Volcanic ash and lignite of the Lower Cretaceous Chaswood Formation, central Nova Scotia

Abstract

Lignites and coals, because of their low sedimentation rates of terrigenous detritus, commonly preserve a record of the input of volcanic ash. A reconnaissance study has been made of lignite from the Lower Cretaceous Chaswood Formation of the Elmsvale basin, central Nova Scotia in order to identify whether volcanic ash is present, and if it is, whether it could be used to correlate the poorly dated terrestrial rocks with better dated marine successions offshore. The bulk mineralogy and geochemistry of lignite and lignitic mudstones has been determined by X-ray diffraction (XRD) and whole-rock geochemical analysis of ashed samples; in addition, selected samples have been examined by electron microprobe (EMP) and scanning electron microscope (SEM). Lignites from the Chaswood Formation have several features indicating the presence of volcanic ash: unusual abundance of high-field-strength elements (HFSE) such as Nb, Ta and Hf; the presence of augite in EMP and XRD analyses; the rare presence of euhedral quartz; and aluminophosphate-sulphate minerals that appear to pseudomorph volcanic ash. Wood or charcoal fragments appear mineralized and diagenetic talc is present. Much of the terrigenous component of the lignites consists of background detrital sediments (characterized by detrital illite) and most of any ash component has been altered to kaolinite. Bulk composition of ash is inferred to range from basaltic to rhyolitic. Ash beds are present in both the lower and the middle member of the Chaswood Formation, but our sampling density was insufficient to correlate single beds from one borehole to another. Several ash beds may be present in a single lignite unit. The closest sources of ash are the early Albian volcanic rocks within the lower Cree Member of the Orpheus graben, in a stratigraphic interval that has been previously correlated with the middle member of the Chaswood Formation, and Berriasian to Barremian volcanic rocks on the SW Grand Banks and Fogo Seamounts. These rocks show similar abundances of HFSE to the Chaswood Formation lignites. Distinctive REE patterns with low La/Nd ratios are found in many lignites from the middle member of the Chaswood Formation. They likely result from concentration in organic matter of relatively unfractionated REE in ash of similar composition to Albian volcanic rocks to the southeast.