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Shocked Quartz, Ir, Sr, and OS Anomalies Found in the Late

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Shocked Quartz, Ir, Sr, and OS Anomalies Found in the Late LPS XXVII 239 Shocked Quartz, Ir, Sr, and 0sanomalies found in the Late Eocene at Massignano (Ancona, Italy): clear evidence of a bolide impact. Aron K. Clymer, Dept. of Geology and Geophysics, University of California, Berkeley, California 94720; Hubert B. Vonhof, Research School of Sedimentary Geology (NSG), Inst. of Earth Sciences, Vrije Universiteit, de Boelelaan 1085, 1018 HV, Amsterdam, Netherlands; Thomas Meisel, Dept. of Geology, University of Maryland, College Park, MD 20742; David M. Bice, Dept. of Geology, Carleton College, Northfield, MN 55057; Alessandm Montanari, Osservatorio Geologico di Coldigioco, 62020 Frontale (MC), Italy. Shock metamorphosed quartz grains coincident with a positive Ir anomaly of 199 +I- 19 ppb, a positive 87Srl86Sr anomaly, and a negative 187W 1880s anomaly of about 0.3 5 have been found in a marly layer of the Late Eocene Scagha Variegata formation at Massignano (Ancona), which clearly indicates a large impact with an interpolated radiometric age of 35.7 +I- 0.4 Ma. The Popigai crater (Siberia, - 100 km diameter) and the recently discovered Chesapeake Bay Crater (Eastern U. S., - 80 km diameter) are the only known giant craters from the Late Eocene and are prime candidates for the event that distributed the shocked quartz and geochemical anomalies found at Massignano. Although the Middle and Late Eocene are characterized by signrficant stepwise extinctions, the only evidence of a biotic response at Massignano to an impact is a cooler water shift in the dinocyst assemblage associated with the impact layer. Therefore the effects of the impact on this region of the Late Eocene Tethys appears mild, and the global ramification s of the Late Eocene impacts on climate and life remains unclear. However, the tight age control of the Massignano impact layer presents a correlation potential that is a critical step in our understanding of the Late Eocene impact scenario. The shocked quartz at Massignano range from 60-100 microns and often display multiple sets of planar deformational features (Figure 1). A pak cancentration of 1.6 +I- 0.4 shocked grabs per cubic centimeter of bulk rock occurs at meter level 5.6 at Massignano, coincident with an Ir anomaly first discovered by Montanari et al. (1993) and subsequently confirmed by the authors (Figure 2). Vonhof has perfonned preliminary Sr measurements on unaltered foraminifera tests extracted from samples taken between meter levels 3.0 and 9.0 at Massignano. The initial results indicate a positive 87Srt86Sr anomaly, just as occurs at the Cretaceous-Tertiary (K-T) boundary worldwide, Meisel has measured 0s isotopes from two marl bulk rock samples within the shocked quartz layer and hdsa 187W1880s ratio of 0.25 as opposed to a normal ration of about 0.6 for seawater in the Late Eocene, strongly suggesting an extraterrestd input. Worldwide impact evidence (i.e. Ir anomalies, micrdrrystites, microtektita, coesite and shocked quartz) in Late Eocene sediments recard a multiple impact scenario. Accelerated stepwise extinctions that occur within the same window of time may be related to the impacts, but the Late Eocene extinctions are mild (-20% genera) compared to the K-T extinctions (-60% genera). The K-T event was the largest mass extinction of the past 250 million years and is probably directly related to the Chicxulub impact crater in the Yucatan (-200-300 km diameter) which in turn is the largest impact crater yet to be discovered on Earth. This suggests that the entity of impact-related extinctions depends on the energy of the impact, although pre-impact global and ecological conditions (i.e. temperature and climatic trends resulting from tectonic andlor volcanic activities) may play a major role in the effects that a large impact has on the biosphere. Reference: Montaaari et al., 1993, Palaios, v. 8, p. 420-437. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LPS XXVII pl .I-' FIGURE 1: Photomicrographs of two shocked quartz grains displaying multiple sets of planar deformational features. White scale bars are 100 pm FIGURE 2 : Simplified m ~2 Shocked Quartz stratigraphic column of the Lithology i (This Paper) Massignano section &om meter co J3 I""1... ""I-." levels 4.0 to 8.0 next to shocked ... ......... ...) ........ j ......... ) ........ 4- ... ... quartz and Lr ctistributions. All ... ....: ........... ;....... i... ......... ...;........ :......... ; ......... ... concentration error bars are 1 o. ... ... ........4 ... ........ :......... : ......... ... ... ... ..:........... : ,-... 2... ........ :......... :......... .......... :... ... ....................................... ...,.. ... ........ ............................... .........................,s -... ..s ... .....s ... .........................,.. ................................ !......... ... 4u ... .........)... ........ 8 .........!...-""' ... .........4 ... ........ :......... ; ......... ..s (I.,s ...,.. ........4 ... ........ :......... :......... .....s $? sf I-' P. (D ... rn ... ....,s 4-....) ,..........+ ......... ) ........ ,.*... ...#.. ........ 4... ........ ;......... ; ......... ... ..................................... .... ..s ,.s ..................................... ,.....,s. 4- ... ........................................ ...,.. I,,, ,I I* II III In 0 1 2 3 ~ith01ogicI # grains1 cm Key calcareous marl marl clay-rich layer with biotite O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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