Early diagenesis of the lower Vectis Formation, Wealden Group Lower Cretaceous, Barremian), Sandown, Isle of Wight

Abstract

The Vectis Formation was deposited in a restricted lagoonal environment that periodically dried up, as indicated by the presence of desiccation surfaces. The fauna indicates that salinities fluctuated significantly during deposition, from fresh to brackish-marine. Pre-burial berthierine-rich clay replaced faecal pellets in the sediment, infilling desiccation cracks, during re-flooding of the lagoon. Concretions formed by early pyrite and apatite cementation during initial burial in lake sediment, with organic debris of fish and wood, acting as nuclei and a trigger for cementation. With subsequent partial or complete exhumation oxidation of the pyrite occurred, prior to cementation by Mn siderite. All further concretion cementation occurred within the oxic to sub-oxic diagenetic zones. Textural relationships indicate that commonly occurring baryte formed after pyrite oxidation and represents the last diagenetic mineral phase. Geological background The Vectis Formation in the Isle of Wight comprises up to 65 m of dark silty clays and subordinate sandstones, and rests with a sharply defined surface upon the pedogenically modified, mottled, slits and clays of the underlying Wessex Formation (White, 1921; Daley and Stewart, 1979; Insole et al., 1998; Radley and Allen, 2012), (Figs. 2-4). Together, the Wessex and Vectis Formations make up the Wealden Group as exposed on the Isle of Wight. The formation is divided into three members; the lowest, the Cowleaze Chine Member (7-10 m), comprises dark silty clays containing bivalves (Filosina) in the lower part. The middle member, the Barnes High Member, is a sandstone (2-7m), which has been interpreted by Stewart (1981) as a delta built into the Vectis lagoon. The uppermost Shepherd's Chine Member (35m) is made up of clays containing thin shelly limestones (Stewart 1981; Radley et al. 1997). The Vectis Formation does not yield age-diagnostic fossils, but magnetostratigraphy (Kerth and Hailwood 1988) can be interpreted as recording the Barremian-Aptian (M0) boundary at the level of the Barnes High Member. The Cowleaze Chine Member is therefore of late Barremian age.

Authigenic berthierine and incipient chloritization in shallowly buried sandstone reservoirs: Key role of the source-to-sink context

“…To form berthierine, specific chemical elements are required such as Fe and Al in a reducing environment (Hornibrook and Longstaffe, 1996). In our case, berthierine appears to have formed from a source material consisting of aluminous and ferric detrital clays (mainly kaolinite, HIMS) and ferruginous-aluminous ooids (Taylor, 1990;Hornibrook and Longstaffe, 1996;Worden and Morad, 2003;Worden and Burley, 2003;Beaufort et al, 2015;Huggett et al, 2018;Worden et al, 2020).…”

“…The crystallization of berthierine has been reproduced in experiments by Harder (1989) from iron hydroxide particles in solution under reducing conditions. In addition, pre-burial berthierine-rich clays have been previously reported in Wealden Group sediments on the Isle of Wight, UK (Huggett et al, 2018) and in the Weald basin, UK (Taylor, 1990). In these cases, berthierine crystallization was interpreted as replacing faecal pellets (Huggett et al, 2018) or from iron oxy-hydroxide ooids and kaolinite formed in soils and transported into the basin.…”

“…In addition, pre-burial berthierine-rich clays have been previously reported in Wealden Group sediments on the Isle of Wight, UK (Huggett et al, 2018) and in the Weald basin, UK (Taylor, 1990). In these cases, berthierine crystallization was interpreted as replacing faecal pellets (Huggett et al, 2018) or from iron oxy-hydroxide ooids and kaolinite formed in soils and transported into the basin. In the present study, we believe the berthierine is unambiguously of diagenetic origin.…”