Robert Rocchia

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

Abstract Magnetic spinels produced by oxidation of extraterrestrial objects in the atmosphere have a composition distinct from terrestrial spinels. They are characterized by a high iron oxidation state, arising from crystallization under high oxygen fugacities, and a high nickel concentration due to the relatively high abundance of this element in extraterrestrial material. The iron oxidation state increases from micrometeorites, to meteoroid ablation material and to impact-generated products. This reflects a progressive increase of the oxygen fugacity, corresponding to decreasing altitudes of crystallization. Spinels found at the Cretaceous-Tertiary boundary are similar to those that crystallized from meteoroid ablation material and impact-generated products, supporting the view that a collisional event did occur at the end of the Cretaceous.

Cosmic markers, 40Ar/39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province

Abstract Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo–French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13–28]. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313–319; Bajpai, J. Geol. Soc. London 157 (2000) 257–260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within uppermost Maastrichtian normal chron C30N at ∼66.5–67 Ma in the Anjar area. Volcanism then stopped at least locally, and lacustrine sediments were deposited over a period that could be in the order of 1–2 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels. This was also responsible for the partial remagnetisation of the underlying flows. These findings generally confirm and complement those of Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424], and are compatible with the occurrence of the K/T impact at the paleontological K/T boundary, and of Deccan trap volcanism straddling the boundary and starting before the impact. Anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors, though still within normal chron C30N. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet. Sci. Lett. 80 (1986) 361–374; Vandamme et al., Rev. Geophys. 29 (1991) 159–190].

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

Abstract Ni-rich spinels have been found throughhout the world at the Cretaceous-Tertiary boundary. These minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris, suggesting that the Cretaceous-Tertiary boundary spinels result from an extraterrestrial event. Their stratigraphic distribution, which has been observed at the Cretaceous-Tertiary boundary sites of El Kef (Tunisia), Caravaca (Spain) and Hole 761C (ODP Leg 122, Indian Ocean), are markedly different from site to site and are all much less extended than the Ir anomalies. Regional differences in Ni-rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions. The wider stratigraphic dispersion of Ir is attributed to postdepositional chemical processes. At El Kef, the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate, the 1–2 mm thick Ni-rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years. This upper limit is consistent only with the collision of an asteroid or a comet at the close of the Cretaceous.

Deposition of channel deposits near the Cretaceous-Tertiary boundary in northeastern Mexico: Catastrophic or normal sedimentary deposits?

Investigation of Cretaceous-Tertiary (K-T) boundary channel deposits in three sections in northeastern Mexico reveal three lithologically and mineralogically distinct units separated by three or more disconformities. The units include a spherule-rich layer (unit 1), laminated sandstone (unit 2), and rippled sand and shale layers (unit 3). On the basis of paleontological, mineralogical, and geochemical evidence, the K-T boundary is just above the top of unit 3. Although the deposits have previously been interpreted as a tsunami deposit related to a presumed K-T impact at Chicxulub on Yucatan, we have found no evidence of a nearby impact. We suggest that deposition occurred in an incised valley setting and over an extended period of time.

Magnetostratigraphy and biostratigraphy of Cretaceous-Tertiary continental deposits, Ager Basin, Province of Lerida, Spain

Abstract At Fontllonga (Northeast Spain, Province of Lerida) a nearly continuous outcrop covers the stratigraphic interval from the Maastrichtian to the Thanetian in continental deposits. Magnetostratigraphy of the section was determined from 72 samples. The magnetic polarity sequence was satisfactorily correlated with chrons 32R to 26R (top of Lower Maastrichtian to Lower Thanetian). Chron 29R, in which the K/T boundary occurs, is unambiguously recognizable considering the biostratigraphic data (charophyte, ostracod, palynomorph). The K/T boundary occurs in the uppermost part of the Microchara cristata subzone, which includes Late Cretaceous and Early Palaeocene species. This subzone is interpreted as an interval of crisis, following the Late Maastrichtian extinctions and preceeding the Palaeocene radiation. No Ir-rich horizon has been yet found in this section. The stratigraphic position of the last dinosaur remains, found in the chron 31N interval, leaves open the question of a causal link between the extinction of Cretaceous reptiles and the K/T event.

Extraterrestrial Ni-rich spinel in upper Eocene sediments from Massignano, Italy

Evidence of a major cosmic event at the end of the Eocene is given by the finding of at least one, possibly two or more, impact horizons containing microtektites, microkrystites, shocked quartz, and unusually high iridium concentrations. We report here the discovery, in the global stratotype of the Eocene-Oligocene boundary at Massignano in Italy, of Ni-rich spinel, a mineral that forms by the interaction of meteoritic bodies with the Earths atmosphere. A maximum concentration of ≈800 crystals mg−1, corresponding to a flux of 2 × 107 crystals cm−2, is observed in a well-dated layer (35.7 ± 0.4 Ma) that also contains shocked quartz and iridium. The crystals are found clustered in flattened spheroids, the probable relic of their host bodies. The chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects, as proposed for the spinel-bearing spheroids found at the Cretaceous-Tertiary boundary. The most likely explanation is that the dust particles were generated by a large cometary impact. Whether this impact is the one that produced microtektites and/or microkrystites, or another one that may have occurred shortly after or before, is still questionable. Additional searching for Ni-rich spinel at other upper Eocene sections may help to answer this question.

