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
National Quaternary Geology
National Geologic Synthesis
Argon Geochronology
Ozark Dome-Arkoma Basin-Ouachita Transect
Geologic Framework of the Intermountain West
Surficial geologic map database of the Aztec 1-degree by 2-degree quadrangle, northern New Mexico and southern Colorado: Contributions to the National Geologic Map
Surficial geologic map database of the Durango 1-degree by 2-degree quadrangle, southern Colorado: Contributions to the National Geologic Map
Detrital zircon geochronology and geochemistry data from the seafloor of the Bering Sea and adjacent river systems
Thermochronologic data from the southern Stillwater Range, Nevada
The National Cooperative Geologic Mapping Program’s U.S. GeoFramework Initiative — Delivering a digital database of geologic map information by 2030
The Seamless Integrated Geologic Mapping (SIGMa) extension to the Geologic Map Schema (GeMS)
Mountain rivers reveal the earthquake hazard of geologic faults in Silicon Valley
Continental shelves as detrital mixers: U-Pb and Lu-Hf detrital zircon provenance of the Pleistocene–Holocene Bering Sea and its margins
Detrital signals of coastal erosion and fluvial sediment supply during glacio-eustatic sea-level rise, Southern California, USA
A Bayesian nonparametric approach to unmixing detrital geochronologic data
Detrital zircon record of magmatism and sediment dispersal across the North American Cordilleran arc system (28-48°N)
Identifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources
The grass is not always greener on the other side: Seasonal reversal of vegetation greenness in aspect-driven semiarid ecosystems
Timing of Cenozoic extension in the southern Stillwater Range and Dixie Valley, Nevada
Coexisting seismic behavior of transform faults revealed by high-resolution bathymetry
Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.)
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.
Science and Products
- Science
National Quaternary Geology
Quaternary geologic deposits represent important reservoirs of hydrologic, aggregate, and other resources and preserve a record of landscape evolution and the distribution and frequency of geologic hazards. These deposits are cataloged in thousands of geologic maps produced over many decades, but many are inconsistent with their neighbors, decades out of date, only available as static...National Geologic Synthesis
The National Cooperative Geologic Mapping Program (NCGMP) Strategic Plan of 2020 calls for the creation of a “variable-scale, national, integrated 2D and 3D geologic-framework model that enables the seamless construction of geologic maps within user-defined regions of interest across the United States by the year 2030” (USGS Open-File Report 2021-1013*). Congress recently directed NCGMP to “bring...Argon Geochronology
This project supports the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This laboratory develops methodology...Ozark Dome-Arkoma Basin-Ouachita Transect
The Ozark Dome-Arkoma Basin-Ouachita Transect (ODABOuT) project examines the three-dimensional geology of a transect of the southern orogenic margin of North America in its best surface exposure in western Arkansas. The Ozark Dome and Arkoma Basin represent a foreland uplift and foreland basin that formed as a result of the impinging Ouachita orogenic belt. Understanding the geometry and evolution...Geologic Framework of the Intermountain West
The Geologic Framework of the Intermountain West project was launched with the goal of producing a new digital geologic map database and 3D geologic model of a transect from the Rio Grande rift to the Basin and Range, based on a synthesis of existing geologic maps with new targeted new mapping, subsurface data, and other data sets. This database will integrate disparate map data, resolve... - Data
Surficial geologic map database of the Aztec 1-degree by 2-degree quadrangle, northern New Mexico and southern Colorado: Contributions to the National Geologic Map
This data release presents geologic map data for the surficial geology of the Aztec 1-degree by 2-degree quadrangle. The map area lies within two physiographic provinces of Fenneman (1928): the Southern Rocky Mountains province, and the Colorado Plateau province, Navajo section. Geologic mapping is mostly compiled from published geologic map data sources ranging from 1:24,000 to 1:250,000 scale, wSurficial geologic map database of the Durango 1-degree by 2-degree quadrangle, southern Colorado: Contributions to the National Geologic Map
