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At least 847 records · Page 47Linked to original sources

USGS2018

Volcanic history of the northernmost part of the Harrat Rahat volcanic field, Saudi Arabia

We present a detailed geologic investigation of Pleistocene to Holocene mafic volcanism within the northernmost part of the Harrat Rahat volcanic field, proximal to the city of Al-Madinah, Saudi Arabia. Our study area covers ∼570 km 2 , and encompasses lava flows, scoria cones, and shield volcanoes of 32 mapped eruptive units consisting of continental, intraplate alkalic and tholeiitic basalts, hawaiites, and a mugearite that erupted from at least 1014 ± 14 ka to a single Holocene event at 1256 A.D. Typical lava flows are roughly 10–15 km long, although they reach nearly 23 km, 1–3 km wide, and ∼10 m thick. The majority of eruptives in our study area erupted ca. 400–340 ka and ca. 180–100 ka. Despite small individual volumes (<1 km 3 dense rock equivalent), each unit resulted from eruption of a distinct magma batch that was influenced by clinopyroxene, olivine, and plagioclase fractionation. Some of these units are interpreted to have undergone magma mixing prior to eruption. Combining our age determinations, geochemistry, and paleomagnetic data sets indicates that several eruptions were temporally and/or spatially clustered. Aligned scoria cones and elongate vent edifices were constructed atop fissure vent systems that reflect the stress field controlling dike ascent through the middle to upper crust.

Geosphere↗

USGS2021

Geomorphic expression and slip rate of the Fairweather fault, southeast Alaska, and evidence for predecessors of the 1958 rupture

Active traces of the southern Fairweather fault were revealed by light detection and ranging (lidar) and show evidence for transpressional deformation between North America and the Yakutat block in southeast Alaska. We map the Holocene geomorphic expression of tectonic deformation along the southern 30 km of the Fairweather fault, which ruptured in the 1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology, geomorphology, and active faults illustrate both strike-slip and dip-slip deformation styles within a 10°–30° double restraining bend where the southern Fairweather fault steps offshore to the Queen Charlotte fault. We measure offset landforms along the fault and calibrate legacy 14 C data to reassess the rate of Holocene strike-slip motion (≥49 mm/yr), which corroborates published estimates that place most of the plate boundary motion on the Fairweather fault. Our slip-rate estimates allow a component of oblique-reverse motion to be accommodated by contractional structures west of the Fairweather fault consistent with geodetic block models. Stratigraphic and structural relations in hand-dug excavations across two active fault strands provide an incomplete paleoseismic record including evidence for up to six surface ruptures in the past 5600 years, and at least two to four events in the past 810 years. The incomplete record suggests an earthquake recurrence interval of ≥270 years—much longer than intervals <100 years implied by published slip rates and expected earthquake displacements. Our paleoseismic observations and map of active traces of the southern Fairweather fault illustrate the complexity of transpressional deformation and seismic potential along one of Earth's fastest strike-slip plate boundaries.

Alaska↗

USGS1992

Partially melted granodiorite and related rocks ejected from Crater Lake caldera, Oregon

