Jan Smit
Dept Sedimentology, Vrije Universiteit Amsterdam, de Boelelaan 1085, 1081HV Amsterdam, Netherlands. <smit@geo.vu.nl>
The barranco del Gredero (Loma de Solana tectonic unit) section attracted scientists since the 1950’s (a.o. Pablo Fallot,[1]. J. Paquet,[2] G. van Veen[3] and A. von Hillebrandt) because of its exceptional thickness, completeness and richness of well-preserved foraminifers. The section remains in the top 5 of complete K/T sections and is still the subject of detailed investigations, e.g. [4], [5, 6]The Milankovitch-style, well-bedded section from early-Maastrichtian to Lower Eocene, is over 225m thick (Jorquera Fm., van Veen[3]developed in a bathyal, hemipelagic facies. By comparison, the same interval in the famous Gubbio section is only 10.5m thick. Analyses of mesopelagic fishteeth (Pat Doyle pers. comm.) suggest a depth of deposition of more than 1000m near the K/T boundary. The Loma de Solana is usually placed in the allochtonous Subbetic units, but the striking lithological similarities with the Agost, Relleu and Finestrat sections further east, demonstrate that the Loma de Solana unit is part of the par-authochtonous Prebetic, the southern margin of the Iberian continent. Van Veen[3], p.116) was one of the first to recognize that the K/T boundary interval was uniquely well exposed in the barranco del Gredero. Two mastersprojects of C. Hermes, measured the Gredero section in detail. [7] subdivided the Paleocene, and his student [8]recognized the presence of the P. eugubina Zone, but analysed only three samples from the K/T interval. Hermes (pers comm) and [8] mentioned a 10 clayinterval at the K/T boundary, but paid no further attention. [9] analysed the K/T interval in further detail, every cm in the K/T transitional interval, including the boundary clay (BCL). He found the red laminae (sample sm75-503), that most people believe it is the Chicxulub impact ejecta layer. A new biozone was established [9] between the top of the Maastrichtian and the base of the G. eugubina Zone; the G. cretacea or the P0 Zone, whose type locality therefore is in the Gredero section. A paleomagnetic analysis of the Gredero section was performed by G. Brunsmann (Univ Amsterdam) [10]. The magnetostratigraphy of the uppermost 100m of the Maastrichtian and the basal 70m of the Paleocene showed magnetochrons C31 to C27, very well comparable to the classic section at Gubbio, and the Agost section 100km further east [11]).
The same sample set was subjected to analyses by neutron activation in 1977 at Delft University. The results, obtained spring 1978, showed a strongly anomalous content of Cr, Ni, As, Sb, Zn in sample Sm75-503, 15-250 times enriched opposite background values, but not (yet) the iridium anomaly [12]. Nickel of course is often associated with Ni-iron meteorites, and values of 2000ppm (of sm75-503) are rare anywhere on earth. Around the same time, Alvarez cs. published the find of anomalous iridium in Gubbio at the 1978 AGU fall meeting, by a similar, but more sensitive neutron activation analysis. Jan Hertogen of Gent university showed in 1979 the presence of anomalous Ir in Gredero as well, particularly in sample 75-503.
The extremely short extinction interval <0.5cm), already called for a catastrophic extinction because in the preceding >100m thick interval very little happened in terms of planktic foraminiferal changes (only G. gansseri disappeared 10 m below K/T). The strong adaptive radiation of foraminifers within the first 50cm above the boundary clay, further strengthened a catastrophic cause. The iridium anomaly, by pointing at a large impact event, provided a plausible cause for the catastrophe.
The Gredero section remained a key player in the investigations that followed the launch of the impact-extinction theory. [13]discovered the tiny (50-500µm) ‘sanidine’ spherules in sm75-503, that later became known as microkrystites, now best explained as condensation droplets from the hot impact vapour cloud. Smit (1977) initially regarded these as “gypsum nodules”, because gypsum is abundant in these pelagic sediments. Gypsum, unfortunately, has the same refraction index as sanidine, so they went unnoticed for several years. Gerard Klaver (NITG/TNO) recognized the quench texture within the sanidine spherules, because he had observed similar textures in the chilled crust of pillow-lava basalts on the island of Bonaire [14] There remained a problem: K-spar is characteristic of K-rich, very viscous igneous rocks, not the ideal environment for producing quench crystals, on the contrary. Also in Furlo and Petriccio (Italy) were these sandine spherules found in the Ir-rich layer, but in these localities accompanied by dark clay-rich spherules, full of quench crystals of magnesioferrite, rich in iridium. These were much more mafic, consistent with the quench crystallinity, but hard to reconcile with K-spar spherules, assumed that they are from the same source. The Cr-rich magnesioferrite and chromite quench crystals also occur in 75-503 in great abundance, but are seemingly floating in the clay matrix, as if they were formed directly by condensation. Close inspection of 75-503 shows that the spinels occur in clusters, outlining flattened spherules. The same textures in the sanidine spherules were also found in spherules of pure smectite (Fonte d’Olio, Bidart), in arsenopyrite spherules from Zumaya, and goethite from New Zealand and Tetri-Tskaro. It became clear that the Kspar quench crystals were pseudomorph alteration products of another precursor, preferably a mafic mineral. The high d 18O (+25‰) of the sanidine [15] confirmed the low temperature origin. The precursor mineral was discovered at DSDP site 577, Shatsky rise Pacific; a Ca-rich augite, clinopyroxene.
