CARBONATE EJECTA SPHERULES IN CRETACEOUS-TERTIARY BOUNDARY DEPOSITS, BRAZOS RIVER, TEXAS

Small carbonate spherules are abundant in K-T boundary sediments in marine sites across southern and eastern North America and in the Caribbean. While some of these are altered remains of silicate glass spherules, the standard interpretation, most appear to be ejecta spherules formed from carbonate source rock. The dominant spherule type in Brazos River deposits (and other examined sites) is less than 1 mm in diameter, pure white in color, composed of microspar calcite, and shows weak accretionary banding. These are small equivalents of dolomite/calcite spheroids in the Spheroid Bed at Albion Island, Belize and in southern Mexico. Internal banding and probable vesicles indicates formation from melt or by accretion of smaller particles. These are different from silicate glass spherules replaced by coarse crystalline calcite+/-clays, which are oblong to spherical, up to a few mm in diameter, and contain large bubble vesicles. A primary impact origin for the microspar spherules has been shown to be possible by a pathway where carbonate converted to oxide undergoes immediate chemical back-reactions, yielding calcium carbonate with a microspar fabric. Thus, microspar fabric is an expected fabric for carbonate of impact origin. The form of Brazos microspar spherules indicates these are not carbonate rock fragments, another common interpretation. This textural evidence further supports the concept that large amounts of carbonate was released as particles, limiting the amount of carbon dioxide gas released to the atmosphere. It also lends support to the idea that carbonatite melts were produced during impact. The small size of the spherules and the vesicular nature of both silicate and carbonate spherules is inconsistent with a ballistic, tektite model of transport of the particles, suggesting the main means of dispersal and transport was by large, turbulent ash clouds.