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At least 271 records · Page 15Linked to original sources

USGS2001

Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula

The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ???7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so-called base-of-slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low-velocity (??? 2.0-2.5 km s-1) sequence of slope sediment, from an underlying sequence of much higher-velocity (> 4-4.5 km s-1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ??? 3900 m and yielded a carbonate-cemented calcarenitic breccia of early-middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin-wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf-to-littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long-term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post-Paleogene history of crustal thinning and landward trench migration of Costa Rica's Pacific margin. During the past 16-17 Myr the calculated mass removal and landward migration rates are 34-36 km3 Myr-1 km-1 of margin, and 3 km Myr-1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.

Tectonics↗

USGS2011

Recent sediment studies refute Glen Canyon Dam hypothesis

Recent studies of sedimentology hydrology, and geomorphology indicate that releases from Glen Canyon Dam are continuing to erode sandbars and beaches in the Colorado River in Grand Canyon National Park, despite attempts to restore these resources. The current strategy for dam operations is based on the hypothesis that sand supplied by tributaries of the Colorado River downstream from the dam will accumulate in the channel during normal dam operations and remain available for restoration floods. Recent work has shown that this hypothesis is false, and that tributary sand inputs are exported downstream rapidly typically within weeks or months under the current flow regime.

Arizona↗

USGS2006

Overview of the Microscopic Imager Investigation during Spirit's first 450 sols in Gusev crater

The Microscopic Imager (MI) on the Mars Exploration Rover Spirit has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Gusev landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Spirit's instrument arm and can resolve objects 0.1 mm in size or larger. This paper provides an overview of MI operations, data calibration, processing, and analysis of MI data returned during the first 450 sols (Mars days) of the Spirit landed mission. The primary goal of this paper is to facilitate further analyses of MI data by summarizing the methods used to acquire and process the data, the radiometric and geometric accuracy of MI data products, and the availability of archival products. In addition, scientific results of the MI investigation are summarized. MI observations show that poorly sorted soils are common in Gusev crater, although aeolian bedforms have well‐sorted coarse sand grains on their surfaces. Abraded surfaces of plains rocks show igneous textures, light‐toned veins or fracture‐filling minerals, and discrete coatings. The rocks in the Columbia Hills have a wide variety of granular textures, consistent with volcaniclastic or impact origins. Case hardening and submillimeter veins observed in the rocks as well as soil crusts and cemented clods imply episodic subsurface aqueous fluid movement, which has altered multiple geologic units in the Columbia Hills. The MI also monitored Spirit's solar panels and the magnets on the rover's deck.

Journal of Geophysical Research E: Planets↗

USGS2008

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Paleoceanography↗

USGS2008

Surface processes recorded by rocks and soils on Meridiani Planum, Mars: Microscopic Imager observations during Opportunity's first three extended missions

The Microscopic Imager (MI) on the Mars Exploration Rover Opportunity has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Meridiani Planum landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Opportunity's instrument arm and can resolve objects 0.1 mm across or larger. This paper provides an overview of MI operations, data calibration, and analysis of MI data returned during the first 900 sols (Mars days) of the Opportunity landed mission. Analyses of Opportunity MI data have helped to resolve major questions about the origin of observed textures and features. These studies support eolian sediment transport, rather than impact surge processes, as the dominant depositional mechanism for Burns formation strata. MI stereo observations of a rock outcrop near the rim of Erebus Crater support the previous interpretation of similar sedimentary structures in Eagle Crater as being formed by surficial flow of liquid water. Well‐sorted spherules dominate ripple surfaces on the Meridiani plains, and the size of spherules between ripples decreases by about 1 mm from north to south along Opportunity's traverse between Endurance and Erebus craters.

