GEOL 340 Sedimentology and Stratigraphy
Lecture 39
Case Study: KT Impact, Belize and Mexico
Pope, K.O., Ocampo, A.C., Fischer, A.G., Alvarez, W., Fouke, B.W., Webster, C.L., Vega, F.J., Smit, J., Fritsche, E., and Claeys, P., 1999, Chicxulub impact ejecta from Albion Island, Belize: Earth and Planetary Science Letters, v. 170, p. 351-364.
Fouke, B.W., Zerkle, A.L., And Alvarez, W., in press, Paleoenvironmental reconstruction of the KT impact event from proximal sedimentary ejecta, Albion Island, Belize. Sedimentology.
Abstract from Fouke et al. (2002)
The depositional and diagenetic history of Cretaceous-Tertiary
(KT) impact ejecta deposited 360 km from the Chicxulub crater
at Albion Island, Belize, has been investigated using integrated
cathodoluminescence and isotopic analyses. A quarry exposes 26
m of Late Cretaceous Barton Creek Formation dolomitized marine
limestone overlain by 16 m of dolomitized Albion Formation impact
ejecta. The Albion Formation consists of a lower fine-grained
~1-m thick Spheroid Bed and an upper 15-m thick coarse conglomeratic
Diamictite Bed. A 14 event paragenetic sequence has been documented
and utilized as a temporal framework to interpret chemostratigraphic
trends in bulk rock d18O, d13C, and 87Sr/86Sr.
The uppermost surface of the Barton Creek Formation was subaerially
exposed prior to the KT impact, as indicated by a brecciated paleosol
that caps up-section decreases in d13C and d18O. Small 1-cm diameter
spheroids in the Spheroid Bed exhibit vermicular crystalline textures
but lack the concentric zonations common to accretionary lapilli.
These spheroids are hypothesized to have originally been impact
glass or reactive Ca and Mg oxide dusts that adhered via water
vapor condensation in the cooling impact vapor cloud and were
dolomitized soon after deposition. The earliest event of dolomitization
in the matrix sediments of the Albion Formation were also post-depositional,
replacing clays formed by devitrification of impact glass. Dolomite
and clay 87Sr/86Sr exhibit a distinct symmetrical distribution
in the Spheroid Bed that range from 0.707745 to 0.707872. Although
unproven, this may represent primary changes in the chemical composition
of the impact glass. The limestone clasts in the Diamictite Bed
were dolomitized prior to the KT impact and exhibit up-section
decreases in bulk rock 87Sr/86Sr. This suggests that the clasts
were excavated from strata equivalent in age or older than the
Barton Creek Formation at locations closer to, or in, the Chicxulub
crater.