Development of lithic-breccias in the 1982 pyroclastic flow deposits of El Chichَn Volcano, Mexico

Pyroclastic flow deposit F1 (volume 0.02 km3) produced during Phase III of the 1982 eruption of El Chichَn Volcano, Mexico, contains basal lithic breccias. The breccia layers are well exposed in El Platanar gully between 2 and 4 km east of the volcano crater. The lithic breccias are inversely graded as defined by lithics, dense juvenile blocks, and pumice supported in a coarse sandy matrix composed of the same constituents. The contact between the main body of the pyroclastic flow deposit and the lithic breccias is generally sharp and planar but not erosive. In some outcrops it is gradational, and is only shown by an alignment of lithic clasts. The origin of these beds is interpreted to be due to a hydraulic jump in the moving pyroclastic flow formed after a pronounced slope break (from 11° to 3°), at the site where the flow began to be funnelled into the El Platanar gully. We have investigated the possible modes of formation of the lithic breccias with analog laboratory experiments. The experiments show that coarse localized segregations could form through a number of mechanisms. The field observations interpreted with the assistance of the laboratory results suggest that pyroclastic flow 1 (which produced deposit F1) moved as a kind of density stratified flow, with a basal lithic-rich zone transporting larger particles and an upper, less-dense zone transporting smaller particles in suspension. At the slope break, flow 1 lost competence and dumped the largest particles, forming a piled-up breccia. Downstream, somewhat smaller lithic particles may have been deposited as ballistics from a low-angle, jetlike structure comprising a hydraulic jump. This deposit thins with distance downstream. Once the deposit was sufficiently thick on its upstream end, particles may have been re-entrained into the jet, to be deposited further along the flow in a low-angle, downstream prograding, bouldery dune. Reverse grading in the dune beds may result from kinetic sieving. We suggest that planar contacts between the lithic breccias and the associated pyroclastic flow deposit may be the result of strong density and velocity gradients between the two parts of the flow.