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Results for “Special Papers of the Geological Society of London”

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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↗

USGS1992

Miocene intensification of upwelling along the California margin as recorded in siliceous facies of the Monterey Formation and offshore DSDP sites

Diatomaceous sediments and their diagenetic equivalents in the Monterey Formation record a variable history of upwelling along the California margin. Distrinctive dark opal-CT and quartz chertz found in distal basins of the Monterey Formation are the result of burial diagenesis of pure biosiliceous oozes (biosiliceous oozes without significant admixtures of clay) and are therefore evidence of intensified coastal upwelling during the early middle Miocene. Dating of six sections of the Monterey Formation, largely by diatom biostratigraphy, suggests that at the Point Reyes and Point Año Nuevo sections in north-central California, the age of the earliest chert intervals is between 13.8 and 15.0 Ma, and 14.3 and 14.8 Ma, respectively. In south-central California, ages from the Shell Beach, Mussel Rock, and Lions Head sections imply that the age of the base of the chert intervals is between 12.7 and 13.3 Ma. Both ages correlate to an early middle Miocene high latitude cooling step that resulted in more vigorous surface water circulation, upwelling of nutrient-rich waters, and increased biosiliceous sedimentation in the North Pacific. The north-south difference in age of the base of the chert interval probably reflects a progressive intensification of the California Current from 15.0 to 12.7 Ma. The age of the onset of biosiliceous sedimentation at DSDP sites of the northeastern Pacific is also generally younger at the more southern sites; however, these particular DSDP sites were located some distance from the centres of coastal upwelling and are not as reliable indicators of the intensification of upwelling along the California margin.

California↗

USGS2000

Tephrochronology of the Brooks River Archaeological District, Katmai National Park and Preserve, Alaska: What can and cannot be done with tephra deposits

The Brooks River Archaeological District (BRAD) in Katmai National Park and Preserve is a classical site for the study of early humans in Alaska. Because of proximity to the active Aleutian volcanic arc, there are numerous tephra deposits in the BRAD, which are potentially useful for correlating among sites of archaeological investigations. Microprobe analyses of glass separates show, however, that most of these tephra deposits are heterogeneous mixtures of multiple glass populations. Some glasses are highly similar to pyroclasts of Aniakchak Crater (160 km to the south), others are similar to pyroclasts in the nearby Valley of Ten Thousand Smokes, and some are similar to no other tephra samples from the Alaska Peninsula. Moreover, tephra deposits in any one archaeological study site are not always similar to those from nearby sites, indicating inconsistent preservation of these mainly thin, fine-grained deposits. At least 15, late Holocene tephra deposits are inferred at the BRAD. Their heterogeneity is the result of either eruptions of mixed or heterogeneous magmas, like the 1912 Katmai eruption, or secondary mixing of closely succeeding tephra deposits. Because most cannot be reliably distinguished from one another on the basis of megascopic properties, their utility for correlations is limited. At least one deposit can be reliably identified because of its thickness (10 cm) and colour stratification. Early humans seem not to have been significantly affected by these tephra falls, which is not surprising in view of the resilience exhibited by both plants and animals following the 1912 Katmai eruption.

Alaska↗

USGS2022

Subaerial volcaniclastic deposits — Influences of initiation mechanisms and transport behaviour on characteristics and distributions

Subaerial volcaniclastic deposits are produced principally by volcanic debris avalanches, pyroclastic density currents, lahars, and tephra falls. Those deposits have widely ranging geomorphic and sedimentologic characteristics; they can mantle, modify, or create new topography, and their emplacement and subsequent reworking can have an outsized impact on the geomorphic and sedimentologic responses of watersheds surrounding, and channels draining, volcanoes. Volcaniclastic deposits provide a wealth of information about eruptive histories, volcanic processes, and landscape responses to eruptions. The volcanic processes that produce these deposits, and consequently the character and sedimentary structures of the deposits themselves, are influenced by initiation mechanism. Deposit preservation is affected by deposit magnitude, texture, and composition, depositional environment, and climate regime. Innovative analyses of deposits from several modern eruptions and advancements in physical and numerical modelling have vastly improved our understanding of volcanic processes, interpretations of eruptive histories, and recognition of the hazards posed by volcanic eruptions. This contribution highlights and summarizes major advances that have occurred in the past few decades in understanding of volcaniclastic deposits and linkages with volcanic processes.

Special Papers of the Geological Society of London↗

USGS2024

Recent advances in characterizing the crustal stress field and future applications of stress data: Perspectives from North America

The stress field controls patterns of crustal deformation, including which faults are likeliest to cause earthquakes or transmit fluids. Since the 1950s, maps of maximum horizontal stress ( S Hmax ) orientations have advanced dramatically, and the style of faulting (relative principal stress magnitudes) has recently been mapped in some regions as well. This perspectives paper summarizes developments in characterizing stress orientations and (relative) magnitudes, including new seismic and borehole methods, as well as progress in identifying the causes of stress variations. Despite these advances, adding far more spatiotemporal detail would allow geoscientists to address many of today's key challenges regarding natural hazards, energy development, and geodynamics. In particular, it is critically important to characterize stress heterogeneity at multiple scales while also recognizing the coherent variability of the stress field. The second part of the paper considers how more detailed stress datasets could prove essential to addressing some of the grand questions in geoscience, including deciphering the poorly understood feedbacks between crustal dynamics and surface processes, improving earthquake and eruption forecasts, and determining the origins and shared properties of plate boundaries.

Geological Society, London, Special Publications↗

USGS2023

Microfossils and biostratigraphy of the Upper Cretaceous Coon Creek Formation Lagerstätte, Mississippi Embayment, USA

The Upper Cretaceous Coon Creek Formation type-locality in McNairy County, Tennessee, is an exceptional marine invertebrate Lagerstätte that was deposited in a nearshore reentrant into the ancestral continent of Appalachia. Extensive taxonomic analysis of the macrofauna has been done for over 100 years. However, documentation of the microfossil component at the type-locality has largely been ignored. This paper provides analysis of calcareous microfossils (benthic and planktonic foraminifera, calcareous nannofossils and ostracods) and palynomorphs (pollen, spores, algal remains and dinoflagellates) for detailed age control and information regarding environment of deposition. The Coon Creek type-locality is late Campanian (76.8–76.0 Ma) in age and was deposited in 35–60 m water depths. Calcareous nannofossil assemblages suggest that there was an influence of colder water through time, possibly by a connection to the northern Western Interior Seaway. The complex interplay between currents of the Gulf of Mexico and the Western Interior Seaway, upwelling and runoff influenced the palaeoenvironment. Increased influx of freshwater palynomorphs near the top of the section suggests placement proximal to the palaeoshoreline and that weathering increased through time. Changes in the dinoflagellate assemblage may be linked to a mosasaur mass mortality event and the presence of a harmful algal bloom.

Tennessee↗