Blocks of medium-grained granodiorite to 4 m, and minor diabase, quartz diorite, granite, aplite and granophyre, are common in ejecta of the ∼6,900 yr BP caldera-forming eruption of Mount Mazama. The blocks show degrees of melting from 0–50 vol%. Because very few have adhering juvenile magma, it is thought that the blocks are fragments of the Holocene magma chamber’s walls. Primary crystallisation of granodiorite produced phenocrystic pl + hyp + aug + mt + il + ap + zc, followed by qz + hb + bt + alkali feldspar (af). Presence of fluid inclusions in all samples implies complete crystallisation before melting. Subsolidus exchange with meteoric hydrothermal fluids before melting is evident in δ 18 O values of −3.4–+4.9‰ for quartz and plagioclase in partially melted granodiorites (fresh lavas from the region have δ 18 O values of +5.8–+7.0‰); δ 18 O values of unmelted granodiorites from preclimatic eruptive units suggest hydrothermal exchange began between ∼70 and 24 ka. Before eruption, the granitic rocks equilibrated at temperatures, estimated from Fe–Ti oxide compositions, of up to ∼1000°C for c . 10 2 –10 4 years at a minimum pressure of 100–180 MPa. Heating caused progressive breakdown or dissolution of hb, af, bt, and qz, so that samples with the highest melt fractions have residual pl + qz and new or re-equilibrated af + hyp + aug + mt + il in high-silica rhyolitic glass (75–77% SiO 2 ). Mineral compositions vary systematically with increasing temperature. Hornblende is absent in rocks with Fe–Ti oxide temperatures >870°C, and bt above 970°C. Oxygen isotope fractionation between qz, pl, and glass in partially fused granodiorite also is consistent with equilibration at T ≥900°C (Δ 18 O qz-pl = +0.7±0.5‰). Element partitioning between glass and crystals reflects the large fraction of refractory pl, re-equilibration of af and isolation or incomplete dissolution of accessory phases. Ba and REE contents of analysed glass separates can be successfully modelled by observed degrees of partial melting of granodiorite, but Rb, Sr and Sc concentrations cannot. Several samples have veins of microlite-free glass 1–5 mm thick that are compositionally and physically continuous with intergranular melt and which apparently formed after the climactic eruption began. Whole-rock H 2 O content, microprobe glass analysis sums near 100% and evidence for high temperature suggest liquids in the hotter samples were nearly anhydrous. The occurrence of similar granodiorite blocks at all azimuths around the 8 × 10 km caldera implies derivation from one pluton. Compositional similarity between granodiorite and pre-Mazama rhyodacites suggests that the pluton may have crystallised as recently as 0.4 Ma; compositional data preclude crystallisation from the Holocene chamber. The history of crystallisation, hydrothermal alteration, and remelting of the granitic rocks may be characteristic of shallow igneous systems in which the balance between hydrothermal cooling and magmatic input changes repeatedly over intervals of 10 4 –10 6 years.

Oregon↗

USGS1993

Elk Lake in perspective

Elk Lake is located in the forested region of north-central Minnesota at the headwaters of the Mississippi River and occupies one of countless basins left behind as the last great Pleistocene ice sheet retreated northward into Canada. In this respect it resembles many other moderately deep, dimictic, hard-water lakes in the north-central United States, the sediments of which contain a history of postglacial and Holocene climatic and environmental change. Elk Lake is different, however, because the Holocene sediments in the deeper part of the lake form an uninterrupted sequence of annual laminations or varves. The varves are a chronometer for timing precisely the biologic, geochemical, and sedimentological responses in the lake to cyclic and progressive changes in climate. The varves also, through profound changes in their composition, divide the history of Elk Lake into three, sharply defined episodes; a postglacial lake, a prairie lake, and a modern, mesic-forest lake. We use these episodes and the character of the varves as a framework to guide the reader to the chapters and discussions found in this volume.

Minnesota↗

USGS1990

Development of slope valleys in the glacimarine environment of a complex subduction zone, Northern Gulf of Alaska

Morphological, seismic-reflection, and sedimentological evidence indicates that glacial ice tongues cut large sea valleys into the Gulf of Alaska continental shelf during the Pleistocene. During the Holocene, glacially-derived sediments from the Copper River and other meltwater streams have been prograding seaward across the shelf, covering the glacial and glacimarine upper Yakataga diamicts that blanketed the shelf and accumulated on the upper slope seaward of the sea valleys during the Pleistocene. GLORIA imagery near Middleton Island provides a new perspective on the glacimarine depositional environment on the continental slope in a collision zone between the Pacific and North American plates. Southwest of Middleton Island, along the subduction margin, sinuous valleys funnel sediment around shelf-edge-parallel, subduction-created, anticlinal ridges that have deflected and locally trapped glacimarine sediment. The slope south and southeast of Middleton Island where oblique convergence occurs, is incised by dendritic, erosional gulley systems, contains no compressional ridges, and thus, the apparently active sediment pathways to the trench are unrestricted. However, below the sea valley mouths, apparently both glacial and glacimarine sediments blanket the upper slope, covering any dendritic gulley systems that may have formed during or since the Pleistocene low stands of sea level.

Alaska↗

USGS1985

The role of erosion by fish in shaping topography around Hudson submarine canyon.

