Francisca Martínez-Ruiz¹, Miguel Ortega-Huertas², Inmaculada Palomo² and ODP Leg 171B Shipboard Scientific Party

¹ Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias. Avda. Fuentenueva s/n, 18002 Granada, Spain (fmruiz@ugr.es)
² Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain (mortega@ugr.es, ipalomo@ugr.es)

The evidence in the early nineties for the occurrence of the K/T meteorite impact in Chicxulub focused an intense debate on this structure as well as on the impact scenario.

Addressing this line of research, the Ocean Drilling Program (ODP) included in its drilling objectives the recovery of K/T boundary sediments in the North American Atlantic margin. Thus, at the Blake Nose Plateau, the ODP Leg 171B drilled in 1997 a detailed record of the K/T event at ODP Site 1049 in three adjacent holes: 1049A, 1049B, and 1049C. The excellent Cretaceous-Tertiary (K/T) boundary interval recovered provided evidence on the deposition of K/T impact-generated material and the location of crater site at Chicxulub. The K/T boundary is marked in the three holes by a single bed of spherules that occurs at the biostratigraphic boundary between the Cretaceous and the Paleocene. This spherule bed varies from 7 to 17 cm in thickness at the three holes drilled at Site 1049, which suggests reworking of the ejecta material (Klaus et al., 2000). Despite this, the spherule bed confirms that the impact generated material from the Chicxulub crater is well preserved at the Blake Nose Plateau. The spherule bed is capped by a 1– 3-mm-thick orange limonitic layer which was initially (during on-board analysis) considered to be a candidate for the so-called fireball layer, but the usual extraterrestrial markers, such as Ni-rich spinels and a strong iridium anomaly, are conspicuously absent. The burrow-mottled ooze overlying the spherule bed contains some reworked Cretaceous planktonic foraminifera, but typical early Danian species (foraminiferal assemblage indicative of the Pa zone) are also present (Norris et al., 1998, 1999). The uppermost Maastrichtian sediments comprise light gray nannofossil-foraminifer ooze (Abathomphalus mayaroensis Zone and Micula prinsii Zone) that is slumped (Norris et al., 1998, 1999; Klaus et al., 2000). The deformation of Cretaceous sediments is a general feature at sites close to Chicxulub, related to the seismic energy input from the Chicxulub impact (Alvarez et al., 1992; Smit, 1999; Norris et al., 2000). A sharp contact separates this ooze from the overlying bed of spherules.

The uppermost Cretaceous and lowermost Danian materials are composed of carbonates, clay minerals, and quartz, and minor quantities of feldspars and traces of heavy minerals and pyrite. Clay mineral assemblages consist of smectite, which is dominant, kaolinite, and illite. Regarding the chemical composition of Cretaceous and Tertiary sediments, a precise chemical stratigraphy of the uppermost Cretaceous sediments cannot be established due to slumping, but their chemical composition is very homogeneous except for some variations related to detrital mineral abundances and redox conditions (Martínez-Ruiz et al., 2001a). The lowermost Danian stratigraphy is undisturbed. Some changes observed in the burrow-mottled ooze are mainly related to diagenetic alteration. The Mn content increases above the boundary bed, indicating diagenetic remobilization of Mn. Smit et al. (1997) reported the maximum Ir concentration in Blake Nose sediments just above the spherule bed, which may suggest that diagenetic remobilization may also have affected extraterrestrial elements. The spherule bed is represented by a coarse, graded, and poorly cemented unit mostly composed of spherules and some Cretaceous foraminifera and clasts. Mineralogical analyses reveal that this bed mostly consists of clays and minor proportions of calcite partially derived from the Cretaceous material. Dolomite, quartz, and zeolites are also present in minor proportions, and trace amounts of rutile, biotite, and some lithic fragments. Clays are mostly smectites; occasional traces of illite and kaolinite are probably derived from contamination by Cretaceous material. The contact of the spherule bed with sediments above and below is very sharp, suggesting very rapid deposition. Stereomicroscope and SEM observations reveal that the morphologies of the Blake Nose spherules are mainly perfect spheres with lesser proportions of oval spherules that contain bubble cavities. Sizes usually range from 100 µm to 1000 µm. Different types of spherules have been distinguished on the basis of color, morphology, and surface texture. They are light green, dark green, or pale yellow, with nodular, smooth or rough surfaces (Martínez-Ruiz et al., 2001b). Spherules are mainly composed of smectite but there is evidence for preserved unaltered glass relics. In addition, TEM microanalyses show some compositional variations between dark green spherules and pale yellow spherules. Smectites from dark green spherules are richer in Fe and Si/Al usually ranges from 3.1 to 3.3; however, in some Si-rich areas, the Si/Al range of 3.5 to 5 does not correspond to a true smectite composition but to the altering glass. This suggests that a Si-rich glass has been their precursor. Smectites from pale yellow spherules have lower Si/Al, usually ranging from 2.1 to 2.5, and originated from a Ca-rich material. Some calcite crystals are also observed in the Ca-rich matrix that could be an original, unaltered phase. Light green spherules smectites have lower Si/Al (2–2.5) than those from dark green spherules. These compositional differences are derived from different precursor glass types. Two end-member types of glass, black andesitic and CaO-poor, and honey-colored CaO-rich, are present in the proximal K/T ejecta from Haitian sections, and Mimbral, Mexico (Izett et al., 1991; Sigurdsson et al., 1991; Smit et al., 1992; Koeberl and Sigurdsson, 1992). The differences between Blake Nose spherules therefore indicate that they were derived from the alteration of compositionally different impact glasses. Variations in smectite octahedral cations also support a compositionally variable precursor because impact-generated glass was only briefly melted, so there was not enough time for the elements to mix and homogenize (e.g., Alvarez et al., 1992). The existence of Ca-rich and Si-rich phases is consistent with the preimpact target stratigraphy (Koeberl, 1993; Hough et al., 1998). All this evidence supports that the K/T boundary material from Blake Nose spherule layer was derived from Chicxulub target rocks, which is further supported by the chemical composition of the K/T boundary sediments. Thus, the REE concentrations significantly decrease in the spherule bed relative to overlying and underlying sediments, mainly due to mobilization during diagenesis. However, C1- normalized REE patterns can be considered informative, and patterns from Blake Nose spherule bed are similar to those of upper crustal rocks and to Cretaceous and Tertiary sediments, suggesting inheritance from upper crustal rocks. Regarding extraterrestrial elements, Cr, Co, Ni, and Ir appear in low concentrations in the spherule bed at Blake Nose, and little evidence for significant extraterrestrial contribution is observed, supporting therefore that the spherule-bed material mainly originated from the alteration of target-rock-derived material.

