Evolution of Holocene sedimentary bentonite in a shallow-marine embayment, Firth of Thames, New Zealand

Abstract

Firth of Thames, northern North Island, New Zealand, is a low-wave energy, shallow (< 30 m depth) marine embayment that has accommodated at least 10 m of mud during the Holocene. Cores from the upper 5 m of this Holocene deposit have radiocarbon ages as old as 5300 yr B.P. and show that the muds consist mainly of silicic volcanic glass, smectite, volcanic rock fragments (ignimbrite, rhyolite, and andesite), and halloysite. A downcore decrease in the abundance of volcanic glass (from 55 to 10 wt% of bulk sediment) is matched by a downcore increase in the abundance of smectite (from a few to 42 wt% of bulk sediment). The smectite has a montmorillonite composition, and exhibits a crenulated, flake-like morphology. The degree of textural development and crystallinity of the smectite increase with sub-bottom depth. Sediments sampled from rivers draining the Hauraki Lowland, a catchment dominated by silicic pyroclastic and volcaniclastic deposits, are compositionally similar to the Firth of Thames marine muds, except that they lack smectite. This, together with the absence of smectite in adjacent shelf sediments and the downcore antipathetic relationship between glass and smectite content, supports an early diagenetic origin for the smectite from glass. The transformation is described by a two-stage model involving a combination of parabolic and linear kinetics, reflecting the hydration of glass and the formation of smectite, respectively. The model indicates the half-life for volcanic glass in the Firth of Thames is only 1500 years. Despite the fact that these smectite-rich muds have not evolved from the alteration of primary pyroclastic deposits, but rather from detrital reworked volcanic glass, we classify them as sedimentary bentonite. We suggest the mode of emplacement of the volcanic material is irrelevant to the name bentonite. Moreover, in contrast to many other studies of bentonite deposits, the Firth of Thames occurrence is different because it is thick (> 5 m), has a young (Holocene) age, and is forming in a shallow-marine setting (< 30 m) from the rapid alteration of glass to smectite, within only a few 1000 years. The abundance of detrital silicic volcanic glass supplied from the Hauraki Lowland catchment, currently up to 65% by weight of total sediment input, determines the abundance of smectite in the Firth of Thames.