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1. Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA

   https://doi.org/10.1130/B35701.1  

   Padgett, Jason S. ; Engelhart, Simon E. ; Kelsey, Harvey M. ; Witter, Robert C. ; Cahill, Niamh ; Hemphill-Haley, Eileen ( February 2021 , GSA Bulletin)

   Abstract Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate that at least four abrupt mud-over-peat contacts are recorded across three sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern Humboldt Bay, California, USA (∼44.8°N, −124.2°W). The stratigraphy records subsidence during past megathrust earthquakes at the southern Cascadia subduction zone ∼40 km north of the Mendocino Triple Junction. Maximum and minimum radiocarbon ages on plant macrofossils from above and below laterally extensive (>6 km) contacts suggest regional synchroneity of subsidence. The shallowest contact has radiocarbon ages that are consistent with the most recent great earthquake at Cascadia, which occurred at 250 cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and ca. 1620 cal yr B.P. For each of the four earthquakes, we analyze foraminifera across representative mud-over-peat contacts selected from McDaniel Creek. Changes in fossil foraminiferal assemblages across all four contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with submergence lasting from decades to centuries. To estimate subsidence during each earthquake, we reconstructed RSL rise across the contacts using the fossil foraminiferal assemblages in a Bayesian transfer function.more »The coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ± 0.47 m for the ca. 1120 cal yr B.P. earthquake, and ≥0.93 m for the ca. 1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment likely was above the upper limit of foraminiferal habitation. The subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than (<50%) the subsidence estimates for other contacts and suggests that subsidence magnitude varied over the past four earthquake cycles in southern Cascadia.« less
2. Minimal stratigraphic evidence for coseismic coastal subsidence during 2000 yr of megathrust earthquakes at the central Cascadia subduction zone

   https://doi.org/10.1130/GES02254.1  

   Nelson, Alan R. ; Hawkes, Andrea D. ; Sawai, Yuki ; Horton, Ben P. ; Witter, Rob C. ; Bradley, Lee-Ann ; Cahill, Niamh ( December 2020 , Geosphere)

   Abstract Lithology and microfossil biostratigraphy beneath the marshes of a central Oregon estuary limit geophysical models of Cascadia megathrust rupture during successive earthquakes by ruling out >0.5 m of coseismic coastal subsidence for the past 2000 yr. Although the stratigraphy in cores and outcrops includes as many as 12 peat-mud contacts, like those commonly inferred to record subsidence during megathrust earthquakes, mapping, qualitative diatom analysis, foraminiferal transfer function analysis, and 14C dating of the contacts failed to confirm that any contacts formed through subsidence during great earthquakes. Based on the youngest peat-mud contact’s distinctness, >400 m distribution, ∼0.6 m depth, and overlying probable tsunami deposit, we attribute it to the great 1700 CE Cascadia earthquake and(or) its accompanying tsunami. Minimal changes in diatom assemblages from below the contact to above its probable tsunami deposit suggest that the lower of several foraminiferal transfer function reconstructions of coseismic subsidence across the contact (0.1–0.5 m) is most accurate. The more limited stratigraphic extent and minimal changes in lithology, foraminifera, and(or) diatom assemblages across the other 11 peat-mud contacts are insufficient to distinguish them from contacts formed through small, gradual, or localized changes in tide levels during river floods, storm surges, and gradual sea-levelmore »rise. Although no data preclude any contacts from being synchronous with a megathrust earthquake, the evidence is equally consistent with all contacts recording relative sea-level changes below the ∼0.5 m detection threshold for distinguishing coseismic from nonseismic changes.« less
3. Loess in the Lodeve? Exploring the depositional character of the Permian Salagou Formation, Lodeve Basin (France).

   https://doi.org/doi:10.1130/abs/2018AM-317628  

   Pfeifer, Lily S. ( October 2018 , Geological Society of America Abstracts with Programs)

   Permocarboniferous strata of basins proximal to the Central Pangaean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (late Paleozoic Pangaean assembly, ice age collapse, megamonsoon inception). The voluminous (estimated 2 km) succession of exclusively fine-grained redbeds that composes the Permian Salagou Formation (Lodéve Basin, France) has been interpreted as recording either lacustrine or fluvial settings. We present preliminary field data to explore the hypothesis that these deposits record eolian transport, and ultimate deposition as either loess or in a shallow lacustrine environment. Fieldwork includes ~1000 m of section described at dm-scale, and magnetic susceptibility measured at 0.5 m intervals, from sections strategically located in both proximal and distal areas, and from all stratigraphic levels of the unit to assess spatial and temporal variations. These data indicate that the lower and middle Salagou Formation is dominated by internally massive, red mud-siltstone with no evidence of channeling. Up-section, a higher frequency of ripples, rare hummocky cross stratification, and mudcracks record the presence of shallow water, but with no channeling, nor units of grain size exceeding very fine-grained sand. Randomly-oriented slickensides at various localities in the mid-upper Salagou may represent incipient pedogenesis. The lack of evidence formore »channels and other fluvial features casts doubt on a fluvial interpretation. A lacustrine interpretation is consistent with local evidence of shallow water. However, in the absence of fluvial transport indicators, large volumes of entirely fine-grained material that were delivered to the Lodéve basin call for eolian transport, and thus a loess or shallow lacustrine interpretation. The documentation of voluminous paleoloess in eastern equatorial Pangea during the Permian could reflect the influence of glaciation associated with the Variscan highlands. Together with previous studies that detail Permian loess in western equatorial Pangea, this work impacts our understandingof the global Late Paleozoic climate system and presents a need to reevaluate modeling parameters (e.g. equatorial mountain glaciation, atmospheric dust loading).« less
4. Mixed carbonate–siliciclastic sedimentation at the margin of a late Miocene tidal strait, lower Colorado River Valley, south‐western USA

