Millimeters-size clay spherules are found at the base of the KT clastic bed, in arc of over 2500 km in length from Alabama to Haiti. The spherules are usually rounded but many display elongated dumbbell or tear drop shapes very similar to splash form tektites or microtektites [ Izett, 1991; Kring et al., 1991; Sigurdsson et al., 1991a; Smit et al., 1992; Claeys et al., 1993].
Glass fragments have been found preserved within the cores of the clay spherules at Beloc
in Haiti [ Izett, 1991; Sigurdsson et al., 1991a; Sigurdsson et al., 1991b] and at
two sites in Northeastern Mexico: Mimbral and La Lajilla [ Smit et al., 1992; Bohor
and Betterson, 1993; Claeys et al., 1993]. A few pieces of glass < 50 microns in size
have also been found at the KT boundary in Deep Sea Drilling Project Sites 536 and 540
between Yucatan and Florida [ Alvarez et al., 1992]. The glass particles range in size from
0.1 to 5 mm and can be classified based on their color into black and yellow types. The black
type is roughly andesitic in composition with around 63 wt% SiO
, the yellow glass has a
more unusual composition and contains of up to 28 wt% CaO.
A broad range of evidence supports the impact origin of the KT boundary glass from the Gulf of Mexico region. This evidence far outweight the proposed volcanic and/or in situ diagenetic origin of the glass suggested by Lyons and Officer [1992] and Stinnesbeck et al., [1993]. Some of the most striking lines of evidence are 1) the similarity of the clay spherule shapes, and morphologies and relict glass surface features to those observed on tektites and microtektites; 2) the lack of high temperature microliths in the glass; 3) the presence of schlieren of yellow glass in the black glass indicative of rapid mixing of two parent composition; 4) a range of major and trace elements unlike volcanic glasses; 5) the high CaO content (28 wt%) of the yellow glass which compatible with impact melting of carbonate sediments but extremely uncommon for igneous processes; 6) the low water content (<0.05 wt%) of the glass, at least an order of magnitude lower than that of volcanic glasses but in the range of impact glasses, 7) large variability in oxygen isotope ratios of the Haiti glasses (4.6 per mil) which is too great to be achieved by igneous processes; and 8) the detection (using Re/Os isotopic system) of a small meteoritic component in the Beloc glass [ Izett, 1991; Sigurdsson et al., 1991a; Sigurdsson et al., 1991b; Blum and Chamberlain, 1992; Smit et al., 1992; Blum et al., 1993; Claeys et al., 1993; Koerberl et al., 1994].