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

Abstract

Silcrete is a type of duricrust formed by the near-surface accumulation of secondary silica within a soil, sediment, rock or weathered material. A variety of models of formation have been put forward, involving silicification in both pedogenic and non-pedogenic settings.The resulting silcrete types differ in terms of their macroscale characteristics, micromorphology, areal extent and behavioural properties. Such differences have significant implications in a range of geological, geomorphological, archaeological and engineering contexts, making the correct identification of silcrete type of considerable importance. This paper reviews the properties of pedogenic, groundwater, drainage-line and pan/lacustrine silcretes, and identifies many characteristics that may be diagnostic. It also discusses a number of more problematic macro-and micro-scale features common to both pedogenic and non-pedogenic silcretes. It concludes with a short checklist to aid the future identification of different silcrete types in the landscape and geological record.

Effect of Deccan lava flows on the sedimentological evolution of Gurmatkal intertrappeans Karnataka, Southern India

“…The abundance of silica veins and the chalcedony as void fills (Figure ) in the upper intertrappean silicified horizon attest downward percolation of silica. These pedogenetic silcretes/calcretes within soil profiles may transect a number of formations on stable basement or basin marginal areas (Callen, ; Milnes & Thiry, ;Thiry, ) and confirm a hiatus or unconformity in the area after the deposition of the upper intertrappean and the lava flow, contrary to the nonpedogenic silcretes (Ullyott & Nash, ). These hiatuses occur typically over a period of 10–10 4 years (Jerram & Widdowson, ).…”

What if none of the Building Stones at Stonehenge Came from Wiltshire?

“…The host deposits have been subject to later geological erosion processes, removing much, if not all, of any un‐silicified sediments to leave cemented boulders and cobbles free. Subsequent periglacial action, and the boulders’ long exploitation by people, result in the discontinuous present‐day distribution (Huggett , 298; Ullyott and Nash , 311–12). The ‘natural’ distribution of sarsen stone in the UK is on the whole an archaeological concept, referring to the geographic availability of the dispersed sarsen fields to people from the Mesolithic onwards.…”

Puddingstones and related silcretes of the Anglo-Paris Basin – an overview

“…Lovell (2016) suggests that silicification occurred beneath the MidLambeth Group land surface (the Mid-Lambeth hiatus), at the time of the PETM. Ullyott and Nash (2016) looked at silcretes not just in the AngloParis Basin but also across the world and show how their structure, both at a macro scale and in thin section, may indicate formation both through pedogenesis at the land surface or by groundwater in the sub-surface. They discuss in detail the four main forms of silcrete: (1) pedogenic, (2) groundwater, (3) drainage-line, and (4) pan/lacustrine.…”

“…Further complication results when composite profiles are formed as a result of more than one period of silicification: most commonly groundwater silcretes form at the base of older pedogenic silcrete. Ullyott and Nash (2016) emphasise that all features, both macroscopic and microscopic should be taken into account when attempting to determine the origin of a silcrete. Non-pedogenic silcrete generally, but not in all cases, lacks the complex macroscopic structure of pedogenic silcrete, while the micromorphological features, principally caps above pebbles, geopetal structures below pebbles, and colloform structures, that were once thought to occur only in pedogenic silcrete are now known to form in sub-surface silcretes (e.g.…”