U33A-01 INVITED
The 35.4 Ma Chesapeake Bay Impact: Effects on Post-Impact Sedimentation
The late Eocene (35.4 Ma) Chesapeake Bay impact structure (CBIS) is a well-preserved, large (85 km, 7th largest known) crater with an `inverted sombrero' shape. The International Continental Scientific Drilling Program (ICDP) and the USGS completed three coreholes at Eyreville, VA to a composite depth of almost 1.8 km into the CBIS in the fall of 2005 and the spring of 2006. A total of 444 m of post-impact sediments were cored along with a 1,322 m impactite section which consists (in descending order) of sediment-clast breccia, sediment megablocks, a large granite megablock, smaller rock blocks in sediment, suevite and lithic breccia, and a section of brecciated mica schist and pegmatites with veins of different breccia types. Ongoing studies of the impactite section will test hypotheses including the source and formation of the North American tektite strewn field, the type of impactor, relationships with the late Eocene Popigai impact, implications of shock-pressure variations for constraining kinetic energy and cratering mechanics, and marine crater excavation and modification processes. Other than the immediate effects of resurge and a megatsunami indicated by the sediment clast breccia, regional and global environmental and stratigraphic effects of this large impact were surprisingly minimal as exemplified by results from Eyreville and backstripping of previously drilled crater coreholes. A thick, deep-water upper Eocene section is partly explained by excess accommodation produced by the impact due to compaction of the rapidly deposited impactites, with little evidence of thermal resetting of subsidence by impact. Possible tectonic effects continue into the early Oligocene. Oligocene and lower Miocene sections are thin regionally, both in the crater and outside in Virginia and Maryland, indicating relative uplift compared to NJ and Delaware apparently unrelated to impact. Middle to lower upper Miocene sequences correlate with sections outside the crater in Delaware and NJ, indicating a dominant control by eustatic change overprinted by regional subsidence in the Delmarva region. An uppermost Miocene-Pliocene marine section is well represented in Virginia, especially in the Eyreville C corehole, versus NJ and Delaware, where it is thin and largely nonmarine or absent. Though the Virginia sediments often are cited as evidence for a Pliocene high sea level (>25 meters) during a period of global warmth, preservation can be attributed to excess regional subsidence, again unrelated to impact. Thus, the dominant effects of this large impact were: 1) excavation; 2) collapse, block slumping, immediate catastrophic sedimentation due to the resurge; 3) rapid creation of accommodation due to compaction and subsequent uplift; 4) an apparent return to `normal' passive margin sedimentation dominated by sea-level and regional tectonic changes, though comparison of backstripping of the Eyreville corehole with coreholes from inside and outside of the crater are still needed.
U33A-02
Global Effects Of Late Eocene Impacts
Two of the three largest impact craters found on Earth since 200 Ma (Popigai and the Chesapeake Bay Impact Structure or CBIS) are late Eocene (~35.4-35.5 Ma based on radiometric ages) and are well preserved, yet the environmental response to these near synchronous and large impacts is poorly understood. No extinction events are recorded in coccolithophorids, planktonic or benthic foraminifera at this time, and terrestrial biota appear unaffected. The late Eocene global temperature history and carbon budget are poorly constrained because of sparsely sampled δ18O and δ13C records. We present new microfauna and nannoplankton evidence and stable isotopic data that show: 1) minimal biotic and temperature response associated with the impacts; and 2) a large and transient carbon isotope excursion associated with the impacts that reflect a major change in the global carbon budget. Southern Ocean ODP Site 1090 provides an exceptional record to test if a carbon isotopic anomaly is associated with the late Eocene impacts because benthic foraminifera are well preserved, the identified ejecta horizon is marked by an Ir anomaly (~950pg/g; Kyte and Liu, 2002), and the magnetostratigraphic age control is excellent (Channell et al., 2003). A first-order correlation to the geomagnetic polarity time scale at Site 1090 places the impact ejecta horizon in Chron C16n.1n (279.01 mbsf) with a corresponding age of ~35.4 Ma, consistent with published radiometric ages. We generated a high-resolution carbon and oxygen stable isotope record of benthic foraminifera across the impact ejecta layer from 34.6-35.8 Ma (8 kyr sampling) and 33.7-36 Ma (16 kyr sampling). Our results show that a transient carbon isotope decrease (277-278 mbsf) of 0.4-0.5‰ is associated with the impact horizon. The δ13C anomaly persists for ~250 kyr; then the signal returns to ‘pre-impact' values. Following recovery from the transient excursion there were no long-term changes in global carbon isotopic values associated with the impacts. Our results show no significant changes in benthic foraminifera oxygen isotope values across the impact ejecta layers, implying that no major changes in deep-water temperatures occurred. Coeval benthic foraminiferal records also show the carbon isotopic excursion: 1) new benthic data from New Jersey slope Site 612 show a 0.5‰ change, though this record is partially truncated due to a hiatus; 2) Southern Ocean Site 689 shows a larger excursion (1.0‰) (Vonhof et al., 2000); and 3) Pacific Ocean Site 1218 shows a smaller anomaly (0.4‰) (Lear et al., 2004). We suggest this δ13C perturbation was global and directly related to the late Eocene impactor(s).
