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A.R. Nelson

Publications and source records attributed to A.R. Nelson.

24 records · Page 2Linked to original sources
USGS1993

Diatom zonation in southern Oregon tidal marshes relative to vascular plants, foraminifera, and sea level

Diatom assemblages across estuarine marshes show a three-part vertical ecologic zonation of the intertidal zone similar to zonations of foraminiferal and vascular plant assemblages. Gradual changes in the compositions of all three types of assemblages reflect gradational zone boundaries, 5-40m wide. Modern mudflat, low marsh, and high marsh zones can be distinguished from one another with diatom assemblage data at three silty marshes in the middle parts of two river-dominated estuaries. The compositions of vascular plant and diatom assemblages on a transect at a sandy site near the mouth of a third estuary differ from those of the silty transects. A particularly distinct diatom subzone (the marsh border subzone) is marked by a dominance of fresh-to-brackish-water diatoms in a few samples from the upper part of the high marsh and lowest part of the upland zone on all four transects, but the vertical range of the marsh border subzone is large (c0.7m) and its range may vary from site to site depending on the amount of freshwater seepage and runoff into marshes. -from Authors

Journal of Coastal Research↗
USGS1992

Holocene coseismic and aseismic uplift of Isla Mocha, south-central Chile

During the past 6000 years Isla Mocha, a 12 km-long island 30 km off the coast of south-central Chile, experienced a 38 m fall of relative sea level caused primarily by rapid tectonic uplift of the island. As many as 18 raised shorelines (strandlines) record this uplift. Historic accounts of uplift during the great earthquakes (M > 8) of 1835 and 1960 suggest some of the more prominent prehistoric strandlines also emerged during great earthquakes on the interface between the Nazca and South America plates. But the close elevational spacing of strandlines, subdued morphology of strandline beaches, scarcity of exposed bedrock wave-cut platforms, and the extremely high rates of aseismic uplift (ca. 70 mm/yr) of the island since the last great earthquake suggest that many strandlines were raised by aseismic rather than coseismic uplift. Strandline heights and 14 new radiocarbon ages on marine shells show that the present-day uplift rate is more than three times the net rate (ca. 20 mm/yr) of the past 1000 years. The recent high rate probably reflects increased aseismic slip on an inferred thrust fault in the overriding South America plate. Isla Mocha overlies an area of high stress concentration between two major segments of the Chilean subduction zone. The inferred high rate of slip on the thrust fault may be a response to stress changes on the plate interface near the boundary between the segments. ?? 1992.

Quaternary International↗
USGS1992

Lithofacies analysis of colluvial sediments - an aid in interpreting the recent history of Quaternary normal faults in the Basin and Range Province, western United States

Inferring the frequency and magnitude of past earthquakes from the stratigraphy in exposures of normal-faulted sediments is difficult because colluvial lithofacies assemblages adjacent to faults are complex. Similarities in facies assemblages adjacent to young fault scarps in arid to semiarid areas, such as the Basin and Range province, allow lithofacies to be grouped into two genetic architectural elements: debris and wash elements. Upper and lower facies associations can commonly be recognized within each element. A lithofacies code scheme, similar to those used in the analysis of fluvial and glacial lithofacies sequences, provides a concise way of illustrating lithofacies relations in fault exposures. The source lithology of colluvial lithofacies is shown in the code, and soil-horizon symbols can be included. The architecture of lithofacies assemblages near fault scarps in semiarid areas is explained by a model of colluvial sedimentation in response to a single surface faulting event. Analysis of lithofacies assemblages exposed in three trenches across normal faults in the eastern Basin and Range shows how the model can be used to interpret fault histories. Similar facies analysis methods may be useful in interpreting colluvial sequences formed by non-tectonic processes.

Journal of Sedimentary Petrology↗
USGS1991

The Wasatch fault zone, utah-segmentation and history of Holocene earthquakes

The Wasatch fault zone (WFZ) forms the eastern boundary of the Basin and Range province and is the longest continuous, active normal fault (343 km) in the United States. It underlies an urban corridor of 1.6 million people (80% of Utah's population) representing the largest earthquake risk in the interior of the western United States. We have used paleoseismological data to identify 10 discrete segments of the WFZ. Five are active, medial segments with Holocene slip rates of 1–2 mm a −1 , recurrence intervals of 2000–4000 years and average lengths of about 50 km. Five are less active, distal segments with mostly pre-Holocene surface ruptures, late Quaternary slip rates of <0.5 mm a −1 recurrence intervals of ≥10,000 years and average lengths of about 20 km. Surface-faulting events on each of the medial segments of the WFZ formed 2–4-m-high scarps repeatedly during the Holocene; latest Pleistocene (14–15 ka) deposits commonly have scarps as much as 15–20 m in height. Segments identified from paleoseismological studies of other major late Quaternary normal faults in the northern Basin and Range province are 20–25 km long, or about half of that proposed for the medial segments of the WFZ.

Journal of Structural Geology↗