits in the Blue Ridge indicates a debris-flow return interval of not more than 2‐4 k.y. in mountainous river basins. This finding, combined with measurements of basin denudation, suggests that approximately half of the long-term denudation from mechanical load occurs episodically by debris-flow processes. Although floods of moderate magnitude are largely responsible for mobilizing sediment in lowgradient streams, our data suggest that high-magnitude, low-frequency events are the most significant component in delivering coarse-grained regolith from mountainous hollows and channels to the lowland floodplains.

Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

Abstract A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains.

Venezuelan debris flow and flash flood disaster of 1999 studied

Alluvial fans in urban and rural areas are sites of episodic, rainfall-induced natural hazards [Garner 1959; Campbell, 1975; Wieczorek et al., 2001]. Debris flows, hyper-concentrated flows, and flash floods that occur episodically in these alluvial fan environments place many communities at high risk during intense and prolonged rainfall. Although scientists have become better able to define areas of high natural hazard, population expansion and development pressures in such areas have put more people at risk than ever before. Recognition of the magnitude and distribution of debris-flow and flash-flood hazards is therefore a critically important area of natural hazard research.

Chemistry and mineralogy of garnet pyroxenites from Sabah, Malaysia

Garnet pyroxenites and corundum-garnet amphibolites from the Dent peninsula of eastern Sabah (North Borneo) occur as blocks in a slump breccia deposit of late Miocene age. The earliest formed minerals include pyrope-almandine garnet, tschermakitic augite, pargasite, and rutile. Cumulate textures are present in two of the six specimens studied. The earlier fabric has been extensively brecciated and partly replaced by plagioclase, ilmenite, and a fibrous amphibole. The bulk composition and mineralogy of these rocks are similar to those of garnet pyroxenite lenses within ultramafic rocks. Estimated temperature and pressure for the origin of the Sabah garnet pyroxenites is 850±150° C and 19±4 kbar.

Major Structural Break between Paleozoic and Mesozoic Rocks in the Eastern Sierra Nevada, California

A major structural break lies between the Paleozoic and Mesozoic rocks in the eastern Sierra Nevada. Tight folds in the Paleozoic section are overturned to the west and faulted against an apparently downthrown steeply dipping sequence of Mesozoic metavolcanic rocks on the west. This fault is a segment of a large regional high-angle fault that separates the Paleozoic and Mesozoic strata in the eastern Sierra Nevada from the Pine Creek pendant northward approximately 50 mi (80 km) to Mount Dana. The development of this fault preceded the widespread intrusions of the Upper Cretaceous magmas of the John Muir sequence of the Sierra Nevada batholith.