Provenance evolution of the English Lower Cretaceous Weald Basin and implications for palaeogeography of the northwest European massifs: constraints from heavy mineral assemblages

摘要

The first comprehensive provenance studies of the English Lower Cretaceous facies based on detailed analyses of heavy minerals is presented from 11 locations across the Weald, southeast England. These samples were sub-jected to QEMSCAN? analysis which provided detailed comprehensive datasets on the heavy minerals within the sediments. Four heavy mineral assemblages were identified in the Hastings Beds: rutile, olivine, tourmaline and apatite dominated assemblages in decreasing order. The Weald Clay Formation is also dominated by four heavy mineral assemblages: apatite, rutile, tourmaline and pyroxene-garnet-epidote dominated assemblages in decreasing order. The high concentrations of rutile, tourmaline and zircon in these samples show that bulk of the sediments were recycled from older sediments within the provenance and show a high degree of reworking of sediments into the basin. These provenances appeared to be primarily granitic with metamorphic interruptions at different times in the basin. Heavy minerals indicate little or no secondary modifications to the sediments. Drastic changes in the values of ZTR index suggested repeated retreat and advance of these provenances. Pearson correlation coefficients revealed that four distinct provenances (Londinia, Armorica, Cornubia and Boreal) supplied varying quantities of sediments into the basin at different times. The evolution of these four prove-nances revealed the complex nature of the palaeogeography of these massifs which had significant implications on other north-western European basins. The wider implication of this study shows that QEMSCAN? method can be successfully and effectively used for analysing heavy minerals for provenance studies when traditional petrographic/heavy mineral methods are not available or unreliable.