A small cliff‐bound estuarine environment: Sandyhaven Pill in South Wales

Abstract

Sandyhaven Pill is a ‘drowned valley’ type of estuary. Thus the deposits differ from most other described estuarine deposits which are of ‘tidally influenced river’ type. The surface sediments may be divided broadly into wave‐dominated deposits (22% of area), tide‐dominated deposits (65%), deposits related to marginal cliff collapse (12%) and river‐dominated deposits (1%). Further subdivision shows that the subenvironments are nested in a progression up the estuary with trends to finer sediment size, reduced sorting and increased biogenic activity. The latter relates to a marine to estuarine faunal change and a strong relationship between the distributions of biota and depositional subenvironments. Over a 29 day period, reduction in wave height was reflected in wave‐dominated areas by shoreward movement of some subenvironment boundaries and by improved definition of symmetrical ripples. The tidal cycle had only a limited effect on the tide‐dominated sediments. The most reliable indicators of estuary trend are channels and asymmetrical ripples; but coring shows that ripples and other minor structures are rarely preserved. Heavy mineral analysis indicates that most of the estuarine sand came from offshore. Gradual sediment build‐up will result in a regressive sequence. If this were preserved under a later transgression, the resultant deposit would be an elongate sediment body bounded laterally by a coarse marginal facies. The sediment sequence would be inward and upward fining with diachronous facies boundaries sloping upwards towards the offshore end and towards the axis of the body.

Sedimentation in a river dominated estuary

“…The overlying material accumulated in a lagoonal setting, marginal to the main estuary channel. This former lagoon is infilled by silts and clays in its lowest part but passes upwards into more frequent horizons of coarse grained sand and gravel (King, 1962;King & Maud, 1964). Some of the lagoonal fill under Durban is pre-Holocene while that under the modern estuary is entirely Holocene, earlier deposits having been scoured from the narrow bedrock channel.…”

“…Sedimentary models of transitional fluvio-marine environments, other than deltas, are poorly developed (Nichols & Biggs, 1985). This is especially true of estuaries, where slow progress in the synthesis of sedimentary models stems largely from the limited number of descriptions of estuarine sedimentation from a restricted number of environmental settings (King, 1980). Present sedimentary models of estuaries 980 J .…”

Inter‐ and supratidal sedimentology of a fjord‐head estuary, south‐western British Columbia

“…Furthermore, these dynamic systems are not accurately described by generalized spatially static models because of changes in the relative influence of river and tides on sedimentation along the axis of the estuary. Despite this, few papers have attempted to characterize the sedimentology of fluvial-marine transitions within contemporary estuaries (King, 1980), and sedimentary models of these transitional environments are poorly developed (Nichols & Biggs, 1985). This paper presents results of an examination of the inter-and supratidal deposits of the Squamish River estuary in south-western British Columbia, Canada.…”

Sedimentary environments and facies of the subtropical mgeni estuary, Southeast Africa

“…Few descriptions of modern estuarine deposits exist and those available are mainly restricted to temperate latitudes (Evans 1965;Allen 1971;King 1980). A few reports of sediments in tropical areas have-also been published (Tucker 1973;Barusseau et al 1985) but sedimentation in subtropical areas remains poorly known.…”