WESTPHALIAN-B MARINE BANDS AND THEIR SUBSURFACE RECOGNITION USING GAMMA-RAY SPECTROMETRY

Palaeogeographic reconstructions, marine band deposition and the varia tions of uranium enrichment in sediments enable marine bands to be div ided into four types: (1) Namurian marine bands, which represent marin e anoxic black shale events, with thick ammonoid acme phases that conc entrate uranium; (2) Vanderbeckei marine bands have thin ammonoid acme zones and abundant benthos, resulting from shallower water depths and poorly developed anoxic events with negligible uranium enrichment; (3 ) Westphalian B/C marine bands are more marginal and have an abundance of land-derived plant fragments with adsorbed uranium, as well as ura nium entrapped within phosphatic tests of Lingula; (4) brackish water Lingula beds with abundant terrigenous matter and negligible uranium r esponse. This classification scheme provides a means of predicting the uranium response of individual marine bands which is attributed prima rily to the type and amount of organic matter, and the salinity of the waters responsible for deposition. This approach allows marine bands to be recognized and identified in the subsurface from their gamma ray and spectral gamma response, and together with palynological analysis allows the marine band to be placed more accurately within the region al stratigraphic framework.