The K-T boundary interval at Barranco del Gredero, Caravaca, Spain, consists of three units: a light gray Cretaceous marl, a 1- to 3-mm-thick ferruginous clay called the “K-T boundary impact layer” because of its content of shock-metamorphosed quartz grains, and an overlying greenish-gray clay. A conspicuous feature of the impact layer is its content of potassium-feldspar particles called “sanidine spherules” by some authors. The particles consist of a porous mesh-work of prismatic potassium-feldspar crystals clearly of authigenic origin. Their shape, distribution, and texture suggest that they are not altered basaltic melt droplets formed during impact of an extraterrestrial object as advocated by some proponents of the impact-extinction theory of Alvarez. Moreover, authigenic spherules also occur in a thin kaolinitic claystone layer at the K-T boundary in continental sedimentary rocks of the Raton Basin, Colorado and New Mexico. This layer is overlain by another claystone (5–8 mm thick) that records the abrupt appearance of shock-metamorphosed quartz grains in the local stratigraphic section. Four compositional types of spherules (kaolinite, goyazite, layered goyazite/kaolinite, and jarosite) occur in the lower of the two claystone layers. Because the spherules are (1) not in a bed containing shock-metamorphosed minerals, (2) common at some and rare at other localities, and (3) hollow and form peculiarly shaped intergrown masses suggest that they are not altered microtektites or melt droplets.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.