Clastic Polymict Dikes in the “megablock” Sequence of the Icdp-chicxulub Drill Core Yax-1

Introduction: The Yaxcopoil borehole (Yax-1) penetrates below the suevitic units through 600 m of sediments (894.9 m – 1510.9 m). These rocks are most likely of Cretaceous age and are regarded as a displaced “megablock”. The stratified sequence was intruded by suevitic dikes (910 m, 916 m), one impact melt dike (1347 m) and several clastic, polymict dikes. The latter are the focus of this study. We present our preliminary petrographic and structural investigations of the clastic dikes obtained by macroscopical, microscopical and SEM inspection. Occurrences: Polymict clastic dikes occur at a depth interval of 1300 m to 1400 m. They have a finely recrystallized dolomite groundmass. Host rock lithologies of these dikes are interlayered and intergrown dolomite and anhydrite as well as brecciated dolomites. Well developed examples of dikes are present at depths of 1314-1316 m (Figs. 1a, b, d) and 1341 m (Fig. 1c). The dikes at 1314-1316 m are an assemblage of incoherent, steeply dipping (> 70°) dikes with several veins that branch off. Dikes in this part reach thicknesses of up to 5 cm. Bulk rock geochemical analysis reveal an enrichment in SiO2, TiO2, Al2O3 Fe2O3 MgO and K2O with respect to the anhydrite-dolomite host rock and systematic decrease in CaO and SO3 from the outer contact to the center of the dike (Tab. 1). The dike at 1341 m represents a coherent layer with a thickness of about 40 cm and has a similar composition but is more abundant in coarser clasts than the upper dikes (Fig. 1c). One additional thin dike (< 5 mm) with Fig. 1: Sample YAX-1_1341,59 m, thin section b/w photograph, width 1,9 cm..