Late Ordovician mass extinction in the Selwyn Basin, northwestern Canada: geochemical, sedimentological, and paleontological evidence

1993 ◽  
Vol 30 (9) ◽  
pp. 1870-1880 ◽  
Author(s):  
K. Wang ◽  
B. D. E. Chatterton ◽  
M. Attrep Jr. ◽  
C. J. Orth
Keyword(s):  
Stable Isotope ◽  
Sea Level ◽  
Mass Extinction ◽  
Facies Analysis ◽  
Isotope Geochemistry ◽  
Late Ordovician ◽  
Cold Climate ◽  
Photic Zone ◽  
Toxic Material ◽  
Short Period

We present a detailed study of the trace element and stable isotope geochemistry, sedimentology, and fossil distributions in two Avalanche Lake (AV4B, AV1) Ordovician–Silurian boundary sections in the Selwyn Basin. Trilobites and conodonts indicate a profound extinction at the end of the Ordovician, which is constrained stratigraphically within a <60 cm interval at AV4B. Facies analysis suggests that the extinction interval coincides with the maximum shallowing (low stand of sea level), which was probably caused by a galcioeustatic regression induced by the Late Ordovician Gondwanan glaciation. The extinction crisis is also signalled by the change in carbonate δ13C: a sudden "Strangelove ocean" δ13C excursion (>3‰ in magnitude) is recorded in the extinction interval. Iridium abundances (<0.051 ppb) in the extinction interval are low and fail to provide evidence for an impact. The highest Ir abundance is found to be associated with reduced sedimentation in a condensed horizon. Cerium anomalies indicate a short period of basin ventilation in the otherwise anoxic Selwyn Basin. The extinction occurred during the time of this basin ventilation, which was probably caused by the cold climate during the glaciation. The ventilation may have triggered upwelling of the deep water through vertical advection, bringing up toxic material, poisoning the upper-water photic zone, and causing the extinction.

Hirnantia Fauna from the Condroz Inlier, Belgium: another case of a relict Ordovician shelly fauna in the Silurian?

2021 ◽  
pp. 1-27
Author(s):  
Sofia Pereira ◽  
Jorge Colmenar ◽  
Jan Mortier ◽  
Jan Vanmeirhaeghe ◽  
Jacques Verniers ◽  
...  
Keyword(s):  
Sea Level ◽  
Mass Extinction ◽  
Late Ordovician ◽  
Lake District ◽  
Low Diversity ◽  
Hirnantia Fauna ◽  
And Sea Level

Abstract The end-Ordovician mass extinction, linked to a major glaciation, led to deep changes in Hirnantian–Rhuddanian biotas. The Hirnantia Fauna, the first of two Hirnantian survival brachiopod-dominated communities, characterizes the lower–mid Hirnantian deposits globally, and its distribution is essential to understand how the extinction took place. In this paper, we describe, illustrate, and discuss the first macrofossiliferous Hirnantia Fauna assemblage from Belgium, occurring in the Tihange Member of the Fosses Formation at Tihange (Huy), within the Central Condroz Inlier. Six fossiliferous beds have yielded a low-diversity, brachiopod-dominated association. In addition to the brachiopods (Eostropheodonta hirnantensis, Plectothyrella crassicosta, Hirnantia sp., and Trucizetina? sp.), one trilobite (Mucronaspis sp.), four pelmatozoans (Xenocrinus sp., Cyclocharax [col.] paucicrenulatus, Conspectocrinus [col.] celticus, and Pentagonocyclicus [col.] sp.), three graptolites (Cystograptus ancestralis, Normalograptus normalis, and ?Metabolograptus sp.), together with indeterminate machaeridians and bryozoans were identified. The graptolite assemblage, from the Akidograptus ascensus-Parakidograptus acuminatus Biozone, indicates an early Rhuddanian (Silurian) age, and thus, an unexpectedly late occurrence of a typical Hirnantia Fauna. This Belgian association may represent an additional example of relict Hirnantia Fauna in the Silurian, sharing characteristics with the only other known from Rhuddanian rocks at Yewdale Beck (Lake District, England), although reworking has not been completely ruled out. The survival of these Hirnantian taxa into the Silurian might be linked to delayed post-glacial effects of rising temperature and sea-level, which may have favored the establishment of refugia in these two particular regions that were paleogeographically close during the Late Ordovician–early Silurian.