An 800-km 2 area of rough topography around the head of Hudson Canyon off the eastern United States is attributed to erosion by tilefish ( Lopholatilus chamaeleonticeps ) and associated species of crustaceans. The rough topography has a relief of 1-10 m, occurs in water depths of 120-500 m, and has been cut into a semilithified, silty clay substrate since the onset of the Holocene transgression. Commercial fishing activity indicates that a large population of tilefish, which dig burrows in the sea floor, occupy the area of the rough topography. Average tilefish burrows are 1.6 m in diameter and 1.7 m in depth. They have a clustered, not uniform, distribution, and their average density is 2,500 per km 2 . The close match of areas of rough topography and high tilefish populations, the active burrowing of the sea floor, and the clustered distribution of the burrows suggest that the hummocky topography in this area may be the result of continuous erosion by tilefish and associated crustaceans during the Holocene. An erosion rate of 13 cm per 1,000 years is necessary to create this topography during the past 13,000 years--and 18 cm per 1,000 years if(as is more likely based on the depths at which tilefish presently are found) the erosion started 9,000 years ago.

New York↗

USGS1977

Pleistocene barrier bar seaward of ooid shoal complex near Miami, Florida

An ooid sand barrier bar of Pleistocene age was deposited along the seaward side of an ooid shoal complex southwest of Miami, Florida. The bar is 35 km long, about 0.8 km wide, elongate parallel with the trend of the ooid shoal complex and perpendicular to channels between individual shoals. A depression 1.6 km wide, interpreted as a back-barrier channel, isolates the bar from the ooid shoals. During sea-level fall and subaerial exposure of the bar, the ooid sand was cemented in place, preventing migration of the barrier. No Holocene analogue of this sand body is recognized, perhaps because of the relative youthfulness of Holocene ooid shoals. This Pleistocene ooid shoal complex, with its reservoir-size barrier bar, may serve as a refined model for exploration in ancient ooid sand belts.

Florida↗

USGS1991

High-energy carbonate-sand accumulation, the Quicksands, southwest Florida Keys

High-resolution seismic-reflection profiles of the Quicksands, located along a broad ridge on the platform shelf west of Key West, Florida, indicate a significant deposit of non-oolitic carbonate sand occurs in a belt 47 km long by 28 km wide. The surface of the belt is ornamented by large (5 m), migrating tidal bars, oriented in a north-south direction, on which sand waves, oriented in an east-west direction, are superimposed. Some of the sand waves are awash at low tide. The sand waves are formed by strong reversing tidal currents flowing between the Gulf of Mexico and the Straits of Florida. The waves migrate directly over Pleistocene bedrock to the east, but the deposit thickens to the west and sand waves there overlie non-oolitic Holocene accumulations as thick as 12 m. Westward-dipping accretionary bedding indicates that net migration of the sands is to the west, despite north-south movement of tidal currents. The westward edge of the accumulation has accreted over deeper, muddier deposits. Although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the Bahamas and elsewhere, no oolitically coated grains were found in this study. Thin-section analyses show the principal component (average 48%) of the sands is fragmented plates of species of the green alga Halimeda , followed by particulate coral (average 17%), which increases off the flanks of the main sand body. Short vibracores confirm the presence of cross-bedding.

Florida↗

USGS1997

Complex response of a midcontinent north America drainage system to late Wisconsinan sedimentation

The geomorphic evolution of Mud Creek basin in eastern Iowa, U.S.A. serves to illustrate how geomorphic influences such as sediment supply, valley gradient, climate, and vegetation are recorded in the alluvial stratigraphic record. Sediment supply to the fluvial system increased significantly during the late Wisconsinan through a combination of periglacial erosion and loess accumulation. Subsequent evolution of the Holocene alluvial stratigraphic record reflects long-term routing of the late Wisconsinan sediment through the drainage basin in a series of cut-and-fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation. When viewed in a regional context, the alluvial stratigraphic record appears to reflect a long-term complex response of the fluvial system to increased sediment supply during the late Wisconsinan. Hydrologic and sediment-supply changes accompanying the spread of Euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the Holocene.

