Origin and Clay-Mineral Genesis of the Cretaceous/Tertiary Boundary Unit, Western Interior of North America

Abstract

Abstraet--A 3-cm-thick, two-layered clay unit that records mineralogic 'and textural evidence of a catastrophic event that occurred at a time now marked as the end of the Cretaceous Period was preserved in ancient peat-forming environments of the Western Interior Basin of North America. The two layers of this unit consist of altered distal ejecta and are easily distinguished by their distinctive texture and impact components from other clay beds, mainly tonsteins and detrital shales, occurring within the sequence of rocks enclosing the Cretaceous/Tertiary (K/T) boundary interval.The lower claystone layer of the K/T boundary unit represents melted silicic target rock that has altered mainly to kaolin minerals. Impact components and signatures of this lower layer include a relict imbricate fabric of glass fragments, shards, bubbles, hollow spherules (altered microtektites), small amounts of shocked mineral grains, and a subdued iridium anomaly. These components and textures, combined with the layer's restricted areal distribution, indicate that this layer, called the "'melt ejecta layer," is the distal part of an ejecta blanket deposit. We interpret the melt ejecta layer to be an altered deposit of mostly impact-derived, shock-melted, silicic target material that traveled through the atmosphere within a detached ejecta curtain and on other ballistic trajectories.The upper laminated layer of the K/T boundary unit consists mostly of altered vitric dust and abundant shocked minerals whose size and amounts decrease away from the putative crater site in the Caribbean area. High-nickel magnesioferrite crystals, high iridium content, geochemical signature, and worldwide distribution all suggest this upper layer originated from a cloud of vaporized bolide and entrained targetrock materials ejected above the atmosphere. The components of this layer, called the "fireball layer," settled slowly by gravitational processes from an Earth-girdling vapor cloud and were deposited immediately on top of the already-emplaced melt ejecta layer.The clay minerals that formed in the two layers are largely a function of composition and the highly unstable, shock-modified state of the fallout materials altered in acidic, organic-rich waters of ancient peat swamps. The fireball layer is mostly altered to smectitic clay from a mafic glass condensed from the vaporized chondritic bolide, along with some kaolinite formed from blebs of melted silicic target material entrained in the vapor plume cloud during ejection. In contrast, the melt ejecta layer is mainly kaolinitic, derived from silicic glass formed from melted target rocks. In this layer, the glass rapidly altered to mostly disordered, micrometer-sized "cabbage-like" or submicrometer-sized embryonic forms of spherical halloysite, probably from an allophane precursor. These crystallization characteristics of the melt ejecta layer are much different than those which formed coarse vermicular aggregates and platy kaolinite crystals in tonsteins from outside the K/T boundary interval througho...

A dual-layer Chicxulub ejecta sequence with shocked carbonates from the Cretaceous–Paleogene (K–Pg) boundary, Demerara Rise, western Atlantic

“…In the terrestrial North American Western Interior KPg sites (e.g., Izett, 1990;Pollastro and Bohor, 1993;Bohor and Glass, 1995;Sweet and Braman, 2001), the ejecta deposition took place in low-energy swamp-like and fluvial plain terrestrial environments. These settings precluded postdepositional reworking, yielding preservation of primary depositional features: the K-Pg sites show a 1-to 3-cm thick lower spherule-rich layer capped by a mm-thick layer rich in (shocked) tectosilicates that has a more Fe-rich clay mineralogy and contains an iridium anomaly (e.g., Izett, 1990;Pollastro and Bohor, 1993;Sweet and Braman, 2001).…”

“…These settings precluded postdepositional reworking, yielding preservation of primary depositional features: the K-Pg sites show a 1-to 3-cm thick lower spherule-rich layer capped by a mm-thick layer rich in (shocked) tectosilicates that has a more Fe-rich clay mineralogy and contains an iridium anomaly (e.g., Izett, 1990;Pollastro and Bohor, 1993;Sweet and Braman, 2001). Due to the acidic, swamp-like deposition environments, however, carbonate clasts -if present originally at all -were likely dissolved during diagenesis.…”

“…The distinct dual partition of the ejecta deposit in proximal to intermediate regions from Chicxulub in a lower, melt-ejecta and spherule-rich bed overlain by a thin layer enriched in shocked tectosilicates and iridium was explained by (i) rapid deposition of the ballistically ejected impact-melted target rocks from Chicxulub after the impact that resulted in spherule beds and (ii) mixture of molten and vaporized projectile material in the expanding ejecta plume together with entrained target material followed by global deposition over a considerably longer period (e.g., Smit et al, 1992a;Smit, 1999;Pollastro and Bohor, 1993). This scenario also explains that in the distal marine K-Pg boundary layers from the Pacific and Tethyan realm, the ejecta consists of shocked quartz and altered spherules of mafic composition, the so-called microkrystites (e.g., Smit et al, 1992a;Smit, 1999).…”

“…These trends indicate a concentration of melted material from relatively shallow target rocks in the early ejecta, and prevalence of deeper crustal components (including shocked minerals) together with material of the projectile concentrated in the so-called fireball layer (Fig. 1 and Izett, 1990;Pollastro and Bohor, 1993;Bohor and Glass, 1995;Kring and Durda, 2002). Of crucial importance for understanding the catastrophic effects of the Chicxulub event on the biosphere is the impact-related input of carbon dioxide and sulfur oxides in the atmosphere by devolatilization of the up to 3 km thick carbonate and evaporite platform sediments that covered the target area at the time of the impact (Pope et al, 1997;Toon et al, 1997).…”

Spherules associated with the cretaceous–paleogene boundary in Poland

“…This thin layer is distributed globally from the impact site and contains, as significant components, altered spherules of impact melt (Montanari et al 1983;Sharpton et al 1992;Pollastro and Bohor 1993;Bohor and Glass 1995;references in Sharpton andWard 1990, Ryder et al 1996).…”