Cap structures as diagnostic indicators of silcrete origin

Abstract

pedogenic silcretes indicate that this may no longer be appropriate. This paper presents 17 the first systematic investigation of the micro-fabric, geochemistry and mineralogy of cap 18 structures in groundwater silcretes, through an analysis of conglomeratic varieties 19 (puddingstones) from the southern UK. Our results suggest that cap structures in 20 groundwater silcretes fall within a spectrum of types, related to the degree of sorting in the 21 inter-gravel host sediment. At one end of this spectrum are well-defined caps within 22 otherwise well-sorted, overgrowth-dominated silcretes. These caps exhibit a grain-23 supported fabric, are cemented by micro-and/or cryptocrystalline silica, and contain 24 floating silt-sized quartz and Ti-oxide grains. We propose that these structures developed 25 mainly as a result of in-washing of fine sediments that were subsequently silicified. At the 26 other end of the spectrum are silcretes with caps defined by concentrations of Ti-oxide 27 grains, as opposed to cement type and grain size. These formed mainly as a result of the 28 remobilisation and precipitation of Ti during the silicification of gravels containing interstitial 29 clay-rich sandy sediment. Between these end-members are silcretes with cap structures 30 formed by a combination of in-washing and redistribution of fines plus some local 31 remobilisation of Ti. Overall, the cap structures in this study exhibit a simple 32 micromorphology, lacking the alternating Ti-and silica-rich lamellae typical of pedogenic 33 silcrete. We conclude that the presence of cap structures alone should not be considered 34 diagnostic of pedogenic silicification unless accompanied by other indicators such as a 35 differentiated profile and abundant, complex, way-up structures within the micro-fabric. 36 37

Distinguishing pedogenic and non-pedogenic silcretes in the landscape and geological record

“…Geopetal features are uncommon but can occur (Callender, 1978). Unlaminated titaniferous caps, for example, have been documented on clasts within groundwater silcretes in eastern Australia (Taylor and Ruxton, 1987) and in sarsens and puddingstones from the southern UK (Ullyott et al, 2004;Ullyott and Nash, 2006;Ullyott et al, 2015) (Fig. 5d).…”

“…5d). Colloform structures with cuspate titania-rich laminae have been documented in groundwater silcretes from Australia (Taylor and Ruxton, 1987;Webb and Golding, 1998), but are rare; poorly developed colloform structures are also known from the UK (Ullyott et al, 2015) (Fig. 5c).…”

“…5d), so cannot be considered diagnostic of pedogenic silicification. Instead, Ullyott et al (2015) suggest that non-pedogenic silcrete caps may be distinguished from their pedogenic equivalents by their simpler form, which typically lacks laminated colloform features.…”

“…Colloform structures, often consisting of alternating laminae of silica and titania, are also described in nonpedogenic silcretes (Taylor and Ruxton, 1987;Webb and Golding, 1998;Ullyott et al, 2015), although such features appear far less abundant and well developed than in pedogenic silcretes. Finally, microscale glaebules or ped-like structures may also be found in non-pedogenic silcretes.…”

“…Summerfield and Goudie, 1980;Jones, 1999;Lovell, 2015) have inferred, by analogy with southern African and Australian pedogenic silcretes, that they formed as part of an extensive stable palaeosurface under tropical or sub-tropical conditions. However, other studies (Ullyott et al, , 2004(Ullyott et al, , 2015Ullyott and Nash, 2006) indicate that the majority of sarsens and puddingstones are non-pedogenic in origin, and hence could have formed in different palaeoclimatic contexts. Further, detailed studies of sarsen boulders and debris in plough soil, older buildings and in archaeological contexts suggest that, at least for some areas (e.g.…”