The next important discovery made at the Gredero section, are the chondritic ratios of all the platinum group elements (PGE) [16], an important indicator that the Ir anomaly was not from a terrestrial (i.e. volcanic) source, as suggested by some. It is remarkable that the magnetic residues of 75-503, consisting primarily of chromite and magnesioferrite, are highly enriched in Ir and Cr.
[17] determined an anomalous amount of soot in 75-503, indicative of large wildfires just after impact. It is curious that at the “twin” site Agost not a trace of soot has been determined, while all other characteristics (lithology, biotic changes, geochemistry) are almost identical.
Last but not least, sample 75-503 yielded an important clue to the composition of the impacting bolide, by the finding of the anomalous Cr-isotope values [18]. Compiled data from all the K/T sites where both Cr and Ir were measured, the concentration profile of both is almost identical, with the strongest peak in the ejecta layer. Sm75-503 holds the K/T boundary record in terms of Cr concentration. It is safe to say that whatever was the source of the iridium, is also the source for the excess Cr. The 53Cr/52Cr ratio on earth is everywhere the same, whether from the mantle or crust. Extraterrestrial matter has a different 53Cr/52Cr. 75-503 yielded a strong negative Cr ratio, (Earth=0) identical to carbonaceous chondrites, unlike ordinary chondrites and asteroids that have a positive ratio. This finding excludes any hypothesis for a terrestrial origin of Cr and Ir at K/T.
Stable isotope analyses have played a major role in establishing the magnitude of the K/T mass-extinctions. T. Romein [19] was the first to analyse stable isotopes in detail across the K/T boundary in the Gredero section. He was the first to find the short-term strong negative(2.5 ‰ d 13C) anomaly, that later became known as the “Strangelove ocean” condition. In these conditions the vertical d 13C gradient in the oceans temporarily disappeared during deposition of the boundary clay. The d 18O profile of Caravaca [20] suggests that seasurface temperatures have risen considerably at the base of the boundary clay. [21] and others confirmed these anomalies in detail, and showed that the vast majority of Cretaceous species above the K/T boundary are reworked specimens.
Benthic foraminifers [22] demonstrate a sudden collapse of ecosystems directly above the K/T boundary, with only a few taxa blooming under anoxic conditions. The benthic taxa must have temporarily migrated elsewhere, because the majority returned to the Gredero seafloor when oxygenation improved. Burrowing organisms likewise temporarily disappeared in the region, probably due to lack of oxygen on the seafloor. The ejecta layers in the sections of Agost, Relleu and Gredero are almost completely intact, not bioturbated. Frequent burrows can be observed in the top Maastrichtian (zoophycos, chondrites, thalassinoides), but those, scavengers of the seafloor, are filled with dark boundary clay, not with ejecta layer debris, which showed that they became active substantially after deposition of the ejecta layer, in contrast with similar borrows from Italy (Furlo, Petriccio) and Bidart., that often contain the debris of ejecta (spherules).
The detailed pattern of extinction of the planktic foraminifers at the K/T boundary was first unraveled at Caravaca [8, 9] [23]. Later research [24] [25] [21] confirmed this extinction pattern, and differ only in minor detail. The Abathophalus mayaroensis Zone is about 100m thick in the Gredero section. In this long biozone (3.5 Ma) few changes occur. A single species disappears (G. gansseri), and a few appear in the upper part of the zone (Pseudoguembelina hariaensis and Plummerita hantkeninoides). The latter datums have been used to subdivide the A. mayaroensis Zone in four. Yet up to- and including the higest sample in the Maastrichtian, all species range up to the ejecta layer (75-503). The presumed decline in species abundence prior to the ejecta layer remains an artifact: The top 10cm of the Maastrichtian is usually partially to dissolved, and only an intensified search yields rare “missing” species. The ejecta layer is free of specimens, as expected. The boundary clay is more than an order of magnitide impoverished in specimens and species. Yet a few species are relatively more abundant, in particular Guembelitria cretacea (and Globigerinelloides, Hedbergella), and may have survived. Heterohelix spp. are also relatively abundant, but it is hard, if not impossible to infer from abundances alone that these are survivors or reworked. Stable isotope analyses do not help either, because of the light isotope values of the diagenetic calcite infilling. The top of the boundary clay is sheared by bedding plane slip. The slip has slickensided the top of the clay and the base of the G. eugubina (Pa) Zone, creating an artificial hiatus of unknown magnitude, represented in the Agost section by a 0.5cm transition zone. The first undisturbed sample of the Paleocene already contains an abundant fauna of Pa, consisting of at least six species, increasing to about ten in Pa [23]. Scavenging through later literature, it seems that the basalmost Paleocene would at least contain over 23 species. This is certainly due to oversplitting, in particular in the genera Eoglobigerina and Parvularugoglobigerina.
In the Gredero section also for the first time acmes of planktic foraminifers and nannofossils were observed in the basal Paleocene (Smit, 1982, [26]. These peaks occur in a short interval only, mainly in Pa, but do not exactly coincide. It was known that a bloom of Thoracosphaera and Braarudosphaera occurs in the basal Paleocene [27], and these species were therefore considered survivor species. These blooms indicate an unstable adaptive radiation, an important argument for the sudden catastrophic extinction at the K/T boundary, as such radiation presumably takes place only in a vacant, empty ecospace.
The Gredero section has provided numerous discoveries of the K/T boundary event that demonstrate the intimate relation between the biotic events and the K/T impact event at Chicxulub. The section is still well accessible, but increasing urban and industrial developments threaten to cover and destroy this unique section.
References
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