U33A-03
Chicxulub Impact and K-T Mass Extinction in Mexico and Texas
New cores and outcrops from El Penon, NE Mexico, and the Brazos River, Falls County, Texas, reveal the stratigraphic and temporal separation between the Chicxulub impact, the sandstone complex (commonly interpreted as "impact-tsunami") and the K-T mass extinction. In NE Mexico, where deposition occurred in about 500 m water depth, the original Chicxulub impact ejecta was discovered in a 1.8 m thick impact glass spherule layer within undisturbed pelagic marls more than 4 m below the base of the sandstone complex. At Brazos, Texas, where deposition occurred in shallow waters (20-80 m), the original spherule ejecta layer was found in a 3 cm thick clay-altered impact spherule layer within undisturbed late Maastrichtian claystones, about 60 cm below the sandstone complex. In both localities, the base of the sandstone complex contains spherules and clasts from shallow nearshore areas, which were eroded from the original impact spherule layer and transported into deeper waters during the latest Maastrichtian sea level lowstand. The K-T mass extinction and Ir anomaly occurred at a much later time. The Chicxulub impact is dated at 300 ky before the K-T boundary and the sea level lowstand about 100 ky before. These data reveal that the K-T mass extinction was not directly related to either the Chicxulub impact, or the sea level lowstand. The discovery of the original Chicxulub impact ejecta spherule layer in Mexico and Texas permits evaluation of the biotic effects of this large impact upon marine faunas and floras in both deep and shallow water environments at 1000 km and 1700 km from the impact crater, respectively. Quantitative analysis of planktic foraminifera reveals a major surprise: No species extinctions or significant species population changes occurred at the time of the Chicxulub impact. The impact coincides with greenhouse warming associated with Deccan volcanism, but appears to have caused no significant environmental stress even within 1000 km, let alone globally. Current estimates thus vastly overestimate the biotic effects of large impacts. The mass extinction at the K-T boundary coincides with a renewed pulse of major Deccan volcanism and a global Ir anomaly that suggests another large impact for which no crater is known to date. Thus, the K-T mass extinction appears to have been caused by the combined effects of volcanism and a second large impact superimposed on long-term biotic stresses.