This data release presents geologic map data for the surficial geology of the Durango 1-degree by 2-degree quadrangle and adjacent areas along the northern boundary of the quadrangle. The map area lies within two physiographic provinces of Fenneman (1928): the Southern Rocky Mountains province, and the Colorado Plateau province, Navajo section. Geologic mapping is mostly compiled from published geDetrital zircon geochronology and geochemistry data from the seafloor of the Bering Sea and adjacent river systems
This dataset archives geochronology and geochemistry measurements from detrital zircons collected from samples taken from the Bering Sea and its major feeder rivers.Thermochronologic data from the southern Stillwater Range, Nevada
This dataset contains apatite and zircon U-Th(He)data, 4He/3He thermochronologic data, and apatite fission-track data from the southern Stillwater Range, Nevada - Multimedia
- Publications
Filter Total Items: 18
The National Cooperative Geologic Mapping Program’s U.S. GeoFramework Initiative — Delivering a digital database of geologic map information by 2030
The U.S. Geological Survey's National Cooperative Geologic Mapping Program's U.S. GeoFramework Initiative responds to Congressional direction to implement the National Geologic Map Database Phase Three. The U.S. GeoFramework Initiative aims to create and deliver a new digital geologic framework of the United States by 2030. This Fact Sheet outlines the collaborations needed and plans taken to achiAuthorsJenna L. Shelton, William Andrews, Joseph Colgan, Samuel Johnstone, David R. Soller, Richard Berg, Donald Sweetkind, Kristine Zellman, John Brock, Kate RitzelThe Seamless Integrated Geologic Mapping (SIGMa) extension to the Geologic Map Schema (GeMS)
Geologic maps are the fundamental building blocks of surface and subsurface three-dimensional geologic framework models of the Earth’s crust. However, as the production and availability of geologic map databases continues to increase, inconsistent data models and the lack of synthesized, national geologic map data at scales appropriate for informed decision making negatively affect the functionalAuthorsKenzie J. Turner, Jeremiah B. Workman, Joseph Colgan, Amy K. Gilmer, Margaret E. Berry, Samuel Johnstone, Kathleen F. Warrell, Marieke Dechesne, D. Paco VanSistine, Ren A. Thompson, Adam M. Hudson, Kristine Zellman, Donald S. Sweetkind, Chester A. RulemanMountain rivers reveal the earthquake hazard of geologic faults in Silicon Valley
The 1989, Mw = 6.9 Loma Prieta earthquake resulted in tens of lives lost and cost California almost 3% of its gross domestic product. Despite widespread damage, the earthquake did not clearly rupture the surface, challenging the identification and characterization of these hidden hazards. Here, we show that they can be illuminated by inverting fluvial topography for slip-and moment accrual-rates—fAuthorsFelipe Aron, Samuel Johnstone, Andreas Mavrommatis, Robert M. Sare, Frantz Maerten, Jack Loveless, Curtis W Baden, George E. HilleyContinental shelves as detrital mixers: U-Pb and Lu-Hf detrital zircon provenance of the Pleistocene–Holocene Bering Sea and its margins
Continental shelves serve as critical transfer zones in sediment-routing systems, linking the terrestrial erosional and deep-water depositional domains. The degree to which clastic sediment is mixed and homogenized during transfer across broad shelves has important implications for understanding deep-sea detrital records. Wide continental shelves are thought to act as capacitors characterized by tAuthorsMatthew A. Malkowski, Samuel Johnstone, Glenn R. Sharman, Colin J. White, Daniel Scheirer, Ginger BarthByCoastal and Marine Hazards and Resources Program, National Cooperative Geologic Mapping Program, Geology, Minerals, Energy, and Geophysics Science Center, Geosciences and Environmental Change Science Center, Pacific Coastal and Marine Science Center, Deep Sea Exploration, Mapping and CharacterizationDetrital signals of coastal erosion and fluvial sediment supply during glacio-eustatic sea-level rise, Southern California, USA