Journal of Geophysical Research E: Planets↗

USGS2010

Global climate changes recorded in coastal wetland sediments: Empirical observations linked to theoretical predictions

Whether coastal areas are experiencing, and responding to, an accelerated rate of global sea-level rise (GSLR) is critically important for the ∼2 billion people living near Earth's oceans. Accretion rates from a suite of physiographically diverse coastal wetlands surrounding Long Island, NY accelerated during the 20th century at 2.3 ± 0.2 × 10 −2 mm yr −2 , which is comparable to reported rates of GSLR acceleration and global temperature changes. Wetlands varied in tidal range, salinity and geomorphic setting, and were located in embayments with limited human impacts in a region with limited and constant rates of subsidence. From geochronologies with temporal resolutions of 2–5 yr, we constructed new composite histories of sediment accretion and mineral deposition. Wetland dynamics are consistent with predictions from sedimentology and a numerical model of ecogeomorphic response, suggesting that these systems, and likely others worldwide, are responding to accelerated GSLR and related climatic changes.

New York↗

USGS2011

A reevaluation of the Pallett Creek earthquake chronology based on new AMS radiocarbon dates, San Andreas fault, California

The Pallett Creek paleoseismic record occupies a keystone position in most attempts to develop rupture histories for the southern San Andreas fault. Previous estimates of earthquake ages at Pallett Creek were determined by decay counting radiocarbon methods. That method requires large samples which can lead to unaccounted sources of uncertainty in radiocarbon ages because of the heterogeneous composition of organic layers. In contrast, accelerator mass spectrometry (AMS) radiocarbon dates may be obtained from small samples that have known carbon sources and also allow for a more complete sampling of the section. We present 65 new AMS radiocarbon dates that span nine ground‐rupturing earthquakes at Pallett Creek. Overall, the AMS dates are similar to and reveal no dramatic bias in the conventional dates. For many layers, however, individual charcoal samples were younger than the conventional dates, leading to earthquake ages that are overall slightly younger than previously reported. New earthquake ages are determined by Bayesian refinement of the layer ages based on stratigraphic ordering and sedimentological constraints. The new chronology is more regular than previously published records in large part due to new samples constraining the age of event R. The closed interval from event C to 1857 has a mean recurrence of 135 years ( σ = 83.2 years) and a quasiperiodic coefficient of variation (COV) of 0.61. We show that the new dates and resultant earthquake chronology have a stronger effect on COV than the specific membership of this long series and dating precision improvements from sedimentation rates.

California↗

USGS2022

Mars Science Laboratory CheMin data from the Glen Torridon region and the significance of lake-groundwater interactions in interpreting mineralogy and sedimentary history

The Glen Torridon (GT) region is positioned in terrains with strong clay mineral signatures, as inferred from orbital spectroscopy. The GT campaign confirmed orbital distinctions with in situ measurements by the Mars Science Laboratory rover, Curiosity , and the CheMin X-ray diffraction instrument with of some of the highest clay mineral abundances to date. Additionally, GT is unique because of distinct phase identifications for the first time by CheMin, including: (i) Fe-carbonates, and (ii) a novel peak in the XRD patterns of some GT samples, with an interplanar spacing of at 9.2 Å. Fe-carbonates have been previously suggested from other instruments onboard, but this is the first definitive reporting by CheMin of multiple samples with Fe-carbonate. This new phase has never been observed in Gale crater with the CheMin instrument and may be a new mineral for Mars, but discrete identification still remains enigmatic because no single phase on Earth is able to account the mineralogical, geochemical, and sedimentological constraints in the GT region. Here, we modeled XRD profiles and propose an interstratified clay mineral, specifically greenalite-minnesotaite (G-M), as a reasonable candidate. The coexistence of Fe-carbonate and Fe-rich clay minerals in the GT samples, supports a conceptual model of a lacustrine groundwater mixing environment in the ancient Gale crater lake. Groundwater interaction with percolating lake waters in the sediments is common in terrestrial lacustrine settings, and the diffusion of two distinct water bodies within the subsurface can create a geochemical gradient and unique mineral front in the sediments. Ultimately, the proximity to this mixing zone controlled the secondary minerals preserved in the Jura, Knockfarril Hill, and Glasgow members of the Mt. Sharp group exposed in GT.