U33A-04
Chicxulub Impact and the Stratigraphy, Nature and Origin of Near-K-T Breccia
Breccias with altered impact glass and located at or near the K-T boundary in Texas (USA), northern and southern Mexico, Belize, Guatemala, Haiti and Brazil are investigated to determine their age, stratigraphy and origin. Ages are variable. The oldest breccia deposit is within the uppermost Maastrichtian in the southern USA (Brazos, Texas), NE Mexico (e.g., Loma Cerca, El Penon) and in the Chicxulub impact crater cores on Yucatan (e.g., cores Yaxcopoil-1, Y6, C1). In all these sections, the geochemistry of glass within the breccias is identical and consistent with Chicxulub impact ejecta. The K-T boundary, Ir anomaly and mass extinction is located well above these impact breccia layers. This strongly supports a pre-K-T age for the Chicxulub impact, as also determined based on sedimentology, stratigraphy and paleontology. In NE Mexico and Texas the oldest Chicxulub impact spherule ejecta layer is interbedded in normal marine sedimentation in the upper Maastrichtian (base of CF1 Zone), about 300'000 year prior to the K-T boundary. All stratigraphically younger spherule ejecta layers represent repeated episodes of reworking and transport of the original layer during a sea-level regression and re- deposition in incised valleys in shallow environments (e.g., Brazos, Texas, La Popa Basin NE Mexico) and submarine canyons in deeper environments via mass flows and turbidites (e.g. Mimbral, Penon, Loma Cerca and many other section throughout NE Mexico). In southern Mexico, Belize and eastern Guatemala, the widespread thick microspherule and larger spheroid deposits are interbedded with breccia, microbreccias and conglomerates in the early Danian as a result of erosion in shallow carbonate platform sediments. The presence of early Danian planktic foraminifera in the matrix of the breccia, as well as within spherule clasts, indicate that redeposition occurred during the early Danian Parvularugoglobigerina eugubina (P1a) zone. In Haiti (Beloc sections), spherule deposits and microbreccias are also reworked together with late Maastrichtian microfossils and redeposited during the early Danian zone P1a. In NE Brazil (Poty Quarry) and Argentina (Neuquen Basin), the breccia layers identified as K-T age are also younger and deposited in the early Danian P1a and P1c zones, respectively. No extraterrestrial markers, such as glass, glass spherules or shocked quartz are present. These breccia and sandstone deposits thus represent normal sedimentary processes with deposition primarily linked to sea-level changes. However, an Ir anomaly is detected in the Early Danian P1a(1) subzone (100-200ky after the KT boundary) in southern Mexico (Coxquihui, Bochil), Guatemala (Actela), Haiti (Beloc) and Brasil (Poty). This suggests that the K-T transition was a time comet showers with current evidence of two large impacts, the pre-K-T Chicxulub impact and K-T impact, and smaller impacts in the early Danian and late Maastrichtian (Boltysh crater). The distribution of the K-T impact breccia is consistent with a multi-impact scenario.
U33A-05
Heterogenity Effects In Thermal Borehole Measurements In The Chicxulub Impact Crater
Boreholes drilled in impact structures are especially suited for investigations of the influence of heterogeneities on petrophysical properties and thermal field. In the scientific well Yaxcopoil-1 drilled within the frame of the International Continental Deep Drilling Program (ICDP) and as part of the Chicxulub Scientific Drilling Project (CSDP) high resolution temperature measurements and a dense petrophysical profile measured on core samples at ~2.2 m depth intervals were recorded. From the calculated vertical component of the thermal gradient and the thermal conductivity measured on the core samples a mean heat flow density of 70.5 +/-1.9 mW/m2 in the depth interval 400 - 1400 m was determined. On the basis of a simple purely conductive heterogeneous 2D thermal model the effect of the refraction of heat caused by heterogeneities is demonstrated. A statistical investigation shows that if the scales of the heterogeneities influencing the values of the measured thermal conductivity and the calculated thermal gradient are small compared to the length of the borehole the effect of the heterogeneities on the vertical heat flow densitiy can be interpreted as thermal noise. Additionally a local 2D profile was modelled to explain the large scale features of the measured temperature profile.