References

Alvarez,W., Smit, J., Lowrie,W., Asaro, F.,Margolis, S.V., Claeys, P., Kastner, M. and Hildebrand, A.R. 1992. Proximal impact deposits at the Cretaceous- Tertiary boundary in the Gulf of Mexico: A restudy of DSDP Leg 77 Sites 536 and 540. Geology, 20, 697–700.

Hough, R.M., Wright, I.P., Sigurdsson, H., Pillinger, C.T. and Gilmour, I. 1998. Carbon content and isotopic composition of K/T impact glasses from Haiti. Geochimica et Cosmochimica Acta, 62, 1285–1291.

Izett, G.A., Dalrymple, G.B. and Snee, L.W. 1991. 40Ar/39Ar Age of Cretaceous- Tertiary boundary tektites from Haiti. Science, 252, 1539– 1541.

Klaus A., Norris R.D., Kroon D. and Smit J. 2000. Impact-induced mass wasting at the K-T boundary: Blake Nose, western North Atlantic. Geology, 28, 319–322.

Koeberl, C. 1993. Chicxulub Crater, Yucatan: Tektites, impact glasses, and the geochemistry of target rocks and breccias. Geology, 21, 211–214.

Koeberl, C. and Sigurdsson, H. 1992. Geochemistry of impact glasses from the K/T boundary in Haiti: Relation to smectites and a new type of glass. Geochimica et Cosmochimica Acta, 56, 2113–2129.

Martínez-Ruiz, F., Ortega-Huertas, M., Kroon, D., Smit, J., Palomo, I. and Rocchia, R. 2001a Geochemistry of the Cretaceous-Tertiary boundary at Blake Nose (ODP Leg 171B). Geological Society Special Publication 183, 131–148.

Martínez-Ruiz, F., Ortega-Huertas, Palomo, I. and Smit, J. 2001b. K/T boundary spherules from Blake Nose (ODP Leg 171B) as a record of the Chicxulub ejecta deposits. Geological Society Special Publication 183, 149–161.

Norris, R.D., Firth, J., Blusztajn, J.S. and Ravizza, G. 2000. Mass failure of the North Atlantic margin triggered by the Cretaceous-Paleogene bolide impact. Geology, 28, 1119–1122.

Norris R.D., Huber B.T. and Self-Trail, J. 1999. Synchroneity of the K-T oceanic mass extinction and meteorite impact: Blake Nose, western North Atlantic. Geology, 27, 419–422.

Norris, R.D., Kroon, D., Klaus, A. and others 1998. Initial reports, Ocean Drilling Program, Leg 171B: College Station, Texas, Ocean Drilling Program, 749 pp.

Sigurdsson, H., D’Hont, S., Arthur, M.A., Bralower, T.J., Zachos, J.C., van Fossen, M. and Channell, E.T. 1991. Glass from the Cretaceous/Tertiary boundary in Haiti. Nature, 349, 482–487.

Smit, J. 1999. The global stratigraphy of the Cretaceous-Tertiary boundary impact ejecta. Annual Review of Earth and Planetary Sciences, 27, 75–113.

Smit, J., Alvarez, W., Montanari, A., Swinburne, N., Kempen, T.M.V., Klaver, G.T. and Lustenhouwer, W.J. 1992. Tektites and microkrystites at the KT boundary: Two strewn fields, one crater. Proceedings of Lunar and Planetary Science, 22, 87–100.

Smit, J., Rocchia, R., Robin, E. and ODP Leg 171B Shipboard Party 1997. Preliminary iridium analyses from a graded spherule layer at the K/T boundary and late Eocene ejecta from ODP Sites 1049, 1052, 1053, Blake Nose, Florida. Geological Society of America Abstracts with Program, 29, p. A141.