Mineralogy and Petrology of the Cretaceous-Tertiary Boundary Clay Bed and Adjacent Clay-Rich Rocks, Raton Basin, New Mexico and Colorado

Abstract

ABSTRACT The Cretaceous-Tertiary (K-T) is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T occurs at the top of a kaolinitic claystone commonly referred to as the boundary clay layer, in an interval of coal and carbonaceous shale. The is defined by the disappearance of certain fossil-pollen taxa. The clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the clay bed; however, their origin is ncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the cl y bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the clay bed first altered to cabbage-like microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The facto s that produced these unique fabrics in the clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.