   https://doi.org/10.1111/sed.12834  

   Gardner, Kevin ; Dorsey, Rebecca J. ; Betzler, ed., Christian ( January 2021 , Sedimentology)

   The processes and deposits of tide‐dominated river deltas and estuaries are well‐understood, but the sedimentary dynamics of tide‐dominated straits and seaways are relatively little studied. Although recent depositional models have started to fill this gap, many aspects of tidal strait sedimentation such as interaction with strait‐margin alluvial fans and marginal marine systems remain poorly understood. This paper presents a study of the late Miocene basal carbonate member of the Bouse Formation exposed along the lower Colorado River, where prior studies have suggested a tidal influence on deposition. This study explores the applicability of tidal strait models in a continental oblique‐rift setting, and tests hypotheses for depositional processes and environments through detailed analysis of sedimentary facies, cross‐bedding architecture and palaeocurrent data. Mixed carbonate–siliciclastic facies on the west margin of the southern Blythe Basin record sedimentation in alluvial fans and fan‐fringing tidal flats at the retreating margin of a transgressive tidal strait. Pre‐Bouse normal faults established a narrow, tectonically confined basin architecture that led to amplification of tidal currents. Basin‐margin deposits pass laterally and up‐section into high‐energy cross‐bedded grainstone facies that record southward migration of compound dunes in the dune‐bedded strait zone of a shallow (ca25 m) tidal strait. These findings providemore »the basis for recognition of strait‐margin facies in other settings where complex facies associations result from along‐strike variations in palaeobathymetry and current velocity. The results support a tidal origin for the southern Bouse Formation and provide new evidence for post‐Miocene uplift of the lower Colorado River Valley and adjacent areas.

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5. Late Holocene environmental change in Celestun Lagoon, Yucatan, Mexico

   https://doi.org/10.1007/s10933-021-00227-4  

   Hardage, Kyle ; Street, Joseph ; Herrera-Silveira, Jorge A. ; Oberle, Ferdinand K. J. ; Paytan, Adina ( December 2021 , Journal of Paleolimnology)

   Epikarst estuary response to hydroclimate change remains poorly understood, despite the well-studied link between climate and karst groundwater aquifers. The influence of sea-level rise and coastal geomorphic change on these estuaries obscures climate signals, thus requiring careful development of paleoenvironmental histories to interpret the paleoclimate archives. We used foraminifera assemblages, carbon stable isotope ratios (δ¹³C) and carbon:nitrogen (C:N) mass ratios of organic matter in sediment cores to infer environmental changes over the past 5300 years in Celestun Lagoon, Yucatan, Mexico. Specimens (> 125 µm) from modern core top sediments revealed three assemblages: (1) a brackish mangrove assemblage of agglutinatedMiliamminaandAmmotiumtaxa and hyalineHaynesina(2) an inner-shelf marine assemblage ofBolivina,Hanzawaia, andRosalina,and (3) a brackish assemblage dominated byAmmoniaandElphidium. Assemblages changed along the lagoon channel in response to changes in salinity and vegetation, i.e. seagrass and mangrove. In addition to these three foraminifera assemblages, lagoon sediments deposited since 5300 cal yr BP are comprised of two more assemblages, defined byArchaiasandLaevipeneroplis,which indicate marineThalassiaseagrasses, andTrichohyalus,which indicates restricted inland mangrove ponds. Our data suggest that Celestun Lagoon displayed four phases of development: (1) an inland mangrove pond (5300 BP) (2) a shallow unprotected coastline with marine seagrass and barrier island initiation (4900 BP) (3) a protected brackish lagoon (3000 BP), and (4) amore »protected lagoon surrounded by mangroves (1700 BP). Stratigraphic (temporal) changes in core assemblages resemble spatial differences in communities across the modern lagoon, from the southern marine sector to the northern brackish region. Similar temporal patterns have been reported from other Yucatan Peninsula lagoons and fromcenotes(Nichupte, Aktun Ha), suggesting a regional coastal response to sea level rise and climate change, including geomorphic controls (longshore drift) on lagoon salinity, as observed today. Holocene barrier island development progressively protected the northwest Yucatan Peninsula coastline, reducing mixing between seawater and rain-fed submarine groundwater discharge. Superimposed on this geomorphic signal, assemblage changes that are observed reflect the most severe regional wet and dry climate episodes, which coincide with paleoclimate records from lowland lake archives (Chichancanab, Salpeten). Our results emphasize the need to consider coastal geomorphic evolution when using epikarst estuary and lagoon sediment archives for paleoclimate reconstruction and provide evidence of hydroclimate changes on the Yucatan Peninsula.

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