Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar

Abstract

66The term 'Budleighensis' river was first proposed to account for the deposition of thick, regionally-67 significant, fine to medium-grained, red-bed sandstone-dominated facies of Triassic (Olenekian - 68Anisian) age across south, central and north-east England (Wills 1970;Audley-Charles 1970). 69Interpreted to flow from south to north, and with deposits apparently related to this system 97The sandstones targeted in this study form a regionally important aquifer and a proven reservoir 98 for hydrocarbons in a number of sedimentary basins in the area, including the Wessex and East 99Irish Sea basins (McKie et al. 2007;Meadows and Beach 1993). Provenance analysis of the 100 drainage system supplying these sands is important because its scale is relatively poorly 101 constrained (<200 km or >500 km?) and it is uncertain as to how far it extends northward, 119In petrographic terms, the nature of these sandstones poses some additional questions. It is 128The Pb-in-K-feldspar provenance tool is particularly applicable to addressing some of the above 149One of the major advantages of the Pb K-feldspar provenance tool is that, in contrast to 150 provenance approaches that utilise signals in robust grains (e.g. U-Pb zircon), it provides a 151 means of assessing first-cycle sand-grain provenance. Furthermore, where it occurs as a 152 significant framework component (~20% modal abundance), K-feldspar must presumably be 153 more representative of the source area than relatively minor components such as zircon which 154 typically make up << 1% of the mode. In these circumstances, K-feldspar can be a proxy for the 155 source of a large portion of the detrital quartz as there is a reasonable probability both minerals 156 are derived from the same source/s. As fresh detrital K-feldspar is unlikely to survive more than 157 one sedimentary cycle, these grains can be tracked back directly to their basement source, 158allowing the scale and geometry of the drainage system to be constrained. These types of 159 insights can improve prediction of reservoir sandstone distribution and quality in the subsurface. 160Importantly, there is much published Pb data from potential basement source areas in the North 161Atlantic Region with which detrital data can be compared. In this study, additional basement data 162 were collected to provide further constraints on possible sources. 164This study is specifically focussed on Middle Triassic sandstones. Although there are 195Backscatter electron and cathodoluminescence imaging: 196After initial petrographic assessment, sections of ~300μm thickness were prepared from which K- 212Two different collector configurations were used during analysis. An ion counter collector 213 configuration was implemented at MUN, allowing Pb analysis from ~30 μm laser ablation pits, 214 which is only slightly larger than spot sizes used to collect Pb isotopic data using secondary ion 219Repeat analyses of the same grains replicated the initial data within error (Table 1). 7 221The analytical technique is d...

Heavy-mineral, mineral-chemical and zircon-age constraints on the provenance of Triassic sandstones from the Devon coast, southern Britain

“…Variscan granites are widespread in the Armorican Massif of northern France and equivalent parts of the Variscan mountain belt in central Europe (Tischendorf et al, 1995;Brown & Dallmeyer, 1996;Ballèvre, 2009;Tyrrell et al, 2012). These granites are known to have been available as a sediment source as early as the Late Carboniferous, on the basis of the presence of detrital zircons in Westphalian sandstones of the UK (Hallsworth et al, 2000;Morton et al, 2010), and on the basis of evidence for rapid exhumation of such granites in the Variscan mountain belt (Schulmann et al, 2002).…”

“…The scarcity of Late Carboniferous-Early Permian (Variscan) zircons indicates that the Devon coast succession was not supplied by a large river system draining the Massif Central, Brittany, Normandy and possibly the Pyrenees (Tyrrell et al, 2012) (Fig. 8).…”

“…Pb-isotopic data from K-feldspars in the Sherwood Sandstone Group in the Wessex and East Irish Sea Basins show that similar grain populations are present, and that their compositions can be matched with granites of Neoproterozoic-Cambrian (Cadomian) granites and Late Carboniferous-Permian (Variscan) granites in northern France, the French Massif Central, the Pyrenees and the Cornubian area of SW Britain (Tyrrell et al, 2012). The Sherwood Sandstone Group of the Wessex Basin was the subject of some of the earliest heavy-mineral provenance studies ever undertaken in the UK (Thomas, 1902(Thomas, , 1909.…”

“…Apart from the pioneering work on this area in the very early part of the last century (Thomas, 1902(Thomas, , 1909, the only data are general information on heavy-mineral abundances that were used in order to calibrate a chemostratigraphic study of the Otter Sandstone Formation by Svendsen & Hartley (2002), and similar information on a small number of samples collected as part of a regional provenance study by Jeans et al (1993). In view of the importance of this section as a key point in evaluating the role of the 'Budleighensis River' system (Tyrrell et al, 2012), an integrated heavy-mineral study, combining determination of provenance-sensitive heavy-mineral ratios, mineral-chemical data on tourmaline and garnet, and detrital-zircon ages, has been undertaken in order to constrain the provenance of the sandstones and conglomerates in the succession and to provide a framework for comparison with equivalent sandstones in the linked basins to the north.…”

Determination of lead isotope compositions of geological samples using femtosecond laser ablation MC-ICPMS

“…The spot size is between 1-65 μm based on 13 adjustable apertures. The sampling cell is a two-volume chamber that consists of a large, efficient sample chamber (5000 cm 3 ) and a super-high-sensitivity coneshaped sample cell (<0.5 cm 3 , home-made). The instrumental parameters are summarized in Table 1.…”