Bolide Impacts on Wet Targets
John Warme, Coordinator, Department of Geology and Geological Engineering, Colorado School of Mines
Christian Koeberl, Geochemistry Institute, University of Vienna
Philippe Claeys, Department of Geology, Vrije Universiteit, Brussels
Walter Alvarez, Department of Geology and Geophysics, University of California, Berkeley
This meeting brought together experts on wet impact processes and products to study outcrop examples, present case studies, exchange ideas, generate discussions and debate, and integrate knowledge of impacts on various wet targets, such as standing water of different depths, saturated ground, and ice. Platforms for field discussion were the proven impact-related Alamo Breccia in southern Nevada and the controversial Upheaval Dome in eastern Utah. The 38 participants included 11 from outside the United States and eight U.S. and foreign students. We convened on Sunday, April 22, in Las Vegas, Nevada, and officially ended on Saturday, April 28, in Moab, Utah. Twenty hardy individuals remained in Moab to traverse the Upheaval depression on April 29. The Alamo Breccia Research Page, http://talus.mines.edu/students/m/mmorgan, contains complete information, plus photos contributed by Steve Dutch.
The forum included daily field traverses to Alamo Breccia localities mixed with inhouse orientation talks on the Breccia (Kuehner, Morgan, Morrow, Warme), five keynote addresses on various topics (Dypvik, King, Melosh, Milkereit, Simonson), 29 posters on specific topics or case studies, open discussions, and field team reports.
An overview of the Alamo Breccia and of interpretation problems that were presented as challenges to the attendees appear on the Alamo Breccia Research Page and in Warme and Kuehner (1998). After two days of field stops and orientation sessions, the expert workforce was divided into teams and given a list of problems on which to focus in the field. The controlling question was whether we can see through the Breccia to solve major problems of its genesis. Sample topics for investigation included: mechanisms of Breccia emplacement (ejecta curtain, fallout, tsunami, slide, or?); identification and preservation of primary ejecta, matrix, and carbonate impact lapilli ("spherules"); formation and preservation of the lapilli; origin of the widespread basal Breccia detachment surface and of megaclasts over it; possibility for compound origins (e.g., direct ejecta, deposits from one or more slides, or resurge gullying); evidence for fluidization and/or liquefaction of clasts and matrix; estimates of crater size, geometry and location; and size and composition of projectile.
|On the Alamo Breccia at the type locality near Hancock Summit, Nevada. Photo by Goeff Notkin.|
The teams took on these overlapping tasks: search for evidence of shock or ground motion within or under the Breccia; characterize wet impact signatures and impact stratigraphy exhibited in the Breccia; interpret the paradoxical distribution of carbonate impact lapilli within the Breccia; and develop a comprehensive scenario for the position and proportions of the crater, character of the projectile, and sequence of Breccia formation.
The Shock Team presented a model whereby near-surface seismic waves could account for the detachment horizon along the base of the Breccia (Dutch, Masaitis, Melosh). The lack of shattercones suggested that the Breccia localities we visited were distal with respect to the crater, although the wet environment may have subdued shattercone development (Dutch, Gaffney, Ryder). The Crater Stratigraphy Team contributed an important hypothesis, generated by comparison of the Alamo Breccia with impact breccias associated with the Lockne impact structure in Scandinavia (Ormo, Von Dalwigk) and other examples. Multiple, delayed resurge events into a crater or into a slump-scar trench could explain the multiple-graded units toward the top of the Breccia and allow time for the carbonate impact lapilli beds to harden and then fragment as the isolated lapilli clasts that occur mixed into the Breccia. Thus, the impact event and the formation of the Breccia may be related but separated by perhaps many days (Kenkmann, Ormo). A recurring idea is that there was more than one impact, which could explain many of the puzzles contained in the Breccia (McElvain). Although a new model was presented that could account for rapid cementation of the lapilli beds by impact calcining and rapid dehydration and cementation (Morgan, Kuehner, Warme), the scattered lapilli clasts in the Breccia could be more easily explained if sufficient time elapsed between impact with its lapilli formation and some fashion of collapse and lapilli bed breakup and redistribution. The Lapilli Team suggested that the beds may have been in the form of rafts, similar to pumice rafts, that broke up, cemented, and sank during the formation of the Breccia, accounting for the disjunct distribution of lapilli clasts (Bell).
Several workers suggested that the Alamo impact occurred on or near a slope (Masaitis) and that the crater may have been destroyed by slumps that created tsunamis and/or resurge deposits, possibly filling a depression from different directions and at different times and accounting for the stacked graded beds that comprise the Breccia (Ormo).
The Grand Scenario Team was reluctant to present a detailed synthesis for the genesis and evolution of the Alamo Breccia because of uncertainties about Late Devonian paleogeography in Nevada (Warme). A major problem is whether the current Breccia represents its original distribution or is the product of significant post-Devonian thrusting, in addition to some measure of Cenozoic extension. Crustal shortening could result in east-west scrambling of the outcrops visited. The plan of the epiplatform Alamo Breccia now forms an eastward-extending semicircle, although deep-water equivalents have been identified as mass flows 100 km or more west of the carbonate platform study area (Morrow).
