Abstract

A 3-cm-thick, two-layered clay unit that records mineralogic and textural evidence of a catastrophic meteorite-impact event marking the end of the Cretaceous Period was preserved in ancient peat-forming environments of the Western Interior of North America. The two layers of the Cretaceous/Tertiary (K/T) boundary unit are comprised of altered distal ejecta and are easily distinguished by their unique texture and impact components, thereby differing from other thin clay beds, mainly tonsteins and detrital shales, that occur within Cretaceous and Tertiary rocks enclosing the K/T boundary interval.

The lower layer of the K/T boundary unit, designated the melt-ejecta layer, is a clay stone about 2 cm thick comprised mostly of authigenic kaolinitic clay minerals. It was deposited from a turbulent ejecta curtain of target-rock melt-glass. This shock-melted glass was somewhat less stable than the volcanic glass ashfalls that altered to produce nearby tonsteins. In contrast, the upper, thinner (up to 1.0 cm thick), laminated layer of the K/T boundary unit, called the fireball layer, contains mostly authigenic smectitic clay minerals formed from a gravitational airfall deposit of material condensed from the vaporized mafic bolide.

The clay minerals in the layers of the K/T boundary unit were initially formed from the diagenetic alteration of these impact-derived glasses by acidic pore waters of the peat-forming environment. Silicic target-rock glass of the K/T melt-ejecta layer rapidly altered to microspherules of disordered halloysite. A relict imbricate fabric of glass fragments, shards, bubbles, and hollow spherules (altered microtektites), small amounts of shocked mineral grains, a subdued iridium anomaly, and restricted areal distribution all suggest that the melt-ejecta layer is the distal part of a semicontinuous ejecta-blanket deposit of mostly shock-melted silicic target material that traveled through the atmosphere within a detached ejecta curtain.

Smectite in the fireball layer formed from more mafic glass derived from the vaporized chondritic bolide. High-nickel magnesioferrite crystals contained in mafic spheroids, high iridium content, and worldwide distribution of the fireball layer all suggest that this upper layer originated from an Earth-girdling cloud of vaporized bolide and entrained target rock materials (including abundant shocked mineral grains) ejected above the atmosphere. The condensing vapor cloud and its entrained target material slowly settled back through the atmosphere by gravitational processes and was deposited as a laminated layer directly on top of the already-emplaced melt-ejecta claystone. This fireball layer is not related to the Manson impact, which has been found to be some 9 m.y. older than K/T.

During subsequent burial and diagenesis, the early-formed metastable halloysite and smectite of the two distinct K/T clay layers reacted to form kaolinite and mixed-layer illite/smectite, respectively. Both the crystal Unity of kaolinite and illitization of smectite varies locally as a function of the degree of diagenetic maturity, probably in response to local temperature variations related to maximum depth of burial (burial diagenesis).