Buried salt-marsh edges and tide-level cycles in the mid-Holocene of the Caldicot Level (Gwent), South Wales, UK

Abstract

Combining stratigraphical, sedimentological and biofacial (foraminifera) approaches, a high-resolution, time-constrained study has been made of a mid-Holocene ( c. 5800–4900 14C yrs BP) estuarine succession ( c. 2 m) continuously exposed for 1.75 km between two major, late-Holocene palaeochannels. The succession displays great lateral variation in bed thickness and facies; bounded by two laterally extensive peats, it consists of silts split by an impersistent, thin peat. A depositional hiatus gradually rises southwestward through the silts from the top of the lower extensive peat to the base of the higher bed, dividing the succession into two depositional sequences. Sequence I, restricted to the southwestern part of the section, ranges from the lower peat to the sharp top of a thin, laterally impersistent, silty peat (cockle-bed peat). It presents vertically symmetrical patterns of texture and environment, broadly from organic marsh and high salt marsh to middle marsh and back to high marsh. Sequence II, overlying the depositional break, is best developed in the northeastern and central parts of the section, beginning in the latter with an erosively based silt (cockle bed) with a life-assemblage of Cardium edule, Hydrobia ventrosa and ostracods. Environmentally, Sequence II is vertically symmetrical only in the northeastern section, where a high salt marsh was succeeded by a low/middle marsh followed by a high marsh overlain by an organic marsh. Asymmetrical patterns of texture and environment prevail in the central section, where the cockle bed appears. Through annually banded deposits that progressively coarsen, a mudflat or low marsh gave way upward to a middle marsh and finally to high and then organic marshes. Sequence I registers a rise of tide level lasting several hundred radiocarbon years. It appears to represent an estuarine marsh with a depositional edge which overlooked to the northeast a largely exposed shelf formed by the lower extensive peat. Sequence II arose much more rapidly, during a further tide-level rise that lasted only 100–200 years. A new marsh formed in the northeastern part of the section, but a wide, brackish, rapidly infilling embayment lay between this marsh and the previous one to the southwest. Differential autocompaction strongly influenced the rate of accumulation of both sequences, creating bed thickness variations of twofold or more. The mid-Holocene estuary experienced subtle, local geographical changes in addition to the gross changes indicated by the silt-peat alternation.