Journal of Sedimentary Research↗

USGS2011

Comparative phylogeography of a coevolved community: Concerted population expansions in Joshua trees and four Yucca moths

Comparative phylogeographic studies have had mixed success in identifying common phylogeographic patterns among co-distributed organisms. Whereas some have found broadly similar patterns across a diverse array of taxa, others have found that the histories of different species are more idiosyncratic than congruent. The variation in the results of comparative phylogeographic studies could indicate that the extent to which sympatrically-distributed organisms share common biogeographic histories varies depending on the strength and specificity of ecological interactions between them. To test this hypothesis, we examined demographic and phylogeographic patterns in a highly specialized, coevolved community - Joshua trees (Yucca brevifolia) and their associated yucca moths. This tightly-integrated, mutually interdependent community is known to have experienced significant range changes at the end of the last glacial period, so there is a strong a priori expectation that these organisms will show common signatures of demographic and distributional changes over time. Using a database of >5000 GPS records for Joshua trees, and multi-locus DNA sequence data from the Joshua tree and four species of yucca moth, we combined paleaodistribution modeling with coalescent-based analyses of demographic and phylgeographic history. We extensively evaluated the power of our methods to infer past population size and distributional changes by evaluating the effect of different inference procedures on our results, comparing our palaeodistribution models to Pleistocene-aged packrat midden records, and simulating DNA sequence data under a variety of alternative demographic histories. Together the results indicate that these organisms have shared a common history of population expansion, and that these expansions were broadly coincident in time. However, contrary to our expectations, none of our analyses indicated significant range or population size reductions at the end of the last glacial period, and the inferred demographic changes substantially predate Holocene climate changes.

PLoS ONE↗

USGS2013

Summit crater lake observations, and the location, chemistry, and pH of water samples near Mount Chiginagak volcano, Alaska: 2004-2012

Mount Chiginagak is a hydrothermally active volcano on the Alaska Peninsula, approximately 170 km south–southwest of King Salmon, Alaska (fig. 1). This small stratovolcano, approximately 8 km in diameter, has erupted through Tertiary to Permian sedimentary and igneous rocks (Detterman and others, 1987). The highest peak is at an elevation of 2,135 m, and the upper ~1,000 m of the volcano are covered with snow and ice. Holocene activity consists of debris avalanches, lahars, and lava flows. Pleistocene pyroclastic flows and block-and-ash flows, interlayered with andesitic lava flows, dominate the edifice rocks on the northern and western flanks. Historical reports of activity are limited and generally describe “steaming” and “smoking” (Coats, 1950; Powers, 1958). Proximal tephra collected during recent fieldwork suggests there may have been limited Holocene explosive activity that resulted in localized ash fall. A cluster of fumaroles on the north flank, at an elevation of ~1,750 m, commonly referred to as the “north flank fumarole” have been emitting gas throughout historical time (location shown in fig. 2). The only other thermal feature at the volcano is the Mother Goose hot springs located at the base of the edifice on the northwestern flank in upper Volcano Creek, at an elevation of ~160 m (fig. 2, near sites H1, H3, and H4). Sometime between November 2004 and May 2005, a ~400-m-wide, 100-m-deep lake developed in the snow- and ice-filled summit crater of the volcano (Schaefer and others, 2008). In early May 2005, an estimated 3 million cubic meters (3×106 m3) of sulfurous, clay-rich debris and acidic water exited the crater through tunnels at the base of a glacier that breaches the south crater rim. More than 27 km downstream, these acidic flood waters reached approximately 1.3 m above normal water levels and inundated a fertile, salmon-spawning drainage, acidifying the entire water column of Mother Goose Lake from its surface waters to its maximum depth of 45 m (resulting pH ~2.9), and preventing the annual salmon run in the King Salmon River. A simultaneous release of gas and acidic aerosols from the crater caused widespread vegetation damage along the flow path. Since 2005, we have been monitoring the crater lake water that continues to flow into Mother Goose Lake by collecting surface water samples for major cation and anion analysis, measuring surface-water pH of affected drainages, and photo-documenting the condition of the summit crater lake. This report describes water sampling locations, provides a table of chemistry and pH measurements, and documents the condition of the summit crater between 2004 and 2011. In September 2013, the report was updated with results of water-chemistry samples collected in 2011 and 2012, which were added as an addendum.

