LATE DEVONIAN EVENTS and MASS EXTINCTIONS. C. A. Sandberg , W
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Catastrophic Events Conference 3016.pdf 1 2 3 LATE DEVONIAN EVENTS AND MASS EXTINCTIONS. C. A. Sandberg , W. Ziegler , and J. R. Morrow , 1U.S. Geological Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225, U.S.A., [email protected]; 2Forschungsinstitut Senckenberg, D-60325 Frankfurt am Main, Germany; 3Department of Earth Sciences, University of Northern Colorado, Greeley, CO 80639, U.S.A. Introduction: The Late Devonian Epoch, one of the conodonts that were blasted from the crater depths most intensively studied of all the Paleozoic Epochs, and redeposited in the resulting chaotic megabreccia was a time of major sea-level changes, catastrophic and tsunami-related turbidites [5], along with events, some of which were impact related, and two carbonate impact spherules, shocked-quartz grains, mass extinctions, one of which was impact related. and a locally high iridium concentration [6, 7]. This These many events are plotted herein against the impact occurred in the middle of the punctata Zone generally accepted sea-level curve [1, 2]. Detailed and coincides with a major transgression and the knowledge and dating of Late Devonian events demise of the first mudmound level in Belgium. resulted from a high-resolution biochronology, based Because of the current controversy in methodology primarily on conodont zonations, but supported in for radiometric dating, we are now uncertain whether part by ammonoid, ostracod, and spore zonations. the Siljan Impact in southern Sweden coincides with This detailed knowledge was gained by intensive the Alamo Impact, with the late Frasnian mass biostratigraphic studies during the past two decades, extinction, or was part of an intervening comet inspired by the IUGS Subcommission of Devonian shower. Stratigraphy and by IGCP Projects on Bio-Events The late Frasnian mass extinction, and Mass Extinctions. Many Late Devonian events commonly known as the Kellwasser Event, occurred produced changes in the environmentally sensitive not at the highstand of transgression [8] but within mudmound levels in Belgium [3]. the regressive part of a thin transgressive-regressive The initial Late Devonian Stage, the black shale bed. Where well developed, this shale Frasnian, was a time of general transgression during contains a detailed record of several stepped the Taghanic onlap that began in the late Middle extinctions and introductions of shallow-water biota, Devonian. The final Famennian Stage was a time of culminating in an abiotic layer at its top. Researchers general regression, probably due to Southern are still trying to locate an impact exactly coincident Hemisphere glaciation, interrupted by four major with this position. Possibly the Siljan Impact was transgressions, probably related to interglacial the source of microtektites found within the episodes. Both the late Frasnian and late Famennian extinction shale in Belgium. However, it seems more mass extinctions occurred during, not at the start of, likely that the extinction occurred as a result of a rapid regressions that closely followed rapid series of sub-critical impacts or comet showers during transgressions. The stepwise late Frasnian mass the Frasnian. This scenario is supported by the extinction, one of the five greatest in EarthÕs history stepwise extinction of several animal groups [9] and [4], is believed to have occurred as a result of by the decreasing size and biotic diversity of the first environmental stresses that were related to not just three Belgian mudmound levels. one but to a series of multiple, non-critical impacts. Famennian: To date, no direct evidence of any The late Famennian mass extinction, on the other impacts has been found in Euramerica during the less hand, is believed to have occurred at the culmination turbulent Famennian Stage. A possible, closely post- of stresses produced by alternating glacial and extinction time of impact, however, is within the late interglacial episodes. part of the earliest Famennian Early triangularis Frasnian: Some Frasnian events are closely related Zone, when normal sedimentation was interrupted to or even may have produced significant globally by deposition of coarse tsunami-related transgressions. The enigmatic Amšnau Event, which breccias. This event has been attributed, alternatively, locally produced a megabreccia, can now be tied to to collapse of platform margins during continuing the onset of volcanism across a 75-km transect of the regression [2]. Only one local Famennian mudmound Rheinisches Schiefergebirge in Germany and to a level, the Baelen [10] is known in Belgium, and this eustatic rise in sea level. It is dated as occurring lacks the stromatoporoids and corals that were within the Early falsiovalis conodont Zone at the important to Frasnian mudmound construction. The first occurrence of Ancyrodella rotundiloba, the Baelen mudmound was constructed during the later conodont used to define the start of the Frasnian. We part of the second major Famennian interglacial? are now trying to determine whether this volcanism transgression. The late Famennian mass extinction, resulted from an oceanic impact. The oceanic, sub- commonly known as the Hangenberg Event, occurred critical Alamo Impact in southern Nevada is at during a severe sea-level drop within the Middle present the best evidenced Late Devonian impact, praesulcata Zone, near the end of the Devonian. even though its crater is now buried or obliterated. Evidence of its ~1.5-km-deep crater derives from Middle Ordovician to possibly Late Cambrian Catastrophic Events Conference 3016.pdf LATE DEVONIAN EVENTS AND MASS EXTINCTIONS. C. A. Sandberg, W. Ziegler, and J. R. Morrow References: [1] Johnson, J. G., Klapper, G. and 51, 451Ð460. [6] Warme, J. E. and Sandberg, C. A. Sandberg, C. A. (1985) GSAB 96, 567Ð587. [2] (1995) CFS 188, 31Ð57. [7] Warme, J. E. and Sandberg, C. A., Ziegler, W., Dreesen, R. and Kuehner, H.-C. (1998) IGR 40, 189Ð216. [8] Hallam, Butler, J. L. (1988) CFS 102, 263Ð307. [3] A., and Wignall, P. B. (1999), ESR 48, 217Ð250. [9] Sandberg, C. A., Ziegler, W., Dreesen, R., and Schindler, E. (1990) GAGP 46, 1Ð115. [10] Dreesen, Butler, J. L. (1992) CFS 150, 1Ð87. [4] McGhee, G. R., Bless, M. J. M., Conil, R., Flajs, G. and R. (1996) CUP, 1-303. [5] Morrow, J. R., Sandberg, Laschet, C. (1985) ASGB 108, 311-359. C. A., Warme, J. E. and Kuehner, H.-C. (1998) JBIS LATE DEVONIAN EVENTS AND MASS EXTINCTIONS STAGE Tournaisian (Southern Hemisphere glaciation?) LATE FAMENNIAN General RegressionInterglacial MASS EXTINCTION Episodes? 4th Early expansa Transgression annulata Transgression Mudmound Levels 3rd (Belgium) FAMENNIAN 4th (Baelen) 2nd Early marginifera Transgression 1st Cheiloceras Transgression Global collapse of Microtektites (Belgium and China) platform margins 3rd (Beauchâteau) LATE FRASNIAN semichatovae Transgression MASS EXTINCTION ? SILJAN IMPACT 2nd (Lion) (368 + 1.1 Ma) FRASNIAN ALAMO 1st (Arche) IMPACT Amönau Event Start of Taghanic onlap GIVETIAN (General late Givetian- (part) Frasnian transgression) REGRESSION TRANSGRESSION NOTE: Late Devonian mass extinctions occured within rapid regressions (sea-level falls), following maximum transgressions (highstands). Sea-level curve after Johnson et. al., 1985, as modified by Sandberg et al., 1988..