Journal of Geophysical Research E: Planets↗

USGS2022

Tectonic subsidence modeling of diachronous transition from backarc to retroarc basin development and uplift during Cordilleran orogenesis, Patagonian-Fuegian Andes

Backstripped tectonic basin subsidence histories are critical for interpreting phases of lithospheric deformation and paleoenvironmental change from the stratigraphic record. This study presents new subsidence modeling of the Rocas Verdes Backarc Basin (RVB) and Magallanes-Austral retroarc foreland basin (MAB) of southernmost South America to evaluate along-strike changes in tectonic subsidence related to the Late Jurassic through Miocene history of the Southern Andes. We compiled composite stratigraphic sections for seven basin localities that span 47°–54°S from published sedimentological records of paleoenvironment, paleobathymetry, and geochronology. Modeling results resolve regional trends in basin tectonic subsidence, uplift, and sedimentation rate that influenced the depositional environment during five broad phases of RVB-MAB development: (a) Late Jurassic tectonic subsidence and basin deepening associated with rift-related backarc extension that postdated regional diachronous rift-related magmatism. (b) Southward younging of Early to Late Cretaceous pronounced acceleration in tectonic subsidence interpreted as the initiation of flexural loading and development of the MAB foreland basin system. (c) Late Cretaceous (ca. 85–70 Ma) tectonic uplift within the central foredeep ∼49° to 52°S, coeval with a shift from slope to shelf deposition at these latitudes. (d) A protracted period of low-magnitude basin uplift and relative tectonic quiescence during the Paleogene, with the exception of southernmost localities; and (e) Synchronous latest Oligocene-early Miocene tectonic subsidence linked to basin deepening and transgression across the northern and central basin sectors. Backstripped tectonic subsidence analysis corroborates existing interpretations for orogenic development in the RVB-MAB and sheds new light on complex polyphase basin histories where extension precedes convergence.

Andes Mountains, Magallanes-Austral retroarc forel↗

USGS2024

Repeated coseismic uplift of coastal lagoons above the Patton Bay Splay Fault System, Montague Island, Alaska, USA

Coseismic slip on the Patton Bay splay fault system during the 1964 M w 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines as much as 11 m on Montague Island in Prince William Sound (PWS). Sudden uplift of 3.7–4.3 m caused coastal lagoons along the island's northwestern coast to gradually drain. The resulting change in depositional environment from marine lagoon to freshwater muskeg created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology across the 1964 earthquake contact and three similar prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that the contacts signal instances of abrupt coastal uplift that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS—1964 Common Era, 760–870 yr BP, 2500–2700 yr BP, and 4120–4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift. Our observations provide evidence for four instances of combined megathrust-splay fault ruptures that have occurred in the past ∼4,200 years in PWS. The possibility that 1964-style combined megathrust-splay fault ruptures may have repeated in the past warrants their consideration in future seismic and tsunami hazards assessments.

Alaska↗

USGS2012

Varve calibrated records of carbonate and organic carbon accumulation over the last 2000 years in the Black Sea

