Geology topics

S. Bennett

Publications and source records attributed to S. Bennett.

8 recordsLinked to original sources

USGS2021

Oases: Finding hidden biodiversity gems in the southern Sonoran Desert

In the arid southern Sonoran Desert, the rugged canyons of the Sierra El Aguaje contain numerous freshwater oases. These habitats are supported by small springs which are usually located along geologic faults in volcanic and granitic bedrock. Genetic evidence from freshwater-obligate species (e.g., fish and frogs) suggests these or similar spring-fed habitats have persisted for thousands to millions of years. Though biologists are just beginning to study these habitats, at least 210 species of aquatic invertebrates have been documented, along with several species of fishes, amphibians, and semi-aquatic reptiles. Additionally, euryhaline fishes occasionally colonize freshwater habitats when hurricane-induced floods connect oases with the sea. At least six new, potentially endemic, species of aquatic invertebrates have been found in recent years, but much work remains to be done to fully document the biota of these oases. Groundwater pumping, introductions of nonnative species, and unmanaged human recreation all threaten the biodiversity of these desert oases. We hope this chapter will draw attention to these beautiful habitats and promote conservation of their unique biota

Sonora↗

USGS2020

Documentation of Surface Fault Rupture and Ground‐Deformation Features Produced by the 4 and 5 July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence

The M w "> M w Mw 6.4 and M w "> M w Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground‐deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground‐deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.

California↗

USGS2020

Global physical controls on estuarine habitat distribution during sea levelchange: Consequences for genetic diversification through time

Determining the extrinsic (physical) factors controlling speciation and diversification of species through time is of key interest in paleontology and evolutionary biology. The role of sea-level change in shaping species richness patterns of marginal marine species has received much attention, but with variable conclusions. Recent work combining genetic data and Geographical Information Systems (GIS)-based habitat modeling yielded a framework for how geomorphology of continental margins mediates genetic connectivity of populations during sealevel change. This approach may ultimately yield insights on how distinct lineages, species, and biodiversity accumulate in coastal settings. Here, we expand this GIS work globally to different geomorphic settings to model estuarine habitat in a larger geographic framework and test how tectonic setting, oceanographic setting, climate, and margin age affect habitat distribution during sea-level change. In addition, independent of estuaries we explore paleobiologic (e.g. Olsson, 1961) and neontolologic effects of sea-level change on evolution, and test the relation between overall shelf area and species richness using data of 1721 fish species. We find 82% global reduction of estuarine habitat abundance at lowstand relative to highstand, and find large habitats change in size much more than small habitats. Consistent with prior work, narrow continental margins have significantly less habitat at highstand and lowstand than wide margins, and narrow margins significantly associate with fore-arc settings, effectively linking tectonic setting to habitat abundance. Surprisingly, narrow margins host greater species richness, a finding which violates the canonical species-area relation. This finding can be explained if: 1) the physical isolation imposed by narrow margins facilitates the formation of new species over time; 2) the sizestability of small habitats, which disproportionately occur on narrow margins, accumulate and retain species extirpated in the more variable habitats on wide margins; or 3) the smaller habitats on narrow margins facilitate greater species richness through greater habitat heterogeneity. These results are generally at odds with prior interpretations, but the combination of richness data and population genetic principles offer a different perspective on these long-studied questions. Finally, we emphasize that the nuance of Pleistocene-Holocene sea level oscillations should be more explicitly considered in genetic studies.

Global and Planetary Change↗

USGS2020

Fault trace mapping and surface-fault-rupture special study zone delineation of the Wasatch Fault Zone, Utah and Idaho

The Wasatch fault zone (WFZ) is a 220-mile-long (350-km) fault zone divided into 10 structural segments extending from southeastern Idaho to central Utah. The central five segments of the WFZ underlie the densely populated Wasatch Front region, where the majority of Utah’s population and economy are proximal to the fault zone. The West Valley fault zone (WVFZ) is an antithetic structure related to the WFZ and runs through the Salt Lake Valley. Communities on or adjacent to the WFZ are at risk of earthquake damage, due to their proximity to the fault zones. During 2016–2018, the Utah Geological Survey and a U.S. Geological Survey collaborator performed updated fault mapping of 39 7.5' quadrangles along the WFZ using recently acquired high-resolution topographic data derived from airborne light detection and ranging (lidar) elevation data. Previous geologic mapping, paleoseismic investigations, historical aerial photography, and field investigations were also used to identify and map surface fault traces and infer fault locations. Special study zones were delineated around fault traces to facilitate understanding of the surface-rupturing hazard and associated risk. Defining these special study zones encourages the creation and implementation of municipal and county geologic-hazard ordinances dealing with hazardous faults. We identified potential paleoseismic investigation sites where fault scarps appear relatively pristine, are located in geologically favorable settings, and where additional earthquake timing data would be beneficial to the continued earthquake research of the WFZ. The fault geometries, attributes, and special study zones were published in the online Utah Geologic Hazards Portal simultaneously with this Report of Investigation (RI). This report contains supplementary material describing the data and methods used to perform the mapping and in locating potential paleoseismic investigation sites in the study area. This work is critical to raise awareness of earthquake hazards in areas of Utah experiencing rapid growth.

