Tomas Villamil

Elemental abundance anomalies in the late Cenomanian extinction interval: a search for the source(s)

Elemental abundances have been measured by neutron activation methods across the Cenomanian-Turonian (late Cretaceous) extinction interval in samples collected from sixteen sites in the Western Interior Basin of North America and from twelve widely separated locations around the globe, including six ODP/DSDP sites. In most Western Interior Basin sites, in Colombia, and in western Europe (weaker), two closely spaced elemental abundance peaks occur in the upper Cenomanian (∼ 92 m.y.), spanning the ammonite zones ofSciponoceras gracile throughNeocardioceras juddii. Elements with anomalously high concentrations include Sc, Ti, V, Cr, Mn, Co, Ni, Ir, Pt and Au. The lower peak coincides with the disappearance (extinction) of the foraminiferRotalipora cushmani. In North American sectionsR. greenhornensis also disappears at or just below this horizon, but in Europe it disappears considerably earlier thanR. cushmani. A series of molluscan extinction and speciation or migration events also begins near the stratigraphic level of the lower elemental abundance peak. The well-documented positiveδ13C excursion begins just before the extinctions and the elemental anomalies, and continues into the lower Turonian, well above the upper anomaly. This carbon isotope excursion has been observed in East European sections where we find little or no evidence of the elemental anomalies, suggesting that the two phenomena may not be tightly coupled. Elemental abundance ratios in the anomalies closely resemble those of Mid-Atlantic Ridge basalt or Hawaiian lava (tholeiitic), but not those of C1 chondrite, black shale, average crustal rocks, or lamproite and kimberlite of roughly similar age in southeastern Kansas. The excess Ir and other siderophiles hint at possible large-body impact(s) for the source. However, we have not located microspherules (other than biogenic calcispheres) or shocked mineral grains in any of our samples. Furthermore, Sc, Ti, V and Mn are not enriched in differentiated Solar-System bodies. Although the weak geochemical signal from comet impact(s) could be masked by the strong terrestrial-like overprint, these anomalies more likely resulted either from intense seafloor spreading activity or merely from increased circulation of deep, metal-rich water associated with the large late Cenomanian through early Turonian eustatic rise and deep-water opening of the South Atlantic. The flooding of continental seaways and margins also could have contributed to the anomalies by preventing much continental detritus from diluting the normal background marine geochemical component.

Decapod and Stomatopod crustaceans from mass mortality Lagerstatten; Turonian (Cretaceous) of Colombia

Mass mortality surfaces and bioclastic lags in Turonian siliceous shales of the San Rafael and La Frontera formations in Colombia have yielded two species of decapod crustaceans, Opthalmoplax spinosus new species and Pinnotheres? species indeterminate, as well as one stomatopod. Sculda sp. All represent first records for the country. Remains of the swimming carcineretid crab, Opthalmoplax spinosus, in association with fish fragments and pelagic ammonites, cover mass mortality surfaces. The occurrence suggests that the high density living population of the crabs, exploiting abundant food resources in areas of topography-controlled upwelling, was periodically reduced catastrophically, perhaps by advection of toxic trace elements or by oxygen starvation. Two other taphofacies, characterized by current oriented appendage fragments and scattered skeletal elements represent normal accumulations of fossil material in the San Rafael and La Frontera formations.

Paleoecology of giant Inoceramidae ( Platyceramus ) on a Santonian (Cretaceous) seafloor in Colorado

Abstract Giant Middle Coniacian to Lower Campanian Platyceramus Seitz is among the largest Cretaceous bivalves, commonly reaching an axial length of over 1 m, and occasionally over 2–3 m in size. The genus is characterized by its large size, very low convexity, normal inflation limited mostly to the umbonal area, and flattened flanks. It is especially common in moderately deep calcareous shale facies, as well as in chalks and limestones of the Niobrara Formation and equivalents. Preferred facies contain abundant pyrite, elevated total organic carbon (TOC), and very low biotic diversity. The genus maintains its giant size in these facies, and becomes more abundant. It clearly prefers dysoxic facies. As such, it probably is chemosymbiotic; photosymbiosis is almost ruled out because of inferred water depths of 200–350 m. It is also found more sparsely, and of smaller size, in oxygenated facies, including shoreface sandstone. The study area contains over 81 giant-sized Platyceramus platinus (Logan, 1898) on a single bedding plane; there are very few small ones. This allows spacing, orientation, and size analysis to be performed on an adult population.