Sedimentologic and geochemical studies of box and gravity cores recovered from the Black Sea during the first leg of a multileg international Black Sea expedition in 1988 allow reconstruction of the basinwide Holocene environmental history of the Black Sea. In the deeper parts of the basin, box cores typically recovered a flocculent surface layer (“fluff”), laminated coccolith marls of Unit I (25–45 cm thick), and the upper 5–10 cm of finely laminated, dark‐colored sapropels of Unit II. Fine‐grained, homogeneous mud turbidites are interbedded with Units I and II over much of the basin, but the stratigraphie position of these turbidites differs, from site to site. The deposition of individual turbidites up to 15 cm thick does not appear to have significantly disturbed underlying laminae. Sediment trap deployments in the Black Sea suggest that light and dark laminae couplets represent annual increments of sedimentation (i.e., varves); we have therefore constructed a varve chronology for the sequence in order to correlate and date distinctive sedimentation and paleoenvironmental events. Distinctive groups of laminae in Unit I can be correlated across the entire deeper basin (a distance of more than 1000 km). This implies a remarkable homogeneity in production, accumulation, and preservation of biogenic material over much of the Black Sea during deposition of Unit I. The change from deposition of finely laminated, organic carbon‐rich sapropels (Unit II) to laminated, more calcareous, coccolith‐rich marls (Unit I) is thought to represent the crossing of a salinity threshold for Emiliania huxleyi. The varve chronology sets this change at about 1.63 ka (1633±100 yr B.P.), but the record of magnetic secular variation measured in several cores produces an age estimate of about 2.0 ka for the base of Unit I, or about 1.2 times the varve age. The average of six calibrated accelerator mass spectrometry radiocarbon ages for the base of Unit I is 2.7 ka, or about 1.7 times the varve age. Following the initial change to coccolith‐dominated sedimentation, deposition of sapropel resumed for at least one significant period, 1.56–1.25 ka. Since 1.25 ka, cycles of carbonate deposition with quasi‐decadal periodicities have produced characteristic darker and lighter assemblages of laminae. These cycles may have been climatically driven. Geochemical analyses coupled with the varve ages adopted herein indicate that accumulation rates of carbonate are nearly an order of magnitude higher in Unit I (averaging 35–45 g m −2 yr −1 ) than in sapropelic Unit II. which contains primarily detrital carbonate. The accumulation of lithogenic components in parts of Unit I is only 1.5 times the rate in Unit II. Deepwater organic carbon accumulation rates are somewhat higher in Unit I (3.5–4.5 g m −2 yr −1 ) than in the upper part of Unit II. Organic carbon accumulation rates in Unit I are somewhat antithetic to those of carbonate, and on the basis of this and additional constraints placed by pyrolysis and carbon isotopic analyses of organic material, it appears that terrestrial organic matter is an important component (perhaps >25%) of total organic carbon burial in the basin. Unit I in the western part of the Black Sea has a higher terrestrial organic component and higher accumulation rates of terrigenous clastic material than Unit I in the eastern part. This difference between eastern and western Black Sea is to be expected because of the major rivers that empty into the western Black Sea from eastern Europe, Ukraine, and Russia. Shallow slope sites, but still within euxinic bottom waters, have lower organic carbon accumulation rates and lower pyrolysis hydrogen indices than deepwater basinal sites, suggesting selective resuspension and oxidation of organic matter at basin margins and focusing of organic matter deposition toward the basin center. A comparison of the Black Sea data with those from several open ocean sites with similar water depths showed no significant difference between organic carbon accumulation rates under oxic and anoxic conditions. For a given bulk accumulation rate the organic carbon accumulation rates, normalized to primary productivity, are about the same in both settings.

Global Biogeochemical Cycles↗

USGS1996

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Journal of Geophysical Research B: Solid Earth↗

USGS1989

Comparative facies formation in selected coal beds of the Powder River Basin

Petrologic studies of thick coal beds [Warwick, 1985; Moore, 1986; Moore and others, 1986; Moore and others, 1987; Warwick and Stanton, in press], which build on sedimentological interpretations [Flores, this volume] of associated units, provide data to interpret and contrast the varieties of peat formation in the Powder River Basin. Detailed analyses of the composition of coal beds lead to more complete interpretations regarding the depositional environment on a regional and local scale. Our efforts in the Powder River Basin [areas A-D in fig. 1 of Flores, this volume] have resulted in a series of site-specific studies that interpret the types of peat formation from the arrangement of different facies which comprise the coal beds and from the spatial form of the coal beds. Our approach was to use a combination of megascopic criteria for facies sampling, and where only core was available, to analyze many interval samples to discriminate facies by their maceral composition. Coal beds in the Powder River Basin are composed of laterally continuous, compositional subunits of the bed (facies) that can be discerned most easily in weathered highwall exposures, less readily in fresh highwalls, and very poorly in fresh-cut core surfaces. In general, very low ash (

