Jared R. Morrow

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.

EFFECTS OF DATA CATEGORIZATION ON PALEOCOMMUNITY ANALYSIS: A CASE STUDY FROM THE PENNSYLVANIAN FINIS SHALE OF TEXAS

Abstract Paleocommunity research efforts have explored a multitude of faunal assemblages using a wide range of sampling and analytical methods to infer a paleoecological signal. Here, we derive six secondary datasets from a single stratigraphic series of faunal assemblages in the Finis Shale (Pennsylvanian) of Jacksboro, Texas, USA, using a variety of data categorization decisions (i.e., abundance versus calcified biomass, all taxa versus selected indicator taxa, and generic versus higher clade resolution). Biomass- and abundance-derived datasets were not significantly different in terms of evenness, Shannons information index, or Simpsons diversity index. Using Bray-Curtis and nonmetric multidimensional scaling ordinations, with Sorenson and relative Sorenson distance measures, ordination axis scores of the six derived datasets were all significantly correlated with one another, suggesting little difference in their respective paleoecological signals. Three potential explanations for this consistent paleoecological signal, regardless of which data categorizations are employed, include: (1) the dominance of a few brachiopod taxa overwhelmingly influenced the community structure, (2) relatively constrained environmental conditions limited community variation, and (3) low variation in specimen size minimized potential differences among abundance and calcified biomass categorizations. We suggest that other datasets with greater diversities, greater evenness, or from a wider range of paleoenvironments might not show this consistency. Thus, to the degree possible and appropriate, paleoecological investigators should test the effects of these data categorization decisions on a paleoecological signal, regardless of the analytical method employed.

Evolution of Devonian carbonate-shelf margin, Nevada

The north-trending, 550-km-long Nevada segment of the Devonian carbonate-shelf margin, which fringed western North America, evidences the complex interaction of paleotectonics, eustasy, biotic changes, and bolide impact–related influences. Margin reconstruction is complicated by mid-Paleozoic to Paleogene compressional tectonics and younger extensional and strike-slip faulting. Reports published during the past three decades identify 12 important events that influenced development of shelf-margin settings; in chronological order, these are: (1) Early Devonian inheritance of Silurian stable shelf margin, (2) formation of Early to early Middle Devonian shelf-margin basins, (3) progradation of later Middle Devonian shelf margin, (4) late Middle Devonian Taghanic onlap and continuing long-term Frasnian transgression, (5) initiation of latest Middle Devonian to early Frasnian proto-Antler orogenic forebulge, (6) mid-Frasnian Alamo Impact, (7) accelerated development of proto-Antler forebulge and backbulge Pilot basin, (8) global late Frasnian semichatovae sea-level rise, (9) end-Frasnian sea-level fluctuations and ensuing mass extinction, (10) long-term Famennian regression and continent-wide erosion, (11) late Famennian emergence of Antler orogenic highlands, and (12) end-Devonian eustatic sea-level fall. Although of considerable value for understanding facies relationships and geometries, existing standard carbonate platform-margin models developed for passive settings elsewhere do not adequately describe the diverse depositional and structural settings along the Nevada Devonian platform margin. Recent structural and geochemical studies suggest that the Early to Middle Devonian shelf-margin basins may have been fault bound and controlled by inherited Precambrian structure. Subsequently, the migrating latest Middle to Late Devonian Antler orogenic forebulge exerted a dominant control on shelf-margin position, morphology, and sedimentation.

REEF RECOVERY FOLLOWING THE FRASNIAN–FAMENNIAN (LATE DEVONIAN) MASS EXTINCTION: EVIDENCE FROM THE DUGWAY RANGE, WEST-CENTRAL UTAH

ABSTRACT The temporally extensive late Middle through Late Devonian biotic crisis involved at least three distinct peaks of elevated extinction intensity during an interval spanning ∼25 myr and resulted in the preferential elimination of certain shallow-marine, warm-water taxa, especially members of reef communities. By the end of the second peak, delimited by the Frasnian–Famennian (F–F) boundary, the stromatoporoids, members of the dominant constructor guild in mid-Paleozoic reefal ecosystems, had ceased building reefs in most parts of the world. The northern Dugway Range in west-central Utah, United States, however, represents one of the few locations globally where stromatoporoids continued reef building into the Famennian. Two measured sections there, which are constrained biostratigraphically using conodonts, indicate that the biohermal sequences occur within the middle Palmatolepis crepida biozone and are early Famennian in age. The post-F–F extinction Dugway reefal faunas are depauperate and dominated by labechiid and stylostromid stromatoporoids, as is characteristic of other early Famennian reefs. In this region, evidence for reefal development is episodic, with stromatoporoid-bearing units interbedded with peloidal and coated-grain carbonate units lacking evidence of reef construction. The stromatoporoid survivors, although fairly minor constituents of Frasnian reef communities, belong to long-ranging clades and may represent so-called extinction-resistant taxa that flourished, albeit locally in Laurentia, following the F–F mass extinction.

IMPACTS AND MASS EXTINCTIONS REVISITED

On several occasions at impact geology- and paleontology-related meetings during the past 10 years, I have heard the statement from colleagues that major extinctions must be linked to large asteroid or comet impact events because “what else could it be?” (i.e., what other known mechanisms, terrestrial or extraterrestrial, could drive the geologically rapid loss of a majority of Earths species?) Although I am strongly pro-impact in my view of Earth history, I think that it is time to reappraise our state of knowledge within impact geology and mass-extinction research, and to examine the widely postulated link between impacts and mass extinctions more critically. It has been over 25 years since the landmark study of Alvarez et al. (1980) launched the global-scale search by the geological and paleontological communities for quantitative, testable evidence tying impact to extinction. Where then do we stand today? In this SPOTLIGHT, I highlight some recent advances in impact geology that have direct bearing on our ability to recognize the signature of ancient impact events and on our ability to link these events to mass killings. The continuing challenge (and frustration!) facing any hypothesized link between impact and extinction is well illustrated in Figure 1. Major biotope events documented in the fossil record may have been driven by a number of very different, potentially interacting causes, both telluric (terrestrial) and cosmic (extraterrestrial) in ultimate origin. These could lead via very different pathways to results that are, at least superficially, very similar. Of the proposed cosmic mechanisms responsible potentially for driving biotic crises, large-body hypervelocity impact remains the most important and the most readily testable in the geological record. FIGURE 1 —Flow chart showing the possible driving mechanisms and complex interactions that could lead ultimately to a global bioevent (after Walliser, 1996) Jared Morrow (doing his …

Impact Cratering and Its Planetary and Environmental Effects: Large Meteorite Impacts and Planetary Evolution IV; Vredefort Dome, South Africa, 17–21 August 2008

The Fourth Conference on Large Meteorite Impacts and Planetary Evolution (LMI IV) was held near the town of Parys in the Vredefort Dome, the center of Earths oldest and largest preserved impact structure. The Vredefort Dome, approximately 120 kilometers southwest of Johannesburg, South Africa, presents a superb cross section through deep levels of the impact structure. The Dome also provides exposures of the exceptionally well preserved Archean and Paleoproterozoic (>3.1 to 2.1 billion year old) rocks of the Kaapvaal craton. In July 2005, the northwestern part of the Dome was declared a World Heritage Site. Work is under way to strengthen the tourism infrastructure at the site, including construction of a visitor center.