U33A-06
Interpretation And Mathematical Modelling Of Temporal Changes Of Temperature Observed In Borehole Yaxcopoil-1 Within The Chicxulub Ipact Structure, Mexico
The geothermal research of the Chicxulub impact structure on the Yucatan Peninsula, Mexico, included repeated temperature logs of the 1.5 km deep borehole Yaxcopoil-1, which were done following 0.3-0.8, 15, 24, 34 and 50 months after shut-in of drilling operations. A gradual distortion of the linear temperature profile by a cold wave of the 0.8 -1.6°C amplitude was detected propagating downward from 145 m to 317 m within the observational period of 50 months (March 2002 - April 2006). As an explanation of this unusual phenomenon, the hypothesis of a downward migration of the drilling mud, accumulated within the overlying and cooler highly porous and permeable karstic rocks during the drilling, was proposed. Velocity of the downward propagation of the cold wave decreased appreciably between the last two logs (December 2004 - April 2006). It may indicate that the mud migrating downward through the system of interconnected caverns and conduits reached a bottom of the secondary porosity zone. We present results of simulations of thermal effects of the downward migrating drilling mud, obtained by a numerical solution of the heat transfer equation in a set of geothermal models of the borehole and its surroundings.
U33A-07
Deep Drilling Into the Chicxulub Impact Crater: Pemex Oil Exploration Boreholes Revisited
The Chicxulub structure was recognized in the 1940´s from gravity anomalies in oil exploration surveys by Pemex. Geophysical anomalies occur over the carbonate platform in NW Yucatan, where density and magnetic susceptibility contrasts with the carbonates suggested a buried igneous complex or basement uplift. The exploration program developed afterwards included several boreholes, starting with the Chicxulub-1 in 1952 and eventually comprising eight deep boreholes completed through the 1970s. The investigations showing Chicxulub as a large impact crater formed at the K/T boundary have relayed on the Pemex decades-long exploration program. Despite frequent reference to Pemex information, original data have not been openly available for detailed evaluation and incorporation with results from recent efforts. Logging data and core samples remain to be analyzed, reevaluated and integrated in the context of recent marine, aerial and terrestrial geophysical surveys and the drilling/coring projects of UNAM and ICDP. In this presentation we discuss the paleontological data, stratigraphic columns and geophysical logs for the Chicxulub-1 (1582m), Sacapuc-1 (1530m), Yucatan-6 (1631m) and Ticul-1 (3575m) boreholes. These boreholes remain the deepest ones drilled in Chicxulub and the only ones providing samples of the melt-rich breccias and melt sheet. Other boreholes include the Y1 (3221m), Y2 (3474m), Y4 (2398m) and Y5A (3003m), which give information on pre-impact stratigraphy and crystalline basement. We concentrate on log and microfossil data, stratigraphic columns, lateral correlation, integration with UNAM and ICDP borehole data, and analyses of sections of melt, impact breccias and basal Paleocene carbonates. Current plans for deep drilling in Chicxulub crater focus in the peak ring zone and central sector, with proposed marine and on-land boreholes to the IODP and ICDP programs. Future ICDP borehole will be located close to Chicxulub-1 and Sacapuc-1, which intersected the impact breccias at about 1 km and the melt and melt- rich breccias at some 1.3-1.4 km.
U33A-08
Impact-generated Hydrothermal Activity at the Chicxulub Crater
Borehole samples recovered from PEMEX exploration boreholes and an ICDP scientific borehole indicate the Chicxulub impact event generated hydrothermal alteration throughout a large volume of the Maya Block beneath the crater floor and extending across the bulk of the ~180 km diameter crater. The first indications of hydrothermal alteration were observed in the crater discovery samples from the Yucatan-6 borehole and manifest itself in the form of anhydrite and quartz veins. Continuous core from the Yaxcopoil-1 borehole reveal a more complex and temporally extensive alteration sequence: following a brief period at high temperatures, impact- melt-bearing polymict breccias and a thin, underlying unit of impact melt were subjected to metasomatism, producing alkali feldspar, sphene, apatite, and magnetite. As the system continued to cool, smectite-series phyllosilicates appeared. A saline solution was involved. Stable isotopes suggest the fluid was dominated by a basinal brine created mostly from existing groundwater of the Yucatan Peninsula, although contributions from down-welling water also occurred in some parts of the system. Numerical modeling of the hydrothermal system suggests circulation occurred for 1.5 to 2.3 Myr, depending on the permeability of the system. Our understanding of the hydrothermal system, however, is still crude. Additional core recovery projects, particularly into the central melt sheet, are needed to better evaluate the extent and duration of hydrothermal alteration.