A NEW CARCINERETID CRAB (UPPER TURONIAN, CRETACEOUS) OF COLOMBIA

Abstract A single large specimen of carcineretid crab from the Cretaceous Puerto Romero unit of Colombia constitutes the basis for a new species, Ophthalmoplax triambonatus. A re-diagnosis of the Carcineretidae results in the assignment of five genera to the family: Carcineretes Withers, 1922; Branchiocarcinus Vega, Feldmann, and Sour-Tovar, 1995; Mascaranada Vega and Feldmann, 1991; Ophthalmoplax Rathbun, 1935; and Woodbinax Stenzel, 1952. The cross-sectional profile of O. triambonatus suggests that it was a back-burrower, rather than an active swimmer.

Biostratigraphic Techniques for Analyzing Benthic Biofacies, Stratigraphic Condensation, and Key Surface Identification, Pliocene and Pleistocene Sediments, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

The northern deep Gulf of Mexico is a geologically complex province consisting of Neogene intraslope minibasins created by sediment loading onto and evacuation of allochthonous salt. Sedimentary fill in the minibasins consists of bathyal turbidite systems with highly variable facies distribution. In this paper, we present three biostratigraphic techniques developed to increase our understanding of the geologic evolution of the northern Green Canyon and Ewing Bank lease areas. The first two techniques address the nature of stratigraphic condensation in minibasins and the relative areal extent of the condensed sections. The third technique uses benthic foraminifers to improve the stratigraphic knowledge of the area. (1) The first technique is concerned with sediment accumulation plots from wells helping to identify possible areas of condensation based on variations in the rate of accumulation. By displaying several wells together, the areal extent of the condensed section can be identified, and the causes for the condensation can be evaluated. (2) The second technique addresses condensation within one minibasin during a 2.5-m.y. interval. Relative abundance values are plotted from a series of wells at selected time surfaces and tied to seismic horizons that are correlated throughout the basin. This allows plotting a series of maps that show how stratigraphic condensation changes throughout a basin. (3) The third technique develops benthic biofacies maps that more accurately reflect the highly irregular paleobathymetry of the slope at the time of deposition. These maps were created by plotting the benthic biofacies of the sediments recovered from a well at the stratigraphic extinction of 29 selected planktonic foraminifer and calcareous nannoplankton species. The maps are integrated with lithostratigraphic and structural palinspastic reconstructions. The resulting maps are interpreted to more accurately reflect benthic biofacies of the continental slope at specific geological times during the Pliocene and Pleistocene.

USE OF CROSS-SECTION RESTORATION TO DETERMINE PALEOBATHYMETRY AND SEA-FLOOR PALEOTOPOGRAPHY IN THE GULF OF MEXICO BASIN

A systematic technique for sequential restoration of salt-related extensional structures incorporates Airy isostasy to quantify both the loading subsidence and the change in water depth during any time interval. The results are used to generate paleo-sea-floor templates for restoration and to reconstruct evolving salt and sediment geometries. Paleobathymetries calculated by the restoration technique are compatible with paleobathymetric environments derived from biofacies analysis of benthic foraminifers sampled in five wells along a reflection seismic profile from western Eugene Island, offshore Louisiana, northern Gulf of Mexico. Furthermore, both aggradation and progradation rates measured from the restorations are consistent with regional depositional patterns. The results demonstrate that the restoration method adequately determines paleobathymetry and sea-floor paleotopography. Restoration of multiple sections can provide a framework for the estimation of sediment transport and deposition geometries and facies development. 26 refs., 3 figs.