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

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.

Outcrop gamma-ray spectrometry: Applications to the Sinemurian–Pliensbachian organic-rich facies of the Lusitanian Basin (Portugal)

“…6) (see also Parkinson, 1996). Comparing with the S. Pedro de Moel section, the obtained curve is more regular and concordant with a marly sedimentary sequence.…”

“…The transgressive phase corresponds to almost the entire Vale das Fontes Fm (unit 4) as suggested by Duarte et al (2010), and is also observed in the two well logs. At Peniche, Parkinson (1996) suggests two cycles of upwardsincrease in clay content during the Pliensbachian interval, with the most clay-rich sediments being found at the base of the Ibex Zone and at the top of the Margaritatus Zone. A similar pattern is also observed in the GR well logs, even if not so pronounced, with the limit between the two cycles of upwards-increase being found at 2260 and 2615 m of the 14 A-1 and Alj-2 wells respectively.…”

“…In this systems, it is possible to obtain a linear relationship between the total organic carbon (TOC) and U (Lüning and Kolonic, 2003). This technique, when associated with the direct analysis of the rocks (biostratigraphy, sedimentology, petrography, geochemistry) can be very helpful, not only to characterize a potential source rock (e.g., Bodin et al, 2011;Lüning et al, 2005;Lüning and Kolonic, 2003;Myers and Wignall, 1987), but also as an important tool for basin analysis, particularly in sequence stratigraphy studies (e.g., Catuneanu, 2006;Collins et al, 2006;Pawellek and Aigner, 2003;Parkinson, 1996), lithostratigraphic characterization (e.g., Raddadi et al, 2005;Martinius et al, 2002;Hadley et al, 2000;Bessa and Hesselbo, 1997) and correlation with subsurface data (e.g., Aigner et al, 1995;Leslie et al, 1993).…”

Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK

“…Measurements were always taken at least 50 cm above the beach level and wet areas of cliff were avoided. Furthermore, cliff sections flat in the vertical plane over at least 1 m were chosen whenever possible, avoiding irregularities such as overhangs and recesses, to ensure that the same volume of rock was measured at each reading; such variations in rock volume are the most significant source of error in the collection of spectral gamma-ray data (Parkinson, 1996).…”

Gamma-ray Spectrometry in Geothermal Exploration: State of the Art Techniques

“…The photoelectric effect [34,35] is utilised to detect gamma-rays [36] with many detectors made from material which undergo scintillation; i.e., visible light is produced when struck by gamma-rays (the use of this effect is described further in Section 3). Detectors made from sodium-iodine are typically used in in-situ surveys [20,26,37,38]. Many other materials are used for scintillators such as bismuth germinate; ceasium-iodide detectors may also be used but these have poor resolution.…”