The development of an atypical Hirnantia-brachiopod Fauna and the onset of glaciation in the late Ordovician of Gondwana

2001 ◽  
Vol 92 (1) ◽  
pp. 1-14 ◽  
Author(s):  
Owen E. Sutcliffe ◽  
David A. T. Harper ◽  
Abdallah Aït Salem ◽  
Robert J. Whittington ◽  
Jonathan Craig
Keyword(s):  
Continental Shelf ◽  
Sea Level ◽  
North Africa ◽  
Mass Extinction ◽  
Ice Sheet ◽  
Late Ordovician ◽  
Shallow Marine ◽  
Marine Deposits ◽  
Brachiopod Fauna ◽  
Glaciomarine Sediments

ABSTRACTThe development of an atypical Hirnantia Fauna in the late Ordovician of Gondwana was coeval with a slow eustatic fall induced by the abstraction of water into a growing ice sheet. This event is dated as early Hirnantian in age and occurred in tandem with the start of a major mass extinction. A tectonic episode in the Caradoc-Ashgill of North Africa differentiated the continental shelf into highs and lows and may have formed the land required for the accumulation of a permanent snow cover. Depositional lows were filled by regressive shallow-marine deposits in the early Hirnantian. During the mid-Hirnantian, advance and retreat of an ice sheet on the continental shelf resulted in the deposition of glaciomarine sediments above these regressive deposits. The demise of an atypical Hirnantia Fauna is attributed to deglaciation and the associated flooding of the continental shelf by a stratified anoxic water column. This glacioeustatic sea-level rise occurred in the late Hirnantian.

scholarly journals Spatial distribution of water level impact to back-barrier bays

2018 ◽  
Author(s):  
Alfredo L. Aretxabaleta ◽  
Neil K. Ganju ◽  
Zafer Defne ◽  
Richard P. Signell
Keyword(s):  
Water Level ◽  
Sea Level ◽  
Water Levels ◽  
Analytical Models ◽  
Barnegat Bay ◽  
Sea Level Fluctuations ◽  
Flooding Hazard ◽  
Short Period ◽  
Inlet Geometry ◽  
Spatial Estimates

Abstract. Water level in semi-enclosed bays, landward of barrier islands, is mainly driven by offshore sea level fluctuations that are modulated by bay geometry and bathymetry, causing spatial variability in the ensuing response (transfer). Local wind setup can have a secondary role that depends on wind speed, fetch, and relative orientation of the wind direction and the bay. Inlet geometry and bathymetry primarily regulate the magnitude of the transfer between open ocean and bay. Tides and short-period offshore oscillations are more damped in the bays than longer-lasting offshore fluctuations, such as storm surge and sea level rise. We compare observed and modeled water levels at stations in a mid-Atlantic bay (Barnegat Bay) with offshore water level proxies. Observed water levels in Barnegat Bay are compared and combined with model results from the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system to evaluate the spatial structure of the water level transfer. Analytical models based on the dimensional characteristics of the bay are used to combine the observed data and the numerical model results in a physically consistent approach. Model water level transfers match observed values at locations inside the Bay in the storm frequency band (transfers ranging from 70–100 %) and tidal frequencies (10–55 %). The contribution of frequency-dependent local setup caused by wind acting along the bay is also considered. The approach provides transfer estimates for locations inside the Bay where observations were not available resulting in a complete spatial characterization. The approach allows for the study of the Bay response to alternative forcing scenarios (landscape changes, future storms, and rising sea level). Detailed spatial estimates of water level transfer can inform decisions on inlet management and contribute to the assessment of current and future flooding hazard in back-barrier bays and along mainland shorelines.

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