Alaska↗

USGS2017

Geology of Kasatochi Volcano, Aleutian Islands, Alaska

Kasatochi is a small, isolated island volcano in the center of the Aleutian Island chain. It consists of a roughly circular cone ~3 km in diameter with a lake-filled central crater that is 1.2 km in diameter and extends from the highest point on the island to sea level. The earliest unit recognized is a thick series of mid-Pleistocene glaciovolcanic deposits consisting of autobrecciated lava, lahars, and volumetrically minor lava masses that we believe to have been emplaced underneath a regional ice cap. This unit is unconformably overlain by several massive Holocene lavas, above which lies a thick sequence of latest- Holocene pyroclastic deposits likely deposited during the crater-forming eruption. The 2008 eruption enlarged the preexisting crater, and produced pyroclastic density currents, surges, and fall that blanketed the entire island except for steep, seaward-facing cliffs on the flanks and the crater wall. 2008 deposits initially extended the shoreline seaward by up to 500 m. Kasatochi lava and scoria are porphyritic basalt, basaltic andesite, and andesite, all of which bear trace-element evidence for prolonged crustal residence and equilibration with an amphibole-rich gabbroic residue. Lavas from individual effusive eruptions have limited compositional range, whereas juvenile scoriae from explosive eruptions span the majority of the compositional range of the entire volcano. 2008 pyroclastic deposits contain texturally diverse amphibole gabbro clasts and smaller, less abundant, plagioclase-free pyroxenitic and peridotitic cumulate inclusions. We infer that the gabbroic inclusions are from the margins of regions of crustal magma storage and evolution and that equilibration with the amphibole plays an important role in the evolution of mafic and intermediate magmas.

Alaska↗

USGS2005

Neotectonics of the offshore Oak Ridge fault near Ventura, southern California

The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200-400 ka ago and that the fault tip is buried by ??? 1 km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reflection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reflection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.

Bulletin of the Seismological Society of America↗

USGS2013

Low footwall accelerations and variable surface rupture behavior on the Fort Sage Mountains fault, northeast California

The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (<20 cm) during an M L 5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault ∼5.6 ka resulted in surface displacements of at least 0.8–1.5 m, implying earthquake moment magnitudes (M w ) of 6.7–7.1. An older rupture at ∼20.5 ka displaced the ground at least 1.5 m, implying an earthquake of M w 6.8–7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface‐rupture trace of this Holocene‐active normal fault. Ground‐motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0.2–0.3g for the 1950 rupture and 0.3–0.5g for the ∼5.6 ka paleoearthquake one kilometer from the fault‐surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0.1–0.3g. We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid‐Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near‐fault footwall ground motions at this site.

California↗

USGS2019

Shallow structure and geomorphology along the offshore northern San Andreas Fault, Tomales Point to Fort Ross, California

We mapped a poorly documented 35-km-long section of the northern San Andreas fault zone (NSAF) between Tomales Point and Fort Ross, California. Mapping is largely based on high-resolution seismic-reflection profiles (38 fault crossings), multibeam bathymetry, and onshore geology. NSAF strike in this section is nearly parallel to plate motion, characterized by a slight (~2°) northerly (transtensional) bend in the south between Tomales Bay and the Bodega isthmus, and a northwesterly (transpressional) ~5° bend in the north between the Bodega isthmus and Fort Ross. The southern transtensional bend is the northern part of the now-submerged, linear, ~50-km-long and 1- to 2-km-wide, "Tomales-Bodega valley." The valley floor is cut by a complex zone of subparallel, variably continuous fault strands and the deformed valley fill is an inferred mix of late Quaternary marine and nonmarine strata. In the northern part of this elongate valley, Holocene fault offset occurred on two fault strands about 740 m apart. The northern transpressional bend is characterized by narrow, elongate, asymmetric basins containing as much as 56 m of inferred latest Pleistocene to Holocene sediment. Between Bodega Head and Fort Ross, the gently dipping (~0.8°) shelf includes two large (4.8 and 5.9 km2) zones of sediment failure that we speculatively correlate with the 1906 San Francisco NSAF earthquake. Similar sediment-failure zones should be common along offshore reaches of the NSAF and other nearshore fault zones, but have apparent limited preservation potential. Onland geomorphic impacts of the mainly offshore NSAF include: (1) Northward upwarping of uplifted marine terraces in the transpressional zone north of Bodega Bay; and (2) Blocking of littoral sediment transport by uplifts on the west flank of the NSAF at Bodega Head and Tomales Point, resulting in rapidly accreting beaches and large coastal sand dune complexes.

