Gamma-ray spectrometry as a tool for stratigraphical interpretation: examples from the western European Lower Jurassic

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Abstract
Portable gamma-ray spectrometry provides an objective and quantitative method of studying sedimentary cyclicity in otherwise-cryptic mudrock successions. Spectral gamma-ray data are presented from the Lower Jurassic sections of the Yorkshire and Dorset Coasts (England), from Peniche (Portugal) and from inland exposures in southern Germany, including a recent road-cut. In Yorkshire, where proximal-distal relationships are readily demonstrated from sedimentological evidence, there is good correspondence between more proximal facies and elevated Th/K ratios. This relationship may be extended to the Dorset succession where the sequence-stratigraphical interpretation of Lower Lias mudrocks has been a source of some controversy. Th/K ratio data here suggest a distal, starved, Hettangian-earliest Sinemurian (Blue Lias) and a prograding Sinemurian interval (Black Ven Marls and Shales-with-‘Beef’). Flooding and further progradation in the Early Pliensbachian (Belemnite Marls) is also suggested. These results provide support for common ‘second order’ stratigraphical forcing mechanisms between the Dorset and Yorkshire successions. Data from mainland Europe suggest that there are systematic regional variations in Th/K ratio, upon which local temporal variations are superimposed. These may reflect climatic or regional sediment transport effects.