Idaho, Utah↗

USGS2019

Neotectonic and paleoseismic analysis of the northwest extent of Holocene surface deformation along the Meers Fault, Oklahoma

TheMeers fault (Oklahoma) is one of fewseismogenic structures with evidence for Holocene surface rupture in the stable continental region of North America. The 37-kilometer-long southeast section of the full 54-kilometer-long Meers fault is interpreted to be Holocene active. The 17-kilometer-long northwest section is considered Quaternary active, but not Holocene active.We reevaluate surface expression and earthquake timing of the northwest Meers fault to improve seismic source characterization.We use airborne light detection and ranging and historical stereopaired aerial photos to evaluate the fault scarp and local faultzone geomorphology. In the northwest, complex surface deformation includes fault splays, subtle monoclinal warping, and a minor change in fault strike. We interpret that the alongstrike transition from surface faulting on the southeastMeers fault to surface folding on the northwest Meers fault occurs at the lithologic contact between Permian Post Oak conglomerate and Hennessey shale. We excavated a paleoseismic trench to evaluate the timing of surface-deforming earthquakes on the northwest section of the fault. The excavation revealed weathered Permian Hennessey shale and an ∼1–2-meter-thick veneer of Holocene alluvial deposits that were progressively deformed during two surface-folding earthquakes likely related to blind fault rupture beneath the site. Repeated onlapping to overlapping stratigraphic sequences and associated unconformities are intimately related to folding events along the monocline. OxCal paleoearthquake age modeling indicates that earthquakes occurred 4704–3109 yr B.P. and 5955–4744 yr B.P., and that part of the northwest section of the Meers fault is Holocene active. We find the Holocene-active section of the Meers fault should be lengthened 6.1 km to the northwest, to a total Holocene-active fault length of 43 km. Empirical scaling relationships between surface rupture length and magnitude reveal that the fault could generate an Mw 7.0 earthquake.

Oklahoma↗

USGS2019

Mesozoic to Cenozoic sedimentation, tectonics, and metallogeny of Sonora, Mexico

We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.

Sonora↗

USGS2018

Holocene surface rupture history of an active forearc fault redefines seismic hazard in Southwestern British Columbia, Canada

Characterizing the hazard associated with Quaternary‐active faults in the forearc crust of the northern Cascadia subduction zone has proven challenging due to historically low rates of seismicity, late Quaternary glacial scouring, and dense vegetation that often obscures fault‐related geomorphic features. We couple lidar topography with paleoseismic trenching across the Leech River Fault on southern Vancouver Island to produce the first detailed surface rupture history of an onland forearc fault in British Columbia, Canada. The results indicate that this fault produced three surface‐rupturing earthquakes in the last ∼9 kyr and is therefore capable of producing large (Mw>6) earthquakes in the future. We provide new constraints on the fault's length (∼130 km) and Holocene slip rate (≥0.2–0.3 mm/year) that, together with the earthquake ages, should be incorporated into new seismic hazard assessments and building code practices relevant to urban centers in southwestern British Columbia (Canada) and northwestern Washington State (United States).

British Columbia↗

USGS2016

A microbial arsenic cycle in sediments of an acidic mine impoundment: Herman Pit, Clear Lake, California

The involvement of prokaryotes in the redox reactions of arsenic occurring between its +5 [arsenate; As(V)] and +3 [arsenite; As(III)] oxidation states has been well established. Most research to date has focused upon circum-neutral pH environments (e.g., freshwater or estuarine sediments) or arsenic-rich “extreme” environments like hot springs and soda lakes. In contrast, relatively little work has been conducted in acidic environments. With this in mind we conducted experiments with sediments taken from the Herman Pit, an acid mine drainage impoundment of a former mercury (cinnabar) mine. Due to the large adsorptive capacity of the abundant Fe(III)-rich minerals, we were unable to initially detect in solution either As(V) or As(III) added to the aqueous phase of live sediment slurries or autoclaved controls, although the former consumed added electron donors (i.e., lactate, acetate, hydrogen), while the latter did not. This prompted us to conduct further experiments with diluted slurries using the live materials from the first incubation as inoculum. In these experiments we observed reduction of As(V) to As(III) under anoxic conditions and reduction rates were enhanced by addition of electron donors. We also observed oxidation of As(III) to As(V) in oxic slurries as well as in anoxic slurries amended with nitrate. We noted an acid-tolerant trend for sediment slurries in the cases of As(III) oxidation (aerobic and anaerobic) as well as for anaerobic As(V) reduction. These observations indicate the presence of a viable microbial arsenic redox cycle in the sediments of this extreme environment, a result reinforced by the successful amplification of arsenic functional genes ( aioA , and arrA ) from these materials.

California↗