Montana, Wyoming↗

USGS2018

Holen et al. reply

Contrary to our hypothesis that the Cerutti Mastodon (CM) site represents a 130,000-year-old archaeological site, in the accompanying Comment Ferraro et al . argue that the site formed through ‘common’ geological and taphonomic processes. As a source for the cobbles that we interpreted as hammerstones and anvils, they postulate a previously unrecognized alluvial fan, later removed by fluvial winnowing that somehow left our five cobbles, refitting flakes, and fragments of stone, mastodon bone and teeth in place. There is no sedimentological or geomorphic evidence of an alluvial fan, and their scenario leaves unexplained a number of taphonomic features, including the two discrete concentrations in which were found cobbles, refit stones and bones, impact-fractured bones, side-by-side femoral heads and a tusk oriented vertically.

Nature↗

USGS2021

Performance of bedload sediment transport formulas applied to the Lower Minnesota River

Despite limitations in reproducing complex bedload sediment transport processes in rivers, formulas have been preferred over collection and analysis of field data due to the high cost and time-consuming nature of bedload discharge measurements. However, the performance of such formulas depends on the hydraulic and sedimentological conditions they attempt to describe. The availability of field measurements provides a unique opportunity to test bedload transport formulas to better guide formula selection. Hydraulic parameters and bedload discharge data from the Lower Minnesota River and two of its tributaries were used to evaluate nine bedload transport formulas using three different indices. The bedload data for the different sites were collected by the United States Geological Survey (USGS) from 2011 through 2014, with bed material varying from very coarse to medium sand. The formulas calculated higher bedload rates than were measured due to a combination of site-specific physical characteristics, including the presence of bed forms (dunes), and sampling uncertainties. Because of the lack of reproducibility of the tested formulas, five power functions, based on the relation between the specific unit power (independent hydraulic variable) and the USGS measured data (dependent variable), were derived as provisional equations to estimate the bedload discharge on the Lower Minnesota River and tributaries.

Minnesota↗

USGS1999

The insect trace fossil Tonganoxichnus from the middle Pennsylvanian of Indiana: Paleobiologic and paleoenvironmental implications

The ichnogenus Tonganoxichnus , produced by one or more monuran insect taxa, is now recorded from the Middle Pennsylvanian Mansfield Formation of Indiana. Tonganoxichnus is a resting trace that has three important implications. First, it represents a recurrent behavioral pattern in Upper Carboniferous to Lower Permian marginal marine environments of North America. Second, it provides finely resolved anatomical information for axial and appendicular body structures and behaviors that are difficult to determine from body‐fossil material alone. Third, integrated sedimentologic and ichnologic observations indicate that the Tonganoxichnus assemblage, inclusive of other ichnotaxa, is common in tidal rhythmites that were developed under freshwater conditions, probably in the innermost part of estuarine systems, close to or at the fluvioestuarine transition.

Ichnos: An International Journal for Plant and Ani↗

USGS2008

Trace fossils and sedimentary facies from a Late Cambrian‐Early Ordovician tide‐dominated shelf (Santa Rosita Formation, northwest Argentina): Implications for ichnofacies models of shallow marine successions

