ORIGIN AND CLAY-MINERAL GENESIS OF THE CRETACEOUS-TERTIARY BOUNDARY UNIT, WESTERN INTERIOR OF NORTH-AMERICA

A 3-cm-thick, two-layered clay unit that records mineralogic and textu ral evidence of a catastrophic event that occurred at a time now marke d as the end of the Cretaceous Period was preserved in ancient peat-fo rming environments of the Western Interior Basin of North America. The two layers of this unit consist of altered distal ejecta and are easi ly distinguished by their distinctive texture and impact components fr om other clay beds, mainly tonsteins and detrital shales, occurring wi thin the sequence of rocks enclosing the Cretaceous/Tertiary (K/T) bou ndary interval. The lower claystone layer of the K/T boundary unit rep resents melted silicic target rock that has altered mainly to kaolin m inerals. 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 gr ains, and a subdued iridium anomaly. These components and textures, co mbined with the layer's restricted areal distribution, indicate that t his layer, called the ''melt ejecta layer,'' is the distal part of an ejecta blanket deposit. We interpret the melt ejecta layer to be an al tered deposit of mostly impact-derived, shock-melted, silicic target m aterial 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 a bundant shocked minerals whose size and amounts decrease away from the putative crater site in the Caribbean area. High-nickel magnesioferri te crystals, high iridium content, geochemical signature, and worldwid e distribution all suggest this upper layer originated from a cloud of vaporized bolide and entrained target-rock materials ejected above th e atmosphere. The components of this layer, called the ''fireball laye r,'' settled slowly by gravitational processes from an Earth-girdling vapor cloud and were deposited immediately on top of the already-empla ced 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-ric h waters of ancient peat swamps. The fireball layer is mostly altered to smectitic clay from a mafic glass condensed from the vaporized chon dritic bolide, along with some kaolinite formed from blebs of melted s ilicic target material entrained in the vapor plume cloud during eject ion. 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 ''cabbag e-like'' or submicrometer-sized embryonic forms of spherical halloysit e, probably from an allophane precursor. These crystallization charact eristics of the melt ejecta layer are much different than those which formed coarse vermicular aggregates and platy kaolinite crystals in to nsteins from outside the K/T boundary inter-val throughout the Western Interior. The contrast in the incipient formation of dominantly kaoli nitic clay minerals in the basal melt ejecta layer and of smectitic cl ay minerals in the overlying fireball layer reflect silicic versus maf ic starting materials, respectively, and also supports the proposed tw o-phased meteorite impact ejection and dispersal model. During subsequ ent burial and diagenesis of the K/T boundary unit, the metastable hal loysite and smectite aggraded to kaolinite and mixed-layer illite/smec tite, respectively. Both the ordering of kaolinite and illitization of smectite varies locally as a function of the degree of diagenetic gra de or maturity, probably in response to local variations in temperatur e due to maximum burial depth (burial diagenesis).