Sam A Johnstone
(he/him)I am a Research Geologist interesested in understanding how processes of erosion and faulting interact to shape Earth’s surface. I study these phenomena through geologic maps, laboratory techniques that measure the history of rocks being exhumed toward the surface, and with measurements and models of Earth's surface topography.
I came to the USGS as a Mendenhall Postdoctoral Fellow in 2016 to study landscape evolution in the Southern Rocky Mountains and to help complete a facility for measuring the cooling histories of minerals as part of the Mineral Resources Program. In 2018 I transitioned to a job as a Research Geologist funded by the National Cooperative Geologic Mapping program, where I work now. In this capacity I conduct regional-scale geologic mapping and synthesis of Quaternary sedimentary deposits with the goal of ultimately establishing a seamlesss national-scale geologic map database that can aid in the characterization of geologic hazards and natural resources. This work is currently focused in two regions, in Arkansas and in a large transect spanning the Intermountain West, for which I also serve as the lead of the surficial geologic working group.
In addition to these roles I conduct research into the mechanisms of landscape evolution in an effort to better interpret the geologic record of natural hazards and past environmental change, focusing on two broad classifications of problems. First, I seek to develop numerical and statistical models to refine geologic inferences made from geochronologic data and to quantify uncertainty in those interpretations. Second, I try to understand the connection between earth surface processes, landscape form, and recent geologic deposits by developing numerical models that enable simulation of these phenomena. In these efforts I am interested in developing approaches for characterizing landscape evolution through paired analysis of topographic and geologic map data.
Prior to starting at the USGS I obtained my PhD from Stanford University in 2016, where I studied how sediment transport and erosion processes shaped hillslopes and river networks, and how some micro-climatic gradients enduced by aspect differences influenced these processes. In 2011 I obtained my MSc from UC Santa Cruz, where I had also obtained my BS a few years earlier. It was at UC Santa Cruz that I first got excited about geologic mapping and where I developed an interest in using low-temperature thermochronology to study histories of faulting.
Education and Certifications
Stanford University, PhD in Geology
UC Santa Cruz, MS in Geology
UC Santa Cruz, BS Earth Science
Science and Products
A stratigraphic approach to inferring depositional ages from detrital geochronology data
A supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system
Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA
Interpretation of low‐temperature thermochronometer ages from tilted normal fault blocks
Controls on submarine channel-modifying processes identified through morphometric scaling relationships
Sediment unmixing using detrital geochronology
Non-USGS Publications**
**Disclaimer: The views expressed in Non-USGS publications are those of the author and do not represent the views of the USGS, Department of the Interior, or the U.S. Government.
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Filter Total Items: 18
A stratigraphic approach to inferring depositional ages from detrital geochronology data
With the increasing use of detrital geochronology data for provenance analyses, we have also developed new constraints on the age of otherwise undateable sedimentary deposits. Because a deposit can be no older than its youngest mineral constituent, the youngest defensible detrital mineral age defines the maximum depositional age of the sampled bed. Defining the youngest `defensible' age in the facAuthorsSamuel Johnstone, Theresa M. Schwartz, Christopher S. Holm-DenomaA supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system
Establishing temporal constrains on major volcanic eruptions is limited by the precision of existing geochronometers. Prior work on the La Garita caldera, created by the eruption of the Fish Canyon Tuff, failed to resolve temporal differences between pre-, syn-, and post-collapse eruptive units. Here, we report 40Ar/39Ar geochronologic data supporting a ca. 100 ka eruptive history of the La GaritaAuthorsLeah E. Morgan, Samuel Johnstone, Amy K. Gilmer, Michael A. Cosca, Ren A. ThompsonEstablishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA
On active alluvial fans, debris-flow deposits and frequent avulsions produce a rough topographic surface. As is the case in many initially rough landforms produced by catastrophic processes, the topography of alluvial fans is progressively smoothed, producing textural differences useful in establishing relative age criteria for fans. Here, we outline an approach for defining a quantitative, numeriAuthorsSamuel Johnstone, Adam M. Hudson, Sylvia Nicovich, Chester A. Ruleman, Robert M. Sare, Ren A. ThompsonInterpretation of low‐temperature thermochronometer ages from tilted normal fault blocks
Low‐temperature thermochronometry is widely used to measure the timing and rate of slip on normal faults. Rates are often derived from suites of footwall thermochronometer samples, but regression of age vs. structural depth fails to account for the trajectories of samples during fault slip. We demonstrate that in rotating fault blocks, regression of age‐depth data is susceptible to significant errAuthorsSamuel Johnstone, Joseph ColganControls on submarine channel-modifying processes identified through morphometric scaling relationships
Submarine channels share morphological similarities with rivers, but observations from modern and ancient systems indicate they are formed under processes and controls unique to submarine settings. Morphologic characteristics of channels—e.g., width, depth, slope, and the relationships among them—can constrain interpretations of channel-forming processes. This work uses morphometric scaling relatiAuthorsLauren E. Shumaker, Zane R. Jobe, Samuel Johnstone, Luke A. Pettinga, Dingxin Cai, Jeremiah D. MoodySediment unmixing using detrital geochronology
Sediment mixing within sediment routing systems can exert a strong influence on the preservation of provenance signals that yield insight into the influence of environmental forcings (e.g., tectonism, climate) on the earth’s surface. Here we discuss two approaches to unmixing detrital geochronologic data in an effort to characterize complex changes in the sedimentary record. First we summarize ‘toAuthorsGlenn R. Sharman, Samuel JohnstoneNon-USGS Publications**
Johnstone, Samuel A., and George E. Hilley. "Lithologic control on the form of soil-mantled hillslopes." Geology 43.1 (2015): 83-86.Johnstone, Samuel, Jeremy Hourigan, and Christopher Gallagher. "LA-ICP-MS depth profile analysis of apatite: Protocol and implications for (U–Th)/He thermochronometry." Geochimica et Cosmochimica Acta 109 (2013): 143-161.Johnstone, S.A., Chadwick, K.D., Frias, M., Tagliaro, G., and Hilley, G.E., 2017, Soil development over mud-rich rocks produces landscape-scale erosional instabilities in the northern Gabilan Mesa, California: Geological Society of America Bulletin, p. B31546.1, doi: 10.1130/B31546.1.Finnegan, N.J., Klier, R.A., Johnstone, S., Pfeiffer, A.M., and Johnson, K., 2017, Field evidence for the control of grain size and sediment supply on steady-state bedrock river channel slopes in a tectonically active setting: Earth Surface Processes and Landforms, doi: 10.1002/esp.4187.Johnstone, S.A., Finnegan, N.J., and Hilley, G.E., 2017, Weak bedrock allows north-south elongation of channels in semi-arid landscapes: Earth and Planetary Science Letters, v. 478, p. 150–158, doi:10.1016/j.epsl.2017.08.037.Zhuang, G., Johnstone, S.A., Hourigan, J., Ritts, B., Robinson, A. and Sobel, E.R., 2018. Understanding the geologic evolution of Northern Tibetan Plateau with multiple thermochronometers. Gondwana Research, 58, pp.195-210.Hilley, G.E., Porder, S., Aron, F., Baden, C.W., Johnstone, S.A., Liu, F., Sare, R., Steelquist, A. and Young, H.H., 2019. Earth’s topographic relief potentially limited by an upper bound on channel steepness. Nature Geoscience, 12(10), pp.828-832.**Disclaimer: The views expressed in Non-USGS publications are those of the author and do not represent the views of the USGS, Department of the Interior, or the U.S. Government.