William B. N. Berry

Late Ordovician mass extinction: A new perspective from stratigraphic sections in central Nevada

Integrated sequence stratigraphic, biostratigraphic, and chemostratigraphic analyses of three stratigraphic sections in central Nevada indicate that Late Ordovician glaciation-induced sea-level fall produced diachronous, stepwise faunal turnover in graptolites, conodonts, chitinozoans, and radiolarians, and also triggered a strong, but transient, positive δ13C excursion. This pattern is very different from that described for most mass extinction events.

Glacio-Eustatic Control of Late Ordovician–Early Silurian Platform Sedimentation and Faunal Changes

Late Ordovician–Early Silurian stratigraphic sequences across the worlds platforms bear evidence for onlapping relations in the early part of the Silurian. In general, Late Ordovician marine benthic faunal communities and faunas found in platform rocks differ from those characteristic of the early part of the Silurian. Planktonic graptolites are also conspicuously different in Early Silurian strata from those in Late Ordovician strata. The impressive evidence for Late Ordovician–earliest Silurian continental glaciation in Africa and significant portions of South America and evidence for only restricted orogenic activity in the Late Ordovician-Early Silurian interval suggests that glaciation was the principal agent behind the changes seen among faunas as well as in the stratigraphic record. Shallowing of marine waters across the Late Ordovician platforms was probably related to “locking up” oceanic waters in the glaciers during glaciation, and onlapping relations took place as the glaciers melted and sea level rose. Restriction of platform marine environments and shallowing as well as cooling of the oceans were primary environmental factors behind the changes in marine faunas. Llandovery deglaciation coincides with a time interval during which animal communities were widely spread areally.

Destabilization of the oceanic density structure and its significance to marine “extinction” events

Abstract Areally extensive overturn of deep toxic or biologically unconditioned water, at the beginning of climatic change, is suggested as a possible contributing factor to mass extinction events in the oceans. The overturn and transfer to the photic zone of such waters is due to disruption of the stable stratification of the oceans as the source of oceanic deep water shifts from middle to high latitudes during climatic cooling. In such a weakly stratified ocean, normal geophysical phenomena such as internal tides, heat flow, etc., could provide the uplift. Comparison of extinction events, climate, sulfur-isotope data, pyrite burial rates, sea-level changes, with times of potential overturn since the Late Precambrian suggest that overturn was a contributing factor in extinction events reported for the Late Ordovician, Late Devonian, Late Triassic, and Late Cretaceous; and for marine faunal change in the Late Precambrian and Middle Silurian.

New perspectives on graptolite distributions and their use as indicators of platform margin dynamics

Graptolite distributions in Ordovician shelf, slope, and basinal facies in the Great Basin indicate that graptolites were scarce in open oceanic waters oceanward of the Cordilleran shelf margin and that they thrived in waters above the margin. This pattern is consistent with that of most zooplankton in modern oceans. It follows from these observations that the depositional setting of typical graptolitic shale was the area of the sea floor under continental-margin upwelling zones where graptolites flourished and within the oxygen-minimum zone where their rhabdosomes were preserved. With changes over time in relative sea level, deep oceanic circulation, and wind-driven surface circulation, the upwelling and oxygen-minimum zones may have thickened or thinned, migrated landward or oceanward, and expanded laterally, contracted, or even disappeared. The observed graptolite occurrences suggest that the primary graptolite biotope—that is, the habitat of diverse and abundant faunas—was a relatively narrow belt of upwelling waters along, and extending somewhat open oceanward from, the continental margin. Provinces were maintained only to the extent that species could disperse along continental margins. Distribution of typical graptolitic strata may be used to interpret development of continental margins, because such distribution incorporates a signal of sea-level rise or fall, oceanographic changes (especially upwelling), and tectonic events that led to creation and deterioration of upwelling conditions in which graptolites flourished.

Stratigraphy, Zonation, and Age of Schaghticoke, Deepkill, and Normanskill Shales, Eastern New York

Graptolite-bearing black shales in eastern New York have served as the standard of reference for Ordovician graptolite successions in North America. The Schaghticoke, Deepkill, and Normanskill shales do not include a continuous sequence of Early and Middle Ordovician graptolite zones, as previously thought. The Schaghticoke Shale bears a fauna characterized by Dictyonema flabelliforme flabelliforme and Staurograptus dichotomous. An earliest Ordovician graptolite zone encompasses it. Four zonal assemblages can be delimited in the Deepkill Shale. Two are latest Early Ordovician; one is earliest Middle Ordovician; the highest is Middle Ordovician and is separated from the others by a structural discordance and a faunal gap of one zone. The Normanskill Shale bears graptolite assemblages typical of two zones and is of late Middle Ordovician age. It can be divided into four lithic members. A lithic unit comprised of huge blocks of Normanskill Shale and other older formations in a black shale matrix, which bears graptolites typical of the Canajoharie Shale, overlies the Normanskill Shale on the eastern side of the Hudson River throughout Rensselaer County and the southern part of Washington County, New York. Its exposure belt may be the western limit of the “Taconic Klippe.” The unit separates apparent autochthonous from allochthonous portions of the Normanskill Shale.

