Microaccretionary and Accretionary Carbonate Spherules of the Chicxulub Impact Event from Brazos River, Texas, and Bass River, New Jersey

Accretionary carbonate spherules of small size are a common component of distal Chicxulub impact ejecta deposits and are documented at Brazos River, Texas, and Bass River, New Jersey, located about 1000 km and 2500 km respectively from the crater. This spherule type is also present in northeast Mexico and is related to accretionary carbonate lapilli present in proximal Chicxulub ejecta deposits. Accretionary carbonate spherules are very common throughout the strewn field, but have been confused with altered silicate glass spherules and not recognized as a distinct spherule type having primary carbonate composition. Accretionary carbonate spherules in distal deposits range in size from less than 50 microns up to 2.5 mm in diameter. On broken margins, the spherules show the presence of small darker spots scattered throughout that appear to be areas of higher clay content. Spherules are generally rounded, but many have a subangular, blocky form as a result of post-fallout reworking. Unstained spherules have a snow white color, a result of being composed of minute, light-scattering crystals and have an accretionary, clotted fabric at both micro and mesoscopic scale that distinguishes them from other carbonate grains. The aggregate texture characteristic of these spherules can be recognized in petrographic thin sections and is well displayed in BSE (backscattered electron) images (Figure 1). The smallest aggregates present within spherules are 1020 micron diameter rounded clusters of crystals and composed of micron-sized calcite crystals. There are also aggregates in the 50-70 micron diameter range and more heterogeneous aggregates up to 0.5 mm occur in the larger spherules. The smallest component units are one micron size crystallites of low-Mg calcite and another population of grains consisting of uniform calcite crystal masses of 5-10 microns diameter occurs interspersed among the aggregate masses. The spherules show little zoning apart from an outer rim that is darker in color (in transmitted light), composed of smaller aggradational masses than in the interior and is a little more indurated. Crystals in the outer rim are often slightly different in elemental composition than crystals present in the interior of spherules. Calcite crystals in the outer rim are often part of larger plate units that are aligned parallel with the spherule surface. Small masses of clay ranging in size from 10 micron to 0.5 mm occur interspersed with the calcite aggregates in spherules and are probably areas of altered silicate glass. The smallest (10 micron) clay masses are round and uniformly of clay composition, or contain other mineral components only in submicron particles. Larger clay masses (100 microns or more) are heterogeneous in composition and contain small calcite crystals scattered through the clay at varying concentrations. SEM imagery of Brazos spherules reveals an interior with much porosity, consisting of pore space between the calcite crystals. The crystal units are mostly equidimensional and semi-rhombic in shape, with a range from rhombic to anhedral in form. There are small areas of multicrystal units corresponding to BSE slightly brighter areas in spherules. Pore spaces