Post-eruptive mechanical sorting of pyroclastic material — an example from Jamaica

Summary

The unfossiliferous Maestrichtian Summerfield Formation of Central Jamaica is composed entirely of hornblende andesite. A measured section shows a 90 m-thick lower unit of greenish-grey, crystal-rich, pumice-poor sandstones and rare mudstones, which interdigitate with shallow-marine rudist limestone. The upper 370 m comprise pink-coloured, pumice-rich fluviatile sandstones and conglomerates with devitrified ignimbrites at two horizons. Although no lavas are exposed it is likely that the erupted magma is represented by the hornblende andesite boulders of the conglomerates. Whole rock chemical analyses show the lava boulders to have around 65% SiO² the ignimbrites 62%, the shallow-marine crystal-rich sandstones 58% and the interstratified mudstones 69%. The marine sandstones are concentrations of phenocrysts and the mudstones represent the finest glassy ash — originally the liquid which coexisted with the phenocrysts before eruption. These variations illustrate that after leaving the vent the pyroclastic material lost much fine glassy ash during the pyroclastic flow stage to produce crystal-enriched ignimbrites. Where the pyroclastic material was deposited in a shallow- marine environment it underwent further mechanical separation with loss of pumice by flotation, lithics and separation into crystals (sandstones) and glassy ash (mudstones). Subtraction of glass (rnudstone) from the magma (conglomerate boulders) shows that the ignimbrite composition can be obtained by loss of around 40 wt. % of glassy ash. The crystal-rich marine sandstones – although no longer containing the phenocryst minerals in their original magmatic proportions – represent a loss of about 60 wt. % glassy ash.