Geology topics

R. S. Lewis

Publications and source records attributed to R. S. Lewis.

At least 19 recordsLinked to original sources

USGS2011

Isotopic evolution of the idaho batholith and Challis intrusive province, Northern US Cordillera

The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the ∼98 Ma to 54 Ma Idaho batholith and ∼51 Ma to 43 Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith's history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap. The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith. These data suggest that the early suites of metaluminous magmatism (98–87 Ma) represent crust–mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83–67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66–54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust.

Idaho batholith↗

USGS2010

Detrital zircon analysis of Mesoproterozoic and neoproterozoic metasedimentary rocks of northcentral idaho: Implications for development of the Belt-Purcell basin

The authors analyzed detrital zircon grains from 10 metasedimentary rock samples of the Priest River complex and three other amphibolite-facies metamorphic sequences in north-central Idaho to test the previous assignment of these rocks to the Mesoproterozoic Belt-Purcell Supergroup. Zircon grains from two samples of the Prichard Formation (lower Belt) and one sample of Cambrian quartzite were also analyzed as controls with known depositional ages. U-Pb zircon analysis by laser ablation - inductively coupled plasma - mass spectrometry reveals that 6 of the 10 samples contain multiple age populations between 1900 and 1400 Ma and a scatter of older ages, similar to results reported from the Belt- Purcell Supergroup to the north and east. Results from the Priest River metamorphic complex confirm previous correlations with the Prichard Formation. Samples from the Golden and Elk City sequences have significant numbers of 1500-1380 Ma grains, which indicates that they do not predate the Belt. Rather, they are probably from a relatively young, southwestern part of the Belt Supergroup (Lemhi subbasin). Non-North American (1610-1490 Ma) grains are rare in these rocks. Three samples of quartzite from the Syringa metamorphic sequence northwest of the Idaho batholith contain zircon grains younger than the Belt Supergroup and support a Neoproterozoic age. A single Cambrian sample has abundant 1780 Ma grains and none younger than ~1750 Ma. These results indicate that the likely protoliths of many high-grade metamorphic rocks in northern Idaho were strata of the Belt-Purcell Supergroup or overlying rocks of the Neoproterozoic Windermere Supergroup and not basement rocks.

Canadian Journal of Earth Sciences↗

USGS2003

Responses of infaunal populations to benthoscape structure and the potential importance of transition zones

Relationships between population abundance and seafloor landscape, or benthoscape, structure were examined for 16 infaunal taxa in eastern Long Island Sound. Based on analyses of a side-scan sonar mosaic, the 19.4-km 2 study area was comprised of six distinct large-scale (> km 2 ) benthoscape elements, with varying levels of mesoscale (km 2 -m 2 ) and small-scale (< m 2 ) physical and biological habitat heterogeneity. Transition zones among elements varied from ~50 to 200 m in width, comprised ~32% of the benthoscape, and added to overall benthoscape heterogeneity. Population abundances of nine taxa varied significantly among the large-scale elements. Most species were found at high abundances only in one benthoscape element, but three had several foci of elevated abundances. Analyses of population responses to habitat heterogeneity at different spatial scales indicated that abundances of eight taxa varied significantly among spatial scales, but the significant scales were mixed among these species. Relatively large residual variations suggest significant amounts of mesoscale spatial variation were unaccounted for, varying from ~1 km 2 to several m 2 . Responses to transition zones were mixed as well. Abundances of nine taxa varied significantly among transition zones and interiors of benthoscape elements, most with elevated abundances in transition zones. Our results show that infaunal populations exhibit complex and spatially varying patterns of abundance in relation to benthoscape structure and suggest that mesoscale variation may be particularly critical in this regard. Also, transition zones among benthoscape features add considerably to this variation and may be ecological important areas in seafloor environments.

New York↗

USGS2002

Geological framework data from Long Island Sound, 1981-1990: A digital data release

Introduction Since 1980 the Coastal and Marine Geology Program of the U.S. Geological Survey and Connecticut Department of Environmental Protection have conducted a joint program of cooperative geologic research in Long Island Sound and its vicinity. As part of this program, a highly successful regional-scale study of the geologic framework was completed. Reconnaissance high-resolution seismic reflection data were collected and used to establish the basic stratigraphy within the Sound and to map the major geologic units; field verification of the geologic interpretations of the seismic profiles was primarily accomplished with vibratory cores. These interpretations were in turn used to produce basin-wide syntheses of the late Quaternary depositional history. Unfortunately, the original seismic records and core logs were generated only in analog form. These unique paper documents, which are still under demand for industrial applications and academic research, are fragile and have become ragged from frequent use. The purpose of this report is to preserve these data by converting the seismic profiles, core descriptions, and ancillary reports into digital form, and to organize these files into a product that can be more readily accessed and disseminated. Not all of the existing high-resolution seismic-reflection surveys, collected in Long Island Sound through cooperatives with the U.S. Geological Survey and the Connecticut Department of Environmental Protection, have been incorporated into this report. These surveys, whose records are still in need of preprocessing and annotation, generally cover smaller areas along the Connecticut coast and were originally intended to provide additional detail to the larger, more regional data sets presented herein. The digital release of the omitted data sets is planned as part of a future product.

Connecticut, New York↗

USGS2002

Processes controlling the remobilization of surficial sediment and formation of sedimentary furrows in north-central Long Island Sound

Sidescan sonar, bathymetric, subbottom, and bottom-photographic surveys and sediment sampling have improved our understanding of the processes that control the complex distribution of bottom sediments and benthic habitats in Long Island Sound. Although the deeper (>20 m) waters of the central Sound are long-term depositional areas characterized by relatively weak bottom-current regimes, our data reveal the localized presence of sedimentary furrows. These erosional bedforms occur in fine-grained cohesive sediments (silts and clayey silts), trend east-northeast, are irregularly spaced, and have indistinct troughs with gently sloping walls. The average width and relief of the furrows is 9.2 m and 0.4 m, respectively. The furrows average about 206 m long, but range in length from 30 m to over 1,300 m. Longitudinal ripples, bioturbation, and nutclam shell debris are common within the furrows. Although many of the furrows appear to end by gradually narrowing, some furrows show a "tuning fork" joining pattern. Most of these junctions open toward the east, indicating net westward sediment transport. However, a few junctions open toward the west suggesting that oscillating tidal currents are the dominant mechanism controlling furrow formation. Sedimentary furrows and longitudinal ripples typically form in environments which have recurring, directionally stable, and occasionally strong currents. The elongate geometry and regional bathymetry of Long Island Sound combine to constrain the dominant tidal and storm currents to east-west flow directions and permit the development of these bedforms. Through resuspension due to biological activity and the subsequent development of erosional bedforms, fine-grained cohesive sediment can be remobilized and made available for transport farther westward into the estuary.

Journal of Coastal Research↗

USGS2001