California↗

USGS2020

Post-glacial Mw 7.0-7.5 earthquakes on the North Olympic fault zone, Washington

Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west‐northwest‐striking crustal faults herein named the North Olympic fault zone, which extends for ∼ 80    km "> ∼ 80 km ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for > 14    km "> > 14 km >14 km west‐northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface‐rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp‐derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek–Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single‐event dextral displacement of 4 ± 1    m "> 4 ± 1 m 4±1 m on the Sadie Creek fault. Inferred maximum rupture length and single‐event slip imply earthquake magnitudes M w "> M w Mw 7.0–7.5. Dextral slip rates of 1.3 – 2.3    mm / yr "> 1.3 – 2.3 mm / yr 1.3–2.3 mm/yr and the ∼ 11 , 000    yr "> ∼ 11 , 000 yr ∼11,000 yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore‐arc.

Washington↗

USGS2016

Preliminary assessment of a previously unknown fault zone beneath the Daytona Beach sand blow cluster near Marianna, Arkansas

We collected new high‐resolution P ‐wave seismic‐reflection data to explore for possible faults beneath a roughly linear cluster of early to mid‐Holocene earthquake‐induced sand blows to the south of Marianna, Arkansas. The Daytona Beach sand blow deposits are located in east‐central Arkansas about 75 km southwest of Memphis, Tennessee, and about 80 km south of the southwestern end of the New Madrid seismic zone (NMSZ). Previous studies of these sand blows indicate that they were produced between 10,500 and 5350 yr B.P. (before A.D. 1950). The sand blows are large and similar in size to those in the heart of the NMSZ produced by the 1811–1812 earthquakes. The seismic‐reflection profiles reveal a previously unknown zone of near‐vertical faults imaged in the 100–1100‐m depth range that are approximately coincident with a cluster of earthquake‐induced sand blows and a near‐linear surface lineament composed of air photo tonal anomalies. These interpreted faults are expressed as vertical discontinuities with the largest displacement fault showing about 40 m of west‐side‐up displacement at the top of the Paleozoic section at about 1100 m depth. There are about 20 m of folding on reflections within the Eocene strata at 400 m depth. Increasing fault displacement with depth suggests long‐term recurrent faulting. The imaged faults within the vicinity of the numerous sand blow features could be a causative earthquake source, although it does not rule out the possibility of other seismic sources nearby. These newly located faults add to a growing list of potentially active Pleistocene–Holocene faults discovered over the last two decades that are within the Mississippi embayment region but outside of the historical NMSZ.

Arkansas↗

USGS2017

Quaternary displacement rates on the Meeman‐Shelby fault and Joiner ridge horst, eastern Arkansas: Results from coring Mississippi River alluvium

This research used coring and optically stimulated luminescence (OSL) dating of displaced, deeply buried Quaternary alluvium to determine vertical displacement rates for the Meeman‐Shelby fault and the Joiner ridge horst, two structures in northeastern Arkansas that have no modern seismicity associated with them. We drilled continuous cores of the entire alluvial section in the hanging wall of each structure, performed detailed core descriptions and analyses, and obtained three OSL ages from each core. The Meeman‐Shelby fault core consists of 36 m of 4.3–5.2‐ka Holocene alluvium overlying 4 m of 14.3‐ka Kennett alluvium that in turn overlies the upper part of Eocene Claiborne Group sediments at a depth of 41 m. Seismic reflection indicates that the basal (Kennett) alluvium at the Meeman‐Shelby fault is displaced ∼28 m across the Meeman‐Shelby fault, which equates to a time‐averaged vertical displacement rate of 2 mm/yr within the last 14.3 ka. The Joiner ridge horst core consists, in descending order, of 11 m of 6.3‐ka Holocene alluvium, 14 m of 11.5‐ka Morehouse alluvium, a paleosol, 6 m of Kennett alluvium, and 4 m of 20.3‐ka Sikeston alluvium that in turn overlies the upper part of Eocene Claiborne Group sediments at a depth of 36 m. Lignite exploration drilling conducted in the 1970s indicates that basal (Sikeston) alluvium is displaced ∼20 m across the eastern bounding fault of the Joiner ridge horst, resulting in a time‐averaged vertical displacement rate of ∼1 mm/yr within the last 20.3 ka. These late Quaternary displacement rates are comparable to time‐averaged displacement rates of faults within the active New Madrid seismic zone.

Arkansas↗