Anoxia pre-dates Frasnian-Famennian boundary mass extinction horizon in the Great Basin, USA

Abstract Major and trace metal results from three Great Basin stratigraphic sections with strong conodont biostratigraphy identify a distinct anoxic interval that precedes, but ends approximately 100 kyr before, the Frasnian–Famennian (F–F, mid-Late Devonian) boundary mass extinction horizon. This horizon corresponds to the final and most severe step of a more protracted extinction period. These results are inconsistent with data reported by others from the upper Kellwasser horizon in Europe, which show anoxia persisting up to the F–F boundary in most sections. Conditions returned to fully oxygenated prior to the F–F boundary in the study area. These data indicate that the worst part of the F–F extinction was not related directly to oceanic anoxia in this region and potentially globally.

Iridium Abundances Across the Ordovician-Silurian Stratotype

Chemostratigraphic analyses in the Ordovician-Silurian boundary stratotype section, bracketing a major extinction event in the graptolitic shale section at Dobs Linn, Scotland, show persistently high iridium concentrations of 0.050 to 0.250 parts per billion. There is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years. Iridium correlated with chromium, both elements showing a gradual decrease with time into the middle part of the Lower Silurian. The chromium-iridium ratio averages about 106. Paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event.

Palaeo-oceanography and biogeography in the Tremadoc (Ordovician) Iapetus Ocean and the origin of the chemostratigraphy of Dictyonema flabelliforme black shales

High concentrations of vanadium, molybdenum, uranium, arsenic, antimony with low concentrations of manganese, iron and cobalt heretofore restricted to Dictyonema flabelliforme-bearing Tremadoc black shales in Balto-Scandia, have been found in coeval black shales in the Saint John, New Brunswick area. Prior palaeogeographic reconstructions place these areas about 400 km. apart in high southern latitudes in the Iapetus Ocean, with New Brunswick in proximity to Avalonia (southeastern Newfoundland). These geochemical similarities are not found in coeval Tremadoc black shales of Bolivia, New York, Quebec, Wales, and Belgium. Palaeo-oceanographic reconstructions of Iapetus support the proximity of Balto-Scandia and the Saint John area during the early Tremadoc and Geesx (1981) suggestion that the signature is a feature of eastern Iapetus. Furthermore, first-order modelling of the major surface currents and related primary productivity in the Tremadoc Iapetus Ocean explain the apparent wide latitudinal range of D. flabelliforme (Fortey, 1984) and the anomalous trace metal content of certain black shales of that time. Variations in the elemental content of these black shales is produced by oceanographic and geologic conditions unique to the geographic site. The distinctive Balto-Scandic geochemical signature resulted from the coincidence of anoxic waters transgressing the shelf at latitudes of high organic productivity at the polar Ekman planetary divergence. This produces the conditions for concentrations of V, U, and Mo in the shales. Metal enriched anoxic bottom waters produced by leaching of volcanics or through hydrothermal activity may be the source of the other enhanced signature elements such as As and Sb. The absence of this geochemical signature in younger non-D. flabelliforme Tremadoc and later black shales in Balto-Scandia and other areas suggests that the closing of Iapetus moved the depositional sites into less productive oceanic areas.

Siluro-Devonian Boundary in North America

The writers note a convergence of opinion on a world-wide scale favoring placement of the Siluro-Devonian boundary at the base of the Gedinnian. This boundary is coincident with the base of the Monograptus uniformis and Icriodus woschmidti zones. It is recognizable in the brachiopod-coraltrilobite succession by the disappearance of pentamerids, Atrypella, Gracianella, halysitids, and Encrinurus and by the incoming of terebratulids, Cyrtina, and common Schizophoria. Insofar as the Pridoli-Lochkov boundary in Bohemia can be revised to correspond to this boundary, the Pridoli is recognized as the uppermost Silurian Stage. In North America, the base of the Devonian, defined as the base of the Gedinnian, is located at what is probably the most satisfactory level because it lies at or near the base of the Helderbergian, which has traditionally been regarded as the lower-most Lower Devonian stage in eastern North America. Seven regions in North America are chosen for discussion because they are representative and form standards of a sort, with which most other fossiliferous sections in North America can be compared. Only in one region does the Siluro-Devonian boundary appear to coincide with a formation boundary, namely, at the base of the St. Alban Formation of eastern Gaspe. It falls within the Stonehouse Formation of Arisaig, Nova Scotia; the Rondout Formation of New York and New Jersey; the Keyser Limestone of western Maryland and vicinity; the Roberts Mountains Formation of central Nevada; the Prongs Creek Formation of Yukon Territory; and the Devon Island and Cape Phillips Formations in the Canadian Arctic Islands.

Lower Llandovery of the Northern Appalachians and Adjacent Regions

Rocks of clearly dated early Llandovery age, as well as rocks that can logically be classed as early Llandovery from their regional relationships, appear to be more widespread than recognized, heretofore, in the northern Appalachians and adjacent regions. Their areal distribution and lithology permit a generalized reconstruction of the paleogeography, which consisted, in general, of three source areas alternating from east to west with three belts of clastic sedimentation. The westernmost clastic belt grades laterally westward into the carbonate rocks of the North American platform. The Central Clastic Belt encloses a belt containing impure carbonates with clastic detritus and clastic interbeds, and, locally, relatively clean carbonate deposits. Llandovery age rocks of the platform include the Manitoulin Dolomite and the Ellis Bay Formation. In the deposits to the east, coeval rocks occur, in part or in whole, within the limy and clastic deposits of the Carys Mills Formation and the Matapedia Group, as well as in the following clastic rock formations: Grimsby, Shawangunk, Tuscarora, Massanutten, Clinch, Smyrna Mills, Perham, Cabano, Weir, Beechhill Cove, Ross Brook, and White Rock.