Emeishan Basalts, SW China: Reappraisal of the formation's type area stratigraphy and a discussion of its significance as a large igneous province

Abstract

The late Permian Emeishan Basalt Formation of SW China is one of Earth's LIPs (large igneous provinces), yet its basic geology remains poorly documented. Recent work on sections close to the type area in Sichuan Province enable us in part to rectify this. Descriptions of the formation and associated units at two areas, one on the lower flanks of Mt Emei and another from a series of outercrops in Ebian County, 50-70 km to the SW, are presented. The basalt pile is 180-270 m thick and in both areas comprises 12 flows that were erupted in relatively quick succession. It rests conformably upon shallow-marine limestones/lignites suggesting emplacement close to sea level. The upper half of the youngest basalt was intensively weathered, but not eroded, prior to it being conformably succeeded by complex body of rocks c. 30 m thick, that includes thin basalts, pyroclastic rocks, tuffs and organic-rich terrestrial sediments. This unit, which has previously been described as a sedimentary package, presumably because intense weathering has obscured the primary lithological fabric in key outcrops, is considered to mark the volcanic waning phase. Uppermost Permian and Triassic terrestrial sediments conformably overlie the terminal volcanic rocks. The sub-regional stratigraphy is compared, as best it can be, with that described from two sections 400 km to the SE; one section matches reasonably well, the other does not, indicating that regional correlations need to be developed carefully. The information is discussed in the context of LIP generator models; several key features of the Emeishan Basalt terrain are at odds with those commonly encountered in LIP's. The most important conclusion is that the unit marks a prematurely terminated system in which full bloodied rifting leading to the development of an ocean basin never started.