Sam A Johnstone
Sam uses geologic mapping and computational methods to characterize and understand earth’s history.
Sam’s primary work is part of the National Geologic Synthesis project, where he works to develop geoinformatics-inspired solutions to integrating and synthesizing geologic data characterizing the nation’s geology. In this work Sam is interested in developing databases and software tools that can facilitate wide-ranging geologic assessments and studies of national scope. Sam is also engaged in efforts to conduct geologic mapping focused on Quaternary deposits (including machine-learning aided mapping of these deposits) and in developing statistical and simulation models to explore geochronology and topographic data.
Prior to starting at the USGS, Sam obtained his PhD from Stanford University in 2016. There he studied how sediment transport and erosion processes shaped hillslopes and river networks. In 2011 Sam obtained his MSc from UC Santa Cruz, where he had also obtained my BS a few years earlier. It was at UC Santa Cruz that Sam first got excited about geologic mapping and where he developed an interest in programming and 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
Quaternary fault mapping of the Zapata and Blanca sections of the Sangre de Cristo fault zone from high-resolution 3DEP topography Quaternary fault mapping of the Zapata and Blanca sections of the Sangre de Cristo fault zone from high-resolution 3DEP topography
This data release contains a dataset that depicts fault scarps along the Zapata and Blanca sections of the Sangre de Cristo fault zone located in the San Luis basin of southern Colorado. The Zapata and Blanca sections extend from the Great Sand Dunes National Park and Preserve to the Blanca Peak massif and are differentiated by a sharp change in fault zone orientation from north-south to...
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 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...
Surficial geologic map database of the Durango 1-degree by 2-degree quadrangle, 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
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...
Detrital zircon geochronology and geochemistry data from the seafloor of the Bering Sea and adjacent river systems Detrital 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 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
Filter Total Items: 20
Detrital signals of coastal erosion and fluvial sediment supply during glacio-eustatic sea-level rise, Southern California, USA Detrital 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...
Authors
Glenn R. Sharman, Jacob A Covault, Daniel F. Stockli, Zack Sickmann, Matthew A. Malkowski, Samuel Johnstone
A Bayesian nonparametric approach to unmixing detrital geochronologic data A 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...
Authors
John R. Tipton, Glenn R. Sharman, Samuel Johnstone
Detrital zircon record of magmatism and sediment dispersal across the North American Cordilleran arc system (28-48°N) Detrital 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...
Authors
Theresa Maude Schwartz, Kathleen D. Surpless, Joseph P. Colgan, Samuel Johnstone, Christopher S. Holm-Denoma
Identifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources 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 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...
Authors
Clark Gilbert, Zane R. Jobe, Samuel Johnstone, Glenn R. Sharman
The grass is not always greener on the other side: Seasonal reversal of vegetation greenness in aspect-driven semiarid ecosystems The 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...
Authors
Nikul Kumari, Patricia M. Saco, Jose F. Rodriguez, Samuel Johnstone, Ankur Srivastava, Kwok P. Chun, Omer Yetemen
Timing of Cenozoic extension in the southern Stillwater Range and Dixie Valley, Nevada Timing 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...
Authors
Joseph P. Colgan, Samuel Johnstone, David L. Shuster
Non-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.
Science and Products
Quaternary fault mapping of the Zapata and Blanca sections of the Sangre de Cristo fault zone from high-resolution 3DEP topography Quaternary fault mapping of the Zapata and Blanca sections of the Sangre de Cristo fault zone from high-resolution 3DEP topography
This data release contains a dataset that depicts fault scarps along the Zapata and Blanca sections of the Sangre de Cristo fault zone located in the San Luis basin of southern Colorado. The Zapata and Blanca sections extend from the Great Sand Dunes National Park and Preserve to the Blanca Peak massif and are differentiated by a sharp change in fault zone orientation from north-south to...
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 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...
Surficial geologic map database of the Durango 1-degree by 2-degree quadrangle, 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
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...
Detrital zircon geochronology and geochemistry data from the seafloor of the Bering Sea and adjacent river systems Detrital 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 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
Filter Total Items: 20
Detrital signals of coastal erosion and fluvial sediment supply during glacio-eustatic sea-level rise, Southern California, USA Detrital 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...
Authors
Glenn R. Sharman, Jacob A Covault, Daniel F. Stockli, Zack Sickmann, Matthew A. Malkowski, Samuel Johnstone
A Bayesian nonparametric approach to unmixing detrital geochronologic data A 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...
Authors
John R. Tipton, Glenn R. Sharman, Samuel Johnstone
Detrital zircon record of magmatism and sediment dispersal across the North American Cordilleran arc system (28-48°N) Detrital 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...
Authors
Theresa Maude Schwartz, Kathleen D. Surpless, Joseph P. Colgan, Samuel Johnstone, Christopher S. Holm-Denoma
Identifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources 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 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...
Authors
Clark Gilbert, Zane R. Jobe, Samuel Johnstone, Glenn R. Sharman
The grass is not always greener on the other side: Seasonal reversal of vegetation greenness in aspect-driven semiarid ecosystems The 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...
Authors
Nikul Kumari, Patricia M. Saco, Jose F. Rodriguez, Samuel Johnstone, Ankur Srivastava, Kwok P. Chun, Omer Yetemen
Timing of Cenozoic extension in the southern Stillwater Range and Dixie Valley, Nevada Timing 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...
Authors
Joseph P. Colgan, Samuel Johnstone, David L. Shuster
Non-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.