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

Abstract The K-T boundary at Beloc in Haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite. A small fraction of these objects still contains well-preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the Caribbean area. However, Beloc glasses are markedly different from tektites and microtektites: their structure and composition suggest that they are not impact-generated products, but are likely of volcanic origin. On the other hand, in sections which have probably preserved their original stratigraphy, the globule bed, showing well-developed grading, appears first in the sedimentary column followed by the ubiquitous Cretaceous-Tertiary boundary (KTB) clay layer containing Ir- and Ni-rich spinels: granulometric considerations indicate that the globule bed and the Ir- and spinel-rich clay layer correspond to two distinct events. We conclude that the globules at the KTB depositional sequence of Beloc represent a local volcanic event slightly preceding the great KTB cosmic catastrophe. Our observations do not provide any unequivocal evidence that a cratering event occurred nearby.

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

Abstract The classical Gubbio (Italy) Cretaceous-Tertiary boundary section has been resampled for both magnetostratigraphy and iridium. Paleomagnetic samples were taken over 7 m in the Maastrichtian and 6 m in the Paleocene. Previous results obtained a decade ago are confirmed. The reversal sequence is well defined and the individual reversals are somewhat more precisely located. A noticeable difference is the location of the position of the 29N/29R limit which may be lowered by 20–30 cm in the stratigraphic column. This would imply that the KTB occurs near the middle of chron 29R. Iridium measurements were made on samples from both shales and surrounding limestone beds from 2 m below to 3 m above the boundary: these measurements indicate that Ir is associated with clay minerals. Concentrations in the two types of samples are indeed compatible when reduced to a carbonate-free basis. Iridium concentrations stand above background over almost 3 m of section, corresponding to half a million years based on magnetostratigraphy. This is likely to indicate a protracted duration of the (external or internal) source of iridium, on top of which the main (short-lived) KTB anomaly proper stands.

The cretaceous/paleogene boundary and planktonic foraminifera in the flyschgosau (Eastern Alps, Austria)

The studied interval extends from 2.5 m below to 1 m above the Cretaceous/Paleogene (K/P) boundary and comprises the uppermost Maastrichtian marly limestone overlain by a boundary (rusty) layer—a dark yellow orange clay 5–10 mm thick, followed by a turbidite sequence of very fine sandstones and grey-brown marls. The following planktonic foraminiferal zones are distinguished: Abathompalus mayaroensis, PO (subzones: POa— Guembelitria cretacea and POb — Globoconusa conusa), Pa or Parvularugoglobigerina eugubina and P1 (subzone: P1a or Subbotina pseudobulloides). The distribution of iridium shows a relatively sharp rise to a maximum value of 7.176 ppb about 1.2 cm above the rusty layer, followed by a rapid drop to normal background levels. The Abathomphalus mayaroensis Zone exhibits a moderately diverse planktonic foraminiferal assemblage. The main extinction episode occurs within the rusty layer; only a few species survived. Survivors are small primitive forms. The first new Paleocene species evolved immediately after the major Cretaceous extinctions. Rapid extinction of planktonic foraminifers coincides with the iridium anomaly which suggests that, at this site, the source(s) of the iridium anomaly was (were) probably responsible for K/P extinctions.

The K/T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers

At Beloc, Haiti the Cretaceous-Tertiary boundary (KTB) is characterized by a spherule bed containing glass particles. These particles are considered by some authors as remains of tektites resulting from a nearby impact. However, because of the stratigraphic complexity of the Beloc sections the genetic link between the KTB cosmic event and the spherule bed is not obvious. In this paper, we report new data on shocked quartz and Ni-rich spinels at Haitian KTB sites. The detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence. The first and largest shocked quartz is found in the upper part of the spherule layer. They are abundant and size graded over the 25–30 cm of carbonate-rich sediments overlying this layer. The first Ni-rich spinels, which are also rich in Cr, appear in the carbonate sediments. The size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event. Although different from typical impact glasses (tektites), Beloc glass particles must be considered as impact-derived products. The enormous fluence of shocked quartz (≈104 grains/cm2) is consistent with a proximal event. The upper part of the sequence is more complex. A second distribution of shocked quartz associated with Ni-rich spinels of different compositions appears in the layer containing the maximum Ir concentration. We propose that these features, which are not easily explained by a sedimentary artifact, result from a second collisional event.