Coastal erosion, including sea-cliff retreat, represents both an important component of some sediment budgets and a significant threat to coastal communities in the face of rising sea level. Despite the importance of predicting future rates of coastal erosion, few prehistoric constraints exist on the relative importance of sediment supplied by coastal erosion versus rivers with respect to past seaAuthorsGlenn R. Sharman, Jacob A Covault, Daniel F. Stockli, Zack Sickmann, Matthew A. Malkowski, Samuel JohnstoneA Bayesian nonparametric approach to unmixing detrital geochronologic data
Sedimentary deposits constitute the primary record of changing environmental conditions that have acted on Earth’s surface over geologic time. Clastic material is eroded from source locations (parents) in sediment routing systems and deposited at sink locations (children). Both parents and children have characteristics that vary across many different dimensions, including grain size, chemical compAuthorsJohn R. Tipton, Glenn R. Sharman, Samuel JohnstoneDetrital zircon record of magmatism and sediment dispersal across the North American Cordilleran arc system (28-48°N)
As zircon U-Pb geochronology has become a leading method in sediment provenance studies and basin analysis over the past 20 years, the volume of detrital zircon data made available in published literature has enabled researchers to go beyond source-to-sink provenance studies to explore increasingly complex geologic problems. In this review, we utilize the growing body of detrital zircon data acquiAuthorsTheresa Maude Schwartz, Kathleen D. Surpless, Joseph Colgan, Samuel Johnstone, Christopher Holm-DenomaIdentifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources
The San Gabriel and Canton faults represent early stages in the development of the San Andreas fault system. However, questions of timing of initiation and magnitude of slip on these structures remain unresolved, with published estimates ranging from 42-75 km and likely starting in the Miocene. This uncertainty in slip history reflects an absence of appropriate piercing points. We attempt to betteAuthorsClark Gilbert, Zane R. Jobe, Samuel Johnstone, Glenn R. SharmanThe grass is not always greener on the other side: Seasonal reversal of vegetation greenness in aspect-driven semiarid ecosystems
Our current understanding of semiarid ecosystems is that they tend to display higher vegetation greenness on polar-facing slopes (PFS) than on equatorial-facing slopes (EFS). However, recent studies have argued that higher vegetation greenness can occur on EFS during part of the year. To assess whether this seasonal reversal of aspect-driven vegetation is a common occurrence, we conducted a globalAuthorsNikul Kumari, Patricia M. Saco, Jose F. Rodriguez, Samuel Johnstone, Ankur Srivastava, Kwok P. Chun, Omer YetemenTiming of Cenozoic extension in the southern Stillwater Range and Dixie Valley, Nevada
The Dixie Valley fault bounds the east side of the Stillwater Range in west‐central Nevada and last ruptured in 1954. Offset basalts indicate that slip began more recently than ~14 Ma, and prior work has interpreted the southern segment as an active low‐angle normal fault. Oligocene igneous rocks in the southern Stillwater Range were steeply tilted during large‐magnitude extension prior to ~14 Ma.AuthorsJoseph Colgan, Samuel Johnstone, David L. ShusterCoexisting seismic behavior of transform faults revealed by high-resolution bathymetry
Transform faults are known to have anomalously low rates of seismicity, but no direct observations reveal why this is the case. We use new, autonomous underwater vehicle high-resolution seafloor mapping to image the morphology of and offsets along transform fault segments in the Gulf of California. Fault splays display a varied history of activation and deactivation of individual fault strands oAuthorsGeorge E. Hilley, Robert M. Sare, Felipe Aron, Curtis W Baden, Dave Caress, Christopher M. Castillo, Stephen C. Dobbs, Jared T Gooley, Samuel Johnstone, Frances Liu, Tim McHargue, Josie M Nevitt, Charles K. Paull, Lauren E. Shumaker, Miles M Traer, Holly H YoungDilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.)
Integrated, multi-method provenance studies of siliciclastic sedimentary deposits are increasingly used to reconstruct the history of source-to-sink transport, paleogeography, and tectonics. Invariably, analysis of large-scale depositional systems must confront issues regarding how to best sample the system and adequately cope with the details of sediment mixing. Potential biases including variatAuthorsMatthew A. Malkowski, Glenn R. Sharman, Samuel Johnstone, Marty J. Grove, Dave L. Kimbrough, Stephen A. GrahamNon-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.