Recent thrust-belt models for the late Paleozoic of the Great Basin of Nevada suggest that crustal shortening was significant. If so, stacked thrust sheets may still cover the crater, or significant uplift and erosion may have destroyed some or all of it. Impact modelers require reliable information about the original area of the Breccia and distance from the periphery to the center. Their conclusion: Structural problems must be resolved before modeling can confidently predict the size and position of the Alamo crater and the character of the impactor.
On Thursday, Huntoon and Koeberl introduced the Upheaval Dome and the forum moved to Moab, Utah, where we closed in on structure for the next three days. The dome is emphatically interpreted as both a salt piercement structure (e.g., Jackson et al., 1998) and as an impact crater (e.g., Huntoon, 2000). The wet impact theme applies there because stratigraphic features we visited can be interpreted as having formed by impact on layers of variably saturated rock. Friday, we visited two outlying areas that contain deformation features that potentially support the impact hypothesis (Alvarez et al., 1998). Alvarez and Shimabukuro led us through portions of Arches National Park, ~40 km from Upheaval Dome, where the boundary between the Jurassic Carmel and Entrada formations is deformed. In some localities, the upper Carmel mudstone beds are segmented and appear thrusted or dragged upward to pierce the overlying Entrada, which locally appears fluidized and loaded into the Carmel to form tongue-shaped bodies with fluid escape structures. Chan showed us the Dubinky Well area in Canyonlands, ~25 km from Upheaval, that exhibits sandstone pipes penetrating the Carmel (Alvarez et al., 1998). Synsedimentary deformation in both areas draw attention because of their unusually large scale, suggesting that they may have formed through extraordinary processes such as impact at Upheaval Dome.
On Saturday, Huntoon led us to Roberts Rift, ~30 km from Upheaval. This unusual feature, filled with debris that includes propant fragments from underlying formations, is proposed as a radial fracture from Upheaval Dome (Huntoon and Shoemaker, 1995). The last formal stop of the forum was Upheaval Dome, now an erosional depression. Huntoon and Plescia introduced the feature, and a roundtable discussion ensued. Unfortunately, the pinched-off salt diapir proponents declined to join the forum; they would have had eager listeners and perhaps concurrence from within our group. We traversed part of the depression rim to view the spectacular exposures in the central uplift. On Sunday, Huntoon and Plescia led an optional 15-km-long foot traverse into the center of the depression.
The forum catalyzed new ideas, acquaintances, after-dinner discussions, and field debates that furthered our understanding of the geological effects of wet impacts. In retrospect, however, the days and nights were too full and the participants too numerous to effectively wring out all of what we observed in the field and brought to the table through our varied career experiences with impact structures. No synthesis was achieved or possible. One post-forum comment provides perspective: "I submit there are an almost unlimited variety of shapes, compositions, sizes, and densities of potential impactors traveling around within the gravitational influence of Earth and the solar system. Impactors of radically different natures would cause a variety of different types of craters and other geomorphic and formational effects that could be preserved in the geologic record." We learned that the inventory is not complete, and the interpretations ongoing, as with any live branch of science.
We gratefully acknowledge the following cosponsors and organizations for financial support that contributed to the size and success of this Field Forum: for student support, The Pretorious Fund of the GSA Foundation and the NASA Planetary Geology and Geophysics Program; for sponsoring foreign speakers, the donors of the Petroleum Research Fund of the American Chemical Society; and for important discretionary funds, the Barringer Company, the Colorado School of Mines, and the Global Impacts Studies Program.
Alvarez, W., Staley, E., O'Connor, D., and Chan, M.A., 1998, Synsedimentary deformation in the Jurassic of southeastern UtahA case of impact shaking?: Geology, v. 26, p. 579-582.
Huntoon, P.W., 2000, Upheaval Dome, Canyonlands, Utah: Strain indicators that reveal an impact origin, in Sprinkel, D.A., et al., eds., Geology of Utah's parks and monuments: Utah Geological Association Publication 28, p. 619-628.
Huntoon, P.W., and Shoemaker, E.M., 1995, Roberts Rift, Canyonlands, Utah: A natural hydraulic fracture caused by comet or asteroid impact: Ground Water, v. 33, p. 561-569.
Jackson, M.P.A., Schultz-Ela, D.D., Hudec, M.R., Watson, L.A., and Porter, M.L., 1998, Structure and evolution of Upheaval Dome: A pinched-off salt diapir: Geological Society of America Bulletin, v. 110, p. 1547-1573.
Warme, J.E., and Kuehner, H.-C., 1998, Anatomy of an anomaly: The Devonian catastrophic Alamo impact breccia of southern Nevada: International Geology Review, v. 40, p. 189-216.
Mary Sue Bell
Ilka Von Dalwigk