DEVELOPMENT OF LITHIC-BRECCIAS IN THE 1982 PYROCLASTIC FLOW DEPOSITS OF EL-CHICHON VOLCANO, MEXICO

Pyroclastic flow deposit F1 (volume 0.02 km(3)) produced during Phase III of the 1982 eruption of El Chichon Volcano, Mexico, contains basal Lithic breccias. The breccia layers are well exposed in El Platanar g ully between 2 and 4 km east of the volcano crater. The lithic breccia s are inversely graded as defined by lithics, dense juvenile blocks, a nd pumice supported in a coarse sandy matrix composed of the same cons tituents. The contact between the main body of the pyroclastic flow de posit and the Lithic breccias is generally sharp and planar but not er osive. In some outcrops it is gradational, and is only shown by an ali gnment 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 degrees to 3 degrees), at the site wh ere the flow began to be funnelled into the El Platanar gully. We have investigated the possible modes of formation of the lithic breccias w ith analog laboratory experiments. The experiments show that coarse lo calized segregations could form through a number of mechanisms. The fi eld observations interpreted with the assistance of the laboratory res ults suggest that pyroclastic flow 1 (which produced deposit F1) moved as a kind of density stratified flow, with a basal lithic-rich zone t ransporting larger particles and an upper, less-dense zone transportin g smaller particles in suspension. At the slope break, flow 1 lost com petence 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 hydraul ic jump. This deposit thins with distance downstream. Once the deposit was sufficiently thick on its upstream end, particles may have been r e-entrained into the jet, to be deposited further along the flow in a low-angle, downstream prograding, bouldery dune. Reverse grading in th e dune beds may result from kinetic sieving. We suggest that planar co ntacts between the lithic breccias and the associated pyroclastic flow deposit may be the result of strong density and velocity gradients be tween the two parts of the flow. (C) 1998 Elsevier Science B.V. All ri ghts reserved.