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" . . . bangs have replaced whimpers and the geological record has become much more exciting than it was thought to be. " Derek Ager (1993) The New Catastro phism. Cambridge University Press, Cambridge, p xix Scientific and public interest in asteroids, comets, and meteorite impacts has never been more intense than right now. Much of this interest stems from the fervent debates surrounding the causes of the Cretaceous-Tertiary mass extinctions and their possible relationships to a giant bolide impact in Mexico's Yucatan Penin sula. Recent spectacular impacts on Jupiter, and several near misses of our own planet by Near-Earth Objects have intensified professional and popular discussion of society's imperative need to understand the process and effects of bolide im pacts. In the United States, the scientific community and the public, as well, were startled to learn, in 1994, that the largest impact structure in this country had been detected beneath Virginia's portion of the Chesapeake Bay. Seismic surveys and deep coring revealed a huge crater, 85 kilometers in diameter and more than a kilometer deep, stretching from Yorktown, Virginia, to 15 kilometers out onto the shallow continental shelf. Several of Virginia's major population centers, includ ing Norfolk, Hampton, and Newport News, are located on the western rim of the crater, and still experience residual effects of the original collision, 36 million years after the impact took place. Exploration and documentation of the Chesapeake Bay impact structure has proceeded in three phases.
Sommario
1 Introduction.- 2 Geological Framework of Impact Site.- 2.1 Crystalline Basement Rocks.- 2.2 Coastal Plain Sedimentary Rocks.- 2.3 Sequence Stratigraphy.- 2.4 Paleogeography of Impact Site.- 2.5 Subsidence of Virginia Continental Margin.- 2.6 Initial Evidence of East Coast Impact.- 2.7 Onshore Borehole Evidence.- 3 Geophysical Framework of Impact Site.- 3.1 Seismic Investigations of Virginia Coastal Plain.- 3.2 Seismic Signature of Crystalline Basement Rocks.- 3.3 Chesapeake Bay Seismic Reflection Profiles.- 3.4 Depth Conversion of Seismic Two-way Traveltimes.- 3.5 Gravity Evidence.- 4 The Primary Crater.- 4.1 Crater Structure and Morphology.- 5 Secondary Craters.- 5.1 Location and Identification.- 5.2 Secondary Craters on Profile T-1-CB.- 5.3 Secondary Craters on Profile T-11-PR.- 5.4 Implications of Secondary Crater Record.- 6 Synimpact Crater-Fill Deposits.- 6.1 Oldest Breccia Unit.- 6.2 Displaced Megablocks.- 6.3 The Exmore Breccia.- 7 Initial Postimpact Deposits.- 7.1 Depositional Setting.- 7.2 Dead Zone.- 7.3 Chickahominy Formation.- 8 Age of Chesapeake Bay Impact Crater.- 8.1 Biochronology.- 8.2 Radiometric Chronology.- 8.3 Magnetochronology.- 8.4 Correlation with Other Craters and Impactites.- 9 Geological Consequences of Chesapeake Bay Impact.- 9.1 General Nature of Consequences.- 9.2 Reconfigured Basement Structure and Morphology.- 9.3 Disruption of Preimpact Sedimentary Column.- 9.4 Source of North AmericanTektite Strewn Field.- 9.5 Far-Field Seismic Effects.- 10 Comparisons with Other Impact Craters.- 10.1 Terrestrial Craters.- 10.2 Extraterrestrial Craters.- 10.3 Comparison with Chicxulub Multiring Impact Basin.- 11 Comparisons Between Impactites.- 11.1 Terrestrial Impactites.- 11.2 Flowin, Fallout, and Dead Zone.- 11.3 Other Intrabreccia Bodies.- 11.4 Continuous Ejecta Blankets.- 11.5 Secondary Breccias.- 11.6 Strewn Fields.- 11.7 Impact Melt Rocks.- 12 Implications for Impact Models.- 12.1 General Conceptual Models and Scaling Relations.- 12.2 Conceptual Model for Chesapeake Bay Crater.- 12.4 General Conceptual Model of Crater-Fill Deposition.- 12.4 Differentiating Crater-Fill Lithofacies at Chesapeake Bay.- 12.5 Comparison of Models.- 13 Biospheric Effects of Chesapeake Bay Impact.- 13.1 Local Paleoenvironmental Effects.- 13.2 Possible Global Paleoenvironmental Effects.- 14 Residual Effects of Chesapeake Bay Impact..- 14.1 Hypersaline Groundwater.- 14.2 Near-Surface Compaction Faults.- 14.3 Surface Expression of Crater.- 14.4 Altered River Courses.- 14.5 Relative Change of Sea Level.- 15 Summary and Conclusions.- 453.- 461.- 489.
Riassunto
" . . . bangs have replaced whimpers and the geological record has become much more exciting than it was thought to be. " Derek Ager (1993) The New Catastro phism. Cambridge University Press, Cambridge, p xix Scientific and public interest in asteroids, comets, and meteorite impacts has never been more intense than right now. Much of this interest stems from the fervent debates surrounding the causes of the Cretaceous-Tertiary mass extinctions and their possible relationships to a giant bolide impact in Mexico's Yucatan Penin sula. Recent spectacular impacts on Jupiter, and several near misses of our own planet by Near-Earth Objects have intensified professional and popular discussion of society's imperative need to understand the process and effects of bolide im pacts. In the United States, the scientific community and the public, as well, were startled to learn, in 1994, that the largest impact structure in this country had been detected beneath Virginia's portion of the Chesapeake Bay. Seismic surveys and deep coring revealed a huge crater, 85 kilometers in diameter and more than a kilometer deep, stretching from Yorktown, Virginia, to 15 kilometers out onto the shallow continental shelf. Several of Virginia's major population centers, includ ing Norfolk, Hampton, and Newport News, are located on the western rim of the crater, and still experience residual effects of the original collision, 36 million years after the impact took place. Exploration and documentation of the Chesapeake Bay impact structure has proceeded in three phases.