The Santa Rosita Formation is one the most widely distributed lower Paleozoic units of northwest Argentina. At the Quebrada del Salto Alto section, east of Purmamarca, Jujuy Province, it is represented by four sedimentary facies: thick‐bedded planar cross‐stratified quartzose sandstones (A), thin‐bedded planar cross‐stratified quartzose sandstones and mudstones (B), wave‐rippled sandstones and bioturbated mudstones (C), and black and greenish gray shales (D). Paleocurrent data, sandstone architecture, and sedimentary structures from faciès A and B indicate bipolar/bimodal paleoflows, suggesting the action of tidal currents. The succession is interpreted as that of a tide‐dominated shelf, with only secondary influence of wave processes. Trace fossils are restricted to facies B and C. The Cruziana ichnocoenosis is preserved on the soles of thin‐bedded planar cross‐stratified quartzose sandstones (faciès B). This ichnocoenosis consists of Conostichus isp., Cruziana omanica, C. semiplicata, C. cf. tortworthi, Cruziana isp. Helminthopsis abeli, Monomorphichnus bilinearis, M. multilineatus, Palaeophycus tubularis, Rusophycus carbonarias, R. latus , and R. isp. The occurrence of Cruziana semiplicata , C. omanica, C. cf. tortworthi , and Rusophycus latus supports a Late Cambrian‐Tremadoc age. Slabbing of Cruziana shows complex interactions between biologic and sedimentologic processes, and suggests a predominance of exhumed traces, washed out and recast by tractive sand deposition. Sandstone soles are densely packed with biogenic structures and exhibit distinctive clusters of Rusophycus isp. that most likely represent trilobite nesting burrows. The Cruziana ichnocoenosis records the resident fauna of a protected, lower intertidal to subtidal interbar setting. The Skolithos ichnocoenosis is represented by high to low density vertical burrows of Skolithos linearis , which extend downwards to the quartzose sandstone soles of faciès B and cross the Cruziana ichnocoenosis. The Skolithos ichnocoenosis represents colonization by suspension‐feeding organisms following a major change in environmental conditions, related to the migration of lower intertidal to subtidal sandwaves. The Planolites ichnocoenosis consists exclusively of Planolites montanus within mudstones overlying wave‐rippled sandstones (facies C). The Planolites ichnocoenosis records opportunistic colonization by inf aunal deposit feeders that mined the organic‐rich fine‐grained sediment during the waning phase of storms that scoured organic detritus from the sea bottom. The section records, from base to top, a Cruziana‐Skolithos ichnofacies zone, a Skolithos ichnofacies zone and an unbioturbated zone typified by the thick‐bedded cross‐stratified quartzose sandstone (fades A). This trend reflects progressively higher energy conditions linked to the establishment of a large sand wave complex. The presence of a mixed Cruziana‐Skolithos ichnofacies in the lower interval reflects changes in substrate and energy levels, rather than water depth. Accordingly, contrasting ichnocoenoses from interbars (Cruziana) and sandwaves (Skolithos) must be considered an example of ichnofacies controlled by local parameters instead of general bathymétrie trends. Conversely, the vertical replacement of the Cruziana ichnofacies by the Skolithos ichnofacies towards the middle interval of the section reflects the environmental changes associated with the transition between the intertidal and subtidal zones. As overall tidal energy increases from supratidal to subtidal settings, the Skolithos ichnofacies tends to occur seaward of the Cruziana ichnofacies in tide‐dominated shallow marine environments. Therefore, onshore‐offshore ichnofacies replacement in tide‐dominated shallow seas is opposite to that in wave‐dominated marine settings.

Ichnos: An International Journal for Plant and Ani↗

USGS2025

Sea-level driven isolation of glacial plant refugia revealed by submerged lake sediment from the Bering Land Bridge and St. Matthew Island

Bering Land Bridge (BLB) climate and vegetation during the Last Glacial Maximum (LGM) remains largely understudied, given challenges associated with collecting records from the submerged BLB. Previous records, confined to the margins of the modern land area and adjacent shelf, reveal conflicting interpretations of Beringian vegetation during the LGM. Here, we reconstruct LGM vegetation, sedimentology, and stable isotopes from a central BLB paleo-lake (Lake Knebel, LK) and compare it with a Holocene peat record from nearby St. Matthew Island (SMI). Results show strong similarities between LGM and late Holocene pollen assemblages, although with differences in relative taxonomic abundance. LGM communities are consistent with a cold and dry steppe or herb tundra environment but suggest the possibility of localized Betula presence in low-lying areas. LK’s bedded lacustrine stratigraphy transitions into undisturbed marine sediments by ~19 ka, providing a maximum limiting age of the transgression. Shrub absence on SMI today and during the Holocene is consistent with island isolation before ~14 to 15 ka, when Betula expanded rapidly at most sites with Bølling-Allerød warming. The combined vegetation evidence indicates preservation of LGM tundra and steppe vegetation assemblages on SMI, suggesting that island vegetation communities may provide additional constraints on the timing of sea level transgression.

Alaska↗